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CARNEGIEINSTITUTE 
OF  TECHNOLOGY 

LI  Bit  All  Y 


PRESENTED  BY 

i..  Williams 


Electrical 
Engineering  Papers 


By  Benjamin  Q.  Lamme 


This  volume  contains  a  collection  of  the  author's 

more  important  engineering  papers  presented 

before  various  technical  societies  and 

published  in  engineering  journals 

and  elsewhere  from 

time  to  time 


79/9 


Published  by 

Westinghouse  Electric  &  Mfg.  Co. 
East  Pittsburgh,  Pa. 


Permission  to  reprint  these  papers  has  been  granted 
by  the  owners  of  the  copyrights  and  individual 
credit  is  given  in  the  foreword  of  each  papei 


PREFACE 

The  papcis  of  Beniamm  G.  Lamme  have  always  interested  American 
engineers  Distributed  in  many  publications,  some  quite  macessible  to 
readers,  it  is  indeed  a  fortunate  circumstance  which  now  makes  available  a 
collection  of  these  papers  for  engineers,  prcfessors,  instructors,  and  students 
and  all  those  interested  in,  and  able  to  understand,  the  progress  of  electrical 
engineering. 

Besides  his  achievements  in  the  art  of  engineering,  Mr.  Lamme  has  been 
gifted  with  the  faculty  for  clear  expression  and  explanation,  which  is  one  of  the 
rarest  to  be  found  in  the  engineering  profession  The  collection  begins  with 
his  early  paper  on  the  Polyphase  Induction  Motor,  which,  in  its  time,  was  a 
pnmer  of  the  characteristics  and  operation  of  such  motois  in  the  hands  of  the 
numerous  users  of  these  machines.  Then  follows  a  period  in  which  he  prepared 
few  papers,  but  which  was  one  of  great  personal  activity  Then  comes  his 
epoch-making  paper,  in  1902,  before  the  Ameiican  Institute  of  Electrical 
Engineers  in  New  York  on  the  Single-Phase  System  of  the  Washington 
Baltimore  and  Annapolis  Railway.  Up  to  this  time,  the  development  of  the 
electric  railway  systems,  as  a  whole,  was  at  a  point  of  complete  stagnation, 
in  the  utilization  of  600  volts  direct  current,  and  this  paper  represented  the 
first  great  and  successful  attempt  to  break  away  from  established  practice 
toward  materially  higher  trolley  voltages.  Its  advent  gave  an  impulse  to 
the  entire  subject  of  the  electrification  of  railroads  greater  than  any  other  it 
had  ever  received,  leading  to  the  complete  abandonment  of  old  and  apparently 
well  established  standards,  as  well  as  to  later  attempts  to  meet  the  new  con- 
ditions with  higher  direct-current  voltages. 

In  1904,  there  are  two  papers,  one  on  a  10,000  Cycle  Alternator,  and 
another  on  the  Synchronous  Motor  for  Regulation  of  Power  Factor.  In  the 
same  year,  he  contnbuted  a  discussion  to  the  subject  of  Single-Phase  Motors 
which  ranks  as  one  of  the  clearest  and  most  suggestive  descriptions  of  this  type 
of  motor,  which  owes  to  him  its  development  and  use. 

Recognizing  the  importance  of  closer  relations  with  the  American  Insti- 
tute of  Electrical  Engineers,  we  find  him  contributing,  from  time  to  time, 
papers  on  Commutation,  on  the  Homopolar  Dynamo,  on  Rotary  Converters, 
on  Turbo  Generators,  on  Losses  in  Electrical  Machinery,  and  on  Engineering 
Education,  etc.  It  is  safe  to  say  that  these  papers  will  be  read  in  their  present 
form  by  many  who  enjoyed  them  when  they  came  out  originally,  and  their 
contents  will  perhaps  be  more  appreciated  today  than  at  the  time  when  they 
were  written. 

To  all  those  who  have  followed  the  development  of  electrical  engineering 
in  America  during  the  past  thirty  years,  and  to  all  those  who  would  like  to 
know  the  histoncal  development  of  electrical  apparatus,  the  senes  of  papers 
which  appeared  in  the  Electric  Journal,  on  the  History  of  the  Railway  Motor, 
of  the  Direct-Current  Generator  and  of  the  Alternating-Current  Generator. 


and  the  History  of  the  Frequencies,  now  collected  for  the  first  time,  will  form 
most  interesting  reading  Here  Mr,  Lamme  had  an  opportunity  to  recount 
the  work  of  himself  and  of  his  associates,  adding  to  it  the  clarity  and  lucidity 
which  have  always  marked  his  style. 

I  think  those  who  have  known  Mr.  Lamme 's  interest  in  education  and  in 
his  instruction  of  young  engineers  will  be  glad  to  find  reprinted  in  this  volume 
two  of  his  contributions  on  the  subject  of  engineering  education.  The  whole- 
some, sound  sense  which  permeates  these  papers  cannot  fail  to  appeal  tc  all, 
and  to  impress  the  reader  with  the  sound  judgment  of  their  author. 

Although  these  papers  represent  a  work  of  thirty  years,  during  which 
time  Mr.  Lamme  has  been  continuously  associated  with  the  great  company 
which  bears  the  name  of  Mr.  Westinghouse,  yet  I  believe  they  do  not  com- 
plete his  whole  life  work  Those  of  us  who  have  had  the  good  fortune  to  have 
known  him  for  a  score  of  years,  or  more,  well  know  that  many  contributions 
will  yet  be  made  by  him  to  the  art  and  science  of  electrical  engineering  The 
publication  by  the  Westinghouse  Company  of  this  collection  of  Mr.  Lammc's 
engineering  papers  on  the  anniversary  of  his  first  connection  with  this  com- 
pany, thirty  years  ago,  represents  a  most  dignified  appreciation  of  his  services 
to  the  entire  engineering  profession. 


Boston  Mass, 
April  2,  1919. 


CONTENTS 


THE  POLYPHASE^MOTOR .          1 

WASHINGTON,  BALTIMORE  &  ANNAPOLIS  SINGLE-PHASE  RAILWAY 37 

SYNCHRONOUS  MOTORS  FOR  REGULATION  OF  POWER  FACTOR  AND  LINE 

PRESSURE.          .  ..  .  ..        .          S3 

DATA  AND  TESTS  ON  10,000  CYCLE  PER  SECOND  ALTERNATOR  65 

THE  SINGLE-PHASE  COMMUTATOR  TYPE  RAILWAY  MOTOR 77 

COMPARISON  OF  SERIES  AND  REPULSION  TYPE  A.C.  COMMUTATOR  MOTORS  .  .97 
COMPARATIVE  CAPACITIES  OF  ALTERNATORS  FOR  SINGLE  AND  POLYPHASE 

CURRENTS  .  .  .  .  .  ....  Ill 

DAMPERS  ON  LARGE  SINGLE-PHASE  GENERATORS  139 

DEVELOPMENT  OF  A  SUCCESSFUL  DIRECT- CURRENT  2000  Kw  UNIPOLAR 

GENERATOR  145 

COMMUTATING  POLES  IN  SYNCHRONOUS  CONVERTERS    .  .  .     .    171 

THEORY  OF  COMMUTATION  AND  ITS  APPLICATION  TO  COMMUTATING  POLE 

MACHINES   .  201 

PHYSICAL  LIMITATIONS  IN  DIRECT-CURRENT  COMMUTATING  MACHINERY  .  .247 
REGULATION  CHARACTERISTICS  OF  COMMUTATING  POLE  MACHINES  AND 

PARALLEL  OPERATION  WITH  OTHER  MACHINES  303 

HIGH  SPEED  TURBO  ALTERNATORS — DssiGNa  AND  LIMITATIONS 313 

TEMPERATURE  AND  ELECTRICAL  INSULATION 353 

TEMPERATURE  DISTRIBUTION  IN  ELECTRICAL  MACHINERY  . ,          365 

SOME  PRACTICAL  CONSIDERATIONS  IN  ARTIFICIAL  VENTILATION  FOR  ELEC- 
TRICAL MACHINERY.  .  387 

SOME  ELECTRICAL  PROBLEMS  PRACTICALLY  CONSIDERED       393 

SOME  CONTROLLING  CONDITIONS  IN  THE  DESIGN  AND  OPERATION  OF  RO- 
TARY CONVERTERS    441 

SIXTY-CYCLE  ROTARY  CONVERTER 469 

IRON  LOSSES  IN  DIRECT-CURRENT  MACHINES 487 

IRON  COMMUTATORS  ....  513 

POLYPHASE  INDUCTION  MOTOR  WITH  SINGLE-PHASE  SECONDARY 519 

A  PHYSICAL  CONCEPTION  OF  THE  OPERATION  OF  THE  SINGLE- PHASE  IN- 
DUCTION MOTOR  525 

SINGLE-PHASE  LOADS  FROM  POLYPHASE  SYSTEMS -559 

THE  TECHNICAL  STORY  OF  THE  FREQUENCIES 569 

THE  DEVELOPMENT  OF  THE  ALTERNATING- CURRENT  GENERATOR  IN  AM- 
ERICA  591 

THE  DEVELOPMENT  OF  THE  DIRECT-CURRENT  GENERATOR  IN  AMERICA.  . .  645 

THE  DEVELOPMENT  OF  THE  STREET  RAILWAY  MOTOR  IN  AMERICA 721 

TECHNICAL  TRAINING  FOR  ENGINEERS 755 

ENGINEERING  BY  ANALYSIS .          ...765 


THE  POLYPHASE  MOTOR 

FOREWORD— This  paper  was  prepared  in  the  early  part  of  1897, 
or  over  twenty-two  years  ago.  It  was  presented  at  the  twentieth 
convention  of  the  National  Electnc  Light  Association  at  Niagara 
Falls  on  June  10,  1897,  and  was  prepared  for  the  purpose  of 
illustrating  the  characteristics  and  properties  of  the  Westing- 
house  Type  C  motor  which,  at  that  time,  was  beginning  to  at- 
tract much  attention.  This  motor  was  radically  new  in  that 
it  had  a  "cage"  type  secondary  winding  for  large,  as  well  as 
small,  sizes,  whereas,  it  was  generally  believed  that  the  cage 
type  was  only  suitable  for  small  power  machines,  due  to  lack 
of  starting  torque. — (ED.) 


INTRODUCTION 

THE  polyphase  motor  is  usually  treated  from  the  theoretical 
standpoint,  and  the  results  obtained  are  of  interest  mainly  to 
designers  and  investigators.  Such  treatment  has  been  principally 
of  a  mathematical  nature,  the  object  being  to  show  how  the 
various  characteristics  of  the  motor  may  be  predetermined.  In 
the  following  treatment  of  the  subject,  the  general  operation  of  the 
motor  will  be  explained  in  a  non-mathematical  way  by  the  use 
of  diagrams  which  illustrate  its  characteristics  under  different 
conditions.  Only  the  non-synchronous  type  of  motors  will  be 
considered,  and  no  distinction  will  be  made  between  two-and 
three-phase  motors ;  for,  if  properly  designed,  they  are  practically 
alike  in  operation. 

It  is  necessary  to  understand  the  characteristics  of  the  poly- 
phase motor  in  order  to  consider  properly  its  application  to  the 
different  classes  of  work  to  be  met  with  in  practice.  These 
characteristics  can  be  presented  in  the  most  intelligible  manner  by 
means  of  curves,  which  represent  the  relations  between  the  speed, 
torque  or  turning  effort,  horse  power  expended  and  developed, 
amperes,  etc.  The  speed-torque  curve,  which  represents  the 
speed  in  terms  of  the  torque,  is  the  most  important  one,  as  upon 
this  depends  the  adaptability  of  the  motor  to  the  various  kinds  of 
work.  The  starting  conditions  also  depend  upon  the  speed-torque 
characteristics.  The  other  curves  that  are  of  importance  in  prac- 
tice are  the  current,  efficiency  aad  power  factor.  As  these  are 
dependent,  to  some  extent,  upon  the  speed-torque  curve,  this  will 
be  considered  first.  Before  treating  of  its  characteristics  a  short 
description  of  the  motor  itself  will  be  given. 


2  ELECTRICAL  ENGINEERING  PAPERS 

CONSTRUCTION  AND  WINDING 

The  polyphase  motor,  like  a  direct-current  motor,  consists 
primarily  of  two  parts,  one  stationary  and  the  other  rotating,  each 
of  which  carries  windings.  The  inside  bore,  or  face,  of  the  sta- 
tionery part  is  generally  slotted,  and  carries  windings  that  resemble 
those  of  the  rotating  part,  or  armature  of  an  ordinary  direct-current 
motor  without  commutator.  The  rotating  part  is  also  slotted  on 
its  outside  face,  and  there  are  windings  in  the  slots.  Both  cores, 
or  bodies,  are  built  up  of  thin  iron  or  steel  plates.  The  general 
arrangement  is  shown  in  Fig.  1.  One  of  these  windings,  generally 


FIG.  1— ARRANGEMENT  OP  WINDINGS  OF  THE  POLYPHASE  MOTOR 
AND  THE  MAGNETIC  FIELDS  WHICH  ARE  PRODUCED. 


that  on  the  stationary  part,  receives  current  from  a  two  or  three- 
phase  supply  circuit.  The  coils  of  this  winding,  although  dis- 
tributed symmetrically  over  the  entire  face  of  the  core,  are  really 
connected  to  form  distinct  groups  which  overlap  each  other.  These 
windings  form  the  two  or  three  circuits  in  the  motor.  When  al- 
ternating electro-motive  forces  are  applied  to  these  circuits,  currents 
will  flow  which  set  up  magnetic  fields  in  the  motor.  These  alter- 
nating fields  in  turn  generate  electro-motive  forces  in  the  windings. 
Part  of  the  current  flowing  in  the  windings  represents  energy  ex- 
pended usefully,  or  in  heating,  and  part  serves  merely  as  magnetiz- 
ing current.  The  latter,  like  the  magnetizing  current  of  a  direct- 
current  machine,  is  dependent  upon  the  dimensions  of  the  mag- 
netic circuit  and  upon  the  magnetic  density  in  the  various  parts. 
Even  when  the  motor  is  running  with  no  load  the  magnetizing  cur- 
rent is  required. 


THE  POLYPHASE  MOTOR  3 

The  second  part  of  the  motor,  generally  the  rotating  part, 
receives  no  current  from  the  supply  circuit.  The  magnetic  fields 
set  up  by  the  first  set  of  windings  pass  through  the  second  windings, 
and,  tinder  certain  conditions,  generate  electro-motive  forces  in 
them.  If  the  second  windings  are  arranged  to  form  closed  circuits, 
currents  will  flow  in  them.  These  currents  are  entirely  separate 
from  those  of  the  supoly  circuits. 

SPEED  AND  SLIP 

When  running,  the  motor  has  a  maximum  speed  that  is  ap- 
proximately equal  to  the  alternations  of  the  supply  circuit  divided 
by  the  number  of  motor  poles  in  each  circuit.  This  is  the  no-load 
speed.  As  the  motor  is  loaded,  the  speed  falls  off  almost  in  pro- 
portion to  the  load.  The  drop  in  speed  is  sometimes  called  the 


FIG.2— DIAGRAM  OF  TWO-PHASE  ALTERNATING-CURRENT  GEN- 
ERATOR. ROTATING-FIELD  TYPE. 

"slip."  This  is  usually  expressed  in  percent  of  the  maximum 
speed.  If,  for  instance,  a  motor  has  a  maximum  speed  of  1000 
revolutions  and  drops  fifty  revolutions  below  this  at  full  load, 
it  has  then  a  slip  of  five  percent. 

TORQUE  AND  ARMATURE  CURRENT 

With  this  type  of  motor,  a  drop  in  speed  is  necessary  for  de- 
veloping torque.  A  fairly  simple  illustration  of  this  action  may  be 
obtained  by  considering  the  operating  of  an  alternating  current 
generator  tinder  certain  conditions.  We  will  take  a  type  of  alter- 
nator having  a  stationary  armature  and  a  rotatable  field  magnet, 
which  can  be  driven  at  various  speeds.  Leads  are  carried  out  from 
the  armature  to  adjustable  resistances.  To  avoid  complexity,  the 
armature  circuits  and  the  resistances  are  considered  as  non-induc- 
tive. The  field  coils  are  excited  by  direct  current.  Fig.  2  shows 
this  arrangement. 


4  ELECTRICAL  ENGINEERING  PAPERS 

When  the  field  is  rotated  at  a  certain  speed,  with  the  field  coils 
charged,  there  is  an  alternating  electro-motive  force  set  up  in  the 
armature  winding.  When  the  armature  circuit  is  closed  through 
a  resistance  a  current  will  flow  and  the  armature  will  develop 
power.  The  power  developed  by  the  armature  is  slightly  less 
than  the  power  expended  on  the  field  shaft,  which  is  proportional 
to  the  product  of  the  speed  and  the  turning  or  driving  effort — i.  e  , 
torque  on  the  shaft.  Consequently,  at  a  given  speed,  a  driving 
effort  is  required  at  the  field  shaft,  corresponding  to  the  power  de- 
veloped by  the  armature  If  the  armature  current  is  increased  or 
decreased,  the  power  developed  is  increased  or  decreased  also,  and 
the  driving  effort  will  vary  in  proportion. 

Let  the  field  now  be  rotated  at  one-half  the  above  speed.  The 
armature  electro-motive  force  becomes  what  it  was  before  Re- 
ducing the  resistance  in  the  armature  circuit  also  to  one-half,  the 
same  current  as  before  will  flow.  The  power  developed  by  the 
armature  is  now  one-half  and  the  speed  of  the  field  is  one-half,  con- 
sequently, the  driving  effort  or  torque  is  the  same  as  before.  Re- 
ducing the  speed  further,  and  decreasing  the  resistance  in  the  ar- 
mature circuit  in  proportion,  to  keep  the  armature  current  con- 
stant, we  find  the  driving  effort  on  the  field  remains  constant. 
Finally,  if  we  reduce  the  speed  so  much  that  the  external  armature 
resistance  is  all  cut  out,  and  the  armature  is  short  circuited  on  itself 
with  the  same  current  as  before,  the  same  driving  effort  is  still 
required. 

The  field  is  now  rotating  very  slowly,  and  the  alternations  in 
the  armature  are  very  low,  being  just  sufficient  to  generate  the 
electro-motive  force  required  to  drive  the  armature  current  against 
the  resistance  of  the  windings.  Any  further  reduction  in  speed 
will  diminish  the  armature  electro-motive  force,  and  hence  the 
armature  current  must  fall,  the  power  developed  be  dimmivshed 
and  the  driving  effort  also  fall  in  proportion.  An  increase  in 
speed  will  increase  the  armature  current,  and  thus  increase  the 
driving  effort  required. 

If  but  one  armature  circuit  is  closed,  the  power  developed  will 
pulsate  as  the  armature  current  varies,  from  zero  to  a  maximum 
value,  and  the  driving  effort  will  also  vary,  But  if  the  armature 
has  two  or  more  circuits  having  different  phase  relations,  it  may 
develop  power  continuously  and  the  driving  effort  will  then  be 
continuous. 


THE  POLYPHASE  MOTOR  5 

ILLUSTRATION  OF  "SLIP" 

The  armature  has  been  considered  as  stationary  and  develop- 
ing power  while  a  certain  driving  effort  was  applied  to  the  field. 
According  to  the  well  known  law  that  any  force  is  met  by  an  op- 
posing force,  the  armature  must  have  a  certain  resisting  effort. 
The  armature  really  tends  to  rotate  with  the  field,  and  the  resist- 
ing effort  is  exerted  to  prevent  this. 

Assume  the  armature  to  be  arranged  for  rotation,  but  locked, 
in  the  above  operations.  Release  the  armature,  attach  a  brake, 
and  adjust  for  a  torque  equal  to  the  resisting  effort  of  the  armature. 
The  armature  just  remains  stationary.  Speed  up  the  field,  and  the 
armature  will  speed  up  also,  keeping  a  certain  number  of  revolu- 
tions behind  the  field.  This  difference  in  speed  is  that  required 
for  generating  the  electro-motive  force  necessary  for  sending  the 
current  through  the  armature.  The  alternations  in  the  arma- 
ture will  remain  constant  for  a  given  armature  current,  indepen- 
dent of  the  speed  at  which  the  armature  is  running. 

If  the  brake  be  tightened,  the  armature  must  drive  more  cur- 
rent through  its  windings  to  develop  the  required  effort,  the 
armature  alternations  must  hence  increase,  and  the  armature  will 
therefore  lag  behind  its  field  more  than  before,  or  the  "slip"  is 
increased.  If  the  brake  be  loosened,  the  armature  will*  run 
nearer  the  speed  of  the  field.  If  the  field  be  driven  at  a  constant 
speed  and  the  brake  be  released,  the  armature  will  run  at  practic- 
ally the  same  speed  as  the  field. 

If  the  winding  consist  of  but  one  closed  circuit,  the  torque 
developed  by  the  armature  varies  periodically,  and  that  developed 
by  the  brake  will  vary  also,  but  to  a  less  extent,  as  it  is  steadied 
by  the  inertia  of  the  rotating  armature.  But  with  two  or  more 
circuits  having  different  phase  relations,  arranged  for  constant 
power  developed  in  the  armature  windings,  the  torque  developed 
is  also  constant  at  all  times.  Consequently,  for  constant  torque 
at  the  brake,  there  should  be  two  or  more  phases  in  the  armature 
windings. 

DIFFERENCE  BETWEEN  ILLUSTRATION  AND  ACTUAL  CASE 

This  explanation  of  the  development  of  torque  in  the  short- 
circuited  armature  is  merely  an  attempt  to  illustrate  certain  of  the 
actions  in  the  polyphase  motor  armature  by  a  comparison  with  the 
operations  of  other  apparatus,  that  is,  in  general,  much  better 
understood.  We  cannot  infer,  from  the  above  illustration,  that  an 
alternating-current  generator  would  run  as  a  motor  tinder"  the 


ELECTRICAL  ENGINEERING  PAPERS 


assumed  conditions,  for,  in  the  above  operations,  mechanical 
power  is  supplied  to  the  field  shaft,  and  mechanical  power  is  de- 
livered by  the  rotating  armature  to  the  brake.  There  is  no  true 
electro-motor  action;  that  is,  there  is  no  transformation  of  elec- 
trical power  supplied  to  mechanical  power  developed. 


No   I 

Circuit  1  at  Maximum 

Current 
Circuit  2  at  Zero  Current 


Circuit  1  Decreasing 

Current 
Circuit  2  with  Increasing  Current 


Circuit  l  at  Zero 

Current 
Circuit  2  at  Maximum  Current 


No  4 


Circuit  1  Increasing  m 

Reverse  Direction 
Circuit  2  Decreasing 


HP'S 

Circuit  1  at  Maximum  In 
Reverse  Direction 
Circuit  2  at  Zero 


Circuit  1  Decreasing 

Circuit  2  Increasing  in  Reverse 

Direction 


PIG   3— DIAGRAM  SHOWING  PRODUCTION  OF  ROTATING  MAGNETIC 
FIELD  BY  TWO-PHASE  CURRENTS 

The  action  of  the  short-circuited  armature  of  the  above  gen- 
erator and  that  of  the  polyphase  motor  are  very  similar  in  regard 
to  drop  in  speed  for  developing  torque  But  in  the  polyphase 
motor,  instead  of  the  mechanically  rotated  field  magnet,  there  is 
a  stationary  core  provided  with  two  or  more  windings  which  cany 
currents  having  different  phase  relation.  These  windings  are 
placed  progressively  around  the  core,  either  overlapping  or  on 
separate  poles.  When  the  currents  flow  in  the  windings,  resultant 
magnetic  poles  or  fields  are  formed,  which  are  progressively  shift- 
ing around  the  axis  of  the  motor.  The  closed  or  short-circuited 
armature,  rotating  in  this  field,  develops  torque  by  dropping  in 


THE  POLYPHASE  MOTOR  7 

speed,  in  the  same  way  that  It  developed  torque  with  mechanically 
rotated  field  magnets.  But  electrical  power,  instead  of  mechanical, 
is  now  supplied  to  produce  the  shifting  of  rotating  field,  and  the 
conversion  from  electrical  power  supplied  to  the  field  windings,  to 
mechanical  power  developed  by  the  armature  shaft  is  a  trans- 
former action  which  does  not  appear  in  the  above  illustration. 

ROTATING  MAGNETIC  FIELD  ELECTRICALLY  PRODUCED 

Fig.  3  shows  diagrammatically  a  progressively  shifting  field, 
with  two  overlapping  windings  arranged  for  two-phase  currents. 
Coils  1-1,  etc.,  form  one  circuit,  while  coils  2-2,  etc.,  form  the  other. 
Starting  with  the  instant  when  the  current  in  1  is  at  its  maximum 
value,  the  magnetizing  force  of  this  set  of  coils  must  be  at  its  maxi- 
mum. The  current  and  magnetizing  force  of  circuit  2  are  at  zero 
value.  Four  poles  or  magnetic  fields,  alternating  N-S-N-S  around 
the  core,  are  formed  directly  over  coils  1.  As  the  current  in  one 
begins  to  decrease,  that  in  2  rises.  We  then  have  the  combined 
magnetizing  forces  of  the  two  overlapping  windings.  These  two 
magnetizing  forces  act  together  at  some  points  and  oppose  at 
others.  The  resultant  magnetic  field  shifts  to  one  side  of  the 
former  position.  As  the  current  in  1  gradually  falls  to  zero  and 
2  rises  to  its  maximum  value  the  magnetic  field  shifts  around  until 
it  is  directly  over  coils  2.  If  the  current  in  1  should  next  increase 
in  the  same  direction  as  before,  while  2  diminished,  the  magnetic 
poles  would  shift  back  again  to  their  former  position.  But  the 
current  in  1,  after  reaching  zero  value,  rises  in  the  opposite  direc- 
tion, while  that  in  2  falls.  This  shifts  the  resultant  poles  forward 
instead  of  backward,  and  they  gradually  shift  ahead  until  they 
are  again  directly  over  coils  1.  But  the  "N"  poles  have  shifted 
around  until  they  now  occupy  the  former  position  of  the  "s" 
poles.  Thus,  with  the  current  in  1  passing  from  a  maximum  in 
one  direction  to  a  maximum  in  the  opposite,  the  poles  have  shifted 
forward  the  width  of  one  polar  space.  Current  in  2  next  rises  in  a 
reversed  direction  and  the  poles  shift  forward  until,  when  the  cur- 
rent in  2  is  a  maximum,  they  are  over  coils  2. 

In  the  diagrams,  Nos.  1,  2,  3,  etc  ,  show  the  positions  of  the 
shifting  field  under  certain  conditions  of  current  in  the  two  circuits. 
In  No.  2,  the  position  shown  is  an  arbitrary  one,  for  it  depends  upon 
the  relative  values  of  the  currents  in  the  two  circuits.  With  the 
two  currents  equal,  the  position  of  the  line  N-N  would  be  half-way 
between  coils  1  and  2. 


8  ELECTRICAL  ENGINEERING  PAPERS 

These  diagrams  show  that  the  magnetic  field  due  to  two-phase 
currents  in  properly  arranged  windings  shifts  progressively  around 
the  axis,  just  as  if  the  field  were  rotated  mechanically. 

SPEED-TORQUE  CURVE 

In  polyphase  motors,  the  part  that  resembles  the  field  in  the 
above  description  and  which  receives  the  current  from  the  line,  is 
usually  called  the  primary,  on  account  of  its  electrical  resemblance 
to  the  primary  of  a  transformer.  The  equivalent  of  the  armature 
in  the  preceding  description  is  called  the  secondary.  If  the  alter- 
nations of  the  supply  circuit  are  constant,  the  reversals  of  the  cur- 
rents in  the  field  or  primary  will  occur  at  a  uniform  rate  and  the 
magnetic  field  will  shift  around  its  center  at  a  definite  speed,  de- 
pending upon  the  rate  of  alternation  of  the  supply  circuit  and  the 
number  of  poles  in  each  circuit  of  the  motor.  If  the  armature  or 
secondary  rotates  at  the  same  speed  as  the  field  shifts,  there  will  be 
no  reversals  or  alternations  in  its  magnetism,  and  there  will  be  no 
currents  and  consequently,  no  torque.  If  a  load  is  thrown  on,  the 
speed  will  drop  and  the  resultant  alternations  in  the  secondary 
will  generate  electro-motive  forces  which  will  drive  currents 
through  the  windings,  and  thus  develop  torque.  The  speed  will 
continue  to  fall,  and  the  secondary  electro-motive  forces  will  con- 
tinue to  increase  until  a  torque  sufficient  for  the  load  is  developed. 


TORQUE 

PIG.  4— SPEED  TORQUE  OF  POLYPHASE  MOTOR 

Increasing  the  load  on  the  motor,  the  speed  should  fall  and  the 
torque  increase  until  zero  speed  is  reached.  The  speed-torque 
curve  would  then  be  of  the  form  shown  in  Pig.  4,  curve  "A."  But 
the  shape  of  this  curve  is  modified  to  a  great  extent  in  actual 
motors  by  certain  effects  which  cannot  be  entirely  eliminated. 

PRIMARY  RESISTANCE  REDUCES  MAGNETIZATION  AT  HEAVY  LOADS 
In  the  case  of  the  revolving  field,  the  magnetization  was  sup- 
posed to  remain  constant  under  different  conditions.     But  in  the 
motor  primary,  the  magnetism  of  the  primary  is  not  constant 
under  all  conditions  and  it  does  not  all  pass  through  the  secondary 


THE  POLYPHASE  MOTOR  9 

circuits.  y  The  primary  windings  necessarily  have  some  resistance, 
and  a  certain  electro-motive  force  is  required  to  drive  the  primary 
current  through  the  windings.  With  a  constant  applied  electro- 
motive force,  the  primary  counter-electro-motive  force  will  dimin- 
ish as  the  drop  in  primary  resistance  increases,  and  the  magnetic 
field  required  will  diminish  also.  Consequently,  to  develop  the 
required  secondary  electro-motive  force  for  driving  the  secondary 
current  through  the  windings  the  speed  must  drop  more  than 
shown  by  curve  "A"  in  Pig  4.  This  gives  a  speed-torque  curve 
as  shown  by  curve  "B,"  in  Fig.  4.  Instead  of  being  a  straight  line 
it  is  somewhat  curved. 

MAGNETIC  LEAKAGE  LIMITS  MAXIMUM  TORQUE 

But  there  is  a  still  more  important  effect  in  the  motor.  The 
primary  and  secondary  currents,  and  their  consequent  magnetizing 
forces,  are  opposed  to  each  other.  The  result  is  that  part  of  the  pri- 
mary magnetism  threads  across  between  the  primary  and  second- 
ary windings  without  passing  into  the  secondary.  Thus,  the 
electro-motive  force  of  the  secondary  is  reduced,  or,  for  a  required 
secondary  electro-motive  force,  the  secondary  alternations  must  be 
increased.  This  means  a  further  drop  in  speed. 

The  secondary  currents  also  tend  to  form  local  magnetic  fields 
around  their  own  coils.  These  local  fields  are  alternating  and  set  up 
electro-motive  forces  in  the  secondary  circuits.  In  consequence, 
the  electro-motive  forces  generated  by  the  magnetism  from  the 
primary  have  to  drive  currents,  not  only  against  the  resistance  of 
the  secondary  windings,  but  also  against  these  local  electro-motive 
forces.  This  necessitates  a  further  drop  in  speed  for  the  required 
torque.  These  local  electro-motive  forces  depend  upon  the 
secondary  alternations  and,  therefore,  vary  with  the  drop  in  speed, 
and  are  greatest  at  zero  speed.  This  introduces  a  very  complicated 
condition  in  the  secondary  circuits.  These  magnetic  fields  which 
thread  around  only  the  primary  or  secondary  windings  are  called 
the  magnetic  leakages,  or  stray  fields,  or  the  magnetic  dispersion. 

If  the  magnetic  leakage  is  relatively  large,  that  is,  twenty  to 
twenty-five  percent  of  the  total  induction,  and  the  secondary  re- 
sistance is  low,  the  speed-torque  curve  will  have  the  peculiar  shape 
shown  in  Fig.  5,  This  curve  shows  the  torque  increasing  as  the 
speed  falls,  until  a  certain  maximum  is  reached.  Beyond  this 
point  the  torque  diminishes  with  further  drop  in  speed.  If  the 
motor  is  loaded  to  the  maximum  torque,  a  slight  increase  in  load 
causes  a  further  drop  in  speed,  the  torque  diminishes  and  the  motor 


10 


ELECTRICAL  ENGINEERING  PAPERS 


stops.  As  a  consequence,  the  normal  rating  of  the  motor  must  be 
considerably  below  this  "pulling-out  point.7'  The  margin  neces- 
sary depends  upon  the  nature  of  the  load  to  be  carried. 


TORQUE 


PIG.  5— SPEED  TORQUE  OP  POLYPHASE  MOTOR,  SHOWING  EFFECT  OP 
MAGNETIC  LEAKAGE 

The  starting  torque,  speed  regulation,  etc.,  of  the  polyphase 
motor  depend  upon  the  form  of  the  speed-torque  curves.  The 
different  methods  of  varying  the  form  of  these  curves  will  be 
considered  next. 

EFFECT  OF  SECONDARY  RESISTANCE  ON  SPEED  CURVE 
As  the  secondary  electro-motive  force  is  that  necessary  to 
drive  the  secondary  currents  through  the  windings,  it  follows  that 
the  electro-motive  force  required  must  depend  upon  the  resistance 
of  these  windings.  A  larger  resistance  means  a  larger  electro- 
motive force  for  the  required  current,  and,  therefore,  a  greater 
number  of  secondary  alternations,  or  a  greater  drop  in  speed. 
The  torque  being  held  constant,  any  variation  of  the  secondary 
resistance  requires  a  proportionate  variation  in  the  slip.  If  the 
slip  with  a  given  torque  is  10  percent,  for  instance,  it  will  be  20 
percent  with  double  the  secondary  resistance,  or  50  percent  with 
five  times  the  resistance.  This  is  true  only  with  the  primary  con- 
ditions of  constant  applied  electro-motive  force  and  constant  alter- 
nations. The  secondary  resistance  may  be  in  the  windings  them- 
selves, or  may  be  external  to  the  windings  but  part  of  the  secondary 
body,  or  it  may  be  entirely  separate  from  the  machine  and  con- 
nected to  the  windings  by  the  proper  leads. 

Fig.  6  shows  the  speed-torque  curves  for  a  motor  with  different 
resistances  in  the  secondary  circuit.  In  curve  "  A"  the  secondary 
resistance  is  small  In  curve  "B"  the  secondary  resistance  is 
doubled.  The  maximttm  torque  remains  the  same  but  the  slip 
for  any  given  torque  is  doubled.  This  motor  starts  much  better 


THE  POLYPHASE  MOTOR 


11 


than  that  in  curve  "A."  In  curve  "c,"  the  resistance  is  again 
doubled  and  the  slip  is  also  doubled.  The  starting  torque  is  in- 
creased but  the  slip  is  rather  large  at  the  rated  torque,  "x."  In 
curve  "D,"  the  slip  is  again  doubled.  In  this  case  the  torque  is 
high  at  start  and  falls  rapidly  as  the  speed  increases.  In  curve 
"E,"  the  maximum  torque  is  not  yet  reached  at  zero  speed.  Con- 
tinuing these  curves  below  the  zero-speed  line,  that  is,  running  the 
motor  in  the  reverse  direction,  we  get  the  general  form  of  these 
different  speed-torque  curves.  They  are  all  of  the  same  general 
shape,  and  all  have  the  same  maximum  torque. 


PIG.  6— SPEED-TORQUE  AND  CURRENT-TORQUE  CURVES  OF  POLYPHASE 
MOTOR  WITH  SECONDARIES  OF  DIFFERENT  RESISTANCE 


12 


ELECTRICAL  ENGINEERING  PAPERS 


So  far  as  torque  is  concerned,  curve  * '  D  ' '  is  the  best  for  starting. 
But  for  running,  curve  "A"  gives  the  least  drop  in  speed.  Con- 
sequently, if  a  resistance  is  introduced  at  start  that  will  give  the 
speed-torque  curve  "D,"  it  should  be  cut  out  or  short-circuited  for 
the  running  condition.  This  is  one  method  of  operation  that  has 
been  much  used. 

CURRENT  CURVE 

In  determining  the  best  starting  condition,  the  current  sup- 
plied to  the  primary  must  be  considered  in  connection  with  the 
speed-torque  curves.  This  current  is  plotted  with  the  series  of 
speed-torque  curves  shown  in  Fig,  6.  Referring  to  this  figure, 
curve  "A*'  represents  the  primary  amperes  in  terms  of  torque. 
Starting  at  the  point  "B,"  of  no-load,  or  zero  torque,  it  rises  at 
a  nearly  uniform  rate  until  maximum  torque  is  approached;  that 
is,  below  the  point  of  maximum  torque  the  current  is  nearly  pro- 
portional to  the  torque,  but  beyond  this  point  the  current  continues 
to  increase  and  reaches  a  maximum  at  the  torque  represented  by 
zero  speed.  At  reversed  speed  this  current  is  further  increased. 
This  one  current  curve  holds  true  for  all  the  speed-torque  curves, 


1  A 
A, 


'c,"  "D,"  etc. 


P  f 

Fig.  7— STARTING  CONDITIONS  WITH  VARIABLE  SECONDARY  RESISTANCE 


THE  POLYPHASE  MOTOR  13 

Comparing  the  different  curves,  we  see  that  "A"  takes  the 
most  current  at  start,  and  gives  low  torque;  "B"  takes  less  cur- 
rent than  "  A,  ' '  and  gives  more  torque ;  "  c  "  takes  less  current  than 
"B";  "D"  takes  less  current  than  "c"  and  gives  the  maximum 
torque  at  start ;  "  E  "  takes  less  current  than  "  D,  ' '  and  develops  less 
torque;  but  the  current  and  torque  are  very  nearly  in  proportion 
over  the  whole  range.  From  this  we  see  that  a  speed-torque  curve 
of  the  form  of  ' '  D  "  or ' '  E  "  is  decidedly  better  for  starting  than ' '  A  ' ' 
or  "B."  But  for  running  at  less  than  the  maximum  torque  there 
is  no  advantage,  so  far  as  current  is  concerned,  in  curve  "D"  over 
curve  "A,"  and  the  speed  regulation  of  "D"  is  poor. 

STARTING  WITH  VARIABLE  SECONDARY  RESISTANCE 

Fig.  7  represents  the  conditions  of  speed,  current,  etc.,  when 
a  variable  secondary  resistance  is  used  at  start.  The  motor 
starts  at  "F"  on  curve  "D,"  and  takes  a  current  "G."  The  cur- 
rent falls  to  "H,"  while  the  speed  rises  to  "i,"  which  corresponds 
to  the  normal  torque  "T"  at  which  the  motor  will  run  under  the 
given  conditions  as  long  as  the  motor  operates  on  curve  "D." 
The  speed  will  remain  at  this  point.  If  the  resistance  in  the 
secondary  is  now  short-circuited,  and  the  load  thus  shifted  to  the 
speed-torque  curve  "A,"  the  torque  at  the  speed  "i,"  increases  to 
"K"  on  torque  curve  "A."  The  current  corresponding  to  this  is 
"L."  As  the  torque  at  "K"  is  greater  than  the  normal  torque 
"T,"  the  motor  speed  will  increase  until  normal  torque  is  reached 
again  at  "M,"  while  the  current  falls  from  "L"  to  "H." 

At  the  moment  of  cutting  out  the  secondary  resistance  there 
was  a  very  considerable  increase  in  the  current.  By  arranging  the 
starting  resistance  in  the  secondary  so  that  the  motor  will  start  at 
some  curve  intermediate  between  "A"  and  "D"  and  thus  take 
more  current  at  start,  somewhat  less  would  be  required  upon 
switching  to  curve  "A."  If  curve  "E"  is  used  for  starting,  and  if 
the  torque  required  when  speeding  up  is  greater  than  that  at  the 
point  where  curves  "A"  and  "E"  cross  each  other,  the  motor  will 
not  pull  up  because  in  switching  from  "E  "  to  "A,"  the  torque  falls, 
and  the  motor  will  stop.  The  current  on  switching  over  increases 
to  "  N,"  and  then  rises  to  "  o  "  as  the  motor  stops.  In  this  case  the 
resistance  that  gives  curve  "E"  is  too  great,  and  a  lower  starting 
resistance  is  required;  with  a  large  number  of  resistance  steps 
small  variation  of  current  is  secured. 

By  making  several  steps  of  the  secondary  resistance,  so  that 
it  may  be  cut  out  gradually,  the  motor  may  be  made  to  pass 


14 


ELECTRICAL  ENGINEERING  PAPERS 


through  a  series  of  speed-torque  curves  with  much  smaller  varia- 
tions of  current  than  shown  in  the  preceding  diagrams.  This 
method  has  been  used  to  some  extent,  but  requires  collector  rings 
or  a  complicated  switching  arrangement  in  connection  with  the 
motor  secondary. 


PIG,  8— STARTING  CONDITIONS  WITH  FIVE  SECONDARY  RESISTANCE  STEPS, 

Fig.  8  shows  the  conditions  for  starting  and  speeding  up  with 
five  speed-torque  curves.  The  motor  starts  on  curve  "  E  "  at  "p." 
The  speed  rises  to  "G."  The  motor  is  then  switched  to  curve 
"D,"  the  torque  rising  to  "H."  The  speed  then  rises  to  "i."  In 
this  way  the  motor  passes  successively  from  "D"  to  "c,"  "B"  and 
"A,"  until  the  full  speed  is  reached.  The  currents  at  no  time  reach 
very  high  values. 

Plotting  the  current  in,  terms  of  speed,  the  use  of  a  large 
number  of  steps  is  shown  to  better  advantage.  This  is  shown  in 
Pigs.  9  and  10.  Fig.  9  shows  the  same  starting  conditions  as  Fig. 
7  with  curves  "D"  and  "A."  The  current  starts  at  "A"  and  falls 
to  "B."  The  resistance  is  then  short-circuited  and  the  current 
rises  to  " c "  and  then  falls  to  "D,"  which  is  the  same  as  "B,"  If 


THE  POLYPHASE  MOTOR 


15 


"A"  had  been  higher  at  start,  "c"  would  have  been  lowered 
slightly.  But  as  the  time  required  for  passing  from  "A"  to  "B" 
is  generally  greater  than  that  from  "c"  to  "D,"  "c"  may  be 
higher  than  "A."  If  the  motor  is  not  required  to  develop  such  a 


SPEED 


FIG.  9— CURRENT  SPEED  CURVE  FOR  MOTOR  STARTING,  AS  IN  FIG.  7. 

large  torque  when  pulling  up,  then  " c "  may  be  lowered  while  "A " 
is  left  unchanged. 

In  Fig  10,  the  currents  in  terms  of  speed  are  shown  for  five 
steps  with  the  five  speed-torque  curves  of  Fig  8.  The  starting 
current  "A"  is  low,  and  none  of  the  currents,  when  switching  from 


SPEED 


FIG.  10— CURRENT  SPEED  CURVE  FOR  MOTOR  STARTING,  AS  IN  FIG.  8 
DOTTED  LINES  SHOW  SAME  CURVE  AS  FIG   9 

one  curve  to  another,  is  large.    The  dotted  lines  show  the  cor- 
responding currents  for  two  steps,  as  in  Fig.  9. 

MOTORS  FOR  VARIABLE  SPEED  WORK 

For  variable  speed  work,  such  as  cranes,  elevators,  etc.,  the 
series  of  curves  in  Fig.  6  shows  one  method  of  regulating  the  speed 
By  varying  the  secondary  resistance  over  a  wide  range,  any  speed 
from  zero  to  maximum  may  be  obtained  with  any  torque  up  to 


10 


ELECTRICAL  ENGINEERING  PAPERS 


the  maximum.  This  requires  the  use  of  collector  rings  and  ad- 
justable rheostats.  The  vanations  in  speed  are  obtained  by 
wasting  energy  in  resistance.  For  a  given  torque  the  same  power 
is  expended  on  the  motor  whether  the  speed  is  zero  or  maximum. 
To  obtain  a  certain  torque  at  start  requires  as  much  power  as 
when  running  at  full  speed. 

An  analysis  of  the  motor  shows  another  way  in  which  the  speed- 
torque  curves  may  be  varied.  In  Fig.  6,  all  the  curves  show  a 
certain  maximum  torque  which  is  the  same  in  all  cases;  but  this 
is  with  the  condition  of  constant  primary  electro-motive  force. 
By  varying  the  electro-motive  force  applied  to  the  primary  we 
may  obtain  a  quite  different  series  of  curves.  Taking,  for  ex- 


TORQUE 


PIG    11— SPEED-TORQUE  AND  CURRENT-TORQUE  CURVES  FOR  POLYPHASE 
MOTOR  WITH  DIFFERENT  VOLTAGES  APPLIED 

ample,  a  speed-torque  curve  of  the  form  "A"  in  Fig.  11,  and 
applying  a  higher  electro-motive  force  to  the  primary,  a  curve  is 
obtained  of  the  same  shape  as  "A,"  but  with  a  much  higher  point 
of  maximum  torque.  Lowering  the  applied  electro-motive  force, 
the  maximum  torque  is  lowered.  The  torques  at  any  given  speed 
are  raised  or  lowered  in  the  same  proportion  as  the  maxima  are 
varied.  At  any  given  speed  the  torques  are  proportional  to  the 
square  of  the  electro-motive  forces  applied.  This  relation  holds 
good  for  any  form  of  the  torque  curve,  whether  of  the  shape  "A," 
"D,"  or  "B,"  shown  in  Fig.  6. 

The  current  curves  are  also  shown  in  Fig.  11.  They  all  have 
the  same  general  shape,  but  have  different  maximum  values,  these 
being  proportional  to  the  electro-motive  forces  applied.  The 
speed-torque  curve  "A"  in  Fig.  11  has  the  same  shape  as  "D"  in 
Fig.  6,  which  gave  too  great  a  drop  in  speed.  In  Fig.  11,  curve 
"B,"  which  is  the  same  form  as  "A,"  gives  less  speed  drop  for  the 
same  torque.  Curve  "c"  gives  less  than  "B,"  and  has  fairly 
good  speed  regulation  from  no-load  up  to  normal  torque  "T."  But 


THE  POLYPHASE  MOTOR  17 

this  result  is  obtained  at  the  expense  of  increased  induction  in  the 
iron,  and  large  no-load  or  magnetizing  current  due  to  the  higher 
electro-motive  force,  is  required.  If  it  is  possible  to  obtain  a 
speed-torque  curve  like  "c"  in  Fig  11  with  the  normal  electro- 
motive force  applied,  we  can  obtain  good  speed  regulation  from 
no-load  up  to  the  rated  torque,  and  shall  be  able  to  start  the  motor 
with  the  maximum  torque  it  can  develop.  Then,  by  lowering  the 
applied  electro-motive  force,  the  same  form  of  speed-torque  curve 
will  be  retained,  but  the  starting  torque  and  starting  current  may 
be  lowered  to  any  extent  desired. 

VARIABLE  SPEED  BY  VARYING  VOLTAGE 

Returning  to  Fig  5,  it  was  stated  that  the  peculiar  shape  of 
this  curve,  with  the  torque  falling  rapidly  after  reaching  a  maxi- 
mum value,  was  due  mainly  to  magnetic  leakage  between  the 
primary  and  secondary  windings.  But  if  the  motor  is  so  pro- 
portioned that  the  leakage  is  very  small  compared  with  the  useful 
field,  the  speed-torque  curve  takes  a  quite  different  shape.  The 
maximum  torque  is  increased  directly  as  the  magnetic  leakage  is 
diminished.  This  is  shown  in  Fig.  12.  Here  "A"  is  similar  in 
shape  to  curve  "A"  in  Fig.  6;  "B"  represents  the  speed-torque 
curve  with  the  magnetic  leakage  reduced  one-half;  "c"  repre- 
sents it  with  about  one-half  the  leakage  of  "B,"  and  "D"  with  one- 
half  that  of  "c." 

In  comparing  Figs  6  and  12,  it  may  be  noted  that  "A"  in  one 
is  the  same  form  as  "A"  in  the  other,  although  drawn  to  a  dif- 
ferent scale.  In  Fig.  12,  "B  "  has  the  same  shape  as  in  Fig.  6,  but 
has  a  different  maximum  value.  The  same  is  true  of  curves  "c" 
and  "D"  in  the  two  figures.  By  lowering  the  applied  electro- 
motive forces  for  curves  "D,"  "  c  "  and  " B  "  of  Fig.  12,  so  that  the 
maximum  torques  are  equal  to  that  of  "A,"  as  shown  by  the  dot- 
ted curves,  we  get  practically  the  same  curves  as  in  Fig.  6. 

Curve  "D,"  in  Fig.  12,  gives  as  good  running  conditions  as 
curve  "A"  in  Fig.  6,  having  about  the  same  drop  in  speed  at  the 
normal  torque  "T."  We  have,  then,  in  "D"  a  curve  which  starts 
at  the  point  of  maximum  torque,  and  which  also  has  a  small  drop 
in  speed  at  the  normal  load.  The  objection  to  this  curve  is  that 
the  starting  current  and  starting  torque,  although  in  the  proper 
proportion  to  each  other,  are  both  much  greater  than  is  necessary 
or  desirable.  By  reducing  the  applied  electro-motive  force  at 
start,  however,  lower  torques  and  currents  are  obtainable.  In  this 
way  we  may  combine  good  starting  and  running  conditions  in  one 


18 


ELECTRICAL  ENGINEERING  PAPERS 


motor  without  the  use  of  starting  resistances,  and  with  a  secondary 
that  has  no  resistance  except  that  of  its  own  windings.  Fig.  13 
shows  the  speed-torque  and  current  curves  of  such  a  motor  with 
the  applied  electro-motive  force  varied  over  a  considerable  range. 


PIG.  12— SPEED-TORQUE  CURVES  OP  POLYPHASE  MOTOR  SHOWING  EF- 
FECT OF  MAGNETIC  LEAKAGE.    DOTTED  LINES  SHOW  CURVES  b,  c,  d 
WITH  REDUCED  VOLTAGE 

If  but  one  electro-motive  force  is  desired  for  starting  and 
speeding  up,  and  the  motor  is  then  to  be  transferred  to  the  work- 
ing electro-motive  force,  the  speed-torque  curves  should  preferably 


TORQUE 

FIG   13— SPEED-TORQUE  AND  CURRENT-TORQUE  CURVES  FOR  POLY- 
PHASE MOTOR  WITH  ELECTRO-MOTIVE  FORCE  VARIED  OVER  A 
WIDE  RANGE. 


have  the  shape  shown  in  Fig.  14.  The  motor  starts  with  the  de- 
sired torque  at  reduced  e.  m.  1,  and  comes  up  to  almost  rated 
speed  before  switching  over.  This  is  suitable  for  constant  speed 
work.  In  Fig.  14  are  shown  both  the  starting  and  running  speed- 
torque  curves,  and  the  currents  both  in  the  motor  and  the  line. 
The  line  currents  are  smaller  than  the  motor  currents  in  the  ratio 
of  reduction  of  electro-motive  force  in  the  regulating  transformers. 


THE  POLYPHASE  MOTOR 


19 


SPEED-TORQUE  CURVES  FOR  VARIABLE  SPEED  WORK 
For  cranes,  elevators,  and  variable  speed  work  in  general, 
curves  of  the  form  shown  in  Fig.  15  are  preferable.    The  line  cur- 
sents  are  also  shown  in  this  figure.    This  series  of  speed-torque 


TORQUE 

FIG.   14— B  EST  SHAPE  OP  SPEED-TORQUE  CURVE  FOR  MOTOR  STARTED 

AND  SPEEDED  UP  WITH  A  SINGLE  REDUCED  VOLTAGE,  BEFORE 

BEING  TRANSFERRED  TO  WORKING  VOLTAGE. 

curves  shows  that  a  wide  range  of  speed  may  be  obtained  by  proper 
variations  of  the  applied  electro-motive  force.  The  line  currents 
"A,"  "B,"  etc.,  practically  overlap  each  other.  This  means  that 
the  line  current  required  with  this  method  of  control  is  very  nearly 


TOHQUE 

FIG.  IS— SPEED-TORQUE  AND  CURRENT-TORQUE  CURVES  OF  MOTOR  FOR 
CRANES,  ELEVATORS  AND  SIMILAR  VARIABLE  SPEED  WORK  WITH  VOL 
TAGE  CONTROL.  CURVES  a,  b,  c,  d,  er  f,  ARE  SPEED-TORQUE  CURVES  WITH 
VARIABLE  VOLTAGE.  CURVES  A,  B,  C,  D,  E,  F.  SHOW  CORRESPONDING 
LINE  CURRENT. 

constant  for  any  given  torque,  independent  of  the  speed.  The 
same  is  true  of  the  method  of  control  by  varying  the  secondary 
resistance.  It  may  be  noted  that  the  current  for  starting,  as  on 
curve  "c,"  for  instance,  is  slightly  greater  than  that  required  for 
running  at  the  same  torque  on  "B"  or  "A."  This  is  due  to  the 
speed-torque  curve  being  somewhat  curved  at  its  outer  end.  With 
a  somewhat  higher  resistance  of  the  secondary  the  curves  are  more 


20  ELECTRICAL  ENGINEERING  PAPERS 

nearly  straight,  but  the  drop  in  speed  is  somewhat  increased  on  the 
speed  torque  for  any  given  electro-motive  force  In  practice,  a 
compromise  is  made  between  the  best  possible  starting  condition 
and  a  condition  of  less  speed  drop. 

A  comparison  of  the  methods  of  control  by  varying  the 
secondary  resistance  and  by  varying  the  applied  electro-motive 
force  shows  that  they  give  practically  the  same  results  in  regard 
to  starting,  speed  regulation,  etc.  But  a  motor  that  has  been 
designed  for  regulation  by  varying  its  secondary  resistance,  will 
generally  give  very  poor  results  when  an  attempt  is  made  to 
operate  it  by  the  variable  electro-motive  force  method  A  motor 
must  be  especially  proportioned  for  small  magnetic  leakage  when 
this  method  of  control  is  to  be  used.  The  proportions  and  the  ar- 
rangement of  the  parts  are  such  as  may  class  this  as  a  practically 

distinct  type  of  motor. 

EFFICIENCY  AND  POWER  FACTOR 

We  come  now  to  the  other  characteristics  of  the  polyphase 
motor,  the  most  important  of  which  are  the  efficiency  and  the 
power  factor.  The  importance  of  efficiency  is  generally  apprec- 
iated, but  the  question  of  power  factor  in  most  cases  appears  to  be 
not  thoroughly  understood  or  else  is  entirely  overlooked. 

The  efficiency  of  a  polyphase  motor  is  the  ratio  of  the  power 
developed  to  the  true  power  expended,  as  in  any  other  kind  of  a 
machine.  The  power  developed  may  be  obtained  from  the  speed- 
torque  curves.  If  the  torques  are  given  for  one  foot  radius,  and 
the  speed  in  revolutions  per  minute,  then  the  product  of  any  given 
torque  by  the  corresponding  speed,  divided  by  5,250,  will  give  the 
power  developed  in  horse-power;  or  torque  multiplied  by  speed, 
divided  by  seven,  gives  the  power  developed  in  watts.  This 
power,  plus  the  iron,  copper  and  friction  losses,  gives  the  true 
power  expended. 

The  power  factor  is  the  ratio  of  the  true  power  to  the  apparent 
power  expected.  This  apparent  power  is  proportional  to  the 
products  of  the  primary  currents  by  the  electro-motive  forces. 
If  there  is  magnetizing  current,  and  if  the  motor  has  magnetic 
leakage,  the  primary  currents  are  not  in  phase  with  their  electro- 
motive forces  and  their  products  represent  an  apparent  power 
which  is  greater  than  the  true  energy  expended.  The  current  of 
each  circuit  can  be  considered  as  made  up  of  two  currents,  one  of 
which  is  in  phase  with  the  applied  electro-motive  force,  represent- 
ing true  energy,  and  the  other  at  right  angles  to  the  electro-motive 
force,  representing  no  energy.  The  right-angled  component  is"  the 


THE  POLYPHASE  MOTOR  21 

one  that  has  an  injurious  effect  on  the  regulation  of  the  generator, 
transmission  lines,  transformers,  etc. 

The  size  of  this  component,  compared  with  the  useful  current, 
may  be  shown  by  a  table : 

Useful  90  Degree 

Power  Factor      Total  Current      Component        Component 
100  100  100  0. 

99  100  99  14.2 

98  100  98  19.9 

95  100  95  31.2 

90  100  90  43  6 

80  100  80  60.0 

70  100  70  71  4 

60  100  60  80  0 

50  100  50  86  6 

40  100  40  91  6 

EFFECTS  OF  LAGGING  CURRENT 

At  90  percent  power  factor,  for  instance,  the  current  that  is 
lagging  90  degrees  behind  the  electro-motive  force  is  equal  to  43  6 
percent  of  the  total  current  flowing.  This  lagging  current  reacts 
on  the  generator,  affecting  the  regulation.  In  an  alternating- 
current  generator,  a  90-degree  lagging  current  in  the  armature 
coils  directly  opposes  the  field  magnetization.  When  delivering 
a  current  at  90  percent  power  factor  there  is  over  43  percent  of  this 
current  opposing  the  field,  and  at  80  percent  power  factor  60  percent 
is  opposing  the  field.  If  the  armature  ampere  turns  are  normally 
20  percent  as  great  as  the  field  ampere  turns,  then  a  load  of  80 
percent  power  factor  will  give  an  opposing  magnetization  in  the 
armature  of  about  60  percent  of  the  total  armature  ampere  turns, 
or  about  12  percent  of  the  total  field,  and  the  armature  electro- 
motive force  will  be  lowered  approximately  that  percent  more  than 
with  a  load  of  100  percent  power  factor. 

The  inductive  effects  of  the  lagging  current  in  the  transmission 
circuits  and  transformers  are  much  more  serious  than  those  from 
a  current  that  is  in  phase  with  the  electro-motive  force.  The 
generator,  transformers,  lines  and  motors  also  have  increased 
losses,  due  to  the  large  current  required  when  the  power  factor  is 
low.  An  80  percent  power  factor  in  a  system  means  losses  due  to 
heating  of  conductors  more  than  50  percent  greater  than  those 
with  100  percent  power  factor.  These  figures  indicate  the  im- 
portance of  good  power  factors  in  an  alternating-current  system. 


22  ELECTRICAL  ENGINEERING  PAPERS 

MAGNETIZING  CURRENT  AND  MAGNETIC  LEAKAGE  DETERMINE  POWER  FACTOR 
The  lagging,  or  90-degree  component,  of  the  current  in  a 
motor  depends  upon  the  amount  of  the  no-load,  or  magnetizing, 
current  and  upon  the  magnetic  leakage  Let  this  lagging  com- 
ponent be  expressed  in  percent  of  the  total  current  Also  express 
the  magnetizing  current  in  percent  of  the  total  current,  and  the 
total  magnetic  leakage  in  percent  of  the  total  primary  induction. 
Then  the  sum  of  the  percents  of  magnetizing  current  and  magnetic 
leakage  represents  very  closely  the  percent  of  the  lagging  com- 
ponent of  the  primary  current.  If,  for  example,  the  magnetizing 
current  is  30  percent  and  the  leakage  is  14  percent,  the  resulting 
lagging  component  is  about  44  percent.  From  the  preceding 
table,  this  indicates  about  90  percent  power  factor  A  low  leakage 
and  a  high  magnetizing  current  may  give  the  same  power  factor  at 
full  load  as  a  high  leakage  and  low  magnetizing  current;  but  at 
half  load,  the  percent  magnetizing  current  is  practically  doubled, 
while  the  percent  magnetic  leakage  is  halved.  Hence,  a  low  mag- 
netizing current  is  of  great  importance  in  maintaining  a  high  power 
factor  If  a  high  value  of  power  factor  over  a  wide  range  is  de- 
sired, then  both  the  leakage  and  the  magnetizing  current  must  be 
low. 

VOLTAGE  CONTROL  VERSUS  RIIEOSTATIC  CONTROL 
The  method  of  control  by  varying  the  primary  electro-motive 
force  is  dependent  upon  the  fact  that  the  motor  has  a  low  magnetic 
leakage.  By  using  certain  proportions  and  arrangements  of  the 
windings  on  the  primary  and  secondary,  the  magnetizing  current 
may  be  made  comparatively  low.  Thus  both  conditions  for  good 
power  factor  are  obtained. 

With  the  method  of  control  by  varying  the  secondary  resist- 
ance, good  power  factors  may  be  obtained.  But  the  form  of 
secondary  winding  required  when  variable  resistances  are  used 
tends  to  reduce  both  the  power  factor  and  the  maximum  torque. 

BEST  FORM  OF  SECONDARY  WINDING 

An  elaborate  series  of  tests  was  made  to  determine  the  best 
type  of  winding  for  the  secondary  of  a  polyphase  motor.  First, 
two  circuits  were  arranged  to  give  secondary  phases  ninety  degrees 
apart.  The  starting,  running  and  maximum  load  conditions  were 
determined.  Then  a  three-phase  secondary  winding  was  used. 
This  gave  a  higher  pulling-out  torque  and  better  power  factor  than 
the  two-phase  Pour  phases  were  tried  and  were  bottcr  than 
three;  and  six  were  better  than  four.  Then  twelve  phases  were 


THE  POLYPHASE  MOTOR  23 

tried,  with  a  gain  over  six  in  maximum  torque,  but  not  much  gain 
in  efficiency.  The  power  factor  was  somewhat  improved.  Finally 
the  winding  was  completely  short-circuited  on  itself,  all  coils  being 
connected  to  a  common  ring.  This  gave  a  further  increase  in  - 
maximum  torque  and  power  factor  over  the  preceding  arrange- 
ment, but  there  was  very  little  gain  in  efficiency.  The  same 
primary  was  used  in  all  these  tests.  Each  time  the  number  of 
secondary  circuits  was  increased  the  power  factor  was  somewhat 
improved.  This  was  due  to  the  fact  that  the  secondary  currents 
were  able  to  so  distribute  themselves  that  the  local  electro-motive 
forces  in  the  coils,  due  to  leakage,  were  diminished;  or,  the  mag- 
netic leakage  may  be  considered  to  have  been  diminished.  This 
would  necessarily  give  higher  pulling-out  torques  and  higher-power 
factors. 

BEST  FORM  OF  PRIMARY  WINDINGS 

Very  complete  tests  were  also  made  to  determine  the  best 
form  of  primary  winding,  and  a  certain  method  of  distribution  of 
the  coils  was  found  to  diminish  the  primary  magnetic  leakage  very 
considerably.  This  somewhat  increased  the  maximum  torque 
and  the  power  factor.  Utilizing  the  arrangements  of  the  primary 
and  the  secondary  windings  just  described,  and  otherwise  pro- 
portioning for  small  magnetic  leakage,  a  motor  may  be  obtained 
that  has  a  comparatively  low  total  induction,  and  yet  has  a  mag- 
netic leakage  of  but  a  few  percent.  The  low  induction  allows  a 
small  magnetizing  current  and  comparatively  low  iron  losses.  The 
low  leakage  gives  a  high  pulling-out  torque,  and  thus  allows  a  good 
speed  regulation,  and  also  good  starting  conditions,  by  varying  the 
applied  electro-motive  force. 

TYPE  C  MOTORS 

Motors  that  are  adapted  for  operation  under  the  conditions  of 
variable  applied  electro-motive  forces  with  constant  secondary 
resistance  must  have  the  special  forms  of  speed-torque  curves 
shown  in  Figs.  12  to  15,  and  they  may  therefore  be  considered  as 
forming  a  distinct  type.  This  type  has  received  the  name  Type  C. 
The  Type  C  motor  is  always  characterized  by  low  magnetic  leak- 
age and  consequent  high  pulling-out  torque.  The  secondary  has 
no  adjustable  resistance  and  all  regulation  is  obtained  by  varying 
the  adjustable  electro-motive  force.  The  secondary  is  made  the 
rotating  part,  on  account  of  the  type  of  winding  used,  which  con- 
sists of  copper  bars  placed  in  tunnels  or  slots  in  the  core  and 
bolted  to  two  end  rings.  There  are  no  bands,  and  the  question  of 


24  ELECTRICAL  ENGINEERING  PAPERS 

insulation  is  of  very  little  importance  for  the  maximum  secondary 
electro-motive  force  does  not  exceed  three  volts  in  a  500  horse- 
power motor  and  is  less  with  smaller  sizes. 

ADVANTAGES  OF  TYPE  C  MOTORS 

This  type  of  motor  possesses  several  distinct  advantages  over 
other  forms  of  polyphase  motors.  The  method  of  control,  by 
varying  the  electro-motive  forces  applied  to  the  motor,  leads  to 


WESTINGHOUSE 
75  H.P.TYPE  C  MOTOR 

3000  ALTS..       POLES 


FIG.  16— PERFORMANCE  CURVE  OF  75  H,  P.  TYPE  C  MOTOR 

two  very  important  advantages,  one  of  which  is  mechanical  and 
the  other  electrical.  With  this  method  of  control  there  are  no 
regulating  appliances  on  the  motor  and,  in  consequence,  it  may  be 
of  the  simplest  possible  form.  The  electrical  advantage  is  that 
the  motor  may  be  started  and  controlled  from  a  distance.  Thus 
it  may  be  placed  entirely  out  of  reach  of  the  operator.  On  travel- 
ing cranes,  for  example,  this  is  of  special  advantage,  for  in  this  case 
only  the  primary  wires  need  be  run  from  the  operator's  cage  to  the 
motor.  If  there  are  several  motors  on  the  crane,  there  may  be  one 
wire  common  to  all  the  motors  and  but  two  additional  wires  per 


THE  POLYPHASE  MOTOR 


25 


motor  are  required.     Thus  for  the  three  motors,  a  minimum  of 
eleven  trolley  wires  may  be  used. 

If  the  variable  electro-motive  forces  are  obtained  from  trans- 
formers, the  switches  for  operating  several  motors  may  be  wired 
to  one  set  of  transformers  and  the  motors  may  be  started  and 
regulated  independently.  For  traveling  cranes,  only  one  set  of 
transformers  is  used  for  the  hoisting,  bridge  and  traveling  motors, 


WESTINGHOUSE 
400  H.P.TYPEC  MOTOR 

3COO  ALTS.,  8  POLES 


3000 


4000    5000    COCO    7000    8000    0000  10000 11000  1200^ 
Pqunds  Torque  at  i  Foot  Radius. 


FIG.  17— PERFORMANCE  CURVES  OF  400  H  P.  TYPE  C  MOTOR 

and  this  set  may  supply  currents  at  different  electro-motive  forces 
to  all  the  motors  at  the  same  time.  A  further  advantage  possessed 
by  this  motor  lies  in  the  high  pulling-out  torque.  If  a  heavy  over- 
load, or  a  load  having  great  inertia,  is  suddenly  thrown  on  a  motor 
that  has  a  speed-torque  curve  like  "A"  in  Fig.  6,  the  point  of 
maximum  torque  may  be  passed  for  an  instant,  and  the  motor  will 
be  stopped  unless  the  load  is  quickly  removed.  A  Type  C  motor 
in  this  case  would  have  its  speed  pulled  down  for  a  moment,  but 
this  reduction  in  speed  gives  an  increased  torque,  thus  enabling 
the  motor  to  carry  the  overload. 


26  ELECTRICAL  ENGINEERING  PAPERS 

If  the  electro-motive  force  of  the  system  Is  suddenly  lowered, 
the  pulling-out  torque  of  the  motors  is  lowered  very  materially. 
A  reduction  of  twenty  percent  in  the  electro-motive  force  will 
lower  the  pulling-out  torque  to  about  two-thirds  of  its  former  value. 
Even  with  a  temporary  drop  in  the  electro-motive  force,  such  as 
would  be  caused  by  a  momentary  short-circuit  on  the  lines,  this 
may  be  sufficient  to  stop  the  motor  But  a  motor  that  has  a 
pulling-out  point  several  times  as  large  as  its  normal  running 
torque  is  very  rarely  in  danger  of  being  shut  down  from  this  cause. 
This  type  of  motor  has  a  starting  torque  from  two  to  four  times 
as  large  as  the  full-load  running  torque  and  it  is  thus  able  to  start 
any  kind  of  load.  In  practice  the  starting  torque  is  adjusted  to  the 
load  to  be  started  by  applying  a  suitable  electro-motive  force,  as 
will  be  explained  below. 

A  last,  but  not  least,  advantage  of  the  Type  C  motor  is  its 
adaptability  for  large  sizes.  The  larger  the  motor  of  this  type,  the 
lower  in  proportion  can  be  its  magnetic  leakage  and  its  magnet- 
izing current.  In  consequence,  the  power  factors  are  very  high. 
The  efficiencies  are  also  very  good  over  a  wide  range  of  load  The 
curves  for  a  seventy-five  horse-power,  six-pole,  3,000-altcrnntion 
motor  are  given  in  Fig.  16,  also  the  curves  for  a  400-horsc-power, 
2,300-volt,  eight-pole,  3,000-alternation  motor  in  Pig.  17  The 
power  factors  of  these  motors  are  good  examples  of  what  can  be 
obtained  on  large  motors  of  this  type. 

SPEED  VARIATION  WITH  POLYPHASE  MOTORS 

There  are  six  methods  of  varying  the  speed  of  polyphase 
motors,  but  some  of  them  are  applicable  only  in  special  cases. 
These  methods  are: 

(1) — Varying  the  number  of  poles. 

(2) — Varying  the  alternations  applied. 

(3) — Motors  in  tandem,  or  series-parallel. 

(4) — Secondary  run  as  single-phase. 

(5) — Varying  the  resistance  of  the  secondary. 

(6) — Varying  the  electro-motive  force  of  the  primary, 

with  constant  secondary  resistance. 

Some  of  these  methods  are  efficient,  while  some  are  very  in- 
efficient if  the  speed  is  to  be  varied  over  a  wide  range. 

VARYING  THE  NUMBER  OF  POLES 

The  first  method,  varying  the  number  of  poles,  is  efficient  to  a 
certain  extent,  but  is  limited  in  the  number  of  combinations  of 


THE  POLYPHASE  MOTOR  27 

poles  obtainable.  But  if  combined  with  some  of  the  other  methods 
it  may  be  made  fairly  effective  over  a  wide  range.  It  consists  in 
varying  the  arrangement  of  the  primary  coils  in  such  a  way  that 
the  number  of  resulting  poles  is  varied.  This  may  be  accom- 
plished by  having  two  or  more  separate  windings  on  the  primary; 
or  one  winding  may  be  used,  it  being  rearranged  for  different  speed. 
With  this  method  of  varying  the  speed,  a  secondary  of  the  "cage " 
type  is  the  only  practical  one.  With  a  "grouped"  or  "polar" 
winding  on  the  secondary,  this  would  need  rearranging  for  the  dif- 
ferent speeds,  just  as  in  the  case  of  the  primary.  But  the  cage 
winding,  being  short-circuited  on  itself  at  all  points,  is  adapted  to 
any  number  of  poles.  In  general,  this  method  of  regulation  will 
allow  for  only  two  speeds  without  great  complications,  and  the 
ratio  of  the  two  speeds  is  preferably  two  to  one,  although  three  to 
one  may  be  obtained.  The  simplest  arrangement  of  winding  con- 
sists of  two  separate  primary  windings;  one  for  one  number  of 
poles,  and  the  second  for  the  other.  In  combination  with  a  var- 
iable primary  electro-motive  force,  the  speed-torque  curves  being 
of  such  shape  that  this  method  may  be  used,  the  variable-pole 
method  of  regulation  may  be  made  fairly  efficient  over  a  wide 
range  of  speed.  But  the  two  windings  considerably  increase  the 
size  of  the  motor,  while  the  one-winding  arrangements  are  rather 
complicated.  Consequently,  we  may  consider  that  this  method 
of  speed  variation  will  be  used  only  in  special  cases. 

VARYING  THE  NUMBER  OF  ALTERNATIONS 

The  second  method,  variable  alternations,  is  theoretically  the 
ideal  method;  but  it  is  practically  limited  to  a  few  special  applica- 
tions, for  we  have  as  yet  no  commercial  alternation  transformer. 

In  a  few  cases,  where  but  one  motor  is  operated,  the  generator 
speed  may  be  varied.  If  the  generator  is  driven  by  a  water-wheel, 
its  speed  may  be  varied  over  a  wide  range,  and  the  motor  speed  will 
also  vary.  If  the  generator  field  be  held  at  practically  constant 
strength,  then  the  motor  speed  may  be  varied  from  zero  to  a 
maximum  at  constant  torque  with  a  practically  constant  current. 
This  is  a  convenient  method  of  operating  a  motor  at  a  distance 
from  the  generator.  The  speed  of  the  motor  may  be  completely 
controlled  by  an  attendant  at  the  generating  station. 

Fig.  18  shows  the  speed-torque  and  other  curves  of  a  motor 
when  operated  at  7200,  3600, 1800  and  720  alternations  per  minute, 
or^at  100,  SO,  25  and  10  percent  of  the  normal  alternations.  The 
speed-torque  curves,  corresponding  to  the  above  alternations  are, 


28 


ELECTRICAL  ENGINEERING  PAPERS 


"a,"  "b,"  "c"  and  "d."  The  current  curves  are  "A,"  "&>"  "c" 
and  "D."  This  figure  shows  that  for  the  rated  torque  "T,"  the 
current  is  practically  constant  for  all  speeds,  but  the  electro-motive 
force  varies  with  the  alternations.  Consequently,  the  apparent 
power  supplied,  represented  by  the  product  of  the  current  by 


FIG    18— PERFORMANCE  CURVES  OF  POLYPHASE  MOTOR  WITH  DIF- 
FERENT ALTERNATIONS  AND  ELECTRO-MOTIVE  FORCES. 

electro-motive  force,  varies  with  the  speed  of  the  motor,  and  is 
practically  proportionate  to  the  power  developed 

MOTORS  IN  TANDEM  OR  SERIES-PARALLEL 

The  third  method  is  to  run  motors  in  tandem  or  series-parallel- 
In  this  arrangement,  the  secondary  of  one  motor  is  wound  with  a 
grouped  or  polar  winding  to  give  approximately  the  same  electro- 
motive force  and  number  of  phases  as  the  primary.  The  secondary- 
is  connected  to  the  primary  of  a  second  motor,  The  secondary 
of  the  second  motor  may  be  closed  on  itself,  with  or  without  a 
resistance,  or  may  be  connected  to  the  primary  of  a  third  motor, 
etc.  The  arrangement  with  two  motors  is  shown  in  Fig.  19.  At 
start,  motor  No.  1  receives  the  full  number  of  alternations  on  its 
primary,  and  its  secondary  -delivers  the  same  number  to  the  prim- 
ary of  motor  No.  2.  Both  motors  will  start.  As  motor  No,  1 
speeds  up,  its  secondary  alternations  fall.  At  about  one-half 
speed,  its  secondary  alternations  are  about  one-half  its  primary, 
and  motor  No.  2  receives  one-half  the  alternations  of  motor  No.  1 ; 
it  also  tends  to  run  at  half-speed,  Therefore,  if  both  motors  ate 


THE  POLYPHASE  MOTOR  29 

coupled  to  the  same  load,  this  half  speed  is  a  position  where  the 
two  motors  tend  to  operate  together.  By  connecting  both 
primaries  across  the  line,  both  motors  will  be  run  at  full  speed. 
Thus,  with  two  motors,  two  working  speeds  may  be  obtained. 
This  method  always  requires  at  least  two  motors.  Its  application 
is  limited  to  a  few  special  cases. 

SECONDARY  WITH  ONLY  A  SINGLE  CIRCUIT  CLOSED 

The  fourth  method — the  secondary  run  with  a  single  circuit 
closed  — will  give  a  half-speed,  and  with  two  or  more  circuits 
closed,  will  give  full  speed.  But  the  power  factor  at  the  half -speed 
is  very  low,  and  the  efficiency  is  not  nearly  so  good  as  when  run 
at  full  speed.  This  may  have  a  few  special  applications.  Fig  20 
shows  this  arrangement. 


FIG.  19— DIAGRAMMATIC  ARRANGEMENT    OF  THE  TWO  POLYPHASE 
MOTORS  CONNECTED  IN  TANDEM  OR  SERIES  PARALLEL 

VARYING  THE  RESISTANCE  OF  THE  SECONDARY 

The  fifth  arrangement  is  by  varying  the  resistance  in  the 
secondary.  This  method  was  considered  before  when  the  speed- 
torque  characteristics  were  shown.  This  will  not  give  constant 
speed  except  with  constant  load,  as  the  speed-torque  curve,  with  a 
relatively  large  resistance  is  a  falling  curve.  At  heavy  torques,  the 
motor  will  run  at  very  low  speeds,  while  with  light  loads  it  will  run 
at  almost  full  speed.  The  speed  regulation  will  be  similar  to  that 
of  a  direct-current  shunt  motor  with  a  resistance  in  circuit  with  the 
armature.  To  hold  constant  speed  with  variable  load,  this  resis- 
tance requires  continual  adjustment. 

VARYING  PRIMARY  VOLTAGE 

The  sixth  method — that  in  which  the  primary  electro  motive 
force  is  varied  while  the  secondary  resistance  is  held  constant — 
gives  the  same  results  as  the  fifth  method,  as  the  speed-torque 
curves  axe  similar.  To  hold  a  constant  low  speed,  the  electro- 
motive force  must  be  varied  continually  if  the  load  is  changing. 


30  ELECTRICAL  ENGINEERING  PAPERS 

Like  the  fifth  method,  it  is  not  efficient  at  low  speeds,  as  the  reduc- 
tion in  speed  is  obtained  by  means  of  a  corresponding  loss  of  energy 
in  the  secondary  circuits.  / 


PIG   20— POLYPHASE  MOTOR  WITH  ONLY  ONE  SECONDARY  CURRENT 

CLOSED 

For  crane  work,  hoisting,  etc  ,  where  it  is  necessary  to  run  at 
reduced  speed  for  but  a  portion  of  the  time,  either  of  the  methods 
five  or  six  is  satisfactory,  but  method  five  requires  the  use  of  a  vari- 
able secondary  resistance,  and  there  must  be  a  set  of  secondary 
leads  earned  out  to  a  rheostat  if  the  speed  changes  are  to  be 
gradual.  This  introduces  complication,  especially  on  a  crane  where 
several  motors  are  to  be  controlled  In  this  case  there  must  be 
trolley  wires  for  both  the  primary  and  the  secondary  circuits  of  each 
motor.  But  by  method  six,  the  control  is  effected  in  the  primary 
circuit  and  only  primary  trolley  wires  are  needed,  and  these  may 
be  controlled  from  one  pair  of  transformers,  as  explained  before. 
The  sixth  method  is  therefore  the  simplest  and  most  practical  one 
to  use  for  hoisting,  etc  ,  and  will  be  found  to  present  many  advan- 
tages for  all  classes  of  work,  whether  speed  regulation  is  important 
or  not. 

METHOD  OP  VARYING  PRIMARY  ELECTRO-MOTIVE  FORCE 

There  are  several  methods  of  varying  the  electro-motive  force 
applied  for  starting  and  varying  the  speed  on  the  Type  C  motor. 
These  may  be  classified  under  three  headings : 

(1) — Varying  the  electro-motive  force  from  the  genera- 
tor. 

(2) — Varying  the  electro-motive  force  by  transformers. 
(3) — Varying  the  motor  connections. 

VARYING  ELECTRO-MOTIVE  FORCE  FROM  THE  GENERATOR 
A  variable  electro-motive  force  may  be  obtained  from  the 
generator  in  several  ways.     The  generator  may  be  run  at  low 


THE  POLYPHASE  MOTOR 


31 


speed,  with  the  field  charged.  This  gives  lower  electro-motive 
force  and  lower  alternations  at  the  same  time.  This  is  adapted 
only  to  places  where  all  the  motors  are  to  be  started  at  once. 

The  generator  may  be  run  at  normal  speed  and  its  field  charge 
lowered.  This  gives  the  normal  alternations  with  lower  electro- 
motive force.  This  is  practicable  only  where  all  the  motors  are  to 
be  started  at  once. 

A  third  method  is  to  so  arrange  the  generator  windings  that 
two  or  more  electro-motive  forces  for  each  phase  may  be  obtained. 
A  lower  electro-motive  force  may  be  used  at  start,  and  a  higher 
for  running. 

The  different  arrangements  of  the  generator  windings  for  this 
purpose  are  as  follows: 

If  the  armature  has  but  one  winding  closed  on  itself,  like  a 
direct-current  machine,  two  or  three  phases  may  be  taken  off.  For 
two  phases  four  leads  are  used,  Pig.  21  illustrates  this  Between 


MOTOR 


STARTING  WITH 
REDUCED  VOLTAGE 


RUNNING  WITH 
FULL  VOLTAGE 

FIG.  21— CONNECTIONS  FOR  TWO-PHASE  MOTORS  STARTING  ON  SIDE 

CIRCUITS. 

1-3  and  2-4  is  the  maximum  electro-motive  force,  and  between  1-2, 
2-3,  3-4  and  4-1  there  is  0.7  the  electro-motive  force  of  1-3.  The 
electro-motive  force  1-2  is  at  quarter  phase  to  that  of  4-1  and  2-3, 
and  the  electro-motive  force  3-4  is  at  quarter  phase  to  that  of  2-3 
and  4-1.  Therefore,  across  any  two  adjacent  side  circuits  we  have 
quarter  phase  circuits  of  0.7  the  electro-motive  force  of  the  main 
circuit.  A  motor  may  thus  be  started  on  any  adjacent  side  cir- 
cuit and  then  switched  to  the  main  circuit.  This  method  is  wdl 
adapted  for  local  plants  where  the  generator  electro-motive  force 
is  200  or  400  volts.  If  there  are  many  motors  to  be  started,  and 


32  ELECTRICAL  ENGINEERING  PAPERS 

the  starts  are  numerous,  it  is  advisable  to  wire  the  starting  switches 
so  that  the  various  motors  are  started  on  different  side  circuits. 

If  the  generator  winding  is  of  the  " open  coil"  type,  a  similar 
arrangement  may  be  obtained  for  two  phases  The  two  windings 
may  be  connected  to  the  middle  point,  thus  giving  side  circuits  of 
0.7  electro-motive  force  This  is  shown  in  Fig  22. 


PIG  22— WINDINGS  OP  '  'OPEN  COIL,"  TWO-PHASE  GENERATORS  CONNECTED 
TOGETHER  AT  MIDDLE  POINT  TO  ALLOW  STARTING  OP  MOTORS  FROM 
"SIDE  CIRCUITS'1, 

Three-phase  connections  do  not  allow  any  convenient  com- 
binations with  the  generator  winding.  A  fourth  wire  may  be  run, 
however,  which  will  give  about  0  58  electro-motive  force  for 
starting. 


FIG    23— CONNECTIONS  OF  TRANSFORMERS  ON  TWO-PHASE  CIRCUITS 
TO  GIVE  .7  AND  .5  NORMAL  VOLTAGE  FOR  STARTING  MOTORS 

VARIABLE  ELECTRO-MOTIVE  FORCE  FROM  TRANSFORMERS 
The  method  of  varying  the  electro-motive  force  by  means  of 
transformers  admits  of  many  different  combinations.     Several  of 
the  simpler  forms  will  be  given. 

(1)  The  transformers  may  be  so  connected  that  two  or  more 
electro-motive  forces  may  be  obtained. 

For  two-phase  circuits,  the  secondaries  may  be  connected 
together  at  the  centre,  as  shown  in  Fig.  23  This  gives  two  main 
circuits,  and  four  side  circuits  of  lower  electro-motive  force  If  an 
extra  wire  be  carried  out  from  the  point  5,  then  1-5,  2-5,  will  form 
a  two-phase  combination  for  0  5  voltage,  while  1-2,  2-3  form  a 


THE  POLYPHASE  MOTOR  3S 

two-phase  combination  for  0.7  voltage,  and  1-3  and  2-4  give  full 
voltage. 

Another  method  is  to  connect  the  secondaries  at  one  side  of 
the  centre,  as  shown  in  Fig.  24.  Then  3-5  and  4-5  give  one  electro- 
motive force;  1-5  and  2-5  give  a  higher  electro-motive  force,  and 
1-3  and  2-4  give  full  electro-motive  force. 

l 


S 


PIG.  24-CONNECTIONS  OP  SECONDARIES  OP  TRANSFORMERS  ON  TWO-PHASE 
CIRCUIT  AT  A    POINT  ONE   SIDE  OP  THE   CENTER,   TO   OBTAIN  LOWER 
ELECTRO-MOTIVE  FORCES  FOR  STARTING  MOTORS. 

These  combinations  are  useful  in  certain  cases,  but  are  not  as 
general  in  their  application  as  the  following  method . 

(2)  Auto-transformers  with  loops  brought  out  for  lower 
electro-motive  forces. 


FIG.  25— DIAGRAMMATIC  ARRANGEMENT  OF  AUTO-TRANSFORMERS  AND 
CONTROLLERS  FOR  REGULATING  SPEED  OF  TWO-PHASE  MOTOR  BY  VAR- 
IANCES OF  VOLTAGE. 

In  this  method,  no  special  combinations  of  the  lines,  lowering 
transformers  or  generators  are  made,  but,  in  connection  with  each 
motor,  a  small  pair  of  auto,  or  one-coil  transformers,  is  used  for 


34  ELECTRICAL  ENGINEERING  PAPERS 

auto-transformers  are  made  larger  From  these  auto- transformers 
several  loops  or  connections  are  brought  out.  For  regulating  the 
speed  these  are  connected  to  the  contact  plates  or  dials  of  a  con- 
troller, as  shown  in  Figs  25  and  26  But  for  starting  purposes  only, 
when  but  one  loop  from  each  transformer  is  used,  a  pair  of  switches 


8 

vvwvwT 

vwvwd 

FIG  26— DIAGRAMMATIC  ARRANGEMENT  OP  AUTO-TRANSFORMERS  AND 
CONTROLLER  FOR  REGULATING  SPEED  OF  THREE-PHASE  MOTOR  BY 
VARYING  THE  VOLTAGE 

are  used  in  connection  with  the  transformers.  With  the  switches 
open,  the  motor  is  disconnected.  Throwing  one  direction  starts 
the  motor  at  reduced  voltage  and  brings  it  up  to  almost  full  speed. 
The  switches  are  then  thrown  over  to  full  electro-motive  force. 

Two  small  transformers  in  a  case  with  one  four- jaw,  throw- 
over  switch,  form  what  is  called  an  "auto-starter."  This  is 
readily  arranged  for  either  two  or  three-phase  circuits  and  motors. 
This  makes  a  most  flexible  arrangement  for  starting,  as  the  motor 
may  be  put  at  any  location,  and  the  auto-starter  may  be  put  in  the 
most  convenient  position.  It  also  loads  all  the  line  wires  equally 
at  start,  and  each  motor  and  starter  really  form  a  unit  separate 
from  all  the  others  One  pair  of  transformers  may  be  connected 
to  several  sets  of  switches  and  thus  be  used  for  starting  several 
motors. 

Where  motors  are  close  to  reducing  transformers,  the  second- 
aries of  the  transformers  may  have  loops  brought  out,  to  which 
one  or  more  switches  are  connected.  The  primaries  of  the  trans- 
formers may  have  loops  connected  to  proper  switches,  and  the 
number  of  primary  turns  in  the  circuit  may  be  varied  instead  of 
the  secondary.  This  is  applicable  when  the  transformers  supply 
only  one  motor,  or  when  several  motors  are  started  at  the  same 
time.  A  regulator  with  secondary  movable  with  respect  to  the 
primary  may  be  used  Regulators  of  this  type  vary  the  electro- 


THE  POLYPHASE  MOTOR  35 

motive  forces  without  any  "make"  or  "break"  devices,  and  con- 
sequently have  no  sparking  tendency.  But  they  are  in  general 
too  complicated  and  costly  to  compete  with  the  transformer  with 
loops. 

VARYING  THE  MOTOR  CONNECTIONS 

This  is  not  a  method  for  changing  the  electro-motive  force 
applied,  but  for  varying  the  number  of  turns  in  series  with  a  given 
electro-motive  force,  and  the  effect  is  the  same  as  varying  the  ap- 
plied electro-motive  force.  This  method  is  rather  limited  in  its 
application  owing  to  the  complication  involved.  The  simplest 
case  for  two-phase  motors  is  a  series-parallel  combination  of  the 
windings  of  each  phase.  This  is  equivalent  to  using  0.5  electro- 
motive force  at  start.  For  three-phase  motors,  series-parallel  may 
be  used  or  the  winding  may  be  thrown  from  the  star  system  of 
connection  at  start  to  the  delta  system  for  running.  This  is  equiv- 
alent to  using  about  0.6  electro-motive  force  for  start.  But,  as 
the  star  connection  is  preferred  for  the  running  condition,  this  com- 
bination is  not  advisable. 

CHOKE  COILS  OR  RESISTANCE  IN  THE  PRIMARY 
There  is  a  fourth  method  of  regulation  which  may  be  men- 
tioned, but  which  is  not  advisable  in  general  practice.  This  is  the 
use  of  choke  coils  or  resistance  in  the  primary  circuits  of  the  motor, 
to  reduce  the  electro-motive  force.  These  really  give  varying 
electro-motive  forces.  With  choke  coils,  the  power  factor  at  start 
is  lowered,  with  correspondingly  bad  effect  on  the  generator  and 
system.  With  ohmic  resistance  in  the  primary  circuit,  the  reduc- 
tion of  electro-motive  force  is  accompanied  by  a  consumption  of 
energy  in  the  primary  circuit  which  in  no  way  represents  torque. 


WASHINGTON,  BALTIMORE  &  ANNAPOLIS  SINGLE- 
PHASE  RAILWAY 

FOREWORD — This  paper  was  presented  before  the  American  Insti- 
tute of  Electrical  Engineers,  September,  1902.  It  was  the  very 
first  information  given  out  for  publication  regarding  the  single- 
phase  alternating-current  railway  system  as  developed  and  in- 
stalled so  extensively  since  that  time. 

Before  the  publication  of  this  paper,  it  was  generally  as- 
sumed that  the  difficulties  in  the  commutation  of  alternating 
current  were  so  great  that  only  motors  of  relatively  small  capac- 
ity could  be  built.  Following  its  publication,  many,  of  the  larger 
companies  throughout  the  world  began  work  on  such  motors 
and  produced  operating  railway  equipments  with  more  or  less 
success  — (ED.) 


THE  Washington,  Baltimore  and  Annapolis  Railway  is  a  new 
high-speed  electric  line  extending  from  the  suburbs  of  Wash- 
ington to  Baltimore,  a  distance  of  about  31  miles,  with  a  branch 
from  Annapolis  Junction  to  Annapolis,  a  distance  of  about  15 
miles.  The  overhead  trolley  will  be  used,  and  schedule  speeds 
of  over  40  miles  per  hour  are  to  be  attained.  This  road  is  to  be 
the  scene  of  the  first  commercial  operation  of  an  entirely  new 
system  of  electric  traction. 

The  special  feature  of  this  system  is  the  use  of  single-phase  alter- 
nating current  in  generators,  transmission  lines,  trolley  car  equip- 
ment and  motors  It  constitutes  a  wide  departure  from  present 
types  of  railway  apparatus.  The  standard  D.  c.  railway  equipment 
possesses  several  characteristics  which  fit  it  especially  for  railway 
service.  These  characteristics  have  been  of  sufficient  importance  to 
overbalance  many  defects  in  the  system.  In  fact,  a  far  greater 
ambunt  of  effort  and  engineering  skill  has  been  required  for  over- 
coming or  neutralizing  the  defects,  than  for  developing  the  good 
features  possessed  by  the  system.  By  far  the  most  important 
characteristic  possessed  by  the  D.  c.  system  is  found  in  the  type 
of  motor  used  on  the  car.  The  D.  c.  railway  motor  is  in  all  cases 
a  series-wound  machine.  The  series  motor  is  normally  a  variable 
field  machine  and  it  is  this  feature  which  has  adapted  the  motor 
especially  to  railway  service.  Shunt-wound  motors  have  been 
tried  and  abandoned.  All  manner  of  combinations  of  shunt, 

37 


3S  ELECTRICAL  ENGINEERING  PAPERS 

series  and  separate  excitation  have  been  devised  and  found  want- 
ing, and  in  many  casesthe  real  cause  of  failure  was  not  recognized 
by  those  responsible  for  the  various  combinations.     They  all 
missed  to  a  greater  or  less  extent  the  variable-field  feature  of 
the  straight  series  motor.     It  is  true  that  a  variable  field  can  be 
obtained  with  shunt  or  separate  excitation,  but  not  without  con- 
trolling or  regulating  devices,  and  the  variation  is  not  inherently 
automatic,  as  in  the  series  motor      Polyphase  and  single-phase 
induction  motors  do  not  possess  the  variable  field  feature  at  all, 
as    they    are    essentially    constant-field  machines      They   are 
equivalent  to  direct  current  shunt  or  separately  excited  motors 
with  constant  field  strength,  which  have  been  unable  to  compete 
successfully  with  the  series  motor     The  variable  field  of  the 
series  motor  makes  it  automatically  adjustable  for  load  and 
speed  conditions.     It  also  enables  the  series  motor  to  develop 
large  torques  without  proportionately  increased  currents      The 
automatically  varying  field  is  accompanied  by  corresponding 
variations  in  the  counter  e.m  f.  of  the  armature,  until  the  speed 
can  adjust  itself  to  the  new  field  conditions      This  feature  is  of 
great  assistance  in  reducing  current  fluctuations,  with  a  small 
number  of  steps  in  the  regulating  rheostat      Any  increase  in 
current,  as  resistance  is  cut  out,  is  accompanied  by  a  momentary 
increase  in  the  counter  e.rn  f  ,  thus  limiting  the  current    increase 
to  a  less  value  than  in  the  case  of  constant  field  motor 

Next  to  the  type  of  motor,  the  greatest  advantage  possessed  by 
the  D,  c.  system  lies  in  the  use  of  a  single  current  or  circuit,  thus 
permitting  the  use  of  one  trolley  wire  The  advantages  of  the 
single  trolley  are  so  well-known  that  it  is  unnecessary  to  discuss 
them  For  third  rail  construction,  the  use  of  single  current  is  of 
even  greater  importance  than  in  the  case  of  overhead  trolley 
It  is  seen,  therefore,  that  it  is  not  to  the  direct  current  that 
credit  should  be  given  for  the  great  success  of  the  present  railway 
system,  but  to  the  series  type  of  motor  and  the  fact  that  up  to 
the  present  time  no  suitable  single-phase  A  c.  motor  has  been 
presented 

Some  of  the  undesirable  features  of  the  D  c  railway  system 
should  also  be  considered.  The  speed  control  is  inefficient.  A 
nominally  constant  voltage  is  supplied  to  the  car,  and  speed  con- 
trol is  obtained  by  applying  variable  voltage  at  the  motor  ter- 
minals This  variation  is  produced  by  the  use  of  resistance  in 
series  with  the  motors,  with  a  loss  proportional  to  the  voltage 
taken  up  by  the  resistance  By  means  of  the  series-parallel 


SINGLE-PHASE  RAILWAY  39 

arrangement,  the  equivalent  of  two  voltages  is  obtainable  at  the 
motor  terminals  without  the  use  of  resistance.  Therefore,  with 
•series-parallel  control,  there  are  two  efficient  speeds  with  any 
given  torque,  and  with  multiple  control  there  is  but  one  efficient 
speed  with  a  given  torque  All  other  speeds  are  obtained 
through  rheostatic  loss,  and  the  greater  the  reduction  from 
either  of  the  two  speeds,  series  or  parallel,  the  lower  will  be  the 
efficiency  of  the  equipment.  At  start,  the  rheostatic  losses'  are 
always  relatively  large,  as  practically  all  the  voltage  of  the  line 
is  taken  up  in  the  rheostat  For  heavy  railroad  service,  where 
operation  for  long  periods  at  other  than  full  and  half  speeds  may 
be  necessary,  the  rheostatic  loss  will  be  a  very  serious  matter. 

The  controlling  devices  themselves  are  also  a  source  of  trouble. 
An  extraordinary  amount  of  time  and  skill  has  been  expended' 
on  the  perfection  of  this  apparatus  The  difficulties  increase 
with  the  power  to  be  handled  The  controller  is  a  part  of  the 
equipment  which  is  subjected  to  much  more  than  ordinary 
mechanical  wear  and  tear,  and  it  can  go  wrong  at  any  one  of 
many  points.  The  larger  the  equipment  to  be  controlled,  the 
more  places  are  to  be  found  in  the  controller  which  can  give 
trouble.  The  best  that  can  be  said  of  the  railway  controller  is 
that  it  is  a  necessarv  evil. 

Another  limitation  of  the  D.  c.  system  is  the  trolley  voltage 
Five  hundred  volts  is  common  at  the  car  and  650  volts  is  very 
unusual.  By  far  the  larger  number  of  the  railway  equipments 
in  service  to-day  are  unsuited  for  operation  at  600  volts,  and  700 
volts  in  normal  operation  would  be  unsafe  for  practically  all 
The  maximum  permissible  trolley  voltage  is  dependent  upon 
inherent  limitations  in  the  design  of  motors  and  controllers 
The  disadvantages  of  low  voltage  appear  in  the  extra  cost  of  cop- 
per and  in  the  difficulty  of  collecting  current.  In  heavy  railroad 
work  the  current  to  be  handled  becomes  enormous  at  usual 
voltages.  A  2400  h  p.  electric  locomotive,  for  example,  will 
require  between  3000  and  4000  amperes  at  normal  rated  power 
and  probably  6000  to  8000  amperes  at  times.  With  the 
overhead  trolley  these  currents  are  too  heavy  to  be  collected  in 
the  ordinary  manner,  and  it  is  a  serious  problem  with  any  form 
of  trolley  or  third  rail  system  which  can  be  used.  It  is  evident 
that  for  heavy  service,  comparable  with  that  of  large  steam  rail- 
ways, a  much  higher  voltage  than  used  in  our  present  D.  c.  sys- 
tem is  essential,  and  the  use  of  higher  voltage  is  destined  to  come, 
provided  it  is  not  attended  by  complications  which  more  than 


40  ELECTRICAL  ENGINEERING  PAPERS 

overbalance  the  benefits  obtained.  A  £111 1 her  disadvantage  of 
the  D.  c.  system  is  the  destructive  action  known  as  electrolysis 
This  may  not  be  of  great  importance  in  interurban  lines,  chiefly 
because  there  is  nothing  to  be  injured  by  it.  In  city  work  its 
dangers  are  well-known,  and  very  expensive  constructions  are 
now  used  to  eliminate  or  minimize  its  effects. 

From  the  above  statements  it  is  evident  that  an  A  c  railway 
system,  to  equal  the  D.  c.,  should  possess  the  two  principal 
features  of  the  D.  c.  system,  viz  A  single  supply  circuit  and  the 
variable  field  motor,  and  to  be  an  improvement  upon  the  D.  c. 
system,  the  A.  c.  should  avoid  some  of  the  more  important  dis- 
advantages incident  to  the  present  D.  c.  railway  apparatus, 

The  system  must,  therefore,  be  single- phase.  The  importance 
of  using  single-phase  for  railway  work  is  well  known  The  diffi- 
culties and  complications  of  the  trolley  construction  are  such 
that  several  A.  c.  systems  have  been  planned  on  the  basis  of 
single-phase  supplied  to  the  car,  with  converting  apparatus  on 
the  car  to  transform  to  direct  current,  in  order  that  the  standard 
type  of  railway  motors  may  be  used  Such  plans  are  attempts 
to  obtain  the  two  most  valuable  features  of  the  present  D  c. 
system.  The  polyphase  railway  system,  used  on  a  few  European 
roads,  employs  three  currents,  and  therefore  does  not  meet  the 
above  requirement.  The  motor  for  the  A  c  railway  service 
should  have  the  variable  speed  characteristics  of  the  series  D.  c. 
motor.  The  polyphase  motor  is  not  suitable,  as  it  is  essentially 
a  constant  field  machine,  and  rloes  not  possess  any  true  variable 
speed  characteristics.  Therefore  it  lacks  both  of  the  good  fea- 
tures of  the  D.  c.  railway  system.  A  new  type  of  motor  must, 
therefore  be  furnished,  as  none  of  the  alternating  current  motors 
in  commercial  use  is  adapted  for  the  speed  and  torque  require- 
ments of  first-class  railway  service.  Assuming  that  such  a 
motor  is  obtainable  for  operation  on  a  single-phase  circuit,  the 
next  step  to  consider  is  whether  the  use  of  alternating  instead 
of  direct  current  on  the  car,  will  allow  some  of  the  disadvan- 
tageous features  of  the  D.  c.  system  to  be  avoided.  The  D.  c. 
limits  of  voltage  are  at  once  removed,  as  transformers  can  be 
used  for  changing  from  any  desired  trolley  voltage  to  any  con- 
venient motor  voltage.  Electrolysis  troubles  practically  disap- 
pear. As  transformers  can  be  used,  variations  in  supply  voltage 
are  easily  obtainable.  As  the  motor  is  assumed  to  have  the 
characteristics  of  the  direct-current  series  motor,  speed  control 
without  rheostatic  loss  is  practicable  when  voltage  control  is 


SINGLE-PHA  SR  RAILWAY  41 

obtained.  This  combination,  therefore,  allows  the  motor  to 
operate  at  relatively  good  efficiency  at  any  speed  within  the 
range  of  voltage  obtained  If  the  voltage  be  varied  over 
a  sufficiently  wide  range,  the  speed  range  may  be  car- 
ried from  the  maximum  desired  down  to  zero,  and  there- 
fore, down  to  starting  conditions.  With  such  an  arrange- 
ment no  rheostat  need  be  used  under  any  conditions,  and  the 
lower  the  speed  at  which  the  motor  is  operated,  the  less  the  power 
required  from  the  line.  The  least  power  is  required  at  start,  as 
the  motor  is  doing  no  work  and  there  is  no  rheostatic  loss.  The 
losses  at  start  are  only  these  in  the  motor  and  transforming 
apparatus,  which  are  less  than  when  running  at  full  speed  with 
an  equal  torque.  Such  a  system,  therefore,  permits  maximum 
economy  in  power  consumed  by  motor  and  control.  This 
economy  in  control  is  not  possible  with  the  polyphase  railway 
motor,  as  this  motor  is  the  equivalent  of  the  D.  c.  shunt  motor, 
with  which  the  rheostatic  loss  is  even  greater  than  with  the 
series  motor. 

The  use  of  alternating  current  on  the  car  allows  voltage  control 
to  be  obtained  in  several  ways.  In  one  method  a  transformer 
is  wound  with  a  large  number  of  leads  carried  to  a  dial  or  con- 
troller drum.  The  Stillwell  regulator  is  a  well-known  example 
of  this  type  of  voltage  control.  This  method  of  regulation  is 
suitable  for  small  equipments  with  moderate  currents  to  be 
handled.  The  controller  will  be  subject  to  some  sparking,  as  in 
the  case  of  D.  c.  apparatus,  and  therefore  becomes  less  satisfactory 
as  the  car  equipment  is  increased  in  capacity.  Another  method 
of  control  available  with  alternating  current  is  entirely  non- 
sparking,  there  being  no  make-and-break  contacts.  This  con- 
troller is  the  so-called  "  induction  regulator,"  which  is  a  trans- 
former with  the  primary  and  secondary  windings  on  separate 
cores.  The  voltage  in  the  secondary  winding  is  varied  by  shift- 
ing its  angular  position  in  relation  to  the  primary.  With  this 
type  of  voltage  controller,  very  large  currents  can  be  handled, 
and  it  is  especially  suitable  for  heavy  equipments,such  as  loco- 
motives It  is  thus  seen  that  there  is  o^ie  method  of  control, 
available  with  alternating  current,  which  avoids  the  troubles 
inherent  to  the  D.  c.  controller.  The  induction  regulator  is 
primarily  a  transformer,  and  all  wear  and  tear  is  confined  to  the 
supports  which  carry  the  rotor.  Therefore  the  objectionable 
controller  of  the  standard  D.  c.  system  can  be  eliminated,  pro- 
vided a  suitable  A.  c.  motor  can  be  obtained.  This  ideal  type 


42  ELECTRICAL  ENGINEERING  PAPERS 

of  controller  is  not  applicable  to  the  polyphase  railway  motor,  in 
which  speed  control  can  be  obtained  only  through  rhcostatic 
loss.  The  polyphase  control  system  is  even  more  complicated 
than  the  D.  c  ,  as  there  must  be  a  rheostat  for  each  motor,  and 
two  or  three  circuits  in  each  rheostat  It  is  thus  apparent  that 
by  the  use  of  single-phase  alternating  current  with  an  A.  c 
motor  having  the  characteristics  of  the  D  c.  series  motor,  the 
best  features  of  the  D  c.  system  can*  be  obtained,  and  at  the 
same  time  many  of  its  disadvantages  can  be  avoided. 

This  portion  of  the  problem  therefore  resolves  itself  into  the 
construction  of  a  single-phase  motor  having  the  characteristics 
of  the  D  c.  series  motor  There  are  several  types  of  single  phase 
A.  c.  motors  which  have  the  series  characteristics.  One 
type  is  similar  in  general  construction  to  a  D.  c.  motor,  but  with 
its  magnetic  circuit  laminated  throughout,  and  with  such  pro- 
portions that  it  can  successfully  commutate  alternating  current 
Such  a  motor  is  a  plain  series  motor,  and  can  be  operated  on 
either  alternating  or  direct  current  and  will  have  the  same  torque 
characteristics  in  either  case.  Another  type  of  motor  is  similar 
in  general  construction  to  the  above,  but  the  circuits  are  ar- 
ranged in  a  different  manner.  The  field  is  connected  directly 
across  the  supply  circuit,  with  proper  control  appliances  in  series 
with  it.  The  armature  is  short-circuited  on  itself  across  the 
brushes,  and  the  brushes  are  set  at  an  angle  of  approximately 
45°  from  the  ordinary  neutral  point  The  first  of  these  two  types 
of  motors  is  the  one  best  adapted  for  operation  in  large  units. 

This  is  the  type  of  motor  which  is  to  be  used  on  the  Washington 
Baltimore  and  Annapolis  Railway.  Several  motors  have  been 
built  and  tested  with  very  satisfactory  results,  both  on  the  test- 
ing stand  and  under  a  car.  The  results  were  so  favorable  that 
the  system  was  proposed  to  the  Cleveland  Engineering  Company, 
representing  the  Washington,  Baltimore  and  Annapolis  Railway, 
and  after  investigation  by  their  engineers,  the  system  was 
adopted.  A  description  of  the  apparatus  to  be  used  on  this  road 
will  illustrate  the  system  to  good  advantage. 

Single-phase  alternating  current  will  bo  suppled  to  the  car  at 
a  frequency  of  16 J  cycles  per  second,  or  2T000  alternations  per 
minute.  The  current  from  the  overhead  trolley  wire  is  normally 
fed  in  by  one  trolley  at  approximately  1,000  volts.  Within 
the  limits  of  the  District  of  Columbia  two  trolleys  are  employed, 
as  by  Act  of  Congress  the  use  of  rails  as  conductors  is  prohibited 
in  this  District,  presumably  on  account  of  electrolysis.  In  this 


SINGLE-PHA  SE  RAIL  WA  Y  43 

case  the  trouble,  of  course,  will  not  exist,  but  the  contracting 
company  has  been  unable  to  obtain  permission  for  the  grounded 
circuit. 

The  alternating  current  to  the  car  is  carried  through  a  main 
switch  or  circuit  breaker  on  the  car,  to  an  auto-transformer 
connected  between  the  trolley  and  the  return  circuit.  At 
approximately  300  volts  from  the  ground  terminal,  a  lead  is 
brought  out  from  the  auto-transformer  and  passes  through  the 
regulator  to  one  terminal  of  the  motors.  For  starting  and  con- 
trolling the  speed,  an  induction  regulator  is  used  with  its  second- 
ary winding  in  series  with  the  motors.  This  secondary  circuit 
of  the  regulator  can  be  made  either  to  add  to,  or  substract  from 
the  transformer  voltage,  thus  raising  or  lowering  the  voltage 


VVWVW^ 1 

<v*wvw> — I j 

rOflO 


Fio.  1  — a    Auto-Transformer     b.  Induction  Regulator     c   Reversing  Switch,     d    Fta  d 
of  Motors      e    Armature  of  Motors      f    Equalizing  Transformer. 

supplied  to  the  motors.  The  regulator  therefore  does  double 
duty.  The  controller  for  D.  c.  motors  merely  lowers  the  voltage 
supplied  to  the  motors  but  cannot  raise  it,  but  an  A.  c.  regulator 
can  be  connected  for  an  intermediate  voltage,  and  can  either 
raise  or  lower  the  motor  voltage.  In  this  way  the  regulator  can 
be  made  relatively  small,  as  it  handles  only  the  variable  element 
of  the  voltage  and  the  maximum  voltage  in  the  secondary  wind- 
ing is  but  half  of  the  total  variation  required. 

In  the  equipments  in  question,  the  range  of  voltage  at  the 
motor  is  to  be  varied  from  approximately  200  volts  up  to  400 
volts  or  slightly  higher.  The  transformer  on  the  car  will  supply 
315  volts,  and  the  secondary  circuit  of  the  regulator  will  be 


44  ELECTRICAL  ENGINEERING  PAPERS 

wound  to  generate  slightly  more  than  100  volts  when  turned  to 
the  position  of  its  maximum  voltage.  This  voltage  of  the  regu- 
lator is  about  one-fourth  of  that  of  the  motors  at  full  voltage. 
The  regulator  can  consequently  be  made  relatively  small,  in 
comparison  with  the  motor  capacity  of  the  equipment.  It  has 
been  found  unnecessary  to  use  much  lower  than  200  volts  in  this 
installation,  as  this  voltage  allows  a  comparatively  low  running 
speed,  and  approximately  200  volts  will  be  necessary  to  start 
with  the  required  torque  The  greater  part  of  this  voltage  5s 
required  to  overcoine  the  e  m  1.  of  self-induction  in  the  motor 
windings,  which  is  dependent  upon  the  current  through  the 
motor  and  is  independent  of  the  speed  ot  the  armature. 

There  will  be  four  motors  of  100  h.p.  on  each  car  The  full 
rated  voltage  of  each  motor  is  approximately  220  volts  The 
motors  are  arranged  in  two  pairs,  each  consisting  of  two  arma- 
tures in  series,  and  two  fields  in  series,  and  the  two  pairs  are 
connected  in  parallel  The  motors  are  connected  permanently 
in  this  manner  As  voltage  control  is  used,  there  is  no  necessity 
for  series  parallel  operation,  as  with  D.  c.  motors.  To  ensure 
equal  voltage  to  the  armatures  in  scries,  a  balancing  or  equalizing 
action  is  obtained  by  the  use  of  a  small  auto-transformer  con- 
nected permanently  across  the  two  armatures  in  series  with  its 
middle  point  connected  between  them.  The  fields  are  arranged 
m  two  pairs,  with  two  fields  in  scries  and  two  pairs  in  multiple 
This  parallels  the  fields  independently  of  the  armatures,  which 
was  formerly  the  practice  with  D  c  motors.  It  was  a  defective 
arrangement  with  such  motors,  as  equal  currents  in  the  field  did 
not  ensure  equal  field  strengths  in  the  motors,  and  the  armatures 
connected  in  parallel  would  be  operating  m  fields  of  unequal 
strength,  with  unequal  armature  currents  as  a  direct  result. 
With  alternating  currents  in  the  fields,  the  case  is  different 
The  voltage  across  the  fields  is  dependent  upon  the  field  strengths, 
and  the  current  supplied  to  the  fields  naturally  divides  itself  for 
equal  magnetic  strengths  The  chief  advantage  m  paralleling 
the  fields  and  armatures  independently  is,  that  one  reversing 
switch  may  serve  for  the  four  motors  and  one  balancing  trans- 
former may  be  used  across  the  two  pairs  of  armatures  The 
usual  D.  c.  arrangement  of  armatures  m  series  with  their  own 
fields  can  be  used,  with  a  greater  number  of  switches  and  con- 
nections. 

The  general  arrangement  of  the  auto-transformer,  regulator, 
jnotors,  etc  ,  is  shown  in  Fig.  1 


SINGLE-PHA SE  RAILWAY  45 

The  induction  regulator  or  controller,  resembles  an  induction 
motor  in  general  appearance  and  construction.  The  primary 
winding  is  placed  on  the  rotor,  and  the  secondary  or  low  voltage 
winding  on  the  stator.  The  rotor  also  has  a  second  winding 
which  is  permanently  short-circuited  on  itself.  This  function 
of  this  short-circuited  winding  is  to  neutralize  the  self-induction 
of  the  secondary  winding  as  it  passes  from  the  magnetic  influ- 
ence of  the  primary.  The  regulator  is  wound  for  two  poles,  and 
therefore  is  operated  through  180°  for  producing  the  full  range 
of  variation  of  voltage  for  the  motors.  One  end  of  the  primary 
winding  of  the  regulator  is  connected  to  the  trolley,  and  the 
other  to  a  point  between  the  regulator  and  the  motors.  It  thus 
receives  a  variable  voltage  as  the  controller  is  rotated.  There 
are  several  advantages  in  this  arrangement  of  the  primary  in 
this  particular  case.  First,  the  regulator  is  worked  at  a  higher 
induction  at  start,  and  at  lower  induction  when  running,  the 
running  position  being  used  in  these  equipments  for  much  longer 
periods  than  required  for  starting  Second,  when  the  motors 
are  operating  at  full  voltage  the  current  in  the  primary  of  the 
•regulator  passes  through  the  motors  but  not  through  the  auto- 
transformer  or  the  secondary  of  the  regulator.  This  allows  con- 
siderable reduction  m  the  size  of  auto-transformer  and  regulator. 
The  motors  on  the  car  are  all  of  the  straight  series  type.  The 
armature  and  fields  being  connected  in  series,  the  entire  current 
of  the  field  passes  through  the  armature  as  in  ordinary  series 
D  c.  motors.  The  motor  has  eight  poles,  and  the  speed  is 
approximately  700  revolutions  at  220  volts.  The  general  con- 
struction is  similar  to  that  of  a  D.  c.  motor,  but  the  field  core  is 
laminated  throughout,  this  being  necessary  on  account  of  the 
alternating  magnetic  field.  There  are  eight  field-coils  wound 
with  copper  strap,  and  all  connected  permanently  in  parallel. 
The  parallel  arrangement  of  field-coils  assists  in  the  equalizing 
of  the  field  strength  in  the  different  poles,  due  to  the  balancing 
action  of  alternating  circuits  in  parallel.  This  arrangement  is 
not  really  necessary,  but  it  possesses  some  advantages  and 
therefore  has  been  used.  With  equal  magnetic  strength  in  the 
poles,  the  magnetic  pull  is  equalized  even  with  the  armature  out 
of  center.  The  armature  is  similar  in  general  construction  to 
that  of  a  D.  c.  motor.  The  fundamental  difficulty  in  the  opera- 
tion of  a  commutator  type  of  motor,  on  single-phase  alternating 
current  lies  in  the  sparking  at  the  brushes.  The  working  current 
passing  through  the  motor  should  be  practically  no  more  difficult 


46  ELECTRICAL  ENGINEERING  PAPERS 

to  commutate  than  an  equal  direct  current,  and  it  is  not  this  cur- 
rent which  gives  trouble.  The  real  source  of  trouble  is  found  in 
a  local  or  secondary  current  set  up  in  any  coil,  the  two  ends  of 
which  are  momentarily  short-circuited  by  a  brush.  This  coil 
encloses  the  alternating  magnetic  field,  and  thus  becomes  a 
secondary  circuit  of  which  the  field-coil  forms  the  primary.  In 


40         ,10         00         TO         80         UO        300        110        130       130        140 
HORSE  POWER 

PIG  2  — 'Westinghouse  Alternating  Current  Railway  Motor     No,  91, — Single-Phase  — 220 

Volts. 

the  motors  of  the  Washington,  Baltimore  and  Annapolis  Rail- 
way, this  commutation  difficulty  has  been  overcome  by  so  con- 
structing the  motor  that  the  secondary  -or  short-circuit  current  in 
the  armature  coil  is  small,  and  the  commutating  conditions  so 


SINGLE-PHASE  RAILWA  Y  47 

perfect  that  the  combined  working  and  secondary  currents  can 
be  commutated  without  sparking.  This  condition  being  ob- 
tained, the  motor  operates  like  a  D.  c,  machine  and  will  give  no 
more  trouble  at  the  commutator  than  ordinary  D.  c.  railway 
motors.  Experience  covering  a  considerable  period  in  the  opera- 
tion of  motors  of  100  h.p.  capacity  indicates  that  no  trouble  need 
be  feared  at  the  commutator. 

An  extended  series  of  tests  were  made  at  the  Westinghouse 
shops  at  East  Pittsburg,  both  in  the  testing  room  and  under  a 
car.  Fig.  2  shows  curves  of  the  speed,  torque,  efficiency  and 
power  factor  plotted  from  data  from  brake  tests. 

It  should  be  noted  that  »the  efficiency  is  good,  being  very 
nearly  equal  to  that  of  high-class  D.  c.  motors.  The  power 
factor,  as  shown  in  these  curves,  is  highest  at  light  loads  and 
decreases  with  the  load.  This  is  due  to  the  fact  that  the  power 
developed  increases  approximately  in  proportion  to  the  current, 
while  the  wattless  component  of  the  input  increases  practically 
as  the  square  of  the  current.  The  curve  indicates  that  the 
average  power  factor  should  be  very  good.  The  calculations 
for  the  W.  B.  and  A.  Railway  show  that  the  average  power  factor 
of  the  motors  will  be  approximately  96  per  cent. 

The  average  efficiency  of  these  equipments  will  be  much 
higher  during  starting  and  acceleration  than  that  of  correspond- 
ing D.  c.  equipments,  and  rheostatic  losses  are  avoided.  When 
running  at  normal  full  speed,  however,  the  efficiency  will  be 
slightly  less  than  with  D.  c.  This  is  due  to  the  fact  that  the  A.  c. 
motor  efficiency  is  slightly  lower  than  the  D.  c.,  and  in  addition 
there  are  small  losses  in  the  transformer  and  the  regulator.  The 
A.  c  equipments  are  somewhat  heavier  than  the  D.  c.,  thus  re- 
quiring some  extra  power,  both  in  accelerating  and  at  full  speed. 
Therefore,  for  infrequent  stops  the  D.  c,  car  equipment  is  more 
efficient  than  the  A.  c.,  but  for  frequent  stops  the  A.  c.  shows  the 
better  efficiency.  Tests  on  the  East  Pittsburg  track  verified 
this  conclusion.  But  the  better  efficiency  of  the  D.  c.  equipment 
with  infrequent  stops  is  offset  with  the  A.  c.  by  decreased  loss  in 
the  trolley  wire,  by  reason  of  the  higher  voltage  used,  and  the 
elimination  of  the  rotary  converter  losses..  The  resultant  effi- 
ciency for  the  system  will  therefore  be  equal  to  or  better  than 
that  of  the  D.  c. 

In  the  W.  B.  and  A  Railway  contract  the  guarantee  given  by 
the  Westinghouse  Electric  and  Mfg.  Co.  states  that  the  efficiency 
of  the  system  shall  be  equal  to  that  of  the  D.  c.  system  with  rotary 
converter  substations 


48  ELECTRICAL  ENGINEERING  PAPERS 

There  is  one  loss  in  the  A.  c.  system  which  is  relatively  much 
higher  than  in  the  o  c  This  is  the  loss  in  the  rail  return.  Tests 
have  shown  that  at  2,000  alternations  this  is  three  to  four  times 
as  great  as  with  an  equal  direct  current  This  would  be  a 
Serious  matter  in  cases  where  the  D.  c  rail  loss  is  high.  But  the 
higher  A.  c  trolley  voltage  reduces  the  current  so  much,  that 
the  A.  c.  rail  loss  is  practically  the  same  as  with  direct  current 
at  usual  voltages  In  many  city  railways  the  D.  c.  rail  loss  is 
made  very  low,  not  to  lessen  waste  of  power,  but  in  order  to 
reduce  electrolysis.  In  such  cases  the  A.  c.  rail  loss  could  be 
higher  than  D.  c  ,  thus  decreasing  the  cost  of  return  conductors. 
More  frequent  transformer  substations,  with  copper  feeders 
connected  to  the  rails  at  frequent  intervals  will  enable  the  rail 
loss  to  be  reduced  to  any  extent  desired.  As  a  frequency  of 
2,000  alternations  per  minute  is  used,  the  lighting  of  the  cars  and 
the  substations  was  at  first  considered  to  be  a  serious  difficulty, 
due  to  the  very  disagreeable  winking  of  ordinary  incandescent 
lamps  at  this  frequency.  Two  methods  of  overcoming  the 
winking  were  tried,  both  of  which  were  successful.  One  method 
was  by  the  use  of  split  phase.  A  two-phase  induction  motor 
(was  run  on  a  single-phase  2,000  alternating  circuit,  and  current 
was  taken  from  the  unconnected  primary  circuit  of  the  motor, 
This  current  was,  of  course,  at  approximately  90°  from  the  cur- 
rent of  the  supply  circuit.  A  two-phase  circuit  was  thus  obtained 
on  the  car.  Currents  from  the  two  phases  wore  put  through 
ordinary  incandescent  lamps,  placed  close  together.  The 
resulting  illumination  a  few  feet  distant  from  the  lamps  showed 
about  the  same  winking  as  is  noticed  with  3,000  alts.  With  two 
filaments  in  one  lamp  the  winking  disappears  entirely.  A  three- 
phase  arrangement  would  work  in  the  same  way. 

A  muoh  simpler  method  was  tried  which  worked  equally  well. 
This  consisted  in  the  use  of  very  low-voltage  lamps.  I^ow  volt- 
age at  the  lamp  terminals  allows  the  use  of  a  thick  filament  with 
considerable  heat  inertia.  Tests  were  made  on  lamps  of  this 
type  at  a  frequency  of  2,000  alts.,  and  the  light  appeared  to  be  as 
steady  as  that  from  the  ordinary  high-frequency  incandescent 
lamp.  The  low  voltage  is  not  objectionable  in  this  case,  as  a 
number  of  lamps  can  be  run  in  a  series,  as  in  ordinary  street 
railway  practice,  and  any  voltage  desired  can  readily  be  obtained. 
as  alternating  current  is  used  on  the  car. 

There  will  be  an  air  compressor,  driven  by  a  series  A.  c.  motor, 
on  each  car,  for  supplying  air  to  the  brakes  and  for  operating 


SINGLE-PHASE  RAILWAY  49 

the  driving  mechanism  of  the  controller.  The  details  of  this 
mechanism  are  not  near  enough  to  completion  to  permit  a  de- 
scription of  it  The  method  used  will  be  one  which  readily 
allows  operation  on  the  multiple-unit  system. 

The  generating  station  contains  some  interesting  electrical 
features,  but  there  is  no  great  departure  from  usual  A.  c,  prac- 
tice. There  will  be  three  1,500  k  w.  single-phase  alternators. 
These  are  24-pole  machines  operating  at  83  revolutions  and 
wound  for  15,000  volts  at  the  terminals.  They  are  o,f  the 
rotating  field  type,  with  laminated  magnetic  circuits  aftd  field- 
coils  of  strap  on  edge.  The  field-coils  are  held  on  the  pole-tips 
by  copper  supports,  which  serve  also  as  dampers  to  assist  in  the 
parallel  running  The  armatures  are  of  the  usual  slotted  type. 
The  armature  coils  are  placed  in  partially  closed  slots.  There 
are  four  coils  per  pole  The  proportions  of  these  machines  are 
such  that  good  inherent  regulation  is  obtained  without  saturation 
of  the  magnetic  circuit.  The  rise  in  potential  with  non-inductive 
load  thrown  off  will  be  approximately  4  per  cent.  An  alterna- 
tive estimate  was  furnished  for  the  generators  proposing  20,000 
volts  instead  of  15,000.  The  simplicity  of  the  type  of  winding 
used,  and  the  low  frequency,  are  both  favorable  for  the  use  of 
very  high  voltage  on  the  generator.  As  15,000  volts  was  con- 
sidered amply  high  for  the  service,  the  engineers  for  the  railway 
considered  it  unadvisable  to  adopt  a  higher  voltage. 

There  are  to  be  two  exciters,  each  of  100  k  w.  capacity  at  250 
revolutions  The  exciters  are  wound  for  125  volts  normal.  The 
armature  of  each  exciter  has,  in  addition  to  the  commutator, 
two  collector  rings,  so  that  single-phase  alternating  current  can 
be  delivered  It  is  the  intention  to  use  the  exciters  as  alter- 
nators for  .supplying  current  to  the  system  for  lighting  when  the 
large  generators  are  shut  down  at  night.  The  main  station 
switchboard  comprises  three  generator  panels,  one  load  panel, 
and  three  feeder  panels.  High-tension  oil-break  switches  are  to 
be  provided,  operated  by  means  of  controlling  apparatus  on  the 
panels.  The  switches,  bus-bars  and  all  high-tension  apparatus 
will  be  in  brick  compartments  separate  from  the  board.  In 
each  generator  circuit  there  are  two  non-automatic  oil-break 
switches  in  series;  and  on  each  feeder  circuit  there  are  two  over- 
load time-limit  oil-break  switches  in  series.  The  two  oil-break 
switches  in  series  on  the  same  circuit  can  be  closed  separately 
and  then  opened  to  test  the  switches  without  closing  the  circuit. 
With  the  switches  in  the  closed  position  they  are  both  operated" 


50  ELECTRICAL  ENGINEERING  PAPERS 

at  the  same  time  by  the  controller,  to  ensure  opening  of  the  cir- 
cuit, and  to  put  less  strain  on  the  switches,  although  either  one 
is  capable  of  opening  the  load  There  will  be  nine  transformer 
substations  distributed  along  the  railway  line.  Each  station 
will  contain  two  250  k.w.  oil-cooled  lowering  transformers, 
supplying  approximately  1 ,000  volts  to  the  trolley  system.  The 
transformers  are  used  in  each  station  so  that  in  case  of  accident 
to  one  transformer  the  station  will  not  be  entirely  crippled.  It 
is  the  intention  of  the  railway  company  to  operate  a  n,  c.  road 
already  equipped  with  the  direct-current  system  The  present 
D.  c  car  equipments  arc  to  be  retained,  but  the  current  will  be 
supplied  from  a  rotary  converter  substation  fed  from  the  main 
system  of  the  W  B  and  A.  Railway.  As  this  system  is  single- 
phase,  it  is  necessary  that  single-phase  rotarics  be  used  in  the 
substations.  There  are  to  be  two  k.w.  550-volt  rotary  con- 
verters. These  are  4-pole,  500-rcvolution  machines.  The 
general  construction  of  these  machines  is  very  similar  to  that  of 
the  Westmghouse  polyphase  rotary  converters.  The  armature 
resembles  that  of  a  polyphase  rotary  except  in  the  number  of 
collector  rings,  and  in  certain  details  of  the  proportions  made 
necessary  by  reason  of  the  use  of  single-phase.  The  commutat- 
ing  proportions  are  so  perfect  that  any  reactions  due  to  the  use 
of  single-phase  will  result  in  no  injurious  effect.  The  field  con- 
struction is  similar  to  that  of  a  polyphase  rotary.  .  The  lamin- 
ated field-poles  are  provided  with  dampers  of  the  "  grid  "  or 
11  cage  "  type,  a  form  used  at  present  in  the  Westinghouse  poly- 
phase rotary  converters.  This  damper  serves  to  prevent  hunt- 
ing, as  in  the  polyphase  machines,  and  also  to  damp  out  pulsa- 
tions due  to  single-phase  currents  in  the  armature.  The  damper 
acts  to  a  certain  extent  as  a  second  phase.  Each  rotary  con- 
verter is  started  and  brought  to  synchronous  speed  by  a  small 
series  A.  c.  motor  on  the  end  of  the  shaft.  The  voltage  at  the 
motor  terminals  can  be  adjusted  either  by  loops  from  the  lower- 
ing transformer  or  by  resistance  in  series  with  the  motor,  so  that 
true  synchronous  speed  can  be  given  to  the  rotary  converter, 
before  throwing  it  on  the  A.  c.  line. 

From  the  preceding  description  of  this  system  and  the  appar- 
atus used  on  it,  some  conclusions  may  be  drawn  as  to  the  various 
fields  where  it  can  be  applied  to  advantage.  It  is  evident  that  ?i 
good  field  for  it  will  be  on  intcrurban  long-distance  lines  such  as 
the  W  B.  and  A,  Railway.  On  such  railways,  high  trolley 
voltage  and  the  absence  of  converter  substations  are  very 
important  factors. 


SINGLE-PHASE  RAILWAY  51 

For  heavy  railroading  also,  this  system  possesses  many  ideal 
features..  It  allows  efficient  operation  of  large  equipments  at 
practically  any  speed  and  any  torque,  and  also  avoids  the  con- 
troller troubles- which  are  ever  present  with  large  direct  current 
equipments.  It.  also  permits  the  use  of  high  trolley  voltage, 
thus  reducing  the  current  to  be  collected  Tn  this  class  of  serv- 
ice the  .advantages  of  this  A  c  system  are  so  great  that  is  it 
possible  that  heavy  railroading  will  prove  to  be  the  special  field 
for  it. 

For  general  city  work,  this  system  may  not  find  a  field  for  some 
time  to  come,  as  the  limitations  m  the  present  system  are  not  so 
great  that  there,  will  be  any  great  necessity  for  making  a  change. 
It  is  probable  that  at  first  this  system  will  be  applied  to  new 
railways,  or  in  changing  over  steam  roads  rather  than  in  replac- 
ing existing  city  equipments.  One  difficulty  with  which  the 
new  system  will  have  to  contend,  is  due  to  the  fact  that  the 
A.  c  equipments  cannot  conveniently  operate  on  existing  city 
lines,  as  is  the  present  practice  where  interurban  lines  run  into 
the  cities.  It  will  be  preferable  for  the  A.  c.  system  to  have  its 
own  lines  throughout,  unless  very  considerable  complication  is 
permitted.  When  the  A.  c.  system  applied  to  interurban  and 
steam  railway  systems  finally  becomes  of  predominant  import- 
ance, it  is  probable  that  the  existing  D.  c.  railways  will  gradually 
be  changed  to  A.  c.  as  a  matter  of  convenience  in  tying  the  vari- 
ous railway  systems  together 

As  was  stated  above,  A.  c  equipments  cannot  conveniently  be 
operated  on  direct  current  lines.  It  does  not  follow  that  the 
motor  will  not  operate  on  direct  current.  On  the  contrary,  the 
motor  is  a  first-class  direct  current  machine,  and  if  supplied  with 
suitable  control  apparatus  and  proper  voltage  it  will  operate 
very  well  on  the  D.  c.  lines.  This  would  require  that  the  motors 
be  connected  formally  in  series,  as  the  voltage  per  motor  is  low. 
A  complete  set  of  D.  c.  control  apparatus  would  be  needed  when 
the  A.  c.  equipment  is  to  be  run  on  direct  current,  and  con- 
siderable switching  apparatus  would  be  necessary  for  disconnect- 
ing all  the  A.  c.  control  system  and  connecting  in  the  D.  c.  The 
complication  of  such  a  system  may  be  sufficient  to  prevent  its 
use,  at  least  for  some  time  to  come 

In  some  cities,  very  strict  laws  are  in  force  in  regard  to  the 
voltage  variations  in  various  parts  of  the  track  system.  The 
permissible  variations  are  so  small  in  some  cases,  that  an  enor- 
mous amount  of  copper  is  used  for  return  conductors;  and  in 


52  ELECTRICAL  ENGINEERING  PAPERS 

some  cases  special  boosters  are  used  in  the  return  circuits  to 
avoid  large  differences  of  potential  between  the  various  parts 
of  the  track  system.  The  object  in  limiting  the  conditions  in 
this  manner  is  to  avoid  troubles  from  electrolysis.  The  A.  c. 
system  will,  of  course,  remedy  this. 

For  city  work,  it  is  probable  that  voltages  of  500  or  600  would 
be  employed  instead  of  1,000  or  higher.  The  transformers  and 
controllers  can  be  designed  to  be  readily  changed  from  full  to 
half  voltage,  so  that  low  voltage  can  be  used  on  one  part  of  the 
line  and  high  voltage  on  another  As  the  car  equipments  of 
such  railways  arc  usually  of  small  capacity,  it  is  probable  that 
speed  control  will  be  obtained  by  means  of  a  transformer  with  a 
large  number  of  leads  carried  out  to  a  control  drum,  rather  than 
by  means  of  the  induction  regulator,  as  the  latter  device  is  much 
more  expensive  in  small  units.  This  is  chiefly  a  question  of  cost, 
and  if  the  advantages  of  the  induction  regulator  are  found  to 
over-weigh  the  objection  of  high  first  cost,  then  it  will  be  used 
even  on  small  equipments. 

In  the  W.  B.  and  A.  Railway,  the  generators  are  wound  for 
single-phase,  In  the  case  of  large  power-stations  with  many 
feeders,  the  generators  may  be  wound  for  three-phase,  with 
single-phase  circuits  carried  out  to  the  transformer  substation, 
or  three-phase  transmission  may  be  used,  with  the  transformers 
connected  in  such  a -manner  as  will  give  a  fairly  well-balanced 
three-phase  load. 

There  are  many  arrangements  and  combinations  of  apparatus 
made  possible  by  the  use  of  alternating  current  in  the  car  equip- 
ments, which  have  not  been  mentioned,  as  it  is  impracticable 
to  give  a  full  description  of  all  that  can  be  done.  But  enough 
has  been  presented  to  outline  the  apparatus  and  to  indicate  the 
possibilities  of  this  new  system  which  is  soon  to  see  the  test  -of 
commercial  service. 


SYNCHRONOUS  MOTORS  FOR  REGULATION  OF 
POWER  FACTOR  AND  LINE  PRESSURE 

FOREWORD— In  1890,  the  author  discovered,  during  certain  ex- 
periments in  the  Westinghouse  testing  room,  that  a  synchronous 
motor  could  affect  the  power  factor  of  the  supply  system,  by 
variations  in^its  field  strength.  Later,  he  proposed  the  use  of 
such  a  machine  for  regulating  the  pressure  of  a  supply  system 
and  for  changing  the  relation  between  e.m.f.  and  current  in 
alternating-current  systems.  However,  even  as  late  as  1904, 
when  the  paper  was  presented,  the  value  of  this  method  of 
operation  was  but  little  appreciated. 

This  paper  should  be  read  from  the  viewpoint  of  the  time 
when  it  was  written.  Hand  regulation  was  the  ordinary  prac- 
tice. Consequently,  alternators  with  inherently  good  regula- 
tion, that  is,  which  would  give  three  to  four  times  full  load 
current  on  sustained  short  circuit,  were  preferred,  in  order  that 
much  hand  regulation  would  not  be  needed.  This  paper  was  pre- 
sented at  a  meeting  of  the  American  Institute  of  Electrical 
Engineers  in  June,  1904. — (ED.) 


IT  is  well  known  that  the  synchronous  motor,  running  on  an 
alternating-current  circuit,  can  have  its  armature  current 
varied  by  varying  its  field  strength.  A  certain  adjustment  of  field 
strength  will  give  a  minimum  armature  current.  Either  stronger 
or  weaker  fields  will  give  increased  current.  These  increased  cur- 
rents are  to  a  great  extent  wattless.  If  the  field  is  weaker  than 
the  normal  (or  field  for  minimum  armature  current),  the  increased 
armature  current  is  leading  with  respect  to  the  e.m.f.  waves  in  the 
motor  and  lagging  with  respect  to  the  line  e.m.f.  For  stronger  than 
the  normal  field,  the  current  is  to  a  great  extent  lagging  and 
tends  to  lessen  the  flux  in  the  motor  and  the  current  is  leading 
with  respect  to  the  line  e  m  f  A  synchronous  motor  therefore 
has  an  inherent  tendency  to  correct  conditions  set  up  by  im- 
proper adjustment  of  its  field  strength  The  correcting  current 
in  the  motor  being  drawn  from  the  supply  system  has  a  correct- 
ing effect  on  sucli  system,  tending  to  produce  equalization  between 
generated  pressures  in  the  motor  and  the  supply  pressure.  This 
characteristic  of  the  synchronous  motor  can  readily  be  utilized 
for  two  purposes;  namely,  for  varying  the  amount  of  leading 
or  lagging  current  in  a  system  for  producing  changes  in  the 
power-factor  of  the  system  (including  transmission  line,  trans- 
formers, and  generators),  or  a  synchronous  motor  can  be  utilized 
for  pressure  regulation  in  a  system. 

53 


54  ELECTRICAL  ENGINEERING  PAPERS 

As  the  synchronous  motor  can  be  made  to  impress  a  leading 
current  upon  the  system,  and  as  the  amount  of  this  leading 
current  will  depend  upon  the  field  adjustment  of  the  synchronous 
motor,  it  is  evident  that  this  property  can  be  used  for  neutral- 
izing the  effects  of  lagging  current  due  to  other  apparatus  on 
the  system.  The  resultant  leading  or  lagging  current  can  be 
varied  and  the  power-factor  controlled  over  a  fairly  wide  range 
depending  upon  the  location  of  the  synchronous  motor  or  motors, 
and  upon  the  current  capacity  of  the  motor,  etc. 

As  the  wattless  current  in  the  motor  is  primarily  a  corrective 
current,  it  is  evident  that  for  most  effective  purposes  for  ad- 
justing power-factor  on  the  system  the  corrective  action  of 
this  current  on  the  motor  should  not  be  too  great.  When 
used  for  such  purpose  the  synchronous  motor  should  therefore 
be  one  which  would  give  a  comparatively  large  current  if  short- 
circuited  as  a  generator  Also  the  motor  should  preferably  be 
one  in  which  the  magnetic  circuit  is  not  highly  saturated,  for 
in  the  saturated  machine  the  limits  of  adjustment  in  the  field 
strength  are  rather  narrow. 

As  has  been  noted  above,  if  the  field  strength  bf  the  motor 
be  varied,  a  leading  or  lagging  current  can  be  made  to  flow  in 
its  armature  circuit,  this  current  being  one  which  tends  to 
adjust  the  pressure  of  the  armature  and  that  of  the  supply 
system.     It  is  evident  that  if  the  armature  pressure  is  held  con- 
stant   and   the  supply  pressure  varied,  a  leading  or  lagging 
current  would  also  flow.     If  for  instance  the  line  pressure  were 
dropped  below  that  of  the  motor,  then  a  lagging  current  would 
flow  in  the  motor  tending  to  weaken  its  field,  and  a  leading 
current  would  flow  in.  the  line,  tending  to  raise  the  pressure, on 
the  line.1    If  the  line  pressure  should  be  higher  than  that  of 
the  synchronous  motor,  then  the  current  in  the  motor  woftld 
be  leading,  tending  to  raise  its  pressure;  while  it  would  be  lag- 
ging with  respect  to  the  line,  tending  to  lower  it§  pressure. 
The  resultant  effect  would  be  to  equalize  the  pressures  of  the 
line  and  motor,  and  there  would  thus  be  a  teadency  to  regulate 
the  line  pressure  to  a  nioro  nearly  constant  value,     It  is  evident 
that  the  less  the  synchronous  motor  is  affectecL  by  the  <correc-' 
tive  current  and  the  more  sensitive  the  line  is  Jo  such  corrective 
action,  the  greater  the  tendency  will  be  toward  constant  pres- 
sure on  the  line      It  is  therefore  evident  that  the   synchronous 
motor  Which  gives  the  largest  current  on  short  circuit  as  n  gen- 
erator would  be  the  one  which  gives  the  greatest  corrective 
action  as  regards  pressure  regulation  of  the  system. 


POWER-FA  CTOR  REG ULA  TION  55 

For  such  regulation,  the  synchronous  motor  which  gives  a  com- 
paratively large  leading  or  lagging  current  with  small  change  to 
the  pressure  of  the  system  is  the  most  suitable  one.  Or,  the 
motor  which  gives  the  greatest  change  in  the  leading  or  lagging 
current  is  the  one  which  gives  best  regulation.  It  is  the  change 
in  the  amount  of  wattless  current  which  produces  the  regulation. 
This  current  could  vary  from  zero  to  100  leading,  for  example, 
or  could  change  from  50  leading  to  50  lagging,  or  could  change 
from  100  lagging  to  zero  lagging.  Any  of  these  conditions 
could  produce  the  desired  regulating  tendency,  but  all  would1 
not  be  equally  good  as  regards  the  synchronous  motor  capacity. 
If  in  addition  to  the  regulating  tendency  it  is  desired  to  correct 
for  lower  power-factor  due  to  other  apparatus  on  the  circuit, 
it  would  probably  be  advisable  to  run  a  comparatively  large 
leading  current  on  the  line  due  to  the  synchronous  motor,  and 
the  regulating  tendency  would  be  in  the  variations  in  the 
amount  of  leading  current,  and  not  from  leading  to  lagging, 
or  vice  versa.  A  larger  synchronous  motor  for  the  same  regu- 
lating range  would  be  required  than  if  the  motor  were  used 
for  pressure  regulation  alone.  It  is  evident  that  the  current 
capacity  of  a -motor  regulating  from  50  leading  to  50  lagging 
need  be  much  less  than  for  current  regulating  from  100  leading 
to  zero.  It  is  evident  therefore  that  if  there  is  to  be  compensa- 
tion for  power-factor  as  well  as  regulation  of  pressure,  that 
'additional  normal  current  capacity  is  required. 

In  case  such  synchronous  motors  are  required  for  regulation 
purely,  it  may  be  suggested  that  such  machines  be  operated 
at  very  high  speeds  compared  with  ordinary  practice.  At  first 
glance  it  would  appear  that  such  a  synchronous  motor  could 
be  operated  at  the  highest  speed  that  mechanical  conditions 
would  allow,  but  there  are  other  conditions  than  mechanical 
ones  which  enter  into  this  problem.  For  instance,  it  is  now 
possible  to  build  machines  of  relatively  large  capacity  for  two- 
poles  for  60-cycle  circuits,  and  for  very  large  capacities — say 
1500  kilowatts — having  four  poles.  Therefore  mechanical  con- 
ditions permit  the  high  speeds,  and  the  electrical  conditions 
should  be  looked  into  carefully  to  see  whether  they  are  suitable 
for  such  service.  As  such  synchronous  motors  should  give  rela- 
tively large  currents  on  short  circuit  the  effect  of  high  speeds 
and  a  small  number  of  poles  on  short-circuit  current  should  be 
considered. 

In  order  to  give  full-load  current  on  short  circuit,  the  field 


56  ELECTRICAL  ENGINEERING  PAPERS 

ampere-rums  of  such  a  machine  should  be  practically  equal  to 
the  armature  ampere-turns,  taking  the  distribution  of  windings, 
etc.,  into  account  By  armature  turns  in  this  case  is  not  meant 
the  ampere  wires  on  the  armature,  but  the  magnetizing  effect 
due  to  these  wires  Therefore  to  give,  for  instance,  five  or  six 
times  full-load  current  on  short  circuit,  the  field  ampere-turns 
should  be  relatively  high  compared  with  the  armature 
This  means  that  the  field  ampere-turns  per  pole  should  be  very 
high,  or  the  armature  ampere-turns  per  pole  very  low  Ex- 
perience shows  that  for  very  high  speed  machines,  such  as  used 
for  turbo-generators,  there  is  considerable  difficulty  m  finding 
room  for  a  large  number  of  field  ampere-turns,  and  therefore 
in  such  machines  it  is  necessary  to  reduce  the  armature  ampere- 
turns  very  considerably  for  good  inherent  regulating  charac- 
teristics This  in  turn  means  rather  massive  construction,  as 
the  magnetic  circuit  m  both  the  armature  and  field  must  have 
comparatively  large  section  and  the  inductions  must  be  rather 
high.  This  in  turn  means  high  iron  losses  in  a  relatively  small 
amount  of  material  compared  with  an  ordinary  low-speed  ma- 
chine, and  abnormal  designs  are  required  for  ventilation,  etc  , 
and  for  mechanical  strength 

An  increase  in  the  number  of  poles  usually  allows  increased 
number  of  field  ampere-turns  without  a  proportionate  increase 
in  the  number  of  armature  ampere-turns  This  condition  is 
true  until  a  large  number  of  poles  is  obtained  when  the  leakage 
between  poles  may  become  so  high  that  the  effective  induction 
per  pole  is  decreased  so  that  there  is  no  further  gain  by  increas- 
ing the  number  of  poles,  unless  the  machine  is  made  of  abnormal 
dimensions  as  regards  diameter,  etc.  Experience  has  indi- 
cated that  in  the  case  of  very  high-speed  and  very  low-speed 
alternators,  it  is  more  difficult  to  obtain  a  large  current  on  short 
circuit  than  with  machines  with  an  intermediate  number  of 
poles  For  example,  it  is  rather  difficult  to  make  a  600  kilovolt- 
ampere,  3600-rev.  per  min.,  2-pole  machine  which  will  give 
three  times  full-load  current  on  short  circuit  A  4-pole,  1800 
rev.  per  min.  machine  can  more  easily  be  made  to  give  three 
times  full-load  current  on  short  circuit  and  with  comparatively 
small  additional  weight  of  material  The  material  in  the  ro- 
tating part  of  the  four-pole  machine,  while  of  greater  weight, 
may  be  of  considerably  lower  cost  per  pound.  The  stationary 
part  of  the  four-pole  machine  may  have  a  sorhewhat  larger  in- 
ternal diameter,  but  the  radial  depth  of  sheet -steel  will  be  less 


POWER- FACTOR  REGULATION  57 

than  in  a  two-pole  machine.  The  total  weight  of  material  in 
the  armature  of  a  four-pole  machine  may  be  practically  no 
greater  than  in  a  two-pole  machine.  Therefore  a  two-pole 
machine  of  this  capacity  should  cost  more  than  a  four-pole 
machine,  if  designed  to  give  the  same  current  on  short  circuit. 
A  six-pole  machine  would  show  possibly  a  slight  gain  over  the 
one  with  four  poles,  but  not  nearly  as  much  as  the  four-pole 
machine  would  over  the  one  with  two  poles  The  real  gain  of 
the  six-pole  over  the  four-pole  construction  would  be  in  ob- 
taining a  machine  which  would  give  more  than  three  times 
full-load  current  on  short  circuit.  It  would  possibly  be  as 
easy  to  obtain  four,  times  full  load  current  on  short  circuit 
with  a  six-pole  machine  as  to  obtain  three  times  full  load  cur- 
rent on  four7pole  machine.  An  eight-pole  machine  would  be 
in  the  same  way  somewhat  better  than  the  six-pole  machine 
Therefore  if  a  600  kilovolt-ampere  machine  giving  six  times 
full-load  current  on  short  circuit  is  desired,  it  would  be  advan- 
tageous to  make  the  machine  with  possibly  eight  to  twelve 
poles.  The  question  of  which  would  be  the  cheaper  would  de- 
pend upon  a  number  of  features  in  design. 

If  very  large  short-circuit  currents  are  desired,  then,  as  in- 
dicated above,  the  number  of  poles  for  a  given  capacity  should 
be  increased,  or  the  normal  rating  of  the  high-speed  machine 
should  be  decreased.  If,  for  example,  the  600  kilovolt-ampere, 
3600  rev.  per  min.  machine,  mentioned  above,  should  be  rated 
at  200  kilovolt-amperes,  then  it  could  give  nine  times  full-load 
current  on  short  circuit ;  but  such  a  method  of  rating  is  merely 
dodging  the  question. 

In  general,  the  following  approximate  Umits  for  speeds  and 
short  circuit  currents  for  40-cycle  apparatus  can  be  given. 
These  limits  are  necessarily  arbitrary,  and  are  intended  to  rep- 
resent machines  which  could  probably  be  made  without  using 
too  abnormal  dimensions, 

600  kilovolt-amperes,  3600  rev.  per  min.,  two  to  three  times 
full-load  current  on  short  circuit. 

1000  kilovolt-amperes,  1800  rev.  per  min.,  three  to  four 
times  full-load  current  on  short  circuit. 

1500  kilovolt-amperes,  1200  rev.  per  min.,  four  to  five  times 
full-load  current  on  short  circuit. 

2500  kilovolt-amperes,  900  rev.  per  min  ,  four  to  five  times 
full-load  current  on  short  circuit. 

For  25  cycles  it  is  more  difficult  to  give  limiting  conditions, 


58  ELECTRICAL  ENGINEERING  PAPERS 

as  the  choice  of  speeds  is  very  narrow.  If,  for  example,  a  1500 
kilovolt-ampere,  2-pole,  1500  rev  per  min.  machine  can  be 
made  to  give  three  times  full-load  current  on  short  circuit, 
then  as  machines  of  smaller  rating  cannot  run  at  higher  speed, 
the  limiting  condition  of  such  machines  must  be  the  amount 
of  current  which  they  will  give  on  short  circuit  In  the  same 
way  a  4-pole  machine  running  at  750  rev  per  min.  may  be 
made  for  5000  kilovolt-amperes  for  three  times  full-load  current 
as  the  limiting  rating,  and  there  is  no  choice  of  speeds  for 
ratings  between  1500  kilovolt-amperes  and  5000  kilovolt-amperes. 

It  should  be  noted  that  the  above  speeds  are  very  high  com- 
pared with  ordinary  alternator  practice  and  are  up  to  high- 
speed turbo -generator  practice,  but  machines  with  the  above 
short-circuit  ratings  and  speeds  are  probably  more  costly  to 
build  than  machines  of  corresponding  ratings  at  somewhat 
lower  speeds  It  will  probably  be  found  therefore  that  for 
the  above  maximum  current  on  short  circuit  the  cheapest 
synchronous  motors  for  the  given  ratings  will  have  somewhat 
lower  speeds  than  those  indicated  above  It  is  certain  that 
tae  lower-speed  machines  will  be  easier  to  design  and  will  be 
slightly  quieter  in  operation  Probably  best  all-round  condi- 
tions will  be  found  at  about  half  the  above  speeds. 

The  above  limiting  conditions  are  given  as  only  approxi- 
mate and  are  based  upon  machines  having  ventiliation  as  is 
usually  found  on  rotating  field  generators  for  high  speed.  Arti- 
ficial cooling,  such  as  obtained  with  an  air-blast  or  blowers 
could  modify  the  above  figures  somewhat;  but  in  general  it 
has  been  found  that  high-speed  alternators  can  be  worked  up 
to  the  limit  imposed  by  saturation  before  the  limit  imposed 
by  temperature  is  attained.  Therefore  if  higher  saturation  is 
not  permissible,  then  there  may  be  relatively  small  gain  by 
using  artificial  cooling. 

One  of  the  principal  applications  of  such  regulating  syn- 
chronous motors  would  be  for  controlling  or  regulating  the 
pressure  at  the  end  of  a  long  transmission  line  for  maintaining- 
constant  pressure  at  the  end  of  the  line,  independent  of  fluc- 
tuations of  load  or  change  of  power-factor.  In  this  case,  in- 
creased output  of  the  transmission  line  may  more  than  con- 
pensate  for  the  cost  of  the  regulating  synchronous  motor.  In 
such  a  case  the  synchronous  motor  not  only  acts  as  a  regulator 
on  the  system  but  costs  nothing  in  the  end.  In  general,  the 
more  current  that  such  a  synchronous  motor  will  give  on  short- 


POWER-FA  CTOR  REG  ULA  TION  50 

circuit,  the  better  suited  it  will  be  for  its  purpose  at  the  end 
of  a  long  transmission  line 

Where  a  number  of  such  synchronous  motors  are  installed 
in  the  same  station,  the  field  adjustment  must  be  rather  care- 
fully made,  to  avoid  cross-currents  between  machines,  and  the 
saturation  characteristics  of  the  various  machines  should  be 
very  similar.  The  better  such  machines  are  for  regulating 
purposes,  the  poorer  they  are  for  equalizing  each  other  by 
means  of  cross-currents 

As  to  the  use  of  dampers  with  such  synchronous  motors, 
it  is  difficult  to  say  just  what  is  required  A  synchronous 
motor  on  a  line  with  considerable  ohmic  drop  is  liable  to  hunt 
to  some  extent,  especially  if  the  prime  mover  driving  the  gen- 
erator has  periodic  variations  in  speed  If  the  synchronous 
motor  gives  very  large  current  on  short  circuit,  then  its  syn- 
chronizing power  is  high ,  this  will  tend  to  steady  the  operation 
of  the  motor  and  decrease  the  hunting.  The  writer  believes 
that  such  motors  in  practice  will  be  found  to  operate  better 
and  have  better  regulating  power  for  constant  pressure  if  pro- 
vided with  rather  heavy  copper  dampers  effectively  placed  on 
the  field  poles.  With  such  heavy  dampers  reaction  of  the 
armature  on  the  field  is  retarded,  and  therefore  the  armature 
may  give  a  larger  momentary  current  than  would  flow  it  there 
were  no  damping  effect;  in  other  words,  the  motor  is  more 
sluggish  than  one  without  dampers  Therefore  the  addition 
of  heavy  dampers  on  such  a  machine  may  produce  the  same 
regulating  effect  which  would  be  obtained  by  a  machine  without 
dampers  which  gives  a  larger  current  on  short  circuit.  Also 
a  machine  with  heavy  dampers  will  usually  be  the  one  with 
the  least  hunting  tendency  and  therefore  will  have  the  least 
effect  on  the  transmission  line  due  to  hunting  currents. 

In  the  above,  the  synchronous  motor  has  been  considered 
only  as  a  regulator  and  not  as  a  motor.  It  may  be  worth 
considering  what  would  be  the  effect  if  the  synchronous  motor 
can  do  useful  work  at  the  same  time  that  it  regulates  the 
system.  In  this  case,  with  a  given  rated  output,  one  com- 
ponent of  the  input  will  be  wattless,  and  the  other  part  will 
be  energy.  The  ratio  of  these  two  components  could  be  varied 
as  desired.  For  example,  considering  the  input  as  100,  the 
wattless  component  could  be  60  when  the  energy  component 
is  80;  or  the  synchronous  motor  could  carry  a  load  of  80% 
of  its  rated  capacity,  this  load  including  its  own  losses,  and  could 


60  ELECTRICAL  ENGINEERING  PAPERS 

have  a  regulating  component  of  60%  of  its  rated  capacity. 
If  the  motor  is  used  as  a  regulating  machine  only,  then  its 
wattless  component  caa  be  practically  100  It  appears  there- 
fore" that  the  machine  could  be  used  more  economically  as  both 
motor  and  regulator  than  as  a  regulator  alone,  but  in  such  case 
it  would  probably  be  advisable  to  run  the  motor  at  somewhat 
lower  speed  than  if  operated  entirely  as  a  regulator.  This 
reduction  in  speed  may  practically  offset  the  gain  in  apparent 
capacity  by  using  the  machine  for  a  double  purpose,  Also 
there  is  comparatively  limited  use  for  large  synchronous  motors 
for  power  purposes,  as  better  results  are  usually  obtained  by 
subdividing  the  units  and  locating  each  unit  nearest  to  its- 
load.  If  a  load  could  be  provided  which  would  permit  very 
high-speed  driving,  then  it  would  probably  be  of  advantage 
to  utilize  the  synchronous  motor  for  driving 

As  the  synchronous  converter  is  one  form  of  synchronous 
motor,  the  question  of  utilizing  such  machines  for  regulators- 
should  be  mentioned*  Upon  looking  into  the  question  of  dis- 
tribution of  losses  in  the  converter,  it  will  be  noted  that  the 
losses  in  the  armature  winding  are  not  uniform  Investigations 
show  that  at  100%  power-factor,  the  lowest  heating  in  copper 
is  obtained,  and  that  any  departure  from  this  power-factor 
shows  considerably  increased  loss  in  the  copper,  such  loss  being 
very  high  in  certain  portions  of  the  winding  Next  to  the 
taps  which  lead*  to  the  collector  there  are  strips  of  winding 
which  at  times  are  worked  at  a  very  high  loss.  Experience 
shows  that  it  is  not  advantageous  to  operate  converters  at  a 
low  power-factor,  and  that  if  so  operated  continuously,  or 
for  any  considerable  periods >  the  winding  should  he  made  much 
heavier  than  for  higher  power-factors,  Also  in  the  usual  de- 
sign of  converters  the  field  is  not  made  as  strong  compared 
with  the  armature  as  in  alternator  practice,  and  therefore  the 
regulating  tendency  of  the  converter  compared  with  a  generator 
or  ordinary  synchronous  motor,  is  low.  Synchronous  con- 
verters can  and  do  act  as  regulators  of  pressure  for  sudden 
changes  of  the  supply  pressure,  but  such  correcting  or  regu- 
lating action  should  not  be  continual;  that  is,  the  pressure 
supplied  to  a  converter  from  a  line  should  nominally  be  that 
required  by  the  converter  for  best  operation  as  a  synchronous 
converter.  Unless  designed  for  the  purpose,  a  synchronous 
converter  should  not  be  used  to  correct  low  powgr-factors. 
due  to  other  apparatus  on  the  circuit. 


PO  WER-FA  CTOR  REG  ULA  TION  61 

In  the  above  considerations  only  general  reference  has  been 
made  to  the  cost  of  synchronous  motors  for  regulating  pres- 
sure and  power-factors  It  is  difficult  to  give  even  approxi- 
mate figures  for  relative  costs  of  such  apparatus  As  inti- 
mated before,  there  is  some  mean  speed  or  number  of  poles 
which  will  be  the  most  suitable  for  giving  a  certain  maximum 
current  on  short  circuit.  For  speeds  slightly  above  or  below 
such  mean  speed,  the  cost  of  the  synchronous  motor  should 
vary  almost  in  proportion  to  the  speed,  provided  the  maximum 
short-circuit  current  can  be  diminished  somewhat  at  the  same 
time  If  the  speed  is  further  increased  or  further  decreased, 
the  cost  will  tend  to  approach  a  constant  figure  As  the  ex- 
treme conditions  are  approached,  the  cost  will  begin  to  rise. 
The  above  assumptions  are  on  the  basis  of  continuous  opera- 
tion at  a  given  current  capacity,  this  being  the  same  in  all  cases. 
The  above  assumption  is  on  the  basis  of  decrease  in  the  max- 
imum short-circuit  current,  as  the  machine  departs  from  the 
mean,  or  best  speed.  If  the  same  maximum  current  is  re- 
quired, then  the  lowest  cost  should  be  at  the  mean  or  best 
speed,  while  at  either  side  the  cost  should  rise. 

It  is  evident  that  it  would  be  difficult  to  give  any  figures  on 
relative  costs  of  such  apparatus.  The  machine  for  the  best 
or  mean  condition,  should  cost  practically  the  same  as  an  alter- 
nating-current generator  of  the  same  speed,  output,  and  short- 
circuit  characteristics.  As  this  speed  would  probably  be  some- 
what higher  than  usual  generator  speeds,  the  cost  of  such 
machine  would  therefore  be  somewhat  lower.  This  cost  would 
be  to  a  considerable  extent,  a  function  of  the  current  on  short 
circuit  for  a  given  rated  capacity  of  machine.  As  mentioned 
before,  in  giving  a  table  of  limiting  speeds  and  short  circuits, 
it  is  probable  that  one-half  this  limiting  speed  would  be  near 
the  best  condition.  Such  machines  would  probably  cost  from 
60%  to  80%  as  much  as  similar  machines  for  usual  commercial 
high-speed  conditions,  neglecting  turbo-generator  practice.  The 
frequency  has  considerable  effect  on  this,  as,  for  example,  there 
is  small  choice  of  speed  as  regards  high-speed  25-cycle  machines. 
Taking  very  general  figures  only,  it  is  probable  that  in  the 
case  of  a  given  capacity  of  machine  for  say  three  or  four  times 
full-load  current  on  short  circuit  the  cost  cannot  be  expected 
to  be  lower  than  one-half  that  of  machines  of  similar  rating  at 
ordinary  commercial  speeds,  turbo-generator  practice  being  ex- 
cluded. The  costs  in  general  should  approximate  more  nearly 


62  ELECTRICAL  ENGINEERING  PAPERS 

those  of  turbo -generators,  but  again,  an  exact  comparison 
cannot  be  made  because  in  usual  practice  the  turbo-generators 
do  not  give  three  to  four  times  full-load  current  on  short  circuit. 

There  are  a  number  of  other  conditions  in  this  general  problem, 
such  as  advantage  or  disadvantage  of  placing  synchronous 
motors  in  the  main  power-house,  or  distnbuting  them  m  a 
number  of  sub-stations  Also  there  is  the  question  of  the  eSect 
of  the  cost  on  the  generating  plant  when  used  with  such  regu- 
lating synchronous  motors.  If  higher  power-factors  are  main- 
tained on  the  transmission  system  and  generator,  a  cheaper 
form  of  generator  can  probably  be  used.  The  high  power- 
factor  permits  a  larger  output  from  the  transmission  system 
and  thus  represents  a  gam.  If  the  synchronous  motor  can  be 
operated  at  its  best  speed  and  also  do  work,  then  there  is  a 
further  gam  If  the  synchronous  motor  should  be  located  *  at 
the  center  of  power  distribution,  and  the  power  is"  distributed 
through  induction  motors,  then  there  is  a  possibility  of  re- 
ducing the  cost  of  such  motors  by  lowering  the  power-factor, 
this  being  compensated  for  by  the  synchronous  motor  deliver- 
ing leading  currents  As  the  cost  per  horse  power  of  small 
motors  will  be  much  greater  than  the  cost  per  horse  power  of 
a  large  regulating  motor,  there  is  a  possibility  of  gain  from  this 
source.  If  the  induction  motors  are  distributed  over  wide 
territory,  this  gain  would  "be  lessened  and  might  disappear. 

It  should  be  mentioned  that  the  powers-factor  of  a  system 
as  influenced  by  difference  in  wave  form  has  not  been  con- 
sidered in  the  preceding  discussion.  It  is  obviously  impossible 
to  neutralize  by  a  synchronous  motor  the  effect  of  currents 
in  a  system  due  to  difference  in  wave  form.  Such  currents  will 
in  general  be  of  higher  frequency  than  the  fundamental  wave 
of  the  system,  and  the  synchronous  motor  obviously  could  not 
correct  for  them,  unless  it  impressed  upon  the  system  opposite 
waves  of  the  same  frequency.  This  would  mean  a  synchronous 
motor  with  a  different  wave  form  from  that  of  the  system. 

The  power-factor  of  a  system  will  also  be  affected  by  any 
hunting  of  the  apparatus  on  the  system.  It  is  evident  that 
the  synchronous  motor  could  not  correct  or  neutralize  such 
•effects,  except  through  exerting  a  damping  effect  on  the  system 
And  other  apparatus  on  the  system.  A  synchronous  motor 
with  heavy  dampers  can  reduce  the  hunting  in  a  system,  but 
such  hunting  can  also  be  damped  by  induction  motors  with 
low -resistance  secondaries,  especially  if  of 'the  cage  type.  This 


POWER- FACTOR  REGULATION  63 

correcting  effect  should  therefore  be  credited  to  the  damper 
rather  than  to  synchronous -motor  action.  There  are  a  number 
of  other  questions  which  arise  in  connection  with  this  regu- 
lating feature  of  the  synchronous  motor,  but  the  subject  is  too 
broad  to  permit  even  mention  of  them. 

The  substance  of  the  preceding  statements  can  be  summarized 
as  follows: 

1.  A  synchronous  motor  can  be-used  to  establish  leading  or 
lagging  currents  in  its  supply  system  by  suitable  field  adjust- 
ment, and  can  thus  affect  or  control  power-factor    or   phase 
relations  of  the  current  in  the  alternating  current  system 

2.  A  synchronous  motor  will  set  up  leading  or  lagging  cur- 
rents in  its  supply  system  if  its  field  strength  is  held  constant, 
and  the  pressure  of  the  supply  system  is  varied  above  or  below 
that  generated  by  the  synchronous  motor      Such  leading  or 
lagging  currents  in  the  supply  system  will  tend  to  vary  the 
pressure  of  the  system      A  synchronous  motor  can  thus   act 
as  a  regulator  of  the  pressure  of  its  supply  system. 

3.  This  regulating  action  is  greatest  with  synchronous  motors 
which  have  the  closest  true  inherent  regulation  (as  indicated  by 
high    field   magnetomotive  force  compared  with  the  armature 
magnetomotive  force)  in  distinction  from  machines  which  have 
close  apparent  regulation  obtained  by  saturation  of  the  mag- 
netic circuit. 

•4.  If  the  synchronous  motor  is  used  both  for  regulating  the 
power-factor  for  neutralizing  the  effect  of  other  apparatus  on 
the  circuit,  and  for  regulating  or  steadying  the  pressure  of 
the  supply  system,  its  normal  capacity  for  regulating  will  be 
diminished. 

5.  The  most   suitable   speeds   for  best   electrical   conditions 
will  in  general  be  considerably  below  highest  possible  speeds 
as  limited  by  mechanical  conditions. 

6.  Heavy  dampers  will  increase  the  effectiveness  of  the  reg- 
ulating tendency. 

7.  If  the  synchronous  motor  can  be  used  for  power  purposes 
as  well  as  for  regulation,  its  apparent  capacity  is  increased. 
This  is  due  to  the  fact  that  the  regulation  is  obtained  by  means 
of  a  wattless  component  and  the  power  from  the  energy  com- 
ponent, and  the  algebraic  sum  of  these  two  is  greater  than  their 
resultant  which  fixes  the  current  capacity  of  the  machine. 

8.  Synchronous  converters  in  general  are  not  suited  for  reg- 
ulating the  pressure  or  controlling  the  power-factor  of  an  alter- 

t  svstem. 


64  ELECTRICAL  ENGINEERING  PAPERS 

9.  The  costs  of  synchronous  motors  for  regulating  purposes 
will  in  general  be  lower  than  for  alternating-current  motors 
or  generators  of  customary  speeds,  and  will  approach  more 
nearly -to  turbo-generator  practice 


DATA  AND  TESTS  ON  10  000  CYCLE  PER  SECOND 
ALTERNATOR 

FOREWORD — In  1902,  the  author  undertook  the  construction  of 
10  000  cycle  per  second  alternator.  This  problem  was  a  very 
new  and  radical  one  at  that  time  and  it  was  considered  worth 
while  to  put  the  record  of  results  m  permanent  form.  There- 
fore, this  paper  was  prepared  on  the  subject  and  presented  before 
the  American  Institute  of  Electrical  Engineers  in  May,  1904. 
This  is  interesting  merely  as  a  record  of  a  relatively  early 
construction. — (ED ) 


IN  the  early  part  of  1902,  M.  Leblanc,  the  eminent  French 
engineer,  was  in  this  country,  and  spent  considerable  time  at 
the  Westinghouse  Electric  &  Manufacturing  Company's  works  at 
East  Pittsburg.  M.  Leblanc  was  very  much  interested  in  cer- 
tain special  telephone  work,  and  in  connection  with  such  work 
he  desired  for  experimentation  a  current  of  very  high  frequency. 
He  took  up  with  the  writer  the  question  of  building  a  successful 
alternator  for  generating  current  at  frequencies  between  5000 
and  10  000  cycles  per  second.  He  was  informed  that  the  ma- 
chine would  necessarily  be  of  very  special  construction,  but  that 
it  was  not  an  impossible  machine.  Later  he  took  up  the 
matter  with  Mr.  Westinghouse,  who,  upon  receiving  satisfactory 
assurance  that  such  a  machine  was  possible,  advised  that  the 
generator  be  built.  A  preliminary  description  of  the  general 
design  was  given  M.  Leblanc  before  he  returned  to  Paris.  He 
was  somewhat  surprised  at  certain  of  the  features  proposed, 
especially  at  the  fact  that  an  iron-cored  armature  was  consid- 
ered feasible  for  a  frequency  of  10  000  cycles  per  second. 

The  machine  was  designed  and  built  on  practically  the  lines 
of  the  preliminary  description  furnished  M.  Leblanc.  The  fre- 
quency being  so  abnormal,  the  writer  believes  that  many  features 
in  the  machine,  with  the  results  obtained,  will  be  of  scientific 
interest,  and  therefore  the  data  of  the  machine,  and  the  tests 
obtained  are  presented  herewith 

The  starting  point  in  this  machine  was  the  sheet-steel  to  be 
used  in  the  armature.  No  direct  data  were  at  hand  showing 
losses  in  sheet-steel  at  such  high  frequencies,  nor  was  there 'at 

65 


66  ELECTRICAL  ENGINEERING  PAPERS 

hand  any  suitable  apparatus  for  determining  such  losses.  As 
preliminary  data,  tests  at  frequencies  up  to  about  140  cycles 
per  second  were  used  and  results  plotted  in  the  form  of  curves; 
these  results  were  plotted  for  different  thicknesses  of  sheet-steel. 
Also,  tests  were  obtained  showing  the  relative  losses  due  to 
eddy  currents  and  hysteresis,  and  these  were  plotted,  taking 
into  account  the  thickness  of  the  sheets.  These  data  were  not 
consistent  throughout;  but  the  general  shape  of  the  curves  was 
indicated,  and  in  this  way  the  probable  loss  at  the  frequency 
of  10  000  cycles  per  second  was  estimated  for  the  thinnest 
sheet-steel  which  could  be  obtained.  The  steel  finally  obtained 
for  this  machine  was  in  the  form  of  a  ribbon  about  2  in.  wide, 
and  about  0.003  in.  thick,  which  was  very  much  thinner  than 
any  steel  used  in  commercial  dynamos  or  transformers,  which 


varies  from  0,125  to  0.0280  inch.  Therefore  the  machine  had  to 
be  designed  with  the  intention  of  using  this  narrow  ribbon  of 
steel  for  the  armature  segments. 

A  second  consideration  of  great  importance  in  the  construc- 
tion of  such  a  machine  is  the  number  of  poles  permissible  for 
good  mechanical  construction.  For  instance,  at  3000  revolu- 
tions— which  was  adopted  as  normal  speed — the  number  of 
poles  required  is  400  for  10  000  cycles  per  second.  The  fre- 
quency, expressed  in  terms  of  alternations  per  minute,  multi- 
plied by  the  pole-pitch  in  inches,  gives  the  peripheral  speed  in 
inches.  At  1  200  000  alternations  per  minute  (or  10  000  cycles 
per  second)  and  a  pole  pitch  of  0.25  in.,  for  example,  the  peri- 
pheral speed  of  the  field  will  be  25  000  feet  per  minute.  It 
was  therefore  evident  that  either  a  pole  construction  should  be 


10000  CYCLE  ALTERNATOR  67. 

adopted  which  would  stand  this  high  peripheral  speed,  or  the 
pole-pitch  should  be  less  than  0.25  in.  It  was  finally  decided 
that  an  inductor  type  of  alternator  would  be  the  most  convenient 
construction  for  this  high  frequency;  with  the  inductor  type 
alternate  poles  could  be  omitted,  thus  allowing  200  pole  projec- 
tions, instead  of  400.  The  field  winding  could  also  be  made 
stationary  instead  of  rotating,  which  is  important  for  such  a 
high  speed.  This  construction  required  a  somewhat  larger  ma- 
chine for  a  givan  output  than  if  the  usual  rotating  type  of 
machine  were  adopted;  but  in  a  machine  of  this  type  where 
everything  was  special,  the  weight  of  material  was  of  compara- 
tively little  importance,  and  no  attempts  were  made  to  cut  the 
weight  or  cost  of  the  machine  down  to  the  lowest  possible  limits. 

The  following  covers  a  general  description  of  the  electrical 
and  magnetic  features  of  the  machine. 

Armature. — The  armature  was  built  up  in  two  laminated 
rings  dovetailed  into  a  cast-iron  yoke,  as  indicated  in  Fig.  1. 


^"  Ffe-B 

The  laminations  were  made  in  the  form  of  segments  dovetailed 
to  the  cast-iron  yoke  (Fig.  2).  Special  care  was  taken  that  the 
laminations  made  good  contact  with  the  cast-iron  yoke,  as  the 
magnetic  circuit  is  completed  through  the  yoke. 

The  armature  sheet-steel  consisted  of  plates  of  0.003  in. 
thickness.  The  sheet-steel  was  not  annealed  after  being  re- 
ceived from  the  manufacturer;  it  was  so  thin  that  to  attempt 
annealing  was  considered  inadvisable.  To  avoid  eddy  currents 
between  plates  each  segment  was  coated  with  a  thin  paint  of 
good  insulating  quality.  This  painting  was  a  feature  requiring 
considerable  care  and  investigation,  as  it  was  necessary  to  obtain 
a  paint  or  varnish  which  was  very  thin,  and  which  would  adhere 
properly  to  the  unannealed  laminations.  These  laminations 
had  a  bright  polished  appearance  quite  different  from  that  of 
ordinary  steel.  They  were  so  thin  that  the  ordinary  paint  or 
varnish  used  on  sheet-steel  made  a  relatively  thick  coating, 
possibly  almost  as  thick  as  the  plates  themselves.  A  very  thin 
varnish  was  finally  obtained  which  gave  a  much  thinner  coating 
than  the  plate  itself,  so  that  a  relatively  small  part  of  the  arma- 
ture space  was  taken  up  by  the  insulation  between  plates. 


ELECTRICAL  ENGINEERING  PAPERS 


Each  armature  ring  or  crown  has  400  slots.  Each  slot  is 
circular  and  0  0625  inch  diameter  "(Fig  3)  There  is  0.03125 
inch  opening  at  the  top  of  the  slot  into  the  air-gap,  and  the 
thickness  of  the  overhanging  tip  at  the  thinnest  point  is  0  03125 
inch 


g.  4 


The  armature  winding  consists  of  No  22  wire,  B.  &  S  gauge, 
and  there  is  one  wire  per  slot.  The  entire  winding  is  con- 
nected in  series  (Fig  4)  The  measured  resistance  of  the  wind- 
ing is  1.84  ohms  at  25°  cent. 

After  the  sheet-steel  was  built  up  in  the  frame,  it  was  ground 
out  carefully.  The  laminations  were  then  removed,  all  burred 
edges  taken  off  and  the  laminations  again  built  up  in  the  frame. 
The  object  of  this  was  to  remove  all  chances  of  eddy  currents 


10000  CYCLE  ALTERNATOR  69 

between  the  plates  due  to  any  filing  or  grinding.    The  finished 
bore  of  the  armature  is  25.0625  inch. 

Field  or  Inductor.  —  This  was  made  of  a  forged-steel  disc 
25  in.  diameter  turned  into  the  proper  shape,  and  the  poles 
were  formed  on  the  outside  by  slotting  the  periphery  of  the  ring. 
The  general  construction  is  indicated  in  Figs,  1  and  5.  The 
poles  were  0  125  in  wide  and  about  0  75  in.  long  radially  and 
were  round  at  the  pole-face.  Fig.  6  shows  the  general  dimen- 
sions of  a  pole. 

The  field  winding  consisted  of  No  21  wire,  B.  &  S.  gauge. 
There  were  600  turns  total  arranged  in  30  layers  of  20  turns  per 
layer.  The  field  coil  after  being  wound  was  attached  to  a  light 
brass  supporting  ring.  The  general  arrangement  of  the  field  or 
inductor,  armature  yoke,  and  bearings,  is  as  indicated  in  Fig  1. 
The  measured  resistance  of  the  field  winding  is  53  8  ohms  at 
25°  cent. 

Tests. — The  machine  was  designed  primarily  for  only  a  small 
output,  but  was  operated  on  temporary  test  up  to  2  kw.  A 
series  of  curves  were  taken  at  500,  1000,  1500,  2000,  2500,  and 
3000  revolutions,  giving  frequencies  from  1667  to  10  000  per 
second.  At  each  of  the  above  speeds,  saturation  curves,  iron 
losses,  and  short-circuit  tests  were  made.  Friction  and  wind- 
age were  also  measured  at  each  speed. 

On  account  of  the  high  frequency,  the  machine  was  worked 
at  a  very  low  induction;  consequently  there  is  an  extremely 
wide  range  in  pressure,  the  normal  operating  pressure  being 
taken  at  approximately  150  volts. 

On  curve  sheet  No.  1,  the  saturation  curves  for  the  various 
speeds  are  given.  These  curves  check  fairly  well,  the  pressure 
being  practically  proportional  to  the  speed  with  a  given  field 
charge.  This  is  to  be  expected  at  the  lower  speeds,  but  it  was 
considered  possible  that  at  3000  revolutions  the  air-gap  might 
be  slightly  lessened,  due  to  the  expansion  of  the  rotor  under 
centrifugal  action;  and  it  was  also  thought  that  eddy-current 
loss  due  to  the  high  frequency  might  affect  the  distribution  of 
magnetism  at  the  armature  face,  but  the  armature  iron  losses 
were  comparatively  small,  and  there  appeared  to  be  no  such 
effect.  Also  there  appears  to  be  no  effect  due  to  expansion  at 
high  speed.  The  air-gap  specified  for  this  machine  is  0  03125  in/ 
on  each  side  or  0.0625  in  total  gap.  A  very  small  varia- 
tion in  the  diameter  of  the  inductor  or  the  bore  of  the  armature 


70 


ELECTRICAL  ENGINEERING  PAPERS 


would  make  a  relatively  large  per  cent,  in  the  effective  air-gap, 
Therefore  no  reliable  calculations  can  be  made  on  the  saturation 
curves  of  this  machine  based  upon  the  specified  air-gap. 

Curve  sheet  No.  2  shows  the  iron  losses  at  various  speeds 
from  500  to  3000  rev.  per  min. — 1667  to  10  000  cycles  per  second. 
These  losses  are  plotted  in  terms  of  watts  for  a  given  exciting 

400 


37 


LO       1.2       1.4 
Field  Amperes 

current.  These  curves  show  a  rather  unexpected  condition  as 
regards  the  losses.  According  to  the  original  data  showing  the 
•  relative  losses  due  to  eddy  currents  and  hysteresis,  the  eddy- 
current  loss  even  with  these  thin  plates  should  have  been  much 
higher  than  the  hysteresis  loss,  but  these  iron-loss  curves  sho^v 


10000  CYCLE  ALTERNATOR 


71 


losses  with  a  given  field  charge  almost  proportional  to  the  fre- 
quency, which  is  the  ratio  that  the  hysteresis  loss  alone  should 
show.  As  the  eddy-current  loss  varies  as  the  square  of  the 
frequency,  the  writer  expected  this  to  be  a  large  element  in 
the  total  iron  loss,  especially  at  the  higher  inductions. 
The  six  curves  shown  on  this  test-sheet  are  fairly  consistent 


1 

1400 
1300 
1200 
1100 
1000 
900 
800 
700 
000 
000 
400 
300 
SJOO 
IOC 


HIGH-  FREQUENCY  ALTERNATOR 

10000  Cycles  per  Second 

Iron-Loss  Tests 


0.2       0.4 


0.8       LO        lj>       1.4 
Field  Amperes 


1.8 


with  each  other,  but  it  should  be  remembered  that  in  making 
measurements  of  such  abnormal  apparatus  little  discrepancies 
in  the  curves  could  easily  creep  in.  For  instance,  in  the  satu- 
ration curve  a  series  of  experiments  were  first  made  to  find 
whether  usual  types  of  voltmeters  were  satisfactory,  and  a  num- 


72 


H&IELECTRICAL  ENGINEERING  PAPERS 


ber  of  different  methods  for  checking  these  readings  were  used; 
In  determining  the  iron  losses  in  curve  sheet  No.  2,  the  machine 
was  driven  by  a  small  motor  and  the  losses  measured  with  difn 
ferent  field  charges.  Under  most  conditions  of  test  the  iron  los$ 
was  a  small  element  of  the  total  loss,  and  therefore  slight  varia-. 
tions  in  the  friction  loss  would  apparently  show  large  variations 


HIGH-FREQUENCY  ALTERNATOR 

10000  Cycles  per  Second 

Short-Circuit  Tests 


<U 
Field  Amperes 

in  the  iron  losses.  Also  the  fly-wheel  capacity  of  the  rotating 
part  of  the  alternator  was  comparatively  high.  Therefore,  if 
there  are  any  variations  in  the  circuits  supplying  the  driving 
motor,  there  would  tend  to  be  considerable  fluctuations  in  the 
power  supplied.  Considering  all  the  conditions  of  test,  the 
curves  appear  to  be  remarkably  consistent. 


10000  CYCLE  ALTERNATOR 


73 


Curve  sheet  No.  3  shows  the  short-circuit  curves  at  speeds 
of  1000,  2000,  and  3000  rev.  per  min.,  or  frequencies  of  3333, 
6667,  and  10  000  cycles  per  second,  respectively.  It  should  be 
noted  that  at  a  given  frequency  the  short-circuit  current  is  pro- 
portional to  the  field  current  over  the  entire  range  measured 


1000 


1500 
rev.  permia. 


2000 


2500 


9000 


tut  that  the  short-circuit  current  is  not  the  same  for  the  same 
field  current  at  the  various  frequencies.  According  to  these 
curves  the  current  on  short  circuit  increases  somewhat  with  the 
given  field  charge  as  the  frequency  is  increased. 

Curve  sheet  No.  4  shows  the  measured  windage  and  friction 
losses  plotted  at  speeds  from  500*to  3000  rev.  per  min.    This 


ELECTRICAL  ENGINEERING  PAPERS 


curve  indicates  clearly  that  the  windage  is  the  principal  friction 
loss  at  the  higher  speeds.  The  writer  has  added  two  curves, 
one  showing  the  estimated  bearing  friction  loss,  and  the  other 
the  estimated  windage,  based  upon  the  assumption  that  the 
bearing  friction  varies  directly  as  the  revolutions  and  the  wind- 
age loss  with  the  third  power  of  the  revolutions.  The  small 
circles  lying  close  to  the  measured  loss  curve  show  the  sum  of 
these  estimated  losses,  and  the  agreement  with  the  measured 
loss  is  fairly  close  over  the  entire  range. 


II 

0,8150 
117  11*? 

HIGH-FHEQUENCr  ALTERNATOR 
10000  Cycles  per  Second 
Regulation  Test  artOOOO  Cycles  per  Second 

No  2 

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Ampei-es  Load 

Curve  sheet  No.  5  shows  regulation  tests  made  at  ISO  volts. 
The  power-factor  of  the  load  on  this  test  was  not  determined, 
and  it  was  extremely  difficult  to  make  accurate  measurements. 
The  load  consisted  of  incandescent  lamps  and  the  wiring  from 
the  machine  to  the  lamps  was  non-inductive  for  the  usual  fre- 
quencies; but  at  the  abnormal  frequency  of  10  000  cycles  per 
second  it  is  more  difficult  to  obtain  a  true  non-inductive  load 
with  ordinary  apparatus.  The  tested  regulation  indicates  that 
the  load  was  practically  non-inductive. 

In  first  undertaking  tests  on  this  machine  there  was  consid- 


10000  CYCLE  ALTERNATOR  75 

erable  difficulty  in  measuring  the  pressures.  It  was  found  that 
at  a  frequency  of  10  000  cycles  per  second  the  Weston  voltmeter 
did  not  work  satisfactorily.  Practically  the  same  deflection 
was  obtained  on  the  high  and  low  scales  of  a  60-120  volt  Weston 
alternating-current  direct-current  voltmeter  with  the  same 
pressure. 

Very  good  results  were  obtained  by  the  use  of  a  form  of 
static  voltmeter  devised  by  Mr.  Miles  Walker.  This  voltmeter 
is  of  the  same  form  as  the  static  wattmeter  described  by  Mr. 
Walker  before  the  AMERICAN  INSTITUTE  OF  ELECTRICAL  EN- 
GINEERS, May  1902,*  Tests  were  also  made  with  the  Cardew 
hot-wire  voltmeter  with  the  high  frequencies,  and  the  results 
checked  very  satisfactorily  with  the  static  voltmeter. 

For  measuring  the  current  a  current  dynamometer  was  used 
which  had  wood  upright  supports  and  a  celluloid  dial.  The 
only  metal  parts  outside  of  the  copper  coils  were  brass  screws. 
It  was  found  that  the  current  dynamometer  is  not  affected  by 
frequency,  unless  there  are  adjacent  metal  parts  in  which  eddy 
currents  can  be  generated  which  react  upon  the  moving  element. 
The  dynamometer  used  had  but  a  few  turns  in  order  to  reduce 
the  pressure  drop  across  it.  This  dynamometer  "was  checked 
very  carefully  at  different  frequencies  and  apparently  gave 
similar  results  for  any  frequency  between  25  and  10  000  cycles. 

Several  temperature  tests  were  made  on  this  machine.  The 
heaviest  load  on  any  test  was  13.3  amperes  at  150  volts,  or 
2-kw.  output.  This  test  was  of  two  hours'  duration,  and  at  the 
end  the  armature  iron  showed  a  rise  of  16°  cent.;  the  armature 
copper  21°  cent,  by  resistance,  and  the  field  copper  17.3°  cent. 
Air  temperature  19°  cent.  The  machine  showed  a  relatively 
small  increase  in  temperature  at  this  load  over  the  temperature 
rise  with  one-third  this  load.  This  was  probably  due  to  the 
fact  that  the  windage  loss  was  so  much  higher  than  the  other 
losses  of  the  machine  that  the  temperature  was  but  little  affected 
by  the  small  additional  loss  with  increase  in  load. 

Attempts  were  made  to  utilize  the  current  from  this  machine 
for  various  experiments,  but  difficulty  was  at  once  found  in 
transforming  it.  At  this  high  frequency  no  suitable  iron-cored 
transformer  was  available.  Transformers  with  open  magnetic 
circuits  were  tried  and  operated  better  than  those  with  iron  cores 
but  were  still  rather  unsatisfactory.  It  was  decided  that  nothing 
could  be  done  in  this  line  without  building  special  transformers. 
[TRANSACTIONS  of  the  A.  I.  E.  E^  Vol.  xix.  p.  1035J 


76  ELECTRICAL  ENGINEERING  PAPERS 

Among  the  few  experiments  made  was  that  of  forming  an  arc 
with  current  at  this  high  frequency.  This  arc  appeared  to  be 
like  an  ordinary  arc  so  far  as  the  light  was  concerned,  but  had 
a  very  high-pitched  note  corresponding  to  the  high  frequency. 
This  note  was  very  distressing  to  the  ears. 

This  machine  is  in  reality  of  the  nature  of  a  piece  of  labora- 
tory apparatus;  and  at  present  it  has  no  commercial  value.  It. 
was  designed  primarily  for  scientific  investigation,  and  appears* 
to  be  a  very  good  machine  for  that  purpose. 


THE  SINGLE-PHASE  COMMUTATOR  TYPE  RAILWAY 

MOTOR 

FOREWORD — This  paper  was  presented  before  the  Philadelphia 
Section  of  the  American  Institute  of  Electrical  Engineers  in 
February,  1908.  It  describes,  as  simply  as  possible,  the  general 
construction  and  characteristics  of  compensated  series  single- 
phase  motors  — (ED.) 


THE  broad  statement  may  be  made  that  it  is  no  more  difficult 
to  commutate  an  alternating  current  than  an  equal  direct 
current  Such  a  statement  would  appear  to  be  entirely  contrary 
to  the  usual  experience,  but  a  little  study  of  the  matter  will  show 
where  the  apparent  discrepancy  lies.  In  commutator  type  alter- 
nating-current motors,  as  usually  built,  a  relatively  large  number 
of  commutator  bars  pass  off  under  the  brush  during  one  alternation 
of  the  supply  current.  While  the  current  supplied  is  varying 
from  zero  to  maximum  value  and  back  to  zero,  possibly  50  bars 
have  been  passed  under  the  brush,  and  therefore  50  coils  in 
the  armature  have  been  reversed  or  commutated.  Some  of 
these  reversals  occur  at  the  top  of  the  current  wave  which  has 
a  value  of  about  40%  higher  than  the  mean  or  effective  value 
which  is  read  by  the  ammeter.  The  motor  is  therefore  at  times 
commutating  40%  higher  current  than  that  indicated  by  the 
instruments.  It  is  thus  evident  that  in  comparing  the  com- 
mutation of  100  amperes  direct-current  with  100  amperes 
alternating-current  we  should  actually  compare  the  direct- 
current  with  141  amperes  alternating.  In  other  words,  for  com- 
mutating equal  currents  alternating-current  or  direct-current, 
the  alternating-current  ammeter  should  register  only  71%  as 
much  current  as  the  direct-current.  Another  way  of  expressing 
it  is  that  we  have  to  commutate  the  top  or  maximum  of  the 
alternating-current  wave,  while  our  instruments  only  record 
the  mean  value. 

If  the  above  represented  the  only  difference  between  the 
alternating  current  and  direct  current  the  problem  to  be  solved 
in  commutation  of  alternating  current  would  not  be  serious. 

77 


78 


ELECTRICAL  ENGINEERING  PAPERS 


However,  the  current  to  be  commutated  by  an  alternating- 
current  motor  is  not  merely  the  working  current  supplied  tc 
the  motor  and  measured  by  the  ammeter,  but  there  is,  in  addi- 
tion, a  current  which  is  generated  m  the  motor  itself,  both  a1 
standstill  and  during  rotation,  which  has  to  be  reversed  or  com- 
mutated along  with  the  working  current.  It  is  this  latter  cur- 
rent, usually  called  the  local  or  short-circuit  current,  which  has 
been  the  source  of  greatest  trouble  in  commutating  alternating 
current;  for  this  short-circuit  current  may  have  a  value  any- 
where from  three  to  ten  times  the  working  current,  depending 
on  the  design  of  the  machine.  Therefore  in  comparing  the  com- 
mutation of  an  alternating  current,  as  indicated  by  an  ammeter, 


/ 

0 

\ 
1 

0 

\ 

I 

1 

0 

o 

r 

f 

1                      *     !     f  ; 

O 

o 

1 

1 

FIG.  1 

with  an  equal  direct  current,  we  should,  in  reality,  consider 
that  the  alternating-current  motor  is  commutating  a  maximum 
current  from  five  to  ten  times  the  value  of  the  indicated  current 
Furthermore,  it  would  not  do  to  reduce  the  ammeter  current  to 
one-fifth  or  one-tenth  value  in  order  to  compare  commutation 
with  direct  current,  because  by  so  doing  we  would  simply  be 
reducing  the  small  applied  component  of  the  total  current 
commutated  by  the  brushes,  the  local  or  short-circuit  current 
still  retaining  a  rather  high  value.  In  order  to  compare  with 
direct-current  commutation,  it  would  be  necessary  for  the 
total  maximum  of  the  combined  supply  and  the  short-circuit 
current  to  be  reduced  to  the  same  value  asr  direct  current. 


SINGLE-PHASE  RAILWAY  IfOTOR  79 

It  is  the  local  current  in  the  armature  turn  short-circuited 
by  the  brush  which  is  the  source  of  practically  all  the  trouble  in 
commutating  alternating  currents  Fig  1  illustrates  a  portion 
of  the  field  and  armature  structure  of  a  commutator  type 
alternating-current  motor.  It  will  be  noted  that  the  armature 
conductor,  which  is  in  the  neutral  position  between  poles,  sur- 
rounds the  magnetic  flux  from  the  field  pole,  just  as  the  field 
turns  themselves  surround  it.  The  field  flux  being  alternating, 
this  armature  turn  will  have  set  up  in  it  an  electromotive  force 
of  the  same  value  as  one  of  the  field  turns.  Short-circuiting 
the  two  ends  of  this  armature  turn  should  have  the  same  effect 
as  short-circuiting  one  of  the  field  turns,  which  is  the  same 
thing  as  short-circuiting  a  turn  on  a  transformer.  Such  a  short- 
circuited  turn,  if  of  sufficiently  low  resistance,  should  have  as 
many  ampere-turns  set  up  in  it  as  there  are  field  ampere-turns 
In  single-phase  motors  of  good  design  the  field  ampere-turns 
per  pole  are  about  twelve  to  fifteen  times  the  normal  ampere- 
turns  in  any  one  armature  coil.  Therefore,  if  the  armature  coil 
in  the  position  shown  in  this  Fig.  1  should  have  its  ends  closed 
on  themselves  the  current  in  this  coil  would  rise  to  a  value  of 
twelve  to  fifteen  times  normal  In  reality,  it  would  not  rise 
quite  this  much,  because  this  armature  turn  is  placed  on  a 
separate  core  from  the  field  or  magnetizing  turns  with  an  air- 
gap  between,  so  that  the  magnetic  leakage  between  the  primary 
(or  field  winding)  and  this  armature  (or  secondary  winding) 
would  tend  to  protect  this  coil  somewhat,  just  as  leakage  between 
the  primary  and  secondary  windings  of  a  transformer  tends  to 
reduce  the  secondary  electromotive  force  and  current.  Also, 
this  armature  coil  is  embedded  in  slots,  thus  adding  somewhat 
to  its  self-induction,  and  tending  further  to  reduce  the  short- 
circuit  current.  In  consequence,  with  its  ends  closed  together 
the  current  in  this  armature  coil  would  probably  not  rise  more 
than  ten  to  twelve*  times  above  normal  value  under  any  con- 
dition. It  is  evident,  therefore,  that  if  the  brush  shown  in 
Fig.  1  as  bridging  across  two  commutator  bars  to  which  the 
ends  of  this  coil  are  connected  is  of  copper  or  other  low-resistance 
material,  then  there  could  be  an  enormous  local  current  set  up 
in  the  coil  when  thus  short-circuited  by  the  brush.  This  local 
current  of  about  ten  times  the  normal  working  current  would 
have  to  be  commutated  as  the  brush  moves  from  bar  to  bar, 
and  therefore  the  operation  of  the  machine  would  be  similar  to 
that  of  a  direct-current  motor  if  overloaded  about  ten  times 
in^ current.  In  other  words,  there  would  be  vicious  sparking. 


80  ELECTRICAL  ENGINEERING  PAPERS 

Even  if  the  low-resistance  brush  were  replaced  by  one  of 
ordinary  carbon,  the  short-circuiting  current  would  still  be  rela- 
tively high,  due  to  the  fact  that  it  is  not  possible  to  make  the 
brush  contact  of  very  high  resistance  by  reducing  the  size  or 
number  of  the  brushes,  because  these -same  brushes  must  carry 
the  working  current  supplied  to  the  motor,  and  there  must  be 
brush  capacity  sufficient  to  handle  this  current.     This  brush 
capacity  will,  in  practice,  be  of  such  amount  that  the  resistance 
in  bridging  from  one  bar  to  the  next  is  still  rather  low,  although 
much  higher  than  if  a  copper  brush  were  used.     Experience 
shows  that  with  not  more  than  four  or  five  volts  generated  in 
this  short-circuited  coil  by  the  field  flux,  the  resistance  of  the 
carbons  at  the  contact  with  the  commutator  would  be  such  that 
a  short-circuit  current  of  three  to  fouf  times  the  normal  working 
current  in  the  coil  can  still  flow.     Therefore,  if  the  motor  were 
equipped  with  carbon  brushes  and  had  but  four  or  five  volts 
generated  in  the  short-circuit  coil,  the  motor  would  have  to- 
comrnutate  the  main  or  working  current  and  also  a  short-circuit 
current  of  possibly  three  times  the  amount.     This  short-circuit 
current  would  also  have  a  maximum  or  top  of  its  current  wave. 
Assuming  100  amperes  as  the  current  supplied  to  the  motor, 
the  machine  therefore  actually  commutates  a  supply  current  of 
141  amperes  and  an  additional  short-circuit  current  of  possibly 
three  times  this  value,  or  from  400  to  500  amperes;  therefore, 
the  motor  actually  comtoutates  the  equivalent  of  about  600' 
amperes  direct  current  when  the  alternating-current  ammeter 
is  reading  100,     It  is  evident  from  this  that  any  one  who  tries 
to  commutate  alternating  current  with  an  ordinary  type  of 
commutating  machine  would  at  once  draw  the  conclusion  that 
alternating  current  in  itself  is  very  difficult  to  commutate, 
naturally  overlooking  the  fact  that  it  is  the  excessive  "current 
handled  by  the  brush  that  is  back  of  the  trouble,  and  not  the 
current  indicated  by  the  ammeter. 

From  what  has  been  stated,  it  is  evident  that  the  excessive 
local  current  is  back  of  the  difficulty  in  commutating  alternating 
current.  All  efforts  of  designers  of  alternating-current  com- 
mutator motors  have  been  in  the  direction  of  reducing  or  elimina- 
ting this  local  current.  The  present  success  of  the  motor,  in 
the  various  forms  brought  out,  is  largely  due  to  the  fact  that 
this  current  has  been  successfully  reduced  to  so  low  a  value  that 
it  does  not  materially  add  to  the  difficulties  of  commutating  the 
main  current.  No  successful  method  has  yet  been  practically 


SINGLE-PHA  SE  RA IL  WA  Y  MOTOR  S 1 

developed  for  entirely  overcoming  the  effects  of  this  short-circuit 
current  under  all  conditions  from  standstill  to  highest  speed. 
Some  of  the  corrective  methods  developed  almost  eliminate  this 
current  at  a  certain  speed  or  speeds,  but  have  little  or  no  cor- 
rective effect  under  other  conditions;  other  methods  do  not  effect 
a  complete  correction  at  any  speed,  but  have  a  relatively  good 
effect  at  all  speeds  and  under  all  conditions.  The  former 
methods  would  appear  to  be  applicable  to  motors  which  run  at, 
or  near,  a  certain  speed  for  a  large  part  of  the  time;  the  latter 
method  would  be  more  applicable  to  those  cases  where  the  motor 
is  liable  to  be  operated  for  considerable  periods  with  practically 
any  speed  from  standstill  to  the  highest.  While  several  methods 
have  been  brought  forward  for  correcting  local  current  when 
the  motor  has  obtained  speed,  yet  up  to  the  present  time  but 
one  successful  method  has  been  developed  for  materially  re- 
ducing this  current  at  standstill  or  very  low  speeds.  It  may 
be  suggested  that  the  short-circuit  voltage  per  coil  be  reduced 
to  so  low  a  value,  say  four  or  five  volts,  that  the  local  current 
is  not  excessive  and  does  not  produce  undue  sparking.  This 
would  certainly  reduce  the  sparking  difficulty,  but  is  open  to  the 
very  great  objection  that  the  capacity  of  the  motor  is  directly 
affected  by  a  reduction  in  the  short-circuit  voltage.  This  voltage 
per  turn  in  the  armature  coil  is  a  direct  function  of  the  value  of 
the  alternating  field-flux  and  its  frequency.  Assuming  a  given 
frequency,  then  the  short-circuit  voltage  is  a  direct  function  of 
the  induction  per  field  pole,  and  the  lower  the  short-circuit  volt- 
age the  lower  must  be  the  field  flux.  But  the  output  of  the 
machine,  or  the  torque  with  a  given  speed,  is  proportional  to 
the  product  of  the  field  flux  per  pole  by  the  armature  ampere- 
turns.  In  a  given  size  of  armature  the  maximum  permissible 
number  of  ampere-turns  is  pretty  well  fixed  by  mechanical  and 
heating  considerations,  and  therefore  with  a  given  armature 
the  torque  of  the  motor  is  a  direct  function  of  the  field  flux. 
Using  the  maximum  permissible  armature  ampere-turns,  the 
output  of  a  given  motor  would  be  very  low  if  the  field  flux  were 
so  low  that  the  short-circuit  voltage  would  not  be  more  than 
three  or  four  volts.  Increasing  the  field  induction,  and  there- 
fore increasing  the  short-circuit  voltage,  increases  the  output. 
Experience  shows  that  on  large  motors,  such  as  required  for 
railway  work,  the  induction  per  pole  must  necessarily  be  so  high 
that  the  electromotive  force  in  the  short-circuit  coil  must  be 
about  double  the  figure  just  given;  therefore,  with  such  heavy 


82 


ELECTRICAL  ENGINEERING  PAPERS 


flux  the  short-circuited    current  will  necessarily  be  excessive 
unless  some  corrective  means  is  used  for  reducing  it. 

I  will  consider  the  standstill  or  low-speed  conditions  first 
For  this  condition  only  one  practical  arrangement  has  so  far 
been  suggested  for  reducing  the  local  current  to  a  reasonably 
low  value  compared  with  the  working  current.  This  method 
involves  the  use  of  preventive  leads,  or,  as  they  are  sometimes 
called,  resistance  leads  These  consist  of  resistances  connected 
between  the  commutator  bars  and  the  armature  conductors. 
Fig.  2  illustrates  the  arrangement.  The  armature  is  wound 
like  a  direct-current  machine,  except  that  the  end  of  one  arma- 
ture coil  is  connected  directly  to  the  beginning  of  the  next 


o' 

8 

O 
0 
0 

FIG  2 

without  being  placed  m  the  commutator  Between  these  con- 
nections separate  leads  are  carried  to  the  commutator  bars,  and 
in  these  leads  sufficient  resistance  is  placed  to  cut  down  the 
short-circuit  current.  The  arrangement  is  very  similar  in  effect 
to  the  preventive  coils  used  in  connection  with  step-by-step 
voltage  regulators  which  have  been  in  use  for  many  years.  In 
passing  from  one  step  to  the  next  on  such  regulators,  it  is  common 
practice  to  introduce  a  preventive  coil  or  resistance  in  such  a 
way  that  the  two  contact  bars  are  bridged  only  through  this 
preventive  device. 

In.  an  armature  winding  arranged  in  this  way,  the  working 
current  is  introduced  through  the  brushes  and  the  leads  to  the 
armature  winding  proper.  After  entering  the  winding,  the 


SINGLE-PHASE  RAILWA  Y  MOTOR  S3 

current  does  not  pass  through  the  resistance  leads  because  the 
connections  between  coils  are  made  beyond  these  leads.  In 
consequence,  only  a  very  small  number  of  these  leads  are  in 
circuit  at  any  one  time,  when  the  armature  is  in  motion  all  the 
leads  carry  current  in  turn  so  that  the  average  loss  in  any  one 
lead  is  very  small.  As  the  brush  generally  bridges  across  two 
or  more  commutator  bars,  there  is  usually  more  than  one  lead 
in  circuit,  but  generally  not  more  than  three.  When  the  brush 
is  bridging  across  two  bars,  there  is  not  only  the  working  cur- 
rent passing  into  the  two  leads  connected  ta  these  two  bars, 
but  there  is  the  local  current,  before"  described,  which  passes  in 
through  one  lead,  through  an  armature  turn,  then  back  through 
the  next  lead  to  the  brush.  There  are  losses  in  these  two  leads 
due  to  these  two  currents.  By  increasing  the  resistance,  the  loss 
due  to  the  working  current  is  increased,  but  at  the  same  time 
the  short-circuit  current  is  decreased.  As  the  Toss  due  to  this 
latter  is  equal  to  the  square  of  the  current  multiplied  by  the 
resistance,  it  is  evident  that  increasing  this  resistance  will  cut 
down  the  loss  due  to  the  local  current  in  direct  proportion  as  the 
resistance  is  increased.  When  the  working  current  is  much 
smaller  in  value  than  the  short-circuit  current,  an  increase  in 
the  resistance  of  the  leads  does  not  increase  the  loss  due  to  the 
working  current  as  much  as  it  decreases  the  loss  due  to  the 
short-circuit  current.  Both  theory  and  practice  show  that 
when  the  resistance  in  the  leads  is  so  proportioned  that  the 
short-circuit  current  in  the  coil  is  equal  to  the  normal  working 
current,  the  total  losses  are  u  minimum.  Calculation,  as  well 
as  experience,  indicates  that  a  variation  of  20%  to  30%  at  either 
side  of  this  theoretically  best  resistance  gives  but  a  very  slight 
increase  in  loss,  so  there  is  considerable  flexibility  in  the  adjust- 
ment of  this  resistance.  The  resistance  of  the  brush  contacts 
and  of  the  coil  itself  must  be  included  with  the  resistance  of  the 
leads  in  determining  the  best  value.  In  practice  it  is  found  that 
with  ordinary  medium-resistance  brushes,  the  resistance  in  the 
leads  themselves  should  be  about  four  or  five  times  as  great  as 
the  resistance  in  the  brush  contact  and  the  coil;  that  is,  we 
usually  calculate  the  total  necessary  resistance  required  and 
then  place  about  70%  or  80%  of  it  in  the  leads  themselves. 
When  leads  of  the  proper  proportion  are  added  to  the  motor,  it 
is  found  that  practically  twice  as  high  field  flux  can  be  used  as 
before  with  the  same  sparking  and  burning  tendency  as  when 
the  lower  flux  is  used  without  such  leads.  But  even  with  six 


84  ELECTRICAL  ENGINEERING  PAPERS 

to  eight  volts  per  commutator  bar  as  a  limit,  we  are  greatly 
handicapped  in  the  design  of  the  motors,  especially  when  the 
frequency  is  taken  into  account.  This  limited  voltage  between 
bars  also  indicates  at  once  why  single-phase  railway  motors  are 
wound  for  such  relatively  low  armature  voltages.  Direct- 
current  railway  motors  commonly  use  from  12  to  20  volts  per 
commutator  bar,  or  from  2  to  2.5  times  the  usual  practice  on 
alternating-current  motors.  With  this  low  voltage  between 
bars  in  alternating-current  machines,  with  the  largest  practic- 
able number  of  bars,  the  armature  voltages  become  200  to  250,. 
or  about  40%  of  the  usual  direct  voltages.  The  choice  of  low 
voltage  should,  therefore,  not  be  considered  as  simply  a  whim 
of  the  designers;  it  is  a  necessity  which  they  would  gladly 
avoid  if  possible. 

Assuming  preventive  leads  of  the  best  proportions,  let  us 
again  compare  the  current  to  be  commutated  in  an  alternating- 
current  motor  with  that  of  the  direct-current.  Considering 
the  ammeter  reading  as  100,  the  working  alternating  current 
has  a  maximum  value  of  140  and  in  addition  there  is  a  short- 
circuit  current  of  same  value.  Even  under  this  best  condition, 
the  alternating-current  mo  tor  must  commutate  a  current  several 
times  as  large  as  in.  the  corresponding  direct-current  motor. 
The  design  of  such  a  motor,  therefore,  is  a  rather  difficult  prob- 
lem, even  under  the  best  conditions.  . 

While  resistance  leads  theoretically  appear  to  give  the  most 
satisfactory  method   for  'obtaining   good    starting   and   slow- 
speed  running  conditions,  yet  other  methods  have  been  pro- 
posed.   The  only  one  of  any  practical  importance  is  that  in 
which  the  short-circuit  voltage  is  reduced  at  start  and  at  slow 
speed  by  sufficiently  reducing  the   field  induction.     As  this 
reduced  field  induction  would  give  a  proportionately  reduced 
torque,  it  is  necessary  at  the  same  time  to  increase  the  armature 
ampere-turns  a  corresponding  amount  above  normal.     This  is 
only  a  part  solution  of  the  problem,  however,  for  the  decrease 
in  short-circuit  current  by  this  means  is  partly  offset  by  the 
increase  in  the  working  current,  so  that  the  total  current  to  be 
commutated  is  not  reduced  in  proportion  to  the  field  flux. 
Where  the  period  of  starting  and  slow  running  is  very  short> 
this  method  is  fairly  successful  in  practice.     However,  with  this 
arrangement  it  is  rather  dangerous  to  hold  the  motor  at  stand- 
still for  any  appreciable  length  of  time,  for  in  such  a  case  the 
large  short-circuit  current  is  confined  to  a  single  coil  and  the 


SINGLE-PHASE  RAILWAY  MOTOR  So 

effect  is  liable  to  be  disastrous  if  continued  for  more  than  a  very 
short  period.  With  this  method  of  starting,  the  total  current 
handled  by  the  brushes  will  usually  be  at  least  two  to  three 
times  as  great  as  when  preventive  leads  are  used. 

The  preceding  statements  refer  mainly  to  starting  or  slow- 
speed  conditions.  When  it  comes  to  full-speed  conditions, 
however,  there  are  various  ways  of  taking  care  of  the  commuta- 
tion. One  of  these  methods  is  based  on  the  use  of  preventive 
leads,  as  described ;  the  other  methods  depend  upon  the  use  of 
commutating  poles  or  commutating  fields  in  one  form  or  another. 

It  is  evident,  from  what  has  been  said,  that  at  start  the  pre- 
ventive leads  which  reduce  the  short-circuit  current  to  low 
values  will  also  be  effective  in  a  similar  manner  when  running 
at  normal  speed  Such  a  motor  with  proper  proportion  of 
leads  will,  in  general,  commutate  very  well  at  full  speed  when 
the  starting  conditions  have  been  suitably  taken  care  of  Nothing 
further  need  be.said  of  this  method  except  that  the  tests  show 
that  the  short-circuit  current  has  considerably  less  value  at 
high  speed  than  at  start. 

The  other  methods  of  commutation  at  speed,  involving  corn- 
mutating  poles  and  commutating  fields,  necessarily  depend  upon 
the  armature  rotation  for  setting  up  a  suitable  electromotive 
force  in  the  short-circuit  coil  to  oppose  the  flow  of  the  short- 
circuit  current.  As  the  electromotive  force  in  the  short-cir- 
cuited coil  is  a  direct  function  of  the  field  flux,  and  is  inde- 
pendent of  speed,  while  the  correcting  electromotive  force  is  a 
function  of  the  armature  speed,  it  is  evident  that  either  the 
commutating  pole  can  produce  the  proper  correction  only  at 
one  particular  speed,  or  the  strength  of  this  commutating  pole 
must  be  varied  as  some  function  of  the  speed  Usually  the 
strength  of  these  poles  is  made  adjustable  with  a  limited  number 
of  adjustments  and  approximate  compensation  only  is  obtained 
on  the  average.  In  the  Siemens-Schuckert  motor  the  corn- 
mutating  poles  are  of  small  size  and  placed  between  the  main 
poles.  These  are  for  the  purpose  of  obtaining  commutation 
when  running.  In  addition  the  armature  is  provided  with  pre- 
ventive leads  for  improving  the  operation  at  start  and  at  slow 
speed.  In  the  Alexanderson  motor,  according  to  published 
description,  no  separate  commutating  poles  are  provided,  but 
the  edges  of  the  main  poles  are  used  as  commutating  poles, 
the  armature  coil  having  its  throw  shortened  until  its  two  sides 
come  under  the  edges  of  the  main  poles.  In  this  motor  the  field 


S6  ELECTRICAL  ENGINEERING  PAPERS 

'is  weakened  and  the  armature  ampere -turns  are  increased 
while  starting  The  commutating-pole  scheme  in  this  motor  is, 
m  some  ways,  not  as  economical  as  in  the  Siemens-Schuckert 
arrangement,  as  the  motor  requires  a  somewhat  higher  mag- 
netization with  a  consequent  reduction  in  power-factor  The 
Winter-Eichberg  motor  is  quite  different  in  arrangement  from 
any  of  those  which  I  have  mentioned  I  will  not  attempt  to 
describe  this  motor  in  full,  but  will  say  that  it  has  two  sets  of 
brushes  in  the  armature,  one  of  which  is  short-circuited  on 
itself,  and  carries  the  equivalent  of  the  working  current  in  the 
types  I  have  described,  while  the  other  carries  the  magnetizing 
or  exciting  current  which  is  supplied  to  the  armature  winding 
instead  of  the  field.  The  arrangement  is  such  as  to  give  prac- 
tically the  same  effect  as  a  commutating  pole  or  commutating 
field.  When  starting,  the  field  flux  is  decreased  and  the  arma- 
ture ampere-turns  increased 

All  of  the  above  motors  are  nominally  of  low  armature  voltage 
and  all  of  them  appear  to  comrnutate  reasonably  well  at  speed. 
Two  of  them  use  the  full-speed  induction  at  start,  while  the 
other  two  use  reduced  induction  and  increased  armature  ampere- 
turns  at  start. 

There  has  been  considerable  discussion  during  the  last  year 
or  two  regarding  the  most  suitable  frequency  for  single-phase 
commutator  type  motors  It  may  therefore  be  of  interest  to 
consider  what  effect  reduction  in  frequency  would  have  on  the 
commutation,  output,  and  other  characteristics  of  the  motor. 

The  short-circuit  voltage,  as  I  have  stated  before,  is  a  function 
of  the  amount  of  field  flux  and  of  the  frequency.  For  a  given 
short-circuit  voltage  the  induction  per  pole  can  be  increased 
directly  as  the  frequency  is  decreased  If  a  certain  maximum 
induction  per  pole  is  permissible  at  25  cycles,  then  with  12.5 
cycles,  for  example,  the  induction  per  pole  may  be  double,  with 
the  same  short-circuit  voltage,  This  would  at  once  permit 
double  output  if  the  saturation  of  the  magnetic  circuit  would 
permit  the  doubling  of  the  induction.  But  on  25-cycle  motors, 
as  usually  built,  we  work  the  magnetic  flux  up  to  a  point  just 
on  the  verge  of  saturation,  so  to  speak,  as  indicated  in  Fig.  3. 
It  is  evident  that  double  induction,  under  such  conditions, 
would  not  be  practicable  unless  the  25-cycle  motor  had  been 
worked  at  an  uneconomically  low  point.  However,  an  increase 
of  30%  to  40%  in  the  induction  would  appear  to  be  obtainable, 
but  a  large  increase  in  excitation  is  required.  With  but  30% 


SINGLE-PHASE  RAILWAY  MOTOR 


87 


to  40%  higher  induction,  and  with  the  frequency  halved,  the 
short-circuit  voltage  would  be  but  65%  to  70%  of  that  with 
25  cycles  or,  in  other  words,  the  voltage  per  turn  in  the  field 
coil  is  but  65%  to  70%.  As  the  higher  induction  raises  the 
armature  counter  electromotive  force  the  field  electromotive 
force  can  be  increased  in  proportion  for  the  same  power-factor, 
or  can  be  30%  to  40%  higher  than  with  25  cycles.  As  the  total 
field  voltage,  therefore,  can  be  30%  to  40%  higher,  and  the 
voltage  per  field  turn  is  but  65%  to  70%,  it  is  evident  that  the 
number  of  field  turns  can  be  doubled  without  changing  the 


<u 

S-l 

c 

o 

I 


FIG.  3 

ratio  of  the  field  inductive  volts  to  the  armature  electromotive 
force.  In  other  words,  the  field  turns  can  be  doubled  if  the 
frequency  is  halved.  With  the  double  field  turns  the  field 
excitation  can  therefore  be  doubled,  which  is  the  requirement 
for  the  increased  induction  shown  in  Pig.  3.,  It  is  thus  evident 
that  halving  the  frequency  will  permit  higher  pole  inductions, 
and  therefore  higher  torque  and  output,  with  lower  short-circuit 
voltage  and  better  commutatmg  conditions  throughout.  Also, 
this  higher  field  induction  is  not  necessarily  accompanied  by  an 
increased  iron  loss,  for  the  lower  frequency  of  the  alternating 


88  ELECTRICAL  ENGINEERING  PAPERS 

flux  compensates  for  this.  On  the  above  basis  it  may  be  asked 
why  a  reduction  to  15  cycles  is  proposed  instead  of  to  12  5,  or 
even  to  10  cycles.  There  are  several  reasons  for  the  choice  of 
15  cycles. 

1.  The  motor  can  be  worked  up  to  so  high  a  saturation  at  15 
cycles  that  there  is  relatively  small  gain  with  a  reduction  to 
12  5  cycles,  which  would  be  about  the  lowest  frequency  to  con- 
sider when  the  transformers  and  other  apparatus  is  taken  into 
account 

2  As  the  torque  of  the  single-phase  motor  is  pulsating  in- 
stead of  being  constant,  as  in  a  direct- current  machine,  there 
is  liability  of  vibration  as  the  frequency  of  the  pulsation  is  de- 
creased. This  effect  becomes  more  pronounced  the  larger  the 
torque  of  the  motor,  and  is,  therefore,  of  most  importance  in  the 
case  of  a  large  locomotive  Expenence  shows  that  this  ten- 
dency to  vibrate  can  be  damped  out  effectively  in  very  large 
motors  with  a  frequency  of  15  cycles,  but  becomes  more  difficult 
to  suppress  as  the  frequency  is  further  reduced.  This  is,  in 
reality,  one  of  the  fundamental  reasons  for  keepmg  up  to  15 
cycles  instead  of  reducing  to  12  5  or  lower. 

3.  The   lower   the   frequency   the   heavier   the   transforming 
apparatus  on  the  car  or  locomotive.      It  is  probable  that  with 
12^  cycles  instead  of  15  cycles,  the  increase  in  weight  and  cost 
of   the   transforming  apparatus   would   about   counter-balance 
the  decrease  in  the  same  items  in  the  motors  themselves,  al- 
though the  efficiency  and  power  factor  of  the  equipment  would 
be  slightly  better  with  the  lower  frequency. 

4.  As  synchronous  converters  will  be  used  to  some  extent  m 
connection  with  the  generating  plants  for  single-phase  systems 
in  order  to  feed  existing  direct  current  railways,  the  frequency 
of  15  cycles  will  be  slightly  more  favorable  than  12.5  as  regards 
cost  of  the  converters  and  the  step-down  transformers      The 
same  will  be  true  if  motor-generators  are  used  for  transforming 
to  direct  current,  also  for  induction  motors. 

Against  the  choice  of  15  cycles  may  be  cited  the  fact  that 
there  are  other  frequencies  which  represent  a  better  ratio  to 
25  cycles  when  frequency-changers  are  to  be  taken  into  account. 
A  low-frequency  railway  generating  plant  may  require  to  tie 
up  with  some  existing  25-cycle  or  60-cycle  plant;  this  can  be 
done  by  interposing  frequency-changers.  Or  it  may  be  desired 
to  obtain  a  lower  frequency  with  a  single-phase  current  from 
some  existing  higher  frequency^  polyphase  plant,  By  inter- 


SINGLE-PHASE  RAILWAY  MOTOR  89 

posing  the  frequency-changer  the  single-phase  railway  load  will 
not  exert  any  unbalancing  effect  on  the  polyphase  supply 
circuit,  and  at  the  same  time  the  railway  circuit  can  be  regulated 
up  or  down  independently  of  the  three-phase  generator  circuit. 
In  case  the  three-phase  plant  is  operated  at  25  cycles,  then  a 
two-to-one  ratio  of  frequencies;  that  is,  12.5  cycles  on  the  rail- 
Way  circuit,  would  give  the  best  conditions  as  regards  choice 
of  poles  and  speeds  in  the  frequency-changer  sets  A  five-to- 
three  relation  is  given  by  15  cycles,  which  is  not  nearly  as  good 
as  the  two-to-one  ratio.  A  frequency  of  16|  cycles  would  give 
a  three-to-two  ratio,  which  represents  considerable  improve- 
ment over  the  five-to-three  ratio  Therefore,  this  slightly 
higher  frequency  may  prove  of  advantage  in  some  cases.  The 
choice  of  this  frequency,  however,  does  not  mean  a  new  line 
of  apparatus;  for  a  well  designed  line  of  15-oycle  motors  tran- 
formers,  etc,  should  operate  very  well  on  a  16§-cycle  circuit 
without  any  change  whatever 

When  transforming  from  60  cycles,  however,  the  15  cycle 
gives  a  four-to-one  ratio  which  is  very  good,  and  neither  12,5 
nor  16f  cycles  is  very  satisfactory.  Therefore  this  15-cycle 
frequency  represents  the  best  condition  in  transforming  from 
60  cycles,  and  fairly  good  conditions  for  transforming  from  25 
cycles;  and  by  operation  of  15-cycle  apparatus  at  16$  cycles  a 
very  good  transformation  ratio  is  obtained  from  25  cycles.  It 
may  be  of  interest  to  recall  that  the  old  Washington,  Baltimore 
and  Annapolis  Railway,  which  was  the  first  road  contracting 
for  single-phase  commutator  motors,  was  laid  out  for  16-J 
cycles.  There  was  considerable  criticisms  at  that  time  of  the 
use  of  this  frequency,  but  the  statement  which  I  have  just  made 
shows  one  very  good  reason  for  this  frequency  A  second  rea- 
son is  that  16f  cycles  per  second  is  2000  alternations  per  minute, 
which  permits  a  steam  turbine  driving  a  two-pole  generator  to 
use  a  speed  of  1000  rev.  per  min.,  which  is  a  very  good  one  for 
large  turbo-generators. 

I  have  gone  into  the  question  of  induction  and  frequency 
as  affecting  the  commutation  and  torque.  I  will  now  take  up 
the  question  of  power-factor  in  the  single-phase  commutator 
motor  In  a  direct-current  motor  we  have  two  electromotive 
forces  which  add  up  equal  to  the  applied  electromotive  force , 
namely,  the  counter  electromotive  force  due  to  rotation  of  the 
armature  winding  in.  the  magnetic  field,  and  the  electromotive 
force  absorbed  in  the  resistance  of  the  windings  and  rheostat. 


90  ELECTRICAL  ENGINEERING  PAPERS 

In  the  alternating-current  motor  there  are  these  two  electro- 
motive forces,  and  there  is  also  another  one  not  found  in  the 
direct-current  machine,  namely,  the  electromotive  force  of  self- 
induction  of  the  armature  and  field  windings  due  to  the  alter- 
nating magnetic  flux  m  the  motor  This  inductive  electro- 
motive force  exerts  a  far  greater  influence  than  the  ohmic 
electromotive  force  for  it  has  much  higher  values 

The  inductive  electromotive  force  lies  principally  in  the  main 
field   or   exciting   winding   of   the    alternating- current   motor 
There  is  a  certain  voltage  per  turn  generated  in  the  field  coils, 
depending  upon  the  amount  of  the  field  flux  and  its  frequency, 
as  stated   before.     This    electromotive  force  per  field   turn  is 
practically  of  the  same  value  as  the  short-circuit  electromotive 
force  generated  in  the  armature  coil,  as  already  referred  to.     I 
have  stated  that  a  short-circuit  voltage  of  three  or  four  volts 
per  armature  turn  gave  prohibitive  designs  and  that  it  was 
necessary  practically  to  double  this     This  means  that  the  field 
coils  also  have  six  to  eight  volts  per  turn  generated  in  them 
The  total  number  of  field  turns  must,  therefore,  be  very  small 
in  order  to  keep  down  the  field  electromotive  force,  for  this 
represents  simply  a  choke-coil  in  series  with  the  armature.     If 
the  armature  counter  electromotive  force  should  be  200  volts, 
for  instance,  which  is  rather  high  m  practice  with  25-cycle 
motors,  then  a  field  self-induction  of  half  this  value  would  allow 
about  14  turns  total  in  the  field  winding.     Compare  this  with 
direct-current  motors  with  150  to  200  field  turns  for  550  volts, 
or  60  to  80  turns  for  220  volts      The  alternating-current  25-cycle 
motor,  therefore,  can  have  only  about  20%  to  25%  as  many 
field  turns  as  the   ordinary  direct-current  motor.     This  fact 
makes  it  particularly  hard  to  design  large  motors  where  there 
must  be  many  poles.     In  the  single-phase  motor  the  induction 
per  pole  being  limited  by  the  permissible  short-circuit  voltage, 
it  is  necessary  to  use  a  large  number  of  poles  for  heavy  torques; 
but  the  total  number  of  field  turns  must  remain  practically 
constant  on  account  of  the  self  induction,  while  in  reality  the 
number  of  turns  should  be  increased  as  the  number  of  poles  is 
increased.     With  a  given  number  of  poles  we  may  have  just 
sufficient  field  turns  to  magnetize  the  motor  up  to  the  required 
point;  but  if  a  large  number  of  poles  should  be  required,  then  we 
at  once  lack  field  turns  and  must  either  reduce  the  field  induc- 
tion, and  thus  reduce  the  output,  or  must  add  more  field  turns 
and  thus  get  a  higher  self-induction  or  choking  action  in  the 


SINGLE-PHASE  RAILWA  Y  MOTOR  91 

field,  with  a  consequent  reduction  in  power- factor  Here  is 
where  a  lower  frequency  comes  in  to  advantage,  fdr,  as  I  showed 
before,  with  the  same  relative  inductive  effect,  the  field  turns 
can  be  increased  directly  as  the  frequency  is  decreased  The 
use  of  15  cycles  thus  permits  67%  more  field  turns  than  25 
cycles  and  raises  our  permissible  magnetizing  limits  enormously 
This  problem  is  encountered  particularly  in  gearless  locomotive 
motors  of  large  capacity.  For  increased  capacity  the  driving 
wheels  are  made  larger,  thus  permitting  a  larger  diameter  of 
motor,  the  length,  axlewise,  being  fixed.  But  with  increased 
diameter  of  drivers,  the  number  of  revolutions  is  decreased  for  a 
given  number  of  miles  per  hour.  With  25-cycle  motors  we 
soon  encounter  the  above  mentioned  limiting  condition  in  field 
turns;  beyond  this  point  the  characteristics  of  the  motor  must 
be  sacrificed,  and  even  doing  this  we  soon  reach  prohibitive 
limits  By  dropping  the  frequency  to  15  cycles,  for  instance, 
we  change  the  whole  situation.  The  induction  per  pole  can  be 
increased  and  the  number  of  poles,  if  desired,  can  also  be  in- 
creased. The  practical  result  is  that,  in  the  case  of  a  high-speed 
passenger  locomotive  with  gearless  motors,  a  700-h  p.  15-cycle 
motor  can  be  got  in  on  the  same  diameter  of  drivers  as  required 
for  a  500-h.p.  25-cycle  motor.  Also  a  500-h.p.  15-cycle  motor 
goes  in  on  the  same  drivers  a.s  a  360-h.p.,  25-cycle  motor.  At 
the  same  time  these  15-cycle  motors  have  better  all  round 
characteristics  than  the  25-cycle  machines  as  regards  efficiency, 
power-factor,  starting,  over-load  commutation,  etc. 

Returning  to  the  design  of  the  motor,  there  is  one  other 
electromotive  force  of  self  induction  which  may  be  considered; 
namely,  that  generated  in  the  armature  winding  and  in  the 
opposing  winding  in  the  pole  face,  usually  called  the  neutralizing 
or  compensating  winding 

Fig.  4  shows  a  section  of  the  field  and  armature  corresponding 
to  the  usual  direct-current  motor,  or  an  alternating-current 
motor  without  compensating  winding.  In  the  direct-current 
motor  the  armature  ampere-turns  lying  under  the  pole  face 
tend  to  set  up  a  local  field  around  themselves,  producing  what 
is  known  as  cross-induction.  This  produces  no  harmful  effect 
except  in  crowding  the  field  induction  to  one  edge  of  the  pole, 
thus  shifting  the  magnetic  field  slightly  and  possibly  affecting 
the  commutation  in  a  small  degree.  But  if  the  armature  is 
carrying  alternating  current  this  cross  flux  will  generate  an 
electromotive  force  in  the  armature  winding,  and  this  will  be 


92 


ELECTRICAL  ENGINEERING  PAPERS 


added  to  the  field  self-induction,  thus  increasing  the  self-induc- 
tion or  choking  action  of  the  machine.  As  the  armature  turns 
on  such  motors  are  much  greater,  in  proportion,  than  the  field 
turns,  it  is  evident  that  the  ampere-turns  under  the  pole  face 
can  exert  a  relatively  great  cross-magnetizing  effect.  This  high 
cross-magnetization  generates  a  high  armature  self-induction 
which  may  be  almost  as  much  as  the  field  self-induction.  Further, 
this  great  cross-induction  would  tend  to  shift  the  magnetic 
field  quite  appreciably,  thus  affecting  the  commutation  to  some 
extent. 

To  overcome  this  serious  objection,  the  neutralizing  winding 
is  added.     This  is  a  winding  embedded  in  the  pole  face  and  so 


o  \o\o\o\o\o  \o  V p IQI  ooj  o  IO/QI  o 


/ 


FIG.  4 


arranged  that  it  opposes  the  armature  cross-magnetizing  action. 
The  arrangement  is  shown  in  Fig.  5.  As  it  opposes  and  thus 
neutralizes  the  cross-induction  set  up  by  the  armature  winding, 
it  eliminates  the  self-induction  due  to  the  cross-magnetization. 
It  also  prevents  shifting  of  the  magnetic  field  and  thus  eliminates 
its  injurious  effect  on  commutation.  As  the  cross-flux  is 
practically  cut  out  the  armature  winding  becomes  relatively 
non-inductive.  There  is,  however,  a  small  self-induction  in 
the  armature  and  neutralizing  windings,  due  to  the  small  flux 
which  can  be  set  up  in  the  space  between  the  two  windings, 
they  being  on  separate  cores  with  an  air-gap  between. 

I  have  stated  that  the  field  turns  of  the  alternating-current 
motor  can  be  only  20%  to  25%  as  many  as  in  ordinary  direct- 


SINGLE-PHASE  RAILWAY  MOTOR 


93 


current  practice.  It  may  be  questioned  how  the  field  caa  be 
magnetized  with  so  few  field  turns.  This  has  been  one  of  the 
most  difficult  problems  in  the  motor.  Obviously,  one  solution 
would  be  the  use  of  a  very  small  air-gap,  but  in  railway  practice 
there  are  objections  to  making  the  air-gap  unduly  small.  Furth- 
ermore, if  the  armature  has  large  open  slots,  as  shown  in  Fig.  6, 
experience  shows  that  a  reduction  in  the  clearance  between  the 
armature  and  field  iron  does  not  represent  a  corresponding  de- 
crease in  the  effective  length  of  the  air-gap,  due  to  the  fact  that 
the  fringing  of  the  magnetic  flux  from  the  tooth  tip  of  the  pole 
face  changes  as  the  air-gap  is  varied.  The  most  effective  con- 
struction yet  used  consists  in  making  the  armature  slots  of  the 
partially  closed  type  as  in  the  secondary  of  an  induction  motor. 
This  is  shown  in  Fig.  7. 


jf 

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s 

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L^^M 

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1 

o  /o  fo  /o  /b't  o\  o\  o\  o^ 

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[     1 

I       1      I       •       1       i             .      \ 

0  \0  \0  \  0  \  0  \0  \0  \pl  O/'  Oy'  O/  O/O  /  O/  0 

\     \    \     \  \    ^_^y    v     X    /    /    / 

FIG.  5 

With  this  construction  practically  the  whole  armature  surface 
under  the  pole  becomes  effective,  and  the  true  length  of  air-gap 
is  practically  the  same  as  the  distance  from  iron  to  iron.  With 
the  increased  effective  surface,  due  to  this  construction,  the 
length  of  air-gap  need  npt  be  unduly  decreased,  which  is  of  con- 
siderable importance  in  railway  work. 

A  further  assistance  in  reducing  the  required  field  turns  is 
the  field  construction  used  in  the  single-phase  motor.  The 
magnetic  circuit  consists  of  laminations  of  high  permeability 
and  usually  without  joints  across  the  magnetic  path.  The  iron 
is  also  worked  either  below  the  bend  in  the  saturation  curve  or, 
at  most,  only  slightly  up  on  the  bend,  except  in  the  case  of  very 
low  frequency  motors  where  more  field  turns  are  permissible. 


ELECTRICAL  ENGINEERING  PAPERS 


Taking  the  whole  magnetic  circuit  into  account,  on  25-cycle 
motors  about  80%  of  the  whole  field  excitation  is  expended  in 
the  air-gap,  while  in  direct-current  motors,  even  with  a  much 
larger  air-gap,  as  much  as  40%  to  50%  of  the  magnetization 
may  be  expended  in  the  iron  and  In  the  joints, 

This  armature  construction  with  the  partly  closed  slots  has 
been  found  very  effective  in  large,  slow-speed,  single-phase 
motors  in  which  a  relatively  large  number  of  poles  is  required. 
This  construction  is  used  on  the  New  Haven  250-h  p.,  25-cycle 


/>  /  >  vj 


FIG.  C 

motors;  also  on  the  500  h.p.,  15-cycle  motor  on  the  Pennsylvania 
locomotive  exhibited  at  Altantic  City  at  the  Street  Railway 
convention,  last  October.  Geared  motors  for  interurban  service 
can  be  constructed  with  ordinary  open  slots  with  bands,  and 
many  have  been  built  that  way.  The  semi-closed  slot,  however, 
allows  more  economical  field  excitation. 

It  may  be  asked  what  the  objection  is  to  low  power-factors 
on  single-phase  railway  motors,  aside  from  the  increased  watt- 
less load  on  the  generating  station  and  transmission  circuits. 
There  is  an  objection  to  the  low  power-factor  in  such  motors, 


SINGLE-PHASE  RAILWAY  MOTOR 


95 


a  very  serious  one.  This  lies  in  the  greatly  reduced  margin  for 
overload  torque  in  case  the  supply  voltage  is  lowered.  In 
railway  work  it  is  generally  the  requirement  of  abnormal  loads 
or  torques  which  causes  a  reduction  in  the  line  voltage ;  that  is, 
the  overload  pulls  down  the  trolley  voltage  just  when  a  good 
voltage  condition  is  most  necessary.  This  is  true  of  direct  cur- 
rent as  well  as  alternating  current.  In  the  direct  current 
motor,  however,  such  reduction  in  voltage  simply  means  reduced 
speed  but  in  the  alternating  current  motor  the  effect  may  be 
more  serious. 

To  illustrate,  assume  a  motor  with  a  power-factor  of  90%  at 
full  load.     The  energy  component  of  the  input  being  90%, 


FIG.  7 

the  inductive  component  is  about  44%  or,  putting  it  in  terms  of 
electromotive  force  the  inductive  volts  of  the  motor  are  44% 
of  the  terminal  voltage,  Neglecting  the  resistance  of  the  motor, 
a  supplied  electromotive  force  of  44%  of  the  rated  voltage 
would  just  drive  full-load  current  through  it  and  develop  full- 
load  torque.  With  full  voltage  applied  the  motor  could  develop 
from  five  to  six  times  full-lgad  torque.  Under  abnormal  con- 
ditions a  drop  of  30%  in  the  line  voltage  would  still  give  suffi- 
cient voltage  at  the  motor  terminals  to  develop  two  and  one  half 
to  three  times  full-load  torque.  Let  us  next  take  a  motor  of 
80%  power-factor  at  full  load.  The  inductive  voltage  would 
then  become  60%  of  the  terminal  voltage,  and  therefore  60% 
Of  the  rated  voltage  must  be  applied  to  send  full-load  current 


96  ELECTRICAL  ENGINEERING  PAPERS 

through  the  motor.  This  neglects  the  resistance  of  the  motor, 
which,  if  included,  means  that-  slightly  more  than  60%  of  the 
voltage  is  required.  With  full  voltage  applied,  this  motor 
would  develop  about  three  or  four  times  the  rated  torque. 
With  30%  drop  in  the  line  voltage  the  motor  could  develop 
from  one  and  one  half  to  two  times  rated  torque,  which  is  hardly 
enough  for  an  emergency  condition 

Taking,  next,  a  motor  with  70%  power-factor  at  full  load  it 
would  require  70%  of.the  rated  voltage  to  send  full-load  current 
through  the  motor;  with  30%  drop  in  line  voltage  the  motor 
could  just  develop  full-load  torque,  and  even  with  15%  drop  it 
would  develop  only  about  one  and  one  half  times  torque.  As 
15%  drop  is  liable  to  occur  on  any  ordinary  system",  this  latter 
motor  would  be  a  very  unsafe  one. 

It  is  evident  from  the  above  that  it  would  be  bad  practice  in 
railway  work  to  install  motors  with  very  low  full-load  power- 
factors.  In  general,  the  higher  the  power-factor  the  more 
satisfactory  will  be  the  service,  other  things  being  equal. 

I  have  endeavored  to  explain  some  of  the  problems  which  have 
been  encountered  in  the  design  of  single-phase  commutator 
railway  motors  of  sizes  suitable  for  all  classes  of  railway  service. 
Here  is  a  type  of  machine  which  has  been  known  for  a  great 
many  years,  but  which,  until  the  last  few  years,  has  been  con- 
sidered utterly  bad.     In  a  comparatively  short  time  it  has  been 
changed  from  what  was  considered  an  unworkable  machine  to 
a  highly  satisfactory  one  and  this  has  been  accomplished,  not 
by  any  radically  new  discoveries,  but  by  the  common-sense 
application  of  well  known  principles  to  overcome  the  apparently 
inherent  defects  of  the  type.     As  an  indication  that  the  motor 
is  making  progress  in  the  railway  field,  I  will  mention  that  the 
first  commercial  single-phase  railway  motors  have  not  been  in 
use  more  than  four  or  five  years,  and  yet  at  the  present  time 
there  have  been  sold  by  the  various  manufacturers  in  this 
country  and  Europe,  a  total  capacity  of  approximately  200,000 
to  250,000  h.p.,  a  very  considerable  part  of  which  has  been  put 
in  operation.     Considering  that  the  motor  was  a  newcomer  in  a 
well  established  field,  the  above  record  is  astonishing.     How- 
ever, it  may  be  safely  predicted  that  what  has  been  done  in  the 
last  five  years  will  hardly  make  a  showing  compared  with  what 
will  be  done  during  the  next  five  years,  for  the  real  field  for  such 
motors,  namely,  heavy  railway  work,  has  hardly  been  touched. 


COMPARISON  OF  SERIES  AND  REPULSION  TYPE 
A.  C.  COMMUTATOR  MOTORS 

FOREWORD — This  is  part  of  a  discussion  by  the  author,  of  papers  by 
Dr.  Steinmetz  and  Mr.  Slichter  before  the  American  Institute 
of  Electrical  Engineers,  January  2d,  1904.  The  major  part  of 
the  author's  discussion  covered  the  comparison  between  the 
series  and  repulsion  type  motors,  in  which,  he  showed  that  the 
repulsion  type  was  simply  a  series  type  motor  with  a  trans- 
former added.  The  several  references  to  Mr.  Shchter's  paper 
given  in  the  discussion  have  but  little  bearing  on  the  technical 
matter  contained  but  could  not  be  eliminated  without  consider- 
able remodeling  of  the  paper. — (ED.) 


DISCUSSION  OF  STEINMETZ  AND  SLIGHTER  PAPERS  n 

IN  the  paper  presented  before  the  American  Institute  of  Elec- 
trical Engineers,  in  September,  1902,  the  speaker  called  atten- 
tion to  the  fact  that  there  were  but  two  types  of  single-phase  al- 
ternating-current motors  having  suitable  characteristics  for  rail- 
way service;  viz.,  that  called  the  "Series  Type/*  and  the  "Repul- 
sion Type."  Attention  was  called  to  the  fact  that  both 
motors  have  suitable  characteristics  for  railway  service,  as 
both  automatically  give  variable-speed  characteristics  with 
changes  in  load.  That  paper  primarily  described  a  single- 
phase  railway  system,  and  the  motor  formed  but  an  ele- 
ment in  the  general  system.  It  was  a  very  general  opinion 
at  that  time  that,  the  success  of  the  commutator  type  of  motor  for 
large  sizes  was  doubtful,  and  the  sparking  feature  was  considered 
a  fundamental  source  of  trouble.  It  was  generally  conceded  that 
if  a  motor  with  series  characteristics  could  be  made  to  operate  suc- 
cessfully, it  would  be  a  great  step  in  advance  in  the  railway  field. 

Since  that  time  single-phase  railway  systems  have  been  more 
fully  developed.  Practically,  no  departures  from  the  general 
system  then  indicated  have  been  furnished,  and  the  types  of 
motors  developed  have  been  along  the  lines  of  the  two  motors 
indicated  in  that  paper. 

Up  to  the  present  time  the  only  suitable  motors  suggested 
for  this  work  have  been  of  the  commutator  type,  and  have  been 
those  having  series  characteristics.  The  speaker  has  suggested 
that  all  these  motors  can  be  considered  broadly  under  the  one 
class  of  series  motors,  as  they  all  have  the  series  characteristics 

97 


98  ELECTRICAL  ENGINEERING  PAPERS 

of  the  direct-current  series  motor.  The  speaker  further  sug- 
gested that  they  be  sub-divided  into  the  "Straight-Series"  type 
and  the  "Transformer-Series"  type.  The  transformer-series 
could  also  be  arranged  in  two  classes;  viz.,  one  in  which  the 
armature  or  field  is  supplied  by  an  external  transformer,  and 
one  in  which  the  transformer  is  placed  in  the  motor  itself,  this 
latter  is  the  repulsion  type  of  motor. 

Figs  1,  2,  and  3  illustrate  the  three  classes.  Fig  1  being  the 
straight-series,  Fig  2  the  transformer-series  and  Fig.  3  the  repul- 
sion motor  Fig  2  would  be  considered  as  a  true  series  motor, 
although  the  armature  and  field  are  not  directly  in  series,  yet  most 
of  the  characteristics  described  as  belonging  in  the  repulsion 


FIG     1. 


PIG     2 


FIG    3 


motor  apply  directly  to  the  transformer  motor  shown  in  the 
figure.  Comparing  the  relations  of  these  motors,  viz.,  the 
straight-series  and  the  repulsion  motor,  we  will  first  take  up  the 
straight-series. 

In  this  motor,  if  properly  designed,  two  pressures  can  be  con- 
sidered; viz,,  that  across  the  field  circuit,  and  that  across  the 
armature  circuit.  The  armature  pressure  can  be  made  practi- 
cally non-inductive  so  that  the  input  of  the  armature  will  repre- 
sent practically  true  energy.  The  pressure  across  the  field 
is  practically  at  right  angles  to  the  armature  pressure,  and  repre- 
sents very  closely  the  wattless  component  supplied  by  the  motor. 
The  resultant  of  these  two  pressures  will  then  be  the  line  pressure. 
The  power-factor  of  the  motor  when  running  is  represented  prac- 


SERIES  AND  REPULSION  MOTORS  99 

ticaUy  by  the  pressure  across  the  armature  winding,  increased 
slightly  by  the  losses  in  the  field-core  and  winding.  Therefore, 
for  high  power-factors  it  is  important  that  the  pressure  across 
the  armature  circuit  be  made  as  high  as  possible,  relatively  to 
the  applied  pressure,  and  that  across  the  field  as  low  as  possible. 

There  are  three  ways  in  which  to  increase  the  pressure  across 
the  armature;  viz.,  by  increase  in  speed,  by  increase  in  the 
number  of  wires  in  series  on  the  armature,  and  by  increase  in  flux 
through  the  armature. 

By  increase  in  speed  and  increase  of  the  wires  in  series,  the 
armature  pressure  will  be  increased  without  affecting  the  field 
pressure,  and  therefore  the  ratio  of  the  armature  pressure  to  the 
line  pressure  is  increased.  Increasing  the  flux  in  the  armature  also 
increases  the  flux  in  the  magnetizing-coil  in  the  field,  and  the 
pressures  of  both  are  increased.  Therefore  this  increase  does  not 
improve  the  power-factor  of  the  machine. 

Instead  of  increasing  the  armature  pressure,  the  pressure 
across  the  field  winding  may  be  decreased ;  this  can  be  done  in  two 
ways;  viz.,  by  reducing  the  turns  in  the  field  coil,  or  by  reducing 
flux  through  the  coil.  Reducing  the  flux  through  the  field  re- 
duces the  flux  in  the  armature  winding  also,  and  therefore  repre- 
sents no  gain;  reduction  in  field-turns,  therefore,  is  the  feasible 
means  of  reducing  the  field  pressure.  Reduction  in  field-turns 
can  be  accomplished  in  two  ways;  viz.,  by  decreasing  the  effec- 
tive length  of  air-gap  in  the  motor,  and  by  increasing  the  cross- 
section  of  gap.  By  making  the  gap  very  small  the  pressure 
across  the  field  could  be  made  very  small  compared  with  the 
line  pressure,  and  extremely  high  power-factors  could  be  ob- 
tained, whether  the  motor  is  of  the  straight-series  or  the  repul- 
sion type.  Also  by  increasing  the  section  of  the  air-gap  the 
turns  of  the  field  can  be  decreased  with  a  given  total  flux  through 
the  coil,  and  the  power-factor  can  thus  be  very  considerably 
increased.  The  first  method,  viz.,  decrease  in  gap,  is  limited  by 
practical  conditions  which  have  been  determined  from  long  ex- 
perience with  direct-current  work.  It  should  be  borne  in  mind 
when  published  descriptions  of  such  motors  are  given,  that  the 
results,  as  regards  power-factor,  generally  depend  upon  data 
which  are  not  given  in  the  description;  such  as  the  magnetic 
dimensions  of  the  armature  and  field,  the  length  of  gap,  etc. 
Therefore,  a  machine  may  be  described  as  showing  an  extremely 
high  power-factor,  which  may  in  practice  not  be  a  commercial 
machine,  from  the  standpoint  of  American  railway  experience. 


100  ELECTRICAL  ENGINEERING  PAPERS 

Increasing  the  section  of  air-gap  without  decreasing  the 
length  of  gap  also  improves  the  power-factor,  but  makes  a  larger 
and  heavier  machine,  as  a  rule. 

Both  these  modifications  reduce  the  ampere-turns  in  the  field. 
The  direction  of  the  improvement  in  the  armature  was  shown 
to  be  in  increased  armature  ampere-turns  with  a  given  speed. 
It  therefore  follows  that  almost  any  result  desired  can  be  ob- 
tained as  regards  power-factor  by  increasing  the  armature  am- 
pere-turns and  decreasing  the  field,  or  exciting  ampere-turns. 
Reference  will  be  made  to  this  point  in  considering  the  repulsion 
motor. 

It  should  be  noted  that  in  all  these  motors  there  should  be  but 
little  saturation  in  the  magnetic  circuit  and  but  few  ampere- 
turns  expended  in  saturation  of  the  iron  under  normal  conditions. 
This  consequent  low  saturation  in  such  motors  leads  to  certain 
characteristics  in  the  torque  curves  which  have  been  cited  this 
evening  as  an  indication  of  superiority  of  alternating-current 
motors  over  direct-current  motors;  namely,  a  torque  increasing 
approximately  as  the  square  of  the  current.  In  fact,  this  superi- 
ority of  torque  should  be  charged  to  the  low  flux-density  of  the 
motor  rather  than  to  the  alternating  current.  If  direct-current 
motors  were  worked  normally  at  as  low  density  as  the  alternating- 
current  motor,  then  the  direct-current  motor  would  show  better 
torque  characteristics,  and  would  be  comparable  with  the  alter- 
nating-current motor.  This  claim  for  a  better  torque  in  the 
alternating-current  motor  compared  with  the  direct-current 
motor  seems  to  be  making  a  virtue  of  a  necessity. 

It  is  evident  from  what  has  been  said  that  the  power-factor  of 
the  straight-series  motor  can  be  made  anything  desired,  it  being 
a  question  of  proportion  between  armature  and  field,  length  of 
air-gap,  amount  of  material  used,  etc.  In  practice  a  compromise 
would  naturally  be  made  among  the  various  characteristics,  and 
a  slight  reduction  in  power-factor  is  probably  of  less  importance 
than  a  corresponding  reduction  in  size  and  weight.  Also  large 
clearance  is  probably  of  more  importance  than  an  extremely  high 
power-factor  at  normal  load.  In  practice  it  will  be  found  that 
the  armatures  of  such  motors  have  a  large  number  of  ampere- 
turns  compared  with  the  fields,  in  order  to  obtain  comparatively 
high  power-factors  with  large  air-gaps.  The  number  of  poles 
need  not  be  made  such  that  the  product  of  the  poles  by  the  normal 
speed  represents  the  frequency  of  the  supply  circuit;  good  series 


SERIES  AND  REPULSION  MOTORS  101 

motors  can  be  made,  and  have  been  made,  in  which  the  number 
of  poles  were  very  much  larger  or  much  smaller  than  represented 
by  this  relation. 

Taking  up  next  the  transformer  type  of  motors — Fig.  2;  the 
field  is  in  series  with  the  primary  of  the  transformer,  the  second- 
ary of  which  is  connected  to  the  terminals  of  the  motor.  I  would 
call  this  a  true  series  motor,  although  it  is  not  a  straight-series 
motor.  In  this  motor  the  pressure  across  the  armature  can  be 
made  practically  non-inductive  and  the  pressure  across  the 
primary  of  the  transformer  will  be  practically  non-inductive. 
The  voltage  across  the  field  winding  will  have  practically  90° 
phase  relation  to  that  across  the  primary  of  the  transformers,  and 
the  magnetic  field,  set  up  by  the  field  winding,  will  have  a  90° 
relation  in  time  to  the  magnetic  field  in  the  transformer,  as  in 
the  repulsion  motor.  In  this  motor  the  voltage  across  the  trans- 
former will  be  highest  at  light  loads  and  will  decrease  with  load 
until  zero  speed  is  reached.  At  start  there  is  lowest  flux  in  the 
transformer  and  highest  flux  in  the  field  winding.  Such  a  motor 
will  have  speed-torque  characteristics  very  similar  to  those  of  a 
straight-series  motor,  except  as  affected  by  the  actions  taking 
place  in  the  transformer  itself.  If  the  transformer  possesses  no 
reactance,  then  at  start  the  current  in  the  armature  should  be 
the  same  as  if  connected  as  straight-series  motor,  and  the  condi- 
tions of  torque  at  start  should  be  the  same.  If  the  transformer 
has  reactance,  then  at  start  the  current  in  the  armature  will  not  be 
quite  equal  to  the  current  which  the  armature  will  receive  if 
coupled  as  a  straight-series  motor,  assuming  the  transformer  to 
have  a  1  to  1  ratio.  Neither  will  the  armature  current  be  ex- 
actly in  phase  with  the  field  current;  therefore  the  starting 
torque  of  a  motor  connected  in  this  way  will  be  slightly  less  than 
the  torque  of  the  same  motor  if  connected  in  straight-series. 
This  is  on  the  assumption  that  the  transformer  is  one  propor- 
tioned for  small  reactance;  but  if  the  primary  and  secondary 
windings  of  the  transformer  should  be  on  separate  cores  with 
air-gap  between,  then  the  reactances  of  the  windings  are  con- 
siderably greater  than  in  the  above  case.  Therefore,  we  should 
expect  a  motor  with  such  a  transformer  to  give  still  lower  torque 
than  the  straight-series  with  the  same  current  supplied  from 
the  line. 

In  a  repulsion  motor  the  transformer  is  combined  with  the 
motor  itself  and  the  primary  and  secondary  windings  are  upon 


102 


ELECTRICAL  ENGINEERING  PAPERS 


different  cores  with  an  air-gap  between.  The  starting  condi- 
tions of  such  a  motor  as  indicated  above  should  be  poorer  than 
the  straight-series  motor,  or  for  the  same  starting  torque  some- 
what greater  apparent  energy  should  be  required.  It  stands  to 
reason  that  applying  the  current  directly  to  the  armature  wind- 
ing should  give  greater  ampere-turns  and  better  phase  relations 
than  generating  this  current  in  a  secondary  circuit,  and  not 
under  ideal  transformer  conditions.  The  tests  which  have  been 
made,  as  well  as  the  results  shown  in  the  curves  of  the  papers 
given  tonight  indicate  this.  It  is  to  be  noted  that  the  torque 
curve  is  not  the  same  shape  near  the  zero  speed  point  as  the 
torque  curve  of  the  series  motor. 


PIG   4. 


FIG   5. 


Series  motors  and  repulsion  motors  may  be  indicated  in  the 
simple  form  shown  in  Figs.  4  and  5.  In  the  diagrams  of  the 
repulsion  motor  (Fig.  5),  two  field-poles  FF,  are  shown,  and  two 
transformer-poles,  TT.  To  obtain  high  power-factors  on  such  a 
motoi  the  ampere-turns  in  T  must  be  very  much  greater  than  in 
F,  which  means  that  the  ampere-turns  in  the  secondary  or  arma- 
ture are  much  greater  than  in  the  exciting  field,  as  in  the  series 
motor.  The  high  power-factor  obtained  with  these  motors  is 
therefore  due  principally  to  the  small  ampere-turns  in  the  field 
and  the  small  pressure  across  the  field. 

For  instance,  with  brushes  set  at  an  angle  of  16°,  from  the 
primary  or  resultant  field,  the  ratio  of  armature  to  exciting  field- 
turns  would  be  almost  5  to  1,  a  ratio  which  will  also  permit  of 
extremely  high  power-factors  in  well-designed  straight-series 


SERIES  ASD  REPULSION  MOTORS  103 

motors  over  wide  ranges  of  speed.  To  this  feature  should  be 
credited  the  good  power-factors  claimed  for  the  repulsion  motor. 
In  either  the  series  or  repulsion  type  of  motors,  high  power 
factors,  especially  at  low  speeds,  are  directly  dependent  upon  this 
fact  of  high  ratio  of  armature  to  field,  and  with  a  high  ratio,  high 
power-factors  should  be  obtained  without  crediting  the  result  to 
leading  currents  in  the  armature.  In  the  diagram  of  the  repul- 
sion motor,  the  line  current  indicated  flows  through  both  the 
field  winding  and  the  transformer  winding.  The  primary  cur- 
rent sets  up  a  magnetic  field  in  the  exciting  windings  in  phase 
with  the  line  current.  If  it  also  set  up  a  field  in  the  transformer 
in  phase  with  the  line  current,  then  the  electromotive  force  gen- 
erated in  the  armature  winding  due  to  rotation  would  have  a  90° 
relation  to  the  electromotive  force  set  up  by  the  transformer,  and 
a  correcting  or  magnetizing  current  would  flow.  This  flow  is 
in  such  direction  that  it  corrects  the  relation  between  the  two 
pressures  in  the  armature  by  shifting  the  transformer  magnetism 
one-quarter  phase  later  than  the  exciting  field  magnetism.  This 
armature  corrective  current  may  thus  be  considered  as  mag- 
netizing the  transformer,  making  the  primary  input  to  the  trans- 
former practically  non-inductive;  but  this  magnetizing  or  cor- 
recting current  may  be  considered  as  flowing  in  a  circuit  at  right 
angles  to  the  field  magnetic  circuit,  and  having  practically  no 
effect  on  the  field  circuit.  Therefore  as  a  rough  approximation, 
the  exciting  field  may  be  considered  to  represent  the  wattless 
component  of  the  input,  and  the  transformer  field  the  energy 
component,  as  in  the  series  motor.  As  to  the  statement  that  the 
magnetizing  current  in  the  armature  reduces  the  wattless  com- 
ponent of  the  exciting  field,  the  speaker  does  not  accept  it  broadly. 
If  this  component  is  reduced,  then  another  component  of  practi- 
cally equal  value  is  introduced  somewhere  else,  for  the  power- 
factors  obtained  with  such  motors  can  be  accounted  for  by  the  high 
ampere-turns  in  the  armature  winding,  compared  with  the  field 
or  exciting  ampere-turns.  If  the  armature  current  improves  the 
power-factor  by  diminishing  the  magnetizing  or  exciting  field, 
then  the  curves  in  Figs.  1  and  4  of  Mr.  Slichter's  paper  should 
show  it.  The  speaker  has  gone  over  both  sets  of  curves  calculat- 
ing the  wattless  components  from  the  power-factors.  From  this 
and  other  data  in  these  curves,  he  finds  that  beginning  near 
synchronous  speed  the  wattless  component  in  the  motor  goes  tip 
.slightly  faster  than  would  be  represented  by  the  field  excitation, 


104  ELECTRICAL  ENGINEERING  PAPERS 

assuming  it  to  be  entirely  wattless.  Therefore,  according  to 
these  curves,  the  power-factors  at  lower  speeds  are  not  quite  as 
good  as  would  be  obtained  by  a  field  entirely  inductive  and  the 
armature  entirely  non-inductive,  in  a  straight  series  motor. 
These  calculations  are  rather  approximate,  as  the  curves  do  not 
check  at  all  well  with  each  other.  For  instance,  the  output  of 
the  motor  as  represented  by  the  input  multiplied  by  the  power- 
factor  and  by  the  efficiency,  does  not  check  with  the  output  as 
represented  by  the  product  of  speed  by  torque,  in  either  set  of 
curves,  the  discrepancies  being  as  high  as  10  percent.  In  Fig.  4, 
for  instance,  either  the  torque  or  the  speed  is  too  high  for  the 
lower  speeds.  Checking  back  on  this  curve,  using  either  the 
speed  and  torque  or  the  power-factor  and  efficiency  for  deter- 
mining the  output,  the  speaker  finds  that  the  wattless  component 
in  the  motor  at  190  revolutions  is  approximately  20  percent 
higher  than  it  would  be  if  the  field  excitation  alone  were  wattless, 
assuming  at  440  revolutions  the  wattless  component  is  represented 
purely  by  field  excitation;  that  is,  from  440  to  190  revolutions  the 
wattless  component  is  increased  20  percent  over  that  which 
would  be  represented  by  field  excitation  alone.  This  indicates 
that  not  only  should  the  field  excitation  be  considered  as  practi- 
cally wattless,  but  that  in  addition  there  is  a  wattless  component 
due  to  reactances  in  the  armature  windings. 

The  armature  current  can  be  split  into  two  components,  one 
of  which  is  partly  magnetizing  and  represents  no  torque.  The 
other  component  is  in  phase  with  the  field  magnetism  and  there- 
fore represents  torque.  The  magnetizing  or  wattless  element 
may  be  comparatively  small,  as  the  number  of  turns  in  the  arma- 
ture is  relatively  large,  but  the  armature  thus  carries  at  times  a 
slightly  larger  current  than  the  straight-series  motor. 

A  further  inspection  of  the  diagram  (Fig.  5)  indicates  how  the 
power-factor  of  the  motor  can  be  made  very  high  at  synchronous 
speed.  At  all  speeds  the  pressure  generated  in  the  armature  due 
to  rotation  in  the  field  of  F,  is  practically  equal  to  the  pressure 
generated  by  the  transformer  T,  thus  making  zero  pressure 
across  the  terminals.  But  also  at  synchronous  speed  the  pressure 
generated  by  the  exciting  field  acting  as  a  transformer,  between 
the  points  a  b,  will  be  practically  equal  to  the  pressure  generated 
in  the  winding  by  rotation  of  the  winding  in  the  transformer 
field.  Therefore  across  a  b  the  pressure  is  practically  zero  with 
these  conditions,  but  the  frequency  remains  the  same  as  that  in 


SERIES  AND  REPULSION  MOTROS  105 

the  field.  If  now  the  magnetizing  current  be  supplied  across  the 
points  a  6,  then  the  required  ampere-turns  for  magnetizing  the 
motor  can  be  supplied  at  practically  zero  pressure,  and  the  turns 
of  the  external  magnetizing  field  can  be  omitted.  Therefore, 
under  this  condition  the  wattless  component  is  practically  zero 
and  the  power-factor  becomes  practically  100  per  cent.  This 
is  the  method  of  excitation  used  on  certain  European  single-phase 
motors  in  which  high  power-factors  are  claimed  for  full-load 
running.  But  this  method  of  excitation  does  not  improve  con- 
ditions at  start,  as  the  same  excitation  will  be  required  at  stand- 
still, whether  the  excitation  be  supplied  to  the  armature  or  to  the 
field.  Therefore  this  method  of  excitation  does  not  help  the 
motor  at  that  condition  of  load  which  is  the  severest  on  the 
generating  and  transmission  system.  It  has  the  advantage  of 
omitting  the  field  exciting  winding,  but  has  the  great  disadvan- 
tage of  requiring  a  double  set  of  brushes  on  the  commutator, 
with  but  half  the  distance  between  the  brushes  found  in  the 
straight-series  or  the  ordinary  repulsion  motor.  I  do  not  believe 
that  such  methods  of  compensation  are  of  sufficient  advantage  to 
overcome  the  complications  attendant  upon  them. 

At  zero  speed,  both  the  straight-series  and  the  repulsion 
motors  have  low  power-factors  and  with  equal  losses  in  the 
motors,  the  repulsion  should  have  slightly  lower  power-factor 
than  the  series.  This  question  of  power-factor  at  start  is  largely 
a  question  of  internal  losses  in  the  motor  at  rest,  ajid  the  repul- 
sion motor  in  individual  cases  may  show  higher  than  the  series 
motor,  because  it  may  be  designed  with  higher  internal  losses. 
The  real  measure  of  effectiveness  is  not  the  power-factor  at  start, 
but  the  apparent  input  or  kilovolt-amperes  at  start  required  for  a 
given  starting  torque.  With  equally  good  designs  of  motors,  the 
speaker's  experience  is  that  the  kilovolt-amperes  will  be  found  to 
be  considerably  less  with  the  straight-series  than  with  the  repul- 
sion motor,  due  to  the  fact  that  the  current  is  fed  directly  into 
the  armature  and  not  by  transformer  action,  and  therefore  the 
conditions  of  phase-relation  and  amount  of  current  in  the  arma- 
ture windings  are  more  favorable.  Therefore  it  follows  that  in 
order  to  have  the  same  kUovolt-ampere  input  for  the  same  start- 
ing torque,  the  repulsion  motor  should  have  a  smaller  length 
of  air-gap  tira-n  the  corresponding  straight-series  motor,  or  should 
have  a  greater  section  of  air-gap,  which  means  greater  weight 
of  motor.  This  is  one  of  the  conditions  which  has  led  the  speaker 


106 


ELECTRICAL  ENGINEERING  PAPERS 


to  the  advocacy  of  the  series  motor  rather  than  the  repulsion 
motor,  as  he  has  considered  this  condition  of  starting  of  more 
importance  than  running;  although  he  is  satisfied  that  many  of 
the  running  conditions  of  a  well-designed  series  motor  will  be 
found  in  practice  to  be  superior  to  those  of  an  equally  well-de- 
signed repulsion  motor. 

Referring  again  to  Fig.  5,  it  will  be  noted  that  two  fields  are 
set  up  in  such  a  motor,  and  that  at  synchronous  speed  these  two 
fields  are  equal.  In  the  straight-series  motor  there  is  but  one 
field  set  up,  the  other  being  omitted.  It  is  evident  that  the 
straight-series  motor  with  the  current  supplied  directly  to  the 
brushes  can  have  a  smaller  section  in  certain  parts  of  the  mag- 
netic circuit  than  is  required  for  the  repulsion  motor,  and  that 
therefore  the  weight  of  material  would  be  less,  and  the  external 


PIG.  1. 

dimensions  can  be  less.  In  Pig.  7  the  heavy  line  represents 
outlines  of  series  and  the  dotted  line  those  of  repulsion  motor; 
therefore,  it  follows  that  for  equally  good  designs  and  same  fre- 
quency, the  straight-series  motor  should  be  more  compact,  and 
should  weigh  less  than  the  repulsion  motor.  It  is  reasonable 
to  expect  this,  as  the  repulsion  motor  contains  a  transformer  in 
addition  to  the  other  parts  found  in  the  straight-series  motor. 
Futherrnore,  the  transformer  found  in  such  a  motor  is  one  with 
an  air-gap,  and  with  the  windings  on  two  separate  elements,  and 
therefore  cannot  be  so  well  proportioned  as  a  separate  trans- 
former could  be.  Also,  there  is  a  transformer  for  each  motor, 
and  in  a  4-motor  railway  equipment,  for  instance,  there  would  be 
four  transformers  of  smaller  size  against  one  largfer  transformer 
used  with  the  series  motor,  this  larger  transformer  having  a 


SERIES  AND  REPULSION  MOTORS 


107 


closed  magnetic  circuit,  and  of  a  highly  efficient  design  com- 
pared with  the  transformers  in  the  motors  themselves. 

A  further  point  should  be  taken  up  in  the  comparison  of  these 
motors ;  viz  ,  the  current  in  the  coil  short-circuited  by  the  brushes. 
This  coil  is  a  secondary  to  the  field  and  the  current  in  it  is  neces- 
sarily greatest  at  the  period  of  strongest  field.  Therefore,  this 
current  will  be  greatest  at  the  time  of  starting.  If  the  repulsion 
motor  and  the  straight  series  motor  have  the  same  field  strength 
at  start,  then  the  short-circuited  current  should  be  the  same  in 
each  But  as  the  current  is  fed  into  the  armature  in  the  repulsion 
motor  through  transformer  action,  it  will  as  a  rule  be  found  that 


CURRENT 


STRAIGHT  SERIES  MOTOR 


REPULSION  MOTOR 


the  starting  field  strength  of  such  a  motor  is  slightly  greater 
and  the  starting  armature  strength  slightly  less  for  a  given  torque 
than  is  found  in  the  straight-series  motor  having  same  ratio  of 
armature  to  field  windings.  Therefore  the  short-circuit  current 
at  start  will  be  somewhat  larger  for  the  repulsion  motor  than  for 
the  corresponding  straight-series  motor.  This  short-circuit  cur- 
rent may  be  somewhat  less  near  full  speed  than  in  the  straight- 
series  motor,  but  it  is  not  the  full-speed  condition  which  is  the 
serious  one.  The  short-circuit  current  at  start  is  one  of  the 
most  serious  conditions  which  confronts  us  in  alterrating-cur- 
rent  motors,  and  is  also  of  great  importance  where  there  is  any 
considerable  operation  on  low  speeds.  The  speaker  advocates 


10S  ELECTRICAL  ENGINEERING  PAPERS 

a  type  wHch  he  considers  gives  the  easiest  condition  in  this  regard. 
This  short-circuiting  cannot  be  entirely  avoided  in  any  of  the 
motors  brought  out  without  adopting  abnormal  and  question- 
able constructions,  although  devices  like  narrow  brushes,  sand- 
wich windings,  etc.,  have  been  proposed.  In  certain  foreign 
motors  the  brushes  used  are  so  narrow  that  they  cover  practically 
the  width  of  one  commutator-bar.  As  such  motors  are  gener- 
ally built  with  a  veiy  large  number  of  bars,  the  brushes  used  are 
extremely  narrow,  being  approximately  0.2  inch  thick  at  the  tip. 
This  will  undoubtedly  lessen  the  short-circuiting,  but  simply 
transfers  trouble  to  another  point;  a  brush  0.2  inch  thick  is  not 
practicable  for  commercial  railway  service;  at  high  speeds, 
with  only  a  moderately  rough  commutator,  such  brushes  will 
be  liable  to  chip  and  break;  further,  the  brush  on  a  street-car 
motor  should  bridge  at  least  two  bars  to  give  good,  smooth, 
brush  operation;  in  practice,  a  0  5  inch  brush  on  motors  of  100  h.p. 
should  be  used. 

The  sandwich  winding,  which  consists  of  two  or  more  wind* 
ings  side  by  side7  will  prevent  short-circuiting  at  the  brushes,  but 
is  only  another  way  of  transferring  trouble  to  another  point;  it 
has  been  found  in  practice  that  it  is  difficult  to  run  a  sandwich 
winding  without  trouble  at  the  commutator  with  direct  current, 
without  a  tendency  to  blackening  and  pitting  the  commutator, 
and  with  alternating  current  this  tendency  to  pitting  and  burn- 
ing of  the  bars  would  be  equally  great. 

As  a  rule,  there  is  little  difference  between  the  operation  of 
repulsion  and  straight-series  motors  as  regards  sparking,  except 
that  the  repulsion  motors  generally  have  greater  current  in  the 
short-circuited  coil  near  zero  speed,  and  therefore  show  greater 
tendency  to  heat  and  spark.  At  or  near  synchronous  speed, 
there  appears  to  be  very  Httle  difference  in  the  commutation, 
although  the  speaker  has  never  given  the  repulsion  motor  the 
same  test  of  long-continued  setvicfe  as  lie  has  in  the  case  of  the 
series  motor.  These  series  motors  have  never  shown  any  tend- 
ency to  give  trouble  on  the  commutator,  and  on  an  exhibition 
car  equipped  with  four  100-h.p.  motors,  the  commutator  nave 
never  been  sandpapered  since  the  equipment  has  been  put  into 
service.  This  exhibition  car  is  used  principally  for  showing  the 
accelerating  properties  of  the  motors;  therefore,  the  speaker 
does  not  hesitate  to  say  that  the  commutation  of  the  straight- 
series  motor  will  prove  to  be  equal  to  that  of  the  direct-current 


SERIES  AND  REPULSION  MOTORS  109 

motor.     Wide  brushes  axe  used  with  it,  such  as  have  been  used 
in  street-railway  motors. 

It  is  well  known  that  with  large  direct-current  motors,  espe- 
cially when  operated  at  very  high  speeds,  there  is  a  tendency  to 
flash  across  the  commutator,  or  to  the  frame  of  the  motor,  if  the 
field  circuit  be  opened  for  a  period  long  enough  to  allow  magnet- 
ism to  drop  to  zero,  and  then  the  field  be  closed  again.  In  this 
case  there  is  a  rush  of  current  before  the  field  has  had  time  to 
build  up,  and  this  rush  of  current,  together  with  field  distortion, 
may  cause  serious  flashing.  In  the  alternating-current  motor, 
whether  of  the  straight-series  or  the  repulsion  type,  this  ten- 
dency should  be  entirely  absent.  In  the  straight-series  motor  the 
magnetism  falls  to  zero  once  in  each  alternation,  and  therefore 
if  this  tendency  existed,  flashing  would  occur  continuously. 
Furthermore,  a  properly  designed  straight-series  motor  can  be 
short-circuited  across  the  brushes  without  injury  to  the  motor, 
and  can  continue  to  operate  in  this  way;  therefore,  if  the  ma- 
chine can  be  short-circuited  in  this  way,  there  is  evidently  no 
tendency  to  maintain  an  arc. 

Returning  to  the  subject  of  power-factors  it  should  be  noted 
that  high-power-factors  are  very  frequently  found  in  motors  of 
low  or  only  moderately  good  efficiency.  This  low  efficiency  to  a 
slight  extent  explains  the  high  power-factor  in  some  motors,  both 
polyphase  and  single-phase.  Low  efficiency  means  higher  true 
energy  expended,  and  with  a  given  wattless  component  it  means 
higher  power-factor.  It  is  the  old  problem  of  increasing  the 
power-factor  by  wasting  energy  in  a  circuit  instead  of  reducing 
the  wattless  component.  The  power-factor  of  any  alternating- 
current  motor  can  be  very  considerably  increased  by  putting 
resistance  in  series  with  it.  Instead  of  this  resistance  the  internal 
losses  of  the  motor  may  be  made  higher,  which  will  accomplish 
the  same  results.  The  motor  will  therefore  appear  to  have  a 
higher  power-factor  than  it  really  deserves,  if  efficiency  of  the 
motor  is  taken  into  account.  If,  for  instance,  the  efficiency  at 
300  rev.  shown  in  Mr.  Slichter's  Fig.  4  would  be  made  as  high  as 
on  direct-current  motors,  then  the  power-factors  with  the  same 
magnetizing  and  other  conditions,  would  have  been  approxi- 
mately four  percent  lower.  This  lower  power-factor  would  not 
have  made  any  harder  condition  on  the  supply  circuit,  but  actually 
would  have  made  a  somewhat  easier  condition,  as  the  supply  sys- 
tem would  have  furnished  about  eight  percent  less  Mlovolt- 


110  ELECTRICAL  ENGINEERING  PAPERS 

amperes.  For  lower  speeds  this  difference  in  power-factor  will 
be  greater,  and  less  for  higher  speeds  A  high  power-factor  at 
start,  obtained  by  the  use  of  resistance  in  series  with  the  motor 
by  high  internal  losses  which  do  not  represent  torque,  is  there- 
fore a  detrimental  condition  rather  than  a  good  one,  as  it  means 
increased  kilo  volt-ampere  expenditure  for  a  given  torque.  This 
is  merely  given  as  an  illustration  showing  that  power-factor  in 
itself  is  not  a  true  indication  of  conditions,  but  must  be  accom- 
panied by  other  data;  this  is  not  a  criticism  of  these  motors, 
but  is  a  general  condition,  found  to  a  greater  or  less  extent  in  all 
alternating-current  motors. 


COMPARATIVE  CAPACITIES  OF  ALTERNATORS  FOR 
SINGLE  AND  POLYPHASE  CURRENTS 

FOREWORD — This  article  was  prepared  many  years  ago  for  the 
information  of  the  younger  technical  men  of  the  Westinghouse 
Company.  It  was  considered  of  sufficient  value  by  the  Com- 
pany to  publish  in  pamphlet  form,  of  which  there  were  several 
editions  from  time  to  time.  It  should  be  considered  as  purely 
educational.  Only  the  types  of  windings  which  were  in  use  up 
to  the  time  the  paper  was  prepared,  are  included. — (ED.) 


THERE  are  a  number  of  popular  misconceptions  regarding  the 
relative  polyphase  and  single-phase  capacities  which  can  be 
obtained  from  a  given  winding.  For  instance,  there  appears  to  be 
a  half -formed  opinion  that  a  given  winding  connected  for  two- 
phase  will  give  a  slightly  less  output  than  when  connected  for  three- 
phase;  but,  on  the  other  hand,  it  seems  to  be  generally  assumed 
that  the  various  three-phase  windings  all  give  the  same  rating. 

Also,  it  is  a  widespread  idea  that  when  any  polyphase  machine 
carries  a  single-phase  load  the  permissible  rating,  with  the  same 
temperature,  is  approximately  71  percent  of  the  polyphase  rating. 
While  there  are  a  few  cases  where  this  may  be  true,  yet,  in  general, 
it  is  far  from  being  the  fact,  as  will  be  explained  below. 

This  fallacy  regarding  single-phase  ratings  arose  partly  from 
early  practice  with  polyphase  machines,  which  were  ofttimes 
designed  with  a  view  to  carrying  single-phase  load  almost  exclu- 
sively. In  consequence,  the  type  of  armature  winding  chosen  was, 
in  many  cases,  that  which  gave  a  high  output  on  the  single-phase, 
with  some  sacrifice  in  the  polyphase  rating,  and  the  single-phase 
rating  in  many  cases  was  a  relatively  large  percent  of  the  polyphase 
rating  simply  because  the  polyphase  rating  was  less  than  could 
have  been  obtained  with  a  different  type  of  winding. 

The  71  percent  (or  70.71  percent)  ratio  of  single-phase  to  poly- 
phase ratings  in  a  given  armature  arose  partly  from  tho  fact  that 
at  these  relative  loads  the  total  armature  losses  were  practically 
equal.  On  old  designs  of  machines,  in  many  cases  it  could  be  as- 
sumed safely  that  with  equal  armature  losses  the  temperature  of 
the  armature  parts  would  be  practically  equal.  This  assumption 

ill 


112 


ELECTRICAL  ENGINEERING  PAPERS 


does  not  hold,  in  general,  on  modern  designs  of  machines  in  which 
each  individual  part  is  proportioned  for  a  specified  result.  The 
distribution  of  the  armature  losses  is  just  as  important  as  the  total 
losses.  If  the  temperature  drop  between  the  inside  of  the  arma- 
ture coil  and  the  armature  core  is  small  compared  with  the  tem- 
perature drop  from  the  core  to  the  air,  then  the  temperature  of  the 
armature,  or  its  rating,  will  depend  largely  upon  its  total  losses, 
equal  ventilation  being  assumed  in  all  such  comparisons  If,  how- 
ever,the  temperature  drop  from  the  coil  to  the  iron,  or  from  the  in- 
side of  the  coil  to  the  outside,  is  relatively  high,  then  the  temper- 
ature limit  may  be  fixed  by  the  loss  in  an  individual  coil  rather 
than  by  the  total  loss.  This  is  particularly  the  case  in  high  voltage 


machines  where  there  is  a  considerable  amount  of  insulation  over 
the  individual  armature  coils.  Also,  in  many  of  the  later  designs 
of  machines  (especially  turbo-generators)  each  armature  coil  is 
practically  separated  from  all  other  coils,  so  that  one  coil  can  have 
but  little  direct  influence  on  the  temperature  of  its  neighboring  coils. 
In  such  an  armature  it  is  possible  to  completely  roast  out  an  indi- 
vidual coil  or  group  of  coils  without  seriously  heating  any  other 
coils  or  groups  of  coils.  It  is  obvious  that  in  such  a  machine  the 
loss  in  the  individual  coils  is  what  fixes  the  rating  of  the  machine, 
and  not  the  armature  loss  as  a  whole.  It  is  evident,  therefore,  that 
when  a  polyphase  machine,  with  such  a  winding,  is  loaded  single- 
phase,  the  maximum  current  which  can  be  carried  in  any  single 
coil  must  be  the  same  for  either  polyphase  or  single-phase  rating. 
As  this  type  of  winding  is  used  in  the  majority  of  large  capacity 
machines  of  the  present  day,  the  following  comparison  will  show 


CAPACITIES  OF  ALTERNATORS 


113 


the  relative  rating  of  such  machines  on  polyphase  and  single- 
phase  loading.  Three-phase  ratings  will  be  considered  first, 
because  the  great  majority  of  modern  machines  are  wound  for 
three-phase. 

THREE-PHASE  WINDINGS 

All  the  various  types  of  commercial  three-phase  windings 
with  their  current  and  voltage  relations  can  be  derived  in  a  very 
simple  manner  from  the  consideration  of  a  ring  armature  with 
its  windings  arranged  in  six  symmetrical  groups,  each  covering 
60  degrees  of  the  ring,  which  may  all  be  closed  together  to  form 
the  ordinary  closed  winding,  or  which  may  be  separated  into  either 
three  or  six  groups  and  connected  to  form  various  delta  and  star 
types  of  windings. 


Let  Fig.  1  represent  such  a  ring  armature  closed  on  itself  and 
with  six  taps  brought  out,  these  being  designated  as  A,  at  B,  b,  C,  c. 

By  connecting  together  the  points  Aa,  aB,  Bb,  etc.,  as  shown 
in  Fig.  2,  a  six-sided  figure  is  obtained,  which  represents  the 
various  voltage  and  phase  relations  which  can  be  obtained  with 
all  commercial  three-phase  (and  six-phase)  windings.  It  will  be 
noted  that  Aa  and  bC  are  of  equal  length  and  are  parallel  in  di- 
rection. The  length  represents  e.  m.  f .  and  the  direction  re- 
presents phase  relation.  Therefore,  these  two  groups  or  legs  are  of 
equal  e.  m  f.'s  and  of  the  same  phase.  The  same  holds  true  of  aB 
and  Cct  and  of  Bb  and  cA.  Beginning  at  Aa,  these  groups  have 
also  been  numbered  consecutively  from  1  to  6,  so  that  in  the 
following  diagrams  a  given  leg  or  group  can  be  identified  by 
number. 


114 


ELECTRICAL  ENGINEERING  PAPERS 


Fig.  3  is  the  same  as  Pig.  2  with  three  leads  carried  out  from 
A,  B  and  C  to  form  the  three  terminals  of  a  three-phase  winding 
The  dotted  lines  from  A  to  B,  B  to  C,  and  C  to  A  represent  the 
voltage   and  phase  relations   obtained  from   this   combination 
This  is  known  as  a  closed  coil  type  of  winding  and  is  the  standard 
arrangement  of  winding  on  a  three-phase  rotary  converter.     The 
comparative  e  m.  f .  values  for  this  and  other  combinations  will  be 
given  later 

By  opening  the  closed  arrangement  of  Fig  2  at  the  points  A,  B 
and  C,  as  shown  in  Fig.  4,  then  an  open  coil  arrangement  is  obtained 
and  the  three  parts  resulting  can  be  recombined  in  several  ways, 
keeping  the  same  voltage  and  phase  relations  of  the  individual 
parts. 


•Fig.  6 


However,  only  one  of  these  combinations, — that  shown  in  Fig. 
5 —  has  been  used  to  any  extent.  This  is  one  form  of  star  winding 
which  is  sometimes  used  to  give  certain  voltage  combinations,  as 
will  be  explained  later. 

By  splitting  Fig.  2  at  six  points  instead  of  three,  as  shown  in 
Fig.  6,  various  other  open  coil  combinations  of  windings  can  be 
obtained  while  keeping  each  group  or  leg  in  its  proper  phase  and 
voltage  relations. 

One  of  these  combinations  is  shown  in  Fig.  7,  in  which  the 
groups  which  are  similar  in  e.  m.  f .  and  phase  are  connected  in 
parallel  and  the  three  resulting  combinations  are  connected  to  form 
a  delta  winding. 

In  Fig.  8  the  two  groups  of  similar  phase  are  shown  in  series 
instead  of  parallel  and  connected  to  form  a  delta.  Obviously 


CAPACITIES  OF  ALTERNATORS 


115 


Pig.  7  and  8  are  equivalent,  except  that  the  terminal  e.  m.  f  of  one 
is  double  that  of  the  other 

By  reconnecting  the  three  components  of  Fig  7  in  the  manner 


shown  in  Fig.  9,  a  parallel  star  winding  is  obtained.  Two  arrang- 
ments  are  shown,  one  with  all  the  legs  connected  together  at  the 
middle  point,  and  the  other  with  the  two  stars  not  connected  at  the 
middle. 

Fig  10  is  equivalent  to  Fig.  9  except  that  the  two  e.  m.  f.'s  of 


\C 

fig.IO 

equal  phase  are  in  series  instead  of  in  parallel,  thus  giving  just  twice 
the  voltage  of  Fig.  9. 

SIX-PHASE  WINDINGS 

The  foregoing  covers  all  of  the  usual  combinations  for  three- 
phase  windings,  open  and  closed  coil  types.  The  same  general 
scheme  may  be  used  to  illustrate  the  usual  six-phase  combinations 
of  windings  which  are  frequently  used  in  connection  with  rotary 
converters. 


116 


ELECTRICAL  ENGINEERING  PAPERS 


In  Fig.  3,  a  three-phase  winding  is  shown  with  terminals  at 
A,  B  and  C.  If  three  other  terminals  be  formed  by  a,  b  and  c, 
then  a  second  three-phase  winding  is  obtained.  The  dotted  lines 
in  Pig.  11  illustrate  the  voltage  and  phase  relation  of  these  two 
windiags.  This  is  the  so-called  double  delta  arrangement  some- 
times used  with  six-phase  rotaries,  the  dotted  lines  representing 
the  voltage  and  phase  relations  of  the  transformers  which  supply 
the  rotary  converters. 


c 
r" 


-/-— y--? 

/  ^    ' 


Fig.  II 


It  is  evident  that  the  voltage  represented  by  ac  is  equal  in 
value  and  phase  to  that  represented  by  BC.  Therefore,  one 
transformer  with,  two  secondaries  of  equal  value  could  be  tapped 
across  these  two  circuits.  Similar  arrangements  can  be  applied  to 
the  other  two-phase  relations  in  this  diagram. 

In  Pig.  2  it  is  evident  that,  if  six  terminals  are  used,  a  voltage 
can  be  obtained  across  Ab  Similar  voltages  can  be  obtained 
across  aC  and  Be.  These  three  voltages  axe  equal  in  value  but 
have  the  60-degree  relation  to  each  other.  It  is  evident  therefore 
that  three  transformers  connected  to  a  three-phase  circuit  can  have 
their  secondaries  connected  to  this  winding  across  the  indicated 


CAP  A  CITIES  OF  ALTERNA  TORS  1 17 

points.  This  arrangement  is  indicated  in  Fig.  12,  and  is  the 
so-called  diametral  connection  of  six-phase  rotaries.  The  middle 
points  of  the  transformer  winding  from  these  three  circuits  can  be 
connected  together,  if  desired. 

RELATIVE  E.  M.  F.  OBTAINED  FROM  THE  FOREGOING 
COMBINATIONS 

From  an  inspection  of  the  above  diagrams,  and  the  application 
of  but  very  little  mathematics,  all  the  e.  m.  f.  relations  of  these 
various  combinations  can  be  readily  obtained.  In  the  following 
comparisons  the  magnetic  field  is  assumed  to  be  of  such  distribu- 
tion that  the  e.  m.  f.  waves  will  be  of  sine  shape,  as  this  greatly 
simplifies  the  various  relations. 

Let  E  represent  the  effective  e.  m.  f .  of  any  one  of  the  six  legs 
or  groups  in  Fig.  2.  Then,  combining  the  various  groups  geo- 
metrically, taking  into  account  the  angular  relation  between  the 
legs  in  the  diagram,  the  various  e.  m.  f.'s  can  be  readily  derived. 
The  results  are  as  follows: 

In  Fig.  3,  the  e.  m.  f .  across  AB,  BC,  etc.,  =  VT  x  E  =  1 .732 
E. 

In  Fig.  5,  the  e.  m.f .  Ad  is  the  same  as  AB  in  Fig.  3  and  is 
therefore  equal  to  -x/lT  x  E,  but  the  e.  m.  f .  AB  in  Fig.  5  is  equal 
to  VlfxAd.  Therefore,  the  e.m.f.  of  AB  —  VlTx  VlTxS^ 
3E. 

In  Fig.  7,  AB  is  evidently  equal  to  one  group  or  side  of  Fig.  2 
and  therefore  the  e.  m.  f .  of  AB  =  E. 

In  the  same  way  the  e.  m.  f .  of  Fig.  8  =  2E. 

In  Fig.  9  the  e.  m.  f .  Ad  is  evidently  equal  to  E  and  the  e.  m.  f . 
AB=  VTxe.  m.  f.  of  Ad.  Therefore  the  e.  m.  f.  of  AB=  i/lT* 
E  =  1.732  E,  or  same  as  Fig.  3.  

In  the  same  way  the  e.  m.  f.  of  ABinFig.  10=  2times  V  3 
x£  =  3.464E. 

For  the  six-phase  combinations  the  following  e.  m.  f.'s  are 
obtained: 

In  Fig.  11  each  of  the  deltas  is  the  same  as  in  Fig.  3  and 
therefore  the  e.  m.  f.'s  are  the  same  and  are  equal  to  1.732  E. 

In  Fig.  12,  Ab  is  geometrically  equal  to  twice  aB  and  the 
e.  m.  f .  Ab  is  therefore  equal  to  2E. 

THREE-PHASE  CAPACITIES 

It  might  be  assumed  from  casual  inspection  that  all  of  the 
different  combinations  of  three-phase  and  six-phase  would  give 


118  ELECTRICAL  ENGINEERING  PAPERS 

the  same  capacities  when  cajrrying  the  same  limiting  current  per 
armature  coil,  or  per  leg.  This  however,  is  not  correct,  as  will  be 
shown  by  the  following: 

Let  A  equal  the  limiting  current  which  can  be  carried  by  one 
coil  or  by  one  group  of  windings.  This  is  not  necessarily  the 
current  per  terminal,  but  it  is  the  current  permissible  in  an  indi- 
vidual coil  without  exceeding  a  certain  prescribed  temperature. 
Then  the  following  ratings  are  obtainable  with  the  above  combina- 
tions of  windings. 

In  Pig.  3  the  rating  =  3A  x  y"~T  x  E  =*  5.196  AE. 

In  Pig.  5  the  current  per  coil  and  per  terminal  =  A.  The 
e.  m.  f .  becomes  A  x  \/ir  x  3E  =  5.196  AE.  Therefore  the  three- 
phase  ratings  of  the  windings  in  Pigs.  3  and  5  are  equal. 

In  Fig,  7  the  current  in  each  leg  of  the  delta  is  2A,  as  there 
are  two  groups  in  parallel,  each  carrying  current  A .  As  the  e.  m.  f. 
across  terminals  is  E,  the  rating  becomes  3  x  2A  x  E  *=  6  AE. 

In  Fig.  8  the  current  per  side  or  leg  of  the  delta  is  equal  to  A 
and  the  e.  m,  f .  is  2E.  The  capacity  therefore  becomes  the  same  as 
for  Fig.  7  or  =  6  AE. 

In  Fig.  9  the  current  per  terminal  is  2A  as  there  are  two 
groups  in  parallel  for  each  terminal.  The  e.  m.  f .  across  the  term- 
inals is  V1T  x  E.  The  capacity  is  therefore  2A  V~  x  V1T  E  = 
6AE. 

The  rating  of  Pig.  10  is  also  6  AE,  the  same  as  Fig.  9. 

In  Figs.  11  and  12  the  ratings  can  be  determined  by  direct 
inspection  from  the  following  method  of  considering  the  problem: 

In  a  closed  coil,  polyphase  machine,  for  example,  such  as 
shown  in  Pig.  2,  one  circuit  can  be  taken  off  from  A  and  a,  a 
second  circuit  from  a  and  B,  etc.,  and  the  total  number  of  circuits 
which  can  be  taken  off  corresponds  to  the  number  of  armature  taps. 
Each  circuit  can  be  considered  as  having  its  own  rating.  There- 
fore, the  effective  voltage  of  each  of  such  circuits  times  the  current 
per  circuit,  times  the  number  of  circuits,  equals  the  rating.  In 
Figs.  11  and  12  six  circuits  can  be  taJsen  off,  each  with  voltage 
E  and  carrying  current  A.  The  rating  therefore  becomes  6  AE. 
The  same  method  could  be  applied  to  any  other  number  of  phases 
from  closed  coil  windings, 

It  is  evident  from  the  foregoing  that  the  same  rating  can  not  be 
obtained  from  the  armature  winding  with  all  methods  of  con- 
nection. In  those  three-phase  arrangements  in  which  two  groups 
of  similar  phase  relations  are  thrown  in  series  or  parallel,  the  high- 


CAPACITIES  OF  ALTERNATORS  119 

est  output  is  obtained.  In  those  cases  where  two  e  m  f.'s  out  of 
phase  with  each  other  are  combined  to  form  one  leg  of  the  three- 
phase  circuit,  it  is  evident  that  the  resultant  e.  m.  f.  is  at  once 
reduced  by  such  combinations  and  that  the  capacity  of  the  ma- 
chine is  therefore  reduced,  simply  because  the  most  effective  use  of 
the  windings  is  not  obtained.  The  three-phase  closed  coil  winding 
is  therefore  not  as  effective  as  the  true  delta  or  star  type  of  winding. 
For  this  reason  the  closed  coil  winding  is  used  in  only  those  cases 
where  some  condition  other  than  the  current  capacity  itself  is  of 
greater  importance.  Otherwise,  delta  and  star  windings  are 
always  used,  the  star  being  preferred  as  it  gives  a  higher  voltage 
with  a  given  number  of  conductors,  or  a  smaller  number  of  conduc- 
tors for  a  given  voltage,  and  is  therefore  somewhat  more  effective 
in  the  amount  of  copper  which  can  be  gotten  into  a  given  space 

SINGLE-PHASE  RATING — Any  three-phase  machine  with  one  of 
the  above  windings  can  be  used  to  carry  single-phase  load  by  using 
two  of  the  three  terminals.  The  single-phase  e.  m  f.'s  obtained  will 
therefore  be  the  same,  in  each  case,  as  the  three-phase.  The 
current  capacity  per  coil,  or  group,  on  single-phase  can  be  no 
greater  than  on  three-phase.  On  this  basis,  therefore,  the  following 
single-phase  ratings  are  obtained  with  the  above  combinations: 

Fig.  3,  calling  A  and  B  the  single-phase  terminals,  then  with 
the  limiting  current  A  per  coil,  the  windings  1  and  2  in  the  diagram 
will  carry  current  A,  and  3,  4,  5  and  6  will  carry  %A.  The  total 
current  at  the  terminals  will  therefore  be  1}^A  and  the  e.  m.  f .  per 
terminal  will  be  V~3~  E.  The  single-phase  rating  then  becomes  1 .5 
A  x  V1T  E  =  2.598  AE.  The  corresponding  three-phase  rating  is 
3A  x  V1TE  =  5.196  AE.  The  single-phase  rating  is  therefore  just 
50  percent  of  the  three-phase  for  this  combination. 

In  Fig.  5,  the  current  per  leg  is  A>  while  the  e.  m.  f.  is  3E. 
The  single-phase  rating  therefore  becomes  3  AE.  The  correspond- 
ing polyphase  rating  is  .A  x  V1T  x  3E  =  5.196  AE.  The  single- 
phase  rating  is  therefore  57.7  percent  of  the  polyphase  rating 

In  Fig.  7  the  total  current  in  two  legs  is  2At  while  in  the 
other  four  legs  of  the  delta  the  total  current  is  A.  The  total 
current  at  the  terminals  therefore  becomes  3 A.  The  e.  rn.  f .  is  E 
and  therefore  the  single-phase  rating  becomes  3  AE.  The  corres- 
ponding three-phase  rating  is  6  AE.  The  single-phase  rating  is 
therefore  50  percent  of  the  polyphase  for  a  true  delta  winding. 

The  same  holds  true  for  Fig.  8. 


120  ELECTRICAL  ENGINEERING  PAPERS 

In  Fig.  9  the  current  per  group  Is  A  and  with  two  groups  in 
parallel  the  current  per  terminal  is  2A.  The  e.  m.  f.  across  the 
terminals  is  VT  E.  The  single-phase  rating  therefore  becomes 
2A  x  VT  E  or  3  464  AE.  The  three-phase  rating  for  the  same 
combination  is  6  AE.  The  single-phase  rating  therefore  becomes 
57.7  percent  of  the  three-phase  when  a  true  star  winding  is  used. 

Pig.  10  gives  the  same  results  as  Fig.  9. 

It  may  be  noted  that  in  the  three-phase  star  arrangement 
two  legs  are  carrying  all  of  the  current,  while  the  third  leg  is  idle 
and  could  be  omitted.  This  means  that  the  active  winding  covers 
two-thirds  of  the  armature  surf ace,  while  an  idle  space  of  one-third 
the  surface  lies  in  the  middle  of  the  winding. 

In  the  delta  winding  it  may  be  noted  that  one  leg,  covering 
one-third  the  surface,  is  directly  in  phase  with  the  single-phase 
e.  m.  f .  and  is  therefore  in  its  most  effective  position  The  other 
two  legs  carry  current  also,  but  are  relatively  ineffective  as  the 
e.  m  f.'s  generated  in  these  two  legs  are  displaced  60  degrees  in 
phase  from  the  single-phase  e.  m.  f .  delivered.  The  delta  arrange- 
ment therefore  has  two-thirds  of  its  winding  acting  in  a  very 
ineffective  manner.  One-third  of  the  winding  is  very  effective. 
In  the  star  arrangement,  two-thirds  of  the  winding  is  almost  in 
phase  with  the  terminal  e.  m.  f .  (being  86  6  percent  effective),  while 
one  leg  is  entirely  idle.  The  star  arrangement  is  about  15  percent 
more  effective  than  the  delta  arrangement. 

The  single-phase  rating  which  can  be  obtained  from  the  two 
six-phase  combinations  shown  in  Figs.  1 1  and  12  should  also  be  con- 
sidered. In  either  of  these  diagrams,  if  two  opposite  terminals, 
such  as  AB,  be  taken  as  the  single-phase  terminals,  then  the  e.  m  f . 
will  be  2E.  As  each  half  of  the  winding  can  carry  the  current  A, 
the  total  which  can  be  handled  is  2A.  The  single-phase  rating 
therefore  becomes  4AE.  The  corresponding  polyphase  rating  is 
6 AE.  The  single-phase  rating  is  therefore  66.7  percent  of  the 
polyphase,  or  is  higher  than  in  any  of  the  other  three-phase  com- 
binations shown.  It  should  be  noted,  however,  that  in  order  to 
obtain  three-phase  from  this  combination,  transformers  are  neces- 
sary in  order  to  transform  from  six-phase  at  the  winding  to  three- 
phase  on  the  line.  Therefore,  while  this  combination  gives  the 
highest  single-phase  and  polyphase  ratings,  yet  if  three-phase  is 
used  on  the  transmission  circuit,  transformers  must  be  interposed. 
Therefore,  the  highest  obtainable  rating  of  single-phase  and  three- 
phase  from  the  same  winding  implies  the  use  of  transformers. 


CAP  A  CITIES  OF  ALTERNA  TORS  121 

The  high  single-phase  rating  obtained  in  this  case  is  due  to  the 
fact  that  the  arrangement  is  equivalent  to  the  star  arrangement 
•with  the  idle  leg  added,  as  illustrated  in  Fig.  13.  The  addition  of 
this  extra  leg  increases  the  terminal  e.  m.  f .  in  the  ratio  of  100 : 86.6, 
while  the  current  per  terminal  remains  the  same.  This  arrange- 
ment, when  used  for  both  single-phase  and  three-phase,  implies 
the  use  of  a  closed  coil  type  of  winding  which,  as  shown  before, 
cannot  give  the  maximum  three-phase  rating  unless  six  terminals 
axe  used. 

It  should  be  noted  that  the  three  legs  shown  in  Fig.  13  have 
the  same  phase  relations  as  a  delta  winding  when  used  on  single- 
phase;  that  is,  one  of  the  three  legs  is  in  phase  with  the  terminal 
voltage,  while  the  other  two  legs  have  a  60-degree  relation,  How- 
ever, these  two  legs,  with  the  60-degree  relation,  carry  the  full  cur- 
rent A;  while  in  the  delta  arrangement  they  carry  one-half  current. 


Therefore,  although  the  voltage  relations  are  the  same,  the  current 
relations  are  quite  different;  which  accounts  for  the  increased 
capacity  with  the  groups  connected  as  in  Fig.  13  or  Fig.  12. 

Fig.  13,  like  Fig.  12,  is  equivalent  to  covering  the  entire  arma- 
ture surface  with  copper  which  is  equally  active  in  carrying  current 
when  the  machine  is  operated  single-phase.  However,  compared 
with  the  three-phase  star  arrangement  where  two  legs  only  are 
active,  it  may  be  seen  that  the  voltage  and  the  output  have  been 
increased  in  the  ratio  of  100:  86.6,  or  about  15  percent,  by  the 
addition  of  33  1-3  percent  in  copper,  and  33  1-3  percent  in  total 
armature  copper  loss.  It  is  evident,  therefore,  that  the  addition 
of  a  third  leg  when  operating  single-phase  does  not  give  results  in 
proportion  to  the  material  used. 


122  ELECTRICAL  ENGINEERING  PAPERS 

COMPARISON  OF  SINGLE-PHASE  AND  THREE-PHASE  RATINGS  ON 
THE  BASIS  OF  EQUAL  TOTAL  ARMATURE  COPPER  Loss 

All  the  foregoing  comparisons  have  been  on  the  basis  of  equal 
losses  in  a  given  coil  or  group;  but  it  has  been  shown  that  with 
some  of  the  windings,  when  operated  on  single-phase,  the  currents 
are  not  divided  equally.  In  consequence,  in  such  cases  the  total 
copper  loss  in  the  windings  must  be  less  than  where  the  current  is 
divided  equally.  In  the  following  comparisons  the  total  copper 
losses  for  three-phase  and  single-phase  are  given,  and  the  possible 
increase  in  single-phase  rating  for  the  same  total  copper  loss  is 
indicated 

Let  r  =  the  resistance  of  one  group. 

Let  A  =  the  limiting  current  per  group,  which  has  been  used 
in  the  above  comparisons. 

Then  in  Fig  3,  for  three-phase,  6Azr  =  the  armature  copper 

loss.    For  single-phase  {  —  jx4r  +  2A2r  =  3AV  =  total  armature 

copper  loss. 

The  three-phase  loss  is  therefore  twice  the  single-phase  on  the 
basis  of  equal  limiting  current  For  equal  total  loss  the  single- 
phase  current  could  therefore  be  increased  as  the  V~2^  as  the  loss 
varies  as  the  square  of  the  current.  As  the  former  single-phase 
output  was  50  percent  of  the  three-phase,  then  for  equal  losses  the 
single-phase  output  "becomes  50  x  V~T  =  70.7  percent  of  the 
corresponding  polyphase  rating. 

In  Fig.  5,  the  three-phase  loss  =  6AV.  The  single-phase  loss 
with  the  same  limiting  current  =  4AV,  as  there  are  but  four  legs  in 
circuit  instead  of  six,  each  leg  carrying  the  same  current  as  when 
operating  three-phase.  The  three-phase  loss  is  thus  6/4  single- 
phase,  and  for  equal  losses  the  single-phase  current  can  be  in- 
creased in  the  ratio  of  -v/6/4-  The  former  single-phase  rating  was 
57.7  percent.  This  therefore  can  be  increased  to  57.7  x  Vs/4  = 
70.7  percent  of  the  corresponding  three-phase  rating. 

In  Figs.  7  and  8,  the  three-phase  loss  =  6AV.  The  single-phase 
loss  =  3A*r,  as  determined  by  direct  inspection  of  currents  and 
resistances,  For  equal  losses,  therefore,  the  single-phase  current 
can  be  increased  as  the  VT-  The  output  then  becomes  50  x  \/~ir 
=  70.7  percent  of  the  corresponding  three-phase  rating. 

In  Figs.  9  and  10,  the  three-phase  loss  =  6A2r.  Single-phase 
loss  =  4A2r.  The  single-phase  output  -  57  7  percent  and  for 


CAPACITIES  OF  ALTERNATORS  123 


equal  loss  this  can  be  increased  in  the  ratio  of  VoTI-  The  output 
then  becomes  70.7  percent  of  the  corresponding  three-phase  output. 

In  Fig.  12,  the  six-phase  loss  =  6AV.  The  single-phase 
loss  =  6A2r,  as  all  the  groups  carry  equal  currents  and  all  are  in 
circuit.  Therefore  the  single-phase  current  cannot  be  further 
increased  and  the  single-phase  output  remains  at  66.7  percent  of 
the  six-phase  output  (or  three-phase  beyond  the  transformers.) 

From  this  it  would  appear  that  most  of  the  above  wind- 
ings would  give,  for  equal  armature  copper  loss,  70.7  percent  of  the 
three-phase  rating.  However,  it  should  be  taken  into  account  that 
the  three-phase  ratings  are  not  all  equal  on  the  basis  of  equal 
copper  loss. 

In  Figs.  3  and  5,  for  instance,  the  three-phase  ratings  are 
equal  to  5.196  AE.  The  three-phase  ratings  with  the  arrangement 
shown  in  Figs.  7,  8,  9  and  10,  are  equal  to  £  4£.  Therefore  Figs.  3 
and  5  have  only  86.6  percent  of  the  three-phase  ratings  of  7,  8,  9 
and  10.  The  single-phase  ratings  of  Figs.  3  and  5  therefore  are 
70.7  percent  of  86.6  percent,  or  61.2  percent  of  the  best  three-phase 
rating  which  can  be  obtained.  Therefore,  on  the  basis  of  6AE 
being  the  best  three-phase  output,  then  with  equal  copper  loss,  the 
arrangements  in  Figs.  3  and  5  give  61.2  x  6AE  =  3.792AE  as  the 
single-phase  rating  with  equal  copper  loss,  while  Figs.  7,  8,  9  and  10 
give  4  243AE  as  the  single-phase  ratings  with  equal  copper  loss, 
and  Fig.  12  gives  4AE  as  the  single-phase  rating  with  the  same 
copper  loss.  Therefore,  the  arrangements  in  Figs.  7,  8,  9  and  10  are 
better  than  any  of  the  others  for  single-phase  rating,  if  total  copper 
loss  is  the  limit  rather  than  the  loss  in  an  individual  coil  or  group. 

However,  if  total  copper  loss  is  the  limit,  then  there  is  still  a 
difference  between  the  true  delta  and  star  windings.  With  the 
delta  winding  the  current  A  is  increased  41  percent,  which  means 
that  one  of  the  groups  will  have  double  the  copper  loss  which  it  has 
on  three-phase,  while  with  the  star  winding  the  current  A  will  be 
increased  slightly  over  22  percent,  which  means  that  two  groups  of 
the  winding  will  have  their  copper  losses  increased  SO  percent.  The 
star  arrangement,  even  with  the  same  total  copper  loss,  works  the 
individual  coils  on  single-phase  easier  than  in  the  delta  arrange- 
ment, 

The  following  table  summarizes  the  above  relationships, 


124 


ELECTRICAL  ENGINEERING  PAPERS 


Haas 


AM 


LIMITI 
PER  COJ 


i 


^  ^    •* 

*%al 


w 


leo 


s 


ftj  W 

CO<J 

'5? 


fe? 


7 


1 


g 


5x2AxE 
«3AE 
5xAx2E 
«=3AE 


JJfl 

M      < 


t-00 


^7 


w    w 


,«|«S 

FH 

B 


S 


CAPACITIES  OF  ALTERNA  TORS  125 

TWO-PHASE  WINDINGS 

w 

The  two-phase  windings  may  be  analyzed  in  a  manner  similar 
to  the  preceding.  Starting  with  a  closed-ring  arrangement,  just  as 
in  the  three-phase,  the  various  relations  may  be  readily  determined. 
Assuming  a  ring,  as  in  Fig.  14,  with  four  taps  brought  out  at  90 
degrees  apart  and  assuming  that  this  winding  is  the  same  in  every 
way  as  that  in  Fig.  1,  then  the  following  e.  m.  f.'s  and  capacities 
are  obtained. 


Fig.  14 

Fig.  15  represents  a  closed  coil  two-phase  winding  correspond- 
ing to  the  three-phase  winding  in  Fig.  3.  Calling  E  the  e.  m.  f.  of 
the  groups  of  legs,  then  the  e.  m.  f.'s  AC  and  BD  =  V1T  x  E. 

Opening  Fig.  15  at  two  opposite  points  as  in  Fig.  16,  the  two 
parts  may  be  rearranged  to  give  Fig.  17.  This  is  an  interconnected 
open  coil  two-phase  winding;  that  is,  the  central  points  are  con- 
nected together  so  that  there  are  fixed  e.  m.  f  .  relations  between  all 
four  terminals.  The  e.  m.  f  .  Ad  is  equal  to  E,  and  the  e.  m.  f  .  across 
AB,  BC,  etc.  =  V!T  x  Ei,  while  the  e.  m.  L  across  AC  and  BD  = 


Splitting  the  winding  of  Fig.  16  at  four  points,  then  the  ar- 
rangements shown  in  Figs.  18  and  19  are  obtained.  These  two 
windings  are  equivalent,  except  that  in  Fig.  18  the  two  legs  which 
are  in  phase  are  connected  in  parallel,  while  in  Fig.  19  they  are  in 
series.  If  the  middle  points  in.  Fig.  19  are  connected  together  tie 
arrangement  becomes  equivalent  to  Fig.  17.  In  Fig.  18,  e.  m.  f.'s 
AC  and  BD  are  equal  to  EI,  while  there  is  no  fixed  e.  m.  f  .  relation 
between  AB,  BC,  etc. 

In  Fig.  19  the  e.  m,  f.'s  ACaad  BD  are  equal  to  2Ei  and  there 
is  no  fixed  relation  between  AB,  BC,  eta,  unless  the  middle  points 


126 


ELECTRICAL  ENGINEERING  PAPERS 


are  interconnected,  in  which  case  the  e.m.f.'s  become  the  same  as 
in  Fig.  17. 

In  Figs.  20  and  21  the  usual  two-phase,  three-wire  arrange- 
ment is  shown.  In  Fig.  20,  AB  =  EI  and  AC  =  \/~2~  x  EI.  In 
Fig.  21  AB  =  2Ei  and  AC  =  2  x  V  2  EI. 


Fig .17 


CAPACITIES  OF  TWO-PHASE  WINDINGS  :  Let  A  equal  the  cur- 
rent per  coil,  this  current  being  the  same  as  for  the  three-phase 
winding.  Then  — 

In  Fig.  15  the  capacity  equals  4 


"    " 
"    " 


18 
19 
20 
21 


It  is  obvious  therefore  that  the  two-phase  capacities  are  equal  for 
all  the  various  windings  which  have  been  commonly  used. 

COMPARISON  OF  TWO-PHASE  CAPACITIES  WITH  THREE-PHASE 

As  the  same  winding  has  been  assumed  for  both  two-phase  and 
three-phase,  it  is  of  interest  to  compare  their  ratings.  Comparing 
E  and  EI  in  Fig.  22,  it  may  be  seen  that  EI  —  -\f~2~  x  E.  Therefore 
the  two-phase  capacities  given  above,  when  put  in  terms  of  three- 
phase  e.m.f.'s  become,  in  all  cases,  4A  x  V1T  x  E  =  5.656  AE. 
The  closed  coil  three-phase  capacity  =  5  196  AE.  The  closed  coij 
six-phase  capacity  =  6  AE.  The  open  coil  (star  or  delta)  three^ 
phase  capacity  =  6  AE.  Therefore,  the  three-phase  closed  coij 
arrangement  gives  the  least  output,  while  the  two-phase,  (which  i$ 


CAPACITIES  OF  ALTERNATORS 


127 


in  reality,  four-phase  with  a  closed  coil  winding)  gives  somewhat 
better  results  and  the  six-phase  closed  coil  gives  still  better  results. 

SINGLE-PHASE  RATING  FROM  TWO-PHASE  WINDING 

Two  of  the  terminals  of  the  two-phase  windings  may  be  used 
for  single-phase.     Assuming  the  same  current  A  per  coil  as  in 


Fig,  19 


two-phase  or  three-phase,  then  the  single-phase  capacity 
In  Fig.  15  =  2A  x  VT  Ei  =  2.828  AEi 
In  Fig.  17  =    A  x     2El  =  2 
In  Figs   18  and  19  =  2  AEl 
In  Figs  20  and  21  =  2 


becomes 


Fig.  2  i 


Fig.  22 


Comparing  the  best  single-phase  obtained  from  the  two- 
phase  with  the  best  single-phase  from  the  three-phase  windings,  EI 
being  equal  to  V^IT  E,  the  following  is  obtained  • 


128  ELECTRICAL  ENGINEERING  PAPERS 

Then  2,828  EiA  =  4  AE,  or  same  as  obtained  from  the 
six-phase  closed  coil  winding. 

Comparing  the  three-phase  closed  coil  winding  with  the  two- 
phase  closed  coil  winding  for  both  polyphase  and  single-phase 
ratings,  the  following  is  obtained  on  the  basis  of  same  loss  per  coil: 
The  3-phase  closed  coil  winding  gives  3-phase  rating  of  5  196  AE. 
"   3       "      "        "  "  "    single"     "       "    2.596 

«   2      "      "        "  "  "    2       "      "       "    5. 656  AE 

«   2      "      "        "          "  "  single"      "       "   4  AE. 

It  is  therefore  apparent  that  with  the  closed  coil  winding  the 
two-phase  arrangement  (or  four-phase  in  reality)  gives  higher  out- 
puts, for  both  polyphase  and  single-phase,  than  the  three-phase 
closed  coil  arrangement  will  give. 

It  may  be  of  interest  to  note  that  in  the  earlier  Westinghouse 
polyphase  machines,  when  the  single-phase  rating  of  a  polyphase 
generator  was  frequently  of  more  importance  than  the  polyphase 
rating,  the  closed  coil  two-phase  winding  shown  above  was  gener- 
ally used.  One  reason  for  the  selection  of  this  type  of  winding  was 
the  high  single-phase  rating  which  could  be  obtained  without  un- 
due sacrifice  in  the  polyphase  rating. 

SPECIAL  CONNECTIONS  FOR  SINGLE-PHASE 

All  of  the  preceding  comparisons  have  had  to  do  with  sym- 
metrical arrangement  of  windings.  However,  by  putting  on  one 
or  more  additional  connections,  which  are  used  for  single-phase 
operation  purely,  the  windings  can  sometimes  be  made  to  give 
larger  single-phase  ratings  than  where  the  straight  polyphase  con- 
nections are  used  for  single-phase  operation.  Two  such  arrange- 
ments will  be  shown  below: — 

It  is  shown  in  Pig.  12  that  by  taking  off  single-phase  at  -A  6,  a 
high  single-phase  rating  can  be  obtained.  For  supplying  three- 
phase  circuits,  however,  it  was  stated  that  transformers  would  have 
to  be  interposed  to  transform  from  six-phase  to  three-phase. 
However,  by  using  A  and  b  as  the  single-phase  terminals  and 
using  A,  B  and  C  as  the  three-phase  terminals,  thus  having  four 
terminals  total  on  the  winding,  as  shown  in  Pig.  23,  the  machine 
can  supply  three-phase  directly  to  the  circuit  and  can  also  deliver 
single-phase  with  the  best  utilization  of  winding.  In  this  case  the 
three-phase  rating  equals  5.196  AE  and  the  single-phase  rating 
-equals  4  AE.  The  single-phase  thus  becomes  approximately 
77  percent  of  the  polyphase.  This  high  relative  rating,  however, 


CAPACITIES  OF  ALTERNATORS 


129 


is  due  to  the  fact  that  the  three-phase  rating  is  only  86.6  percent 
of  the  maximum  three-phase  which  could  be  obtained. 

In  a  similar  way,  with  the  delta  winding  shown  in  Fig.  8,  an 
improved  single-phase  rating  can  be  obtained  by  putting  an 
additional  terminal  at  the  middle  of  one  of  the  legs,  as  shown  in  Fig. 
24.  The  single-phase  is  then  taken  off  at  A  and  6,  while  A,  B  and  C 
are  the  three-phase  terminals.  In  this  case  two  of  the  delta  legs  are 
almost  in  phase  with  the  single-phase,  while  the  third  leg  is  prac- 
tically idle  as  far  as  voltage  is  concerned,  although  it  carries  the 
full  current.  If  the  e.  m.  f .  of  AB  is  2E  then  the  e.  m  f .  of  AB  is 
v"~3~  E.  The  total  single-phase  current  is  2A,  Therefore,  the 


Fig.  24 


single-phase  rating  becomes  3.464  AE.  The  single-phase  rating  in 
this  case  is  therefore  57.7  percent  of  the  three-phase,  instead  of  50 
percent  where  the  single-phase  was  taken  off  at  the  terminals  AB. 
The  above  two  arrangements  are  therefore  more  effective  than 
the  usual  single-phase  from  the  same  types  of  windings.  However, 
as  will  be  shown  later,  the  true  delta  and  the  closed  coil  three- 
phase  windings  are  seldom  used  on  alternating-current  generators 
and  therefore  the  above  special  arrangements  are  of  no  particular 
commercial  advantage. 

COMPARISON  OF  ALTERNATING  AND  DIRECT-CURRENT  RATINGS 
FROM  SAME  ARMATURE  WINDING 

If  direct  current  be  taken  from  the  same  winding  as  described, 
the  limiting  current  per  coil  should  be  the  same  as  the  effective 
(or  square  root  mean  square)  current  when  delivering  alternating 


130  ELECTRICAL  ENGINEERING  PAPERS 

current.  This  is  the  value  A  used  in  the  preceding  comparisons. 
The  direct-current  e.  m.  f .  is  taken  off  from  two  opposite  points  of 
the  armature,  This  e.  m.  f .  therefore  corresponds  to  the  two  op- 
posite terminals  of  either  the  two-phase  closed  coil  or  six-phase 
closed  coil  winding  shown  in  the  preceding  diagrams.  The  direct- 
current  e.  m.  f .  will  be  equal  to  the  maximum  or  peak  value  of  the 
alternating-current  e.  m  f .  taken  off  from  these  two  points  This 
will  be  \J~~z  times  the  effective  value  used  in  the  preceding 
comparison. 

For  the  six-phase  diametral  arrangement,  it  was  shown  that 
the  effective  alternating-current  e  m.  f .  =  IE.  Therefore  the  peak 
value  of  direct-current  e.  m.  f .  will  be  equal  to  VIT  x  IE.  As  the 
limiting  current  is  A,  and  as  there  are  two  direct-current  branches, 
the  total  direct  current  will  be  2£.  The  direct-current  output 
therefore  becomes  4  x  \/~2~  AE  =  5.656  AE. 

The  following  interesting  comparisons  can  therefore  be  made: 

Direct-current  capacity  =  5  656  AE 

1-Phase  closed  coil  capacity  =4  AE          =70.7%  of  D  C. 

3-Phase      "        "         "          =5.192  AE    =91  8%  "      " 

2  "  (4-phase)    "        "          =5656AE    =100%    "      " 
6    "  "        "        "          =6  AE          =1061%u      " 

3  "        open      "        "  =6  AE          =1061%"      " 

From  the  above  it  appears  that  the  two-phase  closed  coil  (and 
two-phase  open  coil)  capacity  is  equal  to  the  direct-current  capacity 
from  the  same  armature  winding.  The  three-phase  closed  coil  is 
less  than  the  direct  current,  while  the  six-phase  is  greater  than  the 
direct  current.  The  three-phase  true  star  or  delta  winding  and  the 
six-phase  closed  coil  winding  are  all  slightly  more  effective  than 
when  the  same  winding  is  used  for  direct-current. 

The  question  may  be  raised  whether  still  higher  ratings  could 
not  be  obtained  from  a  given  winding  by  taking  off  more  phases. 
An  examination  will  show  that  higher  ratings  can  be  obtained  with 
the  number  of  phases  increased,  with  the  dosed  coil  winding;  but 
it  can  be  shown  that  the  possible  increase  over  the  six-phase 
arrangement  is  very  small. 

An  easy  way  of  comparing  the  ratings  of  closed  cofl  windings, 
with  different  numbers  of  phases,  is  to  compare  the  number  of 
circuits  which  can  be  taken  off  between  adjacent  taps  or  terminals 
all  around  the  winding,  as  referred  to  in  first  paragraph  of  page  120. 
This  is  equivalent  to  comparing  the  perimeters  of  the  poly- 


CAPACITIES  OF  ALTERXATORS 


131 


gonal  figures  shown  in  the  diagrams  for  the  various  closed  coil 
combinations  and  is  illustrated  in  Figs.  25,  26,  27  and  28. 
In  Fig.  25,  calling  one  side  E,  than  the  perimeter  =  6E 
In  Fig.^26,  the  perimeter  =  5.656  E.  In  Fig  27  the  perimeter 
=  3  V  3    E  =  5,196  E.    In  Fig.  28,  which  represents  single- 
phase,  the  two  sides  of  the  polygon  coincide,  making  a  straight  line 
Therefore,  double  the  length  of  this  line  should  represent  the 
perimeter,  which  =  4E.  A  comparison  of  these  values  shows  that 
they  are  exactly  in  proportion  to  the  alternating-current  capacities 
given  above 


\ 


Fig.  25 


Fig.  26 


It  is  evident  that  the  greater  the  number  of  phases  obtained 
from  the  closed  coil  winding,  the  more  nearly  the  perimeter  of  the 
polygon  approaches  the  circumference  of  the  circle.  With  an  infinite 
number  of  phases  a  true  circle  would  be  obtained  and  in  this  case 
the  perimeter  becomes  2irE  =  6  283  E.  Therefore,  the  maximum 

/\    983 

possible  polyphase  rating  is ——==   1.047,  or  4.7  percent  greater 

6.0 

than  the  six-phase  closed  coil  rating  or  the  true  star  and  delta  rat- 
ing. Also,  the  greatest  possible  polyphase  rating  is  greater  than  the 
direct-current  rating  in  the  proportion  of  6.283:  5.656,  or  approx- 
imately 11  per  cent. 

FIELD  HEATING 

In  the  above  comparisons  of  the  relative  ratings  of  the  three- 
phase,  two-phase  and  single-phase  windings,  only  the  armature 
copper  losses  have  been  taken  into  account:  but  if  the  problem  is 
to  be  considered  in  its  completeness,  other  armature  conditions  and 
the  field  conditions  must  also  be  taken  into  account. 


132 


ELECTRICAL  ENGINEERING  PAPERS 


A  comparison  of  the  three-phase  and  two-phase  ratings  shows 
that  they  are  usually  so  close^together  that  the  field  conditions 
would  probably  not  exert  a  controlling  influence  on  the  relative 
capacities  In  general,  it  may  be  taken  that  those  combinations 
of  polyphase  windings  which  give  lower  ratings  at  the  same  time 
give  lower  armature  reactions. 

In  comparing  single-phase  with  polyphase  ratings,  however, 
the  field  conditions,  both  as  regards  the  field  winding  and  field  core, 
must  be  taken  into  account  The  armature  reaction  of  the  single- 
phase  winding  is  pulsating  and  tends  to  produce  magnetic  disturb- 


/ 

\ 

X 

\ 
1 

,    1 

\       2 

t 

Fig.  2 7 


Fig.  28 


ances  in  the  field  poles  or  core  which  may  result  in  very  considerable 
iron  losses,  both  eddy  and  hysteretic.  In  general,  these  disturb- 
ances are  relatively  much  greater  on  larger  capacity  machines,  so 
that  provision  must  be  made  on  such  machines  for  suppressing  or 
avoiding  the  ill  effects  of  the  armature  reaction.  This  can  be 
accomplished  to  some  extent,  by  completely  laminating  the  field 
poles.  Another  method  which  has  been  used  on  very  large  ma- 
chines is  the  employment  of  heavy  cage  damper  in  the  pole  faces, 
similar  to  that  of  the  secondary  of  an  induction  motor.  This  damper 
must  have  current  capacity  such  that  when  developing  ampere 
turns  sufficient  to  completely  neutralize  the  armature  pulsations, 
the  heating  effect  in  the  damper  winding,  due  to  the  current  in  it, 
is  relatively  low. 

Field  copper  heating,  in  most  cases,  is  not  a  controlling  con- 
dition, owing  to  the  fact  that  the  single-phase  rating,  defined  by  the 
armature  heating,  as  indicated  above,  is  so  much  lower  than  the 
polyphase  rating  that  the  field  copper  is  usually  worked  some- 
what easier  than  on  the  polyphase  loading.  This  is  particularly 


CAP  A  CITIES  OF  ALTERNA  TORS  133 

true  when  the  rating  is  fixed  by  the  heating  of  individual  armature 
coils.'  However,  if  the  single-phase  rating  is  determined  by  the 
total  armature  loss  and  not  by  the  loss  in  individual  coils,  then  the 
permissible  armature  capacity  on  single-phase  may  be  such  that  in 
some  instances  the  field  copper  is  worked  harder  than  on  polyphase. 
In  such  cases,  if  the  field  copper  is  the  limiting  condition,  then  the 
single-phase  rating  cannot  be  as  high  as  the  armature  would 
permit.  It  may  be  assumed,  however,  that  in  large  machines  the 
armature  conditions,  as  fixed  by  the  loss  in  individual  coils, 
determine  the  safe  single-phase  rating;  and  under  this  assumption 
the  field  conditions,  except  in  regard  to  the  use  of  dampers  or  the 
elimination  of  the  effects  of  armature  reaction,  need  not  be  con- 
sidered. 

APPLICATION  OF  VARIOUS  TYPES  OF  ALTERNATING-CURRENT 

WINDINGS 

The  three-phase  true  star  type  of  winding  is  the  one  which,  in 
general,  lends  itself  to  best  advantage  to  the  various  types  of 
alternating-current  machinery.  It  may  be  a  question  then  as  to 
why  any  other  types  of  windings  are  used.  However,  it  was 
intimated , before  that  where  other  than  the  true  star  winding  is 
used,  there  is  usually  some  condition  other  than  the  output  which 
is  of  first  importance.  In  the  following  will  be  given  some  of  the 
principal  applications  of  the  different  types  of  windings: — 

CLOSED  COIL  TYPES 

The  closed  coil  type  of  winding  is  always  used  with  rotary 
converters.  The  controlling  feature  in  this  case  is  that  the  rotary 
converter  carries  a  commutator,  which  naturally  requires  a  closed 
coil  type  of  winding.  Rotary  converters  are,  in  practice,  wound 
for  three-phase  as  in  Fig.  3,  four-phase  (usually  called  two-phase)  as 
in  Fig.  IS  and  six-phase  as  in  Figs.  11  and  12;  and  the  number  of 
collector  rings  is  3,  4  and  6  respectively.  The  three-phase  winding 
is  generally  used  in  small  capacity  rotaries.  While  the  three-phase 
winding  allows  less  output  than  the  fofttr-phase  or  six-phase,  on 
small  rotaries  the  capacity  is  usually  not  limited  by  the  armature 
copper  loss,  while  the  use  of  three  rings  somewhat  simplifies  the 
machines. 

Four-phase  rotaries  are  used  to  a  very  considerable  extent  in 
connection  with  two-phase  circuits.  However,  where  the  supply 


134  ELECTRICAL  ENGINEERING  PAPERS 

circuit  is  three-phase  it  is  rare  that  the  transformation  is  from  three- 
phase  on  the  supply  circuit  to  the  two-phase  on  the  rotary,  as  there 
are  certain  disadvantages  in  such  transformation  which  more  than 
offset  the  slight  advantage  of  the  four-phase  rotary  over  the  three- 
phase  Moreover,  where  a  higher  number  of  phases  is  of  advantage 
in  a  rotary  converter,  it  is  practicable  to  transform  from  the  three- 
phase  supply  circuit  to  six-phase  for  the  rotary  Two  arrange- 
ments of  such  six-phase  transformation  are  in  use,  as  illustrated  in 
Figs  11  and  12 

One  of  these  is  the  so-called  "  Double  Delta"  arrangement,  in 
which  each  of  the  step-down  transformer  circuits  is  equipped  with 
two  secondaries,  as  indicated  in  Fig  11  These  are  connected  to 
form  two  separate  deltas,  one  being  inverted  with  respect  to  the 
other. 

The  other  arrangement  is  the  so-called  "Diametral"  arrange- 
ment, as  shown  in  Fig  12  This  has  advantages  over  the  double 
delta  in  that  only  one  secondary  circuit  is  required  for  each  phase 
and  the  middle  points  of  these  secondary  circuits  may  be  connected 
together  for  a  neutral  or  middle  wire  between  the  direct-current 
leads  from  the  rotary  converter. 

In  a  rotary  converter  the  armature  copper  loss  is  generally  so 
small,  compared  with  that  of  the  straight  direct-current  or  straight 
alternating-current  machine  with  the  same  winding,  that  all  con- 
siderations of  the  comparative  heating  of  three-phase,  four-phase 
and  six-phase  windings,  as  on  alternating-current  generators,  has 
practically  no  bearing  on  the  rotary  converter  rating.  In  a  rotary 
converter,  an  increase  in  the  number  of  phases  over  six  represents 
a  considerable  reduction  in  the  armature  copper  loss, — much  more 
so  than  in  the  closed  coil  alternating-current  generator  This  is 
due,  in  the  rotary  converter,  to  the  fact  that  one  armature  winding 
carries  both  the  direct  and  the  alternating  currents,  which  are  to  a 
certain  extent,  flowing  oppositely. 

Closed  coil  windings  are  also  occasionally  used  on  the  second- 
aries of  induction  motors  in  order  to  give  a  better  choice  in  the 
number  of  slots  than  would  be  allowed  otherwise  Such  windings 
when  used  on  induction  motors  are  usually  of  the  two-circuit  or 
series  type,  for  the  purpose  of  increasing  the  voltage  as  much  as 
possible  and  at  the  same  time  keeping  the  number  of  conductors 
as  small  as  possible,  while  retaining  the  closed  coil  arrangement. 
A  two-circuit  closed  coil  winding  will  close  upon  itself  symmetrical- 
ly if  the  ntonber  of  turns  or  coils  is  one  more  or  less  than  a  multiple 


CAPACITIES  OF  ALTERNATORS  135 

of  the  number  of  pairs  of  poles.  This  sometimes  allows  the  use 
in  the  secondary  of  an  induction  motor,  of  a  number  of  coils  or  slots 
which  has  no  close  numerical  relation  to  the  number  of  primary 
slots.  For  instance,  if  the  primary  of  a  four-pole  induction  motor 
has  48  slots  with  an  open  coil,  star  or  delta  winding,  then  with 
39  coils  and  slots  in  the  secondary,  a  symmetrical  closed  coil  three- 
phase  winding  could  be  obtained,  while  if  an  open  coil  secondary 
were  used,  the  number  of  slots  should  preferably  be  36  or  42,  which 
might  not  be  as  desirable  as  39  in  some  cases.  This  simply  illus- 
trates an  occasional  use  of  the  closed  coil  winding. 

Closed  coil  windings  were  at  one  time  used  very  extensively  on 
low  voltage,  rotating  armature,  two-phase  generators.  Such  genera- 
tors were  very  satisfactory  for  delivering  a  relatively  large  percent- 
age of  their  rating  as  single-phase.  Furthermore,  with  one  conduc- 
tor per  slot  and  with  bolted-on  end  connectors,  the  potential  bet- 
ween adjacent  end  connectors  was  at  all  points  relatively  low.  The 
symmetrical  arrangement  of  such  windings  also  rendered  them 
very  suitable  for  use  with  supporting  bands  or  end  bells  over  the 
end  winding.  However,  with  the  advent  of  the  rotating  field  ma- 
chines, and  particularly  with  the  use  of  higher  voltages,  the  open 
coil  star  winding  has  entirely  superseded  the  closed  coil  type  of 
generator  winding. 

THREE-PHASE  STAR  WINDINGS 

Two  types  of  star  windings  have  been  shown,  namely,  those  in 
Figs.  5  and  10.  That  of  Fig.  5  gives  less  output  than  that  of  Fig.  10 
in  the  ratio  of  86  6: 100.  There  would  appear  therefore  to  be  no 
use  for  the  Fig.  5  arrangement;  but,  in  certain  cases,  in  using  a 
given  winding  it  may  be  desired  to  reduce  the  voltage  from  12 
percent  to  15  percent  while  retaining  normal  conditions  otherwise. 
In  such  a  case  the  lower  voltage  could  be  obtained,  if  a  new  winding 
were  used,  by  simply  chording  the  winding  one-third  the  pitch. 
On  the  other  hand,  if  an  existing  winding  is  to  be  used,  the  same 
result  could  be  obtained  by  coupling  as  in  Fig.  5. 

In  induction  motors  the  arrangement  shown  in  Fig.  5  may  be 
used  occasionally  where  the  windings  are  arranged  for  coupling  for 
two  different  speeds.  In  some  cases  this  type  of  winding  may  give 
better  average  field  distribution  for  the  two  numbers  of  poles 
than  the  one  shown  in  Fig,  10.  In  this  case  therefore  it  is  the  dis- 
tribution of  the  magnetic  field,  and  not  the  capacity  of  the  winding, 
which  is  the  important  feature. 


136  ELECTRICAL  ENGINEERING  PAPERS 

The  arrangement  shown  in  Fig.  10  is  the  true  star  winding 
which  is  used  almost  universally  on  three-phase  machines.  For  a 
given  voltage  it  requires  fewer  conductors  than  any  other  type  of 
winding,  This  is  of  very  material  advantage  in  allowing,  with  a 
given  number  of  slots,  a  smaller  number  of  conductors  per  slot, 
which,  as  a  rule,  allows  a  better  utilization  of  the  star  space: — 
That  is,  more  copper  can  be  gotten  into  a  given  slot.  Furthermore, 
in  relatively  high  voltage  machines  where  the  conductors  may  be 
very  large  in  number  and  small  in  size,  the  star  winding  with  its 
smaller  number  of  conductors,  each  of  much  larger  size,  gives  more 
substantial  coils  than  any  other  arrangement.  Another  advantage 
of  the  three-phase  winding  is  its  fairly  good  utilization  of  copper 
when  operated  on  single-phase.  When  operated  on  purely  single- 
phase  load,  one  leg  of  the  star  could,  of  course,  be  omitted,  but  if 
it  is  retained  it  becomes  a  reserve  winding  which  may  be  used  in 
case  of  an  accident  to  one  of  the  active  legs  of  the  winding  By 
opening  any  defective  coils  in  an  active  leg  and  connecting  in  the 
reserve  leg  in  place  of  the  defective  one,  the  machine  can  still 
develop  its  specified  rating  on  single-phase. 

Another  advantage  of  the  star  type  of  winding  is  the  readiness 
with  which  the  central  or  neutral  point  can  be  grounded,  which  is 
a  very  considerable  advantage  in  some  high  voltage  systems. 

DELTA  TYPE  WINDINGS 

The  true  delta  type  winding,  as  illustrated  in  Figs.  7  and  8, 
is  not  used  to  any  great  extent  in  either  alternating-current 
generators  or  induction  motors.  For  a  given  voltage  it  requires 
73  percent  more  conductors,  each  of  58  percent  of  the  capacity  of 
those  of  the  true  star  type  of  winding.  As  the  terminals  of  all  three 
legs  are  connected  in  a  closed  circuit  it  is  necessary  that  the  e.  m.  f  .'s 
generated  in  the  three  legs  should  balance  each  other  at  all  instants 
or  there  is  liable  to  be  circulating  current  around  the  windings. 
This  means  that  the  winding  must  be  applied  only  where  the 
conditions  are  favorable,  or  the  conditions  in  the  design  must  be 
made  to  suit  the  type  of  winding.  This  winding  is  occasionally 
used  on  low  voltage  turbo  generators  of  fairly  large  capacity,  due 
to  the  fact  that  the  delta  type  winding  requires  more  conductors 
than  the  star  type.  For  example,  in  a  large  capacity  two-pole 
turbo-generator,  wound  for  relatively  low  voltage,  the  number^of 
conductors  for  the  star  winding  may  be  so  small  that  a  satisfactory 
number  of  slots  is  not  obtained,  even  with  only  one  conductor  per 


CAP  A  CITIES  OF  ALTERNA  TORS  137 

slot  and  even  using  the  double-star  winding,  shown  in  Fig.  9.  In 
such  case  a  double  delta  winding  will  allow  73  percent  more  con- 
ductors and  slots  than  the  double  star  will  give.  Also,  each 
conductor  will  be  much  smaller  than  in  the  star  arrangement, 
which  may  be  of  considerable  advantage  in  the  case  of  low  voltages 
and  very  heavy  current  per  conductor. 

Delta  windings  are  occasionally  used  on  machines  which  are 
arranged  for  connection  for  two  different  voltages,  such  as  6600 
volts  and  11,000  volts.  If  an  armature  is  wound  for  star  connection 
at  11,000  volts,  then  it  can  be  coupled  in  delta  for  6600  volts  with 
practically  the  same  inductions,  losses,  field  currents,  etc.  The 
delta  type  of  winding  is  also  used  occasionally  in  the  primaries 
of  induction  motors  for  special  purposes,  such  as  multi-speed 
combinations  where  the  winding  is  changed  from  one  number  of 
poles  to  another.  In  general,  however,  the  star  type  winding  is 
used  on  induction  motors. 

The  delta  winding  is  not  well  adapted  for  single-phase  opera- 
tion on  account  of  its  low  capacity.  Also,  it  does  not  admit  of 
grounding  of  the  neutral  or  central  point  of  the  system.  Taking 
everything  into  account,  the  true  delta  winding  is  only  used  where 
some  special  condition  is  imposed  upon  the  winding  which  puts 
the  star  arrangement  at  a  disadvantage. 

SINGLE-PHASE  ALTERNATORS 

All  the  foregoing  comparisons  have  been  made  on  the  basis  of 
the  same  armature  winding  being  vised  for  three-phase,  two-phase 
or  single-phase  The  relations  shown  do  not  hold  true  in  general  for 
machines  which  are  wound  initially  for  single-phase  service,  such 
as  for  single-phase  railway  or  electro-chemical  or  electro-fusion 
work.  In  such  cases  the  amount  of  armature  copper  used  and  its 
distribution  are  such  that  the  armature  coils,  either  individually  or 
as  a  whole,  do  not  determine  the  true  Emits  of  output;  but  the 
armature  as  a  rule  can  carry  anything  that  the  field  winding  will 
stand,  so  that  the  field  temperature  becomes  the  true  limit  in  such 
machines.  Also,  very  massive,  well  distributed  cage  dampers  are 
used  with  such  machines  when  they  are  of  large  capacity  and  these, 
in  turn,  have  a  certain  effect  on  the  characteristics,  such  as  the 
regulation,  and  thus  have  an  indirect  influence  on  the  permissible 
capacity.  It  is  well  known  that  if  the  inherent  regulation  of  an  al- 
ternator is  made  poorer,  the  capacity  can  usually  be  increased  with 
the  same  limiting  field  temperature.  In  large  single-phase  gener- 


138  ELECTRICAL  ENGINEERING  PAPERS 

ators,  especially  for  railway  service,  the  capacity  is  increased  by 
sacrifice  in  the  inherent  regulation  of  the  machine.  However,  the 
massive  dampers  greatly  improve  the  regulation  for  quick  changes 
in  load;  while  the  poorer  inherent  regulation  only  affects  the 
regulation  over  considerable  intervals  of  time,  and  automatic 
regulators,  acting  on  the  alternator  excitation,  readily  take  care  of 
the  slow  fluctuations.  In  consequence,  single-phase  generators  of 
large  capacities  may  be  built  for  ratings  which  bear  no  definite 
relation  to  any  of  those  given  above. 

The  armature  windings  of  single-phase  generators,  when  ar- 
ranged for  single-phase  purely,  are  frequently  distributed  over 
only  part  of  the  surface.  Usually  they  cover  considerably  more 
than  half  the  surface,  and  in  extreme  cases  they  cover  80  percent 
or  more.  Of  course,  when  spaced  like  a  true  three-phase  winding 
they  cover  two-thirds  the  surface.  This  arrangement  admits  of  an 
extra  leg  being  added  to  the  winding,  which  is  normally  idle,  if  the 
winding  is  connected  in  star,  this  leg  being  a  reserve  in  case  of 
accident,  as  mentioned  before.  However,  when  such  a  leg  is  not 
added,  the  winding  generally  covers  more  than  two-thirds  the 
surface,  rather  than  less,  but  rarely  covers  the  whole  surface. 


DAMPERS    ON  LARGE  SINGLE-PHASE  GENERATORS 

FOREWORD — This  formed  part  of  the  discussion  of  a  paper  presented 
before  the  Institute  of  Electrical  Engineers,  December,  1908, 
by  Mr.  Murray,  describing  the  operation  of  the  New  Haven 
single-phase  railway.  The  effect  of  the  addition  of  the  massive 
dampers  on  the  rotors  of  the  New  Haven  generators  was  so 
pronounced,  and  the  results  were  so  beneficial,  as  a  whole,  that 
it  was  considered  advisable  to  publish  it  as  new  and  interesting 
material,  in  the  form  of  a  discussion  of  Mr.  Murray's  paper. 
Immediately  after  the  publication  of  this,  heavy  dampers  were 
adopted  very  generally  by  manufacturers  of  large  single-phase 
generators,  throughout  the  world,  who  had  encountered  more  or 
less  trouble  of  the  same  nature  as  found  in  the  New  Haven 
generators.  Practically  all  large  single-phase  generators  since 
then  have  been  built  with  such  dampers  as  part  of  their  con- 
struction.— (ED.) 


WHEN  the  New  Haven  single-phase  generators  were  put  on. 
load  test,  the  first,  and  most  pronounced,  difficulty  was  in 
heating,  not  in  the  winding,  but  in  the  field  or  rotor  structure,  due 
to  the  pulsating  reaction  of  the  armature  winding  when  carrying  a 
heavy  load  in  single-phase  current.  This  reaction  was  known 
previously  to  building  these  machines,  but  on  machines  of  smaller 
capacity  it  had  not  developed  destructive  tendencies.  It  was 
proved  later  that  this  was  simply  because  it  had  not  been  tried  out 
under  the  conditions  which  would  develop  its  most  harmful 
effects.  This  pulsating  armature  reaction  may  be  analyzed  in  the 
following  manner : 

Consider  the  armature  winding  as  a  magnetizing  coil  fixed 
in  (Space  and  carrying  an  alternating  current.  This  coil  may 
be  considered  as  setting  up  an  alternating  field  fixed  in  space. 
For  analysis,  this  alternating-current  field,  fixed  in  space,  may  be 
considered  as  made  up  of  two  constant  fields  of  half  value,  rota- 
ting in  opposite  directions  at  the  synchronous  speed  of  the  machine. 
One  of  these  fields  therefore  rotates  at  the  same  speed  and  in  the 
same  direction  as  the  rotor.  The  other  field  is  traveling  round  the 
rotor  core  in  the  direction  opposite  to  its  rotation.  This  field  may 
therefore  be  considered  as  equivalent  to  one  fixed  in  space  with 
the  rotor  running  in  it  at  double  speed.  This  core  thus  becomes 
an  armature  core  subject  to  a  heavy  induction  at  a  high  frequency* 

139 


140  IELECTRICAL  ENGINEERING  PAPERS 

When  the  first  rotor  was  built,  the  structure  was  laminated  as 
completely  as  mechanical  conditions  would  permit.  However,  in 
the  case  of  high-speed  turbine-generators  of  very  large  capacity,  it 
is  almost  impossible  completely  to  laminate  everything,  due  to  the 
fact  that  the  mechanical  requirements  call  for  rigidity  in  some  of 
the  structural  features.  Upon  testing  the  first  machine  it  was 
found  that  there  was  local  heating,  with  heavy  load,  sufficient  to 
create  hot  spots  in  the  core;  and  in  a  comparatively  short  time  in 
turn  these  hot  spots  damaged  the  insulation  on  the  coils  from  the 
outside,  thus  causing  grounds  on  the  winding.  As  soon  as  this  was 
noticed,  an  effort  was  made  to  eliminate  these  hot  spots ;  but  it  was 
found,  after  several  attempts,  that  as  soon  as  one  was  eliminated 
others  would  show  up  in  some  different  place  as  soon  as  a  higher 
load  condition  was  reached.  It  was  evident,  after  considerable 
work  had  been  done,  that  the  correct  remedy  was  not  being  applied 
to  this  trouble  It  was  then  decided  to  take  a  bold  step  by  at- 
tempting to  eliminate  all  pulsating  reactions  from  the  armature  by 
putting  a  short-circuited  winding  on  the  rotor,  of  such  value  that 
a  very  large  current  could  flow  in  it  with  but  very  little  loss.  It 
was  the  idea  to  damp  out  the  field  in  very  much  the  same  way  that 
the  armature  of  a  polyphase  alternator  will  demagnetize,  or  kill  its 
magnetic  field,  if  the  armature  terminals  are  all  short-circuited 
together.  It  is  known  that  under  this  condition  the  armature 
current  will  rise  to  such  a  value  that  the  field  flux  is  practically 
eliminated.  In  order  to  maintain  this  condition  indefinitely 
without  overheating,  it  is  only  necessary  to  put  enough  copper  on 
the  armature  so  that  the  PR  losses  in  it  under  this  condition  are 
within  the  temperature  capacity  of  the  windings.  Working  on 
this  theory,  a  complete  cage  winding  was  placed  on  one  of  the 
rotors  of  the  New  Haven  generators.  This  rotor  had  not  been 
designed  originally  for  this  purpose,  and  it  was  therefore  difficult 
to  adopt  the  most  suitable  proportions  in  this  winding,  but  what 
was  put  on,  immediately  showed  in  practice  that  a  practicable 
remedy  had  been  applied  for  this  trouble  Meanwhile  the  new 
rotors  designed  for  the  application  of  heavy  cage  windings  were 
under  construction,  and  upon  the  installation  of  these,  the  field  or 
rotor  trouble  all  disappeared.  It  is  interesting  to  note  that  the 
fourth  machine  installed,  which  has  a  4260  kilovolt-ampere  single- 
phase  rating,  has  a  solid  steel  core,  in  the  surface  of  which  the 
copper  cage  winding  is  embedded.  As  this  winding  completely 
eliminated  the  pulsating  armature  reaction,  there  was  no  further 


DAMPERS  ON  SINGLE-PHASE  GENERATORS  141 

occasion  for  laminating  the  field  as  a  protection  from  magnetic 
pulsations. 

I  might  add  that  a  number  of  the  earlier  tests,  leading  up  to 
the  design  of  the  first  New  Haven  rotors,  were  misleading,  in  the 
fact  that  turbine-generators  were  used  for  obtaining  the  prelimin- 
ary data  for  single-phase  operation  and,  in  all  cases,  the  machines 
had  solid  steel  cores.  These  cores  acted  as  dampers  to  a  certain 
extent,  and  this  in  itself  eliminated  part  of  the  pulsation.  It  thus 
developed  afterwards,  that  in  the  very  act  of  lamination  to  avoid  the 
trouble,  we  had  gotten  into  it  deeper. 

Practically  all  this  work  on  the  generators  was  done  before 
the  full  electric  service  was  established,  and  while  only  one  or  two 
generators  were  required  to  be  operated  at  one  time.  With  one 
generator  running,  there  was  apparently  but  little  or  no  disturb- 
ance due  to  short-circuits  on  the  system.  As  the  service  was  in- 
creased and  two  generators  put  in  operation,  the  effect  of  short- 
circuits  became  more  pronounced.  When,  in  June,  1908,  the  entire 
electric  service  was  established,  and  three  generators  were  con- 
nected to  the  system,  it  soon  became  evident  that  there  was  some 
serious  condition  existing  in  the  system,  as  indicated  by  the  ex- 
tremely violent  shocks  to  everything  in  case  of  a  short-circuit. 
This  was  particularly  noticeable  in  the  switching  system,  and,  as 
Mr.  Murray  intimates,  in  the  case  of  a  short-circuit,  all  the 
switches  in  the  system  felt  it  their  duty  to  jump  in  and  open  the 
circuit.  This  indicated  an  abnormal  current  condition.  It  was 
calculated  that  these  machines  would  give  possibly  six  or  seven 
times  full-load  current  on  the  first  rush,  in  the  case  of  a  dead  short- 
circuit,  this  excess  current  dying  down  to  possibly  two  or  three 
times  normal  full-load  current.  All  indications  were,  however, 
that  this  current  was  being  greatly  exceeded,  and  therefore  a 
series  of  oscillograph  tests  were  made  to  determine  the  current 
rush  when  the  lines  were  purposely  short-circuited  tinder  various 
conditions.  These  tests  indicated  that  under  certain  conditions 
each  machine  could  give,  at  the  moment  of  short-circuit,  almost 
5000  amperes  on  one  phase,  the  normal  full-load  current  being  340. 
With  three  machines  in  parallel,  this  would  therefore  mean  that 
approximately  15,000  amperes  could  be  delivered  momentarily. 
This  enormous  current  rush  was  sufficient  to  explain  many  of  the 
difficulties,  but  this  was  not  all  the  explanation.  The  oscillo- 
graph tests  also  showed  that  this  short-circuit  current  would  be 
maintained  at  almost  its  maximum  value  for  a  very  considerable 


142  ELECTRICAL  ENGINEERING  PAPERS 

period,  due  to  the  cage  winding  on  the  rotors  of  the  generators. 
Apparently  this  current  at  the  first  rush,  was  not  appreciably 
greater  than  that  on  the  machine  before  the  dampers  were  added, 
but  without  the  dampers  the  field  was  killed  more  quickly  by  this 
enormous  current,  so  quickly  that  apparently  the  breakers  did  not 
open  until  the  current  had  fallen  somewhat.  However,  with  the 
heavy  cage  winding  on  the  field  structure,  secondary  currents  were 
set  up  in  this  winding,  tending  to  maintain  the  field  strength,  and 
thus  the  current  rush  was  maintained  at  almost  full  value  for 
possibly  20  to  30  alternations.  These  oscillograph  tests  indicated 
very  clearly  that  the  armatures  of  these  generators  did  not  have 
nearly  so  great  internal  self-induction  as  our  calculations  indicated. 

Meanwhile,  the  generators  in  the  power  house  had  been  suf- 
fering from  the  tremendous  shocks  which  accompanied  short- 
circuits  on  the  line.  There  is  necessarily  considerable  local  field 
around  the  end-windings  of  all  these  machines,  and  this  stray 
field  is  especially  large  on  machines  with  a  small  number  of  poles, 
and,  in  consequence,  high  ampere-turns  per  pole.  These  stray 
fields  at  the  ends  tend  to  exert  a  bending  or  distorting  effect  on  the 
end-windings.  In  any  given  machine  the  distorting  force  varies 
as  the  square  of  the  current  carried  by  the  coils.  Our  experience 
with  the  windings  on  these  machines  indicated  that  they  were 
being  subjected  to  enormous  forces  in  the  end-windings.  The 
oscillograph  tests  gave  an  indication  as  to  the  amount  of  this 
force.  As  the  machines  could  give  about  IS  times  full-load  cur- 
rent momentarily  on  short-circuit,  the  force  acting  on  these  end- 
windings  would  be  225  times  normal;  in  this  case,  therefore,  these 
forces  were  so  great  that  it  became  a  serious  problem  to  devise  a 
type  of  bracing  on  the  end-windings  sufficient  to  withstand  such  a 
force.  It  should  also  be  borne  in  mind  that  probably  as  many 
short-circuits  came,  in  one  day,  on  these  generators,  as  the  or- 
dinary high-voltage  power-house  generator  is  called  upon  to 
sustain  in  one  year.  While  ninety-nine  shocks  out  of  a  hundred 
might  not  be  sufficient  to  do  damage,  yet  if  the  shocks  occur  fre- 
quently enough,  the  hundredth  one  will  soon  be  reached.  In  our 
endeavors  to  support  these  windings  against  movement,  probably 
the  most  complete  system  of  bracing  ever  applied  to  alternating- 
current  generators  was  developed  and  used  on  these  machines. 

But  in  spite  of  this  there  was  evidence  of  movement  at  times,' 
It  thus  became  evident  that  some  method  of  limiting  this  short- 
circuit  current  to  the  value  originally  intended;  namely,  about 


DAMPERS  ON  SINGLE-PHASE  GENERATORS  143 

six  times  full-load  current,  would  have  to  be  applied.  This  was 
done  by  placing  an  unsaturated  choke-coil,  or  impedance  coil,  on 
the  trolley  side  of  each  machine  This  coil  takes  up  a  comparatively 
small  voltage  tinder  normal  operation,  but  in  case  of  a  short- 
circuit,  the  electromotive  force  generated  in  it  is  sufficient  to  limit 
the  current  rush  to  less  than  half  the  value  it  would  attain  without 
this  coil  Thus  as  the  shock  on  the  end-windings  of  the  generators 
varies  as  the  square  of  the  current,  it  is  evident  that  cutting  this 
current  in  half  would  cut  the  shock  to  one-quarter  of  its  former 
value,  which,  with  the  method  of  bracing  used  on  these  machines, 
would  mean  the  difference  between  good  and  bad. 

When  these  choke-coils  were  installed,  the  results  on  the 
power  house  were  evident  The  shocks  on  the  machines  were  very 
greatly  reduced,  so  reduced  that  we  do  not  fear  future  trouble 
from  this  source.  It  is  interesting  to  note  that  No.  4  machine; 
that  is,  the  4260  kilovolt-ampere  generator,  referred  to  before, 
was  put  in  service  a  considerable  time  before  the  choke-coils  were 
installed,  and  it  went  through  the  most  severe  short-circuits  ever 
encountered  on  this  system.  Its  armature  winding  has  never 
shown  any  distress.  This  is  partly  because,  in  the  design  of  this 
machine,  the  difficulties  to  be  overcome  were  known,  and  the 
remedies  could  be  applied  in  the  most  suitable  manner. 

An  interesting  point  in  connection  with  the  use  of  the  cage 
windings  on  these  generators,  is  that  the  apparent  regulation  of  the 
system  has  been  improved.  This  was  anticipated,  but  the  actual 
result  in  practice  was  more  pronounced  than  was  expected.  In 
installing  new  rotors  for  these  machines  with  the  heavier  cage 
dampers,  the  inherent  regulation  of  the  generators  was  made 
somewhat  poorer  than  before,  partly  in  order  to  accommodate 
certain  structural  features  in  the  rotor.  It  was  anticipated  that 
the  cage  winding  with  its  damping  effect  would,  to  a  certain 
extent,  mask  this  poor  regulation  by  making  the  machine  sluggish 
as  regards  fluctuation  in  voltage  with  sudden  variations  in  load. 
In  practice  it  was  found  that,  with  the  later  rotors  with  their 
poorer  inherent  regulation,  the  average  regulation  of  the  system 
was  considerably  better  than  before,  thus  indicating  that  most  of 
the  disturbances  in  the  voltage,  when  the  old  rotors  were  used, 
were  due  to  sudden  changes  in  load,  while  the  slow  variations 
were  taken  care  of  by  the  automatic  regulators.  With  the  new 
rotors  the  voltage  changes  are  so  slow,  that  the  Tirrill  regulator 


144  ELECTRICAL  ENGINEERING  PAPERS 

has  plenty  of  time  to  act  before  any  serious  disturbance  can  take 
place. 

It  must  be  borne  in  mind  that  in  one  way  this  New  Haven 
power-house  installation  was  more  difficult  than  anything  under- 
taken heretofore,  and  that  is,  in  the  use  of  11,000  volt  generators 
with  one  terminal  connected  directly  to  ground  Taking  this 
condition  into  account,  together  with  the  enormous  current 
rushes  with  consequent  shocks  on  the  winding,  and  the  single- 
phase  operation  of  units  of  such  large  capacity,  it  may  reasonably 
be  claimed  that  this  was  the  most  difficult  case  of  alternating- 
current  generation  ever  undertaken. 


DEVELOPMENT  OF  A  SUCCESSFUL  DIRECT-CURRENT 
2000  KW  UNI-POLAR  GENERATOR 

FOREWORD— In  1906,  the  Westinghouse  Company  contracted  to 
build  a  2000  kw  uni-polar  type  generator  direct  coupled  to  a 
1200,  revolution  steam  turbine.  Many  difficulties  were  en- 
countered in  shop  tests  on  this  machine,  winch  were  apparently 
corrected,  but  upon  installation  and  operation  on  the  customer's 
premises,  many  new  and  totally  unexpected  difficulties  arose. 

This  paper  illustrates  how  a  responsible  manufacturing  com- 
pany will  throw  its  whole  engineering  and  manufacturing  en- 
deavors into  correcting  serious  difficulties.  It  also  serves  to  give 
student  engineers  a  good  idea  of  the  practical  side  of  manufac- 
turing engineering.  Fearing  the  results  of  the  engineering 
efforts  expended  on  this  machine  would  eventually  be  lost,  the 
author  prepared  them  for  presentation  at  the  twenty-ninth 
annual  convention  of  the  American  Institute  of  Electrical 
Engineers  at  Boston,  June,  1912. — (ED.) 


THIS  paper  is  not  intended  to  be  a  theoretical  discussion  of 
the  principles  of  unipolar  machines;  neither  is  it  a  purely 
descriptive  article.  It  is  a  record  of  engineering  experiences 
obtained,  and  difficulties  overcome,  in  the  practical  development 
of  a  large  machine  of  the  unipolar  type.  Some  of  the  conditions 
of  operation,  with  their  attendant  difficulties,  proved  to  be  so 
unusual  that  it  is  believed  that  a  straightforward  story  of  these 
troubles,  and  the  methods  for  correcting  them,  will  be  of  some 
value. 

Two  theoretical  questions  of  unipolar  design  have  come  up 
frequently;  (1)  whether  the  magnetic  flux  rotates  or  travels 
with  respect  to  the  rotor  of  the  stator;  and  (2)  whether  it  is 
possible  to  generate  e.m.fs.  in  two  or  more  conductors  in  series 
in  such  a  way  that  they  can  be  combined  in  one  direction,  with- 
out the  aid  of  a  corresponding  number  of  pairs  of  collector  rings, 
to  give  higher  e.m.fs.  than  a  single  conductor. 

To  the  first  question  the  answer  may  be  made  that  in  the 
machine  in  question,  it  makes  no  difference  whether  the  flux 
rotates  or  is  stationary;  the  result  is  the  same  on  either  assump- 
tion. To  the  second  it  may  be  said  that  when  the  theory  of  inter- 
linkages  of  the  electric  and  magnetic  circuits  is  properly  con- 
sidered, it  is  obvious  that  the  resultant  e.m.f .  is  always  equivalent 
to  that  of  one  effective  conductor.  It  has  been  proposed  in  the 

145 


146 


ELECTRICAL  ENGINEERING  PAPERS 


past,  by  means  of  certain  arrangements  of  liquid  conductors  in 
insulating  tubes,  to  add  the  e  m  fs  of  several  conductors  in  series 
but  such  a  scheme  does  not  appear  to  be  a  practical  device  There- 
fore, the  theoretical  considerations  being  largely  eliminated,  the 
author  confines  himself  to  the  practical  side  only. 

In  1896  the  writer  designed  a  small  unipolai  generator  of 
approximately  three  volts  and  6000  amperes  capacity  at  a 
speed  of  1500  rev.  per  mm.  This  machine  was  built  for  meter  test- 
ing and  the  occasion  for  its  design  lay  in  the  continued  trouble 
encountered  with  former  machines  of  the  commutator  type 
designed  for  very  heavy  currents  at  low  voltages. 

The  general  construction  of  this  early  machine  is  shown  in 
Fig.  1.  The  rotating  part  of  this  machine  consisted  of  a  brass 
casting,  cylindrical  shaped,  with  a  central  web,  very  similar 
to  a  cast  metal  pulley.  The  two 
outer  edges  of  this  pulley  or  ring 
served  as  collector  rings  for  col- 
lecting the  current  as  indicated 
in  the  figure,  while  the  body  of  the 
same  ring  served  as  the  single 
conductor.  The  object  of  this 
construction  of  rotor  was  to  obtain 
a  form  which  could  be  very  quickly 
renewed  in  case  of  rapid  wear,  as 
this  arrangement  would  allow  a 

small  casting  to  he  made  and  simply  turned  up  to  form  a  new- 
rotor.  However,  this  renewal  feature  has  not  been  of  very 
great  importance  for  the  rotor  of  the  first  machine  was  replaced 
only  after  12  years'  service.  This  period  of  course  did  not 
represent  continuous  service,  for  this  particular  machine  was 
used  for  meter  testing  purposes  or  where  large  currents  were 
required  only  occasionally, 

A  number  of  peculiar  conditions  were  found  in  this  machine. 
In  the  initial  design  the  leads  for  carrying  the  current  away 
from  the  brushes  were  purposely  carried  part  way  around  the 
shaft  in  order  to  obtain  the  effect  of  a  series  winding  by  means 
of  the  leads  themselves.  In  practice,  they  were  found  to  act  in. 
this  manner  and,  in  fact,  they  over-compounded  the  machine 
possibly  30  to  40  per  cent.  In  consequence,  it  was  necessary 
to  shunt  them  by  means  of  copper  shunts  around  the  shaft  in 
ihe  opposite  direction. 


FIG.  1 


UNI-POLA  R  GENERA  TOR  147 

Shortly  after  this  machine  was  put  in  operation  there  was  con- 
siderable cutting  of  the  brushes  and  rings,  especially  at  very 
heavy  currents.  It  was  found  that  block  graphite,  used  as  a 
lubricant,  gave  satisfactory  results.  This  machine  was  operated 
up  to  10,000  to  12,000  amperes  for  short  periods. 

The  description  of  the  above  machine  has  been  gone  into  rather 
fully,  as  it  was  a  forerunner  of  the  2000-kw.  machine  which  will 
be  described  in  the  following  pages.  The  general  principle  of 
construction  and  the  general  arrangement  of  the  two  parts,  or 
paths,  of  the  magnetic  circuit  are  practically  the  same  in  the  two 
machines,  as  will  be  shown. 

In  1904,  due  to  the  rapidly  increasing  use  of  steam  turbines, 
the  question  of  building  a  turbo-generator  of  the  unipolar  type 
was  brought  up,  and  an  investigation  was  made  by  the  writer 
to  determine  the  possibilities.  This  study  indicated  that  a 
commercial  machine  for  direct  connection  to  a  steam  turbine 
could  be  constructed,  provided  a  very  high  peripheral  speed  was 
allowable  at  the  collector  rings  or  current  collecting  surfaces.  It 
appeared  that  the  velocity  at  such  collector  surfaces  would  have 
to  be  at  least  200  to  250  feet  per  second,  in  order  to  keep  the 
machine  down  to  permissible  proportions  of  the  magnetic 
circuit,  and  to  allow  a  reasonably  high  turbine  speed.  Con- 
trary to  the  usual  idea,  the  very  high  speeds  obtainable  with 
steam  turbines  are  not  advantageous  for  unipolar  machines. 
For  example,  while  maintaining  a  given  peripheral  speed  at  the 
current  collecting  surface,  if  the  revolutions  per  minute  of  the 
rotor  are  doubled,  then  the  diameter  of  the  rotor  collecting 
rings  is  halved,  and  the  diameter  of  the  magnetic  core  surrounded 
by  the  collector  rings  is  more  than  halved,  and  the  effective 
section  of  core  is  reduced  to  less  than  one-fourth.  The  e.m.f. 
generated  per  ring  or  conductor,  therefore,  on  the  basis  of  flux 
alone,  would  be  reduced  to  less  than  one-fourth,  but  allowing 
for  the  doubled  revolutions  per  minute,  it  becomes  practically 
one-half. 

On  the  other  hand,  if  the  revolutions  are  reduced,  while  the 
speed  of  the  collector  ring  is  kept  constant,  then  the  e  m.f . 
per  ring  can  be  increased,  as  the  cross  section  of  the  magnetic 
circuit  increases  rapidly  with  reduction  in  the  number  of  revo- 
lutions. But-at  a  materially  reduced  speed,  the  total  material 
in  the  magnetic  circuit  becomes  unduly  heavy.  In  consequence, 
if  the  speed  is  reduced  too  much,  then  the  machine  becomes  too 
large  and  expensive,  while  with  too  great  an  increase  in  speed, 


148 


ELECTRICAL  ENGINEERING  PAPERS 


the  e,m.f.  per  ring  becomes  low  or  the  peripheral  speed  of  the 
rings  must  be  very  high.  It  is  desirable  to  keep  the  number 
of  collector  rings  as  small  as  possible,  for  each  pair  of  rings  handles 
the  full  current  of  the  machine,  and  therefore  any  increase 
in  the  number  of  rings  means  that  the  full  current  must  be  col- 
lected a  correspondingly  large  number  of  times.  Therefore, 
it  works  out  that  the  range  of  speeds,  within  which  the  unipolar 
machine  becomes  commercially  practicable,  is  rather  narrow. 
In  1906,  an  order  was  taken  for  a2000-kw.  1200-rev.permin., 
260-volt,  7700-arnpere  unipolar  generator  to  be  installed  in  a 
Portland  cement  works  near  Easton,  Pa.  The  fact  that  it  is 
a  cement  works  should  be  emphazised,  as  having  a  considerable 
bearing  on  the  history  of  the  operation  of  this  machine,  as  will 
be  shown  later* 


jnnnnnn  nn 


PIG.  2 

This  2000-kw.  machine  does  not  represent  any  theoretically 
radical  features,  being  similar  in  type  to  the  smaller  machine 
already  described,  but  modified  Somewhat  in  arrangement  to 
allow  the  use  of  a  large  number  of  current  paths  and  collector 
rings.  The  general  construction  of  this  machine  is  indicated 
in  Fig.  2. 

The  stator  core  and  the  rotor  body  are  made  of  solid  steel, 
the  stator  being  cast,  while  the  rotor  is  a  forging.  There  are 
eight  collector  rings  at  each  end  of  the  rotor,  the  corresponding 
rings  of  the  two  ends  being  connected  together  by  solid  round 
conductors,  there  being  six  conductors  per  ring,  or  48  con- 
ductors total.  In  each  conductor  is  generated  a  normal 
e.m.f.  of  32.5  volts,  and  with  all  the  rings  connected  in  series, 
the  total  voltage  is  260. 

The  stator  core,  at  what  might  be  called  the  pole  face,  is  built 


UNI-POLAR  GENERATOR 


149 


up  of  laminated  iron,  forming  a  ring  around  the  rotor.  This 
was  laminated  in  order  to  furnish  an  easy  method  for  obtaining 
the  stator  slots  in  which  the  conductors  lie  which  connect  to- 
gether the  brushes  or  brush  holders  for  throwing  the  pairs  of 
rings  in  series.  The  slots  in  the  stator  laminations  were  made 
open,  as-indicated  in  Fig.  3,  in  order  to  readily  insert  the  stator 
conductors.  There  are  16  slots  in  this  ring,  and  in  each  slot 
there  is  placed  one  large  solid  conductor. 

As  first  assembled,  non-metallic  wedges  were  used  to  close 
these  slots,  but  later  these  were  changed  to  cast  iron  for  reasons 
which  will  be  explained  later. 

The  rotor  core  consists  of  one  large  forging,  as  indicated  in 
Fig.  2.  Lengthwise  of  this  rotor  are  12  holes  for  ventilating 


FIG.  3 


FIG.  4 


purposes  originally  2f  in.  diameter.  Each  of  these  holes  con- 
nected to  the  external  surface  by  means  of  nine  If  in.  radial 
holes  at  each  end  of  the  rotor,  these  holes  corresponding  to  mid- 
positions  between  the  collector  rings.  It  was  intended  to  take 
air  in  at  each  end  of  the  rotor  and  feed  it  out  between  the  collec- 
tor rings  for  cooling.  In  addition,  as  originally  constructed, 
there  was  a  large  enclosed  fan  at  each  end,  as  indicated  in  Fig. 
4.  These  fans  took  air  in  along  the  shaft  and  directed  it  over 
the  collector  rings  parallel  to  the  shaft.  The  object  of  this  was 
to  furnish  an  extra  amount  of  air  for  cooling  the  surfaces  of  the 
rings,  and  the  brushes  and  brush  holders,  as  it  was  estimated 
that  the  brushes  and  brush  holders  themselves  could  conduct 
away  a  considerable  amount  of  heat  from  the  rings  by  direct 


150 


ELECTRICAL  ENGINEERING  PAPERS 


contact,  and  that  the  cooling  air  from  the  fans,  circulating  among 
the  brush  holders,  would  carry  away  this  heat.  These  fans 
were  removed  duriag  the  preliminary  tests,  for  reasons  which  will 
be  given  later. 

The  rotor  collector  rings  consisted  of  eight  large  rings  at  each 
end,  insulated  from  the  core  by  sheet  mica,  and  from  each  other 
by  air  spaces  between  them.  Each  ring  has  48  holes  parallel 
to  the  shaft.  These  holes  are  of  slightly  larger  diameter  than 


FIG.  5 


the  rotor  conductors  outside  their  insulation.  Six  holes  in 
each  ring  were  threaded  to  contain  the  ends  of  six  of  the  conduc- 
tors which  were  joined  to  each  ring.  The  six  conductors  con- 
nected to  each  ring  were  spaced  symmetrically  around  the  core. 
Fig.  5  shows  this  construction. 

The  rotor  conductors,  48  in  number,  consist  of  one  in.  copper 
rods,  outside  of  which  is  placed  an  insulating  tube  of  hard  ma- 
terial. Each  conductor,  in  fact,-  consists  of  two  lengths  arranged 
for  joining  in  the  middle.  The  outer  end  of  each  conductor 
is  upset  to  give  a  diameter 
larger  than  the  insulating  tubes,  , 
and  a  thread  is  cut  on  this  ex-  ~~ 
panded  part.  After  th^  rings 
were  installed  on  the  core,  the 
rods  were  inserted  through  the  E 
holes  to  the  threaded  part  of  a 
ring  and  were  then  screwed  pIG.  5 

home. 

At  the  middle  part  of  the  rotor  core,  a  groove  is  cut  as  shown 
in  Fig.  6.  Into  this  groove  the  two  halves  of  each  conductor 
project.  These  two  ends  are  then  connected  together  by  strap 
conductors  in  such  a  way  as  to  giye  flexibility  in  case  of  expan- 
sion of  the  conductors  lengthwise.  This  arrangement  is  also 
shown  in  Fig.  6. 

With  this  arrangement  there  is  no  possibility  whatever  of 
the  conductors  turning  after  once  being  connected.  There  is 


U SI-POLAR  GENERATOR 


151 


a  series  of  holes  from  the  axial  holes  through  the  shaft  to  this 
central  groove,  for  the  purpose  of  allowing  some  ventilating  air 
to  flow  over  the  central  connections. 

As  originally  constructed,  the  conductors  passed  through  com- 
pletely enclosed  holes  near  the  surface  of  the  rotor  core,  as  in- 
dicated in  Fig.  7.  This  construction  was  afterwards  modified 
to  a  certain  extent.  The  face  of  the  rotor  at  this  point  was  also 
solid,  as  originally  constructed.  This  was  afterwards  changed, 
as  will  be  described  later. 

The  collector  rings,  as  originally  constructed,  consisted  of 
a  base  ring  with  a  wearing  ring  on  the  outside,  as  shown  in  Fig. 
8.  Both  rings  were  made  of  a  special  bronze,  with  high  elastic 
limit  and  ultimate  strength.  On  the  preliminary  tests  these 
rings  showed  certain  difficulties  and  required  very  considerable 
modifications,  and  several  different  designs  were  developed 
during  the  preliminary  operation,  as  will  be  described. 


FIG.  7 


iffiSSizzrXTA 


FIG.  9 


FIG.  8 


The  eight  sets  of  brush  holders  at  each  end  are  carried  by 
eight  copper  supporting  rings.  These  supporting  rings  are 
insulated  from  the  frame  of  the  machine  but  are  connected  in 
series  by  means  of  the  conductors  through  the  stator  slots. 
There  are  16  brush  holders  studs  per  ring  and  two  brush  holders 
per  stud,  each  capable  of  taking  a  copper  leaf  brush  f  in.  wide 
by  If  in.  thick.  These. brush  holders  are  spaced  practically 
uniformly  around  the  supporting  rings.  The  supporting  copper 
rings  are  continuous  or  complete  circles,  so  that  the  current 
collected  from  the  brushes  are  carried  in  both  directions 
around  the  ring.  There  are  two  conductors  carried  from  each 
ring  through  the  stator  slots  to  a  ring  on  the  opposite  side  of  the 
machine,  in  order  to  connect  the  various  brush  holders  in  series. 
The  arrangement  is  illustrated  in  Fig.  9, 

The  above  description  represents  the  machine  as  originally 


152  ELECTRICAL  ENGINEERING  PAPERS 

constructed  and  put  on  shop  test.  From  this  point  on,  the 
real  story  begins.  Various  unexpected  troubles  developed,  each 
of  which  required  some  minor  modification  in  the  construction  of 
the  machine  and,  moreover,  these  troubles  occurred  in  series* 
that  is,  each  trouble  required  a  certain  length  of  time  to  de- 
velop, and  each  one  was  serious  enough  to  require  an  immediate 
modification  in  the  machine.  In  consequence,  the  machine 
would  be  operated  until  a  certain  -difficulty  would  develop;  that 
is,  that  trouble  would  appear  which  took  the  least  time  to  de- 
velop. After  it  was  remedied,  a  continuation  pf  the  test  would 
show  a  second  trouble  which  required  a  remedy,  and  so  on. 
Some  of  these  troubles  were  of  a  more  or  less  startling  nature 
as  will  be  described  later. 

This  machine,  after  being  assembled  according  to  its  original 
design,  was  operated  over  a  period  of  several  weeks  in  the  testing 
room  of  the  manufacturing  company.  It  was  operated  both 
at  no  load  and  at  full  load,  and  a  careful  study  was  made  of  all 
the  phenomena  which  were  in  evidence  during  these  tests. 

The  machine  was  first  run  at  no-load  without  field  charge 
to  note  the  ventilation,  balance,  and  general  running  conditions 
of  the  machine.  The  ventilation  seemed  to  be  extremely  good, 
especially  that  due  to  the  fans  on  the  ends  o£  the  shaft.  The 
noise,  however,  was  excessive  — so  much  so  that  anyone  working 
around  the  machine  had  to  keep  his  «ars  padded.  At  first  it 
was  difficult  to  locate  the  exact  source  of  this  noisfe,  but  it  was 
determined  that  the  end  fans  were  responsible  for  a  considerable 
part  of  it. 

On  taking  the  saturation  curve  of  the  machine,  it  was  found 
to  be  extremely  sluggish  in  following  any  changes  in  the  field 
current.  The  reason  for  this  sluggishness  is  obvious  from  the 
construction  of  the  machine,  each  magnetic  circuit  of  the  rotor 
core  being  surrounded  by  eight  continuous  collector  rings  of 
very  heavy  section,  and  also  by  eight  brush  holder  supporting 
rings  of  copper  of  very  low  resistance.  These  rings,  of  course, 
formed  heavy  secondaries  or  dampers  which  opposed  any  change 
in  the  main  flux.  The  total  effective  section  of  these  rings  was 
equivalent  in  resistance  to  a  pure  copper  ring  having  a  section 
of  49  sq.  in.  One  can  readily  imagine  that  such  a  ring  would  be 
very  effective  in  damping  any  sudden  flux  changes.  This  slug- 
gishness of  the  machine  to  changes  in  flux,  however,  was  not 
an  entirely  unexpected  result* 

The  saturation  curve  showed  that  the  machine  could  be  carried 


UNI-POLAR  GENERATOR  153 

considerably  higher  in  voltage  than  originally  contemplated,  for 
apparently  the  magnetic  properties  of  the  heavy  steel  parts 
were  very  good,  and  it  was  possible  to  force  the  inductions  in 
these  parts  to  much  higher  density  than  was  considered  prac- 
ticable in  working  out  the  design.  This  gave  considerable  lee- 
way for  changes  which  later  were  found  to  be  necessary. 

In  taking  the  saturation  curve,  the  power  for  driving  the 
machine  was  measured  and  it  was  found  that  there  were  prac- 
tically no  iron  losses  in  the  machine;  that  is,  at  full  voltage 
at  no-load  the  total  measured  losses  were  practically  the  same 
as  without  field  charge.  This  apparently  eliminated  one  pos- 
sible source  of  loss  which  was  anticipated,  namely,  that  due  to 
the  large  open  slots  in  the  stator  pole  face,  these  slots  being  very 
wide  compared  with  the  clearance  between  the  stator  and  rotor 

After  completion  of  this  test  the  machine  was  then  run  on 
short  circuit  Apparently,  as  there  was  no  iron  loss  shown  in 
the  no-load  full  voltage  condition,  the  short  circuit  test  with  full 
load  current  should  cover  all  the  losses  in  the  rotor  which  would 
be  found  with  full"  load  current  at  full  voltage.  Experience 
afterward  proved  this  assumption  to  be  correct,  for  in  its  final 
form  the  machine  would  operate  under  practically  the  same 
condition  as  regards  temperature,  etc.,  at  full  voltage  as  it  would 
show  at  short  circuit,  carrying  the  same  current,  the  principal 
difference  being  the  temperature  of  the  field  coil. 

It  was  in  this  short  circuit  temperature  run  that  the  real 
troubles  with  the  machine  began.  The  measured  losses,  when 
running  on  short  circuit,  were  somewhat  higher  than  indicated 
by  the  resistance  between  terminals  times  the  square  of  the 
current.  These  extra  losses  were  a  function  of  the  load  and 
increased  more  rapidly  with  heavy  currents.  The  measured 
power  indicated  that  these  excess  losses  were  principally  due  to 
eddy  currents.  However,  the  total  losses  indicated  in  these 
preliminary  tests,  although  somewhat  higher  than  calculated, 
were  still  within  allowable  limits,  as  considerable  margin  had  been 
allowed  in  the  original  proportions  to  take  care  of  a  certain 
amount  of  loss.  It  was  therefore  considered  satisfactory  to  go 
ahead  with  the  short  circuit  tests,  and  in  making  these  it  was 
the  intention  to  operate  long  enough  to  determine  the  neces- 
sary running  conditions  as  regards  lubrication,  heating,  etc. 

As  mentioned  before,  the  original  collector  rings  of  the  machine 
each  consisted  of  a  base  ring  upon  which  was  mounted  a  second- 
ary or  wearing  ring,  it  being  the  intention  to  have  this  latter 


154  ELECTRICAL  ENGINEERING  PAPERS 

ring  replaceable  after  it  was  down  to  the  lowest  permissible 
thickness,  as  it  would  be  rather  expensive  and  difficult  to  replace 
the  base  ring  which  carried  the  rotor  conductors.  As  the  inner 
*ing  was  shrunk  on  the  core  and  the  outer  ring  was  shrunk  on 
over  the  base  ring,  with  a  very  small  shrinkage  allowance,  it 
was  considered  that  the  outer  ring  was  in  no  danger  of  loosening 
on  the  inner  ring,  especially  as  both  rings,  being  of  bronze,  and 
in  good  contact,  should  heat  each  other  at  about  the  same  rate. 
This  assumption,  however,  was  wrong.  The  machine  was  put 
on  short  circuit  load  of  about  8000  amperes  early  one  evening 
and  an  experienced  engineer  was  left  in  charge  of  it  until  about 
midnight.  Up  to  that  time  the  machine  was  working  perfectly, 
with  no  under  heating  in  the  rings  and  no  brush  trouble,  although 
vaseline  lubrication  was  used.  About  midnight  the  engineer 
left  the  machine  in  charge  of  a  night  operator,  and  at  about  three 
o'clock  in  the  morning  this  operator  saw  the  brushes  beginning 
to  spark  and  this  very  rapidly  grew  worse,  so  that  in  a  very  few 
minutes  he  found  it  necessary  to  shut 
the  machine  down.  An  examination 
then  showed  that  several  of  the  outer 
rings  had  shifted  sideways  on  the  base 
ring,  as  indicated  in  Fig.  10.  One  of 
these  rings  had  even  moved  into  contact 
with  a  neighboring  ring  so  as  to  make  FIG.  10 

a  dead  short  circuit  on  the  machine.     It 

was  also  noted  that  all  the  rings  which  loosened  were  on  one  side 
of  the  machine,  and  that  the  surfaces  of  the  rings  exposed  to 
the  brushes  were  very  badly  blistered.     The  brushes  also  were 
in  bad  shape,  indicating  that  there  had  been  excessive  burning 
for  a  short  time.     An  investigation  of  the  loose  rings  showed 
that  they  had  loosened  on  their  seats  on  the  inner  or  base  rings. 
Investigation  then  showed  that  a  temperature  rise  of  70  to  80 
deg.  cent.,  combined  with  the  high  centrifugal  stresses,  would 
allow  the  rings  to  loosen  very  materially.     It  was  then  assumed 
that  as  the  ring  had  heated  up,  bad  contact  had  resulted  be- 
tween the  inner  and  outer  rings  and  this,  in  turn,  had  caused 
additional  heating,  so  that  the  temperature  rose  rather  suddenly 
after  bad  contact  once  formed.     It  developed  later  that  this 
was  probably  not  the  true  cause  of  the  trouble,  but  at  the  time 
it  was  considered  that  the  remedy  for  the  trouble  was  in  the 
tise  of  rings  which  could  be  shrunk  on  with  a  greater  tension. 
It  was  then  decided  to  try  steel  outer  rings  instead  of  bronze 


UNI-POLAR  GENERATOR  155 

on  the  end  where  the  bronze  rings  had  loosened.  However,  upon 
loading  the  machine,  after  applying  the  steel  rings,  a  new  diffi- 
culty was  encountered.  It  was  found  that  the  loss  was  very 
greatly  increased  over  that  with  the  bronze  rings.  This  loss 
was  so  excessive  as  to  be  prohibitive,  as  far  as  efficiency  was 
concerned,  and  also  the  tests  showed  excessive  heating  of  the 
rings  and  of  the  machine  as  a  whole.  Also,  there  were  continual 
small  sparks  from  the  tips  of  the  brushes,  these  sparks  being 
from  the  iron  itself,  as  indicated  by  their  color  and  appearance. 
However,  during  the  time  these  rings  were  operated  there  did 
not  seem  to  be  any  undue  wear  of  either  the  brushes  or  the  rings, 
but  obviously  there  was  continued  burning,  as  indicated  by  the 
sparks.  With  thes,e  steel  rings  it  was  found  to  be  impossible 
to  operate  continuously  at  a  current  of  8000  amperes,  due  to 
the  heating  of  the  steel  rings  in  particular  and  everything  in 
general.  At  a  load  of  6000  amperes  the  loss  was  materially  re- 
duced and  it  was  possible  to  operate  continuously  but  with  very 
high  temperatures.  The  tests  showed  that, 
with  the  steel  rings,  at  full  rated  current,  the 
loss  was  approximately  200  kw.  greater  than 
with  the  bronze  rings,  or  about  10  per  cent 
of  the  output.  With  both  ends  equipped 
_  with  steel  rings,  this  would  have  been  prac- 
FIG  11  tically  doubled. 

While  this  was  recognized  as  an  entirely 
unsatisfactory  operating  condition,  yet  it  allowed  the  machine  to 
be  run  for  a  long  enough  period  to  determine  a  number  of  other 
defects  which  did  not  develop  in  the  former  test.  One  of  these 
defects  was  an  undue  heating  of  the  rotor  pole  face.  This  was 
obviously  not  due  directly  to  bunching  of  the  flux  in  the  air  gap 
on  account  of  the  open  stator  slots,  for  this  heating  did  not  appear 
when  running  with  normal  voltage  without  load.  Further  investi- 
gation showed  that  this  was  apparently  due  to  some  flux  dis- 
torting effect  of  the  stationary  conductors  in  the  stator  slots, 
which  carried  about  4000  amperes  each  at  rated  load.  On 
account  of  ample  margin  in  the  magnetizing  coils  the  air  gap 
was  then  materially  increased,  with  some  benefit.  A  further 
improvement  resulted  in  the  use  of  magnetic  wedges,  made  of 
cast  iron,  in  place  of  the  non-magnetic  wedges  used  before. 
These  wedges  are  illustrated  in  Fig.  1 1 .  This  produced  a  further 
beneficial  effect,  but  there  was  still  some  extra  heating  in  the 
pole  face.  Cylindrical  grooves  alternating  $  in.  and  1  in.  deep 


156 


ELECTRICAL  EXGIXEERIXG  PAPERS 


and  about  J  in.  wide,  with  a  J  ia.  web  of  steel  between,  were 
then  turned  in  the  pole  face.  The  resultant  pole  face  was  there- 
fore crudely  laminated,  as  shown  in  Fig.  12.  Also,  on  account 
of  an  apparent  local  heating  of  the  metal  bridge  over  the  rotor 
slots,  a  narrow  groove  was  cut  in  the  closed  bridge  above  each 
rotor  slot,  thus  changing  it  to  a  partially  open  slot,  as  shown  in  the 
figure.  This  effectively  eliminated  the  excess  loss  in  the  rotor 
pole  face.  This,  however,  led  to  another  unexpected  difficulty, 
which  will  be  described  later. 

After  this  trouble  was  cured,  the  short  circuit  test  was  con- 
tinued with  a  current  of  about  6000  amperes.  After  a  con- 
siderable period  of  operation,  a  very  serious  difficulty  in  the 
operation  of  the  machine  began  to  show  up?  namely,  trouble 
with  lubrication.  At  first  the  lubrication  was  vaseline  fed  on 
to  the  rings  by  lubricating  pads.  This  was  apparently  very 
effective  for  awhile,  but  eventually  it  was  noted  that  slight 
sparking  began,  which,  in  some  cases,  would  increase  very 
rapidly  and,  in  a  comparatively 
short  time,  became  so  bad  that 
the  rings  or  brushes  would  be- 
come badly  scored  or  blistered. 
Examination  of  the  sparking 
brushes  showed  a  coating  of  black 
"  smudge  "  over  the  surface  which 
seemed  to  have  more  or  less  in- 

sulating qualities,  A  series  of  tests  then  showed  that  when- 
ever sparking  began,  the  contact  drop  between  a  brush  and  the 
collector  ring  was  fairly  high  and  this  drop  increased  as  the  spark- 
ing increased.  For  instance,  it  was  found  that  on  good,  clean 
surfaces,  the  voltage  drop  between  the  brushes  and  the  ring 
might  be  0.3  to  0.5  volt.  As  each  brush  carried  about  250 
amperes  at  full  load,  this  represented  75  to  125  watts  per 
brush.  When  this  contact  resistance  rose  to  about  one  volt, 
noticeable  sparking  would  begin-,  the  watts  being,  of  course, 
proportionally  higher,  afid  when  the  contact  drop  became  as  high 
as  two  volts,  representing  about  500  watts  per  brush,  very  bad 
burning  of  the  brushes  and  rings  was  liable  to  occur.  A  series 
of  tests  then  showed  that  vaseline,  or  any  other  lubricating  oil, 
would  tend  to  form  a  coating  over  the  brush  contact  and  this 
coating  would  gradually  burn,  or  be  acted  upon  otherwise  by 
the  current,  so  that  its  resistance  increased  and  the  black 
smudge  was  formed  which  had  more  or  less  insulating  qualities. 


FIG.  12 


VXI-POLAR  GENERATOR  157 

A  great  number  of  tests  were  then  carried  out  with  various 
kinds  of  lubricants  and  it  was  found  that  anything  of  an  dil 
or  grease  nature  was  troublesome  sooner  or  later,  as  the  smudge 
was  formed  on  the  brush  contact.  Then  graphite,  formed  into 
cakes  or  brushes  by  means  of  high  pessure,  was  tried  on  the  rings 
and  the  results  were  very  favorable  compared  with  anything 
used  before.  In  fact,  the  tests  indicated  that  soft  graphite 
blocks  or  brushes  could  furnish  proper  lubrication  for  the  rings. 
The  graphite  is  a  conducting  material,  and  a  coating  of  it  on 
the  brush  contact  does  not  materially  increase  the  resistance 
of  the  contact.  This  was  supposed  to  have  practically  settled 
the  question  of  lubrication  and  brush  contact  trouble,  but  ex- 
perience later  gave  an  entirely  new  turn  to  this  matter. 
While  these  tests  were  being  carried  on,  a  study  of  the  ventila- 
tion of  the  machine  was  being  made.  Tha  tests  indicated  that 
the  end  rings,  that  is,  those  next  to  the  exciting  coils,  were  con- 
siderably cooler  than  those  near  the  center  of  the  machine. 
However,  as  there  were  excessive  losses  and  heating  in  the  steel 
rings  themselves,  it  was  not  possible  to  make  any  material  im- 
provement until  the  rings  were  changed. 

The  steel  rings  at  one  end  of  the  rotor,  and  the  bronze  rings 
at  the  other  end,  were  then  removed  and  a  second  set  of  bronze 
rings  was  tried.  These  rings  were  specially  treated  in  the  manu- 
facture so  that  the  elastic  limit  was  very  high,  and  they  were 
put  on  much  tighter  than  in  the  former  case.  The  load  tests 
were  then  continued  and  the  excess  losses  were  agaia  measured 
at  various  loads.  It  was  found  that  the  losses  were  very  small 
compared  with  those  of  the  steel  rings,  thus  verifying  the  former 
results.  The  temperatures  of  the  rings  were  much  lower  than 
with  the  steel,  but  it  was  found  that  the  heating  of  the  rings  war 
unequal.  It  was  finally  determined  that  this  unequal  heating  was 
due  to  the  large  external  blowers  which  were  driving  the  air  over 
the  rings  in  such  a  way  as  to  heat  those  next  to  the  center  of  the 
rotor  to  a  much  higher  temperature  than  those  at  the  outer 
ends.  It  was  assumed  at  first  that  the  air  entering  the  axial 
holes  through  the  core  and  blowing  out  between  the  rings  as 
shown  in  Fig.  4,  was  more  effective  on  the  outer  rings,  and  that 
this  possibly  caused  the  difference  in  temperatures.  However, 
the  radial  holes  at  the  outer  ends  were  dosed,  and  this  made 
but  little  difference.  The  axial  holes  were  then  closed,  and 
while  the  temperattires  of  the  rings,  as  a  whole,  were  increased, 
about  the  same  difference  as  before  was  found  between  the  end 
rings  and  the  center  ones. 


158  ELECTRICAL  EXGIXEERIXG  PAPERS 

It  was  then  decided  to  remove  the  two  large  bloxvers  to  de- 
termine whether  some  other  method  of  ventilation  would  be 
more  effective.  \Vhen  this  change  was  made  the  windage  of 
the  machine  was  greatly  reduced  and  there  was  greater  uni- 
formity in  the  temperatures  and  the  average  temperature  of 
the  rings  wasonly  about  10  deg.  higher  than  with  the  fans.  More- 
over, the  windage  loss  was  only  about  one-seventh  as  great  as 
before,  although  the  average  temperature  rise  was  not  much 
higher,  which  indicated  that  the  ventilation  through  the  rotor 
holes  was  much  more  effective  than  that  due  to  the  blowers. 
In  consequence,  it  was  decided  to  increase  the  size  of  the  axial 
holes  through  the  rotor  core  from  2f  in.  to  3f  in.  diameter,  and 
to  "  bell-mouth  "  them  at  their  openings  at  the  ends,  in  order 
to  give  a  freer  admission  of  air  to  the  holes.  When  this  was  done 
it  was  found  that  the  temperatures  of  the  rings  were  lower  than 
in  any  of  the  preceding  tests,  and  moreover,  they  were  fairly 
uniform.  Also  after  the  removal  of  the  blowers,  the  objection- 
able noise,  already  referred  to,  was  largely  eliminated,  so  that  it 
was  not  disagreeable  to  work  around  the  machine.  The  graphite 
lubrication  was  continued  "with  the  bronze  rings,  on  this  test, 
and  no  difficulty  was  encountered,  although  the  machine  was 
operated  for  very  considerable  periods  at  approximately  8000 
amperes. 

On  the  basis  of  these  tests,  the  machine  was  shipped  to  its 
destination  and  put  in  service.  Then  the  real  difficulties  began — 
difficulties  which  were  not  encountered  in  the  shop  tests,  princi- 
pally because  the  conditions  under  which  the  machine  operated 
in  service  were  radically  different  from  those  at  the  shop,  and 
also,  because  the  shop  test  had  not  been  continued  long  enough. 
This  machine  was  operated  in  service,  although  not  regularly, 
for  a  period  of  about  two  months,  being  shut  down  at  times  due 
to  difficulties  outside  of  the  generating  unit  itself.  However, 
this  period  of  operation  of  the  generator  was  suddenly  ended 
by  the  stretching  of  one  of  the  outer  collector  rings,  which 
loosened  it  to  such  an  extent  that  it  ceased  to  rotate  with  the 
inner  ring.  This  required  the  return  of  the  rotor  to  the  manu- 
facturer. , 

This  two  months'  operation  gave  data  of  great  practical 
value,  and  in  consequence,  a  number  of  minor  difficulties  were 
eliminated  in  the  repaired  rotor. 

Upon  the  return  of  the  rotor  to  the  shop,  an  examination  of  the 
collector  rings  showed  that  the  separate  shrtmk-on  type  of  ring 


USI-POLAR  GENERATOR 


150 


was  not  practicable  with  any  design  of  nng  then  at  hand.  There- 
fore, it  \vas  decided  to  make  the  collector  rings  in  one  solid  piece 
with  a  very  considerable  wearing  depth.  This  necessitated 
the  removal  of  all  the  base  rings  and,  in  fact,  it  required  a  com- 
plete dismantling  of  the  entire  rotor  winding.  As  the  outer  ring 
had  loosened,  there  was  a  possibility  of  the  base  nngs  loosening 
in  the  same  way,  and  therefore  it  was  considered  necessary  to 
apply  some  scheme  for  preventing  this  loosening  in  case  of  sudden 
heating  and  expansion  of  any  of  the  collector  rings.  It  was  then 
decided  to  apply  some  form  of  spnng  support  underneath  these 
nngs,  which  could  follow  up  any  expansion  in  such  a  way  as  to 
keep  the  rings  tight  under  any  temperature  conditions  liable  to 
be  met  with  in  practice.  The  spnng  support  used  consisted  of 
a  number  of  flat  steel  plates  arranged  around  the  rotor  core,  as 
indicated  in  Fig.  13.  These  plates  were  of  such  length  and  stiff- 
ness that  a  very  high  pressure  was  required  to  bend  them  down  to 

conform  with  the  rotor  surface. 
These  plates  were  arranged 
around  the  rotor  core  and  drawn 
down  with  clamp  rings  until 
they  fitted  tightly  against  the 
mica.  The  collector  ring  was 
highly  heated  and  slipped  over 
the  springs,  the  clamps  being 
removed  as  the  ring  was  slipped 
on.  Tests  were  made  to  find 
at  what  temperature  such  a  ring  would  loosen.  While  the  best 
arrangement  without  springs  would  loosen  at  about  100  to  125 
deg.  cent.,  it  was  found  that  a  ring  supported,  in  the  above 
manner,  was  still  fairly  tight  at  180  deg.  cent.,  which  was  far 
above  any  temperature  which  the  machine  would  attain  under 
any  condition.  It  may  be  said  here  that,  after  several  years' 
operation,  this  construction  still  appears  to  be  first  class,  and 
no  loosening  of  any  sort  has  occurred. 

In  removing  the  winding  from  the  rotor,  it  w£s  discovered 
that  the  insulating  tubes  over  the  rotor  conductors  had  traveled 
back  and  forth  along  the  rods  a  certain  amount*  This  travel, 
if  continued  for  a  long  enough  period,  would  apparently  have 
injured  the  insulation,  although  no  trouble  had  yet  developed. 
Apparently,  during  heating  and  cooling,  the  expansion  and  con- 
traction of  the  rods  would  carry  the  tubes  with  them  lengthwise 
a  very  small  amount.  The  tubes  would  then  seat  themselves  in 


FIG.  13 


160  ELECTRICAL  EXGIXEERIXG  PAPERS 

the  supporting  rings  or  core  and  would  not  return  to  their  original 
positions.  It  was  found  that  in  the  slotted  pole  face  already 
described,  the  webs  or  laminations  of  metal  overhanging  the 
rotor  slots  would  hold  the  tube  when  the  rod  was  traveEng  in 
one  direction,  but  would  sometimes  allow  the  tube  to  move 
slightly  when  the  rod  traveled  in  the  other  direction,  so  that 
there  was  a  sort  of  extremely  slow  ratchet  action  taking  place. 
It  was  evidently  necessary  to  have  the  tubes  fit  rather  tightly 
in  the  retaining  or  supporting  holes  in  the  rings  and  the  core,  and 
to  have  the  rods  fit  rather  loosely  in  the  tubes.  Also,  it  ap- 
peared that  shellac  or  other  "  gummy  "  material  on  the  inner 
surface  of  the  insulating  tubes,  was  harmful,  for  wherever  shellac 
was,  present  the  insulating  tube  always  stuck  to  the  rod  and 
would  tear  at  either  side  of  such  place.  In  consequence,  the 
new  set  of  tubes  was  made  with  a  dry,  hard  finish  on  both  the 
outside  and  the  inside,  and  the  inside  surface  was  also  paraf- 


FIG. 


fined.  This,  when  carried  out  properly,  served  to  remedy  this 
trouble. 

The  reconstructed  rotor,  with  the  solid  collector  rings,  was 
shipped  to  the  customer  and  the  service  was  continued.  After 
operation  for  a  considerable  time,  certain  extremely  serious  dif- 
ficulties appeared.  One  of  these  was  brush  trouble,  and  another 
was  undue  wear  of  the  rings. 

The  brush  trouble  was  a  most  discouraging  one.  The  machine 
was  located  in  an  engine  room  adjacent  to  a  rock-crushing  build- 
ing. Fine  dust  was  always  floating  around  the  machine  and, 
this  dust  continuously  passing  through  the  machine  tended  to 
form  a  deposit  immediately  behind  the  brushes  as  shown  in  Fig. 
14.  This  dust  packed  in  rather  solidly  behind  the  brush,  due  to 
the  high  speed  of  the  rings,  and  eventually  it  tended  to  lift  the 
brushes  away  from  the  rings.  It  also  showed  a  tendency  to  get 
Tinder  the  brush  contact,  with  consequent  increased  resistance 
of  contact.  Frequent  removal  and  cleaning  of  the  brushes 


UXI-POLAR  GENERATOR  Uil 

impracticable,  as  they  were  not  sufficiently  accessible  to  do 
this  readily.  This  rock  dust,  packed  behind  the  brushes, 
also  had  a  scouring  or  grinding  action  on  the  rings  themselves. 
Accompanying  this  was  an  undue  rate  of  wear  of  the  rings.  This, 
however,  was  not  entirely  mechanical  wear,  as  it  appeared  also 
to  be  dependent  upon  the  current  carried  and  was,  to  some  ex- 
tent, due  to  a  burning  action  under  the  brush  which  tended  to 
eat  away  the  surface  of  the  rings.  However,  while  the  undue 
wear  was  not  altogether  due  to  dust  back  of  the  brushes,  yet 
this  accumulation  of  dust  appeared  to  have  a  very  harmful 
action  on  the  machine.  Various  methods  were  considered  for 
overcoming  this  collection  of  dust,  one  of  which  consisted  of 
enclosed  air  inlets  to  the  machine,  fitted  with  screens  for  sifting 
out  the  dust.  This  lessened  the  trouble  to  some  extent,  but 
it  was  evident  that  it  would  not  cure  it  entirely,  as  the  entire 
machine  was  so  located  that  dust  could  come  in  around  the  brush 
holders  without  going  through  the  ventilating  channels. 

The  method  finally  adopted  for  overcoming  the  difficulty  of 
accumulation  of  dirt  was  rather  startling.  It  was  casually  sug- 
gested that  the  copper  leaf  brushes  be  turned  around  so  that 
the  rings  would  run  against  the  brushes,  so  that  the  dirt  or  dust 
over  the  rings  would  be  "  skimmed  off  "  by  the  forward  edge  of 
the  brushes.  This  obviously  would  prevent  the  collection  of 
dirty  but  the  question  of  running  thin  leaf  copper  brushes  on 
a  collector  ring  operated  at  a  speed  of  about  220  feet  per  second 
(or  13,200  feet  per  minute)  looked  like  an  absurdity  to  any  one 
with  experience  in  electrical  machinery,  so  that  we  all  hesitated 
at  first  to  consider  the  possibility  of  it.  However,  as  something 
had  to  be  done,  the  writer  suggested  to  the  engineer  in  charge, 
that  he  change  the  brushes  on  one  of  the  rings  so  that  they  would 
be  inclined  against  the  direction  of  rotation.  This  gave  no 
trouble  and  the  other  brushes  were  then  changed  to  the  same 
direction  and  the  operation  ever  since  has  been  carried  on  with 
this  arrangement.  To  the  writer  this  has  always  seemed  an 
almost  unbelievable  condition  of  operation,  but  as  there  has 
not  been  a  single  case  of  trouble  from  this  arrangement  during 
several  years  of  operation,  one  is  forced  to  believe  that  it  is  all 
right.  This  change  entirely  overcame  the  trouble  from  accumu- 
lation of  dirt.  However,  it  did  not  entirely  cure  the  burning  of 
the  brushes  and  rings  above  described,  but  rendered  the  matter 
of  lubrication  somewhat  easier  than  at  first. 

As  to  the  other  serious  trouble,  it  was  mentioned  that  there 


162  ELECTRICAL  ENGINEERING  PAPERS 

was  a  burning  action  under  the  brushes  which  tended  to  "  eat  " 
or  "  wear  "  away  the  surface  of  the  rings.  This  also  tended  to 
burn  away  the  brush  surfaces,  the  amount  of  burning  in  either 
case  depending,  to  a  considerable  extent  ,  upon  the  direction 
of  the  current.  At  one  side  of  the  machine  the  brushes 
would  wear  more  rapidly,  while  at  the  other  side  the  rings 
would  wear  faster.  The  polarity  of  the  current  was  influential 
in  this  action.  Particles  of  the  metal  appeared  to  travel  in 
the  direction  of  the  current;  that  is,  where  the  current  was  from 
the  ring  to  the  brushes,  the  ring  would  wear  more  rapidly, 
while  the  brush  would  show  but  little  wear,  while  at  the  other 
end  of  the  machine,  the  opposite  effect  would  be  found.  How- 
ever, the  particles  of  metal  taken  from  the  ring  did  not  deposit, 
or  "  build  up,"  on  the  brushes. 

During  all  this  operation,  graphite  had  been  used  for  lubri- 
cation. In  the  earlier  stages,  powdered  graphite  compressed 
into  blocks,  had  been  used.  Later  it  was  found  that  very  soft 
graphite  brushes  in  insulated  holders  would  give  ample  lubri- 
cation for  the  rings.  However,  even  with  this  lubrication  and 
the  removal  of  the  dirt  trouble,  there  was  still  an  appreciable 
burning  of  the  brushes  and  rings  as  indicated  by  the  more  rapid 
wear  of  the  rings  at  one  end  of  the  rotor,  and  of  the  brushes  at 
the  other  end.  Extended  tests  showed  that  this  burning  was 
a  function  of  the  contact  drop  between  the  brushes  and  the  rings. 
Neither  the  nngs  nor  the  brushes  would  burn  appreciably  if 
the  contact  drop  between  the  brushes  and  the  ring  could  be  kept 
very  low.  When  this  drop  became  relatively  high  (about  one 
volt),  the  rings  or  brushes  would  show  an  undue  rate  of  wear.  It 
was  found  also  that,  after  a  considerable  period  of  operation, 
it  was  very  difficult  to  obtain  a  low  brush  contact  drop,  as  the 
brush  wearing  surface  became  coated  with  a  sort  of  "  smudge," 
which  seemed  to  have  resisting  qualities.  An  analysis  of  this 
coating  showed  a  very  considerable  amount  of  zinc  in  it,  and 
it  was  determined  that  the  zinc  in  the  collector  rings  was  burning 
out  and  forming  an  insulating  coating  on  the  brush  contacts. 
The  remedy  for  this  condition  was  the  application  of  some  clean- 
ing agent  which  would  chemically  act  on  the  smudge  and  dis- 
solve it  or  destroy  its  insulating  qualities.  The  right  material  for 
this  purpose  was  found  to  be  a  weak  solution  of  muriatic  acid — 
about  4  per  cent  in  water.  When  this  was  applied  to  the  rings  by 
means  of  a  "  wiper,"  at  intervals,  the  brush  contact  drop  could  be 
reduced  to  a  very  low  figure—  frequently  to  0.1  or  0.2  of  a  volt, 


UXI-POLA  R  GENERA  TOR  1 63 

and  the  rings  would  take  on  a  very  bright  polish.  Also,  while 
this  low  contact  drop  was  maintained  it  was  found  that  the  rings 
showed  an  almost  inappreciable  rate  of  wear.  However,  one 
set  of  rings  continued  to  wear  somewhat  faster  than  the  other. 
This  difficulty  of  unequal  wear  of  the  two  sets  of  rings  was  over- 
come by  arranging  a  switch  so  that  the  polarity  of  the  two  ends 
of  the  machine  could  be  changed  occasionally. 

The  temperature  of  the  machine  was  reduced  by  the  above 
treatment  of  the  rings.  Obviously,  part  of  the  heat  was  due  to 
the  loss  at  the  brush  contacts,  which,  of  course,  was  reduced 
directly  as  the  contact  drop  was  reduced. 

The  machine  was  now  running  quite  decently  with  compara 
tively  heavy  loads,  from  7000  to  10,000  amperes,  and  the  only 
trouble  was  in  several  minor  difficulties  which  were  then  taken 
up,  one  at  a  time,  in  order  to  ascertain  a  suitable  remedy. 
These  difficulties,  however,  were  not  interfering  with  the  regular 
operation  of  the  machine. 

One  of  the  difficulties  which  finally  developed  was  due  to 
stray  magnetic  fluxes  through  the  bearings.  These  fluxes,  pass- 
ing out  through  the  shaft  to  the  shell  of  the  bearing,  consti- 
tuted, in  themselves,  the  elements  of  a  small  unipolar  machine, 
of  which  the  bearing  metal  served  as  collecting  brushes.  The 
e.m.f .  generated  in  the  shaft  was  a  maximum  across  the  two  ends 
of  the  bearing.  Consequently  the  current  collected  from  the 
shaft  by  the  bearing  metal -should  have  been  greatest  near  the 
ends  of  the  bearing,  and  least  at  the  center.  This  was  the  case 
as. indicated  by  the  appearance  of  the  bearing  itself,  which 
showed  evidence  of  pitting  near  the  ends  but  none  at  the  center. 

To  remedy  this  trouble,  a  small  demagnetizing  coil  was  placed 
outside  the  stator  frame,  at  each  end  of  the  rotor,  between  the 
rotor  core  and  the  bearings.  These  coils  were  excited  by  direct 
current  which  was  adjusted  in  value  until  practically  zero  e.m.f . 
was  indicated  on  the  shaft  at  the  two  ends  of  each  bearing.  This 
indicated  that  the  unipolar  action  was  practically  eliminated. 
This  arrangement  has  been  in  use  ever  since  it  was  installed,  and 
no  more  trouble  of  any  sort  has  been  encountered  from  local 
currents  in  the  bearings  or  elsewhere. 

Some  of  the  brushes  did  not  show  as  good  wearing  qualities 
as  desired  and  various  experiments  were  made  with  different 
combinations  of  materials  and  various  thicknesses  and  arrange- 
ment of  the  brush  laminae.  Brass  leaf  brushes  were  tried;  also, 
mixtures  of  copper,  brass,  aluminum  and  various  other  leaf  metals 


164  ELECTRICAL  ENGINEERING  PAPERS 

in  combination.  None  of  these  showed  any  better  than  the  thin 
copper  leaf  brush.  The  tests  finally  showed  that  such  a  brush, 
very  soft  and  flexible,  with  a  suitable  spring  tension,  would 
give  very  satisfactory  results.  Also  instead,  of  two  brushes 
side  by  side,  a.  single  brush,  covering  the  full  width  of  a  ring,  was 
found  to  be  more  satisfactory.  Some  tests  were  also  made  with 
carbon  brushes,  consisting  of  a  combination  of  carbon  or  graphite 
combined  with  some  metal,  such  as  copper,  in  a  finely  divided 
state.  These  brushes  were  claimed  to  have  a  very  high  carrying 
capacity  and  also  to  have  a  certain  amount  of  self -lubrication. 
A  set  of  these  brushes  was  tried  on  one  of  the  rings,  but  lasted 
only  for  a  very  short  time.  The  apparent  wear  was  rapid,  but 
it  is  not  known  whether  this  was  due  to  the  very  high  speed  of  the 
collector  rings,  or  rapid  burning  away  of  the  brush  or4;he  inability 
of  this  type  of  brush  to  quickly  follow  any  inequalities  of  the 
collector  rings.  This  test  was  abandoned  in  a  comparatively 
short  time. 

After  getting  rid  of  the  old  troubles,  a  new  and  unexpected  one 
had  to  appear.  For  some  unknown  reason,  the  insulating  tubes 
on  the  rotor  conductors  began  to  break  down  j  also  grounds  oc- 
curred between  the  collector  rings  and  the  core. 

On  account  of  the  delay  required  in  making  any  changes  in 
the  rings  or  rotor  winding,  the  customer  arranged  with  the 
manufacturer  to  have  a  new  rotor  built  as  a  reserve,  as  it  was 
obvious  that  sooner  or  later  there  would  have  to  be  considerable 
reconstruction  of  the  insulation  on  the  first  rotor  due  to  unex- 
plained short  circuits  and  grounds.    A  new  rotor  was  at  once 
constructed,  embodying  all  the  good  features  of  the  first  rotor, 
with  some  supposedly  minor  improvements.    The  old  rotor'was 
then  removed  for  investigation  and  repairs.     The  cause  of  the 
breakdowns  of  the  insulation  on  the  tubes  was  then  discovered. 
The  air  entering  through  the  axial  rotor  holes  and  passing  out 
through  the  radial  holes  between  the  rings,  carried  fine  particles 
of  cement  or  Crushed  stone  dust  and  this  had  "  sand-blasted  M 
the  under  side  of  the  tubes.     When  the  rotor  had  been  operated 
during  the  preliminary  two  months*  period,  previously  described, 
before  the  replacement  of  the  rings,  no  evidence  of  thfe  sand- 
blasting had  been  visible.     Investigation  showed  that  the  in- 
sulating tubes  in  the  former  winding  had  been  made  with  a  fuller- 
board  base,  which  is  rather  soft  and  fibrous  in  its  construction. 
The  tubes  on  the  second  winding  had  been  made  with  "  fish  " 
paper  instead  of  fullerboard,  in  order  to  give  a  hard  finish  on  the 


UNI-POLAR  GENERATOR 


165 


inside  and  outside.  It  was  due  to  this  hard  material  that  the 
troubles  from  sand  blasting  occurred.  However,  fish  paper 
tubes  were  superior  to  the  fullerboard  in  strength  and  other 
qualities,  and  as  they  were  inferior  only,  in  this  one  character- 
istic, they  were  used  again  in  rewinding  the  rotor,  but  where- 
ever  the  tubes  were  exposed  in  passing  from  one  ring  to  the  next, 
they  were  taped  over  with  several  layers  of  soft  tape  whch  was 
also  sewed.  This  gave  a  soft  finish  which  would  resist  sand- 
blasting, and  no  trouble  from  this  source  has  occurred  for  several 
years. 

From  the  breakdowns  to  ground,  it  was  evident  that  an  entire 
replacement  of  the  rings  was  necessary  in  order  to  repair  the 
mica  bush  or  sleeve  lying  beneath  the  rings.  This  required 
the  removal  of  the  entire  rotor  winding  and  rings.  It  was  found 
that  cement  dust  coming  up  through  the  radial  holes  had  sifted 

in  through  various  crevices-  or  openings 
around  the  holes  and  that,  finally,  con- 
ducting surfaces  and  paths  were  formed 
which  allowed  the  current  to  leak  to 
ground  sufficient  -  to  eventually  burn  the 
insulation.  Therefore,  when  replacing 
the  mica  sleeve  over  the  rotor,  extra  care 
was  taken  to  fit  insulating  bushings  at 
the  top  of  the  radial  holes  in  such  a 
way  as  to  seal  or  -close  all  joints,  thus 
allowing  no  leakage  paths  between 
collector  rings  and  the  body  of  the  cote.  This  is  shown  in  Fig.  15. 
No  further  trouble  has  occurred  at  this  point. 

In  removing  the  collector  rings  for  these  repairs,  it  was  found 
that  the  flat  spring  supports  shown  in  Fig.  13  had  been  entirely 
effective  and  there  was  no  evidence  whatever  of  any  disturbance 
of  the  rings  on  the  core,  and  there  was  no  injury  to  the  mica, 
such  as  would  be  shown  by  any  slight  movement.  The  rings 
were  also  very  tight  so  that  it  took  a  very  considerable  temper- 
ature to  loosen  them  sufficiently  for  removal. 

In  view  of  the  delay  and  expense  of  repairing  one  of  these 
rotors  when  the  collector  rings  had  to  be  removed,  with  the  pos- 
sibility of  damaging  the  insulating  tubes  over  the  conductors, 
and  the  insulating  bush  over  the  core,  it  was  then  decided  that 
a  movable  wearing  ring  was  practically  necessary  in  order  to 
make  this  mgdhine  a  permanent  success.  Therefore,  the 
problem  of  a  separate  outside  wearing  ring,  as  originally  con-' 


FIG.  15 


166 


ELECTRICAL  ENGINEERING  PAPERS 


templated,  was  again  taken  up.  The  difficulty,  already  de- 
scribed, of  the  zinc  burning  from  the  rings  and  forming  a  coating 
on  the  brushes,  indicated  that  some  other  material,  without 
such  a  large  percentage  of  zinc,  should  give  better  results.  The 
difficulty  was  to  obtain  such  a  material,  with  suitable  charac- 
teristics otherwise.  All  data  at  hand  showed  that  rings,  with 
desirable  characteristics  electrically,  did  not  have  the  proper 
elastic  limits,  or  proper  expansion  properties  when  heated.  In 
other  words,  when  such  rings  were  shrunk  on  the  base  or  sup- 
porting ring  they  would  stretch  to  such  an  extent,  when  cooled, 
that  they  would  become  loose  again  with  very  moderate  in- 
crease in  temperature.  The  solution  of  this  problem  of  a  separate 
ring  construction  was  found  in  the  use  of  some  spring  arrange- 
ment -underneath  the  outer  ring  which  would  still  keep  it  tight 
on  the  inner  ring  even  when  hot.  The  spring  arrangement 


PIG.  16 


used  under  the  inner  rings,  as  shown  in  Fig.  13  was  then  applied 
with  certain  modifications.  In  order  to  get  good  contact  be- 
tween the  inner  and  outer  rings  for  carrying  the  current,  each 
of  these  steel  springs  or  plates  was  covered  by  a  thin  sheet  of 
copper  as  shown  in  Pig,  16.  While  each  copper  shest  was  of 
comparatively  small  section,  the  large  number  of  springs  used 
gave  sufficient  total  copper  to  carry  the  current  from  the  outer 
to  the  inner  or  base  ring  without  any  danger  of  current  passing 
through  the  spring  plates  themselves.  This  arrangement  was 
used  in  reconstructing  this  rotor  and  has  proven  entirely 
successful. 

In  order  to  determine  the  effects  of  various  materials  without 
zinc,  or  with  but  a  small  quantity  of  it,  a  number  of  rings  were 
fitted  up  on  a  test  rig  and  were  operated  for  long  periods  with 
currents,  up  to  12,000  amperes  in  some  cases.  In  these  tests, 


UNI-POLAR  GENERATOR  167 

four  different  kinds  of  material  were  used,  all  of  them  representing 
different  mixtures  of  copper  with  a  small  percentage  of  other 
materials  but  with  little  zinc  in  any  of  them.  It  was  feared 
that  copper  brushes  on  the  copper  rings  would  not  work  satisfac- 
torily, but  while  there  was  apparently  some  difference  between 
the  action  of  the  different  rings,  it  was  found  that  copper  brushes 
running  on  copper  were,  in  general,  satisfactory.  The  brushes 
were  in-clined  against  the  rings,  as  in  the  actual  machine, 
during  this  series  of  tests. 

These  tests  were  carried  through  with  various  numbers  of 
brushes,  etc.  It  was  found  that  the  number  of  brushes  could 
be  reduced  to  about  one-third  the  full  number,  and  still  collect 
the  total  rated  current,  but  that  any  great  reduction  from  the  full 
number  of  brushes  made  the  operation  of  the  rings  and  brushes 
more  sensitive,  and  more  attention  was  required  to  keep  them 
in  perfect  condition.  It  was  also  found  that  any  hardness  or 
undue  "springiness"  in  the  brushes,  or  brush  material,  would 
tend  to  give  increased  wear.  Brushes  of  very  thin  leaf  copper, 
eventually  gave  best  results.  It  was  also  shown  by  these  tests 
that  if  a  very  good  polish  could  be  maintained  on  the  rings, 
the  rate  of  wear  from  day  to  day  was  practically  unmeasurable 
on  account  of  its  smallness. 

As  a  result  of  these  ring  tests,  the  rotor  undergoing  repair 
was  equipped  with  outside  copper  wearing  rings,  spring  sup- 
ported. The  material  in  the  rings  was  about  92  per  cent  pure 
copper,  2  per  cent  zinc  and  6  per  cent  tin. 

The  rotor  was  then  installed  in  service  and  has  been  operating 
steadily  for  several  years,  with  entire  success.  The  other  rotor, 
which  had  been  operating  while  this  rotor  was  being  repaired, 
was  then  thoroughly  examined  after  removal,  to  determine  any 
possible  defects.  It  was  noted  that  the  insulating  tubes  over 
the  rotor  conductors  were  badly  cracked  or  buckled  in  a  number 
of  places.  Upon  removal  of  the  rods  or  conductors  it  was  found 
that  the  insulating  tubes  were  stuck  so  tightly  to  the  copper 
rods  that  they  would  be  torn  in  pieces  in  trying  to  remove  them. 
As  it  had  been  intended  that  these  tubes  should  move  freely 
on  the  rods  or  conductors,  as  previously  described,  it  was  evident 
that  there  was  something  radically  wrong.  The  true  cause 
of  the  "trouble  was  then  discovered.  In  first  fitting  this  set  of 
tubes  over  the  rods,  they  had  been  too  tight,  and,  in 
order  to  make  them  fit  easily,  the  men  who  assembled 
the  machine  had  reamed  them*  on  the  inside  to  enlarge 


168  ELECTRICAL  ENGINEERING  PAPERS 

them,  and,  in  doing  so,  had  cut  away  the  inner  hard  sheet  of 
fish  paper  which  had  formed  the  lining,  thus  exposing  a  shellaced 
surface.  As  soon  as  heated,  this  shellac  stuck  the  tube  to  the 
rod  so  that  there  could  be  no  possible  movement  between  the 
two.  In  consequence,  when  the  rods  expanded  or  contracted, 
the  tubes  moved  backward  and  forward  in  the  supporting  holes, 
and  wherever  they  stuck  fast  in  the  outer  holes,  something  had 
to  give,  so  that  eventually  the  tubes  buckled  or  cracked  or  pulled 
open.  This  was  readily  remedied  by  putting  on  new  tubes  prop- 
erly constructed.  As  the  rings  on  this  rotor  were  in  very  good 
condition  with  but  little  worn  away,  the  removable  type  of 
ring  was  not  added,  as  this  would  require  turning  off  a  large 
amount  of  effective  material  on  the  existing  rings  and  replacing 
it  with  new  outer  rings.  It  was  decided  that  as  there  was  several 
years'  wear  in  the  old  rings,  it  would  be  of  no  material  advantage 
to  throw  this  away  when  it  could  be  worn  away  in  service, 
just  as  well  as  it  could  be  turned  off  in  a  lathe.  After  the  rings 
in  this  machine  are  worn  down  the  permissible  depth,  they  will 
be  refilled  by  the  addition  of  the  removable  type. 

This  unipolar  generator  has  now  been  in  service  for  quits 
a  long  period,  with  no  difficulty  whatever,  and  with  an  average 
ring  wear  of  less  than  0.001  in.  per  day,  or  less  than  f  in.  per 
year.     This  may  seem  like  an  undue  rate  of  wear;  but  in  reality 
it  is  an  extremely  low  rate,  if  the  high  peripheral  speed,  and  the 
number  of  brushes,   are  considered.    This  machine   operates 
day  and  night,  seven  days  in  the  week,  and  practically  contin- 
uously during  the  entire  year.     Taking  the   peripheral  speed 
into  account,  the  above  rate  of  Wear  represents  a  total  travel 
of  each,  ring  of  about  3.6  million  miles  for  each  inch  depth  of  wear, 
or  about  150  times  around  the  earth  along  a  great  circle.     Con- 
sidering that  there  are  brushes  bearing  on  each  ring  at  intervals 
of  about  eight  in.,  a  wear  of  one  in.,  for  every  3.6  million  miles 
traveled,  does  not  seem  unduly  large.     If,  at  the  same  time,  it 
is  considered  that  the  brushes  are  collecting  from  7500  to  10,000 
amperes  from  each  ring  on  a  total  ring  surface  of  about  3£  in. 
wide  by  42  in.  diameter,  it  is  not  surprising  that  there  should 
be  more  or  less  "  wear  "  due  to  the  collection  of  this  current. 
In  fact,  the  current  collected  averages  from  16  to  20  amperes 
per  square  inch  of  the  total  ring  wearing  surface.    This  may  be 
compared  with  standard  practice  with  large  d-c.  commutators, 
in  which  H  to  2  amperes  per  square  inch  of  commutator  face 
is  usual  and  3  amperes  is  extreme. 


UNI-POLAR  GENERATOR  169 

On  account  of  the  final  success  of  this  machine,  the  story  of 
its  development  is  a  more  pleasant  one  to  tell  than  is  the  case 
in  some  instances  where  entirely  new  types  of  apparatus  are 
undertaken.  It  might  be  said,  after  reviewing  the  foregoing 
description,  that  many  of  the  troubles  encountered  with  this 
machine  could  have  been  foreseen;  but  such  a  statement  would 
be  open  to  question,  for  the  engineers  of  the  manufacturing 
company  were  in  frequent  session  on  all  the  various  phases  and 
difficulties  which  developed.  The  writer  knows  that  in  many 
cases,  after  any  individual  trouble  was  known,  suggestion  for 
remedies  were  not  readily  forthcoming.  The  writer  does  not 
know  of  any  individual  machine  where  more  engineering  and 
manufacturing  skill  was  expended  in  endeavoring  to  bring  about 
success,  than  was  the  case  with  this  machine.  As  an  example 
of  engineering  pertinacity,  this  machine  is  possibly  without  a 
rival.  A  mere  telling  of  the  story  cannot  give  more  than  a 
slight  idea  of  the  actual  fight  to  overcome  the  various  difficulties 
encountered  in  the  development  of  this  machine. 

The  results  obtained  were  valuable  in  many  ways.  Many 
data  were  obtained  which  have  since  been  of  great  use,  both  from 
a  theoretical  as  well  as  a  practical  standpoint,  in  other  classes 
of  apparatus.  Certain  fundamental  conditions  encountered  in 
this  machine  have  led  to  the  study  of  other  allied  principles 
which  point  toward  possibilities  in  other  lines  of  endeavor. 
Therefore  this  machine,  which  was  very  costly  in  its  develop- 
ment, may  eventually  pay  for  itself  through  improvements  and 
developments  in  other  lines  of  design. 

The  writer  wishes  to  say  a  good  word  for  the  purchaser  of  this 
new  apparatus.  He  was  long-suffering,  and  was  undoubtedly 
put  to  more  or  less  trouble  and  inconvenience,  but  nevertheless 
he  gave  opportunity  to  correct  difficulties.  He  recognized  that 
the  engineers  were  confronted  with  a  new  problem  in  this  ma- 
:hine  and  he  gave  them  an  opportunity  to  cany  it  through  to 
success.  Apparatus  of  this  type  could  only  be  developed  to 
full  success  in  commerical  operation,  as  all  the  difficulties  en- 
countered would  never  have  been  found  on  shop  test.  There- 
fore, the  attitude  of  the  customer  was  of  prime  importance  in 
the  development  of  such  a  machine. 


COMMUTATING  POLES  IN  SYNCHRONOUS 
CONVERTERS 

FOREWORD — About  1909,  the  use  of  commutating  poles  in  syn- 
chronous converters  was  being  studied.  Suggestions  were  made 
from  time  to  time  that  our  usual  slow  speed  rotary  converters 
shoiild  have  interpoles.  The  author,  therefore,  prepared  a  short 
article,  explaining  wherein  commutating  poles  would  be  of  less 
value  to  rotary  converters,  of  the  then  usual  speeds  and  con- 
structions, than  they  would  be  on  direct-current  generators. 

11  Late  in  September,  1910,  the  Chairman  of  the  Papers  Com- 
mittee of  the  American  Institute  of  Electrical  Engineers  asked 
the  author  whether  he  had  any  material  which  could  be  pre- 
pared for  the  Institute  on  very  short  notice,  A  rough  draft 
of  this  article  was  submitted  and  was  at  once  accepted,  with 
instructions  to  complete  it  for  the  following  November  meeting. 
The  author  called  to  his  aid,  Mr.  F.  D.  Newbury,  who  added 
about  one-half  more,  covering  principally  material  on  existing 
types  of  rotaries.  Most  of  this  latter  part  has  been  omitted  from 
this  reprint,  but  the  author's  discussion  at  the  Institute  meeting 
has  been  incorporated  as  it  forms  a  technical  continuation  of  the 
first  part  of  the  paper  and  brings  out  that  the  real  need  for 
commutating  poles  in  rotary  converters  would  come  with  higher 


This  paper  states  that  the  short-circuiting  effect  of  the 
dampers  surrounding  the  commutating  poles  is  considered 
harmful.  However  later  experience  has  shown  that  the  in- 
creased damping  effect  of  this  arrangement  more  than  compen- 
sates for  the  harmful  effects. 

As  this  paper  was  written  before  the  term  "commutating 
pole"  was  adopted  as  standard,  the  term  "interpole"  has  been 
used  throughout. — (ED.) 


SYNCHRONOUS  converters  with  interpoles  have  been  used 
^  but  little  in  this  country  up  to  the  present  time  (1910).  Con- 
sidering that  interpole  generators  and  motors  have  come  into 
extensive  use  in  this  country,  the  question  will  naturally  be  raised 
why  interpole  converters  have  not  come  into  similarly  extensive 
use.  The  reply  might  be  that  the  introduction  of  any  new  type  of 
apparatus  is  a  relatively  slow  process;  but,  on  the  other  hand, 
interpoles  on  direct  current  generators  and  motors  came  into 
general  use  in  a  relatively  short  time,  especially  so  in  railway 
motors.  This  indicates  that  there  has  been  a  more  or  less  pressing 
need  for  interpoles  in  certain  classes  of  apparatus  and  the  greater 
the  need  for  the  change  the  quicker  was  the  change  made. 

Any  important  change  in  design  or  type  must  be  justified 

171 


172 


ELECTRICAL  ENGINEERING  PAPERS 


by  engineering  and  commercial  reasons,  such  as  improved  per- 
formance greater  economy,  or  lower  cost.  In  the  railway  motor, 
placed  under  the  car,  and  more  or  less  inaccessible,  improved 
operation  at  the  brushes  and  commutator,  when  equipped  with 
interpoles,  represented  a  pressing  reason  for  the  change  in  type, 
although  the  cost  and  efficiency  were  not  appreciably  changed. 
In  the  direct-current  generator  with  the  modern  tendency  toward 
higher  speeds  with  lower  cost,  the  interpoles  represented  a 
practical  necessity.  This  has  been  recognized  for  several  years 
and  the  change  to  the  interpole  type  has  been  made  as  rapidly 
as  circumstances  will  permit.  Also,  in  variable-speed  direct- 
curretit  motors  interpoles  have  been  in  general  use  for  a  number 
of  years,  simply  because  the  interpoles  represent  a  very  definite 
improvement  in  a  number  of  ways. 

New  types  of  apparatus  should  only  be  introduced  where  they 
represent  some  distinct  improvement  or  advance  over  existing 
types.  Where  a  new  type  does  not  represent  such  improvement 
and  is  simply  introduced  to  gratify  a  personal  whim  of  the 
purchaser,  or  desire  on  the  part  of  a  manuf acturing  company  to 
produce  something  different  from  other  companies,  the  new 
apparatus,  as  a  rule,  will  not  advance  quickly  into  public  favor 
since  there  is  no  real  necessity  for  it. 

It  is  therefore  a  question 
whether  the  slowness  in  the 
introduction  of  interpoles  in 
synchronous  converters  is  due 
to  lack  of  sufficient  advan- 
tages, or  American  engineers 
do  not  sufficiently  appreciate 
their  advantages.  There  ap- 
pears to  be  room  for  wide 
differences  in  opinion  on  this 
subject.  The  synchronous 
converter  and  the  direct- 
current  generator  are  two 
qttite  different  machines,  in 
their  characteristics,  and  no 
one  can  say  off  hand,  that  interpoles  will  give  the  same  results 
in  both.  In  the  following  is  given  a  partial  analysis  of  the  condi- 
tions occurring  in  the  two  classes  of  machines,  which  will  indicate 
wherein  interpoles  are  of  greater  advantage  on  direct-current 
generators  than  on  converters. 


FIG.  1 


COMMUTATING  POLES  ON  ROTARIES  173 

Taking  up  first,  the  direct  current  generator,  it  may  be 
considered  as  containing  two  sets  of  magnetizing  coils,  namely, 
the  armature  and  the  field  windings.  Considering  the  armatiire 
winding  alone,  the  magnetomotive  force  of  the  armature  winding 
has  zero  values  at  poirxts  midway  between  two  adjacent  brush 
arms  or  points  of  collection  of  current  and  rises  at  a  uniform  rate 
to  the  point  of  the  winding  which  is  in  contact  with  the  brushes. 
This  is  illustrated  in  Fig.  1.  Therefore  the  armature  winding 
has  Its  maximum  magnetizing  affect  or  magnetomotive  foi<~- 
at  that  part  of  the  core  surface  where  the  winding  is  directly 
in  contact  with  the  brushes.  However,  the  magnetic  flux-  set 
up  by  the  armature  winding  will  not  necessarily  be  a  maximum 
at  this  point,  as  this  depends  upon  the  arrangement  of  the  mag- 
netic or  other  material  surrounding  the  armature.  If  this  point 
occurs  midway  between  two  field  poles,  then,  while  the  mag- 
netizing effect  is  greatest  at  this  point,  the  presence  of  a  large 
air-gap  at  this  same  point  may  mean  a  relatively  small  magnetic 
flux,  while  a  much  higher  flux  may  be  set  up  by  the  armature 
winding  at  the  edges  of  the  adjacent  field  poles.  In  the  usual 
direct-current  generator  construction  without  interpoles,  the 
position  of  commutation  is  almost  midway  between  two  adjacent 
poles  and  therefore  the  point  .of  maximum  magnetomotive 
force  of  the  armature  is  also  practically  midway  between  poles. 
The  absence  of  good  magnetic  material  over  the  armature  at 
this  point  serves  to  lessen  the  magnetic  flux  due  to  the  armature 
magnetizing  effect,  but  even  with  the  best  possible  proportions 
there  will  necessarily  be  a  slight  magnetic  flux  set  up  at  this 
point.  While  this  field  is  usually  of  small  value,  yet  unfor- 
tunately it  is  of  such  a  polarity  as  to  have  a  harmful  effect  on  the 
commutation  of  the  machine.  During  the  operation  of  comma* 
tation,  the  coil  which  is  being  commutated  has  its  two  terminals 
short-circuited  by  the  brushes.  If  this  short  circuited  coil  at 
this  moment  is  moving  across  a  magnetic  flux  or  field,  it  will 
have  an  e.m.f .  set  up  in  it  which  will  tend  to  cause  a  lo£al  or 
short  circuit  current  to  flow.  Such  a  current  is  set  up  by  the 
flux  due  to  the  armature  magnetomotive  force  described  above 
and  unfortunately  this  current  flows  in  such  a  way  as  to  give  the 
same  effect  as  an  increased  external  or  working  current  to  be  re- 
versed as  the  coil  passes  from  under  the  brush.  In  other  words, 
the  e.m.f.  set  up  in  the  short  circuited  coil  by  the  above  field 
adds  to  the  e.m.f.  of  self  induction  in  the  coil  due  to  the  reversal 
of  the  working  current. 


174=  ELECTRICAL  ENGINEERING  PAPERS 

Another  cause  of  difficulty  in  the  commutation  of  a  direct 
current  machine  is  the  self  induction  of  the  armature  coils  as  they 
individually  have  the  current  reversed  in  them  in  passing  from 
one  side  of  the  brush  to  the  other.    Each  coil  has  a  local  magnetic 
field  around  itself,  set  up  by  current  in  itself  and  its  c-sighboring 
coils.    The  value  of  this  local  magnetic  field  depends  upon  the 
arrangement  of  the  winding,  the  disposition  of  the  magnetic 
structure  around  the  coil,  the  ampere  turns,  etc.     During  the 
act  of  commutation,  that  part  of  the  local  field  due  to  the  coil 
which  is  being  commutated  must  be  reversed  in  direction.  It  is 
therefore  desirable  to  make  the  local  field  due  to  any  individual 
coil  as  small  as  possible.     This  means  that  the  number  of  tisms 
per  coil  should  be  as  low  as  possible,  the  amperes  per  coil  aiso 
should  be  as  small  as  possible,  while  the  magnetic  conditions  sur- 
rounding the  coil  should  be  such  as  to  give  the  highest  reluctance. 
By  the  proper  arrangement  of  the  various  parts,  it  is  usually 
found  that  the  e.m.f-  of  self  induction,  due  to  the  reversal  of  the 
coil  passing  under  the  brush,  can  be  made  of  comparatively  small 
value  so  that,  if  no  other  conditions  interfere,  good  commutation 
could  be  obtained  under  practically  all  commercial  operating 
conditions.  t  However,  the  magnetic  field  between  the  poles  set 
up  by  the  armature  magnetomotive  force  as  a  whole,  as  described 
above,  adds  very  greatly  to  the  difficulties  of  commutation.     If 
the  armature  magnetomotive  force,  or  the  field  due  to  it,  could 
be  suppressed,  then  one  of  the  principal  limitations  in  the  design 
and  operation  of  direct-current  generators  would  be  removed, 
and  the  commutation  limits  would  be  greatly  extended.     Or, 
better  still,  if  a  magnetic  flux  in  the  reverse  direction  were  estab- 
lished at  the  point  of  commutation,  then  the  e.m.f.  set  up  by  this 
would  be  in  opposition  to  the  e-.m.f.  of  self  induction  of  the 
commutated  coil  and  would  actually  assist  in  the  commutation! 

This  latter  is  what  is  accomplished  by  interpoles.  When  these 
are  used  the  brushes  on  the  commutator  are  so  placed  that  the 
short  circuited  or  commutated  coils  are  directly  under  the  inter- 
pole.  Consequently,  the  maximum  magnetomotive  force  of  the 
armature  is  in  exact  opposition  to  that  of  the  interpoles.  There- 
fore, the  total  ampere  turns  on  the  interpoles  should  be  equal  to 
the  total  ampere  turns  on  the  armature  in  order  to  produce  zero 
magnetic  flux  under  the  interpole  or  at  the  point  of  commutation 
But,  for  best  conditions  there  should  not  be  zero  field,  but  a 
slight, field  in  the  opposite  direction  from  that  which  the  arma- 
ture winding  alone  would  produce.  Therefore,  the  magneto* 


COMMUTATING  POLES  ON  ROTARIES  175 

motive  force  of  the  interpole  must  be  greater  than  that  of  the 
armature  by  an  amount  sufficient  to  set  up  a  local  field  under 
the  interpole  which  will  establish  an  e.m.f .  in  the  short  circuited 
coils  opposite  to  that  set  up  by  the  commutated  coils  themselves 
and  practically  "equal  to  it.  The  excess  ampere  turns  required 
on  the  commutated  poles  is  therefore  for  magnetizing  purposes 
only  and  the  amount  of  extra  ampere  turns  will  depend  upon  the 
value  of  the  commutating  field  required,  depth  of  air-gap  under 
the  commutating  pole,  etc.  The  commutating  field  required  is 
obviously  a  function  of  the  self  induction  of  the  commutated  coil 
and  evidently  the  lower  the  self  induction  the  less  commutating 
field  will  be  required.  It  is  evident  therefore  that  the  commu- 
tating field  under  the  commutating  pole  bears  no  fixed  relation 
to  the  armature  ampere  turns  or  to  the  main  field  ampere  turns, 
but  is,  to  a  certain  extent,  dependent  upon  the  proportions  of 
each  individual  machine. 

It  is  evident  that  the  magnetomotive  force  of  a  given  arma- 
ture varies  directly  with  the  current  delivered,  regardless  of  the 
voltage.  Therefore,  that  part  of  the  interpole  magnetomotive 
force  which  neutralizes  that  of  the  armature  should  also  vary 
directly  in  proportion  to  the  armature  current.  Also,  the  self 
induction  of  the  commutated  coils  will  vary  in  proportion  to  the 
armature  current  carried,  and  therefore  the  magnetic  field  under 
the  interpole  for  neutralizing  this  self  induction  should  also 
vary  in  proportion  to  the  armature  current.  It  is  therefore 
obvious  that  if  the  main  armature  current  be  put  through  the 
interpole  winding,  the  magnetomotive  force  of  this  winding  will 
vary  in  the  proper  proportion  to  give  correct  commutating  con- 
ditions as  the  armature  current  varies,  regardless  of  the  voltage 
of  the  machine.  This  is  on  the  assumption  that  the  entire 
magnetomotive  force  of  the  interpole  winding  is  effective  at  the 
air  gap  and  armature,  which  implies  an  absence  of  saturation  in 
the  interpole- magnetic  circuit.  In  the  usual  construction,  the 
interpole  winding  always  carries  the  main  armature  current 
as  indicated  above. 

One  consequence  of  the  use  of  the  interpole  is  that  somewhat 
less  regard  need  be  paid  to  keeping  the  self  induction  of  the 
commutating  coil  at  its  lowest  value.  In  consequence,  there  is 
somewhat  more  freedom  in  proportioning  the  armature  wading, 
slots,  etc.,  than  in  a  non-interpole  machine,  and  advantage  can  be 
taken  of  this  in  bettering  the  proportions  for  other  characteristics. 


176 


ELECTRICAL  ENGINEERING  PAPERS 


The  conditions  of  design  are  therefore  not  as  rigid  in  the  interpole 
as  in  the  non-interpole  type. 

The  above  description  of  the  interpole  generator  has  been  gone 
into  rather  fully,  as  many  of  the  points  mentioned  ^vill  be  re- 
ferred to  again  in  connection  with  interpoles  on  synchronous 
converters. 

The  synchronous  converter  differs  from  the  direct  current 
generator  in  one  very  important  particular,  namely,  it  may  be 
considered  as  motor  and  generator  combined.  It  receives  cur- 
rent from  a  supply  system  the  same  as  a  motor  and  it  delivers 
current  to  another  system  like  a  direct-current  generator.  The 
magnetomotive  force  of  the  armature  winding  as  a  motor  acts 
in  one  direction,  while  the  magnetomotive  force  of  the  armature 
winding  as  a  generator  acts  in  the  opposite  direction.  As  the 
input  is  practically  equal  to  the  output,  it  is  evident  that  these 
two  armature  magnetomotive  forces  should  practically  neu- 
tralize each  other,  on  the  assumption  that  the  armature  mag- 
netomotive force,  due  to  the  polyphase  current  supplied  has 
practically  the  same  distribution  as -that  of  the  corresponding 
direct-current  winding.  Assuming  that  the  two  practically 
balance  each  other,  then  it  is  evident  that  one  of  the  principal 
sources  of  commutation  difficulty  in  direct  current  generators 


-45°-750-I5° 


FIG,  2 


FIG.  3 


is  absent  in  the  converter  and  therefore  the  limits  in  commuta- 
tion should  be  much  higher  than  those  of  direct-current  ma- 
chines. 


COMMUTATING  POLES  ON  ROTARIES 


177 


The  following  diagrams  show  the  distribution  of  the  alternating- 
current  and  direct-current  magnetomotive  forces  on  a  six-phase 
rotary  converter.  The  magnetomotive  force  distribution  for 
the  alternating-current  input  is  plotted  for  several  different 
positions  of  the  armature.  Three  different  positions  are  shown 
with  the  armatures  displaced  successively  15  electrical  degrees. 
The  general  forms  of  these  distributions  repeat  themselves  for 
further  similar  displacements. 

These  distributions  are  illustrated  in  Figs.  2,  3  and  4.  It  is 
evident  from  these  three  figures  that  the  peak  value  of  the  mag- 
netomotive force  the  armature  varies  as  the  armature  is  rotated, 
as  indicated  by  the  heights  of  the  center  line  in  the  three  figures. 

In  Fig.  5,  the  magnetomotive  force  distribution  of  Fig.  2  and 
the  corresponding  direct-current  distribution  of  Fig.  1  are  both 
shown,  but  in  opposition  to  each  other.  In  this  figure  both  are 
shown  in  proper  proportion  to  each  other,  taking  into  account 
the  alternating  current  amperes  and  the  direct-current  amperes 
output.  The  resultant  of  these  two  distributions  is  also  indi- 
cated in  these  figures. 

In  Fig.  6  the  distributions  correspond  to  Figs.  3  and  1  combined 
and  the  resultant  is  also  shown. 

Fig.  7  combines  Figs.  4  and  1. 


FIG.  4 


PIG. 


It  is  the  resultant  magnetomotive  force  in  these  three  figures 
which  is  important,  as  this  is  the  effective  magnetomotive  force 
which  tends  to  produce  a  flux  or  field  over  the  commutated 


178 


ELECTRICAL  ENGINEERING  PAPERS 


coil.  It  is  evident  from  these  figures,  which  are  drawn  to  scale, 
that  this  resultant  varies  in  height  as  the  armature  is  rotated,  but 
the  maximum  is  only  a  relatively  small  per  cent  of  the  direct- 
current  magnetomotive  force.  Therefore,  it  is  obvious  that 
one  of  the  principal  sources  of  difficulty  in  the  commutation  of 
the  direct-current  generator  is  practically  absent  in  the  converter, 
and  it  is  also  evident  from  this  that  the  commutating  conditions 
in  the  latter  should  be  materially  easier  than  in  the  former. 
This  has  proved  to  be  true  by  wide  experience  in  the  construction 
and  operation  of  converters. 

In  the  above  figures  the  magnetomotive  forces  have  been 
plotted  to  scale  on  the  following  basis: 

The  six-phase  converter  winding  is  connected  to  three  trans- 
formers with  the  so-called  diametral  arrangement;  each  of  the 
three  secondaries  is  connected  across  the  diameter,  or  across 
180  deg.  points  on  the  winding,  the  three  diameters  being  dis- 
placed 60  deg.  with  respect  to  each  other.  Assuming  the  direct 
current  in  the  winding  as  A,  then  the  maximum  value  of  the 
alternating  current  in-  any  one  phase  of  the  alternating-current 
end  will  be  equal  to  f  A,  or  0.667  A,  assuming  100  per  cent 
efficiency.  However,  as  the  alternating-current  input  must  be 
somewhat  greater  than  the  direct-current  output,  due  to  certain 


losses  in  the  machine,  it  is  evident  that  the  maximum  alter- 
nating current  in  any  one  phase  must  be  somewhat  greater  than 
0.667  A.  The  field  copper  losses  may  be  considered  as  part  of 


COMMUTATING  POLES  ON  ROTARIES 


179 


the  output  of  the  rotary.  The  armature  copper  Ipsess  maybe 
considered  as  due  to  an  ohmic  drop  between  the  counter  e.m.f. 
of  the  armature  and  the  transformer  e.m.f.,  and  simply  a  higher 
transformer  e.m.f.  must  be  supplied  to  overcome  this  drop  and 
therefore  it  does  not  effect  the  true  current  input  of  the  rotary. 
However,  the  losses  due  to  rotation,  such  as  iron  loss  and.  the 
friction  and  windage  are  excess  losses  which  represent  extra 
current  which  must  be  supplied  to  the  alternating-current  end 
of  the  rotary.  These  rotational  losses  will  usually  be  relatively 
small  in  a  25-cyde  converter,  being  possibly  4  per  cent  or  5  per 
cent  in  a  small  machine  and  li  per  cent  to  3  per  cent  in  a  large 
machine.  In  the  60-cyde  converters,  where  the  iron  losses  are 
relatively  higher  and  the  speeds  are  somewhat  higher,  giving 
greater  friction  and  windage,  the  rotation  losses  may  be  con- 
siderably greater  than  on  25-cycle  machines.  Assuming  these 
rotation  losses  will  be  3  per  cent,  then  the  maximum  alternating 

rt  ftftrr  A 
current  per  phase  =     '     y  —  =  0.687  A.   The  foregoing  Figs.  5, 

6  and  7  are  worked  out  on  this  assumption  of  97  per  cent  rota- 
tional efficiency  and  on  this  basis  of  -mini-mum  value  of  the 
resultant  magnetomotive  force  of  the  armature  at  the  direct- 
current  brush  is  about  7  per  cent  of  the  direct-current  magneto^ 

motive  force  of  the  same  word- 
ing, while  the  maximum  value  is 
about  20  per  cent.  The  lower  the 
rotational  efficiency  the  smaller 
would  be  these  values,  and  with 
a  rotational  efficiency  of  about 
)  per  cent,  the  mfmrmini  result- 
ant would  fall  to  zero,  while  the 
maximum  value  would  be  about 
13  per  cent. 

The  resultant  magnetomotive 
force  of  a  synchronous  converter 
might  be  compared  with  that  of 
a  direct-current  generator  with 
compensating  windings  in  the 
pole  faces.  It  is  generally  known 
that  such  direct-current  generators  have  much  better  com- 
mutatiag  conditions  than  ordinary  uncompensated  machines. 
If  such  compensating  winding  on  the  field  of  a  direct-current 
machine  covered  symmetrically  the  whole  armature  surface. 


pIG 


180  ELECTRICAL  ENGINEERING  PAPERS 

then  the  armature  reaction  could  be  completely  annulled,  which 
is  not  the  case  in  the  converter.  But  with  compensating  wind- 
ings located  only  in  the  pole  faces,  then  the  armature  magneto- 
motive force  midway  between  the  poles  could  not  be  completely 
annulled,  unless  over-compensation  is  used,  and  the  resultant 
would  be  as  shown  in  Fig.  8,  which  is  not  quite  as  good  as  the 
average  resultant  in  the  converter.  The  commutating  con- 
ditions in  the  converter  can  therefore  be  considered  as  at  least 

In  the  application  of  interpoles  to  the  synchronous  converter 
the  same  principles  should  hold  as  in  a  direct-current  generator, 
namely,  the  interpole  magnetomotive  force  should  be  sufficient 
to  neutralize  that  of  the  armature  winding  and,  in  addition,  should 
set  up  a  small  magnetic  flux  sufficient  to  overcome  the  self  in- 
duction of  the  commutated  coil.  As  the  magnetomotive  force 
the  armature  varies  between  7  per  cent  and  20  per  cent  shown  in 
the  above  figures,  it  is  evident  that  perfect  compensation  of  this 
cannot  be  obtained  and  that  therefore  only  some  average  value 
can  be  applied.  Assuming  that  15  per  cent  will  be  required  on 
the  average  to  compensate  for  this,  then  in  addition  the  inter- 
pole  winding  must  carry  ampere  turns  sufficient  to  set  up  the 
small  magnetic  field  for  commutation.  Thus  the  total  ampere 
turns  on  the  interpole  will  be  equal  to  15  per  cent  of  the  armature 
direct-current  ampere  turns  plus  a  small  addition  for  setting  up 
the  useful  or  commutating  field.  In  the  direct-current  gen- 
erator, the  ampere  turns  on  the  interpoles  must  equal  the  total 
armature  ampere  turns  plus  a  corresponding  addition  for  the 
commutating  field.  It  is  therefore  evident  that  an  interpole 
winding  on  a  converter  will  naturally  be  very  much  smaller  than 
on  a  direct-current  generator,  and  in  general  it  is  between  25  per 
cent  and  40  per  cent  of  the  direct-current. 

In  the  pulsating  resultant  magnetomotive  force  in  the  con- 
verter there  lies  one  possible  source  of  trouble  with  interpoles 
Assume,  for  example,  the  total  ampere  turns  on  the  interpoles 
are  equal  to  30  per  cent  of  the  direct-current  ampere  turns  on  the 
•  rotary  and  that  15  per  cent  of  this  is  for  overcoming  the  average 
value  of  the  resultant  magnetomotive  force,  then  an  average 
of  15  per  cent  will  be  available  for  setting  up  a  commutating 
field;  but,  according  to  the  above  diagrams,  the  resultant  mag- 
netomotive force  of  the  armature  varies  from  7  per  cent  to  20  per 
cent.  With  a  total  interpole  winding  representing  30  per  cent, 


COMMUTATING  POLES  ON  ROTARIES  181 

then  the  effective  or  magnetizing  part  will  vary  from  30-7 
to  30-20;  that  is,  from  23  per  cent  to  10  per  cent.  The  effective 
magnetomotive  force  therefore  tends  to  vary  over  quite  a  wide 
range  so  that  the  commutating  field  would  also  tend  to  vary  up 
or  down  over  a  velry  considerable  range,  which  is  an  undesirable 
thing  for  commutation.  However,  as  this  pulsation  is  at  a 
fairly  high  frequency  it  tends  to  damp  itself  out  by  setting  up 
eddy  currents  in  the  structure  of  the  magnetic  circuit.  If  a 
good  conducting  damper  or  closed  circuit  were  placed  around  the 
interpole,  it  is  probable  that  this  pulsation  would  be  almost 
completely  eliminated,  but  such  a  damper  possesses  certain  dis- 
advantages, as  will  be  shown  later. 

In  practice  this  pulsation  of  the  armature  reaction  under 
the  interpoles  is  apparently  not  noticeably  harmful  in  most 
cases,  as  evidenced  by  the  fact  that  well-proportioned  interpole 
converters  in  commercial  service  show  no  undue  trouble  at  the 
commutator  or  brushes, 

Due  to  the  relatively  small  number  of  ampere  turns  required 
on  the  interpole  of  a  converter  compared  with  those  required  on 
a  direct-current  generator,  the  design  of  the  interpoles  in  the  two 
cases  presents  quite  different  problems.  In  the  direct-current 
generator  the  interpoles  carry  ampere  turns,  which  in  all  cases 
are  greater  than  the  armature  ampere  turns,  as  explained  before. 
As  the  field  ampere  turns  on  the  main  poles  are,  not  infrequently, 
but  little  greater  than  the  armature  ampere  turns,  it  is  evident 
that  the  interpole  winding  may,  in  some  cases,  carry  as  many 
ampere  turns  as  the  main  field  windings.  While  but  a  small 
per  cent  of  these  interpole  windings  is  effective  in  producing 
flux  under  the  pole  tip,  yet  they  are  all  effective  in  producing 
leakage  from  the  sides  of  the  poles.  As  the  interpoles  are  gen- 
erally small  in  section  compared  with  the  main  poles,  and  as 
they  may  carry  ampere  turns  equal  to  the  main  poles,  it  is 
evident  that  the  effect  of  leakage  may  be  relatively  great  on  the 
interpole. 

For  instance,  if  the  leakage  on  the  main  poles  is  15  per  cent  of 
the  useful  flux,  then,  with  the  same  total  leakage  on  the  inter- 
poles, this  may  represent  a  very  high 'value  compared  with  the 
useful  flux,  due  to  the  small  section  of  the  interpole  and  the 
relatively  low  useful  interpole  flux.  In  consequence,  it  is  con- 
siderable of  a  problem  to  proportion  the  interpoles  of  a  direct 
current  generator  so  that  the  leakage  flux  will  aot  saturate  the 
interpoles  at  some  part  of  the  circuit.  If  they  saturate,  then 


182  ELECTRICAL  ENGINEERING  PAPERS 

part  of  the  ampere  turns  on  the  interpole  are  expended  in  such 
saturation  and  the  part  thus  expended  must  be  counted  off  from 
the  extra  or  excess  interpole  ampere  turns.  If,  for  example, 
the  interpole  winding  requires  100  per  cent  for  overcoming  the 
armature  and  there  is  20  per  cent  extra  ampere  turns  for  setting 
up  a  useful  flux,  then  any  saturation  in  the  interpole  circuit  must 
represent  additional  ampere  turns  on  the  field,  as  the  above 
120  per  cent  is  necessary  for  useful  flux  and  for  neutralizing  the 
armature.  With  reduced  current,  and  consequent  lower  satura- 
tion, these  additional  interpole  turns  become  effective  in  mag- 
netizing the  gap  and  thus  the  commutating  flux  is  too  strong. 
At  greatly  increased  load,  more  ampere  turns  are  required  for 
saturation,  and  the  commutating  flux  is  altogether  too  weak. 
It  is  thus  evident  that  a  machine  with  highly  saturated  inter- 
poles  will  not  comtnutate  equally  well  for  all  loads.  Herein 
lies  a  problem  in  the  design  of  interpole  generators,  as  it  is 
difficult  to  maintain  a  relatively  low  saturation  in  the  interpoles 
due  to  their  small  section  and  high  ampere  turns  which  cause 
leakage.  It  is  well  known  that  in  the  main  poles  of  the  generator, 
a  leakage  flux  which  is  higher  than  the  useful  flux  is  objection- 
able, from  the  designer's  standpoint;  and  yet  in  the  use  of  inter- 
poles  this  is  a  normal  condition  rather  than  an  exception. 

In  the  synchronous  converter  the  conditions  are  somewhat 
different  due  to  the  fact  that  the  interpole  ampere  turns  are 
usually  only  25  per  cent  to  40  per  cent  as  great  as  on  a  correspond* 
ing  direct-current  generator.  The  leakage  at  the  -sides  of  the 
poles  becomes  relatively  much  less,  while  the  usefrjl  induction 
remains  about  the  same  as  on  the  direct-current  generator.  In 
consequence,  saturation  of  the  poles  is  not  so  difficult  to  avoid. 
Irl  some  cases,  due  to  the  smaller  ampere  turns  on  the  interpole 
winding,  the  interpole  coils  can  be  located  nearer  the  pole  tip 
and  thus  the  leakage  can  be  further  reduced.  However,  the 
placing  of  the  interpole  coil  over  the  whole  length  of  the  pole 
is  not  as  objectionable  in  the  converter  interpole  as  -it  is  on  the 
direct-current  generator  as  the  ampere  turns  are  less.  It  is  those 
ampere  turns  which  are  located  close  to  the  yoke,  or  furthest 
away  from  the  pole  tip,  which  produce  the  highest  leakage,  while 
those  close  to  the  pole  tip  usually  produce  much  less  leakage, 
but  in  interpole  generators  with  their  high  number  of  ampere 
turns  on  the  interpoles  it  is  often  difficult  to  find  space  for  the 
iaterpole  winding,  even  if  distributed  over  the  whole  pole  length. 
In  some  casess  a  direct-cuirent  machine  may  be  larger  than 


COMMUTATING  POLES  ON  ROTARIES  183 

would  otherwise  be  zequired,  simply  to  obtain  space  for  the  inter- 
pole  winding.  This  is  not  true' to  the  same  extent  in  the  appli- 
cation of  interpoles  to  converters. 

In  the  above  the  leakage  is  referred  to  as  a  function  of  the 
interpole  winding  as  if  the  main  winding  had  little  or  nothing 
to  do  with  it.  The  reason  for  this  nxay  be  given  as  follows: 

Fig.  9  represents  two  main  poles  and  an  interpole  of  a  direct- 
current  generator  or  converter,  with  their  windings  in  place. 
The  direction  of  current  or  polarity  of  each  side  of  each  coil  is 
also  indicated  by  +  or  — .  It  is  evident  that  between  the  inter- 
pole and  one  main  pole,  the  interpole  winding  and  the  main 
field  winding  are  of  the  same  polarity,  while  on  the  opposite  side 
of  the  interpole,  these  two  windings  are  in  opposition.  Let  A 
equal  the  ampere  turns  of  the  interpole  and  B  the  ampere  turns 
in  the  main  coil.  Then,  A+B  will  represent  the  leakage  ampere 
turns  at  one  side  of  the  interpole  and  A  -  B  will  represent  the 
leakage  ampere  turns  at  the  other  side.  Therefore,  the  leakage 
at  the  two  sides  of  the  poles  is  represented  by  (A  +B)  -f  (A  -  B) 
=  2A;  that  is,  the  leakage  could  be  considered  as  due  to  the 
interpole  winding  entirely  and  may  also  be  considered  as  due  to 
double  the  interpole  turns  acting  as  one  side  of  the  interpole 
only.  Another  way  of  looking  at  this  is  to  consider  that  the 
windings  on  the  main  pole  produce  leakage  in'  the  interpoles, 
but  the  leakage  due  to  one  main  pole  acts  radially  in,  one  direc- 
tion in  the  interpole,  while  that  due  to  the  other  main  pole  is 
in  the  opposite  direction. 

Considering  therefore  the  interpole  leakage  as  being  due  to 
the  interpole  ampere  turns  only,  it  is  evident  that  the  syn- 

chronous  converter  will  not  be 
troubled  with  saturation  of  the  in- 
terpoles to  the  same  extent  as  a 
direct-current  generator.  With  the 
same  size  of  interpole  it  is  evident 
that  tke  converter  should  be  able 
to  carry  heavier  overloads  than  the 

direct-current  generator  before  saturation  of  the  interpoles  is 
reached. 

It  was  mentioned  before  that  a  closed  conducting  circuit 
around  the  interpoles  would  be  objectionable.  This  has  be$n 
proved  by  experience  with  irtterpole  generators.  It  is  evident 
from  the  preceding  analysis  that  the  ampere  turns  on  the  inter- 
pole of  a  direct-current  generator  should  always  rise  or  fall  in 


184  ELECTRICAL  ENGINEERING  PAPERS 

proportion  to  the  armature  ampere  turns  in  order  to  give  best 
commutation,  assuming,  of  course,  no  saturation  of  the  poles. 
If  the  interpole  turns  are  directly  in  series  with  the  armature 
winding,  with  no  shunt  across  the  interpole  winding,  it  is  evident 
that  the  interpole  ampere  turns  must  vary  in  direct  proportion 
to  the  armature  ampere  turns.  However,  if  a  non-inductive 
shunt,  for  instance,  were  connected  across  the  interpole  winding 
in  order  to  shunt  part  of  the  current,  then  in  the  event  of  a  sudden 
change  in  load,  the  interpole  winding  being  inductive  due  to  its 
iron  core  and  the  shunt  being  non-inductive,  the  momentary 
division  of  current  during  a  change  in  load  would  not  be  the  same 
as  under  steady  conditions.  In  other  words,  if  the  armature 
and  interpole  current  were  suddenly  increased,  then  a  large  part 
of  the  increase  would  momentarily  pass  through  the  non-induc- 
tive interpole  shunt  until  steady  conditions  were  again  attained. 
In  consequence,  the  interpole  ampere  turns  would  not  increase 
in  proportion  to  the  armature  ampere  turns  just  at  the  critical 
time  when  the  proper  commutating  field  should  be  obtained. 

The  same  condition  is  approximated  when  a  separate  con- 
ducting circuit  is  closed  around  the  interpole.  A  sudden  change 
in  the  current  in  the  interpole  winding,  causes  a  change  in  the 
flux,  and  secondary  currents  are  set  up  in  the  closed  circuit, 
which  always  act  in  such  a  way  as  to  oppose  any  change  in  the 
flux,  whereas,  the  flux  in  reality  should  change  directly  with  the 
current.  The  above  described  non-inductive  shunt  across  the 
interpole  winding  might  be  considered  also  as  completing  a 
closed  circuit  with  the  interpole  winding,  and  therefore  retarding 
secondary  currents  would  be  set  up  in  this  closed  circuit  with  any 
change  in  the  flux  in  the  interpole. 

In  some  cases  it  may  be  impracticable  to  get  exactly  the  right 
number  of  turns  on  the  interpole  winding  to  give  the  correct 
interpole  magnetomotive  force.  For  example,  on  a  heavy 
current  machine,  1.8  turns  carrying  full  current  might  be  re- 
quired on  each  interpole.  If  two  turns  were  used,  with  the 
extra  current  shunted,  the  right  interpole  strength  would  be 
obtained.  A  non-inductive  shunt,  however,  is  bad,  as  shown 
above.  However,  if  an  inductive  shunt  is  used,  instead  of  non- 
inductive,  and  the  reactance  in  this  shunt  circuit  is  properly 
adjusted,  then  it  is  possible  to  get  the  right  interpole  strength  for 
normal  conditions  and  still  obtain  satisfactory  conditions  with 
sudden  changes  in  load.  Also,  by  arranging  the  interpole 
winding  so  that  a  very  considerable  percentage  of  the  current 


COMMUTATING  POLES  ON  ROTARIES  185 

is  shunted  normally  by  an  inductive  shunt  having  a  relatively 
high  reactance  compared  with  the  interpole,  it  should  be  possible 
to  force  an  excess  current  through  the  interpole  winding  in  case 
of  a  sudden  increase  in  load,  in  case  a  stronger  commutating 
field  were  needed  at  this  instant. 

On  the  interpole  synchronous  converter  a  non-inductive  shunt 
across  the  interpole  winding  should  act  very  much  as  on  an  inter- 
pole generator  and  therefore  non-inductive  shunts  are  inad- 
visable. If  any  shunting  is  required  it  should  be  by  means  of  an 
inductive  shunt  in  those  cases  where  the  current  from  the  con- 
verter is  liable  to  sudden  fluctuations,  as  in  railway  service. 
"Where  the  service  is  practically  steady,  a  non-inductive  shunt 
should  prove  satisfactory  for  the  interpoles  of  converters  or 
direct-current  generators. 

Under  extreme  conditions  of  overload  current,  that  is,  in 
case  of  a  short  circuit  across  the  terminals,  it  is  questionable  to 
what  extent  interpoles  are  effective.  It  is  practicable  to  design 
interpoles  on  direcUcurrent  generators  which  will  not  unduly 
saturate  up  to  possibly  three  or  four  times  normal  load.  How- 
ever, in  case  of  a  sudden  short  circuit  the  current  delivered  by 
the  machine  is  liable  momentarily  to  rise  to  a  value  anywhere 
from  15  to  30  times  full  load  current.  With  this  excessive  cur- 
rent the  interpoles  of  the  direct-current  generator  must  -neces- 
sarily be  more  or  less  ineffective.  On  account  of  saturation,  the 
commutating  flux  under  the  interpole  cannot  rise  in  proportion 
to  the  current.  However,  there  should  still  be  some  commutating 
field  present,  which  condition  is  probably  Considerably  better 
than  no  field  at  all,  or  a  strong  field  in  the  opposite  direction 
as  would  be  found  without  commutating  poles.  Therefore,  in 
direct-current  generators  with  well-proportioned  interpoles, 
the  conditions  on  short  circuit  are  generally  less  severe  than  in 
non-interpole  machines. 

If  the  pole  is  highly  saturated  by  the  heavy  current  rush  on 
short  circuit,  then  it  is  evident  that  a  highly  inductive  shunt, 
as  described  above,  which  would  increase  the  interpole  current 
in  a  greater  proportion  than  the  armature  current,  would  simply 
mean  higher  saturation  with  little  or  no  increase  in  the  useful 
flux  tinder  the  interpole. 

In  the  synchronous  converter  at  short  circuit  the  conditions 
may  be  somewhat  different.  When  the  converter  is  short  cir- 
cuited it  can  also  give  extremely  high  currents,  possibly  touch 
greater  than  the  corresponding  direct-current  generator  can  give. 


186  ELECTRICAL  ENGINEERING  PAPERS 

Both,  the  armature  winding  tied  to  an  alternating-current  supply 
system,  and  the  presence  of  the  low  resistance  dampers  on  the 
field  magnetic  circuit,  tend  to  make  the  short  circuit  conditions 
more  severe  in  the  converter.  The  worst  condition,  however, 
would  appear  to  be  in  the  relation  of  the  interpole  ampere  turns 
to  the  armature  ampere  turns  on  short  circuit.  As  shown  before, 
the  normal  ampere  turns  on  the  interpole  winding  will  be  only 
25  per  cent  to  40  per  cent  of  the  direct-current  ampere  turns  on 
the  armature.  In  the  case  of  a  sudden  short  circuit  the  armature 
momentarily  may  deliver  a  very  considerable  current  as  a  direct- 
current  generator,  and  the  armature  reaction,  or  the  resultant 
magnetomotive  force,  may  approach  that  of  a  direct-current 
generator.  In  such  case  the  ampere  turns  on  the  interpole  will 
be  very  much  smaller  than  the*  armature  resultant  magneto- 
motive force  at  this  instant  and  thus  there  will  be  no  commu- 
tating  flux  under  the  interpole,  but,  on  the  contrary,  the  arma- 
ture being  stronger,  there  will  be  a  reverse  flux  which  may  be 
considerably  higher  than  if  no  interpole  were  present,  as  the  iron 
of  the  interpole  represents  an  improved  magnetic  path  for  such 
flux.  While  the  converter  armature  will  probably  never  deliver 
all  its  energy  as  a  direct-current  generator  at  the  instant  of  short 
circuit,  yet  it  may  be  assumed  that  it  will  deliver  some  of  its 
load  thus,  and  it  does  not  require  a  very  large  per  cent  to  be 
generator  action  in  order  to  neutralize,  or  even  reverse  the  effect 
of -the  interpoles.  In  consequence,  on  a  short  circuit  the  con- 
verter may  have  a  reverse  field  under  the  commutating  pole, 
while  the  direct-current  generator  under  the  same  condition 
will  have  a  field  of  the  proper  direction  but  of  insufficient  strength 
which,  however,  is  a  much  better  condition  than  a  field  of  the 
wrong  polarity. 

The  inductive  shunt  mentioned  before,  which  normally  shunts 
a  considerable  portion  of  the  interpole  current,  might  be  more 
effective  in  a  converter  than  in  a  direct-current  generator  in  the 
case  of  a  short  circuit.  In  a  direct-current  generator,  the  inter- 
poles  would  be  so  highly  saturated,  as  described  before,  that  the 
increase  in  current  in  the  interpole  winding  due  to  the  inductive 
shunt  would  be  relatively  ineffective.  In  the  converter,  how- 
ever, the  saturation  of  the  interpole  can  normally  be  very  much 
lower  than  in  the  direct-current  generator  and  it  might  be 
practicable  to  so  proportion  these  interpoles  that  they  do  not 
saturate  highly,  even  on  short  circuit.  In  consequence,  a 
strong  inductive  shunt  might  force  up  the  interpole  ampere 


COMMUTATING  POLES  ON  ROT  ARIES  187 

turns  so  that  the  negative  field  under  the  interpole  would  be 
much  decreased,  or  might  even  be  changed  to  a  positive  field 
and  thus  become  useful  in  commutation.  This  would  be  helpful 
only  during  the  short  circuit.  However,  converters  not  infre- 
quently flash  over  or  "  buck "  when  the  circuit  breaker  is 
opened  on  a  very  heavy  overload  or  a  short  circuit  and  not 
when  the  first  rush  of  current  occurs.  If  the  flash  tends  to 
occur  at  the  opening  of  the  circuit,  then  the  above  mentioned 
inductive  shunt  might  have  just  the  opposite  effect  from  what  is 
desired,  for  it  would  tend  to  develop  or  maintain  a  stronger 

field  under  the  interpole  after  the  armature  reaction  is  removed. 
In  consequence,  the  heavy  inductive  shunt  might  prove  harmful 
in  such  a  case. 

Another  condition  exists  in  a  converter  which  does  not  exist 
in  a  generator*  When  a  short  circuit  occurs  on  a  direct-current 
generator,  the  armature  reaction  tends  to  distort  the  main  field 
very  greatly — so  much  so  that  the  field  of  the  machine  is  very 
greatly  weakened.  This  decreases  the  terminal  voltage  and 
the  resultant  decrease  in  the  shunt  excitation  will  still  further 
tend  to  weaken  the  field.  In  consequence,  the  machine  tends 
to  "  kill  "  its  magnetic  field  and  the  voltage  tends  to  drop  to  a 
low  value.  Therefore,  when  the  breaker  opens  on  a  short  circuit 
the  direct-current  voltage  may  be  falling  rapidly.  When  the 
armature  current  is  removed  from  the  machine  the  voltage  may 
rise  slowly,  depending  upon  the  natural  rate  of  building  up  the 
field.  Consequently,  after  the  breaker  opens  there  is  little  or  no 
tendency  to  flash,  and  practically  all  difficulties  occur  dttting  the 
current  rush,  before  the  breaker  oftens.  In  a  converter,  how- 
ever, the  conditions  are  different.  The  armature  of  the  con- 
verter is  tied  to  an  alternating-current  supply  system  which 
tends  to  maintain  the  voltage  on  the  converter.  The  machine 
cannot  "  kill  "  its  field  in  the  same  way  as  the  direct-current 
generator,  for  the  alternating-current  system  tends  to  maintain 
the  field  by  corrective  currents  which  act  in  such  a  way  as  to  tend 
to  hold  up  the  voltage.  An  enormous  current  may  be  drawn  from 
the  alternating-current  system  momentarily  in  case  of  a  short 
circuit  on  the  direct-current  side  of  the  converter.  This  heavy 
alternating  current  may  cause  a  drop  in  the  alternating-current 
lines,  step-down  transformers,  etc.,  so  that  the  supply  voltage 
does  fall  very  considerably  and  the  direct-current  voltage  does 
drop  materially  in  case  of  a  short  circuit.  However,  the  instant 
the  short  circuit  is  removed  by  opening  the  breaker,  then  the 


188  ELECTRICAL  ENGINEERING  PAPERS 

converter  at  once  tends  to  attain  full  voltage  as  the  alternating- 
current  supply  system  tends  to  bring  the  armature  up  quickly 
to  normal  voltage  conditions.  In  consequence  there  may  be  a 
relatively  heavy  current  flow  in  the  alternating-current  side  of 
the  machine,  while  there  is  no  direct-current  flow  in  the  armature 
Part  of  this  alternating-current  flow  represents  energy  in  bringing 
the  machine  back  to  a  normal  condition,  and  part  is  purely  mag- 
netizing or  wattless  current.  The  energy  component  tends  to 
produce  an  armature  magnetomotive  ibrce  giving  an  active  field 
at  the  point  of  commutation.  This  energy  component  alter- 
nating-current flow,  however,  cannot  be  corrected  by  inter- 
poles,  as  there  is  no  direct  current  flowing. 

A  further  difference  between  the  synchronous  converter  and 
the  direct-current  generator,  in  case  of  a  short  circuit,  lies  in  the 
results  of  field  distortion.  The  enormous  short  circuit  current 
from  the  converter  with  the  armature  acting  partly  as  a  direct- 
current  generator,  may  very  greatly  shift  or  distort  the  field 
flux.  The  dampers  on  the  field  poles  tend  to  delay  this  distor- 
tion. Also,  after  distortion  has  occurred  they  tend  to  maintain 
the  distorted  or  shifted  field  so  that  momentarily  after  the  circuit 
breaker  opens  the  converter  may  be  operating  without  direct- 
current  load  but  with  a  very  badly  distorted  or  shifted  field. 
This  also  tends  to  produce  sparking  or  flashing  after  the  direct- 
current  breaker  has  opened. 

Another  condition  which  may  affect  the  action  of  interpoles 
on  converters,  but  which  does  not  occur  in  direct-current  gen- 
erators, is  hunting.  When  Jaunting  occurs  in  a  converter  the 
energy  current  delivered  to  the  alternating-current  side  of  the 
converter  pulsates,  or  varies  up  and  down  over  a  certain  range, 
which  may  be  either  large  or  small.  At  the  same  time  the  direct- 
current  flow  is  apparently  varied  but  little.  In  consequence, 
the  resultant  magnetizing  effects  of  the  alternating  current  and 
direct  current  do  not  nearly  neutralize  each  other  at  all  times. 
When  the  alternating-current  energy  input  is  least  the  converter 
delivers  part  of  its  direct-current  load  as  a  generator,  the  stored 
energy  in  the  rotating  armature  being  partly  given  up  to  supply- 
ing the  direct-current  power.  In  this  case  the  resultant  magneto- 
motive force  may  be  a  very  considerable  per  cent,  of  the  maxi- 
mum direct-current  magnetomotive  force  of  the  armature  wind- 
ing. Also,  the  magnetic  field  under  the  main  poles  is  distorted 
or  shifted  toward  one  pole  edge.  The  armature  necessarily 
slows  down  during  this  operation,  the  field  polarity  of  all 


COMMUTATING  POLES  ON  ROTARIES  189 

the  poles  being  shifted  toward  one  pole  edge.  The  position 
'of  maximum  e.m.f .  of  the  alternating-current  end  and  also  the 
position  of  maximum  alternating-current  flow  may  be  shifted 
to  a  certain  extent  also.  In  consequence,  the  magnetomotive 
force  due  to  the  alternating-current  flow  will  be  shifted  cir- 
cumferentially  a  certain  amount,  while  the  direct-current  mag- 
netomotive force  cannot  be  shifted,  being  fixed  in  position  by 
the  brushes.  In  consequence,  the  alternating-current  magneto- 
motive force  may  not  be  in  direct  opposition  to  the  direct-current 
at  this  instant,  and  the  resultant  magnetomotive  force  may  be 
much  higher  than  at  normal  condition.  A  moment  later  the 
swing  may  be  in  the  opposite  direction;  that  is,  the  alternating 
armature  current  may  be  greater  than  direct  current  and  the 
energy  being  received  from  the  alternating-current  system  is 
considerably  greater  than  is  given  out  by  the  direct  current* 
Again,  the  two  magnetomotive-forces  will  not  nearly  neutralize 
each  other  and  there  also  will  be  field  distortion,  but  in  the 
opposite  direction,  and  again,  the  two  magnetomotive  forces 
will  not  be  in  direct  opposition  to  each  other  circumferentially. 
If  hunting  is  very  severe,  the  resultant  magnetomotive  force  of 
the  armature  due  to  the  inequality  of  the  input  and  output,  and 
to  the  circumferential  shifting  of  the  magnetomotive  forces 
with  respect  to  each  other,  may  vary  enormously  and  may  pass 
from  positive  to  negative  values  periodically.  It  is  evident  that 
under  such  condition  the  presence  of  an  interpole  may  give  much 
worse  results  than  if  no  interpole  were  present;  for,  as  mentioned 
before,  if  there  is  a  magnetomotive  force  in  the  wrong  direction 
at  the  interpole,  the  interpole  magnetic  circuit  apparently  makes 
conditions  worse.  In  consequence,  an  interpole  synchronous 
converter  should  be  especially  well  designed  to  avoid  hunting* 

All  of  the  above  considerations  have  taken  into  account  only 
the  energy  currents  delivered  to  the  alternating-current  side  of 
the  converter.  Some  consideration  should  be  given  to  the  effect 
of  wattless  currents  in  connection  with  interpoles. 

As  is  well  known,  when  a  synchronous  converter  has  its  field 
strength  improperly  adjusted  for  the  required  alternating-current 
counter  e.m.f.,  alternating  currents  will  flow  in  the  armature  in 
such  a  way  as  to  correct  the  effect  of  the  improper  field  strength; 
that  is,  if  the  field  is  too  weak  wattless  currents  will  flow  in  the 
armature  which  tend  to  magnetize  the  field  of  the  converter. 
These  currents  will  be  leading  in  the  armature,  but  will  be  lagging 
with  respect  to  the  Hue.  On  the  other  hand,  if  the  converter 


190  ELECTRICAL  ENGINEERING  PAPERS 

field  is  too  strong,  these  wattless  or  corrective  currents  will  tend 
to  weaken  the  field  and  will  lag  with  respect  to  the  armature,  but 
will  lead  with  respect  to  the  line.  These  corrective  currents 
will  have  a  lead  or  lag  of  90  deg.  with  respect  to  the  energy 
currents.  Their  magnetomotive  forces  also  will  have  a  lead  or 
lag  of  90  deg.  from  the  magnetomotive  force  of  the  energy  com- 
ponent of  alternating-current  input.  As  this  latter  practically 
coincides  with  the  direct-current  magnetomotive  force,  which  is 
midway  between  the  main  poles,  the  corrective  armatur.e  cur- 
rents will  have  a  maximum  magnetomotive  force  practically 
under  the  middle  of  the  main  poles  and  therefore  become  purely 
magnetizing  or  demagnetizing  due  to  such  position*  Also, 
being  at  right  angles  to  the  energy  component,  the  magneto- 
motive forces  of  the  corrective  currents  will  have  zero  value 
where  the  energy  component  has  maximum,  and  therefore 
should  have  no  direct  effect  upon  the  resultant  magnetomotive 
force  midway  between  the  main  poles,  or  under  the  interpoles 
if  such  are  used.  It  might  be  assumed  therefore  that  the  usual 
wattless  or  corrective  currents,  which  the  converter  may  carry 
on  account  of  improper  field  strength,  will  have  no  direct  harmful 
effect  on  the  commutation-  However,  there  ar6  apparently 
some  indirect  effects  due  to  this  corrective  current,  for  when  a 
converter  is  operated  at  a  bad  power-factor,  either  leading  or 
lagging,  there  is  generally  more  trouble  at  the  commutator  and 
brushes  than  when  a  high  power-factor  is  maintained. 

It  has  been  shown  that  the  maximum  possible  benefit  to  be 
derived  from  interpoles  in  neutralizing  armature  reaction  is  much 
less  in  synchronous  converters  than  in  direct-current  generators. 
In  direct-current  generators  and  motors  interpoles  have  also  been 
oi  great  advantage,  due  to  variable  speed  and  variable  voltage 
requirements-  In  synchronous  converters,  however,  the  re- 
quirement of  variable  speed  is  obviously  absent  and  that  of 
variable  voltage  very  limited.  The  converter  has  constant 
voltage  characteristics  and  variable  voltage  can  only  be  obtained 
through  the  agency  of  such  relatively  expensive  devices  as  in- 
duction regulators,  synchronous  boosters  or  split-pole  construc- 
tions. The  advantages  of  interpoles  in  synchronous  converters 
are  then  to  be  looked  for  only  in  the  direction  of  increased  -outputs 
and  higher  speeds. 


COMMUTATING  POLES  ON  ROTARIES  191 

DISCUSSION 

Some  question  has  been  raised  this  evening  regarding  the 
statements  in  the  paper  that  in  case  of  sudden  overload  or  short 
circuit  the  alternating-current  and  direct-current  magneto- 
motive forces  will  not  balance  each  other  and  that  the  machine 
will  operate  momentarily  as  a  direct-current  generator,  with  a 
correspondingly  high  armature  reaction.  The  basis  of  the 
criticism  is  that  the  converter,  being  a  synchronous  machine, 
cannot  change  its  speed  except  for  a  very  short  period,  namely, 
that  occurring  within  a  small  fraction  of  one  cycle,  otherwise 
the  machine  would  fall  out  of  step.  For  such  a  small  change  in 
speed,  it  Was  argued,  very  little  energy  could  be  given  up  as  a 
direct-current  generator,  as  there  is  not  enough  stored  energy  in 
the  converter  armature  to  giv0  up  much  energy  as  a  direct-cur- 
rent generator  without  falling  OUT;  of  step. 

At  first  thought,  such  an  argument  seemed  reasonable,  but 
one  answer  to  it  is  found  in  the  operation  of  a  synchronous  con- 
verter on  a  single-phase  circuit.  In  such  operation  the  energy 
supplied  to  the  alternating  current  end  falls  to  zero  twice  in  each 
cycle,  while  the  direct-current  output  remains  practically  constant. 
The  alternating-current  input  must  therefore  vary  from  zero 
to  far  above  the  direct-current  output  of  the  machine.  The 
converter  must  therefore  act  as  a  direct-current  generator,  for  a 
brief  period,  twice  during  each  cycle.  When  it  is  considered 
that  such  a  converter  can  operate  with  more  or  less  spkrking  up 
to  three  or  four  times  full-load  current,  or  even  much  more, 
depending  upon  the  design  of  the  machine,  it  is  obvious  that  the 
converter  can  deliver  very  heavy  outputs  momentarily  as  a 
direct-current  machine  without  falling  out  of  step. 

Also,  a  little  calculation  will  show  that  with  an  ordinary  design 
of  synchronous  converter  the  stored  energy  in  the  armature  is 
such  that,  in  dropping  back  as  much  as  45  electrical  degrees  in 
position,  the  armature  could  give  up  an  enormous  energy  com- 
pared with  its  normal  rated  capacity.  If  it  were  not  for  this  it 
would  not  be  possible  to  run  the  machine  on  heavy  load  on  a 
single-phase  circuit. 

That  is  all  I  will  say  in  regard  to  the  points  brought  up  in  the 
discussion.  However,  there  are  several  points  I  want  to  bring-, 
out  in  connection  with  the  paper  itself.  In  the  first  part  of  the 
paper  it  is  stated  that  the  ampere  turns  on  the  interpoles  of  a 
direct-current  generator  are  always  greater  than  the  ampere 


192  ELECTRICAL  ENGINEERING  PAPERS 

turns  of  the  armature  winding.  This  statement  is  not  correct  in 
all  cases  but  in  those  arrangements  which  depart  from  this  rule, 
direct-current  generators  and  converters  would  be  affected  in 
the  same  way  so  that  for  comparative  purposes  the  statement 
in  the  paper  may  be  considered  as  correct. 

When  there  are  as  many  interpoles  as  there  are  main  poles  it  is 
correct  to  say  that  the  ampere  turns  on  the  interpoles  should 
always  be  greater  than  on  the  armature.  However,  in  some 
cases,  especially  on  small  machines,  the  number  of  interpoles  on 
direct-current  machines  is  made  only  half  as  great  as  the  number 
of  main  poles.  There  are  several  advantages  in  this  arrange- 
ment and  they  apply  equally  well  to  generators  and  synchronous 
converters.  Obviously  where  only  half  as  many  interpoles  are 
used  the  commutating  flux  or  field  of  each  interpole  must  be  at 
least  twice  as  strong  as  when  the  full  number  of  interpoles  is  used, 
as  the  opposing  e.m  f .  set  up  by  the  interpoles  must  be  sufficient 
to  overcome  the  e  m.f .  of  self-induction,  regardless  of  the  number 
of  interpoles.  This  opposing  e.m.f.  need  not  be  distributed 
over  the  whole  armature  coil,  but  could  be  located  over  either 
side  of  the  commutated  coils  or  even  along  a  short  portion  of 
its  length.  It  is  only  necessary  that  this  opposing  e.m  f .  should 
have  the  proper  value,  while  the  distribution  of  it  seems  to  be  of 
relatively  less  importance.  It  should  be  understood,  however, 
that  the  use  of  half  the  interpoles  is  permissible  only  with  drum- 
wound  armature  windings,  where  each  armature  coil  spans 
approximately  one  pole  pitch.  Ring-wound  armatures  require 
the  full  number  of  interpoles. 

Experience  shows  that  when  but  half  the  number  of  interpoles 
is  used  the  demagnetizing  ampere  turns,  or  those  which  directly 
oppose  the  armature  magnetomotive  force,  should  have  about 
the  same  value  per  interpole  as  when  the  full  number  is  used. 
However,  the  effective  ampere  turns  which  set  up  the  commu- 
tating flux  must  be  doubled  in  value,  as  just  stated.  Therefore 
the  total  ampere  turns  per  interpole  would  be  greater  than  when 
the  full  number  of  interpoles  is  used,  but  the  total  number  of 
ampere  turns  on  all  the  interpoles  is  much  less  than  with  the 
full  number  of  interpoles.  In  consequence,  there  is  a  very  con- 
siderable saving  in  the  amount  of  copper  required. 

On  account  of  the  increased  number  of  ampere  turns  per 
interpole  when  half  the  number  of  poles  is  used,  the  interpole 
leakage  will  be  increased  in  proportion.  This  is  particularly 


COMMUTATING  POLES  ON  ROTARIES  193 

objectionable  on  large  machines  where  the  design  of  the  inter- 
pole  becomes  difficult  on  account  of  magnetic  leakage.  There- 
fore this  arrangement  is  usually  confined  to  small  machines. 

A  very  considerable  advantage  in  this  arrangement  is  that 
the  ventilating  conditions  are  improved  due  to  the  fact  that  the 
interpoles  and  main  poles  do  not  so  completely  enclose  the  arma- 
ture, for,  with  alternate  interpoles  omitted,  the  circulation  of  air 
between  the  armature  and  the  field  poles  can  be  materially  im- 
proved. 

With  interpole  converters,  with  their  smaller  ampere  turns 
per  interpole,  the  omission  of  alternate  interpoles  will  not  have 
as  much  influence  on  the  general  design  as  in  the  case  of  direct- 
current  generators.  As  the  interpole  ampere  turns  are  only 
about  35  per  cent,  as  great  as  on  a  direct-current  machine,  and 
as  about  half  is  useful  and  half  demagnetizing,  it  is  evident  that 
the  useful  component  would  readily  be  doubled,  thus  doubling 
the  useful  flux,  while  the  total  leakage  would  still  be  far  less  on  a 
direct-current  machine.  Therefore  the  smaller  number  of  inter- 
poles is  much  better  adapted  to  the  synchronous  converter  than 
to  the  direct-current  generator. 

In  the  converter  the  use  of  the  small  number  of  interpoles 
also  possesses  a  further  advantage.  In  the  case  of  a  short  cir- 
cuit, and  assuming  a  negative  field  to  be  set  up  by  the  armature 
reaction,  as  described  in  the  paper,  the  use  of  half  the  number  of 
interpoles  would  cut  this  reverse  field  to  half  value.  In  conse- 
quence, any  flashing  tendency  would  be  proportionately  re- 
duced. Half  the  neutral  spaces  being  without  interpoles,  and 
the  other  half  having  interpoles,  it  is  evident  that  such  an 
arrangement  should  be  practically  midway  between  a  non-inter- 
pole  and  a  full  interpole  converter  as  regards  any  flashing 
tendencies. 

It  is  also  evident  that  with  half  the  number  of  interpoles  the 
ventilating  conditions  will  be  improved  just  as  on  the  direct- 
current  generator. 

The  lower  leakage  in  the  interpoles  of  the  converter  allows 
another  material  difference  between  the  design  of  the  converter 
interpoles  and  those  of  tie  direct-current  generator.  In  ordinary 
direct-current  generators,  especially  those  of  large  capacity, 
the  interpoles,  as  a  rule,  are  made  almost  the  full  width  of  the 
armature  core,  principally  in  order  to  maintain  a  lower  satura- 
tion of  the  interpole  core.  As  the  width  u  i  the  interpoles  is 


194  ELECTRICAL  ENGINEERING  PAPERS 

varied  the  leakage  flux  varies  practically  in  proportion  to  the 
-width,  but  the  total  useful  flux  remains  practically  constant. 
Therefore,  with  wider  interpoles  the  flux  density  due  to  the  com- 
bined leakage  and  useful  fluxes  will  be  lower  than  if  a  narrower 
pole  were  used,  and  the  saturation  will  be  correspondingly  re- 
duced. In  the  interpole  converter,  however,  the  leakage  flux 
being  so  much  lower  than  in  a  direct-current  generator,  it  is 
evident  that  the  useful  flux  could  be  correspondingly  increased 
while  maintaining  no  higher  saturation  than  on  a  direct-current 
machine.  This,  therefore,  permits  a  much  narrower  interpole 
on  the  converter  than  on  a  direct-current  machine.  As  the 
interpole  becomes  narrower  than  the  armature  the  reverse 
field  which  may  be  set  up  on  short  circuit  also  should  be  pro- 
portionately reduced,  so  that  with  interpoles  of  practically 
half  the  width  of  the  armature,  the  conditions  should  be  practi- 
cally equivalent  to  those  where  half  the  number  of  poles  is  used, 
as  far  as  flashing  conditions  are  concerned  The  use  of  narrow 
interpoles  should  also  allow  better  ventilation  than  when  the 
full  width  is  used.  Narrower  interpoles,  of  course,  allow  con- 
siderably less  copper  for  the  same  total  number  of  ampere  turns 
However,  unless  the  interpoles  can  be  made  less  than  half  the 
width  of  the  armature,  the  amount  of  copper  required  for  this 
arrangement  would  be  still  greater  than  would  be  required  with 
only  half  the  number  of  interpoles,  each  of  full  width  of  the 
armature. 

There  are  many  other  points  in  connection  with  the  use  of 
interpoles  on  converters  which  were  not  mentioned  in  this 
paper.  I  will  describe  briefly  a  few  interesting  features  which 
are  encountered  in  the  design  of  such  machines,  but  which  are 
not  found  in  direct-current  machines. 

One  of  these  concerns  the  application  of  dampers  to  interpole 
converters.  It  is  found  that  the  usual  distributed  cage  type  of 
damper  supplied  with  self-starting  converters  is  not  directly 
applicable  to  the  interpole  converter  Dampers  are  supplied 
to  synchronous  converters  for  two  purposes,  namely,  to  prevent 
hunting  and  to  obtain  good  self-starting  conditions.  To  prevent 
hunting  the  damper  should  be  thoroughly  distributed  through 
and  around  the  pole  face  in  the  form  of  numerous  low  resistance 
bars  or  rods  which  are  joined  together  at  each  end  by  low  re- 
sistance connectors.  There  may  or  may  not  be  any  connection 
between  the  dampers  on  adjacent  poles.  In  practice,  with  well 


COMMUTATING  POLES  ON  ROTARIES  195 

proportioned  dampers,  such  connection  between  the  poles  may 
be  of  some  benefit,  but  this  is  difficult  to  determine  as  far  as 
hunting  is  concerned.  Those  conductors  embedded  in  the  pole 
and  immediately  surrounding  it  appear  to  give  all  the  damping 
action  which  is  necessary  if  the  damper  is  well  proportioned. 

However,  when  it  comes  to  self-starting  converters,  that  is, 
those  which  are  started  and  brought  up  to  speed  by  direct 
application  of  alternating-current  to  the  collector  rings,  it  is 
claimed  by  some  designers  that  the  interconnection  between 
the  adjacent  dampers  is  of  benefit  at  the  moment  of  starting, 
by  reducing  the  tendency  toward  dead  points  or  points  of  very 
low  starting  torque.  When  started  in  this  manner  the  armature 
of  the  converter  becomes  the  primary  of  an  induction  motor, 
while  the  cage  damper  in  the  field  poles  becomes  the  equivalent 
of  a  cage  winding  on  the  secondary  of  an  induction  motor.  It 
is  claimed  that  the  interconnection  between  the  dampers  to 
form  a  complete  cage  allows  better  polyphase  action  in  the 
secondary  winding.  Any  beneficial  result  of  this  should  show 
in  more  uniform  torque  at  start,  but  not  to  any  pronounced 
extent  in  the  apparent  input  required  to  start  the  converter 
and  bring  it  up  to  speed. 

When  hunting  occurs  the  magnetic  field  in  the  main  poles  is 
alternately  shifted  or  crowded  toward  one  pole  edge  or  the  other 
and  the  parts  of  the  damper  embedded  in  and  immediately 
surrounding  the  pole  face  aare  particularly  effective  in  preventing 
such  shifting.  Also,  the  lower  the  resistance  and  the  better 
distributed  this  damper,  the  more  effective  it  appears  to  be  in 
general  as  regards  damping. 

On  the  other  hand,  for  self  starting,  the  damper,  acting  as  a 
cage  secondary  of  an  induction  motor,  will  have  the  character- 
istics of  such  secondary  and  therefore  for  best  and  most  uniform 
starting  torque  conditions,  a  relatively  high  resistance  is  de- 
sirable and  a  continuous  cage  is  usually  preferred.  In  conse- 
quence, the  two  conditions  of  best  damping  and  best  starting 
are,  to  a  certain  extent,  opposed  to  each  other. 

In  the  use  of  a  continuous  cage  damper  is  found  a  difficulty 
in  the  application  of  interpoles  to  the  synchronous  converter. 
If  adjacent  dampers  are  connected  together,  as  shown  in  Fig.  1 
then  the  interpole  between  the  two  main  poles  is  actually  sur- 
rounded by  the  low  resistance  damper  circuit,  a  condition  which 
is  very  objectionable,  as  explained  in  the  paper.  Conse- 


196 


ELECTRICAL  ENGINEERING  PAPERS 


quently,  tlie  usual  arrangement  of  the  cage  damper  for 
self  starting  is  not  advisable  on  an  interpole  con- 
verter which  is  subject  to  sudden  fluctuations  in  load.  In 
other  words,  the  continuous  cage  damper  should  not  be  used, 
or  its  design  should  be  modified  very  considerably,  in  the  case  of 
self-starting  converters,  which  are  subject  to  considerable 
fluctuations  in  load  in  service.  If  the  continuous  cage  construc- 
tion is  desired,  the  individual  dampers  might  be  connected 
together  by  high  resistance  connectors. 

A  second  interesting  point  in  the  design  of  interpole  converters, 
but  not  found  in  direct-current  generators,  comes  up  in  connec- 
tion with  the  copper  loss  in  the  tap  coils,  that  is,  those  armature 
coils  which  are  tapped  directly  to  the  collector  rings.  As  is  well 
known  to  those  familiar  with  synchronous  converter  design, 

the  copper  loss  in  the  tap  coils 
of  a  rotary  is  relatively  high 
compared  with  the  average  loss 
in  all  the  coils,  the  loss  per  coil 
falling  ofi  to  a  minimum  value 
between  the  taps.  The  real 
limit  in  carrying  capacity  of  the 
armature  is  fixed  by  the  heating 
of  the  tap  coils  and  not  by  the  armature  copper  as  a  whole. 
It  is  possible  to  overload  an  armature  so  that  the  tap  coils  will 
roast  out  while  the  remaining  coils  will  show  very  much  less 
signs  of  heating.  The  heating  in  these  tap  coils  also  increases 
rapidly  as  the  power-factor  of  the  alternating-current  input  is  de- 
creased, the  output  remaining  constant.  Therefore  by  reducing  the 
power-factor  of  a  converter  while  keeping  the  direct-current 
output  constant  it  is  possible  to  roast  out  the  tap  coils.  The 
true  limit  of  heating  in  a  converter  armature  therefore  is  found 
in  these  coils.  Herein  is  found  a  difference  between  the  inter- 
pole and  the  usual  non-interpole  converter.  In  the  non-inter- 
pole  type,  as  usually  constructed,  the  armature  coils  are  of  the 
fractional  pitch  or  "chorded"  type  in  which  the  "throw"  or 
"span"  of  a  coil  is  one  or  more  slots  less  than  the  pole  pitch. 
The  primary  object  of  this  is  to  improve  commutation.  In  the 
ordinary  direct-current  winding  there  are  two  coils  in  each  slot 
one  above  the  other.  With  a  full  pitch  winding,  when  the 
upper  coil  is  being  commutated  or  reversed  the  lower  coil  in 
the  same  slot  is  also  being  reversed  so  that  the  e.m.f.  of  self- 


FiG.  1 


COMMUTATING  POLES  ON  ROT  ARIES  197 

and  mutual-induction  of  the  commutated  coils  is  due  to  the 
reversal  of  the  local  field  of  both  upper  and  lower  commutated 
coils  in  the  slot.  With  a  fractional  pitch  winding,  the  upper 
coil  which  is  being  commutated  lies  in  a  different  slot  from  the 
lower  one  which  is  being  commutated  at  the  same  instant. 

This  same  arrangement  of  fractional  pitch  winding  puts  the 
upper  tap  coil  in  a  different  slot  from  the  lower  one  so  that  the 
maximum  heating  does  not  occur  in  the  upper  and  lower  coils 
in  the  same  slot,  as  would  be  the  case  if  a  full  pitch  winding  were 
used.  Therefore,  with  a  fractional  pitch  winding  the  heating 
is  somewhat  better  distributed  than  in  the  full  pitch  winding. 
However,  with  interpoles,  a  full  pitch  winding  would  naturally 
be  used,  as  a  fractional  pitch  winding  would  mean  a  relatively 
wide  interpole  with  a  corresponding  increase  in  distance  between 
the  main  poles.  Therefore  with  the  full  pitch  winding  used  with 
interpole  converters  the  heating  due  to  the  tap  coils  will  be  more 
concentrated  than  in  the  non-interpole  type.  In  other  words, 
the  machine  will  have  less  maximum  capacity  unless  more  copper 
is  used  in  the  armature  coils,  or  an  inferior  type  of  interpole 
construction  is  used  in  order  to  allow  a  fractional  pitch  winding. 
This  looks  like  a  minor  point,  but  when  it  is  borne  in  mind  that 
in  modern  converter  designs  the  starting  point  in  the  design  of 
the  armature  winding  is  the  permissible  copper  loss  in  the  tap 
coils,  and  not  the  armature  copper  loss  as  a  whole,  the  import- 
ance of  this  point  may  be  seen. 

A  third  point,  not  mentioned  in  the  paper  but  which  concerns 
design  as  well  as  operation,  is  found  in  self-starting  converters. 
In  such  machines  the  alternating  current  is  applied  directly  to 
the  alternating-current  end  of  the  converter  and  a  rotating  mag- 
netic field  is  set  up,  just  as  in  the  primary  of  an  induction  motor. 
This  field  travels  around  the  armature  at  a  speed  corresponding 
to  the  frequency  of  the  supply  circuit  and  the  number  of  field 
poles  and  all  the  armature  coils  in  turn  are  cut  by  this  traveling- 
field.  Those  coils  which  are  short  circuited  at  the  commutator 
by  the  brushes  form  closed  secondary  circuits  and  secondary 
currents  are  set  up  by  the  alternating  field  just  as  in  commu- 
tating  type  alternating-current  motors  at  start.  As  soon  as  the 
converter  gets  in  motion  the  short  circuit  is  transferred  from  coil 
to  coil  but  the  short  circuit  current  must  be  broken  as  each  coil 
passes  out  from  tinder  the  brushes  and  this  results  in  more  or 
less  sparking,  depe&ding  on  the  size  and  general  proportions  of 


198  ELECTRICAL  ENGINEERING  PAPERS 

the  machine.  It  is  a  question  to  what  extent  this  sparking  is 
dependent  upon  the  normal  commutating  characteristics  of 
the  armature  winding.  Other  things  being  equal,  presumably 
the  better  these  characteristics  the  less  should  be  the  sparking 
and  burning  at  the  brushes  when  the  converter  is  self  started 
from  the  alternating-current  end.  On  this  basis  then,  a  con- 
verter armature  designed  with  poor  commutating  characteristics 
and  in  which  the  commutation  at  synchronous  speed  is  ac- 
complished by  interpoles,  should  spark  considerably  more  when 
starting  than  a  converter  which  has  inherently  very  much 
better  commutating  characteristics.  The  presence  of  com- 
mutatiag  poles  should  in  no  way  help  commutation  at  start  as 
there  is  no  current  in  the  interpole  winding.  However,  as  con- 
verters are  started  very  infrequently,  such  increased  sparking 
at  start  would  probably  do  but  little  real  injury.  This  is  simply 
mentioned  as  one  of  the  points  in  which  the  designer  is  concerned. 

Some  reference  has  been  made  this  evening  to  the  split  pole 
converter  in  connection  with  interpoles.  Some  distinction 
should  be  made  between  the  true  interpole  or  commutating  pole 
arrangement  referred  to  in  this  paper  and  what  is  sometimes 
referred  to  as  the  interpole  in  the  so-called  "split-pole"  con- 
verter. In  the  split  pole  converter,  as  usually  built,  there  is  a 
series  of  wide  poles  alternating  with  narrow  poles,  the  field 
construction  therefore  resembling  somewhat  the  ordinary  inter- 
pole machine.  In  the  split  pole  converter,  however,  the  small 
pole  is  used  primarily  for  the  purpose  of  obtaining  variations 
in  the  direct-current  voltage  and  not  for  the  purpose  of  ob- 
taining a  true  commutating  field.  The  winding  on  this  small 
pole  on  the  split  pole  machine  is  usually  in  shunt  with  the 
armature  instead  of  in  series,  and  its  circuit  is  so  arranged  that 
the  polarity  can  be  varied  from  maximum  down  to  zero  and  to 
maximum  in  the  opposite  direction  regardless  of  the  armature 
current  carried.  In  certain  combinations  this  arrangement 
can  be  made  to  have  the  effect  of  commutating  poles,  but  under 
other  conditions  it  may  have  just  the  opposite  effect. 

The  small  pole  is  usually  placed  dose  to  one  of  the  main 
poles,  thus  allowing  a  fairly  wide  interpolar  space  between  itself 
and  one  of  the  adjacent  large  poles  and  a  very  narrow  space  to 
the  other  large  pole.  Commutation  occurs  usually  in  the  wider 
interpolar  space  and  not  under  the  small  pole  itself  as  is  the  case 
in  the  true  interpole  machine.  The  direct-current  e,m.f.  is 


COMMUTATING  POLES  ON  ROTARIES  199 

generated  by  the  resultant  field  due  to  one  large  pole  and  the 
small  pole  which  is  closest  to  it.  When  these  two  have  the 
same  polarity  the  direct-current  e  m.f .  is  highest  and  when  they 
are  of  opposite  polarity  it  is  lowest.  However,  the  alternating- 
current  e.m  f .  is  due  to  the  flux  of  two  adjacent  poles,  a  large 
and  a  small  one  of  like  polarity.  It  is  evident  therefore  that 
the  maximum  alternating-current  e.m.f .  will  coincide  in  position 
with  the  direct-current  only  at  the  highest  direct-current  e.m.f.; 
that  is,  when  both  fluxes  included  in  one  direct-current  circuit 
are  of  the  same  polarity.  At  lowest  direct-current  e.m.f .  when 
one  direct-current  circuit  includes  two  fluxes  of  opposite  polarity, 
it  is  obvious  that  the  maximum  alternating-current  e.m.f. 
must  be  shifted  tircumferentially  with  respect  to  the  direct- 
current.  The  alternating-current  magnetomotive  force  will 
also  be  shifted  in  like  manner  with  respect  to  the  direct-current 
and  the  resultant  of  the  two  will  vary  both  in  height  and  position 
with  variations  in  the  strength  and  direction  of  the  flux  of  the 
small  pole. 

At  highest  direct-current  e.m.f.  a  coil  which  is  being  corn- 
mutated  lies  midway  between  poles  of  opposite  polarity  and  the 
conditions  resemble  those  in  an  ordinary  converter  as  regards 
commutation.  At  the  lowest  direct-current  e.m.f.  the  commu- 
tated  coil  lies  midway  between  two  poles  of  like  polarity  and 
there  will  be  a  field  flux  in  the  interpolar  space  in  which  the 
armature  coil  must  commutate.  The  direction  of  this  field 
may  be  such  that  it  will  assist  in  commutation;  that  is,  it  will 
tend  to  overcome  the  higher  magnetomotive  force  of  the  arma- 
ture currents  resulting  from  the  alternating-current  and  direct- 
current  magnetomotive  forces  being  shifted  with  respect  to 
each  other,  as  just  mentioned.  Therefore  this  interpolar  field 
flux  may  act  in  a  very  beneficial  manner  tinder  certain  condi- 
tions. However,  if  this  flux  is  in  the  right  direction  for  assisting 
commutation  when  transforming  from  alternating-current  to 
direct-current,  it  will  evidently  be  in  the  wrong  direction  when 
operating  from  direct-current  to  alternating-current.  Also, 
this  field  flux  in  the  interpolar  space  will  vary  with  any  variations 
in  the  strength  of  the  small  pole;  that  is,  with  any  change  in  the 
direct-current  voltage,  although  the  currents  in  the  armature 
may  be  unchanged.  Also,  this  interpolar  field  may  remain  of 
constant  strength,  while  wide  changes  may  occur  in  the  armature 
currents,  and  thus  in  their  resultant  magnetomotive  forces.  It 


200  ELECTRICAL  ENGINEERING  PAPERS 

is  obvious  therefore  that  this  interpolar  flux  can  be  equivalent 
to  a  true  interpole  of  proper  strength  and  polarity,  only  under  a 
very  limited  range  of  operation. 

In  conclusion  I  may  say  that,  as  brought  out  in  the  paper, 
the  real  field  for  interpoles  in  synchronous  converters  is  found 
in  connection  with  higher  speeds  and  large  outputs  per  pole. 
I  am  an  advocate  of  the  highest  speeds  which  the  public  will 
stand,  up  to  the  point  where  no  further  real  gain  in  cost  and 
performance  is  obtained.  If  this  highest  speed  in  converters 
is  such  that  interpoles  are  of  material  benefit,  then  in  such  ma- 
chines we  may  look  forward  to  the  use  of  interpoles.  However, 
for  the  relatively  low  speeds  represented  by  much  of  our  present 
practice  the  use  of  interpoles  can  be  considered  as  only  a  rela- 
tively small  improvement,  concerning  which  there  may  be  honest 
differences  of  opinion  regarding  the  commercial  value. 


THEORY  OF  COMMUTATION  AND  ITS  APPLICATION 
TO  COMMUTATING  POLE  MACHINES 

FOREWORD — This  paper  was  the  result  of  many  years  of  the  author's 
work  on  the  subject  of  commutation.  In  its  presentation,  it 
embodies  a  new  method  of  looking  at  the  problem.  In  this 
method,  the  armature  winding,  as  a  whole,  is  considered  as 
setting  up  a  magnetic  field  in  the  so-called  neutral  zone;  and 
it  is,  primarily,  the  e.m.fs.  set  up  by  the  armature  conductors 
cutting  this  field,  which  are  dealt  with  in  this  theory  of  commuta- 
tion. Before  the  completion  of  the  paper,  the  commutating  con- 
stants of  many  hundreds  of  direct-current  machines  of  various 
kinds  were  checked  to  determine  the  correctness  of  the  method. 
The  paper  was  presented  at  a  meeting  of  the  American  Institute 
of  Electrical  Engineers,  October,  1911. 

Throughout  this  paper,  it  will  be  noted  that  the  term  "inter- 
pole"  was  used  in  place  of  the  present  accepted  term ' '  commutat- 
ing pole"  which  came  later. — (ED.) 


IN  the  usual  theory  of  commutation  it  is  considered  that , 
when  the  current  in  a  coil  is  commutated  or  reversed,  the  local 
magnetic  flux  due  to  the  current  reverses  also,  and  in  so  doing 
sets  up  an  e.m.f.  in  the  coil  which  opposes  the  reversal.  This  is 
the  so-called  reactance  voltage  referred  to  in  commutation  prob- 
lems. The  fact  that  two  or  more  coils  may  be  undergoing 
commutation  at  the  same  time  involves  consideration  of  mutual 
as  well  as  self-induction.  The  relation  of  the  mutual  to  the  self- 
induction,  the  probable  value  of  each,  etc,,  lead  to  such  mathe- 
matical complication  in  the  analysis  of  the  problem,  that  em- 
pirical methods  have  become  the  usual  practice  in  dealing  with 
commutation.  The  usual  analytical  methods  do  not  permit  a 
ready  or  easy  physical  conception  of  what  actually  takes  place. 

According  to  the  usual  theory,  during  the  commutation  of  the 
coil  the  local  magnetic  flux  due  to  the  coil  is  assumed  to  be 
reversed.  However,  in  the  zone  in  which  the  commutation 
occurs,  certain  of  the  magnetic  fluxes  may  remain  practically 
constant  in  value  and  direction  during  the  entire  period  of  com- 
mutation. 

The  fact  of  part  of  the  flux  in  tbe  zone  of  commutation 
remaining  practically  constant  in  value  and  direction,  led  the 
author  to  a  method  of  dealing  with  the  problem  of  commutation 
which  is  based  upon  consideration  of  the  armature  flux  as  a 

201 


202  ELECTRICAL  ENGINEERING  PAPERS 

whole,  as  set  up  by  the  armature  ampere  turns.  The  results 
obtained  by  the  method  were  very  satisfactory,  and  it  was  ap- 
parent that  a  much  better  conception  could  be  obtained  of  some 
of  the  phenomena  of  commutation  than  was  possible  with  former 
methods. 

In  the  following  pages  the  method  is  indicated  in  general, 
and  its  application  to  interpole  machines  is  then  worked  out  in 
greater  detail.  In  non-interpole  machines  the  problem  is  greatly 
complicated  by  the  presence  of  local  currents  under  the  brushes 
which  modify  the  distribution  of  certain  of  the  armature  magnetic 
fluxes,  as  will  be  shown. 

This  theory  of  commutation,  with  the  method  of  calculation, 
is  based  upon  the  broad  principle  of  the  armature  conductors 
cutting  across  the  magnetic  field  %et  up  by  the  armature  "winding  and 
thereby  generating  an  e.mj.  in  the  short  circuited  coils  which  w 
proportional  to  the  product  of  the  revolutions,  the  flux  which  is  cut 
and  the  number  of  turns  in  series.  The  usual  "  reactance  " 
voltage  due  to  reversal  of  the  local  flux  of  an  individual  coil  is 
not  considered,  although  its  equivalent  appears  under  another 
form. 

The  method  in  general  is  therefore  the  same  as  that  used  for 
determination  of  the  main  armature  e.rn.f.,  except  that  the 
magnetic  fluxes  cut  by  the  armature  conductors  are  those  due  to 
the  armature  magnetomotive  force  instead  of  those  due  to  the 
field. 

When  the  armature  winding  is  carrying  current  its  magneto- 
motive force  tends  to  set  up  certain  magnetic  fields  or  fluxes, 
which  have  a  definite  relation  to  the  position  of  the  brushes. 
Considered  broadly,  the  current  after  entering  the  commutator 
or  armature  winding,  at  any  brush  arm,  divides  into  two  paths- 
of  opposite  direction.  As  the  winding  on  each  of  these  paths  is 
arranged  in  exactly  the  same  way,  and  as  the  currents  flow  in 
opposite  directions,  the  armature  windings  in  these  two  paths 
have  magnetomotive  forces  which  are  in  opposite  directions. 
The  resultant  armature  magnetomotive  force  rises  to  a  maximum 
at  points  corresponding  to  the  brush  positions.  Midway  between 
these  points  the  magnetomotive  force  is  zero.  Magnetic  fluxes 
are  set  up  by  these  magnetomotive  forces,  which  are  a  function 
o£  the  force  producing  them,  and  the  proportions,  dimensions, 
and  arrangement  of  the  magnetic  paths;  and  these  magnetic 
fluxes  will  be  practically  fixed  in  position  corresponding  to  the 
brush  setting. 


THEORY  OF  COMMUTATION  203 

The  armature  conductors  cutting  across  these  fluxes  set  up  by 
the  armature  magnetomotive  forces,  will  have  e.m.fs.  generated 
in  them.  In  those  conductors  which  have  their  terminals  short 
circuited  by  the  brushes,  these  e.m.fs.  may  be  called  the  short 
circuit  e.m.fs. 

There  are  three  principal  armature  fluxes  which  are  cut  by  the 
short  circuited  armature  coils.  In  the  order  of  their  usual 
importance  these  are, 

1.  That  which  crosses  from  slot  to  slot.     It  may  be  called  the 
slot  flux. 

2.  The  interpoiar  flux  which  passes  from  the  armature  surface 
to  the  neighboring  poles  or  yoke  surface.     It  may  be  called  the 
interpoiar  flux,  as  distinguished  from  interpole  flux,  ,which  term 
will  be  used  later. 

3.  That  flux  set  up  in  the  armature  end-winding  in  the  zone  of 
the  short  circuited  coil/  due  to  the  magnetomotive  force  of  the 
end  windings  as  a  whole.     It  may  be  called  the  end  flux. 

The  short  circuited  armature  coils  cutting  across  these  three 
fluxes  generate  the  short  circuit  e.m.fs.  The  whole  problem  of 
commutation  may  be  considered  as  depending  upon  the  prede- 
termination of  these  three  fluxes. 

Consider,  first,  an  armature  conductor  approaching  the  poini 
of  current  reversal  or  commutation.  Under  this  condition  the 
current  carried  by  the  coil  always  flows  in  the  same  direction 
as  the  e.m.f.  generated  by  the  conductor  cutting  across  the  magnetic 
field  or  flux  set  up  by  the  armature  winding  is  induced.  When  the- 
terminals  of  an  armature  coil  pass  under  the  brush  and  are  short 
circuited,  it  is  obvious  that  -the  e.m.f.  set  up  in  the  coil  by  the 
armature  flux  is  unchanged  in  direction  for  the  coil  is  still  cutting 
a  field  of  the  same  polarity.  This  e.m.f.  tends  to  maintain  the  cur- 
rent in  the  short  circuited  armature  coil  in  the  same  direction  as 
before  but  the  value  the  current  attains  will  be  dependent  upon, 
the  short  circuit  e.m.f .  and  largely  upon  the  resistance  in  the 
circuit,  which  will  usually  consist  of  the  resistance  of  the  coil 
itself  and  of  the  brush  contact.  As  the  coil  passes  out  of  short 
circuit,  that  is,  as  it  leaves  the  brush,  the  current  must  flow  in 
the  opposite  direction,  but  the  e.m.f .  set  up  by  the  armature 
flux  is  still  in  the  same  direction  as  before.  Therefore,  after 
commutation,  the  armature  current  in  the  coil  is  flowing  in. 
opposition  to  the  e.m.f .  set  up  in  the  coil  by  the  armature  flux. 

The  following  is  a  method  for  calculating  approximately  the 
three  fluxes  before  described  and  the  e^m-fs.,  generated  by  the 


-204= 


ELECTRICAL  ENGINEERING  PAPERS 


armature  conductors  cutting  them.  The  interpolar  fluxes  will 
be  considered  first,  the  end  fluxes  second,  and  the  slot  fluxes 
last,  as  these  latter  are  greatly  complicated  "by  the  problem  of 
local  currents  produced  largely  by  the  interpolar  and  end  fluxes. 

INTERPOLAR  ABMATURE  FLUX 

By- this  is  meant  the  flux  in  the  interpolar  space  between  the 
armature  core  and  the  field  poles  and  yoke,  due  to  the  magneto- 
motive force  of  the  armature  winding,  as  shown  in  Fig.  1.  This 
magnetomotive  force  has  its  highest  value  at  those  parts  of  the 
armature  winding  corresponding  to  the  brush  contacts  on  the 
commutator  and  is  zero  midw;ay  between  such  points.  If  the 
brushes  are  set  -with  zero  lead  then  the  maximum  magneto- 
motive force  of  the  armature  lies  midway  between  adjacent  field 
poles  and  will  taper  off  in  value  from  this  midpoint  toward  the 


\ 


FIG.  1 


FIG    2 


adjacent  edges  of  the  poles.  The  flux  density  between  the  arma- 
ture surface  and  the  sides  of  the  poles  should  therefore  tend  to 
taper  off  as  the  armature  magnetomotive  force  is  reduced  but,  in 
most  types  of  field  construction,  it  tends  to  increase  in  value  due 
to  the  relatively  shorter  magnetic  path  as-  the  edges  of  the  poles 
are  approached.  Usually  this  increase  very  considerably  over- 
balances the  decrease  due  to  the  lower  magnetomotive  forces 
and  in  consequence  the  interpolar  flux  density  due  to  the  arma- 
ture generally  has  a  mini-cntrm  value  midway  between  the  poles 
and  rises  toward  tte  edges  of  the  poles.  This  is  illustrated  by 
Fig.  2. 

The  density  of  this  fltut  in  the  interpolar  space  is  dependent 
-upon  many  conditions  such  as  the  ampere  turns  of  the  armature 
winding  per  poler  distance  between  poles,  conformation  of  the 
poles,  yoke,  etc.  In  Fig.  2  the  ordinates  of  the  dotted  lines 
represent  the  flux  densities  at  the  armature  interpolar  surface 


THEORY  OF  COMMUTATION  20o 

due  to  each  of  the  two  adjacent  poles.  The  resultant  of  these 
two  is  the  full  line  a  c  b  which  represents  the  distribution  of  the 
armature  inter  polar  flux.  This  interpolar  flux  might  be  con- 
sidered as  a  true  magnetic  field  fixed  in  space  with  respect  to  the 
position  of  the  brushes.  This  field  being  fixed  and  the, armature 
conductors  rotating  it  is  obvious  that  any  conductor  moving 
across  this  magnetic  field  must  have  e,m,f.  generated  in  it,  the 
value  of  which  depends  upon  the  flux  which  is  cut  at  any  instant. 
Therefore,  the  e.m.f.  due  to  this  interpolar  field  can  be  de- 
termined directly,  if  the  intensity  of  the  field  itself  can  be  cal- 
culated. 

During  the  period  of  commutation  the  armature  ooil  is  short 
circuited  and  has  the  current  reversed  in  it  under  certain  por- 
tions of  this  field.     The  problem  is  to  determine  the  strength  of 
the  field  corresponding  to  this  point  of  com- 
mutation and  then  by  direct  calculation  the 
corresponding  e.m  f.   can   be  determined. 
In  the  following  analysis  two  cases  will  be 
considered,   namely,    pitch  windings,  and 
"  chorded  "  or  "  fractional  pitch  "  windings. 
Pitch  Windings.    When  commutating  or 
reversing  a  coil  with  a  pitch  winding  it  is 
evident  that  if  there  were  no  lead  at  the 
brushes  such  a  coil  would  commutate,  on 
the  average,  at  the  midpoint  between  two 
poles.     The  e.m.f.  generated  in  the  coil  by  cutting  the  interpolar 
field  would  therefore  be  proportional  to  the  strength  of  the  inter- 
polar flux  at  the  midpoint.     This  flux  can  be  determined  approx- 
imately in  a  fairly  simple  manner  in  the  ordinary  types  of  machines 
in  which  the  poles  are  relatively  long  compared  with  the  distance 
between  adjacent  pole  tips  and  where  the  distance  from  the  arma- 
ture surface  to  the  yoke  is  relatively  great.     The  following  is 
a  method  which  appears  to  give  reasonably  close  results: 
Let  Wt  =  total  number  of  wires  on  the  armature. 
!<,  =the  current  per  conductor. 
p    =  number  of  .poles. 

Then,    the   armature    ampere    turns   per   pole  =•  — C0      *  r 

Ap 

neglectifig  any  change  in  ampere  turns  due  to  the  short  circuit- 
ing action  of  the  brushes. 

In  Fig.  3  let  b  represent  the  length  of  the  mean  flux  path 
corresponding  to  the  mid-interpolar  position.     This  is  assumed 


208  ELECTRICAL  ENGINEERING  PAPERS 

to"  be  a  part  of  a  circle  which  is  poetically  at  right  angles  to  the 
armature  surface  and  the  side  of  the  field  pole,  as  indicated  in 
Pig.  3. 

Let  P  =  widt&  of  body  of  pole. 

Let  .Bt  —  the  flux  density  at  the  midpoint  between  the  poles. 

2X3  197.X  TV* 


Then  E^-- 


2pb 


But  6  =  27ra       QZ.      .   approximately,  as  angle   (90+0)  is 

OOU 

only  approximate. 


Also,  a  =  (  0        —  —  )  approximately. 

\   &  p  £i  I 


Therefore 


Therefore 

2X3.197. 


-B,— 


(0.25£+0.5)  (IT  D-Pp)X2  p 


The  above  gives  the  approximate  flux  density  at  the  midpoint 
tetween  poles.  The  flux  densities  at  points  at  each  side  of  the 
midpoint  can  be  determined  in  a  similar  manner,  taking  into 
account  the  lower  armature  magnetomotive  force  as  the  mid- 
point is  departed  from.  As  the  edge  of  the  pole  is  approached 
the  effect  of  pole  horns  may  complicate  the  flux  distribution  so 
that  the  above  method  of  calculating  interpolar  flux  density  will 
not  apply  for  points  close  to  the  pole. 
E,mj.  Due  to  Interpolar  Flux. 

Let  Ec- The  e.m.f.  due  to  cutting  the  armature  flux. 
D  =  diameter  of  armature. 
L  =  length  of  core  including  ventilating  spaces. 
Tc  =  turns  per  individual  armature  coil. 
Rs  —  revolutions  per  second. 


THEORY  OF  COMMUTATION  207 

Then,  the  e,m  f  .  induced  in  a  coil  cutting  the  field  at  c  (Fig.  2) 
can  be  represented  by  the  formula, 

TcXRs 


c  10 

Or, 


27rDLTcRs 


'      (0.25^+0.5)  (irD-Pp)  ~  108 

Or 

IcWtTcRs      /  ZpXirDL 


L  \ 

D-Pp)  ) 


108  \  (0.25p+0.5)  (irD-Pp) 

Incidentally,  with  this  method  of  dealing  with  the  problem 
the  effect  of  the  addition  of  an  interpole  can  at  once  be  seen. 
The  magnetomotive  force  of  the  interpole  is  superimposed  on 
that  of  the  armature  and  the  resultant  flux  is  then  considered. 
The  armature  conductors  cut  this  flux  and  thereby  generate 
e.m.f.  If  the  interpole  magnetomotive  force  is  stronger  than 
that  of  the  armature,  then  the  flux  established  will  be  in  the 
opposite  direction  in  that  part  of  the  armature  face  which  lies 
under  the  interpole.  Therefore,  the  flux  or  field  over  the  com- 
mutated  coil  in  the  non-commutating  pole  machine  is  replaced 
by  flux  in  the  opposite  direction.  The  presence  of  the  interpole 
does  not  increase  the  reactance  of  the  armature  coil  as  sometimes 
considered,  but,  on  the  contrary,  the  harmful  flux  is  replaced  by 
one  which  is  of  direct  assistance  in  commutation. 

Effect  of  Brush  Width.  In  cutting  across  the  interpolar  flux 
it  is  obvious  that  the  e.m.f.  set  up  in  the  short  circuited  coil  is 
not  a  function  of  the  length  of  time  the  coil  is  short  circuited, 
for  this  interpolar  flux  is  set  up  by  the  armature  winding  as  a 
whole  and  not  by  individual  coils.  If  two  or  more  armature 
•coils  in  series  are  short  circuited  by  the  brush,  then  their  e.m.fs. 
will  be  in  series  while  the  total  resistance  in  the  path  will  be  very 
little  higher  than  in  the  case  of  a  single  coil  short  circuited,  for 
the  principal  part  of  the  resistance  lies  in  the  brush  contact.  It 
is  evident  therefore  that  considerably  higher  short  circuit  cur- 
rents can  be  set  tip  by  the  interpolar  field  when  more  commutator 
bars,  and  more  turns,  are  short  circuited.  It  can  therefore  be 
assumed  that,  as  far  as  the  interpolar  field  is  concerned,  the  more 


208  ELECTRICAL  ENGINEERING  PAPERS 

commutator  bars  the  brush  covers  the  greater  will  be  the  short 
circuit  current  and  the  greater  will  be  the  difficulty  in  commuta- 
tion, assuming  there  is  no  external  field  assisting  commutation 
Chord  Winding.  With  a  pitch  winding,  with  no  lead  at  the 
brushes,  the  commutation  of  a  coil  will  occur  in  the  lowest  part 
of  the  armature  interpolar  flux,  as  a  a  in  Fig.  4.  With  a  chorded 
winding,  as  indicated  at  b  b,  the  commutation  will  occur  under 
somewhat  higher  flux  than  with  a  pitch  winding.  Therefore  in 
considering  the  interpolar  flux  a  full  pitch  winding  commutates 
linder  better  conditions  than  a  chorded  winding. 

END  FLUXES 

The  armature  winding  as  a  whole  sets  up  certain  fluxes  in  the 
end  windings.  These  fluxes  are  fixed  in  position  with  respect 
to  the  brushes,  and  the  armature  coils,  in  cutting  across,  them, 
generate  e.m.fs.  The  only  part  of  these  end  fluxes  concerned  in 


FIG.  4  FIG.  5 

the  present  problem  is  that  which,  the  commutating  coils  cu£ 
during  the  operation  of  commutation. 

Fig.  5  illustrates  an  armature  winding  na  which  the  heavy 
lines  represent  two  coils  in  contact  with  the  brushes  and  there- 
fore at  the  position  of  commutation.  It  is  only  the  end  flux 
density  along  the  shaded  portion  or  zone  of  this  diagram  which 
need  be  considered.  If  the  various  densities  for  this  zone  can 
be  determined,  then  the  e.m.f.  in  the  commutated  coil  can  be 
calculated.  0nly  the  usual  cylindrical  type  of  end  windings 
will  he  considered,  as  practically  all  direct  current  machines  at 
the  present  time  use  this  type.  Such  windings  are  usually  ar- 
ranged iii  two  layers,  the  coils  of  which  extend  straight  out  from 
the  armature  core  for  a  short  distance,  usually  |  in.  to  If  in., 
depending  upon  size  and  voltage  of  the  machine,  and  then  extend 
at  an  angle  to  the  core  of  30  deg.  to  45  deg.  The  conductors  of 
the  upper  and  lower  layers  therefore  usually  He  almost  at  right 
angles  to  each  other 


THEORY  OF  COMMUTATION 


209 


Pitch  Winding  Let  Fig.  6  represent  a  single  coil  of  the  end 
winding  located  in  the  commutating  zone.  Both  theory  and 
test  show  that  the  maximum  flux  density  in  this  zone  is  at  a 
and  tapers  off  slightly  to  6,  then  tapers  off  more  rapidly  from 
b  until  it  reaches  practically  zero  value  at  c.  It  may  be  assumed 
with  but  little  error  that  the  decrease  from  b  to  c  is  at  a  practi- 
cally uniform  rate.  The  flux  density  along  the  commutating 
zone  of  the  end  winding  may  therefore  be  represented  by  Fig.  7, 
in  which  the  ordinates  represent  flux  density.  On  the  above 
assumption  the  total  flux  in  the  commutating  zone  of  the  end 
winding  can  be  determined  with  sufficient  accuracy  if  the  density 
at  b,  for  instance,  can  be  determined  and  the  distances  a  b  and 
c  d  in  Figs.  6  and  7  are  known.  These  latter  can  be  determined 
directly  from  the  winding  dimensions. 


FIG.  6 


FIG.  7 


The  following  is  an  approximate  formula  for  the  flux  density 
at  b,  including  allowance  for  proximity  of  iron  end  plater  core,  etc. 


N  —  number  of  slots  per  pole. 
Ic   =  current  per  conductor. 
Wt  —  total  armature  wires. 
D   =  diameter  of  armature. 
Let  a  b  =  h,  and  c  d  =  m. 

Then  the  flux  cut  by  one  conductor  at  one  end  is 


2.15-1,  WtXlog  2N 


Therefore  the  e.m.f.  per  single  turn  of  the  armature  winding, 


210 


ELECTRICAL  ENGINEERING  PAPERS 


due  to  the  end  flux,  considering  the  end  fluxes  for  both  ends  of 
the  core,  becomes 


m_ \      2.15  lc  Wt  Xlog  2  AT    ir  D  RSX2  Tc 


TT  D  sin  6 


10* 


Or, 


N\ 

X] 


This  formula  is  on  the  basis  of  non-magnetic  paths  around  the 
end  windings,  that  is,  with  no  bands  of  magnetic  material  and  no 
magnetic  supports  under  the  coils,  The  effect  of  bands  over  the 
end  winding  is  approximately  equivalent  to  cutting  the  flux 
path  to  half  length  for  those  parts  of  the 
end  winding  covered  by  the  bands.  There- 
fore, with  bands,  the  diagram  representing 
flux  density  in  the  commutating  zone  of 
the  end  winding  would  be  as  indicated  in 
Fig.  8.  In  this  case  the  total  flux  corresponds  ]\ 

to  the  total  area  of  the  curve  including  the 
dotted  portion      Of  course  the  actual  flux 
distribution  would  not  be  exactly  as  shown       ^ 
in  this  diagram  for  there  would  be  sonie 
fringing  in  the  neighborhood  of  the  bands  FIG.  8 

The  diagram  simply  serves  to  illustrate  the 
general  effect  of  magnetic  bands  and  an  approximate  method  of 
taking  it  into  account. 

The  effect  of  a  magnetic  coil  support  will  be  very  similar  to 
that  of  a  steel  band  in  reducing  the  length  of  path  and  therefore 
increasing  the  flux  in  the  neighborhood  of  the  cpil  support. 
However,  in  case  of  magnetic  bands  over  the  winding  and  coil 
supports  under  it  the  limit  lies  in  saturation  of  the  bands  them- 
selves. This  usually  represents  a  comparatively  small  total  flux. 
The  coil  support,  however,  would  probably  not  saturate  in  any 
case. 

The  above  formula  for  end  flux  can  therefore  be  corrected 
for  magnetic  bands  and  coil  supports  by  multiplying  by  a  suit- 
able constant  to  cover  the  increased  flux". 

It  is  obvious  that  the  determination  of  the  end  flux  is,  to  £ 
certain  extent,  a  question  of  judgment  and  experience.  No 


THEORY  OF  COMMUTATION  211 

iixed  method  or  formula  can  be  specified  for  all  types  of  machines, 
for  this  flux  would  be  influenced  very  greatly  by  the  bands,  if  of 
magnetic  material,  and  by  the  material,  size  and  location  of  the 
coil  supports  and  their  relation  to  the  bands.  Also,  eddy  cur- 
rents may  be  set  up  in  the  coil  supports  which  will  influence  the 
•distribution  of  the  end  flux  in  the  zone  of  the  commutated  coil. 
However,  in  each  individual  case  an  approximation  can  be  made 
whioh  will,  in  general,  be  much  closer  than  would  be  obtained 
from  any  empirical  rule  or  by  neglecting  the  effect  of  the  end 
flux  altogether. 

Chord  Winding.  The  effect  of  chording  the  armature  winding 
is  to  slightly  diminish  the  flux  density  in  the  commutating  zone 
which  results  in  a  slight  reduction  in  the  e.m.f .  of  the  commu- 
tating coil.  But  a  relatively  much  greater  gain  is  obtained 
by  the  consequent  shortening  of  the  distance  c  d  in  Fig.  8  and 
the  corresponding  reduction  of  the  total  end  flux.  Due  to  the 
chording  itself  the  flux  density  at  b  is  reduced  practically  in  the 

ratio  of    i      o  AT  '  where  NI  —  number  of  slots  spanned  by  the 

coil.  For  example,  if  the  full  pitch  is  20  slots  and  the  coil 
spans  18  slots,  then  the  density  at  b  will  be  reduced  in  the 

t  O/* 

ratio  of  .  s   n  =0.971  due  to  the  chording  itself;  and  the  flux 

18 
along  cd,  Fig.  7,  will  be  further  reduced  in  the  ratio   of  ^ 

Z\J 

due  to  the  shorter  end  extension.  The  average  flux  along  c  d 
therefore  will  be  reduced  to  0.9X0.971  =  0.874,  or  about  87  per 
cent  of  that  of  a  pitch  winding. 

Effect  of  Brush  Width.  As  in  the  case  of  the  interpolar  flux  the 
width  of  the  brush,  or  the  number  of  armature  coils  short  cir- 
cuited by  the  brush,  has  practically  no  influence  on  the  e.m.f. 
generated  per  turn.  However,  the  total  effective  armature 
iampere  turns  will  be  reduced  slightly,  if  the  average  current  in 
the  short  circuited  turns  is  less  than  the  normal  current.  This 
will  have  a  very  slight  effect  on  the  e.m.f. 

SLOT  FLUX 

By  this  is  meant  the  magnetic  flux  across  and  over  the  arma- 
ture slots  which  does  not  extend  to  the  yoke  or  field  poles. 

Two  general  cases  will  be  considered;  first,  that  in  which  no 
local  currents  are  present,  which  is  the  case  in  well  designed 
interpole  machines;  and  second,  that  in  which  there  are  local 


212 


ELECTRICAL  ENGINEERING  PAPERS 


currents  set  up  in  the  short  circuited  coils,  which  is  almost 
invariably  the  case  in  machines  without  inter  poles  or  some  other 
form  of  compensation.  Also,  pitch  and  chorded  windings  will 
be  considered. 

SLOT  FLUX  WITH  No  LOCAL  CURRENTS 
Pitch  Winding.  Let  Fig.  9  represent  an  upper  and  a  lower 
coil  in  the  same  slot,  with,  equal  turns  and  currents.  Then  if 
there  is  no  saturation  in  the  adjacent  teeth  the  flux  density  across 
the  slot  will  be  zero  at  the  bottom  of  the  lower  coil  and  will 
rise  to  a  maximum  value  at  the  top  of  the  upper  coil.  There 
will  also  be  a  flux  across  the  slot  above  the  upper  coil  and  also 
from  the  top  of  the  tooth  as  indicated  in  Fig.  9.  The  total  slot 
flux  entering  at  the  bottom  of  the  teeth  is  therefore  equal  to  the 
total  flux  which  crosses  the  two  adjacent  slots,  plus  the  flux 
crossing  at  the  top  of  the  slots.  The  interpolar  fiux  which  ex- 


D 

a 


d 

fc 


FIG    10 


tends  from  the  armature  surface  to  the  poles  or  yoke  is  not  in- 
cluded in  this. 

As  this  slot  flux  is  practically  fixed  in  position  the  armature 
conductor  in 'slot  A,  in  passing  from  a  to  b  must  cut  this  flux. 
It  is  obvious  that  the  flux  which  crosses  above  the  uppermost 
conductor  in  the  slot  is  cut  equally  by  all  the  conductors  in  the 
slot,  as  the  coil  passes  from  position  a  to  position  b;  but  the  flux 
crossing  the  slot  below  the  uppermost  conductor  does  not  affect 
all  the  conductors  equally,  and  therefore,  for  simplicity  of  cal- 
culation, an  equivalent  flux  of  lower  value  can  be  used  which 
may  be  considered  as  cutting  -all  the  conductors  equally. 

Let  d    Fig.  10,  represent  the  depth  of  the  conductors  of  one 

complete  coil. 
/     represent  the  distance  between  the  upper  and  lower 

coils. 

a     represent  the  distance  from  the  upper  conductor  to 
the  core  surface. 


THEORY  OF  COMMUTATION  213 

s     represent  the  width  of  the  slot,  assuming  parallel  sides. 
n    represent  the  ratio  of  width  of  armature  tooth  to  the 

width  of  the  armature  slot,  at  the  surface  of  the  core. 
Tc  represent  turns  per  single  coil,  or  per  commutator  bar, 
Cs  represent  the  number  of  individual  coils,  or  commu- 

tator bars,  per  complete  coil. 
L    represent    the    width    of    armature    core,    including 

ventilating  spaces. 

Ic    represent  the  current  per  armature  conductor. 
Then,  ampere  turns  per  upper  or  lower  coil  =  /c  Tc  G. 

~  .  i  fl  .«  3.19  Ic  TcXCs-L  (2  d+f) 

Total  flux  across  coil  space  =  -  -  —         —  -  --  —*- 

™  1*1.  -1 

Flux  across  slot  above  coil  = 

Flux  from  tooth  top  across  the  slot  is  approximately, 
3.19  Ic  Tc  C,iX2X0.54  ^n 


Total  flux  above-upper  coil=3:19  Ic  Tc  CSL 


The  sum  of  the  two  fluxes  represents  the  total  flux  across  one 
slot  which  enters  at  the  bottom  of  one  tooth.  As  a  similar  flux 
passes  across  the  slot  at  the  other  side  of  the  tooth  the  total 
flux  entering  the  tooth  will  be  double  the  above  and  becomes 

Total  slot  flux=2X3.19  I,  Tf  C.  L   <2  *+*+*  '+L06  '  V"> 

5" 

This  total  flux  cannot  t>e  used  directly  in  the  calculations  as 
it  does  not  affect  all  the  conductors  equally.  It  is  therefore 
necessary  to  determine  equivalent  fluxes  for  the  upper  and 
lower  coils  which  can  be  used  instead  of  the  above  value. 

For  the  lower  coil  the  following  value  has  been  calculated: 

JVM  1          J       A  2X3.19  J.C   TC  CrS  L         ^.,     oOO    J    I     *\ 

The  equivalent  flux=  -  (LSSSa+f) 
And  for  the  upper  coil, 

•r>          -         *         ^      a  2X3.19Xi<T    TC    (*S-Lr  ~   OOQ    i 

Equivalent  flux  =  ---  O<  0.833d 

To  these  equivalent  fluxes  shotild  be  added  the  total  flux 
above  the  upper  coil.  This  gives  the  total  effective  flux  for  the 
upper  and  lower  Coils.  Then,  for  the  lower  coil, 


214  ELECTRICAL  ENGINEERING  PAPERS 

Total  effective  flux 

-2X3I9A7-  C,L 


And  for  the  upper  coil, 
Total  effective  flux 

=  2X3197,  T.  C5Lxt0.833d+2a+108sVB) 

The  average  value  of  the  effective  flux  for  the  upper  and 
lower  coils  then  becomes, 

3i9/.rcc.L(2-67rf+f+4a+2-16aVir) 

o 

(This  average  effective  value  is  approximately  80  per  cent  of  the 
total  slot  flux.) 

On  the  basis  of  a  pitch  winding  and  the  assumption  that  only 
one  armature  coil  is  short  circuited,  that  is,  with  the  brush 
covering  the  width  of  only  one  commutator  bar,  then  the  above 
slot  flux  is  cut  by  all  the  coils  in  the  slot  in  passing  through  one 
slot  pitch.  From  this  the  e.m.f  in  the  commutating  coil  due  to 
the  slot  flux  can  be  calculated  directly  and  may  be  expressed  as 
follows 

&  =  — TTtr-^-X  number  of  slots 


But  CsX  number  of  slots  =  No   of  commutator  bars 

_  total  number  of  conductors       Wt 

~ 


Therefore  the  above  expression  for  e.m.f  may  be  changed  to  the 
form, 


If  it  is  desired  to  compare  this  expression  with  a  certain  well 
known  fonmda  which  has  been  much  used  heretofore*  thea  let 


THEORY  OF  COMMUTATION  215 

the  quantity  in  the  parenthesis  in  the  above  expression  be  repre- 
sented by  cx.     The  formula  can  then  be  changed  to,, 

r?       /ON/Q   IQVX    \v^  Ic  T<?^< number  commutator 
Xi  -  =  (<&  X  o  .  lv  X  cx)  X ™ 


It  contains  the  same  terms  (except  in  the  value  of  the  constant) 
for  the  expression  of  the  e.m.f.  which  has  been  used  heretofore 
in  determining  the  reactance  of  the  commutated  coil. 

Effect  of  Brush  Width  or  Number  of  Commutator  Bars  Covered 
by  Britsh.  The  above  formulas  are  on  the  basis  of  the  brush 
covering  only  the  width  of  one  commutator  bar.  In  this  case 
all  the  conductors  of  one  slot  cut  across  the  entire  slot  flux  in 
passing,  through  one  tooth  pitch.  However,  if  the  brush  covers 
more  than  one  commutator  bar,  then  the  full  slot  flux  is  not  cut 
in  passing  through  one  tooth  pitch,  and  a  movement  greater  than 
one  tooth  pitch  is  required  for  full  cutting.  For  example,  if 
there  is  one  commutator  bar  per  armature  slot  and  the  brush 
covers  a  width  equal  to  two  commutator  bars,  then  the  total 
cutting  of  the  slot  flux  will  take  place  in  two  tooth  pitches. 
Again,  if  there  are  three  commutator  bars  per  armature  slot  and 
the  brush  covers  the  width  of  one  commutator  bar,  then  the  total 
cutting  of  the  total  slot  flux  would  occur  in  one  tooth  pitch,  while 
if  the  brush  covered  two  bars,  the  total  cutting  would  occur  in 
1J  tooth  pitches;  and  if  it  covered  three  bars  If  tooth  pitches 
are  required.  In  other  words,  the  total  cutting  will  occur  in  a 
period  corresponding  to  the  number  of  commutator  bars  per 
slot  plus  one  less  than  the  number  of  commutator  bars  covered 
by  the  brush. 

On  this  basis  the  correction  factor  for  the  slot  e.m.f.  should  be 

C        * 

expressed  by  the  term  ^  ,     *  —  —  f  where  Cs  =  number  of  com- 

~        — 


mutator  bars  per  slot,  and  B  »  =  number  of  commutator  bars 
spanned  by  the  brush.  However,  \*ith  several  coils  per  slot, 
and  with  the  brush  spanning  several  bars,  the  rate  of  cutting  of 
the  tooth  flux  for  the  entire  period  is  not  quite  the  same  as  the 
rate  for  one  tooth  pitch.  Taking  this  into  account  the  correc- 

Q 

tion  factor  should  not  be  equal  to  ~  .  PJ  —  =-,    but   is   slightly 

-        —  I 


greater.     Up  to  four  commutator  bars  per  slot,  and  three  bars 

period  of  coeqsmtatlon  is  obtaitwd 


factorv^     ,    ^  _^  t    the   overage   slot  e,  m.  1  for   the 


216  ELECTRICAL  ENGINEERING  PAPERS 

spanned  by  the  brush  the  correction  factor  can  be  expressed  by 

1  1      * 

the  term  1-i — = — ^-  —   "7^~- 

JJi    txj  L,s 

Taking  the  lengthened  period  of  reversal  into  account,  it 
would  appear  that  a  wide  brush  covering  a  large  number  of 
commutator  bars  should  be  beneficial  in  reducing  the  e.m.f 
generated  by  the  slot  flux.  This  is  true  where  the  local  currents 
ate  very  small,  or  are  absent,  as  is  the  case  in  a  properly  designed 
interpole  machine.  In  a  non-interpole  machine  where  the  local 
currents  in  the  short  circuited  coils  may  be  relatively  high,  this 
condition  does  not  hold,  as  will  be  explained  later. 

The  above  formula  for  e.m.f.  due  to  the  slot  flux  should  there- 
fore be  modified  by  multiplying  by  a  factor  which  takes  into 
account  the  period  of  reversal  as  affected  by  brush  width. 

Chord  Winding.  The  armature  winding  may  be  chorded  one 
or  more  slots  and,  in  some  instances,  where  there  are  several 
coils  side  by  side  there  has  been  chord- 
ing  of  part  of  the  conductors  in  the 
slot.  In  Fig.  11  is  illustrated  the 
conditions  with  one-slot  chording 
The  total  slot  flux  now  occupies  two 
teeth  instead  of  one.  Therefore  the 
e.m.f.  set  up  by  cutting  across  this 
slot  flux  will  be  approximately  one- 
half  that  which  is  obtained  with  a 
full  pitch  winding,  on  the  basis  of  the  brush  covering  the 
width  of  one  ba/  only,  for  the  e.m.f  generated  by  cutting  this 
flux  will  be  reduced  in  proportion  as  the  period  of  cutting  is 
increased  There  is  one  slight  difference  from  the  flux  distribu- 
tion with  a  pitch  winding,  namely,  that  at  the  top  of  the  teeth. 
With  a  chorded  winding  this  flux  will  be  slightly  greater  than 
with  a  pitch  winding,  but  the  total  effect  *of  this  difference  should 
be  relatively  so  small  that  ordinarily  the  value  need  not  be 
changed.  Therefore  equivalent  fluxes  used  with  chord  windings 
can  be  taken  the  same  as  for  pitch  windings.  In  consequence, 
the  e.m.f ,  due  -to  the  slot  flux,  with  one-tooth  chording,  may  be 
taken  as  one-half  that  for  a  pitch  winding,  with  the  brush  cover- 
ing one  commutator  bar  in  both  cases. 

For  two-slot  chording  the  slot  flux  may  be  considered  as  oc- 
cupying the  space  of  two  teeth  only,  while  there  will  be  a  mag- 
netically idle  tooth  at  the  center.  The  e.m.f .  per  coil  actually 
generated  by  cutting  the  slot  flux  will  be,  for  part  of  the  period 

^General  use  of  this  factor,  1  +        B^Cs C^ should,  be  avoided.       It  is  not 

applicable  to  all  tyres  or  ccirbinaticns  of  vir  dings      Use  instead  the  factor  Divine  averaee 


THEORY  OF  COMMUTATION  217 

the  same  as  for  one-slot  chording,  but  there  will  be  an  inter- 
mediate period  where  the  slot  e.m.f.  is  practically  zero,  which 
does  not  occur  with  a  one-slot  chording  or  with  a  pitch  winding. 
The  average  results,  however,  should  be  practically  the  same  as 
if  the  total  slot  flux  were  actually  distributed  over  three  teeth 
instead  of  two. 

Effect  of  Brush  Width  with  Chord  Winding.  In  the  chord 
winding,  when  the  brush  covers  two  or  more  commutator  bars, 
the  period  of  cutting  the  slot  flux  will  be  lengthened  just  as  with 
a  pitch  winding  on  the  assumption  of  no  local  currents  For 
example,  if  there  are  three  commutator  bars  per  armature  slot 
and  the  winding  is  chorded  one  slot,  then  with  the'  brush  covering 
one  .commutator  bar,  complete  cutting  of  the  slot  flux  will  occur 
in  the  space  of  six  commutator  bars.  If  the  brush  covers  three 
commutator  bars  instead  of  one,  then  complete  cutting  will  occur 
in  the  space  of  eight  commutator  bars,  while  in  a  corresponding 
full  pitch  winding  it  would  occur  in  ths  space  of  five  bars.  There- 
fore, the  wide  brush  represents  an  improvement  with  the  chorded 
winding,  but  not  to  the  same  extent,  relatively,  as  with  the  pitch 
winding.  This  is  on  the  assumption  of  absence  of  local  currents 
in  the  short  circuited  coils.  * 

Bands  on  A  rmature  Core.  By  the  preceding  method  of  analysis 
the  effect  of  bands  of  magnetic  material  on  the  armature  core  can 
be  readily  taken  into  account.  This  effect  represents  simply  an 
addition  to  the  total  flux  which  can  pass  up  the  tooth  and  across 
the  top  of  the  slots.  From  the  ampere  turns  per  slot,  the  clear- 
ance between  the  bands  and  the  iron  core,  the  total  section  o±  the 
band,  etc.,  the  flux  due  to  the  band  can  be  calculated  This  flux 
can  either  be  combined  directly  with  the  slot  flu*  already  de- 
scribed and  the  resultant  e.m.f.  can  then  be  calculated;  or,  the 
e.m.f.  can  be  calculated  independently  for  the  baud  flux  alone 
Magnetic  bands  on  the  'armature  introduce  a  complication  into 
the  general  e.m.f  .  formula  due  to  the  fact  that  in  many  cases  the 
flux  into  the  bands  Is  such  as  to  highly  saturate  the  band  material 
at  relatively  low  armature  currents.  This  flux  therefore  is 
usually  not  proportional  to  the  armature  ampere  turns.  If  the 
e.m.f.  due  to  the  band  Sux  is  to  be  calculated  separately,  the 
following  formula  can  be  used: 

4&  represents  the  total  magnetic  flux  in  the  hand  from  the 
armature  core  considering  both  directions  from  the  tooth,  then 

2  <t>b  N  p  Tc  Rs   . 


^Considering  chortling,  the  correction  factor  ^  —  -:--=  -  =  becomes 

-  — 


-=  -  =  - 

&€  —    J,  *-  j   -f    *>t  —    A    -f  «• 

wheie  K  »»  slots  chorded  X  C*.      For  tie  general  case,  where  there  aie  Pf  circuits  and  F 
poles,  the  correction  factor  becomes  -  *+$  - 


218  ELECTRICAL  ENGINEERING  PAPERS 

This  formula  holds  true  for  the  band  flux  which  passes  throug 

the  one  tooth  in  the  pitch  winding.     Proper  allowance  must  be 

made  for  the  effect  of  chord  windings  and  brush  width,  which 

can  be  done  by  the  methods  already  described. 

SLOT  FLUX  WITH  LocAt  CURRENT* 

PitchWinding.  In  the  prsceding  analysis  local  currents  have 
not  been  included,  as  the  method  would  be  greatly  complicated 
by  taking  such  currents  into  account.  In  the  general  method, 
given  below  the  effect  of  local  currents  in  the  short  circuited  coils 
can  be  most  easily  shown. 

As  already  explained,  an  armature  coil,  as  it  approaches  the 
short-circuit  condition,  has  an  e,m.f.  generated  in  it  by  the  inter- 
polar  and  the  end  fluxes.  After  the  coil  is  short  circuited  this 
e.m.f ,  is  still  generated  by  the  coil  and  naturally  a  local  or  short 
circuit  current  tends  to  flow  through  the  coil,  brush  contact 


and  brush.  In  addition,  the  work,  or  supply,  current  is  being 
furnished  to  the  armature  winding  through  the  brushes.  These 
two  currents  are  superimposed  in  the  short  circuited  winding 
in  such  a  way  as,  to  have  a  very  pfonounced  influence  in  the 
distribution  of  the  slot  fluxes.  This  effect  can  be  bast  seen  by 
first  determining  the  distribution  of  the  work  current  in  the 
various  parts  of  the  short  circuited  winding  on  the  assumption 
of  n&  local  current  and  second,  determining  the  distribution 
of  the  local  currents  on  the  assumption  of  no  work  current,  but 
with  the  same  armature  magnetomotive  force  as  in  the  first  assump- 
tion. The  two  distributions  can  then  be  combined  and  the  re- 
sultant currents  in  the  various  parts  of  the  short  circuited  coils 
can  be  obtained. 

Let  Fig.  12  represent  the  first  assumption  in  which  no  local 
currents  are  present.  In  order  to  illustrate  conditions  to  better 
advantage,  four  commutator  bars  are  assumed  to  be  covered  by 
the  brush.  Uniform  distribution  of  current  over  the  brush  con- 
tact can  be  assumed  in  this  case,  as  there  are  no  local  currents* 


THEORY  OF  COMMUTATION 


219 


Tracing  out  the  current  in  each  short  circuited  coil  in  Fig.  12, 
it  will  be  seen  that  the  current  decreases  at  a  uniform  rate  and 
then  rises  in  the  opposite  direction  at  the  same  rate  until  the 
short  circuit  is  removed  The  period  of  commutation  is  the 
longest  possible  with  this  number  of  commutator  bars  short 
circuited,  and  the  brush  conditions  are  ideal,  as  the  current 
density  at  the  brush  contact  is  uniform  at  all  parts  The  above 
are  the  conditions  which  the  designer  endeavors  to  obtain  in  the 
construction  of  good  Interpole  machines,  as  will  be  shown  later. 

In  Fig  13  the  same  arrangement  of  winding  and  brushes  is 
chosen  as  in  Fig.  12  except  that  only  the  local  currents  are  shown 
and  the  values  of  these  are  assumed  as  proportional  to  the 
e.m.fs.  in  the  short  circuited  coils  and  the  resistance  in  circuit.  In 
this  diagram  the  current  is  a  minimum 
in  the  coils  at  the  moment  that  short 
circuit  occurs,  and  rises  to  a  maximum 
value  and  then  diminishes  to  zero  value 
again  at  the  end  of  the  short  circuit 


FIG    14 


FIG    15 


In  Fig  14  the  currents  of  Fig.  12  and  13  are  supenmposed 
The  resultant  currents  in  the  various  parts  of  the  short  circuited 
winding  are  seen  to  rise  after  short  circuit  until  a  maximum  value 
is  reached  and  then  decrease  rapidly  and  reverse  to  normal 
value  m  the  opposite  direction.  Therefore,  the  period  from 
normal  value  of  the  current  to  normal  in  the  opposite  direction 
is  very  much  shorter  than  when  no  local  currents  are  present 
It  may  therefore  be  considered  that  the  period  of  reversal  is 
much  reduced  by  the  presence  of  the  local  currents,  so  that  the 
e  rruf  m  the  short  circuited  armature  conductors  generated  by 
the  slot  flux  is  proportionately  increased,  compared  with  the 
value  it  would  have  in  case  the  local  currents  were  absent 
These  conditions  can  be  shown  possibly  in  a  somewhat  better 
manner  by  curves  a,  b  and  c  in  Fig.  15  The  curve  a  shows  the 
distribution  of  current  in  the  short  circuited  coils  without  any 
local  currents  Curve  b  shows  the  distribution  of  local  currents 


220 


ELECTRICAL  ENGINEERING  PAPERS 


while  curve  c  shows  the  resultant  of  the  two.  The  distance  be- 
tween d  and/  on  curve  c  gives  the  period  of  reversal  from  normal 
current  in  one  direction  to  normal  current  in  the  opposite  direc- 
tion. This  period  is  much  shorter  than  the  full  period  repre- 
sented by  g  f  which  would  be  obtained  without  local  currents. 
The  period  df,  however,  may  not  differ  much  from  the  period 
of  commutation  with  the  brush  covering  the  width  of  only  one 
bar,  when  the  local  current  is  high  compared  with  the  work  cur- 
rent. In  such  case  the  gain  in  the  period  of  commutation  which 
should  be  obtained  by  means  of  the  wider  brush  may  be  practi- 
cally offset  by  the  effect  of  the  local  currents  which  also  increase 
with  the  wider  brush,  so  that  over  a  considerable  r£nge  the 
resultant  of  the  two  effects  may  be  practically  constant.  This 
is  one  indication  why,  in  non-interpole  machines,  the  brush 
width  may  be  varied  over  quite  a  range  with  relatively  small 
noticeable  difference  in  the  commutation.  This  may  be  il- 


FIG.  16 


FIG.  17 


lustrated  by  Pig.  16,  in  which  is  shown  the  current  conditions 
with  two  to  five  bars  spanned.  In  this  figure  a  b,  b  c,  c  d,  etc., 
each  represent  the  width  of  one  commutator  bar.  Therefore, 
curve  A,  extending  over  the  width  ac,  represents  two  bars 
spanned.  The  period  of  reversal  of  the  current  from  normal 
value  in  one  direction  to  normal  in  the  opposite  direction  is 
represented  by  g  c  for  curve  A ,  h  d  f or  curve  B,  i  e  for  C  and 
kf  for  D.  A  comparison  of  these  values  is  interesting.  Calling 
a  b  the  period  of  reversal  with  the  brush  covering  one  bar  only, 
then  g  c  with  two  bars  covered,  is  greater  than  a  b.  kdis  also 
greater  than  a  &,  but  less  than  g  c,  while  i  e  is  slightly  less  than 
ab,  and  kf  is  considerably  less.  However,  the  variation  be- 
tween g  c  and  kf  is  much  less  than  between  a  c  and  af  which 
would  be  the  corresponding  periods  with  no  local  currents. 

It  should  be  borne  in  mind  that  the  above  curves  are  only 
relative,  depending  upon  the  comparative  values  of  the  local 
and  work  currents  and  assuming  a  constant  brush  resistance' 


THEORY  OF  COMMUTATION  221 

which  is  not  correct,  but  they  serve.  Do  illustrate  the  general 
principle  This  method  of  presentation  is  simply  a  skeleton  of 
the  problem  of  commutation  when  local  currents  are  present 
in  ths  short  circuited  coils  and  it  would  be  beyond  the  scope  of 
this  paper  to  attempt  a  full  solution. 

Effects  of  Field  Distortion.  One  of  the  "  bugaboos  "  of  the 
designer  of  commutating  machines  has  been  the  question  of  field 
distortion.  It  has  usually  been  considered  that  when  the  ma- 
chine is  loaded  the  magnetic  field  is  more  or  less  distorted  or 
shifted  from  its  nprmal  no-load  position  and  that  commutation 
is  affected  by  this  distorted  field. 

To  state  the  case  plainly,  the  field  distortion  has  practically 
nothing  to  do  with  the  problem.  The  distorted  field  magnetism 
is  simply  a  resultant  of  the  no-load  main  field  flux  combined 
with  that  due  to  the  armature  winding.  Therefore,  the  two 
components  of  the  distorted  full-load  field  are  the  no-load  main 
field,  which  is  fixed  in  space  and  is  usually  practically  constant, 
and  the  armature  field,  which  is  also  fixed  in  space  but  varies 
with  the  load.  If  the  brushes  are  set  in  a  certain  position  with 
respect  to  the  no-load  field,  then,  as  this  component  of  the  re- 
sultant full  load  field  is  practically  fixed  in  space  and  in  value, 
It  has  no  variable  influence  on  the  commutating  conditions. 
The  true  variable  element  which  does  affect  the  commutation 
is  the  armature  field,  or  flux,  and  it  is  in  this  very  flux  which  is  the 
basis  of  the  preceding  theory  of  commutation.  Therefore,  the 
distorted  resultant  field  of  a  loaded  machine  does  not  present 
any  new  condition  in  the  problem  of  commutation.  Ose  ex- 
ception, however,  can  be  made  to  the  above,  namely,  where 
there  is  any  considerable  saturation  in  the  armature  teeth  or 
in  the  main  field  pole  corners.  The  effect  of  the  armature  mag- 
netomotive force  is  to  strengthen  one  corner  or  edge  of  the  field 
pole  and  to  weaken  the  other  edge,  but  when  saturation  is  pro- 
nounced the  strengthening  action  is  much  less  than  the  weaken- 
ing action.  The  resultant  of  these  actions  is  a  decrease  in  the 
total  value  of  the  main  field  flux.  If,  now,  this  main  field  flux 
be  brought  back  to  its  normal  total  value,  or  higher,  a  very  con- 
siderable addition  to  the  main  field  magnetomotive  force  will  be 
necessary,  which  wilt  be  effective  in  increasing  the  field  flux  at 
the  weaker  pole  corner  to  a  much  greater  extent  than  at  the 
highly  saturated  pole  corner.  In  consequence,  with  load,  the 
main  field  distribution,  or  field  form,  may  be  considered  as  being 
changed  from  its  no-load  form  A,  to  the  fonn  B,  as  indicated 
in  Fig.  17.  It  is,  in  reality,  strengthened  at  a  point  b,  for 


222  ELECTRICAL  ENGINEERING  PAPERS 

example.  In  such  case  the  main  field  will  have  a  variable  in- 
fluence on  the  commutation,  if  the  brush  is  set  with  a  lead,  as 
at  &,  and,  to  a  slight  extent,  the  effect  of  an  interpole  is  thus 

obtained. 

Effect  of  Brush  Lead.  Before  taking  up  the  problem  of  inter- 
poles  on  direct  current  machines  it  might  be  well  to  consider  the 
effect  of  brush  lead,  as  this  gives  a  result  intermediate  between 
true  intsrpole  and  non-interpole  commutating  conditions. 

The  precedmg  formulas  apply  to  non-commutating  pole 
machines  without  brush  lead.  However,  except  in  case  of  re- 
versing machines,  such  as  street  railway  motors,  or  hoist  motors, 
etc.,  it  is  usual  practice  to  give  a  forward  lead  to  the  brushes 
of  direct  current  generators  or  a  slight  backward  lead  to  direct 
current  motors.  The  effect  of  giving  a  lead  at  the  brushes  of  a 
non-interpole  direct  current  machine  may  be  considered  as 
being  equivalent  to  the  effect  of  an  interpole  with  the  exception 


FIG.  18  FIG.  19 

that  correct  flux  conditions  and  proper  commutation,  with  any 
given  brush  setting,  are  obtained  only  for  one  given  load. 

As  described  before,  with  a  non-interpole  machine  the  arma- 
ture winding  sets  up  a  flux  in  the  interpolar  space.  With  no- 
lead  at  the  brushes  this  flux  is  usually  a  minimum  midway  be- 
tween the  poles  and  ris,es  toward  the  polar  edges.  The  flux 
from  the  adjacent  main  poles  has  a  zero  value  midway  between 
the  poles  and  rises  toward  the  polar  edges,  but  has  opposite 
polarities  at  the  two  sides  of  the  midpoint.  This  is  illustrated  in. 
Fig.  18.  The  resultant  of  the  armature  and  field  fluxes  is  indi- 
cated by  the  dotted  line  A.  This  resultant  falls  to  zero  at  one 
side  of  the  midpoint  and  then  rises  in  the  direction  opposite  to 
that  of  the  flux  due  to  the  armature  ampere  turns.  At  the  other 
side  of  the  midpoint  the  two  fluxes  add,  giving  an  increased  re- 
sultant flux  m  the  same  direction  as  the  interpolar  flux  due  to  the 
armature.  Prom  this  figure  it  is  evident  that  if  the  point  of 


THEORY  OF  COMMUTATION  223 

commutation  is  shifted  from  a  to  the  point  of  zero  interpolar 
flux  6,  then  commutation  will  occur  without  any  interpolar  flux 
to  be  taken  into  account,  that  is,  the  e.m.f.  generated  by  the 
short  circuited  armature  conductors  may  be  due  to  the  slot  and 
end  winding  fluxes  only.  If  the  brushes  are  shifted  still  further 
in  the  same  direction  to  c,  then,  not  only  will  the  interpolar  arma- 
ture flux  be  annulled  but  a  flux  in  the  opposite  direction  would  be 
cut  by  the  short  circuited  armature  conductor,  which  will  gen- 
erate an  e.m.f.  in  opposition  to  that  due  to  the  armature  fluxes 
in  the  slots  and  end  windings.  Consequently,  the  commutation 
can  be  materially  assisted  by  such  lead  at  the  brushes. 

The  difficulty  in  the  use  of  this  method  of  commutation  lies 
In  the  fact  that  the  commutating  or  reversing  flux  at  c  is  the 
resultant  of  the  main  field  flux  and  tha  armature  interpolar  flux 
at  this  point,  and  the  latter  flux  varies  with  the  load,  while  the 
former  remains  practically  constant.  Therefore  the  zero  point 
of  the  resultant  field  shifts  backwards  or  forwards  with  change  in 
load  and  the  density  of  the  commutating  field  beyond  the  zero 
point  will  therefore  change  with  the  armature  current.  In  con- 
sequence, if  the  brushes  are  shifted  into  a  suitable  resultant  field 
c  at  a  given  current,  then  with  a  different  load  the  intensity  of 
this  field  at  c  will  be  changed,  and  unfortunately  the  change  will 
be  in  the  opposite  direction  from  that  desired.  In  other  words, 
the  density  of  this  resultant  field  will  decrease  with  increase  in 
load,  whsreas  just  ths  opposite  effect  is  desired  for  good  commu- 
tation over  a  wide  rangs  in  load. 

In  practice,  however,  an  average  condition  is  found  which,  in 
many  cases,  will  give  reasonably  good  commutation  over  a  rela- 
tively wide  range  in  load.  The  brushes  may  be  shifted  at  no- 
load  into  an  active  field  in  such  a  way  as  to  generate  an  e.m.f. 
in  the  armature  coils  of  a  comparatively  high  value.  This 
e.m.f.  will  circulate  considerable  local  current  through  the  brush 
contacts  and  the  amount  of  lead  which  can  be  given  is  dependent, 
to  a  certain  extent,  upon  the  amount  of  local  current  which  can 
thus  be  handled  without  undue  sparking 

As  the  load  is  increased  the  strength  of  the  resultant  field, 
corresponding  to  this  brush  position,  will  be  decreased,  and  with 
some  value  of  the  current  this  field  will  be  reversed  in  direction. 
At  this  point  the  e.m.f  due  to  this  field  is  added  to  the  e.m.f. 
due  to  the  slot  and  end  winding  fluxes.  Obviously  the  limiting 
condition  of  commutation  will  be  reached  at  a  much  higher  cur- 
rent than,  would  be  the  case  if  ao  load  at  all  had  been  given.  This 
condition  is  represented  in  Fig.  19,  in  which  curves  1,  1,  2,  2,  3,  3, 


224  ELECTRICAL  ENGINEERING  PAPERS 

etc.,  represent  the  armature  and  resultant  flux  distributions  with 
various  loads.  In  this  figure  the  brushes  are  given  a  lead  so  that 
commutation  occurs  at  a  point  corresponding  to  b. 

It  is  obvious  that  at  heavy  load  a  still  greater  lead  at  the 
brushes  might  give  improved  commutating  conditions.  How- 
ever, if  the  load  were  suddenly  removed  without  moving  the 
brushes  toward  a,  then  the  short  circuited  coils  would  be  cutting 
the  main  field  at  such  density  that  serious  sparking  or  flashing 
might  occur. 

One  serious  objection  to  this  method  of  commutation  is  that 
the  distribution  of  the  resultant  field  is  practically  such  that 
equally  good  commutation  cannot  be  obtained  for  all  the  coils 
in  one  slot  when  there  are  several  coils  or  commutator  bars  per 
slot.  All  the  coils  of  one  slot  must  pass  under  a  given  position 
or  value  of  the  interpolar  magnetic  field  at  the  same  instant, 
while  the  commutator  bars  to  which  these  coils  are  connected 
must  pass  under  the  brush  Consecut-vely.  If  the  field  intensity 
is  just  right  for  good  commutation  a$  the  first  coil  per  slot 
passes  under  the  brush,  then  it  may  be  entirely  too  great  by  the 
time  the  last  coil  is  commutated.  For  good  commutation  with 
a  number  of  coils  in  one  slot,  the  resultant  interpolar  flux  should 
have  practically  constant-  value  over  the  whole  range  repre- 
sented by  the  period  of  commutation  of  all  the  coils  in  one  slot. 
This  condition,  however,  is  extremely  difficult,  or  is  frequently 
impracticable,  to  obtain  with  the  ordinary  non-interpole  ma- 
chine. 

The  above  treatment  of  the  problem  of  the  effect  of  the  brush 
lead  has  been  based  upon  the  armature  interpolar  magnetic 
field  being  located  in  the  same  position  with  lead  as  when  there 
is  no  lead  at  the  brushes.  It  has  been  assumed  heretofore  that 
the  non-interpolar  flux  due  to  the  armature  winding  has  a  mini- 
mum value  midway  between  the  main  poles  and  rises  uniformly 
toward  two  adjacent  pole  corners.  This,  however,  is  only  true 
when  the  point  of  commutation,  or  brush  setting,  is  midway  be- 
tween the  poles.  When  the  brushes  are  shifted  toward  either 
pole  the  point  of  maximum  armature  magnetic  potential  is 
shifted  in  the  same  way.  This  means  that  the  distribution  of 
the  armature  interpolar  flux  will  be  modified  directly  by  the 
position  of  the  brushes.  Instead  of  rising  uniformly  toward  th3 
two  pole  corners,  with  a  minimum  value  midway  between,  it 
will  have  a  minimum  at  one  side  of  the  midpoint,  this  being  at 
the  opposite  side  from  the  point  of  brush  contact,  and  will  have 


THEORY  OF  COMMUTATION  *  225 

an  increased  value  on  the  side  toward  which  the  brushes  are 
shifted.  This  is  illustrated  in  Fig.  20  in  -which  A  represents  the 
armature  .interpolar  flux  distribution  with  the  brush  at  a,  while 
B  represents  it  with  the  brush  at  b. 

This  increased  armature  interpolar  flux  due  to  the  brush 
shifting  means  that  the  resultant  interpolar  flux  due  to  both  the 
armature  and  main  field  fluxes  will  cross  the  zero  line  at  a  point 
further  removed  from  the  midpoint  than  in  the  case  of  no  lead 
at  the  brushes.  Consequently,  in  order  to  obtain  a  given  useful 
commutating  field  the  brushes  must  be  given  a  greater  amount  of 
lead  and  this  in  turn  shifts  the  zero  point  still  further  Thus, 
the  act  itself  of  shifting  the  brushes  makes  the  commutating  con- 
ditions more  difficult. 

The  calculation  of  the  comrnutating  conditions  with  any  given 
lead  therefore  resolves  itself  into  a  determination  of  the  re- 


FIG    20 

sultant  fluxes  in  which  the  coil  is  short  circuited  or  commutated 
and  the  e.m  fs  generated  by  such  fluxes.  For  the  slot  and  end 
winding  fluxes  the  calculation  will  be  the  same  as  for  no-lead  at 
the  brushes  The  resultant  flux  in  the  interpolar  space  is  the 
only  condition  which  will  introduce  any  variation  from  the  pre- 
ceding formulae  and  methods  of  calculation  This  part  of  the 
problem  resolves  itself  simply  into  the  determination  of  the 
resultant  interpolar  flux  at  the  point  of  commutation  for  any 
given  load  The  corresponding  e.m.f  can  then  be  calculated. 
This,  combined  with  the  e.m.fs.  due  to  the  slot  and  end  windings, 
gives  the  total  short-circuit  e.m.f  The  method  is,  in  principle, 
exactly  the  same  as  given  before,  except  that  the  determination 
of  the  interpolar  flux  will  be  modified. 

Summation  of  Formula  In  order  to  obtain  the  total  voltage  in 
the  short  circuited  coil  a  summation  should  be  made  of  the  four 
separate  voltages  which  have  been  derived  for  the  interpolar, 


226  ELECTRICAL  ENGINEERING  PAPERS 

end,  slot  and  band  fluxes.  In  reality  it  is  the  resultant  fluxes 
which  should  be  combined,  but  as  the  voltages  to  be  derived 
from  these  fluxes  represent  somewhat  different  terms,  a  better 
procedure  appears  to  be  the  summation  of  the  voltages.  Also, 
in  practice  it  is  the  e.m.fs.  generated  by  the  different  fluxes, 
rather  than  the  fluxes  themselves,  which  are  desired. 
The  e.m.f.  derived  from  the  interpolar  flux  is 

WiTcRs  2pirDL 


10s  (0.25  £+0.5)  (jrD-Pp) 

where  Ci  is  a  correcting  factor  for  chord  winding,  etc. 
The  formula  for  the,  end  flux  voltage  is, 

„  _     ^Ic  Wt  TeR>^-4.3(2h+m) 
*~  -CaX         HP x      ihT0 

where  d  represents  the  correcting  factor  for  chord  windings,  etc. 
The  formula  for  the  slot  flux  voltage  is, 

3.19  Ic  Wt  TCRSL     (2.666  d+4a+t+2.16s  Vn) 
&c-c3X  1Q8  - 

where  c3  is  the  correcting  factor  for  the  brush  width,  chord 
winding,  etc.,  and, 
For  the  bands, 


where  c4  is  the  correcting  factor  for  chord  winding,  brush  width, 

etc. 

Therefore, 


<-*-i-**     '*     .   _       7T 
total-         ^          L'    ((0.25/^+0.5) 


2<t>NpTcRs 


THEORY  OF  COMMUTATION  227 

It  is  evident  from  this  last  equation  that  when  there  are  no 
bands  over  the  core  the  total  e.m.f.  in  the  short  circuited  coil 
is  directly  proportional  to  the  current  per  armature  coil  or  con- 
ductor. If  the  bands  saturate,  as  would  usually  be  the  case 
with  any  considerable  load,  then  the  e.m.f.  is  no  longer  directly 
proportional  to  the  current.  Attention  is  called  to  this  point 
as  it  has  some  bearing  in  the  design  of  interpole  machines. 

Condensed  Approximate  Formula.  The  above  formula  can 
be  simplified  very  considerably  by  certain  approximations  which 
introduce  but  little  error  within  the  range  of  ordinary  design 

First,  the  expression,  (025/)+05f  (irD_Pp}  does  not  seem 

to  be  capable  of  any  general  simplification.  In  fact,  as  shown 
from  its  derivation,  it  is  not  a  general  term,  but  applies  only  to 
certain  constructions  and  may  appear  in  a  quite  different  form 
for  other  constructions.  Therefore  this  expression  must  be 
used  with  judgment  in  any  case.  Moreover,  this  term  appears 
only  in  non-interpole  machines  or  in  interpole  machines  only 
when  the  interpoles  are  narrower  than  the  armature  core  or  the 
number  of  interpoles  is  less  than  that  of  the  main  poles.  There- 
fore this  term  may  be  neglected  in  many  cases  where  interpoles 
are  used. 

/O    7L     I     ft-\ 

Second,  the  expression  4.3  -W-:  —  ^p-  log  2  N  can  be  changed 
as  follows: 

4-3      /  •    m     =  —  -  —  *  with  reasonable  accuracy  within  the 
(sin  u)  p 

ordinary  limit  of  design, 

And  log  2  N=  0.9+0.035  N,  with  an  error  of  about  4  per  cent 
within  the  range  of  6  to  24  slots. 


Therefore  4.3  -"  log  2N=---    (0.9+0.035    N}r 

(sin  u)  p 

approximately  . 

This  is  simpler  to  handle,  in  practice,  than  the  original  term. 


Third,  the  expression,  *-™wr'T*-"*  v"  can  ^  sim, 

plified  very  materially. 

Let  the  total  depth  of  slot  be  represented  by  d*,  which  is  equal 
to  2  d+a+1.5  t,  approximately. 

4ds      8  a  — 3  / 

Then,  the  term,  2.666  d+±  a +t  can  be  changed  tc— ^-  ^ ^ — 


228  ELECTRICAL  ENGINEERING  PAPERS 

Assuming  a »0.25  and  J«0.15,  then 

Q  __  O     j 

5 =0.52  approximately. 

o 

_.       ,         2.6Qd+4a+t      id,   .  0  52 
Therefore,  _ -37+— 

This  is  a  very  close  approximation  within  the  ordinary  work- 
ing range  of  slot  dimensions.  Therefore,  the  above  expression 

becomes,  0  *  H — '• 1-2. 16  V#,  which  is  much  simpler  to  use  in 

3  s         $ 

practice. 

Fourth,  in  the  simplified  equation  TT  appears  in  the  first 
and  second  terms,  and  3.19  appears  in  the  third  term.  These 
are  so  nearly  equal  that  TT  may  be  used  as  a  common  factor  for 
the  three  terms. 

The  combined  formulas  for  the  total  voltage  per  armature 
coil  thus  becomes,  in  approximate  form, 

2pDL 


"  10*  (0.25£+0.5)(7rZ>-P/>) 

4  D 

+C*  ^f-  (0.9+0.035  N)  + 
P 

* 

(1.33  &+0.52+2.16*  Vn)1  ,        2  <fr>  N}>  Tc  R, 
-   -        -     - 


. 
c>L 


This  appears  to  be  about  as  simple  a  form  as  the  equation  can 
be  put  into  when  all  the  factors  are  to  be  included-  It  will  be 
shortened  for  machines  without  magnetic  bands  on  the  core 
and  in  many  interpole  machines  the  term  derived  from  the  inter- 
polar  flux  may  be  omitted.  For  a  given  line  of  machines  which 
are  all  of  similar  design,  etc.,  it  is  probable  that  the  terms  can 
be  further  combined  and  simplified. 

INTERFOLAR  MACHINES 

In  the  interpole  machine  a  small  pole  is  placed  between  two 
adjacent  main  poles  for  the  purpose  of  setting  up  a  local  magnetic 
flux  under  which  the  armature  coil  is  commutated.  This  local 

*For  ordinary  working  range  of  slot  dimensions,  216  JV*—  1.07  X  tooth  pitch  at  ar- 
mature surface.  This  formula  may  be  further  simplified  by  substituting  1,07  Pt  for  2.16 
s  v/n.  Pt  being  the  tooth  pitch  at  the  armature  surface. 


THEORY  OF  COMMUTATION  229 

flux,  in  order  to  assist  commutation,  must  be  opposite  in  direction 
to  the  interpolar  flux  set  up  by  the  armature  winding  itself.  To 
set  up  this  flux  in  the  opposite  direction  the  magnetomotive 
force  of  the  interpole  winding  obviously  must  be  greater  than 
that  of  the  armature  winding  in  the  commutating  zone. 

An  armature  coil,  cutting  across  this  interpole  flux,  generates 
an  e.m.f.  proportional  to  the  flux,  the  speed  and  the  number  of 
conductors  in  series.  This  e.m.f.  is  in  opposition  to  the  e.m.f. 
in  the  short  circuited  coils,  generated  by  the  slot  and  end  winding 
fluxes.  For  ideal  commutation  these  e.m.fs.  are  not  only  in 
opposition,  but  they  should  also  be  of  practically  equal  value. 
For  perfect  commutation  the  current  in  a  short  circuited  coil 
should  die  down  to  zero  value  at  about  a  uniform  rate  and 
should  then  rise  to  normal  value  in  the  opposite  direction  by  the 
time  the  coil  passes  out  from  under  the  brush,  as  was  illustrated 
in  Fig.  12.  This  is  the  condition  when  no  local  currents  are  de- 
veloped in  the  short  circuited  coils  and  this  can  only  be  obtained 
when  the  interpole  e.m.f,  at  all  times,  balances  the  armature 
e.m.fs.  in  the  short  circuited  coils. 

Looking  at  the  problem  broadly,  the  resultant  magnetic 
fluxes  and  e.m.fs.  may  be  assumed  as  made  up  of  two  com- 
ponents which  can  be  considered  singly.  One  of  these  com- 
ponents is  that  which  would  be  obtained  with  the  armature 
magnetomotive  forte  alone  acting  through  the  various  flux  paths, 
including  the  interpole.  The  other  would  be  that  which  would  be 
obtained  with  the  full  interpole  magnetomotive  force  alone,  the 
armature  magnetomotive  force  being  absent.  Saturation  is  not 
considered  in.  either  case. 

Considering  the  first  component,  due  to  the  armature  mag- 
netomotive force  alone,  there  would  be  the  slot  and  the  end 
fluxes  with  their  short  circuit  e.m.fs.,  as  already  described, 
and  in  addition,  there  would  be  a  relatively  high  flux,  and  short- 
circuit  e.m.f.  due  to  the  good  magnetic  path  furnished  by  the 
interpole  core.  In  case  the  interpole  does  not  cover  the  full 
width  of  the  armature,  or  the  number  of  interpoles  is  less  than 
the  main  poles,  there  will  also  be  some  interpolar  flux  and  e.m.f. , 
as  already  described. 

Considering  the  second  component,  the  entire  interpole  mag- 
netomotive force  would  set  up  a  relatively  high  flux  through  the 
interpole  magnetic  circuit  and  a.  correspondingly  high  e.m.f. 
would  be  generated  in  a  sfoort  drcuited  armature  coil  cutting 

fhift  flux. 


230  ELECTRICAL  ENGINEERING  PAPERS 

When  these  two  components  are  superimposed,  it  is  seen  that 
the  interpole  flux  due  to  the  armature  magnetomotive  force  is 
in  direct  opposition  to  that  due  to  the  interpole  magneto- 
motive  force  and  therefore  only  the  e.m.f.  due  to  their  dif- 
ference need  be  considered.  As  the  interpole  winding  has  the 
higher  magnetomotive  force,  the  resultant  interpole  e.m.f.  is 
in  opposite  direction  to  the  armature  e.m.fs.,  and  should  be 
sufficient  to  neutralize  them.  This  way  of  considering  the  prob- 
lem avoids  a  number  of  confusing  elements  which  would  com* 
plicate  the  explanation  if  given  in  detail. 

In  practice  it  is  difficult  to  obtain  exact  equality  between  the 
interpole  and  armature  e.m.fs.  That  due  to  the  armature 
fluxes  is  generated  in  all  parts  of  the  coil  including  the  end 
winding,  while  the  e.m.f.  due  to  the  interpole  flux  is  generated 
only  in  that  part  of  the  coil  which  lies  in  the.  armature,  slots, 
However,  it  makes  no  difference  in  what  part  of  the  coil  the  e.m.f. 
due  to  the  interpole  is  generated  provided  it  is  of  such  value  that 
it  properly  opposes  and  neutralizes  the  various  e.m.f  s.,  due  to 
the  armature  fluxes.  Therefore,  in  practice  the  interpoles  need 
not  have  the  same  width  as  the  armature  core  and,  where  space 
and  magnetic  conditions  will  permit,  the  number  of  interpoles 
can  be  made  half  that  of  the  main  poles. 

According  to  the  method  outlined,  the  whole  problem  of  the 
design  of  the  interpole  depends,  first,  upon  the  determination 
of  the  a.m.fs.  due  to  the  armature  fluxes,  and,  second,  upon  the 
determination  of  such  interpole  flux  as  will  generate  an  e.m.f, 
in  the  short  circuited  armature  coils  which  will  equal,  or  slightly 
exceed,  the  armature  e.m.fs. 

Interpole  Calculations.  Assuming  that  all  the  armature 
fluxes,  except  the  interpolar,  are  unaffected  by  the  presence  of 
interpoles,  the  armature  e.m.f.  to  be  balanced  by  the  interpole 
would  be  represented  by  the  formula 


3 


_  _ 
-          (0,25  £+0.5) 

(0.9+0.035  AO  + 
1.88  A  +0.52+2.16  *  Vn 


THEORY  OF  COMMUTATION  231 

However,  the  flux  above  the  slot,  from  the  tooth  top,  is  very 
considerably  modified  by  the  interpolar  flux.  In  fact  most  of 
this  should  be  omitted  It  may  be  assumed  that  the  flux  across 
the  slot,  above  the  upper  coil,  simply  "  bulges  "  up  slightly 
into  the  air  gap,  and  the  remainder  of  the  usual  tooth  top  flux 
is  absent,  except  when  the  interpole  does  not  cover  the  full 
armature  width.  Therefore,  in  the  above  formula,  the  term 
LX2  16  V^  should  be  changed  to  (L-Li)X2  16 Vn  and  the 


]  .33  ck  +  0.52        .       ,u     1.33  & +0.7 

term  ! replaced  by jL_L___ 

«s  s 

Then,   the   corrected   resultant  of  all   the   armature   e.m.fs. 
becomes 

/T          T    \  %DP 

d  (L—L^ 


-Pp) 


4.  n 
+cjX-~ ^  (0.9+0.035  JV) 


In  this  formula 

L          represents  the  width  of  the  armature  core. 

Li  represents  the  effective  width  of  interpole  at  the  gap 
on  the  basis  of  the  full  number  of  interpoles. 

L—Li  is  the  difference  between  the  width  of  the  armature 
core  and  the  interpole  face.  This  term  enters  when  the  interpoie 
is  narrower  than  the  armature  core.  When  alternate  interpoles 
are  omitted  and  the  remaining  interpoles  are  of  the  same  width, 
as  the  armature  core  the  conditions  are  practically  the  same  as 
when  the  full  number  of  interpoles  are  used  but  with  their  width 
equal  to  half  the  core  width.  Other  combinations  should  be 
treated  in  the  same  way  so  that  the  above  formula  can  be  taken 
to  represent  the  general  conditions. 

In  practice  it  is  desired  that  the  resultant  interpole  e.tn.f,, 
and  therefore  the  interpole  flux,  vary  in-  proportion  to  the  arma- 
ture short-circuit  e.m.f.  which  is  to  be  neutralized.  As  shown 
fcy  the  last  ^equation,  this  e.m.f .  is  proportional  to  the  armature 


232  ELECTRICAL  ENGINEERING  PAPERS 

current,  except  where  there  is  saturation  in  the  armature  flux 
path,  as  in  the  case  of  magnetic  bands  over  the  core*  Therefore 
the  interpole  magnetomotive  force  should  vary  in  proportion 
to  the  armature  current,  neglecting  core  bands.  In  consequence, 
in  practice  the  interpole  winding  is  always  connected  in  series 
with  the  armature  winding. 

The  interpole  magnetomotive  force  can  be  considered  as  made 
up  of  two  components,  one  of  which  neutralizes  the  armature 
magnetomotive  force,  and  the  other  component  represents  the 
ampere  turns  which  set  up  the  actual  interpole  flux.  The  first 
component  will  be  referred  to  as  the  neutralizing  ampere  turns 
or  neutralizing  turns  ,  and  the  other  as  the  magnetizing  ampere 
turns  or  magnetizing  turns. 

Let  T  represent  the  total  interpole  turns  for  one  interpole, 

Tv    represent  the  total  magnetizing  interpole  turns  for  one 
interpole. 

Ta    represent   the   total   effective    "  armature   turns   per 

total  eff  .  ampere  turns  of  armature        , 
~~      number  poles  X  total  current       ' 

/    represent  the  amperes  per  interpole  coil. 
Then  ITt  =  IT-I  Ta,  or,  r= 


Let  g    =  effective  air  gap  per  interpole. 

2J»=  flux  density  under  the  interpole,  and 
JS,  =  e.m.f.  in  an  armature  coil  of  turns  Tf  due  to   the 
interpole  flux 
Then, 


The  e.mX  due  to  one  interpole  is  equal  to 


10* 
Or,  for  two  interpoles 

JLxX2r.lt, 


*If  average  slot  e.  rru  f.  is  used  in  calculating'  EC  (See  Note  on  page  215),  this  expression 
should  be  multiplied  by  a  factor  Cp  to  obtain,  average  value  of  Ei.  Cp  is  the  ratio  of  the 
average  flux  density  in  the  commutating  zone,  to  the  marfmijTn  density,  J5». 


THEORY  OF  COMMUTATION  233 

This  e.m.f.  should  be  equal  to  the  e.m.f.  generated  in  the  same 
coils  by  the  armature  flux,  or  Ei=Ec.    Therefore, 

3.19 I  Tt  TT  D  LiX2  Tc  Rs    -IeW,.TfRsir 


,r_r 


gXlO8  10* 

2D  p 


(0.25^+0.5) 
(0.9+0.035 


(L-i02.16  V^  T< 


In  the  second  term  of  this  equation  7t  W(  =  IX  Ta'X2p,  where 
jT0'  =  total  armature  turns  per  pole,  as  distinguished  from  effec- 

tive turns  per  pole  Ta,  and  Ta'  =-  —  ?—  ,  where  &  =  ^'T'  as  will 

1  —  op  Wt 

be  shown  later  under  the  subject  of  "  Effective  Armature  Am- 
pere Turns."  Therefore,  neglecting  magnetic  bands  on  the  core, 
the  above  expression  becomes, 


r  raj>g  r 

1  *~3.19  DZ-!  (l-&^>)  L 


(0.25  ^>+0.5)  (irD-Pp) 


(0.9+0.035  (1'33 


+c»  (L-iO  2.16 


2J>j> 


(vD-Pp) 

(0.9+0.025 . 
+c»  (L— LI)  2.16 


234  ELECTRICAL  ENGINEERING  PAPERS 

If  the  full  number  of  interpoles  is  used,  and  each  covers  the 
full  width  of  the  armature,  then  Z~Z,i  =  0,  and 


(1.33 


Therefore  the  total  interpole  turns  for  one  pole  are  equal  to 
the  effective  armature  turns,  per  pole  multiplied  by  a  constant 
which  is  a  function  of  the'  proportions  of  the  machine.  How- 
ever, this  holds  true  only  for  the  condition  of  no  saturated  path 
for  the  armature  flux,  such  as  magnetic  bands. 

The  above  formula  gives  the  interpole  turns  for  two  inter- 
poles  acting  on  each  armature  coil.  With  but  one  interpole 
per  coil  the  number  of  conductors  per  armature  coil  generating 
the  interpole  e.m.f.  is  halved  so  that  the  flux  density  must  be  at 
least  doubled,  and  the  effect  of  the  armature  flux  in  the  inter- 
polar  space  over  the  other  half  of  the  armature  coil  must  also  be 
taken  into  account.  This  can  be  done  in  the  preceding  formula 
by  using  the  equivalent  value  of  LI. 

With  half  the  number  of  interpoles  the  effective  gap  length,  gr 
will  not  be  the  same  as  with  the  full  number  of  interpoles  with 
the  sameinechanical  gap,  for  the  flux  from  the  interpole  maybe 
considered  as  returning  across  the  gap  of  the  two  adjacent  main 
poles  and  the  value  of  g  must  be  increased  to  represent  the  total 
resultant  gap. 

Let  ge      represent  the  effective  resultant  gap, 

gm     represent  thfc  effective  gap  under  the  main  poles, 
A  i    represent  the  area  of  the  interpole  gap,  and 
Am   represent  the  area  of  one  main  pole  gap. 

These  areas  can  be  derived  from  the  field  distribution  or  "  field 
form  "  of  the  main  and  the  interpoles. 

Then,   the  effective  resultant  gap  g,  =  g-f— p  gmj    and   this 

•"  ./LIT* 

should  be  used  instead  of  g 

With  half  the  number  of  interpoles  and  on  the  basis  of  the 
interpole  flux  returning  through  the  two  adjacent  main  poles, 
it  may  be  assumed  that  this  flux  weakens  the  total  flux  in  one 
pole  and  strengthens  that  of  the  other  pole  a  Ek^  amount.  If 


THEORY  OF  COMMUTATION     '  235 

there  is  no  saturation  in  the  main  poles  or  armature  teeth  under 
them,  then  no  additional  ampere  turns,  other  than  for  the 
increased  gap,  will  be  required  on  account  of  the  main  poles 
carrying  the  interpolar  fluxes.  However,  where  there  is  much 
saturation  of  the  main  poles  or  teeth,  then  additional  interpole 
ampere  turns  will  be  required,  as  will  be  described  later  in  con- 
nection with  effects  of  saturation. 

Chord  Windings  with  Interpoles.  Chorded  armature  windings 
can  be  used  with  interpoles  with  satisfactory  results  provided 
the  interpoles  are  suitably  proportioned.  There  are  apparently 
some  advantages  with  such  an  arrangement,  but  there  are  also 
disadvantages  of  such  a  nature  that  it  is  questionable  whether 
it  is  advisable  to  use  chord  windings  with  such  machines,  except 
possibly  in  special  cases.  When  chorded  windings  are  used  with 
interpoles,  the  e.m.f.  due  to  the  armature  flux  is  usually  much 
smaller  than  with  a  pitch  winding  and  thus  fewer  interpole 
magnetizing  turns  are  required.  Also,  the  effective  armature 
turns  which  must  be  neutralized  by  the  interpole  are  reduced 
somewhat,  which  also  means  a  slight  reduction  in  interpole 
tuftis.  Against  these  advantages  must  be  charged  the  disad- 
vantage of  a  wider  interpole  face.  This  in  itself  would  not  be 
objectionable  where  there  is  space  for  such  wider  pole  face,  but 
if  the  space  between  the  main  poles  must  be  increased  it  may  lead 
to  sacrifice  in  the  proportions  of  the  main  poles  or  changes  in 
the  general  dimensions,  such  that  the  result  as  a  whole  is  less 
economical  than  with  a  pitch  winding. 

Effective  Armature  Ampere  Turns.  The  term  Ta  representing 
the  effective  artnature  ampere  turns  should  be  considered,  as  the 
value  of  this  term  is  influenced  by  a  number  of  conditions,  such 
as  the  number  of  bars  covered  by  the  brush,  the  amount  of 
chording  in  the  armature  winding,  etc.  With  a  full  pitch  wind- 
ing and  neglecting  the  reduction  in  current  in  the  short  circuited 
coils,  the  magnetomotive  force  of  the  total  armature  winding 

is  represented  by  the  expression,  — £^    L ,  and  per  pole  it  is, 

-  .     However,  when  the  brush  spans  several  coils,  so  that 

2p 

a  number  of  armature  coils  are  short  circuited  at  the  same  time, 
the  average  current  in  these  short  circuit  coils  should  be  con- 
siderably less  than  the  normal  value  so  that  the  effective  ampere 
turns  per  pole  is  correspondingly  reduced.  Allowance  must  be 


236  ELECTRICAL  ENGINEERING  PAPERS 

made  for  this  reduction  as  it  has  considerable  influence  in  de- 
termining the  correct  number  of  interpole  turns. 

On  the  basis  of  no  local  currents,  the  average  value  of  the 
current  in  the  short  circuited  coils  is  just  half  that  of  the  work 
currents  per  conductor. 

Let  B    represent  the  total  number  of  commutator  bars, 

BI  represent  the  number  of  bars  spanned  by  the  brush, 
pi  represent  number  of  current  paths,  and 
p    number  of  poles. 

•n   <TT» 

Then,      .    ,"  =  total  number  of  armature  turns  per  pole,  and 
Pip 

7?     7"1 

*     c  =  number  of  turns  by  which  the  total  armature  turns  per 

&  pi 

pole  must  be  reduced  to  obtain  the  effective  turns  per  pole,  or, 


^.—g-,  ..„,.___ 

Let  BI  Te  be  represented  as  a  percentage  of  Wh  or  BI  Tc  «=  b  Wt 
Then,  ro=  0     *      (1 


Wt       IWt 
2p 


T 

Therefore,  Ta/  =  -rr    ' 


Chorded  windings  also  have  an  influence  on  the  effective  arma- 
ture ampere  turns  per  pole-  When  the  winding  is  chorded  one 
slot,  for  example,  then,  in  one  slot  per  pole,  the  upper  and  lower 
coils  will  be  carrying  current  in  opposite  directions  and  tEeir 
magnetizing  effects  will  be  neutralized.  In  consequence,  the 
total  effective  armature  ampere  turns  are  correspondingly  re- 
duced and  this  must  be  allowed  for  in  determining  the  interpole 
turns. 


THEORY  OF  COMMUTATION 


237 


Conditions  Affecting  Interpole  Proportions.  The  foregoing 
formulae  have  been  based  upon  the  use  of  interpoles  of  such  pro- 
portions that  the  interpole  flux  varies  directly  as  the  magnetizing 
current  and  its  distribution  over  the  cojnmutating  zone  is  such 
as  will  give  the  proper  opposing  e.m.f.  at  all  times. 

However,  the  proportionality  of  flux  to  current  can  only  be 
true  as  long  as  there  is  no  saturation,  in  the  interpole  magnetic 
circuit.  Such  saturation  is  liable  to  be  found  in  practice  and  not 
infrequently  it  is  quite  a  problem  df  design  to  avoid  it  within 
the  working  range  of  the  machine. 

Also,  another  difficult  problem  lies  in  so  designing  the  interpole 
face  that  the  flux  distribution  in  the  commutating  zone  is  such 
that  its  e.m.f.  will  properly  balance  the  armature  e.m.fs.  in  the 
short  circuited  coils,  especially  as  the  latter  are  generated  by 
cutting  fluxes  which  may  be  distributed  in  a  quite  different  man- 
ner from  the  interpole  flux. 


FIG.  21 

Shape  and  Proportion  of  Interpole  Face.  As  already  shown, 
the  effective  interpole  flux  under  the  pole  face  is  the  "resultant 
of  the  total  interpole  magnetomotive  force  and  the  opposing 
armature  magnetomotive  force.  As  the  armature  windiag  is 
distributed  over  a  surface  and  the  interpole  winding  is  of  the 
polar  or  concentrated  type,  the  resultant  magnetomotive  force 
would  normally  be  such  as  would  not  tend  to  give  a  uniform  flux 
distribution  under  the  interpole  unless  the  interpole  face  is 
properly  shaped  or  proportioned  for  such  distribution.  The 
conditions  may  be  illustrated  in  Pig.  21.  In  this  figure  the  lines 
A  A  represent  the  armature  magnetomotive  force,  with  a  full 
pitch  winding,  and  the  brash  covering  one  commutator"  bar. 
The  heavier  p&rt  of  the  lines  a  b  c  at  the  peak  of  the  magneto- 
motive force  diagram,  represents  the  armature  magnetomotive 
fdrce  which  would  be  obtained  tinder  the  interpole  face,  a#d  also 
the  fttcc  distribttfioa  wMdi  would  be  obtained,  with  no  interpole 


238  ELECTRICAL  ENGINEERING  PAPERS 

magnetomotive  force,  and  with,  uniform  gap  under  the  pole 
faces.  In  opposition  is  shown  the  interpole  magnetomotive 
force  and  flux  distributions  d  ef  for  corresponding  conditions. 
The  resultant  magnetomotive  force  is  represented  by  g  h  i,  and 
with  a  uniform  gap  under  the  pole,  the  resultant  interpole  flux 
would  have  a  similar  distribution.  Instead  of  this,  either  a 
flat  or,  in  some  cases,  the  reverse  distribution  is  required,  that 
is,  with  a  slight  "  hump  "  in  the  middle  instead  of  a  depression- 
By  prbperly  shaping  the  pole  face  so  as  to  give  an  increased  air 
-gap  toward  the  edges,  the  flux  distribution  can  be  made  practi- 
cally anything  desired.  In  some  cases  a  relatively  narrow 
pole  tip  with  a  very  large  air  gap  will  give  a  close  approximation 
to  the  desired  flux  distribution. 

However,  in  practice  the  above  distribution  of  the  armature 
magnetomotive  force  is  rarely  found.  The  use  of  brushes 
which  cover  more  than  one  commutator  bar  serves  to  cut  off 
or  flatten  out  the  pointed  top  of  the  armature  magnetomotive 
force  diagram,  as  shown  by  the  dotted  line  B,  in  Fig.  21,  and  thus 
lessen  the  depression  at  the  center  of  the  resultant  magneto- 
motive force  distribution. 

As  intimated  before,  this  problem  of  proportioning  the  inter- 
pole face  turns  upon  the  determination  of  the  armature  e.m.fs. 
in  the  short  circuited  coils  which  have  to  be  balanced  by  the 
interpole.  If  the  different  armature  e.m.fs.  are  determined  for 
the  whole  period  of  commutation  and  then  superimposed,  the 
resultant  e.m.f .  indicates  the  flux  distribution  required  under  the 
interpole.  Usually  the  e.m.fs.  due  to  the  end  winding,  and  to 
the  interpolar  flux,  if  any,  will  be  practically  constant  during  the 
whole  period  of  commutation.  If  no  local  currents  are  present 
the  e.m.f.  due  to  the  slot  flux  will  also  be  practically  constant, 
although  it  may  be  slightly  reduced  near  the  beginning  and  end 
of  the  commutation  period.  The  sum  of  these  e.m.fs.  should 
therefore  be  practically  Constant  over  the  whole  commutation 
period  and  therefore,  in  a  well  designed  machine,  the  interpole 
flux,  density  should  be  practically  constant  over  the  whole 
commutation  zone.  As  explained  before,  special  shaping  of  the 
poles  and  pole  face  will  be  necessary,  in  most  cases,  to  obtain 
exactly  this  proper  flux  distribution.  Large  interpole  air  gaps 
are  obviously  advantageous  in  obtaining  such  distribution.  In 
fact*  a  very  small  interpole  gap  makes  the  determination  of  the 
properjnterpole  face  dimensions  very  difficult  in,  many  cases. 
On  accocrat  of  the  interpole  usually  covering  less  than  two 


THEOR  Y  OF  CO  MM  UTA  TION  239 

armature  teeth,  the  ordinarily  accepted  methods  of  determining 
the  effective  length  of  air  gap  under  a  pole  will  not  apply,  iifc 
many  cases,  which  may  lead  to  a  slight  error  in  the  results. 
Practically  the  effective  gap  tinder  the  narrow  interpole  will 
usually  be  longer  than  determined  by  the  ordinary  methods. 
This  partly  explains  the  fact  that,  in  some  cases,  an  increase 
in  mechanical  clearance  between  the  interpole  face  and  the 
armature  core  does  not  require  anything  like  a  corresponding 
increase  in  the  interpole  magnetizing  ampere  turns.  The  effec- 
tive interpole  air  gap  increases,  but  at  a  much  less  rate  than  the 
mechanical  gap. 

The  brush  setting  in  relation  to  the  interpole  is  of  great  im- 
portance. The  point  of  maximum  armature  magnetomotive 
force  is  definitely  fixed  by  the  brush  setting.  With  the  interpole 
fixed  in  position,  any  shifting  of  the  biuishes  backward  or  forward 
will  obviously  change  the  shape  of  the-  resultant  magnetomotive 
fores  distribution  under  the  interpole  face  and  in  consequence 
the  flux  distribution  will  be  changed.  With  but  one  armature 
coil  per  slot  and  the  brush  covering  but  one  commutator  bar, 
good  commutating  conditions  might  be  found  over  a  considerable 
range  of  brush  adjustment,  by  suitably  varying  the  interpole 
ampere  turns.  However,  with  two  or  more  coils  per  slot  and 
with  the  brush  short  circuiting  several  bars,  any  marked  change 
in  the  resultant  interpole  magnetomotive  force  and  flux  distribu- 
tion will  mean  improper  commutation  for  some  of  the  coils. 
Proper  brush  setting  is  therefore  of  first  importance. 

It  has  been  ''assumed  in  the  foregoing  treatment,  that  an  exact 
balance  between  the  interpole  and  armature  e.m.f  s.  will  give  the 
best  conditions.  From  certain  standpoints,  this  is  true,  but  in 
practice  usually  a  slight  excess  in  the  interpole  strength,  or 
"  over-compensation  "  of  the  interpole,  as  it  is  frequently  called, 
is  advantageous.  Reference  to  Fig.  14  shows  that  in  a  machine 
without  interpoles,  and  therefore  without  compensation,  the 
current  flowing  between  the  brush  contact  and  the  commutator 
is  crowded  toward  one  brush  edge,  this  being  the  edge  at  which 
the  commutation  of  a  coil  is  completed,  that  is,  at  the  so-called 
forward  brush  edge.  With  over-compensation  the  opposite 
effect  occurs — that  is,  the  brush  current  density  is  below  the 
average  at  the  forward  edge.  This  is,  to  a  certain  extent,  a  de- 
sirable condition.  Also,  if  there  is  any  saturation  of  the  inter- 
pole circuit  at  overloads,  the  over  excitation  of  the  interpole 
winding  can  take  care  of  the  saturation  ampere  turns,  so  that 


240  ELECTRICAL  ENGINEERING  PAPERS 

normal  compensation  can  be  obtained  at  considerably  higher  load 
than  in  a  machine  with  no  over  compensation.  Furthermore, 
over  compensation  is  desirable  on  account  of  the  effect  of  the 
resistance  of  the  coils  undergoing  commutation,  which  heretofore 
has  been  neglected  as  being  of  minor  importance.  Such  re- 
sistance tends  to  lower  the  current  density  at  the  middle  of  the 
brush  contact,  and  increase  it  toward  the  brush  edges.  Over 
compensation  will  oppose  this  at  the  forward  edge,  but  increase  it 
at  the  back  edge,  which  is  less  objectionable.  Also,  as  shown  in 
Fig.  21,  there  is  liable  to  be  a  depression  at  the  center  of  the 
interpole  flux  distribution,  if  the  pole  face  is  not  properly 
shaped.  This  depression  tends  to  cause  higher  current  densities 
at  the  brush  edges.  Over  compensation  again  tends  to  reduce 
this  density  at  the  forward  brush  edge.  Thus  there  are  several 
good  reasons  for  slight  over  compensation,  and  practical  opera- 
tion bears  this  out,  especially  on  high  voltage  machines,  where 
the  short  circuit  e.m.fs.  average  higher  than  in  other  machines. 

Balanced  Circuits.  It  has  been  assumed  that  the  armature 
ampere  turns  per  pole  have  been  the  same  for  all  poles.  This 
will  be  true  for  the  usual  two-circuit  or  series  type  of  winding, 
or  its  allied  combinations,  but  is  not  necessarily  true  of  the 
parallel  type  of  armature  winding.  Ill  such  a  winding  a  number 
of  circuits  are  connected  in  parallel  at  the  brushes,  and,  unless 
ample  provision  be  made  for  equalizing  the  different  circuits, 
they  may  not  carry  equal  currents  at  all  times.  As  the  re- 
sultant interpole  flux  and  e.m.f.  is  directly  dependent  upon  the 
opposing  armature  ampere  turns,  it  is  obvious  that  any  ine- 
qualities in  the  armature  currents  would  lead  at  once  to  incorrect 
interpole  conditions.  A  poorly  equalized  parallel-wound  arma- 
ture might  furnish  conditions  such  that  the  interpoles  cannot  be 
adjusted  for  satisfactory  operation.  Also  paralleling  of  the 
interpole  windings,  unless  care  be  taken  to  insure  equal  cutrent 
division  among  the  circuits,  is  liable  to  lead  to  trouble, 

Saturation  of  the  Inter.pole  Circuit.  Heretofore  the  interpole 
turns  T,  as  determined,  have  been  only  those  required  for  forcing 
the  resultant  interpole  flux  across  the  effective  interpole  air  gap, 
and  nothing  has  been  allowed  for  any  turns  required  for  magnet- 
izing the  parts  of  the  interpole  circuit  other  than  the  gap. 
Where  such  additional  turns  are  required  they  must  be  added  to 
the  turns  T,  already  determined. 

Saturation  in  the  interpole  magnetic  path  is  the  principle 
cause  for  such  additional  turns,  but  saturation  in  the  various 


THEORY  OF  COMMUTATION  241 

flux  paths  may  occur  in  such  a  way  as  to  be  either  harmful  or 
beneficial,  depending  upon  where  it  is  located.  Beneficial 
saturation  may  be  assumed  to  be  such  as  will  reduce  the  arma- 
ture short  circuit  e.m.fs,,  while  harmful  saturation  tends  to  re- 
duce the  interpole  e.m.f. 

While  the  useful  interpole  flux  passing  into  the  armature  may 
be  relatively  low — say  one-fifth  that  required  for  saturation  of 
the  interpole  material — the  leakage  flux  between  the  interpole 
and  the  two  adjacent  main  poles  is  often  very  much  greater 
than  the  useful  flux  so  that  the  interpole  at  the  part  where  it 
carries  the  highest  total  flux  may  be  worked  up  to  possibly  half 
saturation,  or  higher,  with  normal  load  on  the  machine.  The 
interpole  leakage  flux  is  due  to  the  total  ampere  turns  on  the  inter- 
pole,  while  the  useful  interpole  flux  is  due  only  to  the  magne- 
tizing component  of  the  interpole  ampere  turns,  which  may  be 
as  low  as  15  per  cent  to  25  per  cent  of  the  total  interpole  ampere 
turns.  Ths  leakage  flux  is  thus  liable  to  be  a  high  percentage 
of  the  total  interpole  fluxt 

"While  the  ampere  turns  on  the  interpole  will  rise  in  direct 
proportion  to  the  current,  the  effective  magnetizing  component 
will  rise  in  direct  proportion  only  below  saturation  of  the  inter- 
pole circuit.  Any  ampere  turns  required  for  saturating  this 
circuit  will  be  taken  from  the  magnetizing  component  of  the 
interpole  winding.  Therefore,  when  any  appreciable  saturation 
occurs,  the  effective  magnetizing  component  will  not  vary  in 
proportion  to  the  current,  and  the  interpole  e.m.f.  will  not  vaiy 
in  proportion  to  the  armature  e.m.f s.  As  the  magnetizing  com- 
ponent of  the  interpole  winding  usually  represents  a  relatively 
small  number  of  ampere  turns  per  pole  a  comparatively  slight 
saturation  in  the  interpole  circuit  may  have  an  appreciable  effect  „* 
It  is  therefore  advisable  to  work  at  as  low  a  saturation  as  possible 
in  the  interpole  circuit  so  that  practically  no  saturation  occurs 
within  the  ordinary  working  range  of  the  machine. 

Where  saturation  occurs  in  any  of  the  armature  flux  paths, 
as,  for  instance,  with  saturated  bands  over  the  armature  core, 
the  result  of  such  saturation  will  serve  to  neutralize  the  effect 
of  saturation  in  the  interpole  magnetic  circuit.  In  other  words, 
the  armature  e.m.f.  will  not  rise  in  proportion  to  the  current 
and  thei*efore  the  opposing  interpole  e.m.f.  does  not  need  to 
increase  in  proportion  either. 

The  principal  source  of  saturation  in  the  interpole  circuit  lies 
in  the  magnetic  leakage  from  the  interpole  to  the  adjacent  main 


242  ELECTRICAL  ENGINEERING  PAPERS 

poles.  Serious  trouble  has  often  been  encountered  by  not 
making  due  allowance  for  such  leakage.  However,  there  may 
be  other  causes  for  saturation.  When  the  full  number  of  inter- 
poles  is  used  the  interpole  magnetic  path  or  circuit  is  independent 
of  the  main  pole  magnetic  circuit,  except  in  the  yoke  and  in 
the  armature  core  below  the  slots,  as  indicated  in  Fig.  22. 
In  the  yoke  it  may  be  seen  that  the  interpole  flux  is  in  the  same 
direction  as  the  main  flux  at  one  side  of  the  main  pole  and  is  in 
opposition  to  the  main  flux  at  the  other  side.  The  same  is  true 
in  the  armature  core.  Therefore  the  interpolar  flux  tends  to 
reduce  the  flux  in  one  part  of  the  yoke  and  tends  to  increase  it  in 
the  other  part.  If  the  saturation  in  these  parts  is  relatively  low, 
then  the  magnetomotive  force  required  for  forcing  the  low  and 
the  high  fluxes  through  the  yoke  will  be  but  little  greater  than  if 
these  fluxes  were  equal.  However,  if  the  yoke  is  highly  saturated 
the  increase  in  ampere  turns  required  for  the  high  part  much 
more  than  offset  the  decrease  in  ampere  turns  for  the  low  part, 
so  that,  as  a  result,  additional  am-« 
pere  turns  are  required  for  sending 
the  interpole  flux  through  this  path. 
The  interpole  ampere  turns  therefore 
must  be  increased  on  this  account,  } 


^riien  the  saturation  is  high.  The  same  F      22 

condition  holds  for  the  armature  core. 

A  similar  condition  occurs  -where  half  the  number  of  interpoles 
is  used  and  when  there  is  much  saturation  of  the  main  pole  and 
the  armature  teeth  under  it,  as  already  referred  to.  This  con- 
dition requires  additional  interpole  ampere  turns. 

In  practice,  with  the  ordinary  compact  designs  of  direct  cur- 
rent machines,  it  is  usually  difficult  to  keep  the  total  interpole 
flux  as  low  as  one-third  that  which  gives  any  material  saturation 
and,  not  infrequently,  it  is  much  higher  than  this.  Therefore, 
by  direct  proportion  it  might  be  assumed  that  such  machines 
could  carry  only  double  to  treble  load  without  sparking  badly.  , 
However,  the  resistance  of  the  brushes,  etc.,  will  be  of  such  as- 
sistance that  relatively  higher  loads  may  be  commutated  rea- 
sonably well.  For  instance,  with  the  interpole  worked  at  about 
half  saturation  at  normal  load,  the  machine  may  be  able  to 
commutate  considerably  more  than  double  load  without  undue 
sparking.  It  is  also  of  material  assistance,  where  heavy  over- 
loads are  to  be  carried,  to  over-excite  the  interpolevwinding, 
that  is,  to  make  the  magnetizing  component  somewhat  greater 


THEORY  OF  COMMUTATION  243 

than  required  at  normal  load,  as  described  before.  In  this  case, 
at  light  loads,  the  interpole  e.m.f.  exceeds  the  armature  e.m.f. 
a  certain  amount  which  is  taken  care  of  by  the  brush  resistance 
as  local  currents  will  be  less  harmful  when  the  work  current  is 
low.  As  partial  saturation  is  obtained  at  overload,  the  two 
e.m.fs.  become  equal  but  at  a  higher  load  than  would  be  the  case 
without  ovef -excitation  of  the  interpole. 

Commutating  Conditions  on  Short  Circuit.  When  a  direct 
current  generator  is  short  circuited  across  its  terminals,  either 
through  a  low  external  resistance  or  without  such  resistance,  a 
current  rush  will  occur  which  will  rise  to  a  value  represented 
approximately  by  the  generated  e.m.f.  divided  by  the  resistance 
in  circuit.  This  current  rush  is  only  of  short  duration  as  the 
excessive  armature  current  will  react  to  demagnetize  or  "  kill " 
the  field.  If  the  short  circuit  is  without  external  resistance  the 
current  rush  may  reach  an  enormous  value  as  the  internal  re- 
sistance on  large  machines  is  usually  very  low.  This  means  that 
currents  from  25  to  40  times  full  load  may  be  obtained  on  "  dead  " 
short  circuit.  Experience  shows  that  under  such  current  rushes, 
any  kind  of  direct  current  machine  will  tend  to  flash  viciously  at 
the  brushes. 

By  the  preceding  theory  and  analysis  a  rough  approximation 
to  the  commutating  conditions  on  short  circuit  can  readily  be 
obtained.  Assuming  an  interpole  machine,  the  following  con- 
ditions will  be  found: 

1.  The  interpole  will  be  highly  saturated  so  that  it  is  of  little 
or  no  direct  benefit. 

2.  The  slot  flux  will  rise  to  such  a  value  that  the  armature 
teeth  in  the  commutating  zone  are  practically  saturated. 

3.  There  may  be  some  interpolar  flux  from  the  armature,  as  the 
high  interpole  saturation  may  allow  this. 

4.  The  armature  end  flux,  with  the  exception  of  that  part  due 
to  magnetic  bands,  will  rise  practically  in  proportion  to  the 
current. 

The  following  short  circuit  e.m.f.  conditions  will  be  obtained: 

1.  There  will  be  possibly  a  slight  e.m.f.  due  to  the  armature 
interpolar  flux. 

2.  There  will  be  an  e.m.f.  due  to  the  tooth  flux  which  is  almost 
as  high,  per  conductor,  as  could  be  obtained  by  a  conductor 
cutting  the  flux  under  the  main  field  at  no  load,  for  saturation  of 
the  armature  teeth  may  be  assumed  to  be  the  limit  in  both  cases. 

3.  There  will  be  an  e.m.f.  due  to  the  end  flux  which  may  be 
10  to  20  times  larger  than  at  normal  full  load. 


244  ELECTRICAL  ENGINEERING  PAPERS 

Therefore,  the  total  e.m.£.  in  the  short  circuited  coil  due  to 
cutting  the  armature  flux  on  dead  short  circuit  may  be  higher 
than  would  be  obtained  if  the  brushes  were  shifted  at  no  load  until 
the  commutated  coil  lies  under  the  strongest  part  of  the  main  field. 

As  very  few  machines  of  large  capacity  would  stand  this 
latter  condition  without  flashing,  it  may  be  assumed  that  they 
would  be  no  more  able  to  stand  a  dead  short  circuit  without 
flashing.  In  fact,  8  to  10  times  full  load  current  will  make  an 
interpolar  machine  of  normally  good  design  flash  badly,  as  it  is 
impracticable  to  make  an  interpole  of  the  usual  type  which  will 
not  saturate  highly  at  8  to  10  times  normal  current. 

If,  however,  the  interpole  is  combined  with  compensating 
windings  in  the  main  poles,  the  interpole  leakage  may  be  made 
so  small  that  comparatively  low  saturation  is  obtained  normally 
in  the  interpole  circuit.  In  such  case  the  interpole  may  be  effec- 
tive with  heavier  currents  and  the  flashing  load  may  be  very 
much  higher  than  with  the  usual  type  of  interpole  machines. 

CONCLUSION 

The  foregoing  is  a  general  presentation  of  the  problem  of 
commutation,  which  is  admittedly  crude  and  incomplete  in  some 
points.  In  particular  may  be  mentioned  the  part  describing, 
the  action  of  local  currents.  Also,  the  method  of  considering 
the  resultant  action  in  interpole  machines  as  the  superposition 
of  two  components  does  not  tell  the  whole  story,  but  the  actual 
analysis,  in  detail,  of  a  number  of  these  phenomena  would  be  so 
confusing  and  complicated,  that  a  general  physical  conception 
of  what  takes  place  during  commutation  would  be  lost.  In  the 
ultimate  analysis  it  will  be  found  that  a  number  of  the  methods 
described  are,  In  reality,  simply  illustrations  of  the  conditions 
of  commutation  rather  than  an  analysis  of  the  conditions  them- 
selves. However,  the  method  as  given  throws  light  on  many 
things  which  take  placie  during  commutation.  It  also  includes 
a  number  of  conditions  which  are  not  -covered  in  the  usual 
methods  of  dealing  with  this  problem.  For  example,  the  number 
of  commutator  bars  spanned  by  the  brush  is  an  important  ele- 
ment in  this  method  of  handling  the  problem,  whereas,  in  many 
former  methods,  this  point  was  either  omitted,  or  treated  in  an 
empirical  manner.  In  this  method  the  results  obtained  would  be 
very  greatly  in  error  if  the  brush  span  were  not  included. 

Any  theory  qr  method  of  calculation  is  open  to  question  until 
ti  has  stood  the  proof  of  actual  test.    In  consequence,  the  above 


THEORY  OF  COMMUTATION  245 

"jnethod  has  been  tried  on  a  very  large  number  of  direct  current 
machines,  including  high  speed  direct  current  generators, 
direct  current  turbo-generators,  direct  current  railway  motors  of 
all  sizes,  moderate-  and  low-speed  generators  of  all  capacities, 
industrial  motors  of  various  designs  including  adjustable  speed 
motors  and  machines  with  half  the  number  of  interpoles.  In 
those  cases  where  tfce  actual  test  data  of  the  machines  was  very 
accurately  obtained,  the  agreement  between  the  tests  and  the 
calculated  results  by  the  above  method  was  found  to  be  close. 
In  fact,  the  method  in  som§  cas6s  indicated  errors  or  inaccuracies 
in  the  test  results.  In  a  number  of  cases  of  early  interpole 
machines  there  was  considerable  disagreement  between  the 
results  of  the  calculation  and  the  actual  test,  but,  in  many  of 
these  cases,  later  experience  showed  definitely  that  the  proper 
interpole  field  strength  or  proportions  had  not  been  obtained 
in  the  actual  test  or  that  the  proper  brush  setting  had  not  been 
used.  These  cases  were  thus,  to  a  certain  extent,  a  verification 
of  the  method,  for  in  general  the  greatest  discrepancies  between 
the  calculated  and  the  test  results  corresponded  to  the  ma- 
chines which  eventually  proved  to  have  the  poorest  proportions 
or  adjustment. 

This  theory  of  commutation  looks  complicated  and  cumber- 
some in  its  practical  application,  but  it  should  be  understood 
that  it  is,  in  reality,  an  exposition  of  a  general  method  from 
which  special  and  simpler  methods  may  be  derived  -for  different 
types  and  designs  of  machines.  It  indicates  plainly  that  the 
problem  is  so  complicated  that  no  simple  formulae  or  methods  of 
calculation  can  be  devised  which  will  cover  more  than  individual 
cases,  and  that  such  formulae,  if  applied  generally,  will  lead 
to  errtfr  sooner  or  later.  If,  however,  tt^e  general  derivation 
of  such  simplified  formulae  is  well  understood,  then  they  may 
be  used  with  proper  judgment  and  with  much  less  danger  of 
error  in  the  results.  It  is  evident,  from  the  general  analysis,  that 
the  whole  problem  must  be  handled  with  judgment,  for  new  or 
different  conditions  are  encountered  in  almost  every  type  of 
machine. 

A  great  many  problems,  closely  allied  to  that  of  commutation 
in  interpole  machines,  have  not  been  considered,  because  some 
of  them  represent  special  cases  of  the  general  theory,  while 
others  axe  somewhat  outside  the  subject  of  this  paper.  Of  the 
former  class  may  be  mentioned,  commutation  of  synchronous 
converters,  niachiiies  with  distributed  or  true  compensating 


246  ELECTRICAL  ENGINEERING  PAPERS 

windings,  the  so-called  "  split-pole  "  converter,  and  the  commu- 
tator type  alternating  current  motors,  etc.  In  the  latter  class 
may  be  included  such  problems  as  the  effect  on  commutation  of 
closed  circuits  around  the  interpoles,  losses  due  to  commutation, 
current  distribution  at  the  brush  contact,  etc.  Some  of  these 
subjects  were  included  in  this  paper  as  originally  prepared,  but 
on  account  of  its  undue  length  they  had  to  be  omitted. 


PHYSICAL  LIMITATIONS  IN  DIRECT-CURRENT 
COMMUTATING  MACHINERY 


FOREWORD — This  paper  was  presented  before  the  American  Insti- 
tute of  Electrical  Engineers  in  San  Francisco,  September  16, 
1915,  at  the  Electrical  Congress  at  the  Panama-Pacific  Inter* 
national  Exposition.  It  gives  the  results  of  the  author's  work 
on  determination  of  commutating  limits  covering  the  period  of 
many  years  of  work.  As  regards  commutation,  it  is,  in  reality, 
a  supplement  to  the  paper,  "Theory  of  Commutation  and  Its 
Application  to  Commutating  Pole  Machines."  On  the  subject 
of  flashing,  it  covers  some  very  interesting  limitations,  based 
upon  experience  and  special  tests. 

This  paper  is,  in  reality,  more  or  less  of  a  general  summary 
of  the  author's  experience  in  direct-current  machinery.  Al- 
though usually  looked  upon  as  an  "alternating-current  man," 
he  has  probably  spent  as  much  total  time  on  direct-current 
work  as  on  alternating.  Many  of  the  limiting  conditions  in 
direct-current  machinery,  as  described  in  this  paper,  were  de- 
termined by  the  author  himself.  Many  of  the  early  more  or 
less  radical  developments  and  improvements  in  direct-current 
machinery  resulted  directly  from  his  work.  A  description  of 
some  of  these  is  covered  in  his  historical  papers,  entitled,  "The 
Development  of  the  Direct-Current  Generator  in  America," 
and  "The  Development  of  the  Street  Railway  Motor  in  Amer- 
ica" which  appear  in  the  latter  part  of  this  volume. — (ED.) 


IN     DIRECT-current    commutating    machinery    there    are 
many    limitations    in    practical    design    which    cannot    be 
exceeded    without    undue    risk    in    operating    characteristics. 

Some  of  these  limitations  are  actually  physical  ones,  and,  there- 
fore, cannot  be  avoided  or  over-stepped  without  very  considerable 
departures  from  our  present  methods  of  construction  and  opera- 
tion; others  are  not  wholly  physical,  but  are  fixed  largely  by 
practical  experience,  and  are,  in  consequence,  subject  to  modifica- 
tion, as  our  experience  is  increased.  Seme  of  them  are  quite  defin- 
ite in  nature,  while  others  axe  indefinite.  Some  are  measurable, 
in  a  quantitative  sense,  while  others  may  be  considered  as  quali- 
tative. Noise,  for  instance,  is  a  distinct  limitation,  in  many  cases, 
but  it  is  difficult  to  fix  any  definite  value  where  it  is  prohibitive. 

247 


248  '          ELECTRICAL  ENGINEERING  PAPERS 

Many  of  these  limits  are  not  sharply  defined  in  practise,  due,  in 
many  cases,  to  the  impossibility  of  taking  advantage  of  all"  the 
helpful  conditions  and  of  avoiding  the  objectionable  ones.  There 
are  many  minor  conditions  which  affect  the  permissible  limits 
of  operation,  which  are  practically  beyond  the  scope  of  reliable 
calculation.  Usually,  such  conditions  are  recognized,  and  al- 
lowance is  made  for  them.  It  is  the  purpose  of  this  paper  to 
treat  of  some  of  the  major,  as  well  as  minor,  conditions  which 
must  be  taken  into  account  in  advanced  direct-current  design. 
These  are  so  numerous,  and  are  so  interwoven,  that  it  is  difficult 
to  present  them  in  any  consecutive  order. 

Probably  the  most  serious  limitation  encountered  in  direct- 
current  electric  machinery  is  that  of  commutation.  This  is  an 
electrical  problem  primarily,  but  in  carrying  any  design  of  direct- 
current  machine  to  the  utmost,  certain  limitations  are  found 
which  are,  to  a  certain  extent,  dependent  upon  the  physical 
characteristics  of  materials,  constructions,  etc 

A  second  limitation  which  is  usually  considered  as  primarily 
an  electrical  one,  namely,  flashing,  (and  bucking)  is  in  reality 
fixed  as  much  by  physical  as  by  purely  electrical  conditions. 

A  third  limitation  is  found  in  blackening  and  burning  of  com- 
mutators, burning  and  honeycombing  of  brushes,  etc.  These 
actions  are,  to  a  certain  extent,  electrical,  but  are  partly  physical 
and  "mechanical,  as  distinguished  from  purely  electrical. 

There  are  many  other  limiting  conditions  dependent  upon 
speed,  voltage,  output  per  pole,  quality  or  kind  of  materials 
used,  etc.  As  indicated  before,  these  cannot  all  be  treated 
separately  and  individually,  as  they  are  too  closely  related  to 
other  characteristics  and  limitations. 

COMMUTATION  AND  COMMUTATION  LIMITS 
In  dealing  with  the  limits  of  commutation,  it  is  unnecessary 
to  go  into  the  theory  of  commutation,  except  to  indicate  the 
general  idea  upon  which  the  following  treatment  is  based.  This 
has  been  given  more  fully  elsewhere,*  and  therefore  the  following 
brief  treatment  will  probably  be  sufficient  for  all  that  is  required 
in  this  paper. 

In  this -theory  it  is  considered  that  the  armature  winding  as  a 
whole  tends  to  set  up  a  magnetic  field  when  carrying  current, 
and  that  the  armature  conductors  cutting  this  magnetic  field 

*Theory  of  Commutation  and  Its  Application  to  Comxautatxng  Pole  Machines,  Page  201. 


PHYSICAL  LIMITATIONS  IX  D.C.  MACHINES  249 

generate  e.m.fs.  just  as  when  cutting  any  magnetic  field. 
From  consideration  of  the  armature  magnetomotive  force  alone, 
the  flux  or  field  set  up  by  this  winding  would  have  a  maximum 
value  over  those  armature  conductors  which  are  connected  to 
the  brushes.  If  the  magnetic  conditions  or  paths  surrounding 
the  armature  were  equally  good  at  all  points,  this  would  be  true. 
However,  with  the  usual  interpolar  spaces  in  direct-current 
machines,  the  magnetic  paths  above  the  commutated  coils  are 
usually  of  higher  reluctance  than  elsewhere.  However,  what- 
ever the  magnetic  conditions,  the  tendency  of  the  armature 
•magnetomotive  force  is  to  establish  magnetic  fluxes,  and,  if 
any  field  is  established  in  the  commutating  zone  by  the  armature 
•winding,  then  those  armature  coils  cutting  this  field  will  have 
e.m.fs.  generated  in  them  proportional  to  the  field  which  is  cut. 
As  part  of  this  armature  flux  is  across  the  armature  slots  them- 
selves, and  part  is  around  the  end  windings,  both  of  which  are 
practically  unaffected  by  the  magnetic  path  in  the  interpolar 
space  above  referred  to,  obviously,  then  no  matter  how  poor  the 
•magnetic  paths  in  the  interpolar  space  above  the  core  may  be 
made,  there  will  always  be  e.m.fs.  generated  on  account  of  that 
part  of  the  armature  flux  which  is  not  affected  by  those  paths. 
In  the  coils  short  circuited  by  the  brushes,  these  e.m.fs.  will 
naturally  tend  to  set  up  local  of  short  circuit  currents  during  the 
interval  of  short  circuit. 

In  good  commutation,  as  the  commutator  bars  connected  to 
the  two  ends  of  an  armature  coil  which  is  carrying  current  in  a 
.given  direction,  pass  under  the  brush,  the  current  in  the  coil 
itself  should  die  down  at  practically  a  uniform  rate,  to  zero  value 
at  a  point  corresponding  to  the  middle  of  the  brush,  and  it  should 
then  increase  at  a  uniform  fate  to  its  normal  value  in  the  opposite 
direction  by  the  time  that  the  short  circuit  is  opened  as  the  coil 
passes  from  under  the  brush.  This  may  be  considered  as  the 
ideal  or  straight  line  reversal  or  commutation  which,  however, 
is  only  approximated  in  actual  practice.  This  gives  uniform 
current  distribution  over  the  brush  face. 

If  no  corrective  actions  are  present,  then  the  coil  while  under 
the  brush  tends  to  carry  current  in  the  same  direction  as  before 
its  terminals  were  short  circuited.  In  addition,  the  short  circuit 
current  in  the  coil,  due  to  cutting  the  armature  flux,  tends  to  add 
to  the  normal  or  work  current  before  reversal  occurs.  The 
resultant  current  in  the  coil  is  thus  not  only  continued  in  the 
same  direction  as  before,  but  tends  to  have  an  increased  value. 


250  ELECTRICAL  ENGINEERING  PAPERS 

Thus  the  conditions  at  the  moment  that  the  coil  passes  out  from 
tinder  the  short  circuiting  brush  are  much  worse  than  if  no  short 
circuit  current  were  generated.  The  reversal  of  the  current 
would  thus  be  almost  instantaneous  instead  of  being  gradual  as 
called  for  by  the  ideal  commutation,  and  the  resultant  current 
reversed  much  greater  than  the  work  current  alone.  However, 
the  introduction  of  resistance  into  the  local  circuit  will  greatly 
assist  in  the  reversal  as  will  be  illustrated  later.  The  ideal  condi- 
tion however,  is  obtained  by  the  introduction  of  an  opposing 
e.mi .  into  the  local  short  circuited  path,  thus  neutralizing  the 
tendency  of  the  work  current  to  continue  in  its  former  direction. 

As  this  opposing  e.m.f.  must  be  in  the  reverse  direction  to 
the  short  circuit  e.m.f.  which  would  set  up  by  cutting  the  arma- 
ture magnetic  field,  it  follows  that  where  commutation  is  accomp- 
lished by  means  of  such  an  e.m.f.  it  is  necessary  to  provide  a 
magnetic  field  opposite  in  direction  to  the  armature  field  for 
setting  up  the  commutating  current.  This  may  be  obtained  in 
various  ways,  such  as  shifting  the  brushes  forward  (or  backward) 
until  the  commutated  coil  comes  under  an  external  field  of  the 
right  direction  and  value,  which  is  the  usual  practise  in  non- 
commutating  pole  machines;  or  a  special  commutating  field  of 
the  right  direction  and  value  may  be  provided,  this  being  the 
practise  in  commutating  pole  and  in  some  types  of  compensated 
field  machines.  When  the  commutating  emf.  is  obtained  by 
shifting  the  commutated  coil  under  the  main  field,  only  average 
conditions  may  be  obtained  for  different  loads;  whereas,  with 
suitable  commutating  poles  or  compensating  windings,  suffi- 
ciently correct  commutating  e.m.fs.  can  be  obtained  over  a 
very  wide  range  of  operation. 

In  practise,  it  is  difficult  to  obtain  magnetic  conditions  such 
that  an  ideal  neutralizing  e.mJ.  is  generated.  However,  the  use 
of  a  relatively  high  resistance  in  the  short  circuited  path  of  the 
commutated  coil  very  greatly  simplifies  the  problem.  If  the 
resistance  of  the  coil  itself  were  the  only  limit,  then  a  relatively 
low  magnetic  field  cut  by  the  short  circuited  coil  would  generate 
-sufficient  e.m.f.  to  circulate  an  excessively  large  local  current. 
Since  such  current  might  be  from  10  to  50  times  as  great  as  the 
normal  work  current,  depending  upon  the  size  of  machine,,  it 
would  necessarily  add  enormously  to  the  difficulties  of  commuta- 
tion whether  it  is  in  the  same  direction  as  the  work  current  or  is 
in  opposition.  To  illustrate  the  effect  of  resistance,  assume,  for 
example,  a  short  circuit  e.m.f.  in  the  commutated  coil  of  two 


PHYSICAL  LIMITATIONS  IN  D.C.  MACHINES  251 

volts,  and  also  assume  that  a  copper  brush  of  negligible  resist- 
ance short  circuits  the  coil,  so  that  the  resistance  of  the  short 
circuited  coil  itself  -practically  limits  the  current  to  a  value  20 
times  as  large  as  the  work  current.  Now  replace  this  copper 
brush  with  one  giving  about  20  times  as  large  a  resistance  (some 
form  of  graphite  or  carbon  brush)  then  the  total  resistance  in 
circuit  is  such  that  the  short  circuit  current  is  cut  down  to  a 
value  about  equal  to  that  of  the  work  current.  This  at  once 
gives  a  much  easier  condition  of  commutation,  even  without  any 
reversing  field;  while  with  such  field,  it  is  evident  that  extreme 
accuracy  in  proportioning  is  not  necessary.  Thus  a  relatively 
high  resistance  brush — or  brush  contact,  rather — is  of  very  great 
help  in  commutation;  especially  so  in  large  capacity  machines 
where  the  coil  resistance  is  necessarily  very  low.  In  very  small 
machines,  the  resistance  of  the  individual  armature  coils  has 
quite  an  influence  in  litrilting  the  short  circuit  current. 

It  is  in  its  high  contact  resistance  that  the  carbon  brush  is 
such  an  important  factor  in  the  commutating  machine.  Usually, 
it  is  the  resistance  of  the  brush  that  is  referred  to  as  an  important 
factor  in  assisting  commutation.  In  reality,  it  is  the  resistance 
of  the  Contact  between  the  brush  and  commutator  face  which 
must  be  considered,  and  not  that  of  the  brush  itself,  which  usually 
is  of  very  much  lower  resistance,  relatively.  As  this  contact  re- 
sistance or  drop  will  be  referred  to  very  frequently  in  the  fol- 
lowing, and  as  the  brush  resistance  itself  will  be  considered 
in  but  a  few  instances,  the  terms  "  brush  resistance  rt  and 
"  brush  drop  "  will  mean  contact  resistance  and  contact  drop 
respectively,  unless  otherwise  specified. 

Short  Circuit  Volts  per  Commutator  Bar.  As  stated  before 
the  armature  short  circuit  e,m.f.  per  coil,  or  per  commutator 
bar,  is  due  to  cutting  a  number  of  different  magnetic  fluxes,  such 
as  those  of  the  end  windings-,  those  of  the  armature  slots,  and 
those  over  the  armature  core  adjacent  to  the  commutating  zone. 
Each  of  these  fluxes  represent  different  conditions  and  distri- 
butions, and  therefore  the  individual  e.m.fs.  generated  by  them 
may  not  be  coincident  in  time  phase.  Therefore,  the  resultant 
e,m.f .  usually  may  not  be  represented  by  any  simple  graphical 
or  mathematical  expression. 

When  an  external  flux  or  field  is  superimposed  on  the  armature 
in  the  commutating  zone,  it  may  be  considered  as  setting  up  an 
additional  e.m.f.  which  may  be  added  to,  or- subtracted  from, 
the  resultant  short  circuit  e.nuf.  due  to  the  armature  fluxes. 


252  ELECTRICAL  ENGINEERING  PAPERS 

These  component  e.m.fs.  are  not  really  generated    separately 
in  the  armature  coils,  for  the  external  flux  combines  with  part 
of  the  armature  flux,  so  that  the  armature  coil  simply  generates 
an  e.m.f.  due  to  the  resultant  flux.     However,  as  part  of  the 
armature  short  circuit  e.m.f  is  generated  by  fluxes  which  do  not 
combine  with  any  external  flux,  as  in  the  end  winding,  for  in- 
stance, it  follows  that,  to  a  certain  extent,  separate  e  m.fs    are 
actually  generated  in  the  armature  winding  in  different  parts  of 
the  coil.     For  purposes  of  analysis,  there  are  advantages  in 
considering  that  all  the  e  m.fs  in  the  short  circuited   armature 
coil  are  generated  separately  by  the  various  fluxes.    A  better 
quantitative  idea  of  the  actions  which  are  taking  place  is  thus 
obtained,  and  the  permissible  limitations  are  more  easily  seen. 
In  the  following  treatment,  these  component  e.m.fs    will    be 
considered  separately.     As  that  component,  due  to  cutting  the 
various  armature  fluxes,  will  be  referred  to   very  frequently 
hereafter,  it-  will  be  called  the   "  apparent  "   armature   short 
circuit  e.m  f .  per  coil,  or  in  abbreviated  form,  "  the  apparent 
short  circuit  em.f."     In  practise,  on  account  of  the  complexity 
of  the  separate  elements  which  make  up  the  apparent  short 
circuit  e  m.f.,  it  is  very  difficult,  or  in  many  cases,  impossible, 
to  entirely  neutralize  or  balance  it  at  all  instants  by  means  of  an 
e.m.f.  generated  by  an  extraneous  field  or  flux  of  a  definite  distri- 
bution-.    Therefore,  it  should  be  borne  in  mind  that,  in  practise, 
only  an  approximate  or  average  balance  between  the  two  com- 
ponent e.m.fs.  is  possible.     With  such  average  balance  there  are 
liable  to  be  all  sorts  of  minor  pulsations  in  e.m.f.  which  tend  to 
produce  local  currents  and  which  must  be  taken  care  of  by  means- 
of  the  brush  resistance.     Pulsations  or  variations  in  either  of  the 
component  e.m.fs.  are  due  to  various  minor  causes,  such  as  the 
varying  magnetic  conditions  which  result  from  a  rotating  open 
slot  armature,  from  cross  jnagnetizing  and  other  distorting  effects 
under  the  commutating  poles,  variations  in  air-gap  reluctance 
under  the  commutating  pol^s,  pulsations  in  the  main  field  reluc- 
tance causing  development  of-  secondary  e.m.fs.  in  the  short 
circuited  coils,  etc.     Some  of  these  conditions  are  liable  to  be 
present  in  every  machine;  some  which  would  otherwise  tend  to 
give  favorable  conditions  as  regards  commutation,  are  partic- 
ularly liable  to  -set  up  minor  pulsations  in  the  short  circuit  e.m.f. 
Therefore,  brushes  of  high  enough  resistance  to  take  care  of  the 
short  circuit  e  m.f .  pulsations  are  a  requisite  of  the  present  types 
of  d-c.  machines,  and  it  may  be  assumed  that  there  is  but  little 


PHYSICAL  LIMITATIONS  IN  D  C.  MACHINES  253 

prospect  of  so  improving  the  conditions  in  general  that  relatively 
high  resistance  brushes,  or  their  equivalent,  may  be  discarded. 
It  is  only  on  very  special  types  of  machines  that  low  resistance 
brushes  can  be  used. 

With  ideal  or  perfect  commutation,  the  two  component  e.m.fs 
in  the  short  circuited  coil  should  balance  each  other  at  all  times 
However,  as  stated  before,  this  condition  is  never  actually  ob- 
tained, and  the  brush  resistance  must  do  the  rest.  With  ideal 
commutation,  the  current  distribution  over  the  brush  contact 
face  should  be  practically  uniform,  and  a  series  of  voltage  read- 
ings between  the  brush  tip  and  commutator  face  should  show 
uniform  drops  over  the  whole  brush  face  In  most  cases  in 
practise  however,  such  voltage  readings  will  be  only  averages 
For  example,  instead  of  a  contact  drop  of  one  volt  at  a  given 
point,  the  actual  voltage  may  be  varying  from  zero  to  two  volts, 
or  possibly  from  minus  one  volt  to  plus  three  volts  These 
pulsating  e.m.fs.  will  result  in  high  frequency  local  currents, 
which  have  only  a  harmful  influence  on  the  commutation  and 
commatator  and  brushes  These  pulsations  may  be  assumed 
to  be  roughly  related  in  value  to  the  apparent  short  circuit 
volts  generated  by  the.  armature  conductor.  In  other  words, 
the  higher  the  apparent  short  circuit  volts  per  conductor,  the 
larger  these  pulsations  are  liable  to  be  As  the  currents  set  up 
by  these  pulsations  must  be  limited  largely  by  the  brush  contact 
resistance,  it  is  obvious  that  there  is  a  limit  to  the  pulsation* 
in  voltage,  beyond  which  the  current  set  up  by  them  may  be 
harmful.  A  very  crude  practise,  and  yet  possibly,  the  only 
fairly  safe  one,  has  been  to  set  an  upper  limit  to  the  apparent 
short  circuit  volts  per  bar,  this  limit  varying  to  some  extent  with 
the  conditions  of  service,  such  as  high  peak  loads  of  short  dura- 
tion, overloads  of  considerable  period,  continuous  operation,  etc. 
Experience  has  shown  that  in  commutating  pole  machines,  the 
apparent  short  circuit  voltages  per  turn  may  be  as  high  as  four 
to  four-and-one-half  volts,  with  usually  but  small  evidence  of 
local  high  frequency  currents,  as  indicated  by  the  condition  of 
the  brush  face.  If  this  polishes  brightly,  and  the  commutator 
face  does  not  tend  to  "  smut,"  then  apparently  the  local  currents 
are  not  excessive.  However,  in  individual  cases,  the  above 
limits  have  been  very  considerably  exceeded  in  continuoxis  opera- 
tion, while,  in  exceptional  cases,  even  with  apparently  well 
proportioned  comxnutating  poles,  there  has  been  evidence  of 
considerable  local  current  at  less  than  fo'ur  volts  per  bar. 


254  ELECTRICAL  ENGINEERING  PAPERS 

The  contact  droi>  between  brush  and  commutator  with  the 
usual  brushes  is  about  1  to  1.25  volts  As  is  well  known,  this 
drop  is  not  directly  proportional  to  the  current,  but  increases 
only  slowly  with  very  considerable  increases  in  current  density 
at  the  brush  contact  For  instance,  with  20  amperes  per  sq  in. 
in  a  given  brush » the  contact  drop  may  be  one  volt ;  at  40  amperes 
per  square  inch,  it  may  be  1  25  volts,  while  at  100  amperes  per 
square  inch,  it  may  be  1  4  volts,  and,  with  materially  higher 
currents,  it  may  increase  but  little  further.  This  peculiar  prop- 
erty of  the  brush  contact  is,  in  some  ways,  very  much  of  a  dis- 
advantage For  instance,  if  the  local  currents  are  to  be  limited 
to  a  comparatively  low  density,  then  necessarily  the  voltages 
generating  such  currents  must  be  kept  comparatively  low  With 
the  above  brush  contact  characteristics,  two  volts  would  allow 
a  local  current  of  20  amperes  per  square  inch  to  flow,  (there  being 
one  volt  drop  from  brush  to  commutator  and  one  volt  back  to 
the  brush)  If,  however,  the  local  voltage  is  three  volts  instead 
of  two,  or  only  50  per  cent  higher,  then  a  local  current  of  possibly 
150  to  200  amperes  per  square  inch  may  flow,  and  this  excessive 
current  density  may  destroy  the  brush  contact,  as  will  be  de- 
scribed later 

It  may  be  assumed  in  general  that  the  lower  the  apparent  short 
circuit  voltage  per  armature  conductor,  the  lower  the  pulsations 
in  this  voltage  are  liable  to  be.  Assuming  therefore,  as  a  rough 
approximation  a  50  per  cent  pulsation  as  liable  to  occur,  then, 
from  the  standpoint  of  brush  contact  drop,  the  total  apparent 
voltage  of  the  commutated  coil  in  continuous  service  machines 
should  not  be  more  than  4  to  4J  volts,  which  accords  pretty  well 
with  practise.  For  intermittent  services,  such  as  railway, 
materially  higher  voltages  are  not  unusual 

As  the  main  advantage  pf  the  carbon  brush  is  that  it  determines 
or  limits  the  amount  of  short  circuit  current,  it  might  be  ques- 
tioned whether  such  advantage  might  not  be  carried  much  fur- 
ther by  using  higher  short  circuit  voltages  and  proportionately 
greater  resistance.  However,  there  are  reasons  why  this  cannot 
be  done  The  carbon  brush  is  a  resistance  in  the  path  of  the 
local  current,  but  it  is  also  in  the  path  of  the  work  current  As 
the  brush  resistance  is  increased,  the  greater  is  the  short  circuit 
voltage  which  can  be  taken  care  of  with  a  given  limit  in  short  circuit 
current,  but  at  the  same  time,  the  loss  due  to  the  work  current  is 
increased.  Decreasing  the  resistance  of  the  brush  contact  in- 
creases the  loss  due  to  the  short  circuit  current,  but  decreases 


PHYSICAL  LIMITATIONS  IN  D.C.  MACHINES  255 

that  due  to  the  work  current.  Thus  in  each  individual  case, 
there  is  some  particular  brush  resistance  which  gives  minimum 
loss  However,  this  may  not  always  be  the  resistance  desired 
for  best  commutation,  from  the  operating  standpoint,  but  these 
two  conditions  of  resistance  appear  to  lie  fairly  close  together 
Practise  is  a  continual  compromise  on  this  question  of  brush  con- 
tact resistance  In  some  machines,  a  low  resistance  brush  is 
practicable,  with  consequent  low  loss  due  to  work  current.  In 
other  cases,  which,  to  the  layman,  would  appear  to  be  exactly 
similar,  higher  resistance  brushes  give  better  average  results 
Thus  one  grade  of  carbon  brush  is  not  the  most  suitable  for  dif- 
ferent machines  unless  they  have  similar  commutating  condi- 
tions However,  it  is  impractica*ble  to  design  all  machines  of 
different  speeds,  types,  or  capacities  so  that  they  will  have  equal 
commutating  -characteristics  In  non-commutating  pole  ma- 
chines where  only  average  commutating  fluxes  are  obtainable, 
the  resistance  of  the  brush  is  usually  of  more  importance  than 
m  the  commutating  pole  type,  for.  in  the  latter,  a  means  is  pro- 
vided for  controlling  the  value  of  the  short  circuit  current  How- 
ever, advantage  has  been  taken  of  this  latter  fact  to  such  an 
extent  in  modern  commutating  pole  machines,  that  the  critical 
or  best  brush  resistance  has  again  become  a  very  important 
condition  of  design  and  operation 

"Apparent"  Short  Circuit  em  f  per  Brush.  The  preceding 
considerations  lead  up  to  another  limitation,  namely,  the  total 
e  m  f  short  circuited  by  the  brush.  This  again  may  be  considered 
as  being  made  up  of  two  components, — the  apparent  short 
circuit  e.m  f  per  bar  times  the  average  number  of  bars  covered 
by  the  brush,  hereafter  called  "  The  apparent  short  circuit 
e  m.f  per  brush  *';  and  the  e.m.f.  per  bar  generated  by  the  com- 
mutating field,  times  the  average  number  of  bars  covered  by 
the  brush 

As  has  been  shown,  ordinary  carbon  brushes  can  short  circuit 
2  to  2^  volts  without  excessive  local  current.  Obviously ,  if  the 
resultant  e.m.f.  generated  in  all  the  coils  short  circuited  by  the 
brush, — that  is,  the  resultant  of  the  short  circuit  e.m.fs  ,  due  to 
both  the  armature  and  the  commutating  field  is  much  larger 
than  2J  volts,  large  local  currents  will  flow.  Therefore,  in  a 
commutating  pole  machine,  for  instance,  the  strength  of  the 
commutating  pole  field  should  always  be  such  that  it  also 
neutralizes  the  total  short  circuit  e.m.f-  across  the  brush  within 
a  limit  represented  by  the  brush  contact  drop,  in  order  to  keep 


256  ELECTRICAL  ENGINEERING  PAPERS 

within  the  limits  of  permissible  local  currents.  With  very 
low  resistance  brushes,  the  proportioning  of  the  commutating 
field  for  neutralization  of  the  apparent  brush  e.m.f.  would 
have  to  be  much  closer  than  with  higher  resistance  brushes. 
Moreover,  not  only  should  this  e.m.f.  generated  by  the  com- 
mutating flux  balance  the  total  short  circuit  voltage  across  the 
brush  within  these  prescribed  limits,  but  these  limits  should 
not  be  exceeded  anywhere  under  the  brush. 

It  might  be  assumed  that  if  there  is  a  pulsation  of  t\vo  volts 
per  coil,  for  instance,  then  the  total  pulsation  would  be  equal 
to  this  value  times  the  average  number  of  coils  short  circuited. 
However,  this  in  general  is  not  correct,  as  the  e.m.f.  pulsations 
for  the  different  coils  are  not  in  phase,  and  their  resultant  may 
be  but  little  larger  than  for  a  single  coil 

Based  upon  the  foregoing  considerations,  the  limiting  values 
of  the  apparent  brush  e.m.f.  may  be  approximated  as  follows: 
Assume  ordinary  carbon  brushes  with  1  to  li  volts  drop  with 
permissible  current  densities — that  is,  with  2  to  2|  volts  opposr 
ing  action  as  regards  local  currents.  Also,  assume,  for  example, 
an  apparent  brush  short  circuit  e.m  f .  of  5  volts,  with  brush 
resistance  sufficient  to  take  care  of  2§  volts.  Then  the  total 
e.m  f.  due  to  the  commutating  flux  need  not  be  closer  than  50 
per  cent  of  the  theoretically  correct  value,  with  permissible 
local  currents.  This  is  a  comparatively  easy  condition,  for  it 
is  a  relatively  poor  design  of  machine  in  which  the  commutating 
pole  strength  cannot  be  brought  within  50  per  cent  of  the  right 
value.  Assuming  next,  an  apparent  brush  e.m.f.  of  10  volts, 
then  the  commutating  pole  must  be  proportioned  within  25 
per  cent  of  the  right  value.  In  practise,  this  also  appears  to 
be  feasible,  without  undue  care  and  refinement  in  proportion- 
ing the  commutating  field.  If  this  machine  never  carried  any 
overload,  this  25  per  cent  approximation  would  represent  a 
relatively  easy  condition,  for  experience  has  shown  that  pro- 
portioning within  10  per  cent  is  obtainable  in  some  cases,  which 
should  allow  an  apparent  brush  e.m.f.  of  25  volts  as  a  limit. 
However,  experience  al<o  shows  that  this  latter  is  a  compara- 
tively sensitive  condition,  which,  while  permissible  on  short 
peak  loads,  is  not  satisfactory  for  normal  conditions.  Where 
such  close  adjustment  is  necessary  to  keep  within  the  brush 
correcting  limits,  any  rapid  changes  in  load  are  liable  to  result 
in  sensitive  commutating  conditions,  for  the  commutating  pole 
flux  does  not  always  rise  and  fall  exactly  in  time  with  the  arma- 


PHYSICAL  LIMITATIONS  IN  D.C   MACHINES  257 

ture  flux,  and  ihus  momentary  unbalanced  conditions  of  pos- 
sibly as  high  as  10  or  12  volts  might  occur  with  an  apparent 
brush  e.m.f.  of  25  volts  Also,  very  slight  saturation  in  the 
commutating  pole  magnetic  circuit  may  have  an  unduly  large 
influence  on  unbalancing  the  e  m.f .  conditions.  In  other  words, 
the  apparent  brush  short  circuit  and  neutralizing  e.m.fs.  must 
not  be  unduly  high  compared  with  the  permissible  corrective 
drop  of  the  brushes  Experience  shows  that  an  apparent  e  rn.f 
of  10  volts  across  the  brush  in  well  designed  commutating  pole 
machines  is  usually  very  satisfactory,  while,  in  occasional  cases, 
12  to  13  volts  allow  fair  results  on  large  machines,  and,  in  rare 
cases,  as  high  as  io  to  18  volts  has  been  allowed  on  small  ma- 
chines at  normal  rating.  However,  overloads,  in  some  cases, 
limit  this  permissible  apparent  brush  voltage.  As  a  rule,  30 
volts  across  the  brush  on  extreme  overload  is  permissible, 
but,  usually  this  is  accompanied  by  some  sparking,  usually 
not  of  a  very  harmful  nature  if  not  of  too  long  duration.  Under 
such  overload  conditions,  doubtless  unbalancing  of  three  volts 
or  more  may  tye  permissible,  and  thus,  with  30  volts  to  be 
neutralized,  this  means  about  90  per  cent  theoretically  correct 
proportioning  of  the  commutating  pole  flux.  Cases  have  been 
noted  where  as  high  as  35  to  40  apparent  brush  volts  have  been 
corrected  by  the  commutating  pole  on  heavy  overloads  with 
practically  no  sparking.  This,  however,  is  an  abnormally  good 
result,  and  is  not  often  possible  of  attainment.  Obviously, with 
such  high  voltages  to  be  corrected,  any  little  discrepancies  in 
the  balancing  action  between 'the  various*  e.m.fs.  are  liable  to 
cause  excessive  local  current  flow.- 

Incidentally,  the  above  indicates  pretty '  clearly  why  d-c 
generators  are  liable  to  -flash  viciously  when  dead  short  cir- 
cuited. The  ordinary  large  capacity  machine  can  give  20  to 
30  times  rated  full  load  current'on  short  circuit.  If  this  large 
current  flows,  then,  neglecting  saturation,  the  armature  short 
circuit  e.m.f.  across  the  brush  will  be  excessive.  Assuming, 
for  instance,  a  10-volt  limit  for  normal  rating,  then  with  only 
ten  times  full  load  current,  the  apparent  short  circuit  e.m  f 
would  be  100  volts.  The  commutating  pole,  in  the  normal  con- 
struction, does  not  have  flux  margin  of  10  times  before  high 
saturation  is  reached,  and  in  consequence,  it  may  neutralize 
only  50  to  60  volts  of  the  100.  Therefore  a  resultant  actual 
e.m.f  of  possibly  40  volts  must  be  taken  care  of  by  the  brushes 
This  means  an  enormous  short  circuit  current  in  addition  to 


258  ELECTRICAL  ENGINEERING  PAPERS 

the  10  times  work  current.  Vaporization  of  the  copper  and 
brushes  occurs  and  flashing  results,  as  will  be  described  more 
fully  in  the  treatment  of  flashing  limits. 

Brush  contact  drops  of  1  to  1.5  volts  have  been  assumed 
in  the  preceding,  and  certain  limits  in  the  apparent  short  cir- 
cuit e.m.f.  based  on  these  drops,  have  been  discussed.  How- 
ever, the  conditions  may  be  modifi  ed  to  a  considerable  extent 
by  effects  of  temperature  upon  the  brush  contact  resistance. 
Usually  it  has  been  assumed  that  the  well  known  decrease  in 
contact  resistance  of  carbon  and  graphite  brushes  with  increase 
in  temperature,  is  in  some  ways  related  to  the  negative  tem- 
perature coefficient  of  carbon  and  graphite  The  writer  has 
been  among  those  who  advanced  this  idea,  but  later  experience, 
based  upon  tests,  has  shown  that  the  reduced  drop  with  increase 
in  temperature  does  not  necessarily  hold  any  relation  to  the 
negative  temperature  coefficient  of  the  carbon  brush  itself,  for 
similar  changes  in  the  contact  drop  have  been  found  with  ma- 
terials, other  than  carbon,  which  actually  had,  in  themselves, 
positive  temperature  coefficients.  Moreover,  in  some  tests, 
the  changes  in  contact  resistance  with  increase  in  temperature 
have  proved  to  be  much  greater  in  proportion  than  occurs 
in  the  carbons  themselves.  In  some  cases,  the  measured  drops 
with  temperature  increases  of  less  than  100  deg.  cent  decreased 
to  one-half  or  one-third  of  the  drops  measured  cold 

Obviously,  these  decreased  contact  resistances  or  drops  may 
have  a  very  considerable  effect  on  the  amount  of  local  current 
which  can  flo\v  and,  therefore,  in  such  case  the  foregoing  general 
deductions,  should  be  modified  accordingly  However,  the 
results  are  so  affected  by  the  oxidation  of  the  copper  commutator 
face,  and  other  conditions  also  more  or  less  dependent  upon 
temperature,  that,  as  yet.  no"  definite  statement  can  be  made 
regarding  the  practical  effects  of  increase  in  temperature  except 
the  general  one  that  the  resistance  is  usually  lowered  to  a  con- 
siderable extent  Apparently,  oxidation  of  the  copper  face 
tends  toward  higher  contact  resistance  Ofttimes,  "sanding 
off  "  the  glaze  tends  to  give  poorer  commutation  The  above 
points  to  one  explanation  of  this 

Assuming  any  desired  limits  for  the  apparent  e  m  fs  ,  such  as 
4  to  4|  volts  per  commutator  bar,  it  is  possible  to  approximate 
by  calculation  the  limiting  capacities  of  generators  or  motors  in 
terms  of  speed,  etc  Appendix  I  shows  one  method  of  doing 
this  In  the  writer's  experience,  a  number  of  machines  have  been 


PHYSICAL  LIMITATIONS  IN  D.C.  MACHINES 


259 


earned  up  to  about  the  limits  derived  in  the  appendix,  and  the 
practical  results  were  in  fair  accord  with  the  calculations.  In 
general,  it  may  be  said  that  in  large  machines,  the  upper  limits 
of  capacity  in  terms  of  speed,  etc.  are  so  high  that  they  do  not 
indicate  any  great  handicap  on  future  practise. 

In  the  foregoing,  the  limits  for  the  apparent  short  circuit  e  m.f . 
per  bar  and  per  brush  have  been  based  upon  the  brush  contact 
resistance  However,  it  may  be  suggested  that  something  other 
than  the  brush  contact  resistance  might  be  used  for  limiting  the 
local  current,  and  thus  the  commutating  limits  might  be  raised. 
For  instance,  an  armature  winding  could  be  completely  closed  on 
itself,  with  high  resistance  leads  carried  from  the  winding  to  the 

commutator  bars.  Each  of  such  leads 
would  be  in  circuit  only  where  the 
brushes  touched  the  commutator 
bars.  Thus  there  could  be  very  con- 
siderable resistance  in  each  lead  with- 
out greatly  increasing  the  total  losses; 
and,  unlike  the  brushes,  each  lead 
would  be  in  circuit  only  for  a  very 
small  proportion  of  the  time. 

About  10  years  ago,  the  writer  de- 
signed a  non-commutating  pole  d-c. 
turbo-generator  with  such  resistance 
leads  connected  between  the  winding 
and  the  commutator.  The  leads  were 
placed  in  the  armature  slots  below  the 
main  armature  winding.  The  idea  was 
to  have  enough  resistance  in  circuit 


I  I  I  I ITTT 

FIG.  1 


with  the  short  circuited  coils  that  the  brushes  at  no  load  could  be 
thrown  well  forward  into  a  field  flux  sufficient  to  produce  good 
commutation  at  heavy  load,  even  if  very  low  resistance  brushes, 
were  used.  Tests  of  this  machine  showed  that  the  non-sparking 
range,  with  the  brushes  shifted  either  forward  or  back  of  the 
neutral  point  was  very  much  greater  than  in  an  ordinary  machine. 
In  this  case,  it  developed  that  the  leads  were  of  too  high  resistance 
for  practical  purposes,  as  the  armature  ran  too  hot,  the  heat-dis- 
sipating conditions  in  a  small  d-c.  turbo-armature  not  being  any 
too  good  at  best.  These  tests  however,  indicate  one  possibility 
in  the  way  of  increasing  the  present  limits  of  voltage  per  bar  and 
volts  across  the  brush.  Moreover,  such  resistances  can  have  a 
positive  temperature  coefficient  of  resistance,  instead  of  the 


260  ELECTRICAL  ENGINEERING  PAPERS 

negative  one  of  the  carbon  brushes  and  contacts.  Also,  the 
corrective  action  in  limiting  local  currents  would  vary  directly 
with  the  current  over  any  range,  and  not  reach  a  limit,  as  in  car- 
bon brushes 

Considerable  experience  with  resistance  leads  in  d-c  operation 
has  also  been,  obtained  in  large  a-c,  commutator  type  railway 
motors,  designed  for  operation  on  both  a-c  and  d-c.  circuits. 
Apparently  these  leads  have  a  very  appreciable  balancing  action 
as  regards  division  of  current  between  brush  arms  in  parallel. 
With  but  few  brushes  per  arm,  it  appears  that  very  high  current 
densities  in  the  brushes  can  be  used  without  undue  glowing  or 
honeycombing.  Presumably  the  reduction  in  short  circuit 
current,  when  operating  on  d-c  ,  also  has  much  to  do  with  this. 
Some  special  tests  were  made  along  this  line,  and  it  was  found  that 
a,  very  low  resistance  in  the  leads,  compared  with  that  which  was 
best  for  a-c.  operation,  was  sufficient  to  exert  quite  a  decided 
balancing  between  the  brush  arms 

With  properly  proportioned  resistance  leads  it  should  be  pos- 
sible to  use  very  low  resistance  brushes,  and  relatively  high 
current  densities.  Advantage  of  this  might  be  taken  in  various 
ways.  There  may  prove  to  be  serious  mechanical  objections  to 
such  arrangements  However,  if  the  objections  are  not  too 
serious,  the  use  of  resistance  leads  in  this  manner  may  be  prac- 
tised at  some  future  time  as  we  approach  more  extreme  flesigns 

FLASHING 

One  of  the  limits  in  commtltating  machinery  is  flashing.  This 
tnay  be  of  several  kinds.  There  may  be  a  large  arc  or  fla£h 
from  the  front  edge  of  the  brush,  which  may  increase  in  volume 
until  it  becomes  a  flash-over  to  some  other  part  of  the  machine. 
Again,  a  flash  may  originate  between  two  adjacent  bars  at  some 
point  between  the  brush  arms,  and  may  not  extend  further,  or 
•it  may  grow  into  a  general  flashover.  Different  kinds  of  flashes 
:may  arise  from  radically  different  causes,  some  of  which  may  be 
-normally  present  in  the  machine,  while  others  may  be  of  an 
.accidental  nature. 

Whatever  the  initial  cause,  the  flash  itself  means  vaporized 
conducting  material.  If  the  heat  developed  by  or  in  this  vapor 
arc  is  sufficient  to  vaporize  more  conducting  material — that  is, 
generate  more 'conduct ing  vapor — then  the  arc  or  flash  will  grow 
or  continue.  Thus,  true  flashing  should  be  associated  with 
vaporization,  ,and,  in  sttany  cases,  in  order  to  get  at  the  initial 


PHYSICAL  LIMITATIONS  7.V  D  C    MACHINES  261 

cause  of  flashing,  it  is  only  necessary  to  find  the  initial  cause  of 
vaporization. 

Arcs  Between  Adjacent  Cowrmit-ator  Bars.  This  being  one  of 
the  easiest  conditions  to  analyze,  it  will  be  treated  first,  especially 
as  certain  flashing  conditions  are  dependent  upon  this. 

A  not  uncommon  condition  on  commutators  in  operation  is 
a  belt  of  incandescent  material  around  the  commutator,  usually 
known  as  "ring  fire"  This  is  really  incandescent  material 
between  adjacent  bars,  such  as  carbon  or  graphite,  scraped  off 
the  brush  faces  usually  by  the  mica  between  bars.  As  the  mica 
tends  to  stand  slightly  above  the  copper,  due  to  less  rapid  "wear," 
its  natural  action  is  to  scrape  carbon  particles  off  the  brush. 
These  particles  are  conducting  and  if  there  is  sufficient  voltage,* 
and  current  to  bring  them  up  to  incandescence,  this  shows  as  a 
streak  of  fire  around  the  commutator  In  many  cases,  by  its 
different  intensities  around  the  commutator,  trlis  ring  fire  shows 
plainly  the  density  of  the  field  flux,  or  e  m.f .  distribution  around 
the  machine  It  is  practically  zero  in  the  commutating  or 
neutral  zone,  and  shows  plainly  under  the  main  field.  In  loaded 
machines,  this  often  indicates  roughly  the  flux  distortion.  In 
machines  which  act  alternately  as  motors  and  generators,  as  in 
reversing  mill  work,  the  point  of  highest  incandescence  shifts 
forward  or  backward  over  the  "commutator,  depending  upon  the 
direction  of  field  distortion. 

In  undercut  commutators  (those  with  mica  cut  below  the  cop- 
per surface)  this  ring  fire  is  also  observable  at  times,  due  to  con- 
ducting particles  in  the  slots  between  bars.  Usually  such 
particles  consist  of  carbon  or  graphite,  as  already  stated,  but 
particles  of  copper  may  also  be  present.  Also,  oil  or  grease,  mixed 
with  carbon,  will  carbonize  under  incandescence,  and  will  thus 
add  to  the  ring  fire.  Often  when  a  commutator  is  rubbed  with 
an  oiled  cloth  or  wiper,  ring  fire  will  show  very  plainly,  and  then 
gradually  die  down.  The  burning  oil  exaggerates  the  action,, 
and  also,  the  oil  itself  may  enable  a  conducting  coating  to  adhere- 
to  the  mica  edges,  thus  starting  the  action,  which  disappears* 
when  the  oil  film  is 'burned  away.  However,  when  the  oil  can- 
penetrate  the  mica,  the  incandescence  may  continue  in  spots  and 
at  intervals,  the  mica  being  calcined  or  burned  away  so  that  it 
gradually  disppears  in  spots.  This  is  the  action  usually  called. 
"'  pitting  ",  which  experience  has  shown  to  be  almost  invariably 
caused  by  conducting  material  in  the  mica,  such  as  carbonized 
•oil,  carbonized  binding  material,  copper  and  carbon  particles 
been  carried  in  with  the  oil,  etc. 


'262 


ELECTRICAL  ENGINEERING  PAPERS 


This  ring  fire  is  not  always  a  direct  function  of  the  voltage 
between  bars,  although,  under  exactly  equivalent  conditions  of 
speed,  grade  of  brushes,  etc.,  it  is  closely  allied  with  voltage  condi- 
tions. In  high  voltage  machines,  usually  hard  high-resistance 
brushes  are  used,  which  tend  to  give  off  the  least  carbon  in  the 
form  of  particles;  while  in  low  voltage  machines,  soft,  low-re- 
sistance brushes,  with  a  good  percentage  of  graphite  in  them,  are 
common,  and  these  naturally  tend  to  coat  the  mica  to  a  greater 
extent. 

Under  extreme  conditions,  this  ring  fire  may  become  so  intense 
locally  that  there  is  an  actual  arc  formed  between  two  adjacent 
bars,  due  to  vaporization  of  the  copper.  This  may  show  in  the 
form  of  minute  copper  beads  at  the  edge  of  the  bar,  or  minute 
"pits"  or  "pockets"  may  be  burned  in  the  copper  next  to  the 
mica.  In  extreme  cases,  where  the  voltage  between  bars  is 
sufficient  to  maintain  an  arc,  conical  shaped 
cavities  or  holes  may  be  burned  in  the 
copper.  In  such  cases,  the  arc  is  usually 
explosive,  resembling  somewhat  a  small  , 
"buck-over."  An  examination  of  the  com- 
mutator will  show  melted-out  places,  as  in 
Pig.  2.  Part  of  the  missing  copper  has 
been  vaporized  by  the  arc,  while  part  may 
have  become  so  softened  or  fused  that  it  is 
thrown  off  by  centrifugal  force.  Exper-  ' 
ience  shows  that  sometimes  these  explosive 


IIITITI 


FIG.  2 


arcs  grow  into  general  flashes,  while  at  other  times,  they  are 
purely  local. 

An  extended  study  was  made  of  such  arcs  to  determine  the 
conditions  which  produced  them.  Also,  numerous  tests  were 
made,  the  results  of  which  are  given  below, 
i  It  was  determined  first,  that  these  explosive  arcs  between 
adjacent  bars  were  dependent,  in  practically  all  cases,  upon  a 
fairly  high  voltage  between  bars.  This  was  reasonable  to  expect, 
but  it  was  found  that  the  voltage  between  bars  which  would 
produce  arcs  in  one  case,  would  not  do  so  in  another.  Apparently 
there  were  other  limiting  or  controlling  conditions.  It  developed 
that  the  resistance  of  the  armature  winding  between  two  adja- 
cent bars  has  much  to  do  with  the  arc.  Apparently  an  excessive 
current  is  necessary  to  melt  a  small  chunk  out  of  a  mass  of  good 
heat-conducting  material  like  a  large  copper  commutator;  *  and 
also,  a  certain  amount  of  time  is  required  to  bring  it  up  to  the 


PHYSICAL  LIMITATIONS  IN  D.C.  MACHINES  263 

melting  point  Therefore,  both  time  and  current  are  involved, 
as  \\  ell  as  voltage.  A  series  of  tests  was  made  to  determine  some 
of  the  limiting  conditions. 

The  commutator  of  a  small  machine  (about  20  kw.,  high  speed) 
was  sprinkled  with  iron  filings,  »fme  dust,  etc,  during  several 
days'  operation  under  various  conditions  of  load,  field  distortion, 
etc.   Such  dust,  whether  conducting  or  not;  apparently  would  not 
cause  arcing  between  bars.     Graphite  was  finally  applied  with  a 
special  "\\iper,"  and  with  this,  small  arcs  or  flashes  could  be 
produced  at  50  to  60  volts  maximum  between  commutator  bars. 
It  soon  became  evident  that  this  was  too  small  a  machine  from 
which  to  draw  conclusions.  Then  numerous  other  much  larger  gen- 
erators were  tested     A  slow-speed  engine  type  generator  of  200-kw. 
capacity  at  250  volts,  was  speeded  up  to  about  double  speed, 
in  order  to  obtain  sufficiently  high  e.m.f.  between  commutator 
bars.     With  a  clean  commutator  nothing  was  obtained  at  40 
volts  maximum  per  bar.     The  commutator  was  then  wiped  with 
a  piece  of  oily  waste  which  had  been  used  to  wipe  off  other  com- 
mutators.    Arcs  then  occurred  repeatedly  between  commutator 
bars,  although  all  such  arcs  were  confined  to  adjacent  bars  and 
there  were  no  actual  flashovers  from  brush  holder  to  brush  holder. 
Moreover,  the  arcs  always  appeared  to  start  about  midway 
between  brush  arms  or  neutral  points,  and  lasted  only  until  the 
next  neutral  point  was  reached.     Quite  large  pits  or  cavities 
were  burned  in  the  bars  next  to  the  mica,  as  shown  in  Fig.  2. 
some  of  these  being  possibly  J  inch  in  width,  and  1/16  inch  deep 
or  more  at  the  center      This  indicated  excessively  large  currents. 
These  arcs  would  develop  at  about  32  to  34  volts  between  bars, 
and  they  were  very  vicious  (explosive)  above  35  volts 

Still  larger  machines  were  tested  with  various  speeds,  voltage 
between  bars,  etc  It  was  found  that,  as  a  rule,  the  larger  the 
machine — or  rather,  the  lower  the  resistance  of  the  armature 
winding  per  bar — the  lower  would  be  the  voltage  at  which  serious 
arcing  would  develop  In  these  tests,  it  was  found  that  graphite 
mixed  with  grease  gave  the  most  sensitive  arcing  conditions. 

In  these  various  tests,  no  arcing  between  bars  was  developed 
in  any  case  at  less  than  28  volts  maximum,  while  30  volts  was 
approximately  the  limit  on  many  machines.  However,  the 
results  varied  with  the  speed  Apparently  it  took  a  certain  time 
to  raise  the  incandescent  material  to  the  arcing  point  and  to  build 
up  a  big  arc.  Therefore,  the  duration  of  the  possible  arcing 
period  appeared  to  be  involved.  If  this  were  so,  then  a  higher 


264  ELECTRICAL  ENGINEERING  PAPERS 

voltage  limit  for  a  shorter  time  should  be  possible  with  the  same 
arcing  tendency.  Also,  if  this  were  the  case,  then  with  30  volts 
maximum,  for  instance,  between  commutator  bars  with  an  un~ 
distorted  field  flux,  the  arcing  should  be  the  same  as  with  a  some- 
what higher  voltage  with  a  highly  distorted  narrow  peaked  field. 
In  other  words,  the  limiting  voltage  between  bars  on  a  loaded 
machine  might  be  somewhat  higher  than  on  an  unloaded  machine. 
This  was  actually  found  to  be  the  case,  the,  difference  being  from 
10  per  cent  to  15  per  cent  in  several  instances.  This,  however, 
depended  upon  various  limiting  conditions  such  as  the  actual 
period  within  which  the  arc  could  build  up  to  a  destructive 
point,  etc. 

One  very  interesting  case  developed  which  apparently  illus- 
trated very  beautifully  the  effects  of  lengthening  or  shorten- 
ing the  period  during  which  the  arc  could  occur.     A  high-speed, 
600-volt  generator  of  a  motor-generator  set  was  speeded  up 
about  60  per  cent  above  normal.     Even  at  normal  speed  this 
was  a  rather  high-frequency  machine,  so  that  the  period  of 
time  for  a  commutator  bar  to  pass  from  neutral  point  to  neutral 
point  was  very  short.    At  the  highest  speed  the  graphite-grease 
was  used  liberally  on  the  commutator,  but  without  causing  arc- 
ing, even  when  the  voltage   was  raised  considerably  higher 
than  usually  required  for  producing  arcs  between  bars  in  other 
machines  of  similar   size.     Neither  was   there  much  ring-fire 
at  the  highest  speed  with  normal  voltage.     Finally,    after    an 
application  of  graphite,  without  forming  arcs  or  unusual  ring- 
fire,   the  speed  was  reduced  gradually  with  normal  voltage 
maintained.     The  ring-fire  increased  with  decrease  in  speed,  until 
at  about  normal  speed,  it  was  so  excessive  that  the  on-lookers 
expected  an  explosion  of  some  sort.     However,   the  voltage 
was  now  below  the  normal  arcing  point  and  nothing  happened. 
At  still  lower  speed,  but  with  reduced  voltage  on  account  of 
saturation,  the  ring-fire  gradually  decreased.    Apparently  at 
the  very  high  speeds,  the  tirrie  was  too  short  for  the  ring-fire  to 
reach  its  maximum;  while  with  reduction  in  speed,  even  with 
somewhat  reduced  voltage,  the,  ring-fire  increased  to  a  maxi- 
mum and  then  decreased.     This  test  was  continued  sufficiently 
to  be  sure  that  it  was  not  an  accidental  case.     Only  a  certain, 
combination  of  speed,  frequency,  voltage,  etc.  could  develop 
this  peculiar  condition,  and  it  was  purely  by  accident  that 
this  combination  was  obtained,  for  the  result  was  not  foreseen 
in  selecting  the  particular  machine  used. 


PHYSICAL  LIMITATIONS  IN  D.C    MACHINES  265 

A  summation  of  these  and  other  tests  led  to  th£  conclusion 
that  there  were  pretty  definite  limits  to  the  maximum  volts 
per  bar,  beyond  which  it  was  not  safe  to  go.  These  limits 
however,  involved  such  a  number  of  conditions  that  no  fixed 
rule  could  be  established,  and  apparently,  the  designer  has 
to  use  his  judgment  and  experience  to  a  certain  extent,  if  he 
works  very  close  to  the  limits.  The  grades  and  materials  of 
the  brushes,  the  thickness  of  the  mica,  flux  distortion  from  over- 
loads, etc.  must  be  taken  into  account.  For  instance,  the  above 
tests  were  made  on  machines  with  1/32-inch  mica  between  bars. 
This  thickness  is  fixed,  to  a  great  extent,  in  non-undercut 
commutators,  by  conditions  of  mica  wear,  as  will  be  referred 
to  later.  But  with  undercut  commutators,  thicker  mica  can 
be  used,  and,  while  the  gain  in  permissible  safe  voltage  between 
bars  is  not  at  all  in  proportion  to  the  mica  thickness,  yet  it  is 
enough  to  deserve  consideration. 

The  general  conclusions  were  that  with  1/32-inch  mica, 
large  current  machines  would  very  rarely  flash  with  28  volts 
maximum  between  bars;  while  with  moderate  capacities,  30 
volts  is  about  the  lower  limit;  and  with  still  smaller  machines, 
100  kw.  for  example,  this  might  be  as  high  as  33  to  35  volts, 
the  limit  rising  to  50  or  60  volts  with  very1  small  machines. 

Of  course,  the  brush  conditions  have  something  to  do  with 
the  above  limits,  and  many  exceptions  to  these  figures  will  be 
found  in  actual  practise.  Many  machines  are  in  daily  service 
which  are  subject  to  more  or  less  ring-fire,  but  which  have  never 
developed  trouble  of  any  sort,  and  doubtless  never  will.  Ap- 
parently, ring-fire  in  itself  is  not  harpiful,  as  a  rule.  It  is  only 
where  it  starts  some  other  trouble  tiat  it  may  be  considered  as 
actually  objectionable. 

The  above  limiting  figures  are  interesting  when  compared 
with  the  voltages  necessary  to  establish  arcs  in  general.  An 
alternating  arc  through  air  will  not  usually  maintain  itself  at 
less  than  some  limiting  voltage  such  as  20  to  25  volts,  corres- 
ponding to  peak  values  of  28  to  35  volts.  Moreover,  an  arc 
formed  between  the  edges  of  two  insulated  bodies,  such  as  ad- 
jacent 'commutator  bars,  will  naturally  tend  to  rupture  itself 
due  to  the  shape  of  its  path.  Furthermore,  the  resistance  and 
"reactance  of  the  short  circuited  path,  while  comparatively  low 
in  large  machines,  will  tend  to.  limit  the  voltage  which  main- 
tains the  arc.  In  snr>a-11  machines  with  relatively  high  internal 
drops  in  tlie  sfoort  circuited  coils>  the  current  will  not  reach  a 


266  ELECTRICAL  ENGINEERING  PAPERS 

commutator  vaporizing  value  unless  the  initial  voltage  between 
bars  is  comparatively  high,  and  usually  the  explosive  actions 
are  relatively  small,  and,  in  many  cases,  no  senous  arcs  will 
develop  at  all.  Obviously,  the  less  the  local  current  can  in- 
crease in  the  case  of  short  circuits  between  adjacent  bars,  the 
higher  the  voltage  between  bars  can  be,  without  danger.  In 
machines  having  inherent  constant  current  characteristics,  very 
high  voltages  between  adjacent  commutator  bars  are  possible 
without  serious  flashing  or  burning.  In  consequence,  from  the 
flashing  standpoint,  constant  current  machines  can  be  built  for 
enormously  high  terminal  voltages,  compared  with  constant 
potential  machines.  This  is  a  point  which  is  very  commonly 
overlooked  in  discussing  high-voltage  d-c.  machines. 

Cbming  back  to  the  subject  of  arcs  between  commutator 
bars,  these  are  more  common  than  is  usually  supposed,  for, 
in  ma,ny  cases,  the  operating  conditions  are  such  that*  these 
arcs,  if  very  small,  or  limited,  will  show  no  visible  evidence. 
Only  very  minute 'particles  of  copper  may  be  vaporized.  How- 
ever, these  minute  arcs  may  sometimes  lead  directly  to  more- 
serious  flashing.  If,  for  instance,  they  occur  in  proximity  to- 
some  live  part  of  the  machine,  such  as  an  over-hanging  brush 
holder  which  is  at  a  considerable  difference  of  potential  from 
the  arcing  part  of  the  commutator,  the  conducting  vapor  may 
bridge  across  and  start  a  big  arc  or  flash.  "In  one  instance/ 
which  the  writer  has  in  mind,  a  very  serious  case  of  trouble 
occurred  in  this  way.  This  was  a  very  large  capacity  250- 
volt,  low-speed,  generator,  in  which  the  maximum  volts  per 
bar  were  not  unduly  high.  When  taking  the  saturation  curve 
in  the  shop  test,  this  machine  "  bucked"  viciously  several 
times,  apparently  without  reason.  An  investigation  of  the 
burning  indicated  a  possible  source  of  trouble.  The  brush 
holder  arms  or  supports  to  which  the  individual  holders  were 
attached,  were  located  over  the  commutator  about  midway 
between  neutral  points,  and,  about  one  inch  from  the  com- 
mutator face.  This  was  not  the  normal  position  of  the  brush 
arms,  as  a  temporary  set  of  holders  was  being  used  for  this  test. 
It  was  noted  that  just  before  the  flashovers  occurred,  con- 
siderable ring-fire  developed.  The  conclusion  was  drawn  from 
all  the  evidence  that  could  be  obtained,  that  a  small  arc  had 
formed  between  bars  that  had  reached  to  the  brush  arms,  thus 
short  circuiting  a  high  enough  voltage  to  draw  a  real  flash. 
This  happened  not  once  but  several  times.  The  proper  holders. 


PHYSICAL  LIMITATIONS  IN  D.C.  MACHINES  267 

were  then  applied,  which  put  the  brush  arms  in  a  much  less 
exposed  position,  and  not  a  single  flashover  occurred  in  all  the 
subsequent  tests  and  operation.  In  another  case,  a  large  syn- 
chronous converter  carrying  full  load  on  shop  test  flashed  over 
a  number  of  times,  apparently  without  sufficient  cause.  The 
commutation  was  perfect,  as  evidenced  by  the  fact  that  there 
was  no  perceptible  sparking.  The  maximum  voltage  between 
bars  was  comparatively  low.  At  "first  the  flashovers  were 
blamed  on  drops  of  water  from  the  roof  of  the  building,  but 
this  theory  was  soon  disproved.  An  examination  of  the  brush  - 
holders  showed  that  certain  live  parts,  fairly  close  to  the  com- 
mutator, were  at  a  considerable  difference  of  potential  from  the 
nearest  part  of  the  commutator.  There  was  but  little  ring- 
fire  on  the  commutator,  and  therefore,  minute  arcs  at  first  were 
not  blamed  for  the  trouble.  A  modified  brush  holder  was  tried 
however,  with  a  view  to  decreasing  the  high  difference  of  poten- 
tial between  the  live  parts.  All  flashing  then  disappeared  and 
no  trouble  of  this  sort  was  ever  encountered  in  a  large  number 
of  duplicate  machines  brought  through  afterwards.  Both  the 
above  cases  should  be  considered  as  abnormal,  and  they  have 
been  selected  simply  as  examples  of  what  small  arcs  between 
bars  may  do.  These  two  cases  do  not  in  themselves  constitute 
a  proof  of  this  action,  but  they  serve  to  verify  other  evidences 
which  have  been  obtained.  * 

In  view  of  the  fact  that  small  arcs  of  a  non-explosive  sort 
may  form  at  voltages  considerably  lower  than  the  limits  given 
in  the  preceding  part  of  this  paper,-  it  should  be  considered 
whether  such  small  arcs  can  cause  any  trouble  if  no  other  live 
parts  of  the  machine  are  in  close  proximity.  One  case  should^ 
be  considered,  namely,  thai  of  other  commutator  bars  adjacent 
to  the  arc.  When  conducting  vapor  is  formed  by  the  first 
minute  arc,  this  vapor  in  spreading  out  may  bridge  across  a 
number  of  commutator  bars  having  a  much  liigher  total  differ- 
ence of  potential  across  them  than  that  which  caused  the  initial 
arc.  Assume,  for  instance,  a  very  crowded  design  of  high* 
voltage  corrfmutator.  In  some  cases,  in  order  to  use  high  rota- 
tive speeds,  without  unduly  high  commutator  peripheral  speed* 
the  commutator  bars  are  sometimes  made  very  thin  and  the 
volts  per  bar  very  high,  possibly  up  almost  to  the  limit.  As- 
suming a  thickness  of  bar  and  mica  of  0.2  inch  (or  5  bars  per 
inch)  and  a  maximum  volts  per  bar  of  25,  then  there  is  an  e.m  f . 
of  125  volts  per  inch  circumference  of  the  commutator-  In  such 


268  ELECTRICAL  ENGINEERING  PAPERS 

case,  a  small  arc  between  two  bars  may  result  in  bridging  across 
a  comparatively  high  voltage  through  the  resulting  copper 
vapor  Therefore,  when  considering  the  possible  harmful  effects 
of  minute  arcs,  the  volts  per  inch  circumference  of  the  commuta- 
tor should  be  taken  -into  consideration.  The  writer  observed 
one  high-voltage  commutator  which  flashed  viciously  at  times, 
apparently  without  "  provocation  "  The  only  explanation  he 
could  find  was  that  the  vapor  from  little  arcs  resulting  from 
ringfire  was  sufficient  to  spread  all  over  the  commutator,  the 
bars  being  very  thin  and  the  voltage  per  bar  very  high.  How-, 
ever,  difficullies  from  this  cause  have  not  yet  become  serious, 
probably  because  no  one  has  yet  carried  such  constructions  to 
the  extreme,  in  practical  work 

High  voltage  between  commutator  bars  may  result  in  flash- 
ing due  to  other  than  normal  operating  conditions.  Excessive 
overloads  may  give  such  high  voltages  per  armature  coil  or  per 
commutator  bar,  immediately  under  the  brush,  that  the  terrific 
current  rush  will  develop  conducting  vapors  under  the  brush, 
which  appear  immediately  in  front  of  the  brushes,  as  such  vapors 
naturally  are  carried  forward  by  rotation  of  the  commutator. 
This  short  circuit  condition  under  the  brush  has  already  been 
referred  to  when  treating  of  commutation  limits  It  was  shown 
then  that  an  inherent  short  circuit  voltage  of  4  to  4|  volts'  is 
permissible  in  good  practise  Immediately  under  the  com- 
mutating  pole  this  voltage  is  practically  neutralized  by  the 
commutating  pole  field,  but  immediately  ahead  or  behind  the 
pole  it  is  not  neutralized  usually,  except  to  the  extent  of  the 
commutating  pole  flux  fringe.  Thus,  the  resultant  voltage 
between  two  bars  a  little  distance  ahead  of  the  brush,  is  liable 
to  be  considerably  higher  than  under  the  brush  Assuming, 
for  instance,  3£  volts  per  bar,  due  to  cutting  the  resultant  field 
just  ahead  of  the1  brush,  then  with  10  times  full  load  current, 
for  example,  there  would  be  35  volts  between  bars,  and  this  is 
liable  to  be  accompanied  by  highly  conducting  vapor  formed 
"by  the  excessive  current  at  the  brush  contact,  this  vapor  being; 
carried  forward  by  rotation  of  the  commutator.  Here  are  the 
conditions  for  a  flash,  which  may  or  may  not  bridge  across  to 
.some  other  live  part  If  the  current  rush  is  not  too  great,  this 
flash  will  usually  appear  only  as  a  momentary  blaze  just  in 
front  of  the  brush.  In  many  cases,  if  this  blaze  or  heavy  arc 
were  not  allowed  to  come  in  contact  with,  or  bridge  between, 
any  parts  having  high  difference  of  potential,  it  would  not  be 


PHYSICAL  LIMITATIONS  IN  D.C    MACHINES  269 

particularly  harmful.  In  case  of  "  dead  short  circuiting"  of 
large  moderately  high-voltage  machines  where  the  current  can 
rise  to  25  or  30  times  normal,  it  is  astonishing  how  large  such 
arcs  or  flashes  may  become,  and  to  what  distances  they  will 
reach.  The  arc  will  sometimes  go  in  unanticipated  directions. 
The  conducting  vapor  may  be  deflected  by  magnetic  action 
and  by  air  drafts  Shields  or  partitions  will  sometimes  pro- 
duce unexpected  results,  not  necessarily  beneficial.  Unless 
such  shields  actually  touch  the  commutator  f ac  e  so  that  con- 
ducting vapor  cannot  pass  underneath  them,  the  vapor  that 
does  pass  underneath  may  produce  just  as  harmful  results  as 
if  the  shields  were  not  used.  Trying  to  suppress  such  arcs  by 
covers  or  shields  is  very  much  the  case  of  damming  a  river  at 
the  wrong  end  in  order  to  prevent  high  water 

From  the  preceding  considerations  it  would  appear  that  a 
compensated  direct-current  machine  should  have  some  ad- 
vantages over  the  straight  commutating-pole  type  in  case  of  a 
severe  short  circuit.  With  the  lesser  saturation  in  the  com- 
mutating  pole  circuit  due  to  the  lower  leakage,  the  apparent 
armature  short  circuit  e.m.f.  will  usually  be  better  neutralized 
under  extreme  load  conditions,  and  thus  there  will  be  lower 
local  currents  in  the  brush  contacts  In  addition,  the  armature 
flux  will  be  practically  as  well  neutralized  behind  and  ahead 
of  the  brush,  as  it  is  under  the  brush,  so  that,  with  ten  times 
current  as  in  the  former  example,  there  may  be  only  a  low 
e  m.f.  per  bar  ahead  of  the  brush,  instead  of  the  35  volts  for 
the  former  case  Obviously,  the  initial  flashing  cause,  and  the 
tendency  to  continue  it  ahead  of  the  brush,  will  be  materially 
reduced.  The  compensating  winding  is -therefore  particularly 
advantageous  in  very  high  voltage  generators,  in  which  the 
bars  are  usually  very  thin  and  the  maximum  volts  per  bar  are 
high. 

There  is  a  prevailing  opinion  that  when  a  circuit  breaker 
opens  on  a  very  heavy  overload  or  a  short  circuit,  flashing  is- 
liable  to  follow  from  such  interruption  of  the  current  In  some 
cases,  this  may  be  true  However,  when  a  breaker  opens  on. 
a  short  circuit,  it  is  difficult  for  the  observer  to  say  whether  both 
the  opening  of  the  breaker  and  the  flash  are  due  to  the  excessive 
momentary  current,  or  one  is  consequent  to  the  other.  The 
short  circuit,  if  severe,  will  most  certainly  cause  more  or  less 
of  a  flash  at  the  brush  contacts  by  the  time  the  breaker  is  opened, 
and  if  this  flash  is  carried  around  the  commutator,  or  bridges 


270  ELECTRICAL  ENGINEERING  PAPERS 

across  two  points  of  widely  different  potentials,  then  it  is  liable 
to  continue  after  the  breaker  opens,  and  thus  ^ives  the  im- 
pression that  the  flashing  followed  the  interruption  of  the  cir- 
cuit. In  railway  and  in  mine  work  in  particular,  a  great  many 
flashes  which  are  credited  to  overloads  are  primarily  caused  by 
partial  short  circuits  on  the  system,  or  "  arcing  shorts,"  which 
are  extinguished  as  soon  as  the  main  breakers  are  opened,  so 
that  but  little  or  no  evidence  of  any  short  circuit  remains. 
Such  a  partial  short  circuit  however,  may  be  sufficient  to  open 
the  generator  circuit  and  to  cause  a  flash  at  the  same  time. 
Not  infrequently,  such  flashes  are  simply  credited  to  opening 
of  the  breakers 

There  are  other  conditions,  however,  where  a  flash  is  liable 
to  result  directly  from  opening  the  breaker  on  heavy  overload. 
If  as  referred  to  before,  the  apparent  short  circuit  e  m.f.  per 
brush  on  heavy  overload  is  from  25  to  35  volts,  then  if  the 
armature  magnetomotive  force  could  be  interrupted  suddenly, 
with  a  correspondingly  rapid  reduction  in  the  armature  flux, 
while  the  commutating  field  flux  does  not  die  down  at  an  equally 
rapid  rate,  then  momentarily,  there  will  be  an  actual  short 
circuit  voltage  of  a  considerable  amount  under  the  brushes 
which  may  be  sufficient  to  circulate  large  enough  local  currents 
to  start  flashing..  With  commutating  pole  machines,  this  con- 
dition may  result  from  the  use  of  solid  poles  and  solid  field 
yokes  Laminated  commutating  poles  are  sometimes  very  much 
of  ah  improvement.  However,  the  yokes  of  practically  all 
direct  current  machines  are  of  solid  material,  and  thus  tend  to 
give  sluggishness  in  flux  changes.  The  above  explains  why  non- 
inductive  shunts,  or  any  closed  circuits  whatever,  are  usually 
objectionable  on  commutating  poles  or  their  windings. 

Iri  non-commutating  pole  machines,  where  the  brushes  are 
liable  to  be  shifted  under  the  main  field  magnetic  fringe  in 
order  to  commutate  heavy  loads,  flashing  sometimes  results, 
when  such  heavy  overload  is  interrupted. 

Also,  if  the  rupture  of  the  current  is  very  sudden,  there  \ull 
be  an  inductive  "  kick  "  from  the  collapse  of  the  armature 
magnetic  field.  This  rise  in  voltage  sometimes  is  sufficient  to 
start  a  flash,  especially  in  those  cases  where  flashing  limits  are 
already  almost  reached. 

In  synchronous  converters,  the  conditions  are  materially 
different  from  d-c.  generators  as  regards  flashing  when  the 
load  is  suddenly  broken.  In  such  machines,  the  flash  is  liable 


PHYSICAL  LIMITATIONS  IN  D.C.  MACHINES  271 

to  follow  the  opening  of  the  breaker,  if  simply  a  heavy  over- 
load is  interrupted.  This  is  possibly  more  pronounced  in  the 
commutating  pole  machine  than  in  the  non-commutating  pole 
type  In  a  commutating  pole  converter,  the  commutating 
pole  magnetomotive  force  is  considerably  larger  than  the  re- 
sultant armature  magnetomotive  force,  under  normal  opera- 
ting  conditions,  but  is  much  smaller  than  the  armature  magneto- 
motive force  considered  as  a  straight  d-c.  or  a-c.  machine. 
Normally  the  commutating  pole  establishes  a  commutating 
field  or  flux  in  the  proper  direction  in  the  armature.  However, 
if,  for  any  reason,  the  converter  becomes  a  motor  or  a  generator, 
even  momentarily,  the  increased  magnetomotive  force  of  the 
armature  may  greatly  exceed  that  of  the  commutating  pole, 
so  that  the  commutating  pole  flux  will  be  greatly  increased,  or 
it  may  be  greatly  reduced,  or  even  reversed,  depending  upon 
which  armature  magnetomotive  force  predominates. 

The  above  is  what  happens  when  a  synchronous  converter 
hunts,  and  under  the  accompanying  condition  of  variable 
armature  magnetomotive  force,  the  commutating  pole  con- 
verter, with  iron  directly  over  the  commutating  zone,  is  liable 
to  show  greater  variations  in  the  flux  in  the  commutating  zone 
than  is  the  case  in  the  non-commutating  pole  converter.  Ex- 
perience has  shown  that  uhen  a  synchronous  converter  carry- 
ing a  heavy  overload  has  its  direct-current  circuit  suddenly 
interrupted,  it  is  liable  to  hunt  considerably  for  a  very  short 
.  period,  depending  upon  the  hunting  constants  of  the  individual 
machine  and  circuit.  Apparently,  all  converters  hunt  to  some 
extent  \\ith  such  change  in  load.  This  hunting  means  wide 
variations  in  the  commutating  pole  flux  with  corresponding 
sparking  tendencies.  For  a  "  swing "  or  two,  this  sparking 
may  be  so  bad  as  to  develop  into  a  flash.  Thus  the  flash  follows 
the  interruption  of  the  circuit. 

Curiously,  the  most  effective  remedy  for  this  condition  is 
one  ^hich  has  proved  most  objectionable  in  d-c.  machines, 
namely,  a  low-resistance  closed  electric  circuit  surrounding  the 
commutating  pole.  The  primary  object  of  this  remedy  is"  not 
to  form  a  closed  circuit  around  the  commutating  field,  but  to 
obtain  a  more  effective  damper  in  order  to  minimize  hunting. 
In  a  paper  presented  before  the  Institute  several  years  ago,* 
the  writer,  showed  that  the  ideal  type  of  cage  winding  for  damp* 

*Commtitating  Poles  in  Synchronous  Converters.  PSage  171. 


272  ELECTRICAL  ENGINEERING  PAPERS 

ing  synchronous  converters,  namely,  that  in  which  all  circuits- 
are  tied  together  by  common  end  rings,  was  not  suitable  for 
commutating  pole  converters  due  to  the  fact  that  the  various 
sections  of  this  cage  winding  form  low-resistance  closed  circuits 
around  the  commutating  poles.  This  was  in  accord  \\ith  all 
evidence  available  to  that  time,  and  no  one  took  exception  to 
it  However,  later  experience  has  shown  that  this  was  incor- 
rect, for,  in  later  practise,  it  was  found  that  the  use  of  a  complete 
cage  damper  of  low  resistance  which  decreases  the  hunting 
tendency,  also  greatly  decreases  the  flashing  tendency,  so  that 
today  most  converters  of  the  commutating  pole  type  are  being 
made  with  complete  cage  dampers.  Apparently,  the  flashing 
tendencies  in  converters  due  to  hunting  are  much  worse  than 
those  due  to  flux  sluggishness.  Therefore,  a  sacrifice  can  be 
made  in  one*  for  the  benefit  of  the  other. 

In  the  case  of  a  dead  short  circuit  on  the  d-c  side  of  a  syn- 
chronous converter,  there  is  liable  to  be  flashing,  just  as  in  the 
d-c  machine,  and  the  flash  and  the  breaker  opening  are  liable 
to  occur  so  closely  together  that  an  observer  cannot  say  which 
is  first. 

In  d-c.  railway  motors,  flashing  at  the  commutator  is  not 
an  uncommon  occurrence  One  rather  common  cause  of  flash- 
ing, especially  at  high  speed,  is  due  to  jolting  the  brushes  away 
from  the  commutator,  due  to  rough  track,  etc  This  is  espe- 
cially the  case  with  light  spring  tension  on  the  brushes.  The 
carbon  breaks  contact  with  the  copper,  forming  an  arc  which 
is  carried  around.  Another  prolific  source  of  flashing  is  due 
to  opening  and  closing  the  motor  circuit  in  passing  over  a  gap 
or  dead  section  in  a  trolley  circuit  Here  the  motor  current 
is  entirely  interrupted,  and,  after  a  short  interval,  it  comes  oa 
again,  without  any  resistance  in  circuit  except  that  of  the  motor 
itself.  If  the  current  rush  at  the  first  moment  of  closing  is 
not  too  large,  and  if  the  armature  and  field  magnetic  fluxes 
build  up  at  the  same  rate,  then  there  is  usually  but  small  danger 
of  a  flash,  except  under  very  abnormal  conditions.  The  rapidly 
changing  field  flux  however  generates  heavy  currents  under 
the  brushes,  thus  tending  toward  flashing.  The  reactance  of 
the  motor,  especially  of  the  field  windings,  limits  the  first  cur- 
rent rush  to  a  great  extent.  According  to  this,  closed  second- 
ary circuits  of  low  resistance  around  either  the  main  poles  or 
the  commutating  poles,  should  be  objectionable,  and  experience 
bears  this  out 


PHYSICAL  LIMITATIONS  IN  D.C.  MACHINES  273 

In  railway  armatures,  as  a  rule,  fewer  commutator  bars  per 
pole  are  used  on  the  average  than  in  stationary  machines  of 
corresponding  capacity,  except  possibly,  in  large  capacity 
motors.  This  is  due  largely  to  certain  design  limitations  in 
-such  apparatus,  but  this  has  doubtless  been  responsible  for  a 
certain  amount  of  flashing  in  such  apparatus 

Average  em.f.  and  "  Field  Form"  A  rather  common  prac- 
tise has  -been  to  specify  the  average  volts  per  bar  in  a  given 
machine.  This,  in  itself,  does  not  mean  anything,  except  in  a  very 
general  way;  for  the  lirr.it  is  really  fixed  by  the  maximum  volts 
per  bar,  as  already  shown,  and  there  is  no  fixed  relation  between 
the  average  and  the  maximum  volts  per  bar.  The  ratio  be- 
tween these  two  voltages  is  dependent  upon  the  field  flux  dis- 
tribution,— that  is,  the  "field  form."  In  practise,  this  ratio 
varies  over  a  ^ide  range,  depending  upon  the  preferences  of 
the  designer,  upon  limitations  of  pole  space  available,  etc. 
Also,  TTvith  load,  it  depends  upon  the  amount  of  flux  distortion 
of  the  field,  which,  in  turn  varies  greatly  in  practise.  In  well 
proportioned  modern  machines,  where  space  and  other  limita- 
tions permit,  the  average  e.m.f.  per  bar  is  about  70  per  cent 
of  the  maximum  at  no  load,  and  about  55  per  cent  to  60  per 
cent  with  heavy  load.  This  means  that  about  15  volts  per 
bar,  average,  is  the  maximum  permissible,  in  large  machines 
with  considerable  field  distortion,  if  a  maximum  of  28  volts 
per  bar  is  not  to  be  exceeded.  On  this  basis,  a  600-volt  machine 
should  therefore  have  not  less  than  40  commutator  bars  per 
pole.  However,  this  is  with  considerable  field  distortion.  If 
this  distortion  is  reduced  or  eliminated,  the  average  volts  can 
be  considerably  higher,  as  in  machines  with  high  saturation  in 
the  pole  faces,  pole  horns  and  armature  teeth,  or  with  com- 
pensated fields.  Synchronous  converters  are  practically  self- 
compensated  and  can  therefore  have  higher  limits  than  the 
above,  if  the  normal  rated  e.m.f.  is  never  to  be  exceeded.  How- 
ever, in  600-volt  converter  work,  in  particular,  wide  variations 
sometimes  momentarily  occur,  up  to  700  to  750  volts,  and  such 
machines  should  have  some  margin  for  such  voltage  swings. 
The  ordinary  600-volt  d-c.  generator  also  attains  materially 
Hgher  voltages  at  times,  which  would  be  taken  into  account 
in  the  limiting  voltage  per  commutator  bar  and  the  total  number 
of  commutator  bars  per  pole. 

Obviously,  the  "  fatter  *  the  field  form,  the  nearer  the  aver- 
age voltage  caa  apf>nwb  the  maximum.  With  an  80  per  cent 


274  ELECTRICAL  ENGINEERING  PAPERS 

field  form,  instead  of  70  per  cent,  for  instance,  the  number  of 
bars  per  pole  can  be  reduced  directly  as  the  polar  percentage 
is  increased;  and  35  bars  per  pole  with  80  per  cent  would  be  as 
good  as  40  bars  with  70  per  cent  assuming  the  same  percentage 
of  field  distortion  in  both  cases.  An  increase  in  the  polar  arc 
will  tend  toward  increased  distortion,  but  the  reduced  number 
of  turns  per  pole  should  practically  balance  this,  so  that,  other 
things  being  unchanged,  the  flux  distortion  should  have  prac- 
tically the  same  percentage  as  before. 

In  large  machines  of  very  high  speeds,  large  polar  percentages, 
— that  is,  large  "  field  form  constants,"  are  very  advantageous, 
but  are  not  always  obtainable,  due  to  the  space  required  for  the 
commutating  pole  winding.  In  compensated  field  machines, 
with  their  smaller  commutating  pole  windings,  the  conditions 
are  probably  best  for  high  field  form  constants,  and  high  aver- 
age volts  per  bar;  and  thus  this  type  often  lends  itself  very- 
well  to  those  classes  of  ma- 
chines where  the  minimum 
possible  number  of  commu- 
tator bars  is  necessary.  This 
is  the  case  with  'very  high 
speeds,  and  also  for  very  high 
voltage  machines. 

Usually  it  is  considered  that 
the   commutating   conditions  JG> 

of  a  machine  are  practically  the  same  with  the  same  current, 
whether  it  be  operated  as  a  generator  or  motor.  However, 
when  it  comes  to  flashing  conditions,  there  is  one  very  consider- 
able difference  between  the  two  operations.  In  the  d-c.  gen- 
erator, the  field  flux  distortion  by  the  armature  is  such  as  to 
crowd  the  highest  field  density,  and  thus  the  highest  volts 
per  bar,  away  from  the  forward  edge  of  the  brushes.  In  the 
motor,  the  opposite  is  the  case,  and  therefore  there  is  a  steeply 
rising  field,  and  a  corresponding  e.m.f.  distribution  in  front  of 
the  brushes.  As  the  flash  is  carried  in  the  direction  of  rotation 
it  may  be  seen  that,  in  this  particular,  the  generator  and  motor 
are  different. 

BLACKENING    AND    BURNING — HIGH    MICA — "  PICKING    UP  " 

COPPER 

In  the  preceding,  certain  limitations  of  commutation   and 
flashing  have  been  treated.     There  are,  in  addition,  a  number 


PHYSICAL  LIMITATIONS  IN  D.C   MACHINES  275 

of  other  conditions  which  are  related  closely  to  commutation, 
and  which  have  already  been  touched  upon  to  a  limited  extent. 
One  of  these  is  the  permissible  current  density  in  the  brushes 
or  brush  contacts. 

As  brought  out  before,  there  are  two  currents  to  be  con- 
sidered, namely,  the  work  current  which  flows  to  or  from  the 
outside  circuit,  and  the  local  or  short  circuit  current  which  is 
purely  local  to  the  short  circuited  coils  and  the  brush.  The 
true  current  density  is  that  due  to  the  actual  resultant  current 
in  the  brush  tip  or  face,  which  is  very  seldom  uniform  over  the 
whole  brush  tip.  The  "  apparent  "  current  density  is  that  due 
to  the  work  current  alone — assumed  to  be  uniform  over  the 
brush  tip.  The  current  density  very  commonly  has  been  as- 
sumed as  the  total  work  current,  in  and  out,  divided  by  the 
total  brush  section,  and,  moreover,  this  has  been  considered 
as  the  true  current  density,  the  local  or  short  circuit  currents 
being  neglected  altogether.  This  method  of  considering  the 
matter  has  been  very  misleading,  resulting  in  many  cases,  in 
a  wrong  or  unsuitable  size  of  brush  being  used  just  to  meet 
some  specified  current  density.  In  many  of  the  old,  non-com- 
mutating  pole  machines,  the  local  currents  were  predominant 
under  certain  conditions  of  load,  for  the  brushes,  as  a  rule,  had 
to  be  set  at  the  best  average  position,  so  that  at  some  average 
load,  the  commutating  conditions  would  be  best.  At  higher 
and  lower  loads,  the  short  circuit  currents  were  usually  com- 
paratively large.  The  wider  the  brush  contact  circumferen- 
tially,  the  greater  would  be  the  short  circuit  currents  and  the 
higher  the  actual  current  density  at  one  edge  of  the  brush, 
while  the  apparent  density  would  be  reduced.  Thus,  in  at- 
tempting to  meet  a  low  specified  current  density,  the  true  den- 
sity would  be  gregtly  increased.  The  fallacy  of  this  procedure 
was  shown  in  many  cases  in  which  the  brush  contact  was  very 
greatly  reduced  by  grinding  off  one  edge  of  the  brush.  Very 
often,  a  reduction  in  circumferential  width  of  contact  to  one- 
half  resulted  in  less  burning  of  the  brush  face.  The  apparent^ 
density  was  doubled  but  the  actual  maximum  density  was  . 
actually  reduced.  Many  of  these  instances  showed  very 
conclusively  that  much  higher  true  current  densities  were  prac- 
ticable, provided  the  true  and  apparent  densities  could  be 
brought  more  nearly  together.  This  is  what  has  been  accom- 
plished to  a  considerable  extent  in  the  modern  well  designed 
commutating  pole  machine*  In  such  machines,  the  current  dis- 


276  ELECTRICAL  ENGINEERING  PAPERS 

tribution  at  the  brush  face  is  nearly  uniform  under  all  condi- 
tions of  load      It  is  not  really  uniform,  even  in  the  best  machines ; 
but  the  variations  from  uniformity,  \\hile  possibly  as  much  as. 
50  per  cent  in  good  machines,  is  yet  very  small  compared  \vith. 
the  variation  in  some  of  the  old  non-commutating  pole  machines. 
In  consequence,  it  has  been  possible  to  increase  the  apparent 
current  densities  in  the  brushes  in  modern  commutating  pole 
machines  very  considerably  above  former  practise,  while  still 
retaining  comparatively  wide  brush  faces.     In  fact,  the  width 
of  the  brush  contact  circumferentially  is  not  particularly  limited 
if  the  commutating  field  flux  can  be  suitably  proportioned; 
that  is,  where  a  suitable  width  and  shape  of  commutating  field 
can  be  obtained.     In  many  of  the  old  time  machines,  an  ap- 
parent density  of  40  amperes  per  square  inch  under  normal 
loads  was  considered  as  amply  high,  while  at  the  present  timeT 
with  well  proportioned  commutating  poles,  50  per  cent  higher 
apparent  densities  are   not  uncommon      However,  experience 
shows  that  the  same  brushes,  with  perfectly  uniform  distribu- 
tion of  current  at  the  brush  face,  can  carry  still  higher  currents. 
Therefore,  in  modern  commutating  pole  machines,  the  actual 
upper  limit  of  brush  capacity  is  not  yet  attained.     But  there 
are  reasons  \\hy  this  upper  lirr  it  is  not  practicable.     One  reason 
is  that  already  given,  that  uniform  current  distribution  over 
the  brush  face  is  seldom  found.     This  rreans  that  a  certain 
margin  must  be  allowed  for  variations.    A  second  reason  lies- 
in  the  unequal  division  of  current  between  the  various  brushes- 
and  brush  arms.     This  may  be  due  initially  to  a  number  of 
different  causes.     However,  when  a  difference  in  current  once 
occurs,  it  tends  to  accentuate  itself,  due  to  the  negative  co- 
efficient of  resistance  of  the  carbon  brushes  and  brush  contacts. 
If  one  of  the  brushes,  for  instance,  takes  more  than  its  share  of 
current  for  a  period  long  enough  to  heat  the  brush  more  than, 
the  others,  then  "its  resistance  is  lowered  and  it  tends  to  take 
still  more  current.     If  there  were  no  other  resistance  in  the 
current  path,  it  is  presumable  that  the  parallel  operation  off 
xarbon  brushes  would  be  more  or  less  unsatisfactory.     In  the 
practical   case,  however,    instead   of   the   operation  being  im- 
practicable^ it  is  merely  somewhat  unstable.    Unequal  division 
of  current  between  the  brushes  on  the  same  brush  arms,  is  to 
some  extent,  dependent  upon  the  total  current  per  arm.     Where 
there  are  many  brushes  in  parallel  and  the  total  current  to 
be  carried  is  very  large,  it  is  obvious  that  one  brush  may  take 


PHYSICAL  LIMITATIONS  IN  D.C   MACHINES  277 

an  excessively  large  current  without  materially  decreasing  the 
current  carried  by  the  other  brushes.  As  a  rule,  the  larger 
the  current  per  arm,  the  more  difficult  is  the  problem  of  prop- 
erly balancing  or  distributing  the  current  among  all  the  brushes. 
Schemes  have  been  proposed,  and  patented,  for  forcing  equal 
division,  but,  as  a  rule,  they  have  not  proved  very  practicable, 
although  some  comparatively  simple  expedients  have  been 
tried  out  ^ith  a  certain  degree  of  success. 

In  the  same  way,  the  division  of  current  among  brush  arms 
of  the  same  polarity  is  not  always  satisfactory.  50  per  cent 
variation  of  current  between  different  arms  is  not  very  unusual, 
and  the  writer  has  seen  a  number  of  instances  where  the  varia- 
tion has  been  100  per  cent,  or  even  much  more.  Obviously, 
with  such  variation,  it  is  not  practicable  to  work  the  brushes  up 
to  the  maximum  density  possible,  for  some  margin  must  be 
allowed  for  such  unbalancing. 

Experience  has  shown  that  when  current  passes  through 
a  moving  contact,  as  from  a  brush  to  the  commutator  copper, 
or  vice  versa,  a  certain  action  take  place  which  resembles  elec- 
trolytic action  to  some  extent,  although  it  is  not  really  electro- 
lytic. It  might  also,  be  said  to  resemble  some  of  the  actions 
which  takes  place  in  an  arc.  Minute  particles  appear  to  be 
«aten  or  burned  away  from  one  contact  surface,  and  these  are 
sometimes  deposited  mechanically  upon  the  opposing  surface. 
The  particles  appear  to  be  carried  in  the  direction  of  current 
flow,  so  that  if  the  current  is  from  the  carbon  brush  to  the 
copper,  the  commutator  face  will  tend  to  darken  somewhat, 
•evidently  from  depositation  of  carbon.  If  the  current  is  from 
the  copper  to  the  carbon,  the  brush  face  will  sometimes  tend 
to  take  a  coating  of  copper,  while  the  commutator  face  will 
take  a  clean,  and  sometimes  raw,  copper  appearance.  As  the 
current  is  in  both  directions  on  the  ordinary  commutator,  this 
action  is  more  or  less  averaged,  and  therefore  is  not  usually 
noticed.  With  one  polarity  or  direction  of  current,  the  com- 
mutator face  eats  away,  while  with  the  other  direction,  the 
brush  face  is  eaten  away  and  may  lose  its  gloss. 

The  above  action  of  the  current  gives  rise  to  a  number  of 
limiting  conditions  in  direct-current  practise.  Experience  shows 
that  this  "  eating  away  "  action  occurs  with  all  kinds  of  brushes, 
and  with  various  materials  in  the  commutator.  It  appears  to 
be  dependent ,  to  a  considerable  extent,  upon  the  losses  at  the 
contact  surface.  In  other  words,  it  is  dependent  upon  both  the 


278  ELECTRICAL  ENGINEERING  PAPERS 

current  and  the  contact  drop.  With  reduction  in  contact  drop, 
this  burning  action  apparently  is  decreased,  but  in  commutating 
machinery,  this  reduction  cannot  be  carried  very  far,  in  most 
cases,  on  account  of  increase  in  short  circuit  current,  which 
nullifies  the  gain  in  contact  drop.  In  fact,  in  each  individual 
machine,  there  is  some  critical  resistance  which  gives  least  loss 
and  least  burning  at  the  contact  surfaces. 

Practise  has  shown  that  this  burning  action  is  very  slow  at 
moderate  current  densities  in  carbon  and  graphite  brushes — 
so  slow  as  usually'  to  Be'  unnoted.  However,  if  the  actual 
current  density  in  the  brush  face  is  carried  too  high,  the  burn- 
ing of  the  brushes  may  become  very  pronounced.  With  the 
actual  work  current  per  brush  usual  in  present  practise,  the 
burning  of  the  brush  face  may  usually  be  credited  to  local  cur- 
rents in  the  brushes  This  is  one  pretty  good  indication  of 
the  presence  of  excessive  local  currents.  It  also  indicates  the 
location  and  direction  of  such  currents,  but  is  not  a  very  exact 
quantitative  measure  of  them.  It  is  not  unconr.iron,  in  exam- 
ining the  brushes  of  a  generator  or  motor,  to  find  a  dull  black 
area  under  one  edge  of  the  brush,  which  obviously  has  been 
burned,  while  the  remainder  of  the  brush  face  is  brightly  polished. 
In  severe  cases,  practically  as  good  results  t\ill  be  obtained 
if  the  burned  area  is  entirely  cut  away  by  beveling  the  edge 
of  the  brush. 

This  eating  away  of  either  the  brush  face  or  the  commutator, 
and  the  deposit  upon  the  opposing  face,  leads  to  certain  very 
harmful  conditions  in  direct-current  machinery.  As  stated 
before,  if  the  true  current  density  is  kept  sufficiently  low  in 
the  contact  face,  the  burning  is  negligibly  small  m  most  cases. 
However,  where  the  current  passes  from  the  commutator  to 
the  brush,  it  is  the  commutator  copper  which  eats  away,  while 
the  mica  between  commutator  bars  doe?  not  eat  away,  but  must 
be  worn  away  at  the  same  rate  that  the  copper  is  burnt,  if  good 
contact  is  to  be  maintained.  Let  the  burning  of  the  copper 
gain  ever  so  little  on  the  wear  of  the  mica,  then  trouble  begins. 
The  brush  begins  to  "  ride  "  on  the  mica  edges  and  docs  not 
make  true  contact  with  the  copper.  This  increases  the  burn- 
ing action  very  rapidly,  so  that  eventually  the  mica  stands 
well  above  the  copper  face.  This  is  the  trouble  usually  known 
as  "  high  mica/'  It  is  frequently  credited  to  unequal  rates 
of  wear  of  copper  and  mica.  This  idea  of  unequal  wear  has 
been  partly  fostered  by  the  fact  that  with  relatively  thick 


PHYSICAL  LIMITATIONS  IN  D.C.  MACHINES  279 

mica,  the  action  is  greatly  increased,  or,  with  very  thin  com- 
mutator bars,  with  the  usual  thickness  of  the  mica,  the  high  mica 
trouble  becomes  more  serious.  In  both  these  latter  cases,  it 
is  the  higher  percentage  of  mica, — that  is,  the  relatively  poorer 
wearing  characteristics  of  the  mica  itself,  which  is  at  fault. 
But  the  commutator  copper  does  not  wear  away.  In  fact,  it 
is  not  physically  possible  for  it  to  wear  below  the  mica.  It  is 
"  eaten  away  "  or  burned,  as  described  above.  In  some  special 
cases,  where  this  burning  is  unusually  severe,  the  mica  apparently 
wears  down  about  as  fast  as  the  copper,  so  that  the  commutator 
remains  fairly  clean  and  has  no  particularly  burnt  appearance, 
but  grooves  or  ridges,  showing  undue  wear.  But  this  rapid 
apparent  wear  is  a  pretty  good  indication  that  excessive  burn- 
ing action  is  present  at  times,  usually  due  to  excessive  local 
currents.  In  some  cases,  this  burning  action  may  be  present 
only  during  heavy  or  peak  loads  which  may  be  so  interspersed 
with  periods  of  light  running  that  the  true  wear  of  the  mica 
catches  up  with  the  burning  of  the  copper.  In  such  cases,  the 
commutator  may  have  a  beautiful  glossy  appearance  normally, 
but  may  wear  in  grooves  and  ridges  On  account  of  this  burn- 
ing action,  practise  has  changed  somewhat  in  regard  to  stagger- 
ing of  brushes  on  commutators  to  prevent  ridging  between  the 
brushes.  Formerly,  it  was  common  practise  to  displace  all 
the  positive  brushes  one  direction  axially,  and  the  negatives 
in  the  other  direction,  in  order  to  have  the  brushes  overlap. 
This,  however,  did  not  entirely  prevent  ridging,  for  the  burning 
of  the  copper  occurred  only  under  one  polarity.  It  is  now  con- 
sidered better  practise  to  stagger  the  arms  in  pairs. 

With  commutating  pole  machines,  the  true  current  densities 
in  the  brushes  are  carried  up  to.  as  high  a  point  as  the  non- 
burning  requirements  wall  permit.  Reduction  in  local  currents 
has  been  accompanied  by  increase  in  the  work  current  density. 
Therefore,  conditions  for  burning  and  high  mica  are  still  exist- 
ent, as  in  non-commutating  pole  machines.  In  recent  years, 
a  new  practise,  or  rather  an  extension  of  an  old  practise,  has 
been  very  generally  adopted,  namely,  undercutting  the  mica 
between  bars.  In  early  times,  such  undercutting  was  practised 
to  a  certain  extent,  usually  however,  to  overcome  mica  troubles 
principally.  In  the  newer  practise,  such  undercutting  is  pri- 
marily for  other  reasons,  although  the  mica  problem  is  partly 
concerned  in  it.  During  the  last  few  years,  extended  experi 
ence  ha$  shown  that  graphite  brushes,  or  carbon  brushes  with 


280  ELECTRICAL  ENGINEERING  PAPERS 

considerable  graphite  m  them,  are  extremely  good  for  collect- 
ing current,  but  on  the  other  hand,  are  very  poor  when  it  comes 
to  wearing  down  the  mica»  due  to  their  softness  or  lack  of  ab- 
rasive qualities  Due  to  the  graphite  constituent,  such  brushes 
are  largely  self -lubricating,  and  therefore,  "ride"  more  smoothly 
on  the  commutator  than  the  ordinary  carbon  brush.  They  are 
therefore  much  quieter,  and  this  is  a  very  important  point  \vith 
the  present  high  speeds  which  are  becoming  very  much  the 
practise.  However,  by  undercutting  the  mica,  all  difficulty 
from  lack  of  abrasive  qualities  in  the  brush  is  overcome,  and 
thus  the  good  qualities  of  such  brushes  could  be  utilized.  The 
advantage  of  self-lubricating  brushes  should  be  apparent  to 
anyone  who  has  had  difficulties  from  chattering  and  vibration 
of  brushes,  due  to  lack  of  lubrication.  Such  chattering  may 
put  a  commutator  "  to  the  bad  "  in  a  short  time,  and  the  con- 
ditions become  cumulatively  worse*  Chattering  means  bad 
contact  between  the  brush  and  commutator,  which  in  turn, 
means  sparking  and  burning,  which  means  increased  chatter- 
ing or  vibration. 

The  above  refers  to  burning  of  the  commutator  face.  But 
such  burning  also  may  have  a  bad  effect  on  the  brushes.  When 
the  commutator  copper  burns  away  to  any  extent,  it  may  de- 
posit on  the  brush  face  following  the  direction  of  the  current. 
This  coating  on  the  brush  face  sometimes  leads  to  serious 
trouble,  by  lowenng  the  resistance  of  the  contact  surf  ace.  This 
not  only  allows  larger  short  circuit  current  and  greater  heating 
of  the  brush,  but  it  makes  the  resistance  of  that  particular 
path  lower  than  that  of  other  parallel  brush  paths.  In  con- 
sequence, the  coated  brush  takes  an  undue  share  of  the  total 
current,  as  well  as  an  unduly  large  local  current.  The  result- 
ant heating  may  be  such  that  the  brush  actually  becomes  red 
hot  or  glows.  This  heating  further  reduces  the  resistance, 
and  tends  to  maintain  the  high  temperatures.  This  glowing 
or  overheating  very  frequently  causes  disintegration  of  the  bind- 
ing or  other  material  in  the  brush,  so  that  it  gradually  honey- 
combs at  or  near  its  tip.  This  action  may  keep  up  until  the 
brush  makes  bad  contact.  It  may  be  that  a  similar  action  may 
occur  coincidently  on  other  brushes,  but,  there  is  no  uniformity 
about  it.  This  action  of  transferring  copper  to  the  brush  is 
sometimes  known  as  "  picking  up  copper/'  It  is  not  limited 
to  brushes  of  one  polarity,  except  where  the  metallic  coating 
is  caused  primarily  by  the  work  current.  Where  it  results  from 


PHYSICAL  LIMITATIONS  IN  D.C.  MACHINES  281 

high  local  currents,  it  may  be  on  the  brushes  of  either  polarity, 
for  the  local  currents  go  in  and  out  at  each  brush.  However, 
according  to  the  \\nterTs  experience,  this  coating  is  more  com- 
mon on  the  one  polarity 

Glowing  and  honeycombing  of  brushes  is  not  necessarily 
dependent  upon  the  metallic  coating  on  the  brushes,  although 
this  latter  increases  the  action  Anything  that  will  unduly  in- 
crease the  amount  of  current  m  any  brush  contact  for  a  period 
long  enough  to  result  in  heating  and  lower  contact  resistance, 
with  brushes  in  parallel,  may  start  this  gloxving  and  honey- 
combing. It  is  not  as  common  an  action  in  modern  machines 
as  in  old  time  ones. 

As  an  evidence  that  poor  contact  or  high  contact  drop  tends 
to  produce  burning,  may  be  cited  the  fact  that,  in  many  cases 
of  apparent  rapid  wear  of  the  commutators,  such  wear  has 
been  practically  overcome  by  simply  undercutting  the  mica 
and  thus  allowing  more  intimate  contact  between  brush  and 
copper.  In  some  instances,  this  also  lessened  or  eliminated 
the  tendency  to  pick  up  copper.  Thus  undercutting  has  been 
very  beneficial  in  quite  a  number  of  ways. 

NUMBER  OF  SLOTS,  CONDUCTORS  PER  SLOT,  ETC. 
There  are  certain  limitations  in  direct-current  machines,  de- 
pending upon  the  minimum  number  of  slots  per  pole  which  can 
be  used.  Provided  satisfactory  commutating  conditions  can 
be  obtained,  it  is  in  the  direction  of  economy  of  design  to  use 
a  relatively  low  number  of  slots  per  pole,  with  a  correspond- 
ingly large  number  of  coils  per  slot.  This  is  effective  in  several 
ways.  In  the  first  place,  insulating  space  is  saved,  thus  allow- 
ing an  increase  in  copper  or  iron  sections,  either  of  which  al- 
lows greater  output.  In  the  second  place,  \vider  slots  arc  favor- 
able to  commutation.  Thus  the  natural  tendency  of  d-c.  de- 
sign is  toward  a  minimum  number  of  slots  per  pole.  But  if 
this  is  carried  too  far,  certain  objections  or  disadvantages  arise 
or  become  more  prominent,  so  that  at  some  point  they  over- 
balance the  advantageous  features.  As  the  slots  are  widened 
and  the  number  of  teeth  diminished,  variations  in  the  reluct- 
ance of  the  air  gap  under  the  main  poles,  with  corresponding 
pulsations  in  the  main  field  flux  become  more  and  more  pro- 
nounced. These  may  effect  commutation,  as  the  short  cir- 
cuited armature  coils  form  secondary  circuits  in  the  path  of 
these  pulsations  But  before  this  condition  becomes  objec- 


282  ELECTRICAL  ENGINEERING  PAPERS 

tionable,  other  troubles  are  liable  to  become  prominent,  such 
as  "  magnetic  noises,1'  etc.  If  the  machine  is  of  the  commutat- 
ing  pole  type,  there  are  liable  to  be  variations  in  the  commutat- 
ing  pole  air  gap  reluctance,  so  that  it  may  be  difficult  to  obtain 
proper  conditions  for  commutation.  A  relatively  wide  corn- 
mutating  zone  is  required  if  there  are  many  coils  per  slot;  also, 
all  the  conductors  per  slot  usually  will  not  commutate  under 
equal  conditions,  which  may  result  in  blackening  or  spotting 
|  of  individual  commutator  bars  symmetrically  spaced  around 
the  commutator,  corresponding  to  the  number  of  slots.  Innon- 
commutating  pole  machines,  it  may  be  difficult  to  find  a  suit- 
able field  or  magnetic  fringe  in  which  to  commutate,  and  thus 
the  first  and  last  coil  in  each  slot  will  have  quite  different  fluxes 
in  which  to  commutate. 

Depending  upon  the  relative  weight  of  the  various  advant- 
ages and  disadvantages  of  a  small  number  of  slots  per  pole, 
practise  varies  greatly  in  different  apparatus.     In  small  and 
medium  capacity  railway  motors,  where  maximum  output  in' 
minimum  space  is  of  first  importance,  and  where  noise,  vibra- 
tions, etc.  are  not  very  objectionable,  the  number  of  slots  per 
pole  used  is  probably  lower  than  in  any  other  line  of  d-c.  ma- 
chines, six  to  eight  per  pole  being  rather  common.     In  the 
smaller  and  medium  size  stationary  motors,  where  noise  must 
be  avoided,  a  somewhat  larger  number  of  slots  is  used  in  gen- 
eral, depending  somewhat  upon  the  size  of  the  machine.     On 
still  larger  apparatus,  excepting  possibly,  small  low-speed  en- 
gine type  generators,  10  slots  or  more  per  pole  are  used  in 
most  cases,  and,  in  general,  more  than  12  are  preferred.     In 
the  large  600-volt  machines,  the  number  is  fixed  partly  by  the 
minimum  number  of  commutator  bars  per  pole,  and  the  num- 
ber of  coils  per  slot.     Assuming  three  coils  per  slot,  then  with 
a  minimum  number  of  commutator  bars  of  about  40  per  pole, 
the  minimum  number  of  slots  per  pole  will  be  14,  and  with 
two  bars  per  slot,  will  be  correspondingly  larger.     This  there- 
fore represents  one  of  the  limits  in  present  practise. 

Noise,  Vibration,  etc.  Mention  has  been  made  of  limita- 
tions of  noise  and  vibration  being  reached,  in  considering  the 
minimum  number  of  slots.  This  is  a  very  positive  limitation 
in  design,  especially  so  in  recent  years,  when  everything  is  being 
carried  as  close  as  possible  to  all  limits  in  economies  in  materials 
and  constructions.  All  the  various  conditions  which  cause 
unkltte  noises  in  electrical  apparatus  are  not  yet  well  known, 


PHYSICAL  LIMITATIONS  IN  D  C   MACHINES  283 

and  the  application  of  remedies  is  more  or  less  a  question  of 
"  cut-and-try." 

A  fundamental  cause  of  noise  in  direct-current  machines  lies 
in  very  rapid  pulsations  or  fluctuations  in  magnetic  conditions. 
This  has  been  well  known  for  years,  and  many  solutions  of  the 
problem  of  preventing  such  variations  in  magnetic  conditions 
from  setting  up  vibrations  and  consequent  noise,  have  been 
proposed,  but  many  of  them  appear  to  hold  only  for  the  particu- 
lar machine,  or  line  of  machines,  for  which  they  were  devised. 
A  perfectly  good  remedy  in  one  machine  not  infrequently 
proves  an  utter  failure  on  the  next  one.  There  are  certain 
remedies  for  noise  in  direct-current  machines  which  apply  pretty 
generally  to  all  machines,  but,  as  a  rule,  such  remedies  mean  more 
expensive  constructions  In  general,  large  air  gaps  and  gradual 
tapering  of  the  flux  at  the  pole  edges  tend  toward  quiet  opera- 
tion. A  large  number  of  slots  per  pole  tends  toward  quietness. 
However,  the  trend  of  design  has  been  toward  very  small  air 
gaps,  especially  in  recent  designs  of  small  and  moderate  size 
d-c  motors,  also,  the  aim  has  been  to  use  as  few  armature  slots 
as  possible  Moreover,  newer  designs  with  steel  or  wrought 
iron  frames,  as  a  rule,  have  the  magnetic  material  in  the  frames 
reduced  to  the  lowest  limit  that  magnetic  conditions  will  per- 
mit. Also,  with  the  general  use  of  commutating  poles,  the 
tendency  has  been  toward  "  strong  "  armatures  and  corres- 
pondingly weak  fields,  so  that  the  total  field  fluxes  and  field 
frames  are  relatively  small  compared  with  the  practise  of  a 
few  years  ago.  With  these  small  frames,  resonant  conditions 
not  infrequently  are  encountered,  especially  in  those  machines 
which  are  designed  to  operate  over  a  very  wide  range  in  speed. 
There  is  liable  to  be  some  point  in  the  speed  range  where  the 
poles  or  frame,  or  some  other  part,  is  properly  tuned  to  some 
pulsating  torque  or  "  magnetic  pull  "  in  the  machine.  In  such 
case,  a  very  slight  disturbance  of  a  periodic  nature  may  act 
cumulatively  to  give  a  very  considerable  vibration  and  conse- 
quent noise. 

The  pulsations  in  magnetic  conditions  which  produce  vibra- 
tion may  be  due  to  various  causes,  but,  as  a  rule,  the  slotted 
armature  construction  is  at  the  bottom  of  all  of  them.  Open 
type  armature  slots  usually  arc  much  worse  than  partially 
closed  slots.  Such  open  slots  produce  **  tufting  "  or  "  bunch- 
ing "  of  the  magnetic  flux  l>etween  the  field  and  armature,  and 
it  is  this  bunching  of  flttx  which  usually,  in  one  form  or  another. 


284  ELECTRICAL  ENGINEERING  PAPERS 

produces  a  -magnetic  pulsation  or  pull  which  sets  up  vibration. 
This  bunching  of  lines  may  be  such  as  to  set  up  pulsating  mag- 
netic pulls  at  no-load  as  well  as  full  load  In  other  cases,  the 
ampere  turns  in  the  armature  slots  tend  to  exaggerate  or  accen- 
tuate the  bunching  so  that  the  vibration  varies  with  the  load. 
This  bunching  of  the  flux  may  act  in  various  ways.  The  total 
air  gap  reluctance  betvi  een  the  armature  and  tnain  poles  may 
vary  or  pulsate,  so  that  the  radial  magnetic  pull  between  any 
main  pole  and  the  armature  will  pulsate  in  value.  If  the  re- 
luctances under  all  the  poles  are  varying  alike,  then  these 
pulsating  radial  pulls  will  tend  to  balance  each  other  at  all 
instants.  However,  if  the  reluctances  under  the  different  poles 
do  not  vary  simultaneously,  then  there  are  liable  to  be  un- 
balanced *  radial  magnetic  pulls  of  high  frequency,  depending 
upon  the  number  of  armature  teeth,  speed  of  rotation,  etc. 
If  this  frequency  is  so  nearly  in  tune  with  the  natural  period 
of  vibration  of  some  part  of  the  machine,  such  as  the  yoke, 
poles  or  pole  horns,  armature  core  and  shaft,  that  a  resonant 
condition  is  approximated,  then  vibration  and  noise  are  almost 
sure  to  occur. 

Radial  unbalanced  pulls,  as  described,  are  liable  to  occur  when 
the  number  of  armature  teeth  is  other  than  a  multiple  of  the  num- 
ber of  poles;  and  the  smaller  the  number  of  teeth  per  pole,  the 
larger  will  be  the  unbalancing  in  general  As  a  remedy,  it 
might  be  suggested  that  the  number  of  armature  slots  always 
be  made  a  multiple  of  the  number  of  poles  However,  there  are 
several  objections  to  this  One  serious  objection  is  that,  on 
small  and  moderate  size  d-c  machines,  the  two-circuit  type  of 
armature  winding  is  very  generally  used,  and,  with  this  type  of 
v(  hiding,  the  number  of  armature  coils  and  commutator  bars  must 
always  be  one  more  or  less  in  number  than  some  multiple  of  the 
number  of  pairs  of  poles.  Mathematically  therefore,  with  a  two- 
circuit  winding,  the  number  of  slots  can  never  be  a  multiple  of  the 
number  of  poles  unless  an  unsymmetrioal  winding  is  used, 
that  is,  one  vuth  a  "  dummy  "  coil  A  second  objection  to  using 
a  number  of  slots  which  is  a  multiple  of  the  number  of  poles, 
is  that  there  are  pulsating  magnetic  pulls  "which  may  be  exag- 
gerated by  this  very  construction  There  are  two  kinds  of  mag- 
netic pulls,  a  radial,  which  has  already  been  considered,  and  a 
circumferential,  due  to  the  tendency  of  the  armature  core  to 
set  itself  where  it  will  enclose  the  maximum  amount  of  field 
flux.  Obviously,  if  the  arrangement  of  slots  is  such  that  when 


PHYSICAL  LIMITATIONS  IN  D  C    MACHINES  285 

one  pole  has  a  maximum  flux  into  the  teeth,  another  pole  has 
a  minimum,  then  the  circumferential  puslations  in  torque 
will  be  less  than  if  all  poles  enclosed  the  maximum  or  the  mini- 
mum flux  simultaneously.  This  latter  condition  will  be  produced 
when  the  number  of  armature  slots  is  a  multiple  of  the  number 
of  poles.  Therefore,  in  dodging  unbalanced  radial  magnetic 
pulls  by  using  a  number  of  armature  slots  which  is  a  multiple 
of  the  number  of  poles,  the  designer  is  liable  to  exaggerate  the 
circumferential  variations  in  torque  or  pull,  so  that  he  is  no  better 
off  than  before.  This  circumferential  pulsating  magnetic  pull 
may  act  in  various  ways  to  set  up  vibration,  and  if  there  is  any 
resonant  condition  in  the  machine,  vibration  and  noise  will 
result. 

Several  years  ago,  the  writer  made  some  very  interesting  tests 
on  a  number  of  d-c.  machines  to  discover  the  nature  of  the  vi- 
brations which  were  producing  noise. 
These  machines  had  very  light  frames 
and  were  noisy,  although  not  exces- 
sively so.  The  following  results  were 
noted :  In  certain  four-pole  machines, 
it  was  noted  that  the  frames  vibrated 
in  a  radial  direction,  as  could  be 
easily  determined  by  feeling.  How- 
ever, upon  tracing  around  the  frame 
circumferentially,  nodal  points  were 
pIG  4  noted.  In  some  cases,  there  were 

points   of   practically    no   vibration 

midway  between  the  poles,  as  at  A  in  Fig.  4.  In  other  cases 
the  point  of  least  vibration  was  at  Bt  directly  over  the  main 
poles.  Apparently,  minimum  vibration  at  A  and  maximum  at 
B  occurred  when  the  pulsating  magnetic  pulls  were  in  a  radial 
direction,  while,  with  circumferential  pulls,  the  maximum  vibra- 
tion was  at  A .  It  was  also  noted  in  some  instances  that  a  varia- 
tion in  the  width  of  the  contact  face  of  the  pole  against  the  yoke 
produced  vibrations  and  noise,  and  nodal  points  in  the  yoke, 
the  vibrations  being  a  iBaximum  at  A . 

In  still  other  cases  in  commutating  pole  machines,  vibrations 
and  noise  were  apparently  set  up  by  either  radial  or  circumferen- 
tial magnetic  pulsations  under  the  cominutating  poles  themselves, 
as  indicated  by  'tfoe-f^cfc  tbat  mnoval  of  the  comirrutating  poles, 
or  a  cdnsideraMe  mcr-ease  m  their  air  gaps,  tended  to  overcome 
the  noise.  la  s&cfo  «ases,  tlie  noise  usually  increased  with  the 
load,  m  ooiastant  speed  imchines. 


286  ELECTRICAL  ENGINEERING  PAPERS 

Skewing  of  the  armature  slots,  or  of  the  pole  faces,  has  proven 
quite  effective  in  some  cases  of  vibration  and  noise.  Tapered 
air  gaps  at  the  pole  edges  have  also  proven  effective  in  many 
individual  cases  However,  the  causes  of  the  trouble  and  the 
remedies  to  be  applied  in  specific  cases  are  so  numerous  and  so 
varied  that  at  present  it  is  useless  to  attempt  to  give  any  limita- 
tions in  design  as  fixed  by  noise  and  vibration  due  to  magnetic 
conditions. 

"  FLICKERING"  OF  VOLTAGE,  AND  "  WINKING  "  OF  LIGHTS 
From  time  to  time,  cases  have  come  up  where  noticeable 
11  winking  "  of  incandescent  lights  occur,  this  being  either  of  a 
periodic  or  non-periodic  character,  the  two  actions  being  due  to 
quite  different  causes  In  either  case,  the  primary  cause  of  the 
difficulty  may  be  in  the  generator  itself,  or  it  may  be  'in  the 
prime  mover.  The  characteristics  of  the  incandescent  lamp 
itself  tends,  in  some  cases,  to  exaggerate  this  winking  To  be 
observable  when  periodic,  the  period  must  be  rather  long,  cor- 
responding to  a  very  low  frequency  Periodic  flickering  of 
voltage  may  be  considered  as  equivalent  to  a  constant  d-c 
voltage  with  a  low-frequency  small-amplitude  alternating  e  m  f . 
superimposed  upon  it  In  view  of  the  fact  that  incandescent 
lamps  of  practically  all  kinds  give  satisfactory  service  without 
flicker  at  40  cycles  uith  the  impressed  e  m  f  varying  from  zero 
to  40  per  cent  above  the  effective  value,  one  would  think  that  a 
relatively  small  variation  of  voltage,  of  3  per  cent  or  4  per  cent 
for  instance,  would  not  be  noticeable  at  frequencies  of  5  to 
10  cycles  per  second.  However,  careful  tests  have  shown 
that  commercial  incandescent  lamps  do  show  pronounced 
flicker  at  much  lower  percentage  variations  in  voltage,  de- 
pending upon  the  thermal  capacity  of  the  lamp  filament.  Based 
on  such  thermal  capacity,  low  candle  power  110-volt  lamps,  for 
example,  should  show  more  flicker  than  high  candle  power  lamps. 
Also,  tungsten  lamps  for  same  candle  power  should  be  more  sen- 
sitive than  carbon  lamps,  due  to  their  less  massive  films.  In 
fact,  trouble  from  winking  of  lights  has  become  much  more  pro- 
nounced since  the  general  introduction  of  the  lower-candle  power, 
higher-efficiency  incandescent  lamps. 

In  view  of  the  fact  that  winking  has  been  encountered  with 
machines  in  which  no  pronounced  pulsations  in  voltage  appear 
to  be  possible,  a  series  of  tests  was  made  some  years  ago  to 
determine  what  periodic  variation  was  noticeable  on  ordinary 


PHYSICAL  LIMITATIONS  IN  D.C.  MACHINES  287 

low-candle  power  Tungsten  and  carbon  lamps  A  lamp  circuit 
\vas  connected  across  a  source  of  constant  direct  e  m  f  ,  and  in 
series  with  this  circuit  was  placed  a  small  resistance  which  could 
be  varied  at  different  rates  and  over  varying  range  The 
results  were  rather  surprising  in  the  very  low  pulsations  in  volt- 
age which  showeda  flickering  of  the  light  when  reflected  from  a 
white  surface  With  the  ordinary  frequencies  corresponding  to 
small  engine  type  generators — that  is,  from  5  to  10  cycles — peri- 
odic variations  in  voltage  of  \  per  cent  above  or  below  the  mean 
value  were  sufficient  to  produce  a  visible  wink,  with  16-candle 
power  carbon  lamps;  while  1  per  cent  variation  above  and  below 
was  quite  pronounced  With  corresponding  tungsten  lamps, 
only  about  half  this  variation  is  sufficient  to  produce  a  similar 
wink.  These  tests  were  continued  sufficiently  to  show  that  such 
periodic  fluctuations  in  voltage  must  be  limited  to  extremely 
small  and  unsuspected  limits  This  condition  therefore  imposes 
upon  the  designer  of  such  apparatus  a  degree  of  refinement  in 
his  designs  which  is  almost  a  limitation  in  some  cases. 

It  is  probable  that  non-periodic  fluctuations  in  voltage  do 
not  have  as  pronounced  an  effect  in  regard  to  winking  of  lights 
as  is  the  case  with  periodic  fluctuations,  if  they  do  not  follow 
each  other  at  too  frequent  intervals,  unless  each  individual 
pulsation  is  of  greater  amplitude,  or  is  of  longer  duration 
Possibly  a  momentary  variation  in  voltage  of  several  per  cent 
will  not  be  noted,  except  by  the  trained  observer,  unless  such 
variation  .has  an  appreciable  duration 

A  brief  discussion  of  the  two  classes  of  voltage  variations 
may  be  of  interest,  and  is  given  below. 

Periodic  Fluctuations.  As  stated  before,  these  may  be  due 
to  conditions  inside  the  machine  itself,  or  may  be  caused  by 
speed  conditions  in  the  prime  mover.  Not  infrequently,  the 
two  act  together.  Variations  in  prime  mover  speed  can  act  in 
two  ways;  first,  by  varying  the  voltage  directly  in  proportion 
to  the  speed,  and  secpnd,  by  varying  the  voltage  indirectly 
through  the  excitation,  the  action  being  more  or  less  cumulative 
in  some  cases.  Such  sp^ed  variations  usually  set  up  pulsations 
corresponding  directly  to  tthe  revolutions  per  jraiimte  and  in- 
dependent of  tbe  number  of  pok§  #a  th&?machine$, 

In  the  machine  itself,  p^o^y^^i^  pf  frequency  lower 
than  normal  Creque^v^fSte  !&*£%*  Hw*fj  may  be  caused 
by  magnetic  <fes5p^^fe^|*^^^  s^j,  ®r>  by  unsymmetrical 
windings,  Ustt^$?3fl|^  fluctua^ 


288  ELECTRICAL  ENGINEERING  PAPERS 

lions  at  a  frequency  corresponding  to  the  normal  frequency  of 
the  machine,  and  therefore  will  have  no  visible  effect  unless 
such  normal  frequency  is  comparatively  low,  which  is  usually 
the  case  in  engine  type  d-c.  generators.  In  other  cases,  these 
dissymmetries  may  give  pulsations  corresponding  to  the  rev- 
olutions, and  not  the  poles  For  instance,  if  the  armature 
periphery  and  the  field  bore  are  both  eccentric  to  the  shaft, 
then  magnetic  conditions  are  presented  which  vary  directly 
with  the  revolutions. 

However,  there  have  been  cases  where  no  dissymmetry  could 
be  found,  and  yet  which  produced  enough  variations  to  wink 
the  lights  Usually  in  such  cases,  the  number  of  armature 
slots  per  pole  was  comparatively  small,  and  the  trouble  was 
overcome  by  materially  increasing  the  number  of  slots  per  pole. 
A  second  source  of  winking  has  been  encountered  in  some  three- 
wire  machines  in  which  the  neutral  tap  is  not  a  true  central 
point  In  such  case,  the  neutral  travels  in  a  circle  around  the 
central  point  and  impresses  upon  the  d-c  voltage  a  pulsation 
corresponding  to  the  diameter  of  the  circle  Its  frequency  how- 
ever, is  that  of  the  machine  itself  and  is  therefore  more  notice- 
able on  low  frequency  machines,  such  as  engine  type  generators. 

Non-Periodic  Pulsations  or  Voltage  "  Dips."  In  all  d-c. 
generators,  there  is  a  momentary  drop  or  "  dip"  in  voltage  with 
sudden  applications  of  load,  the  degree  of  drop  depending  upon 
the  character  and  amount  of  load,  etc  The  effects  of  this 
have  been  noted  most  frequently  in  connection  with  electric 
elevator  operation,  in  which  the  action  is  liable  to  be  repeated 
with  sufficient  frequency  to  cause  complaint.  Various  claims 
have  been  made  that  certain  types  of  machines  did  not  have 
such  voltage  dips,  and  that  others  were  subject  to  it  In  con- 
sequence, the  writer  and  his  associates  made  various  tests  in 
order  to  verify  an  analysis  of  this  action  which  is  given  below, 

The  explanation  of  this  dip  in  voltage  is  as  follows.  Assume, 
ior  instance,  a  100-volt  generator  supplying  a  load  of  100  am- 
peres— that  is,  with  one  ohm  resistance  in  circuit.  The  drop 
across  the  resistance  is,  of  course,  100  volts.  Now,  assume 
that  a  resistance  of  one  ohm  is  thrown  in  parallel  across  the 
circuit.  The  resultant  resistance  in  circuit  is  then  one-half 
ohm  However,  at  the  first  instant  of  closing  the  circuit  through 
the  second  resistance,  the  total  current  in  the  circuit  is  only 
100  amperes,  and  therefore  the  line  voltage  at  the  first  instant 
momentarily  must  drop  to  50  volts.  However,  the  em.f. 


PHYSICAL  LIMITATIONS  IN  D.C.  MACHINES  289 

generated  in  the  machine  is  100  volts,  and  the  discrepancy  of 
50  volts  between  the  generated  and  the  line  volts  results  in  a 
very  rapid  rise  in  the  generator  current  to  200  amperes.  If 
the  current  rise  could  be  instantaneous,  the  voltage  dip  would 
be  represented  diagrammatically  by  a  line  only,  that  is,  no 
time  element  would  be  involved.  However,  the  current  can- 
not rise  instantaneously  in  any  machine,  due  to  its  self-induction, 
and  therefore,  the  voltage  dip  is  not  of  zero  duration,  but  has 
a  more  or  less  time  interval.  The  current  rises  according  to  an 
exponential  law,  which  could  be  calculated  for  any  given  ma- 
chine if  all  the  necessary  constants  were  known.  However, 
such  a  great  number  of  conditions  enter  into  this  that  is  it  usually 
impracticable  to  predetermine  the  rate  of  current  rise  in  de- 
signing a  machine,  and  it  would  not  change  the  fundamental 
conditions  if  the  rate  could  be  predetermined,  as  will  be  shown 
later. 

A  rough  check  on  the  above  theory  could  be  obtained  in  the 
following  manner,  by  means  of  oscillograph  tests.  For  example, 
it  was  assumed  in  the  above  illustration  that  with  one  ohm 
resistance  in  circuit,  an  equal  resistance  was  thrown  in  parallel, 
which  dropped  the  voltage  to  one-half.  In  practise,  the  actual 
drop  which  can  be  measured  might  not  be  as  low  as  one-half 
voltage,  as  the  first  increase  in  current  might  be  so  rapid  as  to 
prevent  the  full  theoretical  dip  from  being  obtained.  However, 
an  oscillograph  would  show  a  certain  amount  of  voltage  drop. 
If  now,  after  the  current  has  risen  to  200  amperes  and  the  con- 
ditions become  stable,  the  second  resistance  of  one  ohm  is 
thrown  in  parallel  with  the  other  two  resistances  of  one  ohm 
each,  then  in  this  latter  case,  the  resultant  resistance  is  re- 
duced to  two-thirds  the  preceding  value,  instead  of  one-lialf, 
as  was  the  case  in  the  former  instance.  Therefore,  the  dip 
,  would  be  less  than  in  the  former  case.  Again,  if  "one  ohm  re- 
sistance is  thrown  in  parallel  ^ith  three  resistances  of  one  ohm 
each,  the  restdtant  resistance  becomes  three-fourths  of  the 
preceding  value, — that  is,  the  voltage  dip  is  still  less.  There- 
fore, according  to  the  above  analysis,  if  a  given  load  is  thrown 
on  a  machine,  the  dips  will  be  relatively  less  the  higher  the  load 
the  machine  is  carrying.  Also,  if  the  same  percentage  of  load 
is  thrown  on  each  time,  then  the  dips  should  be  practically  the 
same,  regardless  of  the  load  the  xnadiine  is  already  carrying. 
For  example,  if  the  machine  is  carrying  100  amperes,  and  100 
amperes  additional  is  tha^cmii  o%  ike  dip  shotild  be  the  same  as 


290 


ELECTRICAL  ENGINEERING  PAPERS 


if  the  machine  were  carrying  300  amperes  and  300  amperes 
additional  were  thrown  on. 

Also,  according  to  the  above  theory,  a  fully  compensated 
field  machine,  (that  is,  one  with  a  distributed  winding  in  the 
pole  faces  proportioned  to  correctly  neutralize  the  armature 
magnetomotive  force)  should  also  show  voltage  dips  with  load 
thrown  on.  To  determine  if  this  is  so,  several  series  of  tests  were 
made  on  a  carefully  proportioned  compensated  field  machine.  Two 
series  of  tests  were  made  primarily.  In  the  first,  equal  in- 
crements  of  currents  were  thrown  on,  (1)  at  half  load,  (2)  at  full 
load,  and  (3)  at  1J  load  on  the  armature.  In  the  second  series 
of  tests,  a  constant  percentage  of  load  was  thrown  on;  that  is~7 
at  half  load  the  same  current  was  thrown  on  as  in  the  first  test, 
while  at  full  load,  twice  this  current,  and  at  If  load,  three 
times  this  current  was  thrown  on. 

According  to  the  above  theory,  all  these  should  show  voltage 
dips,  although  the  machine  was  very  completely  compensated. 
Also,  in  the  first  series  of  tests,  the  dips  should  be  smaller  with 
the  heavier  loads  on  the  machine,  while  in  the  second  series 
they  should  be  the  same  in  all  tests.  This  is  what  the  tests 
indicated.  In  the  first  series,  the  dips  in  voltage  varied,  while 
in  the  second  series,  they  were  practically  constant.  The  re- 
sults of  these  tests  are  shown  in  the  following  table.  (The 
oscillograph  prints  were  so  faint  that  it  was  not  considered 
practicable  to  produce  them  in  this  paper.) 

NORMAL  E.M  F— 1200  VOLTS. 


Load  on  generator 

Increase  in  load. 

Dip  in  voltage 

Test. 

(Appro*). 

A 

0  Amps 

417  Amps 

700  Volts 

B 

208 

80 

300       « 

C 

417 

80 

200       * 

D 

625 

80 

150 

B 

417 

160 

300       * 

F 

625 

240 

300       • 

Tests,  B  C  and  D  in  the  table  show  the  dips  for  the  first 
series  of  tests,  while  B,  E  and  F  show  results  for  second  series. 
The  time  for  recovery  to  practically  normal  voltage  was  very 
short  in  all  cases,  varying  from  0.002  to  0.004  seconds  accord- 
ing to  the  oscillograph  curves,  but  even  with  this  extremely 


PHYSICAL  LIMITATIONS  IN  D  C.  MACHINES  291 

short  time,  there  was  very  noticeable  winking  of  tungsten 
lamps,  in  practically  all  tests  The  oscillograph  curves  showed 
practically  no  change  in  field  current,  except  in  test  A. 

The  machine  used  in  these  tests  was  a  special  one  in  some 
ways.  It  was  a  500-kw.,  1200- volt,  railway  generator  with 
compensating  windings  and  commutating  poles,  In  order  to 
keep  the  peripheral  speed  of  the  commutator  within  approved 
practise,  it  was  necessary  in  the  design  to  reduce  the  number 
of  commutator  bars  per  pole,  and  consequently  the  number 
of  armature  ampere  turns,  to  the  lowest  practical  limit.  This 
resulted  in  an  armature  of  very  low  self  induction,  which  was 
very  quick  in  building  up  the  armature  current  with  increase 
in  load,  This  machine  therefore  did  not  show  quite  as  severe 
variations  as  would  be  expected  from  a  normal  low-voltage 
machine  of  this  same  construction.  However,  these  two  series 
of  tests  did  show  pronounced  voltage  dips  which  were  sufficient 
to  produce  noticeable  winking  of  incandescent  lamps.  Presum- 
ably, therefore,  all  normal  types  of  generators  will  wink  the 
lights  under  similar  conditions. 

Data  obtained  on  non-compensated  machines  of  125  and  250 
volts  indicate  the  same  character  of  voltage  dips  as  were  found 
in  the  above  tests.  This  should  be  the  case,  for,  by  the  fore- 
going explanation,  the  compensating  winding  has  no  direct  re- 
lation to  the  cause  of  the  dip. 

It  will  be  noted  in  these  curves  that  the  voltage  recovers  to 
normal  value  very  quickly.  However,  incandescent  lamps 
will  wink,  even  with  this  quick  recovery,  if  the  dip  is  great 
enough.  There  is  some  critical  condition  of  voltage  dip  in 
each  machine  which  would  produce  visible  winking  of  lights. 
Any  increments  of  load  up  to  this  critical  point  will  apparently 
allow  satisfactory  operation.  If  larger  loads  are  to  be  thrown 
on,  then  these  should  be  made  up  of  smaller  increments,  each 
below  the  critical  value,  which  may  follow  each  other  in  fairly 
rapid  succession.  In  other  words,  the  rate  of  application  of 
the  load  is  of  great  importance,  if  winking  of  lights  is  to  be 
avoided  Therefore,  the  type  of  control  for  motor  loads,  for 
instance,  should  be  given  careful  consideration  in  those  cases 
where  steadiness  of  the  light  is  of  first  importance,  and  where 
motors  and  lights  are  on  the  same  circuit. 

An  extended  series  of  tests  has  shown  that,  in  most  cases, 
10  per  cent  to  15  per  cent  of  the  rated  capacity  of  the  generator 
can  be  thrown  on  in  a  single  step  without  materially  affecting 


292  ELCETRICAL  ENGINEERING  PAPERS 

the  lighting  on  the  same  circuit,  and  provided  the  prime  mover 
holds  sufficiently  constant  speed.  However,  judging  from  the 
quickness  of  the  voltage  recovery,  the  prime  mover,  if  equipped 
with  any  reasonable  flywheel  capacity,  cannot  drop  off  materially 
during  the  period  of  the  voltage  dip  as  shown  in  the  curves. 
The  dip  in  voltage  due  to  the  flywheel  is  thus  apparently  some- 
thing distinct  frdm  the  voltage  dip  due  to  the  load.  However, 
if  the  load  is  thrown  on  in  successive  increments  at  a  very 
rapid  rate,  the  result  will  be  a  dip  in  voltage  due  to  the  prime 
mover  regulation,  although  the  voltage  dips  due  to  the  load 
itself  may  not  be  noticeable. 

Thejabove  gives  a  rough  outline  of  this  interesting  but  little 
understood  subject  of  voltage  variations.  Going  a  step  farther, 
a  similar,  explanation  could  be  given  for  voltage  rises  when  the 
load  is  suddenly  interrupted,  in  whole  or  in  part.  This  is 
usually  known  as  the  inductive  kick  of  the  armature  when  the 
circuit  is  opened  This  may  give  rise  to  momentarily  increased 
voltages  which  tend  to  produce  flashing,  as  has  already  been 
referred  to  under  the  subject  of  flashing  when  the  circuit  breaker 
is  opened. 

PERIPHERAL  SPEED  OF  COMMUTATOR 

This  presents  two  separate  limitations  in  d-c.  design,  one 
being  largely  mechanical  and  the  other  being  related  to  voltage 
conditions.  As  regards  operation,  the  higher  the  commutator 
speed,  as  a  rule,  the  more  difficult  it  is  to  maintain  good  contact 
between  brushes  and  commutator  face.  This  is  not  merely  a 
function  of  speed,  but  rather  of  commutator  diameter  and  speed 
together.  Apparently  it  is  easier  to  maintain  good  brush  con- 
tact at  5000  ft  per  minute  with  a  commutator  50  in.  in  di- 
ameter than  with  one  of  10  in.  in  diameter.  Very  slight  un- 
evenness  of  the  commutator  surface  will  make  the  brushes 
"  jump  "  at  high  peripheral  speeds,  and  the  larger  the  dia- 
meter of  the  commutator  with  a  given  peripheral  speed,  the 
less  this  is. 

The  peripheral  speed  of  the  commutators  is  also  limited  by 
constructive  conditions.  With  the  usual  V-supported  com- 
mutators, the  longer  the  commutator,  the  more  difficult  it  is 
to  keep  true,  especially  at  very  high  speeds  and  the  higher 
temperatures  which  are  liable  to  accompany  such  speeds. 
Therefore,  the  allowable  peripheral  speeds  are,  to  some  extent, 
dependent  upon  the  current  capacity  per  brush  arm,  for  the 
ength  of  the  commutator  is  dependent  upon  this.  The  per- 


PHYSICAL  LIMITATIONS  IN  D  C   MACHINES  293 

missible  speed  limits,  as  fixed  by  mechanical  constructions- 
have  been  rising  gradually  as  such  constructions  are  improved, 
At  the  present  time,  peripheral  speeds  of  about  4500  ft.  per 
minute  are  not  uncommon  with  commutators  carrying  800  to 
1000  amperes  per  brush  arm.  In  the  case  of  60-cycle,  600- 
volt  synchronous  converters,  5200-  to  5500-ft.  speeds  are  usual 
with  currents  sometimes  as  high  as  500  to  600  amperes  per  arm, 
In  the  case  of  certain  special  750-volt,  60-cycle  converters,  oper- 
ated two  in  series,  commutator  speeds  of  about  6400  ft.  have 
proved  satisfactory.  These  latter,  however,  had  comparatively 
short  commutators. 

For  the  small  diameter  commutators  used  in  d-c.  turbo- 
generator work,  peripheral  speeds  of  5500  to  6000  ft.  have  been 
common.  However,  such  machines  usually  have  very  long  com- 
mutators and  of  the  so-called  "  shrink-ring  "  construction.  The 
brushes  may  not  maintain  good  contact  with  the  commutator 
at  all  times,  and  in  a  number  of  machines  in  actual  service,  the 
writer,  in  looking  at  the  brush  operation,  could  distinctly  see 
objects  beyond  the  brush  contacts;  that  is,  one  could  see 
"  through  "  the  contact,  and  curiously,  in  some  of  these  cases, 
the  machines  seemed  to  have  operated  fairly  well.  One  ex- 
planation of  this  is  that  the  gaps  between  brushes  and  com- 
mutator were  intermittent,,  and,  with  one  or  more  brush  arms  in 
parallel,  one  arm  would  be  making  good  contact,  while  another 
showed  a  gap  between  brushes  and  commutator.  Appar- 
ently, the  commutators  were  not  rough  or  irregular,  but 
were  simply  eccentric  when  running  at  full  speed  and  the 
brushes  could  not  rise  and  fall  rapidly  enough  to  follow 
the  commutator  face  all  the  time.  Incidentally,  it  may  be  men- 
tioned at  this  point,  that  with  the  higher  commutator  speeds 
now  in  use,  there  has  come  the  practise  of  "  truing  "  commutators 
at  full  speed.  This  is  one  of  the  improvements  which  has  al- 
lowed higher  commutator  speeds. 

The  other  limitation  fixed  by  peripheral  speed,  namely,  that 
of  the  voltage,  is  a  more  or  less  indirect  one.  It  is  dependent 
upon  the  number  of  commutator  bars  that  are  practicable  be- 
tween two  adjacent  neutral  points;  or,  in  other  words,  it  is 
dependent  upon  the  distance  between  neutral  paints.  The 
product  of  the  distance  between  adjacent  neutral  points  and  the 
frequency,  in  Alternations,  gives  the  peripheral  speed  of  the 
commutator,  (distance  between  neiitcal  points  in  feet  times 
per  &&&^]&$^  peripheral  speed  in  feet  per 


294  ELECTRICAL  ENGINEERING  PAPERS 

minute).  With  a  given  number  of  poles  and  revolutions  per 
minute,  the  alternations  are  fixed  Then,  with  an  assumed 
limiting  speed  of  commutator,  the  distance  between  neutral 
points  is  thus  fixed.  This  then  limits  the  maximum  number  of 
commutator  bars,  and  therefore  the  maximum  voltage  which 
is  possible,  assuming  a  safe  limiting  voltage  per  bar  From 
this  it  may  be  seen  that  the  higher  the  penpheral  speed,  the 
higher  the  permissible  voltage  with  a  given  frequency  In  the 
same  way,  if  the  frequency  can  be  lowered  (either  the  speed  or 
the  number  of  poles  be  reduced)  the  permissible  voltage  can 
be  increased  with  a  given  peripheral  speed.  Where  the  speed 
and  the  number  of  poles  are  definitely  fixed  and  the  diameter 
of  commutator  is  limited  by  peripheral  speed  and  other  con- 
ditions, the  maximum  practicable  d-c  voltage  is  thus  very  defi- 
nitely fixed.  This  ^s  a  point  which  apparently  has  been  mis- 
understood frequently  It  explains  why,  in  railway  motors,  for 
high  voltages,  it  is  usual  practise  to  connect  two  armatures  per- 
manently m  series,  also,  why  two  60-cycle  synchronous  conver- 
ters are  connected  in  series  for  1200-  or  1500- volt  service.  In 
synchronous  converter  work,  the  frequency  being  fixed  once  for 
all,  the  maximum  d-c.  voltage  is  directly  dependent  upon  the 
peripheral  speed  of  the  commutator 

CONCLUSION 

The  principal  intent,  in  this  paper  has  been  to  show  that  cer- 
tain limitations  encountered  in  d-c.  practise  are  just  what  should 
be  expected  from  the  known  properties  of  materials  and  electric 
circuits.  The  writer  has  endeavored  to  explain,  in  a  simple, 
non-mathematical  manner,  how  some  of  the  apparently  com- 
plicated actions  which  take  place  in  commutating  machinery 
are  really  very  similar  to  better  understood  actions  found  in 
various  other  apparatus  An  endeavor  has  also  been  made  to 
show  that  a  number  of  the  present  limitations  in  direct  current 
design  and  operation  are  not  based  merely  upon  lack  of  ex- 
perience, but  are  really  dependent  upon  pretty  definite  condi- 
tions, such  as  the  characteristics  of  carbon  brushes  and  brush 
contacts,  etc  Possibly  a  better  understanding  of  the  character- 
istics and  functions  of  carbon  brushes  will  result  from  this  paper. 

The  writer  makes  no  claims  to  priority  for  many  of  the  ideas 
and  suggestions  brought  out  in  this  paper  However,  much  of 
the  material  is  a  direct  result  of  his  own  investigations  and  those 
of  his  associates  during  many  years  of  experience  with  direct- 
currcnt  apparatus 


PHYSICAL  LIMITATIONS  IN  D.C.  MACHINES  295 

APPENDIX 

The  following  method  of  determining  the  maximum  capacity 
which  can  be  obtained  with  given  dimensions  and  for  assumed 
limitations  as  fixed  by  commutation,  flashing  and  other  con- 
ditions, is  based  upon  certain  formulas  which  the  writer  de- 
veloped several  years  ago,  and  which  appeared  in  a  paper  before 
the  Institute.* 

On  page  2389  of  the  1911  TRANSACTIONS  of  the  Institute, 
the  following  general  equation  is  given  : 

„        Ic  WtRs  TC-K   r  _         _  2  Dp  _ 

10*  1C]  (L      L})    (025^+05)    (D+PP) 

9  +  °-035  N)  +  c*-  (JL33  ds  +  °-52  +  2-16  s 


"• 


Where  Ic      =  Current  per  armature  conductor. 

Wt    =  Total  number  of  armature  conductors 
Tc     =  Turns  per  armature  coil  or  commutator  bar. 
L  &  Li  \=  Width  of  armature  core  and  commutating  pole 

faces  respectively. 
D      =  Diameter  of  armature. 
p       =  No.  of  poles. 
N      =  No.  of  slots  per  pole. 
d9      =  Depth  of  armature  slot. 
5        =  Width  of  armature  slot. 
n       =  Ratio  of  width  of  armature  tooth  to  slot  'at 

surface  of  core. 

Ci,  Cz,  £3,  £4  are  design  constants. 

In  order  to  simplify  the  above  equation,  the  following  as- 
sumptions are  made; 

(a)  No  bands  are  used  on  armature  core,  thus  eliminating 
the  last  term  in  the  above  equation.    • 

(b)  Li  =  L,  thus  eliminating  the  first  expression  inside  tha 
bracket  in  the  above  equation, 

B&th  the  above  assumptions  are  in  the  direction  of  increased 
capacity  with  a  given  short  circuit  voltage,  Ec. 
Equation   (1)   then  becomes, 

*Theory  of  Commutatang  and  Its  Application  to  Commutating-Pole  Machines,  Page  201. 


296  ELECTRICAL  ENGINEERING  PAPERS 

[Sept.  16 


+  c3  —  (1  33rfs  +  0.52  +  2  16  s  Vn)  1  (2) 

The  various  terms  in  equation  (2)  should  be  put  in  such  form 
that  limiting  values  can  be  assigned  to  them  as  far  as  possible. 
In  order  to  do  this  the  equation  can  be  condensed  and  simplified 
as  follows,  for  large  machines: 

(a)  Assuming  parallel  type  windings,  — 

7"   2  to  F 

\yt  —       "    r  —  ^  where    F&  =  Average    volts    per    commutator 
l/& 

bar    or    coil. 

where    It  =  Total    current. 

' 


c 

P 

IE  =  Kilowatts    X  10*  =  Kw  103 

Also,  Ra  p  =  2/,  where/  =  Frequency  in  cycles  per  second. 

_,       ,         Ic  Wt  Rs  T,  TT       Kwp  X  4/  rcV    „     u  .      ,....- 
Therefore,  -  j^  -    =  -  V&  X  10s  -  '        p         g 

watts  per  pole. 

(6)     Let  P<  =  Armature  tooth  pitch, 

Then  D 


and  c2  -         (09  +  0.035  N)  =  c2  --  (0.9  +  .035  .Vj  Pt 

p  7T 

In  case  of  a  chorded  winding,  the  term  0.035  N  should  be 
0.035  Nit  where  NI  represents  the  number  of  teeth  or  tooth  pitches 
spanned  by  the  coil. 

(c)  In  the  second  tenn  inside  the  bracket  in  equation  (2), 

the  ratio  •=-  can  be  transformed  into  an  expression  containing 
o 

Pif  as  follows: 

E  =BtStCPR,Wa 
10* 


PHYSICAL  LIMITATIONS  IN  D.C    MACHINES  297 

Bt  =  Flux  density  in  armature  teeth. 

St  —  Section  of  iron  in  armature  teeth. 

Cp  =  Field  form  constant  (percentage  polar  area). 

Rs  =  Revolutions  per  second. 

Ws  =  Wires  in  series. 

St  =  N  T  p  L  c^  where  T  =  Width  of  tooth,  and  c*  =  the 
ratio  of  actual  iron  to  the  core  width  L. 

P  2 
As  an   approximation,    TX9   =  -£•      (This  is  a  fairly  close  ap- 

proximation within  practical  limits  in  the  usual  armature  con- 
structions) . 


Then,  S,= 


4X10*E 


°r        -S  =1 

This  can  further  be  condensed  as  follows. 

W.=  Tc-22-,*rLARtp-2f 


™      t        L  1QS  v*> 

Therefore,  —  =  •=— ^ 


(d)  The    expression    (1.33    d,  +  0.52  +  2.16  $  Vn)    can    be 
modified  as  follows, 

Vw"   =  ^1  T  =  ^(^    =  ^  on  the  basis  that  Txs  =  ^~ 

approx. 

Then,  2.16  sVn  «  1.08  Pt  approx. 
and,  (1.33  d,  +  0.52  +  2.16  sVnj  =  (1  33  ds  +  0.52  +  1.08  Pt) 

Substituting  all  the  above  transformations  in  equation   (2) 
we  get, 

4  (°-9  +  °  °35  Nl}  NPt 


298  ELECTRICAL  ENGINEERING  PAPERS 

„  Ec  Vb  105 

Kwp  =  —  ^  ^., 

P£  (4) 


4c2  (0.9  +0  035tfi)jyyP<a  +  ~^^r  (1  33d.  +  0.52+  1  08  P  t\ 

ntLpL\  C51  c  [ 

Maximum  Kilowatts  per  Pole.     Differentiating  (4)  to  obtain 
Pt  for  maximum  Kwpt 


c»  Nf  (0.9  +  0.035  tf  0  *  '6  (1  33  <fs  +  0  52) 

*  c  •#*  Cp  J\  C$ 
TT  Ci  Vb  10s 


If  Pi  in  equation  (5)  could  be  derived  and  then  substituted 
in  equation  (4),  then  for  any  assumed  value  of  Ec  and  with  the 
other  terms  given  limiting  values,  an  expression  for  the  maximum 
kilowatts  per  pole  could  be  obtained.  The  writer  has  not  been 
able  to  solve  this  directly  in  any  sufficiently  simple  manner, 
although  a  complicated  approximate  expression  can  be  obtained. 
However,  for  practical  purposes,  the  solution  for  an3r  given  con- 
ditions can  be  obtained  by  trial  methods  and  the  results  plotted 
in  curves. 

For  instance,  in  equations  (4)  and  (5),  the  following  terms 
may  be  given  limiting  values  for  a  given  class  of  machines  and 
for  a  specified  voltage 

Tr   -  Turns  per  coil 

c<i     =  End  flux  constant. 

Af    =  Number  of  slots  per  pole      Ari  =  No.  of  teeth  spanned 

by  coil. 

GZ     =  Brush  short  circuit  constant. 
Vi   =  Average  volts  per  bar. 
Cp  =  Field  form  constant     With  max.  volts  per  bar  fixed, 

then  V  max.  X  Cp  =  Vb. 
Bt   =  Flux  density  in  teeth. 
£&     =  Ratio  of  actual  iron  width  to  core  width  L. 

Also,  type  of  armature  winding  can  be  fixed  and  departure 
from  full  pitch  winding,  or  amount  of  chording  can  be  given. 


PHYSICAL  LIMITATIONS  IN  D  C.  MACHINES  299 

There  will  then  remain  for  any  assumed  value  of  Ec,  the  terms, 

Kwp  =  Kilowatts  per  pole. 

Pt     =  Tooth  pitch. 

/        =  Cycles  per  second. 

ds      =  Depth  of  armature  slot. 

All  four  of  these  latter  terms  are  in  equation  (4),  and  the  last 
three  in  equation  (5).  Therefore,  assuming  the  depth  of  slot, 
equation  (5),  the  values  of  Pt  for  different  frequencies  may  be 
determined  by  trial  methods.  The  corresponding  values  of 
Pt,  f  and  d3  can  then  be  substituted  in  equation  (4),  and  the 
kilowatts  per  pole  thus  determined.  Tables  or  curves  can  then 
be  prepared  giving  the  kilowatts  per  pole  for  different  frequencies 
and  for  different  assumed  slot  depths. 

A  series  of  such  tables  have  been  worked  out  for  a  specified 
set  of  Conditions  as  given  below.  The  assumed  limiting  con- 
ditions were  as  follows: 

Ec        =  4.5, — that  is,  one  turn  per  coil  parallel  type  winding 

is  assumed, 
e  m  f   =600  volts. 
Cp        =  0.68 
Vb        =  14.3.     No  of  commutator  bars  per  pole  =  42.     No 

compensating    winding    is    used.    Therefore,     Vb. 

600 


"    42 

14.3  = 
0.68 


and    max.    volts    per  bar  at  no  load   = 
21.     Allowing   25   per   cent   increase   for 


flux  distortion,  and  increased  voltages  at  times, 
gives  26  3  at  full  load. 

r.>         =  1,25     for     average    constructions. 

<-,  =  Vanes  with  the  number  of  coils  per  slot  and  the  aver- 
age number  of  bars  covered  by  the  brush,  but  as- 
suming 2  bars  covered,  then  Cz  =  0.4  approx, 
with  1  slot  chording,  and  with  either  2  or  3  coils 
per  slot. 

Bt  -  150,000  lines  per  sq.  in.  on  the  basis  of  actual  iron 
and  all  flux  confined  to  the  iron. 

r&  =  0.75.  This  allows  for  90  per  cent  solid  iron  and  j 
of  the  total  width  taken  up  by  air  ducts  (aboul 
|"  duct  for  each  2"  of  laminations). 


300 


ELECTRICAL  ENGINEERING  PAPERS 


N 


14  f 


(Two  cases  have  been  assumed,  one  with  3  coils 

per  slot  and  14  slots  per  pole,  and  the  other  with 
2  coils  per  slot  and  21  slots  per  pole. 

14  Slots  per  Pole. — Substituting  the  above  values  in  equations 
(4)  and  (5),  then  for  14  slots  per  pole  equation  (4)  becomes, 


Kw,  =  3767  Ec  [ 


and  equation  (5)  becomes 


jP?  =  18  36  (2.5  d,  +  1)  +  19  Pt 


(6) 


(7) 


Incidentally,  equation  (6)  can  be  simplified  to  a  certain  extent 
by  partially  combining  with  equation  (7),  giving  the  following 
equation : 

p? 


Kiuf  =  99  EC 


3.725  (2.5  d.  +  1)  - 


Equation  (8),  of  course,  can  only  be  used  with  the  values  of 
Pt  determined  from  equation  (7). 

Three  values  for  ds  were  chosen,  1  in.,  1.5  in.,  and  2  in.,  which 
cover  the  practical  range  of  design  for  large  d.-c.  generators. 
Frequencies  from  5  to  60  cycles  were  also  chosen.  The  corres- 
ponding values  for  Pt  and  Kwp  are  tabulated  below, 

TABLE  I. 


f— 

Cycles 

J,  =1* 

d  =  15" 

d  -o* 

per  sec. 

/>!         K~> 

Pt       K»p 

Pi        *«, 

5 

2  85  in.    670 

3  08  m    647 

3  255  in   620 

10 

2  20      453 

2  362     428 

2  504     407 

20 

1  685     2Q9 

1  828     282 

1  945     266 

30 

1  455     235 

1,575     219 

1  680     208 

40 

1  302     197 

1  417     183  5 

1  515     173 

50 

1.20      173  5 

1  305     160 

1  398     151  5 

60 

1  I2o     153 

1  226     143  5 

1  310     135 

21  Slots  Per  Pole.  Substituting  the  proper  values  in  equations 
(4)  and  (6)  for  21  slots  per  pole,  and, one  slot  chording,  and  then 
solving  for  Pt  and  Kwp  for  the  same  slot  depths  and  frequencies, 
the  following  table  is  obtained: 


PHYSICAL  LIMITATIONS  IN  D  C.  MACHINES  301 

TABLE  II. 


/— 

Cycles 

ds  =  1  in. 

<f  <r  =*  1  5  in. 

ds  =  2  in 

per  sec. 

Pt        Kwp 

Pt          Kwp 

Pt          Kwp 

5 

1  985  in.      576 

2  14  in      542 

2  27  in      515 

10 

1  53         380 

1  56        333 

1  77        338 

20 

1  185        249 

1  29        J,52 

1  36        214 

30 

1  022        195 

1  12        181 

1  192       368 

40 

0  922        16,i 

1  003       130 

1  077       141 

50 

0  830        142 

0  932       131 

0  997       123 

60 

0  796        126 

0  874       117 

0  936       110 

SYNCHRONOUS  CONVERTERS 

Two  cases  only  need  be  considered,  namely  25  and  60  cycles. 
For  these  two  cases,  more  definite  limits  can  be  given  than  for 
the  above  rather  general  solution  for  d-c.  machines. 

25  Cycles.  Let  N  ~  21,  and  NI  =  20;  also,  assume  two 
coils  per  slot  for  600  volt  machines. 

ri    =  1.0. 

cz    =  0.37 

ft  =  165,000 

Cp  =  0.7 

Then  for  assumed  values  for  depth  of  slot  of  1  in.,  1.5  in  , 
and  2  in.,  and  for  Ec  =  4.5,  the  following  values  of  Pt   and 
Kwp  are  obtained: 
« 

TABLE  III. 


Depth      of 

Tooth 

Kilowatts 

slot. 

pitch. 

* 

per  pole. 

1  in 

1.09 

278 

1  5 

1   19 

257 

2 

1  275 

243 

60  Cycles.    Let  N  =  15,  and 
per  slot  for  600  volts. 

c3    =  0.4 
Bt  =  150.000 
Cp  =  0.66 


14,    Also,  assume  3  coils 


302 


ELECTRICAL  ENGINEERING  PAPERS 


Then  assuming  s  bt  depth  of  1  in ,  1.25  in.,  and  1  5  in.,  and 
EC  =  4.0,  the  following  values  of  Pt  and  Kwp  result: 


TABLE  IV. 


Depth  of 

Tooth 

Kilowatts 

slot 

pitch. 

per  pole. 

1  in 

1   14 

143 

1  25 

1  195 

137  5 

1  5 

1  24 

132 

The  above  tabulated  results  agree  pretty  well  with  practical 
results  obtained  in  large  generators  and  converters.  There  are 
so  many  possible  variations  in  the  limits  assumed  that  only 
general  results  can  be  shown  For  instance,  in  Table  I,  a 
constant  limiting  induction  in  the  armature  teeth  of  150,000 
lines  per  sq.  in.  is  assumed.  With  low  frequencies  this  can  be 
increased,  while  with  frequencies  of  50  to  60  cycles,  somewhat 
lower  inductions  will  be  used.  Also,  the  commutation  con- 
stant C\  which  is  dependent  upon  the  number  of  bars  covered 
by  the  brush  is  naturally  subject  to  considerable  variation. 

The  results  obtained  are  predicated  upon  parallel  types  of 
windings  and  a  minimum  of  one  turn  per  armature  coil.  If 
types  of  windings  having  the  equivalent  of  a  fractional  number 
of  turns  per  coil  less  than  one,  prove  to  be  thoroughly  satis- 
factory for  large  capacity  machines,  then  the  above  maximum 
capacities  can  be  materially  increased.  However,  accepting 
the  results  as  they  stand,  the  limits  of  capacity  as  fixed  by 
commutation  are  in  general  about  as  high  as  other  limitations 
will  allow 


REGULATION  CHARACTERISTICS  OF  COMMUTATING 

POLE  MACHINES  AND  PARALLEL  OPERATION 

WITH  OTHER  MACHINES 

FOREWORD— About  ten  years  ago,  the  author  found  that  there  was 
considerable  misunderstanding  regarding  the  regulating  charac- 
teristics of  commutating-pole  machines,  and  the  conditions 
which  were  to  be  met  in  parallel  operation.  In  consequence,  he 
prepared  this  brief  article  for  the  use  of  the  engineers  of  the 
Westinghouse  Electric  &  Manufacturing  Company.  It  has 
proved  so  satisfactory  that  it  has  been  kept  in  publication  ever 
since. 

This  paper  was  written  before  the  term  "  comrnutating 
pole"  was  adopted  to  replace  the  term  "interpole"  which  is 
found  throughout  the  article. — (Eo.) 


THE  inherent  regulation  characteristics  of  the  armature  of  a 
direct-current  machine  has  much  to  do  with  its  parallel 
operation  with  other  machines.  When  two  direct-current  ar- 
matures are  coupled  in  parallel  and  delivering  load  to  the  same 
external  circuit,  it  is  necessary,  in  order  to  obtain  stable  conditions, 
for  each  armature  to  tend  to  "shirk"  its  load;  that  is,  it  must 
naturally  tend  to  transfer  load  to  the  other  machine.  This 
tendency  to  shirk  may  be  either  in  bad  speed  regulation  due  to  the 
prime  mover  which  drives  the  armature,  or  in  the  drooping  voltage 
characteristics  of  the  armature  itself.  A  drooping  speed  character- 
istic indirectly  produces  a  drooping  voltage  characteristic  in  the 
armature  and  therefore  both  causes  lead  to  the  one  characteristic, 
namely,  drooping  voltage,  as  the  condition  for  stable  parallel 
operation.  This  drooping  voltage  characteristic  must  be  the  in- 
herent condition.  In  practice,  the  voltage  at  the  armature 
terminals  frequently  rises  with  increase  in  load,  but  its  rise  is  due 
to  some  external  condition,  such  as  increased  field  strength. 

Direct-current  machines,  as  hitherto  ordinarily  constructed, 
naturally  give  drooping  voltage  characteristics  in  the  armature 
windings.  If  two  such  armatures  are  paralleled  they  tend  to 
divide  the  load  in  a  fairly  satisfactory  manner  provided  then- 
prime  movers  regulate  similarly  in  speed.  If  means  are  applied 
for  giving  a  rising  voltage  characteristic  to  the  machines,  such  as 
series  coils  in  the  field,  then  the  armature  terminals  must  be 
paralleled  directly  in  order  to  maintain  stability.  If,  for  instance, 
the  armatures  are  not  paralleled  directly,  but  the  paralleling  is 

303 


304  ELECTRICAL  ENGINEERING  PAPERS 

done  outside  the  series  coils,  then  the  operation  will  be  unstable 
unless  the  machines  still  have  drooping  voltage  characteristics 
If  they  have  rising  characteristics,  then  parallel  operation  is  im- 
practicable. If  either  machine  should  take  an  excess  of  load,  its 
voltage  would  rise,  while  that  of  the  other  machine  would  fall  due 
to  decreased  load.  This  condition  would  naturally  force  the  first 
machine  to  take  still  more  load  and  the  second  one  to  take  still  less 
This  condition  would  continue  until  the  first  machine  actually  fed 
current  back  through  the  other  machine  and  it  would  be  necessary 
to  cut  them  apart  to  avoid  injury.  However,  by  paralleling  the 
two  armatures  inside  the  series  coils,  that  is,  between  the  series 
coils  and  the  armature  terminals,  this  unstable  condition  is  avoided 
due  to  two  reasons,  first,  the  inherent  drooping  voltage  character- 
istics of  the  armatures,  and,  second,  the  fact  that  the  series  coils 
are  paralleled  at  both  terminals,  thus  forcing  them  to  take  pro- 
portional currents  at  all  times  and  thus  compounding  both  ma- 
chines equally. 

If  direct  current  machines  are  so  designed  or  operated  as  to 
give  rising  instead  of  drooping  armature  characteristics,  then 
parallel  operation  is  liable  to  be  unstable.  This  condition  could 
be  obtained  in  ordinary  machines  by  prime  movers  which  tend  to 
speed  up  with  increasing  load,  thus  producing  rising  voltage  on  the 
armature.  Ordinarily,  such  speeding  up  of  the  prime  mover  would 
have  to  be  rather  large,  as  the  normal  drooping  characteristics  of 
the  ordinary  armature  is  fairly  large.  However,  prime  movers  of 
this  character  are  comparatively  rare. 

A  second  condition  which  can  give  a  rising  voltage  is  found 
not  infrequently  in  the  interpole  type  of  direct-current  machine.  The 
interpole  generator  is  similar  to  the  ordinary  type  of  generator, 
except  that  midway  between  the  main  poles  small  poles  are 
placed  which  carry  windings  or  coils  which  are  connected  directly 
in  series  with  the  armature.  The  winding  on  the  interpoles  is 
connected  directly  in  opposition  to  the  winding  in  the  armature. 
The  maxirnurn  magnetizing  effect  of  the  armature  winding  is  found 
at  the  points  on  the  armature  corresponding  to  the  coils  which  are 
being  commutated.  The  interpole  is  intended  to  be  placed 
directly  over  these  points  and  the  interpole  winding  normally  has 
such  a  value  that  it  not  only  neutralizes  the  magnetizing  effect  to 
the  armature  winding  at  these  points,  but  it  also  sets  up  a  small 
magnetic  field  in  the  opposite  direction  whicl^  assists  in  the  com- 
mutation of  the  armature  coil.  Therefore  the  interpole  winding 


PARALLEL  OPERATION  OF  D.C   MACHINES  305 

must  have  a  number  of  ampere  turns  equal  to  the  maximum  ampere 
turns  in  the  armature  winding,  plus  the  excess  ampere  turns 
necessary  to  produce  the  required  commutating  field  strength. 

When  this  interpole  winding  is  placed  directly  over  the  com- 
mutating position  of  the  armature  winding  it  should  have  prac- 
tically no  effect  on  the  armature  characteristics.  If,  however,  the 
interpole  winding  is  not  placed  over  these  positions  it  will  have  an 
effect  on  the  voltage  characteristics  of  the  machine,  tending  to 
either  raise  or  lower  the  voltage,  depending  upon  the  position  of 
the  interpole  with  respect  to  the  commutating  position.  The 
commutating  points  on  the  armature  depend  directly  upon  the 
brush  position  If  the  brushes  are  rocked  backward  or  forward 
from  the  point  corresponding  to  the  mid  position  between  the  poles 
then  the  position  of  the  commutated  armature  coils  moves  back- 
ward or  forward  with  the  brushes.  As  the  commutating  pole  is 
fixed  in  position  it  is  evident  that  the  relation  of  the  commutating 
pole  to  the  coils  undergoing  commutation  can  be  changed  by  the 
different  brush  settings.  Herein  lies  a  possible  trouble  in  parallel 
running,  for  the  commutating  points  can  be  so  shifted,  with 
respect  to  the  commutating  pole,  that  the  armature  winding 
voltage  characteristics  can  be  made  to  rise  instead  of  droop.  As 
explained  before,  this  is  an  unstable  condition  for  parallel  operation. 

This  condition  can  be  illustrated  in  the  following  manner: 


PIG. 


Let  Pig.  1  represent  two  main  poles,  and  interpoles,  with  the 
brushes  set  in  a  position  corresponding  to  the  middle  point  of  the 
interpole.  The  polarity  of  the  interpoles  and  main  poles  is  in- 
dicated in  this  figure.  The  polarity  of  any  interpole,  when  the 
machine  is  running  as  a  generator,  is  always  the  same  as  the 
polarity  of  the  main  pole  immediately  in  front  of  the  interpole. 
When  the  brush  is  placed  in  a  position  corresponding  to  an  exact 
intermediate  point  in  the  interpole  it  is  evident  that  the  aramture 


306  ELECTRICAL  ENGINEERING  PAPERS 

coils  lying  between  two  commutating  points,  that  is,  the  winding 
between  a  and  b  in  Fig.  1,  is  acted  upon  by  induction  from  the 
main  pole  and  by  half  the  induction  from  the  interpoles  adjacent  to 
the  main  pole.  However,  as  these  two  interpoles  are  of  opposite 
polarity,  and  the  induction  is  the  same  from  each,  it  is  evident  that 
they  have  equal  and  opposite  effects  on  the  armature  winding 
between  a  and  b,  and  therefore  do  not  affect  its  voltage 

In  Fig.  2  the  brushes  are  given  a  slight  back  lead  so  that  the 
commutation  is  under  the  trailing  magnetic  flux  from  the  inter- 
pole.  It  is  now  evident  that  between  a  and  b  the  induction  is 
from  the  main  pole  and  from  one  interpole  principally.  With  the 
back  lead  at  the  brushes,  this  interpole  is  the  one  immediately 
behind  the  main  pole  and  therefore  of  the  same  polarity.  This 
interpole  therefore  becomes  a  magnetizing  pole  and  adds  to  the 
e.  m.  f .  generated  between  a  and  b  As  the  strength  of  this  inter- 
pole is  zero  at  no  load  and  rises  with  load,  it  is  evident  that  it  tends 
to  give  an  increased  voltage  between  a  and  b  as  the  load  increases 
and  thus  tends  to  produce  a  rising  voltage  characteristic  instead 
of  a  drooping  one.  The  ampere  turns  in  the  interpole,  as  stated 
before  is  considerably  greater  than  in  the  armature,  but  ordinarily 
the  effect  of  these  ampere  turns  is  almost  neutralized  by  the 
opposing  effect  of  the  armature  winding  However,  with  the 


*"• 


OG00OO© 

%2 

-> 


H \- 


FIG.  2 

back  lead,  as  indicated  in  Fig.  2,  the  opposing  effect  of  the  armature 
winding  is  shifted  to  one  side  of  the  interpole  and  thus  the  inter- 
pole ampere  turns  become  more  effective  in  actually  magnetizing 
the  armature,  but  become  less  effective  in  creating  a  commutating 
field  for  the  coils  which  are  now  being  reversed  by  the  brushes.  On 
account  of  this  less  effective  field  it  may  be  necessary  in  practice 
to  still  further  increase  the  ampere  turns  on  the  commutating  poles 
in  order  to  bring  the  trailing  magnetic  fringe  up  to  a  suitable 
value  for  producing  proper  commutation.  It  is  evident,  that  this 


PARALLEL  OPERATION  OF  D  C    MACHINES  307 

increased  ampere  turns  on  the  commutating  pole  increases  the 
induction  under  other  parts  of  the  commutating  pole  as  well  as 
under  the  trailing  tip,  and  this  increase  under  the  other  parts  of 
the  pole  still  further  increases  the  voltage  between  a  and  b. 

With  a  back  lead  therefore  the  interpole  may  have  the  same 
effect  as  the  series  winding  on  the  main  field;  that  is,  it  may 
compound  the  machine  so  that  the  voltage  at  the  terminals  is  rising 
instead  of  falling,  even  without  any  true  series  winding  on  the 
main  poles.  The  machine  therefore  becomes  an  equivalent  of  a 
compound  wound  machine  and  if  there  is  no  equalizer  between  the 
interpole  winding  and  the  armature  terminal,  the  generator  may 
be  unstable  when  paralleled  with  other  machines. 

Take  the  case,  next,  where  the  brushes  are  given  a  forward 
lead,  as  shown  in  Fig.  3.  Comparing  this  with  Fig.  2,  by  the  same 
reasoning  it  is  evident  that  the  interpole  is  now  opposing  the 
effect  of  the  main  pole,  in  the  winding  between  a  and  b.  The 
interpole  therefore  tends  to  produce  a  drooping  voltage  charac- 
teristic and  has  just  the  opposite  effect  of  the  series  winding.  In 
this  position  of  the  brushes  the  interpole  winding  tends  to  give 
good  characteristics  for  parallel  operation,  but  as  the  effect  of  the 
interpole  is  in  opposition  to  the  main  pole  it  is  evident  that  more 
series  winding  is  required  on  the  main  field  in  order  to  over- 
compound  the  machine  as  a  whole.  Also,  with  the  brushes  in  this 
position  the  interpole  is  not  as  effective  in  producing  gpod  com- 
mutation and  therefore  more  ampere  turns  are  required  on  their 
interpole  winding.  Therefore,  both  the  interpole  winding  and  the 
main  series  winding  must  be  increased  when  the  brushes  are  given 
this  forward  postion.  However,  parallel  operation  should  be 
stable. 

It  is  evident,  therefore,  from  the  above  considerations,  that 
for  best  results  the  brushes  should  be  so  set  that  the  true  point  of 
commutation  comes  midway  under  the  interpole.  If  this  position 


'FIG.  3 


308  ELECTRICAL  ENGINEERING  PAPERS 

is  found  exactly,  then  the  interpole  should  have  practically  no 
effect  on  the  voltage  characteristics  of  the  armature,  and  parallel 
operation  with  other  generators  should  be  practicable  A  very 
slight  forward  lead  is  favorable  to  paralleling,  but  lessens  the 
compounding. 

As  a  back  lead  at  the  brushes,  when  the  machine  is  acting  as  a 
generator,  tends  to  improve  the  compounding  and  lessens  the 
series  winding  required  on  the  main  field,  it  might  be  suggested 
that  this  gives  a  cheaper  and  more  efficient  machine  and  that 
therefore  this  arrangement  should  be  used,  with  some  means  added 
for  overcoming  the  unstable  conditions  of  paralleling.  One 
means  proposed  for  this  is  an  additional  equalizer  connected 
between  the  interpoles  and  the  armature  terminals.  This  has 
been  used  in  one  or  two  instances,  but  in  principle  the  arrange- 
ment is  inherently  wrong.  When  the  interpole  windings  are 
paralleled,  then  the  currents  in  them  must  divide  according  to 
their  resistances.  This  condition  would  not  be  objectionable 
provided  the  armature  currents  also  always  varied  in  the  same 
proportion.  With  slow  changes  in  load  this  condition  might  be 
obtained.  However,  there  are  conditions  of  operation  where  the 
armature  currents  will  not  rise  and  fall  in  proportion  and  there- 
fore the  interpole  windings,  with  this  arrangement,  would  not 
always  have  the  right  value  to  produce  the  desired  commutating 
fields.  By  rights,  each  armature  should  be  connected  directly 
in  series  with  its  own  interpoles  and  the  currents  in  the  two  should 
rise  and  fall  together  for  best  results.  This  condition  will  not  be 
obtained  when  an  equalizer  is  connected  between  the  armatures 
and  interpoles.  This  solution  of  the  problem  should  therefore  be 
avoided  in  general. 

All  the  above  leads  to  the  fact  that  very  accurate  brush  set- 
ting is  required  on  interpole  types  of  machines,  and  furthermore, 
when  such  setting  is  once  obtained  it  should  not  be  capable  of 
ready  adjustment  or  change.  For  this  reason  interpole  machines 
should  not  have  any  brush  rocking  gear.  In  machines  where  such 
gear  is  present  it  would  be  better,  in  general,  if  the  brush  rocking 
mechanism  were  removed  after  the  proper  setting  of  the  brushes  is 
once  obtained,  and  means  should  be  employed  for  locking  the 
brushes  in  this  correct  position. 

The  correct  setting  of  the  brushes  is  rather  difficult  to  obtain 
in  many  cases.  Where  the  armature  conductors  can  be  traced 
from  the  commutator  bars  back  under  the  poles,  it  is  feasible  in 


PARALLEL  OPERATION  OF  D.C  MACHINES      309 

general  to  locate  the  correct  setting  by  the  position  of  the  com- 
mutated  coil  with  respect  to  the  interpoles.  In  standard  practice 
the  throw  or  span  of  the  coil  is  made,  as  nearly  as  possible,  equal  to 
the  pole  pitch.  In  a  parallel  type  of  winding  where  the  number  of 
slots  is  an  exact  multiple  of  the  number  of  poles,  the  space  of  the 
coil  can  be  made  exactly  equal  to  the  pole  pitch.  In  this  case  if 
the  winding  can  be  traced  through,  the  brushes  can  be  so  set  that  a 
coil  or  turn  exactly  under  the  middle  of  the  commutating  poles 
has  its  two  ends  connected  to  the  two  adjacent  commutator  bars 
which  are  symmetrically  short-circuited  by  the  brush;  that  is,  the 
insulating  strip  between  these  two  bars  should  be  under  the 
middle  of  the  brush.  To  carry  this  out  properly  it  is  necessary  to 
trace  the  conductor,  with  absolute  exactness,  through  the  slots 
When  there  are  several  separate  turns  side  by  side  in  one  slot,  it  is 
advisable  to  select  a  middle,  or  approximately  middle,  turn  for 
determining  the  brush  setting. 

In  the  case  of  a  2-circuit  or  series  winding,  it  is  more  difficult 
to  determine  the  brush  setting  by  tracing  out  the  coils,  for  the 
number  of  slots  in  such  windings  is  usually  not  an  exact  multiple 
of  the  number  of  poles  and  therefore  the  span  of  the  coil  is  not 
exactly  equal  to  the  pole  pitch.  In  this  case  the  position  of  the 
coil  must  be  averaged;  that  is,  one  edge  or  half  of  the  coil  may  be 
slightly  ahead  of  the  middle  point  of  its  interpole,  while  the  other 
half  is  slightly  behind  the  middle  of  the  interpole.  Even  if  the 
position  of  the  coil  is  properly  fixed  it  is  not  easy  to  fix  exactly 
the  corresponding  brush  setting,  as  the  two  commutator  bars  to 
which  the  coil  is  connected  do  not  lie  adjacent  to  each  other,  as  in 
a  parallel  type  of  winding,  but  are  two  neutral"  points  apart.  Also, 
the  number  of  commutator  bars  is  not  an  exact  multiple  of  the 
number  of  poles  (except  in  some  rare  cases  where  there  is  an  idle 
bar)  and  therefore  they  do  not  have  a  symmetrical  relation  to  the 
brushes.  The  best  that  can  be  done  therefore  is  to  average  the 
brush  position  as  well  as  possible. 

If  the  winding  is  chorded;  that  is,  if  it  has  a  span  considerably 
shorter  than  the  pole  tip,  then  its  position  will  have  to  be  averaged 
in  the  manner  described  above. 

In  some  cases  it  is  not  practicable  to  trace  out  the  coils  in  the 
above  manner,  as  the  end  windings  may  be  so  covered  that  it  is 
not  possible  to  trace  an  individual  coil  from  the  commutator  to  the 
dot. 


310  ELECTRICAL  ENGINEERING  PAPERS 

On  later  machines  it  is  the  practice  to  put  a  mark  or  "cross" 
on  the  tops  of  two  adjacent  armature  teeth  The  top  conductors 
which  lie  in  the  slot  between  these  two  teeth  are  connected  to 
commutator  bars  which  are  also  marked  with  a  cross  at  their 
outer  ends.  In  this  way  it  is  possible  to  trace  from  the  commut- 
ator to  the  slots. 

When  an  interpole  generator  is  running  alone,  or  where  it  is 
operating  properly  with  other  machines,  and  the  commutation  is 
satisfactory,  it  is  unnecessary,  of  course,  to  look  into  this  question 
of  locating  the  best  brush  setting.  In  those  cases,  however,  where 
the  machine  does  not  parallel  properly  with  others,  and  it  is 
evident  that  the  brush  setting  is  wrong,  then  if  the  above  procedure 
cannot  be  followed,  a  better  brush  setting  can  be  found  by  deter- 
mining the  voltage  characteristics  of  the  armature  This  can  be 
done  by  operating  the  machine  with  various  loads  with  the  series 
winding  cut  out  of  circuit.  If,  under  this  condition,  the  voltage 
either  rises  with  increase  in  load,  or  droops  but  very  little,  then  it 
is  evident  that  a  greater  forward  lead  would  improve  the  opera- 
tion. The  brushes  could  then  be  shifted  slightly  forward  and  the 
regulation  noted.  After  a  brush  setting  has  been  obtained  which 
gives  a  considerably  larger  drop  in  the  voltage,  then  parallel  oper- 
ation should  again  be  tried  with  this  brush  setting,  the  series  coils, 
of  course,  being  connected  in  circuit.  After  proper  paralleling  is 
obtained,  then  it  may  be  necessary  to  re-adjust  the  strength  of  the 
commutating  field.  If  the  machine  has  had  a  considerable  back- 
lead  before  and  is  shifted  to  the  no-lead  condition,  then  it  may  be 
necessary  to  weaken  the  interpole  winding  somewhat.  If  the  new 
brush  setting  however,  should  correspond  to  as  much  forward 
lead  as  it  had  back  lead  before,  then  the  interpole  strength  may  not 
require  readjustment  and  the  commutation  may  be  just  as  good 
as  before.  After  the  proper  conditions  have  been  obtained,  the 
brush  holder  position  should  be  marked  so  that  it  can  be  readily 
found  again  if  necessary. 

There  is  another  feature  wherein  an  interpole  machine  is 
different  from  the  non-interpole  type,  namely,  in  the  amount  of 
series  winding.  In  the  non-interpole  type  the  brushes  are  usually 
given  a  very  considerable  forward  lead.  In  consequence  of  this 
forward  lead  a  part  of  the  armature  ampere  turns  are  actually 
effective  in  demagnetizing  the  field,  and  extra  series  turns  are 
necessary  simply  to  overcome  this  demagnetizing  effect,  without 
accomplishing  any  useful  result. 


PARALLEL  OPERATION  OF  D  C,  MACHINES  311 

On  the  interpole  type,  however,  with  the  brushes  set  property 
there  is  no  lead  at  the  brushes  and  therefore  none  of  the  armature 
turns  are  tending  to  directly  oppose  the  main  field.  In  conse- 
quence of  this  the  number  of  series  turns  may  be  reduced  and  the 
resistance  of  the  series  coils  is  correspondingly  reduced.  When 
operating  interpole  machines  in  parallel  with  other  types  it  may 
be  necessary  to  increase  the  resistance  of  the  series  circuit  in  order 
that  the  interpole  machine  may  take  its  proper  share  of  the  current 
through  the  series  coils.  This  result  is  obtained  best,  in  general, 
by  a  resistance  connected  in  series  with  the  series  coil  and  not  by  a 
shunt  connected  across  the  series  coils  of  the  other  machines.  A 
shunt  across  a  series  coil  of  one  machine  is,  in  reality,  a  shunt 
across  all  the  machines  which  are  operating  in  parallel,  and  it  may 
be  more  effective,  in  one  machine  simply  because  of  the  resistancr 
of  the  leads  connecting  the  various  machines.  These  statements 
apply  to  other  types  of  machines  as  well  as  the  interpole. 


HIGH  SPEED  TURBO-ALTERNATORS—DESIGNS  AND 
LIMITATIONS 

FOREWORD — This  paper  was  prepared  for  the  American  Institute 
of  Electrical  Engineers  at  the  request  of  the  Power  Station 
Committee  of  the  Institute.  It  was  presented  in  January,  1913 
It  contains  a  quite  complete  description  of  the  two  principal 
types  of  turbo  alternator  rotors  up  to  that  time.  In  the  latter 
part  of  the  paper,  it  takes  up  the  problems  of  ventilation,  tem- 
perature and  insulation  from  the  turbo-generator  standpoint. 
Attention  was  called  to  the  high  temperature  liable  to  be  en- 
countered in  very  wide  core  machines,  such  as  turbo-generators, 
due,  to  a  certain  extent,  to  mechanical  limitations.  The  neces- 
sity for  the  use  of  mica  in  such  windings  was  also  brought  out. — 

(ED.) 


THE  real  problems  in  the  design  of  turbo-alternators  did  not 
really  develop  until  the  high-speed,  large  capacity  units  came 
into  demand.  In  the  earlier  work,  the  difficulties  in  design  were 
mostly  those  due  to  lack  of  experience  and  to  insufficient  knowl- 
edge of  the  possibilities  of  materials,  etc.  As  more  data  were 
obtained,  the  speeds  and  capacities  were  gradually  increased 
until  with  the  present  large  capacities  and  high  speeds,  a  number 
of  conditions  are  encountered  which  may  be  considered  as  true 
physical  limitations. 

The  principal  difficulties  in  the  design  of  the  earlier  machines 
were  found  in  the  permissible  weight  on  bearings,  undue  noise 
due  to  the  open  construction  of  the  machines,  and  the  troubles 
incident  to  the  through-shaft  construction  of  the  rotor. 

The  bearing  problem  was  eliminated  by  securing  more  complete 
data,  which  showed  that  the  possibilities  in  this  feature  had 
hardly  been  touched  upon. 

The  solution  of  the  noise  problem  was  largely  one  of  enclosing 
the  machine.  The  noise  was  practically  eliminated,  but  the 
greater  problem  of  ventilation  then  developed. 

In  overcoming  the  difficulties  of  the  through-shaft  construc- 
tion, the  first  great  advance  was  made  in  the  direction  of  larger 
outputs  at  higher  speeds.  In  very  high-speed  machinesr  the 
diameter  of  the  shaft  in  the  rotor  core  is  necessarily  small.  As 
the  overall  diameter  of  the  core  is  comparatively  small,  it  fol- 
lows that,  after  allowing  for  the  slot  depth,  and  the  metal  in  the 

313 


314 


ELECTRICAL  ENGINEERING  PAPERS 


core  necessary  to  withstand  the  high  rotative  stresses,  there  is 
left  but  little  available  space  for  the  shaft.  About  600  kv-a. 
capacity  at  3600  rev.  per  min.  was  the  limit  with  this  construction. 
The  first  great  advance  in  this  problem  was  made  by  the  intro- 
duction of  rotors  without  the  through-shaft.  By  this  means, 
the  parts  of  the  shaft  adjacent  to  the  rotor  core  proper,  could 
be  very  much  heavier1  than  with  the  through-shaft  type, 
and  this  combined,  with  the  solid  rotor  core,  gave  great  stiffness 
or  rigidity  compared  with  the  former  through-shaft  type.  This 
allowed  much  larger  cores,  with  correspondingly  increased  out- 
puts. The  two-pole  parallel  slot  type  of  rotor  with  bolted-on 
shaft  construction,  as  described  later,  was  apparently  a  leader 
in  this  respect,  due  t6  mechanical,  rather  than  electrical,  char- 
acteristics, WKen  this  type  had  proved  to  be  a  successful  one, 
the  possible  capacities  of  two-pole  .3600-revolution,  60-cycle 
machines  at  once  jumped  from  600  to  1000  kv-a.,  and  this  was 


FIG.  1 

quickly  followed  by  1500,  2000  and  3000  kv-a.  units  at  3600 
revolutions.  Since  then,  the  increase  in  capacity  at  this  speed 
has  been  more  gradual,  but  has  been  carried  up  to  5000  kv-a. 
at  present,  with  possibilities  of  a  6250  kv-a.  unit: 

The  radial  slot  type  of  rotor,  also  described  later,  when  con- 
structed with  its  core  and  shaft  in  one  piece,  quickly,  followed  the 
parallel-slot  type  in  the  above  growth,  and  may  eventually  catch 
up  with  its  only  rival  in  the  two-pole,  60-cycle  field  of  construc- 
tion. 

About  the  same  timfe  that  the  through-shaft  type  was  super- 
seded in  the  two-pole,  60-cycle  machine,  a  corresponding  change 
was  made  in  the  two-pole,  25-cyde,  and  in  four-pole  .rotors  for 
both  frequencies,  so  that,  at  the  present  time,  practically  no- 
designs  for  the  highest  speed  machines  use  the  through-shaft 
type  of  construction.  This  latter,  however,  has  been  retained 
in  some  of  the  more  moderate  speed  large  capacity  units. 


TURBO-GENERA  TORS 


315 


On  account  of  the  high,  rotative  and  peripheral  speeds,  the 
general  design  of  large  capacity  turbo-generators  turns  upon  the 
type  and  construction  of  the  rotor,  rather  than  the  stator. 
Various  designs  and  types  of  rotors  have  been  developed  but, 
with  rare  exceptions,  only  two  general  types  are  now  built  in 
this  country.  These  may  be  designated  as  the  radial-slot  and 
the  parallel-slot  types.  Each  has  a  number  of  advantages  over 
its  rival  and  each  has  given  good  results  in  practice. 

RADIAL  SLOT  TYPE  OF  ROTOR 

In  the  radial  slot  type,  as  usually  constructed  for  high-speed 
machines,  the  core  and  shaft  are  forged  in  one  piece  in  the  Smaller 
and  more  moderate  sizes,  but  may  be  built  up  of  a  number  of 
separate  plates  or  disks  bolted  rigidly  together  in  the  larger  sizes. 
In  this  type,  the  core  is  cylindrical  in  all  cases,  and  in  the  outside 
surfaces  are  radial  slots,  usually  arranged  in  groups,  in  which  the 


FIG.  2 

exciting  windings  are  placed.  While  all  radial  slot  types  of 
rotors  bear  a  general  resemblance  to  each  other,  yet  there  are 
marked  differences  in  the  method  of  forming  the  slots  and  teeth 
which  constitute  the  outer  surface.  In  some  types  the  solid 
rotor  core  has  radial  slots  milled  or  slotted  in  the  main  body  of 
the  core.  In  other  cases  the  slots  are  formed  outside  the  main 
core  by  inserted  teeth,  usually  with  overhanging  tips,  between 
which  the  exciting  coils  lie.  These  two  general  constructions 
are  illustrated  in  Fig.  3.  Examples  of  the  inserted-tooth  con- 
struction are  found  in  the  large  moderate  speed  rotors  of  one 
American  company,  and  in  somewhat  higher  speed  machines  of 
a  German  company.  However,  with  the  advent  of  the  high- 
speed, high-capacity  machines,  the  milled-in  construction  of 
the  radial  slots  appears  to  be  taking  the  lead,  due  to  certain 
mechanical  limitations  in  the  inserted-tooth  types. 

On  account'  of  the  radial  slots  and  the  usual  concentric  arrange- 


316 


ELECTRICAL  ENGINEERING  PAPERS 


men!  of  the  exciting  coils,  the  field  or  exciting  turns  cannot  be 
assembled  and  insulated  before  placing  on  the  core,  except  in 
the  inserted-tooth  type  of  construction.  With  the  milled-in-slot 
type,  the  field  conductors,  usually  of  flat  strap,  are  dropped  into 
the  slot  one  at  a  time,  with  insulation  between  individual  turns. 
For  ease  of  winding,  the  ends  are  usually  allowed  to  overhang 
the  core,  and  require  a  very  ample  outside  support  in  the  very 
high  speed  machines  This  is  illustrated  in  Fig.  4.  The  com- 
pleted coils  are  usually  held  in  place  by  strong  non-magnetic 
wedges  in  the  tops  of  the  slots.  These  wedges  are  usually  carried 
by  overhanging  pole  tips,  in  the  inserted-tooth  type,  or  by  grooves 
in  the  sides  of  the  slots  in  the  milled-slot  type  The  design  of 
the  supports  for  the  overhanging  end  windings  has  furnished  one 
of  the  difficult  problems  in  this  type  of  construction  Examples 


FIG.  3 


FIG.  4 


of  radial  slot  end  windings,  and  of  the  rotor  complete,  are  shown 
in  Figs.  5  and  6. 

This  general  construction  of  the  radial  slot  type  of  rotor  is 
obviously  applicable  to  machines  of  any  number  of  poles.  With 
a  two-pole  machine  there  will  be  only  two  groups  of  coil  slots  and 
two  groups  of  concentric  coils,  while  with  four  poles  or  six  poles 
there  will  be  four  or  six  groups  respectively.  It  is  evident  that, 
with  this  construction,  a  cylindrical  rotor  is  obtained,  regardless 
of  the  number  of  poles.  It  is  also  evident  that  the  problem  of 
supporting  the  end  windings  becomes  an  increasingly  difficult 
one,  as  the  number  of  poles  is  decreased  and  the  span  of  the 
end  windings  is  correspondingly  increased. 

The  support  over  the  end  ^findings  usually  consists  of  a  heavy 
ring  which,  in  very  high-spefed  machines,  must  consist  of  material 


TURBO-GENERA  TORS 


317 


FIG.  5 


318  ELECTRICAL  ENGINEERING  PAPERS 

having  extra  good  physical  characteristics,  for  this  ring  must 
not  only  be  able  to  carry  itself,  but  must  also  carry  the  weight 
of  the  underlying  end  windings  which  it  supports.  In  the  German 
inserted-tooth  rotor,  the  end  windings  are  supported  by  steel 
bands  of  many  layers,  instead  of  the  solid  steel  ring.  In  some  of 
the  lower  speed  radial  slot  machines,  such  as  one  American  type 
with  inserted  teeth,  the  end  supports  are  of  ring  form  usually 
made  in  a  number  of  sections,  which  are  bolted  to  an  inner  shelf 
by  numerous  bolts  extending  from  the  outer  ring  between  the 
coils  of  the  end  windings  to  the  inner  shelf.  While  this  construc- 
tion is  satisfactory  for  the  more  moderate  peripheral  speeds,  yet 
with  the  much  higher  speeds  in  some  of  the  later  practise,  this 
construction  has  been  superseded  by  a  solid  ring  type  of  support. 

PARALLEL-SLOT  TYPE  OP  ROTOR 

In  the  parallel-slot  type  of  rotor,  the  slots  for  the  exciting  coils, 
for  any  number  of  field  poles,  lie  in  planes  parallel  to  one  another 
and  to  the  rotor  axis.  The  arrangement  is  illustrated  by  Fig  7. 
As  usually  constructed,  the  slots  are  cut  across  the  ends  of  the 
poles,  as  well  as  in  the  sides,  so  that  the  exciting  coils  are  em- 
bedded in  metal  throughout  their  length.  The  object  of  this 
general  arrangement  of  parallel  slots  is  to  facilitate  the  winding 
of  the  exciting  coils.  The  rotor  can  be  placed  upon  a  turn-table, 
or  similar  device,  and  rotated,  to  wind  the  coils  in  place  under 
tension.  Two  or  more  coils  can  be  wound  at  the  same  time,  as 
is  actually  done  in  practice.  As  the  coils  can  be  wound  under 
tension,  and  as  the  conductors  usually  consist  of  thin  flat  strap, 
which  can  be  wound  in  very  tightly,  the  resultant  winding  is  a 
very  substantial  piece  of  work.  The  finished  winding  is  sup- 
ported by  metal  wedges  over  the  coils. 

It  is  obvious  that,  with  this  construction,  no  external  support 
is  required  for  the  end  windings,  as  the  field  core  proper  furnishes 
the  necessary  support.  It  is  largely  on  account  of  this  feature 
of  well  supported  end  windings  that  the  parallel-slot  type  took 
a  leading  position  during  the  growth  of  .the  larger  two-pole, 
60-cycle  alternators.  With  the  radial-slot  type,  the  support 
of  the  end  windings  presented  a  more  difficult  problem  in  the 
large  capacity,  high-speed,  two-pole  machines,  which,  however, 
is  being  gradually  solved. 

In  the  two-pole,  parallel-slot  construction,  in  order  to  utilize 
the  available  winding  space  to  advantage,  it  is  necessary  for  the 
windings  to  cover  the  central  portion  of  the  core  end  where  the 


TURBO-GENERA  TORS 


319 


FIG.  7 


FIG.  8 


320  ELECTRICAL  ENGINEERING  PAPERS 

shaft  is  usually  attached,  as  shown  before  m  Figs  7  and  8  There- 
fore, with  this  construction,  a  separate  "  head  "  or  driving  flange 
must  be  bolted  to  the  core-  at  each  end,  this  head  carrying  the 
shaft,  as  shown  in  Fig  8.  To  avoid  magnetic  'shunting  of  the 
field  flux,  this  driving  head  must  be  made  of  non-magnetic 
material,  usually  of  some  high  grade  bronze,  to  which  the  shaft 
is  attached  in  such  a  way  as  to  keep  the  magnetic  leakage  as 
low  as  possible.  This  makes  a  good  strong  construction,  but  is 
necessarily  rather  expensive,  due  tp  the  bronze  driving  heads. 
As  these  cost  but  little  more  for  a  long  rotor  than  for  a  short  one, 
the  construction  therefore  tends  toward  relatively  long,  small 
diameter  cores  in  order  to  lessen  the  relative  dimensions  of  the 
bronze  heads. 

In  two-pole,  single  phase  machines  of  this  construction,  the 
copper  cage  damper  for  suppressing  the  armature  pulsating  re- 
action on  the  field,  is  comprised  partly  of  these  bronze  heads, 
which  form  the  "  end  rings  "  for  the  copper  bar&  embedded  in 
the  slots  in  the  rotor  face. 

In  the  four-pole,  parallel-slot  machine,  no  bolted-on  driving 
leads  are  necessary,  for  the  core  proper  and  the  shaft  may  be 
:ast,  or  forged,  in  one  piece,  or  in  two  or  more  pieces,  which  are 
Doited  or  "  linked  "  together  to  form  a  solid  core.  The  principal 
lifference  between  the, two-pole  and  the  four-pole  parallel  slot 
Constructions,  is  that  the  latter  must  have  salient  or  projecting 
Doles,  in  order  to  utilize  the  parallel  construction  for  the  slots, 
vhile  the  two-pole  machine  is  preferably  made  cylindrical.  Fig. 
illustrates  this  feature. 

It  is  evident  that  there  is  considerable  available  space  lost  by 
fie  openings  between  the  projecting  poles,  while  the  sections  of 
he  poles  themselves  are  cut  down  very  materially  by  the  slots 
Dr  the  exciting  winding.  The  limitations  therefore  in  such  a 
otor  are  in  the  magnetic  section  of  the  field  poles  and  in  the 
vailable  copper  space,  and  in  these  features  the  four-pole  parallel 
!ot  rotor  is  inferior  to  the  radial  slot  type.  In  the  two-pole 
lachine,  however,  the  difference  between  the  radial  slot  and  the 
arallel  slot  is  not  nearly  so  pronounced,' as  is  indicated  in  Fig.  10 
here  the  two  arrangements  are  shown  on  one  core  for  compari- 
)n.  It  may  be  seen  from  this  that,  in  the  iwo-pole  form,  the 
*o  constructions  approach  each  other,  to  a  certain  extent, 
>me  of  the  slots  in  the  parallel  construction  being  radial,  while 
,hers  depart  but  little  from  the  radial.  One  disadvantage  in  the 
ro-pole,  parallel-slot  type,  however,  lies  in  the  smaller  amount  of 


TURBO-GENERA  TORS 


321 


copper  space  which  is  obtained,  for  the  slot  space  must  necessarily 
cover  a  less  proportion  of  the  total  circumference  than  is  permis- 
able  with  the  radial  slot  type.  This  winding  space  is  limited  by 
the  physical  requirements  as  regards  bending  and  breaking 

strains  in  the  overhanging  tip  a 
in  Fig.  10.  In  the  radial  slot 
type,  the  slot  space  has  no  such 
limitation.  Also,  on  account  of 
the  grouping  of  the  field  copper 
into  a  narrower  zone  in  the 
parallel-slot  type,  the  heat  con- 
duction from  the  copper  presents 
a  more  difficult  problem  than  in 
the  radial  type. 

At  first  glance,  it  would  appear 
that  the  effective  length  of  the 
field  core  in  the  parallel-slot  type 
is  very  considerably  diminished  by  the  slots  across  the  ends 
of  the  core.  However,  this  is  only  an  apparent  effect,  for  the 
true  length  of  the  core  should  be  taken  as  that  inside  of  the 
winding  slots,  and  it  should  be  considered  that  the  additional 


FIG.  10 


PIG.  11 

length  of  the  core  at  the  pole  face  is  in  the  nature  of  a  coil 
support  which  takes  the  place  of  the  separate  support  in  the 
racial  slot  type.  Therefore,  if  over-all  lengths,  including 
rotor  coil  supports,  are  compared  in  the  two  types,  there 
is  but  little  difference,  as  indicated  by  Kg.  11.  However, 


322  ELECTRICAL  ENGINEERING  PAPERS 

if  the  armature  core  is  made  of  the  same  width  as  the  pole  face 
in  both  types  of  rotors,  then  in  the  parallel-slot  type  it  will  b 
materially  greater  than  in  the  radial,  for  the  over-hanging  pol 
tips  of  the  parallel-slot  machine  are  also  eifective  magneticall 
in  furnishing  flux  to  the  armature.  Therefore,  as  regards  th 
stator,  this  tends  toward  ?i  wider  core  in  the  axial  direction,  an< 
a  shallower  depth  of  iron  back  of  the  armature  slots,  as  indicatfe* 
in  Fig.  12.  Also,  on  account  of  the  relatively  larger  polar  sur 
face,  in  the  parallel  slot  type  of  rotor,  the  magnetic  flux  densit; 
in  the  air  gap  is  usually  rela  tively  smaller  than  in  the  radial  slo 
type,  which  conduces  towards  a  larger  depth  of  air  gap.  Alsc 
on  account  of  the  larger  polar  surface,  the  available  space  fo 
armature  slots  and  teeth  is  correspondingly  increased.  There 
fore,  this  type  of  construction  is  better  adapted  for  the  straigh 
air  gap  method  of  ventilation,  as  will -be  described  later.  Th 
greater  section  available  for  slots  and  teeth  at  the  stator  pol 
face  permits  a  large  number  of  ventilating  ducts.  The  relatively 
large  depth  of  gap  allows  a  large  amount  of  air  to  be  fed  througl 
the  air  gap  to  the  ducts.  Therefore,  the  "  radial  "  type  of  stato 
core  ventilation  has  been  used  very  largely  with  this  type  o 
rotor  construction.  In  the  parallel-slot  type  of  rotor,  it  is  obviou 
that,  due  to  the  large  polar  surface  compared  with  the  minimun 
section  of  the  field  core,  a  limit  in  design  is  found  in  the  magneti< 
saturation  in  the  field  core  itself. 

In  the  four-pole  parallel-slot  rotor,  the  field  section  is  mor< 
limited  than  in  the  two-pole  machine,  due  to  the  fact  that  con 
siderable  magnetic  space  is  lost  by  the  notches  between  the  pro 
jecting  poles.  However,  in  this  type  of  construction,  the  aii 
gap  method  of  ventilation  is  relatively  easy,  due  to  the  fact  thai 
these  interpolar  spaces  furnish  easy  access  of  the  ventilating  aii 
to  the  stator  ventilating  ducts.  In  consequence,  the  problerr 
of  ventilation  is  usually  not  a  serious  one  in  this  type  of  rotor 
Due  to  the  polar  projections,  however,  the  tendency  to  noise  is 
obviously  greater  than  in  either  the  radial-slot  type  or  the  two 
pole  parallel  type,  which  are  always  cylindrical. 

Nothing  has  yet  been  said  as  to  the  peripheral  speeds  obtained 
in  some  of  the  actual  designs  of  th;*  higher  speed  generators. 
These,  in  themselves,  indicate  some  of  the  limitations  which  now 
confront  the  designer. 

In  the  5000  kv-a.,  two-pole,  3600-rev.  per  rnin.,  60-cycle 
generator  already  referred  to,  which  is  of  the  parallel-slot  rotor 
construction,  the  rotor  diameter  is  26  in.  (66  cm.)  This  gives  a 


TURBO-GENERA  TORS 


323 


FIG.  12 


FiO.  21 


324  ELECTRICAL  ENGINEERING  PAPERS 

peripheral  speed  of  408  ft.  (124.3  m  )  per  second,  or  approximately 
24,500  ft  (7468m)  per  minute  The  core  is  designed  for  .a 
very  considerable  margin  of  safety,  and  is  actually  tested  at 
overspecds  which  give  practically  30,000  ft.  (9144  m  )  peripheral 
speed  at  the  surface  pf  the  core 

In  certain  19,000  kv-a  ,  62J-cycle,  four-pole,  1875-rev  per  min. 
machines  now  being  built,  which  are  of  the  radial-slot  rotor 
construction,  the  rotor  diameter  is  49  in.  (124  4  cm  )  This  gives 
a  peripheral  speed  of  24,000  ft.  (7315  m.)  per  minute.  This 
compares  with  a  speed  of  21,600  ft.  (6583  m.)  in  a  21,000  kv-a. 
two-pole,  1500-rev  per  min.,  25-cycle,  radial-slot  machine  also 
being  built,  the  rotor  core  of  which  is  shown  in  Fig.  12.  Obviously 
the  mechanical  limitations  are  being  more  closely  approached  in 
the  60-cycle  machines,  up  to  the  present  capacities. 

If,  a  comparison  is  made  between  the  above  5000  and  19,- 
QOO  kv-a.  rotors,  with  their  parallel  and  radial  type  construc- 
tions, it  is  found  that  their  limitations  lie  in  quite  different 
features.  In  the  radial-slot  type,  the  core  stresses  are  much 
lower  than  in  the  rother,  but  the  supporting  end  ring  is  an  im- 
portant problem,  requiring  for  its  solution,  a  very  high  grade 
steel  for  the  material  of  the  ring.  In  the  parallel-slot  rotor,  the 
maximum  stresses  are  in  the  core  itself,  principally  in  the  parts 
which  overhang  the  slots  at  the  sides  and  ends  of  the  core.  In 
the  radial  slot  core,  there  are  no  such  overhanging  masses.  In 
both  construction's,  the  core  material  is  purposely  made  of  re- 
latively soft  steel,  having  a  high  percentage  elongation,  the  ob- 
ject being  to  obtain  a  material  which  can  yield  sufficiently  to 
transfer  the  strains  from  local  higher  points,  to  adjacent  lower 
parts,  and  thus  equalize  them,  to  a  great  extent. 

The  smaller  diameter  rotor  cores  are  made  of  steel  forgings, 
in  one  piece.  The  larger  cores  are  made  up  of  thick  steel  plates 
assembled  and  bolted  together  to  form  a  solid  jnass  comprising 
the  core  and  shaft  extensions.  By  this  disk  construction,  com- 
mercial material  is  used  which  is  of  uniform  quality  clear  to  the 
center  of  the  disks.  The  fiber  of  the  material  is  in  a  direction 
best  suited  to  the  directions  of  stress.  With  corresponding  sise 
disks  made  in  one  piece,  the  outside,  to  a  certain  depth,  can  be 
given  fair  physical  characteristics,  but  the  center  is  liable  to  be* 
glass-hard,  as  found  by  experience.  However,  this  may  not 
be  a  prohibitive  condition  in  machines  of  more  moderate  per- 
ipheral speeds.  Herein  lies  one  great  difference  between  American 
and  European  limitations.  In  American  practice,  60-cycles, 


T  URBO-GENERA  TORS  325 

calling  for  3600  and  1800-rev.  per  mm  machines,  is  the  standard 
frequency,  while  in  Europe,  50-cycles  is  standard,  giving  3000 
and  1500-rev.  per  min.  machines.  These  lower  speeds  make 
an  enormous  difference  in  the  possibilities  of  design  and  construc- 
tion 

PRESENT  LIMITATIONS  IN  DESIGN 

On  account  of  th§  very  great  capacities,  at  high  speeds,  now 
being  obtained  in  turbo-generator  practice,  a  number  of  problems 
are  being  encountered,  the  solutions  of  which  are  producing 
more  or  less  radical  changes,  both  in  design  alid  in  practise. 
Softie  of  the  limitations  now  encountered  are  in  the  relatively 
high  temperatures  in  certain  parts,  high  losses  in  a  relatively 
small  space,  the  difficulty  of  ventilation,  due  to  the  requirement 
of  endrmous  volumes  of  cooling  air  through  limited  openings 
or-  passages,  the  type  of  insulation,  fire  risks,  regulation  and 
short  circuit  conditions,  etc. 

A  number  of  these  limiting  conditions,  such  as  the  temperature, 
ventilation,  losses,  and  insulation,  are  so  closely  related  to  each 
othfer,  that  ft  is  difficult  to  describe  any  one  of  them  in  detail, 
without  including  the  others  to  a  considerable  extent. 

THE  PROBLEM  OF  VENTILATION 

Ip  the  general  problem  of  ventilation,  four  conditions  must 
be  considered,  namely,  the  total  loss,  or  heat,  developed,  the 
surface  exposed  for  dissipating  this  heat  to  the  &ir,  the  quantity 
of  air  required  to  carry  away  the  heat,  and  the  temperature 
of  the  cooling  air. 

In  the  conduction  of  heat  from  the  surface  of  a  body  into  the 
air,  the  quantity  of  heat  per  unit  $rea  which  can  be  dissipated 
depends  upon  the  difference  in  temperature  maintained  between 
the  surface  of  the  body  and  the  body  of  air  to  which  the  heat  is 
conducted.  The  heat  dissipated  raises  the  temperature  of 
the  adjacent  air  a  certain  amount,  and  thus  tends  to  reduce 
the  temperature  difference,  utoless  the  air  is  renewed  with  suffi- 
cient tapidity.  On  the  other  hand,  if  the  quantity  of  air  is  so 
great,  in  proportion  to  the  heat  dissipated,  that  there  is  but 
little  rise  in  the  air  temperature,  then  any  increased  amount 
of  air  over  the  surface  will  represent  practically  no  gain  in 
ventilation.  In  other  words,  when  the  amount  of  air  passed 
oyar  a  surface  is  sufficient  to  take  up  the  heat  dissipated  from 
the  surface  without  an  undue  rise,  then  a. further  quantity  of 
air  is  wasteful,  and  it  may  even  be  considered  as  indirectly 


326  ELECTRICAL  ENGINEERING  PAPERS 

harmful,  in  those  cases  where  the  total  quantity  of  air  is  limited 
This  has  a  direct  bearing  on  the  size  of  ventilating  ducts  or 
passages  in  a  machine.  If  the  air  path  through  a  duct  is  relatively 
long,  then  a  considerable  width  of  duct  may  be  required  in  order 
to  get  the  necessary  quantity  of  air  through  it.  On  the  other 
hand,  if  the  air  path  is  very  short,  then  a  very  narrow  duct 
may  be  most  effective,  for  a  wider  duct  may  allow  more  air  to 
pass  through  than  can  be  utilized  in  taking  up  the  heat. 

No  matter  how  thoroughly  the  ventilating  air  is  distributed 
through  the  heat-generating  body,  or  however  effective  the 
heat-dissipating  surfaces  may  be,  the  total  air  supplied  must  be 
ample  in  quantity,  or  its  temperature  will  be  raised  an  undue 
amount.  As  the  surfaces  to  be  cooled  must  always  have  a 
higher  temperature  than  the  cooling  air,  any  considerable  rise 
in  the  latter  will  have  a  direct  influence  on  the  ultimate  tempera- 
ture which  may"  be  attained  by  the  body  to  be  cooled.  Con- 
versely, if  an  ample  quantity  of  cooling  air  is  supplied,  but  the 
heat-dissipating  surfaces  are  insufficient,  the  ultimate  tempera- 
ture of  the  body  will  also  be  affected. 

In  large  capacity,  high-speed  turbo-generators,  the  problem 
of  ventilation  is  one  of  the  most  difficult  ones  encountered,  The 
trouble  lies  principally  in  the  large  total  loss  expended  in  a  very 
limited  space.  The  difficulties  of  the  problem  may  be  illustrated 
by  the  following  example : 

Assume,  in  a  1500-rcv.  per  min  ,  25-cycle,  15,000-kv-a 
machine,  a  total  efficiency  of  96  5  per  cent,  including  air  friction 
loss  inside  of  the  machine.  This  means  a  total  loss  in  the  machine 
of  565  kw.,  which  is  not  excessive  for  this  capacity,  but  is  very 
large  for  the  limited  space  in  which  it  is  developed  A  very 
large  volume  of  cooling  air  is  required  for  carrying  away  the 
heat  due  to  this  loss.  A  simple  approximate  rule  for  determining 
the  quantity  of  air  required  is  that  an  expenditure  of  one  kw, 
in  one  minute  will  raise  the  temperature  of  100  cu.  ft  (2.8  cu. 
m.)  of  air  18  deg.  cent.  Therefore,  565  kw.  loss  would  require  a 
supply  of  ventilating  air  of  approximately  50,000  cu.'ft.  (1416 
cu.  m.)  per  minute  for  a  rise  of  the  out-going  air  of  20  deg.  above 
that  of  the  incoming  air.  Assuming  a  velocity  of  3000  ft  (914  m,) 
per  minute,  this  would  mean,  with  a  cylindrical  ventilating 
channel,  a  diameter  of  56  in,  (142.2  cm  ),  which  is  greater  than 
the  rotor  diameter  itself.  However,  as  the  cooling  air  ordinarily 
\vould  be  supplied  to  both  sides  of  the  machine,  the  ventilating 
passage  need  only  be  half  the  above  section  for  each  side. 


TURBO-GENERATORS  327 

Obviously,  such  passages  arc  prohibitively  large,  and  much 
greater  air  velocities  through  the  machine  proper  are  necessary 
Velocities  as  high  as  5000  to  6000  ft.  (1524  to  1828  m  )  per 
minute  are  common,  while,  in  some  cases,  more  than  10,000 
ft.  (3048  m.)  per  minute  has  been  required  m  certain  constricted 
sections  of  the  air  path  inside  the  machines  Therefore,  no 
matter  how  the  problem  is  considered,  it  may  be  seen  that 
the  above  condition  of  the  enormous  volume  of  air  required, 
makes  the  problem,  of  ventilation  a  difficult  one 

There  are  several  methods  of  ventilating  large  turbo  generators, 
depending  upon  the  system  of  applying  the  air.  There  is,  first, 
the  radial  system,  in  which  practically  all  the  cooling  air  passes 
out  radially  through  ventilating  ducts  in  the  stator  core.  This 
radial  system  of  ventilating  can  be  subdivided  into  two  alterna- 
tive methods,  depending  upon  whether  the  air  is  partly  or 
wholly  supplied  through  passages  in  the  rotor,  or  through  the 
air  gap  alone.  These  two  methods  are  illustrated  m  Fig  13 
The  straight  air  gap  arrangement  may  require  a  relatively  large 
air  gap,  combined  with  very  high  velocity  of  the  air  along  the 
gap,  while  the  other  method  permits  a  considerably  shorter  gap. 
The  straight  air  gap  method  of  ventilation  is  used,  to  a 
considerable  extent,  in  all  60-oycle  machines  of  two-pole  con- 
struction, while  it  is  practically  the  only  one  that  has  been  used 
with  the  parallel-slot  type  of  machine  with  either  two  or  four 
poles  In  this  parallel-slot  type  of  rotor,  however,  the  air  gap  can 
be  relatively  larger  than  the  radial-slot  type  of  rotor,  as  explained 
before,  which  compensates,  to  some  extent,  for  the  necessity  of 
depending  upon  this  method  entirely.  In  the  four-pole  parallel- 
slot  rotors,  the  interpolar  spaces  are  also  effective.  Moreover, 
with  parallel-slot  rotors  in  general,  the  openings  from  the  air 
gap  into  the  stator  ventilating  ducts  can  usually  be  somewhat 
larger  in  total  section  than  with  the  radial  type  of  rotor,  as  also 
described  before.  However,  the  relatively  greater  axial  length 
of  the  core  of  the  parallel  slot  type  of  rotor  increases  the  length 
of  the  constricted  air  passages  along  the  air  gap  in  the  two-pole 
machines,  which  is  a  material  disadvantage 

The  straight  air  gap  type  of  ventilation  has  proven  astonish- 
ingly effective  in  cooling  the  rotor  in  both  the  radial  and  parallel- 
slot  types  of  rotors,  and  with  either  type  there  is  usually  no 
great  difficulty  in  forcing  through  enough  air  to  cool  the  rotor 
core  in  a  fairly  effective  manner.  It  must  be  considered,  however,, 
that  the  total  rotor  loss  in  large  turbo-generators  is  possibly 


328 


ELECTRICAL  ENGINEERING  PAPERS 


only  10  per  cent  of  the  total  loss  which  must  be  taken  care  of,  and 
a  relatively  small  proportion  of  the  total  ventilating  air  may 
suffice  to  cool  it.  According  to  actual  measurements,  corrobor- 
ated by  general  experience,  the  cylindrical  surface  of  the  rotor 
core  can  give  off  four  or  five  watts  per  square  inch  (6  45  sq. 
cm.)  to  the  cooling  air,  with  a  temperature  rise  of  the  rotor 
surface  of  about  35  to  40  deg.  cent,  above  the  cooling  air.  To 
those  who  have  had  experience  with  dissipating  heat  from  electric 
apparatus,  this  result  will  appear  to  be  extremely  good. 

The  real  difficulty  with  the  air  gap  method  of  ventilation, 
is  not  so  much  in  getting  enough  air  through  for  cooling  the 


FIG.  13 


FIG.  14 


rotor  itself,  but  it  is  in  the  much  larger  quantity  required  for 
the  stator.  For  instance,  a  one-inch  (2.54  cm.)  depth  of  gap 
from  iron  to  iron)  with  a  50-in.  (127-cm.)  diameter  of  rotor, 
means  a  total  section  of  air  path  into  the  gap  (counting  both 
ends  of  rotor)  of  314  sq.  in.  or  2.18  sq.  ft,  (0.19  sq.  m.)-  At  a 
velocity  of  10  000  ft.  (3048  m )  per  minute,  this  allows  a  flow 
of  only  21  800  cu.  ft.  (617  cu.  m)  per  minute,  which  will  not 
take  care  of  a  large  machine,  from  the  present  standpoint  of 
possible  capacities  with  the  above  diameter  of  rotor.  By 
additional  openings  in  the  rotor  core,  this  might  be  increased  to 
30000  cu.  ft.  (849  cu.  m.)  per  minute,  but  even  this  is  still 


T  URBO-GENERA  TORS  329 

much  less  than  a  machine,  with  a  50-in.  (127-cm.)  diameter  of 
rotor,  would  require  if  built  for  capacities  otherwise  possible. 
Therefore,  on  account  of  this  limitation  in  the  amount  of  cooling 
air,  other  means  of  ventilation  have  received  much  considera- 
tion. Two  other  general  systems  of  ventilation,  in  addition  to 
the  gap  method,  have  been  used,  namely,  the  circumferential 
method,  and  the  axial  The  former  has  been  developed  and 
applied  more  extensively  in  the  past,  but  the  latter  contains 
possibilities  which  are  bringing  it  rapidly  to  the  front. 

In  the  circumferential  method  of  ventilation,  air  is  supplied 
to-one  or  more  points  on  the  outside  circumference  of  the  stator, 
and  is  forced  circumferentially  around  through  the  air  ducts  to 
suitable  outlets,  also  on  the  outside  surface.  Air  gap  ventilation 
is  usually  combined  with  this  circumferential  method,  partly  to 
cool  the  rotor.  The  general  arrangement  is  indicated  diagram- 
matically  in  Fig.  14,  in  its  simplest  form,  namely,  with  one  inlet 
and  one  outlet  diametrically  opposite.  A  serious  objection  to 
this  method  of  ventilation  is  found  in  the  limited  section  of  the 
ventilating  path.  Assuming,  for  example,  a  depth  of  stator 
core  of  20  in.  (50.8  cm.)  outside  the  armature  slots  and  a  total 
of  40  f-in.  (9.5  mm.)  ventilating  ducts,  or  a  total  effective  duct 
space  of  15  in.  (37.1  cm.)  width  then  this  gives  a  total  section 
of  ventilating  path  of  20  X  15  X  2  =  600  sq.  in.,  or  4. 16  sq.  ft. 
(0.386  sq.  in.).  On  account  of  the  relatively  great  length  of  the 
ventilating  path,  air  velocities  of  more  than  6000  to  7000  ft. 
(1828  to  2133  m.)  are  not  desirable  or  economical,  but  even 
with  10,000  ft.  (3048  m.)  velocity,  the  total  quantity  of  air 
would  be  only  41,600  cu.  ft,  (1166  cu.  m.)  per  minute.  Further- 
more, this  method  is  handicapped  in  machines  with  very  high- 
speed rotors,  by  interference  between  the  radial  and  the  cir- 
cumferential systems  of  ventilation,  so  that  the  full  benefit  of 
either  is  not  obtained.  Below  a  certain  rotor  velocity,  apparently 
the  circumferential  action  can  predominate,  and  the  method  is 
fairly  effective  up  to  the  permissible  air  capacity  of  the  stator 
ducts ;  but  at  very  high  speeds  the  radial  ventilation  may  very 
seriously  interfere  with  the  other,  so  much  so,  that  the  radial 
ventilation  alone,  even  with  its  very  restricted  gap  section,  may 
give  as  good  results  as  the  two  methods  acting  together. 

To  avoid  this  interference,  various  methods  have  been  devised, 
such  as  closing  part,  or  all,  of  the  radial  ventilating  ducts  at  the 
air  gap  to  keep  the  radial  effect  from  interfering  with  the  other. 
One  arrangement  which  has  been  used  in  Europe  to  a  considerable 


3£0  ELECTRICAL  ENGINEERING  PAPERS 

extent  is  indicated  in  Fig   15.    In  this,  the  alternate  radial  air 

ducts  are  closed  at  the  outside  surface,  while  all  are  closed  at 

the  air  gap-     The  air  enters  by  the  ducts  open  at  the  back  of 

the  machine,  flows  both  circumferentially  and  toward  the'  gap, 

and  crosses  over  to  the  immediate  ducts  by  axial  opeiiings  back 

of  the  armature  teeth,  and  then  along  these  ducts  to  the  outlet. 

This  scheme  is  effective  in  principle,  but  is  uneconomical  in  the 

sense  that  less  than  the  total  section  of  stator  duqts  is  useful, 

as  regards  the  quantity  of  air  which  can  be  carried.     There  is 

usually  one  large  central  duct  to  allow  an  outlet  for  the  rotor 

ventilating  air.    This  particular  arrangement  of  the  stator  also 

uses  axial  ventilation  in  crossing  over  from  oiie«set  of  ducts  to 

the  other,  which  is  an  effective  arrangement. 

A    modification   of   the    simple   circumferential   method    of 


J 

pnrnonnn 

v 

FIG.  J5 


ventilation  is  to  admit  air  to  the  back  of  the  stator  at  two  oppo- 
site sides  of  the  machine,  and  deliver  it  at  two  outlets  at  inter- 
mediate points  on  thej  surface,  as  shown  diagrammatically  in 
Fig.  16.  By  this  means,  the  cross  section  of  the  ventilating 
path  is  doubled  and  the  length  is  .halved.  Also,  the  interference 
of  the  radial  ventilation  with  the  circumferential  will  be  less 
harmful.  A  serious  disadvantage  in  the  Circumferential  venti- 
lation in  general  is  that  the  ventilating  path  is  relatively  long, 
especially  where  there  is  but  one  inlet  and  outlet,  and  therefore 
the  cooling  air  at  the  outlet  of  the  channel  may  be  considerably 
hotter  than  at  the  inlet,  with  consequent  less  effective  cooling 
action.  This  means  points  of  local  higher  temperature  in  the 
core,  due  to  the  method  ,of  ventilation.  In  the  radial  type  of 
ventilation,  the  coolest  air  is  applied  near  the  seat  of  the  highest 
losses,  namely,  at  the  armature  teeth,  and  immediately  back  of 


TURBO-GENERA  TORS 


331 


them,  and  the  air,  as  it  becomes  heated,  passes  over  the  outer 
part  of  the  iron  which  has  a  diminished  loss,  and  therefore 
normally  less  heat  to  dissipate.  Therefore,  the  effect  of  the  in- 
creased temperature  of  the  cooling  medium  is  offset  by  the  lower 
Loss  and  .consequent  less  necessity  for  ventilation,  in  the  part 
where  the  air  is  hottest.  The  radial  system  of  cooling  is  therefore 
theoretically  the  most  effective,  but  practically,  the  difficulty 
•s  in  applying  it,  due  to  the  limited  air  passages  available. 

BoUk  the  circumferential  and  the  radial  methods  of  cooling 
arre  subject  to  one  serious  defect,  namely,  most  of  the  generated 
heat  in  the  stator  iron  must  be  conducted  across  the  lamina- 
tions to  the  air  ducts.  The  rate  of  conduction  across  the"  lamina- 


FIG.  16 


tions  is  only  from  1  per  cent  to  10  per  cent  as 'great  as  along  the 
laminations  themselves,  according  to  various  authorities. 
Therefore,  if  the  heat  could  all  be  conducted  along  the  lamina- 
tions to  the  ventilating  surfaces,  apparently  much  more  effective 
heat  dissipation  could  be  obtained,  provided  sufficient  surface 
is  exposed  to  the  air,  and  an  ample  quantity  of  air  supplied. 
This  has  led  to  the  development  of  the  axial  system  of  ventila- 
tion, as  distinguished  from  the  radial  and  circumferential. 
In  this  method,  a  large  number  of  axial  holes  are  provided  in 
the  stator  core  which  may  extend  uninterruptedly  from  one 
side  of  the  core  to  the  other,  or  they  may  extend  from  each  side 
to  one  or  more  large  central  radial  channels  which  form  the  outlet. 
The  usual  numerous  radial  ducts  are  omitted,  or  may  be  con- 


332  ELECTRICAL  ENGINEERING  PAPERS 

sidered  as  combined  in  one  central  channel.  This  general 
arrangement  is  illustrated  in  Fig.  17.  The  rotor  cooling  is 
accomplished  by  air  along  the  air  gap,  and  through  the  rotor 
core  to  the  large  central  duct.  In  this  method  of  ventilation 
therefore,  there  is  a  combination  of  two  types,  namely,  the  axial 
and  the  air  gapy  but  there  is  not  the  interference  between  the 
two,  that  is  sometimes  found  where  the  circumferential  method  is 
used. 

From  the  preceding,  it  may  be  seen  that  the  problem  of 
putting  a  sufficient  quantity  of  air  through  the  machine  is  an 
extremely  difficult  one.  In  addition,  in  very  large  machines, 
the  problem -of  supplying  the  required  quantity  of  air  from  a 
suitable  blower  forms  another  serious  problem.  In  smaller 
capacities,  and  in  slower  speed  machines,  it  has  been  the  usual 
practise  to  attach  blowing  fans  to  the  rotor  shaft  or  core,  as 


nonnnn 


FIG.  17 

part  of  the  outfit.  There  is  no  particular  difficulty  in  this 
arrangement,  except,  possibly,  in  the  high-speed  construction 
of  the  fans  required  for  60-cycle,  two-pole  machines.  Such 
fans  can  supply  an  amount  of  air  whieh  is  limited  by  the  diameter 
and  other  dimensions  of  the  fan  itself. 

Assume,  for  example,  that  by  lengthening  the  rotor  core,  or 
by  other  modifications  in  the  construction,  the  capacity  of  the 
machine  can  be  doubled,  and  therefore  double  the  quantity  of 
air  is  required  for  cooling.  If  the  limit  of  the  fan  design  or 
operation  was  reached  before,  then  obviously  some  radical 
change  is  required  with  the  new  capacity  of  the  machine.  This 
condition  apparently  has  been  reached  in  some  of  the  later 
practise  in  large,  high-speed  turbo-alternators.  One  ot>vious 
solution  of  this  difficulty  lies  in  the  use  of  separate  slower  speed, 
large  diameter,  fans  or  blowers.  This  may  appear  to  be  a  step 
backward,  but  when  the  above  conditions  and  limitations  are 


T  URBO-GENERA  TORS  333 

taken  into  account,  it  is  not  so.  The  "  tail  "  must  not  be 
allowed  to  '"wag  the  dog;"  the  blower,  which  is  an  adjunct, 
must  not  be>allowed  to  dominate  the  construction  of  the  machine 
itself.  Moreover,  there  are  a  number  of  meritorious  features  in 
the  use  of  a  separate  blower.  In  the  first  place,  it  can  be  made 
somewhat  more  efficient  than  the  high-speed,  rotor-driven  fans. 
Again,  with  a  suitable  means  to  drive,  variable  speeds,  and 
therefore  different  air  pressures,  can  be  obtained.  This  feature 
may  prove  to  be  very  desirable  or  advantageous  under  peak, 
or  overload,  or  emergency  conditions. 

One  further  condition  keeps  cropping  out  in  the  general 
problem  of  ventilation,  -namely,  that  of  filtering  or  washing,  or 
otherwise  cleaning  the  ventilating  air  With  50,000  to  75,000 
cu.  ft.  (1415  to  2122  cu.  m.)  of  air  per  minute  passing  through 
a  large  machine,  obviously  in  a  year's  time,  an  enormous  quan- 
tity of  foreign  matter  is  carried  through  the  machine  with  the 
ventilating  air.  A  deposit  of  a  very  small  per  cent  of  this  in  the 
machine  will  probably  be  disastrous.  In  fact,  however,  the 
high  velocity  of  the  air  through  the  machine  serves  to  keep  the 
air  passages  clear  if  no  oil  or  moisture  is  allowed  to  enter.  That 
a  large  amount  of  foreign  matter  does  go  through  the  machine 
is  very  soon  shown  in  case  a  little  oil  is  allowed  to  get  into  the 
ventilating  passages.  This  oil  catches  the  dirt  and  in  a  short 
time  the  ventilating  passages  may  be  very  materially  obstructed. 

On  account  of  the  deposit  of  dust,  etc.  in  the  ventilating 
passages,  it  is  necessary  to  clean  certain  types  of  machines  at 
more  or  less  frequent  intervals,  and  it  is  advisable  to  clean  all 
types  occasionally.  With  some  systems  of  ventilation,  where 
such  cleaning  is  difficult,  or  almost  impossible,  such  as  that 
shown  in  Fig.  '16,  provision  must  be  made  for  cleaning  the  air 
before  it  enters  the  machine.  With  the  particular  construction 
shown  in  Fig.  16,  air  filters  are  almost  always  supplied.  In  the 
American  types  of  construction,  however,  such  filters  have  not 
yet  been  used,  except  in  a<  more  or  less  experimental  manner, 
due  probably  to  the  greater  accessibility  of  these  machines  as 
regards  cleaning.  But  such  filtering  processes  possess  consider- 
able merit  in  general.  One  modification  which  is  being  agitated 
at  present  i?  that  of  washing,  instead  of  filtering,  the  air.  This 
serves  the  double  purpose  of  cleaning  and  cooling  the  air,  and 
in  very  hot  weather,  when  the  available  capacity  of  the  machine 
is  at  its  minimum,  this  cooling  effect  may  mean  a  reduction  of 
6  to  10  deg.  in  the  tetnperatuxe  6f  the  machine. 


334  ELECTRICAL  ENGINEERING  PAPERS 

THE  TEMPERATURE  PROBLEM 

In  the  general  problem  of  temperatures  in  electrical  apparatus, 
it  is  not  the  rises,  but  rather  the  ultimate  or  limiting  temperatures 
which  are  of  first  importance.  Furthermore,  the  real  limitation 
in  ultimate  temperature  does  riot  lie  in  the  copper  and  iron, 
but  in  insulating  materials  used;  and  only  insofar  as  the  tem- 
peratures of  the  former  affect  the  latter  do  they  concern  the 
general  problem.  However,  as  insulating  materials  in  themselves 
are  not  usually  sources  of  heat,  but  as  they  receive  most  of  their 
heat  from  adjacent  media,  such  as  iron  or  copper  which  may  be 
generating  loss,  the  real  temperature  problem,  as  regards  insula- 
tion, resolves  itself  into  the  consideration  of  that  of  the  adjacent 
materials.  Therefore,  it  is  one  which,  for  its  full  analysis, 
requires  a  knowledge  of  the  sources  and  amounts  of  heat  gener- 
ated, and  its  conduction  and  distribution  to  other  parts 

Broadly  speaking,  there  is  always  a  flow  of  heat  from  points 
of  higher  to  those  of  lower  heat  potential  and  the  amount  of 
flow  is  also  a  function  of  the  quantity  of  heat  generated,  the 
section  and  length  of  the  paths  through  which  it  can  flow,  and 
the  specific  heat  resistance  of  the  various  materials  which  con- 
duct the  heat.  In  an  electric  generator,  for  example,  heat  is 
generated  in  large  quantities  in  the  armature  teeth  and  in  the 
armature  core  It  is  also  generated  in  the  armature  coils  when 
the  machine  is  carrying  load  Part  of  the  armature  copper  is 
buried  in  the  armature  slots  where  it  is  almost  surrounded  by 
iron,  which,  in  itself,  develops  a  loss,  while  another  part,  such 
as  the  end  windings,  may  be  surrounded  by,  and  thoroughly 
exposed  to,  the  ventilating  or  cooling  air  In  such  end  portions, 
the  flow  of  heat  will  usually  be  from  the  inside  copper,  directly 
through  the  insulation  to  the  cooling  air  The  amount  of  heat 
which  will  flow  from  the  copper  through  the  insulation,  depends 
upon  the  temperature  differences  between  the  copper  and  the 
outside  surface  of  the  insulation,  upon  the  cross  section  of  the 
path  of  flow,  upon  the  thickness  and  4'  make-up  "  of  the  material, 
and  upon  the  heat-conducting  properties  of  the  insulation  itself 
There  is  also  a  considerable  temperature  gradient  from  the 
outside  surface  to  the  air.  If  the  surrounding  air  is  not  renewed 
with  sufficient  rapidity,  the  flow  of  heat  from  the  insulation  to 
the  air  may  raise  the  temperature  of  the  adjacent  air,  so  that 
the  total  temperature  drop  is  decreased,  and  the  amount  of  heat 
dissipated  is  correspondingly  reduced. 

In  the  armature  core,  the  problem  is  much  more  complex 


r  URBO-GENERA  TORS  335 

In  the  copper  buried  in  the  armature  slots,  there  are  usually 
three  paths  along  which  the  heat  can  flow.  First,  it  may  flow 
from  the  copper  directly  through  the  insulation  to  the  fron, 
-provided  the  adjacent  iron  temperature  is  lower  than  that  of 
the  copper.  Second,  it  may  flow  lengthwise  of  the  copper  to 
the  end  windings  to  be  dissipated  directly  into  the  air  from  that 
portion  ot  the  winding,  as  described  above  Third,  in  the  case 
of  open-slot  machines,  one  edge  of  the  coil  may  be  exposed  to 
the  air  in  the  air  gap,  and  there  may  thus  be  a  direct  conduction 
of  the  heat  through  the  insulation  to  the  air  in  the  air  gap.  This 
latter  case,  however,  only  holds  for  the  upper  coil,  or  that  next 
to  the  gap,  in  the  case  of  two  coils  per  slot,  which  is  the  most 
common  construction  In  the  bottom  coil,  the  only  means  of 
conduction  in  the  buried  portion  of  the  coil,  are  to  the  adjacent 
iron  or  lengthwise  to  the  end  windings,  or  to  the  adjacent  upper 
coil,  which,  however,  would  normally  have  at  least  as  high 
temperature  as  the  lower  coil  Therefore,  the  two  effective 
paths  should  be  considered  as  through  to  the  iron  and  thence 
to  the  air,  and  lengthwise  of  the  copper  to  the  end  windings  and 
to  the  air.  It  is  the  relation  of  the  various  factors  of  these  two 
paths  that  control  the  actual  temperatures. 

It  has  usually  been  considered  that,  in  the  buried  copper,  the 
greater  portion  of  the  heat  is  conducted  directly  into  the  sur- 
rounding iron.  This,  however,  is  only  partially  true,  depending 
upon  many  features  in  the  construction  and  type  of  apparatus, 
The  heat  conductivity  of  copper  is,  roughly,  about  six  times  that 
of  laminated  iron  lengthwise  of  the  sheet,  which  is  possibly  ten 
to  twenty  times  as  great  across  the  laminations.  In  an  armature 
which  is  comparatively  narrow  and  which  has  very  open,  well 
ventilated  end  windings,  a  relatively  small  difference  in  tempera- 
ture between  the  copper  at  the  center  of  the  core  and  that  in  the 
end  windings,  may  cause  a  relatively  large  flow  of  heat  from  the 
buried  copper  to  the  end  copper.  Therefore,  in  certain  design?, 
a  great  part  of  the  armature  copper  heat  may  be  dissipated 
through  the  end  windings,  and  not  through  the  armature  core, 
especially  in  those  cases  where  the  armature  core  in  itself  has  a 
considerable  temperature  rise.  There  even  might  be  no  con- 
duction of  heat  from  the  copper  to  the  iron,  or  there  may  be 
conduction  from  the  iron  to  the  copper;  for  it  the  copper  is  at 
the  same  temperature  as  the  iron  at  the  center  of  the  core,  for 
instance,  then  at  each  side  of  the  center,  or  as  the  edges  of  the 
core  are  approached,  the  copper  temperatures  will  be  relatively 


336 


ELECTRICAL  ENGINEERING  PAPERS 


lower  than  at  the  center,  and  therefore  lower  than  the  adjacent 
iron,  on  the  assumption  that  the  iron  temperatures  would  be 
practically  constant  over  the  full  width  of  the  core  The  con- 
ditions would  therefore  be  as  represented  in  Fig.  18.  The  solid 
line  a  in  this  figure  represents  the  iron  temperature  at  uniformly 
40  deg.  cent,  rise,  and  the  dotted  line  b  represents  the  copper 
temperatures  ir6m  the  center  of  the  core  to  the  edges.  Th*  tem- 
peratures at  tne  center  being  assumed  the  same  for  copper  and 
iron,  obviously  there  will  be  a  flow  for  heat  from  the  iron  to  the 
copper  near  the  edges  or  the  core.  The  effect  of  this  additional 
heat  carried  out  by  the  copper  would  be  such  as  to  tend  to  increase 
the  temperature  of  the  copper  at  the  center  of  the  core  by  "  bank- 
ing up  f '  the  copper  heat. 

Again,  if  the  temperature  of  the  copper  at  the  center  is  materi- 


10° 


FIG.  18 


FIG   19 


ally  higher  than  that  of  the  surrounding  core,  the  conditions 
may  be  as  represented  in  Fig.  19  In  this  case,  assuming  the 
core  at  constant  temperature,  there  will  be  heat  flow  from  the 
copper  to  the  iron  at  the  center  of  the  core,  and  from  the  iron  to 
the  copper  at  the  edges 

This  study  of  the  problem  leads  to  certain  very  curious  con- 
ditions which  are  sometimes  found  in  large  machines.  At  no- 
load,  for  instance,  with  practically  no  copper  loss  present,  and 
with  high  iron  loss,  there  may  be  a  very  considerable  flow  of 
heat  from  the  armature  teeth  through  the  insulation  into  the 
copper,  and  thence  to  the  end  windings  and  to  the  air.  In  this 
way  the  temperature -of  the  armature  teeth  at  no-load,  and  with 
normal  voltage  generated,  may  be  considerably  reduced  by  con- 
duction of  the  iron  heat  into  the  copper,  while  the  copper  itself 


TURBO-GENERATORS  337 

may  show  a  very  considerable  temperature  rise.  When  load  is 
placed  upon  such  a  machine,  sufficient  to  raise  the  temperature 
of  the  copper  up  to  that  of  the  iron  in  the  armature  teeth,  the 
latter  is  actually  increased  in  temperature,  due  to  the  prevention 
of  the  heat  conduction  into  the  copper.  In  this  way,  therefore, 
the  copper  may  apparently  heat  the  iron,  although  there  is  no 
direct  flow  of  heat  from  the  copper  to  the  iron,  but  the  reverse 
flow  is  prevented. 

In  high-voltage  windings  requiring  thick  insulation,  the  temp- 
erature drop  from  the  copper  to  the  outside  may  be  relatively 
large;  that  is,  with  a  given  difference  of  temperature  between 
the  copper  and  the  surrounding  air,"  a  relatively  small  amount 
of  heat  may  be  conducted  through  the  insulation.  Experience 
shows  that  the  amount  which  can  be  conducted  is  a  function  of 
the  quality  of  the  material,  the  way  it  is  built  up,  its  thickness, 
and  also  the  pressure  upon  it.  It  is  almost  impossible,  in  a 
machine  in  service,  to  calculate  exactly  the  flow  of  heat,  even  if 
all  the  temperature  conditions  are  known,  for  the  insulating 
material  itself  is  one  of  the  variables  in  the  problem  The  ability 
of  the  insulation  to  conduct  heat  will  change  with  operating 
conditions,  to  some  extent,  as,  for  instance,  it  may  tend  to  ex- 
pand somewhat  under  heat,  and  thus  change  its  heat  conducting 
qualities. 

In  the  armature  iron,  the  problem  of  heat  conduction  is  just 
as  complicated  as  in  the  armature  conductor.  The  principal 
sources  of  heat  lie  in  the  armature  teeth  and  in  the  armature 
core  back  of  the  teeth.  As  a  rule,  the  loss  in  the  portion  of  the 
core  immediately  back  of  the  teeth  is  relatively  greater  than  at 
a  greater  depth,  for  the  magnetic  fluxes  which  cause  the  tempera- 
ture rise,  generally  crowd  close  to  the  teeth,  so  that  the  density 
is  higher  at  such  parts. 

The  heat  from  the  armature  teeth  can  be  dissipated  along 
several  paths.  It  can  flow  lengthwise  of  the  laminations  to  the 
end  of  the  tooth  and  into  the  air  gap,  where  the  ventilation  is 
usually  fairly  good,  but  the  tooth  surface  exposed  is  relatively 
small.  In  the  second  place,  it  can  flow  back  along  the  lamina- 
tions to  the  armature  core  where  it  can  spread  out  through  a  path 
of  much  greater  cross  section  and  be  conducted  partly  to  the  back 
part  of  the  laminations,  and  partly  transversely  to  the  ventilating 
ducts.  A  third  path  from  the  armature  teeth  is  across  the 
laminations  of  the  teeth,  to  the  neighboring  ventilating  ducts. 
This  latter  path,  however,  must  necessarily  be  relatively  poor 


338  ELECTRICAL  ENGINEERING  PAPERS 

in  conductivity  per  unit  section  of  path,  compared  with  the 
others,  but  offsetting  this,  it  is  frequently  of  much  greater  cross 
section  and  of  relatively  small  length.     In  passing  from  plate 
to  plate,  the  heat  must  pass  through  the  insulating  varnish, 
or  other  material  used,  which  is  of  relatively  high  heat  resistance 
compared  with  the  iron  itself.    Nevertheless,  in  machines  with 
radial  ventilation,  a  very  considerable  portion  of  the  heat  due 
to  the  tooth  loss  is  carried  transversely  thrpugh  the  plates  to 
the  air  in  the  ventilating  ducts,  simply  because  that  is  the  path 
of  lowest  total  heat  resistance,  everything  considered.    In  mariy 
cases,  the  temperature  in  the  core  back  of  the  teeth  may  be  as 
high  as  that  of  the  teeth,  themselves,  so  that  the  only  flow 
possible  is  across  the  laminations  to  the  air  ducts,  or  lengthwise 
to  the  tip  of  the  teeth  in  the  air  gap.    Therefore,  the  question 
whether  the  armature  teeth  may  be  hotter  than  the  armature 
core,  or  whether  the  flow  of  heat  is  from  the  teeth  to  the  core, 
or  from  the  core  to  the  teeth,  is  a  very  involved  one;   and  yet 
upon  this  question  depends,  to  a  great  extent,  the  temperature 
rise  in  the  buried  armature  copper.     If  the  armature  core  is 
normally  hotter,  than  the  teeth,  and  a  considerable  amount  of 
heat  in  the  teeth  is  carried  away  by  the  buried  copper  at  no 
load,  then  it  may  happen  that  when  carrying  heavy   load,  the 
heat  in  the  teeth  will  rise  very  considerably  above  the  no-load 
condition,  and  it  may  actually  so  "  bank-up  "  that  there  is  still 
more  or  less  flow  from  the  iron  to  the  copper,  eVen  with  load. 
With  such  a  condition,  therefore,  the  outside  of  the  insulation 
may  reach  a  higher  temperature  than  the  inside,  while  in  those 
cases  where  the  temperature  of  the  copper  rises  above  that  $f 
the  iron  of  the  armature  teeth,  the  inside  of  the  insulation 
will  be  hotter.    Therefore,  the  temperature  to  which  the  insula- 
tion is  liable  to  be  subjected  appears  to  be  largely  a  problem 
for  the  designer  to  determine  from  his-  calculations,  based  upon 
accumulated  data  and  experience.     This  is  especially  the  case 
with  very  wide  armature  cores  and  large,  heavily  insulated 
armature  coils,  such  as  found  in  large  capacity,  high  speed 
turbo  generators.     In  such  machines,   experience  has  shown 
that  various  temperature  conditions  may  be  found,  depending 
upon  the  location  and  relative  values  of  the  losses  in  the  different 
parts  and  the  means  for  conducting  away  the  heat.     Tests 
have  shown  that,  rq.  some  cases,  the  armature  iron  at  the  center 
of  the  core  is  considerably  warmer  than  the  armature  copper, 
while  in  other  cases  the  opposite  is  found  to  be  true. 


T  URBO-GENERA  TORS  339 

In  such  apparatus,  the  temperatures  actually  obtained  are 
liable  to  be  materially  higher  than  the  usual  methods  of  measure- 
ment will  indicate.  These  temperatures  are  inherent  to  the 
conditions  of  design  and  cannot  be  avoided  economically,  in 
certain  types  of  apparatus,  such  as  turbo-generators.  In  such 
machines,  the  limitations  in  speed,  strength  of  material,  etc. 
force  the  designer  to  certain  proportions  which  preclude  larger 
dimensions,  or  lower  inductions  in  the  iron,  or  lower  densities 
in  the  copper,  or  increased  ventilation.  In  such  apparatus 
therefore,  the  development  apparently  lies  in  the  direction  of 
insulations  which  will  stand  the  higher  temperatures  which  may 
be' obtained. 

These  conditions  of  higher  temperatures  in  some  parts  of 
the  machine,  than  indicated  by  the  usual  tests,  have  been 
recognized  for  years  by  designers  and  manufacturers  of  large 
electric  machinery.  A  rough  indication  of  these  temperatures 
can  be  obtained  by  exploring  coils  or  thermo-couples  suitably 
located.  However,  it  is  evident  that  such  coils,  if  located  next 
to  the  copper,  will  not  give  the  correct  temperature  measurement 
if  the  flow  of  heat  is  from  the  iron  to  the  copper,  while  a  coil 
next  to  the  iron  will  not  give  the  correct  result  .with  the  flow 
from  the  copper  to  the  iron.  Experience  has  shown  that  the 
temperatures,  in  corresponding  positions  around  the  core,  may 
not  be  uniform,  due  to  local  conditions.  In  consequence,  it  is 
not  practicable  to  actually  determine  the  true  temperatures  of 
all  parts  of  the  insulation  on  commercial  machines,  except  by 
measurements  of  a  laboratory  nature,  which  wtould  involve  such 
a  number  of  separate  readings  as  to  be  commercially  prohibitive.' 

On  account  of  the  higher  temperatures  which  may  be  found 
in  such  apparatus,  and  the  difficulty  of  making  exact  measure- 
jmcnts,  except  by  laboratory  methods,  manufacturers  very 
generally  have  adopted  the  use  of  mica  as  an  insulating  material 
on  the  buried  part  of  the  coils.  Experience  has  shown  that  such 
material,  when  properly  applied,  can  safely  stand  temperatures 
of  at  least  125  deg.  cent.  How  much  more  has  riot  yet  been 
determined. 

Of  such  machines  it  may  be  said  that  the  manufacturer,  with 
his  'guarantee  of  40  deg.  cent,  by  thermometer,  actually  builds 
for  possible  temperatures  of  70  to  90  deg,  cent,  in  some  parts  of 
the  machine,  for  he  expects  to  find  fairly  high  temperatures  in 
some  cases  with  exploring  devices.  The  usual  guarantee  trf  40 
deg.  cent,  therefore  should  be  considered  as  only  a  relative  indi- 
cation of  a  safe  temperature  in  such  apparatus. 


340  ELECTRICAL  ENGINEERING  PAPERS 

If,  for  instance,  the  exploring  coils  should  show  70  deg.  cent, 
maximum  rise  under  running  conditions,  and  the  permissible 
ultimate  temperature  of  fibrous  or  tape  insulation  is  assumed 
as  90  deg.  cent,  for  continuous  operation,  then  obviously,  with 
air  at  40  deg.  cent,  the  insulation  would  be  considered  as  insuffi- 
cient from  point  of  durability,  except  for  intermittent  service, 
such  as  overloads,  and  such  limited  conditions.  Plainly,  the 
insulation,  for  such  temperatures,  should  be  of  mica,  or  equiva- 
lent material,  for  which  125  deg.  cent,  has  been  found  to  be  safe. 

Furthermore,  it  may  be  stated  that  with  such  mica  insulation, 
a  turbo  generator  which  shows  75  deg.  cent,  rise  by  exploring 
coils,  or  thermo-couples,  has,  in  fact,  more  margin  of  safety 
than  the  ordinary  varnished-tape-insulated  low-voltage  machines 
of  any  type,  which  show  40  deg.  cent,  rise  by  thermometer  or 
SO  deg.  cent,  rise  by  resistance. 

The  foregoing  aims  to  bring  out  clearly  that  the  temperature 
problem  is  a  most  complex  one,  in  all  electrical  apparatus,  and 
especially  so  in  turbo-generators.  It  indicates  that  no  simple 
temperature  test  can  show  all  the  facts,  and  that  all  commercial 
methods  must  be  considered  as  approximations.  It  also  shows 
the  absurdity  of  classifying  a  piece  of  apparatus  as  good  or  bad, 
respectively,  according  to  whether  it  tests  possibly  one  or  two- 
degrees  below  or  above  a  specified  thermometer  guarantee. 
Also,  following  out  the  above  principles  on  heat  flow,  various 
fallacies  in  temperature  measurements  might  be  noted.  For 
example,  it  is  usually  assumed  that,  after  shutdown,  if  a  grad- 
ually rising  temperature  is  shown,  this  is  a  more  accurate  indica- 
tion of  the  true  temperature.  But  this  may  be  entirely  wrong  as 
regards  windings.  If,  for  instance,  the  core  back  of  the  armature 
slots  is  materially  hotter  than  the  armature  teeth  while  carrying 
load,  then,  upon  shut-down,  with  the  air  circulation  stoppedr 
the  teeth  will  rise  to  approximately  the  same  temperature  as  the 
core  back  of  the  teeth,  and  there  may  be  a  flow  of  heat  into  the 
coils,  which  condition  may  not  have  existed  while  carrying  load. 
A  thermo  couple  on  the  coil  or  in  the  teeth  would  thus  indicate 
a  false  temperature  rise  after  shut-down.  This  is  cited  simply 
as  one  of  many  instances,  to  show  the  possibilities  of  entirely 
wrong  conclusions  which  may  be  reached  in  the  problem  of 
temperature. 

THE  INSULATION  PROBLEM 

The  one  fundamental  condition  which  must  be  considered  in 
the  insulation  problem,  is  the  durability  of  the  material  itself,  and 


T  URBO-GENERA  TORS  34 1 

this  must  be  viewed  from  two  standpoints, — the  mechanical,  and 
the  electrical.  From  the  mechanical  standpoint,  the  material 
may  have  its  insulating  qualities  impaired  by  the  action  of 
mechanical  forces  which  tend  to  crack,  or  crush,  or  disrupt  the 
material  itself,  or  it  may  be  affected  by  being  permeated  by 
foreign  materials  or  substances,  or  it  may  be  injured  by  such 
overheating  as  will  partially  or  wholly  carbonize  it,  or  render  it 
brittle  or  otherwise  unsuitable  for  the  desired  purpose. 

From  the  electrical  standpoint,  it  may  be  weakened  by 'deter- 
ioration of  the  quality  of  the  insulating  material  itself  or  some 
of  its  component  parts,  which  may  be  due  to  heating;  or  oxida- 
tion, or  many  other  causes. 

The  effect  of  mechanical  injury,  such  as  cracking  crushing 
or  overheating,  on  the  insulating  qualities,  will  depend  upon 
many  conditions.  In  some  cases,  with  relatively  low  voltage, 
any  effective  mechanical  separation  of  the  parts  is  sufficient  for 
electrical  purposes.  For  higher  voltages,  continuity  of  the  separ- 
ating insulating  medium  is  necessary. 

Experience  has  shown  that,  for  moderate  voltages,  tempera- 
tures which  may  injure,  or  even  ruin,  the  insulating  material, 
from  a  mechanical  standpoint,  may  not  seriously  affect  its 
insulating  qualities.  Many  insuhating  materials  of  a  cellulose 
nature  will  still  retain  good  insulating  qualities  if  maintained 
at  temperatures  as  high  as  150  deg.  cent,  for  such  long  periods 
that  the  material  itself  semi-carbonizes.  Under  such  high 
temperature  conditions,  however,  it  becomes  structurally  bad, — 
that  is,  it  may  become  so  brittle  that  it  tends  to  crumble,  or 
powder,  or  flake  off,  and  thus  its  value  as  an  insulation  is  im- 
paired by  displacement  of  the  material  itself.  In  low  voltages, 
therefore,  it  is  not  a  deterioration  in  the  insulating  qualities, 
but  rather  a  mechanical  breakdown  of  the  material  itself,  which 
is  liable  to  cause  trouble.  With  high  voltages,  however,  the 
conditions  may  be  quite  different.  With  some  insulating  mater- 
ials, the  dielectric  strength  may  be  so  affected  by  long  continued 
high  temperatures  that  the  insulating  quality  becomes  insufficient. 
This  has  a  direct  bearing  on  large  capacity,  high-voltage  turbo- 
generators. 

In  the  problem  of  insulation,  certain  difficulties  have  been 
encountered  in  large  turbo-generators,  which,  while  they  would 
have  developed  eventually  in  other  large  machines,  yet  became 
apparent  more  quickly  and  preeminently  in  the  turbo  type,  due 
to  the  abnormal  conditions  in  its  design.  The  two  most  promi- 


342  ELECTRICAL  ENGINEERING  PAPERS 

nent  difficulties  were,  first,  that  of  relatively  high  temperature 
in  the  buried  copper,  already  described,  and  second,  the  destruc- 
tion of  the  insulation  by  reason  of  static  discharges  between  the 
coils  and  the  armature  iron 

Due  to  the  fact  that  the  ultimate  temperature  reached  in  such 
machines  not  infrequently  exceeds  the  safe  limits  for  insulation 
of  the  fibrous  or  cellulose   type,    such  insulations   will   show 
deterioration  eventually  in  their  insulating  qualities  and  their 
durability.      In   consequence,    with   the   advent   of   the   larger 
machines,  it  became  necessary  to  return  to  the  use  of  mica  for 
insulating  purposes  on  the  buned  part  of  the  coil     This  type 
of    insulation    in    the  form  of  mica  wrappers,  had  been    used 
extensively  on  some  of  the  earlier  large  capacity,  slow-speed 
generators,  but  it  had  not  been  adopted  on  large  turbo-generators, 
due  principally  to   the  difficulty  in  applying  the  very  long 
wrappers  for  the  straight  part  of  the  coil.    However,  when  the 
gradual  deterioration  of  the  fibrous  type  of  insulation  was  noted 
in  large  turbo-generators,  the  mica  wrapper  type  of  insulation 
was  again  taken  up  and,   after  considerable  experiment,   was 
applied  successfully  for  the  outside  insulation  on  the  straight 
parts  of  the  coils     This  use  of  mica  overcame  the  deterioration 
in  the  insulating  qualities  of  the  outside  insulation,    but  for 
a  while  it  was  considered  that  a  fibrous  type  of  insulation  was 
still  effective  between  turns  in  those  coils  where  there  were 
two  or  more  turns  in  series  per  coil.      As  stated  before,  the 
insulating  qualities   of  many   fibrous  materials  will   stand  up 
astonishingly  well   under  low   voltages,  when   the   material   is 
apparently  so  greatly  heated  that  it  is  practically  carbonized, 
Therefore,   temperatures  which  did  not  carbonize,   but  simply 
browned,  or  darkened,  the  material,  had  not  been  considered 
dangerous,    and    undoubtedly    many    thousands    of    electrical 
machines  of  all  kinds  are  today  in  operation,  in  which  the 
insulation  is  in  this  condition,  and  in  which  no  trouble  need  be 
expected.      For  this  reason,  little  or  no  trouble  was  expected 
between  turns  on  the  turbo-generators.     However,  a  new  con- 
dition was  encountered  in  large  capacity  machines,  namely,  the 
insulation  between  turns,  when  it  became  dry  and  brittle  at  the 
higher  temperatures,  was  liable  to  be  injured  by  the  terrific 
shocks  to  which  the  coils  were  subjected  in  such  machines,  in 
case  of  a  short  circuit  across  the  terminals.  The  insulation  would 
be  cracked,  or  so  distributed  that  short  circuits  would  occur  later, 
without  apparent  cause.    These  short  circuits  on  large  machines, 


T  URBO-GENERA  TORS  343 

most  often  appeared  as  breakdowns  to  ground,  even  with  the 
mica  wrapper  insulation  on  the  outside  of  the  coil  Incidentally, 
several  cases  were  discovered  where  arcs  had  occurred  inside 
the  coils  between  adjacent  turns,  and  where  they  had  not  yet 
broken  through  the  outer  insulation  to  ground.  For  many 
months  the  writer,  with  his  associates,  followed  up  this  matter, 
examining  all  available  coils  and  windings.  Eventually  the 
conclusion  was  reached  that  many  of  the  breakdowns  to  ground 
had  actually  started  between  turns  on  the  inside  of  the  coil. 
Moreover,  as  a  corroboration,  it  was  noted  that  in  machines 
with  one  conductor  per  coil,  the  breakdowns  were  practically 
negligible.  This  investigation  led  to  the  practise  of  insulating 
the  individual  turns,  in  each  coil,  from  end  to  end,  with  mica 
tape.  After  the  adoption  of  this  practice,  it  is  noteworthy  that 
the  breakdowns  to  ground  practically  disappeared,  although 
the  outside  insulation  to  ground  had  not  been  changed  in  type 
or  thickness. 

Many  improvements  have  been  made  in  recent  times  in  the 
application  of  this  mica  insulation.  One  of  these  is  the  Haefely 
process,  developed  in  Europe,  but  now  being  used  extensively 
in  this  country.  By  this  process,  the  mica  wrappers  are  s'o 
tightly  rolled  on  the  cofl  that  practically  a  solid  mass  of  insula- 
tion, of  minimum  thickness  and  greatest  heat  conductivity  is 
obtained. 

By  means  of  the  mica  insulation,  the  temperature  difficulties 
in  general  have  been  entirely  overcome,  and  a  durable  and  non- 
deteriorating  construction,  from  an  insulation  standpoint,  has 
been  obtained  with  the  temperatures  which  appear  to  be  more 
or  less  inherent  in  the  large,  high-speed  turbo-generators. 

The  second  trouble,  namely,  that  due  to  static  discharges 
between  the  annature  copper  and  the  iron,  was  also  encountered 
to  a  certain  extent,  on  some  of  the  earlier  machines.  It  was  found 
that  these  discharges  were  apparently  "  eating  "  holes,  or  even 
grooves,  through  the  outside  insulation  of  the  armature  coils. 
This  effect  was  most  pronounced  at  the  edges  of  the  air  ducts 
and  at  the  ends  of  the  armature  core,  where  edges  were  presented 
by  the  iron.  After  a  long  period,  the  holes  or  grooves  would 
become  so  deep  that  the  insulation  was  weakened  or  ruined. 

This  was  a  very  disturbing  condition,  when  it  was  once  fully 
recognized  and  appreciated.  Again,  a  comprehensive  investi- 
gation was  made  to  discover  a  cure  for  this  difficulty.  Various 
types  of  machines  and  windings  were  examined.  It  was  noted 


344  ELECTRICAL  ENGINEERING  PAPERS 

"that  the  action  was  a  function  of  the  voltage  of  the  machine, 
"but  was  noticeable,  in  some  cases,  at  relatively  low  voltages. 
During  the  course  of  the  investigations,  it  was  noted  that  where 
mica, wrappers  were  used  with  an  outside  layer  of  tape,  the  "  eat- 
ing away  "  extended  only  through  the  outside  wrapping  in  as 
far  &s  the  mica,  and  that  no 'apparent  effect  at  the  tnica  was 
visible.  Even  when  examined , with  a  very  powerful  microscope, 
no  evidence  of  any  puncture  of  the  mica  was  found,  in  any  case 
These  investigations  naturally  led  to  th,e-  conclusion  that  the 
#iost  suitable  remedy  for  the  trouble  was  the  use  of  mica  insula- 
fton,  which  was  also  a  remecly  for  the  temperature  conditions 
This  is  one  of  the  rare  cases  in  large  turbogenerators  where  two 
•desirable  conditions  do  not  conflict  With  feach  other.  The  rnica 
insulations  on  the  buried  part  of  the  coil  has  now  been  very 
generally  adopted  in  this  country  on  high-voltage  ipachines, 
whether  of- the  turbine-driven,  or  aiiy  other  type 

This,  static  trouble  was  considered  so  serious  at  one  time  that 
low  voltage  practice  with  step-up  transformers  was  adopted 
by  some  manufacturers  as  the  safest  course,  until  something 
positive  in  the  way  of  a  femedy  was  proved  out.    This  trouble 
promised  to  be  one  of  the  most  serious  encountered  in  high- 
voltage  generator  work,  and  even  threatened  to  revolutionize 
practise  in  winding  generators  for  the  higher  voltages.    However, 
as  consistently  advocated  by  the  writer,  the  us£  of  mica,  suit- 
ably applied,  appears  to  have  entirely  overcome  this  trouble,  as 
evidenced  by  several  year's  experience,  and  'all  indications  now 
are  that  there  need  be  no  fear  from  static  discharges  on  windings 
of  11,000  and  13,000  volts,    Even  in  the  11, 000- volt  New  Hatfejn 
generators  with  one  terminal  grounded,  which  gives  the  equiva- 
lent of  a  I9tOOO-voltT  three-phase  winding  with  the  jieutral 
grouiided,  the  mica  insulation  appears  to-be  successful  and  dur- 
able. 

ROTOR  INSULATION 

In  most  of  the  early  turbo-generators,  the  rotor  winding 
in  the  slots  was  insulated  with  fibrous  material!'"  fish  paper  " 
and  "  horn  M  fiber  having  the  ^reference.  'One  of  the  difficulties 
in  the  rotor  is,  that  the  insulation  between  the  winding  and  the 
slot  is  liable  to  be  crushed  or  cracked  by  the  higlji  •centrifugal 
forces.  In  the  earlier  insulatibns,  before  fish  paper  was  used,  it 
was  found  that  even  at  very  moderate  temperatures,  the  insula- 
tion got  dry  and  brittle,  and  cracked  readily.  Fish  paper,  or 
horn  fiber,  was  then  adopted  pretty  generally.  Such  material 


T  URBO-GENERA  TORS  345 

apparently  stood  much  higher  temperatures  than  the  ordinary 
fibrous  insulations.  However,  experience  also  showed  that 
eventually  this  also  became  brittle,  arid  was  liable  to  be  cracked, 
and  then  displaced,  due  to  the  centrifugal  forces.  There  is 
always  the  possibility  of  a  sinaH  amount  of  movement  in  the 
field  coils  when  a  machine  is  being  brought  up  to  speed,  and  this 
movement,  in  itself,  may  eventually  damage  the  insulation  if 
it  is  at  all  brittle. 

As  the  capacities  and  speeds  of  turbo-generators  were  increased 
and  the  space  limitations  for  the  rotor  windings  became  more 
pronounced,  the  resulting  higher  normal  temperatures  led  to 
the  adoption  of  mica  for  the  insulating  material  in  the  slots  with 
either  mica  or  asbestos  for  the  insulation  between  turns.  As 
the  voltage  between  adjacent  turns  is  always  extremely  low, 
what  is  needed -is  really  a  durable  separating  medium,  rather 
tharf  an  insulation,  this  medium  being  one  which  will  not  become 
crisp  or  brittle  at  fairly  high  temperatures.  Asbestos  has 
served  for  this  purpose  very  effectively,  and  even  has  some 
advantages  over  mica,  as  the  latter  must  be  applied  in  relatively 
small  pieces  in  the  form  of  strap  or  tape,  and  the  individual 
pieces  are  more  readily  displaced  or  shifted  than  is  the  case 
with  asbestos.  Some  very  severe  tests  have  been  made  in 
order  to  determine  the  possibilities  of  such  rotor  insulation 
In  one  case,  a  turbo  rotor  thus  insulated  was  run  at  full  speed  for 
over  40  hours,  with  such  a  current  that  the  rise  by  resistance 
in  the  rotor  copper  was  about  250  deg.  cent  It  was  the  in- 
tention to  continue  this  test  very  much  longer,  but  the  conduc- 
tion of  heat  from  the  winding  to  the  core,  and  thence  through 
the  shaft  to  the  bearings,  was  such  that  finally  the  bearings 
became  overheated  and  gave  out.  After  this  test,  the  winding  was 
carefully  dismantled,  and  no  evidence  of  any  injury^  to  the 
insulation  could  be  discovered.  Of  course,  such  temperatures 
a*e  not  recommended  in  turbo  rotor  practice,  but  this  was 
simply  an  attempt  to  find'  a  temperature  limitation.  If  a 
designer  wants  to  find  the  facts  in  any  apparatus,  he  will  obtain 
the  most  valuable  information  if  he  operates  the  apparatus 
up  to  the  point  of  destruction.  He  thus  fixes  a  limit  which 
•he  must  keep  below. 

The  use  of  thica,  or  mica  and  asbestos,  on  turbo  rotors  has 
been  very  .generally  adopted  in  this  country  at  the  present  time, 
and  it  may  fee  said  that,  within  the  writers  experience,  no  case 
of  .destruction  ot.one  of  these  windings  through  heating,  has 


346  ELECTRICAL  ENGINEERING  PAPERS 

come  to  his  notice,  although  a  great  number  of  them  have 
been  in  service  for  a  relatively  long  time.  In  many  of  the  older 
machines  with  fish  paper  insulation  in  the  rotors,  the  conditions 
of  ventilation  and  the  normal  ratings  of  the  machines  were 
such  that  the  maximum  temperatures  in  the  rotor  windings 
were  relatively  much  less  than  in  present  practise  It  may  there- 
fore be  said  that  the  use  of  mica  in  the  rotor  has  been  largely 
due  to  the  introduction  of  the  larger  capacities  and  higher  speeds. 

LOSSES  IN  TURBO  ALTERNATORS 

The  total  iron  and  copper  losses  in  a  large,  high-speed  turbo- 
alternator  are  in  general  no  higher  than  in  a  corresponding 
capacity  low-speed  machine 

As  far  as  the  iron  losses  are  concerned,  no  further  comment 
need  be  made  than  that  the  magnetic  flux  densities  in  general 
are  somewhat  lower  than  in  lower  speed  machines  of  same 
frequency,  and  therefore  the  losses  per  unit  volume  of  material 
are  no  larger. 

The  total  armature  copper  losses  in  turbo-alternators,  as  a 
rule,  are  considerably  smaller  than  in  corresponding  capacity 
machines  of  the  moderate  or  low-speed  types.  This  is  due  partly 
to  the  use  of  a  smaller  total  number  of  conductors,  and  partly 
to  a  lower  current  density  in  the  armature  conductors.  As 
brought  out  before,  in  a  narrow  core  machine,  a  considerable 
portion  of  the  buried  copper  heat  may  be  conducted  lengthwise 
of  the  conductor  into  the  end  winding,  and  there  dissipated 
into  the  air  In  the  turbo-generator,  with  its  much  wider 
core  and  greater  distance  from  the  buried  copper  to  the  end 
windings,  a  smaller  percentage  of  the  buried  copper  heat  will 
be  conducted  into  the  end  windings.  To  partly  compensate 
for  this,  it  is  usual  to  work  the  armature  copper  in  the  turbo-gen- 
erators at  a  lower  current  density,  and  therefore  at  a  relatively 
lower  total  copper  loss.  This  is  somewhat  of  a  handicap  in  the 
economical  design  of  the  generator,  as  extra  space  is  thus  required 
for  the  armature  winding.  In  some  of  the  earlier  machines,  the 
armature  conductors  were  made  of  solid  copper  bars  of  relatively 
large  section,  partly  for  stiffening  or  bracing  the  end  windings, 
as  will  be  referred  to  later  With  these  solid  conductors  there 
was  a  very  considerable  loss  in  the  buried  copper  due  to  eddy 
currents.  To  compensate  for  this,  the  armature  conductors 
were  made  very  large  in  section,  so  that  the  current  density, 
due  to  the  work  current  alone,  was  very  low  compared  with 


TURBO-GENERA  TORS  347 

practise  in  other  types  of  machines.  On  account  of  the  com- 
paratively large  section  of  armature  conductors,  the  conduction 
of  heat  from  the  buried  copper  to  the  end  windings  was  relatively 
large.  In  some  of  these  earlier,  large  capacity  machines, 
the  nominal  current  density  in  the  armature  conductors  was 
so  low,  and  the  section  of  conductors  so  great,  that  the  total 
buried  copper  loss,  due  to  the  work  current,  could  be  carried 
from  the  buried  paft  of  the  coils  into  the  end  windings  with  a 
comparatively  small  (drop  -in  temperature,  so  that,t  if  there  had 
been  no  eddy  currents  present,  the  buried  copper  would  have 
shown  less  rise  .than  the  iron.  Any  considerable  rise  which 
occurred  was  thtis  chargeable  to  eddy  currents  in  the  buried 
conductors,  rather  than  to  the  work  current.  While  such  con- 
struction was  fairly  effective  for  the  purpose,  yet  it  was  decid- 
edly uneconomical  in  design,  as  indicated  before.  In  fact, 
with  later  proportions  and  methods  of  design,  the  safe  oiitputs 
of  some  of  the  earlier  machines  could  easily  be  50  to  75  per  cent 
greater,  largely  on  account  of  elimination  of  eddy  currents  and 
improvement  in  methods  of  dissipating  heat  from  the  end  wind- 
ings. In  many  of  the  older  machine's,  the  ventilation  of  the 
end  windings  was  not  nearly  as  effective  as  in  modeni  types, 
due  principally  to  the  form  and  arrangement  of  the  end  con- 
nectors, tlsually  $ir  spaces  were  allowed  between  adjacent 
coils  although,  in  some  instances,  these  were  so  small  as  to 
give  but  little  benefit.  Moreover,  in  many  cases,  the  type  of 
fend  winding  employed  rendered  these  air  spaces  between  coils 
rather  ineffective,  unless  special  means  were  taken  td  deflect 
the  air  between  the  coils.  With  later  constructions,  the  end 
windings  lie  more  or  less  agross  the  path  of  the  ventilating  air, 
and  there  are  ample  openings  between  the  coils,  so  that  a  very 
considerate  part  of  the  ventilating  air  will  actually  pass  between 
the  cO^ls  of  thfe  end  windings  in  such  a  way  as  to  give  the  niaxi- 
nuih  possible  ventilation.  When  it  is  considered  that  the  total 
armature  copper  loss  may  be  only  20  per  cent  of  the  total  stator 
loss,  it  will  be  seen  that  an  excessive  amount  of  air  is  Hot  re- 
quired when  the  end  windings  are  properly  arranged  for  most 
effective  ventilation. 

Much  effort  has  been  expended  in  eliminating  or  reducing 
th£  eddy  current,  losses  in  the  buried  copper  of  large  turbo- 
generators, as  well  as  in  othef  types  of  large  capacity  alternators. 
These  eddy  currents  aare  due  to  two  sources,  namely,  the  alter- 
nating magnetic  flux  across  the  slots  djie  to  ttte  armature  ampere 


348  ELECTRICAL  ENGINEERING  PAPERS 

turns  per  slot,  and  secondly,  the  magnetic  fringing  from  the 
rotor  pole  face  into  the  open  armature  slots.    In  some  instances, 
tests  have  indicated  that  the  local  e.m.fs.  set  up  in  the  armature 
conductors  by  the  flux  through  the  slot  opening  is  very  consider 
ably  greater  than  those  due  to  the  flux  across  the  slot.   Obviously 
with  partially  closed  slots,  this  fringing  into  the  top  of  the  slot 
should  be  practically  absent. 

The  simplest  remedy  for  the  eddy  currents  set  up  by  these 
local  e.m.fs.  is  to  sub-divide  the  conductors  into  a  number  of 
wires  or  conductors  in  parallel,  so  arranged  or  connected  that 
the  local  e.m.fs.  oppose  and,  to  a  great  extent,  balance  each 
other.  This  opposition  may  be  obtained  by  special  arrangement 
of  the  conductors  in  each  Individual  slot,  6r  parallel  conductors* 
in  the  two  halves  of  a  complete  coil  may  be  connected  in  oppo- 
sition to  each  other.  Some  of  these  arrangements  do  not  com- 
pletely balance  the  opposing  e.m.fs.,  but  they  include  the  resist- 
ance of  the  complete  coil  in  the  eddy  current  circuit,  so  that  the 
eddy  losses  are  not  only  very  materially  reduced,  but  they  are 
distributed  over  the  entire  coil,  including  the  end  windings,  which 
condition,  in  itself,  represents  a  very  material  improvement. 

PROTECTION  AGAINST  FIRE 

An  important  problem  connected  with  the  insulation  of 
large  turbo-generators,  is  found  in  the  fire  risk,  or  danger  of 
destruction  of  th3  end  windings  due  to  starting  an  arc  at  some 
point.  On  account  of  the  tremendous  ventilation  in  such 
machines,  a  fire,  if  once  started,  may  quickly  ruin  the  entire  end 
winding.  An  extended  investigation  was  made,  with  a  view 
to  providing  an  insulation  which  would  not  burn  rapidly. 
Among  other  tests,  the  end  windings  were  finished  on  the  out- 
side with  an  asbestos  covering  or  tape  However,  such  tape 
requires  some  sort  of  sealing  varnish,  or  material  to  fill  its  pores, 
to  keep  it  from  absorbing  moisture  or  oil.  The  tests  showed  that 
if  a  fire  was  once  started,  combustion  would  be  maintained  by 
the  gases  liberated  by  the  u  gasification  "  of  the  varnishes  and 
other  material  in  the  end  windings,  whether  the  coil  was  covered 
with  asbestos  or  not.  No  covering  which  was  tested  appeared 
to  be  very  effective.  Although  some  outside  covering  might  be 
found  which  would  be  slightly  effective  in  preventing  fire  from 
starting  so  readily*  yet,  if  once  started,  it  appears  that  a  fire 
can  very  easily  maintain  itself  in  such  machines.  Eventually, 
the  conclusion  was  reached  that  the  safest  course  would  be  to 


T  URBO-GENERA  TORS  349 

provide  suitable  closing  doors  or  valves  in  the  air  inlets  to  com- 
pletely shut  off  the  incoming  air  to  the  machine.  In  addition, 
suitable  doors  on  the  air  outlets,  where  they  can  be  applied, 
should  also  be  helpful,  by  retaining  the  smoke  and  burnt  gases 
inside  the  machine,  which  thus  assist  in  smothering  the  flames. 
The  uSe  of  fire  extinguishers  of  the  gaseous  type  will  usually 
be  rather  ineffective,  unless  the  incoming  air  and  ventilation  is 
practically  cut  off.  For  instance,  with  60,000  cu.  ft,  (1698  cu. 
m.)  of  air  per  minute  passing  through  a  large  machine,  the 
addition  of  a  little  gas  for  extinguishing  the  fire  would  hardly 
make  any  impression.  In  one  instance,  in  attempting  to  extin- 
guish a  fire,  an  effort  was  made  to  feed  the  gas  in  against  the 
ventilating  pressure  of  the  fans.  Obviously,  this  would  not 
work,  and  then  a  hose  was  used  in  order  to  get  enough  pressure 
to  counteract  the  fan  action.  Although  the  fire  was  extinguished, 
the  resultant  effect  of  fife  and  the  high  pressure  water  was  that 
new  insulation  was  required. 

REGULATION  AND  SHORT  CIRCUIT  CHARACTERISTICS 
It  has  been  known  for  many  years  to  designers,  that  alterna- 
ting current  generators  can  give,  at  the  instant  of  short  circuit, 
a  much  greater  current  than  that  which  they  will  give  on  con- 
tinued short  circuit.  The  first  emphatic  evidence  of  this,  in  the 
writer's  experience,  was  in  connection  with  the  first  Niagara 
generators  in  1894.  Upon  short  circuiting  one  of  these  machines 
at 'full  speed  and  normal  voltage,  the  results  indicated  a  current 
tush  so  great  that  it  was  apparent  that  it  was  limited  only  by 
the  anhature  self-induction,  and  not  by  the  so-called  synchronous 
reactance.  Later,  after  being  put  into  actual  commercial 
service,  it  was  found  necessary  to  brace  the  end  windings  on  these 
machines.  However,  at  that  time,  no  suitable  instrument, 
such  as  the  oscillograph,  was  available  for  determining  the 
conditions  on  short  circuit,  and  the  phenomena  did  not  permit 
of  fjuich  6*p'erimental  investigation. 

Similar  evidence  was  found  from  time  to  time,  as  in  the  first 
Manhattan  Elevated  engine  type  generators,  which  bent  their 
$nd  windings  out  of  shape  on  a  dead  short  circuit.  But  the  real" 
possibilities  for  trouble  in  this  matter  did  -not  develop  until 
the  large  capacity  turbo-generators  came  into  use.  In  these 
^machines,  the  armature  ampere  turiis  per  pole  are  so  high, 
Compared  with  moderate  speed  alternators,  that  the  stresses 
due  to  the  stray  magnetic  fields  oo^xort  circuit  are  much  greater 


350 


ELECTRICAL  ENGINEERING  PAPERS 


than  the  natural  rigidity  of  the  end  windings  will  withstand. 
The  manufacturer  of  such  apparatus,  without  data  of  any 
quantitative  value  at  hand,  did  not  fully  recognize  the  Veal 
•weakness  in  the  end  windings  until  disaster  overtook  them.  Even 
then  it  was  a  long  and  difficult  undertaking  to  overcome  the 
trouble.  All  kinds  of  designs  of  etid  supports,  and  various  ar- 
rangements of  end  windings  were  tried,  with  more  or  less  success. 
But  each  new  -step  in  the  increase  in  capacity  opened  up  the 
problem  a&ain.  It  was  soon  noted  that  those  armature  windings 
which  were  made  up  of  cable  or  small  wires,  suffered  most  on 
'short  circuit,  and  for  awhile  therq  was  a  tendency  on  the  part 
of  some  manufacturers  to  .use  heavy,  solid  conductors  to  give 
rigidity  in  the  end  windings.  This  was  effective  within  certain 
limits,  but  was  very  expensive  from  the  design  standpoint,  as 
on  account  of  eddy  currents  in  the  buried  copper  It  was  neces- 
sary to  work  at  a  very  low  current  density,  which  was  not 
economical  in  winding  space. 


FIG.  20 

In  this  country,  the  types  of  armature  windings  finally 
narrowed  down  to  the  open-slot  construction,  usually  with  an 
upper  and  lower  coil  per  slot,  with  the  end  winding  arranged 
in  two  layers,  similar  to  d-c.  armature  windings,  or  the  common 
induction  motor  primary  windings.  This  turbo  end  winding 
was  extended  at  various  angles  to  the  axis  of  the  machine  froih 
almost  parallel  up  to  90  deg.,  as  shown  in  Fig.  20.  The  principal 
survivor  of  these  types,  is  one  which  extends  at  some  angle 
between  30  and  60  deg.  to  the  axis.  There  are  several  reasons  for 
this, — first,  it  allows  a  very  'substantial  bracing  to  be  applied  to 
the  end  windings.  Second,  the  stray  fields  around  the  end 
windings  do  not,  to  any  extent,  cut  the  adjacent  solid  parts, 
such  as  the  end  housings,  stator  and  end-plates,  etc.  An  angular 
position  of  approximately  45  deg.  seems  to  be  a  good  compromise 
on  these  points.  Ample  supports,  as  shown  in  Pig.  21  can  be 
applied  for  bracing  the  windings  against  movement  in  any 


T  URBO-GENERA  TORS  35 1 

direction.  Such  end  windings  are  usually  braced  against  metal 
supports  attached  to  the  stator  end-plates.  The  coils  are  so 
clamped  to  the  racks,  and  are  so  braced  against  each  other  that 
the  windings  will  sustain  a  dead  short  circuit  across  the  terminals, 
even  in  the  largest  capacity  machines,  without  injury. 

On  some  recent  large  turbo-generators  the  end  windings  have 
been  further  strengthened  by  double  metal  racks  between  the 
two  layers  of  windings,  so  arranged  as  to  securely  key  these  two 
layers  to  one  another  at  certain  points.  Moulded  mica  troughs 
are  placed  around  the  coils  as  an  extra  insulation  from  the  metal 
racks.  By  this  keying  of  the  two  layers  to  one  another,  the 
winding  as  a  whole  is  stiffened,  quite  irrespective  of  any  other 
clamping  arrangement.  In  fact,  this  is  practically  equivalent  to 
putting  the  end  windings  in  rigidly  held  slots,  thus  approaching 
the  conditions  which  obtain  in  the  buried  part  of  the  coil. 

In  order  to  limit  the  momentary  short  circuit  current,  the 
armature  reactance  is  now  usually  made  as  large  as  the  condition 
of  the  design  will  permit.  This  naturally  means  high  ampere 
turns  per  pole,  which  in  turn  means  high  synchronous  reactance, 
and  consequently  poor  inherent  regulation  of  the  machine, 
especially  on  inductive  loads.  This  can  be  illustrated  by  the 
following  example:  Assume  a  5000-kw.  unit  of  an  earlier  design, 
which  can  give  25  times  full  load  current  on  momentary  short 
circuit.  By  certain  improvements  in  the  design  of  the  armature 
coils,  such  as  the  use  of  deeper  slots,  better  subdivision  of  the 
copper  to  eliminate  eddy  currents,  improved  ventilation  and 
'conduction  of  heat,  etc.,  the  capacity  of  the  machine  is  assumed 
to  be  increased  to  10,000  kv-a.,  the  number  of  armature  turns 
remaining  the  same  as  before.  It  is  evident  that  when  short 
circuited,  the  revised  machine  will  give  the  same  total  current 
as  on  the  former  rating,  which,  however,  is  only  12£  times 
the  rated  current  on  the  new  capacity  basis.  Obviously,  the 
end  winding  stresses  are  no  greater  than  before,  although  the 
nominal  capacity  has  been  doubled,  and  if  it  -were  possible  to 
satisfactorily  brace  the  end  windings  with  the  former  rating, 
the  same  bracing  should  be  effective  on  the  new  rating.  This 
illustrates,  roughly,  what  is  taking  place  in  later  designs,  although 
the  steps  in  the  change  may  not  be  just  those  mentioned.  Again, 
in  the  above  example,  it  is  obvious  that,  with  the  now  rating, 
the  inherent  regulation  at  full  load  is  the  same  as  at  100  per 
cent  overload  on  the  old  rating,  which  means  that  it  is  relatively 
poor.  Another  way  to  express  this  is,  that  the  old  fating  might 


352  ELECTRICAL  ENGINEERING  PAPERS 

give  2|  times  full  load  current  on  steady  short  circuit,  while  the 
new  rating  gives  1  \  times. 

This  condition  of  poorer  regulation  is  inherent  in  the  newer 
practise,  bttt  is  apparently  acceptable  to  the  users  of  such 
apparatus,  for  a  variety  >of  reasons  wliich  do  not  come  within 
the  province  of  this  paper. 

CONCLUSION 

The  foregoing  covers,  in  a  general  way,  fnany  of  the  problems 
encountered  in  large  turbo-generators,  and  defines  the  situation 
as  it  stands  at  present. 

It  anay  be  suggested,  in  connection  with  the  temperature 
problem,  that  the  high  temperatures  obtained  are  due  to  forcing 
the  construction  too  far;  but,  in  answer,  it  may  be  stated  that 
it  is  forced  no  further  in  this  feature  than  in  many  others.  The 
whole  design  has  been  carried  far  beyond  the  most  economical 
construction,  from  the  generator  standpoint  alone.  In  fact,  the 
whole  machine  is  more  or  less  a  compromise  between  desirable 
conditions  as  a  generator,  and  most  economical  conditions  as 
part  of  a  combined  turbine  and  generator  unit.  It  may  £e 
added  that  the  ultimate  limits  in  construction  and  capacity  will 
be  obtained  only  when  the  steam  turbine  conditions  are  satis- 
fied, and  there  are  indications  that  possibly  this  result  is  being 
approached  now  with  the  present  high  speeds. 

There  is  one  small  consolation  in  all  the  confusion  of  develop- 
ment which  has  attended  >  the  turbo-generator  work,  in  the  few 
years  it  has  been  with  us,  namely,  the  question  of  choice  of 
speed  has  been  practically  eliminated.  For  25  cycles,  there 
remains  only  one  speed,  namely  1500  revolutions,  with  two 
poles,  from  the  smallest  unit  up  to  251000  kv-a.  as  a  possible 
upper  limit.  For  60  cycles,  up  to  5000  kv-a  ,  two-pole  machines 
at  3600  revolutions  are  being  furnished,  while  from  this 
capacity  up  to  20,000  kv-a.  four  poles  may  be  used. 

It  will  be  evident  to  any  reader  of  this  paper,  that  the  designers 
of  large  turbo-alternators 'have  had  a  strenuous  time  during  the 
past  few  years — very  much  more  so  than  is  indicated  herein,  for 
their  successes,  rather  than  their  failures  have  been  discussed. 
In  fact,  much  of  the  time  they  have  been  working  ahead  of  their 
data  and  experience.  In  presenting  this  situation  from  the 
design  point  of  view,  it  is  hoped  that  a  better  and  clearer  under- 
standing of  the  turbo-generator  ^problem  will  be  obtained  by  all 
who  are  interested  in  such  apparatus. 


TEMPERATURE  AND  ELECTRICAL  INSULATION 

FOREWORD— In  1911  and  1912,  a  revision  of  the  standardization 
rules  of  the  American  Institute  of  Electrical  Engineers  was  being 
made.  The  problem  of  temperature  guarantees  was  referred  to 
a  sub-committee,  consisting  of  Dr.  Steinmetz  and  Mr.  Lamme. 
It  was  decided  by  the  Standards  Committee  to  hold  a  mid- 
winter convention  of  the  Institute  in  February,  1913.  In  order 
to  furnish  a  basis  for  discussion  of  the  temperature  problem  at 
this  convention,  the  sub-committee  on  temperature  collaborated 
in  the  preparation  of  this  paper. 

It  may   be  noted  that  later  information  has  modified  some 
of  the  figures  for  temperature  limits. — (ED.) 


THE  problem  of  permissible  temperature  limits  in  electric 
apparatus  is  largely  that  of  the  durability  of  the  insulation 
used.  As  this  may  consist  of  materials  of  widely  varying  heat- 
resisting  qualities,  the  probem  resolves  itself  into  one  of  con- 
sideration of  the  properties  of  the  materials  themselves. 

The  durability  of  insulation  may  be  considered  from  two  stand- 
points, the  mechanical  and  the  electrical.  Temperatures  which  may 
ruin  the  insulation,  from  a  mechanical  standpoint,  may  not  radi- 
cally effect  its  dielectric  strength.  This  is  particularly  true  with 
moderate  voltages  where  the  insulation  serves  largely  as  a  separat- 
ing medium.  The  purpose  of  the  insulation  usually  is  two-fold: 
First,  it  must  serve  to  separate,  mechanically,  the  electric  conduc- 
tors from  each  other,  and  from  other  conducting  structures,  and 
second,  it  must  withstand  the  voltage  between  the  electric  con- 
ductors and  between  the  electric  circuits,  and  other  con- 
ducting parts.  In  lower  voltage  apparatus,  usually  only  the 
former  function  applies,  as  the  mechanical  separation  is  more 
than  sufficient  to  withstand  the  voltage  used.  The  dielectric 
strength  of  the  material  is,  however,  of  first  importance  in  high 
voltage  apparatus. 

A  great  majority  of  the  electrical  "breakdowns"  on  low 
voltage  apparatus  is  due  to  mechanical  weaknesses,  as  far  as  the 
temperature  problem  is  concerned;  that  is,  high  temperatures 
may  make  the  insulation  brittle,  or  crisp,  so  that  it  may  flake  off, 
or  powder,  or  crack,  or  be  crushed  by  mechanical  action,  thus 
allowing  the  conductors  to  make  contact  with  each  other  or  with 
adjacent  conducting  material. 


354  ELECTRICAL  ENGINEERING  PAPERS 

The  "  life  of  insulation  "  is  an  indefinite  term  and  must  bo  de- 
fined in  time,  mechanical  strength,  absence  of  foreign  materials 
of  a  conducting  nature,  etc.  Almost  all  insulating  materials 
will  be  somewhat  affected  in  time,  and  many  of  them  tend  to  be- 
come dry  and  brittle.  The  rate  at  which  deterioration  occurs 
\\ith  any  given  material,  is  some  complex  function  of  the  tem- 
perature and  of  other  conditions. 

CLASSES  OF  INSULATIONS 

Insulations  may  be  classified  under  three  headings,  depend- 
ing upon  their  heat-resisting  properties.  However,  all  such 
classifications  must  be  relative,  for  no  absolute  limit  can  be  fixed, 
as  there  is  no  definite  point  at  which  injury  or  destruction  can  be 
said  to  take  place. 

The  usual  insulating  materials  can  be  considered  as  included 
m  three  general  classes: 

Class  A.  This  includes  most  of  the  fibrous  materials,  as 
paper,  cotton,  etc.,  most  of  the  natural  oil  resins  and  gums,  etc. 
As  a  rule,  such  materials  become  dry  and  brittle,  or  lose  their 
fibrous  strength  under  long  continued  moderately  high  tempera- 
ture, or  under  very  high  temperature  for  a  short  time. 

Class  B,  This  includes  what  may  be  designated  as  heat-re- 
sisting materials,  which  consist  of  mica,  asbestos,  or  equivalent 
refractory  materials,  frequently  used  in  combination  with  other 
supporting  or  binding  materials,  the  deterioration  of  which,  by 
heat,  will  not  interfere  with  the  insulating  properties  of  the  final 
product.  However,  where  such  supporting  or  binding  materials 
arc  in  such  quantity,  or  of  such  nature,  that  their  deterioration 
by  heat  will  greatly  impair  the  final  product,  the  material  should 
be  considered  as  belonging  to  class  A 

Class  C.  This  is  represented  by  fireproof,  or  heat-proof 
materials,  such  as  mica,  so  assembled  that  very  high  tempera- 
tures do  not  produce  rapid  deterioration.  Such  materials  are 
used  m  rheostats  and  in  the  heating  elements  of  heating 
appliances,  etc 

All  the  above  are  relative  terms.  The  first  class,  for  instance, 
represents  materials  which  are  really  more  or  less  heat-resist- 
ing, but  which  deteriorate  at  lower  temperatures  than  those  in 
the  second  class,  which  are  defined  as  heat-resisting.  Also,  the 
fireproof  materials  of  the  third  class  are  not  strictly  heat-proof 
or  fireproof,  but  will  simply  withstand  very  high  temperatures 
for  rclativelv  long  pcnods  without  undue  deterioration 


TEMPERATURE  AND  INSULATION 


355 


In  class  A,  the  materials  appear  to  have  a  very  long  life  (or  an 
almost  indefinitely  long  life,  aside  from  mechanical  conditions) 
if  subjected  to  ultimate  temperatures  which  never  exceed  90 
deg,  cent.  Also,  they  appear  to  have  a  comparatively  long  life, 
even  at  ultimate  temperatures  as  high  as  100  deg.  cent.  At 
materially  higher  temperatures  than  100-deg.  cent.,  the  life  is 
very  greatly  shortened,  and  temperatures  of  125  deg.  cent,  will 
apparently  ruin  the  insulation,  from  a  mechanical  standpoint,  in 
possibly  a  few  weeks,  if  such  temperature  is  maintained  steadily 
However,  for  low  voltages,  the  insulating  qualities  may  still  be 
very  satisfactory,  even  at  this  temperature,  and  therefore  the  de- 
struction of  the  insulation  is  purely  one  of  injury  or  breakdown 
from  the  mechanical  standpoint,  as  stated  before.  Tempera- 


10 
8 

W6 

flC 

*. 

2 

o. 

— 

— 

— 

\ 

\ 

\ 

^^ 

ft         75         100         125         130 

DEGREES  C 

FIG.  1 

tures  as  high  as  160  deg.  cent,  on  such  insulations  for  a  con- 
siderable period  may  not  entirely  destroy  their  insulating  qual- 
ities, althoiigh,  mechanically,  such  temperature?  appear  to  be 
impracticable,  except  for  very  short  periods. 

In  order  to  illustrate  the  relation  between  the  possible  life 
and  temperature  of  class  A  insulation,  Fig.  1  is  shown.  This 
must  not  be  taken  as  representing  actual  results,  but  is  simply  in- 
tended to  illustrate,  in  a  very  approximate  manner,  the  very  great 
shortening  of  the  Kfe  of  insulation  by  increase  in  temperature. 

It  may  be  assumed  that  at  very  high  temperatures,  the  insu- 
lation will  have  practically  the  same  life,  in  actual  hours  of  high 
temperature  operation,  whether  the  temperature  is  applied  con- 
tinuously or  intermittently.  For  example,  if  an  insulation  has 
10,000  hours  Kfe  with  a  certain  high  temperature  continuously 


356 


ELECTRICAL  ENGINEERING  PAPERS 


applied,  it  is  assumed  that  it  will  also  stand  the  same  tempera- 
ture for  10,000  hours  in  short  periods,  provided  the  intermediate 
temperatures  are  low  enough  to  represent  an  indefinitely  long 
life.  It  is  probable  that  under  the  intermittent  condition,  the 
life  will  really  be  slightly  greater,  due  to  the  fact  that  depre- 
elation  will  be  largely  mechanical,  and  the  insulation  may  "  re- 
cover/* in  some  of  its  mechanical  characteristics  after  each  period 
of  high  heating. 

If 3  therefore,  higji  temperatures 'are  reached  intermittently, 
with  intermediate  periods  of  lower  value  but  still  high  enough 
to  shorten  the  life  of  the  insulation,  it  may  be  assumed  that  the 
total  life  o£  the  insulation  is  the  resultant  of  the  life  under  the 
two  temperature  conditions. 


10 

3 

t-~i— 

-b 

5 

j 

U 

r 

j 

V 

0 

"x 

*--, 

50        100        150        200        250 
DEGREES  C. 

FIG.  2 


In  heat-resisting  materials,  such  as  those  of  class  B  tempera- 
tures of  125  deg  cent  are  comparable  with  85  deg.  cent  or  90 
deg  cent  in  class  A,  and  150  deg  cent  in  the  tormer  is  comparable 
with  100  deg.  cent  in  the  Utter  Pig  2  illustrates  very  approxi- 
mately the  life-temperature  curve  of  such  insulations  As  in  Fig 
1 ,  this  should  not  be  taken  as  an  exact  representation  of  the  actual 
life  Due  to  the  greater  heat-resisting  qualities  of  such  materials, 
it  appears  that  relatively  higher  temperatures'are  not  as  quickly 
harmful  as  in  the  first  class 

In  class  C  materials,  it  is  difficult  to  give  any  reasonable  indi- 
cation as  to  the  limits  of  temperature,  except  that  very 'high 
temperatures,  (practically  up  to  the  point  of  incandescence)  are 
found  in  some  heating  appliances. 


TEMPERATURE  AND  INSULATION 


357 


TEMPERATURES  AND  FLOW  OF  HEAT 

As  the  insulation,  in  itself,  is  not  usually  the  seat'  of  generation 
of  loss  or  heat,  it  is  the  temperature  of  adjacent  materials  which 
must  be  considered  in  defining  the  conditions  in  the  insulation. 
The  temperatures  of  the  adjacent  materials  should  therefore  be 
considered  only  in  so  far  as  they  affect  the  insulation  itself,  and 
where  such  temperatures  do  not  affect  the  insulation,  or  the  life 
of  the  apparatus,  or  its  normal  perfomance,  they  are  immaterial 
Considering  the  influence  of  the  temperatures  of  the  adjacent 
media,  the  direction  and  amount  of  heat  flow  must  be  taken  into 
account,  as  the  maximum  temperature  in  the  insulatiori  is  de- 
pendent upon  these.  In  the  case  of  armature  windings,  for 
instance,  the  heat  flow  may  be  from  the  buried  portion  of.  the 


coils  toward  the  end  windings.  It  also  may  be  from  the  buried 
copper  through  the  insulation  to  the  armature  teeth,  or  there  may 
be  a  reverse  heat  flow  from  the  iron  to  the  copper,  depending 
upon  the  various  factors  of  construction,  heat  conductivity  of 
the  materials,  amount  of  heat  generated  in  the  various  parts, 
ventilation,  heat  dissipation  etc. 

Depending  upon  conditions  of  heat  flow  and  distribution, 
various  methods  of  temperature  determination  may  be  used. 
No  method  is  accurate,  unless  all  the  conditions  of  heat  flow  are 
accurately  known,  which  is  never  the  case  in  commercial  ma- 
chines. 

The  difficulties  in  the  problem  of  commercial  temperature 
determination  are  illustrated  by  Fig.  3. 


358  ELECTRICAL  ENGINEERING  PAPERS 

In  the  figure,  a  represents  the  temperature  inside  an  armature 
coil,  b  the  temperature  between  the  insulation  and  the  iron  of  an 
armature  tooth,  c  that  in  the  body  of  the  tooth,  and  d  that  in  the 
body  of  the  core  at  some  point  back  of  the  coils  and  teeth.  "Lei 
the  temperatures  at  no  load  be  represented  on  the  ordinate  A 
Then,  at  some  load,  represented  by  ordinate  B,  the  relations 
of  the  various  temperatures  have  changed.  At  C,  D  and  E, 
there  are  still  greater  changes,  depending  upon  the  heat  genera- 
tion and  distribution.  If  the  rated  capacity  of  the  machine  is 
at  E,  for  instance,  then  the  armature  copper  is  hotter  than  the 
iron,  while  if  rated  at  B,  the  reverse  would  be  true.  Obviously, 
no  rule  can  be  formulated  to  cover  these  various  conditions  in 
different  machines,  nor  even  in  a  given  machine,  unless  all  the 
heat  generation,  distribution,  and  dissipation  characteristics  are 
known.  Obviously,  as  far  as  the  insulation  is  concerned,  the 
temperatures  of  a  and  b  are  the  only  ones  which  need  be  consid- 
ered. 

All  temperature  determinations  of  a  commercial  nature,  are 
necessarily  approximations,  or  relative  indications,  upon  which 
proper  margins  must  be  allowed  for  the  ultimate  temperature 
possibly  attained.  Therefore,  in  apparatus  where  there  are 
liable  to  be  discrepancies  of  10  dcg.  between  the  measurable  and 
the  actual  idtimate  temperatures,  a  limit  of  90  deg  cent,  should 
be  allowed  by  conventional  temperature  measurement  on  insu 
lations  in  which  100  deg.  is  set  as  the  maximum  temperature  with 
a  reasonable  length  of  life. 

The  conventional  methods  of  temperature  measurement,  as 
by  resistance,  and  by  thermometer,  do  not  usually  give  the  maxi- 
mum temperature,  but  give  either  the  average,  or  the  outside  sur- 
face, values,  and,  when  measuring  the  temperature  by  these 
methods,  which  are  the  only  ones  generally  applicable,  an  allou  - 
ance  must  be  made  m  windings  for  possible  local  higher 
temperatures.  These  methods  apply  especially  to  those  ma- 
chines of  moderate  or  low  voltages  in  which  the  insulation  is 
relatively  thin,  so  that  the  heat  gradient  from  the  inside  copper 
to  the  outside  surface  is  small.  Also,  they  apply  particularly  to 
those  machines  m  which  the  conditions  of  ventilation  are  not  nor- 
mally difficult,  and  in  which  a  fairly  thorough  distribution  and 
dissipation  of  heat  occurs  among  the  various  parts,  such  as  in 
ordinary  direct-current  armatures,  induction  motors  primaries, 
stators  and  rotors  of  moderate,  speed  alternators  m  which  the 
width  is  relativelv  small  compared  with  the  diameter,  etc 


TEMPERATURE  AND  INSULATION  359 

As  the  ultimate  temperatures  obtained  by  the  apparatus  de- 
pend upon  its  rise  above  the  room  temperature,  or  that  of  the 
cooling  medium,  and  as  such  temperatures  may  vary  over  a  wide 
range,  it  is  not  practicable  to  specify  or  guarantee  ultimate  tem- 
perature of  apparatus  without  also  specifying  the.  elements  upon 
which  it  depends  This,  therefore,  results  in  specifying  the 
temperature  rise  in  relation  to  that  of  the  cooling  medium. 

While  most  apparatus  operates  at  materially  lower  cooling 
temperature  than  35  deg.  cent,  to  40  deg  cent,,  yet  such  tem- 
peratures are  sometimes  reached  for  considerable  periods  of  time 
in  steam  stations,  and  it  appears  therefore  as  justifiable  to  choose 
the  permissible  temperature  rise,  such  that,  at  room  temperature 
of  35  deg.  cent,  to  40  deg.  cent  ,  an  ultimate  temperature  of  85 
deg.  cent  to  90  deg.  cent,  by  conventional  methods  of  measure- 
ment, is  not  exceeded  This  means,  therefore,  a  temperature 
rise  of  50  deg.  cent,  with  conventional  methods  of  testing,  such 
as  by  increase  of  resistance,  or  by  thermometer,  in  those  insula- 
tions which  can  stand  a  continuous  ultimate  temperature  of 
100  deg  cent  with  a  comparatively  long  life.  This  allows  an 
excess  of  10  deg.  cent,  to  15  deg.  cent,  for  local  spots,  or  for  the 
temperature  gradient  through  the  insulation.  A  less  allowance 
should  be  made  for  this  difference  when  methods  of  temperature 
measurement  other  than  the  conventional  are  used,  and  which 
approach  more  closely  to  the  highest  temperature  actuallv  at- 
tained 

When  the  above  temperatures  are  liable  to  be  materially 
exceeded  for  long  periods,  heat-resisting  insulation  of  class  B  is 
recommended  With  such  materials,  a  temperature  of  125  deg. 
cent  is  comparable  with  85  deg  cent  to  90  deg.  cent,  in  the 
materials  of  class  A  Therefore,  on  this  basis  of  a  room  tem- 
perature at  40  deg  cent  or  45  deg  cent.,  rises  of  85  deg.  cent  or  SO 
deg  cent  should  not  be  considered  harmful  However,  in 
those  special  cases  where  the  conventional  methods  may  not 
sufficiently  approximate  local  high  temperatures,  as  may  be 
the  case  in  large  turbo-generators,  or  in  wide  core  alterna- 
tors of  large  capacity,  the  rises. of  80  deg  cent,  or  85  dc« 
cent  should  not  be  specified  by  resistance  or  thermometer, 
but  preferably  some  lower  temperature  such  as  50  deg  cent 
thus  allowing  a  very  considerable  margin  for  local  higher  tem- 
peratures In  such  apparatus  with  the  higher  temperature*, 
which  require  class  B  insulation,  there  fe  liable  to  be  less  uniform- 
itv  of  heat  distribution 


360  ELECTRICAL  ENGINEERING  PAPERS 

If  special  methods  of  temperature  measurement,  such  as  ex- 
ploring coils  or  thermo-couples  are  used  in  such  apparatus,  the 
temperature  limit  of  125  deg  cent,  should  be  considered,  and  not 
the  conventional  50  deg  cent  rise.  In  those  machines  of  this 
class  which  have  relatively  thick  insulation,  and  consequently 
may  have  a  high  heat  gradient  between  the  copper  and  the  iron,, 
(depending  upon  how  much  heat  is  flowing  from  the  copper  to- 
the  iron)  an  ultimate  temperature  of  the  inside  insulation  of 
150  deg  cent  is  considered  as  the  limit,  this  being  comparable 
with  100  deg.  cent  with  insulations  of  class  A. 

In  certain  classes  of  apparatus  which  are  artificially  cooled  by 
air  from  outside  the  room,  the  cooling  is  accomplished  partly  by 
dissipating  heat  to  the  artificial  air  supply,  and  partly  by  dissi- 
pation into  the  surrounding  room.  If  the  temperatures  of  the 
cooling  air  and  of  the  room  are  widely  different,  the  resultant  of 
the  two  temperatures  should  really  be  taken  as  that  of  the  cool- 
ing medium. 

The  variation  of  the  temperature  rise  has  heretofore  been 
considered  as  having  a  definite  relation  to  the  temperature  of  the 
cooling  medium.  However,  it  appears  that  it  does  not  follow 
any  definite  simple  law,  but  it  is  sometimes  positive  and  some- 
times negative,  so  that  no  satisfactory  correction  for  room  tem- 
perature is  possible  at  present.  It  is  therefore  desirable  to  make 
the  temperature  tests  at  a  room,  temperature  as  nearly  as  pos- 
sibte  to  some  specified  reference  temperature,  so  as  to  make  any 
temperature  correction  negligible  The  reference  temperature 
in  the  guarantees  should  therefore  be  such  as  Can  easily  be  secured ; 
that  is,  it  should  be  the  average  temperature  of  the  places  at 
which  the  apparatus  may  be  operated  This  is  from  20  deg. 
:eiit  to  25  deg.  cent ,  and  as  it  is  easier  to  raise  than  to  lower  the 
room  temperature,  the  upper  figure  is  advisable  as  a  reference 
i*alue.  This 'reference  temperature  therefore  should  be  chosen 
is  25  deg  cent., which  is  in  accordance  with  the  previous  A  I  E  E 
standard. 

MEASUREMENT  OF  TEMPERATURE 

In  the  conventional  methods  of  temperature  measurement, 
>y  thermometer,  and  by  resistance,  many  conditions  should  be 
aken  into  account,  and  good  judgment  is  required,  in  all  cases, 
>r  fallacious  conclusions  may  be  obtained 

There  are  many  conditions  which  affect  both  the  accuracy  of 
he  resistance  and  the  thermometer  methods  of  measuring  tern* 
>eraturc  The  resistance  method  measures  only  the  average 


TEMPERATURE  AND  INSULATION  361 

temperature  rise,  and  not  that  of  local  hot  spots.  However,  it 
measures  the  internal  temperature  of  windings,  and  therefore  no 
correction  is  required  for  the  temperature  gradient  through  the 
outside  insulation  The  proposed  margin  between  the  result 
by  the  conventional  method,  and  the  actual  temperature  can 
therefore  be  allowed,  in  the  resistance  measurement,  as  the  dif- 
ference between  the  warmer  and  the  average  temperatures  m 
the  windings.  In  the  resistance  metho<J  of  measurements,  the 
rate  of  transfer  of  heat  from  one  part  of  the  winding  to  another 
will  not  greatly  affect  the  result,  as  the  measurement  indicates 
an  average  temperature,  which  is  that  obtained  if  the  heat  were 
equalized  throughout  the  winding.  However,  the  rate  of  flow 
of  heat  from  the  windings  through  the  outer  insulation  to  other 
parts,  will  affect  the  temperature  measurement  by  resistance,  and 
preferably  the  measurement  by  this  method  should  be  taken 
during  operation  in  those  parts  where  this  is  practicable,  as  in 
field  coils,  and  some  other  instances.  In  those  parts  where  the 
resistance  cannot  be  measured  during  operation,  this  should  be 
done  as  quickly  as  possible  after  shut-down,  and  the  time  taken 
to  shut  down  the  apparatus  should  not  be  unduly  long.  Prefer- 
ably, during  shut-down  of  rotating  apparatus  the  normal  current 
should  be  maintained  on  the  apparatus  until  at  least  a  relatively 
low  speed  is  obtained.  This  would  represent  only  an  average 
condition,  as  the  ventilation  at  lower  speed  is  very  greatly  de- 
creased, while  the  losses  in  the  windings  will  remain  normal, 
thus  tending  to  give  an  increased  temperature  in  the  windings. 
It  would  be  difficult  to  fix  any  definite  rule  which  would  give  the 
exact  temperature  conditions  during  shut-down. 

In  the  measurement  of  temperatui*e  by  thermometer,  con- 
siderable judgment  is  required  Wherever  possible,  the  tern-  " 
perature  should  be  taken  during  operation,  but  the  thermometer 
with  its  pad  should  be  so  placed  that  it  does  not  interfere  with 
'the  normal  air  circulation.  In  thermometer  readings,  as  usually 
obtained  on  windings,  the  heat  gradient  thrdugh  the  insulation 
must  usually  be  allowed  for,  this  being  10  deg.  to  15  deg  as 
previously  defined  However,  depending  upon  the  method  of 
taking  the  temperatures,  this  allowance  should  vary  over  a  con- 
siderable range,  depending  upon  whether  or  not  the  method  of 
measurement  approximates  the  actual  internal  temperature 
For  instance,  the  total  heat  gradient  from  the  inside  copper  to 
the  outside  air  will  be  that  through  the  coil  insulation,  plus  the 
thick  covering  pad  over  the  temperature  bulb  If  the  gradient 


362  ELECTRICAL  ENGINEERING  PAPERS 

through  the  covering  pad  is  very  large  compared  with  that 
through  the  insulation,  the  thermometer  may  indicate  almost 
exactly  the  internal  temperature  of  the  copper;  that  is,  the  heat 
gradient  through  the  insulation  to  the  thermometer,  may  be  rela- 
tively small  compared  with  the  total  gradient  to  the  air.  This 
is  particularly  true  where  the  thermometer  rests  on  a  metallic 
seat  which  covers  a  considerable  portion  of  the  coil  surface.  In 
this  case,  the  heat  which  affects  the  thermometer  bulb  will  pass 
through  a  relatively  large  section  of  surface,  with  a  correspond- 
ingly small  drop  in  temperature,  so  that  the  bulb  more  closely 
approximates  the  temperature  of  the  inside  copper. 

Where  there  is  local  heating  in  the  windings,  and  a  consequent 
liability  of  rapid  transference  of  heat  to  other  parts,  the  results 
obtained  by  the  thermometer  method  will  vary  to  some  extent 
with  the  rapidity  with  which  the  actual  measurement  is  made; 
that  is,  the  more  quickly  the  thermometer  can  be  brought  up  to 
the  full  temperature,  the  more  accurately  the  temperature  of 
the  hottest  part  is  determined.  With  a  very  rapid  method  of 
measurement,  it  may  be  possible  to  measure  practically  the  in- 
ternal temperature  of  the  copper  of  the  winding  before  any  great 
heat  transference  or  dissipation  has  occurred.  In  such  cases, 
obviously,  the  full  allowance  for  the  usual  temperature  margin 
should  not  hold.  It  should  be  fully  understood  that  it  is  the 
ultimate  temperature,  and  not  the  temperature  rise,  which 
should  be  considered  as  the  limiting  condition,  and  that  the 
measured  rise,  plus  the  allowances  for  temperature  gradient, 
plus  the  measured  room  temperature,  is  simply  an  indication  of 
the  possible  ultimate  temperature.  By  whatever  method  the 
temperature  measurement  is  made,  in  all  cases  the  results  may 
TDC  considered  as  more  or  less  approximate,  and  in  the  end,  it  is 
the  manufacturer  who  must  supply  the  necessary  margin  over 
the  approximate  measurement,  in  order  to  make  the  machine 
safe. 

A  "blind  adherence  to  some  particular  rule  or  method  of  taking 
temperatures,  may  lead  to  fallacious  results  in  some  instances. 
In  armature  windings,  in  particular,  incorrect  readings  may  be 
obtained  after  shut-down.  For  example,  if  the  armature  iron 
back  of  the  armature  teeth  were  hotter  than  the  armature  teeth 
and  coils  during  operation,  then  the  temperature  to  which  the 
insulation  is  subject  during  operation  may  be  considerably  lower 
than  that  in  the  hottest  part  of  the  machine,  due  to  the  ventila- 
tion conditions  when  running.  However,  upon  shut-down,  the 


TEMPERATURE  AND  INSULATION  363 

temperature  at  the  insulation  may  rise  to  that  of  the  hottest 
part  of  the  machine,  and  therefore  a  false  temperature,  by  any 
method  of  measurement,  might  be  indicated. 

RECOMMENDATIONS 

That  with  class  A  insulation,  90  deg.  cent,  be  taken  as  the 
ultimate  temperature  limit,  as  indicated  by  conventional  methods 
of  measurement,  or  those  which  give  similar  results,  and  that 
100  deg.  cent,  be  considered  as  the  maximum  ultimate  tempera- 
ture permissible  in  the  insulation,  where  a  comparatively  long 
life  is  a  requirement. 

That  40  deg.  cent,  be  taken  as  the  limiting  temperature  of  the 
cooling  medium,  or  room,  and  that,  therefore,  50  deg.  cent,  be 
the  permissible  rise  by  conventional  methods  of  measurement, 
with  class  A  insulation. 

That  25  deg.  cent,  be  taken  as  tine  reference  air  temperature. 
With  the  permissible  50  deg.  cent  nse,  this  gives  75  deg.  cent, 
as  the  average  operating  condition,  by  conventional  methods  of 
measurement,  or  85  deg  cent,  actual  temperature,  when  the 
usual  margin  represented  by  the  temperature  gradient  is  added. 

An  exception  to  the  rise  of  50  deg.  cent,  can  be  made  in  those 
cases  where  space  or  weight  limitations  are  such  that  higher 
temperatures,  with  consequent  reduced  life,  are  commercially 
economical, such  as  in  railway  motors.  In  such  cases,  with  class 
A  insulation ,  a  nse  of  65  deg.  cent,  with  reference  air  at  25  deg 
cent,  is  at  present  accepted  as  good  practice. 

With  class  B  insulations,  125  deg  cent  be  taken  as  the  ultimat 
temperature  limit,  as  indicated  by  conventional  methods  of 
measurement,  or  by  equivalent  methods,  and  150  deg.  cent,  be 
considered  as  the  maximum  ultimate  temperature  permissible 
in  the  insulation  It  follows  therefore  that  80  deg  cent,  to  85 
deg.  cent,  nse  is  allowable,  with  such  insulations,  by  the  usual 
methods  of  measurement 

No  temperature  correction  should  be  made  for  variation  of 
the  cooling  temperatures  from  the  reference  temperature  of  25 
deg.  cent 

When  the  method  of  temperature  measurement  shows  the 
highest  temperature  actually  obtained  in  the  insulation,  the  maxi- 
mum temperatures  specified  for  the  given  type  of  insulation 
should  hold. 

In  the  final  "decision  on  questions  of  temperature  rise,  the  ulti- 
mate temperature  should  be  the  basis,  rather  than  the  rise. 


TEMPERATURE  DISTRIBUTION  IN  ELECTRICAL 
MACHINERY 


FOREWORD — This  paper  was  presented  at  the  Chicago  Section  meet- 
ing of  the  American  Institute  of  Electrical  Engineers,  November 
27,  1916.  A  number  of  papers  by  the  author  dealing  with  the 
temperature  problem  had  appeared  tbef  ore,  but  the  purpose  of 
this  paper  was  to  put  the  subject  in  more  definite  shape  and 
bring  it  more  nearly  up  to  date.  During  the  discussion  of  the 
paper,  considerable  new  data  was  presented  by  the  author,  and 
it  has,  therefore,  been  included  in  this  reprint. 

This  paper  was  listed  for  a  second  presentation  before  a 
regular  meeting  of  the  Institute  at  Schenectady  in  April,  1917, 
with  a  view  to  obtaining  a  further  discussion,  particularly  by 
engineers  on  design  work.  This  meeting  was  cancelled  due  to 
the  declaration  of  war. — (ED.) 


THE  laws  governing  heat  flow  and  temperature  distribution 
are  so  similar,  in  many  respects,  to  those  governing  electric 
current  flow  and  electric  potentials,  that  it  is  rather  surprising 
that  the  former  have  received  so  little  attention  in  comparison 
with  the  latter.  Some  of  the  laws  of  heat  flow  are  so  well  recog- 
nized that  their  application  to  the  problem  of  temperature  dis- 
tribution in  electric  apparatus  should  have  been  a  leading  feature 
in  the  early  developments  in  such  apparatus;  whereas,  on  the 
contrary,  it  is  only  recently  that  very  careful  study  has  been, 
made  of  such  application. 

One  object  of  this  paper  is  to  indicate,  in  a  comparatively 
simple  manner,  some  of  the  conditions  which  fix  the  tempera- 
tures in  different  parts  of  electric  apparatus.  The  explanations 
given  cannot  be  considered  as  new  or  novel  in  substance,  but  are 
merely  the  application  of  fairly  well  known  principles  of  temperature 
and  heat  flow  to  electrical  machinery.  Before  going  into  the  general 
problem,  certain  simple  conditions  may  be  stated,  such  as: 

1.  The  heat  flow  between  two  points  is  proportional  to  their 
temperature  difference  and  to  the  heat  resistance  of  the  path  or 
paths  between  .them.  Note  the  resemblance  to  Ohm's  law. 

As  a  corollary  to  the  above,  it  should  be  evident  that  between 

365 


366  ELECTRICAL  ENGINEERING  PAPERS 

two  points  at  the  same  temperature,  there  should  be  no  flow  of 
heat. 

2.  The  total  temperature  drop  between  any  two  points  or 
media  of  different  temperatures  will  be  the  same  through  all 
paths  of  heat  flow. 

3.  There  are  no  true  non-conductors  of  heat,  and,  conversely, 
no  perfect  conductors  • 

4  Heat  conduction  and  electric  conduction  bear  some  quan- 
titative relation  to  each  other,  in  the  broad  sense  that  all  electric 
insulators  are  relatively  poor  heat  conductors,  while  good  electric 
conductors  are  correspondingly  good  heat  conductors.     There 
is  apparently  no  rigid  relation  between  the  heat  resistance  and 
electric  resistance  of  the  various  materials  used  in  electric  ma- 
chinery, but  the  general  relation  holds  and  there  are  apparently 
no  radical  exceptions 

5  The  rise  in  temperature  at  any  point,  due  to  generation 
of  heat,  is  dependent  (a)  upon  the  total  heat  generated,  and  (b) 
upon  the  amount  of  heat  which  can  be  earned  away  along  all 
available  paths  per  decree  of  temperature  difference.    The  tem- 
perature will   rise  until  the  heat  dissipation  equals  the  heat 
generation 

6  There  are  two  ways  to  lessen  the  heat  flow  along  any  path, 
(a)    By  interposing   higher  heat  resisting  materials,      (b)    By 
lessening  the  temperature  difference,  as  by  raising  the  tempera- 
ture of  the  part  through  which  the  heat  is  to  be  conducted. 
Conversely,  the  heat  flow  can  be  increased  along  any  path  by 
the  use  of  better  heat  conducting  materials,  or  by  paths  of  lower 
heat  resistance,  and  by  lessening  the  temperature  of  any  part 
to  which  the  heat  is  to  flow. 

What  makes  the  problem  unduly  complicated,  in  electrical 
machinery,  is  the  fact  that  there  are  several  different  sources 
of  heat  generation,  which  may  be,  and  often  are,  all  active  at 
the  same  time.  Moreover,  the  heat  losses  may  be  distributed 
through  the  various  heat  conducting  paths  in  such  a  way  as  to 
render  any  calculation  very  difficult  and  more  or,  less  inexact, 
except  in  a  general  way.  For  example,  there  is  heat  generated 
by  losses  in  the  copper  conductors,  obeying  one  law;  while  there 
is  heat  generated  in  the  iron  parts  under  a  quite  different  law, 
and  there  may  be  heat  generated  by  windage  and  friction, 
according  to  a  third  law.  As  these  different  losses  may  act  in 
different  parts  of  the  heat  conducting  circuit,  it  should  be  evident 
that  the  problem  of  determining  the  exact  heat  distributions, 


TEMPERATURE  DISTRIBUTION 


367 


and  the  temperature,  is  a  very  complex  one.  Such  a  determina- 
tion is  in  the  province  of  the  expert  analytical  designer  of  such 
apparatus,  but  certain  general  conditions  are  of  interest  to  all 
users  of  electric  apparatus. 

Consider  first  the  general  conditions  of  heat  dissipation  from 
an  armature  coil  In  Fig.  1  is  represented  an  armature  slot  with 
the  surrounding  iron,  and  with  two  separate  "coils"  per  slot,  as. 
is  now  the  most  common  practise.  Let  it  be  assumed  that  the 
point  a  represents  the  "hot  spot",  or  part  at  highest  temperature- 
in  the  apparatus.  The  heat  from  this  part  can  flow  along  two 
general  paths,  namely,  longitudinally  through  the  copper"  con- 
ductor itself  to  the  end  windings,  and  thence  to  the  air,  and. 


laterally  through  the  insulation  to  the  surrounding  iron,  or  to 
the  ventilating  ducts.  From  the  iron  the  heat  flow  is  then 
through  various  paths  to  the  external  cooling  air. 

LONGITUDINAL  HEAT  FLOW 

Considering  first  the  longitudinal  conduction  of  heat  in  the 
coil,  then  starting  at  the  point  a,  the  first  unit  of  length  con- 
ductor will  have  a  certain  loss.  If  the  heat  generated  by  this 
first  unit  loss  were  all  that  need  be  considered,  then  the  drop  in 
temperature,  from  the  point  a  to  the  end  windings,  would  be 
simply  a  function  of  the  heat-conducting  properties  of  the  con- 
ductor itself.  But  the  next  unit  length  is  also  generating  its 


368  ELECTRICAL  ENGINEERING  PAPERS 

own  unit  loss,  so  that  the  heat  flow  from  the  second  to  the  third 
unit  length  is  due  to  two  units  loss;   in  the  same  way,  the  flow- 
to  the  fourth  unit  length  will  be  due  to  three  units  loss,  etc 
Therefore,  the  temperature  drop,  or  temperature  difference  per 
unit  length  of  conductor,  increases  more  rapidly  as  the  point  a 
is  departed  from,   and  if  it  is  at  a  considerable  distance    from 
the  end  winding,  and  the  losses  per  unit  length  are  compara- 
tively high,  a  very  high  temperature  may  be  required  at  a  to 
conduct  all  the  heat  longitudinally  to  the  end  windings,     In 
very  wide  core  machines  the  longitudinal  drop  may  be  so  great 
that  the  temperature  at  a  in  practise  will  be  so  far  above  that 
of  the  surrounding  iron,  that  a  very  large  percentage  of  the 
actual  heat  is  conducted  laterally  through  the  insulation  to  the 
iron,  even  if  the  iron  is  at  a  comparatively  high  temperature, 
However,  in  narrow  cores,  the  drop  to  the  end  windings   may" 
be,  in  some  cases,  so  very  low,  possibly  5  to  10  degrees,    that 
with  good  heat  dissipation  from  the  end  windings  themselves, 
the  point  a  may  have,  for  instance,  an  actual  temperature  of 
40  deg.  cent.    If  the  iron  next  to  a  a'so  has  a  temperature  of  40  deg. 
cent,  then  there  would  be  no  flow  of  heat  from  a  to  the  iron.    Fxtr- 
thermore,  in  such  a  case,  as  the  iron  temperature  over  the  whole 
width  of  the  core  may  be  lairly  uniform,  and  as  the  copper 
temperature  decreases  from  a  to  the  end  windings,  obviously 
as  we  'depart  from  the  point  a,  there  would  be  heat  flow  from 
the  iron  to  the  copper,  and  thus  the  windings  would  tend  to 
cool  the  core.    This  is  frequently.,  the  case  with  light  loads  on 
a  machine,  for  in  sucji  conditions  the  coil  loss  is  low,  while  the 
iron  loss  remains  fairly  constant  for  all  loads      In  such  case 
there  may  be  heat  flow  from  the  iron  to  the  copper  along  the 
whole  length  of  the  buried  portion  of  the  coil      At  some  higher 
load,  the  copper  loss  varying  as  the  square  of  the  load,  the  in- 
creased  longitudinal   drop   will   bring    the  copper  temperature 
above  that  of  the  iron  so  that  the  heat  flow  is  from  copper  to- 
iron,    This  condition  is  illustrated  by  Fig  2 

It  must  be  recognized  that  the  lateral  flow  of  heat,  from  the 
coil  to  the  iron,  reduces  the  longitudinal  drop,  such  reduction 
depending  upon  the  relative  percentages  of  Jieat  flow  along  the 
t\\o  paths  ^  It  must  also  be  borne  in  mind  that  in  order  to  have 
such  longitudinal  heat  flow,  the  end  windings  must  be  able  to 
dissipate  their  own  heat  at  lower  temperature  than  would  b^ 
attained  at  a,  or  in  the  core  If  the  end  windings  have  little  or 
no  ventilation,  or  heat  dissipating  capacity,  then  their  own 


TEMPERATURE  DISTRIBUTION 


369 


generated  heat  may  bring  their  temperatures  higher  than  those 
of  the  armature  iron  so  that  the  heat  flow  actually  may  be  from 
the  end  windings  toward  a,  and  then  laterally  through  the  in- 
sulation to  the  core  In  such  case,  the  hottest  spot  will  be  in 
the  end  winding  rather  than  in  the  buried  part  of  the  coil  Obvi- 
ously when  such  condition  occurs  there  is  no  possibility  of  either 
the  end  windings  or  the  buried  part  of  the  coil  being  cooler  than 
the  iron,  for  the  heat  flow  throughout  is  toward  the  iron 

LATERAL  HEAT  FLOW 

Considering  next  the  lateral  flow  of  heat  through  the  insulation 
to  the  iron,  the  amount  of  heat  conducted  is  a  function  of  the 

temperature  difference  and  the 
resistance    of    the    conducting 
path.     Or,  in  other  words,  if  a 
given  amount  of  heat  is  to  be 
conducted    through   a  path   of 
given    resistance,   the  tempera- 
ture in  the  heat  generating  part 
iron  Temp's      will  rise  until  the  required  heat 
copper  Temp's.  is  conducted  away 


Light  Load 


Iron  Temp's 


Copper  Temp  s. 


Medium  Load 


Heavy  Load 


•Width  olCore* 


Copper  Temp  s 
Iron  Temp's. 


^30°C  Drop 

"  —  70«C  -4CTC  Rise  . 
in  Iron  with  Air  at  30*0" 


FIG.  2 


FIG.  3 


To  illustrate  this  problem  more  concretely,  let  Fig.  3  represent 
the  temperature  conditions  in  a  section  of  an  armature  Assum- 
ing, for  example,  the  temperature  of  the  copper  inside  the  coil 
insulation  as  100  deg  cent.,  the  iron  temperature  as  70  deg. 
cent. ,  and  the  air  temperature  as  30  deg  cenfy ,  then  the  following 
-conclusions  may  be  drawn. 

(a)  From  the  outer  coil  (the  one  next  to  the  air  gap)  through 
the  wedge  to  the  air  gap,  the  temperature  drop  will  be  100  — 
30  =  70  deg.  cent.  Obviously,  any  temperature  measurement 
made  outside  the  wedge,  next  to  the  air,  will  approximate  the 


370  ELECTRICAL  ENGINEERING  PAPERS 

> 

temperature  of  the  air  and  not  of  the  copper  Any  temperature 
measurement  made  beneath  the  supporting  wedge  will  measure 
some  intermediate  temperature  between  the  copper  and  the  air 
If  the  temperature  drop  through  the  wedge  should  be  equal  to 
that  through  the  insulation,  then  a  measurement  underneath 
the  wedge  should  show  half  the  temperature  drop  through  in- 
sulation and  wedge,  and  obviously,  the  measured  temperature 
would  be  far  below  that  of  the  copper. 

(b)  If  the  temperature  is  measured  at  the  outside  of  the  coil, 
between  the  iron  and  the  insulation,  it  would  approximate  the 
average  of  the  temperatures  of  the  iron  and  of  the  outside 
.insulation,  or  practically  the  temperature  of  the  iron      If  the 
iron  should  be  at  different  temperatures  at  the  sides  of  the  slot 
and  at  the  bottom,  then  obviously  different  readings  would  be 
obtained,  depending  upon  the  location  of  the  measuring  device. 
It  is  evident  that  such  temperature  measurements  give  no  in- 
dication whatever  as  to  the  true  internal  temperatures  of  the 
coil,  for  the  heat  flow  and  the  resistance  of  the  insulation  are 
nowise  involved  in  the  measurement. 

(c)  At  a  point  a,  between  the  two  coils,  there  should  be  but 
little  heat  flow  through  the  insulation,  unless  the  copper  is 
comparatively  narrow.    If  there  is  but  little  heat  flow  through 
the  insulation  at  this  point,  then  eventually  the  temperature  at 
the  point  a  must  rise  to  approximately  that  of  the  copper  in  the 
two  coils.     Therefore,   a  measuring  device  located  at  a  will 
approximate  the  temperature  of  the  copper  itself,  and  is,  in 
general,  a  good  indication  of  the  h&t  spot  at  that  part  of  the 
winding      Therefore,   as   a   practical   method   of   temperature 
determination,  a  thermo-couple  located  at  a  is  about  the  most 
satisfactory  device  that  we  have.    However,  the  location  of  the 
point  a  along  the  slot  is  also  of  importance  on  account  of  the 
longitudinal  flow  of  heat  in  the  conductor  and  the  consequent 
temperature  drop      In  other  words,  the  direction  of  heat  flow 
in  the  coil  itself,  musl  be  taken  into  account      Therefore,  £ 
thermo-couple  located  as  above,  is  only  satisfactory  when  the 
general  location  of  the  hot  spot  is  known  beforehand      This  is 
usually  determined,  in  a  general  way,  for  a  given  type  or  line 
of  machines,  by  locating  several  thermo-couples  along  the  slots 

With  narrow  slots  and  comparatively  thin  conductors,  and 
especially  with  very  heavy  insulation,  there  is  some  flow  of  heat 
through  the  insulation  which  lies  between  the  two  coils,  this 
heat  passing  out  sidewise  to  the  iron  In  such  case,  the  point  a 


TEMPERATURE  DISTRIBUTION  371 

may  be  of  somewhat  lower  temperature  than  the  copper.  It 
may  happen  also,  in  some  cases,  that,  due  to  unequal  losses  and 
heating  of  the  two  coils  in  the  same  slot,  one  is  at  a  higher  tem- 
perature than  the  other  In  such  case,  due  to  the  heat  flow 
between  the  coils,  the  temperature  indication  at  a  will  not  show 
better  than  an  average  of  the  two  temperatures  Furthermore, 
if  the  temperature  at  c ,  in  a  coil  subdivided  into  many  insulated 
conductors,  is  materially  higher  than  at  by  then  the  temperature 
indication  at  a  may  not  be  a  close  approximation  to  the  maximum 
temperature 

PLOW  THROUGH  IRON  PARTS 

In  the  ordinary  armature,  after  the  heat  passes  from  the 
copper  to  the  iron,  there  is  still  quite  a  problem  involved  m  the 
dissipation  to  the  surrounding  medium,  which  is  usually  the  air 
The  direction  of  the  heat  flow  to  the  iron  will  depend,  to  a  con- 
siderable extent,  upon  the  arrangement  and  location  of  the  heat 
dissipating  surfaces  There  are  two  general  paths  of  heat  con- 
duction in  all  armature  cores;  namely,  a  flow  along  the  lamina- 
tions to  where  their  edges  come  in  contact  with  the  air  or  with 
other  material,  and  a  flow  across  the  laminations  toward  heat 
dissipating  surfaces  The  flow  along  the  laminations  may  be 
calculated  with  fair  accuracy.  Across  them  it  is  difficult  to 
determine  such  flow,  largely  because  the  laminations  are  in- 
sulated from  each  other  by  materials  which  are  poor  conductors 
of  heat  Also  such  flow  is  affected  not  only  by  the  insulation 
between  laminations,  but  by  the  perfection  of  contact  In  other 
words,  the  heat  flow  may  be  affected  by  pressure.  According 
to  the  various  figures  available,  the  heat  flow  per  unit  volume 
of  material  along  the  laminations  is  from  ten  to  one  hundred 
times  as  great,  for  a  given  temperature  difference,  as  across 
them  Obviously,  therefore,  heat  dissipation  from  the  iron  by 
flow  across  the  laminations  should  be  considered  relatively  in- 
efficient, yet  in  the  vast  majority  of  rotating  machines  the  heat 
dissipation  is  largely  across  the  laminations.  The  reason  for 
this  is  that  by  placing  ventilating  passages  or  ducts,  parallel  with 
the  laminations,  at  frequent  intervals  in  the  core,  the  cross 
section  of  the  heat  path  in  the  intervening  iron  sections,  may 
be  made  very  large  compared  with  the  heat  to  be  dissipated, 
so  that  the  density  of  flow  is  very  low  By  the  same  procedure 
the  length  of  the  heat  path  is  made  quite  short  Thus  in  practice, 
the  temperature  drop  through  the  laminations  themselves  may 
be  made  relatively  small  compared  with  other  drops  However, 


£72  ELECTRICAL  ENGINEERING  PAPERS 

not  all  the  heat  in  the  iron  passes  across  the  laminations  to  the 
ventilating  ducts,  for  where  the  length  of  the  path,  along  the 
laminations  to  any  heat  dissipating  surface,  is  not  large,  a  very 
considerable  amount  of  the  heat  may  be  dissipated  from  the 
edges  of  the  laminations  themselves.  In  fact,  in  certain  types 
of  machines  with  very  shallow  iron  cores,  experience  has  shown 
that  the  ventilating  ducts,  parallel  with  the  laminations,  may  be 
omitted,  provided  good  ventilation  is  obtained  over  the  edges 
of  the  laminations.  It  is  evident,  therefore,  that  the  flow  of  heat 
and  distribution  of  temperature  are  dependent  upon  the  arrange- 
ment of  the  iron,  dimensions  and  location  of  the  ventilating 
surfaces  etc. 

HEAT  FLOW  TO  THE  AIR 

After  the  heat  has  passed  from  the  copper  to  the  iron,  the 
resultant  of  the  copper  and  iron  heats  must  be  conducted  to  the 
cooling  medium,  which  is  usually  the  surrounding  air  In  the 
case  of  air,  there  is  usually  a  considerable  drop  in  temperature 
from  the  solid  surface  to  the  cooling  air  itself,  the  amount  of 
such  drop  depending  upon  the  ventilating  conditions.  In  prac- 
tise, there  appears  to  be  a  film  or  layer  of  air  which  adheres  very 
closely  to  the  solid  surfaces.  This  forms  a  sort  of  heat  insulating 
film,  retarding  the  flow  of  heat  to  the  cooling  air  In  air  ven- 
tilation, the  effect  of  any  considerable  air  movement  over  the 
surface  appears  to  be  that  of  scouring  this  hot  film  away  from 
the  surface  and  replacing  it  with  a  film  of  cooler  air  Merely 
scouring  or  rubbing  the  hot  film  away  from  the  surface  is  not 
particularly  advantageous  unless  some  means  is  furnished  at 
the  same  time  for  supplying  an  ample  quantity  of  cooler  air  to 
take  the  place  of  the  removed  hot  film.  Rapid  air  circulation, 
by  means  of  a  supply  of  air  from  the  outside,  appears  to  accom- 
plish both  results  in  one  operation.  Thus,  one  of  the  principal 
actions  of  air  ventilation  appears  to  be  that  of  scouring  away  the 
hot  contact  film,  while  a  second  action  is  to  carry  the  hot  air 
away  without  mixing  it  with  the  incoming  cooler  air  Whatever 
portion  of  the  dissipated  heat  is  absorbed  by  the  incoming  cool- 
ing air  adds  that  much  to  the  temperature  of  the  air  itself  and 
eventually  to  that  of  the  apparatus  to  be  cooled.  Thus  mixing 
the  outgoing  with  the  incoming  air  makes  a  sort  of  Siemens* 
regenerative  furnace  and  the  machine  bocomes  cumulatively 
hottert  and  hotter  until  the  dissipation  through  other  paths  be- 
comes equal  to  the  heat  generated.  In  such  cases  the  ventilation 


TEMPERATURE  DISTRIBUTION  373 

of  the  machine  may  only  be  useful  in  equalizing  or  redistributing 
the  temperatures  in  the  various  parts. 

From  the  preceding  analysis,  it  would  appear  that  the  tempera- 
ture at  the  hottest  part  of  the  coil  is  fixed  principally  by  the  heat 
flow  through  the  copper,  and  its  surrounding  insulation,  directly 
to  the  air,  and  by  the  flow  from  the  copper  to  the  iron,  and  from 
the  iron  to  any  exposed  air  surfaces,  and  then  to  the  air.  Along 
the  first  path,  there  are  three  principal  temperature  drops, 
namely,  in  the  copper  itself,  then  through  the  insulation,  and 
then  from  the  outside  surface  of  the  insulation  to  the  air.  Along 
the  second  path,  there  are  also  three  temperature  drops;  namely, 
from  the  copper  through  the  insulation  to  the  iron,  then  from 
the  iron  to  the  exposed  air  surfaces,  and  then  from  the  surfaces 
to  the  air.  Along  the  first  path  each  part  of  the  copper  path  is 
generating  its  own  heat,  to  be  conducted  away,  in  addition  to 
that  which  is  to  be  conducted  from  other  parts  of  the  path.  In 
the  second  path,  each  part  of  the  iron  path  may  be  generating 
its  own  heat,  which  adds  to  that  coming  from  other  parts. 
The  relative  amount  of  heat  conducted  along  each  path  is  de- 
pendent upon  so  many  conditions,  which  vary  with  the  load, 
that  no  one  but  an  analytical  designer  backed  by  experience 
could  even  approximate  the  values  by  calculation.  However, 
it  should  be  obvious  that  any  measuring  device  applied  to  the 
outside  or  cooling  surface  does  not,  and  cannot,  directly  approxi- 
mate the  temperature  of  the  hottest  part,  except  in  those  rare 
cases  where  the  hottest  part  is  dissipating  heat  directly  to  the 
air.  This  is  true  only  in  very  special  cases  such  as  series  coils 
of  bare  strap,  etc.  In  any  coil  or  part  of  the  apparatus  which  is 
heavily  insulated,  that  is,  which  is  covered  by  poor  heat  conduct- 
ing materials,  an  external  temperature  measurement  is  an  ex- 
tremely poor  indication  of  the  true  internal  temperature,  unless 
many  other  conditions  are  known  which  may  give  an  indication 
of  the  internal  temperature  drops-  In  different  types  and  con- 
structions of  rotating  apparatus,  hot  spots  may  hold  quite 
different  relative  positions  with  respect  to  the  cores  and  wind- 
ings, so  that  no  reasonable  rule  can  be  made  to  cover  all  cases. 
Moreover,  in  some  classes  of  apparatus,  it  is  not  practicable  to 
make  any  temperature  roeasurements  until  after  the  apparatus 
is  shut  down,  and  this  introduces  otter  very  important  errors 
which  should  be  considered,  such  as  cooling  effects  as  a  whole, 
during  the  period  of  shut-down,  equalization  of  temperature 
due  to  internal  condttction,  etc. 


374  ELECTRICAL  ENGINEERING  PAPERS 

EQUALIZATION   OF    TEMPERATURE,  "E'TC. 

When  there  are  hot  spots,  or  zones,  or  areas,  of  different  tem- 
peratures, in  an  armature  winding,  for  instance,  such  difference 
in  temperature  is  maintained  by  the  continual  generation  of 
heat  in  the  various  parts  But  the  moment  that  such  generation 
of  heat  is  stopped  there  is  immediately  a  tendency  for  equaliza- 
tion of  temperatures  by  flow  of  the  stored  heat  from  the  hotter 
parts  to  the  cooler.  In  good  heat  conducting  materials,  as  copper, 
such  equalization  may  be  very  rapid,  so  that  a  temperature 
indicating  instrument  of  a  sluggish  type  may  not  indicate  any- 
thing like  the  true  maximum  temperature  of  the  spot  where  it 
is  placed,  if  applied  after  the  load  is  removed,  especially  if  the 
rate  of  heating  of  the  thermometer  bulb  is  much  less  than  the 
rate  of  heat  transfer  from  one  part  of  the  winding  to  another 
If  located  on  a  hot  spot,  the  reading  may  nse  to  some  interme- 
diate value  and  then  drop  off  as  the  hot  spot  cools  by  heat  con- 
duction to  other  parts.  If  located  upon  a  cool  spot,  it  may  rise 
slowly  for  a  considerable  period,  due  partly  to  sluggishness  of 
the  thermometer  and  partly  to  the  cool  spot  rising  in  temperature 
by  conduction  of  heat  from  some  other  part.  The  conditions 
are  so  varied  that  no  reliable  conclusions  can  be  drawn,  from  the 
action  of  the  "thermometer  alone,  in  regard  to  the  coolest  or 
hottest  spot, 

A  second  condition  which  tends  to  make  such  temperature 
measurements  fallacious,  lies  in  the  cooling  action  in  the  interval 
between  load  removal  and  shut-down  to  take  temperature 
measurements  In  apparatus  which  depends  upon  a  high  degree 
of  artificial  cooling,  such  cooling  effect  may  be  very  considerable. 
This  is  particularly  true  of  high  speed  machines  which  require 
considerable  time  to  come  to  a  standstill.  It  is,  therefore,  de- 
sirable m  such  machines  to  obtain  all  possible  temperature  read- 
ings at  normal  speed  and  with  load  In  rotating  field  machines, 
this  is,  to  a  certain  extent,  practicable,  but  in  most  rotating 
armature  machines,  the  armature  temperatures  usually  are  not 
attainable  until  the  machine  is  brought  to  a  standstill,  and  even 
then  some  error  may  result  from  sluggishness  or  delay  in  taking 
the  readings.  One  method  which  has  been  proposed  at  times, 
for  lessening  the  sluggishness,  is  to  heat  the  thermometers  up  to 
practically  the  normal  operating  temperature  of  the  part  to  be 
measured,  while  the  machine  is  still  carrying  load.  At  the  moment 
of  shut-down  the  heated  thermometer  is  applied.  This,  to  a 
certain  extent,  removes  the  factor  of  sluggishness  in  the  ther- 


TEMPERATURE  DISTRIBUTION  375 

mometer  itself,  but  is  only  a  partial  compensation  It  must  be 
considered  that  the  outside  of  the  insulation  is  at  lower  tempera- 
ture than  the  inside,  and  that,  therefore,  the  body  of  the  insula- 
tion itself  must  ha^e  its  temperature  increased  by  flow  of  heat 
from  other  parts. 

FALLACIES  IN  TEMPERATURE  GUARANTEES  AND  MEASUREMENTS 
In  the  older  methods  of  determining  temperatures,  it  was 
assumed  that  the  thermometer  readings,  obtained  on  a  winding, 
for  instance,  was  a  true  indication  of  the  temperature  of  the 
winding  as  a  whole  The  manufacturers  of  electrical  apparatus 
long  ago  recognized  the  fallacy  of  this  method,  as  they  had  found 
from  bitter  experience  that  there  were  liable  to  be  hotter  parts 
in  the  machine  than  any  thermometer  readings  would  indicate. 
They,  therefore,  designed  machines  with  regard  to  the  possible 
hot  spot  temperatures  as  encountered  in  service,  rather  than 
any  temperature  which  the  exposed  parts  of  the  machine  would 
show  Thus  in  designing  a  certain  machine  for  safety  at  the 
hottest  part,  not  infrequently  the  exposed  parts- of  the  winding 
would  show,  by  thermometer,  comparatively  low  temperatures, 
such  as  25  cleg,  to  33  deg  cent,  rise.  Therefore,  as  the  observable 
temperature  readings  came  so  low  it  became  the  fashion  to  call 
for  35  deg.  cent  guarantees  and,  in  many  cases,  the  operating 
public  lost  sight  of,  or  perhaps  never  knew,  the  real  meaning  of 
such  low  temperatures.  Among  the  designers  of  electrical 
machinery,  it  was  recognized  that  a  temperature  rise  of  35  deg. 
cent  in  itself  was  absurdly  low,  but  that  the  object  in  operating 
at  such  low  temperature  on  a  part  which  could  be  measured  was 
simply  to  protect  the  machine  in  some  inaccessible  hotter  part, 
where  the  temperature  could  not  be  measured.  From  the  present 
viewpoint,  it  is  astonishing  what  reliance  has  been  placed  upon 
temperature  readings  in  the  past.  For  example,  if  a  40  deg. 
cent  machine  showed  41  5  deg  cent  rise  on  test,  it  was  unsafe, 
while  if  "it  showed  38.5  deg.  cent,  rise,  it  was  good.  We  now 
recognize  that  neither  of  these  temperatures  have  any  controlling 
value,  unless  many  other  conditions  are  known  To  the  ex- 
perienced man  they  simply  mean  that  compared  with  the  other 
machines  of  similar  constructions  and  characteristics,  which  have 
proved  satisfactory  in  service,  they  are  reasonably  safe.  To  the 
designer  they  mean  that  when  proper  corrections  have  been 
made  for  the  various  internal  temperature  drops,  the  highest 
temperature  attained,  at  any  point;  Vill  be  within  the  limits  of 


376  ELECTRICAL  ENGINEERING  PAPERS 

durability  of  the  insulating  material  used.  The  whole  problem 
is  a  good  deal  like  that  of  a  determination  of  the  voltage  generated 
in  a  given  power-house,  by  measuring  the  voltage  at  the  end  of 
a  transmission-line.  If  we  know  all  the  constants  of  the  line, 
and  know  the  current  flowing,  etc.,  we  can  figure  back  to  the 
generated  voltage.  Otherwise  the  voltage  at  the  end  of  the  line 
means  but  little.  However,  we  know  that  if  the  system  is  de- 
signed with  reasonable  regard  to  economy  in  general,  there  may 
be  from  ten  to  twenty  per  cent,  voltage  drop  from  power-house 
to  the  end  of  the  line.  Therefore,  by  adding  an  approximate 
correcting  factor  to  this  voltage,  we  can  make  a  reasonable 
estimate  of  the  generated  voltage.  In  the  same  way  in  electrical 
apparatus  of  certain  types,  a  reasonable  internal  temperature 
drop  may  be  approximated,  which  added  to  the  observable  tem- 
perature, gives  a  fair  approximation  to  the  hottest  part,  but 
the  result  is  an  approximation  and  must  be  recognized  as  suck. 
Primarily,  the  manufacturer  must  make  a  safe  machine  for  a 
specified  service  regardless  of  the  temperature  guarantees,  and 
the  temperature  measurements  made  on  most  classes  of  apparatus 
should  be  considered  simply  as  rough  approximations  to  indicate 
that  the  manufacturer  has  made  a  reasonable  attempt  at  a  safe 
machine.  This  may  seem  a  rather  bald  statement,  but  never- 
theless it  is  a  fair  statement  of  the  case. 

* 

ERRORS  IN  TEMPERATURE  MEASUREMENT 

It  has  been  shown  in  the  preceding  that  the  usual  observable 
temperatures  are  in  most  cases  only  crude  approximations  to 
the  real  temperature  conditions.  It  may  now  be  shown  that 
even  the  observable  temperatures,  obtained  by  the  usual  means, 
are  in  themselves  only  crude  approximations  in  many  cases. 
Take,  for  instance,  the  determination  of  temperature  by  in- 
crease in  resistance;  when  the  coil  is  heated  its  temperature  may 
not  be,  and  very  frequently  is  not  nrnforrn  throughout  the  coil. 
As  an  extreme  example,  if  one-fifth  of  the  coil  length  has  a  tern 
perature  of  80  deg.  cent.,  while  four-fifths  of  it  has  a  rise, of 
30  deg.  cent-,  then  the  increase  in  resistance  of  the  coil  as  a  whole 
will  correspond  to  a  rise  of  40  deg.  cent.  Thus,  by  increase  of 
resistance,  the  temperature  may  be  more  than  safe,  while 
actually  one-fifth  of  the  coil  is  far  above  the  safe  temperature 
for  ordinary  fibrous  insulations.  In  other  words,  the  resistance 
method  gives  only  average  results  and  may  be  very  misleading. 
However,  in  those  cases  where  it  is  known,  by  past  experience 


TEMPERATURE  DISTRIB UTION  377 

and  otherwise,  that  there  is  very  little  liability  of  hot-spots,  the 
resistance  method  of  determining  temperature  is  often  quite 
satisfactory  However,  the  method  is  limited  to  comparatively 
few  types  of  windings. 

Considering  next  the  thermometer  method  of  measurement, 
the  theory  of  this  is  quite  simple,  but  apparently  it  has  been  very 
much  misunderstood.  In  windings,  except  in  rare  cases,  the 
thermometer  is  not  applied  directly  to  the  heat  generating 
material  itself,  but  is  applied  outside  of  an  insulating  covering 
Usually  the  temperature  drop  through  this  insulating  covering 
does  not  receive  any  consideration,  and  yet  everything  depends 
upon  this.  Assume,  for  example,  an  insulated  coil,  thermometer 
and  covering  pad,  as  shpwn  in  Fig.  4,  Assuming  the  copper 
inside  the  coil  as  being  of  uniform  temperature,  and  the  cooling 
air  at  a  ajid  b  as  also  at  a  uniform,  but  much  lower,  temperature 
than,  inside  the  coil ;  then  the  temperature  drop  from  the  copper 
to  &  will  be  the  same  as  through  the  insulation,  thermometer 


FIG.  4 

bulb  and  covering  pad  to  the  air  at  a.  Obviously  if  the  tempera- 
ture drops  through  the  insulation  and  through  the  pad  are  equal,, 
then  the  thermometer  bulb  will  show  a  midway  temperature. 
This  is,  of  course,  assuming  that  the  surface  drop  to  the  air, 
previously  referred  to,  is  very  small,  or  that  it  is  included  as  part 
of  the  drop  through  the  pad-  Obviously,  if  the  drop  through  the 
covering  pad  is  made  very  much  higher  than  that  through  the 
insulation  proper,  then  the  thermometer  bulb  more  closely 
approaches  the  copper  temperature.  Thus  it  is  seen  that  all 
kinds  of  results  may  be  obtained,  depending  upon  the  relative 
drops  through  the  pad  and  through  the  insulation.  In  a  low 
voltage  machine,  with  relatively  thm  insulation,  the  p,ad  may 
take  most  of  the  drop.  With  very  heavy  insulation,  the  pad  may 
take  proportionately  less  and  the  thermometer  reading  departs 
accordingly  from  the  copper  temperature.  It  might  be  sug- 
gested that  a  big  thick  pad  of  very  poor  heat  conducting  material 
might  be  used.  This  apparently  would  tend  toward  more  ac- 


378  ELECTRICAL  ENGINEERING  PAPERS 

curate  temperature  readings,  but,  on  the  other  hand,  harmful 
effects  may  be  introduced  by  the  use  of  a  large  pad.  The  resis- 
tance to  heat  dissipation  being  increased  in  the  area  covered  by 
the  pad,  obviously  less  heat  will  be  carried  away  at  this  point 
and,  therefore,  the  heat  generated  under  the  pad  must  be  con- 
ducted to  adjacent  parts  of  the  coil.  This  means  an  increased 
temperature  at  this  point,  due  to  the  use  of  the  pad.  Again,  the 
use  of  the  pad,  in  some  cases,  may  affect  the  normal  ventilation 
of  certain  parts  of  the  coil  not  directly  covered  by  the  pad.  For 
instance,  if  there  is  a  ventilating  space  between  two  adjacent 
armature  coils,  through  which  air  is  normally  driven,  a  pad  which 
covers  this  space  even  partially  may  create  more  or  less  of  an  air 
pocket,  and  thus  materially  affect  the  heat  dissipation,  and  the 
temperature  directly  under  the  pad.  Experience  has  shown  that 
both  of  the  above  conditions  are  obtained  when  good  judgment 
is  not  used  in  the  application  of  the  covering  pad.  This,  of  course, 
applies  particularly  to  those  cases  where  temperature  readings 
are  obtained  while  the  machine  is  in  operation.  Of  course,  after 
shut-down,  most  questions  of  ventilation  and  of  generation  of 
higher  temperature  under  the  pad  need  not  be  taken  into  account. 
There  are  so  many  conditions  entering  into  the  interpretation 
of  the  thermometer  and  resistance  methods  of  determining 
temperature,  that  in  certain  classes  of  apparatus  it  has  been 
very  desirable  to  find  more  accurate  methods.  One  of  these 
is  in  the  use  of  so  called  resistance  coils.  In  this  method  a  coil 
of  fine  wire  of  a  known  temperature  co-efficient,  and  of  known 
resistance  at  a  given  temperature,  is  placed  at  the  place  where 
the  temperature  is  to  be  measured,  and  the  temperature  rise  is 
determined  from  the  increased  resistance  of  the  coil  One  serious 
objection  to  this  arrangement,  is  that  the  resistance  coil  must 
have  considerable  length  and  breadth  so  that  it  really  indicates 
the  average  temperature  of  a  considerable  area  instead  of  a  point. 
When  placed  between  two  coils,  as  indicated  in  Fig.  5,  it  usually 
occupies  so  great  a  proportion  of  the  slot  that  it  indicates  an 
average  temperature  considerably  lower  than  at  a  Furthermore, 
on  account  of  the  length  of  such  coils,  there  may  be  a  consider- 
able difference  between  the  temperatures  at  the  two  ends  Thus 
the  resistance  coil,  like  the  resistance  measurement  of  the  wind- 
ings themselves,  gives  an  average  result,  but  this  average  ma\ 
be  limited  to  a  comparatively  small  area,  whereas,  in  the  resist- 
ance method  in  general  the  indicated  rise  is  an  average  of  the 
whole  winding.  However,  in  'the  resistance  method,  the  tern- 


TEMPERATURE.  DISTRIBUTION 


379 


perature  of  the  conductors  themselves  is  measured,  whereas, 
with  the  resistance  coil  the  temperature  measurement  is  outside 
the  insulation  The  resistance  coil  method  is,  therefore,  a  rel- 
ativ^ly  crude  approximation,  although  when  brought  out  it  was 
really  an  important  step  in  advance  In,  its  early  application, 
many  misleading  results  were  obtained,  due  largely  to  lack  of 
understanding  of  the  principles  governing  temperature  distribu- 
tion and  temperature  drop,  In  some  cases,  the  resistance  coil 
was  placed  under  the  wedge  as  at  b  in  Fig  5  In  other  cases, 
the  coil  was  placed  at  the  side  of  the  slot  next  to  the  iron,  or  at 
the  bottom*  Very  rarely  was  it  placed  midway  between  the  two 
coils,  probably  because  this  was  a  more  difficult  application  and 


u 


\ 


Resistance  Coil 


r"ft"^: Resistance 


Coil 


FIG   5 

also  because  the  greater  accuracy  of  such  location  was  not  rec- 
ognized From  the  use  of  resistance  coils  many  good  engineers 
drew  the  conclusions  that  the  upper  limit  of  permissible  tempera- 
ture for  fibrous  insulations  was  only  80  deg  to  90  deg  cent , 
because  with  the  coils  located  in  certain  ways  and  places,  de- 
terioration of  insulation  at  some  other  point  was  liable  to  begin, 
if  the  above  temperatures  were  exceeded  The  error  was  in  not 
recognizing  the  temperature  drop  between  some  hotter  spot  and 
the  average  location  of  the  resistance  coil  When  this  condition 
•  was  recognized  the  results  obtained  by  resistance  coils  became 
more  consistent  with  the  facts 

A  later  development  than  the  resistance  coil  is  the  thermo- 
couple as  a  practical  device  for  measuring  temperature      One 


3SO  ELECTRICAL  ENGINEERING  PAPERS 

great  advantage  of  the  thermo-couple  is  its  very  small  size,  so 
that  it  can  indicate  the  temperature  at  practically  a  point  instead 
of  a  very  considerable  area  Moreover,  as  it  is  a  zero  current 
method  of  measurement,  when  used  with  a  potentiometer  no 
question  of  size  or  length  of  the  connecting  leads  need  corne  up 
The  thermo-couple  is  so  small  and  has  so  little  mass,  that  it  can 
follow  very  quickly  any  temperature  changes  where  it  is  located 
If  properly  placed  it  furnishes  the  most  accurate  temperature 
indicator  which  we  now  have,  as  it  can  be  located  in  all  sorts  of 
normally  inaccessible  places  However,  its  use  is  practically 
limited  to  stationary  apparatus,  In  rotating  apparatus,  or 
rotating  parts  it  can  be  used  only  after  shut-down,  which  intro- 
duces errors,  as  already  shown. 

MANY-CONDUCTOR  COILS 

In  all  the  preceding  considerations  it  has  been  assumed  that 
the  copper  inside  the  coils  itself  is  at  a  uniform  temperature,  m 
any  given  unit  of  length.    This  is  practically  true,  provided  the 
coil  is  made  up  of  a  single  conductor,  or  of  a  relatively  few  con- 
ductors with  only  a  moderate  amount  of  insulation  between  them 
When  several  coils  or  conductors  are  placed  side  by  side,  as  in 
Pig.  6,  it  would  appear  at  first  glance  that  the  middle  coils  should 
heat  much  more  than  the  outer  ones      But,  in  reality,  unless 
there  are  many  layers  of  coils,  the  temperatures  of  the  different 
coils  will  not  vary  greatly  from  each  other      For  instance,  in 
Pig.  6,  the 'heat  generated  in  the  middle  conductor  is  only 'one- 
third  that  of  the  total  generated  in  the  coil,  and  yet  the  two 
side  surfaces  through  which  this  heat  passes  to  the  adjacent  coils 
aggregate  almost  as  much  as  the  total  outside  dissipating  surface 
of  the  whole  coil,  through  which. all  the  lateral  heat  flow  is  dis- 
sipated.    Considering  further  that  the  insulation  between  the 
middle  coil  and  its  neighbors  is  relatively  thin  compared  with 
the  outside  covering,  it  is  obvious  that  the  temperature  drop 
from  this  coil  to  the  adjacent  ones  will  be  comparatively  small,—- 
possibly  not  over  ten  per  cent  of  the  drop  through  the  outside 
insulation 

However,  with  a  large  number  of  coils  side  by  side,  the  condi- 
tions become  cumulatively  worse.  Here,  the  drop  from  the 
center  conductor  to  the  next  one,  may  be  small  But  the  'drop 
from  the  second  conductor  to  the  third  is  considerably  greater 
due  to  the  heat  of  two  conductors  being  transmitted.  Prom  the 
third  to  the  fourth  thefe  is  a  drop  corresponding  to  the  losses 


TEMPERATURE  DISTRIBUTION 


381 


of  three  conductors,  etc  Thus,  there  is  a  gradually  increasing 
temperature  drop  from  the  center  of  the  coil  toward  the  outside 
surface,  and  if  the  coil  be  very  deep,  that  is,  if  it  consists  of  many 
insulated  layers,  the  sum  total  of  the  drops  may  be  quite  large 
Or,  putting  it  in  another  way,  with  a  comparatively  deep  coil, 
the  temperature  rise  from  the  outside  surface  of  the  coil  itself 
toward  the  center  will  be  very  rapid  at  first,  and  gradually  taper 
off,  as  indicated  in  Pig.  7.  This  is  indicated  very  clearly  m  the 
case  of  an  over-heated  field  of  coil  of  fine  wire.  Here  the  first 
outside  layers  will  usually  be  found  in  a  fairly  good  condition, 
but  at  a  comparatively  little  distance  inside  the  coil  there  may 
be  severe  roasting  or  evidence  of  overheating,  which  may  be 


FIG.  6 


•Temp  at  Center 
N— Temp  at  Edge 


FIG.  8 


1 

FIG.  7 


almost  as  bad  as  at  the  center.  (See  Pig.  8.)  In  such  case,  the 
temperature  measurement  on  the  outside  of  the  coil  is  no  satis- 
factory indication  of  the  hot-spot  temperature  A  temperature 
measurement  by  resistance,  while  a  closer  indication  than  that 
by  thermometer,  also  may  be  very  misleading  It  may  be  stated 
that  modern  design  tendencies  are  toward  comparatively  shallow 
field  coils,  largely  on  account  of  this  condition 

CONCLUSION 

The  whole  object  of  this  paper  is  to  show  the  problem  of  tem- 
perature distribution  and  temperature  measurement,  as  it 
actually  is.  It  is  the  writer's  desire  to  show  that  no  hard  and  fast 
rules  can  be  made  for  determining  the  facts  in  the  case,  and  that 


382  ELECTRICAL  ENGINEERING  PAPERS 

the  best  rules  and  methods  now  practicable  are  only  approximate. 
The  present  limitations  set  for  insulating  materials  are  much 
higher  than  were  considered  practicable  only  a  few  years  ago. 
This  is  not  because  the  limits  have  been  raised,  but  because, 
through  a  better  understanding  of  the  facts,  the  real  upper 
limits  of  temperature  as  fixed  by  durability  of  insulation,  are 
now  known  to  be  considerably  higher  than  was  believed  to  be 
the  case  only  a  short  time  ago  If  the  real  limits  were  in  accord- 
ance with  former  beliefs,  then  all  the  evidence  of  the  more  accu- 
rate modern  tests  and  data  would  indicate  that  the  vast  majority 
of  the  existing  electrical  machines  should  have  "roasted  out" 
comparatively  early  in  their  operation  The  higher  temperature 
limits  were  there,  but  were  not  recognized  Now  we  recognize 
them  and  attempt  to  make  reasonable  allowances  for  differences 
between  the  measurable  temperatures  *and  the  actual  hottest 
parts.  The  present  method  may  be  crude,  but  we  are  not  going 
at  it  blindly,  as  was  formerly  the  case.  Formerly  the  manu- 
facturer took  the  real  responsibility  for  making  a  machine  that 
was  safe  for  the  service,  whatever  the  guarantees  called  for. 
Today  the  responsibility  is  still  his,  but  he  is  attempting  to 
educate  the  public  to  a  knowledge  of  his  real  problems,  and  to  a 
recognition  that  temperature  determination  is  far  from  being 
an  exact  art.  There  is  no  sharply  defined  line  between  good,  and 
bad  in  the  insulating  materials  as  affected  by  temperature,  con- 
sequently there  is  no  sharp  line  between  safe  and  unsafe 
temperatures. 

ABSTRACT  PROM  DISCUSSION 

I  have  made  tip  a  sketch  which  brings  out  much  better  than 
any  description,  some  of  the  fundamental  differences  between 
Class  "A"  and  Class  "B  "  insulations.  These  might  be  called  the  " 
time-temperature  curves  for  these  insulations.  These  must  be 
considered  as  approximations  only,  as,  from  the  very  nature  of  the 
materials  themselves,  no  exact  curves  are  possible.  The  important 
feature  to  be  considered  in  the  curves,  is  the  general  shape  rather 
than  any  absolute  values. 

We  have  made  a  great  many  temperature  tests  of  insulations 
to  determine  their  durability;  also  we  have  made  examinations  of 
a  very  large  number  of  windings  which  have  been  in  service  for 
many  years,  but  for  which  we  had  only  approximate  data  as  to 
temperatures.  Obviously  it  is  impracticable  to  carry  on  an  ac- 
curate life  test  covering  a  long  period  of  years,  so  what  we  did  in 


TEMPERATURE  DISTRIBUTION  383 

most  of  our  tests,  was  to  carry  the  temperatures  up  to  such 
points  that  destruction  was  either  reached  or  indicated  in  a 
comparatively  limited  period  of  time. 

Curve  A  indicates  approximately  the  durability  of  class  A 
insulations  for  various  temperatures  This  should  be  recognized 
as  being  approximate,  but  it  is  optimistic  rather  than  pessimistic. 

Curve  B  applies  to  well  built  class  B  insulations,  as  now  fur- 
nished by  some  of  the  electrical  manufacturing  companies. 
Such  insulations  contain  a  large  percentage  of  heat  resisting 
materials  with  a  comparatively  small  percent  of  binding  material 
and  the  insulation  is  applied  so  tightly  that  deterioration  or  de- 
struction of  the  binder  does  not  appreciably  loosen  up  the  true 
insulating  material 


Considering  curve  A,  taking  105  deg.  cent.,  as  the  ultimate 
temperature  limit  for  long  life  without  undue  deterioration,  then 
with  a  very  slight  increase  in  temperature,  say  to  115  deg.  cent., 
the  life  is  shortened  very  much,  and  at  125  deg.  cent,  such  in- 
sulation is  good  for  only  a  very  few  months  at  the  most.  At 
150  deg.  cent,  it  has  an  exceedingly  short  life. 

Next  considering  curve  B,  our  available  data  indicate  that  for 
over  twelve  months  operation  at  200  deg.  cent.,  the  insulation  is 
in  first  class  shape;  in  fact,  much  better  than  class  A  insulation 
at  110  deg.  cent.,  for  the  same  length  of  time.  At  300  deg. 
cent,  for  six  months,  the  insulation  really  shows  better  than  class 
A  insulation  at  115  deg.  cent,  for  the  same  length  of  time,  and, 
at  400  deg.  cent.,  the  class  B  insulation  for  three  months  is  better 
than  class  A  insulation  at  125  deg.  cent,  for  the  same  length 
of  time.  If  we  now  assume  the  continuous  life  for  the  class  B 
insulation  as  150  deg.  cent.,  then  it  is  seen  that  a  33  percent 
increase  in  temperature  for  one  year  is  no  more  harmful  than  a 
5  percent  increase  in  temperature  over  the  105  deg.  cent,  for 
class  A  insulations  for  one  year.  Also  a  100  percent  increase  in 
temperature  above  its  continuoiis  limit  for  six  months  is  com- 


384  ELECTRICAL  ENGINEERING  PAPERS 

parable  with  a  10  per  cent  increase  in  temperature  for  class  A 
insulation  for  the  same  period.  For  still  higher  temperatures 
the  percentage  is  far  more  in  favor  of  class  B. 

What  I  want  to  bring  out  in  particular  by  means  of  this  dia- 
gram, is  that  the  factor  of  safety  for  overloads  is  vastly  greater 
for  class  B  than  for  class  A  insulations,  on  the  basis  of  continu- 
ous life  being  taken  as  ISO  deg.  cent,  and  105  deg.  cent.,  respec- 
tively. Part  of  this  difference  is  inherently  in  the  characteris- 
tics of  the  materials  themselves,  but  no  doubt  part  of  it  is  due 
to  the  fact  that  the  arbitrary  ISO  deg.  cent,  limit  set  for  properly 
built  class  B  materials  is  considerably  too  low  in  comparison 
with  105  deg.  cent,  for  class  A.  But,  whatever  the  explanation, 
the  difference  is  there. 

In  regard  to  the  very  high  temperatures  for  class  B  insulations, 
such  as  300  deg.  and  400  deg.  cent,  shown  in  curve  B,  attention 
should  be  called  to  the  fact  that  unless  there  is  an  exceedingly 
high  temperature  drop  through  the  insulation  itself,  any  outside 
supporting  layer  or  wrapper  of  fibrous  materials  is  liable  to  be- 
come unduly  heated  and  may  disintegrate.  Therefore,  while  the 
insulation  proper  might  stand  400  deg.  cent.,  for  instance,  yet  if 
this  was  continued  for  any  considerable  length  of  time,  so  that 
the  outside  supporting  material  became  excessively  heated,  such 
material  would  have  to  be  of  something  else  than  the  usual  treated 
tape  or  fibrous  wrappers.  However,  it  so  happens  that  very 
high  temperatures  are  rarely  attained  in  practice,  except  in  the 
case  of  armature  conductors  buried  in  slots.  In  such  case  the 
surrounding  iron  assists  very  materially  in  cooling  the  finishing 
wrapper  on  the  coils,  unless  the  high  temperature  is  maintained 
for  a  very  considerable  period. 

Some  are  inclined  to  look  askance  at  mica  at  150  deg.  to  200 
deg.  cent.,  but  it  must  be  remembered  that  in  certain  heating 
apparatus  mica  is  used  up  to  500  deg.  cent,  and,  in  some  cases, 
even  up  to  750  deg.  cent.  Practically  all  micas  will  stand  up 
to  about  600  deg.  cent ,  without  undue  deterioration,  and  some 
grades  will  stand  up  to  1000  deg  cent.  From  this  viewpoint, 
the  temperature  of  150  deg.  to  200  deg.  cent,  in  armature  coils 
appears  to  be  very  low  and  the  whole  matter  turns  upon  the  way 
such  mica  is  used.  If  the  percentage  of  mica  in  the  insulation  is 
relatively  high  and  the  mica  is  put  on  so  tightly  that  the  binding 
material  can  disintegrate  and  loosen  up  and  yet  the  natural  elas- 
ticity or  springiness  of  the  mica  can  hold  the  insulation  tightly  in 


TEMPERATURE  DISTRIBUTION  385 

place,  then  such  insulation  can  stand  very  high  temperature  with-* 
out  injury.  But,  if  the  mica  is  wound  or  placed  so  loosely  that  this 
disintegration  of  the  binding  or  supporting  material  allows  the  mica, 
part  to  loosen  up  materially,  then  the  insulation  qualities  may  still 
be  very  good  from  the  dielectric  standpoint,  but  may  be  in  such 
poor  shape  mechanically  that  vibration  or  shocks  may  shift  it 
or  displace  it  sufficiently  to  injure  it  as  an  insulator.  This  de- 
fect is  a  mechanical  one  and  not  in  the  quality  of  the  material  itself. 
Mr.  Junkersfeld  has  spoken  of  some  of  his  early  experiences 
with  high  temperature,  and  he  mentioned  that  the  data  which 
he  and  his  associates  obtained  have  had  a  marked  nfluence  in 
leading  the  manufacturers  toward  better  grades  of  insulation. 
This  is  no  doubt  correct,  but  I  wish  to  call  attention  to  the  fact 
that  the  manufacturers  were  also  following  this  matter  inde- 
pendently of  the  operating  companies,  with  the  same  end  in 
view.  For  instance,  the  company  with  which  I  am  associated, 
insulated  the  1894  Niagara  generators  with  mica.  We  did  not 
know  whether  such  insulation  was  required,  but  we  thought  t 
was  good  material  and  so  put  it  on.  Later  tests  showed  that 
this  was  a  very  fortunate  decision,  and  now7  after  twenty  years 
of  operation,  this  insulation  is  still  in  very  good  shape,  although 
subjected  to  very  much  higher  temperatures  than  originally  con- 
templated, 150  deg.  to  200  deg.  cent,  being  not  uncommon  ac- 
cording to  later  tests.  Also  in  1898  and  1899  the  large  engine- 
type  Manhattan  Railway  generators  had  mica  insulation,  in  the 
form  of  wrappers,  on  the  armature  coils.  Following  this,  mica 
insulation  was  used  for  quite  a  number  of  years,  mostly  on  large 
high- voltage  alternators.  About  1904  we  built  some  large  capac- 
ity 60-cycle  turbo-generators  on  which  we  used  mica  wrappers 
on  the  armature  coils.  In  service,  one  of  these  machines  was 
injured  from  some  mechanical  cause  and  we  had  to  rewind  it, 
One  of  the  fads  about  this  time  was  special  oiled-linen  tape 
insulation,  and  quite  a  pressure  was  brought  to  bear  upon  us  to 
rewind  this  machine  with  such  oiled  tape.  With  this  insulation 
the  armature  broke  down  in  a  comparatively  short  time  (within 
a  few  months,  if  I  remember  rightly).  When  the  coils  were 
removed,  the  outside  layer  of  insulation  next  to  the  iron  was 
found  to  be  apparently  in  fair  shape,  but  next  to  the  copper  the 
insulation  showed  indications  of  being  excessively  heated;  in 
fact,  it  was  badly  carbonized  in  some  places.  We  then  reinsti- 
lated  with  tnica  aad  the  machine  was  operated  for  many  years 
without  trouble.  Here  was  a  direct  comparison  between  class  A 


386  ELECTRICAL  ENGINEERJNG  PAPERS 

and  class  B  insulations.  I  do  not  know  how  hot  those  coils  ran, 
but,  judging  from  the  appearance  of  the  oiled-tape  insulation, 
it  must  have  been  materially  above  125  deg.  cent.  Here  was  a 
fortunate  instance  where  the  machine  was  first  insulated  with 
mica  tape  and  then  afterwards  insulated  with  fibrous  materials, 
so  that  actual  comparison  was  obtained  with  the  two  materials 
This  was  ten  to  twelve  years  ago,  so  that  it  cannot  be  said  that 
experience  showing  the  relative  merits  of  these  two  types  of  insu- 
lation is  only  of  recent  date. 

In  the  same  way  similar  experience  was  obtained  with  field 
insulation.    Practically  all  our  early  turbo-generator  fields  were 
insulated  with  fibrous  sheet  materials.    Numerous  instances  oc- 
curred  where    such  insulations  deteriorated  so  much  that  re- 
winding was  required.    This  led  to  numerous  tests  for  tempera- 
ture.   In  some  of  these  earlier  machines  there  was  evidence  of 
practically  uniform  overheating  throughout  the  whole  winding, 
thus  indicating  practically  uniform  temperature.     In  such  cases 
it  was  comparatively  easy  to  approximate  the  ultimate  temper- 
ature from  readings  of  the  field  currents  and  the  field  volts,  thus 
obtaining  the  increase  in  resistance  and  from  this  the  temperature 
rise.     Such  tests  soon  developed  the  fact  that  temperatures  of 
110  deg.  to  125  deg.  cent   were  not  uncommon  on  the  earlier 
turbo-fields,    while   with   the   increased    capacities   and   higher 
speeds,  toward  which  we  were  continually  tending,  the  indica- 
tions were  that  still  higher  temperatures  would  be  attained. 
This  led  to  the  development  of  mica  insulation  for  the  field 
windings  of  turbo-generators     In  1906  and  1907  a  number  of  the 
earlier  hot  fields  were  rewound  with  mica  and  such  fields  have 
been  operating  up  to  the  present  time,  or  until  discarded  in 
favor  of  larger  units.     The  record  with  these  mica  insulated 
fields  has  been  extremely  good.     In  some  of  the  tests  which  we 
made  on  these  earlier  machines  to  determine  the  suitability  of 
mica  for  field  insulation,  we  carried  one  field  up  to  250  deg.  cent, 
for  forty-eight  hours,  and  would  have  continued  the  test  very 
much  longer,  but  the  conduction  of  heat  from  the  core  through 
the  shaft  to  the  bearings  was  sufficient  to  overheat  them    How- 
ever, at  the  end  of  this  test  the  insulation  was  found  to  be  in 
absolutely  good  condition.    This  was  a  very  mild  test,  in  view 
of  our  later  investigations  on  mica,  but  at  that  time  it  was  con- 
sidered wonderful*    I  am  simply  bringing  up  such  points  to 
indicate  that  mica  has  been  used  quite  extensively  on  turbo- 
generators for  many  years. 


SOME  PRACTICAL  CONSIDERATIONS  IN  ARTIFICIAL 
VENTILATION  FOR  ELECTRICAL  MACHINERY 

FOREWORD — The  material  given  was  first  presented  in  a  discussion 
at  one  of  the  meetings  of  the  Association  of  the  Edison  Illumin- 
ating Companies,  September,  1915.  It  was  afterwards  revised 
for  publication  in  the  Electric  Journal. — (ED.) 

IN  the  artificial  cooling  of  power-house  and  sub-station  apparatus 
especially  that  of  large  capacity,  a  number  of  conditions  have 
developed  from  time  to  time  which  have  given  trouble  or  which 
have  provoked  more  or  less  discussion.    A  number  of  these  points 
are  here  presented  briefly. 

QUANTITY   OF  AIR  REQUIRED 

There  is  a  very  definite  physical  relation  between  the  heat 
which  must  be  dissipated  from  a  machine,  the  resultant  temper- 
ature rise,  and  the  quantity  of  air  which  is  passed  through  the 
machine  to  carry  away  the  heat.  The  law  is  that  one  kilowatt  of 
loss  dissipated  into  the  air  will  raise  100  cu.  ft.  of  air  18  degrees  C. 
in  one  minute.  Therefore,  if  there  is  a  definite  loss  to  be 
dissipated  by  the  ventilating  air  and  a  desired  limit  to  the  permis- 
sible rise  in  the  temperature  of  the  air  leaving  the  machine,  then 
there  must  be  a  definite  volume  of  air  per  minute  through  the 
machine.  The  problem  then  resolves  itself  into  getting  this  air 
through  the  machine  or  apparatus.  An  allied  problem  lies  in  the 
means  for  getting  the  heat  from  the  copper  or  iron  to  the  air. 

PRESSURE  REQUIRED  TO  OBTAIN  DESIRED  QUANTITY  OF  AIR 

The  pressure  required  for  a  given  quantity  of  air  is  dependent 
upon  the  size  of  air  passages  or  apertures,  upon  the  shapes  of  the 
ducts,  i.  e  ,  number  of  bends,  abruptness  of  bends,  length  of  ducts, 
etc.,  and  upon  the  velocity  of  the  air.  Too  sma.11  passages  means 
high  velocity  of  the  air,  with  consequent  high  pressure  required. 
However,  with  many  classes  of  artificially-cooled  apparatus,  the 
space  available  for  air  passages  is  comparatively  small  at  some 
places,  so  that  the  air  velocities  are  very  high.  This  means 
ventilation  losses,  but  in  many  cases  these  are  unavoidable  with- 
out radical  changes  in  design  whfth,  in  themselves,  would  mean 
increased  losses  of  other  sorts  equal  to,  or  greater  than,  the  possible 
reduction  in  ventilation  loss. 

3&7 


388  ELECTRICAL  ENGINEERING  PAPERS 

Usually,  the  greater  part  of  the  pressure  developed  by  the 
ventilating  fans  is  used  up  in  the  ducts  or  passages  through  the 
machine  itself.  However,  not  infrequently,  part  of  the  pressure 
is  taken  up  by  restrictions  of  some  sort  in  the  inlet  or  outlet  con- 
duits If  the  machine  is  self-cooling  in  the  sense  that  the  rotor 
carries  its  own  fan,  then  such  restrictions  in  the  conduits  may 
very  seriously  affect  the  quanity  of  air  which  passes  through  the 
machine.  It  is,  of  course,  possible  to  make  the  ventilating  fans 
of  greater  capacity,  just  to  take  care  of  such  contingencies,  but  this 
would  be  penalizing  good  engineering  to  take  care  of  bad,  for  the 
losses  due  windage  would  then  be  unduly  large  where  proper  con- 
duits are  furnished.  Cases  have  been  noted  where  as  much  as  30 
to  40  percent  of  the  available  pressure  has  been  taken  up  by 
improperly  designed  inlet  pipes. 

Where  the  ventilating  fans  are  driven  independently  of  the 
main  rotor,  that  is,  by  motors,  it  is  practicable  to  vary  the  air 
pressure  to  suit  the  requirements,  provided  the  driving  motor  can 
have  its  speed  adjusted  over  a  suitable  range.  This  makes  the 
ventilation  more  or  less  independent  of  restrictions.  It  has  the 
further  advantage  of  allowing  the  quantity  of  air  to  be  varied  to 
suit  the  load  conditions.  By  this  means  an  increased  quantity 
of  air,  but  with  correspondingly  increased  windage  loss,  is  avail- 
able with  heavy  loads,  while  less  air  with  lower  losses  may  be  used 
at  lighter  load  This  arrangement  is  somewhat  more  efficient 
than  the  self-cooled  arrangement  with  the  fans  on  the  main  motor 
shaft,  principally  because  of  the  excessively  high  fan  speeds  com- 
mon in  the  latter  case,  especially  on  turbo-generators  in  which  fan 
speeds  far  above  efficient  operation  are  used.  In  fact,  in  high- 
speed turbo-generators  fan  efficiencies  of  20  to  30  percent  are  not 
unusual.  Much  better  efficiencies  can  be  obtained  from  separate 
slower  speed  fans  (50  to  60  percent).  However,  the  reduction  in 
windage  losses  is  not  proportional  to  the  increased  efficiency  of 
the  fans,  for  part  of  the  windage  loss  is  due  to  ''churning"  of  the 
air  passing  over  the  rotor,  and  this  will  be  present  regardless  of  the 
method  of  supplying  air  to  the  machine  and  is,  to  a  certain  extent, 
a  function  of  the  quantity  of  air  which  passes  through  the  air-gap. 
This  part  of  the  windage  loss  is,  therefore,  greater  in  those  machines 
where  all  the  cooling  air  passes  through  the  air-gap  than  is  the  case 
with  those  types  where  a  considerable  part  of  the  air  passes  di- 
rectly through  the  armature  core,  as  in  axially  ventilated  stators. 
In  either  type  of  ventilation,  variation  in  the  quantity  of  air  with 
load  is  advantageous. 


ARTIFICIAL  VENTILATION  389 

DISPOSAL  OP  HOT  AIR 

This  was  a  matter  of  little  importance  in  the  days  of  small 
capacity  per  unit-area  of  generating  room.  However,  in  these 
latter  days,  where  capacities  of  from  three  to  ten  times  those  of 
former  days  are  developed  in  the  same  space,  the  question  of 
disposal  of  hot  air  from  the  machines  is  becoming  important.  Large 
turbo-generators  may  require  from  50,000  to  100,000  cu.  ft.  of  air 
per  minute.  Five  or  six  such  machines  in  one  generator  room, 
operating  at  full  load,  means  250,000  to  500,000  cu.  ft.  of  air  per 
minute  pouring  into  the  room,  this  air  being  from  20  to  25  degrees 
C.  above  the  normal  air  temperature.  Obviously  some  provision 
should  be  made  for  getting  this  hot  air  out  of  the  room.  In  the 
average  generator  room,  the  total  cubical  capacity  of  the  room  will 
be  only  five  to  ten  times  the  total  volume  of  air  passing  through 
the  machines  per  minute.  This  gives  a  good  quantitative  idea  of 
the  extent  of  ventilation  required  in  order  to  prevent  undue  tem- 
perature rises  of  the  room  air.  In  some  of  the  more  modern 
stations  provision  has  been  made  for  exhausting  the  hot  air  from 
the  machines  into  the  boiler  room. 

DIRT  IN  THE  AIR 

The  enormous  quantity  of  air  required  for  ventilating  large 
turbo-generators  brings  up  the  question  of  amount  of  dirt  carried 
into  the  machines  by  such  air.  Assume,  for  instance,  60,000  cu. 
ft.  of  air  per  minute  through  a  given  machine.  This  weighs  ap- 
proximately 4800  pounds.  The  usual  turbo-generator  will,  there- 
fore, pass  through  itself,  each  thirty  to  forty  minutes,  a  weight  of 
air  equal  to  its  own  total  weight.  Or,  presenting  the  matter  in 
another  way,  45,000,000  cu.  ft.  of  air  passes  through  in  twelve 
hours.  Assuming  as  a  rough  approximation,  that  only  one 
hundredth-millionth  of  the  volume  of  air  consists  of  dust  or  foreign 
particles,  then  the  above  means  that  0.45  cu.  ft.  of  dust  passes 
through  the  machine  in  twelve  hours,  or  45  cu.  ft.  in  100  days  of 
twelve  hours  each.  If  the  air  inlet  is  in  a  dusty  place,  the  above  is 
not  at  all  an  impossibility.  Of  course,  a  considerable  part  of  this 
dust  will  go  directly  through  the  machine,  but  in  the  air  swirls 
and  eddies  inside  the  machine  some  of  it  win  be  deposited,  and 
eventually  this  becomes  a  considerable  handicap  to  the  ventilation. 
This  dust  acts  harmfully  in  two  ways.  In  the  first  place,  it  may 
partially  close  the  ventilating  passages  and  thus  decrease^  the 
quantity  of  veatilatiQ^  air>  la  the  second  place,  it  may  form  a 


390  ELECTRICAL  ENGINEERING  PAPERS 

coating  upon  the  heat-radiating  surfaces  so  that  the  cooling  air 
cannot  come  directly  in  contact  with  such  surfaces.  Ordinarily, 
in  dissipating  heat  from  a  surface  to  the  air,  a  thin  film  of  hot  air 
adheres  to  the  surface,  and  the  heat  is  conveyed  from  the  surface 
through  this  film  to  the  moving  air.  With  high-velocity  air 
striking  the  surface,  this  film  of  hot  air  is  scoured  away  from  the 
surface,  so  that  new  air  continually  comes  in  contact  with  the 
surface.  If  however,  a  coating  of  dust,  or  of  other  heat-insulating 
particles,  gathers  on  the  surface,  then  the  ventilating  air  cannot 
come  in  direct  contact  with  the  surface  and  the  heat-dissipation  is 
at  a  lower  rate.  Dirt  is  particularly  liable  to  adhere  to  the  sur- 
faces in  case  minute  particles  of  oil  are  carried  into  the  machine. 

AIR  WASHERS 

Air  washers  are  now  being  installed  very  generally  in  large 
generating  plants  to  clean  the  air  which  passes  through  the  ma- 
chines. These  washers  have  beneficial  results  in  two  ways.  In 
the  first  place,  they  clean  the  air,  thus  preventing,  to  a  great 
extent,  the  deposit  of  dirt  in  the  machine.  In  the  second  place, 
they  cool  the  air  in  hot  weather,  thus  directly  improving  the 
capacity  of  the  machine  by  allowing  a  greater  temperature  in- 
crease without  exceeding  a  specified  limit  of  temperature.  A 
number  of  attempts  have  been  made  to  cool  turbo-generators  by 
means  of  water  in  suspension  in  the  ingoing  air,  that  is,  by  "fog." 
Such  methods  as  yet  have  shown  no  particular  promise. 

Instances  have  occurred  where  fine,  dry  snow  has  been  drawn 
into  artificially-cooled  apparatus  and  there  melted,  with  the 
formation  of  water  on  the  windings.  This  can  only  happen  in 
cold  weather  and  could  be  avoided  in  several  ways.  An  opening 
from  the  inside  of  the  building  could  be  provided  in  the  air  intake 
so  that  the  ventilating  air  is  not  taken  from  the  outside.  Another 
method  would  be  to  use  the  air  washer  at  such  times,  by  which 
means  the  snow  would  be  abstracted.  It  is  usually  not  con- 
sidered advantageous  to  operate  the  washers  during  extremely 
cold  weather,  but  in  case  of  incoming  snow  there  may  be  con- 
siderable advantage  in  operating  the  washers. 

ZERO  AIR,  AND  WATER  DEPOSIT 

A  number  of  instances  have  been  noted  where,  with  the  in- 
coming air  at  an  extremely  low  temperature  (near  zero  F.),  the 
end  windings  of  turbo-generators  were  found  covered  with  a  film 
of  water.  In  one  case  this  proved  disastrous.  Apparently  this 


ARTIFICIAL  VENTILATION  391 

was  not  condensation  of  water  from  the  air,  in  the  ordinary  sense 
for  the  armature  windings  were  much  hotter  than  the  incoming  air. 
One  explanation  is  that  this  is  due  to  "frozen  fog,"  or  ice  particles 
in  suspension,  which  melt  when  they  come  in  contact  with  the 
heated  parts  of  the  machine.  One  remedy  for  this  trouble  is  in 
the  use  of  doors  from  the  interior  of  the  building  to  the  inlet  pipe, 
which  can  be  opened  in  extremely  cold  weather  to  admit  warmer 
air. 

FIRE   HAZARDS  IN  ARTIFICIALLY-COOLED  MACHINERY 

When  a  fire  is  started  in  such  apparatus  the  artificial  ventila- 
tion tends  to  spread  the  fire  very  quickly,  especially  in  turbo- 
generators. Various  remedies  have  been  proposed,  such  as 
firedoors  or  dampers  in  the  inlet  conduits,  the  use  of  "fireproof1 
end  windings,  chemical  extinguishers,  etc.  Firedoors  have 
proven  only  partially  effective,  possibly  due  to  the  fact  that  it  is 
difficult  to  shut  off  the  air  completely.  For  instance,  in  a  ma- 
chine taking  50,000  cu.  ft.  of  air  per  minute,  if  99  percent  of  the 
air  is  shut  off,  the  remaining  500  cu.  ft  which  can  pass  through  the 
machine  may  be  sufficient  to  maintain  quite  a  destructive  blaze. 
Furthermore,  there  are  liable  to  be  small  leaks  around  the  end 
housings  of  the  machine  which  will  admit  a  little  air. 

It  has  also  been  suggested  that  equally  good  results  would  be 
obtained  by  enclosing  the  outlets  from  the  machine  in  case  of  fire, 
thus  retaining  the  products  of  combustion  inside  the  machine, 
these  forming  a  fairly  good  fire  extinguisher  in  themselves. 

As  regards  chemical  extinguishers,  these  are  practically  useless 
unless  the  incoming  air  can  be  almost  completely  shut  off.  With 
50,000  cu.  ft,  of  air,  for  instance,  passing  through  a  machine,  the 
small  amount  of  gas  which  the  extinguisher  could  furnish  would  be 
so  diluted  as  to  be  worthless.  One  point  to  keep  in  mind  in  apply- 
ing extinguishing  gases  is  that  they  must  be  applied  at  the  incoming 
side  of  the  fan. 

In  some  cases  of  fire  the  operators  have  turned  on  water  from 
high-pressure  mains,  on  the  theory  that,  while  water  may  ruin  the 
insulation,  yet  fire  may  result  in  still  greater  damage. 

Various  attempts  have  been  made  to  produce  *  'fireproof" 
insulations  for  the  end  windings  of  turbo-generators.  The  diffi- 
culty lies  in  the  fact  that  available  fireproof  materials,  such  as  mica 
and  asbestos,  cannot  be  used  alone.  Mica  requires  some  support- 
ing or  binding  material,  while  asbestos  requires  some  filling 


39*  ELECTRICAL  ENGINEERING  PAPERS 

varnish  in  order  to  obtain  suitable  insulating  quality.  It  is  these 
binding  or  filling  materials  that  are  the  real  source  of  trouble,  for 
these  give  off  gases  if  the  temperature  is  sufficiently  high,  and  these 
tend  to  maintain  or  increase  the  blaze.  Thus  the  outlook  is  not 
very  promising. 

When  the  initial  blaze  is  produced  by  a  short-circuit  or  arc 
inside  the  machine,  a  sudden  interruption  of  the  excitation,  by  kill- 
ing the  voltage,  may  extinguish  the  arc  before  a  general  conflagra- 
tion is  established.  If  the  excitation  is  from  a  motor  generator 
across  the  terminals  of  the  machine,  then  a  short-circuit  in  the 
machine  may  automatically  shut  down  the  exciter  set.  In  the  same 
way,  if  the  ventilating  fan  is  driven  by  a  motor  across  the  terminals 
of  the  machine,  the  ventilation  may  decrease  automatically. 

The  above  covers  various  suggested  methods  for  preventing 
damage  by  fire  inside  such  apparatus,  All  of  them  are  admittedly 
defective,  but  each  of  them  possesses  some  merit.  A  simple  satis- 
factory method  of  fire  protection  for  such  apparatus  is  much  to  be 
desired. 

NOISE 

Recent  high-speed  turbo-generator  rotors  are  all  of  the  cylin- 
drical type,  with  relatively  smooth  exterior  surfaces.  Neverthe- 
less, due  to  their  enormously  high  peripheral  speeds  and  the  great 
quantity  of  air  through  the  air  gaps,  there  is  always  very  consider- 
able noise  developed  inside  the  machines  themselves.  As  such 
machines  are  always  very  completely  enclosed,  except  through 
their  outlet  and  inlet  pipes  or  openings,  these  latter  are  usually 
responsible  for  any  complaints  regarding  noise.  Several  cases 
have  developed  where  the  inlet  conduits,  opening  directly  to  the 
outside  of  the  btiilding,  have  permitted  undue  noise.  In  other 
cases,  sheet  metal  conduits  have  acted  as  sounding  tubes  and  ap- 
parently have  exaggerated  the  noise.  Changing  to  plaster-filled 
expanded  metal  conduits  has  helped  in  some  cases.  In  other  cases, 
carrying  the  conduits  up  to  the  roof  of  the  building  has  proved 
effective,  A  secondary  result  of  this  arrangement  is  that  deaner 
air  is  obtained,  unless  the  inlet  is  exposed  to  an  undue  amount  of 
dirt  from  the  chimneys. 


SOME  ELECTRICAL  PROBLEMS  PRACTICALLY 
CONSIDERED 

FOREWORD — This  paper  was  prepared  for  the  eighth  annual  conven- 
tion of  the  Association  of  Iron  &  Steel  Electrical  Engineers  held  at 
Cleveland,  September,  1914.  The  object  of  the  paper  was  to 
present,  in  as  simple  form  as  possible,  certain  problems,  such  as 
insulation,  commutation,  speed  control  of  induction  motors,  etc., 
which  particularly  concerned  iron  and  steel  electrical  engineers. 

"(ED.) 


IN  steel  mill  electrical  work  there  are  a  number  of  subjects  which 
are  of  very  particular  interest  at  present.  In  both  alternating 
and  direct-current  apparatus,  there  is  the  general  subject  of  insul- 
ation troubles,  which  is  always  open  to  discussion.  In  direct- 
current  work,  commutation  and  commutator  troubles  are  subjects 
which  are  always  with  us.  Also,  as  the  induction  motor  is  prob- 
ably used  more  than  any  other  in  mill  work,  the  problem  of  obtain- 
ing variations  and  adjustments  in  speed  with  this  type  of  motor 
has  become  a  very  important  one.  In  alternating-current  work, 
there  is  the  question  of  the  most  suitable  frequency,  which  has 
come  up  prominently  in  the  past  two  or  three  years.  While  these 
various  subjects  may  appear  to  be  more  or  less  disconnected,  yet, 
in  fact,  they  are  already  allied  in  mill  work,  and  all  steel  mill 
electrical  engineers  are  liable  to  be  called  upon  to  deal  with  them. 
In  the  presentation  of  these  subjects,  a  semi-technical  method 
is  followed,  and  all  mathematics,  except  where  masked  under  some 
other  form,  are  omitted.  The  various  subjects  are  treated  in  the 
order  of  their  convenience,  and  without  regard  to  their  relative 
importance. 

THE  INSULATION  PROBLEM 

Practically  all  electrical  apparatus  uses  insulation  in  one  form 
or  another.  Such  insulation  in  general  constitutes  the  weakest 
part  of  the  machine,  both  mechanically  and  electrically.  Insofar 
as  the  generation  or  utilization  of  energy  is  concerned,  its  functions 
are  passive,  it  serving  merely  as  a  protection.  But  in  another  way, 
its  functions  unf ortunately  are  not  passive,  namely,  in  its  effect  on 
heat  flow  and  dissipation.  In  most  cases,  the  parts  which  have  to 
be  insulated  are  heat-generating.  This  is  especially  true  of  the 

393 


394  ELECTRICAL  ENGINEERING  PAPERS 

windings  of  electrical  apparatus.  Experience  shows  that  aH 
electric  insulators  are  heat  insulators  to  a  great  extent,  and  ex- 
tremely good  heat  insulators  in  the  case  of  the  most  practicable 
materials.  It  is  well  known  that  the  best  way  to  apply  heat  in- 
sulations is  in  the  form  of  superimposed  layers,  and  this  happens 
to  be  the  most  practicable  way  of  applying  most  electrical  insula- 
tions. It  is  also  weU  known  that  air  pockets  in  heat  insulations 
improve  their  heat-insulating  qualities.  It  is  partly  on  this 
account  that,  in  the  application  of  electric  insulations,  air  pockets 
are  avoided  as  much  as  possible,  and  endeavor  is  made  to  fill  such 
pockets  with  varnishes  or  impregnating  gums  which  act  as  better 
heat  conductors  than  air  or  gases.  In  general,  it  may  be  said  that 
heat  is  transmitted  more  effectively  by  conduction  through  solid 
bodies,  or  between  solid  bodies  in  contact,  than  by  convection 
through  gaseous  bodies.  Therefore,  the  more  solid,  or  the  better 
filled  is  the  insulation,  the  better  it  will  conduct  heat  as  a  rule,  and, 
in  fact,  there  is  not  such  a  great  difference  between  the  heat-con- 
ducting qualities  of  the  various  commercial  insulations,  on  the 
basis  of  equal  solidity.  The  principal  differences  are  found  in  the 
ways  the  materials  are  applied.  While  some  materials  may  con- 
duct heat  two  or  three  times  as  well  as  others,  yet  this  difference  is 
very  small  compared  with  the  difference  in  heat-conducting 
qualities  between  ordinary  insulations  and  any  of  the  so-called 
electrical  conductors,  such  as  metals.  For  instance,  a  difference 
of  temperature  of  1°C.  between  the  opposite  sides  of  an  inch  cube 
of  copper  will  allow  a  heat  flow  2500  times  as  great  as  with  a  cor- 
responding cube  built  up  of  oil  tape.  And  an  inch  cube  of  wrought 
iron,  which  is  considered  a  poor  electrical  conductor,  will  conduct 
about  400  times  as  much  heat  as  the  block  of  insulation.  There- 
fore, when  compared  with  electrical  conductors,  we  may  say  that 
the  heat-conducting  qualities  of  the  usual  built-up  insulations  are 
fairly  uniform. 

The  heat-conducting  ability  of  insulation  is  a  function  of  the 
thickness  or  distance  the  heat  has  to  traverse,  just  as  in  all  other 
bodies.  Therefore,  when  a  heat-generating  body  is  covered  with 
insulation,  it  is  desirable  to  make  such  insulation  as  thin  and 
compact  as  possible,  where  it  is  desirable  to  keep  the  temperature 
as  low  as  possible.  This  is  an  elementary  fact  which  has  been  very 
much  neglected  and  overlooked  in  the  past. 

In  electrical  apparatus,  it  may  be  said  that  it  is  not  the  tem- 
perature in  the  heat-generating  body  itself  which  is  harmful,  but 


INSULATION  PROBLEMS,  ETC  395 

it  is  the  effects  of  such  temperature  upon  the  enclosing  or  con- 
tiguous insulation  which  must  be  taken  into  account.  Most  of  the 
flexible  insulations  in  every-day  use  do  not  have  high  heat-resisting 
characteristics.  The  effect  of  the  heat  usually  is  more  harmful 
to  the  mechanical  characteristics  of  the  material  than  to  the  elec- 
trical characteristics.  Most  fibrous  insulations,  when  exposed  to 
fairly  high  temperatures  for  long  periods,  or  exceedingly  high 
temperatures  for  much  shorter  periods,  show  a  tendency  to  become 
very  brittle,  and,  in  time,  they  may  even  carbonize  to  a  greater  or 
less  extent.  However,  for  moderate  voltage  stresses,  even  this 
very  dry  or  semi-carbonized  condition  of  the  insulation  does  not 
appear  to  seriously  affect  its  insulating  qualities.  The  real  harm 
lies  in  deterioration  or  possible  injury  of  its  mechanical  properties 
— that  is,  it  may  become  so  brittle  that  it  will  not  stand  mechan- 
ical shocks  or  vibrations,  and  may  crack  or  scale  off  so  that  its 
insulating  qualities  are  impaired  simply  through  mechanical 
defects.  Here  is  where  certain  filling  or  impregnating  varnishes 
or  gums  are  particularly  useful.  As  the  fibrous  insulation  tends 
to  become  brittle  at  high  temperatures,  the  varnish  or  gum  may 
tend  to  soften  at  the  same  temperature,  and  thus  conteract,  to  a 
certain  extent,  the  brittleness  of  the  fibrous  material  itself.  A 
second  function  of  such  gums  or  varnishes  is  to  act  as  fillers  for  all 
spaces  and  interstices,  and  thus  to  assist  in  conduction  of  heat, 
but,  still  more,  to  act  as  a  cushioning  material  to  keep  the  con- 
ductors from  vibrating  under  shocks,  etc.  Of  course,  the  impreg- 
nating gums  or  varnishes  have  a  certain  value  as  insulating 
material,  but  probably  the  above  functions  are  of  far  greater 
value.  For  instance,  the  ordinary  cotton  covering  on  a  wire  will 
stand  far  more  abuse  when  treated  with  some  kind  of  gum  or 
varnish  than  when  used  in  the  dry  condition,  for,  in  the  former 
case,  the  individual  fibers  of  the  covering  are  actually  pasted  in 
place,  and  are  therefore  much  less  liable  to  be  separated  and  thus 
allow  metal  parts  to  come  in  contact.  Usually  what  is  required 
between  adjacent  conductors  in  a  coil  is  a  positive  mechanical 
separation  of  a  very  limited  amount.  In  many  cases,  if  the  bare 
conductors  could  be  maintained  at  a  distance  apart  corresponding 
to  the  thickness  of  the  usual  cotton  covering,  this  would  be  suf- 
ficient for  protection  against  the  voltages  between  the  wires, 
TJie  layer  of  insulation  on  -the  wires  themselves  furnishes  the  sim- 
plest and  easiest  method  of  obtaining  this  mechanical  separation, 
and  the  varnish  or  gum  treatment  makes  this  separating  medium 
of  more  mechanical  and  durable  construction,  and,  at  the  same 
time,  improves  the  heat-conducting  qualities. 


396  ELECTRICAL  ENGINEERING  PAPERS 

There  are  limits  to  the  heat-resisting  qualities  of  all  practic- 
able insulations.  Ordinary  fibrous  materials  of  a  cellulose  nature 
or  base,  will  stand  about  95  °C,  to  100°C.  without  becoming  too 
brittle  to  be  durable.  However,  the  same  materials,  when  treated 
with  suitable  varnishes  or  gums,  apparently  stand  temperatures  of 
about  105  °C.  without  undue  deterioration  mechanically.  At 
this  temperature  the  material  does  not  appear  to  carbonize,  and 
the  varnish  or  gum  assists  in  maintaining  mechanical  continuity 
of  the  material.  At  materially  higher  temperatures,  deterioration 
gradually  takes  place  at  a  rate  depending  upon  the  actual  tem- 
perature attained.  Even  at  150°C.,  treated  fibrous  materials  may 
have  a  total  life  of  several  months  before  the  material  becomes 
unsuited  for  its  purpose.  If  such  high  temperature  exists  only 
for  short  periods,  and  during  the  remaining  time  the  insulation  is 
subjected  to  relatively  low  temperatures,  then  the  life  of  the 
apparatus  measured  in  years,  may  be  fairly  great.  In  other 
words,  high  peak  temperatures  may  not  be  very  harmful,  pro- 
vided the  sum  total  of  such  peak  periods  does  not  add  up  to  as 
long  a  period  as  required  to  injure  the  materials  if  maintained  at 
the  same  peak  temperature  steadily.  However,  the  life  of 
insulation  does  not  decrease  in  direct  proportion  to  the 
increase  in  temperature,  but  at  a  much  faster  rate. 

Other  insulations  in  common  use  are  mica,  asbestos  and 
certain  varnishes  and  gums.  Pure  mica  will  stand  enormously 
high  temperatures,  such  as  700°C.  or  even  higher.  Good  grades 
of  asbestos  stand  at  least  400  °C.  as  shown  by  actual  test,  and 
possibly  very  much  higher.  However,  neither  mica  nor  asbestos, 
in  itself,  is  a  good  material  for  application  to  windings,  due  to 
mechanical  conditions.  In  order  to  obtain  flexibility,  mica  must 
be  built  in  thin  sheets  and  then  assembled  in  the  form  of  a  paper 
or  tape.  This  requires  some  continuous  supporting  base,  usually 
a  thin  tough  paper,  to  which  the  mica  is  attached  by  some  form  of 
binding  gum.  The  result  therefore  consists  of  both  high  and  low 
heat-resisting  materials.  If  the  continuity  and  durability  of  the 
resultant  mica  insulation,  after  application  to  a  coil,  is  dependent 
upon  the  durability  of  the  binding  and  supporting  material,  then 
such  insulation  is  limited  to  temperatures  corresponding  to  fibrous 
materials.  If,  however,  the  binding  and  supporting  material  can 
deteriorate  without  materially  injuring  the  insulation  as  a  whole, 
thai  such  composite  insulation  can  stand  comparatively  high 
temperatures.  In  present  practice,  such  temperatures  are  limited 


INSULATION  PROBLEMS,  ETC.  397 

to  approximately  1SO°C.  for  steady  operation,  not  because  this  is 
an  actual  limit,  but  largely  because  of  lack  of  extended  experience 
at  materially  higher  temperatures.  Apparently,  such  materials, 
when  properly  applied,  will  stand  300°C.  on  peak  service  about 
as  well  as  the  treated  fibrous  materials  will  stand  150°C, 

Asbestos,  as  an  insulation,  is  pretty  poor  material,  but  as  a 
mechanical  separator,  where  high  temperatures  are  obtained,  it 
may  be  very  effective.  Due  to  its  open  fibrous  character,  there 
is  no  true  over-lapping  of  insulating  surfaces,  and,  to  make  as- 
bestos effective  as  an  insulator,  it  must  be  filled  in  with  some 
insulating  filler  or  gum,  in  which  case,  the  gum  is  the  real  insul- 
ator. However,  asbestos  may  answer  for  a  very  good  supporting 
material  for  other  insulations,  such  as  mica,  when  subjected  to 
very  high  temperatures.  Also,  asbestos  may  be  a  suitable  in- 
sulation on  conductors  with  very  low  potential  between  them,  as 
in  field  windings  and  in  armature  windings  with  low  internal 
potentials.  It  should  be  considered  as  essentially  a  separating 
and  supporting  material,  rather  than  as  an  insulation. 

In  recent  years,  a  number  of  synthetic  resins,  such  as  Bakalite, 
have  been  developed,  which  have  a  field  in  insulation  work. 
Such  materials  usually  have  high  heat-resisting  qualities,  but,  in 
their  final  condition,  are  liable  to  be  hard  and  brittle.  Some  of 
them  are  used  extensively  as  impregnating  or  filling  varnishes, 
and  when  so  applied,  are  in  fluid  form,  and  require  further  baking 
to  change  them  to  the  final  form.  Some  very  extravagant 
claims  have  been  made  for  them  by  those  who  were  not  sufficiently 
acquainted  with  the  materials  and  their  properties.  They  are 
very  valuable  in  many  ways,  but,  like  all  other  materials,  they 
have  their  limitations.  In  their  application  to  armature  windings, 
it  is  advisable,  in  many  cases,  to  apply  the  coils  before  being  given 
the  final  baking  on  account  of  the  greater  flexibility  of  the  unbaked 
coils.  But  the  final  treatment  usually  leaves  the  armature 
winding  in  such  rigid  condition  that,  in  case  of  repairs,  it  may  be 
necessary  to  completely  destroy  the  whole  winding.  This  looks 
like  a  bad  feature,  but  to  counter-balance  it,  it  may  be  said  that, 
for  certain  kinds  of  work,  such  prepared  windings  are  less  liable 
to  damage,  and  therefore  the  necessity  for  repairs  is  much  reduced* 
As  impregnating  compounds,  where  stiffness  or  rigidity  is  ad- 
vantageous, such  materials  have  proved  very  satisfactory,  but, 
where  considerable  flexibility  is  desirable,  compounds  of  this 
nature  may  not  pix>ve  so  desirable.  If  the  impregnating  material 


398  ELECTRICAL  ENGINEERING  PAPERS 

is  brittle  and  is  liable  to  cracl:,  tinder  stresses  due  to  change  in 
temperature,  or  movement,  or  shock,  then  it  loses  a  certain  part 
of  its  value.  Where  flexibility  is  important,  gums  or  varnishes 
which  soften  with  heat  are  desirable. 

In  armature  or  field  windings,  it  is  very  unusual  to  find  con- 
stant temperatures  throughout  the  whole  winding,  due  to  the 
different    heat-conducting    and    heat-dissipating    conditions    in 
different  parts.    Therefore,  the  higher  temperature  points  or  ' '  hot 
spots"  must  be  considered  in  fixing  the  insulation  temperature 
limits.     It  is  the  highest  temperature  to  which  the  insulation  is 
subjected  that  must  be  considered  in  fixing  the  limits,  and  only  in 
rare  cases  do  the  ordinary  methods  of  temperature  measurements 
indicate  the  highest  temperatures  actually  attained.     Ordinary 
thermometer   measurements   approximate   the   temperature   at 
some  accessible  point,  but  this  may  not  be,  or,  likely,  is  not  the 
hottest  part.    A  determination  of  the  temperature  by  increase  in 
resistance  gives  only  an  average  value.    Therefore,  by  actual 
measurement  by  the  usual  methods,  the  above  mentioned  tem- 
perature limit  of  105°C.  for  treated  fibrous  materials  is  not  allow- 
able.    For  instance,  the  usual  full  load  guarantee  of  40°C.  rise 
with  a  cooling  air  temperature  of  40°C.  will  give  80°C.  as  the 
temperature  measured,  thus  allowing  a  margin  of  25  °C.  for  some 
higher  internal  temperature — that  is,  for  the  hot  spot.    The  usual 
overload  guarantee  of  55  °C.  by  thermometer,  with  air  at  40  °C., 
will  give  95  °C.  measured,  or  a  margin  of  10°C  for  the  hot  spot, 
which  apparently  is  right  on  the  ragged  edge.    But  then,  this  55  °C. 
guarantee  is  usually  given  only  for  overloads  or  intermittent 
service,  and  it  is  this  condition  which  allows  the  proper  margin. 
If,  however,  an  accurate  means  should  become  practicable  for 
determining  the  actual  hot  spot  temperatures,  then  it  would  be 
practicable  to  rate  machines  at  the  105°C.  measured  temperature. 
As  this  cannot  be  done  at  present,  we  must  fall  back  on  a  lower 
measurable  temperature,  and  allow  a  suitable  margin. 

In  certain  classes  of  apparatus  where  the  higher  temperature 
regions  are  pretty  definitely  known,  it  is  possible  and  practicable 
in  many  cases,  to  insulate,  in  the  hotter  regions,  with  materials 
which  have  higher  heat-resisting  characteristics,  as  already  de- 
scribed. This  is  the  case  in  many  high  voltage  machines,  and  in 
machines  with  very  wide  armature  cores,  such  as  some  turbo- 
generators, high  speed  large  capacity  alternators,  etc.  In  such 
machines,  the  middle  part  of  the  armature  core  is  liable  to  be 


INSULATION  PROBLEMS,  ETC,  399 

much  hotter  than  any  other  part.  Therefore,  it  is  rather  common 
practice  in  such  machines  to  insulate  the  buried  part  of  the  arma- 
ture coils  with  composite  mica  insulation,  which  can  be  easily 
applied  on  the  straight  portions  of  the  coil.  On  the  curved  end 
parts  of  the  coils,  where  taped  insulations  are  required  on  account 
of  the  curvature,  much  lower  temperatures  are  usually  obtained, 
and  thus  fibrous  tape  insulations  are  amply  safe  for  this  part. 

In  apparatus  subject  to  very  heavy  overloads  for  relatively 
short  periods,  excessively  high  temperatures  may  be  attained  by 
the  copper  inside  the  insulation,  but  if  followed  by  much  lighter 
load,  the  high  temperature  may  drop  so  rapidly  that  no  apparent 
damage  occurs.  Experience  has  shown  that,  not  infrequently, 
local  temperatures  of  200°C.  to  300°C.  are  attained  for  a  very 
short  time.  When  such  temperatures  occur  close  to  any  soldered 
connections,  there  is  danger  of  damage  due  to  unsoldering,  for  or- 
dinary commercial  solders  will  soften  at  about  170°C.  to  180°C., 
while  pure  tin  solders  will  soften  at  about  220°C.  to  230°C. 
Therefore,  temperatures  which,  due  to  their  short  duration,  ap- 
parently do  not  harm  the  insulation,  may  actually  unsolder  con- 
nections 

The  above  covers  briefly  the  temperature  part  of  the  insula- 
tion problem.  But  insulating  materials  also  serve  another  pur- 
pose, namely,  to  shield  the  conducting  or  live  parts  of  the  machine 
from  other  foreign  conducting  materials,  such  as  dirt,  grease,  oil, 
water,  etc.  Oils  and  greases  are  usually  considered  as  non-con- 
ducting, but  when  they  are  liable  to  carry  with  them  conducting 
materials,  such  as  copper  and  carbon  particles,  they  become  con- 
ductors in  effect.  Also,  ordinary  dust,  or  dirt,  or  deposit  from  the 
air,  is  a  relatively  poor  conductor*  but  conducts  far  better  than 
the  usual  insulations,  and  is  therefore,  to  a  certain  extent,  danger- 
ous. As  a  conductor,  water  is  considered  as  comparatively  poor, 
and  yet  no  one  would  class  it  as  an  insulator.  Both  water  and  oil 
may  be  directly  harmful  and  may  be  indirectly  injurious  by  their 
actions  upon  the  insulating  materials  themselves.  In  the  case  of 
cloth  tape  insulators,  the  cloth  may  be  considered  as  simply  form- 
ing aKbase  or  reinforcing  structure  for  the  insulation  proper,  which 
is  usually  some  varnish  or  oil  compound.  The  insulation  value 
of  the  material  depends  principally  upon  the  continuity  of  the 
layers  of  varnish  or  oil.  The  cloth  structure  itself  has  no  true 
continuity.  In  applying  such  tapes  or  insulations,  the  layers  over- 
lap each  other  in  such  a  way  as  to  give  the  best  sealed  circuit. 


400  ELECTRICAL  ENGINEERING  PAPERS 

During  the  process  of  taping,  the  surface  may  be  varnished  re- 
peatedly to  further  seal  the  overlapping  joints,  and  to  obtain 
greater  continuity  of  the  insulating  film.  Also,  in  the  composite 
mica  insulations,  the  mica  laminas  are  very  thin  and  arranged  in 
a  number  of  layers  in  such  a  way  as  to  overlap  as  completely  as 
possible  to  form  insulating  films.  The  binding  material  between 
layers  or  films  is  largely  for  the  purpose  of  sticking  or  binding 
the  mica  laminae  to  each  other.  Therefore,  with  either  type  of 
insulation,  continuity  of  the  insulating  film,  is  the  first  requi- 
site, and  any  action  or  treatment  which  tends  to  break  the 
films  will  naturally  tend  to  weaken  the  insulation.  In  high 
voltage  armature  coils,  in  particular,  it  is  of  utmost  importance 
that  the  completed  coils  should  not  be  sprung  or  bent  to  such 
an  extent  that  the  insulation  films  are  liable  to  be  cracked  or 
"buckled"  at  any  point,  for  this  immediately  produces  a  local 
weakness.  In  all  cases,  extreme  care  should  be  taken  in  handling 
such  coils,  especially  in  placing  them  on  the  cores.  Moreover,  in 
machines  which  are  liable  to  carry  excessive  currents,  even  momen- 
tarily, and  which  are  thus  liable  to  distorting  magnetic  stresses, 
the  windings  must  be  so  braced  that  movements  sufficient  to 
crack  or  buckle  the  insulation  are  not  permitted.  There  is  but 
little  real  flexibility  in  such  insulations  when  built  of  any  con- 
siderable thickness.  Insulation  on  cables  might  be  cited  as  an 
exception,  but  here  the  insulating  varnishes  are  soft  and  possibly 
semi-viscous,  so  that  a  certain  amount  of  bending  does  not  break 
the  insulating  films.  To  maintain  this  condition  of  soft  flexible 
insulation,  cables  are  guaranteed  usually  only  for  very  low  maxi- 
mum temperatures,  compared  with  the  temperatures  usually 
found  in  electrical  apparatus. 

The  continuity  of  the  insulating  films  may  be  injured  in  other 
ways  than  by  bending.  If,  for  instance,  a  newly  insulated  coil 
which  has  been  insufficiently  baked  or  "  seasoned,"  is  subjected 
to  a  comparatively  high  temperature  for  even  a  short  time, 
certain  volatile  matter  in  the  varnishes  may  be  given  off  in  the  form 
vapor,  and  these  vapors  may  force  or  break  their  way  through  the 
insulating  films.  The  writer  has  in  mind  one  case  where  a  taped 
insulation  was  used  on  a  rewinding,  with  the  shortest  possible 
time  for  the  baking  before  applying  the  coils  to  the  machine.  The 
insulation  tests  were  high,  but  a  heavy  load  was  thrown  on  the 
machine  at  once  and  carried  for  several  hours.  At  the  end  of  this 
rim,  the  insulation  test  showed  that  the  insulating  material  had 


INSULATION  PROBLEMS,  ETC.  401 

deteriorated  very  greatly,  — so  much  so  that  the  machine  was 
considered  to  be  in  a  dangerous  condition.  Upon  removing  some 
of  the  coils,  an  examination  showed  what  looked  like  little  vol- 
canoes all  over  the  surface  of  the  insulation.  Further  investiga- 
tion showed  that  these  were  real  volcanoes,  for  the  high  inter- 
nal temperature  had  vaporized  some  of  the  original  solvents  which 
had  not  been  entirely  removed  from  the  varnish,  and  such  vapors 
had  actually  erupted  through  the  superimposed  strata  represented 
by  the  insulating  films  or  varnishes.  Therefore,  at  each  one  of 
these  points  of  eruption,  there  was  a  breakdown  of  the  insulating 
strength  of  greater  or  less  depth,  depending  upon  where  the  vapor 
was  formed.  This  is  cited  simply  as  a  very  good  illustration  of 
what  can  happen  in  "green"  insulation. 

Another  source  of  difficulty  which  is  not  unusual,  is  that  due 
to  water,  or  oil,  or  other  foreign  materials  getting  into  the  insula- 
tion. Submersion  of  electrical  apparatus,  due  to  floods,  is  not 
uncommon  in  industrial  plants,  due  to  their  proximity  to  rivers, 
in  many  cases.  In  some  cases,  experience  has  shown  that  a 
flooded  machine  can  be  dried  out  with  apparently  no  harmful 
after  effects,  while  in  other  cases,  it  has  been  found  almost  hope- 
less to  save  the  apparatus.  This  depends  to  some  extent  upon  the 
kind  and  character  of  the  insulation  and  the  means  for  getting  rid 
of  the  water  without  injuring  the  insulation  itself.  If  water  has 
percolated  into  the  coil  and  becomes  sealed  or  trapped  inside,  then 
high  internal  temperatures  obtained  by  any  means  may  simply 
vaporize  the  water  without  getting  rid  of  it.  If  the  insulation  is 
porous,  the  water  may  be  driven  off  readily.  If  the  drying  heat  is 
applied  from  the  outside,  then,  before  the  center  is  heated  suffic- 
iently to  vaporize  the  water,  the  outside  insulating  films  may  seal 
together  under  the  higher  outside  temperature,  so  that  the  internal 
vapors  cannot  escape  except  by  disrupting  the  film  If,  on  the 
other  hand,  heat  is  applied  from  the  inside,  by  means  of  current 
for  instance,  and  the  heating  is  too  rapid,  vapor  may  be  formed 
more  rapidly  than  it  can  percolate  through  the  insulation,  and  it 
may  injure  the  insulation  in  escaping.  fc  Also,  in  the  case  of  elec- 
trical heating,  non-uniformity  of  temperature  must  be  taken  into 
account.  For  instance,  the  armature  winding  of  a  high  voltage 
alternator  might  be  operated  on  a  short  circuit  for  the  purpose  of 
drying  out.  The  drying  out  current  may  be  so  high  that  the 
center  of  the  armature  core  is  considerably  above  100°C.  or  the 
boiling  point  of  water,  while  the  end  windings  may  be  30  percent 


402  ELECTRICAL  ENGINEERING  PAPERS 

or  40  percent  cooler.  In  such  case,  the  water  in  the  hot  part  of  the 
coils  is  simply  vaporized  and  driven  to  the  end  windings  and  there 
condensed.  This  is  not  an  unusual  condition  in  drying  out  high 
voltage  windings  which  contain  moisture.  One  instance  may  be 
cited,  where,  several  years  ago  the  power  house  of  the  Westing- 
house  Electric  &  Manufacturing  Co.  was  flooded  for  several  days, 
and  several  large  2200 -volt  turbo-generators  were  partly  sub- 
merged. One  of  these  machines  was  dried  out  on  short  circuit  for 
about  a  week  at  a  temperature  of  possibly  120°C.  inside  the  coil. 
At  the  end  of  this  time,  no  leak  to  ground  showed  and  the  machine 
was  put  in  service.  A  few  weeks  afterwards,  a  short  circuit 
occured  inside  one  of  the  coils,  in  the  end  winding.  When  dis- 
mantled, this  coil  was  found  to  be  sopping  wet  in  the  end  portion, 
although  the  buried  part  of  the  coil  was  fairly  dry.  The  baking 
process  had  simply  distilled  the  water  from  the  center  to  the  end 
parts.  An  ftTa.Tnina.tinn  of  others  of  the  submerged  coils  showed 
the  same  condition.  It  is  possible  that  untaping  of  the  end  wind- 
ing sufficiently  to  have  allowed  the  escape  of  vapor  would  have 
allowed  this  machine  to  dry  out  properly,  but  apparently  this 
would  not  be  the  case  unless  the  end  windings  in  themselves  could 
have  been  brought  up  to  a  temperature  considerably  above  100°C. 
and  this  might  have  meant  1SO°C.  in  the  buried  portion,  which 
would  probably  have  been  injurious,  except  to  mica  insulations, 
which  did  not  happen  to  be  on  these  machines.  Furthermore,  it 
is  not  always  easy  to  get  rid  of  moisture,  even  at  100°C.  with 
fibrous  insulations.  One  very  effective  manner  of  doing  so  is  by 
means  of  a  vacuum.  Experience  has  shown  that  if  apparatus  to 
be  dried  out  is  heated  to  tne  boiling  point,  in  a  vacuum,  the  moist- 
ure usually  is  removed  very  completely.  For  most  effective  re- 
sults the  water  should  be  vaporized,  for,  under  some  conditions, 
and  with  some  materials,  the  force  of  capillarity  may  approximate 
IS  Ibs.  so  that  a  good  vacuum  alone  may  not  be  able  to  overcome 
the  capillary  action.  From  the  scientific  standpoint,  the  use  of 
vacuums  in  drying  goes  much  further  than  the  above.  For  ex- 
ample, the  boiling  point  of  water  is  very  much  reduced  in  a 
vacuum,  so  that  materially  lower  temperatures  may  be  used  for  re- 
moving water  than  would  otherwise  be  the  case.  For  rapid  drying 
under  ordinary  air  pressures,  considerably  over  100°C.  is  needed, 
while  in  a  fairly  good  vacuum,  100°C.  or  less,  may  allow  a  very 
rapid  evaporation  of  moisture  and  a  correspondingly  rapid  and 
thorough  drying. 


INSULATION  PROBLEMS,  ETC.  403 

In  the  same  flood  which  submerged'  the  generator  above  re- 
ferred to,  vast  quantities  of  other  apparatus  of  various  types  and 
designs  were  also  flooded,  and,  in  drying  out  this  apparatus,  a 
great  deal  of  valuable  experience  and  data  were  obtained.  A 
summation  of  this  and  other  experience  may  be  of  value  and 
interest,  and  is  therefore  given  below. 

Low  voltage  alternating-current  windings,  such  as  induction 
motors  and  alternating-current  generators  for  600  volts  and  less, 
dried  out  very  readily  by  the  application  of  current  to  the  windings. 

In  general,  low  voltage,  direct-current  armature  windings  were 
dried  out  by  the  application  of  current  or  by  baking  in  ovens.  How- 
ever, there  was  great  difficulty  in  drying  out  commutators,  and 
eventually  the  only  real  satisfactory  way  proved  to  be  by  heating 
them  in  a  vacuum.  Therefore,  the  finaldrying  out  of  direct-current 
armatures  was  principally  by  vacuum. 

The  complete  drying  out  of  field  coils  was  very  difficult,  either 
by  current  heating  or  by  ovens.  However,  the  outside  of  the  coils 
could,  in  many  cases,  be  dried  sufficiently  to  show  practically  no 
ground,  while  the  inside  of  the  coil  was  still  wet.  In  most  cases, 
field  coils  could  be  operated  in  this  condition  and  could  eventually 
dry  themselves  out.  This  would  probably  be  satisfactory  for 
drying  out  individual  machines,  but  was  not  considered  satis- 
factory for  stock  apparatus.  Vaccum  drying  under  high  temper- 
ature proved  most  satisfactory,  and  this  was  adopted. 

High  voltage  windings  for  generators  and  transformers  were 
dried  out  in  vacuum,  no  other  methods  proving  entirely  satisfac- 
tory, except  in  individual  instances. 

It  may  be  borne  in  mind  that  his  was  a  situation  where  super- 
ficial correction  was  not  permissible.  During  the  various  tests 
and  methods  which  were  carried  out,  searching  investigations  of 
the  results  were  made  in  order  to  determine  the  sufficiency  of  the 
method  used.  Field  coils  and  armature  coils  were  opened  up  at 
various  stages  of  the  process  for  examination.  For  instance,  one 
of  a  lot  of  street  railway  armatures  which  were  dried  in  an  oven 
until  apparently  all  right,  was  dismantled  for  exa.mina.tion.  The 
windings  appeared  to  be  fairly  well  dried  out,  but  upon  opening 
the  commutator  V-ring,  very  considerable-  moisture  was  found 
tinder  the  commutator  bars  and  IB  the  mica  bushing.  Apparently, 
oven  baiing  would  not  remove  this  satisfactorily.  The  commut- 
ator was  then  sealed  tightly  and  the  armature  was  then  put  in  a 
vacuum  oven  and  dried  for  a  few  hoars.  After  this  all  water  had 


404  ELECTRICAL  ENGINEERING  PAPERS 

disappeared  from  the  commutator.  Another  commutator  was  then 
opened  and  purposely  filled  -with  water  and  then  closed  and  sealed 
as  tightly  as  possible  before  placing  in  the  vacuum  oven.  After 
an  over-night's  treatment,  the  inside  of  the  commutator  was  found 
to  be  entirely  free  from  moisture.  This  test  illustrated  the  ability 
of  the  vacuum  oven  to  remove  water.  It  was  then  adopted  very 
generally  for  drying  out  such  apparatus  as  was  liable  to  have 
water  sealed  or  trapped  inside  the  insulation.  It  must  be  under- 
stood, however,  that  certain  kinds  of  apparatus  were  dried  out 
just  about  as  well  using  temperature  alone.  In  these  cases,  how- 
ever, as  intimated  before,  the  vaporized  water  could  readily 
escape  to  the  air. 

There  is  one  condition,  however,  where  even  vacuum  oven 
drying  may  not  produce  the  desired  result,  for  the  operation  of 
drawing  off  the  water  may  injure  the  insulating  varnish  films.  To 
illustrate,  some  years  ago  one  of  the  large  power  plants  at  Niagara 
Falls  was  flooded  to  a  considerable  depth  by  an  ice  jam,  which 
backed  the  water  up  into  the  power  house.  The  machines  were 
flooded  to  a  depth  of  twenty  or  thirty  feet  for  a  period  of  several 
days,  and  the  windings  were  pretty  thoroughly  impregnated 
throughout  with  water.  Strenuous  attempts  were  made  to  dry  out 
these  windings  by  heating  to  temperatures  of  12S°C.  or  higher. 
The  end  windings  were  untaped  at  points  to  allow  the  moisture  to 
escape.  Also,  attempts  were  made  to  create  a  vacuum  around 
the  machines  by  means  of  air-tight  covers  or  casings  and  vacuum 
pumps,  but  this  latter  was  not  very  satisfactory.  After  a  few  weeks, 
apparently  but  little  progress  had  been  made.  A  chemist  then 
advanced  the  suggestion  that,  IE  linseed  oil  varnishes  had  been 
used  in  the  insulation,  then,  under  the  conditions  of  flooding 
which  had  occurred  in  this  plant,  the  varnish  itself  would  have 
absorbed  water,  and  he  was  of  the  opinion  that  heating  alone, 
unless  carried  up  to  the  destructive  point,  would  not  drive  off 
this  water.  Investigations  were  then  made  along  this  line,  and 
it  was  actually  found  that  the  varnished  films  were  thoroughly 
filled  with  water,  and  moreover,  this  water  could  not  be  removed 
without  more  or  less  injury  to  the  film  itself.  For  moderate  or  low 
voltage  machines,  apparently,  the  removal  of  the  water  would  not 
injure  the  insulation  sufficiently  to  prevent  operation,  but  in  high 
voltages,  such  as  6600  volts  or  higher,  the  insulation  would  be  left 
in  a  relatively  weakened  and  unsafe  condition.  In  the  machines 


INS  ULA  TION  PROBLEMS,  ETC.  405 

in  question,  it  was  found"  advisable  to  remove  the  insulation  en- 
tirely and  replace  with  new. 

As  a  rule,  field  coils  can  be  dried  out  in  a  fairly  satisfactory 
manner  by  heating  with  current  for  a  sufficiently  long  period. 
When  a  field  coil  is  thoroughly  wet  inside,  its  resistance  may  fall 
considerably,  due  to  low  resistance  between  turns  and  layers,  but 
when  current  is  applied,  there  is  but  little  danger  of  burnouts,  as 
the  leakage  of  current  through  the  insulation  is  distributed  over 
such  large  surfaces  that  there  is  no  danger  of  burning  at  any  one 
point,  unless  there  is  some  defectively  insulated  point  in  the  coil. 
Therefore,  after  the  coil  is  sufficiently  dried  so  that  its  leakage  to 
ground,  or  any  metal  supports,  is  sufficiently  low  to  be  safe,  then 
usually  the  coil  can  be  put  in  operation  and  allowed  to  dry  out  in 
regular  service.  If,  however,  the  field  coil  rotates  and  is  subject 
to  centrifugal  or  other  forces,  the  wet  condition  of  the  internal  in- 
sulation may  allow  internal  distortions  or  movements  which  might 
cause  partial  short  circuits. 

COMMUTATION  AND  RELATED  PROBLEMS 

In  the  practical  design  and  operation  of  electrical  apparatus, 
there  is  no  problem  which  is  apparently  more  enshrouded  in  mys- 
tery than  that  of  commutation.  Theoretically  this  problem  has 
been  treated  in  various  ways  and  analyzed  to  various  degrees,  but 
the  practical  results  not  infrequently  disagree  with  the  theoretical, 
principally  because  the  latter  are  predicated  upon  conditions  which 
are  not,  or  can  not,  be  obtained  practically.  Moreover,  even  when 
the  problem  can  be  correctly  analyzed,  and  a  proper  solution 
indicated,  it  may  not  be  practicable  or  feasible  to  apply  such 
solution.  In  other  words,  the  theory  may  show  just  where  a 
trouble  lies,  but  the  application  of  a  suitable  remedy  is  another 
story. 

The  difficulty  is  that  the  theories  of  commutation  are  built 
upon  many  conditions  which  are  inter-dependent.  But  many  of 
these  conditions  differ,  in  different  types,  designs  or  sizes  of 
machines,  and,  even  in  a  given  machine,  a  change  in  one  condition 
may  greatly  modify  other  conditions.  For  instance,  the  local  or 
short-circuit  currents  which  are  present  in  the  coils  short-circuited 
by  the  brushes  during  the  operation  of  commutation,  have  a  pre- 
ponderating influence  on  the  commutation,  and  yet,  these  local 
currents  are  greatly  influenced  by  many  conditions,  such  as  the 
dimensions  and  grade  of  brush,  condition  of  contact  surface, 


406  ELECTRICAL  ENGINEERING  PAPERS 

rigidity  and  type  of  brush  holder,  etc.  Obviously,  with  such 
variable  elements  entering  into  the  problem,  any  rigid  analysis  is 
most  difficult.  In  such  cases,  the  theory  is  valuable  in  locating 
any  probable  causes  of  difficulty. 

A  great  variety  of  conditions  or  phenomena  are  encountered 
in  commutating  machinery,  which  require  more  or  less  knowledge 
of  the  theory  of  commutation  in  order  to  understand  them.  For 
instance,  the  causes  for  sparking,  flashing,  burning  of  brushes, 
undue  wear  of  commutator  copper,  etc  ,  are  all  directly  related  to  the 
commutation  problem.  Even  questions  of  composition,  or  grade 
of  brushes,  type  of  brush  holder,  etc.,  are  related  problems. 

As  it  is  the  writer's  purpose  to  treat  the  above  points  from 
the  practical  side,  he  considers  it  advisable  first  to  give  a  brief 
explanation  of  the  commutation  problem  from  the  standpoint 
which  he  has  found  to  be  simplest  and  most  illuminating. 

THEORY  OF  COMMUTATION 

A  direct-current  armature,  when  carrying  current,  becomes 
a  true  electro-magnet,  with  the  poles  located  on  the  armature  at 
positions  corresponding  to  those  coils  which  are  in  contact  with 


PIG.  1. 

the  brushes.  If  the  armature  were  surrounded  by  a  smooth  ring 
of  iron,  (Fig.  1)  then  a  magnetic  field  would  be  set  up  between  the 
armature  and  the  surrounding  ring,  this  field  having  maximum 
values  at  points  corresponding  to  the  brush  contacts  and  zero 
values  midway  between.  The  magnetic  field  would  rise,  from 
the  zero  points,  uniformly  to  the  highest  values,  because  the  arma- 
ture winding,  which  is  a  magnetizing  winding,  is  uniformly  dis- 
tributed over  the  armature  core.  If,  now,  deep  notches  were  cut 
in  the  surrounding  ring  at  points  corresponding  to  the  highest 
field,  (Fig.  2)  there  would  be  comparatively  weak  field  at  these 
points,  due  to  the  high  reluctance  of  the  gap  or  notch.  The  ex- 


INSULATION  PROBLEMS,  ETC.  407 

ternal  ring  in  this  case  represents  the  field  structure  of  an  ordinary 
D.  C.  machine,  and,  in  such  machines,  the  armature  when  carrying 
current  actually  tends  to  set  up  magnetic  fields  in  this  manner,  and 
the  coils  in  contact  with  the  brushes  are  practically  always  located 
in  the  space  between  poles, — that  is,  in  the  notches  in  the  sur- 
rounding ring  in  the  above  illustration.  Also,  the  coils  in  contact 
with  the  brushes  are  those  in  which  the  current  is  reversed  in 
direction  when  passing  under  the  brushes,  or,  in  other  words,  are  the 
commutated  coils.  Therefore,  it  may  be  seen  that  the  commutated 
coils  always  lie  in  what  would  be  the  strongest  magnetic  field  set 
up  by  the  armature  winding,  if  there  were  no  polar  notches.  But, 
even  with  such  notches  or  interpolar  spaces,  the  armature  winding, 
when  carrying  load,  tends  to  set  up  some  magnetic  field,  part  of 
which  is  in  the  space  over  the  armature,  and  part  of  it  across  the 
armature  slots.  This  latter  flux  from  slot  to  slot,  is  not  influenced 
by  the  notches  in  the  surrounding  iron, — that  is,  by  the  interpolar 
spaces.  In  addition,  the  armature  winding  sets  up  a  magnetic 
field  around  the  end  windings. 


PIG.  2 

The  annature  winding,  when  carrying  current,  therefore 
always  tends  to  set  up  a  magnetic  field,  through  which  the  arma- 
nire  conductors  rotate  and  generate  e.  m.  f.'s,  just  as  when  they 
cut  the  mai'r>  field  set  up  by  the  field  windings.  These  coils  short- 
circuited  by  the  brushes  also  generate  e.  m.  f.'s  and  as  their  ter- 
minals, which  are  conomutator  bars,  are  connected  together  or 
short-circuited  by  the  brushes  on  the  commutator,  the  e.  m.  f.'s 
generated  by  cutting  the  armature  flux  tend  to  cause  local  or  short- 
circuit  currents  to  flow  in  such  coils.  Such  currents  will  be  known 
hereafter  as  the  local  or  short-circuit  currents,  to  distinguish  them 
from  the  useful  or  work  currents  of  the  annature. 

As  an  armature  coil  carrying  current  in  a  given  direction, 
approaches  and  passes  under  the  brush,  the  current  should  die 


403  ELECTRICAL  ENGINEERING  PAPERS 

down  to  zero  value  at  the  midpoint  of  the  brush  and  should  rise 
to  full  value  in  the  opposite  direction  by  the  time  the  coil  leaves 
the  brush.  This  would  be  a  theoretically  perfect  condition,  but 
is  very  difficult  or  almost  impossible  to  attain  in  practice.  The 
coil,  while  under  the  brush,  has  an  e.  m.  f .  generated  in  it  due  to 
cutting  the  armature  field,  as  already  described,  and  a  local 
current  circulates.  This  short-circuit  current  normally  adds  to 
that  of  the  work  current  before  reversal,  and  thus  tends  to  main- 
tain it  right  up  to  the  moment  that  the  coil  passes  out  from  under 
the  brush.  The  reversal  of  the  current  in  the  short-circuited  coil 
is  thus  accomplished  almost  instantaneously  instead  of  gradually. 
If,  however,  this  local  current  could  be  generated  in  ike  opposite 
direction,  then  it  would  tend  to  oppose  the  work  current  as  the  coil 
came  under  the  brush,  so  that  the  resultant  current  would  first  die 
to  zero  and  then  rise  in  the  opposite  direction,  if  the  short-circuit 
current  were  just  large  enough;  so  that  the  work  current  would 
simply  replace  the  short-circuit  current  in  direction  and  value  as  the 
coil  passes  out  from  under  the  brush.  Therefore,  if  the  short- 
circuit  current  were  of  exactly  the  right  value,  the  resultant 
effect  would  be  the  same  as  if  the  work  current  alone  were  present 
and  this  current  died  down  to  zero  value  as  the  coil  passed  the 
middle  of  the  brush,  and  rose  to  full  value  in  the  opposite  direction 
by  the  time  the  coil  left  the  brush.  In  other  words,  a  local  current 
of  the  proper  direction  and  value  in  the  commutated  coils  will  give 
theoretically  ideal  commutation.  Such  local  current  therefore, 
might  be  designated  as  the  commutating  current.  In  practice 
it  is  found  however,  that  a  current  somewhat  higher  than  the 
ideal  value  gives  the  best  general  results  as  will  be  explained 
later.  In  any  case,  however,  this  local  current,  to  be  effective, 
must  be  in  the  reverse  direction  to  that  which  would  normally 
be  set  up  by  the  armature  coil  cutting  the  magnetic  field  due  to 
the  armature  winding  itself.  This  means  therefore,  that  where 
commutation  is  accomplished  by  means  of  short  circuit  or  local 
current,  an  external  field  in  the  opposite  direction  to  the  armature 
field  is  necessary  for  setting  up  such  local  currents.  This  result 
may  be  accomplished  in  several  ways.  The  brush  may  be  rocked 
forward  or  backward  until  the  commutated  coil  comes  under  the 
magnetic  field,  or  fringe  of  the  field,  set  up  by  the  main  field 
winding.  If  rocked  in  one  direction  (forward  in  the  generator, 
backward  in  the  motor)  the  direction  of  the  main  field  will  be  in 
opposition  to  the  armature  field.  Obviously,  if  shifted  into  a 


INS  ULA  TION  PROBLEMS,  E TC  409 

strong  enough  external  field,  the  armature  field  may  be  com- 
pletely neutralized  at  some  given  point,  such  as  that  of  the  short- 
circuited  coil,  or  the  resultant  field  might  be  even  strong  enough 
in  the  opposite  direction  to  set  up  the  desired  local  or  short-circuit 
current  in  the  commutated  coil.  Under  this  condition,  ideal  corn- 
mutating  conditions  should  be  obtained.  However,  as  the 
armature  magnetic  field  at  this  point  tends  to  vary  with  the 
armature  current,  while  the  external  field  tends  to  remain  constant, 
it  is  evident  that  the  ideal  resultant  field  will  only  obtain  at  one 
particular  load.  Therefore,  only  an  average  condition  can  be 
obtained  in  this  way.  However,  by  shifting  the  brushes  backward 
or  forward  under  the  external  field,  the  proper  local  or  commutating 
currents  should  be  obtained  for  any  given  load;  but  brush  shifting 
is  not  usually  considered  a  very  practical  method  of  operation, 
although  required  by  many  non-commutating  pole  machines  in 
service.  What  is  needed  is  an  external  field  directly  over  the 
commutated  coils  which  is  always  of  the  proper  strength  to  set  up 
commutating  currents  of  the  right  direction  and  of  the  proper 
value  with  respect  to  the  work  currents,  so  that  the  resultant  in 
the  short-circuited  coil  will  give  the  effect  of  the  work  current 
reversing  at  the  middle  of  the  brush  at  all  loads.  To  accomplish 
this,  an  external  field  to  produce  this  local  current  should  always 
be  in  opposition,  to  the  armature  magnetic  field,  should  be  some- 
what greater  in  value,  and  should  vary  in  proportion  to  the 
armature  field, — that  is,  to  the  armature  current.  This  result  is 
accomplished  by  the  now  well-known  commutating  pole,  which  is 
simply  a  small  pole  placed  over  the  commutated  coil,  and  usually 
excited  by  a  winding  directly  in  series  with  the  armature,  but 
having  a  somewhat  greater  magnetizing  force  than  the  armature 
winding.  The  function  of  this  pole  is  solely  to  set  up  in  the 
armature  coil  a  local  or  commutating  current  of  the  proper  di- 
rection and  value. 

A  condition  which  makes  the  problem  of  commutation  very 
much  easier  to  solve  is  the  use  of  a  relatively  high  resistance  in  the 
short-circuited  path  of  the  coil  which  is  being  commutated.  Due 
to  the  extremely  low  resistance  of  the  ordinary  armature  coil,  a 
relatively  low  magnetic  field  set  up  by  the  armature  would  generate 
enormous  local  currents  in  the  short-circuited  coil  if  the  resistance 
of  the  coil  itself  were  the  only  limit.  These  .currents  might  be  ten  to 
twenty  times  as  great  as  the  normal  work,  current,  and  would  add 
seriously  to  tfce  difficulties  of  commutation.  Even  if  a,  commut- 


410  ELECTRICAL  ENGINEERING  PAPERS 

ating  field  were  present,  this  would  have  to  be  proportioned  and 
adjusted  so  accurately,  to  get  the  right  value  of  the  commutating 
current,  that  the  construction  would  be  almost  impracticable. 
But  if  considerable  resistance,  compared  with  the  coil  itself, 
could  be  interposed  in  the  short  circuited  path,  this  obviously 
would  so  greatly  assist  in  fixing  the  value  of  the  short-circuit 
current  that  undue  refinement  and  adjustment  would  not  be 
necessitated.  Let  us  suppose,  for  instance,  that  the  short  circuit 
e.  m.  f ,  in  the  commutated  coil  is  two  volts,  and  a  copper  brush  of 
practically  zero  resistance  at  the  contact  is  used,  then  the  resist- 
ance of  the  short-circuited  coil  itself  limits  the  current  which 
flows.  Let  us  assume  that  this  gives  a  local  current  of  ten  times 
the  value  of  the  work  current.  Now,  if,  instead  of  the  zero  resist- 
ance brush  contact,  one  of  about  ten  times  the  resistance  of  the 
coil  is  used,  then  the  total  resistance  in  circuit  becomes  over  ten 
times  as  great,  and  the  short-circuit  current  is  cut  down  to  a 
value  comparable  with  that  of  the  work  current.  Obviously  this 
in  itself  would  represent  an  easier  condition  of  reversal  without 
any  external  reversing  field,  and,  with  such  a  field,  extreme  ac- 
curacy in  proportions  and  adjustment  are  not  necessary  to  give  ap- 
proximately the  right  value  of  the  local  current  which  assists  in 
commutation.  Therefore,  a  relatively  high  resistance  brush, — 
that  is,  one  with  contact  resistance  high  compared  with  the  re- 
sistance of  the  coil — is  of  very  material  aid  in  commutation.  This 
is  wherein  the  carbon  brush  is  such  a  successful,  or  even  necessary 
adjunct  of  the  commutating  machine.  It  serves  such  a  vital 
purpose  that  it  may  be  said  that,  without  the  carbon  brush  or 
its  equivalent,  the  electrical  industry  would  never  have  made 
anything  like  the  progress  it  has  made. 

The  principal  function  of  the  carbon  brush  being  that  of 
limiting  the  local  current,  it  might  be  assumed  that  the  advan- 
tages might  be  increased  indefinitely  by  further  increasing  the 
resistance,  but  there  are  usually  limits  to  all  good  things.  The 
carbon  brush  increases  the  resistance  in  the  path  of  the  local 
currents,  but  it  also  increases  it  in  the  path  of  the  work  current. 
If  the  resistance  is  carried  too  high,  the  losses  due  to  the  work 
current  may  constitute  a  more  serious  objection  than  the  local 
currents.  Consequently,  practice  is  one  continual  compromise  on 
this  point.  In  those  cases  where  the  short-circuit  current  is 
normally  relatively  small  due  to  low  value  of  the  armature  mag- 
netic field,  it  is  obvious  that  a  lower  resistance  in  the  short-circuit 


INSULATION  PROBLEMS,  ETC.  411 

path  can  be  used,  or,  in  other  words,  a  low  resistance  carbon 
brush  is  practicable,  with  consequent  low  loss  due  to  work  cur- 
rent. In  other  cases  with  higher  inherent  local  current,  higher 
resistance  carbon  will  give  better  average  results.  It  is  thus 
obvious  that  one  grade  of  carbon  brush  is  not  the  best  one  for 
different  machines  unless  they  all  have  similar  inherent  commut- 
ating  conditions.  It  is  exceedingly  difficult  to  give  equal  com- 
mutating  characteristics  to  machines  of  different  sizes  and  types, 
and,  in  most  cases,  even  of  the  same  type  or  line.  In  non-com- 
mutating  pole  machines,  the  grade  of  the  brush  is  of  more  im- 
portance than  in  the  commutating  pole  type,  for  in  the  latter,  we 
have  a  means,  in  the  commutating  pole  strength  itself,  of  modi- 
fying or  controlling  the  value  of  the  local  current.  But  the  best 
commutating  pole  gives  only  average  correction, — that  is,  average 


Fig.  3. 

value  of  the  desired  local  current,  and  the  resistance  of  the  brush 
must  be  depended  upon  to  take  care  of  pulsations  or  irregularities 
in  the  local  current,  acting  to  smooth  them  out  to  a  greater  or  less 
extent.  Thus  a  fairly  high  resistance  brush  is  required  on  the 
commutating  pole  machine,  but  its  resistance  tisually  can  be 
lower  than  that  required  in  the  non-commutating  pole  type. 

The  above  gives  a  crude  idea  of  the  phenomena  of  commuta- 
tion. However,  there  are  a  number  of  very  closely  related  condi- 
tions, such  as  burning  and  blackening  of  the  commutator,  causes 
and  effects  of  high  mica,  effect  of  under-cutting  of  mica,  rapid 
and  undue  wear  of  commutator  copper  and  brushes,  etc.,  which 
can  be  explained  more  or  less  directly  by  the  above  theory. 

Blackening  of  the  commutator,  high  mica,  and  rapid  wear  of 
the  commutator  copper  and  brushes  may  aH  be  credited  to  actual 
burning,  or  something  similar  to  electrolytic  action,  occurring  under* 


412  ELECTRICAL  ENGINEERING  PAPERS 

the  brushes.  It  is  not  usually  the  bright  sparks  at  the  brush  tips 
which  cause  trouble,  but  it  is  frequently  on  unnoted  sparking  under 
the  brush  face.  These  sparks  may  be  very  minute, — so  much  so  that 
they  would  naturally  be  assumed  to  be  harmless.  The  apparent 
electrolytic  action  under  the  brushes  may  be  really  a  similar 
sparking  action  which  cannot  be  observed.  Experience  has 
shown  that  usually  there  is  a  tendency  for  minute  particles  of  the 
conducting  material  to  be  burned  or  carried  away  from  the  con- 
tact surfaces,  (Fig.  3)  depending  on  the  direction  of  the  current. 
These  particles  appear  to  travel  in  the  direction  the  current  i&  flow- 
ing, but  they  do  not  always  deposit  on  the  opposite  contact  sur- 
face. If  the  current  is  from  the  brush  to  the  commutator  copper, 
the  brush  surface  tends  to  be  eaten  away,  while  with  the  current 
from  the  copper  to  the  brush,  the  copper  eats  or  burns  away. 


Fig.  4. 

With  ordinary  current  densities  and  very  low  loss  in  the  brush 
contact,  this  action  seems  to  be  very  minute,  but  it  appears  to  in 
crease  rapidly  with  increased  loss  at  the  brush  contact.  There- 
fore, high  current  density  in  the  brush  contact  may  produce 
this  action.  This  does  not  mean  high  density  due  to  the  work 
current  alone,  but  means  the  high  actual  density,  due  to  both 
work  and  local  currents.  In  non-commutating  pole  machines 
and  in  commutating  pole  machines  with  bad  adjustment,  the 
local  current  under  the  brush  may  exceed  in  value  the  work 
current.  As  this  adds  to  the  work  current  at  one  edge  of  the 
brush  and  subtracts  at  the  other  edge,  the  result  will  be  greatly 
increased  density  and  high  watts  tinder  one  part  of  the  brush. 
This  may  result  in  burning  away  one  edge  of  the  brush  surface, 
and  is  frequently  observedin  examination  of  brushes.  This  usually 
is  most  noticeable  where  the  current  passes  from  the  brush  to  the 
commutator,  but  at  the  holders  of  the  other  polarity,  a  similar 
•action  is  tending  to  burn  away  the  commutator  copper.  How- 
ever, the  commutator  mica  does  not  tend  to  burn  away,  and  there- 


INSULATION  PROBLEMS,  ETC. 


413 


fore,  if  the  mica  does  not  wear  down  mechanically  at  the  same  rate 
that  the  copper  burns  away,  eventually  the  mica  stands  an  in- 
finitestimal  amount  above  the  copper  (Fig.  4)  and  the  brushes  will 
make  a  decreasingly  good  contact  with  the  copper  itself.  This 
increases  the  loss  at  the  brush  contact  and  increases  the  burning 
action  which  results  in  still  poorer  contact,  so  that  the  results 
become  cumulatively  worse.  If,  however,  these  periods  of  burn- 
ing are  intermittent,  due  to  variable  load  conditions,  and  there  is 
considerable  operation  at  lighter  loads  or  non-burning  conditions, 
the  mica  may  wear  down  somewhat  and  the  commutator  and 
brushes  may  polish  sufficiently  during  these  periods  to  mask  the 
direct  effects  of  the  burning.  But  the  results  may  show  in 
grooving  and  apparent  rapid  wear  of  the  commutator  face.  There 
may  thus  be  burning  without  blackening,  or  without  direct  evi- 


tl 


Fig.  5. 

dence  of  high  mica.  If,  however,  the  burning  pe±iods  exceed  the 
polishing,  then  visible  evidence  of  burning  and  high  mica  may  be 
found.  The  brushes  may  also  show  this  burning  and,  in  some 
cases,  may  honey-comb  badly  at  one  edge,  or  even  over  the  whole 
surface.  Where  one  edge  burns  over  a  very  distinct  area,  (Fig.  5) 
it  usually  may  be  assumed  that  the  burnt  area  could  just  as  well 
be  cut  away,  with  but  little  harm  to  the  operation,  and  possibly 
some  good,  for  the  fewer  the  commutator  bars  that  the  brush  spans 
the  lower  will  be  the  total  short-circuit  current,  as  a  rule.  And, 
moreover,  by  doing  away  with  the  localized  burning  tinder  the 
brush,  it  may  be  assumed  that  the  commutator  burns  less  also. 
However,  cutting  away  part  of  the  brush  face  will  crowd  the  work 
current  into  the  remaining  portion,  so  that  burning  in  this  portion 


414  ELECTRICAL  ENGINEERING  PAPERS 

may  be  increased.    Therefore,  narrowing  the  brush  face  is  not 
a  general  remedy  for  the  trouble,  but  is  successful  in  some  cases. 

Burning  of  the  commutator  may  also  be  coincident  with  a 
deposit  of  copper  on  the  brush  face.  This  is  usually  known  as 
"picking  up  copper. "  Apparently,  in  some  cases,  where  the 
copper  is  burnt  away  from  the  commutator  face,  it  actually 
collects  or  deposits  on  the  brush  face,  forming  low  resistance  spots 
or  surfaces.  This  gives  the  equivalent  of  low  resistance  brushes, 
with  consequent  increase  in  local  current  and  still  greater  burning 
action.  Moreover,  with  several  brushes  in  parallel,  any  one 
brush  with  copper  on  its  face,  may  tend  to  take  more  than  its 
share  of  the  total  work  current,  which  tends  to  cause  further  heat- 
ing and  burning.  Increased  heating  in  itself  will  cause  a  greater 
tendency  of  the  brush  to  take  an  undue  proportion  of  the  current, 
for  carbon  brushes,  unfortunately,  have  a  negative  coefficient  of 
resistance.  This  means  that  if  any  brush  carries  more  than  its 
share  of  current  and  becomes  heated  thereby,  its  resistance  is 
reduced  and  it  tends  to  take  still  greater  current.  This  is  particu- 
larly the  case  with  a  large  current  per  brush  arm,  with  a  large 
number  of  brushes  in  parallel  on  each  arm.  If  one  brush,  for  any 
reason,  such  as  picking  up  copper,  takes  an  undue  share  of  the 
current,  it  may  take  an  increasing  share  until  the  contact  surface, 
or  the  whole  commutator  end  of  the  brush,  may  become  red  hot 
and  slowly  disintegrate.  Such  action,  if  continued,  will  event- 
ually so  destroy  or  injure  the  brush  contact  that  it  carries  a  de- 
creased current,  the  resistance  increases,  and  eventually  the  current 
falls,  not  only  to  normal,  but  probably  far  below  normal  value, 
due  to  bad  contact,  and  the  other  brushes  must  then  assume  an 
excess.  If  other  brushes  repeat  this  action,  then  the  condi- 
tions become  increasingly  bad  for  the  remaining  brushes,  and 
they  may  repeat  the  same  action.  In  time,  all  the  brushes  may 
thus  become  badly  burned  or  honey-combed. 

Such  conditions  are  sometimes  very  difficult  to  correct. 
In  some  instances,  higher  resistance  brushes  bring  improve- 
ment, while  in  other  cases,  lower  resistance  brushes  are  better. 
If,  for  instance,  the  local  currents  are  relatively  small,  and  the 
burning  or  picking  up  of  the  copper  is  due  principally  to  the  work 
current,  then  a  lower  resistance  brush  may  actually  reduce  the 
watts  at  the  contact,  even  though  the  local  current  may  be  in 
creased.  If,  on  the  other  hand,  the  local  currents  are  high  and 
are  principally  responsible  for  the  burning  action,  then  a  still 


INSULATION  PROBLEMS,  ETC.  415 

higher  resistance  may  actually  reduce  the  loss.  Thus  it  may 
be  seen  that,  in  many  of  these  commutator  and  brush  prob- 
lems, it  is  impossible  to  make  a  definite  statement  regarding 
the  effect  of  any  given  make  of  brush  unless  one  knows  what 
is  actually  taking  place  in  the  machine.  And  every  time  the 
brush  is  changed,  the  conditions  change,  for  they  are  more  or 
less  inter-dependent. 

Another  remedy  for  some  of  the  above  troubles  is  under- 
cutting the  mica  between  commutator  bars.  This  does  not  re- 
move the  primary  cause  of  the  trouble,  namely,  the  large  local 
currents  or  high  current  density  on  the  brush  contact,  but  it 
lessens  the  harmful  effect  of  these  by  allowing  the  brush  to  main- 
tain better  contact  with  the  commutator  copper,  thus  reduc- 
ing the  contact  losses.  In  this  way  the  burning  action  can  be 
diminished,  in  most  cases,  to  such  an  extent  that  the  commut- 
ator face  will  polish,  and  this  in  turn  will  enable  the  brush  to 
make  better  contact  with  the  copper.  Undercutting  in  general 
is  advantageous  where  the  commutator  mica  takes  up  a  large 
percent  of  the  total  surface,  such  as  20%  or  more.  The  larger 
the  percentage  of  mica,  the  less  liable  it  is  to  wear  away  as  rapidly 
as  the  copper,  and  the  greater  the  liability  of  the  brushes  being 
lifted  above  the  copper,  with  consequent  burning  and  blacken- 
ing. Not  only  must  the  percentage  of  mica  be  relatively  small, 
but  the  actual  thickness  between  two  adjacent  bars  must  also  be 
limited,  unless  the  mica  is  undercut.  The  general  practice  at 
present  with  non-undercut  commutators  is  about  1-32  in.  thick- 
ness between  bars;  and  even  considerably  less  than  this,  as^low 
as  .018"  to  .020"  is  not  unusual  in  small  machines  which  are 
not  undercut.  Where  commutators  axe  undercut,  there  is  a  pos- 
sibility, or  liability,  of  carbon  dust  collecting  in  the  slots  and  short- 
circuiting  between  bars,  unless  the  peripheral  speed  of  the  com- 
mutator is  sufficient  to  keep  the  slots  dear.  Therefore,  in  slow 
speed  commutators  which  are  undercut,  it  may  be  necessary  at 
times  to  brush  out  or  dean  the  slots.  In  high  speed  commutators, 
or  in  variable  speed  machines  which  intermittently  reach  high 
speeds,  there  is  usually  but  little  difficulty  from  carbon  collecting 
in  the  slots.  Obviously,  where  a  commutator  is  to  be  slotted, 
there  is  no  necessity  for  maintaining  a  minimum  thickness  of 
mica,  as  the  limitation  in  such  cases  is  in  the  width  of  the  slot, 
which  may  be  as  much  as  1-16  in.  in  some  cases.  Wide  slotting, 
however,  is  liable  to  produce  brush  chattering  in  some  cases. 


416  ELECTRICAL  ENGINEERING  PAPERS 

Slotted  commutators,  while  advantageous  in  some  ways, 
present  certain  operating  objections  in  others.  Except  where 
the  brushes  cover  several  bars,  the  slotted  commutators  are  liable 
to  produce  more  or  less  chattering  of  the  brushes,  unless  fre- 
quently lubricated.  Therefore,  with  such  commutators,  self- 
lubricating  brushes  are  recommended.  Such  brushes  usually 
contain,  or  consist  of  graphite,  and,  in  consequence,  generally 
they  are  of  lower  resistance  than  ordinary  carbon  brushes,  and 
therefore  are  not  as  useful  in  assisting  commutation.  In  com- 
mutation pole  machines  where  the  resistance  of  the  brushes  is  of 
relatively  less  importance,  graphite  brushes  are  often  very  satis- 
factory. As  such  brushes  are  usually  soft  in  texture,  they  are 
not  well  adapted  for  wearing  away  the  mica  in  commutators 
which  are  not  undercut. 

In  the  application  of  the  commutating  pole  to  direct-cur- 
rent machines,  certain  conditions  have  arisen  which  are  pecul- 
iar to  this  construction.  For  instance,  according  to  the  theory 
already  given  above,  the  flux  of  the  commutating  pole  should 
arise  and  fall  directly  in  proportion  to  the  armature  current. 
This  means  that  there  must  be  practically  no  saturation  in  the 
commutating  pole  circuit.  Probably  many  of  the  early  diffi- 
culties with  commutating  pole  machines  were  due  to  a  lack  of 
appreciation  of  this  point.  Also,  in  machines  with  rapidly 
changing  current,  the  commutating  pole  flux  should  change 
at  the  same  rate  as  the  armature  current  or  the  proper  local  cur- 
rent conditions  for  commutation  are  not  obtained.  This  means 
that  the  commutating  field  winding  should  not  be  adjusted  or 
varied  in  strength  by  means  of  a  non-inductive  shunt,  as  is  com- 
mon practice  in  adjusting  series  field  winding.  As  the  com- 
mutating pole  winding  is  normally  inductive,  then  in  the  case 
of  sudden  change  in  current,  an  improper  proportion  of  the 
current  will  flow  through  a  non-inductive  shunt  at  the  time 
that  the  armature  current  is  changing.  Either  no  shunt  should 
be  used,  or,  if  one  is  necessary,  it  should  have  the  same  induct- 
ance as  the  commutating  field  circuit.  The  former  is  prefer- 
able but  requires  most  accurate  designing. 

Another  requirement  in  commutating  pole  machines,  is 
accurate  setting  of  the  brushes.  As  a  certain  definite  value  of 
the  local  or  commutating  current  is  desired  in  the  short-circuited 
coil,  it  is  obvious  that  the  coil  must  be  short-circuited  by  the 
brushes  at  some  very  definite  position  with  respect  to  the  com- 


INSULATION  PROBLEMS,  ETC. 


417 


mutating  pole.  This  is  especially  true  in  reversing  machines. 
Otherwise,  the  commutation  in  the  two  directions  would  not  be 
alike.  Furthermore,  an  incorrect  setting  of  the  brushes,  with 
respect  to  the  commutating  pole,  will  have  a  slight  effect  on  the 
inherent  regulation  of  the  machine.  In  a  direct-current  generator, 
for  instance,  with  the  brushes  set  so  that  commutation  is  exactly 
central  to  the  commutation  pole,  the  magnetic  flux  of  these  poles 
has  no  resultant  effect  on  the  generated  e.  m.  f.  of  the  armature 
winding  as  a  whole,  and  therefore  has  no  effect  on  the  regulation. 
But  if  the  brushes  are  shifted  ahead  of  this  correct  point  in  a  gener- 


Fig.  6. 


ator,  (Fig.  6)  part  of  the  commutating  pole  flux  becomes  effective 
in  generating  e.  m.  f.,  and  in  opposition  to  th.Q  armature  e.  m.  £. 
A  back  lead  in  the  same  way  would  tend  to  increase  the  armature 
e.  m.  f .  Thus  the  inherent  regulation  of  the  generator  is  affected  by 
the  brush  setting.  In  a  motor  with  commutating  poles,  a  for- 
ward lead  of  the  brushes  tends  to  increase  the  counter  e.  m.  f . 
generated  and  thus  tends  to  lower  the  speed  with  increase  in 
load,  while  a  back  lead  tends  to  increase  the  speed.  With  per- 
fect setting  of  the  brushes  with  respect  to  the  commutating 
poles,  and  an  adjustment  of  the  commutating  pole  strength 
just  sufficient  to  give  the  theoretically  ideal  short  circuit  or 
commutating  current,  the  commutating  'poles  should  have  prac- 
tically no  effect  o*r  the  speed/  But  in  actual  practice,  in  direct- 
current  motors,  it  is  found  better  to  actually  over-compensate 


418  ELECTRICAL  ENGINEERING  PAPERS 

slightly — that  is,  to  make  the  commutating  pole  slightly  stronger 
than  the  ideal  value.  This  increases  the  local  or  commutating 
current  above  the  ideal  value  so  that  commutation  is  well  com- 
pleted before  the  coil  leaves  the  brush.  This  gives  less  spark- 
ing, or  "blacker*'  commutation,  at  the  brush  edge,  and  ap- 
parently is  more  satisfactory  from  the  operator's  standpoint. 
But  this  over-compensation  has  an  effect  on  the  speed  character- 
istics of  a  motor.  It  means  that  the  zero  point  of  the  current 
is  shifted  backwards,  and  the  resultant  effect  is  similar  to  shift- 
ing the  brushes  backwards,  and  it  therefore  tends  to  speed  up 
the  machine,  as  described  before.  But  this  speeding-up  ten- 
dency will  vary  with  the  load,  as  the  commutating  pole  strength 
increases  with  the  load.  If  the  motor  normally  has  a  "flat" 
speed  curve,  this  increase  may  be  sufficient  to  bring  the  speed 
with  load  above  the  no-load  speed,  and  this  is  an  unstable  condi- 
tion in  the  operation  of  constant  speed  motors.  For  instance,  if 
a  motor  with  rising  speed  characteristics  has  a  high  inertia  load 
suddenly  thrown  on  it,  the  heavy  current  required  will  tend  to 
speed  up  the  machine,  and  thus  take  a  still  heavier  current. 
But  a  drooping  speed  curve  has  the  opposite  effect.  Therefore, 
in  motors  built  for  general  market  conditions,  where  the  load 
conditions  may  be  of  any  nature,  it  is  desirable  that  so-called 
constant  speed  motors  should  always  have  slightly  drooping 
speed  characteristics  at  least.  But  commutating  pole  motors,  if 
designed  along  ideal  lines, — that  is  with  high  armature  magneto- 
motive forces — and  with  comparatively  flat  inherent  speed 
characteristics,  are  liable  to  be  affected,  in  speed,  to  a  certain 
extent,  by  overcompensation  of  the  commutating  pole.  In  some 
cases,  this  effect  may  be  so  small  that  it  does  not  over-balance 
the  inherent  droop  in  the  speed  curve.  In  other  cases,  it  may 
more  than  over-balance,  so  that  the  actual  speed  curve  rises  with 
load.  This  is  particularly  noticeable  in  adjustable  speed  ma- 
chines for  a  wide  range  in  speed.  Such  machines  have  full  field 
strength  at  lowest  speed,  and  here  the  effect  of  the  local  or  com- 
mutating current  on  the  speed  may  be  very  small.  At  three  or  four 
times  speed,  however,  the  main  field  is  very  weak,  while  the  com- 
mutating current  is  practically  the  same  as  at  low  speed,  and 
therefore  has  three  or  four  times  the  effect  in  increasing  the  speed. 

Therefore,  such  motors  are  liable  to  have  rising  speed  curves 
at  Ijigher  speeds,  although  they  may  be  slightly  drooping  at 
the  lowest  speeds. 


INSULATION  PROBLEMS,  ETC  419 

Obviously,  as  this  effect  is  a  function  of  the  armature  cur- 
rent, it  should  be  corrected  by  means  of  the  armature  current. 
This  is  readily  accomplished  by  adding  to  the  main  field  wind- 
ing a  very  small  winding  in  series  with  the  armature,  and  con- 
nected to  magnetize  in  the  same  direction  as  the  shunt  winding 
on  the  field  poles.  While  this  might  be  looked  upon  as  a  series 
winding,  yet  its  function  is  that  of  compensation  for  commu- 
tating  pole  action.  The  ampere  turns  in  this  compensating 
winding  are  normally  very  small,  being  just  sufficient  to  bal- 
ance the  effect  of  the  excess  short  circuit  current  in  the  corn- 
mutated  armature  coils.  In  adjustable  speed  machines,  at  low 
speed  and  full  field  strength,  this  small  compensating  winding 
has  but  very  little  effect,  as  it  is  so  small  in  proportion  to  the 
shunt  ampere  turns.  At  the  highest  speeds,  however,  where  the 
shunt  ampere  turns  are  very  low  and  the  rise  in  the  speed  curve 
is  liable  to  be  greatest,  this  compensating  winding  has  the  most 
effect.  It  therefore  tends  to  produce  proper  compensation  at 
all  speeds  and  loads. 

Another  phenomenon  which  has  appeared  in  direct  cur- 
rent machines,  and  which,  at  times,  has  been  falsely  credited  to 
commutating  conditions,  is  that  of  "pitting,"  or  "eating  away" 
of  the  mica  between  commutator  bars.  Nearly  all  manufac- 
turers and  operators  have  encountered  this  difficulty  at  some 
time  or  other.  This  has  also  been  credited  to  high  voltage  be- 
tween bars,  too  thin  mica,  quality  of  carbon  brush,  use  of  lubri- 
.cants,  etc.  Some  years  ago,  the  writer  and  his  associates  made 
an  extended  study  of  this  matter,  based  upon  a  very  large  number 
of  cases  of  actual  trouble.  The  results  which  were  derived  from 
the  general  practical  data  from  machines  in  actual  service  were 
so  conflicting  that  no  positive  conclusions  could  be  drawn  di- 
rectly from  such  data.  However,  eventually  the  evidence  pointed 
to  oil  as  apparently  one  of  the  fundamental  conditions  in  this 
trouble.  Extended  tests  were  then  made  to  determine  the 
effects  of  oil  on  the  mica,  and  the  results  indicated  very  clearly 
that  those  insulations  which  absorb  oil  were  liable  to  pit  or  eat 
away  in  time.  Apparently  where  the  oil  could  dissolve  out  the 
binding  material  in  the  tnica^  ininute  particles  of  carbon  or  copper, 
wotild  be  disseminated  through  the  mica,  thus  decreasing  its  re- 
sistance locally.  Combustion  of  these  particles  would  usually  be- 
noticed  as  "ringfire"  around  the  onimmtator.  Ring-fire  is  almost 
always  due  toincaadesc^it  carbon  particles  scrapedoff  the  brushes, 


420  ELECTRICAL  ENGINEERING  PAPERS 

but  is  not  usually  harmful  if  the  mica  adjacent  to  the  spark  does 
not  burn  or  deteriorate.  Experience  shows  that  the  ordinary  good 
grades  of  mica  plate  are  not  affected  by  such  ring-fire,  and  it  is 
only  when  foreign  conducting  particles  penetrate  into  the  plate 
that  this  burning  may  gradually  eat  away  the  mica.  It  was  found 
that  some  binding  materials  used  in  building  mica-plate  were 
much  more  soluble  in  oil  than  others,  and  it  was  noted  that  in 
those  plates  with  soluble  binders,  the  pitting  was  most  pro- 
nounced. In  fact,  in  those  grades  of  mica  plate,  where  the 
binder  was  practically  insoluble  in  oils,  no  pitting  was  found, 
even  under  very  extreme  conditions  of  test  This  led  then  to 
one  solution  of  the  pitting  trouble,  namely,  the  use  of  what 
might  be  designated  as  insoluble  binders  in  the  mica  plate,  with 
very  tight  construction  of  commutator,  so  that  oil  could  not 
penetrate  along  the  sides  of  the  plate,  and  with  care  in  prevent- 
ing oil  from  getting  on  the  commutator.  With  the  first  two  con- 
ditions, the  latter  should  not  be  so  important,  yet  one  never 
knows  whether  the  first  two  conditions  are  perfect,  especially 
after  a  machine  has  been  in  operation  for  a  long  period  and  has 
been  subjected  to  severe  changes  in  temperature  at  the  com- 
mutator. 

After  getting  at  the  probability  of  oil  as  a  pause  of  pitting, 
many  careful  examinations  were  made  of  pitted  mica,  and  in 
general,  there  was  evidence  that  the  mica  binder  had  been  at- 
tacked by  oil.  In  some  instances  where  the  operators  were  ab- 
solutely sure  that  there  had  never  been  any  oil  on  the  commut- 
ator, careful  chemical  and  microscopical  analysis  showed  the  oil- 
In  some  cases  the  mica  was  actually  spongy  with  oil,  and  yet 
it  was  claimed  that  no  evidence  of  oil  had  ever  been  noticed  on 
the  commutator. 

Much*  depends  upon  the  grade  of  mica  used  in  the  plate 
for  building  up  in  commutators.  Certain  micas  seem  to  wear 
much  faster  than  others,  and  yet  be  just  as  good  insulators.  The 
well  known  amber  micas  seem  to  be  by  far  the  most  successful 
for  this  purpose.  Many  attempts  have  been  made  to  use  cheaper 
grades  of  white  mica,  and,  in  some  cases,  with  good  success,  but 
the  difficulty  is  that  it  is  not  uniformly  successful,  and  trouble 
from  high  mica  may  develop  only  after  a  large  number  of  ma- 
etjines  have  been  put  on  the  market.  Many  costly  experiences 
of  this  sort  have  made  the  manufacturers  very  conservative  in 
this  matter*  It  takes  so  long  to  find  whether  a  new  mica  is  good 


INSULATION  PROBLEMS,  ETC.  421 

or  not,  that  it  Is  questionable  iii  most  cases  whether  it  should  be 
tried  out  at  all. 

In  the  early  days,  the  commutator  mica  was  made  com- 
paratively thick,  and  was  punched  out  of  solid  material.  When 
the  slotted  types  of  direct  current  armatures  came  into  use, 
with  their  greater  sparking  tendencies,  the  old  solid  thick  mica, 
used  with  surface-wound  armatures,  immediately  showed  trouble 
due  to  high  mica.  This  soon  led  to  thinner  mica,  which  helped 
the  trouble  somewhat.  Then  somebody  discovered  that,  by 
splitting  the  mica  into  very  thin  plate  and  reassembling,  without 
binder,  the  results  were  still  better,  as  this  split-up  mica  seemed 
to  wear  or  flake  off  at  the  edges  much  better  than  solid  mica. 
Then  someone  discovered  that  stall  better  results  were  obtained 
by  splitting  or  flaking  the  mica  and  building  up  into  plates,  with 
a  suitable  binder  Since  that  time,  practically  no  radical  im- 
provements have  been  made  in  commutator  mica,  except  in  the 
binding  material  possibly,  and  in  the  better  choice  of  the  grades 
of  mica  used,  but  the  mica-plate  of  today  in  general  is  very  similar 
to  the  mica-plate  of  IS  or  20  years  ago.  Of  course,  refinements 
in  manufacture  have  occurred,  which,  in  most  cases,  however, 
have  had  but  little  effect  on  the  quality  of  the  product.  At 
present,  it  does  not  look  as  if  a  more  suitable  material  can  be 
found  for  this  purpose  Many  attempts  have  been  made  to 
substitute  other  materials,  but  these  have  only  proved  successful 
in  certain  applications.  The  built-up  mica  possesses  certain 
physical  qualifications  which  have  not  been  obtained  with  any  other 
material.  For  instance,  under  heating  and  cooling,  the  commut- 
ator changes  in  dimensions  circumferentially,  as  well  as  axially, 
and  under  this  action  the  mica  undergoes  much  more  compression 
at  times  than  at  others.  Therefore,  a  material  of  a  more  or  less 
elastic  nature  is  required  between  bars,  in  order  to  avoid  per- 
manent compression,  with  resulting  eventual  looseness.  Struc- 
turally the  mica-plate  meets  this  condition  very  well.  In  the 
second  place,  the  material  between  bars  should  be  one  which 
wears  down  properly  and  yet  does  not  have  any  cutting  or  grind- 
ing action  on  the  commutator  and  brushes  as  it  wears  off.  Mica 
apparently  meets  this  condition,  while  many  other  mineral  com- 
pounds, such  ate  asbestos,  appear  to  have  a  grinding  action.  In 
the  third  place,  the  material  should  be  more  or  less  heat  and 
spark-resisting.  Again,  it  should  be  a  non-absorbent  of  oil. 
tnica-plate  seems  to  be  the  oaity  material  so  far  which 


422  ELECTRICAL  ENGINEERING  PAPERS 

meets  all  the  requirements  for  general  purposes.  Hard,  inelastic 
materials  of  various  sorts  have  been  tried  and  have  not  proved 
successful.  Asbestos  in  sheets  and  plates  and  in  conjunction 
with  other  materials,  has  not  proved  very  satisfactory.  Fibrous 
and  cellulose  materials  have  given  good  results  in  some  cases, 
but  are  not  sufficiently  heat  and  oil-proof  for  general  purposes. 
The  only  material  departure  has  been  made  in  micas  used  with 
undercut  commutators.  In  such  cases,  white  micas  and  others 
which  have  all  the  good  characteristics  of  the  amber  micas,  except 
their  wearing  characteristics,  can  be  used,  for  the  undercutting 
eliminates  the  necessity  for  good  wearing  qualities.  At  the  same 
time,  undercutting,  as  explained  before,  is  advantageous  in  other 
ways. 

There  are  a  number  of  mechanical  conditions  entering  into 
the  practical  side  of  the  problem  of  commutation.  As  shown 
before,  it  is  very  important  to  maintain  good  contact  between 
the  brush  face  and  the  commutator  in  order  to  keep  down  losses 
and  burning  action.  Moreover,  good  contact  in  general  should 
mean  good  contact  over  the  whole  brush  face  in  order  to  keep  down 
the  current  density.  Therefore,  if  the  brushes  chatter  or  vibrate 
in  their  holders,  or  have  a  rocking  action  tending  to  give  alternate 
""heel-and-toe"  contact,  obviously  the  operation  is  liable  to  be 
affected  thereby.  Vibrating  brush  holders,  vibrating  brushes 
and  chattering  or  movements  of  any  kind  with  respect  to  the 
commutator  face,  are  objectionable  and,  not  infrequently,  very 
harmful. 

Vibrating  brush  holders  may  be  due  to  various  causes,  which 
do  not  show  up  on  the  manufacturer's  test.  The  machine  may 
be  so  located  that  its  environments  are  to  blame  for  vibration. 
Bad  gearing  may  cause  chattering.  The  foundations  or  sup- 
ports may  not  be  as  substantial  as  on  the  shop  test,  so  that  some 
small  disturbance  may  be  exaggerated  and  produce  vibrations  in 
parts  or  in  the  whole  machine.  Sometimes  the  brushes  may 
chatter  due  to  lack  of  lubricant,  and  this  may  set  the  whole  brush 
holder  structure  into  vibration.  Whatever  the  cause,  it  is  always 
best  to  stop  such  vibrations  as  far  as  possible,  especially  in  ma- 
chines handling  large  currents. 

Vibration  of  brushes  in  their  boxes  may  be  due  to  badly 
fitted  brushes,  (Pig.  7)  or,  on  machines  which  have  long  been 
in  operation  with  heavy  currents  per  brush,  the  brush  boxes 
may  be  eaten  away  inside  so  that  they  are  not  of  uniform  di- 


INSULATION  PROBLEMS,  ETC.  423 

mensions.  Low  resistance  shunts  on  carbon  brushes  are  for 
the  purpose  of  carrying  away  the  current  from  the  brush  by 
some  other  path  than  through  the  sides  of  the  brush  box.  How- 
ever, due  to  the  raking  or  dragging  action  of  the  commutator 
on  the  brushes,  they  are  liable  to  bear  rather  heavily  against 
one  side  of  the  box,  especially  at  the  lower  edge  next  to  the  com- 
mutator. Some  current  will  naturally  pass  to  the  box,  and  this  in 
time  will  tend  to  burn  away  the  boxes  and  the  carbons.  How- 
ever, as  the  carbons  burn  away,  they  are  eventually  replaced 
by  new  ones;  but  the  boxes  are  seldom  replaced,  and  in  time 
they  may  burn  away  so  that  they  are  larger  next  to  the  commut- 
ator. The  brush  then  fits  tightly  only  at  the  top  and  is  free  to 
move  or  vibrate  or  chatter  at  the  commutator  end,  which  is  just 
the  place  where  such  movement  should  be  avoided.  Attention 
is  called  to  this  action  in  particular  on  account  of  the  carelessness 
often  exhibited  in  regard  to  the  shunts  on  the  brushes. 


Fig.  7. 

Chattering  is  not  infrequently  due  to  lack  of  lubrication 
on  the  commutator  or  in  the  brushes.  When  the  commutator 
gets  too  dry  and  has  a  high  polish,  a  radial,  or  almost  radial 
brush  may  vibrate  or  chatter  just  as  a  pencil  does  when  moved 
along  a  pane  of  glass.  If  the  commutator  is  lubricated  with 
oil  to  overcome  this,  care  should  be  taken  not  to  use  an  excess 
of  oil  or  the  mica  may  absorb  it.  Frequently  immediately  after 
oiling,  a  commutator  shows  ring-fire,  which  is  due  to  combustion 
of  minute  particles  of  carbon  and  oil  over  the  top  of  the  mica. 
Sometimes  chattering  is  best  overcome  by  the  use  of  self -lubri- 
cating brushes.  In  undercut  commutators,  the  slots  are  liable 
to  cause  chattering  with  non-lubricated  brushes,  giving  out  a 
noise  of  a  pitch  comparable  with  the  product  of  the  revolutions 
by  the  number  of  commutator  bars.  As  oil-lubrication  should 
be  used  with  caution  on  undercut  machines,  practice  now  usually 
calls  for  some  form  of  self-lubricating  brush,  partly  or  wholly 
graphite,  as  has  been  referred  to  under  "under-cutting.1* 

In  communicating  pole  machines  it  is  especially  important 
that  the  brushes  should  not  have  any  heel-and-toe  movement, 


424  ELECTRICAL  ENGINEERING  PAPERS 

for  when  the  brush  makes  contact  at  one  edge  or  the  other,  the 
result  is  equivalent  to  rocking  the  brushes  backward  or  forward, 
which,  as  explained  before,  is  particularly  objectionable  in  such 
machines. 

In  direct-current  machines,  burnt  or  black  spots  will  some- 
times develop  on  the  commutator  at  points  removed  from  each 
other  a  distance  corresponding  to  that  between  holders  of  the 
same  polarity.  This  is  sometimes  very  bothersome,  and  the 
cause  of  the  difficulty  is  not  always  easy  to  find.  Any  condi- 
tion which  produces  one  bad  spot  may  tend  to  produce  similar 
spots  symmetrically  displaced  around  the  commutator.  When 
one  spot  is  formed,  and  this  spot  passes  under  one  brush  arm, 
the  brush  contacts  at  this  arm  are  naturally  poorer  and  the  other 
brush  arms  of  the  same  polarity  tend  to  take  the  load,  and  the 
current  density  in  their  brushes  is  correspondingly  increased 
during  this  short  period.  If  there  is  any  tendency  toward  high 
mica,  for  instance,  then  the  increased  current  at  these  points  will 
produce  increased  burning  away  of  the  copper  and  burnt  spots 
may  develop.  If  once  developed  they  may  gradually  travel 
around  the  commutator  until  the  whole  commutator  is  black. 
A  local  or  high  mica  strip  may  be  the  initial  cause  of  the  trouble, 
or  a  rough  spot  on  the  commutator  may  give  the  same  result. 
But  very  often,  resultant  high  mica,  following  the  initial  cause, 
tends  to  spread  the  trouble.  As  soon  as  such  black  spots  are 
noted,  further  trouble  frequently  can  be  headed  off  by  scraping 
or  cutting  the  mica  below  the  copper  surface  in  the  burnt  regions. 

One  of  the  most  severe  conditions  that  any  direct-current 
generator  can  encounter  is  a  dead  short-circuit  across  its  ter- 
minals, or  in  the  immediate  neighborhood  of  the  machine.  Very 
few  machines  outside  of  those  of  comparatively  small  capacity 
and  of  low  voltage,  can  stand  such  short-circuit  without  severe 
flashing.  Tests  have  shown  that  moderately  large  direct-cur- 
rent generators  will  give,  at  the  moment  of  short-circuit,  from 
20  to  30  times  full  load  current.  No  ordinary  commutating 
machine  can  be  built  to  take  care  of  such  a  current  rush,  and 
vicious  arcing  and  flashing  generally  results.  This  is  an  inherent 
condition  in  the  design.  No  responsible  manufacturer  who 
knows  his  business  will  guarantee  to  overcome  this  character- 
istic. However,  fortunately,  the  great  majority  of  short-circuits 
on  direct-current  power  systems  occur  at  some  distance  from  the 
generator,  and  moreover,  in  many  cases,  such  short-circuits  are  tnade 


INSULATION  PROBLEMS,  ETC.  425 

through  arcs  rather  than  by  dead  contact,  so  that  the  generators 
do  not  get  the  maximum  possible  current  rush. 

It  might  be  suggested  that  quick-acting  circuit  breakers 
would  take  care  of  such  extreme  conditions  by  opening  at  the 
loads  for  which  they  are  set.  But  this  setting  is  that  at  which 
the  tripping  mechanism  works,  and  if  the  current  rises  rapidly 
enough,  it  may  be  far  in  excess  of  the  tripping  value  by  the  time 
that  the  breaker  actually  opens  or  ruptures  the  circuit.  In  fact, 
this  is  just  what  happens  in  the  case  of  a  severe  short-circuit. 
Oscillograph  tests  have  shown  that  the  current  "rush"  on  short- 
circuit  may  reach  its  maximum  value  in  one-fiftieth  of  a  second, 
or  even  less,  while  the  ordinary  commercial  circuit  breakers 
seldom  operate  in  less  than  one-tenth  of  a  second,  which,  in 
reality,  is  pretty  rapid  action  for  a  mechanical  device  There- 
fore, it  will  have  to  be  an  extremely  rapid-acting  breaker  which 
can  get  the  circuit  open  before  the  short-circuit  current  has  risen 
to  several  times  the  full  load  value. 

One  other  subject  might  be  considered  under  commutation, 
namely,  the  influence  of  the  commutating  characteristics  upon 
the  permissible  range  of  speed  variation  and  speed  adjustment  in 
direct-current  motors.  There  are  two  general  methods  for  ob- 
taining speed  variation  in  such  apparatus,  namely,  by  variation 
in  the  e.  m.  f .  supplied  to  the  armature  terminals,  and  by  varia- 
tion in  the  field  strength  or  flux. 

In  the  early  development  of  adjustable  speed  motors,  the 
first  of  the  above  methods  was  used  almost  exclusively,  largely 
on  account  of  the  fact  that  the  commutation  problem  was  more 
easily  handled  with  this  method.  As  the  motor  could  be  given 
full  field  strength  much  of  the  time,  and  as  the  reduction  in  field 
strength  was  not  great  under  any  conditions,  fairly  good  com- 
mutation was  obtainable  in  general.  Where  constant  torque  was 
required,  this  method  was  fairly  satisfactory  and  economical 
However,  where  constant  horse-power  was  required,  obviously,  with 
this  method,  the  armature  current  had  to  be  increased  directly  as 
the  armature  voltage  and  speed  were  reduced.  Thus  the  armature 
had  to  be  designed  for  a  voltage  capacity  corresponding  to  the 
highest  voltage,  and  a  current  capacity  corresponding  to  the 
lowest  voltage.  Thus  it  became  quite  large  for  a  given  horse- 
power rating,  and  was  therefore  very  tineconornical  in  material. 
However,  the  larger  currents  at  lower  vblta^s  did  not  tepresent 
such  a  hardship  m  confutation,  for  %M&  increases  in  current 


426  ELECTRICAL  ENGINEERING  PAPERS 

were  accompanied  by  corresponding  reductions  in  speed,  which, 
made  commutating  conditions  proportionately  easier,  Thus 
the  commutating  problem  was  not  so  serious  with  this  method 
of  speed  control.  However,  for  constant  horse-power  service, 
it  was  obvious,  early  in  the  development,  tliat  the  most  econ- 
omical arrangement  as  regards  material,  would  be  the  use  of  a 
constant  voltage  across  the  armature,  thus  requiring  constant 
armature  current,  speed  control  being  obtained  by  variation  in 
the  field  strength.  But  this  meant  that  the  field  had  its  full 
strength  at  the  lowest  speed,  and  the  field  flux  would  be  decreased 
directly  as  the  speed  was  increased.  This  was  the  ideal  arrange* 
ment,  but,  unfortunately,  commutating  conditions  were  very 
difficult  at  the  weaker  fields, — that  is,  at  the  higher  speeds.  In 
consequence,  a  number  of  more  or  less  freakish  designs  were  put  out, 
with  the  idea  of  overcoming  the  commutation  troubles,  by  using 
variable  field  speed  control.  Some  of  these  designs  were  satis- 
factory from  the  operating  standpoint,  but  this  method  of  speed 
control  did  not  reach  its  full  development,  except  in  the  com- 
mutating pole  type  of  machine,  thus  showing  that  the  commut- 
ation problem  was  a  serious  one  in  this  method  of  operation. 
With  properly  proportioned  commutating  poles,  the  commutating: 
conditions  are  practically  independent  of  the  speed,  so  that, 
with  their  use,  the  real  limitations  in  speed  range  are  found  in 
other  conditions,  such  as  the  instability  of  very  weak  fields,  etc. 

It  is  evident  from  the  above  that,  as  regards  speed  regu- 
lation in  direct-current  work,  a  constant  field  motor  is  at  a  serious 
disadvantage  compared  with  one  in  which  the  field  strength  can 
be  varied.  In  fact,  this  holds  true  for  alternating-current  as 
well  as  for  direct-current  motors,  as  will  be  shown  later.  The 
alternating-current  induction  motor  as  essentially  a  constant  field 
machine,  and  normally  operates  at  constant  speed,  on  a  given  fre- 
quency. In  the  constant  field  direct-current  motor,  it  was  shown 
that  speed  variation  is  accomplished  by  variation  in  the  voltage 
applied  to  the  armature.  The  analogous  condition  in  the  induction 
motor  would  be  in  variation  in  the  frequency  applied,  and  not 
in  the  voltage.  This,  then  leads  up  to  the  next  subject,  namely* 

SPEED  CONTROL  OP  INDUCTION  MOTORS 
Repeating  preceding  statements,   the  induction  motor  is 
primarily  a  constant  speed  machine  when  supplied  with  constant 
frequency  and  e,  m.  f .,  which  is  the  standard  condition  in  all 


INSULATION  PROBLEMS,  ETC.  427 

alternating  power  service.  When  running  at  full  speed,  the 
secondary  frequency  and  e.  m.  f .  are  both  very  small,  the  frequency 
being  only  such  as  will  generate  enough  e.  m.  f.  to  send 
the  required  secondary  currents  through  their  own  windings 
when  closed  upon  themselves.  If  the  secondary  resistance 
is  increased,  with  a  given  current  flowing,  the  secondary 
e.  m.  f.  must  be  proportionately  increased,  and  the  secondary 
frequency,  to  generate  such  e.  m.  f.,  must  also  be  correspondingly 
increased.  This  secondary  frequency,  which  represents  the 
departure  from  synchronous  speed,  or  the  "slip,"  is  therefore 
always  proportional  to  the  secondary  e.  m.  f .  Therefore,  speed 
regulation  of  the  motor,  by  means  of  the  secondary  circuit,  means 
corresponding,  or  proportional  variation  in  the  secondary  frequency 
and  e.  m.  f .,  and  all  methods  of  speed  regulation  or  adjustment  of 
induction  motors  through  secondary  control  are  based  upon  fre- 
quency and  voltage  variation  in  the  secondary  circuits. 

All  methods  of  speed  regulation  of  induction  motors  may  be 
classified  under  two  general  heads;  (1)  primary  circuit  control, 
and  (2)  secondary  circuit  control. 

PRIMARY  CIRCUIT  CONTROL 

Two  general  methods  are  practicable,  namely,  variation  in 
the  number  of  primary  poles,  and  variation  in  the  frequency 
supplied  to  the  primary.  The  former  method  is  very  limited  in 
the  range  of  control  which  is  practicable.  Usually,  two  operat- 
ing speeds  can  readily  be  obtained,  while  three  or  four  lead  to  much 
added  complication,  and  more  than  four  speeds  does  not  appear  to 
be  commercially  practicable  except  in  very  special  cases.  For 
fine  graduations  in  speed,  pole  changing  is  apparently  out  of  the 
question.  c 

By  suitable  change  in  the  primary  frequency  supplied  to  the 
motor,  any  desired  speed,  or  speed  range,  is  obtainable.  But  in 
general,  the  problem  of  furnishing  this  variable  frequency  is  just 
as  serious  as  that  of  speed  adjustment  of  the  motor  itself  on  a 
fixed  frequency.  In  other  words,  it  takes  the  difficulty  away 
from  the  motor  and  transfers  it  elsewhere,  but  does  not  eliminate 
it. 

There  are  various  ways  of  generating  variable  frequency. 
For  instance,  an  alternator  may  be  driven  by  an  adjustable  speed 
motor.  This  should  be  af  direct-current  motor,  for,  if  an  alter- 
nating motor  is  used,  the  problem  of  varying  its  speed  is  just 
the  same  as  that  of  the  induction  motor  which  is  to  be  regulated. 


428  ELECTRICAL  ENGINEERING  PAPERS 

Another  way  is  to  connect  the  alternating-current  generator  to 
an  adjustable  speed  prime  mover,  such  as  an  engine  or  water- 
wheel.  Such  methods  of  regulation  require  one  generating 
outfit  for  each  motor  to  be  regulated,  except  where  two  or  more 
motors  are  to  be  regulated  over  the  same  range  at  the  same  time 
The  method  in  general  is  very  seldom  used. 

Other  possible  methods  of  regulating  the  primary  frequency 
lie  in  frequency  changers  of  certain  types  by  which  a  given  fre- 
quency can  be  converted  to  any  other  frequency  by  commutation 
of  alternating  current.  Various  types  of  such  machines  are 
possible  but  they  possess  certain  very  objectionable  limitations, 
in  that  they  must  commutate  currents  of  frequencies  approxim- 
ating those  of  the  primary  supply  system  At  25  cycles,  this  may 
be  practicable,  in  some  cases,  but  on  60  cycle  supply  circuits  it  is 
out  of  the  question.  One  other  serious  objection  to  regulating 
the  primary  frequency  is  that  the  frequency  controlling  device 
must  have  a  capacity  equal  to  that  of  the  motor  to  be  regulated, — 
that  is,  the  entire  input  of  the  induction  motor  must  be  handled 
by  the  frequency  regulator. 

In  general,  therefore,  regulation  of  induction  motor  speed  by 
change  in  primary  frequency  is  not  advisable,  and  appears  to  be 
practicable  only  in  certain  very  special  applications.  This  then 
brings  us  to  the  alternative  of  secondary  circuit  control. 

SPEED  CONTROL  BY  CHANGE  IN  SECONDARY  FREQUENCY 

As  brought  out  before,  any  speed  variation  of  an  induction 
motor  with  unchanged  primary  frequency  means  accompanying 
change  in  the  secondary  frequency  and  voltage.  At  standstill, 
the  secondary  voltage  is  a  maximum,  and  the  secondary  frequency 
is  100  percent  of  that  of  the  primary, — that  is,  it  is  the  same  as 
the  primary.  At  true  synchronous  speed,  the  secondary  voltage 
and  frequency  are  zero.  At  any  intermediate  speed,  the  secondary 
voltage  generated  and  the  secondary  frequency  are  respectively 
equal  to  the  standstill  voltage,  and  the  standstill  or  primary 
frequency,  multiplied  by  the  slip,  in  percent,  the  slip  being  the 
drop  from  synchronous  speed.  It  should  be  noted  that  the  second- 
ary generated  voltage  is  mentioned,  for  this  is  not  the  same  as  the 
secondary  terminal  voltage,  due  to  a  certain  internal  drop  in  the 
windings  when  current  is  flowing.  This  internal  drop  is  usually 
small  compared  with  the  secondary  standstill  voltage,  usually 
being  from  2  percent  to  3  percent  except  in  small  motors,  and 


INSULATION  PROBLEMS,  ETC  429 

therefore  may  be  neglected  in  any  general  discussion  not  involv- 
ing exact  calculations. 

All  methods  of  secondary  circuit  control  in  induction  motors 
include  some  methods  of  regulating  or  controlling  the  secondary 
voltage  and  frequency.  The  simplest  practical  device  is  the  use 
of  resistance  inserted  in  the  secondary  circuit.  In  order  to  get 
the  required  current,  for  a  given  torque,  through  such  resistance, 
the  voltage  must  be  increased  and  this  requires  increase  in  the 
secondary  frequency, — that  is,  drop  in  speed.  But  with  a  given 
resistance,  if  the  load  or  torque  is  varied,  the  secondary  current 
must  vary,  which  means  corresponding  variation  in  voltage  and 
secondary  frequency, — that  is,  in  speed.  Therefore,  speed 
regulations  by  secondary  resistance  means  variable  speed  with 
variations  in  torque;  and  constant  speed  with  variation  in  torque 
is  only  obtainable  by  varying  the  resistance  inversely  with  the 
current,  in  order  to  obtain  a  constant  voltage  drop.  Such  method 
of  speed  regulation  is  therefore  satisfactory  only  to  a  limited 
extent.  Moreover,  this  method  of  speed  regulation  is  unecon- 
omical, in  that  there  is  a  rheostatic  loss  practically  proportional 
to  the  drop  in  speed  below  synchronism.  At  half  speed,  for 
instance,  half  the  output  of  the  motor  is  wasted  in  resistance. 

Obviously  what  is  needed  is  some  arrangement  which  will 
absorb  the  required  secondary  voltage  in  other  than  resistancef 
and  which  will  automatically  hold  such  voltage  constant,  with 
varying  current,  in  those  cases  where  constant  speed  character- 
istics are  required  for  each  speed  setting  The  difficulty  in  ob- 
taining such  a  device  is  not  simply  in  the  voltage  range  required, 
but  is  largely  on  account  of  the  range  in  frequency  necessary. 
Therefore,  all  such  devices  must  be  of  adjustable  frequency,  and 
therein  lies  the  true  difficulty,  just  as  in  the  case  of  frequency 
changers  in  the  primary  circuit,  as  already  referred  to.  The 
difficulty,  however,  is  not  nearly  as  serious  in  the  case  of  regula- 
tion of  the  secondary  circuit,  for  the  variable  frequency  device 
needs  to  be  of  a  total  capacity  corresponding  to  the  slip,  in 
percent.  Furthermore,  where  the  departure  from  synchronism 
is  not  large,  the  actual  frequency  in  the  frequency  controlling 
machine  is  so  low  that  commutator  type  alternating-current  machines 
are  permissible  up  to  relatively  high  capacity.  The  problem  there- 
fore resolves  itself  into  one  of  variable  frequency,  just  as  in  the 
case  of  primary  circuit  j^gulatibn,  as  already  referred  to,  except 
that  the  frequencies  and  capacities  dealt  \ffith  usually  are  very 


430  ELECTRICAL  ENGINEERING  PAPERS 

much  lower  in  the  case  of  secondary  circuit  control      The  problem 
is  simply  easier,  but  not  of  a  different  nature. 

In  all  methods  of  rating  by  secondary  control,  the  regulating 
device  must  absorb  power  corresponding  in  percent  practically 
to  the  secondary  terminal  voltage,  or  the  secondary  frequency, 
or  slip.  This  power  must  be  utilized  if  economical  operation  is 
required.  There  are  three  general  methods  by  which  it  can  be 
utilized,  namely,  it  may  be  transformed  to  mechanical  power 
and  assist  in  driving  the  motor  shaft,  or  it  may  be  transformed 
to  the  primary  or  line  frequency  and  fed  back  into  the  line,  or  it 
may  be  transformed  to  direct  current  for  use  in  some  other  part 
of  the  system.  Combinations  of  these  three  methods  may  be 
used.  For  instance,  this  secondary  power  may  be  transformed 
to  direct  current  and  then  be  transformed  to  mechanical  power 
by  means  of  a  direct-current  motor  connected  to  the  induction 
motor  load.  Or,  it  may  be  transformed  to  direct  current  and 
then  re-transformed  to  the  primary  frequency  and  fed  back  into 
the  line. 

Three  types  of  variable  frequency  devices  have  been  pro- 
posed for  absorbing  the  secondary  terminal  voltage,  namely, 
A.  C.  commutator  motors,  rotary  converters,  and  commutator 
type  frequency  changers.  In  the  first  named,  the  A.  C.  commut- 
ator motor  either  delivers  its  power  directly  to  the  shaft  of  the 
induction  motor,  or  to  an  A.  C.  generator  which  returns  it  to  the 
line,  or  to  a  D  C.  generator  which  delivers  its  current  to  some 
D.  C.  system  or  load  where  it  can  be  utilized.  In  the  second  type 
mentioned,  a  rotary  converter  absorbs  at  its  collector  rings  the 
secondary  terminal  voltage  and  transforms  it  to  a  proportional 
direct-current  voltage.  The  direct-current  power  is  then  fed  into 
a  direct-current  motor  connected  with  the  induction  motor  load, 
or  is  transformed  to  the  primary  frequency  by  a  suitable  motor- 
generator  set.  In  the  third  type,  a  commutator  type  frequency 
changer  transforms  the  secondary  terminal  voltage  and  frequency 
to  a  proportionate  voltage  at  the  primary  frequency,  and,  by 
means  of  suitable  transformers,  the  secondary  power  is  then  re- 
turned to  the  primary  supply  circuit. 

Each  of  these  arrangements  possesses  some  advantages  over 
the  others,  and  also  some  disadvantages.  The  A.  C.  commutator 
motor  is  a  relatively  expensive  type  of  machine,  especially  for 
very  low  speeds.  Therefore,  when  the  induction  motor  to  be 
regulated  is  of  comparatively  low  speed,  placing  the  commutator 


INSULATION  PROBLEMS,  ETC  431 

motor  on  the  induction  motor  shaft  means  a  relatively  expensive 
commutator  machine.  In  such  cases,  it  may  be  advisable  to 
either  gear  it  to  the  load  or  connect  it  to  a  generator  which  returns 
power  to  the  line  or  delivers  it  to  another  system.  By  such 
means,  a  smaller  and  higher  speed  commutator  type  A.  C.  motor 
may  be  used,  but  at  a  certain  expense  in  auxiliary  apparatus.  For 
a  frequency  of  25  cycles,  the  A.  C.  commutator  motor  does  not 
present  any  undue  inherent  difficulties  if  the  speed  range  of  the 
secondary  control  is  not  too  large.  With  50  percent  drop  in 
speed,  for  instance,  the  frequency  handled  by  the  A.  C.  commut- 
ator motor  is  only  12J^  cycles.  But  with  a  60  cycle  supply 
system,  a  speed  range  of  50  percent  means  that  the  A.  C. 
commutator  motor  must  handle  30  cycles,  which  is  a  much  more 
difficult  and  expensive  proposition. 

With  the  rotary  converter  speed  regulation,  no  new  or  diffi- 
cult problems  are  involved,  either  in  the  transformation  or  utiliza- 
tion of  the  secondary  power.  Where  the  induction  motor  speed 
is  not  too  low,  a  direct-current  motor  connected  to  the  shaft  may 
utilize  the  direct-current  power  from  the  rotary  converter.  How- 
ever, unlike  the  A.  C.  commutator  scheme  above  described,  the 
rotary  converter  arrangement  makes  its  best  showing  in  connec- 
tion with  60  cycle  supply  systems,-  for,  with  the  higher  frequency, 
the  secondary  frequency  of  the  main  motor  is  correspondingly 
higher  for  the  same  speed  range,  which  allows  the  use  of  a  relatively 
smaller  rotary  for  the  same  percentage  of  power  transformed. 
To  illustrate— On  a  25  cyde  supply  system,  with  30  percent  speed 
range,  the  maximum  secondary  frequency  is  7  J^  cycles.  A  4-pole 
rotary  operating  at  this  frequency  will  run  at  225  r.p.m. ;  that 
is,  at  this  speed,  it  transforms  or  utilizes  30  percent  of  the  power 
of  the  induction  motor.  Considering  now,  60  cycles,  with  the 
same  speed  range,  the  secondary  frequency  becomes  18  cydes, 
and  a  4-pole  rotary  of  30  percent  of  the  motor  capacity  will 
operate  at  540  r.p.m.  on  this  frequency.  Obviously,  a  rotary 
converter  of  much  smaller  dimensions  can  be  used,  than  in  the 
former  case.  The  auxiliary  means  for  absorbing  the  direct  cur- 
rent power  from  the  rotary  converter  can  be  practically  the  same 
for  either  frequency.  Therefore,  with  this  method,  60  cydes 
makes  the  better  showing. 

In  the  third  scheme,  (Fig.  8)  the  secondary  frequency  of  the 
induction  motor  is  transformed  directly  to  the  primary  frequency 
in  a  single  machine.  The  auxiliary  means  for  utilizing  the  trans* 


432 


ELECTRICAL  ENGINEERING  PAPERS 


formed  power  consists  of  suitable  stationary  transformers  with 
tap  for  varying  the  voltage.  As  the  frequency  changer  is  a  rather 
unusual  device,  a  brief  description  of  its  principle  may  not  be 
out  of  place  at  this  point.  As  usually  built,  it  consists  of  an 
armature  like  that  of  a  rotary  converter,  equipped  with  both 
commutator  and  collector  rings.  Unlike  the  rotary,  the  field 
may  consist  of  a  simple  " keeper"  or  ring,  (Fig.  9)  without  wind- 
ings, which  encircles  the  armature.  Also,  unlike  the  rotary  con- 


Fig.  8. 


Fig.  9. 


verter,  the  commutator  is  equipped  with  a  double  or  triple  set 
of  brush  holders  for  handling  polyphase  current.  The  ordinary 
spacing  of  the  D.  C.  holders  on  a  direct  current  rotary  would 
correspond  to  one  ph&oe  of  the  frequency  changer.  The  arma- 
ture can  be  driven  by  any  suitable  small-capacity,  adjustable 
speed  device.  Practically  the  only  load  carried  by  the  driving 
device  consists  of  brush  friction  and  windage. 

If  such  a  machine  has  its  collector  rings  connected  to  the 
main  supply  system  through  suitable  transformers,  a  rotating 
field  will  be  set  up  in  the  armature  core  (and  keeper)  which  travels 
around  the  core  at  a  speed  corresponding  to  the  frequency  divided 
by  the  number  of  poles,  just  as  in  an  induction  motor.  If,  now, 
the  core  is  rotated  mechanically  in  the  opposite  direction,  at  a 
speed  equal' to  the  frequency  divided  by  the  number  of  poles,  then 
the  magnetic  field  set  up  in  the  core  will  stand  still  in  space  and 
could  be  replaced  by  an  external  field  excited  by  direct  current, 


INSULATION  PROBLEMS,  ETC  433 

just  as  in  a  rotary  converter.  Under  this  condition,  the  brushes 
on  the  commutator  would  tend  to  deliver  current, — that  is,  alter- 
nating current  having  zero  frequency.  Under  this  condition, 
the  external  stationary  keeper  has  zero  frequency  in  it,  while  the 
armature  core  has  normal  frequency.  Assume  now  that  the  core 
is  rotated  either  faster  or  slower  than  synchronous  speed.  The 
magnetic  field  set  up  by  the  armature  winding  will  travel  back- 
ward or  forward  in  space  at  speed  corresponding  to  the  departure 
of  the  core  from  synchronism  and  the  brushes  on  the  commut- 
ator will  tend  to  deliver  alternating  current  at  a  frequency  pro- 
portional to  the  departure  of  the  armature  core  from  synchronous 
speed.  Thus  by  varying  the  speed  of  the  armature  core  from 
synchronism,  any  desired  frequency  can  be  obtained  at  the  commu- 
tator brushes  But  the  voltage  at  the  commutator  brushes  is 
practically  equal  to  the  voltage  at  the  collector  rings,  regardless 
of  the  speed  of  rotation,  and  by  varying  the  voltage  supplied  to 
the  collector  rings,  the  voltage  at  the  commutator  can  be  varied 
independently  of  the  frequency,  which  is  dependent  solely  upon  the 
spead  of  the  armature.  Thus,  independent  control  of  the  voltage 
and  frequency  is  obtainable,  which  makes  the  device  quite  flexible 
in  its  application.  But  such  a  device  has  other  very  desirable 
characteristics.  As  it  is  primarily  one  form  of  rotary  converter,  we 
should  naturally  expect  that  it  would  show  some  of  the  small-cop- 
per-loss characteristics  of  the  rotary  converter.  Analysis,  however, 
shows  that  it  goes  even  further  than  this.  In  a  6-phase  rotary 
converter,  for  instance,  the  armature  copper  loss  averages  about 
26  percent  of  that  of  a  corresponding  direct-current  winding,  due 
to  part  of  the  alternating  current  being  fed  directly  through  to  the 
direct-current  circuit  without  transformation.  But  as  there  is 
transformation  from  one  kind  of  current  to  another,  the  operation 
is  incomplete,  and  there  are  certain  transformation  losses  which 
are  especially  large  at  and  near  the  so-called  tap  coils,  which  are 
connected  to  the  collector  rings.  But  in  a  6-phase  frequency 
changer  of  the  above  type,  the  transformation  is  from  6-phase 
alternating  current  to  6-phase  current  of  another  frequency,  and  a 
still  larger  percent  of  the  current  passes  through  without  transform- 
ation than  is  the  case  in  a  6-phase  rotary  converter.  In  consequence, 
the  frequency  changer  has  only  about  two-thirds  as  much  copper 
loss  as  a  rotary  converter;  that  is,  for  6-phase,  it  is  less  than  18 
percent  of  that  of  a  corresponding  direct-current  machine.  More- 
over, the  tap-coil  losses  of  the  fotary  converter  are  practically 


434  ELECTRICAL  ENGINEERING  fAPERS 

absent.     It  thus  becomes  an  extremely  effective  transforming  de- 
vice, as  far  as  frequency  is  concerned. 

Such  a  device  is  also  very  economical  as  regards  iron  losses- 
As  it  generates  voltages,  when  connected  to  the  secondary  ter- 
minals of  an  induction  motor,  which  are  proportional  to  the  speed 
range,  obviously,  with  moderate  speed  variations,  the  magnetic 
flux  in  the  armature  core  will  be  small  compared  with  what  would 
be  necessary  to  generate  full  secondary  voltage.  Also,  even  this 
reduced  induction  is  at  a  comparatively  low  frequency  in  the 
surrounding  ring  or  keeper,  and  is  only  at  full  frequency  in  the 
armature  core  proper.  Evidently  therefore,  the  armature  core 
and  armature  teeth  sections  can  be  made  relatively  small  where 
the  range  of  speed  adjustment  is  small,  such  as  25  percent  to  35 
percent  from  synchronism.  This  small  average  core  loss,  together 
with  the  very  small  copper  loss  tends  toward  a  very  economical 
construction  of  machine. 

In  such  a  frequency  changer,  the  problems  of  commutation 
are  very  similar  to  those  in  the  A.  C.  commutator  motor,  and  at 
25  cycles  the  design  becomes  simpler  and  easier  than  at  60  cycles. 
The  machine  is  independent  of  the  speed  of  the  induction  motor 
to  be  controlled,  which  is  not  the  case  with  the  A.  C.  commutator 
motor  in  its  simplest  application,  namely,  direct  connection  to  the 
•main  motor  shaft.  Such  frequency  changer  in  its  simplest  form 
may  be  arranged  to  be  self -compensating,  and  the  conimutating 
conditions  can  be  brought  well  within  those  of  well-proportioned 
A.  C.  commutator  motors. 

In  the  application  of  these  various  speed  regulating  devices, 
two  power  conditions  should  be  given  consideration, — namely, 
whether  the  motor  outfit  is  to  develop  constant  horse  power  at  the 
shaft,  or  constant  torque,  with  the  developed  power  varying  in 
proportion  to  the  speed.  In  most  cases,  in  steel  mill  work,  con- 
stant torque  is  all  that  is  necessary,  while  in  a  few  special  cases 
constant  horse  power  may  be  desired. 

Where  constant  torque  is  preferred,  the  frequency  converter 
should  prove  to  be  most  desirable  in  many  ways,  particularly  on 
account  of  its  flexibility  in  application,  so  that  a  few  suitable 
sizes  should  cover  range  of  application.  In  this  feature,  and  in  a 
number  of  others,  it  has  the  advantage  over  the  rotary  converter 
or  the  alternating-current  commutator  motor  schemes. 

Where  constant  horse-power  is  required,  it  is  questionable 
whether  any  one  scheme  has  the  advantage  in  all  cases.  Where 


INSULATION  PROBLEMS,  ETC.  435 

the  induction  motor  speed  is  fairly  high,  and  the  frequency  is  low, 
the  A.  C.  commutator  motor  directly  connected  to  the  induction 
motor  shaft  is  a  good  arrangement,  as  only  one  regulating  ma- 
chine is  required.  If,  however,  the  speed  is  so  low  that  the  A,  C. 
commutator  motor  connection  to  the  main  shaft  is  inadvisable,  so 
that  power  must  be  returned  to  the  supply  system,  then  this 
arrangement  will  not  compare  favorably  with  the  frequency 
changer  scheme.  The  rotary  converter  scheme,  delivering 
direct-current  power  to  a  motor  on  the  induction  motor  shaft, 
also  makes  a  good  constant  power  outfit,  but  where  the  speed  is 
too  low  to  allow  an  economically  proportioned  direct-current 
motor,  the  scheme  is  also  at  a  disadvantage  compared  with  the 
frequency  changer.  But  where  the  frequency  changer  is  used 
with  constant  horse-power,  the  main  induction  motor  must  be 
large  enough  to  deliver  the  rated  power  to  the  shaft  at  the  lowest 
speed,  the  excess  power  being  transferred  to  the  line  by  the  fre- 
quency changer.  If,  however,  the  main  motor  is  operated  above 
synchronism  at  its  highest  speed,  by  an  amount  corresponding  to 
the  slip  below  synchronism  at  its  lowest  speed,  then  the  increase 
in  capacity  of  the  main  motor  and  of  the  frequency  changer,  to 
give  constant  power  at  the  shaft,  will  be  only  about  half  as  much 
as  if  all  the  speed  variation  were  below  synchronism.  This  brings 
up  a  point  not  yet  brought  out,  namely,  that  some  of  these  ad- 
justable speed  devices  allow  operation  of  the  main  motor  above 
synchronous  speed.  This  is  particularly  true  in  those  cases  where 
the  speed-regulating  or  auxiliary  apparatus  can  impress  its  own 
frequency  upon  the  secondary  of  the  main  motor,  and  where  such 
impressed  frequency  can  be  independently  controlled.  In  such 
cases,  by  gradually  varying  the  frequency  down  to  zero  and  then 
up  in  the  opposite  direction,  the  main  motor  can  have  its  speed 
varied  through  the  synchronous  position. 

Various  other  methods  of  speed  regulation  have  been  pro- 
posed, but  most  of  them  have  not  yet  seen  actual  test.  Several 
schemes  have  been  proposed  for  utilizing  mercury  vapor  rectifiers 
for  transforming  the  secondary  current  of  the  induction  motor  to 
direct  current.  This  is  one  case  where  a  frequency-changing 
controlling  device  does  not  form  a  fundamental  part  of  the  con- 
trol. On  the  other  hand,  such  method  of  control  is  as  yet  more 
theoretical  than  practical,  and  moreover,  the  mercury  rectifier  is 
not  yet  in  general  use  for  power  service.  At  best,  therefore,  this 
method  is  one  of  the  future. 


436 


ELECTRICAL  ENGINEERING  PAPERS 


Correction  of  power  factor  in  induction  motors,  by  means  of 
a  low  frequency  exciter  in  the  secondary  circuit  is  feasible.  In 
connection  with  the  above  described  adjustable-speed  devices,  it 
may  be  said  that  almost  all  such  devices  can  be  designed  to  correct 
power  factor,  as  well  as  to  produce  change  in  speed,  and,  in  many 
cases,  this  involves  practically  no  extra  complication.  For 
instance,  in  the  frequency  changer  method,  where  the  voltage  can 
be  varied  independently  of  the  frequency,  an  increase  in  the 
frequency  changer  voltage  without  change  in  speed  would  simply 
mean  the  transfer  of  wattless  current  from  the  supply  system  to 
the  secondary  circuit  of  the  induction  motor,  and  this  replaces 
the  primary  wattless  magnetizing  current.  By  proper  voltage 
adjustment,  the  primary  wattless  component  could  be  reduced  to 
zero,  or  even  changed  to  a  large  leading,  instead  of  lagging,  com- 
ponent, with  consequent  change  in  power  factor  from  lagging  to 
leading  of  any  desired  value.  This  simply  illustrates  the  general 
method  of  power  factor  correction  by  all  these  various  devices. 


Fig.  10. 

While  on  the  subject  of  power  factor  correction,  it  may  be 
stated  that  only  two  general  methods  of  power  factor  correction 
are  practicable — namely,  by  means  of  static  condensers  connected 
across  the  system,  and  by  means  of  rotating  condensers  of  some 
form.  (Fig.  10). 

The  static  type  of  condenser  is  commercial  on  a  small  scale, 
and,  possibly,  may  become  so  on  a  large  scale  in  the  not  far  dis- 
tant future.  Large  capacity  static  condensers  can  be  built  at 
present,  but  possibly  not  at  a  cost  which  will  compete  with  the 
rotating  type. 


INS  ULA  TION  PROBLEMS,  E TC.  437 

Rotating  condensers  may  be  divided  into  two  sub-classes, — 
namely,  synchronous  and  non-synchronous.  The  synchronous 
type  is  well  known  commercially.  Usually  it  is  simply  a  syn- 
chronous motor  with  over-excited  field.  It  may  or  may  not 
deliver  power  as  a  motor  while  acting  as  a  condenser. 

The  non-synchronous  condenser  is  simply  a  non-synchronous 
or  induction  motor  with  its  secondary  excited,  instead  of  its 
primary.  When  acting  as  a  condenser,  the  secondary  is  over- 
excited. It  is  therefore  somewhat  similar  to  the  synchronous 
coiidenser,  low  frequency  alternating  current,  instead  of  direct 
current  being  used  for  excitation.  The  condenser  also  may  act 
as  a  motor  delivering  power.  This  type  of  condenser  has  not 
been  used  to  any  extent  in  this  country. 

From  the  foregoing  discussion  of  speed  control,  it  is  apparent 
that  the  frequency  of  the  supply  system  has  an  important  bearing 
on  the  induction  motor  problem  in  general.  There  are  only  two 
accepted  standard  frequencies  in  general  use  in  this  country, — 
namely,  60  and  25  cycles,  and  both  are  in  use  in  central  power 
stations.  The  tendency  for  mills  and  factories  to  purchase  power 
from  such  central  power  stations,  instead  of  generating  it  them- 
selves, appears  to  be  increasing.  This  therefore,  leads  to  another 
subject  of  direct  interest  to  steel  mill  engineers,  namely, — 

CHOICE  OP  FREQUENCY 

This  question  is  not  limited  to  mill  work,  but  has  become 
a  very  general  one  in  the  whole  electrical  business.  Some  years 
ago  a  committee  of  steel  mill  engineers  decided  upon  25  cycles 
as  a  standard  frequency  for  steel  mill  work.  The  reasons  for  this 
decision  were  amply  sufficient  at  that  time  and  still  hold  good  to 
a  certain  extent.  But,  in  more  recent  times,  the  general  tendency 
of  the  large  central  station  or  power  companies  toward  60  cycles, 
together  with  the  sale  of  such  power  to  steel  mills  and  other  in- 
dustrial plants,  has  changed  the  situation  somewhat.  At  the 
time  that  the  steel  mill  committee  recommended  in  favor  of  25 
cycles,  there  was  an  apparent  tendency  of  the  large  power  com- 
panies toward  this  frequency.  But,  as  intimated,  that  tendency 
is  now  reversed,  partly  -due  to  improvements  in  certain  types  of 
apparatus,  such  as  rotary  converters.  It  therefore  may  be 
pertinent  to  discuss  this  subject  of  frequency  more  fully,  in  view 
of  its  possible  influence  on  -mill  work. 


438  ELECTRICAL  ENGINEERING  PAPERS 

The  principal  loads  to  be  handled  by  general  power  or  central 
station  alternating  current  plants  are:  first,  lighting,  including 
arc,  incandescent,  etc.;  second,  motor  power  service;  and,  third, 
direct-current  service  for  various  purposes,  such  as  railway,  etc. 

In  general,  there  is  no  particular  question  regarding  the 
better  frequency  for  lighting  service,  for  60  cycles,  for  direct  use 
in  both  arc  and  incandescent  lamps  undoubtedly  gives  better 
results  than  25  cycles. 

When  it  comes  to  motors,  either  synchronous  or  induction, 
60  cycles  present  more  advantages  in  general,  except  for  very  low 
speeds,  and,  even  in  this  case,  with  synchronous  machines,  the 
choice  is  in  doubt.    In  the  case  of  induction  motors,  however, 
there  are  certain  fields  where  25  cycles  will  show  better  results. 
This  is  in  very  slow  speed  work,  or  very  slow  speed  in  proportion  to 
the  capacity.     It  is  a  rule  in  practically  all  types  of  generators 
and  motors  that  the  greater  the  number  of  poles,  the  greater  must 
be  the  total  magnetizing  ampere  turns.     In  windings  excited 
by  direct  current,  the  number  of  exciting  turns  may  be  increased 
with  increase  in  the  number  of  poles,  at  a  certain  expense  in  cop- 
per, so  that  the  actual  exciting  or  magnetizing  current  may  not  be 
excessive,  even  with  a  very  large  number  of  poles — that  is,  in  very 
slow  speed  machines.    But  in  induction  motors,  the  same  turns 
are  used  for  magnetizing  and  for  generating  counter  e.  m.  f .    The 
latter  condition  usually  so  fixes  the  number  of  turns,  in  a  given 
capacity  and  speed  of  machine,   that  the  actual  magnetizing 
current  increases  very  greatly  with  increase  in  the  number  of 
poles, — that  is,  with  decrease  in  speed,  so  that,  with  a  large 
number  of  poles,  this  magnetizing  current  becomes  so  large  in 
comparison  with  the  work  current  that  the  characteristics  of  the 
machine  are  very  seriously  affected.    This  increase  can  be  limited 
to  a  certain  extent  by  increasing  the  dimensions  of  the  machine, — 
that  is,  its  cost.    Herein  is  where  25  cycles  may  give  consider- 
able advantage  over  60  cycles.    For  instance,  a  4-pole,  25  cycle 
motor  will  have  about  the  same  speed  as  a  10-pole,  60  cycle 
motor.    The  4-pole  motor  should,  and  usually  does  have  smaller 
magnetizing  current  than  the   10-pole.    However,   the   4r-pole 
machine  for  the  same  speed  should  require  more  material  than 
the  10-pole,  on  account  of  higher  magnetic  flux  conditions.  There 
fore,  if  the  10-pole  machine  were  made  of  larger  dimensions  than 
the  4-pole,  but  utilizing  the  4-pole  magnetic  material,  its  magnet- 
izing current  might  be  made  fairly  comparable  with  that  of  the 


INSULATION  PROBLEMS,  ETC.  439 

4-pole  machine.  However,  with  the  same  total  useful  material, 
but  arranged  in  larger  dimensions,  the  idle  material,  such  as 
frame,  supports,  etc.,  will  be  somewhat  greater  in  the  10-pole 
machine  of  the  same  speed,  and  therefore,  in  general,  for  equal 
speed  and  equal  characteristics,  the  60  cycle  induction  motor 
should  cost  more  than  the  25  cycle.  However,  for  general  power 
distribution  with  relatively  small  motor  capcities,  it  is  not  correct 
to  compare  a  10-pole  60  cycle  motor  with  a  4-pole,  25  cycle;  for, 
in  most  cases,  60  cycle  motors  of  higher  speed  can  be  used,  such 
as  eight,  six  and  four-pole,  giving  respectivley  900,  1200  and  1800 
r.p.m.,  neglecting  the  small  slip.  These  higher  speed  and  smaller 
number  of  poles  in  general  more  than  offset  the  advantages  of  the 
25  cycle,  4-pole,  750  r.p  m.  motor  as  regards  cost  and  character- 
istics, and  at  the  same  time,  the  greater  choice  in  speeds  is  very 
advantageous.  In  25  cycles,  the  highest  speed  is  1500  r.pm., 
with,  two  poles,  and  experience  has  shown  that,  in  size  and  con- 
struction, a  2-pole  induction  motor  has  very  little  advantage  over 
a  4-pole,  except  possibly  in  very  small  capacities.  Therefore,  60 
cycles,  with  its  4-pole  1800  r.p  m.,  6-pole  1200  r.p.m.,  8-pole  900 
r.p.m.  motors,  has  a  decided  commercial  advantage  over  the  25 
cycle  system  with  its  2-pole  1500  r.p.m.,  and  4-pole  750  r.p.m. 
motors. 

However,  when  we  compare,  for  instance,  a  moderate  capacity 
12-pole,  250  r.p.m.,  25  cyde  with  a  30-pole,  240  r.p.m.,  60  cycle 
motor  we  may  find  the  advantage  considerably  in  favor  of  the  25 
cycle, — so  much  so  that  if  all  the  motors  to  be  used  in  a  given 
plant  were  of  this  speed  or  lower,  and  there  were  no  other  offsetting 
advantages  for  60  cycles,  such  as  lighting,  etc.,  then  the  proposi- 
tion ;would  look  like  a  good  one  for  25  cycles.  However,  if  only  a 
small  percentage  of  the  total  load  is  represented  by  such  low  speed 
motors,  then  the  60  cycle  supply  may  make  otherwise  a  sufficiently 
good  showing  to  warrant  its  use.  If,  however,  we  go  to  the  extreme 
case  of  moderate,  or  even  very  large,  capacity  motors  at  75  to  100 
r.p.m.,  then  we  run  into  almost  prohibitive  constructions  with  60 
cycles,  either  in  size  or  in  operating  characteristics.  At  60  cycles, 
such  motors  are  liable  to  have  such  low  power  factors  that  the 
actual  current  taken  by  the  motors  is  so  large  compared  with  the 
work  current  that,  even  with  poor  performance,  a  very  large 
motor  is  required  for  a  given  capacity.  In  25  cycles  however,  such 
motors  can  make  a  very  much  better  showing.  Therefore,  at  the 
present  time,  25  cycles  prepresents  the  most  suitable  frequency 


440  ELECTRICAL  ENGINEERING  PAPERS 

for  such  motors.  However,  hope  may  be  extended  for  the  60 
cycle  motor.  If  such  motors  are  to  be  operated  at  constant 
speed,  or  even  tinder  variable  or  adjustable  speeds,  as  has  been 
described  under  an  earlier  subject,  it  is  possible  and  practicable 
to  overcome  the  difficulty  of  the  poor  power  factor  and  large 
current  from  the  supply  system  by  connecting  a  special  low  fre- 
quency exciter  in  the  secondary  circuit  of  the  induction  motor, 
which  will  supply  the  magnetizing  current  to  the  secondary 
instead  of  the  primary,  just  as  in  the  non-synchronous  type  of 
condenser  already  referred  to.  This  does  not  eliminate  the 
magnetizing  current  in  the  motor,  but  simply  puts  it  in  the 
secondary  circuit. 

The  above  is  a  considerable  digression  from  the  central  station 
problem,  but  it  has  a  direct  bearing  on  the  purchase  of  power  by 
fmills  from  central  stations.  From  the  above,  it  is  obvious  that 
or  the  general  sale  of  motor  power  to  varied  industries,  the  160 
cycle  central  station  has  a  direct  advantage  over  the  25  cyce, 
in  the  great  majority  of  service. 

When  it  comes  to  the  question  of  delivering  direct  current 
from  an  alternating-current  system,  the  25  cycle  system,  in  con- 
nection with  rotary  converters,  is  generally  assumed  to  have 
considerable  advantage  over  the  60  cycles.  However,  even  that 
advantage  is  disappearing,  due  to  recent  advances  in  the  design 
of  high  speed  apparatus  for  converting  from  alternating  to  direct 
current.  Where  motor  generators  are  used,  60  cycles  in  general 
allow  a  more  satisfactory  choice  of  converting  set;  for,  in  many 
cases,  for  a  given  capacity,  the  60  cycle  set  can  be,  given  a  some- 
what higher  speed  than  the  25  cycle.  Therefore,  the  advantage 
of  25  cycle,  if  such  exists,  must  lie  in  rotary  converters.  But 
recent  advances  in  60  cycle  rotary  converter  construction  have 
made  the  60  cycle  rotary  a  strong,  and  pretty  reliable  competitor 
of  the  25  cycle  rotary, — so  much  so  that,  at  the  present  time,  quite 
a  number  of  electric  railways  have  shut  down  their  own  D.  C. 
generating  stations,  and  are  buying  power  from  60  cycle  central 
stations  through  60  cycle  rotaries.  This  development  has  removed 
one  of  the  most  serious  handicaps  of  the  60  cycle  system,  so  that 
the  present  tendency  of  central  station  work,  and  even  power 
transmission,  is  strongly  toward  60  cycles.  The  steel  mill  en- 
gineers should  therefore  keep  this  tendency  strongly  in  mind. 


SOME  CONTROLLING  CONDITIONS  IN  THE  DESIGN 
AND  OPERATION  OF  ROTARY  CONVERTERS 

FOREWORD-^-This  raper  was  presented  at  the  twenty-eighth  annual 
convention  of  the  Association  of  Edison  Illuminating  Companies 
at  Hot  Springs,  Va.,  September,  1912.  At  that  time,  the  syn- 
chronous booster  type  of  converter  was  becoming  well  estab- 
lished and  it  was  the  author's  purpose  to  show  in  this  paper  some 
of  the  conditions  of  commutation  which  were  encountered  in  the 
synchronous  type  of  machine. — (ED.) 

EXPERIENCE  shows  that  the  rotary  converter  is  one  of  the 
most  satisfactory  and  reliable  of  the  various  types  of  rotating 
electrical  machinery.  In  its  efficiency  of  transformation,  its  com- 
mutation and  temperature  characteristics,  and  its  operating 
characteristics  in  general,  it  makes  an  extremely  good  showing. 
Furthermore,  it  is  a  type  of  machine  which  has  not  changed  greatly 
in  the  last  decade.  The  more  recent  developments  have  been 
principally  in  the  use  of  commutating  poles  and  in  a  very  material 
increase  in  the  rotative  speeds,  these  two  features,  however,  being 
closely  allied,  as  will  be  shown  later. 

In  considering  the  various  characteristics  of  the  rotary  con- 
verter, such  as  its  current  and  voltage  capacities,  e.  m.  f .  regula- 
tion, commutation  and  the  use  of  commutating  poles,  maximum 
speeds  permissible  with  a  given  output,  etc.,  certain  fundamental 
conditions  or  limitations  in  the  design,  are  of  controlling  import- 
ance. In  order  to  obtain  a  fuller  understanding  of  the  possibilities 
and  capabilities  of  such  apparatus,  a  brief  consideration  of  these 
fundamental  conditions  will  be  given. 

COMMUTATION  LIMITS  AND   SHORT-CIRCUIT  E.   M.    F.'s 

One  condition  of  controlling  importance  in  all  commutating 

machinery  is  the  commutation.      If  the  machine  does  not  com- 

mutate  well,  then  perfections  in  other  features  are  overshadowed. 

High  efficiency,  low  temperature  rise,  and  low  first  cost,  do  not 

outweigh  bad  operation  at  the  commutator. 

In  the  ordinary  commutating  machine,  the  armature  winding, 
when  carrying  current,  sets  up  local  magnetic  fields,  or  fluxes, 
across  which  the  armature  conductors  cut  and  thus  generate 

441 


442  ELECTRICAL  ENGINEERING  PAPERS 

e.  m.  f.'s,  just  as  when  they  cut  across  the  main  field  fluxes.  These 
local  fields,  due  to  the  armature  ampere  turns,  usually  have  peak 
values  at  those  points  on  the  armature  where  one  or  more  armature 
coils  are  short-circuited  by  the  brushes  on  the  commutator.  The 
conductors  or  turns  which  are  thus  short-circuited,  have  certain 
voltages  generated  in  them,  and  the  brushes  are  short-circuiting 
across  these  voltages.  It  may  thus  be  said  that  there  is  a  certain 
short-circuit  voltage  per  armature  coil,  or  between  adjacent  com- 
mutator bars,  which  may  be  called  the  inherent  short-circuit  e,  m.  /. 
per  bar.  If  the  brush  is  wide  enough  to  cover  several  bars,  then  it 
short-circuits  the  voltages  of  several  bars.  The  average  value  of 
this  may  be  called  the  inherent  brush  short-circuit  e.  m.  f.  The  value 
of  this  latter  is  of  utmost  importance  in  commutating  machinery, 
for  it  is  upon  this,  and  the  resistance  of  the  brush,  that  the  amount 
of  short-circuit,  or  "local,"  current  depends. 

When  the  work  current,  or  that  which  flows  to  the  external 
circuit,  passes  from  the  commutator  to  the  brush,  it  should  be 
distributed  evenly  over  the  whole  brush  contact,  providing  there 
are  no  disturbing  conditions.  On  the  basis  of  uniform  distribu- 
tion of  current  over  the  brush  contact,  the  minimum  current 
density  at  the  brush  contact  would  naturally  be  obtained,  which 
would  be  an  ideal  condition  in  many  ways.  This  ideal  distri- 
bution of  the  work  current  over  the  brush  contact  area  will  be 
called  the  apparent  current  density  in  the  brush,  to  distinguish  it 
from  the  true  current  density,  which  is  due  to  the  resultant  current 
in  the  brush,  which  is  always  greater  than  the  work  current. 

The  principal  cause  of  the  difference  between  the  true  and  the 
apparent  densities  in  the  brush  lies  in  the  local  or  short-circuited 
current,  due  to  the  brush  short-circuit  voltage  just  described 
This  local  current  distributes  over  the  brush  contact  according  to 
the  short-circuited  voltages  under  the  brush  contact,  and  is  thus 
practically  zero  at  the  middle  of  the  brush,  and  maxitnum  at  the 
edges,  flowing  from  the  commutator  to  the  brush  at  one  side  of 
the  mid-point,  and  from  the  brush  to  the  commutator  at  the  other 
side.  It  thus  adds  to  the  work  current  at  one  side  of  the  brush, 
and  subtracts  from  it  at  the  other  side,  and,  not  infrequently,  the 
local  current  is  so  great,  relatively,  that  the  resultant  current  at 
one  brush  side  will  be  several  times  that  due  to  the  work  current, 
while  at  the  other  edge  it  will  actually  be  in  the  opposite  direction. 

This  condition  can  be  illustrated  by  Figs.  1,  2  and  3. 


SYNCHRONOUS  BOOSTERICONVERTERS 


443 


Fig.  1  represents  the  conditions  where  only  the  work  current 
flows. 

The  height  ab,  which  is  uniform,  represents  the  value  of  the 
work  current. 

Fig.  2  represents  the  conditions  under  the  brush  if  only  the 
local  current  is  considered  (on  the  assumption  that  the  field  due  to 
the  armature  work  current  is  present,  but  the  work  current  itself 
is  absent). 

ac  represents  the  maximum  current  in  one  direction  at  one 
edge  of  the  brush,  while  de  represents  an  equal  and  opposite  current 
at  the  other  edge. 


ftgl 


fig3. 


Fig.  3  represents  the  conditions  when  both  currents  are 
present.  At  one  edge  of  the  brush  the  current  is  excessive  com- 
pared to  the  work  current,  while  at  the  other  edge  the  current  is  in 
the  opposite  direction.  Obviously,  the  part  of  the  brush  between 
d  and  f  in  this  figure  is  not  only  useless,  but  is  worse  than  useless, 
for  it  not  only  does  not  carry  any  current  into  the  armature,  but 
actually  adds  to  the  current  carried  by  the  part  between  a  and  /. 
Therefore,  if  the  part  between  d  and/  were  actually  cut  away,  the 
remaining  part  between  a  and  /  would  not  be  worked  as  hard  as 
before.  This  diagram  represents  a-  somewhat  extreme  condition, 
but  is  not  an  unusual  one,  as  experience  has  shown,  for  in  a  great 
many  commutating  machines  in  actual  service,  improved  results 
have  been  obtained  by  narrowing  "the  brush  contact  a  certain 


444  ELECTRICAL  ENGINEERING  PAPERS 

amount.  Obviously,  the  apparent  current  density  in  the  brush 
would  be  represented  by  the  height  ab,  while  the  true  current 
density  would  be  represented  by  the  maximum  height  ag,  in  Fig-  3, 
which  may  be  several  times  as  great  as  the  height  ab. 

It  is  evident  from  consideration  of  the  above  figures  that  the 
conditions  would  be  greatly  improved  by  any  reduction  in  the 
value  of  the  local  or  short-circuit  current.  Narrowing  the  brush, 
as  mentioned  above,  is,  to  a  certain  extent,  effective.  This  reduces 
the  local  current,  but  at  the  same  time  it  reduces  the  effective  path 
for  the  work  current.  Another  partial  remedy  would  be  in  the  use 
of  higher  resistance  at  the  brush  contact,  such  as  is  furnished  by 
certain  makes  of  brush.  This  would  reduce  the  local  current 
without  reducing  the  area  of  the  brush  contact,  but  at  the  same 
time  it  introduces  resistance  in  the  path  of  the  work  current,  which 
is  practically  equivalent  to  reducing  the  area  of  the  path.  It  is, 
therefore,  to  a  certain  extent,  equivalent  to  narrowing  the  brush. 
A  third  and  more  satisfactory  method  is  to  reduce  the  inherent 
short-circuit  voltage  across  the  brush,  while  at  the  same  time 
retaining  the  full  width  of  the  brush.  This,  however,  is  a  question 
of  design  and  the  proportioning  of  the  machine  itself,  and  obviously 
such  modification  cannot  readily  be  supplied  to  a  machine  al- 
ready constructed.  This  method  of  correcting  trouble  will  be 
referred  to  again. 

The  above  figures  illustrating  the  effect  of  the  local  current, 
do  not  make  the  story  quite  as  bad  as  it  actually  is.  If  the  brush 
contact  resistance  in  a  given  brush  were  of  constant  value,  irres- 
pective of  the  current  in  it,  then  the  above  illustrated  conditions 
would  hold.  But  the  brush  resistance  is  actually  variable  in 
effect;  that  is,  at  ordinary  working  current  densities,  the  e.  m.  f. 
drop  across  the  brush  contact  does  not  increase  directly  with  the 
current,  but  at  a  much  less  rate.  This,  therefore,  is  equivalent 
to  a  decrease  in  the  resistance  of  the  brush  contact  with  increase 
in  current  and,  unfortunately,  this  decrease  is  very  pronounced, 
even  within  the  limits  of  permissible  current  densities.  Therefore, 
with  local  current  in  the  brush,  giving  high  densities  at  the  outer 
edges,  the  resistance  of  the  brush  may  be  so  reduced  as  to  give  even 
worse  distribution  than  indicated  by  Fig.  3. 

Cotisidering  next,  actual  permissible  brush  drops,  it  may  be 
noted  that,  as  the  local  current  enters  at  one  side  of  the  brush  and 
leaves  at  the  other  side,  the  contact  resistance  in  series  with  the 


SYNCHRONOUS  BOOSTER  CONVERTERS  445 

local'cttrrent  path  is  twice  the  ordinary  contact  resistance  between 
brush  and  commutator.  From  an  examination  of  a  large  amount 
of  data  on  brush  drops,  it  appears  that,  with  the  ordinary  com- 
mercial brushes,  there  is  about  1  to  1.25  volts  drop  between  the 
brush  and  the  commutator  when  carrying  currents  of  30  to  SO 
amperes  per  square  inch.  With  the  brush  contact  resistance 
indicated  by  these  drops,  it  is  evident  that  with  a  short-circuit 
voltage  of  2  to  2  ^  volts  across  the  brush,  a  local  current  could 
flow  which  would  have  a  value  at  the  brush  edges  corresponding 
to  a  current  density  of  30  to  50  amperes.  Assuming  a  short-cir- 
cuit voltage  which  would  give  a  density  of  50  amperes  per  square 
inch  at  the  edges,  then  with  a  work  current  flowing  which  also 
gives  an  apparent  current  density  of  50  amperes,  the  resultant 
density  at  one  brush  edge  would  become  zero,  while  at  the  other 
edge  it  would  become  100  amperes  per  square  inch.  With  brushes 
having  a  low  contact  resistance  the  conditions  would  be  worse, 
and  there  would  be  a  current  of  negative  direction  at  one  edge  of  the 
brush. 


In  practice,  an  inherent  brush  short-circuit  e.  m.  f  .  of  2  to 
volts  is  very  seldom  found,  as  it  is  too  low  a  value  for  commer- 
cial designs.  However,  with  much  higher  short-circuit  e.  m.  f  .'s 
the  conditions  would  obviously  be  very  much  worse  than  indicated 
above,  and  yet  in  commutating  machines  of  the  non-commutating 
pole  type,  inherent  short-circuit  e.  m.  f.'s  of  4  or  5  volts  would  be 
considered  relatively  low,  and  even  7  or  8  volts  would  not  be 
considered  unduly  high  in  some  cases.  Evidently,  with  such 
e.  m.  f.'s  actually  across  the  brush,  the  local  currents  in  the  brush 
should  be  excessive  and  there  should  be  severe  sparking  and 
burning  at  the  brushes  and  commutator.  However,  this  im- 
possible condition  is  overcome  to  a  considerable  extent  by  generat- 
ing an  opposing  voltage  in  the  short-circuited  coils.  This  result 
is  obtained  in  non-commutating  pole  machines  by  shifting  the 
brushes  toward  one  of  the  pole  corners  to  such  an  extent  that  the 
short-circuited  coils  are  cutting  across  a  small  part  of  the  main 
field  flux,  which  thus  generates  a  small  e.  m.  f  .  in  them.  The  shift 
of  the  brushes  must  always  be  in  such  a  direction  that  this  e.  m.  f  ., 
due  to  the  main  field,  is  in  opposition  to  the  short-circuit  e.  m.  f  . 

To  illustrate  the  above  case,  let  it  be  assumed  that  the  in- 
herent short-circuit  voltage  across  the  brush  at  full  load,  with  no 
lead  at  the  brushes,  is  six  volts.  This,  if  not  partially  neutralized, 
would  generate  an  unduly  high  local  current,  so  that  the  operating 


446  ELECTRICAL  ENGINEERING  PAPERS 

conditions  would  be  comparatively  bad.  Then,  assume  that  the 
brushes  are  shifted  so  that  the  short-circuited  coils  are  cutting 
across  a  main  field  flux  sufficient  to  give  three  volts.  As  this  is  in 
opposition  to  the  normal  short-circuit  e.  m.  f .,  the  resultant  short- 
circuit  e.  m.  f .  will  be  equal  to  6  —  3  =  3  volts,  which  would  not 
be  anything  like  as  bad  as  before.  If,  now,  the  load  is  removed 
from  the  machine,  the  brushes  still  retaining  their  lead,  the  three 
volts  due  to  the  main  field  will  still  be  generated  in  the  short- 
circuit  armature  coils,  and  there  will  be  a  no-load  short-circuit 
e.  m.  f.  of  three  volts,  which  would  set  up  a  local  short-circuit 
current.  However,  as  no  work  current  is  present  under  this  con- 
dition, the  short-circuit  current  could  obviously  be  practically  as 
great  as  the  maximum  value  of  the  resultant  current  at  the  full 
load  conditions.  Therefore,  if  three  volts  short-circuit  e.  m.  f .  is 
permissible  at  full  load,  then  four  or  five  volts  would  be  permissible 
at  no  load  with  practically  the  same  commutating  conditions  as  at 
full  load.  Therefore,  the  brush  could  be  shifted  forward  into  a 
field  representing  four  volts,  for  instance,  and  thus  at  no  load  the 
short-circuit  voltage  will  be  four  volts,  while  at  full  load  it  would  be 
6  —  4  =  2  volts.  Therefore,  by  this  means  an  impossible  com- 
mutating condition,  represented  by  no  lead  at  the  brushes,  be- 
comes a  possible  and  practicable  condition  by  giving  a  certain 
amount  of  lead.  On  non-commutating  pole  machines  where  a 
slight  amount  of  lead  is  almost  always  required,  a  resultant  short- 
circuit  e.  m.  f .  of  three  volts  across  the  brush  may  be  permissible, 
in  some  cases,  at  full  load,  but  this  cannot  be  assumed  to  be  true  in 
all  cases,  for  there  are  other  conditions,  besides  commutation,  which 
are  dependent  upon  the  amount  and  distribution  of  currents  in  the 
brush.  Of  these  other  effects,  the  principal  ones  may  be  classified 
as,  burning  of  the  commutator  and  brush  faces,  high  mica,  and 
picking  up  of  copper. 

"WEAR"  OR  "EATING  AWAY"  OF  COMMUTATOR  AND  BRUSHES 

An  elaborate  and  long  extended  series  of  tests  has  shown  that 
when  a  relatively  large  current  passes  from  a  brush  to  a  commutator 
or  collector  ring,  or  vice-versa,  there  is  a  tendency  for  undue 
"wear,"  as  it  might  be  called,  of  either  the  commutator  or  brush 
face,  depending  upon  the  direction  of  current.  If  the  current  is 
from  the  commutator  to  the  brush,  then  the  commutator  face 
"wears"  or  is  "eaten"  away,  while  with  the  current  from  the 
brush  to  the  commutator,  the  brush  shows  increased  wear.  This 


SYNCHRONOUS  BOOSTER  CONVERTERS  447 

is  not  a  true  mechanical  wearing  away  of  the  commutator  or 
brush,  but  is  more  like  an  electrolytic  action,  except  that  usually 
the  particles  taken  from  one  surface  do  not  deposit  on  the  other. 
This  rate  of  wear,  as  shown  by  test,  is  a  function  of  the  current 
density,  the  area  of  surface  through  which  the  current  passes,  and 
the  contact  drop.  It  is  not  directly  proportional  to  the  contact 
drop,  or  the  current,  but  increases  in  a  much  greater  proportion 
than  either,  or  possibly  even  more  rapidly  than  the  product  of  the 
two.  However,  this  is  difficult  to  determine  definitely,  for  with 
the  wear  once  started,  the  trouble  tends  to  accentuate  itself.  In 
other  words,  this  wear  will  increase  the  contact  drop  and  in  turn 
the  increase  in  contact  drop  will  exaggerate  the  wear,  so  that  the 
action  is  cumulative.  This  wearing  action  is  apparently  very  slight 
in  amount  at  true  brush  densities  of  SO  to  60  amperes  per  square 
inch,  with  carbon  brushes,  and  if  the  commutating  characteristics 
are  very  good,  even  much  greater  true  densities  are  practicable, 
possibly  up  to  100  amperes  per  square  inch.  If  the  apparent 
density  could  be  brought  up  to  the  true  density;  that  is,  if  no 
current  but  the  work  current  were  present,  then  this  high  current 
density  in  the  brush  might  be  utilized  in  well  designed  machines, 
but  this  implies  the  absence  of  all  local  currents,  also,  perfect 
division  of  the  current  between  the  various  brushes  and  brush 
arms,  as  will  be  referred  to  later.  These  two  conditions  are  rarely 
attained  in  practice,  and  it  would  probably  be  dangerous  to 
attempt  apparent  densities  of  100  amperes  per  square  inch  in  the 
ordinary  carbon  brush;  but  with  commutating  pole  machines, 
where  an  opposing  e.  m.  f .  is  generated  in  the  short-circuited 
armature  coils,  the  condition  of  relatively  small  local  currents  can 
be  obtained  by  very  careful  proportioning  of  the  commutating  pole 
field.  This  means  therefore  that  higher  current  densities  in  the 
brushes  are  feasible  in  commutating  pole  machines  in  general  than 
in  the  non-commutating  pole  type.  This  has  a  direct  bearing  on 
the  synchronous  converter  problem,  as  will  be  shown  later  when 
considering  high  speeds  and  marimum  outputs  with  a  given 
number  of  poles.  However,  the  condition  of  perfect  division  of 
current  between  the  different  brushes  has  not  been  obtained  in  any 
simple,  practical  manner,  and  therefore  some  margin  in  brush 
current  density  must  be  allowed,  even  in  commutating  pole  ma- 
chines. 


448  ELECTRICAL  ENGINEERING  PAPERS 

HIGH  MICA 

When  the  maximum  current  density  in  a  brush  contact  is 
comparatively  high,  due  to  local  currents  or  other  causes,  the 
commutator  and  brush  "wear"  may  be  relatively  rapid  compared 
with  the  mechanical  wear  due  to  friction  of  the  brushes  on  the 
commutator.  Under  this  apparent  wear  the  commutator  copper 
will  be  slowly  eaten  away  by  the  current,  but  the  commutator  mica 
will  not  be  materially  affected.  The  mica  must  wear  down  by  the 
mechanical  friction  of  the  brushes,  If  the  "eating  away"  of  the 
copper  exceeds  the  mechanical  wear  of  the  mica,  then  a  condition 
is  reached  which  tends  to  increase  the  def  ect.  As  soon  as  the  copper 
face  is  burned  even  an  infinitesimal  amount  below  the  mica,  the 
brush  face  tends  to  "ride"  on  the  mica  and  thus  has  a  reduced 
contact  on  the  copper  surface,  or  even  none  at  all.  This  condition 
increases  the  burning  action  and  eventually  results  in  the  so-called 
"high  mica"  where  there  is  an  actual  gap  between  the  brush  and 
the  commutator  face  Such  a  condition,  once  started,  does  not 
tend  to  cure  itself,  except  tinder  certain  special  conditions  of 
operation.  This  high  mica  is  frequently  charged  to  the  use  of 
"hard"  mica,  which  tends  to  produce  a  similar  condition. 

In  some  cases,  this  trouble  from  high  mica  may  not  be  due  to 
either  excessive  local  currents  or  hard  mica,  but  may  be  due  to  a 
relatively  high  proportion  of  mica  to  copper  surface.  Where 
comparatively  thin  commutator  bars  are  used  on  a  machine,  the 
thickness  of  mica  between  the  bars  is  not  reduced  in  proportion, 
so  that  the  percentage  of  mica  may  be  relatively  high.  In  con- 
sequence  of  this  high  percentage,  the  mica  itself  does  not  wear  as 
rapidly  as  where  a  less  total  amount  is  used,  while  the  copper  may 
eat  away  at  the  same  rate.  This  may  therefore  tend  toward  high 
mica,  even  where  the  local  currents  are  relatively  small.  This 
condition  of  high  percentage  of  mica  is  found  particularly  in  high 
voltage  machines  where  the  number  of  bars  is  necessarily  great 
and  the  thickness, of  each  bar  correspondingly  small.  On  the 
other  hand,  with  low  voltage  machines,  the  percentage  of  mica  is 
relatively  less,  but  other  conditions  may  enter  which  partly 
neutralize  this  advantage.  With  lower  voltages,  for  a  given 
capacity,  the  current  is  correspondingly  greater  and,  with  a  given 
contact  drop,  the  losses  are  correspondingly  increased  and  the 
tendency  to  produce  noise  by  the  brushes  is  also  greater.  To 
overcome  these  objectionable  features,  a  soft,  low  resistance  brush 
is  frequently  used.  This,  however,  increases  the  tendency  for 


SYNCHRONOUS  BOOSTER  CONVERTERS  449 

local  currents  and  thus  increases  the  copper  wear,  while  at  the  same 
time  a  softer  brush  has  less  grinding  action  on  the  mica.  There- 
fore, the  low-voltage  machine  may  also  tend  toward  high  mica. 

A  common,  and  very  effective,  remedy  for  this  tendency 
toward  high  mica  is  to  "undercut"  the  mica  so  that,  everywhere 
on  the  brush  wearing  surface,  it  lies  slightly  below  the  copper 
surface.  This  does  not  remove  the  initial  cause  of  the  trouble, 
namely,  the  tendency  to  eat  away  the  copper  surface.  But  it 
must  be  considered  that  this  initial  tendency  is  usually  very  slight, 
and  that  the  major  part  of  the  wear  is  due  to  the  lessening  of  the 
contact  between  the  brushes  and  the  copper,  thus  increasing  the 
burning  tendency.  In  consequence  of  undercutting  the  mica,  the 
brush  can  always  maintain  good  contact  with  the  commutator 
face,  and  thus  the  actual  burning  may  be  so  slow  as  to  be  practically 
negligible.  The  true  gain  from  undercutting  the  mica  thus  lies 
in  the  maintenance  of  more  intimate  contact  between  the  copper 
and  the  brush. 

This  eating  away  of  the  commutator  face  may  occur  in  service 
and  yet  the  commutator  may  polish  beautifully.  This  is  found 
in  some  cases  where  the  burning  action  is  pronounced,  and  yet  the 
conditions  of  operat?  jn  are  such  that  the  mica  can  be  worn  down 
mechanically  as  rapidly  as  the  copper  bums  away.  This  is  not 
infrequently  the  case  with  machines  where  there  are  heavy  peak 
loads  of  relatively  short  duration,  followed  by  very  much  longer 
periods  of  operation  with  but  little  load.  Under  such  conditions 
the  burning  action  during  the  peak  loads,  with  a  consequent 
tendency  to  high  mica,  is  hidden  by  the  grinding  action  of  the 
brushes  on  the  mica  during  the  long  periods  of  operation  at  light 
load,  so  that  the  mica  is  kept  practically  flush  with  the  copper  and 
the  copper  surface  is  polished.  That  real  burning  is  present  is 
often  indicated,  in  such  machines,  by  relatively  rapid  wear  on  the 
commutator  in  grooves  when  the  brushes  are  not  well  staggered. 

"PICKING  UP  COPPER" 

Another  condition  which  sometimes  accompanies  high  current 
density  in  the  brushes,  is  the  so-called  "picking  up  of  copper." 
Apparently,  under  some  conditions,  particles  of  copper,  eaten 
away  from  the  commutator  face,  will  collect  on  the  brush  face. 
This  may  result  in  glowing  at  the  brush  contact,  eventual  burning 
away  or  "honey-combing"  of  the  brush  surface  and  general 
trouble  at  the  commtitator.  This  difficulty  is  possibly  largely 
cumulative  in  its  action.  A  slight  qoating  of  copper,  or  copper 


450  ELECTRICAL  ENGINEERING. PAPERS 

"spots,"  may  form  on  a  brush.  This  gives  a  more  intimate,  or 
lower  resistance,  contact  with  the  commutator  face.  With  many 
brushes  in  parallel,  an  undue  percentage  of  the  total  current  may 
then  pass  through  this  one  point,  or  brush,  or  low  resistance  con- 
tact, and  the  current  density  at  this  point  may  even  become  so 
great  that  the  burning  will  be  excessive.  The  resistance  of  the 
carbon  brush,  in  itself,  does  not  help  this  condition,  for,  un- 
fortunately for  this  case,  carbon  has  a  negative  coefficient  of 
resistance  so  that  heating  lowers  its  resistance  and  thus  accentuates 
the  unequal  division  of  current.  One  remedy  for  this  condition 
is  a  more  uniform  contact  resistance  between  the  brush  and  the 
commutator.  Experience  has  shown  that  undercutting  the  mica 
will  frequently  overcome  this  difficulty  of  picking  up  copper, 
particularly  so  if  the  machine  can  be  "nursed"  until  the  com- 
mutator face  acquires  a  glaze.  In  some  cases,  a  different  grade 
of  brush  will  be  an  improvement,  but  it  is  generally  difficult  to 
predict  the  most  suitable  brush,  unless  the  inherent  commutating 
characteristics  of  the  machine  are  well  known.  This  picking  up 
of  copper  appears  to  be,  to  a  great  extent,  a  function  of  the  cur- 
rent density,  and  is  apparently  somewhat  of  an  electrolytic  action, 
the  copper  eating  away  from  the  commutator  and  depositing  upon 
the  brush.  Whatever  tends  to  materially  reduce  the  tendency 
for  the  commutator  face  to  eat  away,  also  tends  to  reduce  the 
picking-up  effect. 

The  foregoing  features,  while  apparently  minor  in  nature,  are 
all  of  fundamental  importance  in  commutating  machinery  in 
general,  and  particularly  so  in  the  case  of  commutating-pole 
rotary  converters,  especially  in  those  commutating-pole  rotaries 
which  have  what  might  be  called  self-contained  or  "auto"  regula- 
tion of  voltage,  such  as  those  with  synchronous  boosters,  or  with 
regulating  poles. 

COMMUTATING  POLES 

In  the  direct-current  generator  of  large  capacity  and  high 
speed  the  commutating  pole  has  proved  to  be  a  real  necessity. 
In  such  machines,  due  to  the  reduced  number  of  poles  and  high 
armature  ampere  turns  per  pole,  and  consequent  large  fields  or 
fluxes  set  up  by  the  armature,  together  with  the  high  speed,  the 
inherent  short-circuit  voltages  across  the  brush  have  reached 
excessive  values,  such  as  12  to  14  volts  at  normal  load.  Such 
e.  m.  f /s,  unless  largely  neutralized,  would  obviously  set  up  exces- 


SYNCHRONOUS  BOOSTER  CONVERTERS  451 

sive  short-circuit  currents  under  the  brush.  As  a  resultant  short- 
circuit  voltage  under  the  brush  of  about  2  volts  or  less  at  full  load 
is  desirable,  it  is  obvious  that  some  such  device  as  the  commut- 
ating  pole,  which  introduces  an  opposing  e.  m.  f.  in  the  short- 
circuited  armature  coils,  is  practically  a  necessity;  and,  further- 
more, this  opposing  e.  m.  f .  must  vary  practically  in  proportion  to 
the  load,  in  order  to  keep  within  the  permisssible  short-circuit  limits 
across  the  brush  at  all  loads.  Shifting  the  brushes  forward  into 
an  active  field  to  neutralize  12  volts,  for  instance,  is  obviously 
impracticable,  for  if  a  sufficient  opposing  e  m.f.,  such  as  10  volts, 
is  thus  introduced  into  the  short-circuited  coils  at  full  load,  then 
it  is  so  large  that  it  will  give  prohibitive  currents  at  no  load  if  the 
same  brush  lead  is  maintained.  Therefore  with  such  a  machine 
of  the  non-commutating  pole  type,  the  brushes  must  be  shifted 
with  the  load,  which,  in  many  cases,  is  not  a  practicable  condi- 
tion. Consequently,  the  commutating  pole,  with  its  neutralizing 
e.  m.  f .  varying  in  proportion  to  the  load,  is  a  necessary  device 
with  such  machines. 

In  the  rotary  converter,  however,  the  conditions  are  not  so 
severe.  On  account  of  the  alternating  and  direct  currents  in  the 
armature  winding  opposing  each  other,  the  resultant  armature 
magnetizing  effect  is  very  smaU  compared  with  that  of  a  corres- 
ponding D.  C.  generator.  Therefore  the  magnetic  fields  set  up  by 
the  armature  winding  are  relatively  much  smaller,  and  the  inherent 
short-circuit  e.  m.  f.'s  are  also  lessened.  Therefore,  the  speed, 
current,  number  of  poles,  etc.,  being  equal,  the  rotary  converter 
would  naturally  have  a  materially  lower  inherent  brush  short- 
circuit  e.  m.  f.  than  the  D.  C.  generator.  In  many  cases  this 
e.  m.  f .  may  be  within  the  permissible  limits  of  the  6  to  8  volts, 
when  the  brushes  are  to  be  given  a  fixed  lead,  while  the  corres- 
ponding *D.  C.  generator  might  have  10  to  12  volts,  which  cannot 
be  sufficiently  corrected  by  a  fixed  lead.  Therefore,  the  addition 
of  the  commutating  pole  to  the  rotary  converter  usually  will  not 
represent  the  same  gain  or  improvement  as  in  the  D.  C.  generator, 
and  its  use,  in  some  cases,  is  more  in  the  nature  of  a  refinement  of 
operation  than  an  absolute  necessity.  It  may  be  suggested  that, 
by  the  use  of  commutating  poles,  the  inherent  short-circuit  e.  m.  f . 
might  be  made  higher,  or  given  the  same  values  as  in  D.  C.  gen- 
erators, with  a  consequent  gain  in  cost  of  the  machine,  due  to  the 
use  of  higher  speeds  or 'a  reduced  annount  of  material  There 
might  be  some  saving,  wit&«sucli  a  procedure,  but,  on  the  other 


452  ELECTRICAL  ENGINEERING  PAPERS 

hand,  there  are  certain  operating  conditions  in  commutating  pole 
rotaries,  not  encountered  in  D.  C.  generators,  which  make  it 
inadvisable,  in  many  cases,  to  work  at  as  high  commutating 
limits  -as  on  commutating-pole  D.  C.  machines.  In  D.  C.  gener- 
ators the  armature  has  a  definite  magnetizing  action,  depending 
upon  the  current  carried,  and  this  magnetizing  action  is  always 
of  the  same  value  for  the  same  armature  current,regardless  of  speed, 
voltage,  or  any  other  condition.  The  function  of  the  commut- 
ating-pole winding  is  to  overcome  or  neutralize  this  armature 
magnetizing  effect  at  the  point  where  the  armature  coils  are 
short-circuited,  and  in  addition,  to  set  up  a  magnetic  field  in  the 
opposite  direction  to  that  which  the  armature  winding  will  tend 
to  establish.  A  positive  relation  is  thus  established  which  is 
practically  unaffected  by  conditions  of  operation. 

In  the  rotary  converter,  however,  the  conditions  are  somewhat 
different.  As  the  resultant  armature  ampere  turns  are  normally 
very  small,  the  commutating-pole  ampere  turns  required  are  cor- 
respondingly reduced,  and  have  a  much  smaller  value  than  on  a 
corresponding  D  C  machine.  If  the  resultant  armature  ampere 
turns  always  held  a  definite  value,  for  a  given  direct  current 
delivered,  under  all  conditions  of  operation,  then  the  commutating- 
pole  winding  could  readily  be  given  the  necessary  proportions  for 
setting  up  the  desired  commutating  field.  But  the  resultant 
armature  ampere  turns  in  the  rotary  can  vary  over  a  considerable 
range,  while  delivering  a  direct  current  of  practically  constant 
value,  and  consequently  with  a  constant  commutating-pole 
strength.  Obviously,  with  a  constant  commutating-pole  strength 
and  a  resultant  armature  magnetizing  effect  which  can  vary  over  a 
considerable  range,  the  resultant  short-circuit  e.  m.  f .  can  also 
vary  up  or  down,  while  commutating  a  given  current,  and,  if  the 
variation  is  excessive,  bad  commutating  conditions  will  result. 
As  the  average  value  of  the  resultant  ampere  turns  of  the  rotary 
converter  armature  is  only  about  15  percent  of  that  of  the  same 
armature  as  a  D.  C.  machine,  it  is  obviotis  that  a  relatively  small 
unbalancing  of  the  opposing  alternating  and  direct  currents  may 
give  a  great  increase  in  the  resultant  ampere  turns,  which  may 
greatly  disturb  the  commutating-pole  conditions  and  set  up 
relatively  large  resultant  brush  short-circuit  e.  m.  f.'s. 

As  such  disturbances  can  actually  occur  in  rotary  converters 
from  several  causes,  it  is  usually  advisable  to  make  the  inherent 
short-circuit  e.  m.  f  as  small  as  possible,  without  undue  sacrifice  in 


SYNCHRONOUS  BOOSTER  CONVERTERS  453 

the  design  of  the  machine.  One  condition  which  can  produce  the 
above  unbalancing  between  the  alternating  and  direct  currents  is 
"hunting."  When  a  rotary  hunts  it  alternately  stores  energy  in 
the  rotating  parts  and  returns  it  to  the  system,  during  which  the 
speed  of  the  rotary  oscillates  with  respect  to  the  frequency  of  the 
supply  system.  While  storing  energy  in  the  moving  parts  the 
alternating-current  in-put  is  higher  in  value  and,  in  restoring 
power  to  the  line,  is  lower  in  value  than  is  required  for  the  average 
D.  C.  output.  In  consequence,  where  hunting  occurs,  the  result- 
ant armature  ampere  turns  periodically  vary  in  value  and  there  is 
a  corresponding  periodic  short-circuit  voltage  across  the  brush 
which  may  reach  excessive  values  and  cause  vicious  sparking,  or 
even  flashing. 

Another  cause  of  variation  in  armature  reaction  is  found  in 
sudden  changes  of  load  on  a  rotary  converter.  When  a  sudden 
load  is  thrown  on,  the  rotary  may  momentarily  carry  part  of  its 
load  as  a  D.  C.  generator.  This  means  disturbance  of  the  corn- 
mutating  field,  in  the  wrong  direction,  at  the  very  moment  that 
this  field  should  be  at  its  best.  But  by  avoiding  too  high  normal 
short-circuit  voltages  in  the  armature  winding,  the  above  condi- 
tions of  undue  voltages  across  the  brush  can  be  relatively  lessened. 

In  rotaries  with  "  self  -contained "  regulation,  another  dis- 
turbance is  introduced,  which  will  be  described  later. 

RELATION  OP  SPEED  TO  CURRENT  CAPACITY,  ETC. 

In  the  design  of  all  rotating  machines  for  transformation  pur- 
poses, as  high  speeds  should  be  chosen  as  conditions  of  economical 
design  will  allow.  In  D.  C.  generators,  the  speeds  and  the  number 
of  poles  have  no  rigid  relation  to  each  other.  Thus,  a  1000  kw,  500 
r.  p.  m.  generator  could  have  from  4  to  12  poles,  as  desired.  For 
600  volts,  and  corresponding  currents,  it  could  have  6  poles,  for 
instance.  For  half  this  voltage,  with  twice  the  current,  it  could 
have  12  poles,  with  the  same  speed.  There  is  therefore  a  certain 
freedom  in  the  design  of  such  a  machine. 

In  the  rotary  converter,  however,  the  above  condition  is 
absent.  The  frequency  is  fixed,  which  at  once  fixes  the  relation 
of  the  number  of  poles  to  the  revolutions  per  minute,  for  the 
frequency  is  the  product  of  the  two.  Therefore,  if  a  600  volt,  1000 
kw  25  cycle  rotary  converter  would  require  6  poles  at  500  revolu- 
tions, then  a  machine  with  half  this  voltage  and  twice  the  current 
end  with  12  poles,  must  operate  at  250  revolutions,  and  not  500. 


454  ELECTRICAL  ENGINEERING  PAPERS 

In  rotaries  where  the  current  per  brush  arm,  an£  per  pole,  is  at  the 
highest  permissible  limit,  the  number  of  poles  must  vary  directly 
and  the  speed  inversely,  as  the  total  current  to  be  handled.  Thus, 
for  example  a  270  volt  rotary  of  large  capacity  will  inherently  have 
more  poles,  and  will  run  at  a  lower  speed,  than  a  600  volt  rotary 
of  equal  capacity,  which  is  not  necessarily  the  case  with  D.  C. 
generators. 

The  minimum  number  of  poles  in  either  a  rotary  converter  or  a 
D.  C.  generator  is  practically  fixed  by  the  direct  current  to  be 
handled.  There  is  a  practical  limit  to  the  current  per  brush  arm, 
as  fixed  by  the  permissible  current  density  in  the  brushes  and  the 
permissible  breadth  of  the  commutator  face.  There  are  physical 
conditions  which  limit  the  breadth  of  the  commutator  face,  de- 
pending upon  the  speed,  expansion  conditions  under  temperature, 
etc.  The  maximum  breadth  being  determined  for  any  given  case, 
the  circumferential  thickness  of  the  brushes  being  fixed  by  limits 
of  inherent  short-circuit  e.  m.  f .,  and  the  current  density  in  the 
brushes  being  fixed  by  limits  of  brush  and  commutator  wear,  as 
before  described,  it  follows  that  the  maximum  current  per  brush 
arm  is  pretty  definitely  fixed,  with  present  constructions.  For  a 
given  total  output  in  current,  the  limiting  current  per  brush  arm 
thus  fixes  the  total  number  of  brush  arms  and  poles,  and  thus  fixes 
the  speed  for  a  given  frequency.  These  limiting  conditions  are 
pretty  closely  approached  in  recent  25  cycle  rotaries  of  the  com- 
mutating  pole  type. 

LIMITING  CURRENT  PER  BRUSH  ARM 

As  indicated  above,  this  is  a  function  of  the  length  of  the 
commutator,  which  depends,  to  some  extent,  upon  the  peripheral 
speed  of  the  commutator  face.  With  25  cycle  rotaries,  consider- 
ably lower  peripheral  speeds  are  obtainable  than  with  60  cycle 
rotaries,  without  unduly  decreasing  the  distance  between  adjacent 
brush  arms  or  neutral  points  The  peripheral  speed,  in  feet  per 
minute,  of  any  commutator  is  equal  to  the  distance  in  feet  between 
two  adjacent  neutral  points,  multiplied  by  the  frequency  in  alter- 
nations per  minute  \  thus,  with  25  cycles  per  second  (or  3000 
alternations  per  minute)  with  one  foot,  or  12",  between  adjacent 
neutral  points,  the  commutator  peripheral  speed  will  be  3000  ft. 
per  minute.  With  60  cycles  per  second  (7200  alternations  per 
minute)  with  8",  or  2-3  ft.  between  adjacent  neutral  points,  the 
peripheral  speed  of  the  commutator  will  be  two-thirds  of  7  200  = 


SYNCHRONOUS  BOOSTER  CONVERTERS  455 

4800  ft.  per  minute.  Or,  in  other  words,  with  equal  peripheral 
speeds,  the  25  cycle  rotary  can*  have  2.4  times  as  great  distance 
between  neutral  points- as  a  60  cycle  machine.  The  above  relation 
of  commutator  speed  to  frequency  holds  true  regardless  of  the 
number  of  poles.  It  therefore  follows  that,  as  the  25  cyde  ma- 
chine can  have  much  lower  peripheral  speed  at  the  commutator, 
the  difficulties  of  building  the  commutators  should  be  very  much 
less.  It  should  therefore  be  practicable  to  build  much  wider 
commutators  for  25  cycle  rotaries  than  for  60  cycle,  and  experience 
bears  this  out.  With  the  wider  commutators,  at  25  cycles,  the 
brush  bearing  surface  is  increased,  and  thus  with  a  given  width  of 
brush,  the  number  of  brushes  per  arm  can  be  correspondingly 
greater  than  for  60  cycles. 

In  the  second  place,  even  with  considerably  lower  peripheral 
speeds  at  the  commutator,  the  thickness  of  the  commutator  bars 
will  be  considerably  greater,  in  most  cases,  than  can  be  used  on 
60  cycle  machines  of  the  same  rated  voltage.  In  consequence, 
with  a  given  thickness  of  brush,  fewer  bars  will  be  short-circuited 
on  the  25  cycle  machine,  than  on  the  60  cycle,  and  therefore,  in 
general,  somewhat  thicker  brushes  are  permissible  for  given 
inherent  brush  short-circuit  limits.  This,  again,  allows  more 
current  per  brush,  so  that  the  25  cycle  machine  has  an  advantage 
in  total  current  per  arm,  due  to  the  thickness  of  brushes,  and  to 
the  number  of  brushes  which  can  be  used  per  arm.  On  the  basis 
of  a  brush  %"  thick,  and  a  current  density  of  50  amperes  per  square 
inch,  experience  shows  that  a  normal  rated  current  of  about  1000 
amperes  per  brush  arm  is  possible  on  large  25  cyde  rotaries  which 
are  designed  to  carry  heavy  overloads  for  moderate  periods,  such 
as  two  hours.  With  such  brush  thickness,  these  rotaries  can  be 
designed  for  moderately  low  inherent  short-circuit  voltages  and 
abnormal  refinement  in  proportioning  of  the  commutating  pole 
dimensions  is  not  required,  as  extremely  dose  adjustment  of  the 
resultant  short-circuit  voltage  is  unnecessary.  With  thicker 
brushes,  such  as  1"  instead  of  ^",  it  is  possible  to  operate  at 
somewhat  higher  current  per  arm,  possibly  up  to  1200  amperes, 
but  this  is  at  a  certain  expense  in  higher  inherent  short-circuit 
e.  m.  f  ,'s  and  less  all-around  margin  in  general.  With  the  thicker 
brush  there  is  necessarily  a  greater  tendency  for  local  currents, 
and  therefore  doser  proportioning  of  the  commutating  poles  is 
required.  However,  with  equally  careful  proportioning,  with 
the  %"  thickness  of  brush,  the  results  would  be  relatively  better 
also. 


456  ELECTRICAL  ENGINEERING  PAPERS 

One  of  the  possible  troubles  with  very  heavy  currents  per 
brush  arm,  lies  in  the  difficulty  of  obtaining  equal  division  of 
current  among  all  the  various  brushes  per  -arm.  The  possibility 
of  trouble  is  apparently  considerably  increased,  the  greater  the 
current  per  arm,  and  if  this  greater  current  per  brush  arm  is  ob- 
tained by  the  use  of  thicker  brushes  rather  than  by  greater  length 
of  commutator,  then  the  result  is  practically  equivalent  to  working 
the  machine  harder,  or  nearer  the  limit.  If  the  operation  of  two 
commutators  be  compared,  one  with  a  %"  thickness  of  brush 
and  the  other  with  a  1"  brush,  both  having  such  brush  capacity 
that  they  are  worked  at  equal  apparent  current  densities,  then, 
other  conditions  being  equal,  the  commutator  with  the  %" 
brush  will  be  found  in  general  to  give  superior  results.  And  ex- 
perience has  shown  that  in  many  cases  the  1"  brush  can  have  its 
width  cut  down  to  %"  width,  with  apparent  improvement  in 
operation.  However,  if  both  the  %"  and  1"  brush  actually  show 
the  same  true  current  density;  that  is,  including  all  local  currents 
and  unbalancing  of  current  between  brushes,  then  with  equally 
well  proportioned  commutating  poles,  there  should  be  but  little 
difference  in  the  operation  with  the  two  thicknesses  of  brushes. 

Assuming  1000  amperes  as  representing  the  limiting  current 
per  arm  with  %"  brushes  on  25  cycle  machines,  then  on  60  cycle 
rotaries,  which  usually  have  brushes  of  less  than  %"  thickness, 
and  considerably  narrower  commutators  on  account  of  higher 
peripheral  speeds,  the  maximum  rated  current  per  arm  will  be  in 
the  neighborhood  of  600  amperes.  This  smaller  current  per  arm 
should  apparently  handicap  the  60  cycle  machine  compared  with 
the  25  cycle,  but,  in  compensation,  on  the  basis  of  equal  revolu- 
tions per  minute,  a  60  cycle  rotary  will  have  2.4  times  as  many 
brush  arms,  which  more  than  makes  up  for  the  lower  current  per 
arm.  Therefore,  from  this  standpoint  it  should  be  feasible  to 
operate  the  60  cycle  rotary  at  considerably  higher  speed  than  the 
25  cycle.  This,  however,  has  not  been  carried  to  the  limit,  in 
present  practice,  as  the  speeds  which  would  be  obtained  would  be 
so  high,  in  some  cases,  that  present  commercial  conditions  will  not 
allow  them.  This  means,  therefore,  that  we  have  probably  not 
yet  reached  the  possible  maximum  speeds  which  are  practicable 
with  60  cydes. 


SYCHRONOUS  BOOSTER  CONVERTERS  457 

E-  M.  F.  REGULATION  OF  ROTARY  CONVERTERS 

There  are  three  well-known  methods  for  varying  the  D.  C. 
e.  m.  f .  of  rotary  converters,  with  a  fixed  A.  C.  supply  voltage. 
These  three  are  known  as  the  induction  regulator,  the  syn- 
chronous booster,  and  the  regulating-pole  methods  of  control. 
In  the  induction  regulator  method,  an  induction  regulator  varies 
the  A  C.  voltage  up  or  down  over  the  range  necessary  to  give  the 
desired  D.  C.  voltage  change.  In  the  synchronous  booster 
method,  an  A.  C.  generator  of  a  capacity  corresponding  to  half 
the  range  of  control  is  operated  synchronously  with  the  rotary 
converter  and,  by  means  of  direct-current  field  control  of  this 
booster,  the  A.  C.  e.  m.  f .  supplied  to  the  rotary  is  varied  up  or 
down.  In  the  third  method  each  main  pole  of  the  rotary  proper 
is  made  up  of  two  or  more  smaller  poles,  one  or  more  of  which  may 
have  the  excitation  varied  and  by  this  means  the  ratio  of  the  D.  C. 
to  the  A.  C.  e.  m.  f .,  in  the  rotary  converter  armature  itself,  may  be 
changed. 

Each  of  these  three  methods  has  certain  possibilities,  advan- 
tages, and  disadvantages,  depending  upon  the  conditions  of  oper- 
ation The  induction  regulator  method  has  been  used  very  con- 
siderably in  the  past,  but  is  but  little  advocated,  in  more  recent 
work,  due  probably  to  the  fact  that  it  is  more  complicated  and 
expensive  than  other  methods  Both  the  synchronous  booster 
and  the  regulating  pole  methods  of  voltage  regulation  have 
been  used  more  or  less  extensively,  however,  principally  with- 
out commutating  poles.  With  the  introduction  of  the  latter,  a 
new  problem  enters,  which  has  a  very  considerable  bearing  on  the 
design  of  such  apparatus,  especially  in  machines  of  very  large 
current  capacity  where  the  maximum  permissible  current  per 
brush  arm  is  approximated.  This  problem  lies  in  the  variable 
armature  magnetizing  force  of  the  rotary,  with  change  in  D. 
C.  e.  m.  f ,  while  delivering  a  given  current.  Obviously,  if 
the  resultant  armature  ampere  turns  vary,  the  commutating-pole 
ampere  tun^  should  vary  a  corresponding  amount.  But  if  the 
commutating-pole  winding  is  in  series  with  the  direct-current 
armature  current,  which  may  not  be  varied  with  change  in  voltage, 
the  desired  conditions  are  not  met  by  such  an  arrangement.  In 
this  lies  the  real  problem. 

In  the  rotary  converter  with  synchronous  booster,  but  with- 
out commutetmg  poles,  the  dffictaity<of  variable  armature  reaction 


458  ELECTRICAL  ENGINEERING  PAPERS 

such  as  indicated  above,  exists  also,  but  is  usually  not  serious,  as 
indicated  by  the  following: 

In  a  rotary  converter  without  synchronous  booster  or  regulat- 
ing poles,  the  ratio  of  the  alternating  current  to  the  direct  current 
is  in  normal  operation  pretty  definitely  fixed.  The  two  currents 
oppose  each  other  in  the  armature  winding  to  such  an  extent  that 
the  resultant  ampere  turns  vary  between  about  7  percent  and  22 
percent  of  the  value  in  a  D.  C.  machine,  or  with  a  mean  of  about 
17  percent,  when  a  full  pitch  armature  winding  is  used.  When  a 
"fractional  pitch"  or  "chorded"  winding  is  used,  this  value  is 
reduced,  depending  upon  the  amount  of  chording.  This  small 
resultant  acts  in  the  same  direction  as  on  a  D.  C.  machine,  and  sets 
up  a  small  field  which  affects  the  commutation  slightly.  Any- 
thing which  will  increase  the  ratio  of  the  alternating-current 
in-put  to  the  direct  current  will  tend  to  reduce  the  resultant 
armature  ampere  turns,  for  normally  the  D  C.  effect  is  slightly  in 
excess.  Therefore,  if  the  rotary  should  act,  to  a  certain  extent 
as  a  motor,  thus  receiving  some  A.  C.  in-put  which  is  not  trans- 
formed to  direct  current,  the  resultant  armature  ampere  turns  will 
be  reduced,  and  may  even  pass  the  zero  value  and  be  in  the  op- 
posite direction. 

Again,  if  the  rotary  converter  armature  transforms  some 
mechanical  power  received  at  its  shaft,  into  direct  current,  so  that 
the  direct-current  output  is  correspondingly  greater  than  the  A.  C. 
input,  then  the  resultant  armature  ampere  turns  will  be  increased. 

In  the  synchronous  booster  method  of  regulation,  the  above  is 
just  what  happens.  The  normal  A.  C.  e.  m.  f .  corresponds  to  the 
midway  point  on  the  D.  C.  e.  m.  f.  range.  When  the  booster 
neither  "boosts"  nor  "bucks,"  the  alternating  current  supplied 
corresponds  properly  to  the  direct  current  delivered,  and  the 
resultant  armature  ampere  turns  have  a  mean  value  of  17  percent 
approximately,  assuming  a  full  pitch  winding.  If  the  D.  C.  e.  m.  f . 
is  boosted  15  percent,  for  example,  the  A.  C.  supply  e.  m.  f.  re- 
maining constant,  then  obviously  the  current  supplied  to  the" 
alternating  end  is  increased  with  respect  to  the  current  delivered 
by  the  D.  C.  end,  in  the  ratio  of  the  percentage  boost.  Therefore, 
the  normal  resultant  armature  ampere  turns  are  reduced  to 
17  —  15  —  2  percent. 

Again,  when  the  D.  C.  e.  m.  f.  is  reduced  IS  percent,  the 
direct  current  is  increased  IS  percent  relatively  to  the  A.  C.  and 
the  resultant  armature  amperes  are  increased  15  percent,  and 


SYCHRONOUS  BOOSTER  CONVERTERS  459 

become  17  -h  15  =  32  percent.  Therefore,  with  a  boost  and  buck 
of  IS  percent  voltage,  while  carrying  the  same  direct-current  load, 
the  resultant  armature  reaction  would  be  varied  from  2  percent 
to  32  percent  of  that  of  a  D.  C.  armature.  This,  however,  is  not 
serious  in  a  rotary  converter  without  commutating  poles,  as  even 
with  32  percent  armature  reaction,  the  conditions  are  much  better 
than  in  a  D.  C.  machine  where  the  armature  reaction  is  100  percent. 

But  when  commutating  poles  are  introduced  the  conditions 
are  quite  different.  The  commutating-pole  winding  normally 
should  be  equal  to  the  effective  or  resultant  armature  ampere  turns, 
plus  the  magnetizing  ampere  turns  for  setting  up  the  required 
magnetic  field  under  the  commutating  poles  This  latter  com- 
ponent usually  is  small.  Counting  the  effective  armature  ampere 
turns  as  17  percent  of  that  of  a  D.  C.  armature,  and  assuming  the 
magnetizing  component  as  25  percent,  then  normally  the  total 
commutating  pole  turns  would  be  42  percent.  If  this  42  percent 
is  furnished  by  series  excitation  from  the  D.  C.  end  of  the  rotary, 
then  it  will  be  constant  in  value,  with  a  constant  value  of  the  direct 
current,  regardless  of  the  variations  in  the  D.  C.  e.  m.  f. 

Now,  suppose  the  D.  C.  voltage  is  boosted  IS  percent  by 
means  of  a  synchronous  booster,  then  the  resultant  armature 
ampere  turns  fall  to  2  percent,  as  shown  before,  and,  the  commut- 
ating-pole ampere  turns  remaining  at  42  percent,  the  difference, 
which  is  40  percent,  wiH  all  become  magnetizing.  Therefore,  with 
a  boost  of  IS  percent,  the  magnetizing  component  of  the  com- 
mutating-pole winding  is  increased  from  25  percent  to  40  percent, 
although  the  current  to  be  commutated  is  unchanged.  In  the 
same  way,  if  the  D.  C.  voltage  is  bucked  15  percent,  then  the 
armature  ampere  turns  become  32  percent  and  the  magnetizing 
component  of  the  commutating-pole  field  winding  becomes  42  — 
32  =  10  percent,  when  it  should  be  25  percent.  Therefore,  the 
commutating  field  strength  actually  varies  up  or  down  60  percent 
from  the  required  value,  due  to  the  synchronous  booster  action, 
when,  in  reality,  it  should  remain  constant. 

If  a  resultant  short-circuit  e.  m.  f .  of  3  volts  across  the  brushes 
were  aJlowed,  then,  this  60  percent  variation  in  the  commutating- 
pole  strength,  would  mean  that  the  inherent  short-circuit  e.  m.  f . 
is  only  5  volts,  which  is  normally  neutralized  by  the  commutating 
field.  However,  an  inherent  short-circuit  e.  m.  f .  of  5  volts  is  so 
low  that  it  wou!4  require  a  rather  difficult  and  expensive  design, 
and  therefore  seven  to  8  volts  inherent  short-circuit  e.  m.  f .  should 


460  ELECTRICAL  ENGINEERING  PAPERS 

be  considered  in  most  cases.  Obviously,  with  the  above  conditions 
of  variable  armature  reaction,  this  would  lead  to  vicious  sparking 
conditions,  especially  at  heavy  overload,  or  at  no-load  conditions. 
Therefore,  series  excitation  of  the  commutating  pole  by  the  direct 
current  delivered,  should  not  give  satisfactory  results.  What  is 
needed  is  a  variation  in  the  commutating  pole  excitation  in  accord- 
ance with  any  changes  in  the  armature  reaction  of  the  rotary ;  that 
is,  a  reduced  excitation  at  boost  and  increased  excitation  at  buck. 

Looking  at  the  variable  elements,  it  may  be  seen  that  the 
field  current  of  the  synchronous  booster  had  its  current  in  one 
direction  at  boost  and  the  reverse  direction  at  buck.  Herein 
would  appear  to  be  a  solution  of  the  problem,  by  putting  the 
booster  field  current  through  an  auxiliary  winding  on  the  comwmt- 
ating  fole,  so  that  it  opposes  the  series  commutating-pole  coil  at 
boost  and  adds  to  it  at  buck.  At  first  thought,  this  seems  to  fit 
the  conditions  perfectly,  and,  in  fact,  it  does,  at  one  definite  direct 
current  delivered,  but  does  not  do  it  perfectly  at  other  loads. 
This  is  shown  by  the  following  figures.  Assume  the  preceding 
value  of  42  percent  series  ampere  turns  on  the  commutating-pole, 
with  an  additional  auxiliary  winding  having  the  same  percent 
ampere  turns  at  full  load  as  the  percentage  boost  or  buck.  For 
example,  with  IS  percent  boost,  then  at  full  load  the  auxiliary 
winding  has  15  percent  ampere  turns,  which  are  in  opposition  to 
the  42  percent  series  turns,  while  at  15  percent  buck,  at  full  load, 

the  15  percent  auxiliary  winding  acts  with  the  42  percent  series. 

( 
With  15  percent  boost  at  full  load,  the  armature  reaction  is 

lessened  by  15  percent,  and  the  total  commutating  field  excitation 
is  also  reduced  15  percent  by  means  of  the  auxiliary  winding. 
Thus  the  resultant  magnetizing  component  of  the  field  winding 
remains  at  the  required  25  percent.  At  no  boost  or  buck,  where 
there  is  no  current  in  the  booster  field  and  auxiliary  commutat- 
ing-pole circuit,  the  resultant  magnetizing  component  of  the  com- 
mutating-field  winding  remains  at  25  percent,  as  explained  before. 
When  the  booster  field  is  reversed,  in  order  to  buck  the  A.  C.  voltage, 
the  auxiliary  field  ampere  turns  on  the  commutating  pole  also 
are  reversed,  and  at  15  percent  buck  they  add  15  percent  to  the 
series  commutating-pole  winding,  and  thus  give  an  effective  mag- 
netizing value  of  25  percent  instead  of  10  percent,  as  given  before. 
Hence,  with  this  arrangement,  the  resultant  commutating-field 
strength  is  correct  for  all  the  voltages,  at  the  assumed  full  load 
current. 


SYNCHRONOUS  BOOSTER  CONVERTERS  461 

Considering,  next,  the  half-load  condition,  then  the  armature 
Ampere  turns,  both  A.  C.  and  D.  C.  are  halved  and  the  resultant 
armature  reaction  is  also  halved.  However,  for  the  same  percent- 
age boost  or  buck  in  D.  C.  voltage,  the  synchronous  booster  must 
operate  over  the  same  voltage  range  as  at  full  load,  and  therefore,  if 
the  booster  field  current  is  the  same  for  the  same  voltage  range, 
regardless  of  load,  then  the  auxiliary  winding  on  the  commutating 
pole  adds  or  subtracts  15  percent,  when,  for  correct  commutating- 
field  conditions,  it  should  add  or  subtract  only  7%  percent. 
Therefore,  the  excess  field  strength  at  the  two  extremes  of  voltage 
is  7J^  percent,  or  30  percent  of  the  normal  full  load  magnetizing 
component  of  the  commutating-pole  winding  of  25  percent,  which 
was  assumed  as  that  required  to  neutralize  the  assumed  inherent 
brush  short-circuit  e.  m.  f.  of  8  volts.  A  30  percent  component 
of  this  would  mean  2  4  resultant  volts  across  the  brush.  Prac- 
tically the  same  condition  would  also  be  found  at  50  percent  over- 
load. This  apparently  would  not  be  a  prohibitive  condition  if  it 
represented  the  full  range  of  operation.  At  no-load,  however,  the 
excess  effect  of  the  auxiliary  winding  would  be  15  percent  instead 
of  7J^  percent,  giving  a  magnetizing  component  equal  to  60 
percent  of  the  normal  magnetizing  effect  of  the  commutating-pole 
winding,  or  4.8  resultant  volts  across  the  brush,  which  is  higher 
than  desirable.  The  above  arrangement  therefore  fails  for  extreme 
changes  in  load,  if  the  synchronous  booster  excitation  is  constant 
for  a  given  percentage  boost  or  buck,  independent  of  the  load  on 
the  rotary.  What  is  required  with  this  scheme  is  an  excitation  of 
the  synchronous  booster,  which,  for  the  same  range  of  voltage 
variation,  increases  and  decreases  with  the  load  on  the  rotary. 
If,  for  instance,  the  15  percent  boost  or  buck  could  be  obtained 
at  no-load  on  the  rotary,  with  one-half  the  fijeld  excitation  that 
would  be  required  for  full  load,  then  the  excess  ampere  turns  in  the 
auxiliary  winding  on  the  commutating  pole  would  be  only  7% 
percent  total,  at  no  load,  instead  of  the  15  percent  indicated  above, 
and  the  resultant  short-circuit  e.  m.  f .  across  the  brush  at  no-load 
would  be  2.4  volts,  which  is  entirely  practicable. 

From  the  above  analysis,  the  solution  of  this  problem  is  in- 
dicated. It  lies  in  giving  the  synchronous  booster  such 
characteristics  that  its  field  current  varies  greatly  with  change  in 
the  load  on  the  machine.  This  can  be  done  in  various  ways,  but 
most  readily  by  designing  the  synchronous  booster  with  relatively 

ampere  turns  on  its  armature  compared  with  its  field  ampere 


462  ELECTRICAL  ENGINEERING  PAPERS 

turns,  which  is  the  very  construction  needed  for  making  the  most 
efficient  and  least  expensive  booster.  In  such  a  booster,  with  very 
high  armature  reaction,  the  field  current  can  be  made  to  vary  over 
a  relatively  wide  range,  with  a  given  percentage  boost  or  buck, 
with  any  considerable  changes  in  the  armature  current.  With 
this  construction  therefore,  it  is  practicable  to  build  a  synchronous 
booster  type  of  rotary  converter  with  commutating  poles  which 
will  automatically  adjust  its  commutating-pole  exciting  conditions 
to  suit  changes  in  both  load  and  voltage,  and  thus  there  is  no  occasion 
to  revert  to  the  induction  regulator,  or  other  outside  means  of 
control. 

As  a  proof  of  the  correctness  of  the  above  principles,  may  be 
cited  the  largest  capacity  synchronous  booster,  commutating-pole 
rotary  converters  yet  built,  namely  those  recently  furnished  to  the 


PIG  4,    NEW  YORK  EDISON  3500  KW   SYNCHRONOUS  BOOSTER  COMMU- 
TATING POLE  ROTARIES. 

New  York  Edison  Company,  one  of  which  is  shown  in  Fig.  4. 
These  machines  have  a  normal  continuous  rating  of  3500  kw  at 
270  volts,  and  13,000  amperes  D.  C.  They  must  also  carry  50 


SYNCHRONOUS  BOOSTER  CONVERTERS        ~         463 

percent  higher  current  for  two  hours,  or  19,000  amperes  at  270 
volts.  In  addition,  by  means  of  their  synchronous  boosters,  they 
can  vary  the  voltage  from  270  up  to  310  or  down  to  230,  while  still 
carrying  the  rated  current.  As  these  are  the  most  remarkable 
machines  of  this  type  yet  constructed,  a  more  complete  decrip* 
tion  of  them  will  be  in  order. 

The  contract  included  five  machines  of  3500  kw,  of  the 
horizontal  shaft  type,  and  two  machines  of  3000  kw  of  the  vertical 
shaft  type,  these  latter  to  fit  existing  foundation  plans.  Each 
3500  kw  machine  has  a  normal  rating  of  270  volts  D.  C.  at  13,000 
amperes,  and  is  arranged  to  boost  and  buck  approximately  15 
percent.  The  synchronous  booster  therefore  has  a  normal 
capacity  of  about  525  kw.  The  A.  C.  end  of  the  rotary  is  arranged 
for  6-phase  double-delta  connection,  requiring  about  165  volts 
normal.  The  alternating  current  handled  by  each  of  the  six 
collector  rings  is  enormous,  being  approximately  6300  amperes. 
As  the  rotary  has  28  poles,  the  current  from  each  collector  ring  is 
carried  by  14  leads,  through  the  armature  windings  of  the  booster, 
to  the  rotary  converter  armature,  where  it  divides  into  28  paths,  or 
one  per  pole,  in  the  usual  manner.  The  normal  alternating 
current  per  armature  circuit  in  .the  booster  thus  becomes  450 
amperes,  and  in  the  rotary  it  is  225  amperes.  As  the  machine 
has  6  collector  rings,  with  14  leads  per  ring,  there  are  84  windings 
on  the  synchronous  booster.  Each  winding,  however,  simply 
consists  of  a  single  group  of  coils.  As  the  booster  has  28  poles,  the 
same  as  the  rotary  itself,  it  has  therefore  three  groups  of  coils  per 
pole,  the  same  as  an  ordinary  three-phase  generator.  The  booster 
armature  is  therefore  simply  an  ordinary  type  of  three-phase 
generator,  except  that  the  various  groups  of  coils  in  each  phase  of 
the  armature  are  not  connected  in  series,  but  are  in  reality  con- 
nected in  parallel  at  the  collector  rings  and  at  the  main  armature 
winding.  This  arrangement  of  the  booster  armature  between  the 
collector  rings  and  the  rotary  converter  armature  thus  presents  a 
relatively  simple  arrangement,  and  tends  toward  compactmess 
and  symmetry  in  the  complete  armature  ttnit,  as  shown  in  Fig.  5. 

The  brushes  on  the  collector  rings  are  of  a  metal-carbon 
type,  arranged  in  box-holders  somewhat  like  ordinary  carbon 
brushes.  The  type  of  metal-carbon  brushes  used  has  a  very  low 
contact  drop  under  normal  operation,  being  approximately  1-10 
that  of  ordinary  carbon  brushes.  The  total  number  of  brushes 
per  ring  is  20,  and  each  brush  has  a  section  of  2.15  square  inches. 


464  ELECTRICAL  ENGINEERING  PAPERS 

thus  giving  a  normal  current  density  of  147  amperes  per  square 
inch- 
On  the  direct-current  end  there  are  28  brush  arms,  giving  a 
normal  rated  current  per  brush  arm  of  930  amperes,  approximately, 
and,  for  the  two  hours  overload,  of  1400  amperes  approximately. 
There  are  15  brushes  per  arm,  each  of  %"  x  1%"  section,  thus 
giving  an  apparent  current  density  of  47}^  amperes  per  square 
inch. 

The  armature  winding  of  this  rotary  converter  is  thoroughly 
cross  connected  in  order  to  equalize  the  circuits, — a  point  of  very 
considerable  importance  in  commutating  pole  machines.  The 
field  poles  are  also  equipped  with  heavy,  well  distributed  copper 
dampers  in  order  to  destroy  any  tendency  to  hunt,  which  is  a  very 
important  condition  in  commutating-pole  rotaries,  as  previously 
explained. 

The  air  gap  -under  each  main  field  pole  is  one-half  inch.  The 
use  of  this  large  gap  naturally  lessens  any  tendency  for  magnetic 
noises.  As  the  brushes  are  of  a  lubricating  type,  and  as  the  brush 
holders  have  special  devices  for  adjusting  the  brush  tension  very 
accurately,  the  machines  run  very  quietly.  The  commutator 
mica  is  undercut  about  1-32  inch. 

As  these  rotaries  are  equipped  with  both  synchronous  boosters 
and  commutating  poles,  very  careful  designing  as  regards  commut- 
ation characteristics,  had  to  be  done.  The  variable  armature 
reaction,  for  all  the  various  conditions  of  load  and  boost  and  buck 
of  the  D.  C.  e-  m.  f ,  were  carefully  calculated,  and  the  commutat- 
ing field  proportions  for  correcting  these  reactions  were  deter- 
mined. In  the  analysis  and  example  previously  given,  showing 
what  conditions  of  inherent  short-circuit  e.  m.  f.,  etc.,  could  be 
allowed,  and  still  obtain  permissible  results,  and  armature  reaction 
of  17  percent  under  normal  conditions  and  a  magnetizing  compo- 
nent of  commutating-pole  strength  of  25  percent  were  assumed, 
giving  a  total  of  42  percent.  It  was  shown  that,  with  a  suitable 
auxiliary  winding  oti  the  commutating  pole,  satisfactory  conditions 
could  be  obtained  from  no-load  to  50  percent  overload,  with  IS 
per  cent  boost  or  buck,  with  an  inherent  bnash  short-circuit  e.  m.  f . 
as  high  as  8  volts.  But  in  these  3500  kw  New  York  Edison  ma- 
chines, by  very  careful  analysis  of  the  conditions  of  commutation, 
the  inherent  short-circuit  e.  m.  f .  at  full  load  was  gotten  down 
to  6.3  volts  instead  of  8,  while  the  average  armature  reaction  was 
made  as  low  as  llj^  percent,  instead  of  17  percent,  both  of  which 


SYNCHRONOUS  BOOSTER  CONVERTERS 


465 


conditions  are  very  favorable,  compared  with  the  former  assumed 
permissible  limits  The  normal  or  series  commutating  field  ampere 
turns  are  39  percent,  instead  of  42  percent,  so  that  the  magnetizing 
component  is  27^  percent,  the  other  llj^  percent  simply  opposing 
the  normal  armature  reaction.  This  large  magnetizing  component 
is  obtained  by  the  use  of  a  %"  air  gap  under  each  commutating 


PIG.  s, 

pole.  Such  a  large  gap,  in  itself,  is  of  direct  assistance  in  obtaining 
the  desired  distribution  of  the  commutating  field  flux,  and  thus 
makes  the  design  problem  somewhat  easier 

A  brief  description  of  some  of  the  unusual  features  of  these 
machines  may  be  of  interest. 

The  rotary  converter  main  frame  or  field  is  of  cast  steel,  in 
order  to  reduce  somewhat  the  overall  dimensions,  and  keep  inside 
th,e  customer's  requirements.  The  synchronous  booster  field  frame 
is  of  cast  irao.  Both  the  main,  field  and  the  booster  have  bolted-in 
laminat&d 


466  ELECTRICAL  ENGINEERING  PAPERS 

The  commutator,  main  armature,  booster  armature,  and  col- 
lector rings  are  each  assembled  on  separate  spiders.  The  com- 
mutator spider,  however,  is  pressed  on  the  hub  of  the  main  ar- 
mature spider,  so  that  the  shaft  can  be  removed  for  shipment, 
-without  disturbing  the  connection  of  the  armature  winding  to  the 
commutator. 

The  commutator  is  of  the  through-bolt  construction  with 
heavy  steel  "V-rings."  These  rings  are  of  large  section  in  order 
to  avoid  distortion  under  the  heavy  clamping  strains  to  which  they 
are  normally  subjected.  The  commutator  bars  are  designed  to  give 
the  same  deflection  at  all  points.  The  commutator  diameter  is 
120",  and  the  width  of  exposed  face  is  30". 

The  two  3000  kw  vertical  units  for  the  same  company  are  of 
practically  the  same  general  design  as  the  3500  kw  except  that  they 
are  somewhat  smaller,  and  operate  at  higher  speed.  They  have  22 
poles,  and  the  normal  current  per  brush  arm  is  1010  amperes, 
compared  with  930  on  the  3500  kw. 

An  extensive  series  of  tests  were  made  on  both  the  3500  kw 
and  the  3000  kw,  some  of  the  results  of  which  are  as  follows : 

Armature  iron  loss  at  270  volts,  3500  kw,  16.3  kw. 

3000  kw,  13.8  kw. 

That  is,  the  normal  armature  iron  loss  in  both  sets  is  less  than 
0.47  of  one  percent, — a  remarkably  low  figure. 

The  booster  armature  of  the  3500  kw  unit  showed  an  iron  loss 
at  15  percent  boost  or  buck,  of  4.6  kw,  or  less  than  0.9  of  1  percent 
of  its  own  rating,  which  is  only  15  percent  of  that  of  the  unit. 

At  310  volts  D.  C  ,  the  3500  kw  unit  showed  20.1  kw  iron  loss, 
and  at  230  volts,  8  8  kw.  The  total  iron  losses,  including  the 
booster  armature,  thus  varies  between  13  4  kw  and  24.7  kw  over 
the  entire  range  of  voltage  operation,  or  between  0.4  and  0.7  of  1 
percent  of  the  rated  capacity  of  the  machine. 

Under  all  conditions  the  efficiency  of  the  unit  showed  apprec- 
iably higher  than  the  guarantees,  due  partly  to  the  relatively  low 
iron  loss,  as  given  above. 

The  following  temperature  test  results  were  obtained  on  the 
3500  kw  unit: 

Amp.  V.         Hrs.  Run    Arm.Rise  Comm  Rise 

13000  270  10  31.5°C.        37°C. 

12950  231  5  21.5  29.5 

12950  317  6  31.5  30 


SYNCHRONOUS  BOOSTER  CONVERTERS  467 

On  account  of  lack  of  certain  facilities,  a  50  percent  overload 
temperature  test  was  not  made  on  this  unit,  but  this  was  carried 
out  on  the  3000  kw  unit,  as  shown  in  the  following  results  of  tests  : 

Amp.  V.  Hrs.  Run.  Arm.  Rise  Comm.Rise 

"11100  270  13  17S°C.       28.5°C. 

*166SO  270  2  42  45.4 

11100  232  8  165  36 

11000  3125  8  27.5  305 

In  these  temperature  tests,  the  first  run  was  made,  in  each 
case,  for  a  period  long  enough  to  reach  constant  temperature.  The 
other  tests  followed,  while  the  machines  were  hot,  so  that  steady 
temperature  conditions  were  reached  in  a  shorter  time. 

In  the  commutation  tests,  the  results  were  equally  satisfac- 
tory. At  270  volts  the  3500  kw  machine  was  tested  from  no-load 
up  to  19,300  amperes;  also,  at  310  volts  from  no-load  up  to  14,000 
amperes;  and  at  230  volts  from  no-load  to  full  load;  and  at  257 
volts,  up  to  15,550  amperes.  Under  all  these  conditions  the  com- 
mutation was  remarkably  good,  and  this  may  therefore  be  taken 
as  evidence  of  the  correctness  of  the  principles  given  in  the  earlier 
part  of  this  paper.  Furthermore,  as  an  illustration  of  the  accuracy 
that  is  possible  in  the  design  of  such  apparatus,  when  the  funda- 
mental principles  are  sufficiently  well  known,  it  may  be  stated 
that,  in  the  case  of  this  3500  kw  unit,  aH  drawings  were  made  up, 
and  all  the  above  shop  tests  made  on  the  completed  machine, 
without  any  changes  whatever,  in  the  electrical  or  magnetic 
design,  from  the  original  engineering  design  specification.  Also, 
on  shop  test,  absolutely  no  re-adjustments  were  necessary  in  any 
of  those  parts  where  provision  is  usually  made  for  such  adjustment 
by  reason  of  possible  slight  variations  in  material  or  workmanship, 
or  inability  of  the  designer  to  predetermine  certain  characteristics 
with  sufficient  accuracy. 


*These  runs  were  duplicated.    The  results  given  are  the  highest  rises  obtained  from 
either  test. 


SIXTY-CYCLE  ROTARY  CONVERTERS 

FOREWORD— This  paper  was  prepared  for  the  twenty-ninth  annual 
convention  of  the  Association  of  Edison  Illuminating  Companies 
at  Cooperstown,  N.  Y.,  September,  1913.  At  the  time  it  was 
presented,  the  subject  of  60  cycle  ^rotaries  was  becoming  a  very 
"live"  one,  as  improvements  in  this  type  of  machine  were  bring- 
ing it  very  rapidly  to  the  front  as  a  competitor  of  the  25  cycle 
rotaries. — (ED.) 


ONE  of  the  most  significant  developments  in  the  past  year  has 
been  the  greatly  increased  purchase  of  large  60  cycle  rotaries 
by  central  station  plants.  Here  is  an  example  of  a  type  of  ma- 
chine which  has  been  more  or  less  discredited  in  the  past,  but  which, 
all  at  once,  is  coming  prominently  to  the  fore.  The  present 
machine  itself  is  not  radically  different  from  its  older  forms,  but  it 
contains  many  minor  improvements  which,  individually,  do  not 
stand  out  prominently,  yet,  collectively  have  served  to  overcome 
those  little  difficulties  which  formerly  were  just  sufficient  to  put 
the  machine  in  the  questionable  class.  However,  a  number  of 
general  conditions  were  also  involved  in  this  improvement.  It  is 
the  purpose  of  this  paper  to  show  wherein  the  new  machine  is 
superior  to  the  older  type,  and  also  to  indicate  wherein  a  number 
of  modifications,  each  in  themselves  of  a  small  amount,  have 
combined  to  form  a  relatively  large  improvement. 

Shortly  after  the  25  cycle  rotary  began  to  be  prominent  in 
electrical  work,  that  is,  about  15  to  18  years  ago,  the  problem  of 
60  cycle  rotaries  was  also  presented,  as  the  relatively  numerous 
60  cycle  plants  also  had  need  of  economical  means  for  transform- 
ing to  direct  current.  In  consequence,  there  being  a  field  for  60 
cycle  rotaries,  such  machines  were  built  and  installed  in  a  number 
of  places.  These  early  machines  were  in  some  cases,  fairly  suc- 
cessful, while,  in  others,  they  were  failures.  Apparently,  in  some 
of  these  cases  of  failure,  the  rotary  itself  was  not  entirely  to  blame, 
as  it  was  operated  under  conditions  which  would  now  be  considered 
impracticable,  with  our  present  knowledge  and  experience. 

These  early  60  cycle  rotaries  were  very  greatly  handicapped 
in  design  by  the  limitations  of  commercial  and  manufacturing 
practice  of  those  days.  Relatively  low  speeds  were  considered 
necessary  from  the  comnaercial  standpoint,  and  with  60  cycles 

469 


470 


ELECTRICAL  ENGINEERING  PAPERS 


this  meant  a  large  number  of  poles,  even  for  relatively  small  out- 
puts. Also,  manufacturing  limitations  called  for  relatively  low 
peripheral  speed  of  the  commutators.  In  those  days  commutator 
speeds  of  much  in  excess  of  4,000  feet  per  minute  were  considered 
excessive,  and  unduly  dangerous,  both  from  the  manufacturing 
and  operating  standpoints.  Herein  was  a  handicap  of  the  worst 
sort  upon  the  design.  The  peripheral  speed  of  the  commutator  is 
equal  to  the  distance  between  adjacent  neutral  points  multiplied 
by  the  number  of  alternations  per  minute  (revolutions  per  minute 
x  No.  of  poles).  On  this  basis,  3600  feet  peripheral  speed  with  60 


Co/r>  mutating  Zone 
w/ffi  no  /eacf. 


Fig.  1.  Commutating  Zone  with  No  Lead. 

cycles  per  second  (7200  alternations  per  minute),  gave  6  inches, 
between  adjacent  neutral  points.  Even  4200  feet  peripheral 
speed  gave  only  7  inches  between  neutral  points.  It  is 
obvious  therefore  that,  even  with  this  higher  peripheral  speed  of 
the  commutator,  there  was  undue  crowding  of  the  brush  holders, 


60  CYCLE  CONVERTERS  471 

which  in  itself  was  a  bad  feature.  But  the  worst  feature  was  in 
the  fact  that,  with  only  7  inches  between  commutator  neutral 
points,  the  maximum  permissible  number  of  commutator  bars  was 
unduly  limited.  Assuming,  for  example,  a  thickness  of  bar-plus-mica 
of  3-16  inches,  whichis  very  thin,  the  7  inches  between  neutral  points 
would  allow  about  36  commutator  bars  between  neutral  points,  or 
per  pole.  This  number  was  ample  for  250  to  300  volt  machines, 
but  for  600  volts,  experience  indicated  that  it  was  on  the  ragged 
edge,  especially  with  the  field  flux  distributions  obtained  with  those 
early  machines.  In  consequence,  60  cycle  rotaries  for  250  to  300 
volts,  rendered  a  better  account  of  themselves  than  the  600  volt 
machines,  and  the  latter  were  very  much  inclined  to  flash  at  times, 
due  to  the  small  number  of  commutator  bars,  and  a  high  maximum 
voltage  between  bars. 

The  field  flux  distribution  had  something  to  do  with  the 
questionable  operating  conditions.  With  these  earlier  machines, 
very  high  peripheral  speeds  of  the  armature  core  were  considered 
objectionable,  for  several  reasons.  One  was,  that  the  construc- 
tions of  that  time  did  not  allow  very  high  peripheral  speeds  of  the 
armature  windings,  and,  a  second  reason  was  that,  with  the 
relatively  low  speeds,  and  consequent  large  number  of  poles,  the 
armature  dimensions  and  cost  would  have  been  excessive  for  a  given 
output.  In  general,  a  12  inch  pole  pitch  was  considered  as  large  as 
desirable  or  practicable,  which  corresponds  to  7200  feet  per 
minute  at  60  cycles  per  second. 

With  this  small  pole  pitch,  in  order  to  obtain  a  sufficiently  wide 
commutating  zone  between  the  poles,  it  was  necessary  to  make  the 
poles  relatively  narrow.  The  use  of  narrow  poles  led  into  one 
difficulty,  as  regards  flashing,  as  will  be  explained  later,  while 
widening  the  pole  and  narrowing  the  interpolar  space  led  into 
another  difficulty  of  flashing  which  was  equally  serious.  The 
situation  can  be  illustrated  by  Figs.  1  and  2.  In  Fig.  1  the  field 
flux  distribution  is  indicated  for  an  extreme  case  of  a  pole  face  as 
wide  as  8  in.  and  with  only  4  in,  interpolar  space,  the  total  pole  pitch 
being  12  in.,  and  the  poles  without  polar  horns.  The  "field  form,11 
which  indicates  the  flux  distribution,  in  this  case  has  a  relatively 
wide  top,  and  the  proportions  are  such  that  the  maximum  e.  m.  f . 
between  the  bars  is  about  40  percent  greater  than  the  average 
3,  m.  f .  per  bar.  With  36  commutator  bars,  for  instance,  at  600 
volts,  the  average  volts  per  bar  would  be  16  2-3,  and  the  maximum 
voltage  per  bar  almost  24,  which,  in  itself,  may  be  a  safe  figure  if 


472  ELECTRICAL  ENGINEERING  PAPERS 

never  exceeded.  However,  the  flux  distribution  in  the  interpolar 
space,  as  indicated  by  Fig.  1,  is  such  that  there  is  almost  no  width 
to  the  neutral  or  commutating  zone,  and  therefore  the  brushes  are 
short-circuiting  the  armature  coils  in  an  active  field,  even  at  no 
load,  and,  in  some  cases,  this  short-circuiting  action  may  be  so 
great  that  there  are  excessive  local  currents  in  the  brushes.  Fur- 
thermore, with  the  neutral  point  so  narrow,  a  very  slight  forward 
shifting  of  the  brushes,  to  take  care  of  load  conditions,  would  place 
the  brush  in  such  a  strong  field  at  no-load,  that  there  is  danger  of 


Commutotinq  Zone 
w/th  no  /eacf. 


Fig.  2.  Commutating  Zone  with  No  Lead. 

flashing  when  the  load  goes  off,  or  changes  suddenly.  Therefore, 
with  such  proportions,  the  neutral  point  would  be  too  narrow  for 
reasonably  safe  operation.  The  remedy  for  this  particular  condi- 
tion, with  these  former  machines,  was  obviously  in  the  use  of  wider 
interpolar  spaces,  and  consequently  narrower  poles,  the  pole  pitch 
being  limited  to  about  12  inches  as  previously  stated. 


60  CYCLE  CONVERTERS  473 

In  Fig.  2  is  illustrated  the  conditions  with  the  wider  interpolat 
space,  and  narrower  pole  face,  these  being  taken  as  5}^  in.  and  6J^in. 
respectively,  instead  of  4  in.  and  8  in.  Obviously  the  flux  conditions 
in  the  interpolar  space  are  much  better  than  in  Fig.  1 ,  and  it  should 
be  possible  to  shift  the  brushes  slightly  for  full  load  conditions  with- 
out excessively  bad  conditions  as  regards  sparking  and  flashing  at 
no  load.  But  the  same  figure  also  shows  that  the  field  flux  distribu- 
tion as  a  whole  is  considerably  narrower  at  the  peak  value  than 
in  Fig.  1,  and  therefore  the  ratio  of  the  maximum  value  of  the 
e.  m.  f .  per  commutator  bar  to  the  average  e.  m.  f .  is  much  greater. 
In  this  case,  the  maxium  per  bar  is  about  65  percent  greater  than- 
the  average,  and,  with  16  2-3  average  per  bar,  the  maximum  be- 
comes almost  28  volts,  which  is  in  the  danger  zone  as  regards 
arcing  between  bars,  except  in  relatively  small  machines.  There- 
fore, in  overcoming  the  sparking  and  flashing  difficulties  incident 
to  the  narrow  neutral  zone  of  Fig.  1,  an  equivalent  difficulty  is 
encountered,  due  to  the  narrow  field  distribution,  or  field  form. 
Plainly,  in  these  older  machines,  whichever  way  we  turned,  we 
were  in  difficulty. 

An  obvious  remedy  for  the  above  difficulties  was  in  the  use  of 
wider  pole  pitches,  which  would  allow  both  the  commutating  or 
neutral  zone  of  Fig.  2,  and  the  wider  field  form  of  Fig.  1.  But  in- 
creasing the  pole  pitch,  with  a  given  number  of  poles  and  given 
speed,  means  increasing  the  diameter  of  the  armature,  and  even 
though  the  armature  could  thereby  be  narrowed,  the  cost  of  the 
larger  diameter  machine  would  necessarily  be  somewhat  in- 
creased. The  remedy  for  this  condition  was  in  reduction  in  the 
number  of  poles  as  the  pitch  was  increased,  thus  keeping  down  the 
size  of  the  armature  for  a  given  output.  But  reduction  in  the 
number  of  poles  necessarily  means  higher  speeds,  which  were 
formerly  considered  commercially  objectionable,  as  no  one  had 
yet  been  educated  up  to  high  speeds.  Therefore,  between  com- 
mercial limitations,  difficulties  in  design  and  manufacturing  condi- 
tions, the  60  cycle  rotary  was  in  a  bad  way.  Mild  attempts  were 
made  from  time  to  time  to  increase  the  speed  by  decreasing  the 
number  of  poles,  but  this  could  only  be  done  commercially  in 
relatively  small  steps.  In  such  increases  in  speed,  and  decrease 
in  the  number  of  poles,  other  difficulties  began  to  be  encountered, 
such  as  somewhat  poorer  inherent  conrnrutating  characteristics, 
due  to  the  higher  speed  and  greater  current  per  brush  arm  to  be 
commutated.  The  higher  the  speed  was  made,  and  therefore  the 


474  ELECTRICAL  ENGINEERING  PAPERS 

more  commercial  the  machine  became  as  regards  size  and  cost, 
the  greater  were  the  inherent  difficulties  in  the  design.  However, 
with  increased  experience  in  commutator  constructions,  one  great 
advance  was  made  by  increasing  the  commutator  speeds  of  the  60 
cycle  rotaries.  Instead  of  approximately  7  in.  between  points,  the 
distance  was  increased  to  8%  or  9  inches  for  600  volts,  giving  5100  to 
5400  feet  peripheral  speeds  at  the  commutator  face.  This  allowed 
as  many  as  45  to  48  commutator  bars  per  pole,  which  is  well 
within  the  range  of  good  direct-current  600  volt  practice.  This 
increase  in  the  number  of  bars  reduced  the  average  and  the  maxi- 
mum volts  per  bar.  In  this  manner,  one  of  the  principal  weak- 
nesses of  the  former  designs  was  eliminated  Also,  by  improved 
mechanical  design  which  allowed  higher  peripheral  speed  of  the 
armature  windings,  the  pole  pitch  could  be  increased  to  about  16in., 
instead  of  12,  without  an  unduly  large  diameter  of  armature  for 
a  given  output.  This  also  allowed  a  much  better  field  flux  distri- 
bution, or  field  form,  such  as  in  Fig.  1,  and  better  interpolar  space 
of  Fig.  2.  In  consequence,  the  maximum  voltage  per  bar  on  the 
60  cycle  rotaries  has  been  brought  down  well  within  accepted  D.  C. 
practice  for  600  volt  work.  For  lower  voltage  work,  these  limit- 
ations have  never  been  so  prominent,  but  the  same  steps  in  the 
development  have  proved  advantageous  in  lower  voltage  rotaries 
also. 

With  increased  speed  and  decreased  number  of  poles,  the 
current  per  brush  arm  on  the  larger  60  cyde  rotaries  has  gradually 
increased  until  it  is  practically  double  what  it  was  on  former  ma- 
chines of  same  capacity  This  higher  current  per  arm,  with  the 
increased  number  of  commutator  bars  per  pole,  and  the  higher 
speeds,  all  tend  toward  making  the  commutation  problem  more 
difficult.  But  at  this  stage  in  the  development,  the  commutating 
pole  began  to  loom  up  as  a  possibility  in  rotary  .converters,  and 
this  has  furnished  the  latest  important  step  in  the  improvement 
of  these  machines.  By  the  addition  of  commutating  poles,  still 
higher  revolutions  are  permissible  than  formerly.  While  relatively 
high  speed  rotaries  of  large  capacity  can  be  made  without  com- 
mutating poles,  yet  the  addition  of  such  poles  has  rendered  the 
design  less  difficult  and  has  allowed  still  further  increases  in  speed, 
which  are  very  welcome  in  the  way  of  improving  the  standing  of 
the  60  cyde  rotary.  With  these  higher  speeds,  and  greater  out- 
puts with  a  given  diameter  of  machine,  the  losses  have  not  in- 
creased anything  like  in  proportion,  so  that  the  difficulties  cf  the 


60  CYCLE  CONVERTERS  475 

60  cycle  rotaries  have  been  gradually  increasing  until  now  they  are 
treading  on  the  heels  of  the  25  cycle.  In  fact,  when  the  greater 
efficiency  of  the  60  cycle  step-down  transformers  is  taken  into 
account,  the  difference  between  the  efficiencies  of  60  cycle  and  25 
cycle  converting  units  in  large  capacities,  is  not  enough  to  attract 
any  particular  attention.  Thus  the  use  of  the  commutating  poles 
has  been  of  advantage  principally  in  allowing  higher  speeds,  with 
consequent  better  characteristics  in  general. 

The  modifications  above  described  cover  electrical  defects 
principally.  However,  there  were  a  number  of  other  minor  condi- 
tions in  these  earlier  machines  which  might  be  considered  as 
mechanical  defects,  or  mechanical  and  electrical  combined.  These 
were  found  principally  in  the  brush  holder  and  commutator  con- 
structions, and  in  the  materials  in  the  commutator.  On  the 
higher  voltage  machines,  in  which  a  large  number  of  commutator 
bars  per  pole  was  necessary,  the  thickness  of  each  bar  was,  and 
still  is,  very  small,  and  thus  the  proportion  of  thickness  or  mica 
between  bars  to  the  thickness  of  the  bars  themselves  is  a  very 
considerable  percent.  On  this  account,  it  has  been  difficult  to 
obtain,  in  many  cases,  a  wear  or  abrasion  of  the  mica  equal  to  the 
so-called  copper  "wear,"  which,  in  reality,  is  more  in  the  nature  of 
slow  burning  than  actual  wear  from  friction.  No  matter  how 
perfect  the  commutation  may  be  in  appearance,  there  is  always 
a  slight  tendency  to  burn  the  face  of  the  commutator  by  the  cur- 
rent passing  between  the  commutator  and  the  brushes.  This 
burning  normally  may  be  at  an  extremely  slow  rate,  but  if  the 
mica  does  not  wear  down  at  the  same  rate,  the  result  will  be  that, 
after  a  time,  the  mica  lifts  the  brush  surface  away  from  contact 
with  the  copper,  and  thus  an  almost  infinitestitnal  gap  exists 
between  the  brush  and  the  copper  of  the  commutator.  This  gap 
then  exaggerates  the  burning  tendency,  and  the  difficulty  thus 
accentuates  itself."  The  hardness  and  wearing  quality  of  the  mica 
must  be  such  that  it  will  always  wear  down  as  fast  as  the  copper 
burns  away,  so  that  normal  contact  is  maintained  between  the 
copper  and  the  brush.  Where  the  percentage  of  mica  is  high,  and 
where  the  mica  varies  ia  hardness,  as  is  liable  to  be  the  case  in 
practice,  it  is  difficult  to  avoid  more  or  less  tendency  to  high  mica 
and  consequent  trouble.  This  trouble  is  also  accentuated  by  high 
commutator  peripheral  speeds,  as  it  is  more  difficult  to  maintain 
uniform  contact  between  the  brush  face  and  the  commutator.  In 
consequence,  in  60  cycle  rotary  converters  in  geueral,  and  in  high 


476  ELECTRICAL  ENGINEERING  PAPERS 

voltages  in  particular,  experience  has  shown  that  it  is  advisable  to 
undercut  the  mica  slightly,  in  order  to  avoid  any  tendency  toward 
high  mica,  and  also  in  order  to  be  able  to  use  brushes  which  contain 
some  lubricant  such  as  graphite.  It  is  obvious  that  where  any  con- 
siderable grinding  action  by  the  brushes  is  necessary,  to  keep  down 
the  mica,  such  lubrication  is  not  practicable  to  the  same  extent  as 
where  no  grinding  action  is  necessary.  In  consequence,  on  later 
types  of  60  cycle  rotaries,  the  commutator  mica  is  usually  under- 
cut, thus  allowing  good  contact  to  be  maintained,  and  thus 
reducing  any  resultant  burning  action  to  the  minimum.  The  true 
causes  of  the  difficulty  with  high  mica  were  not  thoroughly  ap- 
preciated, in  the  older  60  cycle  rotaries,  and,  in  consequence,  in 
many  cases,  brushes  of  a  hard,  grinding  character  were  used,  with 
consequent  increased  losses  and  other  disadvantages. 

Also,  on  some  of  the  older  machines,  even  with  the  much  lower 
peripheral  speeds  than  at  present,  the  design  and  construction  of 
commutators  were  not  as  nearly  perfected  as  at  present,  and  there 
was  always  more  or  less  danger  from  unevenness,  and  other  defects, 
which,  while  not  showing  in  themselves  any  particularly  harmful 
results,  would  very  often  show  indirect  harm  by  causing  high  mica, 
sparking,  brush  troubles,  etc. 

Furthermore,  in  many  of  the  earlier  machines,  the  brush 
holders  were  not  as  rigid  or  as  well  suited  for  operation  on  high 
speed  commutators  as  in  present  practice.  In  some  cases,  the 
operating  characteristics  of  the  rotary  were  greatly  modified  by 
simply  changing  the  angle  of  inclination  of  the  brush  to  the  com- 
mutator, or  the  direction  of  inclination,  or  the  brush  pressure, 
etc.  Brush  chattering  was  not  uncommon,  and  if  there  is  any- 
thing which  will  surely  cause  bad  commutators  and  commutation, 
it  is  severe  chattering  at  the  brushes,  as  this  prevents  good  contact 
between  the  face  of  the  carbon  and  the  commutator  face. 

On  many  of  the  earlier  machines  the  brush*  holders  were  not 
arranged  with  due  regard  to  harmful  results  from  incipient  arcs 
between  bars,  or  in  the  neighborhood  of  the  brush  holders  and 
brushes.  On  sudden  changes  in  load,  or  partial  short-circuits,  or 
even  in  normal  operation  in  those  cases  where  the  maximum  voltage 
between  bars  is  unduly  high,  the  not  uncommon  "ring-fire  "  around 
the  commutator,  due  to  burning  of  the  carbon  or  graphite  de- 
posited on  the  mica  or  between  bars,  may  develop  into  small  arcs, 
with  consequent  vaporization  of  copper,  the  resultant  vapor  being 
a  good  conductor.  If  the  brush  holder  or  other  parts  axe  in  dose 


60  CYCLE  CONVERTERS  477 

proximity  to  the  point  where  such  small  arcs  may  form,  the  con- 
ducting vapor  may  bridge  across  from  the  commutator  face  to  the 
adjacent  parts,  where  there  is  any  considerable  difference  of 
potential  between  them,  and  may  develop  real  arcs  or  flashes 
which  are  of  a  destructive  nature,  possibly  necessitating  the  shut- 
down of  the  machine  until  the  commutator  can  be  smoothed  up. 
In  many  of  the  older  machines,  with  their  very  small  distances 
between  brush  holders,  and  their  generally  crowded  conditions, 
and  their  voltages  per  bar,  such  arcs  were  much  more  liable  to  occur 
than  in  the  modern  machines. 

In  the  development  of  the  60  cycle  rotary  converter,  there  were 
other  conditions  beside  commutation,  flashing,  etc  ,  which  had  to 
be  taken  into  account.  The  rotary  converter  is  a  synchronous 
machine,  and  must  follow  rigidly  in  step  with  its  source  of  e,  m.  f . 
supply,  or  there  will  be  difficulties  in  the  operation.  The  early 
rotaries,  in  many  cases,  were  operated  from  generators  driven  by 
slow-speed  reciprocating  engines,  which  did  not  run  at  uniform 
rotative  speed,  there  being  pronounced  periodic  speed  fluctuations 
during  each  revolution.  In  some  cases  this  condition  was  so  bad 
that  the  generators  in  the  power  house  would  not  operate  decently 
in  parallel.  As  the  engines  and  generators  varied  in  speed  period- 
ically,  obviously  the  frequency  of  the  electric  circuit  varied  to  the 
same  extent,  and  any  synchronous  apparatus  operated  on  such 
system  would  also  have  to  vary  in  speed  to  the  same  extent,  if  the 
conditions  were  such  that  the  machine  should  follow  the  supply 
system,  as  is  the  case  in  rotary  converters.  If  the  rotary  did  not 
follow  rigidly,  it  would  periodically  either  "under-run"  or  "over- 
run.1' This  action  is  called  hunting,  and  it  was  very  serious  at 
some  of  the  early  plants.  Not  infrequently  the  generators  at  the 
power  house  would  not,  hold  a  rigid  relation  to  each  other,  and 
hunted  badly. 

There  are  causes  of  hunting,  other  than  variations  in  speed 
of  the  prime  mover  on  generating  unit,  but  usually  these  have  been 
of  secondary  importance,  and  will  not  be  considered  further. 
The  action  of  hunting  of  the  rotary,  with  variations  in  speed  of  the 
generator,  may  be  explained  briefly,  as  follows:  The  rotary  gen- 
erates an  alternating  e.  m.  f .  wave  similar  to  that  of  the  generating 
or  supply  system,  but  in  opposition,  or  as  a  counter  e.  m.  f .  If 
the  generator  momentarily  rtms  faster,  then  its  e.  m.  f .  will  be 
ahead  of  that  of  the  rotary.  A  motor  current  flows,  tending  to 
raise  the  speed  of  the  rotary  to  that  of  the  generator.  If  the 


478  ELECTRICAL  ENGINEERING  PAPERS 

generator  now  drops  back  in  speed,  its  e.  m.  f .  wave  drops  back, 
and  the  rotary  tends  to  deliver  current  to  the  generating  system, 
thus  tending  to  slow  the  rotary  converter  speed  down  to  that 
of  the  generator.  The  action  in  the  rotary  is  therefore  one  which 
tends  to  speed  it  up  or  slow  it  down  to  follow  the  generator.  This 
action  of  the  rotary,  acting  alternately  as  a  motor  or  as  a  generator, 
is  what  constitutes  hunting.  Usually  this  action  of  following  the 
generator  speed  is  not  a  serious  one,  as  a  relatively  small  current 
may  produce  the  necessary  accelerating  or  retarding  action.  The 
difficulty  is  that  the  rotary  may  over-run;  that  is,  it  may  speed  up 
too  much  or  drop  back  too  much,  and  thus  have  an  increased 
motor  or  generator  action.  In  other  words,  this  accelerating  or 
retarding  action  may  exaggerate  the  swinging  effect,  just  as  in  the 
case  of  a  swinging  pendulum,  where  a  very  slight  force,  if  timed 
just  right,  may  gradually  increase  the  swing  of  the  pendulum.  In 
those  cases  in  the  early  rotaries  where  hunting  was  most  severe, 
the  periodic  speed  changes  in  the  generating  system  were  usually 
timed  just  right  to  cause  the  rotary  to  over-run,  and  thus  exag- 
gerate the  hunting  action. 

The  direct  result  of  this  hunting  was  visible  in  bad  operation 
at  the  commutator.  In  the  normal  rotary  converter,  when  run- 
ning properly  in  synchronism,  there  is  practically  no  armature 
reaction  in  the  armature  winding,  such  as  is  found  in  direct-cur- 
rent machines,  for  the  alternating  current  supplied  to  the  armature 
winding  is  in  opposition  to,  and  practically  neutralizes,  the  magnet- 
izing effect  due  to  the  direct  current  delivered.  Therefore,  as  far 
as  reactions  on  the  field  are  concerned,  the  rotary  is  quite  different 
from  a  direct  current  machine,  and,  at  full-load,  the  armature  has 
very  little  more  effect  on  the  field  than  at  no-load.  However, 
when  the  rotary  is  hunting,  the  current  due  to  the  hunting  action 
above  described  is  not  balanced  by  the  direct  current  delivered,  so 
that  this  current  acts  like  that  in  a  straight  A.  C.  or  D.  C.  machine, 
and  sets  up  magnetic  fluxes  in  the  interpolar  space,  and  under  the 
edges  of  the  poles,  which  are  harmful  in  character.  These  fluxes 
create  bad  commutating  conditions  by  reason  of  the  armature 
coils  under  the  brushes  being  short  circuited  in  a  periodically 
varying  magnetic  field,  which  is  not  the  case  when  the  rotary  is 
not  hunting.  Therefore,  as  a  rotary  hunts,  there  is  usually  periodic 
sparking  at  the  brushes,  which  is  in  time  with  the  periodic  "beat" 
which  usually  can  be  heard  in  a  machine  when  it  hunts.  This 
sparking  will  get  more  and  more  severe  as  the  rotary  hunts  more, 


60  CYCLE  CONVERTERS  479 

until  it  may  become  so  bad  that  the  machine  flashes  over.  This 
hunting  in  some  of  the  early  machines  was  a  very  puzzling  phen- 
omenon, and  it  was  not  until  its  nature  and  cause  were  determined 
that  an  effective  remedy  was  applied.  The  corrective  now  uni- 
versally applied  consists  in  the  use  of  copper  dampers,  or  "cage 
windings,"  in  the  field  pole  faces  of  the  rotaries,  It  is  not  within 
the  province  of  this  paper  to  explain  the  action  of  these  dampers, 
but  it  may  simply  be  said  that  they  exert,  to  a  certain  extent,  a 
braking  action  on  the  over-running  action  of  the  rotary,  and  also 
they  damp  out  the  field  distortions  due  to  hunting,  such  distortions 
materially  exaggerating  the  hunting  action.  The  dampers  thus 
reduce  one  source  of  accentuation  of  the  hunting,  and  exert  a 
braking  action  to  overcome  the  effects  of  the  others.  Such  damp- 
ers were  used  early  on  60  cycle  rotaries,  but  in  comparatively 
crude  forms.  Moreover,  the  angular  variations  in  speed  with  60 
cyde  generating  units,  were  usually  greater,  in  degrees  per  electrical 
cycle,  than  in  25  cycle  machines,  due  to  the  much  larger  number 
of  poles,  and  this  made  the  hunting  tendencies  of  the  rotaries  much 
greater,  and  the  damping  problem  correspondingly  more  difficult 
than  in  25  cycle  rotaries.  In  consequence,  60  cyde  rotaries  should 
have  had  more  damping  action  than  25  cyde  machines,  while, 
on  the  contrary,  they  actually  had  much  less.  The  60  cycle  rotary 
was  therefore  considered  a  much  more  delicate  machine  as  regards 
hunting,  than  its  25  cycle  brother,  and  yet  the  fault  was  really  in 
the  generating  plant  in  many  cases. 

The  advent  of  the  later  60  cycle  turbo  generating  plants  have 
been  a  large  item  in  the  successful  development  of  the  later  type  of 
60  cyde  rotaries.  The  problem  of  angular  variation  in  speed  of 
the  prime  mover  has  disappeared,  and  therefore  the  dampers  on 
modern  60  cycle  rotaries  have  to  take  care  of  only  those  secondary 
causes  of  hunting,  which  were  present  in  the  old  conditions,  but 
were  masked  by  the  much  greater  cause  in  the  generating  condi- 
tions. Also,  with  the  'newer  high  speed  rotaries,  with  their 
relativdy  wider  poles,  it  is  practicable  to  add  many  more  damper 
bars  per  pole  than  in  the  older  machines,  and,  in  fact,  with  the 
later  machines,  the  problem  of  hunting  is  rardy  encountered. 
However,  a  new  problem  in  connection  with  hunting  has  come 
up  in  connection  with  the  advent  of  the  commutating  pole,  both 
in  25  and  60  cydes.  In  the  commutating-pole  generator,  the 
ampere  turns  in  eadi  commutating-pole  coil  is  sufficient  to  not  only 
neutralize  the  entire  magnetizing  force  of  the  armature  winding 


480  ELECTRICAL  ENGINEERING  PAPERS 

per  pole,  but  also  to  furnish  an  excels  flux  for  conrmutating.    In  the 
connnutating-pole  rotary,  there  is  normally  but  little  resultant 
magnetizing  effect  in  the  armature  winding,  due  to  the  A.  C.  and 
D.  C.  currents  being  normally  in  opposition,  and  therefore  the 
cominutating  pole  winding  must  only  be  strong  enough  to  neutralize 
the  very  small  resultant  armature  reaction,  and  give,  in  addition, 
a  magnetic  flux  sufficient  for  commutation.     In  consequence,  the 
ampere  turns  on  the  commutating-pole  winding  may  be  only  30 
percent  to  40  percent  of  the  total  armature  ampere  turns,  con- 
sidered as  in  an  A.  C.  or  D.  C,  machine,  whereas,  in  a  D.  C.  gen- 
erator, the   commutating-pole  winding  is  usually  at  least  125 
percent  of  the  armature  ampere  turns.    Therefore,  the  rotary 
converter  with  its  30  percent  to  40  percent  commutating-pole 
ampere  turns,  instead  of  125  percent,  cannot  act  as  a  generator 
or  motor  with  good  commutation,  as  its  commutating-pole  strength 
is  then  much  less  than  required.    As  a  generator  or  motor,  the 
armature  reaction  may  not  only  over-power  the  commutating- 
pole  winding,  but  may  set  up  a  strong  magnetic  flux  in  the  wrong 
direction     The  commutating  conditions  may  thus  become  much 
worse  than  if  the  commutating  pole  were  absent     Therefore,  the 
commutating-pole  rotary  converter,  when  acting  as  a  generator  or 
motor,  is  a  much  worse  machine  than  if  the  commutating-pole 
itself  were  omitted.    Herein  lies  a  source  of  possible  trouble  with 
commutating-pole  rotaries.    In  case  there  is  hunting,  the  arma- 
ture will  act  alternately  as  a  generator  and  motor,  and,  under  such 
conditions,  the  magnetizing  force  of  the  armature  may  be  such 
that  it  will  demagnetize  the  commutating  pole,  or  even  reverse  the 
flux  under  it,  so  that  the  machine  acts  in  the  same  way  as  if  it 
were  operating  with  the  current  reversed  in  the  commutating-pole 
winding,   which  would  obviously  give  very  bad  commutating- 
conditions.    Therefore,  when  the  commutating-pole  rotary  hunts, 
it  represents  a  worse  condition  than  when  a  non-commutating- 
pole  machine  hunts  to  an  equal  extent.    In  consequence,  with 
commutating-pole  machines,  it  is  very  important  to  suppress  any 
hunting  tendency,  and  this,  in  general,  requires  somewhat  better 
damping  conditions  than  in  the  non-commutating-pole  machine. 
Therefore,  although  improved  conditions  of  generation,  etc.,  have 
eased  up  on  the  damper  requirements,  yet  the  necessities  of  the 
commutating  poles  have  made  the  damper  requirements  more  rigid. 
In  some  cases,  this  has  led  to  a  very  curious  situation.    It  is  well 
known  that  the  commutating  pole,  whether  on  a  direct-current 


60  CYCLE  CONVERTERS 


481 


machine  or  on  a  rotary,  should  not  have  any  closed  conducting! 
circuit  around  it,  as  such  closed  circuit  acts  as  a  secondary  or  op- 
posing circuit  in  case  of  sudden  change  of  load,  preventing  the 
commutating-pole  flux  from  rising  or  falling  in  step  with  changes 
in  load.  Therefore,-  from  the  standpoint  of  commutating-pole 
construction,  there  should  be  no  closed  circuit  surrounding  the 
commutating  pole  itself.  However,  from  the  standpoint  of  the 
best  arrangement  of  the  damper  to  prevent  hunting,  a  complete 
cage  winding,  tying  all  the  poles  together,  as  shown  in  Fig.  3,  is,  in 
general,  the  most  economical  and  effective.  But  such  a  closed 


\  O  \ 


;  o  i 


o 


b 

FIG.  3. 

winding  forms  a  rather  effective  closed  secondary  circuit  around 
the  commutating  pole.  One  would  therefore  assume  that  it  is  in- 
advisable to  dose  this  damper  circuit  around  the  commutating 
pole,  and  that  a  break  at  a  or  b  in  Fig.  3  for  instance,  would  be  an 
improvement.  However,  in  some  instances,  experience  has  shown 
that  the  improvement  in  the  damping  action  as  a  whole,  in  pre- 
venting hunting,  by  tying  together  at  a  and  6,  more  than  over- 
balances the  harmful  effects  of  the  dosed  secondary  circuit  around 
the  commutating  pole,  caused  by  the  dosed  damper  winding. 
This  is  not  necessarily  always  the  case,  the  results  depending  upon 
individual  and  local  conditions,  to  some  extent.  The  same 
damping  effect  as  tying  together  at  a  and  6  might  be  obtained 
theoretically  by  special  proportioning  of  the  damper  on  each  pole, 
but,  in  some  cases,  especially  on  60  cyde  machines,  space  require- 
ments do  not  permit  such  proportioning  of  the  damper,  so  that 
it  may  prove  betefcer,  ty>  tie  the  dampers,  together  at  a  and  b.  , 


482  ELECTRICAL  ENGINEERING  PAPERS 

A  new  condition  also  developed  in  connection  with  self  starting 
of  mmtrnit.fl.ting.pnle  rotaries.  In  the  older  60  cycle  rotaries,  start- 
ing motors  were  rather  commonly  used,  due,  not  to  the  inability 
of  the  rotary  to  start  itself,  but  to  the  effect  of  the  large  start- 
ing current  upon  the  relatively  small  generating  plants  of  those 
days.  Later  practice  tends  strongly  toward  self -starting,  except 
in  special  cases.  There  are  some  very  considerable  advantages  in 
this  self-starting,  and  at  the  same  time,  there  are  some  disadvan- 
tages, especially  in  the  60  cycle  rotaries.  The  greatest  advantage 
lies  in  the  rapidity  with  which  the  rotary  can  be  started  from  rest 
and  brought  up  to  synchronism,  together  with  the  fact  that  no 
synchronizing  devices  are  required.  With  the  old  starting  motor, 
the  machine  had  to  be  brought  to  synchronous  speed  and  then 
thrown  in  step.  This  was  more  difficult  with  60  cycles  than  with 
25  cycles,  and  self-starting  eliminates  this  trouble.  On  the  other 
hand,  while  starting  and  accelerating,  the  rotary  converter  is 
purely  an  induction  motor  of  a  rather  crude  sort,  and  will  take  a 
relatively  large  starting  current — in  some  cases  approximately  full 
load  current  from  the  line — and  this  current  is  at  very  low  power 
factor ;  that  is,  at  least,  90  percent  to  95  percent  of  it  is  purely  watt- 
less. When  starting  a  large  capacity  rotary,  this  will  represent  a 
relatively  large  inductive  load  thrown  suddenly  on  the  power 
plant. 

However,  the  new  condition  which  developed  with  the 
advent  of  commutating  poles,  lies  in  sparking,  and  not  in  the 
starting  current.  As  the  rotary  converter  at  start  acts  like  an 
induction  motor,  it  has  a  rotating  magnetic  field  flux  set  up,  which 
travels  around  the  armature.  The  armature  coils  short-circuited 
by  the  brushes  form  secondaries  to,  or  are  cut  by,  this  field,  and 
therefore  have  relatively  large  e.  m.  f.'s  set  up  in  them,  which 
develop  large  local  currents.  The  e.  m.  f  's  set  up  in  the  short-circuit- 
ed coils  are  usually  somewhat  greater  in  the  60  cycle  rotaries  than 
in  25  cycle,  due  primarily  to  the  fact  that  there  are  usually  fewer 
conductors  in  series  for  the  normal  voltage  of  the  machine,  and 
therefore  the  normal  voltage  per  conductor  is  relatively  higher 
than  in  the  25  cycle  rotary.  In  consequence,  at  start,  assuming 
that  similar  voltages  are  applied  for  starting  both  60  and  25  cycle 
machines,  the  relative  voltage  per  conductor  generated  by  the 
rotating  field  set  up  by  the  armature  winding  will  also  be  higher 
in  the  60  cycle  rotary.  Also,  the  number  of  commutator  bars 
covered  by  the  brush  will  usually  be  greater  on  the  60  cycle  rotary. 


60  CYCLE  CONVERTERS  483 

In  consequence  of  these  two  conditions,  the  short-circuiting  action 
of  the  brushes  and  the  sparking  will  be  worse  on  the  60  cycle 
rotaries,  but  it  is  liable  to  be  excessive  on  all  large  machines. 

With  the  advent  of  the  commutating-pole  rotary  converter, 
a  still  more  difficult  condition  has  been  encountered  in  self -starting, 
namely,  that  the  flux  conditions  in  the  zone  of  commutation  of  the 
short-circuited  coils  are  materially  higher  than  in  the  non-com- 
mutating-pole  machine.  In  the  latter  type,  while  the  short- 
circuited  coils  cut  an  alternating  flux  and  therefore  have  local 
currents  set  up  in  them,  these  coils,  in  commutating  or  reversing 
these  currents,  lie  midway  between  the  poles,  and  therefore  in  the 
region  where  the  conditions  of  reversal  are  easiest.  But  in 
placing  the  commutating  pole  directly  over  the  short-circuited 
coils,  the  conditions  of  reversal  of  the  short-circuited  current  are 
made  much  more  difficult  during  starting.  In  consequence,  during 
starting  and  accelerating,  the  sparking  conditions  in  the  commut- 
ating-pole rotary,  both  for  60  and  25  cycles,  are  much  worse  than 
in  the  older  non-commutating-pole  type.  In  fact,  in  the  larger 
machines,  the  conditions  are  so  bad  that  it  has  been  found  neces- 
sary to  add  brush  lifting  devices  which  will  lift  all  the  brushes  but 
two,  during  starting  and  bringing  up  to  speed.  This  is  an  added 
complication,  but  it  is  offset,  to  some  extent,  by  the  fact  that, 
with  the  brushes  lifted,  there  is  no  sparking  at  all,  and  therefore 
the  commutator  does  not  suffer  at  all  during  the  operation  of 
starting. 

In  the  earlier  60  cycle  rotaries,  the  question  of  variable  voltage 
came  up  in  connection  with  250  to  300  volt  machines.  The 
general  means  of  voltage  variation  in  these  machines  was  almost 
entirely  by  means  of  induction  regulators,  or  step-by-step  trans- 
formers. It  is  only  in  very  recent  years  that  the  self-contained 
units,  such  as  the  synchronous  booster  rotaries,  and  the  regulating 
pole  type,  have  been  brought  forward.  For  60  cycle,  the  syn- 
chronous booster  appears  to  be  the  only  really  practical  method, 
due  largely  to  limitations  in  design  and  in  space  requirements. 
If  commutating  poles  are  to  be  used,  then  the  regulating  pole  type 
of  machine,  with  main  and  auxiliary  poles,  in  addition  to  the  com- 
mutating poles,  requires  a  very  crowded  design  of  field,  unless  a 
larger  pole  pitch  is  chosen  than  in  the  synchronous  booster  ma- 
chine, in  which  there  are  only  the  commutating  and  the  ma.in  poljes. 

When  synchronous  boosters  are  used  with  commutating 
poles,  the  problem  of  proper  adjustment  of  the  commutating-pole 


484  ELECTRICAL  ENGINEERING  PAPERS 

strength,  with  varying  loads  and  voltages,  comes  in.  This  has 
been  treated  before  rather  fully  in  a  paper  before  the  association, 
and  nothing  further  need  be  said,  except  that  this  problem  of 
adjustment  is  just  as  pronounced  in  60  cycle  machines  as  in  25. 
Where  the  range  of  voltage  is  relatively  small — say,  never  exceed- 
ing 10  percent  up  or  down — it  is  practicable  to  so  proportion  the 
commutating-pole  windings  that,  without  any  automatic  or  hand- 
adjusting  devices,  good  commutation  can  be  obtained  over  the 
whole  working  range.  However,  if  materially  higher  voltages  are 
needed,  such  as  15  percent  to  20  percent  up  or  down,  practice 
indicates  that  some  auxiliary  device  is  required  for  automatically, 
or  by  hand,  adjusting  the  field  strength  at  the  extreme  condition, 
which  appears  to  be  at  no-load  with  maximum  boost  or  buck. 
For  this  condition,  an  automatic  device  has  been  developed,  which, 
when  the  main  current  falls  to  a  relatively  low  value — say,  one- 
fourth  full  load — automatically  short-circuits  that  part  of  the 
commutating-pole  winding  which  is  in  series  with  the  booster  field. 
The  same  operation  cuts  into  the  circuit  a  resistance  equivalent  to 
the  section  of  the  winding  cut  out.  This  latter  is  a  necessity,  due 
to  the  fact  that  any  variation  of  the  resistance  of  the  booster  field 
circuit  will  vary  the  amount  of  boost  or  buck,  and  thus  affect  the 
main  voltage  of  the  machine.  Any  automatic  device  therefore 
should  hold  the  resistance  of  the  booster  circuit  constant.  Such 
devices  have  been  installed  on  a  number  of  synchronous  booster, 
commutating-pole  type  machines.  They  can  be  located  at  the 
rotary,  and,  being  purely  automatic  in  their  action,  require  no 
attention  from  the  switchboard  operators  or  anyone  else.  As 
such  a  device  operates  only  very  infrequently,  but  at  fairly  regular 
intervals,  such  as  once  or  twice  a  day,  it  is  not  liable  to  wear  out 
due  to  excessive  operation,  or  to  stick  due  to  non-use.  Experience 
has  shown  that,  except  for  extremely  wide  ranges  in  D.  C.  voltage, 
only  one  step  is  needed  in  such  automatic  device. 

60  cycle  rotary  converters  are  now  being  manufactured  in 
relatively  large  capacities,  such  as  1000,  1500  and  2000  kw  for  270 
volts  with  synchronous  boosters,  and  up  to  2500  kw  for  higher 
voltages.  Larger  capacities,  for  either  voltage,  can  be  constructed 
without  difficulty,  and  with  as  good  performance  as  in  the  capac- 
ities mentioned.  The  modern  60  cycle  rotary  converter  for  either 
270  or  600  volts,  is  approaching  very  dose  to  the  25  cycle  rotary 
in  its  general  characteristics,  such  as  efficiency  etc.  In  commu- 
tation, it  can  be  fully  equal  to  the  25  cycle.  In  general  retta- 


60  CYCLE  CONVERTERS  485 

bility,  the  modern  machine  is  far  ahead  of  the  older  types.  This 
development  of  the  60  cyde  rotary  therefore  removes  one  of  the 
most  serious  handicaps  formerly  encountered  by  the  large  60  cyde 
generating  systems. 


IRON  LOSSES  IN  DIRECT-CURRENT  MACHINES 

FOREWORD — This  paper  was  presented  before  the  Schenectady  Sec- 
tion of  the  American  Institute  of  Electrical  Engineers,  March, 
1916,  before  an  audience  composed  almost  entirely  of  engineers 
of  the_  General  Electric  Company.  It  is  principally  of  interest 
to  designing  engineers,  in  general,  and  it  brings  out  some  of  the 
problems  actually  involved  in  an  analysis  of  the  loss  conditions 
occurring  in  direct-current  machinery.  Certain  explanations 
of  eddy  current  losses,  due  to  saturation  of  the  armature  teeth, 
are  brought  put  here  for  the  first  time,  the  author  believes,  and 
some  approximate  methods  of  calculating  these  losses  are  given. 
In  fact,  a  careful  study  of  this  paper  will  indicate  wherein  the 
calculation  of  the  no-load  losses  in  any  direct-current  machine 
is,  necessarily,  more  or  less  empirical,  while  the  conditions  with 
load  are  very  much  worse. 

After  the  presentation  of  this  paper,  there  was  a  general 
discussion,  largely  of  a  constructive  and  educational  nature, 
which  appears  in  the  Proceedings  of  the  American  Institute  of 
Electrical  "Engineers. — (ED.) 


IRON  loss  is  a  general  term  to  cover  a  number  of  losses, 
of  various  kinds,  which,  by  the  nature  of  the  tests, 
are  included  in  one  set  of  measurements  and  which,  in  reality, 
should  be  known  as  core  loss.  The  term  has  been  used  so 
promiscuously,  without  indicating  what  it  really  includes,  that 
many  have  come  to  believe  that  it  means  the  true  iron  loss  and 
nothing  else.  In  fact,  however,  the  true  iron  loss,  due  to  magnetic 
conditions  in  the  iron  itself  may  be,  in  many  cases,  only  a  moderate 
percentage  of  the  total  core  loss.  What  might  be  called  the 
normal  hysteretic  and  eddy  current  losses  in  the  iron  itself  may 
be  overshadowed  by  abnormal  losses  due  to  improper  flux  dis- 
tribution and  other  causes  consequent  upon  incorrect  propor- 
tioning of  flux  paths  and  directions.  Also,  usually  no  distinc- 
tion has  been  made  between  the  losses  simply  located  in  the  iron 
itself,  and  those  directly  due  to  magnetic  conditions.  Further- 
more, the  losses  in  non-magnetic  parts  adjacent  to  the  iron,  and 
lying  in  the  flux  paths,  may,  in  some  instances,  even  exceed  the 
total  losses  in  the  iron.  The  readily  practicable  methods  of 
measuring  the  core  losses  show  only  their  sum  and  there  is 
no  true  indication  of  the  relative  values  of  the  various  com- 

487 


488  ELECTRICAL  ENGINEERING  PAPERS 

ponents.  To  separate  the  total  core  loss  into  its  various  com- 
ponents, except  by  complicated  and  expensive  laboratory  meth- 
ods, appears  to  be  alrrost  impossible  However,  it  is  possible 
to  indicate  the  various  components  and  their  probable  causes, 
and  in  some  cases  they  can  be  segregated  very  crudely  by 
calculation. 

In  most  rotating  machinery  the  calculation  of  the  individual 
elements,  which  rrake  up  the  total  core  loss,  is  necessarily  only 
approximate,  in  comrrercial  apparatus.  This  is  due  partly 
to  the  fact  that  there  are  many  possibilities  of  variation  in  loss 
on  account  of  conditions  of  manufacture  and  materials,  as 
will  be  described  later.  This  is  evidenced  by  the  fact  that 
two  machines,  built  at  different  times  from  the  same  draw- 
ings and  the  same  tested  grade  of  materials,  will  ofttimes  show 
materially  different  core  losses.  If  two  such  machines  vary 
twenty -per  cent  from  each  other  in  core  loss,  it  is  obviously 
impracticable  to  expect  any  refinement  in  calculation  closer 
than  twenty  per  cent  Even  if  ^e  always  could  come  within 
twenty  per  cent  by  direct  calculation  and  could  place  any 
great  reliance  upon  the  results,  it  would  be  a  great  step  ahead, 
in  certain  types  of  apparatus.  In  the  discussion  of  the  various 
losses  and  their  causes,  given  throughout  the  following  paper, 
it  \\ill  be  sho\\n  \\hy  it  is  impracticable  to  calculate,  with  any 
exactness,  certain  of  these  losses. 

In  separating  the  total  core  loss  into  its  components,  two 
principal  classifications  of  losses  rray  be  rradc.  Cne  of  these 
is  eddy  current  loss,  either  in  the  iron  laminations  themselves 
or  in  other  conducting  parts  wherein  e.m.fs  are  generated 
during  rotation.  Such  e.m.fs.  will  set  up  local  currents  where 
closed  paths  are  possible,  and  if  such  paths  are  in  the  lamina- 
tions themselves,  instead  of  in  neighboring  solid  parts,  it  is 
simply  incidental  Eddy  current  loss  in  the  laminations  is, 
therefore,  not  a  special  kind  of  loss,  and  it  should  rightly  be 
classed  with  other  eddy  losses  in  the  machine. 

The  second  class  of  losses  includes  those  due  to  changes 
in  the  magnetic  conditions  in  the  iron  itself;  these  are  known 
as  hysteresis  losses.  These  latter  are  dependent  upon  the 
material  itself  and  not  its  structure.  Lamination  is  primarily 
for  increasing  the  resistance  in  the  eddy  current  paths  and  not 
for  the  purpose  of  affecting  the  hysteresis.  In  fact,  lamina- 
tion may  increase  the  hysteretic  losses,  for  a  given  volume  of 
material 


IRON  LOSSES  IN  D  C    MACHINES  489 

The  principal  object  of  this  paper  is  to  show  causes  for  some 
of  the  principal  losses.  These  are  usually  related  to  two  sets 
of  frequencies,  namely,  the  normal  frequency  (revolutions  per 
second  times  number  of  pairs  of  poles),  and  some  very  high 
frequency,  dependent  upon  the  number  of  slots,  commutator 
bars,  etc.  The  hysteretic  losses  are  undoubtedly  affected  by 
these  higher  frequencies  but  apparently  not  to  the  same  extent 
as  the  eddy  losses.  These  high-frequency  losses  are  liable  to 
be  present  in  most  classes  of  rotating  machines,  while  in  some 
instances  they  may  overshadow  all  other  losses  Certain  of 
them  are  characteristic  of  certain  types  of  machines  only,  while 
others  are  liable  to  be  present  in  any  type  of  rotating  machine 

In  most  classes  of  rotating  machines,  only  the  no-load  core 
losses  can  be  measured  with  any  accuracy  by  ordinarily  con- 
venient methods  of  measurement.  However,  if  the  various 
components  of  the  no-load  loss  can  be  approximately  deter- 
mined, then  it  is  possible  to  indicate  in  what  way  these  same 
components'  will  be  affected  by  load.  A  quantitative  deter- 
mination of  the  component  losses  with  load  is,  however,  very 
difficult  to  determine  except  in  a  very  few  classes  of  machines. 

In  direct -current  machines  the  principal  no-load  armature 
core  losses  are  the  hysteresis  loss  in  the  iron,  eddy  losses  in 
the  iron  and  copper,  and  eddy  losses  in  other  adjacent  conduct- 
ing parts,  which  may  be  seats  of  e  m.fs.  The  relative  values 
of  these  losses  are  dependent  upon  many  conditions.  In  a 
thoroughly  well  designed  machine  the  eddy  losses  in  the 
copper  and  any  other  parts  than  the  iron  should  be  relatively 
small  compared  with  the  iron  loss  proper.  Again,  the  pro- 
portion of  hysteresis  to  eddy  loss  in  the  iron  itself  depends 
upon  many  conditions,  such  as  the  various  frequencies  in  the 
machine,  the  grade  of  material,  the  degree  of  lamination,  the 
perfection  of  the  insulation  of  the  laminae  from  each  other, 
the  distortion  of  the  material  in  handling  and  building,  the 
conditions  of  punching,  treatment  during  assembly,  grinding, 
filing,  etc.  Here,  at  once,  so  many  variables  appear  that  one 
cannot  reasonably  expect  any  great  accuracy  in  any  prede- 
termination of  eddy  loss  in  the  iron  itself.  Hysteresis  loss  is 
also  affected  by  some  of  these  conditions- 

It  is  a  fact  well  known  to  designers  that  the  iron  loss  -tables 
used  by  transformer  engineers  do  -not  directly  apply  to  ro- 
tating machinery,  but  that  an  increase,  in  some  cases,  of  one 
hundred  per  cent „  art  more  is  necessary,  depending  upon  the 


490  ELECTRICAL  ENGINEERING  PAPERS 

type  of  machine.  This  increase  is  due  largely  to  additional 
causes  of  loss  which  do  not  occur  to  any  appreciable  extent 
in  transformers.  Some  of  these  additional  losses  are  as  follows: 

(a)  Handling  of  iron.     Experience  shows  that  well  annealed 
armature  iron  will  have  its  losses  very  materially  increased  by 
springing  or  bending      If  a  lamination  is  given  a  decided  bend, 
beyond  the  elastic  limit,  and  then  is  straightened  out,  the  loss 
at  the  part  which  has  been  bent  may  be  increased  as  much  as 
100  per  cent.     This  fact  must  be  taken  into  account  in ^  ma- 
chinery where  armatures  with  many  light  teeth  are  used.     Here 
it  is  almost  impossible  to  prevent  some   abuse  of  the  iron, 
especially  in  the  teeth,  which  are  the  parts  usually  worked  the 
hardest.     Furthermore,  tests  have  shown  that  if  iron  is  bent, 
even  at  a  small  angle,  and  not  beyond  the  elastic  limit,  the 
loss  is  materially  higher  with  the  iron  in  this  strained  con- 
dition, although  the  loss  may  return  to  normal  when  the  iron 
is  allowed  to  spring  back  to  normal  position.     And  if  the  iron 
is  annealed  in  a  curved  or  warped  position,  then  when  straight- 
ened out  in  building  the  strain  is  present,  with  increased  loss. 
In  building  up  armature  cores,  undoubtedly  part  of  the  iron 
is  put  under  stress,  especially  in  the  teeth.     Any  dent  in  the 
iron,  produced  by  hammering  or  otherwise,  also  tends  tor  in- 
crease the  loss. 

(b)  A  second  source  of  increased  loss  in  the  iron  is  due  to 
the  operation  of  punching.     In  shearing  the  iron  a  small  amount 
adjacent  to  the  sheared  part  is  affected  much  in  the  same  way 
as  when  iron  is  bent  beyond  the  elastic  limit.     In  transformer 
plates  this  strip  next  to  the  sheared  edge  represents  but  a  very 
small  percentage  of  the  total  volume  of  each  plate  or  lamina- 
tion.    However,  in  armatures  with  many  comparatively  long 
narrow  teeth,   this   sheared   part   may  represent   a  relatively 
large  percentage  of  the  whole  plate  and,  moreover,  this  is  a 
part  which  often  has  the  largest  losses.     But    this    may    not 
have  as  great  effect  on  the  losses  as  another  result  of  the  shear- 
ing, namely,  the  sharp  burrs  which  are  left  on  the  iron.     These 
may  be  very  small  or  almost  negligible  in  appearance  and  yet 
represent  quite  a  large  percentage  of  the  thickness  of  the  plate. 
For  example,  a  burr  of  two  mils  height,  or  1/500  in.,  seems  to 
be  very  small  indeed,  and  yet  it  is  about  12  per  cent  of  the 
thickness  of  a   17-mil  lamination.     Dies  must  be  maintained 
in  very  good  condition  to  keep  the  burr  below  two  mils.     The 
effect  of  this  burr  is  to  bring  increased  thickness  and  pressure 


IRON  LOSSES  IN  D.C^MACHINES  491 

at  the  edge  of  the  sheets,  particularly  at  the  teeth.  If  the 
laminations  are  all  turned  one  direction  in  building  and  the 
edges  match  perfectly  the  sheets  might  fit -together  so  accur- 
ately that  the  burr  would  cause  no  extra  thickness.  But  it 
is  impossible  to  obtain  such  accuracy  in  practise  and,  there- 
fore, the  burrs  of  one  sheet  "ride"  upon  the  surface  of  the 
next  sheet,  thus  increasing  the  total  thickness  of  the  built- 
up  iron.  In  -practise,  however,  the  iron  is  pressed  down  to 
approximately  uniform  height  throughout.  This  means  that 
the  burrs  carry  considerable  of  the  pressure  at  the  armature 
teeth  and  there  is  more  or  less  of  a  tendency  to  cut  through 
the  insulating  film  on  the  plates,  thus  increasing  the  eddy 
current  losses.  This  is  'obviously  a  variable  condition  depend- 
ing upon  the  accuracy  of  building,  upon  the  condition  of  the 
dies,  etc.,  and  no  method  of  calculation  can  take  this  loss  into 
account  with  any  accuracy.  In  small  machines  with  low 
voltage  per  unit  length  of  core  i  this  loss  usually  is  not  of  great 
importance.  However,  in  high-speed  large-capacity  machines, 
it  becomes  increasingly  .important  and  in  some,  cases  special 
means  are  used  for  removing  tihe  burr  before  insulating  the 
individual  armature  plates, 

(c)  Another  source  of  iron  loss,  and  one  which  also  is  be- 
yond the  scope  of  calculation,  is  found  in  the  filing  of  armature 
slots  and   cores.     In  ideal  armatures  with  perfect  punchings 
and  assembly,  there  should  be  no  occasion  for  filing.     However, 
the  practise,  in  many  cases  where  the  armature  iron  does  not 
build  up  with  perfectly  smooth  surfaces  in  the  slots,  is  for  a 
limited  amount  of  filing  to  be  done.     Usually   this  takes  off 
only  isolated  high  spots,  sp  that  the  adjacent  laminations  are 
not  bridged  over  to  any  great  extent  by  the  burrs  due  to  filing. 
The  tendency  of  most  workmen  is  to  file  down  to    a  nicely 
polished  surface,  whereas  a  coarse  filing  gives  better  results 
as  it  tends  to  break  the  laminations  away  from  each  other. 
Filing  is  most  harmful  in  machines  having  a  relatively  high 
voltage  per  unit  length  pf  core.     A  milling  cutter  for  cleaning 
out  slots  is  usually  worse  than  a  file,  as  it  produces  greater 
burring  of  the  edges.     However,  if  the  milling  is  followed  by 
filing  with  a  very  coarse  file  the  results  may  be  just  as   satis- 
factory as  with  filing  alone.     Obviously,   no  method  of  cal- 
culation can  show  accurately  the  losses  due  to  such  burring- 

(d)  The  iron  losses  are  affected  to  a  certain  extent  by  pres- 
sure, that  is,  by  the  tightness  with  which  the  core  is  clamped. 


492  ELECTRICAL  ENGINEERING  PAPERS 

The  loss  due  to  this  is  probably  closely  related  to  some  of  the 
preceding  losses,  such  as  bending  and  springing  of  plates, 
effect  of  burrs,  etc.  -In  small  machines  the  effect  of  pressure 
apparently  is  of  little  moment,  but  in  large  very  long  cores  it 
may  become  very  appreciable.  It  is  particularly  noticeable 
in  large  turbo-generator  armatures  where  the  cores  are  very 
wide.  In  such  machines,  in  attempting  to  draw  the  core 
down  to  a  sufficiently  solid  condition  as  a  whole,  the  parts 
next  to  the  end  plates  are  liable  to  receive  abnormal  pressure, 
with  consequent  increase  of  loss  -in  those  parts.  For  this 
reason,  it  is  the  practise  in  some  cases  to  add  an  extra  separation 
of  paper  at  frequent  intervals  near  each  end  of  the  core.  Ex- 
perience shows  that  this  equalizes  the  losses  and  temperatures 
very  materially.  That  this  is  due  to  undue  pressure  and  not 
to  stray  field  or  other  conditions,  is  indicated  by  the  fact  that 
when  high  temperatures  are  found  in  the  iron,  at  each  end 
of  the  core,  very  often  the  condition  can  be  relieved  by  s'mply 
lessening  the  pressure  to  a  comparatively  small  extent.  The 
writer  has  known  cases  where  the  temperature  in  the  end  sec- 
tions of  the  iron  has  been  reduced  30  to  50  per  cent  by  "easing 
off"  the  end  plates.  The  total  loss  in  the  core  may  not  be  re- 
duced very  .much,  for  the  reduction  in  pressure  usually  affects 
only  the  end  sections  to  any  great  extent.  Presumably  this 
loss  is  due  to  increased  contact  between  the  adjacent  plates, 
possibly  from  the  burr,  but  not  entirely  so,  for  similar  results 
have  been  found  in  some  cases  where  the  burr  had  been  fairly 
well  removed  before  enameling  the  plates.  The  character  of 
the  enamel  coating  used  for  insulating  purposes  also  has  some- 
thing to  do  with  this.  , 

In  connection  with  pressure,  the  effect  of  heating  of  the  core 
may  be  considered.  Cases  have  been  noted  where  the  effect  of 
high  temperature  of  the  core  has  been  to  increase  the  pressure 
between  the  laminations,  due  to  expansion.  This  in  turn 
increased  the  loss  and  thus  still  further  increased  the  tem- 
perature. This  effect  has  not  been  uncommon,  to  a  minor 
extent,  but  a  few  cases  have  occurred  where  the  combined 
pressure  and  temperature  cumulatively  have  resulted  in  ex- 
cessive core  temperatures.  In  one  case  which  the  writer  has 
in  mind,  a  certain  large  machine  operated  for  about  two  years 
without  any  noticeably  high  temperature  in  the  core.  Then, 
in  a  comparatively  brief  time,  it  showed  evidence  of  increas- 
ing temperature  until  finally  an  entirely  prohibitive  tempera- 


IRON  LOSSES  IN  D.C    MACHINES  493 

ture  showed  at  one  place.  Examination  showed  that  the  core 
was  very  tight  and  all  evidence  indicated  that  increased  tem- 
perature was  causing  increased  pressure  and  thus  further  in- 
creasing the  loss.  In  this  machine,  fortunately,  the  construc- 
tion of  the  armature  core  and  winding  was  such  that  the  end 
plates  could  be  released  very  easily  about  \  in.  on  each  end. 
This  was  tried  as  an  experiment  and  the  temperatures  all 
returned  to  the  former  normal  of  about  30  deg.  cent.  rise. 
As  an  interesting- side  issue,  it  may  be  mentioned  that  on  this 
machine  the  armature  teeth  at  each  end  of  the  core  had  been 
breaking  off,  although  stout  brass  supporting  fingers  had  been 
used.  Apparently  under  the  increased  pressure,  due  to  heating, 
the  fingers  would  be  bent  away  from  the  core,  thus  releasing 
the  tooth  laminations.  Repeated  tightening  of  the  brass 
fingers  did  not  relieve  this  condition.  However,  when  the  end 
plates  were  released  J  in.  at  each  end  of  the  core,  the  brass 
fingers  were  then  sprung  in  against  the  teeth  and  afterwards 
remained  in  position  so  that  no  breakage  of  tooth  laminations 
was  ever  reported  afterwards. 

Obviously,  with  losses  dependent  upon  pressure,  no  extreme 
accuracy  in  calculation  of  such  losses  is  possible.  However, 
in  moderately  small  size  machines,  and  especially  in  those  of 
very  moderate  frequency  and  of  very  low  voltage  per  unit 
length  of  core,  the  effect  of  pressure  is  not  serious,  within  a 
moderate  range  of  practicable  pressures. 

(e)  Another  source  of  iron  loss,  but  which  is  not  in  the  arm- 
ature core,  is  that  of  the  pole  face,  ,due  to  the  tufting  or  bunch- 
ing of  the  flux  between  the  field  pole  and  the  armature  teeth, 
where  slotted  armatures  are  used.  Obviously,  with  all  other 
conditions  the  same,  this  pole  face  loss  will  depend  upon  a 
number  of  variables  in  the  lamination  of  the  material  itself. 
The  effect  of  burrs  from  punching,  the  burring  over  of  the 
surface  due  to  turning,  the  effect  of  pressure,  etc.,  all  appear 
in  the  pole  face  loss.  Therefore,  it  is  evident  that  great  ac- 
curacy in  the  calculation  of  such  loss  is  impossible,  in  com- 
mercial apparatus.  There  are  other  conditions  that  affect 
this  pole  face  loss  which  will  be  considered  later  under  this 
subject. 

Armature  Ring  Loss.  The  true  iron  loss  in  the  aimature 
ring  is  dependent  upon  the  total  flux  per  pole,  distribution  of 
flux,  rate  of  change  of  flux,  etc.  The  problem  is  much  com- 
plicated by  the  fkct  that  the  flux  distribution  in  the  ring  usually 


494  ELECTRICAL  ENGINEERING  PAPERS 

is  not  uniform,  that  is,  certain  parts  of  the  core  have  higher 
maximum  densities  'than  other  parts  However,  in  ordinary 
practise  the  core  densities  used  are  relatively  low,  so  that  the 
losses  can  be  Approximated  by  averaging  the  inductions  in 
certain  parts.  However,  the  rate  of  change  of  flux  in  the 
ring  is  dependent,  to  a  certain  extent,  upon  the  flux  distribution 
in  the  air  gap  and  armature  teeth,  and  this  introduces  some 
error,  always  in  the  direction  of  increased  loss. 

The  distribution  of  flux  in  the  armature  ring  is  also  depend- 
ent upon  the  effective  length  of  the  various  flux  paths.  These 
latter  will  naturally  depend  upon  various  conditions,  such 
as  the  number  of  poles,  diameter  of  armature;  flux  distribution 
in  the  air  gap  and  teeth,  etc.  Therefore,  any  method  which 
does  not  take  this  distribution  into  account  is  necessarily  only 
approximate.  However,  in  practise  there  are  so  many  other 
variables,  as  already  described,  in  connection  with  manufactur- 
ing conditions,  such  as  burring,  filing,  etc.,  that  empirical  rules 
have  been  developed,  based  upon  numerous  tests,  which  ap- 
proximate the  armature  core  loss  in  a  standard  type  of  ma- 
chine about  as-  accurately  as  any  attempt  toward  exact  cal- 
culation. 

Armature  Tooth  Losses  at  No-Load.  Apparently  the  flux 
densities  in  the  armature  teeth  can  be  calculated  with  more 
accuracy  than  in  the  various  parts  of  the  core,  for  in  the  teeth 
the  fluxes  are  limited  to  fairly  definite  paths.  Therefore, 
exclusive  of  the  losses  due  to  manufacturing  conditions,  as 
already  described,  the  tooth  losses  can  be  fairly  accurately 
calculated,  probably  with  much  greater  accuracy  than  many 
other  losses,  as  will  be  described.  The  tooth  losses  may  be 
considered  further  as  follows: 

The  flux  density  in  each  individual  armature  tooth  passes 
through  a  cycle,  indicated  by  the  shape  of  the  field  form.  With 
the  field  form  of  the  shape  illustrated  in  Fig.  1,  the  tooth  den- 
sity will  be  a  maximum  at  A,  and  this  density  will  remain 
practically  constant  as  the  tooth  moves  toward  C  until  the 
point  B  is  reached.  It  will  then  decrease  as  the  ordinate  of 
the  field  form  curve  decreases  and  will  reach  zero  value  at  C, 
The  cycle  of  flux  change  is  not  sinusoidal,  and  therefore,  the 
actual  tooth  iron  loss  should  not  agree  with  that  represented 
by  the  usual  iron  loss  curves  based  upon  sinusoidal  changes 
in  induction.  The  difference,  however,  may  be  relatively 
small  in  the  ordinary  types  of  machines.  The  error  may  be 


IRON  LOSSES  IN  D.C.  MACHINES  495 

taken  care  of  by  some  suitable  correcting  factor,  which  of 
course,  will  be  only  approximate  for  the  average  case 

The  density  in  the  armature  teeth  is  involved  in  the  iron 
loss  This  density  is  not  uniform  over  the  entire  depth  of 
the  tooth,  with  the  usual  parallel-side  slots,  for  the  section 
of  the  tooth  tapers  off.  This  difference  of  section,  in  small 
diameter  machines,  may  be  very  considerable.  However,  a 
higher  density  at  the  base  of  the  tooth,  tending  to  give  higher 
iron  loss,  is  compensated  for,  to  some  extent,  by  the  reduced 
volume  of  material.  In  consequence,  the  mean  section  at 
some  point  from  one-half  to  two-thirds  the  way  down  the 
tooth  may  be  taken  and  the  mean  density  and  volume  of  ma- 
terial, based  upon  this  section,  may  be  used  for  approximating 
the  iron  loss.  The  accuracy  of  this  method  will  be  dependent, 
to  some  extent,  upon  the  actual  density  used.  For  instance, 

if  both  the  minimum  and 
maximum  densities  in  the 
tooth  are  relatively  low,  then 
the  loss  calculated  for  the 

[ |"  mid-point  density,  at  the  mid- 

j  j    point  section,  will  be  closer 

B  A    to  the  true  loss  than  if  the 

maximum  density  is  exces- 
sively high. 

pIG>  i  Armature  Copper  Eddy  Cur- 

rent Loss  at  No-Load.     There 

may  be  a  number  of  eddy  current  losses  in  the  copper,  some  of 
which  are  of  a  minor  nature.  However,  there  may  be  two 
relatively  large  losses,  depending  upon  the  design  of  the  ma- 
chine. One  of  these  is  due  to  the  flux  from  the'  field  poles 
entering  the  armature  slots  and  cutting  the  conductors.  This 
is,  to  a  certain  extent,  a  function  of  the  saturation  of  the  tops 
of  the  armature  teeth.  It  is  also  dependent  upon  the  width 
of  the  slot  opening  Compared  with  the  iron-to-iron  clearance. 
At  first  thought,  one  would  say  that  the  larger  the  air  gap 
the  more  would  the  lines  from  the  pole  pass  into  the  tooth 
top.  However,- the  opposite  is  the  case,  for  the  larger  the  gap, 
the  nearer  do  the  lengths  of  paths  into  the  slot  approach  to 
the  iron-to-iron  clearance,  in  percentage. 

In  moderate  size  machines  with  relatively  small  air  gaps 
and  moderate  slot  widths,  the  eddy  current  loss  from  fringing 
into  the  top  of  the  slot  is  comparatively  ,small,  and,  as  a  rule, 


496 


ELECTRICAL  ENGINEERING  PAPERS 


no  special  precautions  need  be  taken  to  minimize  it.  This 
particular  loss  is  usually  greatest  in  high-voltage,  large-capac- 
ity turbo-alternators,  where  relatively  wide  slots,  up  to  1  5 
in.  or  more,  may  be  used,  and  where  the  air  gaps  are  very  large. 
In  such  cases  lamination  of  the  top  conductors  to  avoid  eddies 
from  this  cause  may  be  desirable. 

The  second  source  of  eddy  current  loss  in  the  copper,  which 
is  liable  to  be  larger  than  all  others  combined,  is  due  to  the 
peculiarities  of  flux  distribution  in  the  armature  teeth.     Let 
Fig.  2  represent  the  magnetic  conditions  in  a  given  machine. 
It  is  evident  from  this  figure  that  under  the  central  flat  part 
of  the  field  form,  the  armature  teeth  are  worked  at  a  uniform 
induction,  assuming  that  there  is  no  field  distortion.     How- 
ever,  at   the   edges  of  the   pole  the  tooth   density  decreases 
slightly.     If  the  saturation  of  the  teeth  under  the  flat  part  of 
the   field   form  is  very  high 
(materially     above      120,000 
lines  per  sq.  in.),  the  ampere- 
turns  required  to  magnetize 
the  teeth  may  be  very  con- 
siderable.    However,    at  the 
edge  of  the  pole  a  compara- 
tively small  decrease  in   the 
flux  density  in  the  teeth  (15 
to  20  per  cent)  will  mean  a 
relatively  enormous  decrease 
in  the  ampere-turns  for  the  teeth.     For  instance,  the  tooth  c 
in  Fig.  2,  under  the  central  flat  part  of  the  field  form,  may 
require  2000  ampere-turns,  while  the  next  tooth  bt  under  the 
pole  edge,  which  is  worked  at  possibly  20  per  cent  lower  den- 
sity, may  require  only  10  to  20  per  cent  as  many  ampere-turns. 
Assuming  such  conditions,  then  the  magnetic  potential  at  the 
top  of  tooth  c  will  be  higher  than  that  at  the  top  of  b  by  1600 
to  1800  ampere-turns.     Therefore,   under  this  condition   there 
will  be  a  very  considerable  flux  across  the  slot  between  c  and 
b.    A  little  earlier  or  a  little  later  in  the  rotation  this  flux  across 
this  slot  will  not  exist  to  any  extent,  for  the  ampere-turns  for 
b  and  c  will  then  both  be  comparatively  low  or  very  high, 
while  the  difference  between  them  will  be  small.     In  conse- 
quence, near  each  pole  edge,  there  is  a  very  rapid  rise  and  fall 
of  flux  across  the  armature  slots.     This  is  illustrated  in  Fig.  2. 
Obviously,  the  armature  conductors  lying  in  the  path  of 


PIG  2 


IRON  LOSSES  IN  D.C.  MACHINES 


497 


this  flux  will  be  the  seat  of  e.m.fs.  which  will  tend  to  set  up 
local  currents,  the  value  of  which  will  be  some  function  of 
the  e  m.f.  producing  the  current,  of  the  dimensions  of  the 
conductor,  etc  If  the  flux  across  the  slot  is  large,  this  e.m.f. 
may  also  be  considerable,  for  the  rate  of  this  flux  change  will 
be  high  compared  with  the  normal  frequency  of  the  machine. 
As  the  e.m  f .  generated  is  a  function  of  the  maximum  difference 
between  the  ampere-turns  required  for  two  adjacent  teeth 
and  as  the  loss  in  any  given  case  will  vary  as  the  square  of  the 
e.m  f  ,  obviously  the  loss  in  one  slot  will  vary  as  the  square  of 
the  maximum  difference  between  the  ampere-turns  of  two 


FIG.  3 


adjacent  teeth.  At  very  high  saturation,  the  maximum  dif- 
ference between  the  ampere-turns  required  for  two  adjacent 
teeth  may  be  relatively  high  and  the  loss  may  be  correspond- 
ingly great.  Due  to  the  shape  of  the  permeability  curve  of 
steel  at  very  high  saturation,  the  difference  between  the  ampere- 
turns  of  two  adjacent  teeth  may  increase  faster  than  the  square 
of  the  terminal  e.m.f.  Therefore,  the  eddy  current  loss  due  to 
this  cause  may  increase  faster  than  the  fourth  power  of  the 
total  induction  per  pole.  Evidently,  therefore,  it  is  desirable 
to  keep  these  eddy  current  losses  at  a  low  value  at  no-load, 
for  the  high  tooth  ampere-turns  under  the  distorted  field  con- 
ditions of  full  load  will  tend  to  increase  the  percentage  of  these 
losses  very  greatly.  Fig.  3 ,  shows  a  characteristic  core  loss 


498  ELECTRICAL  ENGINEERING  PAPERS 

curve  for  a  generator  in  which  the  copper  loss,  due  to  the  above 
cause,  is  very  large  at  the  higher  e.m.fs. 

Several  years  ago,  the  writer  spent  considerable  time  in 
attempting  to  determine  the  value  of  this  eddy  current  loss  at 
no-load.  Neither  sufficient  nor  entirely  satisfactory  data 
were  available.  From  the  data  at  hand,  the  following  em- 
pirical formula  was  derived,-  which  appeared  to  accord  fairly 
well  with  the  facts  in  a  number  of  cases  whjch  were  worked 
out.  This  formula  applies,  however,  only  to  windings  with 
two  conductors  in  depth  per  slot.  This  formula  for  the  loss 
in  conductors  is 


Watts  loss  =  ft  Mi™  + 


a      =  Maximum  ampere-turns  for  one  tooth. 

Ve   =  Total  volume  of  copper,  in  cubic  inches,  in  one  slot. 

R»    =  Revolutions  per  second. 

p     =  Number  of  poles. 

The  values  for  the  watts  eddy  current  loss  in  the  copper 
were  approximated  by  taking  the  iron  loss  curves  at  the  lower 
e.m.L  values  (where  the  above  eddy  current  loss  would  be  very 
low),  and  then  projecting  them  for  the  higher  values  accord- 
ing to  the  laws  which  the  iron  loss  alone  should  follow.  The 
difference  between  this  corrected  iron  loss  and  the  actual  test 
curve  was  assumed  to  consist  largely  of  eddy  current  loss.  As 
this  difference  usually  increased  very  rapidly  at  higher  induc- 
tions, the  above  assumption  was  in  line  with  the  preceding  state- 
ments that  this  eddy  current  loss  may  increase  much  more 
rapidly  than  the  square  of  the  flux.  In  this  determination 
obviously  the  pole  face  loss  would  have  to  be  taken  into  ac- 
count. This  was  taken  care  of  as  far  as  possible,  by  tests 
with  relatively  large  air  gaps,  the  pole  face  loss  thus  being 
very  small. 

It  may  be  noted  that  in  the  above  empirical  formula,  the 
ampere-turns  for  one  tooth  under  the  maximum  field  has  been 
used,  instead  of  the  maximum  difference  between  the  ampere- 
turns  of  two  adjacent  teeth.  However,  the  tests  indicated 
in  general  that  the  maximum  difference  was  approximately 
proportional  to  the  maximum  ampere-turns  in  one  tooth  and, 
therefore,  it  was  simpler  to  use  the  total  turns  for  6ne  tooth. 
Also,  where  the  total  tooth  ampere-turns  are  tapered  off  over 


IRON  LOSSES  IN  D.C    MACHINES  499 

several  teeth,  the  difference  between  the  ampere-turns  for 
adjacent  teeth  is  reduced,  but  more  slots  and  more  copper  is 
involved,  whereas  the  empirical  formula  includes  only  the 
copper  for  one  slot.  Various  attempts  were  made  to  include 
all  the  different  factors,  such  as  ampere-turns  across  each 
slot,  number  of  slots,  number  of  conductors  involved,  counter 
magnetomotive  force  of  the  eddy  currents,  etc.,  but  none  of 
the  resulting  formulas  gave  as  consistent  results  as  the  above. 
It  must  be  admitted  that  this  formula  is  an  extremely  crude 
one,  but  it  happened  to  fit  most  of  the  cases  that  the  \\riter 
was  able  to  analyze.  In  deriving  this  equation,  it  was  found 
that  if  the  loss  was  assumed  to  vary  directly  as  the  square  of 
the  tooth  ampere-turns,  then  it  would  be  too  great  at  very 
high  tooth  saturation.  At  high  tooth  densities,  the  flux  across 
the  slots,  at  the  pole  edge,  is  distributed  over  several  successive 
slots,  so  that  the  maximum  difference  between  the  ampere- 
turns  of  two  adjacent  teeth  bears  a  lower  proportion  to  the 
ampere-turns  for  one  tooth.  Also,  at  very  high  tooth  densi- 
ties there  is  more  or  less  fringing  of  flux  down  through  the  slot, 
in  parallel  with  the  tooth  flux,  and  this  makes  the  determina- 
tion- of  the  actual  tooth  flux  difficult.  In  the  formula,  there- 
fore, the  term  (1000  +  a)2  is  used  in  place  of  az  to  take  care 
of  these  conditions.  This  term,  however,  is  obviously  wrong, 
in  that  it  indicates  a  loss  when  the  tooth  saturation  is  negli- 
gible. However,  this  loss  under  low  saturation  usually  works 
out  from  the  formula  to  be  of  comparatively  small  value,  so 
that  the  error  is  not  of  much  importance. 

A  modified  formula,  which  agrees  with  the  above  fairly 
closely  at  high  saturations,  but  gives  no  'loss  at  zero  saturation, 
is  the  following: 

Watts  loss  =    135F.*.M4000 


The  following  table  shows  the  comparison  of  the  copper 
eddy  loss  compared  with  the  calculated  loss  by  the  first  formula 
above,  for  a  number  of  machines.  It  will  be  noted  that  the 
agreement  is  not  particularly  close,  but  possibly  as  good  as 
could  be  expected,  considering  how  -the  test  losses  were  de- 
rived. It  may  be  stated  that  these  were  all  comparatively 
old  types  of  machines,  for"  in  recent  years  great  pains  have 
teen  taken  to  eUinitiate  large  eddy  losses  of  this  character,  so 


500 


ELECTRICAL  ENGINEERING  PAPERS  \ 


that  it  was  necessary  to  go  to  old  machines  in  order  to  obtain 
exaggerated  cases. 


Eddy  loss 

Eddy  loss 

Kilowatt 

Terminal 

Rev 

No   of 

Calculated 

estimated 

calculated 

rating 

e  m.f 

per 

poles 

ampere  turns 

from 

from 

mm 

in 

test  curve; 

formula; 

tepth 

kw 

kw 

340 

600 

C83 

G 

1400 

2  7 

3  4 

" 

700 

" 

3000 

10  0 

9  7 

500 

600 

22-5 

10 

2500 

7  5 

6  15 

" 

625 

" 

4000 

17  5 

12  5 

750 

250 

514 

10 

1200 

1   5 

2  76 

" 

320 

" 

7200 

32  0 

3Q  3 

750 

550 

514 

8 

1500 

4  5 

3  3 

" 

700 

" 

7000 

33  0 

3G  5 

1000 

250 

514 

12 

000 

2  5 

2  2 

u 

330 

" 

0000 

50  0 

41   3 

1000 

600 

514 

10 

1225 

4      3 

4   C 

M 

700 

" 

2iSO 

10  0 

11   3 

2000 

575 

300 

14 

3000 

12  0 

7    1 

*' 

675 

" 

7000 

'•{(.  0 

28  2 

Losses  at  No-Load.  Il  has  long  been  known  that, 
with  open  slot  armatures,  there  arc  liable  to  be  considerable 
losses  in  the  field  pole  faces  due  to  bunching  of  the  magnetic 
flux  from  the  armature  teeth  to  the  pole  face,  the  armature 
teeth  thus  acting  as  small  poles  of  an  "inductor"  type  alter- 
nator, of  which  the  pole  face,  to 
a  small  depth,  serves  the  function 
of  the  armature  core. 

While  the  effect  of  this  "inductor 
pole  action  has  long  been  known, 
the  amount  of  loss  due  to  it  has 
frequently  been  underestimated, 
especially  in  machines  with  rela- 
tively small  air  gaps  compared 
with  the  width  of  the  armature 
slots. 


—a b- 


FIG.  4 


The  following  crude  description  will  illustrate  the  extent  of 
the  variations  in  flux  in  the  air  gap  due  to  the,  open  armature 
slots.  In  Fig.  4,  a  represents  the  width  of  one  armature  tooth 
and  b  represents  the  width  of  one  armature  slot.  Let  g  rep- 
resent the  single  air  gap  (iron  to  iron). 

In  the  lower  diagram,  which  represents  the  flux  distribution 
in  the  air  gap,  let  Ba  represent  the  flux  density  in  the  air  gap 


IRON  LOSSES  IN  D.C    MACHINES  501 

under  the  armature  teeth  and  Bb  the  minimum    flux  density 
corresponding  to  the  center  of  the  slot. 

Then,  a  X  Ba  —  the  total  flux  under  one  tooth,  for  unit 
width  of  core,  and  b  (Ba  —  Bb)  c  —  the  total  decrease  in  flux 
for  the  space  covered  by  one  slot  represents  the  average 
height  of  the  curve  def  in  Fig.  4.  If  this  curve  be  assumed  to 
be  sine  shaped,  then  c  would  be  0.636  Any  other  shape  which 
would  be  likely  to  be  found  in  practise  would  not  be  far  from 
this  value.  A  V-shape,  as  one  extreme,  would  give  c  =  0  5 
while  a  circular  shape,  as  the  other  extreme,  would  give  c  = 
0.784.  Apparently  the  value  would  lie  somewhere  between 
these  two  extremes. 

In   calculating  the   effective   gap   from   the  above  diagram 
and  assumptions,  the  following  equation  would  be  obtained- 


Increased  gap  g>  =  g  X 


(a  +  _     (Ba  _  Bk)e 


°r'  *'  =  *  X  .        b(B\-Bb)c    =  *  X  ,         (6)  (B.  -  ft)  c 
(a  +  b)  Ba  (a  +  b)  (Ba) 

The  resemblance   of   this   equation   to   Carter's   well-known 
equation  for  the  increased  air  gap  may  be  seen  at  once. 


In  Carter's  equation,  gr  =  g  X  , 


x       a  +  b 

Comparing  these  two  formulas,  it  is  evident  that  k 

* 

(Bq   -  gft)  C 


An   extremely   close   approximation   to   k   can   be   obtained 

J?_ 

frt>m  the  empirical  formula  k  =  — ^    .  This  holds   closely 

V    ,  -f-+"« 


502  ELECTRICAL  ENGINEERING  PAPERS 

to  Carter's  curve  over  almost  the  entire  range.     Equating  the 
above  two  values  of  k,  we  obtain  the  equation 

Eg  -Bb        1        (ft) 
Ba 


This  gives  the  ratio  of  the  flux  density  at  the  middle  of  the 
slot  to  the  flux  density  under  the  tooth. 

As  an  example  of  what  these  relative  values  may  be,  assume 
that  a  =  by  or  the  slot  width  =  the  tooth  width,  and  that 
g  =  0.25  ft,  which  is  extreme  for  large  a-c.  or  d-c.  generators, 
but  not  unusual  for  induction  motors.  Assuming  c  =  0.635, 

•o 

then  ~   =  0.3,  or  the  density  under  the  middle  of  the  slot 

JDa 

is  only  0.3  of  that  under  the  tooth.  With  these  same  values, 
the  value  of  g'  becomes  1.2859,  pr  the  gap  increase  is  28.5  per 
cent,  which  is  not-  unusual  for  some  machinery.  Obviously, 
a  variation  in  the  flux  density  at  the  pole  face  of  70  per  cent 
should  tend  to  give  high  iron  losses  in  the  pole  face  itself.  In 
fact,  some 'of  the  inductor  type  alternators  which  were  in  com- 
mon use  a  few  years  ago  did  not  give  variations  in  armature 
flux  materially  better  than  indicated  by  the  above  value.  Such 
proportions  as  the  above  example  would,  therefore,  be  fairly 
good  for  an  inductor  alternator. 

The  above  analysis  is  given  simply  to  furnish  a  means  for 
determining  the  possible  variations  in  the  flux  density  which 
may  be  obtained  with  open  slots.  This  gives  a  much  better 
conception  of  the  problem  than  can  usually  be  obtained  directly 
from  Carter's  formula  for  the  increased  length  of  gap.  It 
also  gives  a  good  idea  of  the  possibilities  of  tooth  losses  in  those 
cases  where  the  teeth  of  ,one  element  or  member  of  a  machine 
alternately  pass  under  the  teeth  and  slots  of  the  other  member. 

Considerable  work  has  been  done  aL  various  times  to  de- 
termine the  pole  face  losses  due  to  open  armature  slots.  The 
difficulty  in  determining  a  workable  formula  is  very  consider- 
able, as  there  are  many  conditions  which  may  directly  or  in- 
directly affect  this  loss-  For  example,  the  thickness  of  the 


IRON  LOSSES  IN  D.C.  MACHINES  503 

laminations,  or  the  material  in  the  pole  face,  may  have  an  in- 
fluence. Any  general  formula  for  this  loss  would  require 
different  constants  for  different  types  of  pole  faces.  One 
formula  for  this  loss  has  been  given  by  Professor  C.  A.  Adams 
and  his  associates.*  The  formula  is  very  complex  and  some- 
what difficult  to  use. 

A  much  simpler  formula  for  laminated    pole  faces  is  as  fol- 
lows, ior  0  031-in.  laminations: 


Watts  loss 


CfW*gL          R,g 


E  =  Generator  voltage. 

b  —  Width  of  slot. 

g  =  Single  air  gap  (iron  to  iron) . 

Wt  =  Arm  a  lure  wires  in  series. 

L  =  Width  of  pole  face. 

Cf  =  Field  form  constant. 

Se  =  Total  slot  space  =  width  of  slot  X  No.  of  slots. 

It  is  very  difficult  to  obtain  any  reliable  data  on  pole  face 
losses  alone,  for  other  core  losses  are  liable  to  be  included  in 
any  tests.  Variation  of  air  gap.,  with  everything  else  in  the 
construction  unchanged,  gives  a  partial  measure.  However, 
this  changes  the  field  form  somewhat  and  thus  modifies  the 

*Pole  Face  Losses,  by  Comfort  A.  Adams,  A,  C.  Lamer,  C.  C.  Pope 
and  C.  O  Schooley.  P*oc.  A.  I.  E.  E.,  July,  1909,  page  1151. 


(Bo  V  4       /    v     V-66 
"loO     x(lo/ 


x 


Wp  *  Pole  face  loss. 

Sp    =  Section   of  one  pole  face   (average  section  where  the 

density  is  BQ). 
p       «=  Number  of  poles. 
0.000462    =  Constant  for  &-in.  laminations. 

Bg     —  Density  in  the  gap  over  the  section  Sp. 

v        — '  V^ocity  of  the  armature  surface  in  feet  per  second. 

q        =^  Ratio  of  iwidth  of  slot,  to  air  gap. 


504  ELECTRICAL  ENGINEERING  PAPERS 

tooth  saturation  and  the  tooth  and  eddy  losses,  to  a  certain 
extent,  thus  rendering  doubtful  the  pole  face  component. 

The  above  formula  is  necessarily  approximate  and  applies 
only  to  laminated  pole  faces.  The  effect  of  cutting  away 
part  of  the  laminations  in  order  to  produce  high  saturation 
at  the  pole  face  is  not  included.  However,  it  is  possible  that 
this  may  not  influence  the  loss  to  any  great  extent.  The 
greater  part  of  the  loss  is  represented  by  eddy  currents,  and 
cutting  away  part  of  the  laminations  will  tend  to  break  up  the 
losses  between  plates  and  this  may  compensate  to  a  con- 
siderable extent  for  the  higher  densities  in  the  remaining  plates 
It  is  hoped  that  some  time  in  the  future  more  complete  data 
may  be  obtained,  over  a  sufficiently  wide  range  of  conditions, 
to  cover  the  practical  range  of  ordinary  design. 

The  following  table  covers  a  number  of  machines  with  ad- 
justable air  gaps  in  which  the  pole  face  losses  were  worked 
out  according  to  the  above  formula.  Also,  the  total  calculated 
and  the  total  test  losses  are  given,  to  indicate  the  agreement  in 
a  general  way.  The  writer  is  perfectly  willing  to  admit  that 
he  believes  that  the  fairly  close  agreements  between  some  of 
the  calculated  and  test  totals  are  largely  accidental,  and  they 
should  not  be  taken  as  proof  of  any  great  accuracy  of  the 
methods. 

It  is  obvious  from  this  table  that  the  pole  face  losses  may 
be  comparatively  high  in  some  cases,  provided  the  formula  is 
reasonably  correct.  Evidently,  if  these  losses  could  be  cal- 
culated with  any  great  accuracy,  the  design  of  thfe  machine 
might  be  considerably  modified,  compared  with  more  recent 
practise,  with  advantageous  results.  The  pole  face  losses  will 
evidently  be  greatly  increased  by '  field  distortion  when  the 
machine  is  carrying  lo^d.  Eddy  currents  in  the  copper  are 
'also  affected  by  field  distortion,  and  a  correct  method  of  cal- 
culating both  the  eddy  current  and  the  pole  face  losses  with 
various  loads  should  lead  to  considerable  modification  in  the 
proportions  of  d-c.  machines,  in  general, 

Stray  Losses.  Under  this  heading  may  be  included _a  number 
of  no-load  losses  which  are  usually  of  a  minor  nature.  Among 
these  may  be  included  secondary  losses  in  the  armature  wind- 
ing due  to  unsymmetrical  cross-connections  or  unbalanced 
voltages  in  parts  of  the  winding  which  are  connected  in  parallel, 
There  are  various  possibilities  for  losses  from  this  source  and1, 
tn  consequence,  it  is  always  advisable  to  use  armature  wind- 


IRON  LOSSES  IN  D  C   MACHINES 


505 


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506  ELECTRICAL  ENGINEERING  PAPERS 

ings  which  are  as  symmetrical  as  possible.  Also,  the  arrange- 
ment of  the  winding  should  be  such  as  always  to  generate  bal- 
anced e.m.fs.  in  parallel  circuits.  This  condition  is  not  in- 
frequently overlooked  in  the  design  of  direct-current  machines. 

A  second  cause  of  undue  loss  in  the  armature  winding  may 
be  occasioned  by  short-circuiting  one  or  more  of  the  armature 
coils  under  an  active  field.  The  brushes  may  be  shifted  from 
the  magnetic  neutral  point  so  that  some  of  the  armature  con- 
ductors are  short-circuited  under  the  main  field  flux;  or  the 
neutral  point  may  be  so  narrow  and  the  brush  so  wide  that 
some  of  the  armature  turns  are  short-circuiting  in  an  active 
field,  even  when  the  brush  is  set  for  the  no-load  neutral.  An 
armature  winding  which  is  considerably  "chorded"  in  a  field 
with  a  narrow  neutral  point  may  have  two  sides  of  a  coil  short- 
circuited  in  fields  of  the  same  polarity.  The  e.m.fs.  in  the  two 
sides  of  the  coil  should,  therefore,  balance  each  other  if  the 
brush  is  set  at  the  true  neutral.  However,  if  the  brush  short- 
circuits  several  coils  or  turns,  obviously  only  one  of  them  can 
be  at  the  true  neutral  and  have  balanced  e.m.far.  set  up  in  its 
two  halves.  The  other  turns  may  have  more  or  less  local 
current  in  them,  which  may  be* a  source  of  considerable  loss. 

A  third  condition  may  occur  when  there  are  considerable 
pulsations  in  the  reluctance  in  the  air  gap  under  the  main, 
poles  as  the  armature  teeth  move  under  the  poles.  This  vary- 
ing reluctance  usually  gives  varying  main  flux  and  at  a  relatively 
high  frequency.  The  armature  coils  short-circuited  by  the 
brashes  will  act  as  secondaries  to  these  .pulsating  flaxes  and 
in  consequence  there  may  be  some  loss  in  the  short-circuited 
cofils  due  to  this  cause.  Any  solid  parts  of  the  yoke  or  poles 
may  also  have  losses  due  to  this  cause.  Usually,  however, 
such  losses  are  small. 

A  fourth  source  of  loss  may  rise  from  stray  fluxes  fronL'the 
main  fields  to  the  armature,  which  do  not  pass  through  well 
laminated  parts  of  the  armature  core.  For  instance,  the 
ventilating  spacers  may  be  so  dimensioned  and  shaped  that 
eddies  can  be  set  up  in  them.  Also,  the  finger  plates  at  each 
end  of  the  core,  the  end  plates,  etc.,  may  cany  light  fluxes 
which  produce  some  loss.  Bands  on  the  armature  core  or 
at  the  ends  may  also  be  the  seat  of  e.m.fs.  and  will  have  some 
loss  in  them.  Thes'e  losses  are  difficult  to  determine,  and, 
in  practise,  should  be  eliminated  as  far  as  possible. 


IRON  LOSSES  IN  D  C.  MACHINES 


507 


FULL  LOAD  LOSSES 

It  is  evident  from  the  foregoing  that  the  no-load  core  losses 
are  dependent  upon  so  many  variable  conditions  that  there 
can  be  no  great  accuracy  in  predetermining  such  losses  unless 
all  the  details  of  construction,  material,  treatment,  etc.,  are 
known  for  each  individual  machine.  The  impossibility  of 
accurate  calculation  is  shown  by  the  fact  that  the  individual 
machines  built  on  the  same  stock  order  will  vary  considerably 
from  each  other,  especially  in  certain  types. 

While  the  no-load  losses  are  difficult  to  predetermine,  the 
full  load  losses  are  still  much  more  difficult  to  calculate,  as 
will  be  shown  in  the  following  rough  analysis.  Here,  the 
effects  ot  flux  distortion  by  the  armature  magnetomotive  force 
tend  to  exaggerate  the  pole  face  losses  and  those  in  the  arma- 
ture copperj  which  are  the  two  relatively  large  losses  which 

are  most  difficult  to  calculate 
at  no-load.  Also  commuta- 
tion and  brush  losses,  due 
to  load,  now  enter  into  the 
problem.  The  individual  core 
losses  may  be  considered 
briefly  as  follows: 

Armature  Ring  Lo$sy  with 
Load.  This  loss  should  not 
change  greatly  with  load, 
provided  the  total  flux  at  load  is  practically  the  same  as  at 
no-load.  Under  this  condition  a  variation  in  the  distribu- 
tion of  this  flux  is  about  the  only  factor  which  should  pro- 
duce any  material  change  -in  loss.  The  full  load  field  form 
may  be  illustrated  by  Fig.  5.  It -is  evident  from  this  figure 
that  the  flux  is  now  crowded  toward  one  pole  edge  and, 
therefore,  the  major  part  is  concentrated  in  a  narrower 
space.  The  average  length  of  the  flux  path  may,  therefore, 
be  somewhat  greater  than  at  no-load,  but  in  some  cases  this 
may  tend  to  distribute  the  flux  more  uniformly  through  the 
depth  of  the  ring.  However,  where  the  flux  enters  the  core 
at  the  base  of  the  teeth  there  will  *be  slightly  more  crowding 
and,  therefore,  somewhat  ir^creased  loss.  Taking  everything 
into  consideration  it  would  appear  that,  in  general,  the  arm- 
>ature  ring  loss  can  be  considered  as  practically  constant  with 
constant  t£&al ,  ftuix;  apd;  .speed^  independent  of '  the  yariation 
in.  ' 


FIG.  5 


508  ELECTRICAL  ENGINEERING  PAPERS 

In  variable-speed  and  adjustable-speed  d-c.  machines,  the 
armature  ring  loss  may  vary  over  a  wide  range  due  to  changes 
in  total  flux  and  speed.  Such  cases  are  difficult  to  calculate 
with  any  degree  of  accuracy,  although  no  more  so  than  other 
losses  in  the  same  machines. 

Armature  Tooth  Loss,  uiih  Load.  As  shown  by  Pig.  5,  the 
tooth  flux  density  at  one  edge  of  the  pole  is  decreased  and  at 
the  other  edge  is  increased  when  the  field  flux  is  distorted  by 
the  armature  magnetomotive  force.  The  increased  density 
in  the  armature  teeth  means  increased  iron  loss  and,  if  the  dis- 
tortion is  very  great,  the  increase  in  tooth  loss  may  be  very 
large,  being  in  some  cases  even  doubled  or  trebled,  compared 
with  the  no-load  tooth  loss.  No  direct  rule  can  be  given  for 
the  calculation  of  this  loss,  except  that  it  may  be  determined 
approximately  by  calculating"  the  flux  distribution  with  oad 
and  thus  determining  the  flux  densities  in  the  teeth. 

In  variable-speed  and  adjustable-speed  machines,  particu- 
larly in  the  latter,  the  tooth  loss  with  load  will  be  affected  very 
considerably  by  changes  in  both  speed  and  total  flux.  In 
variable-speed  machines  of  the  series  type,  reduction  in  speed 
usually  accompanies  increase  in  total  flux,  so  that,  as  regards 
the  losses,  one  effect  partly  neutralizes  the  other,  so  that  the 
increase  in  tooth  loss  with  load  may  be  less  than  in  a  constant- 
speed  machine.  In  adjustable-speed  machines,  however,  es- 
pecially in  those  of  constant  horse  power  and  constant  voltage, 
the  tooth  losses  will  vary  over  a  very  wide  range  with  change 
in  speed.  Here,  the  armature  magnetomotive  force  is  con- 
stant (assuming  a  constant  horse  power)  and  the  field  flux  is 
varied  from  a  maximum  value  at  lowest  speed  to  one-quarter 
value  at  four  times  speed,  assuming  a  four-to-one  range.  The 
total  flux,  therefore,  varies  inversely  as  the  speed  and  the  two 
effects  should  nearly  compensate  each  other,  as  regards  losses, 
if  it  were  not  for  the  variation  in  flux  distortion.  At  lowest 
speed,  with  considerable  saturation  in  the  pole  horns  and 
armature  teeth,  the  armature  magnetomotive  force,  even  if 
relatively  large  compared  with  the  field  magnetomotive  force, 
may  not  produce  very  large  distortion,  so  that  the  tooth  loss 
is  not  increased  excessively  over  the  no-load  tooth  loss.  How- 
ever, as  the  field  is  weakened,  the  armature  magnetomotive 
force  remaining  constant,  the  distortion  is  relatively  increased, 
so  that  the  peak  value  of  the  distorted  field  may  remain  almost 
constant  in  height.  As  the  armature  tooth  losses  are  dependent 


IRON  LOSSES  IN  D.C   MACHINES 


509 


upon  the  peak  value  of  this  field,  then  obviously  the  combined 
effect  of  this  field  and  the  increase  in  speed  will  mean  very 
greatly  increased  tooth  losses.  With  very  low  field  magneto- 
motive force,  the  distortion  may  be  so  great  as  to  give  a  double 
peak,  as  indicated  in  Fig.  6.  This  double  peak  gives,  to  some 
extent,  the  effect  of  a  double  frequency  and  thus  further  in- 
creases the  loss. 

Eddy  Currents  in  Copper.  When  the  field  form  is  distorted, 
with  load,  the  ampere-turns  in  the  teeth  at  one  pole  corner 
are  greatly  increased,  while  those  at  the  other  corner  are  de- 
creased. Therefore,  there  will  be  an  increased  loss  in  the 
copper  at  one  pole  edge  and  a  decreased  loss  at  the  other  pole 
edge.  However,  as  this  loss  at  high  inductions  will  vary  al- 
most as  the  square  of  the 
ampere-turns  in  the  armature 
teeth,  it  is  evident  that  the  re- 
duction in  the  loss  at  one  pole 
corner  may  be  small  compared 
with  the  increase  in  loss  in  the 
copper  at  the  other  pole  corner. 
The  resultant  loss  can  be  calcu- 
lated approximately  by  using  the 
formula  already  given  for  no- 
load  conditions,  but  with  the 
ampere-turns  in  the  teeth  based 
on  the  load  conditions.  This 
would  give  a  loss  corresponding 
to  no-load  with  the  maximum 
induction  in  the  teeth  raised  to 
peak  value  with  load.  This  would  include  losses  for  the  two 
pole  corners;  therefore,  the  result  should  be  halved,  as  the 
peak  density  occurs  at  only  one  pole  edge. 

If  the  empirical  formula  given  for  the  copper  loss  repre- 
sents the  facts,  even  to  a  merely  approximate  degree,  the  re- 
sults are  very  startling  when  applied  to  some  of  the  old-time 
machines.  The  calculations  show  that  in  some  cases  the 
eddy  current  copper  loss  at  heavy  load  was  several  times  greater 
than  at  no-load.  This  should  be  true,  but  to  a  much  less  extent, 
in  more  modern  types  of  machines.  The  results  indicate  that 
in  many  cases  there  will  be  considerable  gain  by  reducing 
the  field  distortion  through  high  saturation  in  the  pole  face, 
pole  horns,  etc.  This  saturation,  however,  would  have  to  be 


FIG.  6 


510  ELECTRICAL  ENGINEERING  PAPERS 

so  arranged  as  to  give  the  most  beneficial  field  distribution 
with  load,  and  haphazard  methods  of  cutting  off  pole  corners, 
without  regard  to  the  field  form  with  load,  would  have  to  be 
avoided.  In  fact,  in  the  past,  the  cutting  away  of  pole  corner 
laminations,  in  many  cases,  has  been  largely  for  the  purpose 
of  improving  commutation,  and  not  to  obtain  the  best  field 
form  with  load. 

Pole  Face  Losses,  with  Load.  The  pole  face  losses  will  obvi- 
ously be  affected  locally  by  change  in  the  flux  density  in  the 
air  gap  or  at  the  pole  face.  Field  distortion  will  tend  to  increase 
the  loss  at  one  pole  corner  and  decrease  it  at  the  other.  The 
increase  will  usually  considerably  exceed  the  decrease,  but  the 
resultant  will  not  be  increased  in  anything  like  the  same  pro- 
portions as  the  copper  eddy  current  losses  under  the  pole  corners 
are  increased  with  load.  A  rough  approximation  for  the  in- 
creased iron  loss  could  be  obtained  by  comparing  the  squares 
of  the  densities,  at  several  points  along  the  distorted  field 
form,  with  the  squares  of  the  densities  of  the  no-load  field 
form  corresponding  to  the  total  induction. 

As  the  increase  in  pole  face  losses  with  load  will,  in  some 
instances,  be  considerably  less  than  the  increase  in  the  eddy 
current  losses,  it  might  be  advantageous  in  such  cases  to  de- 
crease the  field  distortion  by  pole  face  saturation,  even  at  the 
expense  of  increasing  the  no-load  pole  face  losses.  For  example, 
if,  in  an  extreme  case,  the  air  gap  were  decreased  20  per  cent 
and  the  air  gap  ampere-turns  thus  gained*  were  expended  in 
suitably  saturating  the  pole  face  material,  then  the  full  load 
field  distortion  might  be  much  less  than  with  the  larger  gap, 
with  the  same  total  field  magnetomotive  force.  The  no-load 
eddy  current  copper  losses  would  be  practically  unchanged, 
while  the  no-load  pole  face  loss  would  be  increased.  However, 
the  full  load  pole  face  loss,  due  to  the  reduced  distortion,  might 
be  no  greater  than  with  the  larger  gap,  while  the  eddy  current 
losses  in  the  copper  might  be  very  much  less  than  with  the 
•larger  gap.-  In  consequence,  while  the  total  no-load  losses 
[would  be  increased  somewhat,  the  full  load  loss  would  be  smaller 


than  before,  and  the  carrying  capacity  of  the- machine  would 
be  actually  increased.  This  would  apply,  however,  only  to 
those  machines  where  the  no-load  eddy  current  and  armature 
tooth  losses  are  relatively  high  and  where  the  distortion  is 
rather  large  with  load. 

Stray  Losses^    When  the  machine  is  carrying  load,  the  stray 


IRON  LOSSES  IN  D.C.  MACHINES  511 

losses  given  under  the  no-load  conditions  may  also  exist  and 
at  the  same  time  some  of  these  may  be  greatly  exaggerated. 
Also,  other  losses  may  appear  which  are  not  found  at  no-load. 

Copper  loss  due  to  short-circuiting  the  armature  coils  in  an 
active  field  will  sometimes  be  more  pronounced  than  at  no- 
load,  particularly  in  non-commutating-pole  machines  in  which 
the  brushes  are  shifted  into  an  active  field  to  produce  com- 
mutation. This  field,  as  a  rule,  will  only  be  of  proper  value 
to  produce  proper  commutation  at  some  definite  load,  while 
at  other  loads  there  may  be  very  considerable  local  currents 
in  the  short-circuited  coils  which  may  produce  loss. 

As  the  main  field  flux  is  crowded  toward  one  pole  corner 
and  the  field  form  becomes  more  pointed  in  shape,  the  effect 
of  variable  reluctance  in  the  air  gap  may  become  more  pro- 
nounced than  at  no-load,  and,  therefore,  pulsations  of  the  main 
field  flux  may  cause  more  loss  in  the  short-circuited  armature 
coils. 

Stray  fluxes  from  the  main  poles  will  be  distributed  differ- 
ently from  the  no-load  condition  and  the  densities  of  these 
stray  fields  may  be  considerably  higher  at  certain  points  and 
thus  give  increased  losses. 

Additional  losses  at  full  load  may  be  due  to  fluxes  set  up 
by  the  magnetomotive  force  of  the  armature  winding  itself 
when  carrying  load.  For  instance,  the  armature  winding  will 
set  up  magnetic  fields,  through  the  end  windings,  which  fields 
are  fixed  in  space,  in  a  rotating  armature  machine.  Bands  or 
supporting  parts,  or  other  solid  metal,  rotating  with  the  end 
winding,  may  cut  these  stationary  fields  or  fluxes,  and  thus 
losses  may  be  set  up  which  are  a  function  of  the  load. 

Another  source  of  loss  at  load  may  be  found  in  the  operation 
of  commutation  itself.  A  magnetic  field  or  flux  is  set  up  by 
the  armature  winding  across  the  slots  from  one  commutation 
zone  to  the  next.  At  the  point  of  commutation  this  flux  is  re- 
versed in  direction  with  respect  to  the  armature  conductors, 
and,  therefore,  there  will  be  local  currents  set  up  in  the  arm- 
ature copper  tself,  due  to  this  action.  This,  however,  should 
be  more  properly  charged  to  commutation  loss  rather  than 
to  armature  core  loss. 

The  above  covers  the  principal  core  losses  in  direct-current 
machines.  It  was  the  original  intention  to  analyze  the  core 
losses  in  the  various  types  of  rotating  machines,  but  it  soon 
developed  that  the  subject  was  too  extensive  for  the  scope 


512  ELECTRICAL  ENGINEERING  PAPERS 

of  this  paper,  therefore  it  was  limited  to  d-c.  machines  only. 
However,  many  of  the  conditions  which  hold  for  d-c.  machines 
also  apply,  to  a  certain  extent,  to  many  other  types.  In  ad- 
dition there  are  losses  in  d-c.  machines  which  are  relatively 
large  compared  with  those  in  other  apparatus,  due  to  the  fact 
that  the  tooth  saturation  in  d-c.  machines  is  frequently  carried 
much  higher  than  in  other  apparatus. 

The  foregoing  treatment  of  core  losses  is  qualitative  rather 
than  quantitative,  and  it  deals  with  the  simpler  phenomena 
only.  It  omits  some  very  complex  conditions,  such  as  the 
effect  of  pulsations  in  flux  superposed  on  high  densities,  dis-r 
placed  minor  hysteresis  loops,  etc.,  which  mean  •  additional 
losses.  The  principal  object  of  the  paper  is  to  give  a  better 
idea  of  the  possibilities  and  impossibilities  of  the  problem  of 
core  losses. 


IRON  COMMUTATORS 

FOREWORD— During  the  past  two  years  1916-1918,  frequent  in- 
quiries have  been  made  as  to  why  iron  is  not  used  instead  of 
copper  in  the  construction  of  commutators.  Apparently  it  has 
been  assumed,  in  some  instances,  that  the  present  use  of  copper 
is  more  or  less  of  a  fad  and  that  other  metals,  such  as  iron,  could 
be  used  if  desired. 

This  paper  appeared  in  the  Electric  Journal,  July,  1918. 
—(ED.) 


IN  the  course  of  the  development  of  commutating  machinery 
various  metals  have  been  tried  out  in  commutators,  all  the  way 
from  pure  copper,  both  hard  and  soft,  through  various  alloys  and 
brasses,  cast  copper  of  various  purities,  aluminum,  wrought  iron, 
clear  down  to  cast  iron.  All  such  materials  have  received  con- 
sideration at  some  time  or  other  and  have  been  given  fairly  con- 
clusive tests.  Experience  has  shown  that  all  of  them  could  be  used 
in  commutators  if  one  is  willing  to  pay  the  price,  this  price  being 
in  the  first  cost  of  apparatus,  in  maintenance  or  in  less  satisfactory 
operating  characteristics,  or  a  combination  of  all.  Under  the 
stress  of  war  conditions  it  may  be  necessary  to  pay  any  price,  and 
apparently  this  is  the  condition  which  has  confronted  German 
manufacturers.  In  consequence,  materials  and  constructions  are 
used  simply  as  a  matter  of  necessity  which,  however,  may  not  con- 
form to  conditions  of  even  reasonably  good  design. 

The  use  of  copper  in  modern  commutators  is  a  matter  of 
development  and  not  simply  a  fad.  In  fact,  most  of  the  early 
commutating  machines  used  other  metals  in  their  commutators, 
which  would  now  be  considered  quite  unsuitable.  Cast  copper  and 
various  brasses  and  bronzes  were  used  quite  extensively,  with 
more  or  less  bad  results.  Pure  copper  was  considered  too  expen- 
sive for  general  use  and  it  was  only  after  very  considerable  de- 
velopment that  the  conclusion  was  reached  that  its  apparent 
higher  first  cost  was  more  than  neutralized  by  improved  mainten- 
ance and  operation.  Even  after  pure  copper  had  come  into 
general  use  for  p-nm-miif.fl.tnr  construction,  it  was  not  known,  or 
understood,  why  it  was  so  superior  to  other  metals. 

About  twenty-seven  years  ago  the  writer  made  extended 
tests  on  the  use  of  iron  in  street  railway  commutators.  The  ma- 

513 


514  ELECTRICAL  ENGINEERING  PAPERS 

chines  soon  developed  "high  mica"  and  the  commutators  grad- 
ually blackened,  the  contact  surfaces  blistered  and  sparking 
gradually  increased  until  the  commutating  conditions  became 
practically  impossible  from  the  operating  standpoint.  These 
conditions  repeated  themselves  in  every  test  until  finally  this  con- 
struction was  given  up  as  impracticable.  The  difficulty  was 
blamed  largely  upon  high  mica,  as  it  was  assumed  that,  in  some 
way,  the  metal  wore  below  the  mica,  thus  causing  bad  brush  con- 
tacts, with  resultant  burning  and  blackening.  It  was  not  recog- 
nized that  the  converse  was  really  the  case  and  that  the  high  mica 
was  the  result  of  burning  rather  than  the  cause.  In  all  machines 
of  those  days  there  was  more  or  less  tendency  for  the  commut- 
ators to  "wear "  considerably,  and  it  was  not  recognized  that  such 
was  not  true  mechanical  wear,  but  that  it  was  the  result  of  burn- 
ing away  the  contact  surfaces. 

A  little  later,  the  writer  made  quite  complete  tests  on  the  use 
of  aluminum  on  street  railway  motor  commutators.  This  material 
worked  better  than  iron,  in  the  sense  that  burning  and  blackening 
and  high  mica  did  not  appear  as  quickly  as  with  the  iron.  How- 
ever, like  the  iron  commutator,  there  was  no  tendency  to  polish, 
but  the  commutator  soon  assumed  a  dull  appearance  which 
gradually  changed  to  a  blackened  and  burnt  condition. 

Bronzes  and  brasses  were  tried  on  similar  railway  commut- 
ators, and  while  these  gave  better  results  than  the  aluminum  or 
iron,  yet  they  developed  high  mica  much  more  quickly  than  the 
copper  commutators.  With  such  evidence  at  hand,  the  use  of 
forged  or  drawn  copper  for  commutator  bars  was  a  natural  con- 
clusion. However,  even  with  the  best  copper  obtainable,  there 
was  some  tendency  toward  blackening  and  burning  of  the  com- 
mutators, generally  accompanied  by  high  mica,  and  the  difficulty 
was  blamed  primarily  on  the  mica.  It  was  assumed  that  the  cop- 
per bars  did  not  wear  as  rapidly  under  the  carbon  brush  as  was  the 
case  with  other  metals.  At  the  same  time  it  was  recognized  that 
when  the  machine  was  operated  without  current  none  of  these 
metals  seemed  to  wear  unduly.  It  was  only  when  considerable 
current  was  carried  that  the  wear  was  excessive.  At  that  time, 
the  real  explanation  of  this  difficulty  was  not  fully  appreciated. 

Later  investigations  on  collector  rings  and  commutators,  de- 
veloped the  fact  that  whenever  a  current  is  carried  between  a 
stationary  brush  contact  and  a  moving  surface,  there  is  a  tendency 
to  burn  away  either  the  brush  contact  face  or  the  moving  surface, 


IRON  COMMUTATORS  515 

depending  upon  the  direction  of  the  current  and  upon  the  current 
density.  It  was  found  that  this  burning  action,  which  is  some- 
what similar  to  that  occurring  in  an  arc,  was  to  some  extent  a 
function  of  the  contact  loss.  This  was  indicated  partly  by  the 
fact  that  the  burning  was  a  function  of  both  the  brush  contact 
drop  and  the  current  density.  A  given  current,  for  instance, 
might  produce  very  little  burning  as  long  as  the  contact  drop  was 
quite  low;  whereas,  if  for  any  reason  such  contact  drop  increased 
materially,  noticeable  burning  would  begin.  If  the  current  was 
from  the  brush  to  the  commutator  or  collector,  the  brush  contact 
surface  would  tend  to  burn  away  more  than  the  opposing  surface. 
If,  on  the  contrary,  the  current  was  from  the  collector  or  commut- 
ator to  the  brush,  then  the  collector  surfaces  would  tend  to  burn 
and,  in  some  cases,  deposit  the  burnt  material  on  the  brush  face. 

When  carbon  brushes  are  used,  there  is  usually  a  very  con- 
siderable contact  drop  due,  apparently,  to  the  nature  of  the  ma- 
terials in  the  brush  itself.  This  drop,  in  many  cases,  is  in  the 
nature  of  one  volt  for  each  contact  and  it  is  fairly  constant  over 
quite  a  wide  range  of  current.  In  consequence,  the  contact  loss 
varies  nearly  in  proportion  to  the  current  and  not  as  the  square  of 
the  current.  Due  to  this  very  considerable  loss  with  carbon 
brushes,  there  is  a  tendency  to  burn  away  the  brush  surface  and  to 
burn  and  blister  the  commutator  or  collector  surfaces  with  which 
the  brush  is  in  contact.  This  tendency  to  burn  is  dependent  upon 
the  actual  current  density  in  the  brush  (including  local  or  short- 
circuit  currents),  but  the  resultant  burning  is  largely  a  function  of 
the  material  in  the  commutator  or  collector  face.  As  the  brush 
cannot  make  perfect  contact  with  the  metallic  surface  to  which  it 
is  opposed,  there  are  minute  arcs  at  the  contact  and  these  evi- 
dently burn  away  the  surfaces.  However,  the  real  burning  action 
is  dependent  upon  the  inability  of  the  surface  to  conduct  away 
heat  rapidly,  for  if  the  heat  developed  in  the  surface  film  is  not 
conducted  away  with  sufficient  rapidity,  then  such  surface  is  liable 
to  be  blistered  or  burned  locally,  even  though  moving  with  respect 
to  the  brush.  Such  burning  or  blistering  naturally  roughens  the 
contact  surface  and  increases  the  contact  drop  and  thus  tends  to 
increase  the  arcing  and  burning  action.  Thus,  if  there  is  any 
burning  action  it  tends  to  grow  worse,  cumulatively.  This  burn- 
ing away  of  the  surface  leaves  the  metal  surface  of  the  commutator 
slightly  lower  than  the  mica,  imless  the  latter  wears  mechanically 
at  the  same  rate  that  the  comtiiutator  metal  bums  away.  As  this 


516  ELECTRICAL  ENGINEERING  PAPERS 

is  not  usually  the  case,  high  mica  soon  develops,  simply  by  the 
action  of  burning  away  of  the  metal.  Thus  high  mica  is  a  result 
of  the  trouble,  rather  than  the  cause.  However,  as  even  a  very 
gradual  burning  away  will  eventually  leave  the  mica  above  the 
surface,  modern  practice  has  tended  toward  undercutting  of  the 
mica,  so  that  even  with  a  slight  burning  tendency  the  brush  still 
maintains  contact  with  the  metal,  thus  preventing  accentuation  of 
the  trouble. 

As  mentioned  before,  this  burning  action  is  a  function  of  the 
contact  voltage,  the  current  density,  and  the  non-burning  or  non- 
blistering  qualities  of  the  metal  constituting  the  commutator.  It 
is  in  this  latter  feature  that  copper  has  proven  so  superior  to  other 
metals.  Extended  experience  shows  that  the  heat  conducting 
qualities  of  pure  copper  are  so  good  compared  with  most  other 
metals  or  alloys  that  the  burning  or  blistering  action  of  the  current 
under  the  brush  is  very  small,  except  for  high  current  densities. 
Anything  which  tends  to  decrease  the  heat  conducting  properties 
of  the  commutator  metal,  tends  to  increase  burning  action.  This 
has  been  very  clearly  demonstrated  in  elaborate  tests  of  carbon 
brushes  on  collector  rings,  etc.,  where  questions  of  commutation 
did  not  come  in  to  disturb  the  conclusions.  Such  tests  have  been 
made  covering  copper,  bronzes  and  alloys  of  various  sorts,  wrought 
iron,  cast  iron,  etc.  In  practically  all  cases,  with  high  current 
densities,  the  burning  and  blistering  action  appears  to  be  dependent 
upon  the  ability  to  conduct  the  heat  away  from  the  contact  sur- 
face. By  such  conduction  the  local  heating  of  the  contact  film  of 
metal  is  kept  at  a  low  point  which  results  in  reduced  fusion  of  the 
metal,  and  with  very  good  heat  conducting  materials  the  fusion  of 
the  metal  may  be  so  minute  that  the  polishing  action  of  the  brush 
keeps  the  surface  in  a  smooth  glossy  condition. 

It  is  an  interesting  fact  that  the  electrical  conductivities  of  the 
metals  and  their  mixtures  and  alloys,  bear  a  fairly  dose  relation  to 
their  heat  conductivities.  Experience  shows  that  very  little  im- 
purity in  copper  will  reduce  its  electrical  conductivity  to  possibly 
one-third  or  one-quarter,  and  its  heat  conductivity  will  be  decreased 
nearly  in  proportion.  Most  of  the  alloys  of  copper  have  a  very  low 
conductivity  compared  with  copper  itself,  while  wrought  iron  is 
worse  than  most  of  the  copper  alloys  in  this  regard.  The  series  of 
tests  above  referred  to,  indicated  quite  clearly  that  the  burning 
tendency  varied  very  much  as  the  electrical  resistance  of  the  ma- 
terial, that  is,  with  the  heat  resistance.  Wrought  iron,  having 


IRON  COMMUTATORS  517 

from  eight  to  ten  times  the  resistance  of  copper,  would  burn  or 
blister  and  get  rough  at  very  much  lower  current  densities  than 
copper  commutators  or  rings.  Even  some  of  the  alloys  which  ap- 
peared to  be  good  for  collector  rings,  showed  blistering  effects  at 
very  much  lower  limiting  current  densities  than  copper.  Conse- 
quently, it  developed  that  the  limiting  carrying  capacity  of  dif- 
ferent metals  in  commutators  and  collector  rings,  varied  roughly 
with  the  heat  conducting  qualities,  and  thus  copper  proved  to  be 
superior  to  any  of  its  alloys  or  any  other  available  material.  Ac- 
cording to  this  line  of  reasoning,  silver  should  be  better  than  cop- 
per, but  this  is  not  an  available  metal.  The  above  also  explains 
why  alloys  of  copper  in  which  other  elements  have  been  introduced 
for  the  purpose  of  hardening,  etc.,  usually  do  not  have  the  ultimate 
carrying  capacity  found  in  copper.  Aluminum  has  fairly  good 
heat  conductivity,  if  pure,  but  it  is  so  easily  oxidized  and  the  re- 
sistance of  the  oxidized  surface  rises  so  rapidly,  that  presumably 
this  fact  neutralizes  any  possible  gain  otherwise.  Experience  on 
actual  commutators  showed  that  aluminum  did  not  take  a  polish, 
even  under  moderate  current  densities  and,  in  fact,  it  acted  very 
much  like  some  of  the  higher  resistance  metals  used  in  the  tests. 

It  should  be  evident  from  the  above  that,  when  materials  of 
higher  heat  resistance,  that  is,  with  poorer  heat  conductivity  than 
copper,  are  used  in  commutators,  the  operating  current  densities 
should  be  reduced  accordingly.  Thus,  it  may  be  possible  to  use 
iron  or  steel  for  commutator  bars,  provided  the  brush  current 
densities  are. reduced  sufficiently.  In  very  small  machines,  this 
might  mean  only  an  increase  in  the  dimensions  of  the  commutator 
and  brushholders.  In  larger  machines,  however,  any  material 
modification  in  the  proportion  of  the  commutator  may  lead  to 
radical  changes  in  the  machine  as  a  whole,  so  that  the  total  cost 
would  be  materially  higher  than  in  the  copper  commutator  ma- 
chine. This  depends  entirely  upon  how  much  sacrifice  is  to  be 
made  in  operating  conditions  and  maintenance.  If  these  are  to 
be  kept  at  the  same  high  standard  as  on  present  copper  commut- 
ator machines,  then  it  is  questionable  whether  the  iron  commut- 
ator would  prove  to  be  practicable  under  any  conditions.  The 
same  conditions  hold  true,  to  a  certain  extent,  with  certain  alloys 
instead  of  copper  in  the  commutator.  As  such  alloys,  as  a  rule, 
cost  nearly  as  much  as  copper  itself,  it  should  be  obvious  that  any 
material  increase  in  the  dimensions  of  a  commutator  will  soon 
balance  any  possible  gain. 


51S  ELECTRICAL  ENGINEERING  PAPERS 

In  larger  machines  one  serious  condition  would  be  liable  to  be 
encountered  with  other  than  copper  commutators.  At  present 
these  machines  are  built  for  quite  high  peripheral  speeds  of  the 
commutators,  and  construction  difficulties  are  encountered  winch 
would  make  any  increase  in  their  length  or  diameter  very  objection- 
able. Consequently,  serious  modifications  in  the  general  construc- 
tion of  the  machine,  and  possibly  in  its  speed  conditions,  are  liable 
to  be  necessitated.  In  fact,  in  many  cases  the  whole  design  of  the 
machine  is  predicated  on  the  commutator  construction  In  such 
cases  the  use  of  a  poorer  material  in  the  commutator  would  un- 
doubtedly be  a  backward  step  in  the  development. 

It  is  thus  obvious,  that  the  use  of  iron  in  commutators,  while 
possibly  practicable  under  the  urge  of  necessity,  is  not  in  the  di- 
rection of  an  advance  in  the  art.  In  fact,  it  is  a  big  step  backward. 
It  should  be  assumed  naturally  that  if,  in  the  past  thirty  years  of 
development  in  commutating  machinery,  iron  commutators  have 
not  come  to  the  front,  it  is  for  very  good  reasons,  and  the  pre- 
ceding is  simply  an  attempt  to  bring  out  some  of  the  foremost 
reasons. 


POLYPHASE  INDUCTION  MOTOR  WITH  SINGLE- 
PHASE  SECONDARY 

FOREWORD — Repeated  requests  had  been  made  of  the  author,  from 
time  to  time,  for  a  simple  explanation  of  the  half  speed  charac- 
teristics of  f  a  polyphase  induction  motor  with  single-phase 
secondary  circuit.  The  analysis  presented  here  does  not  require 
more  than  a  working  knowledge  of  the  characteristic  principles 
and  curves  of  the  polyphase  induction  motor. 

This  appeared  in  the  Electric  Journal,  September,  1915. 

—(ED.) 


IT  is  well  known  that  when  a  polyphase  induction  motor  is 
operated  with  only  one  secondary  circuit  closed — that  is,  with 
a  single-phase  group-wound  secondary,  it  will  develop  at  full 
speed  a  maximum  torque  much  less  than  with  polyphase  secondary, 
and  then  with  increasing  load  will  drop  to  approximately  half 
speed,  where  it  will  develop  comparatively  high  maximum  torque. 
However,  a  simple  non-mathematical  explanation  of  the  causes 
of  this  action  has  not  yet  been  put  forward,  to  the  writer's  know- 
ledge. Such  an  explanation  is  possible,  and  the  following  is  one 
which  should  be  grasped  without  difficulty  by  those  who  are 
familiar  only  with  the  general  characteristic  curves  and  actions 
of  induction  motors. 

To  begin  with,  consider  the  relations  between  the  rotation  of 
the  magnetic  field  and  the  mechanical  rotation  of  the  rotor  in  the 
two  cases  of  rotated  and  stationary  primary  windings — that  is, 
with  the  primary  winding  (1)  on  the  rotor,  and  (2) — on  the  stator. 
In  the  case  of  the  primary  winding  on  the  rotor,  let  the  direction 
of  the  magnetic  field  rotation  be  in  the  lefthand  direction  (counter- 
clockwise), as  indicated  in  Pig.  1.  This  rotating  field  tends  to 
pull  the  stator  around  in  the  same  direction  (left  hand).  As  the 
stator  core  and  winding  cannot  rotate,  the  rotor,  due  to  the  torque 
between  the  rotor  and  stator,  turns  in  the  opposite  direction 
(right  hand).  When  starting  from  rest,  the  stator  or  secondary 
has  the  same  frequency  as  the  primary;  then  as  the  rotor  speeds 
up  in  the  right-hand  direction,  the  magnetic  field  set  up  by  the 
primary  winding  rotates  in  the  left-hand  direction,  and  therefore 
its  speed  relative  to  the  stator  becomes  less  and  less  until  synchron- 
ism is  reached.  Thus  the  secondary  or  stator  frequency  generated 

#19 


520  ELECTRICAL  ENGINEERING  PAPERS 

by  the  rotating  primary  field  decreases  until  synchronism  is  reached; 
which  is  the  normal  action  of  the  polyphase  rotor. 

Considering  next  a  stationary  primary  winding  and  a  rotating 
secondary,  and  assuming  right-hand  rotation  of  the  magnetic 
field,  as  shown  in  Fig.  2,  it  is  obvious  that  the  secondary  winding 
and  core  will  be  pulled  around  in  the  same  direction — that  is,  the 
rotor  will  turn  in  the  right-hand  direction.  Thus,  left-hand 
rotation  of  a  primary  magnetic  field  on  the  rotor,  and  right-hand 
rotation  of  a  primary  magnetic  field  on  the  stator,  both  tend  to 
give  right-hand  rotation  of  the  rotor.  These  two  conditions  are 
explained  rather  fully  as  they  enter  into  the  following  explanation, 
of  the  action  of  the  induction  motor  with  single-phase  secondary. 


FIG.  1.  PRIMARY  ON  ROTOR     FIG.  2  PRIMARY  ON  STATOR 

The  next  consideration  is  the  resolution  of  a  single-phase 
field  in  an  induction  motor  into  two  components  rotating  in 
opposite  directions  at  the  same  speed,  each  of  half  the  maximum 
value  of  the  single-phase  field.  Such  component  rotating  fields,  it 
may  be  asserted,  do  not  actually  exist,  but,  nevertheless,  the 
•resultant  of  two  such  fields  of  proper  value  and  rotation  will 
actually  be  a  single-phase  field  corresponding  to  that  set  up  by  a 
single-phase  winding  on  the  motor.  This  assumption  of  two 
oppositely  rotating  fields  as  the  equivalent  of  a  single-phase  field 
is  a  great  aid  in  explaining  the  speed  characteristics  of  the  poly- 
phase motor  with  single-phase  secondary. 

Assume  next  that  the  induction  motor  with  single-phase 
secondary  has  its  primary  winding  on  the  rotor  and  its  secondary 
on  the  stator.*  Consider  first  the  standstill  condition.  The 
primary  field  is  assumed  to  be  rotating  in  the  left-hand  direction, 
as  in  Fig.  1.  This  field,  cutting  the  secondary  winding  at  syn- 
chronous speed,  generates  e.  m.  f .  and  current  in  the  secondary,  of 

*This  particular  arrangement  is  chosen,  as  it  appears  to  the  writer  to  allow  a  somewhat 
dearer  conception  to  be  obtained  of  what  takes  place  in  the  motor. 


INDUCTION  MOTOR— SINGLE  PHASE  SECONDARY         521 

a  frequency  equal  to  the  primary,  but  single-phase,  due  to  there 
being  but  one  closed  circuit.  This  secondary  current  may  be 
considered  as  setting  up  a  single-phase  field  which  is  actually  fixed 
in  space,  but  which  may  be  replaced  by  two  oppositely  rotating 
fields,  each  of  half  value,  traveling  around  the  core  at  a  speed 
synchronous  with  the  frequency  of  ike  secondary  current  generated. 
One  of  these  rotating  fields  travels  in  the  same  direction  and  at  the 
same  speed  as  the  primary  field  set  up  by  the  rotor  windings.  It 
thus  corresponds  to  the  rotating  magnetic  field  set  up  by  the  usual 
polyphase  secondary  winding.  The  action  between  this  rotating 
secondary  field  and  the  primary  is  that  of  a  polyphase  motor,  and 
torque  is  developed  at  all  speeds  from  standstill  up  to  synchronous 
speed,  as  in  the  usual  polyphase  motor,  as  shown  in  Pig.  3.  The 
rotor  under  the  action  of  this  torque  tends  to  rotate  in  the  right- 
hand  direction. 

Considering  next  the  other  component  rotating  field  set  up 
in  the  stator  by  the  single-phase  secondary  current  in  the  rotor, 
this  rotates  in  the  right-hand  direction,  or  opposite  to  the  primary 
rotating  field.  This  secondary  component  field  traveling  around 
the  stator  may  be  considered  as  the  primary  field  of  an  induction 
motor.  For  a  secondary,  it  makes  use  of  the  windings  on  the 
rotor  core,  such  secondary  circuits  being  closed  back  through  the 
primary  transformers,  supply  system,  etc.  This  may  not  be  a 
very  good  secondary  closed  circuit,  but  it  is  all  that  is  available 
and  the  motor  does  the  best  it  can  under  the  circumstances. 

Let  us  now  consider  what  torque  conditions  obtain  with  this 
right-hand  rotating  primary  field  with  its  freak  secondary  circuit. 
Taking  standstill  conditions  first,  it  is  obvious  that  as  the  primary 
field  is  on  the  stator  and  rotates  right-handedly,  the  turning 
effort,  or  torque,  will  tend  to  run  the  rotor  in  the  right-hand  di- 
rection. Therefore,  at  start,  the  torque  is  in  the  same  direction  as 
in  the  case  of  the  left-hand  component.  The  two  torques  there- 
fore add  at  start. 

Considering  next  slow  rotation  of  the  rotor,  as  the  primary 
winding  on  the  rotor  moves  slowly  in  the  right-hand  direction  the 
left-hand  rotation  of  its  flux  decreases  in  speed  with  respect  to  the 
secondary  winding,  so  that  the  secondary  or  stator  frequency  is 
correspondingly  reduced.  Therefore,  the  two  oppositely  rotating 
secondary  component  fields  rotate  more  slowly.  The  left-hand 
one  rotates  at  the  same  speed  in  space  as  the  fundamental  primary 
field,  as  described  before.  The  right-hand  component  travels  in 


522 


ELECTRICAL  ENGINEERING  PAPERS 


the  opposite  direction,  while  its  secondary  circuit,  namely,  the 
primary  coils  on  the  rotor,  are  rotating  in  the-same  direction  (right 
hand) ,  but  at  a  somewhat  lower  speed.  The  torque  exerted  by  this 
right-hand  component  is  therefore  still  in  the  right-hand  direction 
— that  is,  the  same  as  that  of  the  other  component. 


Torque 

FIG  .5. 


O — Torque  of  left-hand  component,  In  synchronism  with  primary  field  and  in  the  same 
direction.  B — Torque  of  nght-hand  component,  rotating  in  opposite  direction  and  in  syn- 
chronism with  A,  relative  to  the  secondary  winding  C — Resultant  of  A  and  B.  D — Torque 
of  the  same  motor  with  polyphase  secondary. 

However,  when  half  synchronous  speed  of  the  rotor  is  reached 
a  new  condition  enters.  At  this  speed  the  secondary  frequency 
has  fallen  to  one-half  that  of  the  primary  or  supply  circuit.  The 


INDUCTION  MOTOR— SINGLE  PHASE  SECONDARY         523 

left-hand  component  of  the  secondary  field  is  now  traveling  in  the 
left-hand  direction  at  half  the  speed  it  had  when  the  rotor  was 
stationary,  but  the  primary  field  is  also  traveling  left-handedly 
at  the  same  speed  in  space,  so  that  the  two  fields  are  still  in 
synchronism. 

Considering  next  the  right-hand  component,  this  is  traveling 
at  half  speed  in  the  right-hand  direction,  while  the  rotor  is  travel- 
ing at  the  same  speed  in  the  same  direction.  Therefore,  the  second- 
ary circuit  for  this  right-hand  field  (the  winding  on  the  rotor)  is 
now  traveling  in  synchronism  with  it,  and  therefore  no  secondary 
current  or  torque  can  be  generated.  This,  therefore,  corresponds 
to  full  speed  on  the  usual  induction  motor.  The  torque-speed 
curve  up  to  half  speed,  therefore,  may  be  indicated  by  the  lower 
half  of  the  curve  shown  in  Fig.  4.  -  At  higher  than  half  speed  the 
stator  frequency  is  still  further  reduced  and  the  right-hand  field 
travels  at  a  still  slower  speed,  while  its  secondary  winding  on  the 
rotor  core  is  now  traveling  faster  than  its  primary  field,  or  it  is 
running  above  synchronism.  It  therefore  develops  a  negative 
torque.  This  negative  torque  above  half  speed  should  show  a 
negative  characteristic  somewhat  similar  to  the  positive  speed- 
torque  curve  below  half  speed.  This  is  indicated  in  Fig.  4. 
However,  there  is  this  difference;  as  the  rotor  speed  approaches 
true  synchronism,  the  frequency  of  the  stator  circuit  approaches 
zero,  until  at  full  speed  of  the  rotor  (synchronous  speed)  the 
secondary  frequency  becomes  zero.  Under  this  condition  the 
stator  field  and  currents  fall  to  zero,  and  there  is  no  torque  from 
either  the  right  or  the  left-hand  component  field.  Therefore,  as 
this  zero  frequency  is  closely  approached,  both  torque  curves 
rapidly  approach  the  zero  value.  By  combining  the  speed-torque 
curves  for  both  component  fields  the  resultant  speed-torque  curve 
may  be  plotted  as  in  Fig.  5.  It  will  be  seen  from  this  resultant 
curve  that  stable  torque  conditions  are  found  at  about  half  speed. 
Also,  the  starting  torque  is  good.  Above  half  speed  the  torque 
conditions  are  somewhat  indefinite,  depending  upon  individual 
circuit  conditions,  etc.  In  general,  the  rotor  will  continue  to  run 
at  full  speed,  if  first  brought  up  to  this  speed  by  means  of  poly- 
phase secondary  operation,  and  will  pull  considerable  load  with- 
out dropping  to  the  lower  stable  speed. 

While  the  torque  at  start,  due  to  the  resultant  of  the  two 
torque  curves  shown  in  Fig.  5,  is  larger  than  that  due  to  the  left- 
hand  component  alone,  yet  it  must  be  borne  in  mind  that  this 


524  ELECTRICAL  ENGINEERING  PAPERS 

latter  may  be  much  smaller  than  the  starting  torque  of  the  motor 
with  polyphase  secondary.  Therefore,  this  method  of  starting 
with  one  secondary  circuit  only  is  not,  in  general,  an  improvement 
on  starting  with  a  polyphase  secondary.  To  illustrate  this, 
another  speed-torque  curve  should  be  added,  representing,  in  a 
general  way,  the  conditions  with  the  same  motor  if  a  symmetrical 
polyphase  secondary  winding  is.  used.  This  is  shown  in  Fig.  6.* 
In  general,  this  will  show  better  starting  and  also  higher  maximum 
or  pullout  torque  than  with  single-phase  secondary. 

To  the  experienced  designer  it  will  be  obvious  that  such  a 
motor  is  but  one  form  of  internal  cascade,  and  a  very  poor  one  at 
that.  A  motor  under  such  conditions  of  operation  carries  exces- 
sive currents,  the  primary  winding  carrying  both  primary  and 
secondary  currents,  these  being  of  different  frequencies,  except  at 
standstill.  The  power-factor  is  also  comparatively  poor. 

*The  curves  C  and  D  in  Fig.  6  have  been  plotted  partly  from  actual  test  data. 


A  PHYSICAL  CONCEPTION  OF  THE  OPERATION  OF 
THE  SINGLE-PHASE  INDUCTION  MOTOR 

FOREWORD — In  the  training  of  young  engineers,  directly  from  the 
technical  schools,  the  author  has  found  that  very  few  of  them 
have  any  conception  of  the  operation  of  the  single-phase  induc- 
tion motor.  In  attempting  to  work  out  a  simple  method  of 
presenting  the  problem,  the  subject  matter  of  this  paper  was 
gotten  together  from  time  to  time,  and  eventually  it  was  written 
in  its  present  form  and  jpresented  before  the  American  Institute 
of  Electrical  Engineers  in  April,  1918.  It  should  be  understood 
that  the  purport  of  this  whole  paper  is  to  illustrate  the  prin- 
ciples of  operation  and  not  to  indicate  a  method  of  calculation. 
The  usual  methods  of  treating  the  single-phase  induction  prob- 
lem are  so  mathematical  that  the  average  engineer  or  student 
cannot  follow  them.  In  the  method  given  in  this  paper  a 
knowledge  of  the  characteristics  of  the  polyphase  motor  is 
necessary,  as  the  entire  method  is  based  upon  the  fundamental 
idea  of  a  pair  of  polyphase  machines  operating  with  variable 
voltages  and  opposing  torques.  From  this  viewpoint,  the 
various  characteristics  of  the  single-phase  motor  are  explained 
in  a  non-mathematical  manner. 

It  may  be  considered,  to  a  certain  extent,  as  supplementary 
to  the  author's  paper  on  "The  Polyphase  Motor,  which  was 
published  twenty-one  years  before,  and  which  is  reprinted  in 
the  first  part  of  this  volume. — (ED.) 


THE  underlying  principles  and  the  operating  characteristics 
of  the  polyphase  induction  motor  are  so  well  understood  that 
it  is  found  desirable  to  consider  the  single-phase  induction  motor, 
simply  as  a  special  case  of  the  polyphase.  The  following  treatment 
of  the  subject  should  not  be  considered  as  an  analysis  of  the  true 
phenomena  of  the  motor  but  should  be  looked  upon  more  as  a 
ready  means  of  visualizing  the  actions  in  the  form  of  well  known 
polyphase  motor  Characteristics.  Also,  it  should  not  be  con- 
sidered as  a  presentation  of  new  material,  for  the  underlying 
methods  used  are  old  and  relatively  well  known.  It  is  simply 
an  attempt  to  describe  the  operation  of  the  single-phase  motor 
in  a  way  which  may  be  most  easily  understood  by  those  not 
versed  in  the  mathematics  of  the  subject. 

Starting  with  the  old  assumption  that  a  single-phase  alter- 
nating magnetic  field  may  be  considered  as  being  made  up  of  two 
constant  fields,  each  of  half  the  peak  value  of  the  single-phase 
field  and  rotating  at  uniform  speed  in  opposite  directions,  then 

525 


526  ELECTRICAL  ENGINEERING  PAPERS 

if  the  single-phase  flux  distribution  is  of  sine  shape  and  varies 
sinusoidally  in  value,  it  may  be  replaced,  or  represented,  by  two 
sine-shaped  fields  of  constant  value  rotating  in  opposite  direc- 
tions. This  is  the  simplest  case  and  allows  a  relatively  easy 
explanation  of  many  single-phase  problems.  However,  when 
the  flux  distribution,  or  field  form,  due  to  the  single-phase 
winding,  is  other  than  of  sine  shape,  then  the  oppositely  rotating 
components  cannot  be  considered  as  of  sine  shape,  but  will 
assume  certain  varying  forms  as  they  rotate,  the  resultant  of 
each  instantaneous  pair  always  giving  the  single-phase  field 
corresponding  to  that  instant. 

As  other  than  sine-shape  fields  tend  toward  complications  in 
the  physical  conception  of  the  single-phase  induction  motor 
actions,  and  lead  more  or  less  into  the  mathematical  conception, 
the  following  analysis  will  be  limited  essentially  to  sine-shape 
distributions. 

As  a  starting  point  and  to  show  reasons  for  certain  later 
analysis,  let  us  assume  a  single-phase  induction  motor  operating 
at  no-load,  full  speed,  with  its  polyphase  secondary  winding 
short-circuited.  The  single-phase  primary  field,  of  assumed  sine 
shape,  is  considered  as  made  up  of  the  two  sine-shape'  equal 
components  of  constant  value,  and  of  half  the  peak  value  of  the 
single-phase  field,  and  rotating  synchronously  in  opposite  direc- 
tions. One  of  these  fields  is  traveling  in  the  same  direction  as, 
and  slightly  faster  than,  the  rotating  secondary.  The  slip  of 
the  secondary  with  respect  to  this  field  is  of  the  same  nature  as 
in  the  ordinary  polyphase  motor.  As  the  machine  is  carrying 
no  load  the  secondary  current  corresponding  to  this  rotating  field 
is  very  small,  being  just  large  enough  to  overcome  the  rotational 
losses  in  the  motor  itself,  and  its  frequency  is  equal  to  the  slip 
frequency  due  to  the  forward  field  component. 

As  there  is  an  assumed  backward  flux  or  field  component  of 
equal  value,  the  rotating  secondary  winding  cuts  this  at  almost 
double  the  frequency  of  the  line.  Stated  exactly,  the  sum  of  the 
backward  and  the  forward  frequencies,  in  the  secondary  winding, 
is  equal  to  exactly  double  the  frequency  of  the  primary  supply 
system.  The  secondary  winding  cutting  the  backward  field  at 
this  high  frequency  tends  to  generate  a  very  considerable  e.  m.  f. 
and,  with  the  winding  closed  on  itself,  short-circuit  currents  will 
flow,  which  tend  to  damp  out  or  suppress  the  flux  which  causes 
them.  This  secondary  current  will  rise  until  its  magnetizing 
effect  is  practically  equal  and  opposite  to  the  magnetomotive 


SINGLE-PHASE  INDUCTION  MOTOR 


527 


528  ELECTRICAL  ENGINEERING  PAPERS 

force  which  produces  the  backward  field,  which  thus  becomes 
almost  zero  in  value.  Consequently  there  are  two  distinct  sets 
of  secondary  currents  flowing,  one  of  very  small  value  and  of  a 
frequency  corresponding  to  that  of  the  forward  rotation,  and  the 
other  of  very  much  larger  value  and  of  almost  double  the  line 
frequency.  Actual  tests  of  the  secondary  circuit  of  a  single-phase 
induction  motor  at  small  load,  taken  with  an  oscillograph,  Fig. 
J ,  show  both  of  these  currents  as  above  described. 

MAGNETOMOTIVE  FORCES  AND  MAGNETIC  FLUXES 

It  is  seen  from  the  preceding  that,  right  at  the  beginning  of  our 
analysis,  a  new  condition  is  encountered,  namely,  the  introduc- 
tion of  a  secondary  opposing  magnetomotive  force  which  reacts 
on  one  of  the  primary  field  components  and  practically  neutral- 
izes it.  Also,  there  is  a  mixture  of  magnetomotive  forces  and 
magnetic  fields,  which  is  liable  to  lead  to  confusion.  Obviously 
the  introduction  of  the  opposing  secondary  magnetomotive  force 
rotating  synchronously  with  the  backward  component  of  the: 
primary  introduces  some  entirely  new  features.  Therefore, 
before  going  any  further  with  the  above  method,  it  is  desirable 
to  set  aside  for  awhile  the  viewpoint  of  two  equal  oppositely 
rotating  fields  and  begin  with  a  preliminary  study  of  the  magneto- 
motive forces  and  the  magnetic  fields  resulting  from  them. 

It  may  be  mentioned  that  while  the  assumption  of  two  op- 
positely rotating  component  fields,  in  place  of  a  single-phase  field, 
is  well  known  and  has  been  used  quite  frequently,  the  correspond- 
ing analysis,  from  the  viewpoint  of  magnetomotive  forces,  ap- 
parently has  been  but  little  used.  When  magnetomotive  forces, 
instead  of  magnetic  fluxes,  are  considered,  then  the  single-phase 
primary  magnetomotive  force,  fixed  in  position,  can  be  replaced 
by  two  equal  components  of  constant  value,  such  as  would  be 
developed  by*  direct  current,  each  of  half  the  peak  value  of  the 
single-phase,  and  rotating  at  synchronous  speeds  in  opposite 
directions. 

Returning  again  to  our  analysis,  let  us  consider  two  funda- 
mental magnetomotive  forces,  namely,  a.  primary  single-phase 
one,  fixed  in  position  and  varying  sinusoidally  and  a  secondary 
one  of  constant  value,  of  half  the  peak  value  of  the  primary 
which  rotates  synchronously  in  one  direction  and  which  is  in 
opposition  to  the  primary  in  the  position  where  the  two  coincide. 

Let  us  assume  that  the  primary  single-phase  magnetomotive 
force  is  split  into  its  two  equal  oppositely  rotating  components,- 


SINGLE-PHASE  INDUCTION  MOTOR 


529 


then  the  results  may  be  illustrated  as  in  Figs,  2,  3,  4,  and  5. 
In  Fig.  2,  C  and  D  represent  the  two  components  forming  the 
single-phase  magnetomotive  force  A.  At  the  position  chosen, 
C  and  D  are  of  equal  value  and  coincide  in  position  and  polarity, 
B,  which  represents  the  secondary  magnetomotive  force,  is  also 
of  half  the  peak  value  of  A,  but  is  of  opposite  polarity.  It, 
therefore,  neutralizes  one  of  the  components  C  or  D,  thus  leaving 
a  resultant  of  half  the  peak  value  of  A . 


In  Fig.  3,  the  component  D  has  shifted  thirty  degrees  to  the 
left,  while  C  has  shifted  an  equal  distance  to  the  right.  The 
secondary  magnetomotive  force  B  is  shifted  thirty  degrees  to  the 
left,  thus  neutralizing  D  and  leaving  only  the  component  C. 

In  Fig  4,  D  and  B  are  shifted  sixty  degrees  to  the  left,  while  C 
is  shifted  sixty  degrees  to  the  right  In  the  same  way,  in  Fig.  5, 
B  and  D  have  shifted  ninety  degrees  to  the  left  and  C  has  shifted 
a  corresponding  amount  to  the  right. 


PIG.  4 

Thus  from  the  above  it  is  seen  that  a  single-phase  magneto- 
motive force,  fixed  in  position  and  varying  sinusoidally,  and  a 
constant  magnetomotive  force  of  half  the  peak  value  of  the 
single-phase,  which  is  in  opposition  at  the  point  of  coincidence 
of  position,  and  which  rotates  synchronously  in  either  direction, 
will  give  a  resultant  constant  magnetomotive  force,  rotating  in 
the  opposite  direction,  but  which  is  of  the  same  polarity  as  the 
single-phase  magnetomotive  force  at  the  position  of  coincidence. 


530 


ELECTRICAL  ENGINEERING  PAPERS 


In  other  words,  a  single-phase  magnetomotive  force,  fixed  in 
position,  and  an  opposing  constant  one  of  half  the  peak  value 
rotating  in  either  direction,  will  give  a  resultant  rotating  magneto- 
motive force  equivalent  to  that  of  a  polyphase  induction  motor. 

As  a  continuation  of  the  above,  the  resultant  magnetomotive 
force  C  could  be  replaced  by  a  magnetic  field  or  flux,  resulting 
from  such  magnetomotive  force.  If  this  magnetic  field  is 
plotted  to  the  same  scale  as  the  magnetomotive  force  which 
produces  it,  then  C,  in  Figs.  2  to  5,  can  represent  a  magnetic 
field.  This  field  will  be  constant  in  value  and  of  half  the  peak 
value  of  the  field  which  the  single-phase  magnetomotive  force 
alone  would  set  up. 

Thus  according  to  Figs.  2,  3,  4  and  5,  by  the  introduction  of  an 
"opposing"  magnetomotive  force,  equal  m  value  to  one  of  the 
component  magnetomotive  forces  of  the  single-phase  and  rotating 
synchronously  with  it,  one  of  the  two  components  of  the  mag- 
netic field  can  be  suppressed  and  only  the  other  component  left, 


FIG.  5 

the  resultant  is  thus  a  rotating  magnetic  .field,  just  as  in  the 
polyphase  induction  motor. 

However,  a  further  modification  of  this  should  be  considered. 
Assuming  again,  that  the  single-phase  primary  magnetomotive 
force  is  replaced  by  its  two  equal  rotating  components,  as  in  Figs, 
2  to  5,  then  by  the  addition  of  an  opposing  magnetomotive  force, 
similar  to  B  in  the  same  figures,  but  of  less  value  than  the  com- 
ponent D,  then  the  resultant  of  this  opposing  magnetomotive 
force  and  the  component  D  is  a  reduced  magnetomotive  force 
of  the  same  polarity  as  D.  There  will  then  remain  two  magneto- 
motive  forces,  each  of  constant  value,  one  of  half  the  peak 
value  of  A  and  the  other  of  some  smaller  value,  depending  upon 
the  opposing  force  B.  These  two  rotating  magnetomotive 
forces  can,  therefore,  set  up  two  oppositely  rotating  fields  of 
unequal  value.  These  are  illustrated  in  Figs.  6,  7  and  8r 

In  Fig.  6,  B  is  assumed  at  some  less  value  than  the  component 
D.    The  resultant  of  D  and  B  is  shown  as  E.     Therefore,  at  this 


SINGLE-PHASE  INDUCTION  MOTOR 


531 


position  C  and  E  represent  the  two  resultant  magnetomotive 
forces  and  the  two  component  fields.  In  Fig.  7,  the  conditions 
are  shown  for  thirty  degrees  shift  and  here  again  E  and  C  repre- 
sent the  two  fields.  In  Fig.  8  the  shift  is  for  sixty  degrees. 

Thus  by  the  introduction  of  a  constant  "opposing"  magneto- 
motive force  of  less  than  either  of  the  components  of  the  single 
phase,  two  oppositely  rotating  fields  of  unequal  value  may  be 
set  up.  As  extreme  cases  of  this,  if  the  constant  opposing' mag- 
netomotive force  is  made  zero  in  value,  the  magnetic  field  corres- 
ponding to  its  position  and  rotation  will  rise  to  the  full  value  of 
the  oppositely  rotating  field;  and,  on  the  other  hand,  if  the 
constant  opposing  magnetomotive  force  is  made  half  the  peak 
value  of  the  single  phase,  the  correspondingly  rotating  field 
becomes  zero.  Both  of  these  cases  are  in  accordance  with  the 
oarlier  assumptions 

The  above  conditions  of  the  single-phase  primary  magneto- 


\ 


FIG.  6 


FIG.  7 


motive  force  and  a  constant  secondary  one,  in  opposition,  which 
may  be  of  half  the  peak  value,  or  some  less  value  down  to  zero, 
and  which  rotates  synchronously  in  one  direction,  resulting  in  two 
magnetic  fields  which  may  be  of  equal  or  unequal  value,  and 
which  rotate  synchronously  in  opposite  directions,  all  form 
essential  parts  in  the  physical  conception  or  visualization  of  the 
actions  of  the  single-phase  motor  which  will  be  given  below. 

It  should  be  observed  that  in  the  above  method  of  considering 
the  production  of  a  rotating  field  in  the  single  phase  induction 
motor,  the  two  primary  components  of  the  single-phase  magneto- 
motive force  and  the  secondary  damping  magnetomotive  force 
all  rotate  synchronously,  and  such  rotation  is  independent  of  the 
speed  of  the  secondary  core.  In  some  methods  of  considering  the 
single-phase  induction  motor  problem^  the  single-phase  primary 
winding  is  assumed  to  generate  a  magnetomotive  force  in  the 
secondary  which,  by  rotation  of  the  core,  is  carried  around  until 
it  generates-  a  second  magnetic  field  or  flux  at  right  angles  to  the 


532  ELECTRICAL  ENGINEERING  PAPERS 

original  primary  flux,  thus  giving  the  equivalent  of  a  polyphase 
magnetic  field.  -  However,  the  above  method  does  not  involve 
such  method  of  treatment. 

It  should  also  be  recognized  that  the  foregoing  analysis  only 
covers  no-load  conditions  and  that  with  the  addition  of  load  new 
conditions  are  brought  in  to  the  problem.  These,  however, 
will  be  brought  out  later,  for  the  no-load  conditions  require 
further  consideration,  especially  as  regards  the  generation  of  the 
primary  counter  e.  m.  f.  by  the  above  descnbed  rotating  fields. 
As  already  shown,  there  may  be  a  single  magnetic  field  rotating 
synchronously,  or  there  may  be  two  component  fields  of  equal 
value  rotating  in  opposite  directions,  or  there  may  be  inter- 
mediate conditions  of  oppositely  rotating  fields  of  unequal  value, 
depending  upon  the  value  of  the  damping  or  opposing  secondary 
magnetomotive  force. 


FIG.  8 

COUNTER  E.  M.  P.  GENERATION  AND  EXCITATION 
Considering  next  the  counter  e.  m.  f .  generated  in  the  primary, 
we  should  first  look  into  the  e.  m.  f.  conditions  produced  by  two 
oppositely  rotating  fields  of  equal  values.  If  the  secondary 
circuits  are  open,  the  two  component  fields  are  both  present  and 
are  concerned  in  the  generation  of  the  counter  e.  m.  f .  This  is 
true  whether  the  secondary  is  stationary  or  is  rotated  at  full 
speed.  If,  however,  the  secondary  is  closed  upon  itself,  then 
when  running  at  speed,  one  of  the  component  fields  is  practically 
damped  out  and  the  other  jnust  generate  the  entire  primary 
counter  e.  m.  f.  Thus,  two  entirely  different  conditions  are 
encountered,  depending  upon  whether  the  secondary  is  open  or 
closed.  To  explain  this  properly,  some  further  analysis  is  re- 
quired, as  follows: 

In  the  first  place,  it  may.be  stated  that  the  e.  m.  f.,  produced 
in  the  primary  winding  by  cutting  its  two  component  fields,  is  the 
same  as  that  generated  by  the  single  phase  sine  shape  field, 
varying  sinusoidally  and  acting  on  the  primary  winding  as  in  a 


SINGLE-PHASE  INDUCTION  MOTOR 


533 


transformer.  Herein  lies  a  simple  illustration  of  the  equivalence 
of  the  transformer  and  the  flux  cutting  methods  for  calculating 
e.  m.  fs.  In  Figs.  9,  10  and  11,  are  shown  several  positions  of 
the  two  oppositely  rotating  fields  and  their  relation  to  the 
primary  winding. 

In  Fig.  9  is  shown  the  magnetic  flux,  or  field,  A,  which  is  set 
up  by  a  primary  winding  a.     This  winding,  of  course,  would 


FIF.  9 

require  a  tapered  distribution  to  give  such  a  field.  This  is 
mentioned  incidentally  as  it  has  no  direct  bearing  upon  the 
explanation,  except  from  the  mathematical  standpoint. 

Assuming  the  single-phase  field  at  its  maximum  or  peak  value, 
then,  at  this  instant,  the  two  component  fields,  B  and  C,  each 
of  half  the  peak  value,  will  coincide  both  in  position  and  polarity. 
From  the  transformer  method  of  calculation,  the  e.  m.  f.  gener- 


Fia  10 

ated  at  this  instant,  in  the  winding,  will  be  zero,  as  the  rate  of 
change  of  the  flux  is  zero.  Also  from  the  flux*  cutting  method, 
the  e.  m'.  f .  in  the  primary  winding  will  be  zero,  for,  as  is  evident 
from  the  figure,  each  belt  or  group  of  the  primary  winding  is 
cutting  fields  which  have  equal  positive  and  negative  areas  or 
values. 

Considering  liext  the  conditions  in  Fig.  10,  in  which  the  two 
rotating  components  have  traveled  ninety  degrees.    The  fields 


534 


ELECTRICAL  ENGINEERING  PAPERS 


are  shown  as  B  and  C  It  is  evident  that  the  resultant  of  these 
two  fields  is  zero  in  value,  that  is,  the  single-phase  field  is  passing 
through  its  zero  value,  and,  accordingly,  is  generating  the  maxi- 
mum e  m  f.  by  the  transformer  method  Also,  considering 
component  B  of  the  rotating  fields,  obviously,  by  the  cutting 
method  it  is  generating  maximum  e  m.  f  in  the  winding:  a  Also, 
component  C  is  generating  maximum  e  m  f  in  winding  a 
However,  as  one  of  these  fields  is  positive  in  this  position  and  is 
traveling  in  one  direction,  while  the  other  field  is  negative  and  is 
rotating  in  the  opposite  direction,  the  two  e  m  fs  will  be  in  the 
same  direction,  and  thus  will  be  added.  Thus,  from  the  figure, 
this  position  will  give  the  maximum  e.  m  f.  in  the  winding  by 
the  cutting  method  It  can  be  shown  by  calculation  that  this 
maximum  value  is  the  same  with  either  the  cutting  or  the  trans- 
former methods  of  considering  e  m.  f  generation. 

shows  that  both  of  the  component  fluxes  must  be  taken 


PIG.  11 

into  account  in  generating  the  total  primary  e.  m.  f  ,  and  if  either 
component  is  decreased  in  value  or  suppressed,  the  total  e  m  f. 
generated  in  the  winding  will  be  decreased  correspondingly, 
unless  the  other  component  is  increased  a  corresponding  amount. 

Pig  11  is  simply  a  continuation  of  the  conditions  of  Figs  9 
and  10,  in  showing  an  intermediate  position  of  the  component 
field.  The  result  is  the  same  as  if  the  two  fields  were  momen- 
tarily replaced  by  the  field  D 

According  to  the  above  analysis,  to  produce  a  given  counter 
e.  m.  f,  in  the  primary,  with  one  of  the  component  fields  damped 
out,  the  other  component  must  be  doubled  in  value  It  was 
shown  before  that  in  the  single  phase  induction  motor,  running 
at.  full  speed  with  no  load,  the  backward  field  is  practically 
damped  out  by  the  secondary  current.  Thus  with  only  the 
forward  component  field  remaining,  either  the  counter  e  m  f. 
will  be  halved  or  the  forward  flux  component  must  be  doubled, 
the  latter  being  the  case  This  means,  in  turn,  that  the  primary 


SINGLE-PHASE  INDUCTION  MOTOR  535 

magnetomotive  force  must  be  doubled  in  value.  In  other  words, 
suppressing  one  of  the  two  rotating  field  components  results 
in  doubling  the  no-load  excitation  of  the  motor.  Furthermore, 
doubling  the  magnetomotive  force  of  the  primary  and  thus  doub- 
ling the  forward  component  of  the  field  also  doubles  the 
backward  component,  which,  in  turn,  is  suppressed  by  doubled 
secondary  current.  The  above  conditions  of  doubled  excitation 
is  on  the  basis  of  sine  flux  distribution.  With  other  distributions 
the  same  result  holds  approximately,  but  not  exactly,  due  to 
conditions  involving  the  shape.of  the  field. 

It  is  evident  from  the  above  that,  with  the  secondary  circuits 
open,  the  excitation  required  is  of  constant  value  regardless  of 
the  speed  of  the  rotor  core  and  windings;  also  when  running  at 
speed,  the  primary  excitation  is  doubled  as  soon  as  the  secondary 
circuit  is  closed.  However,  it  is  not  obvious,  on  first  considera- 
tion, that  even  with  the  secondary  circuits  closed  the  primary 
excitation  falls  to  half  the  full  speed  value,  when  the  motor  is 
brought  to  standstill.  This  involves  load  conditions  which  will 
be  treated  later,  but  nevertheless'  this  feature  may  be  brought 
out  at  this  time.  The  explanation  lies  in  the  fact  that  at  rotor 
standstill  the  damping  action  of  the  secondary  current  will  be 
exerted  equally  on  both  the  forward  and  backward  components 
of  the  primary  field,  so  that  necessarily  these  must  be  maintained 
at  equal  value,  and,  by  the  above  analysis,  this  requires  but  half 
the  excitation,  compared  with  the  no-load  full-speed  condition 
where  the  backward  field  is  practically  completely  suppressed. 

LOAD  CONDITIONS 

When  the  single-phase  induction  motor  is  loaded,  the  total 
input  current  can  be  considered  as  made  up  of  two  components, 
namely,  the  no-load  (practically  all  magnetizing)  and  the  load 
current.  This  latter  is  simply  the  increased  current  in  the  pri- 
mary due  to  the  load  and  does  not  entirely  represent  energy. 
This  load  current,  being  single-phase,  may  be  represented  by  two 
equal  oppositely  rotating  magnetomotive  forces  in  the  primary 
of  the  motor,  just  as  in  the  cas6  of  the  no-load  current.  The 
fields  which  these  two  magnetomotive  forces  tend  to  set  up  are 
both  practically  suppressed  by  two  equivalent  secondary  mag- 
netomotive forces  rotating  in  opposite  directions.  The  forward 
secondary  component  corresponds  to  the  secondary  load  mag- 
netomotive force  in  the  polyphase  motor  and  the  interaction 
between  this  magnetomotive  force  and  the  forward  primary  field 


536 


ELECTRICAL  ENGINEERING  PAPERS 


develops  torque  just  as  in  the  polyphase  motor.  The  backward 
component,  at  first  thought,  would  appear  to  develop  an  opposing 
torque,  corresponding  in  value  to  that  of  the  polyphase  motor  at 
approximately  200  per  cent  slip.  This,  however,  is  not  the  case, 
for  at  this  slip  the  ordinary  polyphase  motor  takes  an  excessive 
primary  current  tending  to  develop  a  large  magnetic  field,  which 
is  suppressed  by  a  correspondingly  large  secondary  magneto- 
motive force.  In  the  single-phase  induction  motor,  howlever, 
the  primary  backward  rotating  magnetomotive  force  component, 
due  to  the  load  current,  can  be  only  of  the  same  value  as  the 
forward.  This  fact  must  be  borne  in  mind  as  it  is  a  very  import- 
ant factor  in  the  later  analysis. 

To  illustrate  the  characteristics  of  thfe  single-phase  induction 


20 

40 

60 

j    80 

I  too 

[120 
140 
160 


200 


A 


iZ 


4- 


10     20     30     40     50     60     70     80 

TORQUE 


FIG  ,12 

motor,  it  may  be  compared  with  the  action  of  two  polyphase 
induction  motors  rigidly  coupled  together,, and  connected  to  the 
line  to  give  opposite  rotations.  Such  a  set  or  unit  has  certain 
characteristics  which  are  so  similar  to  those  of  the  usual  single- 
phase  induction  motor  that  on  first  consideration  one  would 
assume  them  to  be  identical.  However,  a  more  careful  study  of 
the  individual  operating  conditions  shows  that  the  similarity 
is  only  a  general  one,  and  a  number  of  decided  discrepancies  are 
found. 

The  characteristics  of  the  above  two-motor  unit  and  the  single- 
phase  motor  may  be  compared  as  follows: 

(1)  The  speed  torque  characteristics  of  the  two  motors  of  the 
polyphase  unit  may  be  represented  by  A  and  B  in  Fig.  12  and 


SINGLE-PHASE  INDUCTION  MOTOR 


537 


their  resultant  by  curve  C.  According  to  this  latter  curve,  the 
resultant  torque  is  zero  at  standstill,  and  a  slight  change  in  speed 
in  either  direction  will  give  an  effective  torque  tending  to  speed 
up  the  unit  in  whichever  way  it  is  started.  This,  therefore, 
corresponds  to  the  well  known  starting  characteristics  of  the 
single-phase  motor. 

(2)  It  may  also  be  seen  that  the  maximum  torque  the  unit 
develops  is  materially  less  than  that  of  either  of  the  two  com- 
ponent motors      This  fact  is  also  consistent  with  single-phase 
motor  operation  compared  with  the  same  machine  on  poly- 
phase. 

(3)  At  full  speed,  according  to  this  resultant  curve,  the  slip 


20 

40 


60 


120 
. 

140 
160 
180 
200 


7 


I 


L 


7 


0      10     20     30     40     50     60     70    80 
TORQUE 

FIG.  13 


for  a  given  torque  is  very  much  larger  than  that  of  the  corre- 
sponding polyphase  motor.  This  is  not  true  of  the  single-phase 
motor  and  herein  lies  one  of  the  discrepancies  in  this  method  of 
illustrating  the  operation. 

(4)  It  is  well  known  that  in  the'  polyphase  motor  the  maxi- 
mum torque  it  can  develop,  with  constant  voltage  applied,  is 
independent  of  the  secondary  resistance;  while,  in  the  single- 
phase  motor,  in  general,  an  increase  in  the  secondary  resistance 
will  decrease  the  maximum  torque  and  a  decrease  will  have  the 
opposite  effect.  This  may  be  illustrated  by  repeating  the  curves 
of  Fig+  12  with  modified  secondary  resistance  in  the  two  com- 
ponent motors.  In  Fig.  13  the  secondary  resistance  is  increased 
and  in  Fig.  14  is  decreased  relatively  to  that  of  Fig,  12.  The 


538 


ELECTRICAL  ENGINEERING  PAPERS 


resultant  speed-torque  curves  for  the  three  figures  show  that  the 
maximum  torques  are  materially  affected  by  the  secondary  re- 
sistance. The  same  holds  true  for  the  single-phase  induction 
motor. 


10     20     30     40     50     60     70     80 
TORQUE 


FIG.  14 


(5)  However,  this  method  of  illustrating  the  characteristics 
of  the  single-phase  motor  torque  fails  when  -the  conditions  of 
secondary  resistance  is  such  that  the  maximum  polyphase  torque 


o 

20 
40 
60 

_ 

«> 

"0 
120 

140 
160 
180 


200 


0   10  20  30  40  50  60  70  80 
TORQUE 

PIG.  15 


is  developed  at  about  100  per  cent  slip.  Fig.  15  illustrates  this. 
From  this  speed  torque  curve  it  appears  that  the  unit  has  a  very 
low  resultant  torque,  but  this  is  not  the  case  in  the  single-phase 


SINGLE-PHASE  INDUCTION  MOTOR 


539 


induction  motor,  for  with  a  polyphase  motor  developing  its 
maximum  torque  at  100  per  cent  slip,  the  same  machine  on  single- 
phase  will  give  a  very  considerable  maximum  torque.  Herei 
again  is  a  discrepancy  which  the  assumed  equivalent  arrange- 
ment does  not  cover  properly. 

(6)  In  Fig  16,  the  current-torque  curve  D,  for  the  component 
motors  in  the  above  figures,  is  shown.     This  indicates  plainly 
what  a  wide  discrepancy  there  is  between  the  currents  taken  by 
the  primaries  of  the  two  motors  when  running  at  speed.     For 
example,  at  a  given  speed  a,  the  current  taken  by  the  forward 
rotating  motor  is  b,  while  c  represents  the  current  taken  by  the 
backward  motor.     Obviously,  the  current  taken  from  the  line, 
which  is  the  resultant  of  b  and  c,  is  much  greater  than  that  re- 
quired to  produce  the  resultant  torque  and  the  power  factor  of 
such  a  unit  must  necessarily  be  very  poor.     However,  such  is  not 
the  case  with  the   single-phase 

motor,  for  the  inputs  and  the 
power  factors  are  not  greatly 
different  from  those  of  polyphase 
motors  of  the  same  capacity. 
Herein  lies  a  radical  difference 
between  the  single-phase  motor  £ 
and  the  above  assumed  unit. 

(7)  Another    difference    be- 
tween such  a  unit  and  the  true 
single-phase  motor  lies  in    the 
no-load  or  magnetizing    input. 

Obviously,  the  combined  magnetizing  components  for  the  two 
motors  will  be  twice  as  great  as  for  a  single  machine,  whereas, 
in  the  single-phase  motor  the  magnetizing  input  is  practically  the 
same  as  in  the  corresponding  polyphase  machine.  Here  is 
another  pronounced  discrepancy. 

It  is  evident  from  the  above  that  while  this  method  of  illus- 
trating the  action  of  the  single-phase  motor  by  means  of  two 
polyphase  motors,  coupled  for  opposite  rotation,  is  in  the  right 
direction,  some  special  modifying  conditions  must  be  introduced 
to  account  for  the  discrepancies.  The  action  of  this  two-motor 
unit,  therefore,  will  be  followed  up  further,  with  the  introduction 
of  certain  modifications  derived  primarily  from  consideration  of 
certain  characteristics  of  the  single-phase  induction  motor  itself 

In  the  first  place,  curves  A ,  B  and  C  of  Pig.  1 2  were  based  upon 
equal  and  constant  e.  m  fs.  applied  to  the  terminals  of  both 


100 


TORQUE 
PIG.  16 


540 


ELECTRICAL  ENGINEERING  PAPERS 


motors.  That  this  is  not  a  correct  assumption  can  be  deter- 
mined from  the  operating  conditions  in  the  single-phase  motor. 
From  the  analysis  of  the  component  rotating  fields  it  was  shown 
that  at  full  speed  the  backward  component  was  practically 
damped  out  by  a  secondary  ^magnetomotive  force,  thus  leaving 
only  the  forward  component,  which  then  rose  to  practically 
double  value  in  order  to  generate  the  required  e,  m.  f .  However, 
at  standstill,  the  secondary  winding  holds  the  same  rotational 
relation  with  respect  to  both  component  fields  and,  therefore, 
neither  field  can  be  damped  out  more  than  the  other.  Conse- 
quently, at  standstill,  both  component  fields  are  equal  in  value 
and  the  counter  e.  m.  f.  of  the  primary  is  generated  by  the  two 
oppositely  rotating  fields,  instead  of  a  single  one  of  double  value 


180 

200 , . 

0      10    20     30     40     50     60     70     80 
TORQUE 

FIG.  17 

as  is  the  case  at  full  speed.  Therefore,  at  standstill,  the  forward 
field  is  of  only  half  the  value  of  the  full  speed  field.  This  corre- 
sponds to  the  operation  of  the  polyphase  motor  at  half  field 
strength,  that  is,  with  half  the  primary  voltage  applied,  thus  re- 
quiring one-quarter  the  magnetizing  input.  The  same  voltage 
condition  applies  also  for  the  backward  component  at  zero  speed. 
It  would  appear,  therefore,  that  in  the  unit  composed  of  two 
polyphase  motors  coupled  together,  the  voltage  applied  to  the 
terminals  of  the  forward  motor  should  be  at  practically  full  value 
at  synchronous  speed  and  should  fall  to  half  value  at  standstill 
or  100  per  cent  slip,  and  should  have  practically  zero  value  at 
200  per  cent  slip.  Then  assuming,  as  a  first  approximation, 
that  the  decrease  in  voltage  from  full  speed  to  200  per  cent  slip 


SINGLE-PHASE  INDUCTION  MOTOR 


541 


is. a  straight  line  law,  new  speed  torque  curves,  corresponding  to 
Fig.  13,  but  with  the  torques  decreasing  as  the  square  of  the 
voltage,  can  be  illustrated  as  in  Fig.  17.  Here  curves  A  and  B 
correspond  to  Fig.  12,  while  D  and  E  correspond  to  the  above 
proportionate  reductions  in  voltage.  The  resultant  F  of  these 
latter  curves  is  also  shown. 

This  new  resultant  F  is  similar  in  general  shape  to  C  of  Fig.  12, 
but  indicates  some  quite  different  characteristics.  For  instance, 
at  the  higher  speed  values  it  coincides  quite  closely  with  the 
polyphase  speed  torque  curve,  which  is  actually  the  case  in  the 
single-phase  motor.  In  the  second  place,  with  high  secondary 
resistance,  as  shown  in  Fig.  15,  the  speed-torque  curves  are 
modified  a's  in  Fig.  18,  which  shows  both  the  former  characteristic 


20 

40 

60 

§5  80 


140 
160 
180 
200 


X 


A 


FIG.  18 


10     20     30     40     50     60     70    30 
TORQUE 


and  the  new  one.  Here  the  resultant  torque,  under  the  new 
assumption  is  materially  higher  and  more  nearly  conforms  with 
the  condition  in  the  single-phase  motor. 

Under  the  earlier  assumption  of  constant  voltage  on  both 
motors,  it  was  shown  that  the  magnetizing  current  would  be 
twice  as  great  as  in  the  single-phase  motor.  On  this  new  assump- 
tion, however,  at  full  speed,  with  practically  full  voltage  on  one 
motor  and  zero  voltage  on  the  other,  the  total  magnetizing  cur- 
rent will  be  only  half  as  great,  and  will  approximate  that  of  one 
motor  alone,  and,  therefore,  that  pf  the  single-phase 'rtiotor. 

Furthermore,  under  the  new  assumption,  the  current  taken 
by  the  primary  of  the  backwardly  rotating  motor  is  quite  small 
at  high  speed  and,  therefore,  the  resultai^t  current  taken  from  the 


542 


ELECTRICAL  ENGINEERING  PAPERS 


line  is  not  excessive  and  is  more  nearly  consistent  with  actual 
single-phase  motor  conditions. 

Thus,  with  this  new  condition  of  reduced  terminal  voltage  with 
reduction  in  speed,  practically  all  the  conditions  of  the  single- 
phase  motor  are  met,  except  possibly  from  the  quantitative  view- 
point. The  two-motor  combination  thus  serves  as  a  very  good 
illustration.  There  is,  however,  one  further  condition  which 
must  be  rigidly  met  if  the  new  curves  are  to  be  reasonably  exact, 
namely,  the  primary  currents  taken  by  the  two  motors  must  be  equal, 
for,  as  shown  in  the  early  part  of  this  analysis,  the  forward  and 
backward  rotating  components  of  the  primary  current  in  the 
single-phase  induction  motor  are  equal  at  all  times.  Conse- 
quently to  duplicate  this  condition,  the  primary  e.  m.  fs.  im- 


TORQUE 

FIG.  19 

pressed  upon  the  terminals  of  the  two  polyphase  motors  should 
be  varied  in  such  a  way  that  the  primary  currents  will  always  be 
equal.  In  addition,  it  is  assumed  that  the  sum  of  the  two  im- 
pressed voltages  is  constant.  This,  however,  is  only  an  approxi- 
mation. 

The  next  step  is  to  determine  what  is  the  actual  law  of  voltage 
variation  which  will  satisfy  the  above  conditions  of  current  and 
voltage.  A  ready  means  for  obtaining  this  lies  in  the  speed- 
torque  and  current-torque  curves  of  the  polyphase  motor  at 
constant  voltage.  From  the  current-torque  curve  at  constant 
voltage  corresponding  curves  for  any  other  voltage  can  readily 
be  plotted  by  varying  the  abscissae  as  the  square  of  the  voltage 
and  the  ordinates  directly  as  the  voltage.  This  is  illustrated  in 
Fig.  19.  Here  A  is  the  polyphase  motor  speed-torque  curve  at 


SINGLE-PHASE  INDUCTION  MOTOR  543 

constant  voltage  B  represents  the  part  below  the  100  per  cent 
slip  line,  but  turned  above  the  zero  speed  line  for  convenience. 
B  can  also  be  considered  as  the  back  torque  at  full  voltage,  but 
thrown  to  the  right  of  the  zero  torque  line  for  convenience. 
Curve  C  represents  the  primary  current  for  full  voltage  condi- 
tions Then  at  a  speed  a,  for  example,  the  primary  currents 
corresponding  to  the  forward  and  back  torque  will  be  b  and  c 
respectively. 

Assume  next  that  the  voltage  is  halved  for  both  rotations,  then 
the  new  speed-torque  curves  will  be  A\  and  B\  in  which  the  tor- 
ques are  reduced  as  the  square  of  the  voltage.  The  new  current 
curve  will  be  C\.  The  currents  for  speed  a  will  now  be  61  and 
ci,  or  half  of  b  and  c,  as  they  are  varied  as  the  voltage. 

The  above  figure  is  simply  to  illustrate  the  rule  for  variation 
of  the  primary  current  with  the  voltage,  in  the  polyphase  motor, 
and  does  not  represent  the  actual  conditions  which  we  are  after; 
for  in  the  above  the  voltage  reductions  are  the  same  for  both  the 
forward  and  the  back  torques.  But,  according  to  our  former 
analysis,  this  condition  of  equal  voltages,  for  the  two  rotations, 
holds  only  for  the  100  per  cent  slip  point.  For  other  speeds  the 
two  voltages  are  reduced  unequally,  but  with  the  sum  of  the  two 
approximately  constant  according  to  the  assumptions. 

If,  for  any  speed  a,  we  let  x  represent  the  percentage  of  voltage 
reduction  for  the  forward  torque,  then  1  —  x  will  represent  the 
corresponding  reduction  for  the  back  torque.  Let  //  be  the 
primary  current,  corresponding  to  the  forward  torque  for  this 
speed  at  full  voltage,  and  /&  the  current  for  the  back  torque  at 
the  same  speed  and  also  for  full  voltage.  Then  Ifx  will  represent 
the  primary  current  at  the  reduced  voltage  for  the  forward 
rotation  and  Ib  (1  —  x)  will  be  the  primary  current  for  the  back 
rotation.  One  of  the  conditions  of  our  two-motor  unit,  to  make 
it  correspond  with  the  single-phase  motor,  is  that  these  two 
primary  currents  must  be  equal.  Therefore,  Ifx  =*  Ib  (1  —  x)t 
and 


and  (1  - 


The  above  allows  the  determination  of  the  percentage  x  of 
full  voltage  which  must  apply  for  each  speed  between  zero  and 
synchronism,  when  the  values  of  the  current  If  and  Ib  for  full 
voltage  are  known. 

A  second  method  of  determining  the  percentages  of  vqltage 


544  ELECTRICAL  ENGINEERING  PAPERS 

for  the  two  rotations  is  available  when  the  speed-torque  curve 
of  the  motor  on  single  phase  has  been  determined,  by  test  or 
otherwise.  By  our  former  assumption  this  single-phase  torque 
is  the  difference  between  the  speed  torque  curves  for  the  forwar^ 
and  backward  rotations  with  the  respective  voltages  reduced 
the  proper  percentages.  These  torques  for  any  given  speed  vary 
as  the  square  of  the  terminal  voltage.  For  example,  calling  Tj 
the  forward  torque,  at  full  voltage  and  speed  a,  and  T$  the  back 
torque,  and  Ti  the  single  phase  torque  for  the  same  voltage  and, 
speed,  then  Tfoc2  —  Tb  (1  —  x)z  =  TI,  from  which  x  may  be 
determined,  with  Tf,  Tb  and  Ti  known. 

>It  would  appear  from  the  preceding  that,  if  the  assumptions 
made  are  anyways  close  to  the  actual  conditions,  this  method  of 
analysis  shows  a  means  for  deriving  the  single-phase  speed- 
torque  curve  from  the  polyphase  curves  of  the  same  machine. 
Methods  of  calculating  the  primary  current  and  speed-torque 
characteristics  of  the  polyphase  motor  have  been  developed  quite 
completely,  so  that  it  is  not  necessary  at  this  place  to  give  any 
details  of  such  methods.  The  accuracy  of  the  methods  for  calcu- 
lating the  polyphase  curves  depends  almost  entirely  upon  the 
correct  determination  of  the  reactance  and  saturation  constants. 
All  methods  for  the  direct  determination  of  the  single-phase 
speed-torque  characteristics  also  involve  the  use  of  corresponding 
reactance  and  saturation  constants.  Therefore,  the  above 
method  brings  in  no  new  and  more  difficult  conditions.  The 
primary  object  of  this  paper,  however,  is  not  to  develop  a  new 
method  of  calculation,  but  simply  to  give  a  better  conception  of 
the  close  relation  of  the  single-phase  and  polyphase  characteris- 
tics. 

After  development  of  the  above  method,  an  attempt  was  made 
to  check  it  by  applying  certain  existing  test  data,  but  without 
positive  results,  although  the  indications  were  quite  satisfactory. 
It  was  discovered  that  in  all  the  existing  test  data  at  the  writer's 
command,  where  the  polyphase  speed-torque  and  current- 
torque  curves  has  been  obtain  by  actual  test,  constancy  of 
temperature  had  been  more  or  less  disregarded.  The  effect  of 
change  in  the  secondary  resistance  on  the  polyphase  speed- 
torque  curve  is  to  change  the  slips  but  not  the  maximum  torque. 
The  difficulty,  however,  in  the  polyphase  tests  available  was  that 
apparently  the  resistance  had  varied  very  considerably  during 
the  tests,  especially  at  the  points  of  high  slip,  where  the  second- 
ary losses  were  very  large.  As  a  result  the  speed-torque  curves 


SINGLE-PHASE  INDUCTION  MOTOR 


515 


corresponded  to  those  of  motors  in  which  the  resistance  increase^ 
as' the  Ipad  and  slip  increased.  ^  As  a  consequence,  the  torques 
below  the  zero  speed  line  were  considerably  too  large,  which 
meant  that  in  applying  these  curves  to  the  above  method,  the 
back  torques  were  presumably  entirely  too  great,  thus  apparently 
introducing  errors  in  the  derivation  of  the  resultant  single-phase 
curve. 

The  effect  of  these  discrepancies  are  shown  in  Fig.  20.  Here, 
A  shows  the  speed-torque  curve  as  it  should  be  at  constant  tem- 
perature,, whereas,  B  shows  the  curve  with  the  resistance  of  the 
secondary  increasing  with  increased  slip.  The  corresponding 
current-torque  curves  are  also  shown.  A  consideration  of  these 
curves  would  seem  to  indicate 
that  the  resultant  single-phase 
curves  derived  from  A  and  B 
should  differ  somewhat. 

It  was  then  decided  to  make 
a  more  accurate  set  of  tests  on  a 
10  h.p.,  60;cycle  four-pole,  three- 
phase  motor  of  the  wound-  • 
secondary  type,  so  that  the  i 
secondary  resistance  could  be ! 
varied  if  so  desired.  It  was  also  < 
decided  to  obtain  a  test  with  two 
similar  motors  rigidly  coupled 
together,  with  their  individual 
primary  windings  in  series,  but 
with  their  secondaries  indepen- 
dent. As  already  explained,  the 
theory  of  the  foregoing  method 
calls  for  equal  currents  in  the  two  oppositely  rotating  fields.  This 
condition  is  automatically  obtained  by  coupling  two  primaries  in 
series  with  each  other.*  With  this  arrangement,  if  the  power 
factors  of  the  two  motors  were  always  equal,  then  it  should  be 
equivalent  to  the  method  already  described.  However,  these  are 
practically  never  equal  except  at  the  standstill  position,  although 
an  analysis  of  the  problem  shows  that  the  two  primary  voltages, 
with  this  series  arrangement,  are  not  greatly  out  of  phase  with 
each  other  over  a  very  large  part  of  the  working  range.  The 
writer  has  not  yet  sufficiently  analyzed  the  series  arrangement 

*In  reviewing  an  early  draft  of  this  paper,  thfe  suggestion,  with  a  number  of  other  most 
excellent  ones*  was  made  by  Mr.  R.  B.  Hetaond-  Hoover,  it  developed  later  that  tfcfe  9amg 
suggestion  appeared  ab<mt  twenty  years  agoin  Mr.  B.  A.  Behxend'sboofc/^The  Induction  Motor/1 


0      10    20 


30     40     50 
TORQUE 

FIG.  20 


60     70    BO 


546  ELECTRICAL  ENGINEERING  PAPERS 

to  be  sure  that  it  exactly  represents  all  the  conditions  of  the 
two  rotating  fields  in  the  single-phase  motor,  but  is  inclined  to 
think  that  such  is  the  case.     However,  the  approximate  method 
developed  in  this  paper  lends  itself  so  readily  to  calculation,  that 
it  was  considered  worth  while  to  check  it  up  carefully  by  test 
to  see  what  degree  of  accuracy  could  be  obtained. 
The  following  series  of  tests  was  planned: 
(1)     Three-phase  speed-torque  and  primary  current  curves 
at  220  volts  with  one  motor  alone,  with  its  secondary  short- 
circuited  on  itself. 

"(2)  Single-phase  speed-torque  and  primary  current  curves  on 
the  same  motor  as  (1)  at  220  volts  and  with  the  secondary 
short-circuited  on  itself. 

(3)  Three-phase  speed-torque  and  primary  current  curves  on 
the  same  motor  and  at  same  voltage,  but  with  external  resistance 
in  the  secondary  circuits. 

(4)  Single-phase  speed-torque  curves  under  same  conditions 
as  (3). 

(5)  Speed-torque  and  primary  current  curves  with  two  similar 
motors  with  their  primary  windings  coupled  in  series,  and  with 
the  secondaries  independently  short  circuited  on  themselves,  one 
of  these  motors  to  be  that  used  in  tests  (1)  arid  (4) 

(6)  Similar  tests  to  (5),  but  with  resistance  in  the  secondaries 
as  in  (3). 

In  carrying  out  these  tests,  the  torque  was  measured  by  a 
special  dynamometer  brake,  the  power  absorbing  element  of 
which  consists  of  a  special  separately-excited  direct-current 
machine.  Below  zero  speed,  power  was  supplied  to  the  direct- 
current  machine  in  order  to  obtain  negative  rotation. 

Difficulties  in  obtaining  consistent  tests,  especially  at  negative 
speeds,  soon  developed,  due  to  variations  in  temperature.  With 
the  very  heavy  currents  at  low  and  at  negative  speeds,  the 
motor  would  heat  so  rapidly  that  all  kinds  of  speed-torque 
readings  could  be  obtained.  Test  after  test  was  made  and  while 
these  would  agree  very  well  for  the  higher  speed  points  where 
the  heating  was  small,  they  showed  all  kinds  Of  inconsistencies 
for  the  negative  speeds,  in  particular.  The  currents  for  these 
speeds  also  showed  very  wide  discrepancies  Eventually  it 
was  found  that  those  tests  taken  with  extreme  rapidity,  and 
which  covered  only  a  comparatively  small  number  of  points, 
would  plot  in  quite  reasonable  curves  above  zero  speed,  so  that 
the  writer  was  enabled  thus  to  obtain  quite  consistent  curves 


SINGLE-PHASE  INDUCTION  MOTOR  547 

for  both  torque  and  current  between  1800  rev.  per  min.  and 
standstill.  Not  only  were  the  curves,  consistent  in  themselves 
but  those  taken  with  different  secondary  resistances  were  fairly 
consistent  with  each  other.  It  then  remained  to  obtain  rea- 
sonable readings  for  the  negative  speeds.  Obviously  it  was 
wrong  to  take  a  large  number  of  test  points  and  then  draw 
an  average  curve  through  them,  for  it  is  evident  that  the  er- 
rors, due  to  heating,  tend  to  throw  the  torques  and  currents 
to  one  side  of  the  proper  curves.  Consequently  the  correct 
curves  should  really  be  boundary  lines  rather  than  averages.  It 
was  noted,  in  particular,  that  heating  did  not  appear  to  affect 
the  speed  to  the  same  extent  as  the  torque  at  very  large  slips, 
and,  consequently,  by  plotting  the  current  in  terms  of  speed 
rather  than  torque,  less  erratic  curves  were  obtainable,  and  it 
was  possible  to  plot  speed-current  curves  which  were  quite  con- 
sistent for  the  different  conditions  of  secondary  resistance. 
Furthermore,  from  the  speed-torque  and  speed-current  curves 
above  the  zero  line,  which  appeared  to  be  reasonably  correct, 
as  they  were  consistent  with  each  other,  it  was  possible  to  de- 
rive the  constants  for  the  general  equations  for  speed-torque. 
It  was  found  that  such  derived  equations  fitted  these  curves 
quite  accurately  and,  moreover,  they  held  the  proper  relation  of 
constants  for  both  high-  and  low-resistance  secondaries  The 
various  agreements  between  the  calculations  and  the  tests  for  the 
higher  speeds  were  such  that  one  could  assume  that  the  derived 
equations  were  practically  correct  and  that  from  them  the  curves 
for  the  negative  speeds  could  be  plotted  with  fair  accuracy.  In 
this  way  the  curves  for  the  negative  speeds  were  first  obtained 
and  it  then  remained  to  check  them  by  actual  test.  Finally  a 
method  of  testing  was  tried  which  appeared  to  give  quite  good 
results.  This  consisted  in  setting  the  apparatus  at  about  the 
desired  speed  and  torque  conditions ;  then  cooling  the  motor  down 
to  the  required  temperature  preparatory  to  obtaining  the  desired 
test,  the  power  was  then  thrown  on  and  readings  obtained  in  the 
shortest  possible  time,  five  seconds,  for  instance.  Allowing  the 
motor  to  run,  additional  readings  were  obtained  at  five  second 
intervals.  A  series  of  consecutive  readings,  at  definite  intervals 
apart,  was  thus  obtained  and  plotted  in  a  curve.  By  extending 
this  curve  back  to  the  instant  of  starting,  results  were  obtained 
which  were  undoubtedly  quite  close  to  those  corresponding  to 
the  starting  temperatures,  and  were  not  only  quite  consistent 
with  each  other,  but  also  plotted  very  close  to  the  negative  exten- 


548 


ELECTRICAL  ENGINEERING  PAPERS 


sions  of  the  calculated  curves.  As  a  result  of  a  series  of  tests 
extending  over  several  weeks,  data  was  obtained  which  plotted 
in  curves  which  agreed  fairly  well  with  each  other  throughout. 


20 


0  Points  on  Curve      without  External  Resistance 


200          o 


10 


20 


30  40  50 

TORQUE  IN  POUNDS 

FIG.  21 


60 


70 


80 


RESULTS  OF  TESTS 

Polyphase    Speed-Torque,     Speed-Current    and    Current-Torque 

Curves 

In  Fig.  21  are  shown  the  polyphase  speed-torque  and  primary 
current  both  with  the  secondary  short-circuited,  and  with  re- 
sistance added  In  the  speed-torque  curves  the  circled  points 


SINGLE-PHASE  INDUCTION  MOTOR 


549 


torque  test  was  selected  in  which  no  correction  had  been  made 
for  temperature  and  where  the  conditions  were  quite  closely 
comparable  with  those  of  the  single-phase  tests.  From  the 
speed-torque  and  current  data  of  this  polyphase  test,  the  re- 
sultant single-phase  speed-torque  curve  was  calculated,  making 
no  attempt  at  corrections  of  any  sort.  This  speed-torque  curve 
is  represented  by  the  small  squares  in  Fig.  23.  This  lies  much 
closer  to  the  tested  single-phase  curve,  thus  indicating  that  tem- 
perature is  possibly  an  explanation  of  a  considerable  part  of  the 
discrepancy  between  the  calculations  and  the  tests.  This  would 

TABLE  II. 


Primary 

Reduced 

Slip 

Torque  at 

amperes  per  leg 

torque 

Re- 

full  voltage 

at  full  voltage 

X  - 

suit- 

For 

For 

ant 
tor- 

posi- 

nega- 

j 

For- 

que 

tive 

tive 

Tf 

Tb 

If 

Ib 

* 

1-x 

ward 

Back 

speeds 

speeds 

f 

0  02 

1.98 

8.2 

64  3 

12  5 

134  2 

0.915 

0  085 

6  9 

0  5 

6  4 

0  05 

95 

18.9 

64  8 

18  0 

133.8 

0  881 

0  119 

14  7 

0  9 

13.8 

0  10 

90 

33  3 

65  4 

28  0 

133.0 

0  826 

0  174 

22  5 

2  0 

20.5 

0  15 

85 

41  5 

66  1 

37  0 

132.3 

0  781 

0  219 

27.5 

3  2 

24,3 

0  20 

80 

53  1 

66  9 

46  0 

131  5 

0  740 

0  260 

29  3 

4  5 

24  8 

0  25 

75 

59  8 

67  6 

53  0 

130  8 

0  712 

0  288 

30.3 

5  6 

24  7 

0  30 

70 

65  1 

68  4 

60  0 

130.0 

0  684 

0  316 

30  5 

6.8 

23  7 

0  35 

65 

69  2 

69  1 

66.0 

129.0 

0  662 

0  338 

30  4 

7  9 

22  5 

0,40 

60 

72.2 

69  9 

71.2 

128  0 

0  643 

0  357 

29.9 

8  9 

21  0 

0.50 

50 

76  5 

71.4 

81  0 

125.5 

0.608 

0.392 

28.2 

11.0 

17  2 

0  60 

1,40 

78  7 

72  9 

88.0 

122  7 

0  582 

0  418 

26.6. 

12  7 

15  9 

0  70 

1.30 

79.6 

74.4 

94.0 

120  0 

0  561 

0  439 

25  0 

14  4 

10  6 

0.80 

1  20 

79  6 

75  9 

99.5 

118.1 

0  542 

0  458 

23.4 

15.9 

7.5 

0  90 

1.10 

79.2" 

77.8 

104.0 

112.2 

0.519 

0.481 

21.4 

18  0 

3  4 

1  00 

1.00 

78  9 

78  9 

108  2 

108  2 

0  50 

0  50 

19.7 

19  7 

0 

also  indicate  that  heat  effects  as  referred  to  in  connection  with 
Fig.  20  are  not  as  objectionable  as  anticipated  However,  the 
writer  does  not  believe  that  all  the  discrepancy  is  due  to  heating, 
but  considers  that  this  approximate  method  of  dealing  with  the 
problem  makes  the  back  torque  too  small  In  the  arrangement 
with  two  motors  in  series,  as  mentioned  before,  the  voltages  of 
the  two  motors  will  not  usually  add  up  directly  to  give  the  line 
voltage,  and  the  motor  which  represents  the  back  torque,  will 
have  a  relatively  larger  percentage  of  the  total  voltage  than  is 
tlie  case  with  the  above  method  of  considering  the  prpblexn. 
This  will  be  considered  further  under  the  two-motor  tests, 


550 


ELECTRICAL  ENGINEERING  PAPERS 


Unfortunately,  due  to  the  very  short  time  available,  it  was 
not  possible  to  make  any  extended  tests  on  single  phase  with 
a  view  to  correcting  for  temperature  In  consequence,  the 
calculated  single-phase  speed-torque  curve,  which  is  on  the 
basis  of  constant  temperature,  is  compared  with  tested  curves  in 
which  no  temperature  correction  has  been  made.  It,  therefore, 
is  noL  known  in  this  case  how  much  of  the  discrepancy  is  due  to 
temperature. 

In  Table  II  is  shown  data  similar  to  that  of  Table  I,  but  for 
the  tests  with  resistance  in  the  secondary.  It  will  be  noted  that 
the  resultant  of  the  forward  and  back  torques  is  considerably 
lower  than  in  Table  I,  which  is  consistent  with  the  fact  that  in- 


10 


20 


30  40  50 

TORQUE  IN  POUNDS 
FIG.  22 


60 


70 


creased  secondary  resistance  reduces  the  maximum  torque  of  the 
single-phase  motor. 

In  Pig.  23  is  shown  the  calculated  single-phase  speed-torque 
and  the  tested  torques  of  the  motor  with  resistance  in  secondary. 
Here  the  circled  dots  represent  the  actual  test  readings  and  the 
crosses  represent  the  points  obtained  from  the  last  column  of 
Table  II,  The  discrepancies  are  somewhat  smaller  than  in  the 
motor  with  short  circuited  secondary.  This  should  be  the  case, 
if  heating  is  responsible  for  any  considerable  part  of  the  dis- 
crepancy, for  the  currents  are  relatively  smaller. 

In  order  to  get  a  crude  idea  as  to  how  much  of  the  difference 
may  be  due  to  this  feature  of  temperature,  a  polyphase  speed-* 


SINGLE-PHASE  INDUCTION  MOTOR 


551 


represent  actual  test  readings,  while  the  solid  line  covers  the 
points  calculated  from  the  derived  equations. 

In  Table  I,  covering  data  on  the  short-circuited-rotor  tests, 
are  shown  the  forward  and  back  torques  and  the  corresponding 
forward  and  back  currents  for  the  various  speeds  between  zero 
and  200  per  cent  slip,  as  derived  from  Fig.  21;  also  the  calculated 
values  of  the  ratio  of  voltages,  x  and  (1  —  #),  by  which  the  for- 
ward and  back  torques  should  be  reduced  in  order  to  get  the 
equivalent  single-phase  speed-torque  curve.  The  corresponding 
reduced  values  for  the  forward  and  back  torques  are  also  given 
as  calculated  from  the  values  x  and  (1  —  x).  The  last  column 
shows  the  difference  between  the  reduced  forward  and  back 
torques,  which  should  represent  the  single-phase  torque,  accord- 
ing to  the  foregoing  analysis. 

TABLE  I 


Primary 

Reduced 

Slip 

Torque  at 

amperes  per  leg 

torque 

Re- 

full  voltage 

at  full  voltage 

X  - 

sult- 

For 

For 

tor- 

posi- 

nega- 

lh 

For- 

que 

tive 

tive 

Tf 

?b 

*f 

lb 

1  —  » 

ward 

Back 

speeds 

speeds 

f      b 

0  02 

1  98 

20 

35  8 

19  0 

154  8 

0  89 

0.11 

15  8 

0  4 

15  4 

0  05 

1  95 

41 

36  3 

34  0 

154  5 

0  819 

0  181 

27,5 

1  1 

26  4 

0  10 

1  90 

61  7 

36  9 

55.5 

154  0 

0  735 

0.265 

33  3 

2  6 

30.7 

0.15 

1  85 

71  6 

37  7 

71  0 

153  5 

0.684 

0  316 

33  5 

3  8 

29  7 

0  20 

1  80 

77  3 

38  3 

85.0 

153  0 

0  643 

0  357 

31  9 

4  9 

27  0 

0  25 

1  75 

79  4 

39  2 

96  0 

152  5 

0  614 

0  386 

29  9 

5  9 

24  0 

0  30 

1  70 

79,6 

39.9 

104  0 

152  0 

0  594 

0  406 

28  1 

6  6 

21.5 

0.35 

1,65 

78  8 

40  8 

110.0 

151  5 

0  580 

0.420 

26  5 

7  2 

19  3 

0.40 

1  60 

77  6 

41  6 

113.0 

151.0 

0  572 

0.428 

25  4 

7  6 

17  8 

0  50 

1  50 

74  0 

43  6 

121.0 

150  0 

0.554 

0  446 

22  9 

8  7 

14  2 

0  GO 

1  40 

70  0 

45  5 

128.0 

149  0 

0  538 

0.462 

20  3 

9  7 

10  4 

0  70 

1  30 

65.9 

47  8 

133  0 

147  3 

0  526 

0.474 

18  2 

10.8 

7.4 

0  80 

1  20 

62  1 

50.0 

136  5 

145,5 

0  516 

0  484 

16  6 

11  7 

4  9 

0  90 

1  10 

58  8 

52  7 

139  5 

143  5 

0.507 

0  493 

15  1 

12  8 

2  3 

1  00 

1  00 

55  5 

55  5 

141  5 

141  5 

0  50 

0  500 

13  9 

13  9 

0 

In  Fig.  22  are  shown  the  single-phase  speed-torque  and  cur- 
rent-torque curves  with  short-circui ted  secondary,  as  plotted  from 
Table  I,  and  checked  by  actual  test.  The  circled  dots  represent 
actual  test  points,  while  the  crosses  represent  points  plotted  from 
the  last  column  in  Table  I.  The  agreement  of  test  and  calcu- 
lated values  are  as  close -as  can  really  be  expected  considering 
the  difficulties  in  obtaining  the  data,  and  the  possible  errors. 


552 


ELECTRICAL  ENGINEERING  PAPERS 


Two  Motors  in  Series 

In  Table  III  is  shown  the  test  data  and  the  calculations  de- 
rived therefrom,  for  two  motors  with  their  primaries  in  series 
and  with  their  secondaries  short-circuited  independently  In 
this  test  no  external  resistance  was  used  in  the  secondaries. 
Considerable  difficulty  was  encountered  in  making  this  test,  due 
partly  to  bad  alignment  of  the  machines,  as  they  were  rigidly 
coupled  together.  Furthermore,  in  several  of  the  earlier  tests, 
the  effects  of  temperature  were  disregarded  and  all  indications 
were  that  the  secondaries  were  quite  hot  during  the  tests.  There 
was  so  much  discrepancy  between  the  various  results  that  the 


10 


20 


30  40  50 

TORQUE  IN  POUNDS 

FIG.  23 


70 


80 


writer  cannot  feel  sure  of  the  data  shown,  in  this  table,  although 
it  was  obtained  under  quite  careful  conditions  of  test. 

In  the  above  table  the  percentage  of  line  voltage  applied  to 
each  motor  is  shown.  It  is  of  interest  to"  compare  these  per- 
centages with  those  shown  in  Table  I.  This  is  illustrated  in  Fig. 
24.  This  shows  that  the  percentage  of  voltage  on  the  forward 
rotating  motor  is  higher  at  the  higher  speeds,  than  in  Table  I, 
but  is  lower  at  the  low  speeds.  On  the  other  hand,  the  voltage 
on  the  backward-rotating,  motor  is  higher  at  all  speeds  than  in 
Table  I.  Thus,  the  back  torque  has  always  a  higher  value  than 
in  Table  I.  Consequently,  with  the  reduced  forward  torque  at 
the  lower  speeds  and  the  higher  back  torque,  the  resultant  torque 


SINGLE-PHASE  INDUCTION  MOTOR 


554 


ELECTRICAL  ENGINEERING  PAPERS 


derived  from  the  polyphase  curve  will  naturally  be  lower  than 
in  Table  I,  which  appears  to  be  the  case  in  all  the  tests  made. 
The  data  in  Table  III  indicate  that  the  two  motors  have  their 


10   20   30   40   50   60   70   80   90   100 
PERCENT  OF  VOLTAGE  PER  MOTOR 

FIG.  24 


primary  voltages  very  nearly  in  phase  at  all  times.  The  sum  of 
the  two  motor  voltages  is  never  much  greater  than  that  of  the 
line. 


20 


60 


100 


10 


15  20 

TORQUE  IN  POUNDS" 
FIG.  25 


25 


30 


In  Fig.  25  is  shown  the  calculated  and  test  speed-torque  results 
corresponding  to  Table  III.  The  test  result  shows  lower  torques 
at  the  low  speeds  than  can  be  derived  from  the  voltage  percent^ 


SINGLE-PHASE  INDUCTION  MOTOR 


555 


8-3 


n3 

I 


6  • 


ooooooooooooo 


Sg  S  g  S 


oootoiflooooiaooo 


ooooooooooooo 


556 


ELECTRICAL  ENGINEERING  PAPERS 


ages  applied  to  the  polyphase  torques      Part  of  this  difference 
may  be  due  to  temperature  conditions. 


0 
20 

40 
60 
80 
100 
120 

1Af\ 

S* 

<** 

u 

Two-Motor 
nit   Forward   Motor 

\. 

/ 

/ 

/ 

* 

"-» 

i  An 

// 

/ 

'h 

r 

200 

0 

1 

^~~ 
0       2( 

^ 

X^-**1* 

3        30       40       50       60        70       80        90      10( 
PERCENT  VOLTAGE  PER   MOTOR 

FIG.  26 

In  Table  IV  is  shown  the  corresponding  data  for  two  motors 
with  resistance  in  the  secondary.  Under  this  condition  the 
various  tests  made  were  more  consistent  with  each  other  and 


20 


40 


60 


80 


100 


10  15  20 

TORQUE  IN  POUNDS 
FIG.  27 


25 


30 


the  writer  has  more  confidence  in  the  data  than  in  the  case  of 
Table  III. 

In  Fig.  26  is  shown  the  percentages  of  line  voltage  on  each  of 


SINGLE-PHASE  INDUCTION  MOTOR  557 

the  two  motors,  compared  with  those  in'  Table  II  These  show 
the  same  differences  as  in  Fig.  24,  where  there  was  no  external 
resistance. 

In  Pig.  27  is  shown  the  speed-torque  curve  for  both  calculation 
and  test,  as  taken  from  Table  IV.  Here  the  discrepancies  are 
much  smaller  than  in  Fig,  25. 

CONCLUSION 

While  the  data  is  not  as  exact  as  the  writer  would  desire,  yet 
he  feels  that  the  general  results  obtained  from  the  various  tests 
have  indicated  that  the  method  of  analysis  followed  in  this  paper 
is  along  proper  lines  and  that  this  conception  of  the  action  of  the 
single-phase  induction  motor  is  of  considerable  assistance  in 
obtaining  a  proper  understanding  of  the  machine.  As  stated 
before,  the  primary  purpose  of  this  paper  is  not  to  develop  a 
method  of  calculation,  but  is  sinlply  to  illustrate  some  of  the 
characteristics  of  the  single-phase  motor.  It  is  hoped  that  this 
will  bring  out  more  clearly  the  very  intimate  relation  between  the 
polyphase  and  single-phase  induction  motors  in  their  operating 
characteristics. 


SINGLE-PHASE  LOADS  FROM  POLYPHASE  SYSTEMS 

FOREWORD — This  paper  was  presented  at  the  thirtieth  annual 
convention  of  the  Association  of  Edison  Illuminating  Com- 
panies, held  at  White  Sulphur  Springs,  Va  ,  September,  1914. 
Its  purpose  was  to  show  some  of  the  possibilities  of  phase  con- 
version from  polyphase  to  single-phase,  in  view  of  the  increasing 
requirements  for  single-phase  service  for  electric  furnace  work 
and  various  other  special  applications.  The  paper  deals  with 
some  of  the  problems  of  synchronous  phase  balancers,  etc. — 

(ED.) 

THE  broad  statement  may  be  made  that  it  is  not  practicable 
to  transform  a  polyphase  load  to  single-phase  by  means  of 
transformers  alone.  There  is  a  definite,  positive  reason  for  this, 
namely,  a  single-phase  load  represents  power  which  is  pulsating 
or  varying  periodically  from  zero  to  a  maximum,  value,  while  a 
balanced  polyphase  load  represents  continuous  power  of  constant 
value.  It  is  obviously  not  feasible  to  transform  from  continuous 
power  to  pulsating,  or  vice  versa,  without  some  means  of  storing 
and  restoring  power,  which  is  not  practicable  with  transformers. 

Keeping  the  above  statements  in  mind,  it  is  obviously  a  waste 
of  time  to  attempt  to  accomplish  the  result  by  special  transformer 
connections  or  arrangements.  However,  many  attempts  have  been 
made  to  produce  this  result  with  transformers  alone,  and  some  with 
superficial  evidence  of  success — that  is,  in  some  cases,  it  has  been 
possible  to  load  the  three  phases  equally  in  current  when  delivering 
single-phase  load.  But  balanced  currents  in  this  case  do  not  mean 
balanced  power  loads,  nor  do  they,  as  a  rule,  mean  less  total  loss  in 
the  generator  windings.  In  fact,  the  equality  of  the  currents  in  the 
different  leads  is  obtained  simply  by  out-of -phase  currents,  part  of 
them  usually  being  leading  and  part  lagging.  The  resultant  re- 
actions and  unbalancing  effects  of  these  leading  and  lagging  cur- 
rents have  precisely  the  same  effect  on  the  generating  system  as 
the  single-phase  alone  would  have. 

On  the  basis  therefore  of  storing  and  restoring  power  in  order 
to  obtain  balanced  three-phase  loads  when  delivering  single-phase, 
various  possible  methods  of  accomplishing  this  result  may  be  con- 
sidered. The  obvious  method  is  by  means  of  a  motor-generator  in 
which  a  three-phase  motor  drives  a  single-phase  generator,  the 
entire  single-phase  load  being  transformed  from  electrical  to 

1   559  ,  •.     ::  -; 


560  ELECTRICAL  ENGINEERING  PAPERS 

mechanical,  and  then  back  to  electrical*.  Where  entire  inde- 
pendence of  the  single-phase  and  three-phase  currents  is  desired, 
this  of  course,  is  the  ideal  method.  On  the  other  hand,  it  is 
possibly  the  least  efficient  method.  But  where  both  change  in 
frequency  and  change  to  single-phase  load  are  involved  without  dis- 
tortion of  the  polyphase  load  conditions,  then  double  transforma- 
tion of  power  appears  to  be  necessary,  such  as  from  electrical  to 
mechanical  and  back  to  electrical,  or  from  electrical  to  some  other 
form  of  electrical  power,  involving  a  second  complete  transforma- 
tion. The  motor-generator  is  an  example  of  the  first,  while  trans- 
formation from  three-phase  to  direct  current  by  a  rotary  conver- 
ter, arid  from  direct  current  to  single-phase  of  another  frequency 
by  a  second  converter,  is  an  example  of  the  second. 

Where  the  power-factor  of  the  load  is  low,  as  in  some  electrical 
furnace  systems,  one  advantage  of  the  motor-generator  method  is 
that  the  power-factors  of  the  supply  system  and  the  load  are  ab- 
solutely independent  of  each  other. 

However,  where  the  transformation  from  three-phase  to 
single-phase  is  at  the  same  frequency,  it  would  appear  that  part 
of  the  single-phase  load  could  be  delivered  directly  from  one  phase 
of  the  three-phase  system,  while  the  other  part  of  the  load  could 
be  taken  from  the  other  phases  and  re-transformed  in  phase  by 
rotating  apparatus  to  that  of  the  single-phase  load,  so  that  otily 
part  of  the  load  would  thus  need  transformation.  For  instance, 
assume  that  one-third  of  the  single-phase  power  is  taken  from  one 
phase,  and  the  other  two  phases  supply  power  to  a  suitably  wound 
motor,  which  drives  a  single-phase  generator  having  the  same 
phase  relation  as  the  third  circuit  of  the  three-phase  system. 
Obviously,  the  generator  could  feed  its  single-phase  load  in  parallel 
with  the  other  single-phase  circuit.  The  three  generator  circuits 
would  thus  be  equally  loaded  and  the  single-phase  generator  of 
the  motor-generator  set  would  not  be  transforming  the  full  single- 
phase  load.  This  illustrates  the  principle  of  transforming  from 
three-phase  to  single-phase  without  transforming  the  whole  load, 
but  this  particular  arrangement  of  apparatus  is  not  a  very  practical 
one.  But  the  question  naturally  arises  whether  this  cannot  be  done 
in  comparatively  simple  manner  by  means  of  a  single  machine,  con- 
nected across  the  three-phase  circuit,  which  will  serve  to  transfer 
power  from  part  pf  its  circuits  to  others  at  a 'different  phase 
relation.  This  principle  has  been  utilized  in  the  past  to  transform 
from  single-phase  to  polyphase,  and  in  the  same  apparatus  the 


SINGLE-PHASE  FROM  POLYPHASE 


561 


operation  has  proven  to  be  reversible.  It  may,  therefore,  be  con- 
sidered as  settled  that  such  transformation  is  possible  and  practic- 
able. 

Fundamentally,  the  action  of  phase  balancing  is  as  follows:— 
When  a  single-phase  load  is  taken  from  a  polyphase  circuit,  it  tends 
to  distort  the  phase  relations  in  the  latter  circuit.  Any  synchron- 
ous or  induction  type  polyphase  motor  connected  to  a  distorted 
polyphase  circuit  will  act  in  such  a  way  as  to  have  a  balancing 
effect  on  its  supply  system.  Any  such  motor  will  naturally  tend 
to  do  this,  for  the  motor,  with  its  own  balanced  phase  relations  will 
tend  to  take  current  and  load  in  accordance  with  the  supply 
voltages — that  is,  it  will  tend  to  takfe  more  from  the  higher  volt- 
ages, and  if  the  power  taken  from  the  higher  circuits  exceeds  the 
load  or  losses  of  the  motor  itself,  then  the  excess  is  fed  back  into 


PIG    I—SCHEME  OP  CONSTRUCTION  FOR  CONVERTING  FROM 
SINGLE-PHASE  TO  BALANCED  THREE-PHASE 

the  lower  voltage  circuits.  It  thus  has  a  balancing  action  on  the 
supply  circuit.  This  is  the  natural  tendency  of  all  polyphase 
synchronous  and  induction  types  of  rotating  machines  when  con- 
nected to  a  supply  circuit.  However,  in  the  motor  itself,  this 
tendency  to  correct  the  unbalancing  of  the  supply  circuit  will  be 
accompanied  by  a  corresponding  tendency  inside  the  motor  itself 
to  distort  its  own  internal  phase  relations  until  they  match  those 
of  the  supply  system.  But  if  the  distortion  of  the  phase  relations 
inside  the  motor  can  be  prevented  or  neutralized  in  any  manner, 
then  the  motor  will  transfer  loads  between  its  phases  or  circuits 
to  such  an  extent  that  it  wilLcorrect  the  tmbalanting  of  the  poly- 


562 


ELECTRICAL  ENGINEERING  PAPERS 


phase  system.  In  other  words,  if  balanced  three-phase  potentials 
are  held  at  the  point  of  delivery  of  single-phase  load,  then  the 
three-phase  supply  system,  up  to  that  point,  will  be  balanced. 
The  operation  of  the  various  phase-balancing  methods  therefore 
lies  in  correcting  the  effects  of  the  internal  phase  distortions  in  the 
phase-balancing  motor,  whether  it  be  of  the  induction  or  of  the 
synchronous  type. 

The  action  of  a  phase-converting  device  in  a  simple  form  can 
probably  be  shown  best  by  an  arrangement  used  in  railway  work 
for  converting  from  single-phase  to  balanced  three-phase,  and 
from  three-phase  to  single-phase  when  acting  regeneratively. 

Fig.  1  illustrates  such  ah  arrangement,  consisting  of  a  trans- 
former, a  phase  splitter,  and  single  and  three-phase  circuits.  The 
transformer  is  connected  across  the  single-phase  circuit,  which,  for 
simplicity,  also  is  shown  as  one  phase  of  the  three-phase  circuit. 
The  phase  splitter  has  one  phase  connected  across  the  same  phase 


FIG   2— VOLTAGE  CONDITIONS  IN  THE  CIRCUITS  INDICATED  IN  FIG  1 
WHEN  TRANSFORMING  SINGLE-PHASE  TO  THREE-PHASE 

as  the  transformer;  while  its  other  phase,  which  is  wound  in  90- 
degree  relation  to  the  former,  has  one  end  connected  to  some  in- 
termediate point  of  the  transformer,  and  its  other  is  connected  to 
the  third  phase  of  the  three-phase  circuit. 

The  voltage  relations,  both  without  and  with  load,  when 
transforming  to  three-phase,  are  indicated  in  Fig.  2.     In  this 


SINGLE-PHASE  FROM  POLYPHASE 


563 


diagram,  ab  represents  the  single-phase  e.  m.  f.  delivered  to  the 
transformer.  The  line/c  represents  the  e.  m.  f.  generated  in  phase 
2  of  the  phase  splitter,  this  being  86.6  percent  of  ab.  Therefore, 
with  fc  at  right  angles  to  ab,  lines  ac,  and  ab  are  equal,  and  a 
balanced  three-phase  circuit  is  obtained  at  the  three-phase  termin- 
als. 

Next,  assuming  that  a  three-phase  load  is  carried,  then,  due  to 
internal  distortions,  fc  is  both  reduced  in  value  and  shifted  in  phase 
to  the  position  fd.  The  three-phase  voltage  relations  are  then 
indicated  by  ab,  ad  and  bd.  To  correct  this  distorted  condition, 
assume  (1) —  that  the  e.  m.  f.  across  phase  1  of  the  phase  converter 
is  increased  sufficiently  to  increase  the  e.  m.  f .  of  phase  2,  so  that  it 
will  be  represented  byfe,  instead  of  fd,  the  increase  being  such  that 
a  line  connecting  c  and  e  will  be  parallel  with  ab.  Then  assume 
(2)  that  the  connection  at  /  is  moved  along  ab  to  a  point  g  such 


FIG.  3— VOLTAGE  RELATIONS  WHEN  TRANSFORMING  THREE- 
PHASE  TO  SINGLE-PHASE 

that/g  equals  ce.  This  brings  terminal  e  to  the  position  c,  and  the 
internal  phase  relations  will  then  be  such  that  balanced  e.  m.  f.'s, 
corresponding  to  ab,  ac  and  be,  will  be  delivered  to  the  three-phase 
circuit  when  carrying  load,  and  the  three-phase  circuit  will  neces- 
sarily carry  balanced  three-phase  load,  although  the  source  of 
power  is  single-phase. 

In  Fig.  3  is  shown  a  similar  arrangement,  except  that  the 
transfer  of  power  is  from  three-phase  to  single-phase,  using  the 


564  ELECTRICAL  ENGINEERING  PAPERS 

same  apparatus  as  in  Fig.  2.  As  in  Fig.  2,  ab,  ac  and  be  represent 
three-phase  balanced  voltages,  or  the  no-load  condition.  With 
load,  the  conditions  are  the  reverse  of  those  in  Fig.  2.  The  voltage 
fc  is  shifted  in  phase  with  respect  to  ab,  but  in  the  opposite  direction. 
Also  ab  is  shortened  with  respect  to  fc.  The  unbalanced  phase 
relations  can  therefore  be  represented  by  the  triangle  a^  bi,  d. 
Therefore,  if  aj)i  is  to  be  maintained  at  the  value  ab,  then  yd  will  be 
increased  proportionately  tofdi,  and  the  relations  are  represented 
by  the  triangle  abdi.  This  triangle  therefore  has  to  be  corrected  to 
correspond  with  the  balanced  diagram  abc.  This  can  be  done  by 
(1)  reducing  the  e.  m.  f .  of  phase  two  of  the  phase  converter  (by 
reducing  phase  one,  for  instance),  and  by  (2) —  moving /to  g. 
This  brings  di  in  coincidence  with  c  and  a  balanced  three-phase 
condition  then  results. 

It  is  obvious  in  Fig.  2  that  the  addition  of  an  e.  m.  f .  at  the 
terminal  d  corresponding  in  value  and  direction  to  the  line  cd 
would  have  corrected  to  a  balanced  condition  for  the  assumed 
load  and  power-factor.  Also,  in  Fig.  3,  a  correcting  e.  m.  f.  d\c 
would  have  accomplished  the  desired  result.  In  the  actual  dia- 
grams, instead  of  supplying  this  correcting  e.  m.  f .  directly,  it  was 
obtained  indirectly  by  combining  two  right  angle  e.  m.  f  .'s  of  suit- 
able value  and  direction,  these  two  being  readily  obtainable  in  the 
arrangement  shown.  However,  the  illustration  shows  how  a 
single  correcting  e.  m.  f.  of  proper  phase  and  value  can  correct 
from  a  distorted  three-phase  system  to  a  balanced  system. 

Instead  of  the  above  special  arrangement  for  changing  from 
three-phase  to  single-phase,  any  standard  type  of  three-phase 
motor,  either  synchronous  or  induction,  could  be  used  for  phase 
balancing  by  the  addition  of  a  suitable  correcting  e.  m.  f.  in  one 
of  the  phases,  and  if  this  correcting  e.  m.  f .  is  of  such  value  and 
direction  as  to  maintain  balanced  e.  m.  f.'s  across  the  three  termi- 
nals, then  the  phase-balancing  motor  will  correct  the  single-phase 
load. 

If  an  induction  motor  is  used  as  a  phase  balancer  under  the 
above  conditions,  then  it  will  simply  serve  as  a  phase  converter, 
but  has  no  ability  to  correct  or  adjust  the  power-factor.  If  the 
phase  balancer  is  of  the  synchronous  type,  however,  it  can  be 
adjusted  and  controlled  to  act  as  both  a  phase  converter  and  a 
power-factor  corrector.  If  the  single-phase  load  to  be  carried  is 
at  a  relatively  low  power-factor,  then  it  will  exert  a  demagnetizing 
effect  upon  the  phase  balancer  which  must  be  taken  into  account 


SINGLE-PHASE  FROM  POLYPHASE  565 

when  the  e.  m.  f  phase  relations  are  adjusted  for  proper  balancing. 
This  means  that  the  field  excitation  of  the  phase  balancer  must  be 
increased  sufficiently  to  overcome  the  demagnetizing  tendency  of 
the  single-phase  load.  This  increase  in  field  excitation  will  tend 
to  increase  the  e.  m.  f.'s  of  all  the  armature  circuits  but,  as  one 
winding,  when  balanced  conditions  are  obtained,  will  carry 
practically  all  the  wattless  current  corresponding  to  the  single- 
phase  load,  while  the  others  will  be  carrying  power  only  (on  the 
assumption  that  100  percent  power-factor  is  maintained  on  the 
three-phase  system)  the  effect  of  the  internal  self-inductions  of  the 
phase  balancer  will  be  such  that  the  resultant  e.  m.  f.'s  of  some 
of  the  windings  will  be  increased  to  a  greater  extent  than  others 
when  the  field  excitation  is  increased.  Therefore,  when  correcting 
for  inductive  loads,  a  different  value  and  direction  of  the  correct- 
ing e.  m.  f.  is  necessary  than  would  be  required  for  single-phase 
loads  without  power-factor  correction. 

It  is  obvious  from  the  above  that  what  is  needed  for  obtaining 
balanced  conditions  and  corrected  power-factor  on  the  polyphase 
system  when  carrying  a  low  power-factor  single-phase  load,  is  a 
suitable  synchronous  motor  acting  as  a  phase  balancer  in  connec- 
tion with  some  auxiliary  means  for  introducing  a  correcting 
e.  m  f .  which  should  vary  in  value  and  direction  with  the  load  and 
power-factor. 

There  are  various  ways  by  which  this  result  can  be  accom- 
plished. To  illustrate:  It  may  be  assumed  that  the  desired 
correcting  e.  m.  f .  may  be  obtained  by  means  of  a  small  synchron- 
ously running  booster  which  is  connected  in  series  with  one  phase  of 
the  phase  balancer.  The  value  of  this  e.  m.  f .  can  be  varied  by  vary- 
ing the  field  excitation  of  the  booster  field.  The  phase  relation  of 
this  booster  e.  m.  f .  can  be  regulated  in  various  manners,  as,  for 
instance,  by  mechanically  shifting  the  field  structure  circunofer- 
entially  with  respect  to  the  armature.  Or,  the  armature  of  the 
booster  might  have  two  fields  side  by  side,  but  with  their  poles 
displaced  circumferentially  90  degrees  with  respect  to  each  other. 
Then,  by  separate  adjustment  of  the  excitations  of  the  two  fields 
up  and  down,  or  reversed,  the  e.  m.  f .  generated  by  the  booster 
armature  can  be  given  any  desired  direction  or  value.  Or,  instead 
of  two  fields  side  by  side,  a  single  field  structure  can  be  used  in  the 
booster,  with  two  exciting  windings  overlapping  or  displaced  90 
degrees  with  respect  to  each  other,  Kke  the  primary  windings  of  a 
two-phase  induction  motor.  By  proper  adjustment  of  the-  eafcdt- 


566  ELECTRICAL  ENGINEERING  PAPERS 

ing  current  in  these  two  windings,  the  same  results  as  with  two 
fields  side  by  side  may  be  obtained.  With  the  booster  e.  m.  f .  thus 
under  control,  it  is  obvious  that  any  desired  phase  or  voltage 
correction  can  be  obtained  in  the  phase  balancer.  There  are 
various  other  ways  of  obtaining  the  corrective  e,  m,  f.,  such  as  by 
induction  regulators,  etc.,  but  the  above  is  sufficient  to  illustrate 
the  general  arrangement  or  method  of  operation.  Mr.  E  P.  W 
Alexanderson*  has  also  proposed  a  method  of  accomplishing  this 
result. 

The  very  considerable  complication  of  such  methods  of  phase 
balancing  may  be  necessary  where  widely  fluctuating  loads  and 
non-related  variations  in  power-factor  are  encountered.  In  such 
cases,  automatic  voltage  regulations  can  be  used  in  connection 
with  the  main  and  the  booster  fields  to  obtain  the  desired  cor- 
rective action.  However,  combination  of  the  synchronous  ma- 
chine and  its  booster,  or  boosters,  requires,  as  a  rule,  considerably 
less  total  apparatus  than  a  straight  motor-generator,  and  the 
losses  should  also  be  materially  less. 

However,  where  the  single-phase  load  conditions  are  not  too 
widely  fluctuating,  it  is  possible  to  use  much  simpler  arrangements. 
In  electric  furnace  work  the  single-phase  load  and  power-factor 
may  be  almost  constant  when  the  load  is  on.  In  such  cases, 
phase  splitting  may  be  accomplished  in  a  fairly  simple  and  effective 
manner  by  a  single  synchronous  machine,  either  with  or  without  a 
small  additional  autotransformer,  and  with  suitable  taps  and 
switches  for  varying  certain  voltage  relations. 

In  synchronous  phase  balancers  there  is  a  very  considerable 
magnetic  action  on  the  field  poles  and  structure  by  the  armature 
winding  when  carrying  load,  unless  the  field  poles  are  equipped 
with  ample  cage  dampers  similar  to  those  required  on  the  fields  of 
large  single-phase  generators.  If  these  dampers  are  of  proper 
proportions,  tlie  pulsating  eSect  of  the  armature  on  the  field  can 
be  suppressed  with  comparatively  small  loss  in  the  dampers. 
However,  the  alternative  of  such  machine,  namely,  the  straight 
motor-generator,  must  also  have  heavy  dampers  on  its  single- 
phase  element,  so  this  does  not  change  the  relative  efficiencies 
of  the  two  methods. 

When  power-factor  correction  is  required,  as  well  as  phase 
balancing,  then  the  size  or  capacity  of  the  phase  balancer  will 
depend  to  a  certain  extent  upon  the  amount  of  power-factor 

*PBase  Balancer  for  Single-Phase  Load  on  Polyphase  Systems,"  by  Mr.  B.  P.  W.  Alexan- 
derson,  General  Electrical  Review,  December,  1913. 


SINGLE-PHASE  FROM  POLYPHASE 


567 


correction.  As  it  may  be  of  interest  to  know  what  capacity  of 
phase  balancer  is  required  in,  terms  of  single-phase  load,  the  ap- 
proximate curves  shown  in  Fig.  4  have  been  worked  out  for  dif- 
ferent power-factors,  showing  the  capacity  (three-phase)  re- 
quired in  phase  balancers  for  each  1,000  k.v.a.  single-phase  load 
taken  off.  The  ordinates  represent  power-factors  of  the  single-phase 
load,  while  the  abscissae  show  the  k.v.a.  ratings  of  the  phase 
balancers  required  at  various  three-phase  power-factors.  The 
phase  balancing  k.v  a.  is  given  in  terms  of  three-phase  capacity — 
that  is,  the  capacity  which  the  machine  would  have  as  a  three- 
phase  generator,  with  a  current  rating  corresponding  to  the 
largest  of  the  unbalanced  currents  in  its  three  phases.  In  other 
words,  this  rating  is  on  the  basis  of  maximum  local  losses,  instead 
of  averaging,  and  thus  represents  the  most  severe  condition. 
The  capacity  of  the  booster  or  other  apparatus  for  supplying  the 
correcting  e.  m.  f .  is  not  included.  This  can  be  assumed  roughly 
as  about  15  percent  of  that  of  the  phase  balancer,  whether  it  is  a 
separate  piece,  such  as  a  separate  booster  or  transformer,  or  is 
obtained  in  the  balancer  windings. 


— rafer"^ \m *-iK3B 

,va  yhaae  I  aland jg  Cap;  olty     | 


\ 


1350" 


PIG.  4— CURVES  SHOWING  THE  BALANCING  CAPACITY  REQUIRED 
(THREE-PHASE)  FOR  EACH  1  000  K.V.A.  OP  SINGLE-PHASE  LOAD 

These  curves  show  that  usually  there  is  considerable  saving 
in  capacity  of  apparatus  in  the  use  of  a  phase  balancer,  as  com- 
pared with  a  straight  motor-generator  where  power-factor  cor- 
rection is  not  important.  For  example,  assume  a  single-phase 
load  is  at  70  percent  power-factor,  while  the  corresponding  three- 


568  ELECTRICAL  ENGINEERING  PAPERS 

phase  balanced  power  is  to  be  held  at  the  same  power-factor. 
From  the  table,  the  approximate  capacity  of  the  phase  balance  is 
1,000  k.v.a.  Adding  IS  percent  for  the  booster,  gives  1,150  k.v.a. 
as  the  total  balancing  capacity  required.  Comparing  this  with  a 
straight  motor-generator,  the  driving  motor  will  have  a  normal 
capacity  of  700  kw  approximately,  while  the  1,000  kv.a.  single- 
phase  generator  would  approximately  correspond  in  capacity  to  a 
1,500  k.v.a.  three-phase  machine — thus  requiring  a  total  of  2,200 
k.v.a.,  compared  with  1,150  k.va.  for  the  phase  balancer.  The 
latter  means,  therefore,  materially  less  expensive  apparatus — also 
more  efficient.  It  may  be  noted  throughout  that  where  there  is 
no  correction  of  power-factor,  the  balancer  capacity  in  k.v.a/ will 
be  equal  to  the  k  v.a.  of  the  single-phase  load.  If,  however,  in  the 
above  example,  70  percent  single-phase  power-factor  is  to  be  cor- 
rected to  90  percent  in  the  three-phase  circuit,  then  the  balancer 
capacity  will  be  1,390  k.v.a.  Adding  15  percent  for  the  correct- 
ing booster  gives  1,600  kv.a.  against  2,200  k.v.a.  for  the  motor- 
generator. 

The  phase  balancer  therefore  apparently  does  not  correct  for 
power-factor  as  advantageously  as  the  straight  motor-generator. 
Also,  where  automatic  correction  of  power-factor  is  desirable, 
the  motor-generator  arrangement  is  somewhat  less  complicated. 


THE  TECHNICAL  STORY  OF  THE  FREQUENCIES 

FOREWORD — This  paper  was  presented  before  ^the  Washington 
Section  of  the  American  Institute  of  Electrical  Engineers  in 
January,  1918.  It  covers  briefly  the  history  of  the  various 
frequencies  used  in  America  and  the  engineering  and  technical 
reasons  which  have  influenced  their  ultimate  choice  or  rejection. 
The  author  had  in  mind  the  preservation  of  this  in  more  or  less 
historical  form,  in  order  that  it  should  not  eventually  be  entirely 
lost.  Since  the  publication  of  this  paper,  the  author  has 
received  many  favorable  comments  on  it,  as  being  the  only 
reliable  source  of  information  on  the  subject  which  is  now 
available.  It  has  been  reprinted  in  its  entirety  in  a  number  of 
technical  papers  and  the  material  drawn  from  it  has  been  used 
in  a  number  of  technical  lectures  by  various  engineers  and 
educators. — (ED . ) 


IN  the  early  days  of  the  alternating  current,  there  were  no  well 
established  tendencies  toward  any  definite  frequencies,  either 
in  this  country  or  in  Europe.  Each  manufacturer  selected  that 
frequency  which  best  suited  his  particular  style  of  generating 
apparatus,  and  the  greater  the  number  of  manufacturers,  the 
greater  the  number  of  frequencies.  But  quite  early,  in  America, 
there  developed  definite  tendencies  toward  certain  standards. 
Later,  similar  tendencies  in  Europe  operated  to  bring  about  a 
general  adoption  of  a  limited  number  of  definite  frequencies. 

It  is  not  the  purpose  of  the  paper  to  deal  with  the  history  of 
any  but  the  American  tendencies  and  developments,  as  these  form 
a  sufficient  story  in  themselves. 

The  story  of  how  and  why  the  various  commercial  fre- 
quencies came  into  use  and  then  iropped  out  again,  in  most 
cases,  is  not  primarily  the  story  of  the  frequencies  themselves, 
but  of  the  various  uses  to  which  the  alternating  current  has 
been  applied.  In  other  words,  fundamental  changes  in  the 
application  of  alternating  current  have  led  to  radical  changes  in 
the  frequencies.  Some  of  the  applications  which  have  had  a 
determining  factor  on  the  frequency  of  the  supply  system  are 
as  follows;  incandescent  lighting,  transformers,  transmission 
systems,  arc  lighting,  induction  motors,  synchronous  converters, 
constructional  conditions  in  rotating  machinery,  and  operating 
conditions.  A  brief  consideration  of  these  items  individually, 

569 


570  ELECTRICAL  ENGINEERING  PAPERS 

from  the  present  viewpoint,  indicates  that  while  some  of  them 
had,  at  one  time,  very  considerable  influence  in  determining 
frequency  conditions,  yet,  in  a  number  of  cases,  the  original 
reasons  have  disappeared  through  improvements  and  refine- 
ments, as  will  be  described  later. 

At  various  times  the  following  standard  frequencies  have  been 
in  use  in  this  country,  namely,  133H»  125,  83J^,  66%, 
60,  50,  40,  30  and  25  cycles  per  second.  These  did  not  appear 
chronologically  in  the  order  given  above,  and  a  few  odd  fre- 
quencies in  a  few  special  applications  are  omitted. 

In  the  following,  the  various  frequencies  will  be  considered 
more  or  less  in  the  order  of  their  development  and  basic  reasons 
will  be  given  for  their  choice,  and  the  writer  will  endeavor  to 
show  why  certain  of  them  have  persisted,  while  others  have 
dropped  out.  It  will  also  be  shown  why  the  commercial  situa- 
tion has  first  tended  strongly  toward  certain  frequencies  and 
afterwards  swung  toward  others. 

133  AND  125  CYCLES 

In  the  earliest  alternating  work,  the  whole  service  consisted 
of  incandescent  lighting,  and  the  electric  equipment  was  made 
up  of  small  high-speed  belted  single-phase  generators  and  house- 
to-house  distributing  transformers.  As  the  transformers  were 
of  small  capacity  and  as  their  design  was  in  a  very  crude  state, 
it  was  believed  that  a  relatively  high  frequency  would  best  meet 
the  transformer  conditions.  A  choice  of  such  an  odd  frequency 
as  133j/£  cycles  per  second,  is  due  to  the  fact  that  in  those 
early  days  (1886  to  1893)  frequencies  were  usually  designated 
in  terms  of  alternations  per  minute.  One  of  the  earliest  com- 
mercial generating  units  constructed  by  the  Westinghouse 
company  had  a  speed  of  2000  rev.  per  min.  and  had  eight  poles. 
This  presented  a  fairly  convenient  constructional  arrangement 
for  the  surface-wound  type  of  rotating  armature,  which  was  the 
only  one  recognized  at  that  time.  The  speed  of  2000  rev.  per 
min.,  with  eight  poles,  gave  16,000  alternations  per  minute,  or 
133J^  cycles  per  second  according  to  our  present  method  of 
designation.  Thus  the  earliest  frequency  in  commercial  use  in 
this  country  was  fixed,  to  a  certain  extent,  by  constructional 
reasons,  although  the  house-to-house  transformer  problem  ap- 
parently indicated  the  need  for  a  relatively  high  frequency. 
The  Thomson-Houston  company  adopted  a  standard  fre- 
quency of  15  000  alternations  per  minute,  (125  cycles)  instead 


STORY  OF  THE  FREQUENCIES  571 

of  the  Westinghouse  16,000,  but  the  writer  does  not  know  why 
this  difference  was  made.  However,  the  two  frequencies  were 
so  close  together  that  practically  they  could  be  classified  as  one. 
At  this  time,  it  should  be  borne  in  mind,  there  were  no  real 
transmission  problems,  no  alternating-current  arc  lighting,  no 
induction  motors  and  the  need  for  uniform  rotation  of  the  genera- 
tors was  not  recognized.  The  induction  motor,  in  its  earliest 
stages,  came  in  1888  and  considerable  work  was  done  on  it  in 
1889  and  1890,  but  it  required  polyphase  supply  circuits  and  com- 
paratively low  frequency  and,  therefore,  it  had  no  connection 
whatever  with  the  then  standard  single-phase,  133f  and  125 
cycle  systems.  The  synchronous  converter  was  also  unheard 
of  (one  might  say  almost  undreamed  of)  at  that  time. 

60  CYCLES 

In  1889  or  1890  it  was  beginning  to  be  recognized  in  this 
country  that  some  lower  frequency  than  125  and  133 1  cycles 
would  be  desirable.  Also  about  this  time  direct-coupled  and 
engine-type  alternators  were  being  considered  in  Europe  and  it 
was  felt  that  such  construction  would  eventually  coftie  into  use 
in  America.  It  was  appreciated  that  in  such  case^  133f  cycles 
would  present  very  considerable  difficulties  compared  with 
some  much  lower  frequency,  due  to  the  large  number  of  poles 
which  would  be  required.  For  instance,  an  alternator  direct 
driven  by  an  80-rev.  per  min.  engine  would  require  200  poles 
to  give  the  required  frequency  and  such  construction  was  looked 
upon  as  being  practically  prohibitive.  About  this  time  Mr. 
L.  B.  Stillwell,  then  with  the  Westinghouse  company,  made  a 
very  careful  study  of  this  matter  of  a  new  frequency,  in  connec- 
tion with  the  possibilities  of  engine  type  generators,  and  after 
analyzing  a  number  of  cases,  it  appeared  that  7200  alternations 
per  minute  (60  cycles  per  second),  was  about  as  high  as  would 
be  desirable  for  the  various  engine  speeds  then  in  sight.  Trans- 
former constructions  and  arc  lighting  were  also  consideredln 
this  analysis.  While  it  was  deemed  that  a. somewhat  higher 
frequency  might  be  better  for  transformers,  yet  a  lower  fre- 
quency than  60  cycles  was  considered  as  possibly  better  for 
engine  type  generators.  A  compromise  between  all  the  various 
conditions  eventually  led  to  60  cycles  as  the  best  frequency. 
However,  while  ^this  frequency  originated  about  1890,  it  did  not 
come  into  use  suddenly,  for  it  was  impossible  to  introduce  such 
a  radical  change  in  a  brief  time.  Moreover,  the  direct-coupled 


572  ELECTRICAL  ENGINEERING  PAPERS 

or  engine-type  generator  was  slow  in  coming  into  general  use  and, 
therefore,  there  was  not  the  necessity  for  the  introduction  of 
this  low  frequency  in  many  of  the  equipments  sold  from  1890 
to  1892.  However,  by  1893,  60  cycles  became  pretty  firmly 
established  and  was  sharing  the  business  with  the  133i -cycle 
systems.  It  should  be  borne  in  mind  that,  at  this  time,  the 
adoption  of  this  frequency  was  not  considered  as  a  direct  means 
for  bringing  forward  the  polyphase  induction  motor,  for  the 
earlier  60-fcycle  systems,  like  the  125-  and  133^-cycle,  were  all 
single-phase  Also,  it  was  then  thought  that  the  polyphase 
motor  would  possibly  require  a  still  lower  frequency  and,  more- 
over, the  polyphase  system  was  looked  upon  as  in  a  class  by 
itself,  suitable  only  for  induction  motor  work.  At  that  time 
the  introduction  of  polyphase  generators  for  general  service  was 
not  contemplated.  This  followed  about  two  or  three  years  later. 

In  1890  the  Westinghouse  company,  which  had  been  de- 
veloping the  Tesla  polyphase  motor  laid  aside  the  work,  largely 
on  account  of  there  being  no  suitable  general  supply  systems  for 
this  type  of  motor.  The  problem  was  again  revived  in  1892, 
in  an  experimental  way,  with  a  view  to  bringing  out  induction 
motor  which  might  be  applied  on  standard  frequencies  such  as 
could  be  used  in  commercial  supply  circuits  for  lighting  and  other 
purposes.  It  should  be  understood  that  at  this  time  such  cir- 
cuits w^re  not  in  existence  but  were  being  contemplated.  In 
1893,  after  the  polyphase  motor  had  been  further  developed  up 
to  the  point  where  it  showed  great  commercial  possibilities,  the 
best  means  for  getting  it  on  the  market  were  carefully  considered. 
It  was  decided  that  the  best  way  to  promote  the  induction  motor 
business  was  to  create  a  demand  for  it  on  commercial  alternating- 
current  systems.  This  meant  that,  in  the  first  place,  such  sys- 
tems must  be  created.  Therefore,  it  was  decided  to  undertake 
to  fill  the  country  with  polyphase  generating  systems,  which  were 
primarily  to  be  used  for  the  usual  lighting  service  It  was 
thought  that,  with  such  systems  available,  the  time  would  soon 
come  when  there  would  be  a  call  for  induction  motors.  In  this 
way  experience  would  be  obtained  in  the  construction  and  opera- 
tion of  polyphase  generators  and  the  operating  public  would  not 
be  unduly  handicapped  in  the  use  of  such  generators,  compared 
with  the  older  single-phase  types. 

An  early  example  of  this  new  practise  ^as  in  the  2000-kw. 
polyphase  generating  units  used  for  lighting  the  Chicago  World's 
Pair  in  1893.  Here  the  single-phase  type  still  persisted,  as  each 


STORY  OF  THE  FREQUENCIES  573 

generator  unit  was  made  tip  of  two  similar  frames  placed  side  by 
side,  but  with  their  single  phase  armatures  dsplaced  one-half 
pole  pitch  from  each  other  so  that  the  combined  machine  de- 
livered two  single-phase  currents  displaced  90  degrees  from  each 
other.  It  was  considered  that  each  circuit  could  be  regulated 
independently  for  lighting  service,  and  polyphase  motors  could 
be  operated  from  the  two  circuits.  These  generators  (at  that 
time  the  largest  in  this  country)  were  designed  in  1892  and  were 
of  60  cycles.  These,  therefore,  indicate  the  tendency  at  that  time 
toward  lower  frequency  and /polyphase  generation,  although 
commercial  polyphase  motors  were  not  yet  on  the  market. 

25  CYCLES 

At  the  same  time  that  60  cycles  was  selected  as  a  new  standard 
it  was  refcognized  that  at  some  future  time  there  would  be  a 
place  for  some  much  lower  frequency,  but  it  was  not  until  two 
years  later  that  this  began  to  narrow  down  to  any  particular 
frequency.  In  1892  the  first  Niagara  electrification,  after  several 
years  consideration  by  eminent  authorities,  had  centered  on 
polyphase  alternating  current  as  the  most  desirable  system.  The 
engineers  of  the  promoting  company  had  also  worked  out  what 
they  considered  the  most  suitable  construction  of  machine.  This 
involved  5000-h.  p.  units  at  250  revolutions  per  minute.  Prof. 
George  Forbes,  one  of  the  engineers  of  the  company  had  furnished 
the  electrical  designs  for  a  machine  with  an  external  rotating 
field  and  an  internal  stationary  armature.  His  design  used  eight 
poles,  thus  giving  2000  alterations  per  minute,  or  16f  cycles  per 
second.  Quite  independently  of  this,  the  Westinghouse  com- 
pany, in  1892,  had  been  working  on  the  development  of  synchron- 
ous converters,  using  belted  550-volt  d-c.  generators  with  two- 
phase  collector  rings  -added.  The  tests  on  these  machines  had 
shown  the  practicability  of  such  conversion  and  had  even  proved 
at  this  early  date,  that  the  converter  copper  losses  were  much 
lower  than  in  the  corresponding  d-c.  generators.  Thus  it  is  an 
interesting  fact  that  the  first  evidence  of  this  important  principle 
was  obtained  from  a  shop  test  rather  than  by  calculation.  The 
writer,  from  an  analysis  of  the  tests,  which  were  made  under  his 
immediate  direction,  concluded  that  the  armature  copper  losses 
must  be  considerably  lower  than  in  the  same  machine  used  as  a 
d-c.  generator,  He  also  brought  the  matter  to  the  attention 
of  Mr.  R*  D,  Mershon,  -then  with  the  Westinghouse  company, 
and  the  problem  was  then  worked;  gut  mathematically  by  him 


574  ELECTRICAL  ENGINEERING  PAPERS 

and  the  writer,  in  two  quite  different  ways,  but  with  similar 
results,  showing  that  the  converter  did  have  actually  very  much 
reduced  copper  losses. 

As  a  result  of  this  work  of  the  Westinghouse  company  on  the 
synchronous  converter,  it  was  decided  that,  to  make  such  ma- 
chines practicable,  some  suitable  relatively  low  frequency  was 
required.     This  appeared  to  be  about  30  cycles.    About  this 
time  the  construction  of  the  Niagara  generators  was  taken  up 
with  the  Westinghouse  company  to  see  whether  it  would  con- 
struct these  machines  according  to  the  designs  submitted  by  the 
promoting  company's  engineers.     These  designs  were  gone  over 
as  carefully  as  the  knowledge  of  such  apparatus,  at  that  time, 
permitted,  and  many  apparent   defects  and  difficulties  were 
pointed  out.     The  Westinghouse  company  then  proposed,  as  a 
substitute,  a  16-pole,  250-rev.  per  min.  machine  (the  speed  being 
definitely  fixed  at  250  rev.  per  xnin.).     This  gave  33f  cycles  or  as 
near  to  the  Westinghouse  proposed  30  cycle  system,  as  it  was 
possible  to  get.     Then  many  arguments  were  brought  forward, 
pro  and  con,   for  the  two  machines   and  frequencies.     Prof. 
Forbes1  preference  for  l&f  cycles  was  based  partly  on  the  pos- 
sibilities it  presented  for  the  construction  and  operation  of  com- 
mutator type  motors,  just  as  with  direct  current  circuits.     The 
Westinghouse  contention  was  that  this  frequency  was  too  low 
for  any  kind  of  service  except  possibly  commutator  type  ma- 
chines.    Tests  were  made  with  incandescent  lights  and  it  was 
found  that  at  33|  cycles  there  was  little  or  no  winking  of  light, 
while  at  16f  cycles,  the  winking  was  extremely  bad.     Tables 
were  also  made  up,  showing  the  limited  number  of  speed  com- 
binations at  16f  cycles  for  induction  motors,  in  case  such  should 
come  into  use.     This  showed  how  superior  the  33£  cycles  would 
be  as  regards  such  apparatus.    It  was  also  brought  cut  that 
synchronous  converters,  when  such  became  commercial,  would 
be  much  better  adapted  for  the  higher  frequency,  as  the  choice 
of  speeds  would  be  much  greater.     Prom  the  present  viewpoint 
the  arguments  appear  to  have  been  much  in  favor  of  the  West- 
inghouse side  of  the  case. 

As  a  consequence  of  all  this  discussion  the  suggestion  was 
advanced  by  some  one,  that  a  12  pole,  250-revolution  machine, 
(that  is,  3000  alternations,  or  25  cycles),  might  meet  sufficiently 
the  good  qualities  of  both  of  the  proposed  frequencies  and  would 
thus  be  a  good  compromise.  In  consequence  a  12-pole,  25-cycle 
machine  was  worked  up  by  the  Westinghouse  company  and 


STORY  OF  THE  FREQUENCIES  575 

eventually  this  frequency  was  adopted  for  the  Niagara  genera- 
tors. Afterwards,  while  these  generators  were  being  constructed 
it  was  brought  out  pretty  strongly  that  the  great  advantage  of 
this  frequency  would  be  in  connection  with  synchronous  con- 
verter operation,  but  that  it  was  also  extremely  well  adapted  for 
slow-speed  engine  type  generators,  which  were  then  coming  into 
use.  In  consequence  of  the  prominence  given  this  frequency  it 
was  soon  adopted  as  a  standard  low  frequency,  especially  in  those 
plants  where  synchronous  converters  were  expected  to  form  a 
prominent  part  of  the  system. 

However,  while  60  and  25  cycles  came  into  use;  as  described 
above,  it  must  be  recognized  that  they  had  competitors.  For 
instance,  66f  cycles  (8000  alternations  or  one-half  of  16,000) 
was  used  to  a  considerable  extent  by  one  of  the  manufacturing 
companies.  Also  50  cycles  came*  into  use  in  certain  plants  and, 
to  a  certain  extent,  is  still  retained,  but  has  become  the  standard 
high  frequency  of  Europe.  Instead  of  25  cycles,  the  Westing- 
house  company  advocated  30  cycles  for  some  of  its  plants,  largely 
because  with  the  25  per  cent  higher  speeds  permissible  with  such 
frequencies,  the  capacities  of  induction  motors  could  be  cor- 
respondingly increased  and  also  incandescent  lighting  was  more 
satisfactory.  However,  it  was  soon  recognized  that  the  66f  and 
30  cycle  variations  from  the  two  leading  frequencies  of  60  and  25 
cycles  were  hardly  worth  while,  and  they  were  gradually  dropped, 
except  in  plants  already  installed.  A  brief  attempt  was  made 
at  a  somewhat  later  period  to  place '40  cycles  upon  the  market 
as  a  substitute  for  both  25  and  60  cycles.  This  was  done  under 
the  impression  that  40  cycles  would  give  a  universal  system  for 
arc  and  incandescent  lighting,  transmission,  induction  motors, 
synchronous  converters  and  about  everything  else.  This  fre- 
quency possessed  many  merits  and  it  was  thought,  at  One  time, 
that  it  might  win  out,  but  apparently  the  other  two  frequencies 
were  too  well  established,  and  the  40  cycle  system  eventually 
lost  ground. 

The  problem  of  the  frequencies  finally  narrowed  down  to  the 
two  standards,  and  these  two  were  accepted  because  it  was 
thought  that  they  covered  such  entirely  different  fields  of  ser- 
vice that  neither  of  them  could  ever  expect  to  cover  the  whole. 
In  other  words,  two  standards  were  required  to  cover  the  whole 
range  of  service.  It  was  recognized  that  25  cycles  would  not 
take  care  of  alternating-current  arc  lighting  and  that  it  was 
questionable  for  incandescent  lighting  in  general.  In  other  tfays, 


576  ELECTRICAL  ENGINEERING  PAPERS 

such  as  suitability  for  engine-type  construction,  application  to 
induction  motors  and  synchronous  converters  and  transmission 
of  power  to  long  distances,  it  tnet  the  needs  of  an  ideal  system, 
as  then  understood.  Also,  in  parallel  operation  of  engine-type 
alternators,  which  was  one  of  the  serious  problems  of  those  days, 
the  25-cycle  machines  were  unquestionably  superior  to  the  60- 
cycle  ones,  due  to  the  lesser  displacement  of  the  e.  m.  f.  waves 
with  respect  to  each  other  with  a  given  angular  variation  in  the 
engine  speeds.  However,  although  the  25-cycle  system  pre- 
sented so  many  advantages,  it  could  not  take  care  of  the  lighting 
business,  and,  therefore,  could  not  entirely  dominate  the  situa- 
tion. 

As  regards  60  cycles,  it  was  felt  that  this  could  handle  the  direct 
lighting  situation  in  a  very  satisfactory  manner  and  was  pos- 
sibly better  suited  for  transformers  than  25  cycles,  although 
there  were  differences  of  opinion  in  this  matter,  especially  when 
it  came  to  the  larger  capacities.  It  was  reasonably  well  adapted 
for  induction  motors  in  general,  but  not  for  very  low  speeds.  In 
matters  of  transmission  and  in  the  operation  of  synchronous 
converters  it  was  thought  to  be  vitally  defective. 

From  the  above  consideration  it  would  appear  that  the  25- 
cycle  systems  presented  the  stronger  showing  as  a  whole  and, 
therefore,  there  was  a  decided  tendency  toward  this  frequency, 
except  in  those  cases  where  lighting  directly  from  the  alternating- 
current  system  was  considered  of  prime  importance.  In  those 
systems,  such  as  many  of  the  Edison  companies,  where  low- 
voltage  three-wire  direct  current  was  used  from  synchronous 
converters,  the  tendency  was  almost  solidly  toward  the  25-cycle 
system.  In  those  days  the  central  station,  which  had  gotten 
itself  committed  to  the  60-cycle  system  so  deeply  that  it  could 
^not  change,  was  looked  upon  with  commiseration.  Sixty-cycle 
plants  were  looked  upon,  to  a  certain  extent,  as  a  necessary  evil. 
In  fact,  so  strong  was  the  tendency  toward  25  cycles  that  in  many 
cases  25-cycle  plants  were  installed  for  industrial  purposes,  where 
60  cycles  would  have  been  better.  The  25-cycle  synchronous 
converter  development  advanced  by  leaps  and  bounds  and  the 
machines  were  so  good  in  their  operation  that  it  was  believed 
that  60-cycle  converters  could  never  be  really  competitive  with 
them. 

On  the  other  hand,  in  those  large  plants,  which  were  so  "un- 
fortunate" as  to  have  60  cycles  installed,  many  apparent  make- 
shifts were  adopted  to  meet  the  various  service  requirements. 


STORY  OF  THE  FREQUENCIES  577 

In  arc  lighting,  incandescent  lighting,  transformers  and  motors 
there  was  no  need  for  makeshifts.  However,  in  conversion  to 
direct  current,  one  of  the  greatest  difficulties  appeared.  There 
were  many  who  advocated  motor-generators  for  this  purpose, 
largely  because  the  60-cycle  converter  was  thought  to  be  im- 
practicable, in  spite  of  the  fact  that  the  manufacturing  companies 
were  putting  them  on  the  market.  The  60-cycle  converter  at 
that  time  bore  a  bad  name.  It  is  now  recognized  that  many  of 
the  faults  of  the  early  60-cycle  synchronous  converter  operation 
were  not  in  the  converters  themselves,  but  were,  to  a  consider- 
able extent,  in  the  associated  apparatus.  Low-speed  engine- 
type,  60-£ycle  generators  were  not  always  adapted  for  operation 
of  synchronous  converters.  In  fact,  in  numerous  cases  such 
generators  would  not  operate  in  an  entirely  satisfactory  manner 
in  parallel  with  each  other,  and  yet  when  it  was  attempted  to 
operate  synchronous  converters  from  these  same  generators  the 
unsatisfactory  results  were  not  blamed  upon  the  generating 
system  but  upon  defects  of  the  converters  themselves.  Unfor- 
tunately, defects  in  the  generating  and  transmission  systems 
usually  appeared  in  the  converters  as  sparking  and  flashing, 
and  such  troubles  naturally  would  be  credited  to  defects  in  the 
construction  of  the  converters  themselves.  In  fact,  in  those 
days,  60-cycle  converters  were  expected  to  do  things  which  now 
are  considered  as  absurd.  For  instance,  in  one  case  in  the  writ- 
er's knowledge  a  60-cycle  synchronous  converter  was  criti- 
cized as  being  a  very  badly  designed  piece  of  apparatus,  due  to 
serious  flashing  at  times.  Investigation  developed  that  this 
converter  was  expected  to  operate  on  either  one  of  two  indepen- 
dent 60-cycle  systems  with  no  rigid  frequency  relation  to  each 
other.  The  converter  in  service  was  thrown  from  one  system  to 
the  other  indiscriminately,  and  sometimes  it  flashed  in  the  trans- 
fer and  sometimes  it  did  not.  The  machine  was  considered  to 
be  "no  good"  because  it  would  not  always  stand  such  switching. 
At  one  time  the  writer  stood  almost  alone  in  his  belief  that 
the  60-cycle  synchronous  converter  presented  commercial  pos- 
sibilities sufficient  to  make  it  a-  strong  future  contender  with 
the  25-cycle  machine,  provided  proper  supply  conditions  were 
furnished  and  certain  difficulties  in  the  proportions  of  the  con- 
verter itself  were  overcome.  One  basis  for  his  contention  was 
that  in  some  of  the  60-cycle  plants,  where  the  generator  rotation 
was  quite  uniform,  the  converters  were  evidently  much  superior 
in  their  operation  to  other  plants,  using  slow-speed  engine-type 


578  ELECTRICAL  ENGINEERING  PAPERS 

generators  with  considerable  periodic  variations.     In  such  plants 
the  hunting  tendency  of  the  converters  was  very  greatly  re- 
duced, with  consequent  improvement  in  sparking  and  general 
operation.    It  was  early  recognized  that  hunting  was  a  very 
harmful  condition,  both  in  60-  and  25-cycle  synchronous  con- 
verters, but  whereas  it  was  a  relatively  rare  condition  in  25-cycle 
plants  it  was  much  more  common  with  60  cycles.     However, 
the  operating  public  was  not  particularly  concerned  whether 
the  trouble  was  in  the  generating  plant  or  in.  the  converters 
themselves,  as  long  as  such  trouble  existed  and  was  not  overcome. 
Very  early  in  the  synchronous  converter  development  it  was 
found  that  hunting  would  produce  sparking  or  flashing  at  the 
commutators  of  the  converters.     However,  even  in  those  plants 
where  there  was  no  hunting  apparent,  there  was  difficulty  at 
times  due  to  flashing,  especially  with  sudden  change  of  load,- 
which  resulted  in  temporary  increase  in  the  d-c.  voltage.     This 
was  a  difficulty  which  was  inherent  in  the  converter  itself  and 
could  not  be  blamed  entirely  upon  the  generating  or  transmitting 
conditions,  for  25-cycle  machines  were    practically  free  from 
this  trouble  under  similar  conditions  of  operation.     Investiga- 
tion developed  the  fact  that  this  flashing  trouble  was  due  Jargely 
to  unduly  high  value  of  the  maximum  volts  between  commutator 
bars.     This  difficulty  was  recognized  long  before  it  was  over- 
come, simply  because  certain  physical  limitations  in  construction 
had  to  be  removed.     There  were  two  ways  in  which  the  maximum 
volts  per  bar  could  be  reduced,  namely,  by  increasing  the  number 
of  commutator  bars  per  pole  and  by  decreasing  the  ratio  of  the 
maximum  volts  to  the  average  volts  per  bar,  that  is,  by  increas- 
ing the  ratio  of  the  pole  width  to  the  pole  pitch,  but  both  of 
these  involved  structural  limitations  in  the  allowable  peripheral 
-speeds  of  the  commutator  and  the  armature  core.     Here  is 
where  a  little  elementary  mathematics  comes  in.      The  per- 
ipheral speed  of  the  commutator  is  directly  proportional  to  the 
distance  between  adjacent  neutral  points  on  the  commutator, 
and  the  frequency.     Therefore,  with,  a  given  frequency  the 
distance  between  the  adjacent  neutral  points  is  directly  propor* 
tional  to -the  peripheral  speed.    Thus,  a  commutator  speed  of 
4500  ft.  per  min.  which  was  then  considered  an  upper  limit, 
the  distance  between  adjacent  neutral  points  on  a  60-cycle 
converter  is  only  7$  in.  (19  cm.)     This  distance  is  thus  fixed 
mathematically  and  is  independent  of  the  number  of  poles  or 
revolutions  per  minute,  or  anything  else,  except  the  peripheral 


STORY  OF  THE  FREQUENCIES  579 

speed  and  the  frequency.  With  this  distance  of  1\  in.,  (19  cm  ), 
about  the  only  choice  in  commutator  bars  per  pole  was  36, 
giving  an  average  of  16f  volts  per  bar  on  a  600-volt  machine, 
and  nearly  20  volts  per  bar  with  momentary  increase  of  voltage 
to  700,  which  is  not  uncommon  in  railway  service. 

However,  it  is  not  this  average  voltage  which  fixes  the  flashing 
conditions,  but  it  is  the  maximum  voltage  between  bars,  and 
this  is  dependent  upon  the  average  voltage  and  upon  the  ratio 
of  the  pole  width  to  the  pole  pitch.  Here  is  where  one  of  the 
serious  difficulties  came  in.  As  mentioned  above  the  pole  pitch 
is  directly  dependent  upon  the  peripheral  speed  of  the  armature 
core  and  the  frequency.  Therefore,  in  a  60-cycle  machine,  if 
the  peripheral  speed  is  fixed,  the  pole  pitch  is  at  once  fixed. 
For  example,  with  an  armature  peripheral  speed  of  7200  ft.  per 
min.,  which  was  considered  high  at  that  time,  the  pole  pitch 
becomes  12  in  (30,48  cm.) ,  regardless  of  any  other  considerations, 
and  here  was  where  a  most  serious  difficulty  was  encountered. 
If  a  sufficiently  wide  neutral  zone  for  commutation  was  allowed 
the  interpolar  space  became  so  wide  that  there  was  not  enough 
left  for  a  good  pole  width.  For  instance,  if  the  interpolar  space 
was  made  6  in.  (15  24  cm.)  wide,  in  order  to  give  a  sufficiently 
wide  commutating  zone  to  prevent  sparking  or  flashing,  due  to 
fringing  of  the  main  field,  then  this  left  only  6  in.  for  the  pole 
face.  With  this  relatively  narrow  pole  face  the  ratio  of  the 
maximum  volts  to  the  average  volts  was  so  high  that  with  the 
36  commutator  bars  per  pole  the  machine  was  sensitive  to  arcing 
between  commutator  bars  thus  resulting  in  flashing.  By  widen- 
ing the  pole  face  this  difficulty  would  be  lessened  or  overcome 
but  with  the  fixed  pole  pitch  of  12  in.  (30.48  cm.)  the  neutral 
zone  would  be  so  narrowed  as  to  make  the  machine  sensitive  to 
sparking  and  flashing  at  the  brushes.  Thus,  no  matter  which 
way  we  turned  we  encountered  trouble.  Obviously  there  were 
two,  directions  of  improvement,  namely,  by  increasing  the 
number  of  commutator  bars,  thus  reducing  the  average  voltage, 
and  by  increasing  the  pole  pitch,  thus  allowing  relatively  wider 
poles  with  a  given  interpolar  space.  These  two  conditions  look 
simple  and  easy,  but  it  took  several  years  of  experience  to 
attain  them.  When  we  have  reached  apparent  physical  limita- 
tions in  a  given  construction,  especially  when  such  limitations 
are  based  upon  long  experience,  we  have  to  feel  our  way 
quite  slowly  toward  higher  limitations.  For  instance,  in  the 
case  of  the  60-cy"cte  converters  we  could  ndt  boldly  jump  our 


580  ELECTRICAL  ENGINEERING  PAPERS 

peripheral  speeds  20  to  25  per  cent  higher  and  simply  assume 
that  everything  was  all  right.  We  first  had  to  build  apparatus 
and  try  it  out  for  a  year  or  so.  Troubles,  due  to  peripheral 
speed,  do  not  always  become  apparent  at  once,  and  thus  time 
tests  are  necessary  Therefore,  while  the  peripheral  speeds  of 
the  60-cycle  synchronous  converters  were  actually  increased  20 
to  25  per  cent  practically  in  one  jump,  yet  it  took  two  or  three 
years  of  experimentation  and  endurance  tests  before  the  manu- 
facturers felt  sure  enough  to  adopt  the  higher  speeds  on  a  broad 
commercial  scale.  Thus,  while  the  change  from  the  older  more 
sensitive  type  of  60-cycle  converter  to  the  later  type  occurred 
commercially  within  a  comparatively  short  period  yet  the  actual 
development  covered  a  much  longer  period. 

Let  us  see  now  what  an  increase  of  25  per  cent  in  the  peripheral 
speeds  actually  meant.  As  regards  the  commutator,  the  number 
of  bars  could  be  increased  25  per  cent,  that  is,  from  36  to  45  per 
pole,  which  was  comparable  with  ordinary  d-c,  generator  practise. 
In  the  second  place,  an  increase  of  25  per  cent  in  the  peripheral 
speed  of  the  armature  core  meant  a  15-in.  (38.1-cm.)  pole  pitch, 
where  12  in.  (30.8  cm.)  was  used  before.  Assuming,  as  before, 
a  6-in.  (15.24-cm.)  interpolar  space,  then  the  pole  face  itself 
became  9  in.  (22,8  cm,)  in  width  instead  of  6  in.  (15.24  cm.) 
or  an  improvement  of  50  per  cent,  In  fact,  this  latter  improve- 
ment was  so  great  that  some  manufacturers  did  not  consider  it 
necessary  to  increase  the  number  of  commutator  bars,  although 
in  the  Westinghouse  machines  both  steps  were  made. 

The  above  improvements  so  modified  the  60-cycle  converter 
that  it  began  to  approach  the  25-cycle  machine  in  its  general 
characteristics.  It  was  still  quite  expensive  compared  with  the 
25-cycle,  due  to  the  large  number  of  poles,  and  its  efficiency  was 
considerably  lower  than  its  25-cycle  competitor,  on  account  of 
high  iron  and  windage  losses.  However,  due  to  the  need  for 
such  a  machine  it  was  gradually  making  headway,  in  spite  of 
handicaps  in  cost  and  efficiency. 

Almost  coincident  with  the  initiation  of  the  above  improve- 
ments in  the  60-cycle  converter,  came  another  factor  which  has 
had  much  to  do  with  the  success  of  this  type  of  machine.  This 
was  the  advent  of  the  turbo-generator  for  general  service.  As 
stated  before,  one  of  the  handicaps  of  the  60-cycle  converter  was 
in  the  non-uniform  rotation  of  the  engine  type  generators  which 
were  common  in  the  period  from  1897  to  about  1903  or  1904. 
But,  about  this  latter  date,  the  turbo-generator  was  making 


STORY  OF  THE  FREQUENCIES  5S1 

considerable  inroads  on  the  engine-type  field  and  within  a  rela- 
tively short  period  it  so  superseded  the  former  type  of  unit,  that 
it  was  recognized  as  the  coming  standard  for  large  alternating 
power  service.     With  the  turbo-generator  came  uniform  rotation 
and  this  at  once  removed  one  of  the  operating  difficulties  of  the 
60-cycle  converters.     However,  in  the  early  days  of  the  turbo- 
generator, 25  cycles  still  was  in  the  lead  and  many  of  the  earlier 
generators  were  made  for  this  frequency,  especially  in  the  larger 
units.    But  it  was  not  long  before  it  was  recognized  that  60 
cycles   presented   considerable   advantage   in   turbo-generator 
design  due  to  the  higher  permissible  speeds.     In  the  earlier  days 
of  turbo-generator  work,  this  was  not  recognized  to  any  extent, 
as  the  speeds  of  all  units  were  so  low  that  the  effect  of  any  speed 
limitations  was  not  yet  encountered.      For  instance,  a  1500-kw  , 
60-cycle  turbo-generator  would  be  made  with  six  poles  for  1200 
revolutions,  while  a  corresponding  25  cycle  unit  would  be  made 
with  two  poles  for  1500  revolutions.      This  slightly  higher  speed 
at  25  cycles  about  counterbalanced  the  difficulties  of  the  two- 
pole  construction  compared  with  the  six-pole.     However,  before 
long,  more  experience  enabled  the  six  pole,  60-cycle  machine  to 
be  replaced  at  1800  revolutions,  and  a  little  later  by  two  poles  at 
3600  revolutions.     This,  of  course,  turned  the  scales  very  much 
in  the  other  directioA.     In  larger  units,  however,  the  advantage 
still  appeared  to  be  in  favor  of  25  cycles,  but  in  the  course  of 
development,  1500  revolutions  was  adopted  quite  generally  for 
25-cycle  work,  and  this  was  the  limiting  speed,  as  such  machines 
had  only  two  poles,  or  the  smallest  number  possible  with  ordinary 
constructions.    On  the  other  hand,  for  60  cycles,  1800  revolutions 
was  adopted  quite  generally  for  units  up  to  almost  the  extreme 
capacities  that  had  been  considered,   consequently  the  con- 
structional conditions  in  the  large  machines  swung  in  favor 
of  60  cycles.    Therefore,  with  the  coming  of  the  steam  turbine 
and  the  development  of  high-speed  turbo-generator  units,  the 
tendency  has  teen  strongly  toward  60  cycles.    This,  with  the 
greater  perfection  of  the  60-cycle  converter,  had  much  to  do  with 
directing  the  practise  away  from  the  25  cycles. 

'However,  there  were  other  conditions  which  tended  strongly 
toward  60  cycles.  In  the  early  development  of  the  induction 
motor,  the  25-cycle  machines  were  considerably  better  than  the 
60-cycle  and  possibly  little  or.ao  more  expensive.  However, 
as  refinements  in  design  and  practise  came  in,  certain  important 
advantages  of  the  60-cycle  began  to  crop  out.  For  instance, 


582  ELECTRICAL  ENGINEERING  PAPERS 

with  25  cycles  there  is  but  little  choice  in  speed,  for  small  and 
moderate  size  motors.  At  this  frequency  a  four-pole  motor  has 
a  synchronous  speed  of  only  750.  The  only  higher  speed  per- 
missible is  1500  revolutions  with  two  poles,  and  it  so  happens 
that  in  induction  motors  the  two-pole  construction  is  not  mate- 
rially cheaper  than  the  four  pole,  consequently  the  principal  ad- 
vantage in  going  to  1500  revolutions  was  only  in  getting  a  higher 
speed  where  such  was  necessary  for  other  reasons  than  first  cost. 
However,  in  60  cycles  the  case  is  quite  different,  where  a  four- 
pole  machine  can  have  a  speed  of  1800  revolutions,  synchronous, 
a  six  pole  1200,  an  eight  pole  900  and  a  ten  pole  720  revolutions. 
In  other  words,  there  are  four  suitable  speed  combinations  where 
a  25  cycle  motor  had  only  one.  Moreover,  with  the  advance  in 
design  it  developed  that  these  higher  speed  60-cycle  motors  could 
be  made  with  nearly, as  good  performances  as  with  the  25-cycle 
motors  of  same  capacity,  and  at  somewhat  less  cost.  However, 
leaving  out  the  question  of  cost,  the  wider  choice  of  speeds  alone 
would  be  enough  to  give  the  60-cycle  motor  a  pronounced  prefer- 
ence for  general  service. 

However,  there  is  one  exception  to  the  above.  Where  very 
low-speed  motors  are  required,  such  as  100  rev.  per  rain.,  the  60- 
cycle  induction  motor  is  at  a  considerable  disadvantage  com- 
pared with  25  cycles,  or  this  has  been  the  case  in  the  past.  It 
is  partly  for  this  reason  that  the  steel  mill  industry,  through  its 
electrical  engineers,  adopted  25  cycles  as  standard  some  ten  or 
fifteen  years  ago.  At  that  time,  it  was  considered  that  in  mill 
work,  in  general,  there  would  be  need  for  very  low-speed  motors 
in  very  many  cases.  However,  due  to  first  cost,  as  well  as  other 
things,  there  has  been  a  tendency  toward  much  higher  speeds  in 
steel  mill  work,  through  the  use  of  gears  and  otherwise,  so  that 
part  of  this  argument  has  been  lost.  However,  there  still  remain 
certain  classes  of  work  where  direct  connected  very  low-speed 
induction  motors  are  desirable  and  where  25  cycles  would  ap- 
pear to  have  a  distinct  advantage. 

In  view  of  the  above  considerations,  steel  mill  work  has  hereto- 
fore gone  very  largely  toward  25  cycles,  particularly  where  the 
mills  installed  their  own  power  plants.  However,  in  recent 
years  there  has  been  a  pronounced  tendency  toward  purchase  of 
power,  by  steel  mills,  from  central  stations,  and  the  previously 
described  tendency  of  central  stations  toward  60  cycles  has 
forced  the  situation  somewhat  in  the  steel  mills,  particularly  in 
those  cases  where  the  central  power  supply  company  can  furnish 


STORY  OF  THE  FREQUENCIES  583 

power  at  more  reasonable  rates  than  the  steel  mill  can  produce  in 
its  own  plant.  This,  therefore,  has  meant  a  tendency  toward  60 
cycles  in  steel  mill  work,  even  with  the  handicap  of  inferior  low- 
speed  induction  motors.  But,  on  the  other  hand,  remedies  have 
been  brought  forward  even  for  this  condition.  The  great  diffi- 
culty in  the  construction  of  low-speed,  60-cycle  induction  motors 
is  in  the  very  large  size  and  cost  if  constructed  for  normal  power 
factors,  or  the  very  low  power  factor  and  poor  performance  if 
constructed  of  dimensions  and  costs  comparable  with  25  cycles. 
In  the  latter  case  the  extra  cost  is  not  entirely  eliminated  because 
a  low  power  factor  of  the  primary  input  implies  additional  gener- 
ating capacity,  or  some  means  for  correcting  power  factor  on  the 
primary  system.  However,  in  some  cases  it  is  entirely  practi- 
cable to  correct  the  power  factor  in  the  motors  themselves  by  the 
use  of  so  called  "phase  advancers"  of  either  the  Leblanc  or  the 
Kapp  type.  Such  phase  advancers  are  machines  connected  in 
the  secondary  circuits  of  induction  motors  and  so  arranged  as  to 
furnish  the  necessary  magnetizing  current  to  the  rotor  or  second- 
ary instead  of  to  the  primary  In  this  way  the  primary  current 
to  the  motor  will  represent  largely  energy  and  the  power  factors 
can  be  made  equal  to,  or  even  much  better  than  in,  the  corre- 
sponding 25-cycle  motor;  or,  in  some  cases,  the  conditions  may 
be  carried  even  further  so  that  the  motor  is  purposely  designed 
with  a  relatively  poor  power  factor,  in  order  to  further  reduce  the 
size  and  cost,  and  the  phase  advancers  are  made  correspondingly 
larger.  In  those  cases  where  the  cost  of  the  phase  advancer  is 
relatively  small  compared  with  the  main  motor,  there  may  be  a 
considerable  saving  in  the  cost  of  the  main  motor  and  then  add- 
ing part  of  the  saving  to  the  cost  of  the  phase  advancer. 

One  difficulty  in  the  use  of  phase  advancers  is  found  in  the 
variable  speeds  required  in  some  kinds  of  mill  work.  In  those 
cases  where  flywheels  driven  by  the  main  motors  are  desirable 
to  take  up  violent  fluctuations  in  load,  it  is  necessary  to  have 
considerable  variations  in  the  speed  of  the  induction  motor,  in 
order  to  bring  the  stored  energy  of  the  flywheel  into  play. 
T/nfortunately  this  variable  speed  in  the  induction  motor  is  one 
of  the  most  difficult  conditions  to  take  care  of  with  a  phase 
advancer,  so  that  here  is  a  condition  where  the  60-cycle  motor 
is  at  a  decided  disadvantage. 

Thus  it  may  be  seen  from  the  above  that  ev^n  in  the  steel 
mill  field,  where  the  induction  motor  lias  the  most  extreme  appli- 
cations, there  is  quite  a  strong  pendency  towax<J  60  cycles,  due 
to  the  purchase  of  power  from  central  sttpply  systems. 


584  ELECTRICAL  ENGINEERING  PAPERS 

There  remains  one  more  important  element  which  has  had 
something  to  do  with  the  tendency  toward  60  cycles,  namely, 
the  transmission  problem.     In  the  earlier  days  of  transmission 
of  alternating  current,  25  cycles  was  considered  very  superior 
to  60  cycles  due  to  the  better  inherent  voltage  regulation  con- 
ditions.   At  one  time,  it  was  thought  that  60  cycles  had  a  -very 
limited  field  for  transmission  work.     However,  a  number  of 
power  companies  in  the  far  west  had  installed  60-cycle  plants, 
principally  for  local  service  and  with  the  growth  of  these  plants 
came  the  necessity  for  increased  distance  of  transmission  through 
^  development  of  water  powers.     At  first  it  was  thought  they  were 
badly  handicapped  by  the  frequency,  but  gradually  the  apparent 
disadvantages  of  their  systems  were  overcome  and  the  distances 
of  transmission  were  extended  until  it  became  apparent  that 
they  could  accomplish  practically  the  samfe  results  as  with  25 
cycles.     Part  of  this  result  has  been  obtained  by  the  use  of 
regulating  synchronous  condensers.     It  is  a  curious  fact  that 
the  possibility  of  synchronous  motors  used  as  condensers  for 
correction  of  disturbances  on  transmission  systems,  has  been 
known  for  about  25  years,  but  it  is  only  within  quite  recent  years- 
that  they  have  come  into  general  use  as  a  solution  of  the  trans- 
mission problem,  and  largely  in  connection  with  60-cycle  plants. 
In  1893  the  writer  applied  for  a  patent  on  the  use  of  synchronous 
motors  as  condensers  for  controlling  the  voltage  at  a^iy  point 
on  a  transmission  system  by  means  of  leading  or  lagging  currents 
in  the  condenser  itself.    A  broad  patent  was  obtained,  but  there 
was  no  particular  use  made  of  it  until  it  had  practically  expired. 
Another  improvement  came  along  which  still  further  helped 
to  advance  60  cycles  to  its  present  position,  namely,  the  use  of 
commutating  poles  in  synchronous  converters.     The  principal 
value  of  commutating  poles  in  the  60-cycle  converters,  has  not 
been  so  much  in  an  improvement  in  commutation  over  the  older 
types  of  machines,  as  in  allowing  a  very  considerable  reduction 
in  the  number  of  poles  with  corresponding  increase  in  speed, 
resulting  in  reduction  in  dimensions.    As  a  direct  result  of  this 
increase  in  speed  the  efficiencies  of  the  converters  have  be&n 
increased.     If,  for  instance,  the  speed  of  a  given  60-cycle  con- 
verter can  be  doubled  by  cutting  its  number  of  poles  to  one-half, 
while  keeping  the  same  pole  pitch  and  the  same  limiting   per- 
ipheral speed,  then  obviously  the  amount  of  iron  in  the  armature 
core  is  practically  halved  and,  at  the  same  magnetic  densities 
the  iron  loss  is  also  practically  halved.    Also  with  the  same 


STORY  OF  THE  FREQUENCIES  585 

peripheral  speed  and'  half  diameter  of  armature  the  windage 
losses  can  be  decreased  materially.  Thus  the  two  principal 
losses  in  the  older  converters  have  been  very  much  reduced. 
There  have  also  been  reductions  in  the  total  watts  tor  field 
excitation,  and  in  other  parts,  so  that,  as  a  whole,  the  efficiency 
for  a  given  capacity  60-cycle  converter  has  been  brought  up  quite 
close  to  that  of  the  corresponding  25-cycle  machine,  even  when 
the  latter  is  equipped  with  commutating  poles.  This  gain  of 
the  higher  frequency  compared  with  the  lower  is  due  to  the 
fact  that  the  lower-frequency  machine  was  much  more  handi- 
capped in  its  possibilities  of  speed  increase,  and  furthermore, 
the  iron  losses  and  windage  represented  a  much  smaller  propor- 
tion of  the  total  losses  in  the  low-frequency  machine.  This 
improvement  in  the  efficiency  of  the  60-cycle  converter  together 
with  the  lower  losses  in  the  60-cycle  transformer  as  compared 
with  the  25-cycle,  has  brought  the  60-cycle  equipment  almost 
up  to  the  25-cycle,  so  that  the  difference  at  present  is  not  of 
controlling  importance.  This  development  has  given  further 
impetus  toward  the  acceptance  of  60  cycles  as  a  general  system. 

Formerly  a  serious  competitor  with  the  60  cycle  converter 
was  the  60-cycle  motor-generator.  This  was  installed  in  many 
cases  because  it  was  considered  more  reliable  and  more  flexible 
in  operation  than  the  synchronous  converter.  Both  of  these 
claims  were  true  to  a  certain  extent.  However,  with  improve-* 
ments  in  the  synchronous  coverter  the  difference  in  reliability 
practically  disappeared,  but  there  remained  the  difference  in 
flexibility.  In  the  motor-generator  set,  the  d-c.  voltage  could 
be  varied  over  quite  a  wide  range,  while  in  the  older  60-cycle 
rotaries  the  d-c.  voltage  held  a  rigid  relation  td  the  alternating 
supply  voltage.  However,  with  the  development  and  perfection 
of  the  synchronous  booster  type  of  converter,  flexibility  in 
voltage  was  obtained  with  relatively  small  increase  in  cost  and 
minor  loss  in  economy.  This  has  been  the  last  big  step  in  putting 
the  60-cycle  converter  at  the  front  as  a  conversion  apparatus, 
so  that  today  it  stands  as  the  cheapest  and  most  economical 
method  of  converting  alternating  current  to  direct  current, 
Moreover,  while  the  25-cycle  synchronous  converter  has  appar- 
ently reached  about  its  upper  limit  in  speed,  there  are  still 
possibilities  left  for  the  60-cycle  converter. 

In  line  with  the  above  it  is  of  interest  to  note  that  for  units  of 
1000  kw,  and  less,  the  60-cycle  converter  has  nearly  driven  the 
25-cycle  out  of  business  from  the  manufacturing  standpoint. 


586  ELECTRICAL  ENGINEERING  PAPERS 

For  the  very  large  size  converters,  25  cycles  still  has  the  call, 
but  largely  in  connection  with  many  of  the  railway  and  three- 
wire  systems,  which  have  been  installed  for  many  years;  that  is, 
the  growth  of  this  business  is  in  connection  with  existing  genera- 
ting systems.  However,  the  60-cycle  converter,  in  large  capacity 
units,  is  gaining  ground  rapidly  and  it  is  of  interest  to  note  that 
the  largest  converters  yet  built,  namely,  5800  kw.,  are  of  the 
60-cycle  type. 

One  most  interesting  point  may  be  brought  out  in  connection 
with  the  above  described  "battle  of  the  frequencies",  namely, 
it  was  fought  out  in  the  operating  field,  and  between  conditions 
of  service,  and  not  between  the  manufacturing  companies. 
This  is  a  very  good  example  of  how  such  matters  should  be 
handled.  Here  the  engineers  of  the  manufacturing  companies 
were  expending  their  efforts  to  get  all  possible  out  of  both 
frequencies,  and  consequently  development  proceeded  apace. 
When  60-cycle  frequency  seemed  to  be  overshadowed  by  its 
25-cycle  competitor,  the  engineers  took  a  lesson  from  the  latter 
and  proceeded  to  overcome  the  shortcomings  of  the  former. 
It  was  no  innate  preference  of  the  designing  engineers  that  has 
brought  the  higher  frequency  to  the  fore ;  it  was  the  recognition 
that  it  had  greater  merits  as  a  general  system,  if  its  weak  points 
could  be  sufficiently  strengthened;  and,  therefore,  the  engineers 
turned  their  best  efforts  toward  accomplishing  this  result. 

It  must  not  be  assumed,  for  a  moment  even,  that  because  60 
cycles  appears  to  be  the  future  frequency  in  this  country,  that 
25  cycles  was  a  mistake.  Decidedly  it  was  not.  In  reality  it 
formed  a  most  important  step  toward  the  present  high  develop- 
ment of  the  electric  industry  Many  things  we  are  now  ac- 
complishing with  60  cycles  would  possibly  never  have  been 
brought  to  present  perfection,  if  the  success  of  the  corresponding 
25-cycle  apparatus  had  not  pointed  the  way.  The  success  of 
the  25-cycle  converter,  and  the  high  standard  of  operation  at- 
tained, gave  ground  for  belief  that  practically  equal  results  were 
obtainable  with  60  cycles.  Therefore,  the  25-cycle  frequency 
served  a  vast  purpose  in  electrical  development;  it  was  a  high 
class  pacemaker,  and  it  isn't  entirely  out-distanced  yet. 

There  has  been  considerable  speculation  as  to  what  two  stand- 
ard frequencies  would  have  met  the  needs  of  the  service  in  the 
best  manner,  and  would  have  resulted  in  the  greatest  develop- 
ment in  the  end.  It  has  been  claimed  by  some,  that  50  and  25 
cycles  would  have  been  better  than  60  and  25.  In  the  earlier 


STORY  OF  THE  FREQUENCIES  587 

days  possibly  the  former  would  have  been  better,  but  as  a  result 
both  standards  might  ha\te  persisted  longer.  In  any  case,  the 
general  advantages  would  have  been  small.  In  one  class  of  ma- 
chines, namely,  frequency  changers,  consisting  of  two  alternators 
coupled  together,  the  25-50  combination  would  certainly  have 
been  advantageous. 

Again  it  has  been  questioned  whether  30  and  60  cycles  would 
not  have  been  a  better  choice.  This  was  the  original  Westing- 
house  choice  of  frequencies,  but  not  on  account  of  frequency 
changers.  As  stated  before,  it  was  felt  that  30  cycles  could  do 
about  all  that  25  cycles  could,  and  would  give  an  advantage  of 
25  per  cent  higher  speed  in  motors  and  converters,  with  corre- 
spondingly higher  capacities.  Also  for  direct  coupled  alterna- 
tors, the  two-to-one  ratio  of  frequencies  would  fit  in  nicely  with 
engine  speeds,  in  most  cases.  Possibly,  from  the  present  view- 
point, the  choice  of  thirty  cycles,  would  have  longer  retained  the 
double  standard. 

Something  further  may  be  said  regarding  the  40-cycle  system* 
brought  out  by  the  General  Electric  Company.  This  contained 
many  very  good  features,  for  the  time  it  was  brought  out.  It 
was  then  believed  that  if  the  60  cycle  frequency  was  retained, 
the  double  standard  was  necessary.  The  40-cycle  system  was 
an  attempt  to  eliminate  this  double  standard.  It  apparently 
furnished  a  better  solution  than  60  cycles  then  promised  for  the 
synchronous  converter  problem,  and  was  a  fair  compromise  in 
about  everything  else.  But  it  came  too  late,  for  the  25-cycle 
system  was  too  firmly  entrenched,  and  for  further  development, 
the  designing  engineers  preferred  to  expend  their  energies  in 
seeing  what  could  be  accomplished  with  60  cycles,  as  this  seemed 
to  present  greater  possibilities  than  either  25  or  40,  if  it  could  be 
sufficiently  perfected.  Thus  the  40-cycle  system  probably 
missed  success  due  to  being  just  a  little  too  late. 

As  to  50  cycles,  it  was  stated  that  this  is  still  in  use  to  a  limited 
extent.  Most  of  the  50-cycle  plants  in  this  country  are  in  Cali- 
fornia. Such  plants  were  started  during  the  nebulous  period  of 
the  frequencies,  and  have  persisted,  to  a  certain  extent,  partly 
because  certain  60-cycle  apparatus  could  be  easily  modified  to 
meet  the  50-cycle  requirements.  Also,  as  50  cycles  is  the 
standard  in  many  foreign  countries  to  which  this  country  exports 
equipment,  the  use  of  50  cycles  in  some  home  plants  has  not  been 
unduly  burdensome  from  the  manufacturers'  standpoint. 

In  addition  to  the  preceding,  there  have  been  certain  classes 


588  ELECTRICAL  ENGINEERING  PAPERS 

of  electric  service  which  have  depended  upon  frequency,  but 
which  have  not  been  a  determining  factor  in  fixing  any  par* 
ticular  frequency,  Among  these  may  be  considered  commutat- 
ing  types  of  a-c.  apparatus.  The  first  a-c.  commutating  mo- 
tors of  any  importance,  which  appeared,  were,  of  course,  the  25- 
cycle,  single-phase  railway  motors.  These  as  a  rule  have 
operated  from  their  own  generating  plants,  or  from  other  plants 
through  frequency-converting  machinery.  One  exception  in  the 
railway  work  may  be  noted  in  the  use  of  15  cycles  on  the  Visalia 
plant  in  California,  There  is  a  pretty  well  defined  opinion  among 
certain  engineers  experienced  in  such  apparatus  that  some  low 
frequency,  such  as  15  cycles,  would  present  very  considerable 
advantages  in  the  use  of  single-phase  railway  motors  in  very 
heavy  service,  such  as  on  some  of  the  western  mountain  roads. 
Here  the  problem  is  to  get  the  largest  possible  motor  capacity 
.on  a  given  locomotive,  and  the  main  advantage  of  the  lower  fre- 
quency would  be  in  allowing  a  very  materially  higher  capacity 
within  a  given  space.  This  does  not  imply  reduced  weight  or 
cost  compared  with  the  25  cycles,  but  simply  means  greater  motor 
capacity.  With  the  modern,  more  highly  developed,  single- 
phase  types  of  railway  motors,  it  would  appear  that  there  may  be 
very  considerable  possibilities  in  15  cycles. 

Outside  of  the  railway  field,  there  has  been  more  recently  a 
development  of  various  types  of  a-c.  commutating  apparatus, 
principally  in  connection  with  heavy  steel  mill  electrification 
work.  Such  apparatus  has  been  largely  in  the  form  of  three 
phase  commutating  machines  and  these  have  been  used  prin- 
cipally in  connection  with  speed  coritrol  of  large  induction  mo- 
tors. As  these  regulating  machines  are  usually  connected  in  the 
secondary  circuits  of  induction  motors,  the  frequency  supplied  is 
represented  by  the  slip  frequency.  Consequently  where  the  slip 
frequency  never  rises  to  a  large  percentage  of  that  of  the  primary 
system,  such  commutating  motors  ate  applicable  without  undue 
difficulties.  Such  motors,  presumably  are  better  adapted  for 
25-cycle  mill  equipments  than  for  60-cyde,  but  due  to  the  ten- 
dency, already  described,  for  steel  mills  to  go  to  60  cycles  on  pur- 
chased power,  it  has  been  necessary  to  build  these  three-phase 
commutating  motors  for  the  regulation  of  60-cycle  main  motors, 
in  many  cases. 

There  is  still  another  class  of  service,  which  has  come  in  re- 
cently, where  the  choice  of  frequency  is  of  much  importance, 
but  where  there  is  no  great  necessity  for  adhering  to  any  standard. 


STORY  OF  THE  FREQUENCIES  589 

namely,  in  heavy  ship  propulsion  by  electric  motors.  As  each 
ship  equipment  is  a  complete  system  in  itself,  and  as  it  cannot  tie 
up  with  other  systems,  there  is  not  any  controlling  need  for  main- 
taining any  definite  frequency  or  voltage.  Except  in  similar 
vessels,  there  is  little  chance  for  duplication  in  parts,  as  the 
various  equipments  vary  so  much  in  size  and  capacity.  In  conse- 
quence it  has  been  found  advisable,  at  least  up  to  the  present 
time,  to  design  each  propulsion  equipment  for  that  frequency 
which  best  suits  the  generator  and  motor  speeds,  taking  into  ac- 
count the  various  operating  conditions  and  limitations,  such  as 
the  different  running  speeds,  steaming  radius,  etc.  In  conse- 
quence, different  maufacturers  bidding  on  such  equipments 
may  specify  different  frequencies,  depending  upon  the  construc- 
tional features  of  their  particular  types  of  apparatus.  At  the 
present  time  with  the  relatively  small  amount  of  experience  ob- 
tained with  the  electrical  propulsion  of  ships,  it  looks  as  if  it 
would  be  a  considerable  handicap  to  attempt  to  adopt  some 
standard  frequency  for  all  service.  Later,  with  wide  experience, 
it  may  be  possible  to  adopt  some  compromise  frequency,  which 
will  not  unduly  handicap  any  of  the  service. 

CONCLUSION 

It  has  been  the  writer's  intention  to  show  that,  as  a  rule,  the 
choice  of  frequency  has  been  a  matter  of  most  serious  considera- 
tion, based  upon  service  conditions  at  the  time.  Moreover,  in 
view  of  the  wide  range  of  conditions  encountered,  it  is  surprising 
how  few  frequencies  have  been  seriously  considered  in  this  coun- 
try. Occasion  has  arisen,  times  without  number,  where  an 
obvious  solution  of  a  given  problem  would  lie  in  modification 
of  the  frequency  to  allow  the  use  of  apparatus  and  equipment 
already  designed,  but  the  engineers  of  the  manufacturing  or- 
gahization  have  steadily  held  out  against  such  policy,  regardless- 
of  the  apparent  need  of  the  moment.  The  swing  of  the  pendulum 
from  60  cycles  to  25  cycles  and  back,  has  covered  a  period  jof 
many  years  and,  therefore,  cannot  be  considered  as  a  fad  of  the 
moment,  but  is  the  result  of  well  defined  tendencies,  backed  by 
the  best  engineering  experience  available.  As  a  rule  no  manu- 
facturer has  made  any  particular  frequency  his  "pet,"  but  all 
have  worked  to  develop  each  system  to  its  utmost. 


THE  DEVELOPMENT  OF  THE  ALTERNATING- 
CURRENT  GENERATOR  IN  AMERICA 

FOREWORD — The  following  article,  which  first  appeared  in  the 
Electric  Journal,  contains  a  fairly  complete  brief  history  of  the 
evolution  of  the  alternating-current  generator  in  so  far  as  the 
Company,  with  which  the  writer  is  connected,  is  concerned. 
Reference  is  made  incidentally  to  the  work  of  other  manufactur- 
ing companies,  but  this  is  not  very  complete,  as  the  writer  did 
not  have  the  necessary  material  available  for  describing  such 
developments,  except  m  a  very  general  way. — (ED.) 


IN  the  early  days  of  the  alternating-current  generator,  it  was 
constructed  in  almost  as  many  types  as  there  were  designers. 
The  principal  endeavor  of  each  designer  appeared  to  be  toward  the 
development  of  a  new  alternator  which  would  bear  his  name.  A 
few  of  these  early  types  were  of  the  rotating  field  construction, 
while  a  much  greater  number  were  of  the  rotating  armature  type. 
Some  had  iron  core  armatures,  while  others  had  coreless  armatures, 
and  there  were  many  discussions  as  to  whether  the  core  or  the 
coreless  type  was  superior  and  would  survive.  Many  of  the  early 
predictions  would  now  form  quite  interesting  reading,  in  view  of 
the  fact  that  present  practice  is  so  far  removed  from  the  early 
anticipations.  Here  and  there  among  the  early  machines  was  one 
which  contained  some  of  the  important  elements  of  recent  ap- 
paratus, but  in  many  cases  such  machines  disappeared  in  the 
general  course  of  development,  the  meritorious  features  being 
insufficient  to  save  the  type. 

SURFACE  WOUND  ARMATURES 

In  America,  the  principal  early  type  of  alternator  had  a 
rotating  armature  with  surface  windings  and  an  external  cast 
iron  multipolar  field.  This  type  was  used  very  considerably  or, 
in  fact,  almost  exclusively,  from  1886  to  1890.  This  was  the  type 
built  by  the  Westinghouse  and  the  Thomson-Houston  Companies. 
There  were  only  minor  differences  in  the  construction  of  the  ma- 
chines built  by  these  two  companies  which,  however,  at  that  time, 
appeared  to  be  very  great.  These  differences  consisted  principally 
in  the  way  the  end  windings  of  the  armature  coils  were  supported, 
in  the  construction  of  the  end  bells  and  ventilating  openings  in  the 
armature  core,  in  the  method  of  attaching  th  e  armature  core  to  the 

591 


592  ELECTRICAL  ENGINEERING  PAPERS 

shaft,  in  the  winding  of  the  field  coils  in  metal  bobbins,  etc.  Both 
machines  had  surface  -windings  with  concentric  coils,  one  layer 
deep  in  the  radial  direction.  In  the  Westinghouse  construction, 
the  end  windings  were  turned  down  toward  the  shaft  and  were 
supported  by  radial  wooden  clamps,  as  indicated  in  Fig.  1.  In  the 
Thomson-Houston  armature,  the  end  windings  were  arranged  in 
an  axial  instead  of  a  radial  direction,  and  were  supported  by  bands 
or  external  clamps.  This  construction  is  also  indicated  in  Fig.  1. 
The  Slattery  machine,  which  was  also  on  the  market  at  that  time, 
was  of  the  same  general  type  as  the  above  machines.  Presumably 
these  two  different  methods  of  end  winding  were  used  on  account 
of  the  patent  situation.  At  that  time  there  was  much  discussion 
of  the  respective  merits  of  the  two  constructions. 

These  early  machines  were  built  principally  for  frequencies  of 
15,000  and  16,000  alternations  per  minute  (125  and  133  cycles  per 
second).  In  those  days,  everything  was  rated  in  alternations  per 
minute,  as  this  represented  the  product  of  the  number  of  poles  by 
the  number  of  revolutions.  Such  high  frequencies  were  selected, 
mainly,  on  account  of  transformer  conditions,  and  not  alternator 
design.  Practically  all  alternating  service  consisted  of  house  to 
house  lighting,  and  in  relatively  small  units,  and  the  higher  fre- 
quency was  supposed  to  be  of  great  advantage  in  transformer 
design  and  operation,  which  presumably  was  the  case  with  the  very 
small  amount  of  data  and  experience  available  at  that  time. 

About  the  only  commercial  voltage  for  alternating  work  at 
that  time  was  1000  or  1100  volts.  This  was  supposed  to  be  ex- 
cessively high  and  dangerous,  and  there  was  much  question 
whether  such  an  excessive  voltage  should  be  permitted.  This- 
matter  was  actually  taken  before  a  number  of  the  state  legislatures 
for  the  purpose  of  obtaining  laws  prohibiting  or  limiting  the  use  of 
such  voltage.  Another  reason  why  no  higher  voltage  was  used 
was  in  the  construction  of  the  alternators  and  transformers.  With 
the  experience  and  materials  available  at  that  time,  together  with 
the  high  speed  rotating  armature  construction  ajid  the  surface 
windings,  even  1100  volts  was  a  very  serious  problem  in  the  gen- 
erator. About  1889  or  1890,  there  appeared  some  slight  demand 
for  higher  voltages,  and  a  few  2000  or  2200  volt  surface-wound 
alternators,  of  the  then  standard  type,  were  built.  However,  even 
then  it  was  recognized  that  the  surface-wound  type  of  alternator 
was  not  well  adapted  for  higher  voltages,  and  there  was  much, 
question  whether  a  different  type  winding  should  not  be  developed 


DEVELOPMENT  OF  THE  A  C.  GENERATOR 


593 


for  1100  volts.  This  gradually  led  to  the  next  big  step,  namely, 
the*development  of  the  "  toothed7'  type  of  alternator  with  one  big 
tooth  per  pole,  in  distinction  from  the  slotted  type  of  armature 
with  a  number  of  slots  per  pole,  which  was  a  considerably  later 
development. 

TOOTHED  ARMATURES 

The  first  commercial  toothed  type  of  armature  appears  to  have 
been  gotten  out  by  the  Westinghouse  Company.    These  first  ma- 


PIG.  1— SURFACE  WOUND  ARMATURE  WITH  RADIAL  CLAMPS  (UPPER) 
AND  WITH  AXIAL  CLAMPS  (LOWER) 

chines  were  radically  different,  in  details  of  construction,  from  the 
later  toothed  armature  types  of  machines  which  came  into  general 
use.  The  first  toothed  armatures  were  small  air-gap  machines. 
In  the  surface-wound  armatures,  the  clearance  between  the  arma- 
ture surface  and  the  field  poles  was  comparatively  small,  although 
the  total  air-gap  (iron  to  iron)  was  large  on  account  of  the  surface 
winding.  In  constructing  the  new  toothed  armature,  the  actual 
clearance  (iron  to  iron)  between  armature  and  field  was  kept  about 
the  same  as  in  the  surface-wound  alternators  (bands  to  iron),  but 
this  clearance  actually  represented  the  total  air-gap  in  the  toothed 
type.  Moreover,  in  sinking  the  windings  below  the  surface,  it  was 
endeavored  to  maintain  a  practically  uniform  outside  surface,  so 
that  overhanging  tooth  tips  were  used  with  relatively  narrow  slots 
for  putting  in  the  windings.  The  general  construction  was  similar 
to  Fig.  2.  On  account  of  the  sma.11  clearance,  and  consequent 
higher  magnetic  additions,  it  was  fottpd  necessary  to  use  lamin- 


594  ELECTRICAL  ENGINEERING  PAPERS 

ated  poles  with  these  machines  in  order  to  avoid  excessive  field 
heating. 

The  self-induction  of  the  armature  windings  on  these  machines 
was  very  high  compared  with  the  old  surface-wound  armatures  and 
therefore,  in  order  to  obtain  passably  good  regulation,  fewer 
armature  turns  had  to  be  used,  with  correspondingly  higher  induc- 
tions, and  this  made  the  use  of  solid  poles  impracticable  on  account 
of  heating.  Furthermore,  on  account  of  the  overhanging  tooth 


FIG  2. 

tips,  the  small  air-gap  and  the  high  induction  per  pole,  this  early 
type  of  toothed  armature  was  very  noisy.  In  one  instance,  it  was 
credibly  reported  that  one  of  these  machines  could  be  heard  two 
miles  away  on  a  quiet  night.  However,  several  machines  of  this 
construction  were  put  out  by  the  Westinghouse  Company,  and 
operated  for  many  years. 

Meanwhile,  the  possibilities  of  the  toothed  armature  con- 
struction in  the  old  cast  iron  field  were  being  given  consideration, 
The  writer  made  a  special  study  of  this  matter,  and  finally  decided 
that,  in  order  to  make  this  construction  possible  with  solid  cast 
iron  poles,  it  would  be  necessary  to  work  at  relatively  low  induc- 
tions per  pole,  and  with  a  very  large  air-gap,  (fully  as  large  as  on 
the  old  surface-wound  machines)  and  with  a  shape  of  tooth  tip 
which  did  not  have  such  great  width  compared  with  the  pole  tip  as 
shown  in  Fig.  2.  This  meant  that  a  pole  tip  and  air-gap  as  shown 
in  Fig.  3,  should  be  used.  "  With  this  arrangement,  the  armature 


DEVELOPMENT  OF  THE  A.C.  GENERATOR  595 

self-induction  would  still  be  relatively  high,  and  the  regulation 
correspondingly  bad,  necessitating  some  form  of  compounding  for 
regulating  the  voltage,  similar  to  the  compounding  of  a  direct- 
current  generator.  This  armature  construction  was  worked  out 
in  detail  for  a  37.5  kilowatt  field  (that  is,  for  the  standard  field  of 
the  37.5  kw  surf  ace-wound  type  of  machine) .  The  armature  teeth 


PIG.  3 

were  similar  to  those  in  Fig.  3,  in  shape,  and  the  air-gap  or  clear- 
ance from  iron  to  iron,  was  made  %  inch  on  each  side  of  the 
machine.  The  field  was  also  compounded.  When  this  machine 
was  put  on  test,  it  was  found  at  once  that  it  could  be  loaded  to  60 
kilowatts  without  undue  heating  of  the  armature  and  field  iron, 
and  the  problem  of  perfecting  this  machine  then  became  one 
merely  of  increasing  the  amount  of  armature  copper  to  carry  the 
current  at  the  60  kilowatt  rating.  This,  therefore,  represented  a 
big  step  in  the  development  of  the  American  type  alternator.  It 
was  found  that  all  the  other  Westinghouse  standard  cast  iron 
machines  of  the  rotating  armature  type  could  readily  be  changed 
in  line  with  the  above  improvement. 

COMPOUNDING  ALTERNATORS 

The  compounding  of  the  60  kilowatt  machine  was  not  a  new 
feature,  for  already  some  of  the  laminated  field  toothed-armature 
type  of  machines  had  been  compounded,  in  order  to  improve  their 
regulation.  Two  different  methods  of  compounding  alternators 
had  been  developed  by  the  Westinghouse  ajsd  Thomson-Houston 
Companies,  respectively.  In  the  Wesftiaghottse  armature  constrtic- 


596 


ELECTRICAL  ENGINEERING  PAPERS 


tion,  the  armature  discs  were  punched  in  single  pieces,  with  spokes, 
and  were  threaded  directly  on  the  armature  shaft,  no  spider  being 
used.  This  construction  is  illustrated  in  Fig.  4.  In  the  assembled 
armature,  the  spokes  were  therefore  of  laminated  material.  These 
laminated  spokes  were  utilized  as  the  core  of  a  compounding  trans- 
former. One  lead  from  the  armature  winding  was  carried  around 
the  spokes  of  the  armature  before  passing  to  the  collector  ring. 
This  winding  formed  the  primary  of  a  series  transformer.  The 
secondary  was  also  wound  on  the  spokes,  and  the  two  ends  were 
carried  to  the  bars  of  a  rectifying  commutator  on  the  shaft.  The 
number  of  commutator  bars  was  equal  to  the  number  of  poles. 
The  alternating  current  from  the  secondary  winding  was  by  this 
means  changed  to  a  pulsating  direct  current. 

In  the  Thomson-Houston  method  of  compounding,  the  main 
armature  current  was  carried  directly  to  a  rectifying  commutator, 
and,  after  being  commutated,  was  passed  to  the  field-compound 
winding,  and  back  to  the  commutator,  and  then  to  a  collector  ring. 


FIG.  4-SKETCH  OP  COMPENSATED  TYPE  OP  WINDING 

The  main  armature  current  therefore  passed  directly  through  the 
field,  while  in  the  Westinghouse  method  the  secondary  current 
from  a  series  transformer  was  passed  through  the  field.  Both 
methods  represented  series  compounding,  and  gave  practically 
equal  results,  but  there  was  much  discussion  as  to  the  merits  of  the 
two  methods.  Both  of  these  methods  delivered  pulsating  direct 
current  to  the  field  winding.  There  was  considerable  inductive 
e.  m.  f.  set  up  in  the  field  windings  by  this  pulsation,  and  this 
tended  to  cause  inductive  discharges  across  the  rectifying  com- 


DEVELOPMENT  OF  THE  A  C   GENERATOR 


597 


mutators.  In  the  Thomson-Houston  method  this  trouble  was 
overcome  to  a  considerable  extent  by  the  use  of  a  non-inductive 
shunt  in  parallel  with  the  rectifying  commutator,  i.e.,  across  the 
compound  winding.  In  the  Westinghouse  method  a  similar  result 
was  accomplished  by  saturating  the  series  transformer  (or  armature 
spokes)  to  such  a  high  point  that  the  inductive  kick  from  the  field 
could  readily  discharge  through  the  secondary  winding  of  the 
transformer  without  giving  high  enough  voltage  to  flash  across  the 
commutator. 


FIG.  5— EARLY  WESTINGHOUSE  60  KILOWATT  ALTERNATOR  WITH 
COMPENSATING  WINDING 

The  details  of  this  method  of  compounding  have  been  gone 
into  rather  fully,  as  this  compounding  was,  at  that  time,  an  im- 
portant step  in  our  progress.  The  fact  that  of  all  these  machines 
were  built  for  single  phase,  allowed  us  to  use  such  compounding. 
With  the  advent  of  polyphase  generators,  such  methods  of  com- 
pounding soon  disappeared,  principally  for  the  reason  that  a  great 
majority  of  the  early  polyphase  machines  handled  separate  single- 
phase  loads  on  the  different  phases,  and  it  was  not  practicable  to 
compound  for  these  independently. 

The  above  toothed  type  generator  came  into  use  about  1890 
and1  lasted  for  several  years,  or  practically  until  polyphase  gener- 
ators actually  came  into  fairly  general  use  before  true  polyphase 
loads  became  common.  These  toothed  type  generators  allowed 


598 


ELECTRICAL  ENGINEERING  PAPERS 


the  use  of  relatively  high  voltages,  as  far  as  the  armature  winding 
was  concerned,  so  that  2200  volts  became  comparatively  common, 
and  even  3300  volts  or  higher  was  used  in  some  cases.  In  fact, 
the  limit  in  such  machines  appeared  to  be  at  the  collector  rings, 
rather  than  in  the  armature  winding. 

Something  may  be  said  regarding  the  type  of  winding  used 
on  the  armatures  of  these  machines.  In  the  Westinghouse  con- 
struction the  armature  coils  were  machine-wound  and  taped  before 
placing  on  the  armature  core.  Each  coil  was  made  wide  enough 
to  slip  over  the  top  of  the  armature  tooth,  as  shown  in  Figs.  6  and 
7.  This  made  the  coil  considerably  wider  than  the  body  of  the 
armature  tooth,  so  that,  after  slipping  over  the  tooth  top  the  coil 
had  to  be  reduced  in  width  by  special  clamping  tools.  Supporting 
wedges  were  then  driven  in  between  adjacent  coils. 

Something  may  be  said  regarding  the  temperatures  of  these 
early  alternators,  both  of  the  surface-wound  and  of  the  toothed 
types.  In  those  days  temperature  measurements  were  very 
crude  compared  with  present  practice,  which  is  admittedly  still 
only  approximate.  In  some  of  the  surface-wound  armatures 
excessively  high  temperatures  must  have  been  encountered  in 
many  instances,  judging  from  the  appearance  of  the  insulation  on 
the  individual  wires,  after  a  year's  service,  for  instance.  However, 


FIG.  6— SKETCH  SHOWING  METHOD  OP  PUTTING  MACHINE  WOUND 
COILS  ON  THE  POLES 

it  was  difficult  to  obtain  reasonably  correct  temperature  of  the 
armature  windings,  for  the  actual  temperature  of  the  conductors 
was  undoubtedly  reduced  very  greatly  before  the  armature  could 
be  brought  to  a  standstill.  Even  after  this,  temperature  rises  of 
50  or  60  degrees  C.  were  not  considered  as  excessively  high.  With- 
out doubt,  some  of  these  early  machines,  at  times,  attained  actual 
internal  temperatures  of  120  to  130  degrees  C.,  or  even  higher, 
with  insulation  on  the  conductors  consisting  of  untreaded  cotton 
fibre.  No  overloads  were  possible,  for  each  size  of  machine  was 
rated  in  as  many  "lights  "  as  it  could  carry  on  a  shop  test  without 


DEVELOPMENT  OF  THE  A.C.  GENERATOR  599 

breaking  down.  The  first  few  minutes,  while  starting  up  a  new 
alternator  in  the  testing  room,  were  always  anxious  ones  for  the 
operators,  especially  so  if  any  "improvements  "  had  been  made  on 
the  armature  winding.  Any  defect  in  winding,  or  wrong  con- 
nection usually  resulted  in  a  stripped  armature  and  much  flying 
copper.  If  nothing  happened  within  the  first  few  minutes  after 
the  machine  was  put  on  load,  the  attendants  all  came  out  from 
behind  posts  and  other  protections  and  went  on  with  their  work. 

When  the  toothed  armature  came  into  use  the  above  condi- 
tions were  much  alleviated.  Defects  in  construction,  or  short- 
circuits,  could  not  strip  such  armatures,  and  thus  the  danger  and 
excitement  were  removed.  However,  it  was  found  that  the  first 
short  run  did  not  tell  the  story  of  excessive  heating  as  promptly 
as  in  the  case  of  the  surface-wound  type.  Experience  showed  that 
the  toothed  construction  apparently  could  stand  a  severe  shop 
test  and  still  go  wrong  under  similar  loading  within  a  short  time 
after  being  installed.  It  was  found  that  a  given  size  of  conductor 
would  not  carry  as  much  current  in  the  concentrated  coils  of  the 
toothed  construction  as  was  the  case  in  surface-wound  coils. 
However,  the  method  of  testing  the  temperature  did  not  show  this, 
as  the  main  part  of  the  toothed  armature  coil  was  so  embedded  and 
so  covered  with  insulation  that  the  thermometer  readings  did  not 
indicate  nearly  the  true  temperatures.  The  size  of  wire  and  the 
amount  of  copper  in  the  coils  then  had  to  be  increased  until  the 
machines  did  stand  up  in  service.  The  true  explanation  of  the 
discrepancies  was  not  well  understood  at  that  time.  In  these 
toothed  alternators,  as  in  the  surface-wound  machines,  the  first 
machines  were  rated  in  "lights,"  but  gradually  the  kilowatt  rating 
came  into  use  and  became  standard  practice. 

INTRODUCTION  OP  POLYPHASE  ALTERNATORS 

In  1892  and  1893,  polyphase  alternators  began  to  be  con- 
sidered seriously.  In  1889  and  1890,  a  few  such  alternators  had 
been  built  for  the  operation  of  Tesla  induction  motors.  These 
early  polyphase  alternators  were  of  the  surface-wound,  rotating 
armature  type*  These  machines  were  very  special  in  construction, 
and,  like  the  Tesla  motors,  did  not  find  much  of  a  market.  How- 
ever, in  1892  and  1893,  it  began  to  be  recognized  that  the  best  way 
to  encourage  the  development  of  the  induction  motor  would  be 
by  creating  a  demand  for  it,  and  it  was  decided  that  a  good  way  to 
create  a  demand  would  be  by  encouraging  the  general  adoption  of 


€00 


ELECTRICAL  ENGINEERING  PAPERS 


polyphase  alternators  and  supply  circuits,  with  the  idea  that,  when 
a,  suitable  supply  circuit  was  available,  there  was  eventually  bound 
to  be  a  demand  for  motors  to  operate  upon  such  circuits.  With 
this  general  policy  in  view,  there  was  great  activity  in  the  develop- 
ment of  polyphase  generators.  ^This  very  quickly  led  to  a  very 
considerable  departure  in  armature  construction  from  the  usual 
toothed  armature  as  used  in  single-phase  machines.  The  poly- 
phase winding,  requiring  two  or  more  coils  per  pole,  naturally 


FIG.  7— VIEW  OP  ARMATURE  IN  PROCESS  OF  PLACING  COILS  AND 
CLAMPING  THEM  INTO  SHAPE  IN  THE  SLOTS 

tended  toward  the  slotted  armature  construction  with  two  or  more 
slots  per  pole.  It  was  soon  recognized  that,  in  general,  the  larger 
the  number  of  slots  per  pole  and  the  smaller  the  number  of  con- 
ductors per  slot,  the  better  would  be  the  general  characteristics  of 
the  machine,  so  that  the  construction  naturally  tended  toward  the 
modern  slotted  type.  Moreover,  practically  all  the  development 
in  polyphase  alternators  was  at  relatively  low  frequency,  com- 
pared with  former  practice.  It  so  happened  that  there  had  been  a 
well-defined  tendency  toward  lower  frequency  in  the  period  from 
1890  to  1892.  This  tendency  was  largely  independent  of  the 
induction  motor  problem  for,  at  the  time  it  became  most  pro- 
nounced, there  was  no  true  induction  motor  problem.  It  was 
becoming  recognized  that  125  to  133  cycles  per  second  was  too  high 
for  certain  classes  of  work  and  for  engine  type  generators,  and 
that,  in  general,  a  very  considerably  lower  frequency  must  event- 
ually be  adopted.  A  great  many  lower  frequencies  were  tried  by 
the  different  manufacturing  companies,  ranging  from  50  to  85 
cycles.  However,  60  cycles  seemed  to  have  the  preference  at  the 
time  polyphase  alternators  began  to  come  in. 

The  early  polyphase  generators  were  mostly  of  the  rotating 
armature  type,  and  usually  with  a  fairly  large  number  of  slots  per 


DEVELOPMENT  OF  THE  A  C.  GENERATOR  601 

pole.  One  notable  exception  was  tlie  "monocycle"  machine 
which  usually  had  only  two  slots  per  pole,  one  large  slot  for  the 
main  armature  winding,  and  one  smaller  slot  for  the  so-called 
"teaser"  winding.  Also,  the  early  two-phase  alternators  of  the 
"inductor"  type,  built  by  the  Stanley-Kelly  Company,  frequently 
had  only  two  slots  per  pole.  However,  it  may  be  said  that,  from 
1893  to  about  1898,  the  great  majority  of  the  American  built 
alternators  were  of  the  rotating  armature  type  with  distributed 
armature  windings.  The  principal  exceptions  were  the  Stanley 
inductor  type  of  machines  and  a  few  special  " rotating  field" 
machines,  as  distinguished  from  the  inductor  type. 

The  rotating  armature  machines  were  usually  of  1100  or  2200 
volts,  although  a  few  of  considerably  higher  voltage  were  con- 
structed. A  few  cases  may  be  cited  where  special  constructions 
were  used.  For  instance,  the  principal  lighting  plant  at  the  Chic- 
ago World's  Pair  in  1893,  consisted  of  a  large  number  of  Westing- 
house  "twin"  type  generators.  Each  unit  had  two  single-phase, 
standard  toothed  type  armatures  side  by  side  on  the  same  shaft. 
The  teeth  of  the  two  armatures  were  staggered  90  electrical 
degrees  with  respect  to  each  other,  so  that  the  two  together  could 
deliver  currents  having  90  degrees  relation  to  each  other.  The 
object  of  this  construction  was  to  obtain  polyphase  current  with 
standard  single-phase  types  of  machines  without  any  radically  new 
development.  This  type  of  unit  did  not  persist  and,  in  fact,  was 
simply  an  expedient  for  this  particular  occasion. 

THE  FIRST  NIAGARA  GENERATORS 

Also,  in  1892  and  1893,  the  first  large  Niagara  electrical  de- 
velopment was  worked  out.  The  advisory  engineers  of  this  plant, 
proposed  5000  horse-power  generators,  having  stationary  internal 
armatures  and  rotating  external  fields  to  obtain  large  flywheel 
capacity.  In  fact,  the  construction  was  not  unlike  the  usual 
rotating  armature  machine  of  that  period,  as  far  as  general  ap- 
pearance of  the  armature  and  field  cores  and  windings  were  con- 
cerned. However,  the  method  of  supporting  and  rotating  the 
heavy  external  field  at  a  speed  which,  at  that  time,  was  con- 
sidered excessively  high,  required  an  "umbrella"  type  of  field 
support,  which  gave  these  machines  a  distinctive  appearance. 
This  type  of  construction  did  not  persist,  although  these  early 
machines  are  still  in  operation. 


602  ELECTRICAL  ENGINEERING  PAPERS 

A  further  distinctive  feature  in  these  first  Niagara  machines 
was  in  the  frequency  employed.  A  speed  of  2SO  revolutions  per 
minute  was  decided  upon.  The  engineers  of  the  power  company 
proposed  eight-pole  machines,  giving  2000  alternations  per  minute 
or  16  2-3  cycles  per  second.  The  Westinghouse  Company  pro- 
posed, as  an  alternative,  16  poles,  giving  33  1-3  cycles,  the  advan- 
tages claimed  for  this  frequency  being  that  it  was  better  suited 
for  motors  and  rotary  converters,  which  were  then  promising  to 
become  of  importance.  One  advantage  claimed  for  the  16  2-3 
cycle  machine  was  that  it  would  permit  the  use  of  commutator 
type  alternating-current  motors.  After  much  discussion,  and 
weighing  and  balancing  of  all  the  various  arguments  for  and  against 
these  two  frequencies,  it  was  finally  decided  to  use  12  poles,  giving 
3000  alternations  per  minute,  or  25  cycle  polyphase  current  and, 
as  fax  as  the  writer  knows,  this  was  the  origin  of 'the  present  25 
cycle  standard. 

Considering  what  a  radical  departure  from  ordinary  construc- 
tion was  made  in  these  first  Niagara  generators,  it  is  self-evident 
that  many  curious  and  interesting  conditions  developed  during  their 
design,  construction  and  tests.  As  far  as  the  writer  knows,  these 
were  the  first  large  alternators  which  were  deliberately  short-cir- 
cuited at  their  terminals  when  running  at  full  speed  and  at  normal 
field  charge.  There  were  no  instruments  available  to  measure  the 
first  current  rush,  but  it  was  obvious  that  this  current  was  far 
greater  than  the  steady  short-circuited  current  of  the  machine 
tinder  similar  field  charge,  for  there  were  ample  evidences  of  a  ter- 
rible shock  at  the  moment  of  short-circuit.  It  was  suspected  at 
that  time  that  the  first  rush  of  current  was  only  limited  by  the 
armature  impedance,  and  not  by  the  so-called  synchronous  im- 
pedance which  fixes  the  value  of  the  steady  short-circuit  current. 

This  also  was  the  earliest  machine  of  which  the  writer  pre- 
determined the  field  form  and  wave  form  by  analysis  of  the  flux 
distribution.  Later,  when  making  shop  tests  on  one  of  these 
machines,  the  e.  m.  f .  wave  form  was  measured  directly  by  rotating 
the  field  at  normal  field  charge  at  such  an  extremely  low  speed 
that  a  voltmeter  connected  across  the  armature  terminals  showed 
such  gradual  variations  in  e.  m.  f .  that  readings  taken  at  regular 
intervals  could  be  plotted  to  form  the  voltage  wave.  Slow 
rotation  was  obtained  by  means  of  a  steel  cable  wrapped  about  the 
outside  of  the  external  field  and  with  one  end  of  the  cable  attached 
to  a  small  diameter  spindle  around  which  it  was  wrapped  at  a  very 


DEVELOPMENT  OF  THE  A  C.  GENERATOR  603 

slow  rate.  This  was  a  very  crude  method,  but  the  wave  form  thus 
obtained  checked  very  accurately  with  tests  made  some  years  later. 

Also,  the  early  Niagara  machines  embodied  one  of  the  first 
distinct  attempts  to  ventilate  alternators  artificially.  Early  belted 
machines  had  had  small  ventilating  bells  on  each  end.  But  these 
Niagara  machines  were  designed  primarily  with  a  view  to  setting 
up  an  abnormal  air  circulation  by  means  of  special  "scoops"  or 
ventilators  on  the  umbrella  supports.  Very  much  thought  and 
discussion  were  given  to  this  subject  of  artificial  ventilation.  The 
results  of  our  tests  led  to  the  arrangement  of  the  scoops  so  that 
they  acted  as  exhaust  pipes. 

Also,  water  cooling  of  the  armature  spider  was  tried  on  some 
of  these  early  machines,  but  proved  ineffective,  due  to  the  fact 
that  the  cooling  medium  was  applied  too  far  away  from  the  point 
of  development  of  the  larger  part  of  the  armature  iron  and  copper 
losses. 

INFLUENCE  OF  DIRECT-CURRENT  DESIGN 

It  must  be  kept  in  mind  that  the  general  trend  of  direct- 
current  development  had  a  certain  influence  on  alternating-current 
generator  work.  For  example,  there  had  been  a  slow,  but  positive 
tendency  in  direct-current  generators,  toward  the  engine  type 
construction.  Also,  from  1890  to  1893,  direct-current  generator 
armature  construction  had  changed  from  the  surface  wound  to  the 
slotted  type.  This  doubtless  had  some  influence  in  changing  al- 
ternator design  toward  the  slotted  type,  especially  when  the  poly- 
phase type  of  windings  came  into  use.  Also,  there  was  a  pro- 
nounced tendency  toward  the  engine  type,  slow  speed  alternator, 
accompanying  direct-current  practice.  Practically  all  of  these 
early  engine  type  alternators,  except  the  inductor  type,  had 
rotating  armatures.  Meanwhile,  an  interesting  development  took 
place  in  the  armature  construction  of  some  of  these  machines. 
In  most  of  the  smaller  belted  machines,  open  armature  slots  were 
used  with  machine-wound  armature  coils.  However,  many  of  the 
early  larger  machines,  especially  of  the  engine  type,  were  built  for 
relatively  low  voltage,  such  as  440  volts,  two  or  three  phase.  This 
admitted  in  many  cases  of  simple  bar  windings  with  one  or  two 
conductors  per  slot.  This  allowed  partially  closed  armature  slots 
with  shoved-through  straight  conductors,  and  bolted-on  end 
windings,  giving  a  very  strong  substantial  type  of  winding  for  re- 
sisting the  rotational  Stresses.  The  partially  closed  slot  became  a 


604  ELECTRICAL  ENGINEERING  PAPERS 

sort  of  standard  in  Westinghouse  machines,  and  endured  for  a 
number  of  years,  and  was  even  carried  into  the  stationary  armature 
type  of  machine  when  rotating  fields  came  into  general  use.  This 
partially  closed  slot  arrangement  was  a  very  good  one  as  long  as 
the  generator  voltages  were  relatively  low.  The  same  may  be 
said  of  the  rotating  type  of  armature  as  a  whole.  However,  when 
high  voltages  came  into  more  general  use,  a  different  construction 
was  preferable. 

In  reviewing  the  period  of  the  rotating  armature,  slotted  types 
of  machines,  the  monocyclic  system  should  be  briefly  described. 
Apparently  this  was  gotten  out  with  the  idea  that  it  avoided  the 
patented  features  of  the  Tesla  polyphase  system,  The  armature 
circuits  on  this  monocyclic  system  were  so  arranged  that,  when 
dairying  load,  one  phase  carried  nearly  all  of  the  energy  load, 
while  both  phases  supplied  magnetizing  current  for  the  operation 
of  induction  motors.  During  the  period  when  this  machine  was  in 
vogue,  single-phase  lighting  work  represented  the  principal 
service,  while  induction  motor  loads  were  relatively  small.  With 
increased  use  of  polyphase  loads,  and  with  the  elimination  of  the 
patent  situation,  the  monocyclic  system  gradually  dropped  out. 

It  was  early  recognized  that  a  stationary  armature  winding 
would  be  an  ideal  one  in  some  respects,  but  it  was  thought  that  any 
rotating  field  construction  was  bound  to  be  a  difficult  and  expen- 
sive one.  The  inductor  type  construction  was  supposed  by  some 
engineers  to  overcome  the  objections  to  the  rotating  field,  but 
many  others  considered  that  this  type  was  not  a  final  one,  as  it  did 
not  use  the  magnetic  material  in  the  machine  to  the  best  advan- 
tage. In  the  earlier  alternators,  with  insufficient  ventilation 
through  the  armature  core,  relatively  low  magnetic  densities  were 
necessary  to  avoid  excessive  iron  heating,  and  the  inductor  alter- 
nator, with  its  non-reversal  of  armature  flux,  was  worked  at  almost 
double  the  induction  of  the  rotating  armature  type  of  machine, 
and  thus  the  disadvantages  of  the  non-reversal  of  flux  of  the  in- 
ductor type  were  masked.  In  other  words,  the  inductor  alter- 
nator was  worked  well  up  toward  saturation,  while  the  other 
types  were  worked  at  only  about  half  saturation.  However,  with 
improvements  in  ventilation  due  to  radial  ventilating  ducts,  im- 
provements in  iron  by  better  annealing  and  painting  of  the  lam- 
inations, etc.,  the  flux  densities  in  the  rotating  armature  machines 
were  gradually  increased  until  high  densities,  approaching  satura- 
tion, were  reached.  A  corresponding  increase  in  flux  densities 


DEVELOPMENT  OF  THE  A  C   GENERATOR  605 

in  the  inductor  type  was  not  possible,  on  account  of  saturation. 
Therefore  the  rotating  armature  type  of  machine,  in  the  later 
designs,  was  much  more  economical  than  the  inductor  type,  al- 
though the  latter  had  a  very  considerable  advantage,  especially 
at  high  voltages,  in  its  stationary  armature  construction.  Due  to 
the  merits  of  the  stationary  armature  construction,  the  present 
rotating  field  type  of  machine  was  gradually  evolved,  which  pos- 
sesses the  advantages  of  the  stationary  armature  of  the  inductor 
type  machine  and  the  reversing  flux  of  the  rotating  armature  al- 
ternator. It  was  the  development  of  this  type  of  machine  which 
sounded  the  death-knell  of  the  inductor  type.  However,  the 
Westinghouse  Company,  about  1897,  decided  to  bring  out  a  line 
of  inductor  type  alternators  to  meet  market  conditions,  although 
such  decision  was  contrary  to  the  recommendations  of  the  design- 
ing engineers  of  the  company,  whose  recommendation  in  particular 
was  in  favor  of  the  rotating  field  construction  as  a  more  permanent 
type.  However,  as  the  rotating  field  type  was  not  yet  established, 
except  in  a  very  minor  way,  and  as  the  inductor  type  had  been  on 
the  market  for  years,  it  was  decided  to  build  the  inductor  type, 
although  the  desigfi  adopted  was  somewhat  different  from  the 
Stanley  type.  Three  sizes  of  these  machines  were  built,  two 
belted  and  one  engine  type,  but  the  inductor  type,  as  a  commercial 
proposition,  soon  died  out. 

One  of  the  interesting  peculiarities  of  the  inductor  type 
alternator,  as  usually  built,  was  in  the  enormous  stray  field  appear- 
ing in  the  shaft,  bearings,  bedplate,  and  sometimes  in  the  engine- 
governing  mechanism  in  engine  type  units,  necessitating  in  at 
least  one  case,  the  use  of  brass  governor  balls.  In  the  usual  con- 
struction of  inductor  alternator,  there  was  but  one  exciting  wind- 
ing. The  magnetic  circuit  and  the  field  winding  were  arranged  as 
in  Fig,  8,  which  shows  both  the  Stanley  and  the  Westinghouse 
constructions.  The  normal  or  useful  path  of  the  magnetic  flux 
is  indicated  by  the  dotted  lines  a,  a.  Obviously,  the  field  coil 
which  set  up  flux  through  these  paths  could  also  send  magnetic 
fluxes  through  the  shaft,  bearings  and  bedplate  along  the  dotted 
lines  b,  b.  Moreover,  if  the  two  bearings  were  not  connected  by  a 
magnetic  bedplate,  as  might  be  the  case  in  engine  type  machines, 
then,  in  two-crank  engines  the  engine  cylinders  and  other  parts 
became  opposite  poles  of  a  very  powerful  electro-magnet,  when 
the  field  coil  was  excited.  The  stray  magnetic  field  set  up  in 
engine  type  units  was  sometimes  so  strong  as  to  interfere  with  the 


606 


ELECTRICAL  ENGINEERING  PAPERS 


governing  mechanism.  Also,  with  a  strong  unidirectional  flux 
between  the  bearings  and  shaft,  each  bearing  became  part  of  a  small 
unipolar  generator,  of  which  the  bearing  surfaces  formed  the 
brushes.  In  some  machines,  quite  heavy  currents  were  generated 
in  the  bearings,  sufficient  to ' '  eat  away  "  the  bearing  surfaces  or  to 
pit  them  so  that  bad  bearing  operation  resulted.  As  this  was 
primarily  a  magnetic  trouble,  insulating  the  bearings  from  their 
pedestals  would  not  stop  the  action.  To  overcome  this  trouble, 


PIG.  8— SKETCH  OP  MAGNETIC  CIRCUIT  OP  AN  INDUCTOR 
ALTERNATOR 

the  Stanley  Company  added  a  "bucking  "  coil  placed  around  the 
shaft  at  one  side  of  the  generator,  this  coil  being  in  series  with  the 
main  field  winding  and  magnetizing  in  the  opposite  direction.  The 
ampere-turns  of  this  bucking  coil  being  made  equal  to  those  of  the 
field  coil,  the  resultant  ampere-turns  between  the  two  bearings 
would  be  zero.  Obviously,  in  an  alternator  with  a  bedplate  and 
two  bearings  in  which  the  armature  frame  rested  directly  on  the 
bedplate,  a  single  bucking  coil  at  one  side  of  the  machine  would 
not  neutralize  the  stray  field  through  both  bearings. 

ROTATING  FIELD  GENERATORS 

Considering  next  the  rotating  field  type  of  machines,  possibly 
the  earliest  example  was  the  Niagara  type,  mentioned  before.  This 
had  an  internal  stationary  armature,  with  windings  on  its  outer 
periphery  like  the  ordinary  rotating  armature.  Outside  this  was  the 
rotating  field,  consisting  of  a  heavy  forged  steel  ring  with  inwardly 
projecting  poles.  However,  this  type  of  construction  was  relatively 
expensive,  and  was  never  adopted  generally.  The  more  modern 


DEVELOPMENT  OF  THE  A.C   GENERATOR  COT 

rotating  field  type  of  alternator,  with  external  stationary  armature, 
was  a  rather  gradual  development  and,  during  this  period,  there 
was  much  heated  discussion  as  to  the  relative  advantages  of  the 
rotating  field  and  rotating  armature  types.  The  rotating  field 
gradually  superseded  the  rotating  armature  construction  for  a  num- 
ber of  reasons,  the  principal  one  having  to  do  with  the  armature 
windings  and  voltages.  In  the  rotating  armature,  the  end  wind- 
ings were  more  difficult  to  support  than  in  the  stationary  armature. 
Also,  with  the  gradual  advent  of  higher  voltages,  the  stationary 
winding  proved  to  be  far  superior.  However,  as  a  goodly  pro- 
portion of  the  alternators  built  during  this  transition  period  were 
of  the  engine  type  and  for  low  voltage,  in  which  heavy  bar  wind- 
ings could  be  used,  (such  being  conditions  tinder  which  the  rotat- 
ing armature  made  its  best  showing,)  this  type  persisted  for  several 
years  after  the  rotating  field  type  became  commercial.  Gradually 
increasing  voltages,  however,  necessitated  the  use  of  stationary 
armature  machines,  for  at  least  part  of  the  business.  The  manu- 
facture of  two  types  of  apparatus  for  the  same  general  purpose 
could  not  persist,  and  eventually  that  type  was  adopted  exclusive- 
ly, which  allowed  both  high  and  low  voltages.  By  1900,  the  rot- 
ating field  alternator  had  come  into  very  general  use,  and  the 
rotating  armature  type  was  disappearing.  This  rotating  field  type 
has  persisted  until  the  present  time,  although  many  minor  modi- 
fications have  been  brought  out  from  time  to  time,  due  largely  to 
change  in  speed  conditions,  etc. 

In  the  rotating  field  development,  the  tendency  for  a  number 
of  years  was  strongly  toward  the  engine  type  construction  and 
relatively  low  speeds  in  many  cases.  The  construction  was  carried 
to  the  extreme,  in  some  cases,  where  the  usual  flywheel  capacity 
required  for  the  slow  speed  engines  was  incorporated  in  the  field 
structure  of  the  alternator  itself.  In  some  cases,  this  meant 
enormously  large  machines  for  the  output.  A  prominent  example 
of  this  is  found  in  the  seventeen  6  000  kilowatt  engine-type  ma- 
chines designed  in  1899  and  1901  respectively,  and  installed  in 
the  Fifty-ninth  and  Seventy-fourth  Street  power  stations  of  the 
Interboro  Rapid  Transit  Company  of  New  York  City.  As  an 
indication  of  the  changes  taking  place  in  the  electrical  field,  it  may 
be  stated  here  that  arrangements  have  been  made  recently  to  take 
out  a  number  of  these  machines  atid  install  in  their  place  30  000  kw 
turbo-generator  units*  The  existing  engine  type  machines  are 
probably  in  as  good  condition  mm  as  wbuea  first  installed,  and  are  , 


608 


ELECTRICAL  ENGINEERING  PAPERS 


being  replaced  simply  because  they  occupy  too  much  space  in 
proportion  to  their  output. 

The  rotating  field  alternator  of  the  early  days  was  not  radically 
different  from  the  rotating  field  alternator  of  today,  the  principal 


PIG   9— SHOP  VIEW  OP  5  000  HORSE-POWER  TWO-PHASE  NIAGARA 
ALTERNATOR 

differences  being  in  the  type  of  armature  windings,  methods  of 
ventilation,  etc. 

FIELD  CONSTRUCTION 

In  the  types  of  field  windings  there  has  been  but  little  change. 
In  many  of  the  old  stationary  field  machines  of  large  capacity,  the 
field  windings  consisted  of  strap  wound  on  edge,  one  layer  deep. 
For  smaller  machines,  either  square  or  round  wire  was  commonly 
used.  In  the  latter  rotating  field  machines,  similar  constructions 
are  used.  In  the  construction  of  the  field  itself,  there  have  been 
some  variations  and  modifications.  In  many  Of  the  older  ma- 
chines the  poles  were  laminated  as  at  present.  The  method  of 


DEVELOPMENT  OF  THE  A  C   GENERATOR 


609 


attaching  the  poles  varied  in  different  constructions.  In  many 
of  the  earlier  Westinghouse  rotating  fields  the  laminations  were 
punched  with  two  or  more  poles  in  one  piece,  the  poles  having 
no  overhanging  tips,  and  the  field  coils  being  held  in  place  by 
metal  wedges  between  pole  tips,  fitted  into  notches  or  grooves  at 
the  pole  tips,  each  pole  being  attached  to  the  field  ring  or  yoke 
by  means  of  bolts  or  dove-tails.  This  latter  construction  possesses 
numerous  advantages,  in  that  cheap  dies  can  be  used,  and  the 
same  pole  punchings  can  be  used  for  a  number  of  different  designs, 
in  which  either  the  diameter  or  the  number  of  poles  is  varied. 

WATER  WHEEL  TYPE  GENERATORS 

With  the  advent  of  the  turbo-generator  on  a  large  scale,  the 
engine  type  rotating  field  alternator  almost  disappeared  from  the 
manufacturing  field,  except  in  the  smaller  size  units.  However, 
during  this  period  there  has  been  a  gradual  development  in  the  use 


yj?Unfiri.Mri.MMJlf>i.f  ti  I  "•>'  (*?t1M 


PIG.  10— STATOR  OR  ARMATURE  OP  NIAGARA  ALTERNATOR 


of  water  powers,  and  water-wheel  driven  generators  have  come 
into  much  greater  prominence  in  the  past  few  years.  In  this  line 
of  development,  speeds  and  capacities,  unheard  of  in  the  earlier 
days,  have  become  accepted  practice  with  the  development  of  both 
high-head  and  low-head  water  powers.  In  the  former  the  ten- 
dency has  been  toward  very  high  speeds  for  a  given  capacity  such 


610  ELECTRICAL  ENGINEERING  PAPERS 

as  the  17  000  k  v.a.,  375  r.p.m.,  Westinghouse  machines,  built  for 
the  Pacific  Light  &  Power  Company,  and  the  10  000  k.v.a  ,  600 
r.p.m,,  Westinghouse  generators,  built  for  the  Sao  Paulo  plant  in 
Brazil.  Typical  examples  of  low-head,  slow-speed  practice  are 
found  in  the  60  cycle,  75  r.p.m.,  96  pole,  2  700  k.v  a.  Westinghouse 
generators  for  the  Stevens  Creek  development,  and  the  25  cycle, 
58  r.p.m  ,  52  pole,  9  000  k  v  a.  General  Electric  generators  for  the 
Keokuk  plant.  The  former  are  abnormal  in  the  very  large  number 
of  poles  required  for  moderate  output,  while  the  latter  are  ab- 
normal in  the  very  low  speed.  Both  of  the  above  machines  are  of 
the  vertical  type,  and  are  examples  of  a  very  pronounced  tendency 
toward  vertical  machines,  which  has  been  apparent  in  the  later 
water  wheel  practice. 

On  account  of  the  high  speeds  of  some  of  the  modern  rotating 
field  alternators,  mechanically  stronger  spiders  have  come  into 
general  use.  Even  in  moderate  speed  units  the  usual  high  run- 
away over-speed  of  100  percent  has  necessitated  the  use  of  very 
substantial  spiders. 

During  the  past  few  years,  some  very  interesting  spider  con- 
structions for  the  rotating  fields  of  large  high  speed  alternators  have 
been  built  to  meet  the  severe  speed  requirements.  Some  of  these 
have  been  made  up  of  cast  steel  centers  or  spiders  with  cylindrical 
rims  built  up  of  overlapping  laminated  punchings,  thoroughly 
bolted  together  and  attached  to  the  spider  by  dove-tails.  The  outer 
periphery  of  the  laminated  ring  carries  dove-tail  grooves  for  poles. 
In  another  construction,  the  entire  spider  consists  of  thick  rolled 
iron  plates,  bolted  together,  and  with  dove-tail  grooves  on  the  out- 
side for  the  poles.  In  still  other  constructions,  the  rim  of  the 
spider  consists  of  a  heavy  steel  ring  in  one  or  more  sections  to 
which  the  cast  spider  is  bolted.  Usually  with  this  cast  rim  the 
poles  are  bolted  to  the  spider.  In  some  cases  the  rim  forms  an 
integral  part  of  the  spider  itself,  being  cast  with  the  spokes  and 
hub.  The  type  of  construction  adopted  in  each  case  is,  to  a  large 
extent,  dependent  upon  the  stresses  to  be  taken  care  of,  so  that  no 
one  type  seems  to  fit  all  cases  to  best  advantage 

THE  PROBLEM  OF  VENTILATION 

In  the  later  rotating  field  alternators  the  problem  of  ventila- 
tion has  received  much  consideration,  especially  in  the  case  of 
machines  operating  at  abnormal  speeds:  In  very  high  speed 
machines  of  very  large  output  the  armature  and  field  cores  have 


DEVELOPMENT  OF  THE  A  C.  GENERATOR  611 

a  ratio  of  width  to  diameter  which  is  relatively  much  greater  than 
in  ordinary  machines,  and  this  has  necessitated  abnormal  conditions 
of  ventilation.  Something  may  be  said  here  regarding  the  general 
problem  of  ventilation  of  alternators  and  its  influence  on  the  evolu- 
tion. Back  in  1891  or  1892,  radial  ventilating  ducts  or  passages 
came  into  use  commercially  on  certain  direct-current  machines. 
The  results  being  quite  satisfactory,  it  was  natural  that  alternators 
should  have  the  same  method  of  ventilation.  The  use  of  such 
ducts  was  in  reality  one  of  the  great  steps  forward  in  the  evolution 
of  dynamo-electric  machinery,  although  but  little  recognition  has 
been  given  to  this  fact  in  electrical  literature.  The  use  of  radial 
ventilating  ducts  has  continued  to  the  present  time  with  little 
change  except  in  the  construction  of  the  spacers  themselves,  which 
have  been  many  and  varied  in  design  and  materials.  With  the 
change  from  the  rotating  armature  to  the  rotating  field  construc- 
tion of  alternators  this  feature  was  retained  in  full.  In  some  of  the 
earlier  Westinghouse  rotating  field  machines  the  field  structure 
also  had  numerous  ventilating  ducts,  principally  for  the  purpose  of 
supplying  ample  air  to  the  armature  ducts.  Also,  about  ten  years 
ago,  special  ventilating  end  bells  and  vanes  began  to  be  used  on 
rotating  fields,  in  order  to  set  up  an  extra  air  circulation  through 
the  armature  end  windings,  etc.,  due  largely  to  the  fact  that  the 
slow-speed  engine-type  machines  of  that  period  did  not  have  much 
natural  blowing  action.  Following  this,  and  partly  as  an  out- 
growth of  turbo-generator  enclosing,  came  the  semi-enclosed 
alternators,  mostly  for  high-speed  water-wheel  driven  units,  and 
this  practice  is  not  uncommon  at  present. 

The  proper  ventilation  of  an  alternator  or,  for  that  matter,  of 
any  dynamo-electric  machine,  is  very  much  of  a  problem,  for  no 
two  cases,  in  different  sizes  or  types  of  machines,  are  quite  alike. 
The  problem  Hes  first,  in  furnishing  the  proper  quantity  of  air  to 
carry  away  the  heat  developed,  and  in  then  distributing  such  air 
in  proper  proportion  through  the  complex  multiple  paths  in  the 
machine.  The  proper  distribution  of  the  ventilating  air  is  usually 
the  most  serious  part  of  the  problem.  The  present  solutions  of  the 
problem  are  based  largely  upon  past  experience,  and  no  really  work- 
able rules  have  yet  been  developed.  In  arriving  at  the  present 
practice  many  disheartening  experiences  have  been  undergone  by 
all  designing  engineers.  The  writer  has  known  many  cases  where 
totally  unexpected  i^esiaits,  both  good  and  bad,  have  been  devel- 
oped and,  ia  soma  <o£  these  cases,  no  logical  explanation  was  forth- 


612 


ELECTRICAL  ENGINEERING  PAPERS 


coming,  so  that  the  results  could  not  be  taken  advantage  of,  with 
any 'assurance,  in  future  work.  This  has  been  one  of  the  most 
discouraging  features  in  the  general  problem  of  ventilation. 

ARMATURE  WINDINGS 

Something  might  be  added  here  on  the  subject  of  armature 
windings.  There  have  been  probably  as  many  types  of  armature 
windings  developed  as  there  have  been  types  of  alternators.  The 
windings  for  the  earliest  smooth  body  and  the  toothed  armature 
constructions  have  already  been  described.  In  the  early  West- 
inghouse  polyphase  alternators,  two-phase  was  used  mostly,  due 
principally  to  the  fact  that  single-phase  lighting  circuits  formed 
the  principal  load,  and,  with  two-phase  machines,  there  were  only 
two  circuits  from  a  machine  instead  of  three  circuits  with  the  three- 
phase  winding.  Moreover,  many  of  these  very  early  polyphase 


ff^xfff^ 


FIG.  11— VARIOUS  TYPES  OF  EARLY  STATIONARY  ARMATURE  WINDINGS 


alternators  were  used  in  reality  as  straight  single-phase  machines, 
taking  current  off  one  phase  only.  For  this  purpose  a  closed 
coil  armature  winding,  like  that  of  a  direct-current  generator  or 
rotary  converter,  with  four  taps  for  taking  off  the  two  phases, 
gave  about  the  most  economical  type  of  winding,  as  far  as  arma- 
ture copper  losses  were  concerned.  When  such  an  armature  is 
used  for  single-phase  it  can  deliver  seven-tenths  as  much  output 


DEVELOPMENT  OF  THE  A  C   GENERATOR  613 

as  a  single-phase  machine  as  it  can  give  two-phase  with  the  same 
total  copper  loss  per  coil.  It  was  partly  for  this  reason  that  many 
of  the  early  Westinghouse  polyphase  machines  had  a  single  closed 
coil  winding.  Another  reason  for  such  winding  was  that  there 
were  no  definite  phase  groups  and  no  high  potential  between 
phases.  Furthermore,  the  arrangement  of  the  end  windings  was 
such  that  the  coils  tended  to  interlock  and  support  each  other, 
thus  assisting  in  resisting  centrifugal  forces.  This  winding  was 
used  mostly  for  two-phase  machines,  but  was  also  used  to  a  con- 
siderable extent  on  three-phase  armatures. 

With  the  advent  of  the  rotating  field  type  of  machine,  this 
closed  coil  type  of  armature  winding  was  not  used  to  any  great 
extent,  open-coil  two-phase  and  star-connected  three-phase  taking 
its  place.  Delta-connected  three-phase  was  used  in  very  rare 
cases,  as  there  was  danger  of  circulating  current  with  such  wind- 
ings. 

In  the  construction  of  armature  windings  possibly  more 
radical  changes  have  taken  place  than  in  the  types  of  windings. 
Many  of  the  larger  low  speed  rotating  armature  alternators  had 
bar  windings  with  separate  end  connectors,  soldered  or  bolted  on. 
Many  of  the  earlier  stationary  type  armatures  had  either  built-up 
bar  or  strap  windings,  or  concentric  type  windings  in  which  each 
phase  winding  was  arranged  in  a  concentric  group,  and  the  groups 
of  (the  different  phases  overlapped  each  other.  Some  of  these 
were  made  with  partially  closed  slots  and  others  with  open  slots. 
The  built-up  bar  or  strap  windings  were  frequently  of  the  partially 
closed  slot  type,  while  the  concentric  windings  were  more  usually 
of  the  open  slot  type.  Gradually,  however,  both  these  types  of 
windings  were  superseded  by  the  "duplicate  coil'*  type  of  winding, 
similar  in  appearance  to  the  usual  direct-current  armature  and 
induction  motor  primary  windings.  This  later  type  of  alternator 
winding  was  arranged  in  two  layers  of  coils  at  the  ends,  in  either 
one  or  two  layers  in  the  slots,  The  two-layer,  two-coil  per  slot 
arrangement  is  now  practically  the  standard.  These  types  are 
illustrated  in  Fig.  11. 

In  the  rotating  field  machines,  partially  closed  slot  construc- 
tion was  carried  to  comparatively  high  voltages.  For  instance,  the 
6  000  kilowatt,  75  r.p.xn.,  11  000  volt,  three-phase  generators  built 
for  the  Manhattan  Elevated  Railway  in  1900  had  three  bars  side 
by  side,  in  each  slot,  with  soldered-on  end  collectors. 


t>14 


ELECTRICAL  ENGINEERING  PAPERS 


As  the  partially  closed  slot  and  the  open  slot  constructions  are 
radically  different  from  each  other,  something  should  be  said 
regarding  the  reasons  which  prompted  the  use  of  either  type.  As 
already  indicated,  the  partially  closed  slot  type  came  in  with  the 
larger  rotating-armature  low-voltage  alternators  in  which  bar 
windings  could  be  used  This  construction  gave  good  mechan- 


PJG.  12— BAR  AND  END  CONNECTOR  TYPE  OP  WINDING  WITH 
PARTIALLY  CLOSED  SLOTS 


ical  support  for  the  bars  in  the  slots,  thus  avoiding  the  use  of 
bands.  Moreover,  with  the  very  narrow  slot  openings  at  the  top 
of  the  slots,  there  was  very  little  "bunching"  of  the  magnetic  flux 
at  the  armature  tooth  tips  with  the  consequent  low  pole  face 
losses,  even  with  very  small  air-gaps,  and  high  gap  flux  densities. 
The  disadvantages  of  the  partially  closed  slot  is  found  largely  ia 
the  type  of  windings  required. 


DEVELOPMENT  OF  THE  A  C   GENERATOR 


615 


In  these  early  machines,  it  was  found  practicable,  in  general, 
to  use  completely  formed  and  insulated  coils  with  such  slots,  and 
therefore,  either  hand  windings  or  built-up  types  of  windings  were 
used.  While  these  were  possible  and  practicable  in  a  manufactur- 
ing establishment,  yet  such  types  of  windings  are  usually  difficult 
to  repair  by  the  ordinary  operator  inexperienced  in  the  refinements 
of  armature  winding.  When  it  comes  to  repairs,  the  usual  ma- 
chine-wound coil,  which  is  completely  insulated  before  being 
placed  on  the  armature  core,  is  very  superior  but,  in  general,  this 
type  of  winding  requires  an  open  slot  construction.  However,  when 


PIG.  13— DUPLICATE  COIL  TYPE  OP  WINDING,  TWO  COILS  PER  SLOT 

the  stationary  armature  construction  came  into  general  use,  the 
advantages  of  the  overhanging  tooth  tips  in  supporting  the  coils 
largely  disappeared.  There  remained  therefore  the  disadvantages 
of  the  flux  bunching,  against  the  advantage  of  coil  construction, 
if  open  slots  were  used.  However,  the  use  of  laminated  poles,  and 
the  judicious  proportioning  of  the  air-gaps  and  flux  densities,  to  a 
great  extent  eUmtaated  the  losses  dtia  to  open  slots*  Ip. 


616  ELECTRICAL  ENGINEERING  PAPERS 

quen.ce,  the  open  slot  construction  and  the  duplicate  type  of 
armature  coil,  have  apparently  come  to  stay,  in  this  country. 

Various  attempts  have  been  made  to  obtain  the  advantages 
of  both  the  open  and  the  partially  closed  slot  arrangements. 
Probably  all  large  manufacturers  of  alternators  have  tried  some 
form  of  magnetic  wedge,  instead  of  the  usual  fibre  or  wood  wedges 
which  serve  to  retain  the  coils  in  the  slots.  Another  arrangement 
is  the  equivalent  of  combining  two  or  more  open  slots  in  one,  with 
an  over-hanging  tooth  tip,  which  covers  the  slot  with  the  exception 
of  the  widths  of  one  coil.  Two  or  more  completely  insulated  coils 
are  fed  successively  into  the  slot  opening  and  arranged  side  by 
side.  This  does  not  give  any  narrower  slot  opening  than  with  the 
open  slot  construction,  but  the  number  of  openings  is  reduced  to 
one-half,  or  one-third.  This  construction  is  used  rather  extensive- 
ly in  the  rotors  of  large  induction  motors,  but  apparently  is  but 
little  used  in  generators. 

Bracing  of  the  end  windings  against  short-circuit  shocks  has 
been  a  comparatively  recent  practice.  The  necessity  for  such 
bracing  has  been  dependent  to  a  considerable  extent  upon  the 
output  per  pole,  and  the  old  time  machine  seldom  had  such  a  large 
output  per  pole  that  the  short-circuit  current-rushes  were  suffi- 
cient to  cause  dangerous  distortions  of  the  end  windings.  How- 
ever, such  bracing  was  used  on  the  Niagara  machines,  previously 
described,  and  also  on  the  Manhattan  generators  above  referred 
to.  These,  however,  were  very  rare  instances.  However,  with 
the  recent  high-speed,  high-output  water-wheel  generators,  the 
outputs  per  pole  have  become  such  that  some  form  of  end  bracing 
has  become  rather  common. 

Modern  Westinghouse  machines  of  this  kind  are  braced  to 
stand  a  dead  short-circuit  across  the  terminals  without  damage  to 
the  windings.  Under  this  condition,  these  large  machines  may  give 
a  momentary  current  rush  of  from  ten  to  twenty  times  the  rated 
full-load  current.  However,  the  bracing  required  on  the  end 
windings  of  such  machines  is  of  relatively  much  less  importance 
than  on  turbo-generators  of  corresponding  capacity,  due  to  the 
fact  that,  in  the  former  class  of  machines,  the  end  windings  are 
relatively  short  compared  with  those  of  turbo-generators. 

The  above  description  brings  us  practically  up  to  date,  as  far 
as  the  ordinary  synchronous  alternator  is  concerned.  No  de- 
scription of  the  development  of  the  turbo-generator  has  yet  been 
given.  This  forms  a  rather  distinct  development  which  should 


DEVELOPMENT  OF  THE  A  C   GENERATOR  617 

follow  at  this  point  presumably,  but  it  is  thought  advisable  to 
interpolate  here  some  description  of  the  problems  of  parallel 
operation,  e.  m.  f.  wave  form,  regulation,  etc.,  which  came  into 
prominence  and  were  practically  taken  care  of  previous  to  the 
advent  of  the  turbo-generator  on  a  large  scale. 

PARALLEL  OPERATION  OF  ALTERNATORS 

One  of  the  great  problems  which  developed  in  the  operation 
of  alternators  was  that  of  the  parallel  running  of  two  or  more  units. 
At  one  time  this  was  a  very  serious  question,  but  in  recent  years, 
it  is  very  seldom  heard  of.  Considering  the  almost  universal 


PIG.  i*— DETAIL  VIEW  OP  THREE-PHASE  CONCENTRIC  WINDING 

practice  of  paralleling  alternators,  which  holds  at  the  present  time, 
one  might  be  led  to  wonder  why  there  ever  was  any  trouble.  Far 
back,  in  the  days  of  the  high-frequency  surface-wound  alternators, 
paralleling  was  attempted  in  many  cases  and,  not  infrequently, 
with  considerable  success.  However,  a  failure  in  an  attempt  to 
parallel,  in  those  days,  usually  meant  the  destruction  of  the  ap- 
paratus. Those  old  time  surface-wound  alternators  usually  had 
very  low  self-induction,  so  that,  in  case  of  sudden  short-circuit,  an 
enormous  current  could  flow, '  sufficient'  usually  to  strip  the  arm* 


618 


ELECTRICAL  ENGINEERING  PAPERS 


ature  winding  from  the  core,  by  bursting  the  bands,  or  otherwise. 
A  failure  in  an  attempt  to  parallel  two  machines  was  practically 
equivalent  to  a  short-circuit,  and  this  usually  meant  destruction 
of  the  apparatus.  However,  if  once  paralleled  successfully,  the 
machines  usually  did  not  act  badly.  One  favorable  condition,  not 
then  appreciated,  was  that  all  these  early  machines  were  belt- 


PIG.  15—75  K.V  A.,  THREE-PHASE.  60  CYCLE,  2  300  VOLT,  150  R.  P.  M. 
ROTATING  FIELD  ENGINE  TYPE  ALTERNATOR 


driven.  It  may  be  said,  however,  that  in  those  days  parallel  opera- 
tion, while  considered  possible,  was  also  considered  more  or  less 
risky.  In  the  period  immediately  following  the  surface-wound 
alternator,  parallel  operation  was  very  much  the  exception,  rather 
than  the  rule  and,  when  engine-type  alternators  came  into  use, 
paralleling  was  considered  for  several  years  as  very  questionable. 
At  this  time  the  situation  was  as  follows: — Belted  alternators 
could  be  paralleled  in  many  cases.  Direct-coupled  alternators, 


DEVELOPMENT  OF  THE  A.C   GENERATOR  619 

if  f  flexibly  driven,  could  be  paralleled  almost  as  well  as  belted 
machines,  while  direct-coupled  or  engine  type  generators,  without 
flexible  coupling  or  drive,  could  not  be  relied  on  to  parallel  with- 
out hunting.  It  thus  became  recognized  that  some  flexibility 
between  the  generator  and  its  prime  mover  was  an  important 
adjunct  to  parallel  operation.  This  led  to  the  consideration  that 
the  engine  might  be  back  of  the  difficulty  in  many  instances,  and 
it  was  then  assumed  that  inequalities  in  the  regular  rotation  resiolt- 
ing  from  insufficient  flywheel  or  from  hunting  governors,  tended 
to  cause  hunting  in  the  generators.  Investigation  showed  that 
such  conditions  did  tend  to  produce  hunting,  but  that  the  magnetic 
conditions  in  the  machine  itself  would  oftentimes  maintain,  or  even 
accentuate,  the  hunting.  Obviously,  therefore,  the  trouble  was 
both  in  the  prime  mover  and  in  the  generator,  It  was  noted  fur- 
ther 'that  if  the  angular  fluctuations  in  the  driving  power  were 
relatively  small,  hunting  usually  would  be  very  small,  or  would  not 
be  apparent  at  all.  It  was  further  recognized  that,  with  belt  or 
flexible  drive,  which  tended  to  smooth  out  the  speed  fluctuations 
due  to  the  prime  mover,  tjie  hunting  tendency  tended  to  disappear. 
Attention  was  then  turned  toward  improvement  of  the  prime 
movers,  especially  in  engine-type  machines,  in  order  to  reduce 
fluctuations  in  angular  velocity  by  means  of  heavy  flywheels,  -and 
by  means  of  dampers  of  some  sort,  such  as  dashpots,  on  the 
governing  mechanism  of  the  engine.  Much  improvement  was 
accomplished  in  this  way, 

THE  INTRODUCTION  OF  DAMPERS 

During  this  period  many  attempts  were  made  to  lessen  the 
tendency  of  the  alternator  to  maintain  hunting.  Investiga- 
tion showed  that,  during  hunting,  the  magnetic  flux  in  the  field 
poles  shifted  back  and  forth  across  the  pole  faces  in  time  with 
the  hunting,  while  such  action  did  not  occur  when  there  was 
no  hunting.  This  at  once  led  to  the  theory  that  a  lo^1  resist- 
ance winding  on  the  pole  face,  or  imbedded  in  the  poles,  would 
prevent  or  oppose  this  flux  shift,  and  thus  assist  in  overcoming 
hunting.  However,  about  this  time,  rotary  converters  were 
coming  into  use,  and  it  was  found  that,  in  such  machines, 
hunting  was  usually  more  severe  than  in  alternators,  so  that,  in 
this  country,  the  first  true  application  of  damping  windings  or 
devices  to  stop  hunting  were  applied  on  rotary  converters*  It  was 
also  noted  at  this  time  that  solid  pole  generators  and  rotary  con- 
verters did  not  tti&t  to  the  sam®  extent  as  did  laminated  pole  ma-^ 


620 


ELECTRICAL  ENGINEERING  PAPERS 


chines,  and  it  was  correctly  assumed  that  the  solid  pole  faces  gave 
an  effect  similar  to  that  of  low  resistance  damping  windings.  How- 
ever, as  it  was  desirable  to  use  laminated  pole  tips,  copper  dampers 
on  the  poles  gradually  came  into  use.  Some  of  these  early  dampers 
were  very  crude  in  form  and  type  compared  with  present  construc- 


FIG.  16— VARIOUS  FORMS  OF  DAMPERS 

tions.  However,  imperfections  in  the  construction  of  the  dampers 
were  balanced  to  some  extent  by  the  large  section  of  copper  used 
and  consequent  low  resistance.  The  earliest  form  of  damper  used 
in  this  country  consisted  of  copper  rings  surrounding  the  poles  and 


PIG.  17— GRID  DAMPERS  ON  FIELD  POLES 

copper  tips  overhanging  the  beveled  pole  edges.  This  was  the  form 
most  commonly  used  on  converters.  On  alternators,  in  some  cases 
the  damper  consisted  simply  of  a  low  resistance  ring  around  each 
pole.  In  still  other  cases  the  damper  consisted  of  a  heavy  copper 
plate  covering  the  pole  face.  This  latter  construction  was  only 


DEVELOPMENT  OF  THE  A.C.  GENERATOR  621 

possible  in  machines  with  large  air-gaps  and  very  narrow  or  partial- 
ly closed  armature  slots.  These  crude  forms  of  dampers  were 
gradually  superseded  by  the  so-called  "grid"  damper  which  con- 
sisted of  a  copper  grid  surrounding  the  pole  and  with  ribs  which  lay 
in  slots  in  the  pole  face.  These  various  types  of  dampers  are 
shown  in  Fig  16.  In  very  few  cases  were  these  old  types  of 
dampers  so  interconnected  as  to  form  a  complete  cage  winding 
around  the  field. 

Many  tests  were  made  at  various  times  to  determine  the 
effect  of  interconnecting  the  grids  on  the  different  poles  to  form 
one  complete  cage.  As  a  rule,  there  was  no  appreciable  gain, 
and  it  was  then  assumed  that  such  interconnection  had  no  material 
advantages.  However,  it  later  developed  that  the  reason  why 
interconnection  of  the  dampers  did  not  improve  the  damping 
action  very  materially,  was  due  largely  to  the  very  great  amount 
of  copper  in  those  parts  of  the  grid  dampers  m  lying  between  the 
poles.  The  grid  damper  was  very  effective,  but  was  expensive  in 
material,  and  was  not  easily  applied  on  poles  with  overhanging 


PIG.  18— CAGE  WINDING  TYPE  OP  DAMPERS 

pole  tips.  This  type  of  damper  was  gradually  superseded  by  one 
similar  to  the  usual  cage  winding  on  the  secondaries  of  induction 
motors,  and  this  is  the  type  which  is  in  most  general  use  at  the 
present  time.  This  construction  has  practically  the  same  effect- 
iveness as  the  old  grid  type,  but  is  much  more  economical  in 
material  and,  being  placed  in  partially  closed  slots,  it  does  not  as 
greatly  affect  the  iron  losses  in  the  machine,  as  was  liable  to  be  the 
case  with  the  open  slots,  generally  used  with  the  grid  damper. 

With  the  gradual  introduction  of  dampers  and  improvements 
in  angular  rotation  of  the  prime  movers,  hunting  troubles  in  alter- 
nators practically  disappeared,  and  parallel  operation  presented 


622  ELECTRICAL  ENGINEERING  PAPERS 

no  difficulties,  except  under  very  abnormal  conditions.  Apparent- 
ly these  dampers  or  "amortisseurs,"  as  they  are  sometimes  called, 
were  first  proposed  by  the  French  engineer,  Maurice  LeBlanc, 
about  1891.  However,  they  were  "rediscovered"  in  this  country 
by  engineers  who  were  not  familiar  with  the  above  engineer's 
work. 

VOLTAGE  WAVE  FORM 

The  e  m.  f .  wave  form  of  alternating-current  generators  has 
been  a  matter  of  much  discussion  since  the  early  days  of  alter- 
nator design.  The  old  surface-wound  machines  gave  a  very  close 
approximation  to  a  perfect  sine  shape,  due  to  the  arrangement  of 
the  winding  and  to  the  very  large  air-gap.  The  first  toothed  ar- 
matures, with  their  very  small  air-gaps,  gave  e.  m.  f.  waves  which 

I [    | I 


JLJ 


A 


FIG    19— VOLTAGE  WAVE  FORMS 

Of  early  toothed  armature  machines  and  of  Later  toothed  armatures  with  larger  air  gaps  and 
beveled  poles. 

departed  very  widely  from  a  true  sine.  In  fact,  this  had  about 
the  worst  wave  form  of  any  of  the  alternators  which  have  been  put 
out  by  the  Westinghouse  Company.  Its  shape  was  somewhat 
like  that  shown  in  Fig.  19,  as  would  now  be  expected  when  the 
configuration  of  the  armature  tooth  tips  is  taken  into  account. 
The  later  toothed  armatures  with  large  air-gaps  and  beveled  tooth 
tips  gave  much  better  wave  shapes. 

With  the  advent  of  the  true  polyphase  windings  and  the 
slotted  armatures  with  several  slots  per  phase  per  pole,  fairly 
close  approximation  to  sine  shaped  e.  m.  f .  waves  became  common, 
In  the  first  Niagara  Falls  5  000  horse-power,  two-phase  alter- 


DEVELOPMENT  OF  THE  A  C.  GENERATOR  623 

nators,  the  voltage  wave  was  slightly  flattened  on  the  top  due  to 
the  fact  that  the  pole  face  width  was  somewhat  greater  than  the 
width  of  each  phase  group  in0 the  armature.  When  very  high 
voltages  came  into  general  use,  and  especially  in  machines  with 
small  pole  pitch,  the  number  of  armature  slots  per  phase  per  pole 
was  reduced  to  a  minimum  in  order  to  lessen  the  total  insulation 
space.  In  extreme  cases,  but  one  slot  per  phase  was  used,  giving 
but  two  slots  per  pole  for  two-phase  and  three  slots  per  pole  for 
three-phase.  Such  windings  required  special  shaping  of  the  field 
pole  tips  in  order  to  approximate  even  roughly  a  smooth  wave 
form  of  the  sine  shape.  Later  practice,  however,  has  tended 
toward  the  equivalent  of  at  least  two  slots  per  phase  per  pole,  in 
order  to  obtain. better  results.  Sometimes  the  desired  result  is 
obtained  by  the  use  of  one  extra  idle  or  "hunting"  tooth  per 
phase. 

In  the  early  days  of  parallel  operation  of  engine  type  alter- 
nators which,  as  described  before,  represented  the  most  difficult 
conditions,  great  stress  was  laid  upon  the  question  of  wave  form 
in  some  of  the  discussions  of  parallel  operation,  and  particularly, 
in  the  operation  of  rotary  converters  without  hunting.  Grad- 
ually, however,  this  question  disappeared  and  it  became  recog- 
nized that  all  the  cases  of  hunting  encountered  could  be  explained 
in  some  other  way  than  by  the  e.  m.  f .  wave  forms,  and  it  is  now 
generally  accepted  that  about  the  only  effect  on  parallel  operation 
due  to  wave  form  lies  in  possible  circulating  currents  of  higher 
frequency  than  the  fundamental. 

At  the  present  time,  a  very  close  approximation  to  the  sine 
shaped  wave  is  considered  preferable  for  general  purposes,  es- 
pecially in  transformation  and  transmission  work.  There  have 
been  some  instances  of  telephone  disturbances  due  to  wave  form 
but,  as  a  rule,  some  local  peculiarities  of  the  distribution  circuits 
have  been  involved  in  this  trouble,  for,  in  other  cases,  similar  or 
even  worse  shaped  waves  have  given  absolutely  no  telephone  dis- 
turbances. 

REGULATION  AND  COMPOUNDING 

Something  should  be  said  on  the  subject  of  regulation  of 
alternators,  for  this  is  a  very  important  characteristic,  and  has 
had  considerable  influence  on  types  and  designs.  The  old  surface- 
wound  alternators  had  extremely  good  regulating  characteristics 
due  to  their  low  amiature  self-induction  and  low  armature  reac- 


624  ELECTRICAL  ENGINEERING  PAPERS 

tion  consequent  upon  their  large  air-gaps.  The  writer  does  not 
know  what  valite  the  current  rose  to,  on  steady  short-circuit, 
compared  with  the  normal  rated  current,  but  it  was  probably  four 
or  five  times  full  load.  The  current  rush  on  short-circuit  was 
probably  five  times  as  great  as  the  steady  value.  It  is  not  to  be 
wondered  at  that  such  armatures  not  infrequently  wrecked  them- 
selves in  case  of  a  dead  short-circuit.  In  the  later  toothed  arma- 
ture types,  the  armature  self-induction  and  reaction  on  the  field 
were  very  much  larger,  proportionately,  than  in  the  surface- 
wound  machines.  This,  however,  spoiled  the  regulation  and  some 
method  of  compounding  was  used,  as  already  described.  This 
compounding  was  common  practice  until  larger  capacity  machines, 
especially  the  engine  type,  came  into  use.  Even  some  of  these 
latter  were  compounded  by  commutating  the  armature  current 
(either  directly  or  from  a  series  transformer)  and  compounding 
the  exciter  field  by  means  of  the  commutated  current.  A  few  of 
the  smaller  size  alternators  were  both  self -excited  and  compounded 
by  commutating  derived  alternating-current  circuits  from  the 
armature.  This,  however,  was  found  to  be  very  delicate,  as  the 
excitation  and  compounding  were  greatly  affected  by  changes  in 
the  power-factor  of  the  load,  and  by  changes  in  speed. 

One  early  attempt  was  made  to  compound  single-phase 
alternators  to  correct  for  power-factor.  In  this  case  the  com- 
mutated armature  current  was  sent  through  the  series  or  compound 
winding  of  the  exciter.  The  brushes  on  the  alternating-current 
commutator  were  so  set  that  at  100  percent  power -factor  they  were 
commutating  about  the  middle  of  each  voltage  wave.  In  con- 
sequence, the  current  delivered  to  the  brushes  was  not  a  true 
direct  current  but  consisted  of  a  double  number  of  half  waves, 
half  of  which  were  inverted,  and  the  direct-current  component  of 
this  commutated  current  was  small  and  had  but  little  compound- 
ing effect.  However,  with  change  in  power-factor  of  the  load,  the 
phase  of  the  current  shifted,  so  that  at  some  reduced  power-factor, 
commutation  occurred  at  the  zero  point  of  the  current  waves  and 
the  resultant  current  was  all  effective  for  magnetizing  the  exciter 
field.  The  total  commutated  voltage  was  very  low  and  the  com- 
mutator bars  were  shunted  by  a  resistance  so  that  there  was  no 
bad  sparking,  even  when  commutating  at  the  middle  of  the  current 
wave.  This  method  did  actually  compound  fairly  well  for  change 
in  power-factor,  but  the  field  for  such  method  proved  to  be  very 
limited,  for  compounding  of  alternators  fell  into  disuse  shortly 
after  this. 


DEVELOPMENT  OF  THE  A  C.  GENERATOR  625 

The  usual  method  of  compounding  on  the  early  alternators 
was  simple  series-current  compounding,  just  as  in  direct-current 
apparatus.  Where  the  commutated  current  was  supplied  directly 
to  the  field  compound  winding,  voltages  of  about  30  to  60  volts 
were  most  common  at  rated  full  load.  With  much  higher  than  60 
volts,  there  was  a  liability  of  short-circuiting  the  compounding  by 
arcing  between  bars  on  the  commutator  There  was  also  a  liability 
of  arcing  or  flashing  when  the  phase  of  the  current  shifted  due  to 
change  in  power-factor. 

When  polyphase  rotating  armatures  came  into  use,  similar 
methods  of  compounding  were  resorted  to.  However,  the  second- 
ary current  was  a  resultant  of  the  two,  or  three  primary  currents, 
for  each  of  the  primary  phases  was  carried  around  the  compensat- 
ing transformer  (or  spokes  of  the  armature)  and  the  secondary 
winding  carried  a  current  in  phase  with  the  resultant  of  the  primary 
ampere-turns.  In  the  case  of  three-phase  windings,  the  direction 
of  one  lead  was  reversed  around  the  compensating  transformer. 
Some  curious  conditions  arose  from  the  phase  relations  of  the 
secondary  current  when  parallel  operation  was  practiced.  It  was 
necessary,  when  paralleling  the  main  winding,  to  parallel  also  the 
compound  winding.  As  the  compounding  current  from  each 
machine  pulsated  from  zero  to  maximum  value  in  each  alternation, 
it  was  necessary  to  so  parallel  the  terminals  that  all  the  commut- 
ated currents  had  zero  value  at  the  same  instant,  otherwise,  the 
brushes  on  one  commutator  would,  at  times,  short-circuit  the  cur- 
rent from  the  other  commutator. 

With  the  advent  of  larger  belted  machines,  and  of  engine-type 
machines  in  particular,  the  compounding  of  polyphase  machines 
was  more  or  less  unsatisfactory  and  was  practically  abandoned. 
To  compensate  for  the  lack  of  compounding,  bettei  inherent  regu- 
lations were  aimed  at  in  the  designs.  This  meant,  primarily, 
machines  which  would  give  comparatively  large  currents  on  steady 
short-circuit,  three  to  four  times  full  load  being  rather  common,  and 
even  six  times  full  load  being  attained  in  some  machines.  The 
momentary  current  rush  at  the  instant  of  short-circuit  must  have 
been  excessive  on  some  of  these  machines,  due  to  their  very  low 
armature  self-induction.  However,  due  to  the  relatively  small 
ampere-turns  per  pole,  no  very  destructive  distortions  were  found 
in  practice.  This  characteristic  of  the  short-circuit  currents  was 
carried  into  the  rotating  field  construction,  and  even  into  the 
early  turbo-generator  work. 


626  ELECTRICAL  ENGINEERING  PAPERS 

This  practice  o£  giving  the  alternators  good  inherent  regula- 
tion was  expensive  in  a  number  of  ways,  as  it  usually  meant 
higher  iron  losses  and  less  output  than  was  possible  otherwise,  with 
a  given  size  machine,  or  a  given  amount  of  material.    Even  at  this 
early  date,  it  was  recognized  that  some  form  of  automatic  field 
current  regulator  which  would  maintain  the  terminal  voltage  con- 
stant, regardless  of  the  inherent  regulation  would  be  a  very  useful 
piece  of  apparatus.    Some  form  of  regulation  which  would  take 
care  of  change  in  power-f  actor,  as  well  as  load,  was  the  aim  of  many 
designers.    Among  the  different  schemes  brought  out,  the  Rice 
method  of  compounding,  brought  out  by  the  General  Electric 
Company,  is  of  interest.    This  was  used  principally  with  rotating 
field  alternators.    In  this  scheme,  the  exciter  was  usually  placed 
on  the  same  shaft  as  the  alternator  field,  and,  in  such  case,  had 
the  same  number  of  poles  as  the  alternator.    The  leads  from  the 
alternator  armature  were  carried  through  the  exciter  winding  in 
such  a  way  that  a  lagging  current,  carried  by  the  alternator, 
tended  to  strengthen  the  field  of  the  exciter  by  shifting  the  arma- 
ture reaction  with  respect  to  the  exciter  field  poles.     In  this  way  a 
compounding  action  on  the  exciter  was  obtained  which  was  prac- 
tically in  proportion  to  the  demands  of  the  alternator  field  with 
varying  power-factor.    In  the  case  of  engine-type  machines  of 
comparatively  low  speed,  the  exciter  was  geared  to  the  alternator 
shaft,  so  that  it  ran  at  a  considerably  higher  speed  and  the  number 
of  poles  in  the  exciter  was  correspondingly  reduced 

This  method  of  compounding  was  effective,  but  the  whole 
combination  was  apparently  unduly  complicated  and  expensive. 
Furthermore,  it  did  not  give  the  desired  compensation  under  all 
conditions  of  operation,  as  it  would  not  correct  for  changes  in 
speed. 

A  later  method  of  compensation  for  power-factor  was  devised 
by  Alexanderson,  and  was  used  on  a  limited  number  of  General 
Electric  machines.  In  this  scheme  a  derived  current  from  the 
alternator  itself  was  commutated  in  such  a  manner  that  compensa- 
tion, proportional  to  the  power-factor,  was  obtained.  This  was  a 
purely  self-excited  alternator  scheme  and,  like  all  self-exciting 
schemes  in  such  apparatus,  it  was  sensitive  to  speed  changes, 
probably  to  a  much  greater  extent  than  the  Rice  arrangement 
above  described.  A  fundamental  defect  in  all  self -exciting  ^com- 
pensated alternator  schemes  lies  in  the  fact  that  stability  of  ex- 
citation is  dependent  upon  having  considerable  saturation  in  the 


DEVELOPMENT  OF  THE  A  C   GENERATOR  627 

alternator  magnetic  current  and,  coincidently,  if  there  is  such 
saturation,  the  compound  current  has  no  direct  relation  to  the 
load  or  power-factor.  Thus  such  machines  are  either  sensitive  to 
speed  changes,  or  their  compounding  is  only  approximate. 

Following  these  schemes  came  the  use  of  automatic  regulators 
of  which  the  Tirrill  is  best  known.  This  regulator  acts  directly  on 
the  exciter  field  by  short-circuiting  a  resistance  in  series  with  the 
field  winding,  the  range  of  exciter  voltage  being  controlled  by  the 
length  of  time  the  rheostat  is  short-circuited.  Instead  of  cutting 
the  resistance  out  in  steps,  which  tends  to  give  sluggish  action  in 
the  fields,  the  Tirrill  regulator  cuts  the  whole  resistance  out  each 
time,  and  the  length  of  time  is  varied.  This  results  in  quick 
action.  As  the  regulator  tends  to  hold  constant  voltage  at  the 
alternator  terminals,  or  on  the  line,  change  in  power-factor  or  in 
speed  does  not  modify  the  action.  This  type  of  regulator  has 
proven  very  effective,  especially  in  the  case  of  alternators  sub- 
jected to  sudden  and  violent  changes  in  load,  power-factor  and 
speed. 

With  the  advent  of  larger  alternator  units,  in  proportion  to 
the  changes  in  load,  the  inherent  regulation  has  been  made  relat- 
tively  poorer,  primarily  because  better  machines  otherwise  are 
thus  obtained.  The  short-circuit  currents  are  reduced,  and 
relatively  lower  iron  losses,  and  lower  temperatures  or,  higher  out- 
puts with  a  given  temperature,  are  obtained.  This  has  been  car- 
ried further  in  turbo-generator  design  than  in  any  other  class  of 
alternators,  due  partly  to  fundamental  limitations  in  design. 
However,  this  poorer  inherent  regulation  has  proven  to  be  of  no 
practical  importance,  where  suitable  automatic  regulators  have 
been  used  with  the  machines. 

One  fallacy  which  was  frequently  found  in  the  past,  and  which 
still  persists  to  some  extent,  is  that  alternators  should  have  equal 
inherent  regulation  to  parallel  properly.  This  is  based  partly  on 
the  feeling  that  the  field  currents  of  the  alternators  should  vary 
over  equal  range  when  carrying  their  proper  proportion  of  load, 
together  with  the  knowledge  that  the  variations  in  field  current 
are  dependent,  to  some  extent,  upon  the  inherent  regulation. 
However,  the  fact  that  the  shape  of  the  saturation  curve,  in  a 
given  alternator,  may  have  much  more  influence  on  the  excitation, 
especially  at  high  saturations,  is  usually  overlooked. 


628  ELECTRICAL  ENGINEERING  PAPERS 

TURB  O-GENERATORS 

The  advent  of  the  turbo-generator  has  had  a  predominant 
influence  on  alternator  design.  After  the  turbo-alternator  once 
became  established  commercially  in  this  country,  it  quickly  revo- 
lutionized conditions  by  driving  the  large  engine-type  alternators 
out  of  the  field.  The  evolution  of  all  electrical  apparatus  has  been 
comparatively  rapid,  but  that  of  the  turbo-alternator  has  possibly 
exceeded  anything  else  in  the  electrical  field.  This  evolution 
therefore  merits  a  fairly  complete  description. 

The  first  turbo-alternators  built  by  the  Westinghouse  Com- 
pany, were  installed  in  the  power  plant  of  the  Westinghouse  Air 
Brake  Company  about  1898.  These  were  three  rotating  armature 
machines  of  300  kilowatts  capacity,  which  ran  at  a  speed  of  3  600 
r.  p  m  ,  giving  7  200  alternations  per  minute,  or  60  cycles  per 


PIG.  20— FIELD  OF  EARLY  ROTATING  ARMATURE  TURBO-GENERATOR 

second.  They  were  coupled  to  Parsons  turbines,  built  by  The 
Westinghouse  Machine  Company.  The  Parsons  Company  in 
England  had  been  building  rotating  armature  alternators  for  a 
number  of  years,  and  the  Westinghouse  Company  simply  followed 
the  Parsons'  precedent.  These  first  machines  were  operated  for 
several  years,  but  it  was  obvious,  soon  after  their  installation, 
that  the  rotating  armature  type  of  machine  would  not  serve  for 


DEVELOPMENT  OF  7 HE  A  C    GENERATOR 


629 


general  turbo-alternator  purposes.  It  was  evident  that,  for 
voltages  even  no  higher  than  2  200,  the  rotating  armature  con- 
struction, at  the  necessary  turbo-generator  peripheral  speeds, 
would  become  almost  impracticable.  Attention  therefore  was 
soon  turned  toward  a  3  600  revolution,two-pole,  rotating  field  type, 


FIG.  21.— ARMATURE  FOR  TURBO-GENERATOR  OF  THE  TYPE  SHOWN 

IN  FIG.  20 

and  a  very  large  number  of  possible  constructions  were  con- 
sidered. Finally  one  like  that  shown  in  Fig.  23  was  worked  out 
and  built  in  1899.  This  had  the  field  windings  completely  em-, 
bedded  in  a  number  of  parallel  slots,  with  supporting  metal  wedges 


FIG.  22—1  000  KW  OPEN  TYPE  TURBO-GENERATOR 

at  the  tops  of  the  grooves  or  slots.  One  machine  of  this  type  was 
built  and  tested.  It  operated  in  a  satisfactory  manner,  except  as 
regards  windage  and  noise.  The  machine  was  not  closed  at  the 
ends,  like  modem  turbo-alternators,  and  thus  any  noise  generated 
in  the  machine  could  be  readily  transmitted  to  the  outside.  The 
noise  was  caused  largely  by  the  two  flat  sides  of  the  rotor.  It  was 


630 


ELECTRICAL  ENGINEERING  PAPERS 


so  shrill  and  penetrating  that  it  was  very  disagreeable  to  be  around 
the  machine,  and  was  even  painful  to  the  ears  after  a  short  time. 
This  construction  was  therefore  abandoned  temporarily,  but  after 
a  few  months  it  was  taken  up  again  and  a  new  rotor  was  built 
which  was  entirely  round,  as  shown  in  Fig.  24,  but  was  otherwise 
very  similar  to  that  shown  in  Fig.  23.  This  new  rotor,  although 
noisy  compared  with  modern  machines,  was  so  quiet,  compared 
with  the  first  construction,  that  it  was  immediately  adopted  as  a 
standard  construction.  This  is  the  now  well-known  parallel  slot 
construction  which  has  been  used  very  extensively  by  the  Westing- 
house  Company,  although  many  very  radical  changes  have  been 
made  in  the  constructive  features  of  the  rotor  itself.  This  type  of 
rotor  was  used  originally  only  for  the  400  kilowatt  size  at  60  cycles. 
In  the  earlier  machines  of  this  type  a  number  of  very  curious 
conditions  developed.  In  the  first  machines  the  rotors  were  built 
of  a  number  of  thick  discs  or  "cheeses"  side  by  side,  which  were 
put  on.  the  shaft  at  high  pressure.  The  two  end  discs  were  thicker 
than  the  others  in  order  to  accommodate  the  grooves  in  which  the 
rotor  end  windings  lay.  The  discs  were  made  of  high  grade  f org- 


o 


PIG.  23— -EARLY  TWO-POLE  ROTATING  FIELD 


ings.  After  some  of  these  machines  had  been  in  operation  for  a 
•considerable  period  it  was  found  that  sotne  of  the  discs  in  the  field 
core  were  traveling  axially,  i.e.,  quite  appreciable  gaps  or  spaces 
were  showing  between  adjacent  discs.  In  one  instance  they  trav- 
eled to  such  an  extent  that  the  field  windings  were  stretched 
longitudinally  at  the  openings  between  the  discs,  until  the  con- 
ductors were  actually  attenuated  to  an  extent  visible  to  the  eye. 
Obviously,  the  stretching  force  must  have  been  enormous. 


DEVELOPMENT  OF  THE  A  C   GENERATOR  031 

Eventually,  the  construction  was  changed  on  these  two-pole 
rotors  to  a  single  disc  of  forged  steel.  Still  later,  steel  castings 
were  used  quite  extensively  instead  of  forgings  although,  later 
still,  the  castings  were  abandoned  in  favor  of  forgings.  There  was 
much  adverse  opinion  regarding  the  advisability  of  using  castings 
for  the  3  600  revolution  machines,  as  some  engineers  held  that 
they  were  more  liable  to  contain  flaws  than  would  be  the  case  with 
forgings.  An  interesting  fact  in  connection  with  this  is  that,  while 
a  number  of  these  early  high  speed  machines  "  exploded,7'  gener- 
ally during  runaways,  yet  in  no  instance  was  a  cast  steel  field  wreck- 
ed from  this  cause.  This,  however,  does  not  constitute  a  proof 
of  the  superiority  of  cast  steel,  for  it  so  happened  that  all  the 
serious  runaways  were  on  machines  with  forged  rotors.  However, 


o 


PIG.  24— ROUND  TYPE  TWO-POLE  ROTOR 

the  record  is  a  clear  one  as  far  as  cast  steel  fields  are  concerned, 
for,  of  all  the  sizes  and  speeds  of  steel  rotors  which  the  Westing- 
house  Company  has  put  out,  not  a  single  cast  steel  disc  has  burst. 
Present  speed  and  output  requirements  have  now  carried  the 
construction  up  to  a  point  where  special  forged  materials  are  the 
accepted  practice. 

Soon  after  the  two-pole,  400  kilowatt  rotating  field  machine 
was  put  on  the  market,  a  four-pole,  750  kilowatt,  1  800  revolution 
machine  was  built.  The  rotor  of  this  machine  had  four  salient  poles 
bolted  on.  These  poles  were  provided  with  overhanging  pole  tips, 
and  the  field  winding  consisted  of  four  coils  wound  with  strap-on- 
edge.  In  fact,  this  first  construction  was  very  similar  to  the 
present  type  of  rotor  fields  now  used  for  other  than  turbo  work. 
This  construction  proved  difficult  and  expensive,  but  was  applied 
to  a  number  of  six-pote,  1 200  revolutiop  machines.  However, 


632 


ELECTRICAL  ENGINEERING  PAPERS 


the  parallel  slot  construction  used  in  the  two-pole  machines  was  so 
satisfactory  that  it  was  soon  adopted  for  the  four  and  six-pole 
machines,  as  shown  in  Fig.  25 .  In  the  six-pole  machine  it  was  not 
possible  to  make  the  poles  integral  with  the  central  core,  on 
account  of  the  inability  to  machine  the  parallel  slots  in  the  sides  of 


PIG,  25— PARALLEL-SLOT  FOUR-POLE  AND  SIX-POLE  FIELD 
CONSTRUCTION 


the  poles,  or  to  put  in  the  windings.  Therefore,  separate  poles  were 
constructed,  with  parallel  slots,  and  these  were  first  wound  and 
then  bolted  into  place  on  the  central  core  which,  in  this  case,  waa 
made  integral  with  the  shaft.  The  four-pole  machine  was  con- 
structed for  750  and  1 000  kilowatts  capacity,  and  the  six-pole  con- 
struction was  made  for  1  500  to  3  000  kilowatts. 

Meanwhile,  there  had  grown  up  some  demand  for  moderate 
capacity  25-cyde  machines  at  1  500  revolutions.  These  were 
constructed  along  exactly  the  same  lines  as  the  two-pole,  3  600 
revolution  machines  above  described. 

In  this  early  work  one  order  for  four  5  500  kw,  four-pole,  1  000 
revolution  machines  was  taken.  This  was  entirely  beyond  the 
constructions  undertaken  before  by  the  Westinghouse  Company. 
The  parallel  slot  type  of  rotor  was  adopted.  An  attempt  was  made 


DEVELOPMENT  OF  THE  A  C.  GENERATOR 


633 


to  get  forgings  in  a  single  piece  large  enough  for  these  rotors,  but 
they  were  found  to  be  glass  hard  and  brittle,  except  at  the  outer 
surface.  As  very  large  steel  castings"  were  frowned  upon,  it  was 
decided  to  make  these  rotors  of  discs  turned  out  of  very  thick  steel 
plates,  somewhat  like  the  early  400  kw  machines  already  de- 
scribed. Parallel  slots  were  used  as  in  the  smaller  four-pole 
machines.  This  construction  proved  to  be  feasible  but  was  very 
expensive,  and  shortly  after  this,  large  cast  steel  discs  were  used, 
two  discs  side  by  side  being  used  to  form  one  rotor.  This  con- 
struction was  satisfactory,  and  was  used  for  many  years 


7 


7 


FIG.  26— TWO-POLE  FIELD  OF  THE  BOLTED  ON  CONSTRUCTION 

Shortly  after  turbo-generators  came  into  general  use,  there 
was  considerable  complaint  regarding  the  noise  due  to  windage. 
All  these  machines  were  equipped  with  some  form  of  ventilating 
device,  which  either  formed  part  of  the  normal  construction  of  the 
rotor  or  consisted  of  some  special  blowing  device  at  the  ends  of  the 
rotor.  Both  the  high  speed  and  the  large^quaixtity  of  cooling  air 


634  ELECTRICAL  ENGINEERING  PAPERS 

required,  tended  to  make  a  noise  which  was  very  objectionable. 
A  series  of  experiments  with  covers  over  various  parts  of  the 
machines,  showed  that,  by  completely  enclosing  the  two  ends  of 
the  machine  and  by  enclosing  the  field  frame  except  at  the  top  and 
bottom,  (in  a  horizontal  machine)  the  windage  noise  could  be  so 
deadened  as  to  be  practically  unobjectionable.  However,  the 
tests  also  showed  that  artificial  ventilation  was  necessary  under 
this  condition.  This  very  quickly  led  to  the  practice  of  enclosing 
and  artificially  cooling  turbo-generators,  which  practice  has  been 
maintained  to  this  day.  The  first  Westinghouse  enclosed  ma- 
chines  were  built  about  1903. 

The  use  of  artificial  cooling  marked  a  great  step  in  advance  in 
turbo-generator  work,  for  the  results  indicated  that,  by  supplying 
a  sufficient  quantity  of  air  and  properly  distributing  it  through 
the  machine,  very  marked  increase  in  capacity  was  possible,  and  a 
point  was  soon  reached  where  the  possible  capacities  were  beyond 
the  mechanical  limitations  of  the  construction.     This  led  to  radical 
modifications  in  the  type  of  rotor,  with  a  view  to  taking  advantage 
of  the  increased  capacity.      Apparently  all  manufacturers  did 
more  or  less  development  work  along  such  lines.    In  the  Westing- 
house  constructions,  the  use  of  a  through  shaft  was  found  to  be 
one  of  the  serious  limitations,  and  this  led  to  types  of  rotors  with- 
out any  through  shaft.     In  the  two-pole  machines,   this  was 
particularly  important,  and  the  problem  was  especially  difficult 
with  the  parallel-slot  construction,  provided  ample  space  was  al- 
lowed for  the  field  winding.    The  old  through-shaft  two-pole  con- 
struction lost  considerable  winding  space,  due  to  the  shaft-  space, 
as  shown  before  in  Fig,  24.    Attempts  to  construct  such  a  machine 
with  the  shaft  forming  part  of  the  core,  resulted  in  still  less  ef- 
ficient use  of  the  possible  winding  space.     It  was  obvious  that  if  the 
whole  possible  winding  space  were  taken  up  with  slots,  then  the 
capacity  of  the  field  winding  would  be  greatly  increased.     In 
consequence,  a  rotor  construction,  such  as  shown  in  Pig,  26,  was 
designed  and   constructed.    In   this,   bronze   end  supports  or 
"heads"  were  bolted  to  each  end  of  the  field  core,  and  the  shaft 
proper  was  attached  to  these  bronze  heads.    Bronze,  or  a  similar 
non-magnetic  material,  -was  necessary  to  prevent  magnetic  short- 
circuiting  of  the  field  flux,     This  design  was  constructed  and  tested 
on  a  1  000  k.v.a.,  3  600  revolution  machine,  and  then  was  built 
successively  for  1  500,  2  000,  3  000,  4  000  and  5  000  k.v.a.  machines, 
all  at  3600  r.  p.  m.    The  same  construction  was  also  applied  to 


DEVELOPMENT  OF  THE  A  C   GENERATOR  635 

two-pole  machines  of  25  cycles,  up  to  12  000  k.v.a.  capacities. 
This  construction  of  rotor  has  given  an  extremely  good  account  of 
itself.  However,  it  proved  to  be  expensive  on  small  capacity  ma- 
chines, as  the  bronze  heads  formed  an  undue  proportion  of  the 
cost  of  material.  For  higher  capacities  of  3  600  r.  p.  m.  machines, 
increase  in  capacity  is  obtained  largely  by  increasing  the  length  of 
the  rotor  core,  and  thus  the  bronze  heads  form  a  relatively  lower 
percentage,  and  the  construction  becomes  more  reasonable  in  cost. 

From  the  preceding,  it  may  be  seen  that  only  two  types  of 
turbo-generators  have  been  used  very  extensively,  namely,  the 
parallel-slot  type  and  the  radial-slot  type.  Each  of  these  types 
has  some  very  pronounced  advantages.  The  principal  advantage 
of  the  parallel-slot  type  is  in  the  arrangement  and  support  of  the 
field  coils.  Each  coil  can  be  wound  directly  in  place,  with  the 
conductor  under  tension,  and  the  finished  winding  is  completely 
encased,  and  is  thoroughly  protected  against  dirt,  movement  of  the 
conductors,  etc.  Against  this,  the  radial-slot  machine  allows  more 
room  for  copper,  and  is  magnetically  more  economical  in  material. 
However,  the  field  windings  are  more  difficult  to  apply  and  must 
be  supported  at  the  ends  by  auxiliary  means,  such  as  separate 
external  steel  rings. 

The  enormous  increase  in  output  of  turbo-generators,  within 
very  recent  years,  has  made  the  electric  and  magnetic  proportions 
of  the  rotors  a  feature  of  first  importance  in  the  design,  so  that  the 
radial-slot  type  for  two-pole  machines  has  become  the  standard 
construction,  almost  universally.  This  will  be  referred  to  again 
under  the  four-pole  construction. 

While  the  two-pole  parallel  slot  construction  was  being  de- 
veloped for  larger  capacities,  the  four-pole  construction  for  60 
cycle  machines  has  been  pushed  up  to  capacities  of  about  12  000 
k.v.a.  with  the  parallel  slot,  cast  steel  rotors.  In  order  to  do  away 
with  the  through-shaft  construction,  the  rotor  was  made  of  two 
castings  or  discs,  each  of  which  was  cast  solid  with  the  shaft,  as 
shown  in  Fig.  27.  The  two  discs,  after  machining,  were  bolted 
together  by  a  number  of  very  heavy  bolts  located  near  the  pole 
tips  and,  in  some  cases,  shrink  links  were  placed  in  the  pole  face, 
connecting  the  two  halves  together.  The  parallel  grooves  were 
then  machined  in  the  steel  core,  just  as  in  the  through-shaft  type. 
In  this  four-pole  construction  the*  problem  of  armature  ventila- 
tion was  comparatively  simple.  Air-gap  ventilation  (that  is,  all 
air  through  the  armature  core  supplied  froqa  air-gap)  was  easily 


636 


ELECTRICAL  ENGINEERING  PAPERS 


accomplished,  due  to  the  open  spaces  between  the  poles,  which 
could  admit  an  ample  air  supply.  However,  the  same  construction 
tended  toward  high  windage  losses  due  to  air  "churning." 


T                       ^  -*  " 

r--r--r:-rzr[ 

«___._«._  p< 

r 

PIG.  27— FIELD  CONSTRUCTION  WITH  TWO  HALVES  HELD  TOGETHER 
BY  HEAVY  BOLTS 

This  problem  of  ventilation  has  had  much  to  do  with  the 
evolution  of  turbo-generator  design.*  In  the  two-pole,  parallel  slot 
machine  for  3  600  r.  p.  m.,  in  which  the  diameter  of  the  rotor  is 
relatively  small,  the  amount  of  air  which  can  be  forced  into  the 
air-gap  from  each  end  is  rather  limited.  Assuming,  for  example, 
a  rotor  diameter  of  24  inches,  which  is  almost  as  large  as  we  can 
go  for  a  3  600  r.  p.  m.  machine,  then,  with  an  air-gap  (iron  to  iron) 
of  %-inch,  which  is  also  a  fairly  large  gap,  the  total  cross-section  of 
the  air  inlet  at  the  air-gap  at  both  ends  of  the  rotor  will  be  1 12  sq. 
in.  With  the  very  high  air  velocity  of  10  000  ft.  per  minute,  this 
means  a  total  air  supply  of  less  than  8  000  cu.  ft.  per  minute.  This 
may  be  sufficient  for  a  moderate  capacity  turbo-generator,  but 
for  machines  of  high  capacities,  such  as  3  000  to  5000  k.v.a.,  this  is 
not  nearly  enough  cooling  air.  Obviously,  either  much  larger 
inlets  through  the  air-gap  are  required,  or  some  additional  method 
of  cooling  is  necessary.  Larger  air-gaps  usually  mean  either  more 
expensive  machines,  or  reduced  output  with  a  given  machine,  due 
to  lower  flux  densities.  Therefore,  the  tendency,  in  machines  of 
the  very  high  capacities,  and  very  high  speeds,  has  been  toward  a 
combination  of  air-gap  with  other  methods  of  ventilation.  In  the 
25  cycle,  two-pole  machine  with  a  maxicnum  speed  of  1  500  r.p.m., 
rotors  of  larger  diameter  are  possible  and,  as  a  rule,  much  larger 
air-gaps  are  practicable  than  in  60  cycle  machines.  In  conse- 
quence, air-gap  ventilation  comes  nearer  being  practicable  but  in 


*A  more  complete  exposition  of  the  subject  of  "Turbo-Alternator   Ventilation,  "etc.r 
Is  contained  in  the  paper  on  page  313. 


DEVELOPMENT  OF  THE  A.C.  GENERATOR  637 

the  larger  capacities,  even  this  is  insufficient  and  auxiliary  methods 
have  been  necessary  in  some  cases. 

This  need  for  auxiliary  methods  of  ventilation  led  to  the  axial 
method  of  ventilating  armature  cores  in  distinction  from  the 
radial  method,  in  which  the  air  was  carried  out  through  numerous 
radial  air  ducts  or  passages.  In  the  axial  method,  a  large  number 
of  ventilating  holes  are  arranged  in  the  armature  core  parallel  to 
the  axis  of  the  machine.  These  form  ventilating  paths  in  parallel 
with  the  air-gap  path.  With  the  small  diameter  long  cores 
necessary  for  3  600  r.  p,  m.,  high  capacity  machines,  the  develop- 
ment of  this  method  of  ventilation  was  contemporaneous  with  the 
development  of  the  higher  capacities.  The  same  has  proved  to  be 
the  case  for  the  later  types  of  Westinghouse  four-pole,  60  cycle, 
1  800  r.  p.  m.  machines,  which  departed  very  considerably  in  rotor 
construction  from  the  four-pole  cast  steel  type  already  described. 

As  the  capacities  of  the  3  600  r.  p.  m  ,  60  cycle  machines  were 
gradually  pushed  up,  a  corresponding  development  occurred  in  the 
1800  r.  p.  m.  machines.  At  10  000  to  12  000  k.v.a.,  the  four-pole 
cast  steel  construction  was  apparently  approaching  its  limits. 


FIG.  28— MODERN  ROTATING  FIELD  ON  BALANCING  WAYS 

For  larger  sizes,  therefore,  a  different  construction  was  adopted 
which  allowed  more  suitable  material  to  be  obtained.  For  the 
largest  diameters  and  highest  speeds,  a  plate  construction  was 
adopted  by  the  Westinghouse  Company,  in  which  the  end  discs 
and  the  shaft  ends  were  forged  as  units,  and  the  intermediate  discs 
were  made  of  rolled  plate  material,  the  whole  construction  being 
bolted  together  permanently  to  form  a  solid  core.  This  core  was 
then  slotted  with  radial  slots,  and  the  usual  radial  slot  type  of  field 
winding  is  used.  A  similar  construction  was  adopted  on  the  larger 
25  cyde  machines.  For  intermediate  capacities,  both  60  and  25 


638 


ELECTRICAL  ENGINEERING  PAPERS 


cycles,  solid  discs  are  used  in  some  cases  instead  of  the  plate 
construction,  This  brings  the  larger  turbo  development  up  to 
the  present  date. 

In  the  comparatively  small  60  cycle  turbo-generators,  where 
the  parallel  slot  construction  with  the  bronze  driving  heads  was 
relatively  expensive,  as  already  described,  the  later  development 
has  been  towards  core  and  shaft  forged  in  one  piece,  and  with 


FIG.  29— STATOR  OF  6Z5  K.V.A.,  2  300  VOLTS,  3  600  R.  P.  M.. 
TURBO-GBNERATOR 

With  aerial  ventilation  and  central  duct.    Supporting  ring  both  inside  and  outside  thfc 
end  windings.    Typical  method  of  bracing  smaller  machines. 

radial  slots,  and  eventually  this  construction  may  be  carried  up 
to  the  largest  practicable  size  of  3  600  r.  p.  m,  machines.  It  is  diffi- 
cult to  predict  the  limit  in  capacity  which  may  be  reached  even- 
tually in  3  600  r.  p.  m.  generators,  but  6  250  k.v.a.  appears  to  be 
practicable. 

Some  special  radial-slot  machines  had  been  developed  for  the 
New  Haven  Railroad  about  1907.  As  these  machines  were  de- 
signed to  deliver  25  cycle  single-phase  current,  and  as  the  pulsating 
armature  reaction  of  *such  machines  would  be  relatively  high,  the 
rotors  were  designed  with  laminated  cores,  with  a  view  to  lessening 
core  losses.  The  rotors  were  made  of  single  disc  laminations  shrunk 
on  the  shaft.  The  discs  were  provided  with  radial  slots.  The  con- 
struction was  very  similar  to  the  later  radial-slot  rotors,  except 
that* the  rotor  end  windings  were  also  embedded  in  slots,  and 


DEVELOPMENT  OF  THE  A.C   GENERATOR  630 

supported  by  wedges  embedded  in  the  periphery  of  the  core, 
whereas,  in  the  later  radial-slot  rotors,  the  end  windings  are  sup- 
ported by  external  rings.  These  early  radial-slot  rotors  showed 
very  considerable  overheating  in  single-phase  operation,  and  it 
was  found  necessary  to  apply  a  very  complete  cage  damper  em- 
bedded in  the  periphery  of  the  rotor.  Later  experience  showed  that 
the  solid-core  parallel-slot  rotor  with  an  equal  damper  applied  to 
its  surface  was  just  as  effective,  and  many  of  the  later  single-phase 
machines  were  built  in  this  manner.  However,  some  recent  1 1  250 
k.v.a.  single-phase  generators  are  being  built  of  the  plate  construc- 
tion already  described, 

REGULATION  AND  SHORT-CIRCUIT  CURRENTS  OP  TURBO-ALTER- 
NATORS 

Like  the  ordinary  synchronous  generator,  the  modern  turbo- 
alternator  is  designed  with  a  comparatively  high  inherent  regula- 
tion. In  fact,  in  order  to  avoid  excessive  short-circuit  currents, 
the  inherent  regulation  must  be  made  comparatively  poor  by 
making  the  armature  self-induction  as  high  as  practicable.  Even 
under  the  best  condition,  such  machines  are  liable  to  give  12  to 
15  times  rated  current  during  the  first  current  rush.  Furthermore, 
the  solid  plates  or  discs,  of  which  most  turbo-rotors  are  now  made, 
tend  to  prolong  the  period  of  maximum  short-circuit  current.  The 
consequence  of  these  conditions  is  a  tremendous  racking  force 
acting  on  the  end  windings  during  a  short-circuit  current  rush, 
which  tends  to  distort  the  winding  badly  unless  it  is  very  strongly 
braced.  The  Westinghouse  Company  encountered  such  a  diffi- 
culty on  some  of  their  earliest  turbo-alternators  and  there  has  been 
a  practically  continuous  development  along  the  lines  of  more 
substantial  bracing  which  has  kept  pace  with  the  increased  require- 
ments of  the  higher  speeds  and  the  higher  capacities.  The  bracing 
used  on  the  modern  machines  is  designed  to  resist  distortion  of  the 
end  windings,  under  dead  short-circuit,  without  reactances  inter- 
posed, and  each  new  size  as  it  is  developed  is  given  such  a  short* 
circuit  test.  A  20  000  k.v.a*  60  cycle,  1 800  r.  p.  m,  high  voltage 
alternator  was  recently  subjected  to  such  short-circuit  tests  at  full 
voltage  without  injtiry. 

The  preceding  gives  a  brief  history  of  tba  developmeixt  of  the 
turbo-geaerator,  insofar  as  carried  out  by  the  Westinglxouse  Com- 
pany. The  General  Electric  Company  went  through  a  correspoxid- 


640 


ELECTRICAL  ENGINEERING  PAPERS 


ing  course  of  development,  in  general,  although  not  in  the  specific 
constructions  described,  and  a  number  of  interesting  types  or  con- 
structions have  been  brought  out.  The  gradual  increase  in  speed 
has  undoubtedly  had  much  to  do  with  the  evolution  of  their  various 
types,  just  as  in  the  case  of  the  Westinghouse  evolution.  One  of 
the  most  radical  steps  which  the  General  Electric  Company  has 
made  in  the  past  few  years  is  in  the  change  from  the  vertical  to  the 
horizontal  type  of  machines.  Presumably  the  very  high  speeds 
which  later  came  into  use  have  had  much  to  do  with  this  change. 
In  the  earlier  turbo-generator  practice,  the  speeds  of  the  General 
Electric  Company's  machines  were  relatively  lower 'than  the 


PIG.  30— SAME  MACHINE  AS  SHOWN  IN  PIG,  29  BEFORE  WINDING 


Westinghouse,  presumably  on  account  of  the  type  of  steam  turbine 
used.  Many  of  the  early  rotor  constructions  were  of  the  salient 
pole  type  for  four  poles  and  higher.  Gradually  these  were  super- 
seded by  constructions  leading  up  to  a  radial-slot  type  iti  which 
the  slots  were  formed  by  teeth  inserted  in  dovetail  grooves  in  the 
rim  of  the  spider.  This  type  was  very  similar  in  appearance  to  the 
later  types,  except  that  the  slots  had  overhanging  tooth  tips,  thus 
giving  a  partially  closed  slot  construction.  More  recently,  with 
greatly  increased  speeds,  this  construction  has  been  superseded  by 
solid  forged  cores  with,  shaft  forged  on,  and  with  radial  grooves 
milled  in  the  surface  for  the  field  winding.  These  latter  rotors  are 
used  largely  in  the  horizontal  type  high  speed  machines, 


DEVELOPMENT  OF  THE  A.C   GENERATOR  641 

In  the  Allis-Chalmers  construction,  in  the  larger  machines 
having  four  or  more  poles,  the  earlier  construction  of  the  rotor 
consisted  of  forged  discs  with  through  shafts.  These  discs  had 
radial  slots  for  the  windings  very  similar  to  the  present  construc- 
tion of  all  manufacturers.  The  smaller  machines  generally  had 


PIC.  31—STATQR  OP  LARGE  MODERN  TURBO-ALTERNATOR 


cores  forged  in  a  single  piece  with  the  shaft,  and  with  radial 
windings.  As  regards  methods  of  ventilation,  both  the  General 
Electric  and  Allis-Chalmers  Companies  went  through  a  course 
of  development  leading  up  to  their  present  practices.  " 

As  regards  parallel  operation,  turbo-generators  have  been 
particularly  free  from  this  old  time  difficulty,  due  largely  to  the 
uniform  rotative  effort  of  the  steam  turbine,  and  partly  to  the  high 
flywheel  capacity  of  the  turbo-generator  and  turbine  rotors,  which 
tends  to  limit  any  speed  oscillations,  due  to  the  governors,  to  a 
relatively  low  period,  such  as  would  not  tend  to  accentuate  the 
hunting  action  in  the  generators  themselves*  Moreover,  in  those 
rotors  which  have  been  built  with  solid  cores  or  of  thick  plates,  the 
solid  material  tends  to  act  as  a  damper  circuit.  As  a  consequenoe, 
hunting  in  toto-gaaemtosrs,  or  <3ttfficulties  ia  parallel  nmnaag, 
have  been  eattremely  rare* 


642  ELECTRICAL  ENGINEERING  PAPERS 

INDUCTION  TURBO-GENERATORS 

This  type  of  turbo-generator  has  been  proposed  commercially 
a  number  of  times  during  the  past  ten  years,  and  a  few  installations 
have  been  built.  The  first  of  any  importance,  consisting  of  a  1  250 
kilowatt  generator  of  30  cycles,  1  800  r.p.  m.,  two  poles,  was  in- 
stalled in  the  plant  of  the  Baltimore  Copper  Smelting  &  Rolling 
Company.  This  generator  was  a  true  polyphase  induction  motor, 
direct  connected  to  a  steam  turbine.  The  construction  of  the 
generator  was  exactly  the  same  as  would  have  been  used  at  that 
time  in  a  two-pole,  1  800  r.  p.  m.  induction  motor  of  the  same 
capacity.  The  entire  load  of  this  machine  consisted  of  a  1  200 
kilowatt  rotary  converter  connected  directly  to  the  terminals  of 
the  generator.  The  generator  and  converter  were  brought  up  to 
speed  separately  and  the  rotary  converter,  with  its  field  excited, 
was  connected  directly  to  the  generator  and  furnished  the  excita- 
tion for  the  generator.  There  was  no  other  synchronous  apparatus 
in  circuit  except  the  converter. 

Several  years  ago,  a  number  of  much  larger  induction  gener- 
ators were  built  by  the  General  Electric  Company  for  the  Inter- 
borough  Rapid  Transit  Company  of  New  York  City.  These 
machines  are  of  6  000  k.v.a.  nominal  capacity  and  operate  in 
parallel  with  the  11 000  volt,  three-phase,  engine-type  generators 
previously  installed  in  the  same  power  house.  The  engine-type 
generators  furnish  the  excitation  for  the  induction  generators.  The 
entire  load  of  this  station  is  represented  by  rotary  converters. 
There  have  been  no  prominent  instances  of  the  use  of  induction 
generators  for  other  than  steam  turbine  drive.  Apparently  this 
type  of  generator  has  no  very  wide  field. 

CONCLUSION 

This  history  is  admittedly  far  from  complete,  in  that  it  has 
not  mentioned  the  work  of  some  of  the  earlier,  and  also  some  of 
the  later  manufacturing  companies.  The  field  is  far  too  large  to 
permit  everything  to  be  covered.  Moreover,  no  attempt  has  been 
made  to  describe  European  constructions  and  developments  in 
alternating-current  generators.  It  may  be  stated,  however,  that 
in  some  very  important  features,  European  engineers  antedated 
the  Americans,  while  in  other  equally  important  constructions 
American  designers  were  first  in  the  field.  As  a  rule,  the  represen- 
tatives of  the  electrical  manufacturing  companies  have  been  so 


DEVELOPMENT  OF  THE  A.C.  GENERATOR  CM 

wide  awake  and  ready  to  adopt  new  principles  when  they  con- 
tained any  promise,  that  it  is  sometimes  very  difficult  to  give  any 
company  or  individual  proper  and  deserved  credit  for  being  first 
in  any  given  development.  Furthermore,  no  attempt  has  been 
made  to  give  credit  to  the  various  engineers  who  have  been  closely 
identified  with  alternator  development,  for  it  would  be  impossible 
to  do  justice,  or  give  deserved  credit  to  all  of  them. 


THE  DEVELOPMENT  OF  THE  DIRECT-CURRENT 
GENERATOR  IN  AMERICA 

FOREWORD — This  history  is  not  merely  a  collection  of  facts  or  near 
facts  drawn  up  from  old  records  or  from  second-hand  leports, 
but  is  an  original  story  prepared  by  one  who  has  been  in  the 
thick  of  the  battle  almost  from  the  early  skirmishes,  almost 
thirty  years  ago.  This,  therefore,  might  be  called  a  reminis- 
cence, as  well  as  a  history.  Being  written  almost  entirely  from 
personal  observation  and  experience,  obviously  it  cannot  be 
considered  as  a  complete  history  of  direct-current  generator 
and  motor  development,  but  probably  no  other  individual 
in  the  country  could  write  as  complete  an  account  of  the  de- 
velopment, from  his  own  observation  and  contact  with  the 
work  itself.  This  subject,  broadly  considered,  should  cover  all 
kinds  of  direct-current  rotating  machines,  including  constant- 
current  arc  lighting  generators,  unipolar  generators,  etc.  How- 
ever, as  the  development  of  the  railway  motor  is  to  appear  in  a 
separate  article,  and  as  the  constant-current  generator  has  now 
become  commercially  obsolete,  or  practically  so,  the  scope  of 
the  following  article  is  limited  to  the  development  of  the  con- 
stant-potential generators  and  motors, 

This  article  was  first  published  in  the  Electric  Journal. — 
(ED.) 


HE  history  of  the  direct-current  machine  goes  so  far  back  that 
-*•  it  is  not  within  the  scope  of  this  paper  to  cover  the  earliest 
developments.  Many  of  the  earliest  machines  were  of  the  con- 
stant current  type  for  series  arc  lighting,  Doubtless  the  peculiar 
types  which  appeared  in  the  constant  current  arc  machines  im- 
pressed themselves  upon  the  early  constant  potential  generators, 
for  these  latter  were  about  as  numerous  in  type  and  construction 
as  the  arc  machines.  One  of  the  characteristic  features  in  the 
early  direct-current  design  was  the  radical  differences  in  construc- 
tion of  the  machines  built  by  different  designers  or  manufac- 
turers. In  fact>  every  designer  appeared  desirous  of  getting  out 
a  new  type  which  could  bear  his  name.  In  consequence  freak 
designs,  from  the  present  viewpoint,  were  much  more  common 
than  those  built  upon  sensible  principles  as  understood  to  a 
limited  extent  in  those  days*  Real  development  toward  the 
present  almost  universal  standard  types  did  not  take  place  until 
the  early  "cut  and  try'1  methods  of  design  were  superseded 
partly,  or  wholly,  by  calculations  based  upon  the  principles  of 
the  electric  and  magnetic  circuits. 

Aside  from  the  desire  of  each  particular  designer  to  have  his 
name  connected  with  some  new  or  special  type  of  machine,  many 

645 


646  ELECTRICAL  ENGINEERING  PAPERS 

of  the  freakish  characters  of  these  early  machines  were  due  prim- 
arily to  an  incomplete  or  wrong  conception  of  the  magnetic 
circuit.  As  soon  as  the  magnetic  circuit  became  sufficiently  well 
ttnderstood  to  permit  fairly  accurate  calculations  of  the  magnetic 
conditions,  then  the  design  of  direct-current  machines  began  to 
take  a  definite  trend  toward  certain  constructions,  When  the 
"figures*1  showed  that  a  certain  construction  was  magnetically 
better,  and  considerably  cheaper,  than  other  known  constructions, 
the  manufacturer  naturally  favored  it,  In  the  direct-current 
machine,  as  in  other  types  of  electrical  apparatus,  the  real  develop- 
ment and  eventually  the  standardization  of  general  types  was  a 
result  of  the  development  of  the  calculating  engineer  as  distin- 
guished from  the  experimental  and  the  "cut-and-try"  designer. 

From  the  present  viewpoint,  some  very  absurd  constructions 
appeared  in  the  early  machines.  There  were  some  very  ponderous 
arguments  put  forward  for  and  against  such  construction,  both 
sides  usually  being  wrong  according  to  our  present  ideas.  For 
example,  the  early  Edison  bipolar  field  construction  used  two^or 
more  magnet  cores  attached  to  each  pole  piece,  each  core  carrying 
a  field  winding.  Other  manufacturers  pointed  out  the  absurdity 
of  such  field  construction,  but  as  a  rule,  they  did  not  recognize  that 
they  were  using,  in  many  cases,  similarly  absurd  magnetic  condi- 
tions. Two  magnet  cores  per  pole  piece,  or  per  pole,  were  found  in 
the  "  Weston"  type,  as  shown  in  Fig.  1-&,  and  in  the  "  Brush  "  or 
later ' '  Short "  type ;  and  each  magnet  limb  carried  its  own  exciting 
coil,  just  as  in  the  early  Edison  machine.  This  peculiar  Edison 
construction  was  soon  abandoned,  while  the  same  feature  was 
retained  for  some  years  afterwards  by  many  other  manufacturers, 
while  they  were  still  laughing  at  the  Edison  absurdity. 

THE  BIPOLAR  GENERATOR 

The  first  tendency  toward  any  very  definite  types  for  general 
use  appeared  in  railway  generators.  There  were  then  four  leading 
types  of  railway  equipment,  namely,  the  Edison  with  the  Sprague 
motor  system,  the  Thomson-Houston,  the  Westinghouse,  and  the 
Short  manufactured  by  the  Brush  Company,  Each  of  these  com- 
panies put  out  its  own  type  of  bipolar  railway  generator. 

The  Edison  railway  generator  was  practically  a  duplication 
of  the  Edison  lighting  generator.  The  general  arrangement  of  the 
magnetic  circuit  was  as  shown  in  Fig.  l-£.  The  field  cores,  yoke 
,and  pole  pieces  were  usually  of  wrought  iron.  The  construction 


DEVELOPMENT  OF  THE  D  C   GENERATOR 


647 


was  comparatively  massive,  .  The  armature  was  of  the  surface- 
wound,  two-pole  "  drum  "  type,  and  hand  wound.  On  some  of  the 
earlier  generators,  copper  brushes  were  used,  but  carbon  brushes 
were  adopted  later.  Many  of  these  machines  were  compound- 
wound,  the  same  as  in  present  standard  railway  practice. 

Occasionally  some  very  weird  engineering  was  used  in  the 
early  days  in  connection  with  compound  windings,  when  operating 
two  or  more  machines  in  parallel.  For  instance,  in  one  railway 
plant  of  about  1889,  which  the  writer  examined  personally,  the 
machines  were  properly  installed  as  far  as  armature  and  field  leads 
and  equalizer  leads  were  concerned,  but  the  main  ammeters  were 
connected  in  the  series  coil  circuits,  beyond  the  equalizer  leads,  so 
that  they  indicated  the  current  in  the  series  coils  and  not  in  the 
armature.  Moreover,  each  series  field  was  provided  with  an  ad- 
justable shunt  so  that  the  currents  of  the  different  series  coil  cir- 
cuits could  be  properly  equalized.  A  very  noticeable  characteristic 
of  this  plant  was  that  some  of  the  armatures  heated  and  sparked 


FIG.  1— EARLY  GENERATOR  FRAMES 

A— The  Edison  type.    B— The  United  States  (Western)  type.    C— Later  Bdiaon  type,  xrith 
magnetically  insulated  base.    D — The  Thomson-Houston  type. 

much  more  than  others  and  had  to  be  rewound  frequently,  while 
others  never  had  to  be  rewound.  The  engineer  in  charge  was  much 
worried  over  this  situation  until  the  writer,  in  discussing  the  in- 
stallation of  a  Westinghouse  generator  in  this  plant,  jokingly 
asked  him  whether  he  wanted  the  ammeter  of  the  Westinghouse 
machine  placed  in  the  armature  circuit,  or  in  the  series  field 
circuit.  The  engineer  immediately  "saw  something,"  for  over 
night  he  revised  the  arrangement  of  his  existing  circuits,  although 
the  former  arrangement  was  in  accordance  with  the  manufac- 
turer's drawings.  This  case  is  cited  simply  as  an  illustration 
of  the  mistakes  which  were  not  uncommon  in  those  days,  and 
were  not  confined  to  any  one  manufacturer.  ,  . 


648  ELECTRICAL  ENGINEERING  PAPERS 

The  Edison  type  of  generator  had  one  serious  handicap  from 
the  magnetic  standpoint,  namely,  the  pole  pieces  had  to  be  in- 
sulated magnetically  from  the  bedplate,  as  indicated  in  Fig,  1-c. 
This  insulation  was  of  some  non-magnetic  metal,  such  as  zinc  or 
brass.  It  had  to  be  of  considerable  thickness  to  prevent  undue 
shunting  of  the  magnetic  field,  for  this  shunt  path  was  in  parallel 
with  the  air-gap  between  the  armature  and  field,  which  was 
usually  quite  long,  due  to  the  heavy  surface  windings  on  the 
armature  core.  This  Edison  type  of  machine,  however,  survived  in 
railway  work  as  long  as  the  bipolar  type  lasted. 

One  of  the  early  Thomson-Houston  constant  potential  gener- 
ators was  modeled  after  the  characteristic  Thomson-Houston  arc 
generator.  It  had  a  globular  type  armature,  and  the  general  ar- 
rangement of  the  field  type  armature,  and  the  general  arrangement 
of  the  field  structure  was  similar  to  that  of  the  arc  generator. 
This  machine  had  a  demagnetizing  or  "compensating'*  coil  over 
the  armature,  with  a  view  to  compensating  for  armature  reaction. 
Apparently,  this  machine  was  used  but  little,  if  at  all,  in  railway 
work.  The  principal  type  of  Thomson-Houston  generator  for 
railway  work  was  practically  equivalent  to  the  Edison  machine 
turned  upside  down,  Fig.  1-rf.  One  of  the  best  known  machines 
of  this  type  was  designated  as  the  "D-62."  This  had  a  normal 
rating  of  about  80  horse-power,  or  60  kilowatts.  Magnetically 
the  construction  of  the  machine  was  superior  to  the  Edison  bi- 
polar, in  that  the  pole  pieces,  being  at  the  top  of  the  machine,  did 
not  have  any  undue  leakage  to  the  supporting  parts.  The  arma- 
ture of  this  machine,  like  the  Edison  bi-polar,  was  of  the  drum 
type,  with  the  coils  wound  on  the  surface  by  hand.  Some  of  these 
machines  also  had  "compensating "  coils  over  the  armature.  This 
machine  was  in  great  repute  at  one  time,  and  was  undoubtedly  a 
good  operating  machine,  for  those  times.  Like  the  Edison,  the 
Thomson-Houston  machine  was  belt-driven.  The  writer  was 
much  impressed  back  in  the  90's,  upon  seeing  a  generating  station 
containing  what  was  said  to  be  80  of  these  D-62  machines  in  one 
generating  room,  all  belt-driven  from  a  system  of  line  shafting 
overhead.  The  forest  of  belts  was  exceedingly  impressive.  Like 
the  Edison,  this  type  of  generator  persisted  as  long  as  bi-polar 
generators  were  used  in  railway  work. 

A  third  type  of  bi-polar  generator  which  was  used  in  the  early 
railway  work,  was  the  "  Weston"  type,  built  by  the  United  States 
Electric  Co.  (controlled  by  the  Westmghouse  Electric  Co.)  Or- 


DEVELOPMENT  OF  THE  D  C.  GENERATOR 


G49 


iginally,  this  type  of  machine  was  arranged  with  horizontal  mag- 
nets, as  indicated  in  Fig.  2-a.  In  the  smaller  machines  the  bear- 
ings were  carried  by  bronze  brackets  connecting  the  pole  pieces. 
In  larger  machines,  separate  pedestals  carried  the  bearings. 
In  the  railway  generators,  most  of  the  machines  of  this  type  were 
arranged  with  vertical  instead  of  horizontal  magnets,  as  shown 
in  Fig.  2-fe.  Separate  pedestals  carried  the  bearings  in  these  ver- 
tical machines.  Like  the  Edison  and  Thomson-Houston  machines, 
above  described,  this  United  States  machine  had  a  surface-wound, 
drum  type  of  armature,  hand  wound.  This  bi-polar  railway  ma- 
chine was  used  to  a  less  extent  than  the  Edison  or  Thomson- 
Houston  machines,  because  the  Westinghouse  Company  did  not 
get  into  electric  work  as  early  as  the  Edison  and  Thomson- 
Houston  Companies.  However,  this  type  persisted  as  long  as  any 
of  the  other  bi-polars. 

A  fourth  type  of  bi-polar  generator  which  was  used  exten- 
sively in  railway  work,  was  the  "  Short "  generator.  This  was 
modeled  after  the  general  lines  of  the  Brush  arc  generator,  the 
Short  railway  system  being  manufactured  by  the  Brush  Company. 


FIG.  2— EARLY  GENERATOR  FRAMES 

A— The  United  States  (Weston)  type,  with  horizontal  magnets.     B— The  United  States 
(West on)  type,  with  vertical  magnets.     O— The  Short  (Brush)  type, 

Like  the  Brush  arc  machine,  the  Short  railway  generator  had  a 
ring  armature  with  the  armature  coils  lying  between  teeth  on  the 
two  side  faces  of  the  armature  core,  the  pole  pieces  being  presented 
toward  the  sides  of  the  armature.  The  armature,  therefore,  was 
really  of  the  "disc"  type.  The  field  magnets  were  arranged  on 
each  side  of  the  armature,  just  as  in  the  Brush  arc  machine.  The 
commutator  was  placed  outside  the  frame  on  one  end  of  the  shaft 
aad  the  armature  leads  were  carried  down  radially  from  the  ar- 
mature winding  to. the  shaft  arid  along  the  shaft  to  the  commut- 
ator* The  whole  construction  of  the  armature  was  very  awkward* 


650  ELECTRICAL  ENGINEERING  PAPERS 

from  the  present  viewpoint.  The  general  construction  is  in* 
dicated  in  Fig.  2-c.  The  armature  required  a  non-magnetic  spider 
as  was  the  case  in  all  bi-polar  ring  armatures.  Possibly  one  of  the 
worst  defects  of  this  type  of  machine  was  the  liability  of  a  strong 
side  pull  due  to  inequality  of  air-gaps  on  the  two  sides  of  the  ring 
armature.  Any  inequality  meant  an  unbalanced  magnetic  pul] 
with  a  tendency  for  an  axial  movement  of  the  shaft.  Thrust 
bearings  were  necessary  to  prevent  this.  In  this  machine  were 
evidences  of  the  later  slotted  armature  construction.  However, 
it  is  questionable  whether  the  armature  teeth  on  these  early  Short 
generators  were  designed  primarily  for  magnetic  purposes  or  for 
mechanical  reasons.  They  did  not  constitute  a  toothed  armature, 
as  we  design  it  nowadays.  However,  the  machine  must  have 
acted,  to  a  certain  extent,  as  a  toothed  type.  Like  all  the  others, 
this  machine  was  belt-driven.  It  also  persisted  as  long  as  the  bi- 
polar machines  were  used  for  railway  service. 

From  the  above  it  may  be  seen  that  all  of  the  leading  bi-polar 
railway  generators  were  quite  different  from  each  other  in  their 
general  appearance  and  construction.  There  were  many  warm 
discussions  regarding  the  merits  and  demerits  of  each  type.  When 
equipped  with  carbon  brushes,  all  of  them  operated  reasonably 
well.  All  of  them  ran  rather  hot  in  the  armature,  due  to  the  fact 
that  little  provision  was  made  for  ventilation.  In  this  respect  the 
Short  armature  was  probably  better  than  the  others,  as  it  was  of 
a  fairly  open  ring  construction.  Being  surface-wound  in  practic- 
ally all  cases,  the  commutation  was  not  difficult,  as  the  self-induc- 
tion was  low.  When  the  multi-polar  types  of  railway  generators 
came  in,  there  were  lots  of  "  stand-patters  "  who  insisted  that  the 
old  two-pole  machines  were  good  enough,  and  that  we  were  foolish 
in  trying  to  do  away  with  them. 

In  these  various  types  of  bi-polar  generators,  the  Edison  was 
carried  up  to  ISO  kw  capacity,  or  possibly  somewhat  larger;  the 
United  States  was  carried  up  to  ISO  kw  capacity.  Apparently 
the  Short  and  Thomson-Houston  were  not  carried  up  to  such 
large  sizes,  the  Thomson-Houston  "D-62"  being  the  "crack"1 
machine,  to  the  end  of  the  bi-polar  dynasty. 

There  were  a  number  of  other  railway  generators  at  this  time 
in  this  country,  but  they  were  not  as  well  known  as  the  above  and 
did  not  persist  as  long,  presumably  because  the  railway  systems  of 
which  they  usually  formed  a  part,  did  not  persist. 


DEVELOPMENT  OF  THE  D  C    GENERATOR  651 

MULTIPOLAR  TYPES 

About  1890,  the  Thomson-Houston  Company,  in  bringing  out 
a  larger  capacity  railway  generator,  adopted  a  multipolar  design, 
in  which  an  external  octagonal-shaped  yoke  supported  four  in- 
ternally projecting  poles,  each  of  which  carried  a  field  winding. 
This  arrangement,  shown  in  Fig.  3-a,  was  thus  an  approach  to  the 
present  almost  universal  type  of  field  construction.  The  armature 
was  of  the  ring  type,  with  four  circuits,  one  per  pole,  and  there 
were  four  brush  arms  and  four  sets  of  brushes  on  the  commutator. 
This  machine  was  separately  excited,  an  exciter  being  provided. 
The  opinion  was  somehow  spread  broadcast  that  separate  excita- 
tion was  necessary  in  general,  and  for  parallel  operation  in  par- 
ticular, in  multipolar  generators.  No  real  explanation  was  forth- 
coming as  to  why  this  was  so,  but  apparently  almost  everybody 
accepted  it  as  a  fact  without  explanation.  Shortly  after  the  intro- 
duction of  this  multipolar  type  of  generator,  the  writer  examined 
several  such  machines  which  had  just  been  installed  in  the  Du- 
quesne  Traction  power  house  in  Pittsburgh.  The  conclusion 


A  B 

FIG.  3— EARLY  MULTIPOLAR  GENERATORS 
A — Octagonal  frame.    B— Cylindrical  frame. 

drawn  was  that  separate  excitation  was  necessary  only  because 
the  machines  were  not  worked  sufficiently  high  on  the  saturation 
curve  to  give  stability  when  self-excited.  With  the  surface- 
wound  armatures  and  consequent  large  air-gap,  together  with  the 
multipolar  construction,  a  relatively  low  saturation  was  used  ap- 
parently. Possibly  the  writer  drew  a  wrong  conclusion  in  this 
case,  but  nevertheless  he  made  up  his  mind  that  self-excitation 
was  just  as  practicable  in  multipolar  railway  generators  as  in 
bipolar. 


652  ELECTRICAL  ENGINEERING  PAPERS 

As  stated  before,  the  United  States  bi-polar  machines  were 
furnished  by  the  Westinghouse  Company  for  railway  work. 
These  were  built  at  the  United  States  Works  at  Newark,  New 
Jersey.  But  early  in  1890,  the  Westinghouse  Company  con- 
sidered the  construction  of  larger  generators  at  its  Pittsburgh 
shops,  and  a  contract  was  taken  for  a  250  horse-power  generator 
for  railway  work.  The  electrical  design  for  this  machine  was 
prepared  by  the  writer.  A  cylindrical  external  field  ^with  in- 
wardly projecting  poles,  as  shown  in  Fig.  3-fc,  similar  to  the  West- 
inghouse alternators  of  that  time,  was  chosen  as  the  ideal  type. 
The  four  poles  were  laminated  and  were  cast  into  the  yoke  similarly 
to  the  Westinghouse  laminated  field  alternators  of  that  date. 
This  field  was  also  a  close  approach  to  the  present  standard  con- 
struction, being  a  slight  step  ahead  of  the  Thomson-Houston 
machine  described  above,  in  the  use  of  laminated  poles.  The 
armature  was  of  the  ring  type,  surface-wound.  At  this  time  the  air 
was  filled  with  talk  that  the  ring  armature  was  the  coming  type. 
There  were  some  good  reasons  for  this.  In  the  older  bi-polar 
drum  armatures,  most  of  the  over-heating  had  been  in  the  piled 
up,  unventilated  end  windings.  As  the  ring  armature  had  no 
end  windings  to  speak  of,  it  was  naturally  supposed  that  all 
troubles  from  heating  would  be  overcome  by  the  adoption  of  this 
type.  Thus  the  last  weak  point  in  the  railway  armature  was  sup- 
posed to  be  done  away  with. 

Being  convinced  that  multipolar  generators  would  operate 
satisfactorily  when  self -excited,  if  worked  high  enough  on  the 
saturation  curve  to  give  stability  in  excitation,  the  writer  deliber- 
ately designed  this  first  four-pole  machine  for  self -excitation,  but 
this  was  not  known  to  anyone  but  the  Company's  superintendent 
Mr.  Albert  Schmid,  who  was  fully  in  accord  on  this  point.  How* 
ever,  when  the  machine  was  about  completed  and  ready  for  test, 
the  information  leaked  out  that  it  had  been  designed  for  self- 
excitation,  which,  of  course,  was  entirely  contrary  to  all  good 
and  accepted  practice.  The  writer  was  criticised  from  all  sides 
for  his  temerity  and  for  his  lack  of  good  judgment.  However,  the 
machine  was  put  on  test,  and  did  operate  in  an  entirely  satisfactory 
manner  when  self-excited.  But  some  of  the  wise  ones  still  shook 
their  heads,  for  we  were  violating  all  known  "laws."  Neverthe- 
less, we  stuck  to  self-excitation,  and  it  is  still  with  us. 

When  testing  this  first  machine,  considerable  new  experience 
was  obtained.  For  instance,  when  moiling  at  normal  voltage, 


DEVELOPMENT  OF  THE  D  C.  GENERATOR  653 

the  machine  was  dead  short-circuited  across  the  outside  terminals 
with  a  somewhat  surprising  display  of  fire-works.  Also  when, 
after  this  short-circuit  test,  the  surface-wound  ring-armature 
winding  was  found  to  be  shifted  about  two  inches  circumferentially 
at  certain  points,  some  of  us  had  doubts  regarding  the  desirability 
of  this  type  of  winding.  This  was  one  of  the  points  that  led  to  the 
slotted  armature  construction  described  later. 

This  first  Westinghouse  multipolar  machine  was  considered  a 
"giant,"  and  railway  people  all  over  the  country  were  invited  to 
witness  its  operation  in  the  Company's  Pittsburgh  works.  Quite 
a  number  of  visitors  did  come  long  distances  to  see  this  250  h.p. 
machine,  and  some  comments  were  made  that  this  was  probably 
the  upper  limit  in  size  that  would  ever  be  made.  It  was,  at  that 
time,  hard  to  conceive  that  any  electric  railway  would  ever  need 
anything  larger  than  this  capacity.  Moreover,  it  was  thought  by 
some  that  the  limit  in  belting  had  been  reached.  As  everything 
was  belted  in  those  days,  it  was  difficult  to  see  that  any  other 
method  of  drive  was  possible.  Nevertheless,  this  machine  was 
quite  a  wonder,  compared  with  what  had  preceded  it. 

SLOTTED  ARMATURE  TYPES. 

Several  machines  of  this  type  and  capacity  were  built  and  put 
out*  However,  the  writer  was  not  entirely  satisfied  with  the  ring 
winding  in  particular,  nor  the  surface  winding  in  general.  At 
this  time,  in  alternating-current  work,  there  was  a  strong  tendency 
toward  "toothed"  armature  constructions,  with  the  windings 
completely  embedded  below  the  surface  of  the  core.  These  alter- 
nating-current armature  types  with  one  tooth  per  pole  did  not 
lend  themselves  to  direct-current  work,  but  the  idea  of  embedding 
the  windings  persisted.  In  the  summer  of  1890,  while  scheming  on 
a  new  railway  motor,  a  slotted  armature  construction  was  worked 
out  by  the  writer,  with  a  view  to  improving  the  magnetic  condi- 
tions so  greatly  that  a  slow-speed  single-reduction  railway  motor 
would  be  possible.  The  calculations  for  the  magnetic  condition 
(crude  as  they  were  in  those  days)  showed  such  astonishingly  good 
results  that  the  same  construction  was  considered  in  connection 
with  the  design  of  a  large  Westinghouse  slotted  armature  railway 
generator  which  in  tnany  ways  may  be  considered  the  forerunner 
of  present  practice.  These  calculations  on  the  railway  motor 
resulted  in  the  well  known  Westinghouse  No,  3  single  reduction 
motor,  which  was  practically  'the  f  orefatlter  of  the  present  tmi* 


654  ELECTRICAL  ENGINEERING  PAPERS 

versal  type  of  railway  motor,  This  was  not  actually  the  first  single 
reduction  motor,  but  was  the  first  which  anyways  nearly  ap- 
proached the  present  type. 

It  was  while  making  the  original  calculations  on  the  slotted 
armature  for  the  single  reduction  motor  that  the  "two-circuit"  or 
"series"  type  of  armature  winding  was  devised  for  multipolar  ma- 
chines. In  working  out  the  railway  motor,  it  soon  became  evident 
that  a  four-pole  design  was  necessary.  With  any  of  the  then  known 
armature  windings,  either  four-brush  arms  were  required  on  the 
commutator,  or  the  commutator  had  to  be  cross-connected  at 
every  bar,  in  order  to  allow  the  use  of  two-brush  arms  only.   The 
writer  deliberately  set  about  to  devise  an  arrangement  of  connec- 
tions which  would  allow  the  use  of  two-brush  arms  on  a  multipolar 
winding  without  any  other  cross  connections  than  the  normal  con- 
nections of  the  winding.    The  two-circuit  winding  was  the  result, 
and  the  law  of  the  winding  was  worked  out  for  various  combina- 
tions of  poles,  etc.,  while  still  closing  on  itself  properly.    This 
winding  was  included  in  the  design  of  the  trial  single  reduction 
motor  then  being  designed.  Question  was  raised  regarding  this  new 
winding,  when  it  was  first  proposed,  by  those  who  appreciated  that 
something  radically  new  was  involved  in  its  use.  The  writer  had  to 
"swear  up  and  down"  that  it  was  absolutely  correct  in  principle, 
even  if  it  was  new  and  untried,  but  he  apparently  convinced  the 
others  more  by  his  vehemence  than  by  his  theories,  for  most  people 
in  those  days  had  very  little  conception  of  the  theory  of  armature 
windings.  However,  when  the  first  single  reduction  railway  motor 
armature  came  on  test,  the  theory  proved  to  be  all  right — at  least 
the  armature  was  all  right  and  had  but  two  brush  arms  on  a  four- 
pole  machine.   This,  as  far  as  the  writer- knows,  was  the  original 
two-circuit  multipolar  winding  in  this  country.   Application  for  a 
patent  was  refused  however,  on  the  basis  of  a  certain,  until  then 
unknown,  foreign  patent — that  is,  unknown  as  far  as  the  writer 
and  any  of  his  colleagues  were  concerned.  It  may  be  worth  mention- 
ing that  at  the  time  this  two-circuit  winding  was  first  used,  the 
criticism  brought  against  it  was  that  it  was  entirely  too  com- 
plicated to  be  adopted  generally.    This  is  interesting  in  view  of 
the  fact  that  at  the  present  time  this  is  our  simplest  direct-current 
winding,  and  it  is  used  probably  to  a  greater  extent  than  all  other 
windings  together.  Prophecies  in  those  days  were  no  more  reliable 
than  they  are  at  present. 


DEVELOPMENT  OF  THE  D.C   GENERATOR  655 

Returning  to  the  slotted  type  of  railway  generators,  as  stated 
above,  it  was  decided  to  build  an  armature  of  this  type.  Slotted 
armatures  had  previously  been  used  in  America  by  the  United 
States  Company  and  others  on  comparatively  small  capacity 
generators  and  motors,  but  apparently  on  nothing  "within  gun- 
shot "  of  the  size  which  we  were  contemplating.  Therefore,  opinions 
were  obtained  from  a  number  of  then  eminent  engineers  who  had 
had  experience  of  some  sort  with  slotted  armatures.  All  opinions 
were  unanimous  that  we  could  not  make  a  250  horse-power  rail- 
way armature  of  the  slotted  type  which  could  commutate  suc- 
cessfully without  rocking  the  brushes  with  change  in  load.  This 
was  very  discouraging,  but  it  was  then  decided  that  this  was  pos- 
sibly a  case  where  everyone  was  wrong,  and  that  we  had  better 
build  one  armature  in  order  to  obtain  some  positive  information. 

This  new  railway  armature  was  designed  with  95  slots  and  95 
turns  and  95  effective  commutator  bars,  the  new  two-circuit  drum 
winding  being  used,  although  it  was  intended  that  there  should  be 
four  brush  arms.  As  95  turns  were  all  that  could  be  used,  with  the 
desired  saturation  of  the  existing  field  frame,  and  as  there  did  not 
seem  to  be  enough  bars  for  a  four-pole,  525  volt  machine,  the 
actual  number  of  commutator  bars  was  made  190,  but  only  every 
other  bar  was  connected  to  the  armature  winding.  The  intervening 
bars  were  idle  (or  '  'dummies  ")>  and  really  constituted  broad  insul- 
ating or  separating  segments  between  the  active  bars.  The 
brushes  covered  practically  four  bars,  so  that  they  touched  two 
active  bars  all  the  time. 

There  was  considerable  discussion  regarding  the  type  of  ar- 
mature slot  to  be  used,  namely,  whether  it  should  be  partially 
opened  at  the  top  or  should  be  entirely  closed*  It  was  finally  de- 
cided to  make  the  slots  entirely  closed,  on  the  principle  that  the 
dosed  slot  would  be  the  ideal  one  if  it  prove  satisfactory,  and  if 
it  didn't  prove  satisfactory,  the  slots  could  then  have  openings  cut 
in  the  top,  without  rewinding. 

When  this  first  machine  came  on  test^  some  most  remarkable 
results  were  obtained,  due  apparently  to  both  the  idle  segments 
in  the  commutator  and  the  entirely  closed  armature  slots,  Spark- 
ing was  present  at  all  times,  even  with  only  the  exciting  current  as 
a  load.  Rocking  the  brushes  was  tried  for  curing  this,  but  when 
any  considerable  load  was  carried  uo  point  of  sparkless  commut- 
ation could  be  fouaxi  However,  a  curious  feature  of  the  results  was 
that  the  brushes  cottld  be  locked  practically  anywhere  cm  the 


656  ELECTRICAL  ENGINEERING  PAPERS 

mutator  without  causing  flashing.  Copper  brushes  were  tried  with 
like  results.  Also,  copper  brushes  were  tried  behind  the  carbon 
brushes,  and,  in  some  cases,  such  heavy  local  currents  were  gener- 
ated between  the  copper  and  carbon  brushes  that  the  brushes  got 
red  hot  over  almost  their  full  length,  and  yet  we  did  not  produce  a 
single  flash  from  this  machine,  although  we  explored  around  over 
almost  the  entire  commutator  with  the  experimental  brushes. 
Mr.  Chas.  F.  Scott  and  the  writer  spent  almost  an  entire  night 
experimenting  with  the  brushes  and  brush-holders,  with  our  faces 
at  times  almost  against  the  commutator  and  brushes,  where  a 
flash  would  probably  have  caused  permanent  injury  to  us.  In 
view  of  the  vicious  flashing  which  occured  at  times  on  later  ma- 
chines, it  is  still  somewhat  of  a  puzzle  why  this  first  machine  was 
so  absolutely  non-flashable.  Apparently,  the  ' '  dummy ' '  commut- 
ator bars  should  receive  most  of  the  credit  for  this  result. 

Finding  that  the  closed  slot  construction  was  a  failure,  the 
armature  slots  were  then  cut  open  as  wide  as  possible  while  still 
retaining  sufficient  overhanging  tips  for  holding  the  armature 
conductors  in  place.  The  first  test  after  this  showed  a  marvelous 
improvement.  The  armature  now  would  carry  from  no-load  up 
to  considerably  above  its  rated  load  without  serious  sparking  and 
without  shifting  the  brushes.  The  machine  was  then  put  on 
exhibition  and  many  prominent  people  saw  it  put  through  its 
"  stunts."  The  only  serious  defect  that  developed  was  in  slight 
spotting  or  burning  of  alternate  commutator  bars.  In  an  attempt 
to  overcome  this,  each  idle  bar  was  connected  to  an  adjacent 
active  bar,  thus  reducing  the  total  number  of  bars  to  95.  Under 
this  condition  the  spotting  was  stopped,  and  the  commutation  ap- 
peared to  be  just  as  good  as  before;  therefore  it  was  decided  that 
95  bars  were  sufficient  for  a  four-pole,  525  volt  railway  generator 
of  this  general  construction. 

This  slotted  armature  railway  generator  turned  all  future 
construction  of  large  machines  toward  the  slotted  type.  However, 
this  first  machine  by  no  means  fixed  the  final  design  of  this  type  of 
armature,  but  merely  set  the  general  type.  Many  variations  ap- 
peared in  the  following  years,  but  all  contained  the  slotted  con- 
struction and  the  drum  type  of  armature  winding.  As  far  as  the 
armature  itself  was  concerned,  the  principal  variations  were  in  the 
shape  of  the  armature  slots  and  in  the  form  or  construction  of  the 
armature  coils.  Furthermore,  some  experience  was  required  in. 
order  to  determine  the  best  proportions  of  armature  teeth,  air-gap, 


DEVELOPMENT  OF  THE  D.C   GENERATOR  567 

etc.,  in  order  to  produce  good  commutation  over  a  wide  range  of 
load  without  brush  shifting,  a  very  necessary  condition  in  railway 
work.  While  in  fact  this  first  large  generator  did  not  require  brush 
shifting,  yet  the  reasons  for  this  were  not  fully  appreciated  at  the 
time,  and  therefore  the  good  results  obtained  were,  to  a  certain 
extent,  accidental.  The  right  conditions  were  aimed  at  in  making 
up  the  design,  but  as  it  was  largely  a  question  of  how  far  to  carry 
certain  proportions,  it  was  partly  accidental  that  these  conditions 
were  carried  just  far  enough  to  obtain  such  satisfactory  results. 
In  this  first  machine  the  armature  teeth  were  made  comparatively 
thin  and  short  and  were  saturated  to  an  excessive  degree.  Also, 
the  air-gap  was  purposely  made  quite  large,  with  the  idea  that  the 
air-gap  and  tooth  saturation  together  would  require  very  high 
magnetizing  ampere-turns  compared  with  the  armature  ampere- 
turns,  which  was  considered  as  a  very  desirable  condition  for  com- 
mutation. However,  the  extremely  beneficial  effect  of  high  tooth 
saturation  in  holding  the  brush  lead  constant  was  not  then  fully 
appreciated.  This  was  discovered  when  a  somewhat  larger  machine 
was  built  a  few  months  later.  In  this  larger  machine  the  tooth 
saturation  was  considerably  lower  than  in  the  first  armature,  al- 
though the  air-gap  ampere-turns  were  comparatively  high.  This 
later  machine  was  found  to  be  much  more  sensitive  to  shifting  of 
lead  than  the  former  machine.  A  careful  study  was  made  of  the 
influence  of  tooth  saturation  in  holding  the  lead  constant,  and  from 
this  study  a  scheme  for  saturating  the  pole  face  instead  of  the 
armature  teeth  was  suggested,  which  will  be  referred  to  later. 
The  Thomson-Houston  earliest  slotted  type  railway  generators 
were  also  subject  to  change  in  lead,  but  evidently  the  cause  of  the 
difficulty  was  soon  discovered,  for  the  later  machines  were  not  so 
sensitive  in  this  regard.  Also,  apparently  their  earlier  slotted 
machines  had  weaker  fields  compared  with  the  armatures  than  was 
the  case  with  the  Westinghouse  machines. 

The  early  Westinghouse  slotted  armatures  for  railway  gen- 
erators all  had  partially  closed  slots,  while  some  of  the  early 
Thomson-Houston  machines  had  rectangular  open  slots  and  others 
had  partially  closed  slots.  The  slot  construction  depended,  to  a 
certain  extent,  upon  the  the  type  of  armature  winding  used.  The 
early  Westinghouse  machine  had  only  two  round  conductors  per 
slot,  which  were  threaded  through  two  stout  insulating  tubes  made 
of  rolled  up  paper  and  shellac,  as  shown  in  Pig,  4-a,  These  tubes 
had  walls  of  1-16  to  3-32  of  an  inch  in  thickness.  The  winding 


658  FLECTPICAL  ENGINEERING  PAPERS 

consisted  of  either  round  solid  conductors  or  of  twisted  cable, 
threaded  through  these  tubes.  The  conductors  were  first  cut  in 
lengths  corresponding  to  one  complete  turn.  All  the  conductors  of 
the  lower  layer  were  threaded  through  the  lower  tubes  and  then 
bent  down  at  each  end  in  an  involute  shape,  as  indicated  in  Fig. 
4-i  and  c.  At  the  front,  or  commutator  end,  the  conductors 
extended  just  sufficiently  to  furnish  the  front  end  connection  to  the 
commutator.  At  the  rear  end,  the  conductors  extended  far  enough 
to  furnish  the  return  or  upper  layer.  The  lower  layer,  after  being 
bent  down  at  each  end  to  an  insulated  support  over  the  shaft,  was 
banded  or  clamped  down  solidly  to  the  support.  The  extended 
rear  ends  of  the  coils  or  conductors  were  then  bent  outward  to  the 
periphery  of  the  core,  in  an  involute  form  and  were  shoved  through 
the  upper  layer  of  tubes,  and  carried  directly  to  the  commutator, 


PIG.  4— EARLY  WINDING  SCHEMES 

thus  completing  the  winding.  The  various  steps  in  this  construction 
are  indicated  in  Fig.  4-d     In  the  smaller  machines,  that  is,  from 
about  ISO  kw  down,  solid  conductors  were  used,  these  being  in 
some  cases  as  large  as  No.  2  B,&  S.  gauge  wire.  The  maximum  size 
of  solid  conductor  was  determined  by  the  ability  of  the  winders 
to  handle  it  without  undue  difficulty.    For  larger  sizes,  twisted 
cables  were  used,  made  up  of  fairly  large  wires,  such  as  No.  10  or 
No.  12  B.  and  S.  gauge.    Considerable  stiffness  was  preferred  itx 
order  to  give  mechanical  strength  to  the  winding.    This  type  of 
armature  winding  was  used  for  a  number  of  years,  and  was  some- 
times modified  to  the  extent  of  four,  or  even  six  tubes  and  con- 
ductors per  slot,  arranged  radially.  Also,  in  some  cases,  the  section 
of  the  tubes  was  made  elongated  to  take  two  or  three  parallel  con- 
ductors forming  one  turn..     In  the  larger  machines  with  this 
winding,  with  the  very  high  tooth  inductions  used,  undoubtedly 
the  use  of  cable  materially  lessened  eddy  currents  in  the  conduc- 
tors.   This,  however,  was  not  a  prime  reason  for  the  use  of  cable, 
ease  of  winding  being  the  principal  reason-. 


DEVELOPMENT  OF  THE  D.C   GENERATOR  650 

Some  of  the  early  Thomson-Houston  slotted  armatures  had, 
open  slots,  while  others  had  partially  closed  slots,  of  rectangular 
shape  in  both  cases.  Straight  copper  straps  or  bars  of  rectangular 
section  were  either  laid  in  or  shoved  through  the  slots  from  one 
end,  depending  upon  whether  the  slots  were  of  the  open  or  partially 
closed  type.  Separate  strap  end  connectors  of  the  involute  type 
were  riveted  or  soldered  to  the  armature  bars  at  each  end.  This 
general  construction  is  shown  in  Pig.  S-a.  The  writer  does  not 
know  whether  this  was  the  earliest  Thomson-Houston  winding 
for  slotted  armature  generators,  but  it  was,  at  least,  a  very  early 
one.  In  some  cases,  two  or  even  three  bars  were  placed  side  by 
side  in  one  slot,  forming  two  or  three  separate  turns.  This  general 
type  of  winding  was  retained  by  the  Thomson-Houston  Company 
(G.  E.)  for  several  years. 


PIG.  5— EARLY  COIL  FORMS 

FORMED  COILS 

After  two  or  three  years'  use  of  the  Westinghouse  "shoved- 
through"  type  of  armature  winding  above  described,  the  develop- 
ment of  a  type  of  armature  coil,  which  could  be  com- 
pletely formed  and  insulated  before  placing  in  the  slots, 
was  taken  up.  Various  schemes  to  accomplish  this  were  under- 
taken* The  first  one  attempted  consisted  in  forming  the  rear  end 
and  the  two  straight  parts  of  the  coil  as  a  complete  coil  which  was 
then  shoved  through  the  partially  closed  slots  from  the  rear  end, 
the  front  end  connectors  being  formed  of  copper  strap  which  were 
then  riveted  or  soldered  on,  after  the  conductors  had  been  shoved 
through  the  slots,  as  shown  in  Fig.  5-b.  One  armature  of  the  sort 
was  actually  constructed.  It  was  appreciated  however  that  if  open 
armature  slots  were  used,  instead  of  partially  closed,  the  entire  coU, 
including  both  front  and  rear  end  connections,  could  be  constructed 
in  one  piece*  This  construction  was  then  tried  out  and  adopted,  as 
illustrated  in  Pig.  5-c,  It  may  be  noljed  that  both  end  windings, 
in  this  early  one  piece  coil,  were  of  the  involute  type.  This  con- 
struction was  retained  by  the  Westiugbotiae  Company  for  some 
years,  and  was  eacteaded  to  tv<ro,  three,  and  even  four  and  five, 


660  ELECTRICAL  ENGINEERING  PAPERS 

separate  conductors  side  by  side  in  one  slot.  Then  gradually  the 
involute  winding  at  the  front  end  was  replaced  by  axially  ar- 
ranged end  windings  between  the  armature  core  and  commutator, 
just  as  in  present  practice.  The  rear  end  was  later  straightened 
up  in  the  same  way,  so  that  the  present  type  of  winding  was  thus 
attained. 

Incidentally,  it  should  be  mentioned  that  the  completely 
wound  coil,  with  involute  end  connections,  was  developed  and 
applied  by  Rudolph  Eickemeyer,  to  surface-wound  armatures, 
several  years  previous  to  this.  In  this,  as  in  several  other  things, 
Eickemeyer  was  considerably  in  advance  of  his  time.  In  some  of 
the  earlier  armatures  with  open  slots,  the  windings  embedded  in 
the  armature  core  were  supported  by  bands  over  the  core.  This 
appeared  to  be  the  usual  Thomson-Houston  practice,  (some  of 
the  above  developments  occurring  during  the  formation  of  the 
General  Electric  Company  from  the  Thomson-Houston  and 
Edison  Companies).  With  open  slots,  the  preferred  Westinghouse 
construction  consisted  of  fiber  wedges  over  the  embedded  parts  of 
the  winding,  but  the  writer  does  not  know  whether  this  construc- 
tion originated  with  the  Westinghouse  Company  or  not*  There 
was  considerable  discussion  as  to  the  relative  merits  of  the  two 
arrangements,  but  apparently  both  were  entirely  successful. 

END  WINDING  SUPPORTS 

As  regards  supports  for  the  end  winding,  the  early  shoved- 
through  winding  on  the  Westinghouse  machines,  shown  in  Pigs. 
4-fc  and  c  had  good  supports  at  each  end  against  centrifugal 
force.  The  slots  being  partly  closed,  this  type  of  winding  the  re- 
fore  had  no  bands  or  wedges  on  any  part  of  it.  With  the  develop- 
ment of  the  strap  coil,  as  shown  in  Pig.  5-c,  it  soon  developed 
that  in  high-speed  machines  the  rear  end  of  the  winding  required 
some  sort  of  support.  This  was  first  made  in  the  form  of  bronze 
end  bells,  which  braced  the  end  winding,  both  axially  and 
radially,  as  shown  in  Fig.  6.  The  front  end  winding  had  no 
support.  However,  in  the  early  years  of  this  winding,  very  low 
speed  armatures  were  the  rule,  for  this  was  the  age  of  engine- 
type  generators,  which  will  be  referred  to  more  fully.  Except  iti 
rare  cases,  the  armature  end  connections  were  rigid  enough  to  be 
self-supporting  without  belts  or  bands.  When  rotary  converters 
and  motor-generators  came  into  general  use,  with  their  very  much 
higher  peripheral  speeds,  the  end  bell  over  the  rear  end  was 
eventually  replaced  by  axial  end  windings  with  heavy  bands,  as  in 


DEVELOPMENT  OF  THE  D.C.  GENERATOR 


(561 


present  practice.  The  Thomson-Houston  or  General  Electric 
Company  preceded  the  Westinghouse  Company  in  the  use  of 
bands  on  the  end  windings,  probably  in  part  because  their  built-up 
end  windings  had  more  need  for  some  external  support.  Also,  as 
they  abandoned  the  built-up  end  construction  in  favor  of  complete 
coils,  they  changed  to  the  axial  construction  on  the  rear  end, 
which  also  necessitated  the  use  of  bands.  Thus  both  companies 
eventually  came  to  the  same  construction,  through  different 
courses  of  development. 


PIG.  6— METHOD  OF  BRACING  THE  END  WINDINGS  BY  A  METALLIC 

END  BELL 

The  writer  has  gone  into  this  history  of  the  slotted  construc- 
tion and  the  armature  windings,  because  these  constitute  probably 
the  most  radical  points  in  the  history  of  direct-current  generator 
design. 

OTHER  MULTIPOLAR  TYPES 

Something  should  be  said  regarding  other  types  of  multipolar 
generators  developed  during  this  time.  The  Edison  Company  did 
not  go  into  the  multipolar  design  for  railway  generators,  as  far  as 
the  writer  knows,  for  about  this  time,  the  Thomson-Houston  and 
Edison  Companies  were  combined  into  the  G.  E,  Company.  The 
Edison  Company,  however,  had  designed  and  built  some  large 
low-voltage  generators  for  the  Edison  licensee  companies.  These 
had  large  cylindrical  external  fields  with  inwardly  projecting  poles 
of  cast  steel  or  wrought  iron.  The  armatures  were 'of  the  ring  type, 
surface  wound,  and  were  fitted  with  radial  commutators  on  one 
face  of  the  armature  core,  the  commutator  bars  forming  part  of 
the  armature  winding*  These  machines,  as  a  rule,  used  metal 
brushes.  They  were  manufactured  up  to  quite  large  capacities 
by  the  General  Electric  Company  in  continuing  the  Edison  design. 

The  Short  bi-polar  type  of  railway  generator  was  simply  ex- 
panded into  a  multipolar  type,  having  the  same  general  con- 


662  ELECTRICAL  ENGINEERING  PAPERS 

structional  features.  This  apparently  presented  no  advantage 
over  the  Short  bi-polar  machine,  except  that  it  permitted  machines 
of  larger  capacity  to  be  built.  At  one  time  it  was  claimed  by  some 
authorities  that  the  Short  multipolar  generator  was  the  coming 
type.  However,  it  was  apparently  too  expensive,  for  later,  in  or- 
ganizing the  electrical  work  of  the  Walker  Company,  Prof  Short 
abandoned  this  construction  in  favor  of  one  similar  to  the  Westing- 
house  and  G.  E.  machines. 

ENGINE-TYPE  GENERATORS 

All  the  earlier  generators  were  of  the  belted  type,  This  was 
eventually  carried  up  to  comparatively  large  capacities  in  either 
belt  or  rope  drive,  500  kw  machines  being  not  uncommon.  How- 
ever, shortly  after  the  slotted  type  of  armature  construction  came 
into  general  use,  a  tendency  was  manifested  toward  direct  driving 
from  the  engine.  Once  well  started,  direct  driving  soon  became 
almost  exclusive  practice,  except  for  extremely  small  units.  Two 
methods  of  direct  driving  were  used  about  equally  in  the  earlier 
practice,  namely,  direct  coupling  of  complete  generator  units  to 
the  engines,  and  straight  engine-type  machines  in  which  the  gen- 
erator armature  was  placed  on  the  engine  shaft.  The  former  might 
be  said  to  be  an  adaptation  of  the  belted  type  to  direct  driving. 
The  engine-type  machine,  however,  might  be  considered  a  distinct 
type,  as  the  units  were  designed  primarily  in  connection  with  the 
prime  movers. 

Designers  of  direct-current  machines  rather  welcomed  the 
engine  type,  even  if  it  did  make  much  of  their  former  work  ob- 
solete, for  the  engine-type  machine  very  much  simplified  some  of 
the  problems  which  had  been  encountered  in  the  larger  capacity 
belted  machines.  For  example,  the  commutation  problem  became 
very  much  easier,  due  to  the  much  lower  speed,  larger  commut- 
ators, etc.  The  heating  problem  was  also  temporarily  solved,  for 
the  engine-type  machines  were  comparatively  large  and  massive 
for  a  given  output,  and  thus  could  dissipate  their  heat  rather 
easily. 

The  direct-coupled  and  engine-type  practice,  once  started, 
came  in  rather  quickly.  The  first  railway  units  appeared  about 
1892,  and  by  1893  they  had  made  great  progress  commercially. 
A  number  of  large  engine-type  and  direct-coupled  railway  gener- 
ators were  exhibited  by  various  manufacturers  at  the  Qhicag, 
Worlds  Fair  in  1893.  The  power  house  for  the  Intramural  Rail- 


DEVELOPMENT  OF  THE  D  C.  GENERATOR  0(33 

way  at  the  Fair  contained  practically  only  such  machines,  if  the 
writer  remembers  rightly.  The  Worlds  Fair  engine-type  generators 
were  presumably  all  exhibition  machines;  however,  they  were 
but  little,  if  any,  ahead  of  the  times  for,  during  the  same  year 
both  the  Westinghouse  and  G.  E.  Companies  contracted  for  a 
number  of  very  large  machines  of  the  engine  type.  It  might  be 
said  therefore  that  the  engine-type  railway  generator  was  well 
established  commercially,  in  1893,  or  within  about  two  years  after 
the  first  slotted  armature  for  large  railway  generators  was  devel- 
oped. Without  the  slotted  construction,  it  is  doubtful  whether 
such  rapid  and  enormous  development  could  have  taken  place. 

There  was  one  exception  to  the  slotted  armature  construction 
in  large  machines  for  railway  work,  namely,  the  Siemens-Halske 
generator,  which  was  exploited  in  this  country  for  several  years 
from  about  1895.  This  was  the  well-known  external  armature 
construction,  in  which  a  ring  wound  armature  surrounded  a 
stationary  multipolar  internal  field  structure.  The  armature  was 
ring-wound,  the  inner  surface  cutting  the  field,  while  the  outer 
surface  formed  the  commutator.  The  brushholder  thus  sur- 
rounded the  entire  armature.  When  first  introduced  into  America, 
these  generators  used  metal  brushes,  this  being  possible  due  to  the 
surface  type  of  winding,  low  voltage  per  bar,  wide  neutral  zone,  etc. 
However,  in  American  railway  practice,  metal  brushes  did  not 
prove  entirely  satisfactory,  especially  in  case  of  short-circuit,  as 
they  burned,  and  burred  and  "welded"  badly.  Carbon  brushes 
were  used  later,  but  the  general  construction  was  not  very  suit- 
able for  such  brushes,  This  type  of  machine  as  a  whole  was  not 
competitive  with  the  rugged,  well-protected  armature  and  com- 
mutator construction  of  other  American  makes,  and  it  dropped 
out  when  the  American  Siemens-Halske  Company  went  out  of 
business.  It  is  interesting,  however,  as  a  late  survivor  of  the 
surface-wound  type  of  railway  generator  armature. 

The  engine-type  construction  in  general  soon  spread  into  all 
fields  of  electric  generator  work,  such  as  lighting,  electrolytic  work, 
etc.,  and  was  a  standard  construction  for  many  years,  before  it 
suffered  a  decline.  In  small  lighting  work,  the  type  persists  today, 
but  has  now  almost  disappeared  in  railway  work,  due  largely  to 
the  general  introduction  of  the  polyphase  alternating-current 
system  of  generation  and  transmission  of  power,  with  conversion 
to  direct  current  by  rotary  converters  and  motor-generators.  The 
period  ^of  decline  b^g&a  about  1898  to  1900,  whan  large  capacity 
rotary  converters  began  to  take  the  field. 


664  ELECTRICAL  ENGINEERING  PAPERS 

When  the  engine-type  practice  was  in  vogue,  some  very  large 
units  were  constructed  for  comparatively  low  speeds — 1  000,  1  500 
and  2  000  kw  units  at  75  to  80  r.  p.  m.  were  common,  and  quite  a 
number  of  railway  units  of  3  000  kw  were  built.  For  lighting  service, 
some  units  of  still  larger  capacity  were  built. 

The  engine  type  machine  in  its  prime  was  a  magnificient  piece 
of  apparatus.  On  account  of  its  low  speed,  it  was  of  comparatively 
large  dimensions  for  a  given  output.  In  the  largest  capacity,  low- 
speed  engine-type  generators,  overall  dimensions  of  25  to  27  feet 
were  attained.  This  is  very  large,  compared  with  present  practice, 
which  is  confined  almost  entirely  to  relatively  high-speed  machines 
However,  large  as  they  were,  they  were  midgets,  both  in  size  and 
capacity,  alongside  some  of  the  alternating-current  engine-type 
generators  at  their  maximum.  The  latter  were  constructed  up  to 
capacities  of  5  000  to  6  000  kw  compared  with  3000  kw  for 
direct  current,  while  the  engine-type  alternators  attained  overall 
diameters  as  high  as  42  feet.  Incidentally,  as  regards  capacity  alone, 
the  race  between  alternators  and  direct-current  machines  has  been 
very  much  one-sided,  almost  since  the  polyphase  system  became 
thoroughly  commercial,  the  earliest  Niagara  generators  (constructed 
in  1893),  of  3  750  kw,  being  practically  of  as  large  capacity  as  the 
largest  direct-current  machine  ever  built;  while  in  later  poly- 
phase work,  generators  of  the  usual  multipolar  construction  have 
been  built  up  to  17  000  kw,  and  turbo-generators  up  to  30  000  and 
35,000  kw.  Obviously,  as  regards  marimum  capacity,  the  direct- 
current  machine  makes  but  a  poor  comparison,  for  reasons  which  do 
not  come  within  the  scope  of  this  paper.  Nevertheless,  this  should 
in  nowise  detract  from  the  direct-current  generator,  as  an  engineer- 
ing accomplishment. 

In  general,  the  engine-type  constructions  of  different  manu- 
facturers were  very  similar,  except  in  details.  The  principal  dif- 
ferences were  in  the  way  the  field  yoke  was  split,  in  the  construc- 
tion of  the  field  poles  and  field  winding,  and  in  the  details  of  the 
armature  winding,  as  already  described.  In  the  earlier  Westing- 
house  machines,  the  field  yoke  was  split  vertically,  so  that  the  two 
halves  could  be  moved  away  from  the  armature  in  a  direction  at 
right  angles  to  the  shaft.  The  G.  E.  construction,  in  general,  was 
horizontally  split,  and  access  to  the  armature  was  obtained  either 
by  sliding  the  field  parallel  to  the  shaft  or  by  removing  field  poles 
or  by  lifting  off  the  top  half  of  the  field.  There  was  much  argument 
regarding  the  respective  merits  of  these  two  constructions. 


DEVELOPMENT  OF  THE  D.C.  GENERATOR  665 

FIELD  POLES  AND  WINDINGS 

On  the  subject  of  field  poles  and  field  windings,  something  may 
be  said,  because  this  part  of  the  direct-current  machine  underwent 
many  modifications  in  type,  materials,  etc.  In  the  early  bi-polar 
machines  already  described,  the  pole  pieces  and  poles  varied  with 
the  different  types.  The  Edison  and  T-H  constructions  used 
wrought  iron  or  cast  steel  in  both  the  poles  and  yoke,  as  far  as  the 
writer  knows.  The  field  cores  in  the  Short  machines  were  of  wrought 
iron  or  cast  steel,  and  presumably  similar  material  was  used  in  the 
pole  pieces.  All  these  machines  had  cylindrical  magnet  cores  with 
cylindrical  field  coils  surrounding  them.  The  United  States 
(Westinghouse)  bi-polar  machine  had  cast  iron  fields  throughout. 
The  magnet  cores  were  oval  in  shape  instead  of  circular. 

When  the  multipolar  generators  came  in,  various  constructions 
of  poles  and  pole  pieces  were  used  by  different  manufacturers. 
The  Westinghouse  Company  used  poles  of  rectangular  shape,  of 
laminated  steel,  which  were  cast  into  the  yoke.  The  field  coils 
were  of  rectangular  shape  and  were  slipped  over  the  poles  from 
the  air-gap  end.  The  rectangular  shape  of  magnet  core  and  the 
laminated  construction  has  been  retained  throughout  by  the 
Westinghouse  Company  in  their  multipolar  generators,  except 
in  some  early,  relatively  small  capacity  belted  and  engine-type 
generators,  in  which  cast-iron  poles  were  cast  integral  with  the 
yoke.  These  also  were  rectangular  in  shape. 

Many  of  the  other  manufacturing  companies,  in  their  early 
multipolar  machines,  used  wrought  iron  and  steel  very  extensively 
in  the  magnet  cores  and  pole  pieces  and,  in  some  cases,  in  the  yoke. 
Frequently  the  magnet  cores  were  made  cylindrical,  while  the  pole 
pieces  or  caps  were  rectangular.  The  theory  was  that  the  cylin- 
drical core  was  the  most  economical  shape  for  both  iron  and  copper. 
This  of  course,  was  true  where  the  armature  diameter  was  the  lim- 
iting dimension  in  the  machine  and  where,  in  consequence,  there 
was  plenty  of  field  space  for  use  of  the  cylindrical  poles.  For  a 
given  section  of  field  iron,  obviously  the  cylindrical  type  of  core 
and  winding  required  more  room  rircumf  erentially  around  the  ar- 
mature, than  rectangular  poles  of  equivalent  section. 

The  solid  pole  face  was  not  very  objectionable  on  the  eajrly 
machines^  especially  where  the  air-gaps  were  large,  and  the  arma- 
ture slots  were  relatively  narrow.  However,  the  tendency  of  design 
was  towaxd  widecr  armature  slots  with  several  bars  side  by  side  in, 
each'  slot,  as  this  allowed  considerable  increase  i#  capacity  for  a 


«66  ELECTRICAL  ENGINEERING  PAPERS 

gjven  armature  diameter,  and  also  the  wider  slot  permitted  better 
oommutating  conditions.  Also,  especially  in  engine-type  machines 
with  many  poles,  the  design  tended  towards  smaller  air-gaps 
Consequently,  conditions  were  soon  reached  where  there  was  con- 
siderable "bunching "  of  the  magnetic  flux  in  the  pole  faces,  due  to 
the  relatively  wide  armature  slots".  This  meant  loss  and  heating  in 
solid  pole  faces,  especially  under  flux  distortion  with  load.   With 
laminated  poles,  this  heating  was  apparently  very  small,  but  with 
solid   poles  it  was  sometimes  excessive — so  much  so  that,    in 
some  cases,  the  mantifacturers  of  machines  with  such  solid  pole 
tips  would  turn  circumferential  grooves  in  the  pole  faces  to  "semi- 
laminate  ' '  them    In  some  cases,  solid  magnet  cores  were  used  with 
laminated  pole  tips.    The  Bullock  Company,  like  the  Westing- 
house,  used  laminated  poles,  but  its  successor,  the  Allis-Chalmers 
Company,  adopted  solid  poles  in  some  of  its  large  machines,  but 
eventually  "returned  to  the  laminated  construction.     The  T-H 
Company  and  later  the  G.  E.  Company  used  solid  poles  and  pole 
tips  for  many  years.  In  many  cases,  however,  their  magnet  cores 
were  rectangular  in  shape  just  as  in  present  practice.   Unlike  the 
early  Westinghouse  machines,  the  G.  E,  poles  were  bolted  to  the 
yoke  which  was  sometimes  of  cast  steel  and  at  other  times  of  cast 
iron,  while  the  early  Westinghouse  poles  were  laminated  and  cast 
into  the  yoke,  as  already  described,  the  yoke  being  cast  iron.  Thus, 
both  constructions  contained  some  of  the  elements  of  present 
standard  practice,  which  embodies  laminated  poles  of  rectangular 
section,  bolted  to  either  cast  iron  or  cast  steel  yokes. 

In  the  earlier  generators,  the  Crocker-Wheeler  Company  used 
cylindrical  poles  with  solid  pole  tips,  but  with  somewhat  larger 
air-gap  than  used  by  other  manufacturers,  thus  avoiding,  to  a  con- 
siderable extent,  any  undue  losses  in  the  pole  faces. 

FIELD  WINDINGS 

The  construction  of  field  windings  is  so  closely  related 
to  that  of  the  pole  pieces  that  a  brief  account  of  their 
development  may  be  given  at  this  point.  Practically  all  the 
early  field  coils  were  wound  in  metal  bobbins  or  shells.  They  were 
usually  very  heavily  insulated,  both  inside  and  outside.  The  metal 
shells  were  first  lined  with  paper  or  other  insulation  to  a  consider- 
able thickness;  the  wire  was  then  wound  in,  usually  with  much 
paper  or  cloth  between  the  layers,  and  thea  the  outside  surface 
was  covered  possibly  }^  i&-  deep  with  a  finishing  layer  of  rope. 
The  whole  construction  was  a  most  excellent  one  for  keeping  in  the 


DEVELOPMENT  OF  THE  D  C.  GENERATOR  667 

heat.  If  a  coil  ran  too  hot,  more  copper  and  insulation  were  added, 
instead  of  improving  the  heat  dissipating  and  ventilating  condi- 
tions. Naturally,  in  following  such  lines,  the  field  coils  eventually 
became  very  massive.  Shunt  field  coils  on  railway  generators  were 
not  infrequently  four  or  five  inches  deep.  When  one  of  these  coils 
roasted  out  it  was  usually  found  that  the  first  half  inch  of  wire  next 
to  any  heat-dissipating  surfaces  was  usually  in  fair  condition, 
while  deeper  in  the  winding  was  progressively  worse.  To  overcome 
this,  in  some  cases  the  field  coils  were  made  in  two  concentric  parts 
with  a  narrow  space  between.  This  was  the  first  step  towards  im- 
proving the  ventilation. 

In  the  construction  of  the  early  field  coils,  the  writer  ob- 
jected often,  and  strenuously,  to  the  enormous  amount  of  insula- 
tion embedded  between  layers  in  such  coils,  and  also  to  the  great 
depth  of  insulation  in  the  metal  shells.  This  great  depth  in  the 
shells  was  due  largely  to  the  fact  that  the  various  parts  of  the 
insulation  were  "butted"  instead  of  being  overlapped,  so  that 
great  thickness  was  required  to  give  sufficient  creepage  distance 
One  early  improvement  was  in  the  use  of  overlapped  insulation  at 
the  joints,  which  allowed  a  great  reduction  in  thickness.  Also,  the 
introduction  of  coils  without  metal  shells,  which  followed  from  the 
use  of  similar  coils  by  the  Westinghouse  Company  in  railway 
motors,  allowed  the  outside  surfaces  to  be  insulated  after  the  coil 
was  completed.  This  was  another  step  in  the  direction  of  reduced 
insulation,  for  this  type  of  coil  could  be  insulated  more  satisfactor- 
ily and  with  less  danger  of  bad  joints,  than  when  the  shells  were 
used.  But  still  enormous  quantities  of  insulation  were  used  between 
layers.  The  writer  arranged  a  "  horrible  example  "  of  this  one  day 
when  tearing  down  a  large  field  coil.  The  insulation  between  layers 
was  carefully  piled  up  as  the  coil  was  unwound,  until,  at  the  finish, 
the  pile  of  insulation  from  the  inside  of  the  coil  was  several  times 
larger  than  the  original  coil,  due  of  course  to  being  loosely  piled, 
But  it  was  hardly  believable  to  the  observer,  that  all  that  "stuff" 
came  from  the  inside  of  the  coil.  Gradually,  however,  it  was 
found  that  much  of  this  internal  insulation  could  be  omitted.  Its 
only  use  originally  was  to  prevent  short-circuits  between  layers 
while  winding  the  coil,  as  the  wire  was  hammered  pretty  hard  while 
winding,  in  order  to  take  out  the  "bulge/* 

In  series  field  coils,  originally  the  Westinghouse  Company 
used  round  wire  for  the  woxding,  and  as  the  size  of  machines  in~ 
creased,  two  or  more  wires,  or  two  or  more  field  coils,  were 


668  ELECTRICAL  ENGINEERING  PAPERS 

leled.  In  all  cases  the  series  winding  was  placed  beside  the  shunt 
winding,  and  generally  next  the  yoke  in  the  earlier  machines. 
Later,  strap,  wound  flatwise,  was  used  in  some  cases;  but  about 
1895  the  strap  on  edge  alternator  field  winding  was  developed, 
and  almost  immediately  the  Westinghouse  Company  used  this 
same  winding  for  series  field  coils.  Incidentally,  it  may  be  men- 
tioned that  the  writer  applied  for  a  patent  on  this  edge-wound 
field  coil  construction  but,  to  his  surprise,  found  that  it  had  been 
covered  by  a  patent  about  50  years  before,  in  connection  with 
electro-magnets. 

In  the  earlier  Thomson-Houston  (and  G.  E.)  machines,  the 
field  coils  were  wound  in  metal  bobbins,  and  this  construction  was 
retained  somewhat  longer  than  by  the  Westinghouse  Company. 
In  many  cases  the  series  winding  consisted  of  strap  or  ribbon,  wound 
flatwise,  outside  the  shunt  winding.  The  merits  of  this  construc- 
tion, compared  with  the  strap-on-edge,  were  much  discussed,  but 
apparently  both  were  sufficiently  good  constructions  for  those 
times. 

As  heat-conducting  and  radiating  conditions  and  ventilation 
became  better  understood,  the  outer  insulation  on  the  coils  was 
reduced  materially,  and  precautions  were  taken  to  ventilate  the 
field  wmdings  more  thoroughly.  Series  windings  were  better  ex- 
posed to  the  air,  and  shunt  windings  were,  in  some  cases,  sub- 
divided in  order  to  increase  the  effective  ventilating  surfaces.  Also, 
in  view  of  the  fact  that,  with  heavy  deep  coils,  the  center  portion 
would  be  roasted  out,  while  the  outside  part  would  be  comparatively 
good,  practice  gradually  tended  toward  comparatively  shallow  coils, 
arranged  for  good  air  circulation  over  them.  In  series  coils  and  in 
coirnnutating-pole  windings,  where  comparatively  heavy  strap  or 
bar  conductors  are  used,  the  individual  turns  are  now  separated  by 
air  spaces  in  many  cases.  In  other  words,  in  modern  design,  low 
temperatures  are  obtained  not  by  piling  on  material,  but  by 
improvements  in  heat  dissipation. 

COMMUTATION 

The  problem  of  commutation  and  the  conditions  which  in- 
fluenced it  were  of  paramount  importance  in  the  early  days.  The 
theory  of  commutation  was  understood  crudely,  and  the  conditions 
which  gave  good  commutation  were  more  or  less  appreciated.  It 
was  known  that  a  surface-wound  armature  should  commutate  bet- 
ter than  the  slotted  type,  with  the  same  number  of  turns  per  com- 


DEVELOPMENT  OF  THE  D.C   GENERATOR  669 

mutated  coil,  and  with  the  same  current  per  coil.  It  was  well  under- 
stood that  embedding  the  coil  in  the  slot  would  increase  the  self- 
induction,  and  thus  render  commutation  more  difficult.  The 
advantages  of  the  slotted  construction  were  pretty  well  appreciated 
before  its  adoption,  but  everybody  feared  the  commutation.  It 
was  not  appreciated  that,  in  adopting  the  slotted  construction,  the 
number  of  armature  turns  in  general,  and  the  number  of  turns  in 
series  per  coil  in  particular,  could  be  reduced  sufficiently  to  over- 
come the  inherently  higher  self-induction  of  the  slotted  construc- 
tion. As  soon  as  the  slotted  construction  proved  practicable  in 
large  machines,  a  new  era  began  in  the  commutating  problem. 
Designers  studied  and  analyzed  the  comtnutating  conditions  and 
limitations  much  more  closely  than  ever  before,  and  many  tests 
were  made  solely  for  the  purpose  of  getting  commutation  data. 
In  this  study  it  was  soon  determined  that  high  saturation  of  the 
armature  teeth  was  beneficial  in  maintaining  a  fixed  lead  at  the 
brushes.  At  that  time,  in  preparing  a  brief  written  analysis  of 
commutating  conditions  in  slotted  machines  for  Mr.  Albert 
Schmid,  then  superintendent  of  the  Westinghouse  Company,  the 
writer  showed  the  beneficial  effects  of  high  armature-tooth  satur- 
ation and  explained  the  reason  why  this  was  so,  as  well  as  the 
theories  of  that  time  would  permit,  and  he  furthermore  showed 
that  saturation  of  the  pole  face  in  general,  and  the  pole  corners  or 
edges  in  particular,  should  accomplish  similar  results.  For  then 
apparently  good  reasons  (but  which  afterward  proved  to  be  en- 
tirely wrong)  it  was  decided  that  it  was  not  worth  while  trying  for 
a  patent.  A  year  later,  however,  Mr.  N.  W.  Storer  applied  for  and 
obtained  a  patent  covering  cutting  away  part  of  the  laminations 
in  the  field  pole  corners  in  order  to  produce  high  saturation.  Mr. 
Wm.  Cooper  (formerly  with  the  Bullock  Company,  and  afterward 
with  the  Westinghouse  Company),  also  obtained  a  patent  on  cut- 
ting away  the  laminations  across  the  whole  pole  face.  These  two 
patents  led  into  certain  expensive  lawsuits,  but  both  arrangements 
were  considerably  antedated  by  the  author's  writtea  analysis  re- 
ferred to  above. 

The  advantages  of  a  "stiff"  field  in  preventing  shifting  of  the 
armature  neutral  point  was  known  comparatively  early.  With  the 
big  air-gaps  on  the  surface-wound  machines,  there  was  not  much 
difficulty  in  getting  the  field  ampere-turns,  ot  field  strength,  touch 
higher  than  that  of  the  armature*  But  with  the  adoption  of  the 
slotted  armature  construction,  there  was 


670  ELECTRICAL  ENGINEERING  PAPERS 

toward  reduction  of  the  air-gap  in  order  to  obtain  more  economical 
designs.  Experience  soon  indicated  that  it  was  much  more  econ- 
omical to  obtain  a  "stiff"  field  by  saturating  the  armature  teeth 
or  the  field  pole  tips  or  pole  face,  than  by  putting  the  excitation  in 
the  air-gap  alone.  Thus  saturation  in  the  path  of  the  armature 
cross  ampere-turns  soon  became  the  regular  practice.  Saturating 
the  armature  teeth  meant  more  slot  or  copper  space,  but  meant 
higher  iron  losses.  Saturating  the  pole  face  or  pole  comers  gave 
much  lower  iron  losses,  but  slightly  less  copper  space  and  copper. 
However,  in  general,  saturating  the  pole  corners  appeared  to  give 
better  all  around  results,  and  this  method  eventually  became 
standard  practice  with  practically  all  manufacturing  companies. 

Another  important  condition  in  the  problem  of  commutation 
was  the  armature  self-induction.  In  the  early  days  much  was  talked 
and  written  about  mutual  induction  in  commutation  After  the  ad- 
vent of  the  slotted  construction  experience  soon  began  to  point  out 
that  the  important  factor  in  limiting  commutation  was  the  self- 
induction  of  the  individual  coils,  rather  than  their  mutual  induc- 
tion. Therefore,  slot  construction  soon  tended  toward  lower  self- 
induction,  that  is,  toward  wide  slots.  At  first,  on  account  of  the 
imaginary  large  effect  of  mutual  induction,  it  was  not  considered 
advisable  to  place  two  or  more  separate  coils  in  one  slot,  and  there- 
fore a  large  number  of  comparatively  narrow  slots,  corresponding 
to  the  number  of  commutator  bars,  was  common.  However,  with 
the  recognition  of  self-induction,  and  not  mutual  induction,  as  the 
controlling  factor,  practice  soon  tended  toward  two  and  three  coils 
per  slot,  with  correspondingly  fewer  slots  and  relatively  better  slot 
proportions.  The  results  in  general  were  favorable,  and  at  the  same 
time,  with  fewer  slots  and  more  conductors  per  slot,  the  total  insul- 
tion  space  was  decreased  and  the  copper  space  was  correspond- 
ingly increased.  This  was  one  of  the  really  big  steps  in  increasing 
the  capacity  and  decreasing  the  dimensions  of  generators.  How- 
ever, like  many  other  good  things,  this  had  to  be  carried  too  far 
before  the  best  proportions  could  be  found,  and  in  quite  a  number 
of  cases  too  few  slots  and  too  many  bars  per  slot  were  tried,  result- 
ing in  special  commutating  troubles,  due  to  improper  magnetic 
conditions. 

In  working  over  the  problem  of  reducing  the  self-induction, 
the  writer  conceived  the  idea  of  purposely  so  arranging  the  arma- 
ture winding  that  the  upper  and  lower  coils  in  the  same  slot  would 
not  be  commutated  or  reversed  at  the  same  moment.*  This  was 

*U.  S.  Patent  No.  588,279 


DEVELOPMENT  OF  THE  D  C,  GENERATOR  071 

accomplished  by  changing  the  throw  of  the  coil  from  full  pitch  to 
one  or  more  slots  more  or  less  than  the  full  pitch.  In  two-circuit 
windings  with  one  turn  per  coil,  the  end  connector  at  one  end  ne- 
cessarily has  to  span  more  than  full  pitch  if  the  end  connector  at  the 
other  end  spans  less.  This  scheme  of  " fractional  pitch,"  or 
"chordcd"  winding  was  soon  tried  out  and  proved  to  be  quite 
beneficial,  except  in  those  cases  where  the  neutral  or  commutating 
zone  was  too  narrow.  This  arrangement  was  very  widely  adopted, 
and  remained  in  general  use  until  the  commutating  pole  came  in. 
With  this,  at  first,  full  pitch  windings  were  used,  but  now  the 
" fractional  pitch"  or  "chorded"  armature  winding  has  come  into 
extended  use  in  some  types  of  commutating  pole  machines. 


EQUALIZING  CONNECTIONS  ON  ARMATURE  WINDINGS 

The  various  types  of  armature  windings  and  their  effects 
should  be  considered.  As  indicated  bef  ore,  the  series  or  two-circuit 
winding  was  used  principally  on  the  early  slotted  armature  ma- 
chines of  moderate  capacity.  The  parallel  drum  type  winding  on 
multipolar  machines  was  well  known  at  this  time,  but  most  of  the 
machines  built  were  not  large  enough  to  require  this  winding. 
However,  as  larger  capacities  came  in,  it  was  recognized  that  the 
armature  winding  would  have  to  be  subdivided  into  more  paths, 
principally  on  account  of  commutation,  and  the  parallel  type  of 
winding  began  to  be  used.  With  this  type  of  winding  it  was  soon 
noticed  that  the  commutating  conditions  were,  not  infrequently, 
considerably  poorer  than  in  the  two-circuit  winding,  on  the  basis 
of  equivalent  windings  and  commutator  bars.  This  was  particu- 
larly true  in  machines  with  more  than  four  poles.  It  was  soon  dis- 
covered that  there  was  unequal  division  of  current  among  the 
various  parallel  circuits,  and  tests  indicated  that  this  was  due 
primarily  to  unequal  e.  m.  f/s  generated  in  the  different  parallel 
circuits  by  inequality  in  field  strengths  of  the  different  poles. 
This  necessitated  very  careful  adjustments  of  the  air-gaps  around 
the  machine  and,  where  the  discrepancy  was  apparently  due  to  the 
magnetic  material  itself,  such  as  the  poles  or  yoke,  it  was  in  some 
cases  the  practice  to  adjust  the  individual  field  coils  to  give  the 
required  equality  of  field  magnetic  strengths.  This  was  a  prac- 
ticable but  not  very  satisfactory  situation.  In  some  cases,  the 
unbalancing  was  so  bad  that  some  of  the  parallel  circuits  would 


672  ELECTRICAL  ENGINEERING  PAPERS 

feed  back  through  others,  so  that  the  relative  current  unbalancing 
was  actually  increased.    This  action  appeared  to  be  as  follows . — 
When  any  armature  circuit  carried  a  current,  it  tended  to  "cross 
magnetize"  the  field  pole,  strengthening  the  flux  at  one  pole  edge 
and  weakening  it  at  the  other.    Without  saturation,  these  two 
actions  should  balance  each  other,  so  that  the  total  pole  strength 
remained  practically  constant  regardless  of  the  flux  distortion. 
In  consequence,  the  armature  e.  m.    f.  per  pole  should  remain 
practically  constant.    However,  with  any  considerable  saturation 
in  the  path  of  the  cross  flux,  the  increased  flux  at  one  pole  corner 
was  not  equal  to  the  reduction  at  the  other  corner,  so  that  the 
resultant  total  flux,  and  the  e.  m.  f.  were  decreased.     Assume, 
for  instance,  a  ten-pole  parallel-wound  armature  in  a  field  in  which 
one  pole,  or  one  magnetic  circuit,  was  much  weaker  than  the  others. 
The  stronger  circuits  tended  to  feed  current  back  through  the 
weaker.     There  would  be  distortion  under  all  the  poles,  but  if 
eight  circuits  fed  current  through  two  circuits,  then  the  distor- 
tion in  the  two  circuits  would  be  much  greater  than  in  the  eight. 
If  there  were  high  saturation  in  the  path  of  the  cross  magnetic 
circuits,  then  all  the  magnetic  fields  would  be  weakened  to  a 
certain  extent,    but  the  two  normally  weaker  ones    would  be 
weakened  much  more  than  the  others,  due  to  the  larger  currents. 
Thus  their  e.  m.  f  's  would  be  still  further  reduced  and  more  cur- 
rent would  flow  through  them.    The  action  thus  would  become  cu- 
mulative, and  might  increase  until  destructive  local  currents  would 
flow  in  some  of  the  circuits.    In  certain  of  the  early  parallel-wound 
machines,  the  writer  observed  some  extreme  cases  of  this  action, 
in  which  the  current  gradually  increased  to  such  values  that  the 
carbon,  brushes  became  red  hot  over  the  whole  length,  and  the 
sparking  at  the  commutator  was  terrific.     Measurements  of  the 
armature  voltages  in  such  cases,  with  the  brushes  raised,  always 
showed  considerable  unbalancing. 

The  problem  of  unbalanced  circuits  in  parallel-wound  multi- 
polar  armatures  was  known  and  the  conditions  accepted  for  several 
years  before  a  true  remedy  was  found  for  it.  As  in  many  other 
cases,  this  remedy  resulted  from  an  unusually  severe  case  of  trouble. 
The  Westinghouse  Company  had  sold  a  number  of  high  current 
machines  for  electrolytic  work.  These  were  built  with  14-pole 
fields  and  parallel-wound  armatures.  The  then  usual  methods  of 
balancing  the  magnetic  circuits  were  relied  upon.  However,  on  test, 
the  first  of  these  machines  developed  undue  difficulty  in  maintain- 


DEVELOPMENT  OF  THE  D.C.  GENERATOR  073 

ing  balanced  magnetic  circuits.  The  finest  possible  adjustments 
were  necessary  to  obtain  reasonably  good  operating  conditions. 
Also,  due  possibly  to  the  dimensions  of  the  frame,  and  the  lack  of 
rigidity  in  the  temporary  foundation,  the  machine  would  get  out 
of  magnetic  alignment  very  easily,  and  would  get  into  vibration. 
As  soon  as  vibration  began,  commutation  troubles  would  com- 
mence and  would  grow  worse.  Mr.  Philip  Lange,  then  superin- 
tendent, gave  this  machine  his  personal  attention  with  a  view  to 
finding  how  to  adjust  for  permanently  good  results.  After  several 
days  of  adjustment,  he  became  somewhat  discouraged  and  told  the 
writer  that  he  did  not  believe  that  mechanical  adjustment  was  a 
satisfactory  solution  of  this  difficulty.  In  discussing  the  matter, 
the  writer  suggested  that,  as  two  or  more  separate  armatures,  oper- 
ating in  parallel,  would  have  their  field  strengths  equalized  by 
tying  them  together  electrically  through  polyphase  alternating- 
current  connectors,  therefore,  as  a  parallel-wound  multipolar 
armature  was  equivalent  to  a  number  of  separate  armatures  feed- 
ing into  a  common  direct-current  circuit,  it  was  theoretically  pos- 
sible to  balance  the  different  field  fluxes  by  tying  all  the  parallel 
armature  circuits  together  by  polyphase  connections.*  The  writer 
at  that  time  was  very  familiar  with  such  action,  through  extended 
tests  of  alternators  in  parallel,  rotary  converters,  etc. ;  also,  having 
applied  for  a  broad  patent  covering  the  principle  of  controlling 
the  field  strength  of  synchronous  generators  or  motors  by  leading 
or  lagging  alternating  currents.!  After  making  this  suggestion,  as 
to  a  possible  cure  for  the  above  trouble,  he  then  proposed  that  it  be 
tried  on  this  machine  in  a  comparatively  simple  manner  by  wind- 
ing three  insulated  copper  bands  over  the  front  end  of  the  armature 
winding  between  the  commutator  necks  and  the  armature  core, 
and  then  carrying  suitable  insulated  connectors  from  these  bands 
to  the  tops  of  the  commutator  necks,  using  seven  such  connectors 
per  ring.  In  this  way,  a  crude  but  easily  accessible  set  of  poly- 
phase connections  was  added  to  the  machine.  This  work  was 
cai'ried  far  into  the  night  before  being  ready  to  test.  Mr.  Lange  left 
word  that  the  results  of  the  tests  be  telephoned  him  as  soon  as  ob- 
tained. About  midnight  he  received  a  message  that  the  commuta- 
tion was  perfect  and  there  was  no  vibration.  He  then  suggested 
that,  possibly,  unusually  good  mechaucial  adjustment  had  been 
obtained  in  setting  up  the  field,  and  that,  therefore,  the  cross  CCH> 

*0,  $,  Patent  No.  573,009.  ttl.  S.  Puteftt  No,  582,131, 


674  ELECTRICAL  ENGINEERING  PAPERS 

nections  might  not  be  responsible  for  the  results.   As  a  check  on 
this,  he  asked  that  the  cross  connections  be  opened,  without  dis- 
turbing the  machine  otherwise  in  any  manner,  and  the  test  then 
be  repeated.    Under  this  condition,  the  sparking  and  vibration 
were  as  bad  as  ever,  or  possibly  worse  than  the  average  former 
results,  for  in  setting  up  the  field  after  adding  the  cross  connec- 
tions, no  particular  attempt  had  been  made  to  obtain  good  adjust- 
ment of  the  magnetic  circuit.  The  results  therefore  appeared  to  be 
conclusive,  and  the  several  machines  on  this  order  were  then  fitted 
with  three-phase  cross-connections,  and  all  showed  up  equally 
well  on  shop  test.    A  curious  condition  developed  in  one  of  these 
machines  after  installation  in  the  customer's  plant.   It  was  found 
that  one  of  the  generators  did  not  commutate  as  well  as  the  others, 
and  the  reason  for  this  was  not  discovered  for  several  weeks     It 
was  then  found  that  this  machine  did  not  have  the  same  shunt 
field  current  as  the  others.    Investigation  then  showed  that  two 
field  coils  had  -wrong  polarity,  namely,  those  marked  No,  6  and 
No.  9.    In  assembling  the  field  coils  on  the  poles,  the  numerals 
painted  on  the  coils  had  apparently  been  read  upside  down,  and 
the  coils  thus  interchanged.   In  those  days,  the  field  coils  had  the 
connections  "open"  or  " crossed'*  inside  the  coil  so  that  one  could 
not  determine  from  its  outside  appearance  what  its  polarity 
would  be.  Here  was  a  case  where  a  14-pole  parallel-wound  arma- 
ture had  operated  for  several  months  with  two  field  coils  reversed, 
and  yet  the  armature  cross-connections  actually  neutralized  the 
wrong  polarity  and  established  fields  of  the  correct  polarity  in 
their  place.    This  was  very  good  evidence  as  to  the  effectiveness 
of  the  cross-connections. 

As  soon  as  this  method  of  balancing  parallel  circuits  was 
developed,  it  was  applied  to  all  new  machines  being  built  by  the 
Westinghouse  Company,  and  was  also  applied  to  a  large  number 
of  parallel-wound  armatures  already  in  operation.  It  was  soon 
found  that  with  three  cross-connections,  there  was,  in  some  cases, 
a  tendency  to  "spot*1  the  commutator  at  as  many  points  or 
regions  as  there  were  cross-connecting  taps  to  the  commutator. 
These  spots  or  regions  were  always  between  the  cross-connecting 
taps  to  the  commutator.  To  overcome  this,  four  cross-connecting 
rings  were  tried  on  one  flagrant  case,  and  the  spots  were  still 
found,  but  of  less  width  than  before.  Six  cross-connections  were 
then  tried  on  the  same  machine,  with  still  evidence  of  a  little  spot- 
ting. A  total  of  nine  cross-connections  was  then  tried  and  the  spot- 


DEVELOPMENT  OF  THE  D.C.  VENERATOR  675 

ting  entirely  disappeared.  This  was  then  tried  on  various  ma- 
chines, with  equally  good  results,  and  therefore  a  comparatively 
large  number  of  cross-connections  was  adopted  as  standard  West- 
inghouse  practice.  Sometime  later,  it  was  common  practice  to 
cross-connect  all  the  commutator  bars,  but  it  was  found  that  this 
apparently  gave  no  better  results  than  a  considerably  less  number 
of  cross-connections.  Present  experience  seems  to  indicate  that  one 
cross-connection  per  armature  slot  is  as  far  as  it  is  necessary  to  go, 
even  in  the  most  extreme  cases. 

Cross-connection  of  parallel-wound  armatures  was  grad- 
ually adopted  by  other  companies,  until  now  it  is  practically 
universal.  Doubtless,  most  of  the  manufacturers  have  gone 
through  the  same  course  of  development  a*s  indicated  above,  in 
regard  to  the  number  of  cross-connections  used.  The  first  G.  E, 
cross-connected  armature  that  the  writer  saw  had  only  two  cross- 
connecting  rings,  but  later,  this  was  changed  to  11  rings,  the  ma- 
chine having  22  slots  per  pole.  This  was  one  of  the  things  that 
everybody  had  to  find  out  by  actual  experience.  The  use  of 
cross-connections  on  parallel  windings  has  undoubtedly  had  a  very 
great  influence  on  direct-current  development,  and  yet  normally 
it  has  appeared  to  be  simply  one  of  the  incidental  features  of 
direct-current  design. 

In  equalizing  the  c.  m.  f  .'s  by  equalizing  the  pole  fluxes,  natur- 
ally the  magnetic  pulls  around  the  machine  should  be  equalized. 
This  was  recognized  in  the  first  place,  but  an  actual  proof  of  it  was 
obtained  very  early  in  the  practice.  In  one  case,  with  a  large  engine- 
type  machine,  one  of  the  engine  pedestals  slipped,  the  armature 
being  thrown  to  one  side  until  it  was  practically  touching  the  field 
poles.  With  a  non-equali/,ed  armature  winding,  the  unbalanced 
magnetic  pull  under  this  condition  would  have  been  so  great  that 
the  field  would  have  "hugged"  the  armature  presumably  to  the 
point  of  destruction.  But  in  this  case,  the  armature  ran  freely  in 
the  field,  and  there  was  no  evidence  of  any  magnetic  pull  or  un- 
balancing, except  possibly  a  slight  sparking  at  the  commutator, 

In  the  early  days,  numerous  attempts  were  made  to  retain 
the  good  features  of  the  two~drcuit  winding,  and  still  increase  the 
number  of  armature  circuits.  This  was  mostly  before  the  parallel 
winding  had  been  perfected  by  the  development  of  cross-connec- 
tions. The  most  obvious  method  of  extending  the  field  of  the  two- 
circuit  winding  was  by  using  two  or  more  two-circuit  windings  on 
the  same  armature  core  and  arranging  them  to  operate  normally 


676  ELECTRICAL  ENGINEERING  PAPERS 

in  parallel  at  the  brushes.  When  the  several  two-circuit  windings 
were  entirely  independent  of  each  other,  they  were  known  as 
"sandwich  windings."  In  other  modifications  of  these  windings, 
the  various  circuits  closed  on  each  other  and  formed  the  so-called 
' '  re-entrant "  windings.  The  writer  had  some  early  experience  with 
both  these  arrangements,  but  neither  proved  very  satisfactory. 
In  the  first  case  tried,  a  direct-connected  machine  of  moderate  size 
had  a  sandwich  winding,  consisting  of  two  entirely  independent 
two-circuit  windings  connected  to  alternate  commutator  bars.  This 
was  run  on  test  for  24  hours  with  excellent  results.  Fortunately,  in 
order  to  do  some  special  testing,  it  was  then  operated  a  few  hours 
longer,  and  burning  of  alternate  commutator  bars  developed. 
The  burnt  bars  were  marked,  the  commutator  was  turned  down, 
and  the  tests  were  repeated.  Burning  again  occurred.  This  opera- 
tion was  repeated  several  times  with  like  results,  except  that  it 
was  noted  that  the  same  set  of  bars  did  not  always  burn.  After 
possibly  a  week  of  testing,  this  winding  was  abandoned  as  im- 
practicable. Apparently,  similar  results  were  found  by  other 
manufacturers,  except  that  in  some  cases  they  got  their  machines 
on  the  market  before  they  discovered  the  difficulty  of  burning 
alternate  bars.  Some  years  later,  Prof.  E.  E,  Arnold  of  Karlsruhe, 
Germany,  developed  a  system  of  cross-connections  for  such  arma- 
ture windings  which,  to  a  great  extent,  overcame  this  burning 
action  at  the  commutator.  This  system  consists  in  interconnecting 
various  points  of  equal  potential  in  the  different  circuits.  The 
writer  at  an  earlier  date  similarly  cross-connected  some  sand- 
wich type  two-circuit  closed  coil  windings  on  some  large  multi- 
polar  low  voltage  alternators.*  The  arrangement  of  armature  wind- 
ing and  cross-connections  was  the  same  as  Prof.  Arnold's  patent 
of  later  date.  If  commutators  had  been  connected  to  these  alter- 
nator windings,  the  arrangement  would  have  "been  the  same 
as  Prof  Arnold's.  These  same  machines  are,  the  writer  believes, 
in  operating  service  at  the  present  time. 

COMMUTATOR  MICA 

While  considering  questions  of  commutation,  the  effect  of 
windings  on  commutation,  etc.,  the  story  of  commutator  mica 
should  not  be  overlooked,  for  much  interesting  history  is  involved 
in  this.  The  use  of  mica  as  an  insulating  material  between  com- 
mutator bars  dates  far  back.  In  the  latter  Ws,  the  use  of  mica  in 

*IT.  S.  Patent  No.  680,793. 


DEVELOPMENT  OF  THE  D.C   GENERATOR  1>7T 

this  way  was  well  established,  although  red  fiber  and  similar  ma- 
terials were  still  used  occasionally.  However,  the  mica  practice  in 
those  days  was  not  at  all  standardized  or  uniform.  Various  thick- 
nesses of  mica  were  used  from  possibly  1-32  up  to  %  in,  A  thick- 
ness of  1-16  in.  seemed  to  have  the  preference  in  the  larger 
machines.  With  the  surface  armature  windings,  used  entirely  in 
those  days,  one  thickness  of  mica  appeared  to  serve  as  well  as  any 
other.  Also,  practically  all  mica  was  punched  out  of  solid  sheets* 
and  was  unsplit  as  far  as  possible.  In  railway  motors,  1-16  in. 
mica  was  also  in  fairly  common  use. 

In  designing  the  first  slotted  armature,  single  reduction  rail- 
way motor  in  1890,  as  already  described,  the  mica  between  the 
commutator  bars  was  made  1-32  in.  thick,  for  some  reason  which 
the  writer  does  not  now  recall.  On  test,  the  commutators  of  the 
first  two  machines,  which  were  made  with  this  thin  mica,  showed 
no  mica  trouble  whatever.  However,  this  very  thin  mica  was  noted 
in  particular  by  the  railway  people,  and  a  "howl  went  up  "  against 
it.  In  consequence  of  the  great  criticism,  later  motors  were  made 
with  much  thicker  mica,  although  not  nearly  as  thick  as  the 
general  criticism  called  for.  However,  when  these  later  motors 
went  into  commercial  service,  trouble  soon  developed  at  the  com- 
mutators. Very  bad  blackening  and  burning  of  the  commutator 
face  occurred  in  all  motors.  It  was  soon  noted  that,  in  all  cases  of 
such  burning,  the  commutator  mica  stood  above  the  copper  sur- 
face. It  was  not  evident  at  first,  however,  whether  the  burning 
was  inherent  in  the  slotted  type  of  machine,  with  the  high  mica 
merely  as  a  result  of  such  burning,  or  whether  insufficiently  rapid 
wear  of  the  mica,  compared  with  the  copper,  started  the  trouble. 
However,  it  was  found  that  scraping  down  the  mica,  below  the 
surface  of  the  copper,  stopped  the  burning  action.  It  thus  ap- 
peared that  insufficiently  rapid  wear  of  the  mica  was  back  of  this 
trouble.  Undercutting  of  the  mica  was  resorted  to  for  awhile,  but 
was  considered  as  only  a  temporary  remedy.  In  looking  for  a 
permanent  remedy,  the  action  of  the  first  two  motors  built,  with 
1-32  in.  mica,  was  carefully  studied.  It  was  noted  that,  although, 
these  two  motors  had  been  in  service  longer  than  any  of  the  others,, 
yet  no  tendency  for  burning  or  high  mica  had  ever  developed!, 
This  was  a  pretty  fair  indication  of  what  to  do,  and  therefore,  a 
number  of  additional  commutators  were  built  with  1-32  in.  mica, 
and  installed  in  places  where  there  was  comcnutator  blackeoang* 
All  these  new  comtrmtators  were  &  graat  improvement,  so  that 


678  ELECTRICAL  ENGINEERING  PAPERS 

many  of  the  old  commutators  with  thicker  mica  were  changed  to 
the  1-32  in.  mica.  This  practically  eliminated  the  trouble,  but 
occasionally  there  were  cases  where  black  spots  or  regions  devel- 
oped on  the  commutators  which  could  apparently  be  traced  to  local 
hard  places  in  the  mica.  Thus  the  problem  of  more  rapidly  wear- 
ing mica  came  up,  and  various  experiments  were  made  to  find  such. 
The  first  effective  step  consisted  in  splitting  the  mica  into  a  number 
of  fine  sheets  before  building  it  in  the  commutator,  with  the  idea 
that  its  rate  of  wear  would  be  increased  in  that  way.  This  did  prove 
fairly  effective,  particularly  in  large  generators.  As  the  slotted  type 
of  railway  generator,  however,  followed  about  a  year  after  the 
motor,  fortunately  the  investigations  of  the  motor  troubles  were 
then  under  way  and  their  temporary  remedies  were  applicable  very 
early  in  the  generator  work.  Sub-dividing  the  mica  as  described 
almost  eliminated  the  motor  mica  troubles  and  did  help  the  gen- 
erators very  considerably;  but  the  generator  trouble  was  harder 
to  eliminate,  because  everyone  was  opposed  to  the  use  of  1-32  in. 
mica  in  generator  commutators,  and  therefore  a  considerably 
thicker  mica  was  retained.  It  was  recognized  that  soft  quick 
wearing  mica  was  most  desirable,  and  it  was  soon  found  that  some 
kinds  of  mica  were  better  than  others.  Thus  the  softer  "amber" 
micas  were  soon  chosen  in  preference  to  the  harder  "  white"  micas 
for  use  between  commutator  bars.  Incidently,  in  connection  with 
this  question  of  soft  mica,  the  writer  recalls  an  incident  which  hap- 
pened in  1892.  A  fire  occurred  in  one  of  the  Pittsburgh  car  barns 
which  destroyed,  among  other  equipment,  a  car  containing  two 
Westinghouse  single-reduction  motors.  An  examination  of  these 
motors  was  made  by  the  writer  after  the  fire,  and  it  was  found 
that  the  commutators  had  been  apparently  red  hot.  In  dismant- 
ling one  of  these  commutators,  the  mica  between  commutator  bars 
which  formerly  had  been  in  solid  pieces,  came  out  in  the  original 
form,  but  was  semi-calcined  to  a  white  appearance,  and  was  split 
into  extremely  thin  laminae  and  appeared  to  be  very  soft.  The 
writer  suggested  at  the  time  that  here  was  the  kind  of  mica  that  we 
ought  to  have  in  commutators.  However,  it  was  considered  of  in- 
sufficient strength  to  use  in  commutators,  the  use  of  built-up  mica 
with  shellac  or  other  binding  material  not  being  then  well  known. 

The  use  of  finely  split  mica  without  any  binder,  in  commuta- 
tors, lasted  for  a  year  to  two.  Such  construction,  however,  re- 
quired relatively  large  sheets  of  mica,  which  were  unduly  expen- 
sive. Then,  "Micanite"  which  consisted  of  small  thin  laminae  of 


DEVELOPMENT  OF  THE  D.C.  GENERATOR  670 

mica  built  up  into  sheets,  with  a  suitable  binder,  came  into  use,  and 
this  eventually  solved  the  mica  problem,  at  least  partially,  as  far 
as  the  material  was  concerned.  But  several  grades  of  this  built-up 
mica  were  developed,  and  various  binding  materials  were  used  by 
different  manufacturers,  which  eventually  led  to  very  serious 
trouble  from  "pitting,"  etc.  In  this  pitting,  sparks  between  ad- 
jacent commutator  bars  would  gradually  eat  away  the  edges  of  the 
mica,  and  this  action  would  follow  the  burnt  edges  until  deep  places 
were  eaten  down  between  the  bars.  All  manufacturers  of  electrical 
machinery  encountered  more  or  less  trouble  from  this  pitting 
which,  in  many  cases,  was  charged  against  the  design  of  the  ma- 
chine. Eventually  the  cause  was  found  to  lie  principally  in  the 
binding  material  in  the  mica  plate,  and  with  improvement  in  this 
point  and  care  in  keeping  free  oil  off  the  commutators,  pitting  grad- 
ually disappeared. 

Recognizing  the  fact  that  mica  is  not  an  unmixed  blessing  in 
commutators,  various  attempts  have  been  made  from  time  to  time 
to  find  substitutes.  Electrically,  substitutes  may  be  found  without 
difficulty,  but  up  to  the  present  time,  none  of  them  have  shown 
suitable  physical  properties,  except  in  limited  applications.  "  Red 
fiber"  and  paper  were  used  many  years  ago  on  commutators  of 
surface-wound  machines,  apparently  with  very  good  success. 
Such  materials  wore  down  rapidly  under  the  brush,  especially  if 
any  sparking  occurred.  Therefore,  if  high  fiber  should  lift  the 
brush  away  from  the  commutator  face,  the  resultant  sparking 
would  soon  burn  the  fiber  down  flush  with  the  copper  so  that  good 
contact  between  brush  and  copper  would  again  result.  This 
action  in  itself  is  an  ideal  one.  However,  all  fibers  or  papers  are 
subject  to  more  or  less  expansion  or  contraction,  due  to  moisture 
conditions,  and  this  was  a  serious  objection.  If  any  considerable 
expansion  of  the  fiber  occurred  in  its  thickness,  that  is,  in  the  cir- 
cumferential direction,  of  the  commutator,  then,  when  shrinkage 
followed,  due  to  drying  out  of  the  moisture,  there  would  necessarily 
be  some  slight  looseness  circumf erentially  and  oil  or  other  foreign 
material  could  penetrate  between  commutator  bars,  which  would 
eventually  lead  to  pitting.  What  was  required  was  a  material  with 
some  elasticity,  so  that  the  space  between  commutator  bars  would 
always  be  filled  solidly  regardless  of  expansion  or  contraction  of 
the  cjotnmutator*  The  finely  laminated  structure  of  inica  plate, 
furnishes  the  necessary  elasticity,  but  apparently  no  other  mater- 
ial yet  available  fc&$  satisfactorily,  *&et  this  condition.  The 


680  ELECTRICAL  ENGINEERING  PAPERS 

ical  conditions  required  however,  were  not  as  well  understood  many 
years  ago,  and  hence  the  many  attempts  to  use  other  materials 
than  mica  in  commutators.  The  Westinghouse  Company  tried 
''fish  paper"  quite  extensively  years  ago,  in  a  number  of  low 
voltage  machines.  In  the  preliminary  tests  the  material  used  ap- 
peared to  be  influenced  but  little  by  atmospheric  conditions,  and 
excellent  results  were  obtained.  However,  in  attempting  to  extend 
its  use,  very  considerable  difficulty  was  encountered  in  finding 
material  which  was  as  little  affected  by  moisture  as  that  used  in 
the  first  tests.  It  developed  that  the  particular  qualities  desired 
for  this  service  were  exceptional  rather  than  normal,  in  the  manu- 
facture of  this  material.  Thus  it  soon  became  evident  that  this 
material  would  not  meet  the  requirements  in  general.  Various 
other  materials  were  tested,  such  as  asbestos  and  paper  built  up 
in  alternate  layers,  mica  and  paper  built  up  in  alternate  layers,  etc. 
While  all  of.  these  were  operative,  yet,  in  general,  they  did  not 
show  sufficient  promise  to  extend  their  use.  Such  experience  and 
developments  were  not  confined  to  the  Westinghouse  Company, 
for  presumably  all  enterprising  manufacturers  have  had  more  or 
less  the  same  experience. 

Within  recent  years,  practice  has  tended  very  largely  toward 
undercutting  the  mica.  This  has  not  been  due  to  mica  trouble 
primarily,  but  has  arisen  from  brush  conditions.  Experience  has 
shown  that  certain  grades  of  brushes  are  electrically  and  mechanic- 
ally very  desirable,  with  the  exception  that  they  do  not  have  suf- 
ficient grinding  or  wearing  action  on  the  mica.  In  order  to  ob- 
tain the  full  benefits  of  such  brushes,  in  many  cases,  it  is  necessary 
to  undercut  the  mica,  and  thus  eliminate  entirely  the  problem  of 
mica  wear.  This  is  especially  the  case  in  those  modern  relatively 
high-speed  machines  with  comparatively  thin  commutator  bars, 
in  which  the  thickness  of  mica  represents  a  fairly  large  percentage 
of  the  bar  thickness.  In  such  machines,  undercutting  was  first 
adopted,  but  as  the  beneficial  effects  of  the  better  brushes  became 
recognized  undercutting  has  been  very  generally  adopted  on  all 
high-speed  machines,  in  order  to  use  such  brushes.  This  under- 
cutting has  been  the  latest  important  step  in  the  mica  problem. 

BRUSHES  AND  BRUSHHOLDERS 

The  types  and  constructions  of  brushes  and  brushholders  have 
held  a  not  unimportant  place  in  the  development  of  direct-current 
apparatus.  On  the  very  early  apparatus,  copper  or  metal  alloy 


DEVELOPMENT  OF  THE  D  C.  GENERATOR 


681 


brushes  were  used  almost  exclusively.  These  were  even  used  on 
early  railway  motors,  but  the  results  could  not  be  considered  as 
highly  satisfactory.  It  is  hardly  conceivable  that  electric  traction 
could  have  reached  its  present  high  development  if  the  metal 
brush  had  been  retained.  About  1887  and  1888,  the  carbon  brush 
began  to  come  into  use  and  it  is  still  with  us,  with  no  prospects  of 
being  displaced  by  anything  else,  except  in  very  special  applications, 
such  as  extremely  low  voltage  work.  It  may  be  said  that  no  one 
thing  has  had  a  greater  influence  on  direct-current  development 
than  the  carbon  brush. 

By  the  time  the  larger  railway  generators  began  to  come  into 
•use,  as  described  in  the  first  part  of  this  article,  the  carbon  brush 


PIG    7— THE  DEVELOPMENT  OF  THE  BRUSHHOLDER 
A — The  sliding  type     B — The  swivel  type.     C — Parallel  motion  type.     D— Reaction  type 


was  well  established.  However,  the  manufacture  of  such  brushes 
in  those  days  was  rather  crude  compared  with  present  practice, 
which  also  may  be  said  of  everything  else  in  the  electrical  business. 
There  was  no  wide  range  of  varieties  or  grades  to  choose  from, 
and  the  carbon  was  a  carbon  brush  only.  If  there  had  been  many 
grades,  we  probably  would  not  have  known  how  to  apply  them. 
We  found  what  brush  gave  us  good  results  in  a  certain  case,  and 
we  stuck  to  it "  through  thick  and  thin. '  *  When  there  were  troubles, 
they  were  frequently  blamed  on  the  mica  or  the  brushholders. 
The  latter  were  thus  subject  to  continual  change.  Types  came  and 
went  and  then  came  back  again.  However,  out  of  the  multiplicity 
of  brushholders,  several  comparatively  distinct  types  arose,  Fig.,7, 
such  as  the  "sliding,"  the  "reaction"  (which  was  one  form  of  the 
sliding  type),  the  "swivel"  and  the  *' parallel-motion'1  types. 
Each  had  its  good  points  as  weU  as  bad.  All  of  these  types  are  still 
in  use  to  a  certain  extent,  but  certain  of  them  predominate  in 
present  manufacture.  It  so  happens  that  the  one  which  is  now  fur- 
nished almost  exclusively  with  the  larger  apparatus,  namely,  the 
sliding  type,  was  alao  one  6f  the  earliest  developed. 


682  ELECTRICAL  ENGINEERING  PAPERS 

The  early  Weston  type  railway  generator  built  by  the  U.  S. 
Co.  (described  in  the  early  part  of  this  article),  had  a  sliding  type 
carbon  brushholder.  When  the  Westinghouse  Company  took  tip  the 
manufacture  of  the  multipolar  generator,  sliding  carbon  holders 
were  used.  As  far  as  the  writer  knows,  the  early  T-H  generators 
also  had  sliding  holders.  The  railway  motors  of  that  time,  built  by 
the  Edison  (Sprague  system),  the  T-H,  the  Westinghouse,  and  the 
Short  Companies,  all  had  some  form  of  sliding  holder,  and  us- 
ually with  the  carbons  standing  radially  to  the  commutator.  In 
railway  motors,  there  was  a  definite  reason  for  this,  for,  as  such 
motors  operated  in  either  direction,  the  radial  carbon  was  as- 
sumed to  give  the  best  average  working  conditions.  Also  radial 
carbons  were  used  on  some  of  the  early  railway  generators,  but  it 
was  soon  found  that  a  slight  inclination  of  the  brushes  to  the  face  of 
the  commutator  allowed  a  somewhat  smoother  action,  there  being 
less  chattering  and  less  "screeching"  in  the  case  of  highly  polished 
commutators.  It  was  a  good  deal  like  moving  a  pencil  over  a 
sheet  of  glass.  With  the  pencil  held  too  nearly  vertical  to  the 
glass,  there  is  liable  to  be  chattering  and  screeching.  Thus  it  soon 
became  standard  practice  to  incline  the  brushes  when  the  sliding 
type  was  used. 

In  the  early  sliding  type  holders,  it  was  found  that  when 
heavy  currents  per  brush  were  carried,  there  was  a  tendency  to- 
wards burning  of  both  the  brush  and  the  inside  of  the  brush  box. 
In  some  forms  of  sliding  holders  it  was  attempted  to  overcome 
this  by  clamping  each  carbon  tightly  in  a  metal  box,  and  arranging 
for  the  boxes  to  slide  up  and  down  on  the  holders.  The  transfer  of 
current  was  thus  between  metal  surfaces.  But  eventually  this  de- 
veloped trouble.  The  obvious  remedy  for  all  these  cases  was  to 
attach  flexible  shunts  from  the  carbon  itself  to  the  solid  frame  of 
the  brushholder.  However,  the  designers  had  many  trials  and 
tribulations  before  this  remedy  was  well  developed.  In  the  early 
Westinghouse  generators,  before  the  use  of  shunts  became  general, 
it  was  found  that  very  long  carbons  were  effective  in  preventing 
burning  of  the  carbons  in  their  boxes,  and  this  practice  was  ad- 
hered to  pretty  faithfully  for  several  years.  Of  course,  the  bene- 
ficial effects  of  the  long  carbon  were  largely  in  increasing  the  con- 
tact surface,  and  in  lessening  any  tendency  to  chatter,  as  a  long 
carbon  would  not  "rattle  about"  in  its  box,  as  readily  as  a  short 
one. 


DEVELOPMENT  OF  THE  D,C\  GENERATOR  (>S3 

Due  to  the  difficulty  in  transferring  current  from  the  carbon 
to  the  holder,  in  the  sliding  shunt  type,  the  Westinghouse  Com- 
pany (and  presumably  all  other  companies,  also)tried  out  various 
forms  of  the  swivel  type  holder.  In  this  type,  the  brush  was 
clamped  in  a  metal  box  which  was  attached  to  an  arm  which 
swiveled  about  a  holder  rod  or  pin.  This  soon  showed  trouble  at 
the  swivel  contacts,  due  to  the  current,  and  therefore,  shunts  had 
to  be  attached  from  the  swivel  arms  to  the  holder  frame  in  order  to 
protect  the  swivel  joints  or  bearings.  However,  as  this  shunt  con- 
nected metal  to  metal,  it  was  not  difficult  to  apply.  This  type  of 
holder  therefore,  seemed  to  solve  the  problem.  But  another  dif- 
ficulty came  up.  About  this  time,  engine-type  generators  came  into 
general  use,  and  it  was  soon  found  that  the  swivel  type  holder  was 
liable  to  give  trouble  on  engine-type  machines,  owing  to  the 
"  weaving'*  action  of  the  armature  and  commutator  due  to  move- 
ment of  the  shaft  in  the  bearings,  resulting  from  the  engine 
crank  action.  As  long  as  the  commutator  ran  perfectly  true  with 
respect  to  the  brushholder,  the  brush  faces  would  follow  the 
commutator  perfectly.  But,  with  the  brushholders  hung  from 
the  generator  field  frame,  or  pedestals  on  the  generator  base,  it 
may  be  seen  that  with  any  weaving  action  due  to  the  engine 
cranks,  the  brush  faces,  with  swivel  holders  could  not  possibly 
remain  in  intimate  contact  with  the  commutator  face,  and  a 
periodic  "hcel-and-toc"  contact  would  result.  This  necessarily 
meant  sparking  under  the  brush  face,  with  consequent  gradual 
burning  away  of  the  brush  face  and  the  commutator  copper. 
Therefore,  the  commutators  gradually  "smutted";  that  is,  they 
got  "dirty,"  and  would  show  no  polish.  This,  of  course,  was  a 
fatal  defect,  and  eventually  put  the  swivel  holder  out  of  business, 
as  far  as  the  large  engine-type  generator  was  concerned.  However, 
this  type  of  holder  had  much  greater  success  on  self-contained 
machines,  in  which  the  commutator  and  brushholder  could  be 
made  to  run  perfectly  true  with  respect  to  each  other. 

Another  type  of  holder  which  later  came  into  use,  and  for 
which  great  claims  were  made,  was  the  parallel-motion  holder. 
This  was  somewhat  similar  to  the  swivel  type,  except  that  the 
brush  box  moved  up  and  down  parallel  to  itself  through  a  parallel- 
motion  arrangement.  This  parallel-motion  part  usually  consisted 
of  two  parallel  arms  of  flexible  material,  which  were  connected  at 
one  end  to  the  brushholder  frame  and  at  the  other  to  the  brush 
box.  The  flexible  axtns  were  made  of  laminae  of  copper,  bronase  or 


684  ELECTRICAL  ENGINEERING  PAPERS 

steel,  and  were  flexible  enough  to  allow  a  slight  up  or  down 
motion,  but  were  not  flexible  enough  to  make  the  holder  unduly 
flimsy.  In  prmtiplej  this  holder  seemed  a  very  good  one,  and  it 
held  its  own  for  some  years.  The  Crocker-Wheeler  Company 
probably  used  this  type  to  a  greater  extent  than  any  other  manu- 
facturer. It  appears  to  be  applied  but  little  at  present,  presum- 
ably due  to  questions  of  cost  and  space  requirements. 

A  fourth  form  of  brushholder,  namely,  the  reaction  type, 
is  in  reality,  one  form  of  the  sliding  type  holder,  for  the  brush 
slides  up  and  down  parallel  to  itself.  In  this  holder  the  sliding 
brush  is  inclined  to  the  commutator  and  holder  at  such  an  angle 
that  the  reacting  forces  tend  to  make  it  hug  the  brushholder  face, 
and  thus  give  contact  between  the  carbon  and  holder.  This  type  of 
holder  was,  at  one  time,  applied  quite  extensively  to  railway  gen- 
erators by  the  Bullock  and  the  Walker  companies,  and  is  still 
applied  to  small  machines,  to  some  extent. 

During  all  these  departures  and  variations  in  brushholder 
design,  the  straight  sliding  carbon  type  was  still  going  through 
a  course  of  development,  which  consisted  principally  in  simplify- 
ing and  "improving  "  the  construction  of  the  holder  itself,  and  the 
application  of  shunts  between  the  holders  and  carbons.  This 
latter  was  no  simple  matter,  and  almost  as  much  effort  has  been 
expended  in  suitably  attaching  shunts  to  carbon  brushes  as  in 
developing  carbon  brushholders  themselves.  One  great  difficulty 
was  that  any  new  method  of  attaching  the  shunt  to  the  carbon  had 
to  be  tried  out  in  actual  service  for  a  comparatively  long  period 
before  it  could  be  accepted  or  condemned,  and  usually  it  was 
condemned.  A  certain  shunt  attachment  might  prove  perfectly 
satisfactory  in  one  class  of  service,  and  would  be  almost  worthless 
in  another  class.  One  principal  difficulty  was  that  the  current,  in 
passing  from  the  carbon  to  the  shunt,  or  vice  versa,  would  tend 
gradually  to  eat  away  the  points  of  contact  so  that  eventually  the 
shunts  would  loosen  or  lose  contact.  This  was  a  pretty  big  prob- 
lem, and  in  the  early  part  of  the  development,  the  manufacturers 
of  the  electric  machines  usually  attached  the  shunts,  developing 
various  methods  of  doing  this.  Later,  however,  the  carbon  manu- 
facturers took  up  the  matter  and,  in  general,  were  able  to  produce 
simpler  and  better  methods  by  attaching  the  shunt  during  the 
formation  or  manufacture  of  the  carbon,  the  shunt  thus  forming 
an  integral  part  of  the  carbon  instead  of  being  an  after  attachment. 
With  the  greater  perfection  of  the  shunt  attachments,  the  sliding 


DEVELOPMENT  OF  THE  D.C   GENERATOR  685 

type  carbon  holders  began  to  dominate  the  field  until,  today,  this 
type  is  most  generally  used. 

There  were,  of  course,  many  variations  in  the  construction  of 
the  sliding  holders  themselves,  such  as  in  the  types  and  arrange- 
ments of  springs,  the  materials  and  methods  used  in  the  manu- 
facture of  the  holders,  the  sizes  and  shapes  of  carbons,  but  these 
have  apparently  had  no  very  controlling  effect  on  the  general  de- 
velopment. In  generators,  the  inclination  of  the  carbon  either 
toward  or  against  the  direction  of  rotation,  was  at  one  time  a  much 
mooted  point,  but  apparently  it  has  never  been  definitely  decided 
which  practice  is  better,  as  both  are  used  at  the  present  time,  and 
apparently  the  choice  depends  upon  local  conditions,  such  as 
commutator  speed,  lubricating  quality  of  the  carbons,  and  a 
number  of  other  conditions.  In  many  cases,  changing  the  holder 
from  either  direction  of  inclination  to  the  opposite  direction,  has 
apparently  helped  the  operation. 

GRADES  OF  BRUSHES 

In  recent  years,  much  more  attention  has  been  paid 
to  the  various  grades  of  carbons,  as  regards  their  conduct- 
ing and  lubricating  qualities,  softness,  etc.  Graphite  brushes, 
or  the  use  of  graphite  in  carbon  brushes,  was  long  ago  recognized 
as  furnishing  some  very  good  qualities.  However,  it  was  soon  noted 
that  brushes  with  much  graphite  in  them  were  liable  to  give 
41  smutty"  or  burnt  commutators,  at  least  in  railway  work.  This 
was  blamed  largely  on  the  brush,  which  was  possibly  true  to  some 
extent,  but  it  was  later  recognized  that  the  fault  was  partly  in  the 
inability  of  such  lubricating  brushes  to  wear  the  mica^down 
rapidly  enough.  In  some  cases,  two  grades  of  brushes  were  used 
on  a  machine  at  the  same  time,  part  having  high  abrasive  qualities 
and  the  others  being  of  a  graphite  nature  and  furnishing  good 
lubrication.  With  a  better  understanding  of  the  problem  has  again 
come  the  use  of  graphite  types  of  brushes,  with  undercut  com- 
mutators, and  they  appear  to  be  very  successful  in  many  cases. 
Thus  a  type  of  brush  which,  at  one  time,  was  condemned  for  rail- 
way work,  has  later  come  into  extended  use,  due  partly  to  changes 
in  constructive  conditions. 

Where  better  conducting  qualities  in  the  brushes  were  de- 
sired, the  so  called  carbon-gauze  brushes  have  been  tised  at 
times.  These  originated  probably  as  early  as  1892.  They  were 
used  on  large  railway  generators  to  some  extent,  but  principally 
in  connection  with  lower  voltage  machines.  In  these  brushes, 


686  ELECTRICAL  ENGINEERING  PAPERS 

sheets  of  fine  wire  gauze  were  embedded  in  the  carbon  during  the 
manufacture. 

The  question  of  plated  versus  unplated  brushes  came  up  very 
early  in  the  application  of  carbon  brushes,  and  is  not  satisfactorily 
settled  yet.  One  theory  was  that  plating  assisted  in  the  transfer  of 
current  between  the  carbon  and  the  brush  box,  reducing  the 
burning  action.  Another  theory  was  that  there  should  be  little  or 
no  flow  of  current  between  the  carbon  and  box,  and  therefore, 
plating  was  harmful,  especially  on  carbons  with  good  shunts. 
Again,  where  the  shunts  have  been  attached  simply  to  the  outside 
surface  of  the  carbon,  it  has  been  claimed  that  plating  assisted 
in  getting  the  current  to  the  shunt.  And  the  question  is  still  open 
for  discussion.  In  many  cases  probably  it  is  merely  a  matter  of 
personal  opinion.  There  are  so  many  variable  conditions  in 
each  machine  that  one  can  get  quite  different  results  at  different 
times,  or  under  different  conditions  of  service. 

BURNING  OF  BRUSHES,  "PICKING  UP  COPPER,"  ETC. 
Ever  since  carbon  brushes  came  into  use  there  has  been  more  or 
less  trouble  from  burning  of  the  brush  faces,  honeycombing  of  the 
carbon  structure  and  picking  up  of  copper.  These  do  not  always  go 
together,  but  there  are  certain  common  causes  for  all  these  actions. 
In  the  very  early  slotted  armature  generators  ample  brush  capa- 
city was  usually  furnished.  However,  gradually  the  ratings  of  such 
machines  were  increased,  without  radical  changes  in  the  propor- 
tions, due  largely  to  improvement  in  ventilation,  so  that  eventually 
the  carbon  brushes  were  worked  at  very  high  apparent  current 
densities  (densities  due  to  work  current  only).  It  was  soon  evi- 
dent that  the  brushes  were  worked  too  hard,  and  steps  were  taken 
to  improve  this  condition  by  increase  in  brush  size,  etc.  Brushes 
were  made  thicker  circumferentially,  with  a  view  to  reducing  the 
current  density,  the  fact  being  overlooked,  or  not  recognized,  that 
the  local  or  cross  currents  of  the  brush  face  were  back  of  a  consider- 
able part  of  this  brush  trouble.  In  many  cases  these  thicker  brushes 
did  not  improve  conditions,  or  were  even  harmful.  This  latter 
was  proved  to  be  the  case,  in  many  instances,  by  simply  beveling 
the  "toe"  of  the  thick  brush  in  order  to  reduce  the  breadth  of 
contact.  Very  often,  much  better  operation  was  obtained  with 
these  beveled  brushes,  although  the  apparent  current  density  in 
the  brush  was  increased,  but  in  fact,  the  actual  current  density 
was  decreased.  This  led  up  to  an  appreciation  of  the  fact  that  the 
local  current  in  the  brush  in  many  cases,  was  actually  greater  than 


DEVELOPMENT  OF  THE  D  C   GENERATOR  «S7 

the  useful  or  work  current.  The  writer  very  early  reached  the  con- 
clusion that  the  apparent  current  density  in  carbon  brushes  was  of 
no  real  importance  in  designs  or  guarantees,  unless  other  conditions, 
such  as  thickness  of  brush,  etc.,  were  also  apecified. 

Due  largely  to  too  high  actual  current  density  in  the  brush, 
there  was  much  trouble  in  some  of  the  early  machines  from  burn- 
ing of  the  brush  faces.  This  burning  would  begin  at  one  edge  of  the 
brush  and  gradually  travel  across  the  whole  brush  face,  until  the 
entire  face  had  been  burned  away  and  the  brush  tip  badly  honey- 
combed in  some  instances.  This  honeycombing  was  usually  co- 
incident with  "  glowing,"  that  is,  red  hot  spots  would  appear  in  the 
brush  tips.  Many  attempts  were  made  to  cure  such  conditions  by 
substitution  of  a  different  kiiad  of  brush,  and  sometimes  with  suc- 
cess, due  principally  to  change  in  brush  resistance,  with  conscqunt 
change  in  local  currents.  But  this  was  largely  "cut-and-try".  It  was 
also  found  that  improvement  in  the  inherent  commutating  con- 
ditions would  also  lessen  the  brush  trouble.  Obviously,  this  simply 
reduced  the  local  currents,  which  consequently  helped  both  the 
commutator  and  the  brushes, 

POLARITY 

It  was  noted  very  early  in  direct-current  work  that 
the  positive  and  negative  carbon  brushes  did  not  act  exactly  alike. 
Those  of  one  polarity  would  some  times  take  a  good  polish  at  the 
brush  face,  while  those  of  the  other  polarity  would  show  but  little 
polish.  Also,  the  brush  faces  would  sometimes  have  a  coating  of 
copper  formed  on  them,  or  small  particles  of  copper  would  embed 
themselves  in  the  brush  face.  This  action  was  not  the  same  for 
both  polarities.  It  was  quite  a  long  time  before  this  unequal  polish- 
ing and  picking  up  of  copper  was  even  partially  understood.  Even- 
tually it  was  found  that  when  a  current  passed  through  a  moving 
contact,  such  as  that  between  a  brush  and  a  moving  commutator, 
or  collecting  ring,  there  was  a  tendency  for  minute  particles  of  the 
material  of  the  contact  faces  to  be  eaten  or  burned  away,  depend- 
ing upon  the  direction  of  current.  When  the  current  passed  from 
the  brush  to  the  commutator,  the  brush  face  tended  to  eat  away, 
while  with  current  in  the  reverse  direction,  the  commutator  copper 
showed  this  effect.  With  low  current  densities  in  the  points  of  con- 
tact, it  was  noted  that  this  action  was  very  slight.  Also,  the  better 
the  contact,  that  is,  the  lower  the  resistance  of  contact,  the  less 
this  action  was.  In  some  cases- the  material  burned  away  from  one 
surface  was  deposited  on  the  opposing  fape,  possibly  mechanically. 


688  ELECTRICAL  ENGINEERING  PAPERS 

For  instance,  with  a  carbon  brush  and  current  passing  from  the 
carbon  to  the  commutator,  the  commutator  conditions  were 
averaged,  and  it  was  only  in  the  brushes  that  any  difference  would 
show.  Long  before  this  action  was  well  appreciated  it  was  indi- 
cated on  the  collector  rings  of  certain  rotating  armature  alter- 
nators and  rotary  converters.  In  some  of  these  machines,  with 
copper  brushes  on  the  rings,  but  with  very  high  current  densities 
at  the  brush  contacts,  it  was  found  that  the  rings  tended  to  wear  out 
of  round,  with  half  as  many  low  places  as  there  were  current  alter- 
ations per  revolution,  or  number  of  poles  in  the  field.  This  was 
quite  pronounced  in  some  cases,  but  was  usually  blamed  on  loose 
brushes,  because  increasing  the  brush  contact  pressure  usually 
helped  it  temporarily.  The  fact  was,  that  this  was  a  true  burning 
action,  as  above  described,  and  it  was  only  in  every  other  alterna- 
tion that  the  current  was  in  the  direction  which  would  burn  the 
collector  ring.  The  fact  that  the  different  collector  rings  did  not 
have  their  low  spot  in  phase  with  each  other  was  not  appreciated 
for  a  long  time. 

With  the  carbon  brushes,  this  action  between  the  commut- 
ator and  brushes  became  much  more  pronounced  as  the  apparent 
current  densities  were  increased.  Also,  picking  up  of  copper  became 
more  pronounced.  Both  brush  polarities  suffered  a  great  deal  from 
burning.  One  polarity  would  have  the  brush  face  burned  away  due 
to  the  direction  of  current,  this  action  being  cumulative  for,  as  the 
face  burned  away  at  one  edge,  due  to  the  sum  of  the  local  and  work 
currents,  the  contact  arc  would  be  decreased,  and  the  contact 
would  continue  to  burn  away,  although  the  local  currents  would  be 
lessened.  On  the  other  polarity,  where  such  burning  should  not  be 
expected,  a  coating  of  copper  would  form  in  some  cases,  and  this 
would  tend  to  lower  the  brush  contact  resistance,  and  thus  in- 
crease the  local  currents,  which  depended  upon  the  contact  resist- 
ance. Thus  the  brushes  of  this  polarity  would  also  be  burned,  due 
to  the  excessive  current.  Moreover,  as  the  copper  deposit  was 
frequently  very  irregular,  the  reduction  in  brush  contact  resist- 
ance would  be  local  only.  At  the  spots  of  lower  resistance,  an  excess 
part  of  the  work  current  would  flow,  tending  to  produce  local  heat- 
ing. As  the  temperature  coefficient  of  resistance  of  carbon  is 
negative,  any  local  heating  would  mean  still  lower  local  resistance, 
a  larger  percentage  of  the  total  current  concentrated  at  this  point, 
and  thus  more  heating,  the  action  becoming  cumulative,  until 
glowing  occurred  at  times.  This  abnormal  local  heating  tended  to 


DEVELOPMENT  OF  THE  D  C   GENERATOR  i>89 

disintegrate  the  brush,  so  that  cavities  formed  at  or  near  the  brush 
tip  and  the  carbon  became  "honeycombed."  This  action  was  not 
always  coincident  with  "picking-up-copper,"  for  anything  which 
produced  unequal  division  of  current  among  the  brushes  or  over  the 
brush  contact,  tended  toward  glowing  and  honeycombing.  This 
action  apparently  was  more  closely  connected  with  high  current 
densities,  either  locally  or  as  a  whole,  than  with  any  other  cause. 
Obviously,  with  the  brushes  worked  normally  very  close  to  the 
limit  as  regards  permissible  current  densities,  any  little  inequal- 
ities in  current  were  liable  to  have  a  more  pronounced  effect. 

This  action  has  been  dealt  with  rather  fully,  as  it  was  one  of 
the  very  serious  troubles  in  old  time  machines.  Many  attempts 
were  made  to  overcome  this  trouble  by  changing  the  kind  of 
brush,  the  type  of  brushholdcr,  proportions  of  the  armature,  etc., 
and  with  varying  success.  Burning  of  the  brushes  was  frequently 
accompanied  by  high  mica  on  the  commutator,  and  this  in  turn 
exaggerated  the  burning.  Undercutting  the  mica,  by  allowing 
more  intimate  contact  between  the  brushes  and  copper,  quite  fre- 
quently alleviated  this  contttion.  But  where  the  actual  current 
densities  were  very  high,  even  undercutting  did  not  cure  the 
trouble.  Eventually,  it  was  recognized  that  lower  actual  current 
densities  must  be  had,  and  when  this  was  thoroughly  appreciated 
and  embodied  in  the  designs,  burning  of  brushes  and  picking  up  of 
copper  were  of  much  less  usual  occurrence.  In  the  commutating- 
pole  machine,  referred  to  later,  the  local  currents  are  under  partial 
control,  and  thus  the  apparent  current  densities  in  the  brush  can 
be  brought  up  much  nearer  to  the  actual  limiting  densities,  so  that  • 
today  considerably  higher  apparent  densities  are  used  regularly. 

The  above  covers  one  principal  cause  of  brush  trouble.  How- 
ever, there  were  many  cases  of  trouble  in  which  the  brush  densities 
were  not  unduly  high,  considering  the  size  of  brush  face,  but  where 
the  effective  brush  contact  area  was  reduced  by  bad  mechanical 
conditions,  such  as  chattering  of  the  brushes,  commutators  out  of 
round,  etc.  It  was  not  always  possible  to  distinguish  between  the 
various  causes  of  brush  burning  and,  not  infrequently,  a  remedy 
which  worked  in  one  case  was  an  entire  failure  in  the  next.  Un- 
equal division  of  current  between  different  brush  arms  in  parallel, 
and  also  between  the  different  brushes  on  the  same  arm,  also 
greatly  complicated  the  problem. '  Back  of  this  is  the  negative 
temperature  coefficient  of  resistance  of  the  carbon  brush,  as 
mentiotxed  before*  With  a  positive  coefficient,  any  local  increase 


6SO  ELECTRICAL  ENGINEERING  PAPERS 

in  current  would  be  opposed  by  a  local  increase  in  resistance,  thus 
tending  to  equalize  the  current  distribution  between  the  various 
brushes.  However,  with  the  negative  coefficient,  a  more  or  less 
unstable  condition  exists.  This  unfortunate  condition  has  been 
recognized  for  many  years  and  presumably  it  has  been  a  serious 
handicap  in  direct-current  design  and  development  Various 
devices  for  overcoming  it  have  been  suggested  from  time  to  time, 
but  no  very  practical  one  has  yet  been  produced. 

BRUSHHOLDER  SUPPORTS 

Having  dealt  with  brushes  and  brushholders,  the  brushholder 
arms  and  supports  should  be  given  consideration,  as  there  is  some 
interesting  history  connected  with  this  feature  of  design.  In  the 
earlier  machines  the  brushholder  arms  or  supports  were  carried  by 
brackets  attached  to  or  surrounding  the  bearing.  This  was  com- 
mon practice  in  all  early  belted  machines.  Provision  was  usually 
made  few  some  easy,  quick  method  for  rocking  the  brushes  forward 
or  backward  to  suit  the  commutation.  Even  on  the  railway  gener- 
ators, where  the  point  of  commutation  was  supposed  to  be  fixed, 
such  brush-rocking  devices  were  alwayff furnished,  so  that  the  best 
average  position  of  commutation  could  readily  be  found. 

When  the  engine-type  generator  came  in,  new  problems  were 
encountered.  In  the  first  place  the  engine  bearing  was  not  always 
a  suitable  place  to  attach  a  brushholder,  and  in  the  second  place, 
with  large  diameter  commutators,  this  made  a  rather  flimsy  sup- 
port for  a  large  diameter  of  holder.  Also,  a  new  problem  came  up 
in  the  weaving  action  of  the  armature  of  the  engine-type  generator, 
'as  already  referred  to.    If  the  brushholder  frame  was  held  sta- 
tionary, then  the  weaving  action  of  the  commutator  meant  con- 
tinual motion  of  the  brushes  up  and  down  in  their  holders,  wHch 
was  considered  undesirable.    In  one  early  Westinghouse  engine- 
type  generator,  an  attempt  was  made  to  make  the  brushholder 
follow  the  commutator  by  suspending  it  directly  on  the  engine 
shaft  by  a  sleeve  or  bushing  which  fitted  over  the  shaft,  thus  form- 
ing a  bearing.  This  was  prevented  from  rotating  with  the  shaft  by 
means  of  a  brace  to  some  stationary  part  of  the  engine  frame.  This 
worked  for  awhile,  until  one  day  the  bearing  "froze"  on  the  shaft, 
the  brace  broke,  and  the  brushholder  started  to  rotate  around  the 
commutator.    This  ended  the  history  of  that  particular  type  of 
support. 

The  next  step  in  the  development  of  the  brushholder  support 
consisted  in  hanging  it  from  the  field  yoke,  either  centering  it  in 


DEVELOPMENT  OF  THE  D  C   GENERATOR  m»I 

the  yoke  itself  (early  Wcstinghouse  practice)  or  centering  it  on  a 
number  of  rigid  arms  extending  out  from  the  yoke  toward  or  over 
the  commutator,  (early  G.  E.  practice).  Of  course,  these  two 
methods  were  practically  equivalent.  Eventually,  for  purely  con- 
structive reasons  apparently,  centering  in  the  yoke  itself  became 
the  general  practice  and  is  standard  practice  at  the  present  time 
in  the  larger  machines.  This  method  of  support  did  not  eliminate 
the  difficulty  from  weaving  action  of  the  commutator,  but  in  fur- 
nishing a  rigid  brush  support,  the  resultant  troubles,  due  to  weav- 
ing action,  were  partly  overcome  and  the  development  of  good  slid- 
ing type  brushholders  took  care  of  the  rest. 

Another  trouble  developed  occasionally,  principally  in  con- 
nection with  brusliholders  for  long  commutators,  that  is,  wide 
commutator  faces.  The  individual  brushholder  arms  would  some- 
times vibrate  or  chatter  badly.  At  first,  it  was  attempted  to  make 
the  individual  arms  rigid  enough  to  take  care  of  this,  but  as  each 
arm  had  to  be  insulated  from  the  brushholder  frame,  it  was  dif- 
ficult to  obtain  sufficient  rigidity  without  undue  complication 
and  expense.  As  an  alternative,  the  practice  was  adopted  of  tying 
adjacent  arms  to  each  other  at  their  ends  by  means  of  insulated 
supports,  so  that  the  entire  system  of  brush  arms  thus  formed  one 
rigid  body.  This  was  very  effective  and  is  standard  practice  today. 

On  the  earlier  machines,  the  brush  arms,  to  which  the  brush- 
holders  were  attached,  were  made  of  brass  or  some  other  fairly 
good  conducting  material.  For  some  reason  or  other,  iron  was  con- 
sidered objectionable,  and  it  was  many  years  before  it  came  into 
general  use  for  the  brushholder  arms.  Now  it  is  standard  practice. 
Possibly,  commercial  reasons  may  have  influenced  this  delay  in  the 
use  of  iron  in  brush  arms,  for  it  was  criticised  as  being  "cheap  "  in 
appearance, 

COMMUTATORS 

Commutators  and  commutator  constructions  also  have  a 
history,  but  it  is  rather  difficult  to  trace  this  systematically.  Very 
early  in  direct-current  generator  practice,  the  present  "V"  con- 
struction for  supporting  the  commutator  bars  was  developed  and, 
with  various  minor  modifications,  it  has  come  through  to  the 
present.  This  construction,  Pig,  8-a,  was  adopted  on  the  earliest 
Westtnghouse  tnultipolar  generators  and,  with  only  one  exception, 
namely,  the  shrink-ri&g  oonstruction  in  ttirbo-getierator  commut- 
ators, it  foas  been  retained  on  these  machines  throughout.  The 


692  ELECTRICAL  ENGINEERING  PAPERS 

angle  of  the  V's,  the  shape  and  construction  and  material  of  the 
insulation  have  varied  from  time  to  time,  but  this  general  method 
of  supporting  the  bars  has  remained  unchanged. 

Another  early  method  of  supporting  the  commutator  bars, 
which  was  used  considerably  by  some  manufacturers,  including 
the  Thomson-Houston,  if  the  writer  remembers  rightly ,was  as 
shown  in  Fig.  8-6.  This  was  apparently  a  fairly  satisfactory  con- 
struction in  the  early  days,  but  was  abandoned  later  by  practically 
everybody,  in  favor  of  the  V  construction.  When  built-up  mica 
insulating  bushing  came  into  use  for  insulating  the  commutator 


'  A  '  B 

PIG.  8— TYPES  OF  COMMUTATOR  BAR  CONSTRUCTION 

bars  from  their  supports,  apparently  the  V-ring  construction  was 
simpler  as  regards  mica  bushings  than  any  other  construction,  and 
this  may  have  been  enough  to  turn  the  manufacture  toward  this 
one  construction. 

In  the  early  days,  there  was  a  great  variety  of  methods  of 
attaching  the  armature  windings  to  the  commutator*  In  some 
cases,  the  commutator  was  made  without  " necks"  in  the  modern 
sense  of  the  word,  the  windings  being  carried  directly  to  the  com- 
mutator face  and  attached  thereto  by  solder  or  screws.  When 
railway  generators  began  to  come  in,  the  Westinghouse  Company 
used  comparatively  long  necks  to  which  the  armature  conductors 
were  attached  by  means  of  slots  in  the  end  of  the  necks  in  which 
the  conductors  were  laid  and  then  soldered.  In  some  cases,  these 
necks  fitted  tightly  against  each  other  with  mica  between,  as 
between  commutator  bars,  as  in  Fig.  9-6.  In  other  cases,  especially 
where  the  commutator  bars  were  comparatively  wide,  thin  separ- 
ate necks  with  air  spaces  between  them  were  used,  Fig.  9-a.  Us- 
ually these  necks  were  made  of  copper  strap,  riveted  or  soldered 
to  the  end  of  the  bars.  In  other  cases,  the  neck  and  bar  was 
sawed  out  of  one  piece.  The  open  neck  was  most  common  in  the 
larger  Westinghouse  generators.  In  practically  all  cases,  these 
necks  were  made  so  stiff  that  they  were  self-supporting  and  re- 
quired no  insulation  between  them. 


DEVELOPMENT  OF  THE  D.C   GENERATOR 


693 


In  the  Thomson-Houston  and  early  General  Electric  large 
railway  generators,  these  necks  were  frequently  made  of  flexible 
material,  and  were  usually  insulated  throughout  their  length  in 
consequence.  With  the  built-up  type  of  strap  winding  used  on 
these  early  machines,  as  has  already  been  described,  presumably 
these  flexible  commutator  necks  were  of  material  advantage  in 
winding  and  connecting.  In  many  cases,  these  necks  were  at- 
tached to  small  brass  or  copper  blocks  or  terminals,  of  rectangular 
shape  which,  in  turn,  were  attached  to  the  commutator  bars  by 
means  of  screws  so  that  they  could  be  disconnected,  if  desired,  Fig. 
9-c  and  d.  Eventually  the  rigid  neck  construction  came  into 
general  use. 


PIG.  9— METHODS  OF  CONNECTING  COMMUTATOR  BARS  TO  ARMATURE 

WINDINGS 

MATERIAL 

There  was  quite  a  variety  of  materials  tried  out  in 
the  earlier  commutators.  For  the  bars,  copper,  either  drawn  or 
rolled,  has  been  used  from  almost  the  earliest  times,  but  many 
attempts  have  been  made  to  get  away  from  this  material,  largely 
on  account  of  the  expense.  Various  brasses  and  bronzes,  and  even 
cast  copper,  have  been  used  quite  extensively,  and  not  entirely  on 
account  of  lower  cost*  for  some  of  these  were  about  as  expensive  as 
drawn  copper.  One  idea  was  that  the  rapid  "wear"  of  copper 
commutators  which  was  sometimes  encountered,  was  due  to  the 
softness  of  the  material  and  that,  therefore,  some  much  harder 
kind  of  material  would  give  less  wear.  Of  course,  it  was  not  known 
then  that  the  rapid  wear  in  those  cases  was  not  true  frictional  wear, 
but  was  due  to  burning  under  the  brushes,  to  high  and  hard  mica, 
etc.  This  wear  was  a  principal  reason  for  using  the  various  brasses 
and  bronzes.  After  long  trials  of  each  of  these  materials,  the  con- 
clusion was  usually  reached  that  the  average  results  were  no  better 
than  with  copper.  In  some  of  the  early  Westinghouse  experience, 
cast  segments  were  used,  with  apparently  good  results.  However, 
in  the  larger  bara,  blow-hotes  were  liable  to  be  found  near  thfc 


664  ELECTRICAL  ENGINEERING  PAPERS 

center  of  the  bar.    This  was  taken  up  with  the  manufacturer  of 
these  bars,  but  the  trouble  was  not  entirely  overcome. 

One  rather  amusing  case  of  trouble  came  within  the  writer's 
experience  in  connection  with  one  of  these  early  cast  copper  com- 
mutators. This  was  a  fairly  large  capacity,  low  voltage  belted  ma- 
chine, with  very  thick  commutator  bars.  The  commutator  ran 
very  hot  in  the  early  service,  due  largely  to  brushholder  troubles, 
and  the  writer  was  surprised  to  find  solder  was  being  thrown  over 
everything  in  the  neighborhood.  He  looked  the  commutator  necks 
over,  but  could  not  find  that  any  solder  was  missing  at  these 
points.  Throwing  of  solder  still  continued,  and  in  comparatively 
large  quantities.  Then  the  man  who  had  charge  of  the  building 
of  the  commutator  was  questioned,  and  he  asked,  innocently 
enough,  whether  this  could  have  resulted  from  filling  blow-holes  in 
the  commutator  bars  with  solder.  He  then  explained  that  the 
heavy  cast  bars  had  developed  so  many  and  such  large  blow- 
holes some  distance  below  the  wearing  surface  that  he  had  thought 
it  best  to  fill  them  up  with  solder.  It  may  be  added  that  eventually, 
this  commutator  ran  all  right,  either  due  to  lower  temperature  or 
to  the  escape  of  all  the  solder  that  could  find  an  outlet. 

As  the  mica  and  brushholder  troubles  were  gradually  elimin- 
ated, it  became  much  better  recognized  that  pure  copper  rolled, 
drawn  or  hammered — was  about  the  best  possible  material  for 
commutator  purposes.  It  took  time  and  additional  experience  to 
prove  that  this  was  the  best  polishing  material.  Various  tests  were 
made  with  iron,  aluminum  and  other  materials,  in  comparison 
with  copper,  and  it  was  found  that  copper  polished  best  of  all 
practicable  materials,  under  heavy  load  conditions.  It  was  found 
that  other  materials  under  sparking  conditions  developed  minute 
globules  or  "beads"  on  the  commutator  face,  and  thesev beads 
were  liable  to  be  very  hard  in  some  cases,  so  that  they  destroyed  the 
brush  polish,  and  also  prevented  the  commutator  face  from  polish- 
ing. The  conclusion  drawn  eventually  was  that  the  copper  was  so 
much  better  a  conductor  of  heat  that  these  tiny  metal  beads 
would  not  be  formed,  as  the  heat  would  be  conducted  away  too 
'  rapidly.  Iron  was  particularly  bad  in  this  regard.  Since  the  good 
characteristics  of  copper  have  been  more  thoroughly  recognized, 
this  material  has  been  used  almost  exclusively. 

With  the  exception  of  the  bars,  about  the  only  materials 
which  need  be  considered  are  the  insulation  and  the  supporting 
rings.  In  the  early  days,  the  supporting  insulation,  under  the 


DEVELOPMENT  OF  THE  D  C.  GENERATOR  695 

metal  clamps,  was  made  of  almost  any  kind  of  sheet-insulating- 
material,  such  as  paper,  fiber,  oiled  canvas,  or  sheet  mica.  When 
built-up  and  moulded  mica  came  into  use,  this  was  quickly  adapted 
to  commutator  purposes,  and  is  still  standard  practice. 

One  of  the  most  serious  problems  in  commutator  insulation, 
in  general,  has  been  that  of  keeping  out  oil.  Where  oil  could  creep 
into  the  commutator,  it  was  very  liable  to  carry  copper  and  carbon 
dust  with  it,  and  incipient  short  circuits  or  arcs  sometimes  resulted 
which  developed  into  more  serious  trouble.  One  of  the  great  prob- 
lems has  been  to  obtain  "tight"  commutators.  Modern  practice 
seems  to  be  pretty  successful  in  this.  In  the  question  of  tight  com- 
mutators, there  have  long  been  two  schools,  (or  two  sets  of  advo- 
cates), one  favoring  the  so-called  " arch-bound"  construction  and 
the  other  the  "  drum-bound."  These  terms  practically  define 
themselves.  In  the  arch-bound  construction  the  commutator  is 
drawn  down  until  the  circumferential  pressure  is  the  limiting  re- 
sistance. In  the  drum-bound,  the  commutator  is  drawn  down  until 
it  binds  upon  a  central  drum  or  support  which  is  the  commutator 
bush.  One  advantage  claimed  for  the  drum-bound  construction  is 
that  the  commutator  is  affected  less  by  temperature,  as  the  cir- 
cumferential pressure  is  not  a  controlling  condition.  Moreover,  it  is 
claimed  that  it  is  easier  to  assemble  such  a  commutator.  Against 
this,  the  claim  for  arch-bound  is  that  it  gives  greater  tightness  than 
the  drum-bound,  and  tightness  is  a  most  essential  characteristic  in 
commutators.  For  many  years  the  writer  has  favored  tight  com- 
mutators, as  his  experience  with  pitting,  (dealt  with  under  the  sub- 
ject of  mica)  indicates  that  tightness  is  a  necessary  condition.  At 
the  present  time,  the  so-called  V-bound  construction  which  might 
be  considered  as  intermediate  between  the  arch  and  drum-bound 
constructions,  seems  to  be  the  most  satisfactory,  in  general. 

It  is  not  necessary  to  say  much  about  the  supporting  rings  for 
commutators.  On  the  early  machines,  these  were  frequently  made 
of  cast  iron.  When  engine-type  machines  came  in,  the  supporting 
rings  were  usually  made  segmental  as  they  had  to  be  split  to  get 
them  over  the  shaft,  the  low  commutator  speeds  allowing  segmental 
construction  without  danger.  However,  as  higher  speed  machines 
began  to  come  in,  such  as  rotary  converters  and  motor-generators, ' 
solid  supporting  rings  of  cast  steel  or  wrought  iron  came  into  very 
general  use.  This  was  not  due  altogether  to  the  higher  speeds,  but 
was  partly  due  to  the  necessities  for  making  tighter  commutators 


696  ELECTRICAL  ENGINEERING  PAPERS 

than  formerly,  which  necessitated  stronger  materials  in  the  sup- 
porting rings. 

WEARING  DEPTH  OF  COMMUTATORS 

Practice  has  changed  greatly  in  this  feature,  especially  in 
recent  years,  due  partly  to  improvements  in  design,  and  partly  to  a 
recognition  of  consistency  in  proportions.  On  very  early  gener- 
ators and  motors,  the  wearing  depth  of  commutators  was,  rightly, 
very  large.  As  commutators  "wore"  rapidly,  due  to  high  mica, 
poor  commutation,  etc,,  they  had  to  be  sandpapered  or  turned 
down  rather  frequently.  A  good  part  of  the  commutator  was  thus 
wasted.  But  when  engine-type  generators  came,  with  their  much 
better  commutating  characteristics,  the  great  wearing  depth  was 
retained  in  general,  apparently  largely  for  traditional  reasons. 
Some  of  these  engine-type  generators  did  not  require  even  sand- 
papering once  in  two  years,  and  yet  any  proposed  reduction  in 
wearing  depth  was  looked  at  askance. 

To  illustrate  the  above,  the  following  incident  is  cited: — The 
writer  broached  the  subject  of  commutator  wear  with  the  en- 
gineer of  a  large  railway  system,  in  which  2J^  in.  wearing  depth 
of  commutators  was  standard  practice  with  his  larger  commu- 
tators. He  was  asked  how  much  the  large  commutators  had  worn 
down  in  the  previous  nine  years,  of  pretty  steady  operation.  The 
answer  was,  "About  one-sixty-fourth  of  an  inch.*'  "  Then,  at  that 
rate,  how  long  will  your  commutator  last  ? ' '  After  a  little  figuring, 
— "About  1500  years.'1    "And  how  long  will  the  rest  of  the  ma- 
chine last?"  After  a  little  thought, — "Not  over  50  years  at  most." 
By  actual  figures,  a  3-32  in,  wearing  depth  in  this  case,  correspond- 
ing to  50  year's  life,  would  have  been  an  absurdity  of  the  opposite 
extreme.    But  a  factor  of  safety  of  ten  would  have  given  a  total 
available  wearing  depth  of  one  inch,  while  the  commutators  ac- 
tually had  iy%  times  this.     The  extra  material  is  thus  useless 
during  the  actual  life  of  these  machines.   As  the  inconsistency  of 
abnormal  wearing  depths,  usually  specified  for  commutator 
became  better  realized,  they  were  gradually  decreased.    This,  of 
course,  had  to  be  recognized  by  the  users  of  such  apparatus,  as  well 
as  the  manufacturers.  With  the  advent  of  the  commutating-pole 
machines,  with  their  better  commutating  characteristics,  the 
wearing  depth  of  commutators  has  been  reduced  to  a  fairly 
reasonable  amount,  still  allowing  a  wide  factor  of  safety.    It  is 
somewhat  saddening  to  think  of  the  thousands  of  tons  of  copper 
tied  up  uselessly  in  abnormal  commutator  proportions,  but  then 


DEVELOPMENT  OF  THE  D.C.  GENERATOR  697 

one  has  only  to  look  in  various  other  directions  to  see  what  pos- 
sibly may  be  similar  extravagances  some  of  which  are  in  full  force 
at  the  present  day,  especially  in  methods  of  rating  and  applying 
electrical  apparatus, 

TEMPERATURE  AND  VENTILATION 

One  important  general  subject  has  not  yet  been  touched  upon 
very  fully,  namely,  that  of  temperature,  together  with  the  related 
subject  of  ventilation.  In  the  early  days,  all  electric  machinery  ran 
hot  and  the  manufacturers,  as  a  rule,  knew  why  the  apparatus  ran 
hot,  but  did  not  know  just  how  to  remedy  the  case.  Armature 
cores  were  ventilated  to  a  limited  extent,  but  the  windings  were 
very  poorly  ventilated.  The  temperature  of  the  windings  was  high 
(how  high  nobody  knew  or  appreciated)  but  as  no  one  had  any 
particular  experience  with  lower  temperatures,  the  high  tempera- 
tures were  taken  as  a  matter  of  course.  This  was  particularly  true 
of  some  of  the  early  railway  generators,  On  continuous  full-load 
run,  some  of  these  would  reach  125  degrees  C.  by  thermometer,  but 
as  railway  load  in  those  days  was  far  from  continuous,  this  ap- 
parently did  not  make  any  difference.  A  rise  of  60  to  75  degrees 
C.  was  considered  fairly  good  on  continuous  temperature  test. 
However,  it  was  decided  about  1892  that  some  lower  standard, 
such  as  40  degrees  rise,  should  be  adopted.  When  this  went  into 
effect  in  the  Westinghouse  testing  room,  some  very  amusing  in- 
cidents occurred.  The  testing  room  men  who  had  been  accustomed 
to  rises  of  60  or  70  degrees  would  be  much  worried  over  nominal 
40  degree  machines  which  actually  showed  42  to  45  degrees  rise, 
as  they  feared  the  machines  might  burn  up  on  test.  They  seemed  to 
accept  the  newly  set  40  degree  limit  as  an  absolute  limit  of  safety, 
regardless  of  past  experience. 

After  the  40  degree  limit  was  adopted,  it  has  stayed  with  us 
more  or  less  constantly  until  the  present  time*  The  writer  does 
not  know  where  this  exactplimit  originated,  nor  who  was  back  of  it. 
It  just  came  and  stayed. 

In  those  early  days  temperature  measurements  could  be  made 
by  thermometer  about  as  accurately  as  at  the  present  time,  but 
people  did  not  know  how  to  hunt  for  hot  spots  and,  in  conse- 
quence, were  liable  to  put  the  thermometer  on  the  coldest  part  of 
the  winding,  and  then  wonder  why  such  a  cool  machine  (60  to  70 
degrees  C.  rise)  should  burn  out  so  readily.  But  what  they  did 
e,  they  aimed  to  measure  carefully* 


698  ELECTRICAL  ENGINEERING  PAPERS 

In  those  days  all  temperatures  were  measured  after  shut 
down  of  test,  and  usually  by  covering  the  thermometer  bulb  with 
a  great  wad  of  cotton  waste.  But  the  thermometers  used  were  not 
particularly  accurate,  and  variations  of  five  degrees  or  more 
between  different  thermometers  tested  at  the  same  air  temper- 
attire  were  found  by  the  writer  in  a  number  of  cases. 

In  this  early  work,  some  almost  unbelievable  incidents  oc- 
curred. For  instance,  one  of  the  routine  testing  men  one  day 
announced  to  the  writer  that  he  had  found  a  method  of  cutting 
the  temperatures  of  railway  motors  (old  double  reduction  surface- 
wound  armatures)  to  about  one-half.  He  claimed  he  had  accom- 
plished this  repeatedly  and  was  sure  of  his  results.  As  this  ap- 
peared to  be  a  very  valuable  idea,  he  was  urged  to  divulge  his 
method.  After  a  good  deal  of  coaxing,  he  stated  that  he  had 
attained  this  result  by  leaving  the  waste  off  the  thermometer  while 
taking  the  temperatures.  This  was  a  case  of  absolute  faith  in 
what  the  thermometer  said.  A  little  explanation  of  the  functions 
of  the  covering  pad  of  waste,  and  of  the  principles  of  temperature 
measurement,  soon  put  this  man  in  the  right. 

As  soon  as  the  necessity  for  lower  temperatures  was  recog- 
nized, the  problem  of  ventilation  became  very  active.   Armature 
windings  were  arranged  for  more  or  less  effective  air  circulation, 
and  special  ventilating  ducts  were  placed  in  the  armature  cores 
at  intervals.    The  writer  does  not  know  who  first  introduced  venti- 
lating ducts  in  the  armature  cores,  but  they  did  not  originate  in 
the  Westinghouse  Company.   With  the  surface-wound  armatures 
of  course,  there  was  little  or  no  occasion  for  radial  ventilating 
ducts,  as  the  armature  surface  was  pretty  thoroughly  covered  up. 
Openings  or  holes  parallel  to  the  shaft  were,  however,  rather  com- 
mon in  surface-wound  armatures  for  alternators,  but  in  direct- 
current  machines,  except  of  the  ring  type,  even  such  ventilation 
was  usually  impracticable.     However,  with  the  advent  of  the 
multipolar  railway  generators,  with  the;  drum  armature  windings 
and  slotted  armature  cores,  there  was  an  opportunity  to  use 
radial  ventilating  ducts  effectively,  and  they  soon  cafne  into 
general  use.      The  Thomson-Houston  Company  preceded  the 
Westinghouse  Company  in  the  use  of  such  ducts,  according  to  the 
writer's  memory.    However,  -by  the  time  that  engine-type  rail- 
way generators  had  practically  monopolized  the  field,  radial  ven- 
tilating ducts  in  armature  cores  were  standard  practice  with  prac- 
tically everybody,  and  this  is  standard  construction  at  the  present 


DEVELOPMENT  OF  THE  D.C.  GENERATOR  M9 

time.  Many  varieties  and  constructions  of  ventilating  ducts  have 
been  devised  and  tried  out.  About  the  only  important  departure 
from  this  construction  has  been  in  armatures  equipped  with 
ventilating  fans  at  one  end  in  which  the  air  is  drawn  axially 
through  the  armature  instead  of  radially.  This  has  been  used 
mostly  on  recent  railway  motors,  and  on  certain  lines  of  industrial 
motors. 

The  ventilation  of  direct-current  armature  windings  has 
varied  much,  depending  upon  other  controlling  conditions.  For 
instance,  the  end  windings  of  railway  motors  were  formerly  much 
better  ventilated  than  at  present,  and  with  very  beneficial  results 
as  regards  temperature.  However,  the  railway  motor,  being  an 
enclosed  machine,  circulated  its  own  dirt  (carbon  and  copper  dust,) 
to  such  an  extent  that  the  windings,  especially  back  of  the  commu- 
tator, would  become  so  coated  that  surface  leakage  became  serious. 
To  overcome  this,  practice  gradually  tended  toward  unventilated 
end  windings,  that  is,  they  were  so  completely  boxed  in  that  the 
dust  trouble  was  pretty  thoroughly  eliminated.  But  the  railway 
motor  may  be  considered  as  an  exceptional  case,  due  to  its  normal 
inaccessibility  for  cleaning,  and  the  tendency  in  other  classes  of 
machines  has  been  toward  better  ventilation,  rather  than  the 
reverse. 

In  commutators,  the  subjects  of  temperature  and  ventilation 
have  always  been  with  us,  and  probably  are  destined  to  stay  with 
us  as  long  as  the  business  lasts.  Primarily,  the  reason  for  this  is 
that  the  commutator,  in  large  machines,  is  so  costly  and  sometimes 
so  difficult  to  construct,  that  the  natural  tendency  is  toward 
crowding  it  down  in  dimensions,  with  a  resulting  tendency  to  in- 
crease the  temperature.  Thus  the  battle  between  size  and  temper- 
ature is  always  on. 

In  the  very  early  railway  commutators,  in  belted  type  ma- 
chines, the  temperature  of  the  commutator,  like  all  the  rest  of  the 
machine,  was  usually  fairly  high.  However,  these  early  commu- 
tators were  not  very  large,  so  that  expansion  troubles  were  not  very 
serious-  After  the  multipolar  generators  came  in,  it  was  soon  found 
that  long  commutator  necks,  with  air  spaces  between  them,  Fig. 
£-a,  were  quite  effective  in  cooling  the  commutator.  The  writer 
had  this  impressed  upon  him  particularly  in  connection  with  an 
early  direct-connected  railway  generator  in  which  the  necks  and 
tnica  were  solid  deaor  up  to  the  outer  periphery  where  the  winding 
joined  the  coftmtftetor,  Fig.  9*c.  Tlie  speed  of  the  armature  was 


700  ELECTRICAL  ENGINEERING  PAPERS 

only  140  r.  p.m.,  and  the  diameter  of  the  commutator  was  small,  so 
that  the  ventilation  was  comparatively  poor.  This  commutator 
ran  very  hot  on  test.  It  was  concluded  that  the  solid  necks  had 
much  to  do  with  this,  as  these  did  not  permit  the  usual  heat  dissip- 
ation which  probably  occurred  with  open  necks  The  writer  then 
had  long  radial  slots  milled  in  the  center  of  the  bars,  as  shown  in 
Fig.  9-a.  This  set  up  a  slight  air  draft  across  the  commutator.  The 
reduction  in  temperature  was  so  pronounced  that  the  writer 
became  a  thorough  convert  to  the  use  of  ventilated  necks,  and  he 
adhered  to  such  construction  as  far  as  possible  on  all  large  com- 
mutators thereafter.  However,  when  engine-type  generators  came 
in,  the  commutators  were  usually  made  so  large,  compared  with 
belted  machines,  that  the  temperature  problem  practically  dis- 
appeared as  far  as  commutators  were  concerned.  However,  when 
rotary  converters  and  motor-generators  became  the  prevailing 
practice,  with  the  general  introduction  of  the  polyphase  system 
in  railway  and  central  station  work,  this  problem  of  temperature 
again  became  active,  and  has  become  more  important  as  the  speeds 
and  ouputs  have  increased,  so  that  today  the  problem  is  a  "real 
live"  one.  In  modern  commutators,  auxiliary  "necks'1  or  ventilat- 
ing vanes  are  sometimes  attached  to  the  outer  ends  of  the  commut- 
ator bars,  or  ends  farthest  from  the  winding.  Like  the  open  com- 
mutator necks,  these  are  quite  effective  in  dissipating  the  commut- 
ator heat. 

Various  rules  have  been  developed  from  time  to  time  for 
determining  the  size  of  commutator  required  for  a  given  current, 
without  overheating.  However,  all  such  rules  have  proved  to  be 
only  crudely  approximate,  for  the  heating  is  dependent  upon  the 
commutation  and  friction  losses,  which  are  extremely  variable  in 
different  types  and  sizes  of  apparatus.  The  modern  cotnmutating- 
pole  construction  of  machine,  which  furnishes  a  means  for  partially 
controlling  the  commutation  losses,  has  been  a  great  help  in  the 
commutator  heating  problem.  This  however,  has  simply  al- 
lowed higher  speeds  with  correspondingly  smaller  diameters  of 
commutator  for  a  given  output,  and  thus  the  battle  between  size 
and  temperature  goes  on. 

In  the  modern  high-speed  motor-generators,  probably  the 
ventilation  has  been  carried  farther  than  in  any  other  class  of 
direct-current  apparatus.  In  the  modern  machine,  two  funda- 
mental conditions  in  the  problem  of  ventilation  are  recognized, 
namely:  supplying  a  sufficient  quantity  of  air  to  carry  away  the 


DEVELOPMENT  OF  THE  D.C.  GENERATOR  701 

heat,  and  so  distributing  this  air  that  it  can  take  up  the  heat  with 
the  least  temperature  drop  in  the  various  parts  of  the  machine. 
The  whole  modern  theory  of  ventilation  is  built  up  upon  these 
two  conditions. 

SPECIAL  CLASSES  OP  DIRECT-CURRENT  MACHINES 

In  the  foregoing,  direct-current  generators  and  motors  in 
general  have  been  considered.  However,  there  are  several  rather 
special  classes  or  types  of  machines  which  merit  separate 
consideration,  in  some  of  their  features.  Among  these  are  double- 
commutator  machines,  turbo-generators,  unipolar  generators,  and 
double-current  generators  (a.c,-d.c.  machines).  Also,  commut- 
ating-pole  machines  in  general  have  not  been  taken  up,  but  as 
these  represent  practically  the  latest  great  step  in  the  direct-cur- 
rent development,  and  therefore  are  newer  in  history  than  the  above 
special  types,  the  latter  will  be  considered  first. 

DOUBLE-COMMUTATOR  MACHINES 

By  this  is  meant  an  armature  with  two  separate  commutators* 
usually  placed  at  opposite  ends  of  the  armature  core.  Such  ma- 
chines usually  have  been  designed  only  for  very  special  purposes, 
such  as  the  collection  of  very  heavy  currents,  or  where  two  separate 
voltages  are  desired  from  the  same  armature  core.  Obviously, 
where  the  current  to  be  handled  is  too  large  to  come  within  the 
limits  of  a  single  commutator,  the  first  suggestion  would  be  to  add 
a  like  commutator  on  the  other  end  of  the  armature,  and  prefer- 
ably connected  to  the  same  armature  winding.  This  looks  like  a 
simple,  easy  solution  of  the  problem,  and  so  it  proved  to  be  from 
the  mechanical  or  constructional  standpoint  purely.  Prom  the 
electrical  standpoint,  it  sometimes  proved  to  be  a  very  unsatis- 
factory construction. 

The  earliest  machine  of  this  type  that  the  writer  had  any 
practical  experience  with  was  built  by  the  United  States  Company 
about  1890,  this  being  of  the  Weston  type,  previously  described. 
This  was  a  two-pole,  200  kw,  60  volt  machine  and  had  a  commut- 
ator at  each  end  connected  to  a  common  winding  of  the  surface- 
wound  type.  This  machine  was  not  very  successful  and  was  later 
provided  with  a  slotted  type  of  armature,  which  was  also  not  en- 
tirely successful,  due  largely  to  the  fact  that  copper  brushes 
required  to  handle  the  large 


702  ELECTRICAL  ENGINEERING  PAPERS 

In  1893,  the  Westinghouse  Company  built  some  60  volt,  3600 
ampere,  six-pole  belted  generators  with  two  commutators  con- 
nected to  the  same  winding.  These  had  carbon  brushes.  No 
particular  trouble  was  encountered  in  the  operation  of  these 
machines,  except  it  was  found  that  rather  careful  adjustment  of 
the  two  sets  of  brushholders  was  necessary  in  order  to  produce 
proper  division  of  load.  After  this,  from  time  to  time,  double 
commutator  machines  of  moderate  size  were  built,  and  it  was  found 
in  some  cases  that  it  was  difficult  to  divide  the  load  equally  be- 
tween the  leads  from  the  two  commutators,  ana  at  the  same  time 
obtain  good  commutation  at  both  sets  of  brushes.  Various  schemes 
were  introduced  for  overcoming  this  trouble.  In  some  cases,  the 
leads  from  the  two  commutators  were  tied  solidly  together,  and  the 
ammeter  was  connected  in  the  combined  circuit.  The  brushes  on 
the^two  commutators  were  then  shifted  until  best  commutation 
conditions  were  obtained.  Later  experience  showed  pretty  clearly 
that,  under  this  condition  of  best  commutation,  the  two  commut- 
ators were  usually  supplying  quite  unequal  currents,  especially 
where  both  were  connected  to  one  armature  winding.  This  cured 
the  trouble  simply  by  hiding  it.  In  one  plant,  where  some  4  000 
ampere,  250  volt;  double  commutator  generators  were  installed, 
it  was  found  that  with  the  best  commutation,  one  commutator 
supplied  3  000  amperes,  while  the  other  furnished  only  1  000.  No 
adjustment  of  the  brush  lead  would  overcome  this  and  maintain 
good  commutation.  When  one  commutator  carried  3000  amperes 
without  sparking,  then  the  brushes  on  the  other  one  could  not  be 
rocked  from  the  1  000  ampere  non-sparking  position  into  a  position 
where  it  would  divide  current  equally  with  the  other  commutator, 
without  sparking.  There  appeared  to  be  no  non-sparking  position 
which  would  give  equal  current  division.  Finally,  a  low  resistance 
in  the  form  of  a  heavy  cast  iron  grid,  was  introduced  into  the  leads 
from  the  higher  current  commutator,  with  the  brushes  on  both 
commutators  set  for  the  best  commutation.  This  forced  a  larger 
percentage  of  the  current  to  pass  through  the  other  commutator, 
and  thus  equalization  was  obtained.  It  was  surprising  how  little 
was  required  to  balance  the  two  commutator  loads.  This  case  illus- 
trates the  general  difficulty  which  appeared  in  double  commut- 
ator machines  of  larger  capacity  when  one  winding  only  was  used. 
This  led  the  Westinghouse  Company  to  advocate  two  independent 
armature  windings  when  double  commutators  were  required. 
With  this  arrangement,  any  equality  of  current  would  mean  in- 


DEVELOPMENT  OF  THE  D.C.  GENERATOR  703 

equality  in  resistance  drops,  which  would  tend  to  equalize  the  loads 
A  number  of  armatures  of  this  type  were  actually  built. 

It  may  be  of  interest  to  note  that  the  largest  capacity,  highest 
speed  machine  ever  built  by  the  Wcstinghouse  Company  had  two 
commutators  connected  to  a  single  armature  winding.  This  was 
the  generator  end  of  a  flywheel  type  motor-generator  set  for 
furnishing  current  to  a  reversing  mill  at  the  Illinois  Steel  Com- 
pany's plant  in  South  Chicago,  and  was  rated  as  a  3  000  kw,  375 
r.  p.  in  ,  600  volt  machine.  In  this  case,  however,  the  commutators 
feed  into  separate  loads,  which  are  adjusted  to  divide  approxi- 
mately equally.  The  armature  of  this  machine  has  some  unusual 
constructive  features.  On  account  of  the  large  output  and  high 
speed,  and  the  fact  that  the  load  varies  with  great  rapidity,  it 
was  desired  to  obtain  the  effect  of  one-half-turn  armature  coils 
instead  of  the  one-turn  coils,  which  are  usual  practice.  The  arma- 
ture winding  was  made  of  the  usual  parallel-drum  type  with  one 
turn  per  coil,  but  with  only  half  as  many  coils  as  there  are  bars  in 
each  commutator.  The  commutators  were  connected  to  the  wind- 
ing at  each  side  of  the  core  in  the  usual  manner,  except  that  only 
alternate  commutator  bars  were  connected.  Then,  from  the  ac- 
tive bars  on  one  commutator,  strap  connections  or  conductors 
were  carried  under  the  armature  core  to  the  idle  or  intermediate 
bars  of  the  other  commutator,  and  from  the  active  bars  of  this 
second  commutator  similar  conductors  were  carried  through  to 
the  intermediate  bars  of  the  first  commutator.  Thus  the  potential 
of  any  intermediate  commutator  bar  was  midway  between  the 
potentials  of  the  two  adjacent  active  bars,  and  the  result  was 
equivalent  to  the  use  of  a  half -turn  winding  on  the  armature.  As 
far  as  the  writer  knows,  this  is  the  only  large  machine  ever  built 
with  this  type  of  winding.  It  has  been  in  successful  operation  for  a 
number  of  years. 

Another  type  of  very  large  capacity,  high-speed  double  com- 
mutator generator  was  built  by  the  G.  E.  Company  for  the 
Niagara  Falls  plant  of  the  Aluminum  Company  of  America*  These 
machines  are  of  3  500  kw  capacity,  600  to  700  volts,  300  r,  p.  m. 
and  are  direct-coupled  to  waterwheels.  Each  commutator  carries 
brushes  of  one  polarity  only.  The  number  of  poles  in  each  ma- 
chine is  comparatively  large  and,  in  consequence  of  this  and  the 
high  speed,  the  distance  between  commutator  neutral  points  is 
relatively  small,  and,  presumably  to  avoid  crowding  the  bruafct- 
holders  too  much,  alternate  holders  are  omitted.  Thus  one  ccro* 


704  ELECTRICAL  ENGINEERING  PAPERS 

mutator  has  only  positive  brushes,  and  the  other  only  negative. 
This  increased  the  distance  between  brushliolders,  but,  of  course, 
did  not  increase  the  distance  between  adjacent  neutral  points  on 
the  commutator.  Therefore,  as  regards  flashing  around  the  com- 
mutator, this  double  spacing  of  the  holders  may  be  considered  as 
more  or  less  fallacious.  Due  to  this  arrangement  of  brush  holders, 
some  unusual  commutator  and  brushholder  operating  conditions 
were  found  The  two  commutators  of  each  machine  did  not  polish, 
or ' '  wear, ' '  equally,  due  largely  to  the  fact  that  in  one  commutator 
the  current  flow  was  entirely  from  the  brushes  to  the  commutator, 
while  in  the  other  it  was  the  reverse,  the  effect  of  which  has  al- 
ready been  described  under  brushes  and  commutation.  However, 
these  machines  have  been  in  operation  for  a  number  of  years,  and 
the  above  is  not  intended  as  a  criticism  of  this  particular  design, 
but  is  merely  to  call  attention  to  a  very  unusual  construction. 

The  Bullock  Company,  some  years  ago,  built  some  large 
capacity  double-commutator  machines  for  the  Massena  plant  of 
the  Aluminum  Company  of  America.  In  these  machines,  unbalan- 
cing of  the  commutator  current  was  encountered.  This  condition 
was  corrected  by  the  use  of  brushes  of  different  resistance  in  the 
two  polarities  of  each  commutator. 

It  should  be  evident  from  the  preceding  that  the  double-com- 
mutator machines,  in  large  capacities,  as  built  by  various  manu- 
facturers, have  necessitated  either  special  operative,  or  special 
balancing  conditions.  The  trouble  is,  to  a  certain  extent,  an  inher- 
ent one.  The  advent  of  the  commutating  pole  apparently  has  not 
improved  the  position  of  the  double-commutator  machine,  and 
at  the  present  time  such  machines  are  only  recommended  where 
some  very  special  conditions  require  such  construction. 

DIRECT-CURRENT  TURBO-GENERATORS 

Direct-current  generators,  driven  by  steam  turbines,  were 
introduced  in  this  country  many  years  ago  by  the  DeLaval  Com- 
pany. However,  these  generators  were  geared  to  the  steam  tur- 
bines and  operated  at  only  comparatively  high  speeds.  They  were 
not  turbo-generators  in  the  modern  meaning  of  generators  direct- 
coupled  to  the  turbine.  It  is  in  this  latter  type  that  special  features 
are  involved. 

Probably  the  first  true  turbo-generator  which  was  tried  in 
commercial  service  in  this  country  was  one  designed  by  the  Wes- 
inghouse  Company  in  1896  for  direct  connection  to  a  Parsons 


DEVELOPMENT  OF  THE  D.C.  GENERATOR 


705 


5  000  r.  p.  m.  turbine.  This  generator  was  designed  for  a  capacity 
of  120  kw  at  160  volts.  When  one  considers  that  this  machine  had 
a  speed  of  practically  double  that  of  the  modern  turbo-generator 
of  corresponding  capacity,  it  may  be  appreciated  that  this  was 
quite  a  problem  for  a  first  machine.  This  was  one  of  Mr.  N.  W. 
Storer's  early  "pets,"  and  it  required  an  extraordinary  amount  of 
petting  to  make  it  behave. 

The  real  operating  difficulties  in  this  machine  were  due  to  the 
exceedingly  high  speed.  There  was  no  undue  difficulty  in  making 
an  armature  which  would  hold  together.  The  armature  had  par- 
tially closed  slots  with  two  rectangular  straps  shoved  through 
from  one  end  and  then  carefully  formed  at  each  end,  over  sup- 
porting shelves  or  brushes.  There  was  a  commutator  at  each  end. 
The  commutator  peripheral  speed  was  over  8  000  feet  per  minute, 
and  herein  occurred  the  real  troubles  with  the  machine.  There 


FIG.  10— RADIAL  TYPE  COMMUTATOR 

were  four  poles  and  four  brush  arms,  with  carbon  brushes  origin- 
ally, and  graphite  brushes  later.  Neither  carbon  nor  graphite  was 
successful,  as  intimate  contact  between  the  brushes  and  commut- 
ator apparently  could  not  be  maintained  at  the  high  commutator 
speed.  Fine  V-shaped  grooves  were  then  turned  in  the  commut- 
ators, and  brushes  of  parallel  brass  wires  were  used,  similar  to  those 
on  some  machines  in  England.  These  brushes  maintained  pretty 
fair  contact,  but  with  the  slotted  type  armatures  used,  the  corn- 
mutating  conditions  were  not  satisfactory  enough  to  allow  the  us© 
of  metal  brushes.  There  was  always  more  or  less  sparking  at  the 
brushes,  so  that,  after  a  certain  amount  of  service,  this  machine 
was  taken  out.  It  was  redesigned  later  for  a  speed  of  3  600  r.  p.  m., 
but  apparently  was  never  completed. 

Turbo-geuemtor  work  was  then  dropped  by  the  Westing- 
house  Compaay,  turtil  1904,  except  that,  for  several  years  previous 


706  ELECTRICAL  ENGINEERING  PAPERS 

to  this,  exciters  had  been  built  from  time  to  time  for  direct-con- 
nection to  turbo-alternators  at  both  1  800  and  3  600  r.  p.  m.  These, 
however,  were  usually  standard  machines  of  small  capacity, 
simply  modified  for  these  high  speeds.  Previous  to  1904  the  G.  E, 
Company  built  and  put  in  service  a  number  of  turbo-generators  of 
moderate  size,  which  operated  with  such  success  as  to  encourage 
the  growth  of  this  business.  In  1904  the  Westmghouse  Company 
again  took  up  this  work,  and  three  units  were  designed  of  100  kw, 
200  kw  and  500  kw  capacity,  the  latter  for  600  volts  for  railway 
work. 

The  100  kw  unit  was  designed  for  a  speed  of  about  2  000  r.p.m. 
and  did  not  prove  such  a  difficult  machine  to  build  or  operate.  The 
200  kw  was  designed  for  250  volts.  Two  of  these  machines  were 
put  in  operation  and  eventually  developed  commutator  trouble. 
New  commutators  were  then  furnished  of  the  radial  type,  such  as 
the  British  Westinghouse  Company  had  been  building.  In  this 
radial  type  commutator,  as  shown  in  Fig.  10,  the  brushes  were 
located  in  grooves  in  the  commutator  and  bore  on  opposite  faces 
of  the  grooves.  By  this  construction,  much  higher  commutator 
speeds  were  allowable,  as  radial  vibration,  or  radial  inequalities  of 
the  commutator  did  not  affect  the  brush  contact.  The  only  two 
machines  of  this  type  which  were  built  are  still  in  service.  The 
radial  commutator  construction,  however,  as  used  on  these  two 
machines,  proved  unduly  expensive,  and  no  more  were  built  for 
service. 

The  500  kw,  600  volt  turbo-generators  operated  at  a  speed  of 
1  500  r.  p  m.  The  commutator  speed  was  about  5  500  feet  per 
minute.  The  shrink-ring  type  of  commutator  construction  was 
used.  As  tlje  metal  of  these  rings  was  very  close  to  the  commu- 
tator face,  any  little  arcing  or  sparking  was  liable  to  bridge  over  to 
the  rings  and  thus  cause  the  machines  to  flash  over.  It  was  there- 
fore necessary  to  insulate  these  rings  completely  in  order  to  prevent 
flashing.  This,  however,  was  successfully  accomplished.  In  service, 
these  Westinghouse  machines  operated  fairly  well,  with  the  ex- 
ception of  certain  mechanical  difficulties,  due  primarily  to  high 
speed.  In  specific  applications,  they  operated  very  well,  but  they 
proved  too  delicate  to  send  out  broadcast  and  their  manufacture 
was  dropped  for  a  while. 

In  1909,  turbo-generators  were  again  taken  up  by  the  West- 
inghouse Company.  In  this  case,  however,  the  construction  was 
limited  to  sizes  of  150  kw  and  lower.  A  large  number  of  these 


DEVELOPMENT  OF  THE  D.C.  GENERATOR  707 

small  units  were  put  out  and  have  been  quite  successful  from  the 
operating  standpoint,  but  they  were  expensive  to  build,  from  the 
generator  standpoint,  compared  with  machines  of  similar  capaci- 
ties at  much  lower  speeds  and  low  in  economy  on  account  of  low 
turbine  speeds. 

It  was  long  ago  recognized  that  the  turbo-generator  unit  was  a 
bad  compromise  between  the  most  desirable  turbine  and  generator 
speeds.  In  general,  the  highest  practicable  speed  for  the  gener- 
ator was  much  below  the  desired  speed  for  the  steam  turbine.  In 
practically  all  cases  of  actual  turbo-generators,  the  engine  was 
therefore  operated  at  too  low  and  the  generator  at  too  high  speed. 
In  turbo-alternators,  the  capacities  and  speeds  were  continually 
being  increased,  while  in  direct-current  work  the  tendency,  es- 
pecially in  larger  capacities,  was  toward  lower  speeds.  Obviously, 
this  was  going  in  the  wrong  direction,  and  there  appeared  to  be 
little  or  no  hope  of  carrying  the  turbo-generator  into  the  large 
capacities.  Obviously,  the  solution  of  this  problem  lay  in  some 
mechanical  or  electrical  form  of  drive  which  would  allow  higher 
turbine  speeds  and  lower  generator  speeds.  In  the  electrical  drive,  a 
solution  was  obtained  by  substituting  for  the  generator  a  very 
much  higher  speed  alternator,  the  current  from  which  was  supplied 
directly  to  a  rotary  converter  of  any  preferred  speed.  In  a  500  kw 
unit,  for  instance,  a  3  600  r.  p,  xn.  turbo  alternator  was  used  in- 
stead of  1  500  r.  p.  m.  required  with  the  direct-current  unit.  This 
enormous  gain  in  speed  was  sufficient  to  offset,  in  cost  3nd  per- 
formance, the  additional  apparatus  required.  Moreover,  the  com- 
bination was  much  less  delicate  than  the  straight  turbo-generator. 
A  number  of  such  sets  was  supplied,  of  capacities  from  300  up  to 
2  000  kw.  However,  a  mechanical  solution  of  the  problem  was  then 
brought  forward  in  the  Westinghouse  floating  gear,  which  solved 
the  gear  problem  for  very  large  capacities,  and  allowed  high-speed 
turbines  to  drive  moderate  speed  generators*  In  this  way,  turbines 
built  for  turbo-alternator  units,  which  were  about  as  high  speed  as 
practice  would  permit,  could  be  connected  to  generators  designed 
tor  motor-generator  sets  which  were  also  usually  of  as  high  speed 
as  best  design  would  permit,  as  regards  cost  and  good  operation. 
Thus,  with  this  gear,  the  best  speed  conditions  for  both  engine  and 
generator,  could  be  obtained,  and  moreover,  standard  turbines  and 
generators  designed  for  other  purposes  could  be  combined  in  one 
unit — a  very  desirable  condition  from  both  the  inantifacturiag 
and  the  commercial  standpoint. 


703  ELECTRICAL  ENGINEERING  PAPERS 

Quite  a  number  of  these  geared  sets  in  capacities  from  500  to 
3  750  kw  have  been  built  and  put  in  operation.  The  results  ob- 
tained have  sealed  the  doom  of  the  large  direct-connected  turbo- 
generator. In  fact,  they  are  so  favorable  that  there  is  good  reason 
to  believe  that  eventually  the  geared  sets  will  be  carried  down  to 
sizes  much  less  than  300  kw  with  very  considerable  gain  in  econ- 
omy at  least.  This  looks  like  a  logical  line  of  development, 

UNIPOLAR  GENERATORS 

The  saying  that, ' '  Happy  is  the  country  which  has  no  history  " 
may  be  paraphrased  in  the  electrical  manufacturing  business,  into 
*  *  Happy  is  the  company  which  has  no  unipolar  history."  Yet  there 
was  a  time  when  the  unipolar  generator  appeared  to  fill  a  long-felt 
want,  namely,  as  large  capacity  turbo-generators.  It  was  long  ago 
recognized  that  the  turbo-generator  could  not  be  carried  to  large 
capacities  unless  unduly  low  turbine  speeds  were  used.  Against 
this,  the  unipolar  generator  appeared  to  furnish  a  satisfactory 
means  for  getting  direct-current  in  large  quantities,  and  at  com- 
mercial voltages,  from  very  high-speed  generators. 

From  the  standpoint  of  size,  weight  and  cost,  the  higher  the 
speed  of  the  unipolar  generator,  the  more  commercial  it  becomes. 
But  from  the  current-delivering  standpoint,  which  is  the  import- 
ant function  of  the  machine,  the  higher  the  speed  the  more  difficult 
does  successful  operation  become.  The  difficulty  is  not  in  making 
a  machine  which  will  hold  together  at  very  high  speeds,  but  is 
almost  entirely  in  making  the  current-collecting  devices  work 
satisfactorily  at  such  speeds.  In  the  unipolar  generator,  the  voltage 
obtainable  per  conductor,  that  is,  per  pair  of  collector  rings,  is  a 
direct  function  of  the  section  of  the  magnetic  circuit  and  of  the 
number  of  revolutions  of  the  machine.  But  the  collector  rings 
must  surround  this  magnetic  circuit,  and  therefore,  the  peripheral 
speed  of  the  collector  rings  is  also  a  function  of  the  section  of  the 
magnetic  circuit  and  the  revolutions.  Therefore,  the  voltage  per 
pair  of  collector  rings  is  related  to  the  peripheral  speed  of  the  col- 
lector rings ;  and,  unless  a  relatively  large  number  of  collector  rings 
is  used  for  ordinary  commercial  voltage,  the  unipolar  turbo- 
generator is  bound  to  have  extremely  high  collector-ring  speeds, 
practically  between  12  000  and  20  000  feet  per  minute.  By  making 
the  section  of  the  magnetic  circuit  larger  and  reducing  the  speed 
in  proportion,  the  peripheral  speed  of  the  collector  rings  may  be 
reduced  somewhat,  as  the  periphery  of  the  magnet  does  not  in- 


rEVELOPMRXT  OF  THE  D.C   CBXERATOR  709 

crease  as  fast  as  its  section.  However,  this  cannot  be  carried  very 
far,  in  large  capacity  machines,  without  running  into  abnormal 
weights  and  costs.  Therefore,  in  large  capacity  unipolar  turbo- 
generators, it  is  not  commercially  practicable  to  get  below  certain 
collector  ring  speeds. 

As  already  mentioned,  about  1904  there  was  a  considerable 
demand  for  large  turbo-generators.  The  Westinghouse  Company 
put  out  nothing  above  500  kw,  but  the  General  Electric  Company 
put  out  a  few  much  larger  si  zes,  such  as  800  kw  and  even  one  of 
1  700  kw  for  railway  work.  These  larger  machines,  however,  re- 
quired such  low  turbine  speeds  as  to  be  commercially  unsatisfac- 
tory. Moreover,  the  generators  themselves  were  not  particularly 
satisfactory,  being  beyond  the  limits  of  practical  design  of  those 
days.  Therefore,  the  General  Electric  Company,  apparently  in 
looking  for  a  substitute,  took  up  the  unipolar  design  for  turbo- 
generator work.  This  did  not  appear  to  present  any  great  diffi- 
culties, as  the  unipolar  principle  had  been  well  proven  out  years 
before,  A  number  of  unipolar  generators  in  capacities  up  to  500  kw, 
and  even  one  unit  of  2  000  kw  were  built.  These  machines  appar- 
ently were  very  promising  at  first,  but  later  a  number  of  unexpected 
difficulties  developed. 

In  1896,  and  later,  the  Westinghouse  Company  built  and  dis- 
posed of  a  number  of  small  three-volt  unipolar  generators  for  meter 
testing;  but  about  1906,  a  contract  was  obtained  for  its  only  large 
unipolar  turbo-generator.  This  was  a  2  000  kw  machine,  260  volts, 
at  1  200  r.  p.  m.  This  differed  from  the  General  Electric  machine 
in  a  number  of  respects,  having  a  considerably  lower  peripheral 
speed  at  the  collector  rings,  and  using  special  bronze  instead  of  steel 
in  the  rings.  It  turned  out  that  these  two  differences  were  more  or 
less  fundamental  in  their  effect  on  the  operation.  This  machine  was 
finally  put  into  successful  service  after  discouraging  experiences,* 
and  operated  satisfactorily  for  several  years.  It  has  been  shut 
down  very  recently,  and  is  now  held  as  reserve  to  a  rotary  con- 
verter transforming  plant,  which  has  taken  its  load.  This  change 
was  not  on  account  of  any  operating  defects  of  this  unipolar  unit, 
but  on  account  of  very  cheap  power  rates,  a  condition  which  has 
recently  been  responsible  for  putting  many  industrial  and  railway 
generating  plants  out  of  business. 

The  General  Electric  2  000  kw  unipolar  generator  showed 
difficulty  in  current  coUectic>&.  The  collector  rings  were  of  steel 

p*g»  H5. 


710  ELECTRICAL  ENGINEERING  PAPERS 

and  the  peripheral  speed  was  about  40  percent  higher  than  on  the 
Westinghouse  machine.  These  conditions  introduced  certain  fun- 
damental difficulties  which  could  not  be  overcome,  and  eventually 
this  generator  was  replaced  by  an  alternator  which  furnished 
current  to  a  rotary  converter. 

These  two  cases  practically  end  the  history  of  the  large 
capacity  unipolar  generator  in  America,  for  the  alternator,  with 
rotary  converter,  and  the  geared  generator,  as  referred  to  under 
turbo-generators,  have  practically  put  the  unipolar  generator  out 
of  business. 

AC-DC  GENERATORS 

Both  alternating  and  direct  currents  have  been  required 
from  the  same  generating  station  at  times,  such  as  for  railway 
service  directly  from  the  station,  and  for  lighting  or  for  trans- 
mission to  remote  rotary  converter  or  motor-generator  substa- 
tions. Therefore,  ever  since  both  alternating  and  direct  current 
have  been  generated  in  the  same  station,  there  have  been  pro- 
positions that  both  services  be  supplied  from  one  generator.  As 
early  as  1893,  the  Westinghouse  Company  took  a  contract  for  two 
150  kw  generators  capable  of  delivering  SO  cycle  alternating  and 
525  volt  direct  current  from  the  same  armature.  These  were  8-pole, 
750  r.  p.  m.  belted  machines,  and  therefore  were  not  unlike  other 
belted  machines  of  that  time,  in  speed  and  capacity.  The  larger 
number  of  poles  somewhat  complicated  the  conditions,  and  made 
the  commutation  more  difficult,  due  to  the  comparatively  narrow 
neutral  zones.  It  was  recognized  in  the  design  of  these  early  ma- 
chines that  separate  excitation  would  be  required  on  account  of 
the  alternating-current  load,  as  it  was  well  known  at  that  time 
that  self-exciting  machines  were  very  unstable  when  carrying 
inductive  loads.  Also,  on  account  of  the  two  classes  of  service 
from  the  same  machine  it  was  considered  useless  to  compound  the 
machine  for  direct-current  load.  In  fact,  it  was  pretty  thoroughly 
recognized  that  in  general  the  two  classes  of  service  could  not  be 
very  consistently  handled  from  one  machine. 

When  these  machines  were  put  on  test,  some  very  interesting 
results  were  obtained.  For  instance,  when  an  alternating  inductive 
load  was  thrown  off,  the  direct-current  load  conditions  remaining 
unchanged,  the  voltage  would  rise  possibly  30  to  40  percent,  due 
to  the  fact  that,  as  alternating-current  machines,  the  magnetic 
circuits  could  not  be  highly  saturated,  as  was  then  common 


DEVELOPMENT  OF  THE  AC  GENERATOR  711 

practice  in  direct-current  work.  With  the  high  resultant  voltage, 
there  was  liability  of  considerable  flashing  at  the  commutator. 
Also,  when  the  direct-current  terminals  were  short-circuited,  the 
commutator  developed  the  largest  fireworks  that  the  writer  has  ever 
seen  from  a  small  machine.  This  was  due  primarily  to  the  separate 
field  excitation  and  the  high  field  ampere-turns  compared  with  the 
armature.  In  the  ordinary  self -excited  machine,  there  will  usually 
be  vicious  fireworks  momentarily  in  case  of  a  short-circuit,  but  the 
machine  quickly  "kills0  its  excitation.  In  this  separately  excited 
machine,  the  field  excitation  was  not  killed  in  this  manner,  and  if 
flashing  or ' '  bucking '  'was  once  started,  it  kept  up  the  performance. 
This  trouble  was  finally  reduced  by  means  of  a  special  field  circuit 
breaker  which  killed  the  field  in  case  of  excessive  load. 

These  machines  were  so  sensitive  on  the  direct-current  end 
that  the  writer  advised  against  their  shipment,  However,  they 
were  so  badly  needed  that  they  were  sent  out  to  help  matters  tem- 
porarily, and  astonishing  as  it  may  seem,  additional  machines, 
exact  duplicates  of  the  first,  were  ordered  from  time  to  time.  It 
developed  that  the  power  company  used  them  principally  for 
alternating-current  work,  the  direct-current  end  representing 
reserve  or  emergency  conditions  for  a  railway  plant.  They  were 
perfectly  satisfactory  as  straight  alternators,  and  they  could 
have  been  used  to  supply  direct-current  if  any  emergency  had 
occurred. 

About  two  years  later,  the  Westinghouse  Company  sold 
some  belted  a.c.-d.c.  machines,  in  which  the  principal  service  was 
to  be  direct-current  with  some  alternating-current  to  be  taken  for 
operating  a  rotary  converter.  This  was  not  such  a  difficult  condi- 
tion, and  proved  satisfactory  in  service.  The  tendency  in  railway 
work,  however,  was  toward  engine-type  generators  almost  ex- 
clusively and,  of  course,  the  engine-type  a.c,-d.c.  generator  had  to 
receive  some  attention.  But  here  a  stumbling  block  occurred  in  the 
frequencies  and  the  usual  engine-type  speeds.  With  60  cycles.,  for 
instance,  an  engine  speed  of  120  r.  p.  m.  (which  was  high  for  a  500 
kw  unit,  for  example)  would  require  60  poles  to  give  the  desired 
frequency.  But  a  60-pole  direct-current  machine  of  500  kw  was 
coniinercially  impracticable.  The  same  held  true  for  practically 
all  engine  speeds  and  generator  capacities,  so  that  the  a.c,-d.c« 
60  cycle  engine-type  machine  was  never  considered  commercial. 
A  few  60  cycle  high-speed  belted  a,c,-dc.  generators  were  sold, 
these  being  usually  60  cycle  rotary  converters  modified  into 
erators. 


712  ELECTRICAL  ENGINEERING  PAPERS 

In  25  and  30  cycles,  however,  there  were  possibilities  in  engine- 
type  generators  and  in  consequence,  a  few  of  them  were  built. 
The  Westinghouse  Company  furnished  four  1 250  kw,  550  volt 
a.c.-d.c.  generators  with  32  poles  for  94  r.  p.  m.  Some  500  kw 
slow-speed  machines  for  25  cycles  were  also  built.  Quite  a  number 
of  12-pole,  250  r.  p.  m  ,  25  cycle  and  14-pole,  257  r.  p.  m.,  30  cycle, 
500  volt  a.c.-d.c.  direct-coupled  generators  were  built  and  put  in 
service.  Most  of  these  machines  were  intended  for  railway  service 
with  both  kinds  of  current,  the  alternating  current  being  trans- 
mitted to  rotary  converter  substations. 

All  the  above  were  good  operating  machines,  but  more  or  less 
at  the  expense  of  abnormally  large  designs.  They  had  to  be  made 
with  exceptionally  good  commutating  characteristics,  for  the 
armature  reaction  and  field  distortion  were  dependent  upon  the 
alternating  as  well  as  the  direct-current  loads. 

Apparently,  experienced  designing  engineers  have  never  been 
wildly  enthusiatic  over  the  a.c,-d  c.  generator,  for  its  interfering 
characteristics  were  against  the  machine.  In  consequence,  other 
methods  of  accomplishing  the  same  result  in  a  more  satisfactory 
manner,  were  given  very  careful  consideration.  With  improvements 
in  the  straight  alternator  and  in  the  rotary  converter,  it  gradually 
developed  that  an  alternator  and  a  rotary  converter  could,  in 
many  cases,  be  built  as  cheaply  as  a  corresponding  a.c.-d.c.  gen- 
erator. When  this  stage  was  reached,  the  a.c.-d.c.  generator,  in 
most  cases,  had  no  real  excuse  for  existing,  and  so  it  dropped  out  of 
sight  commercially.  Here  is  a  case  of  a  type  of  machine  which  at 
one  time  had  good  commercial  prospects,  but  which  is  now  en- 
tirely obsolete  from  the  manufacturing  standpoint.  If  anyone 
imagines  there  is  any  discredit  attached  to  this  situation,  then  the 
Westinghouse  Company,  which  put  out  most  of  the  a.c.-d,a  ma- 
chines, will  have  to  shoulder  the  greater  part  of  it.  The  enormous 
development  of  the  alternating-current  power  stations,  with  rotary 
converter  substations,  has  put  a  number  of  very  good  lines  of  ap- 
paratus out  of  business. 

COMPENSATING  AND  COMMUTATING-POLE  MACHINES 

The  cotnmutating-pole  machine  is  the  latest  big  step  ia 
direct-current  development.  It  is  very  closely  allied  to  the  com- 
pensated machine,  although  it  may  be  noted  that  the  latter  came 
into  use  many  years  ago  while  the  straight  commutating-pole  type 
was  comparatively  modern  in  its  application.  Crude  forms  of 


DEVELOPMENT  OF  THE  D.C.  GENERATOR  713 

compensating  windings  were  proposed  comparatively  early  in  the 
electrical  work.  However,  the  more  complete  form  in  which  the 
distributed  compensating  winding  was  used  appeared  about  1893. 
This  was  the  Thompson-Ryan  compensating  winding.  A  line  of 
direct-current  generators,  known  as  the  Thompson-Ryan,  was  put 
on  the  market.  However,  compensating  windings  did  not  come  into 
general  use,  apparently  because  the  conditions  under  which  they 
make  their  best  showing  did  not  exist  at  that  time.  The  compen- 
sating winding  possessed  three  advantages :  ( 1) ,  it  helped  commut- 
ating  conditions;  (2),  It  reduced  the  maximum  voltage  between 
commutator  bars,  with  a  given  number  of  bars  per  pole,  and  (3),  it 
allowed  a  higher  no-load  induction  in  the  armature  teeth.  However, 
when  the  first  compensating  machines  were  brought  out,  none  of 
these  advantages  were  controlling — ( 1) ,  due  to  the  fact  that  engine- 
type  generators  then  practically  had  the  field,  and  in  such  gener- 
ators as  a  rule,  commutation  was  not  a  limit ;  (2) ,  there  were  usually 
more  commutator  bars  than  were  actually  needed  as  regards  maxi- 
mum voltage  between  bars;  (3),  those  slow-speed,  low  frequency 
machines  usually  had  their  armature  teeth  worked  so  very  high 
that  the  elimination  of  the  cross-magnetizing  effect  of  the  armature 
would  not  mean  a  very  large  gain  in  permissible  flux,  in  many  cases. 
The  real  field  for  compensating  windings  in  generators  was  in 
high-speed,  high-frequency  machines  with  a  small  number  of  com- 
mutator bars  and  with  comparatively  low  teeth  saturations,  such 
as  in  the  generators  of  motor-generator  sets.  But  it  so  happened 
that  when  these  began  to  reach  their  limits  in  capacity  and  speed, 
commutation  was  the  first  serious  difficulty  encountered,  and  the 
adoption  of  commutating  poles  overcame  this,  so  that  -the  field 
for  the  compensating  winding  was  again  narrowed.  The  field 
seems  now  to  be  limited  largely  to  those  machines  in  which  cross 
induction  is  objectionable  and  where  there  is  need  for  reducing  the 
number  of  commutator  bars  per  pole  below  what  has  heretofore 
been  permissible  practice.  In  consequence,  the  compensating 
winding  has  recently  extended  only  to  two  new  classes  of  appar- 
atus, namely,  very  large  motors  and  generators  for  reversing  mills 
and  steel  plants,  and  high  voltage  generator  work,  in  which  there 
is  difficulty  in  finding  room  for  the  requisite  number  of  commut- 
ator bars  without  excessively  high  commutator  peripheral  speeds. 

The  ajmrnui^tibog  pole  which  teas,  during  the  past  few  years, 
come  into  such  general  ttse,  is  &ot  a  modem  idea,  as  it  was  pro- 
-Dosed  about  1 889  to  1890  m  EagtaxwL  Ttes  was  a  field  for  it  i& 


714  ELECTRICAL  ENGINEERING  PAPERS 

the  early  belted  multi-polar  railway  generators,  but  before  practical 
designs  progressed  up  to  the  point  of  commercially  using  such  a 
device,  the  engine-type  generator  came  in,  and  this,  to  a  great 
extent,  did  away  with  the  necessity  for  the  commutating  pole. 
Thus  the  commutating  pole  idea  dropped  out  of  sight,  and  did  not 
come  back  again  until  a  real  need  for  it  developed.  This  was  ap- 
parently in  connection  with  adjustable  speed  motors  having  speed 
ranges  of  three  or  four  to  one.  The  occasion  for  such  a  speed 
range  developed  in  connection  with  machine-tool  electric  drives. 
In  some  of  the  earlier  machine-tool  drives,  a  two-voltage,  three- 
wire  supply  was  used.  With  such  a  system,  a  shunt  motor  having 
an  adjustable  speed  range  of  two  to  one  by  means  of  a  shunt  field 
could  be  given  a  four  to  one  total  speed  range  by  means  of  the 
three-wire  supply  system.  This,  however,  was  a  rather  complex 
arrangement,  taken  as  a  whole,  and  was  not  of  general  application, 
and  it  soon  became  evident  that  a  four  to  one  speed  range  in  the 
motor  itself,  with  the  standard  two-wire  or  otic-voltage  supply 
system,  would  be  much  simpler  and  of  more  general  application. 
This,  therefore,  led  to  the  four  to  one  adjustable  speed  motor  for 
such  service. 

Various  schemes  were  tried  for  building  such  motors,  among 
others  being  commutating  poles.  The  great  difficulty,  of  course, 
was  to  maintain  good  commutation  at  the  highest  speeds  and 
weakest  fields.  Eventually  the  commutating  pole  furnished  the 
simplest  solution  from  the  design  standpoint  and  was  adopted  by  a 
number  of  the  different  manufacturing  companies,  the  Electro- 
Dynamic  Company  apparently  being  earliest  in  the  field.  How- 
ever, the  use  of  commutating  poles  was  limited  principally  to  such 
special  service  until  occasion  arose  to  apply  it  on  railway  motors. 
The  railway  motor  had  been  developed  into  a  well-established 
piece  pf  apparatus,  with  apparently  only  a  few  serious  defects, 
among  which  was  an  occasional  tendency  to  "buck  over,"  ap- 
parently without  sufficient  reason  in  some  cases.  This  was  credited 
partly  to  "breaking-and-making"  the  circuits  when  passing  over 
gaps  in  the  trolley  system,  etc.  Under  such  conditions  it  was 
found  that  the  motors  would  take  a  comparatively  heavy  current 
rush  which  sometimes  would  cause  flashing.  Early  tests  de- 
veloped the  fact  that  a  railway  motor  equipped  with  commutating 
poles  was  apparently  much  less  sensitive  to  flashing  than  the 
standard  type.  This  was  a  promising  idea  and  was  quickly  fol- 
lowed up.  This  was  one  of  the  reasons  for  adoption  of  commutating 


DEVELOPMENT  OF  THE  D.C.  GENERATOR  715 

poles  in  railway  motors,  but  there  was  quite  a  number  of  other 
reasons,  possibly  none  of  them  controlling,  but,  taken  together, 
they  made  quite  a  showing.  In  an  extremely  short  time  after  corn- 
mutating  poles  began  to  be  talked  about  for  railway  motors  they 
became  pretty  much  a  "fad,"  and  this  assisted  in  their  adoption. 
This  revolution  was  accomplished  very  quickly,  and  within  about 
two  years  after  they  were  first  considered  seriously,  practically 
everything  in  the  way  of  new  railway  motors  was  of  the  commut- 
ating-pole  type.  But  this  was  one  of  the  fads  that  lasted,  for  there 
was  much  more  merit  in  the  commutating  poles  in  railway  motors 
than  the  public  first  appreciated. 

The  quick  revolution  in  railway  work  accomplished  by  com- 
mutating poles  had  its  effect  upon  all  other  classes  o£  moderate 
and  large  sized  generators  and  motors.  In  some  classes  of  appar- 
atus where  the  commutating  pole  was  embodied  it  was  not  actually 
needed,  such  as  in  small  slow-speed  generators.  However,  in  other 
classes  of  machinery,  such  as  high-speed  generators,  it  practically 
revolutionized  design  in  a  few  years1  time  by  allowing  the  use  of 
much  higher  speeds  for  a  given  output,  or  much  larger  output  for 
a  given  speed.  This  was  true  particularly  in  those  cases  whore  com- 
mutation was  the  real  limit.  The  commutating  poles,  in  removing 
this  limit,  or  greatly  increasing  it,  allowed  radical  changes  in  the 
design. 

It  has  long  been  known  that  maximum  outputs  for  a  given 
amount  of  material  meant  high  armature  ampere-turns  for  a  given 
size  of  armature.  Nevertheless,  such  constructions  could  not  be 
carried  to  their  logical  limits  in  the  usual  non-com-mutating  pole 
machines,  due  to  limitations  of  commutation.  However,  the  use  of 
commutating  poles  overcame  this  difficulty,  and  thus,  to  a  con- 
siderable extent,  revolutionized  the  design  of  moderate  and  high- 
speed motors  for  general  purposes*  A  leading  example  of  this  is 
found  in  the  Westinghouse  "SK"  line  of  motors,  which  was  de- 
signed with  commutating  poles  to  use  the  material  throughout  to 
the  very  best  possible  advantage,  This  line  was  designed  with  a 
view  toward  future  tendencies,  and  was  apparently  considerably 
ahead  of  the  times  when  it  first  came  out,  judging  from  some  of  the 
criticisms  which  were  brought  against  it.  At  the  present  time  the 
commutating-pole  construction  is  used  in  practically  everything 
in  direct-current  apparatus  that  the  Westinghouse  Company 
builds,  with  the  exception  of  very  small  apparatus.-  Marry  of 
other  manufacturing  companies  can  say  the  same. 


716  ELECTRICAL  ENGINEERING  PAPERS 

Since  the  commutating  pole  has  come  into  general  use,  gener- 
ators and  motors  for  some  very  extreme  applications  have  been 
carried  out.  Very  large  capacities  of  generators  at  speeds  tin- 
dreamed  of  with  non-coirunutating-pole  machines  have  been 
carried  through  successfully.  For  example,  a  3000  kw,  600  volt, 
375  r.  p.  m.  generator,  forming  part  of  a  flywheel  motor-gener- 
ator set,  was  furnished  by  the  Westinghouse  Company  for  the 
Illinois  Steel  Company  for  operating  a  large  reversing  mill.  Also, 
a  number  of  3  500  kw,  300  r.p.m.,  watershed  driven  generators  of 
600  to  700  volts  were  furnished  by  the  General  Electric  Company 
for  an  aluminum  plant  at  Niagara  Falls.  These  two  examples 
represent  about  the  extremes  in  direct-current  design  that  have  yet 
been  built;  for,  while  larger  capacity  machines  have  been  or  are 
being  built,  yet  they  are  for  much  lower  speeds.  In  these  two 
extreme  examples,  furnished  by  two  different  manufacturers,  there 
are  certain  superficial  resemblances.  For  example,  in  both  cases, 
the  armatures  are  of  the  double  commutator  type,  both  com- 
mutators being  connected  to  the  same  winding. 

In  slower  speed  machines,  some  3  750  kw,  270  volt  Westing- 
house  machines  at  180  r.  p.  m.  have  been  geared  to  1  800  r.  p*  m. 
steam  turbines.  These  are  possibly  the  largest  current-capacity 
machines  yet  built.  The  field  for  large  direct-current  generators 
now  appears  to  be  limited  principally  to  electro-chemical  or  electro- 
metallurgical  work  where  direct  current  is  necessary,  or  to  special 
applications,  such  as  certain  classes  of  mill  work,  etc.  Some  com- 
paratively large  generators  have  been  built  for  motor-generator 
sets;  but  the  synchronous  booster  rotary  converter  now  seems  to 
be  making  considerable  headway  in  that  special  field  where  the 
motor-generator  formerly  stood  alone,  namely,  where  fairly  wide 
variations  in  direct-current  voltage  are  required, 

HIGH  VOLTAGE  GENERATORS  WITHOUT  COMMUTATORS 

This  is  a  chapter  which  should  possibly  be  recorded  simply 
on  account  of  the  persistency  of  its  subject,  and  of  the  vast  amount 
of  unproductive  work  which  has  been  expended  upon  it.  This 
work,  however,  has  been  of  more  or  less  educational  value,  on  the 
theory  that  as  much  is  learned  through  failures  as  from  successes. 
The  production  of  unidirectional  high  voltage  in  generators  with- 
out commutators  or  a  multiplicity  of  collector  rings,  has  been  one 
of  the  "will-o'-the-wisps  "  of  the  electrical  field.  Apparently  there 
are  only  a  few  engineers  and  analysts  who  have  gone  into  this 


DEVELOPMENT  OF  THE  D  C  GENERATOR  717 

problem  so  thoroughly  that  they  recognize  certain  fundamental 
reasons  why  such  machines  cannot  be  built. 

This  problem  may  be  compared,  in  some  ways,  with  the  per- 
petual motion  fallacy.  A  perpetual  motion  scheme  may  be  made 
so  complicated  and  may  involve  so  many  principles  and  combina- 
tions that  it  is  very  difficult  to  put  one's  finger  on  the  real  fallacy; 
yet,  if  the  law  of  conservation  of  energy  is  brought  to  bear  upon  it, 
the  details  of  the  scheme  need  not  be  considered  at  all,  for  this 
one  fundamental  law  condemns  it  utterly,  regardless  of  methods 
or  means  involved.  In  the  same  way,  in  the  direct-current  ma- 
chine without  a  commutator,  certain  fundamental  principles  of 
flux  cutting  and  e.  m.  f.  generation  are  sufficient  to  condemn  all 
machines  of  this  sort,  regardless  of  their  type  or  construction.  In 
the  final  analysis  they  usually  come  down  to  one  effective  con- 
ductor, or  turn,  per  pair  of  collector  rings,  which  is  the  well-known 
unipolar  generator.  However,  the  fundamental  principles  are 
difficult  to  make  clear  to  the  inexperienced,  just  as  it  is  hard  to 
convince  some  people  that  the  law  of  conservation  of  energy  holds 
good  over  the  whole  finite  scale,  from  the  practical  standpoint. 
Therefore,  the  writer  anticipates  passing  upon  such  schemes  in 
future,  just  as  he  has  done  for  some  25  years  past,  and  he,  coin- 
cidentally,  will  be  obliged  to  dampen  many  bright  hopes. 

As  indicated,  the  fallacies  are  principally  due  to  misunder- 
standing of  fundamental  principles.  For  instance,  &  favorite 
scheme  is  to  have  the  "  magnetic  lines  "  of  the  field  move  across  the 
conductors,  or  vice  versa,  generating  e.  m.  f .  in  one  direction,  and 
then  have  the  lines  closed  back  on  themselves  through  some  path 
which  the  conductors  do  not  cut;  not  recognizing  that,  as  the 
magnetic  lines  are  closed  circuits  and  the  electric  circuit  is  also  a 
closed  circuit,  the  lines  cannot  be  cut  once  unless  they  are  cut 
twice,  if  the  action  is  to  be  continuous*  The  second  cutting  is 
always  such  as  will  generate  e.  m.  f .  in  opposition  to  the  first,  and 
the  only  way  to  avoid  cutting  twice  is  to  interpose  some  relatively 
non-moving  or  non-cutting  part,  which  means  two  sliding  con- 
tacts for  each  effective  turn.  This,  of  course,  leads  at  once  to  the 
usual  unipolar  generator  with  one  turn  for  each  pair  of  collector 
rings. 

As  another  instance,  a  common  mistake  has  been  to  assume 
that,  as  the  movement  of  the  coil  across  fluxes  or  fields  of  alternate 
directions  or  polarities  will  give  an  alternating  e.  m.  f .,  therefore, 
by  revwsing  tbe  fields  at  the  proper  rate,  the  armature  e.  m.  1  will 


718  ELECTRICAL  ENGINEERING  PAPERS 

be  correspondingly  reversed,  thus  making  it  unidirectional; 
whereas,  in  fact,  it  is  still  alternating,  but  of  double  frequency. 

Other  schemes  involve  "inductor"  alternator  constructions, 
with  a  view  to  obtaining  half  waves,  or  those  of  only  one  polarity, 
thus  giving  a  pulsating  unidirectional  e.  m.  f .  In  such  schemes  the 
various  transformer  actions  and  the  reverse  cutting  of  the  flux  by 
the  conductors  are  usually  overlooked.  Still  other  schemes  are 
dependent  upon  the  assumption  that  magnetic  lines  may  be  open 
or  discontinuous;  or,  if  continuous,  may  be  stretched  or  length- 
ened indefinitely.  Again,  combinations  of  several  or  all  of  these 
ideas  may  be  involved  in  the  same  proposed  device,  thus  making  a 
lucid  explanation  almost  an  impossibility.  One  of  the  most  amus- 
ing schemes,  which  not  infrequently  appears,  is  where  the  apparatus 
generates  a  true  alternating  e.  m.  f .,  but  in  which  the  inventor  has 
followed  the  action  only  through  one  e.  m.  f.  wave,  and  over- 
looked the  rest  of  the  cycle. 

All  told,  probably  hundreds  of  thousands  of  dollars  have  been 
expended  on  this  general  fallacy,  and  doubtless  many  more 
thousands  will  be  expended,  just  as  in  the  case  of  perpetual 
motion.  Moreover,  much  valuable  time  has  been  expended  by 
those  who  did  not  believe  in  the  possibility  of  such  apparatus  in 
showing  wherein  individual  schemes  submitted  to  them  are  not 
operative.  The  writer  probably  has  been  requested,  two  or  three 
times  per  year  on  an  average,  to  make  a  careful  analysis  and,  in 
some  cases,  a  full  written  report  on  schemes  of  this  nature,  which 
have  been  submitted  to  the  Westinghouse  Company.  *  In  fact,  he 
has  repeatedly  threatened  to  prepare  a  printed  form  which  could 
be  used  as  a  "blanket"  report  for  all  such  cases.  However,  the 
fact  that  many  of  these  cases  come  from  "very  good  friends" 
apparently  precludes  such  procedure. 

CONCLUSION 

As  will  at  once  be  noted  by  any  "old-timer/*  this  history  is 
far  from  being  a  complete  one.  The  writer's  endeavor  has  been  to 
cover  those  points  within  his  direct  knowledge  and  experience 
which  have  had  an  important  effect  on  direct-current  generator 
and  motor  development ;  in  other  words,  he  has  attempted  only  to 
hit  the  "high  spots."  If  all  the  interesting  sidelights,  incidents, 
etc.,  within  his  own  experience  were  to  be  included  it  would  have 
extended  this  article  to  triple  length,  possibly,  and  it  would  have 
become  much  more  reminiscent  than  historical.  This  article  has 


DEVELOPMENT  OF  THE  D.C  GENERATOR  719 

been  made  very  broadly  impersonal,  as  due  credit  cannot  be  given 
to  all  who  have  expended  so  much  time  and  energy  in  bringing  the 
development  up  to  its  present  high  stage.  Occasionally  personal 
references  have  been  included,  partly  to  break  up  the  historical 
tenor,  and  partly  to  counteract  any  impression  which  may  have 
been  created,  wholly  unintentionally,  that  the  writer  personally, 
or  the  company  with  which  he  is  associated  has  been  the  only 
active  participant  in  this  great  development.  Much  of  the  history 
covering  the  inside  story  of  direct-current  generator  development 
has  never  been  recorded  and  will  eventually  be  lost.  If  a  few  of  the 
ancient  mariners  of  this  sea  could  be  induced  to  tell  their  tales  it 
would  be  a  boon  to  the  younger  generation.  If  the  writer  has  aided 
even  a  little  in  preserving  this  early  history,  he  feels  thoroughly 
repaid  for  his  effort* 


THE  DEVELOPMENT  OF  THE  STREET  RAILWAY 
MOTOR  IN  AMERICA 

FOREWORD — The  author  took  an  active  part  in  some  of  the  earliest 
commercial  successful  electric  railway  developments  and  has 
been  in  close  touch  with  much  of  the  later  work.  His  direct 
knowledge  regarding  details  of  development  by  companies 
other  than  the  one  with  which  he  is  identified  is,  therefore, 
necessarily  limited,  to  a  certain  extent.  No  claim  is  made  that 
this  article  covers  the  history  of  all  railway  motors.  It  is 
rather  the  history  of  the  author's  own  experience  in  this  very 
interesting  field.  The  present  article  is  limited  to  street  railway 
motors  and  no  attempt  is  made  to  cover  heavier  service  as 
represented  by  interurban  and  main  line  railways.  Neither  are 
controllers  and  control  systems  more  than  merely  touched  upon. 
This  article  was  first  published  in  the  Electric  Journal. — 
(ED.) 


RAILWAY  motor  development  in  America  began  back  in  the 
early  80 's  but  much  of  this  was  of  a  purely  pioneer  nature 
and,  while  it  left  its  impress,  in  most  cases  it  was  not  a  lasting  one. 
On  the  other  hand,  certain  of  this  early  pioneer  work  led  directly  to 
the  commercial  railway  motor  of  the  later  80's. 

Principal  among  the  pioneers  in  this  work  may  be  mentioned 
— Van  Depoele,  Henry,  Daft,  Bentley-Knight,  Sprague  and  Short. 
Some  of  the  railway  systems  brought  out  by  the  early  inventors 
simply  flashed  up  for  a  short  time  and  then  disappeared.  Others 
came  and,  through  merit,  stayed  until  forced  out  of  the  field  by 
later  developments,  many  of  their  good  points  being  embodied  in 
the  later  systems.  The  Van  Depoele  system,  with  its  under- 
tunning  trolley,  left  its  impress  on  the  future  systems  in  the  form 
of  the  tinder-running  trolley  itself,  which  has  been  used  almost 
universally  since.  Professor  Short,  with  his'series  system  attracted 
some  attention  for  awhile,  but  being  defective  in  certain  funda- 
mental principles,  this  system  disappeared  in  favor  of  the  parallel 
system,  which  Short  himself  later  adopted,  The  Sprague  system, 
which  came  a  little  later  than  some  of  the  others,  was  along  more 
nearly  correct  lines.  It  contained  certain  good  fundamental  prin- 
ciples; it  persisted  longer  than  the  other  early  systems,  and 
eventually  established  electric  propulsion  as  the  coming  system  of 
traction  for  street  railways,  etc.  This  will  be  referred  to  more 
completely  -under  the  description  of  railway  motors. 

721 


722 


ELECTRICAL  ENGINEERING  PAPERS 


RAILWAY  MOTORS 


Practically  all  the  early  railway  motors  which  were  com- 
mercially successful  were  of  the  double-reduction  gear  type,  i.  e., 
there  were  two  sets  of  gears  between  the  armature  shaft  and  the 
car  axle.  There  were  two  reasons  for  this,  namely,  the  compar- 
atively slow  speed  of  the  cars  of  those  days,  and  the  high  speed  of 
the  motors,  necessitating  something  like  a  ten-to-one  speed  re- 
duction. In  most  of  these  designs  the  motors  themselves  were 
suspended  from  the  car  axle  and  were  connected  thereto  by  means 
of  spur  gearing.  In  a  few  special  instances  attempts  were  made  to 
drive  the  axles  through  bevel  gears,  one  motor  being  connected  to 
two  axles.  None  of  these  survived.  Also,  chain  drive  was  used  on 
the  early  Van  Depoele  system. 


FIG.  1— SPRAGUE  DOUBLE  REDUCTION  MOTOR    1889 

By  1889  the  electric  railway  had  become  quite  firtnly  estab- 
lished. Even  at  this  early  day  the*  most  successful  systems  had 
certain  points  of  similarity,  which  apparently  had  some  bearing  on 
their  success,  At  this  time,  the  Thomson-Houston  (a  development 
of  the  Van  Depoele  system),  the  Sprague  (Edison  Company)  and 
the  Short  (Brush  Company)  systems  were  at  the  fore  and  all  were 


STREKf  RAILWAY  MOTOR  IN  AMERICA  723 

apparently  quite  successful.  Early  in  1890,  the  Westinghouse 
Company  entered  the  field  with  a  street  railway  system,  thus, 
making  four  principal  manufacturers.  Thereafter  for  several  years 
these  four  systems  were  the  leading  ones  on  the  market.  Gradually 
two  of  these  dropped  out,  or  combined  with  others,  leaving  the 
General  Electric  (Thomson-Houston  and  Edison)  and  the  West- 
inghouse as  the  only  large  manufacturers.  Therefore,  the  follow- 
ing description  will  be  confined  largely  to  the  motors  of  the  four 
earlier  systems  and  the  two  later  ones. 

SPRAGUE  RAILWAY  MOTOR 

The  Sprague  electric  railway  motor  system  of  1888  to  1890 
was  unquestionably  the  most  perfect  one  of  that  time  from  the 
standpoint  of  control  and  economy  of  operation.  This  was  due 
principally  to  certain  fundamental  features  of  design,  which  had 
been  carried  to  the  utmost.  This  motor  was  of  the  two-pole 
type.  The  armature  was  of  the  surface-wound  type  with  several 
layers  of  wire.  It  is  obvious  that  such  a  motor  was  inherently 
poorly  protected  and,  from  the  present  standpoint  would  be  con- 
sidered an  extremely  doubtful  piece  of  mechanism  to  place  tinder 
a  car.  However,  in  those  days,  all  other  makes  of  motors  were  just 
as  questionable  and,  therefore,  this  motor  did  not  suffer  by  com- 
parison. 

The  interesting  f  eature  about  this  motor  was  in  the  method  of 
starting  and  speed  control.  The  field  structure  was  made  of  a  good 
grade  of  wrought  iron  of  high  magnetic  permeability.  The  field 
coils  were  wound  in  three  sections  of  different  sizes  of  wire  and 
different  numbers  of  turns  and  the  field  windings  were  so  propor- 
tioned that,  with  all  the  field  coils  in  series  at  start,  a  heavy  torque 
was  obtainable  with  a  very  small  starting  current,  thus  avoiding 
overheating  the  fields  without  the  use  of  a  starting  rheostat, 
However,  it  should  be  said  that,  with  all  the  field  coils  in  series, 
the  combined  resistance  of  the  armature  and  field  was  sufficient  to 
fix  the  starting  current  at  a  relatively  small  value.  Following 
the  series  starting  position,  by  series-paralleling  of  the  field  coils, 
various  combinations  of  speed  were  obtainable  up  to  the  maximum 
desired.  Here  was  a  system  where  all  starting  and  controlling  was 
done  without  external  rheostats,  a  very  economical  method  of 
operation  and  one  which  has  possibly  not  been  exceeded  in  any  of 
the  ^later  commercial  direct-current  methods  of  operation.  This 

due  largely  to  the  relatively  high  speed  of  the  armatttee  of  thfe 


724  ELECTRICAL  ENGINEERING  PAPERS 

double-reduction  type  and  to  the  fact  that  the  field  magnetic  flux 
could  be  worked  over  a  very  wide  range,  while  the  total  motor 
capacity  was  small  compared  with  modern  practice.  These  favor- 
able conditions  disappeared  largely  in  the  later,  lower  speed,  single- 
reduction  motors. 

While  this  early  Sprague  motor  was  a  very  fine  one  from  the 
viewpoint  of  economy  of  power,  yet  according  to  the  writer's  ex- 
perience, it  did  not  have  the  ruggedness  for  emergencies  found  in 
some  of  its  competitors.  The  very  element  which  made  it  so  econ- 
omical, namely,  the  series-parallel  field  windings  and  the  absence 
of  a  rheostat,  made  it  more  delicate  in  emergency  conditions  which 
required  abnormal  currents  for  prolonged  periods;  such  as  push- 
ing snow  plows,  for  instance,  during  severe  storms.  In  some  cases 
the  Sprague  motor  proved  very  inferior  to  some  of  its  competitors, 
due  to  overheating  when  running  at  low  speeds.  Nevertheless, 
with  all  of  its  weaknesses,  this  Sprague  double-reduction  motor 
must  be  considered  as  the  high  class  one  of  its  day, 

THE  THOMSON-HOUSTON  MOTOR 

In  general,  the  Thomson-Houston  motor  was  of  the  same 
general  type  as  the  Sprague.  The  magnet  core  was  of  wrought  iron, 
or  equivalent  material.  The  armature  was  of  the  usual  surface- 
wound  type.  Unlike  the  Sprague  motor,  speed  control  was  only 
partially  obtained  by  varying  the  field  strength.  The  field  was 
wound  with  loops  or  taps  brought  out  near  the  middle  of  its 
length.  For  starting  and  acceleration,  the  full  field  winding  was 
used  with  a  rheostat  in  series.  To  accelerate,  the  rheostat  was  cut 
out  gradually  and  for  still  higher  speed,  only  part  of  the  field  wind- 
ing was  used,  the  other  part  remaining  idle.  Thus  there  was  no 
true  series-paralleling  of  the  field  windings.  This  method  of  opera- 
tion, therefore,  was  less  economical  than  the  Sprague  arrangement 
but,  on  the  other  hand,  the  proportions  of  the  field  winding  and  the 
rheostat  were  such  that  the  motor  could  stand  more  severe  con- 
ditions during  starting  and  acceleration.  The  field  magmetic  cir- 
cuit was  apparently  much  more  highly  saturated  than  that  of  the 
Sprague  motor,  resulting  in  a  flatter  speed  curve.  In  consequence 
this  motor  would  run  somewhat  faster  than  the  Sprague  on  heavy 
load,  and  was  considered  by  many  operators  as  a  better  hill 
climber,  simply  because  it  ran  faster  up  hill.  Due  to  its  lower 
saturation,  the  Sprague  motor  tended  to  drop  off  very  considerably 
in  speed  on  heavy  grades  and  this  was  considered  an  evidence  of 


STREET  RAILWA  Y  MOTOR  IN  AMERICA  72o 

weakness,  that  is,  of  lack  of  power;  whereas,  in  fact,  it  was  a  real 
merit  in  those  days  of  limited  power  supply.  The  range  of  current 
taken  by  this  Thomson-Houston  motor,  due  to  its  flatter  speed 
characteristics,  was  apparently  considerably  greater  than  that  of 
other  types  of  railway  motors.  The  commutation  on  this  motor 
was  apparently  very  good  compared  with  the  Sprague  motor.  In 
fact,  the  latter,  according  to  the  writer's  experience,  appeared  to 
be  one  of  the  poorest  commutatitig  motors  on  the  market.  Never- 
theless, due  to  its  special  method  of  control  and  the  consequent 


FIG.  2— THOMSON-HOUSTON  DOUBLE  REDUCTION  MOTOR— P-30 

smaller  currents  required,  this  poorer  commutation  did  not  seem 
to  have  as  harmful  effects  as  one  would  infer  from  looking  at  it. 
In  other  words,  the  commutator  of  the  Sprague  motor  had  about 
as  good  life  as  any  of  the  others. 

One  thing  that  counted  against  good  commutation  on  these 
early  motors  was  the  extremely  heavy  mica  between  commutators 
bars.  One-sixteenth  inch  mica  was  not  at  all  uncommon  on  such 
motors  and  when  trouble  developed  at  the  commutator,  there  was 
frequently  a  cry  for  thicker  mica  and,  as  a  consequence,  the  thicker 
the  mica  the  greater  the  trouble.  This  persisted  up  into  the  later 
motor  practice  and  was  a  source  of  much  trouble  for  several  years. 

THB  SHORT  MOTOR 

In  construction,  the  Short  railway  motor  was  a  close  relative  of 
the  Brush  arc  machine,  that  is,  its  magnetic  circuit  and  other  parts 
were  arranged  very  similarly  to  that  of  the  arc  machine.  A  disc 
armatttre  was  used  with  polefaces  presented  at  the  sides  of  thearma- 
tuxe,  The  early  machines  were  of  a  two-pole  type  and  later 


726  ELECTRICAL  ENGINEERING  PAPERS 

general  construction  was  developed  in  four  poles  in  connection 
with  later  Short  systems.  The  armature  of  this  Short  motor  was  of  a 
.toothed  type,  this  also  being  apparently  a  development  from  the 
Brush  arc  machine,  It  is  questionable  whether  the  teeth  on  this 
armature  were  proportioned  for  magnetic  purposes  or  for  mechan- 
ical. The  teeth  were  few  in  number  and  the  slots  between  were 
quite  wide.  Magnetically  the  arrangement  might  be  considered 
as  some  improvement  over  the  surface-wound  type,  but  the  pro- 
portions were  not  such  as  would  be  considered  effective,  even  in  the 
true  slotted  types  of  armatures  which  followed  two  or  three  years 
later. 

This  Short  type  railway  motor  contained  a  number  of  more  or 
less  fundamental  defects,  which  in  the  end  were  sufficient  to  rule 
out  the  type.  In  the  first  place,  due  to  the  disc  type  of  construc- 
tion and  side  poles,  there  was  a  tendency  for  strong  unbalanced 
side  pull  between  the  armature  and  the  pole  pieces,  and  strong 
thrust  collars  were  necessary  to  overcome  this.  On  account  of  this 


PIG.  3— SHORT  DOUBLE  REDUCTION  MOTOR— 1890 

arrangement  no  end  play  was  permissible,  as  in  ordinary  railway 
motors.  In  the  second  place,  the  method  of  connection  between  the 
commutator  and  armature  winding  was  a  very  awkward  one, 
since  the  armature  leads  had  to  be  carried  radially  to  the  shaft  and 
then  along  the  shaft  to  the  commutator.  In  the  third  place,  with 
this  general  construction,  a  non-magnetic  spider  had  to  be  used,  as 
a  rule.  This  meant  a  construction  which  was  not  as  solid  6r  as 
durable  as  was  obtainable  with  the  cylindrical  drum,  type  of  arma- 


STREET  RAILWA  Y  MOTOR  IN  AMERICA  727 

ture  with  the  laminations  pressed  directly  ou  the  shaft  or  upon 
a  cylindrical  supporting  spider. 

Even  with  all  these  defects  this  type  of  machine  was  continued 
for  several  years  and  was  carried  into  the  single  reduction  type  and 
into  the  gearless,  when  the  construction  was  somewhat  simplified 
by  the  use  of  four  and  six  poles  respectively.  However,  the  type 
was  destined  to  disappear  due  to  fundamental  defects,  and  ap- 
parently only  the  persistency  of  Professor  Short,  who  originated 
it,  kept  it  going  as  long  as  it  did.  Eventually  Professor  Short  him- 
self abandoned  the  type,  when  he  put  out  the  Walker  motor,  which 
will  be  mentioned  later. 

WESTINGHOUSE  MOTOR 

The  remaining  double-reduction  motor,  which  made  any  con- 
siderable impression  on  the  railway  field,  was  the  Westinghouse. 
This  was  brought  out  in  the  Spring  of  1890,  somewhat  later  than 
the  other  systems  mentioned.  In  general  type,  this  motor  was  quite 
similar  to  the  Sprague  and  the  Thomson-Houston.  However,  the 
field  core  was  of  cast  iron  and  the  motor  was,  therefore,  somewhat 
heavier  than  its  competitors.  The  armature  was  surface-wound 
and  similar  to  almost  all  railway  motors  of  that  time.  The  field 
winding  was  arranged  in  two  coils  without  metal  "bobbins,"  with 
different  sizes  of  wire  and  different  numbers  of  turns.  For  starting, 
all  field  windings  were  in  series  and  the  rheostat  was  connected  in 
series.  For  higher  speed  the  smaller  winding  was  cut  out.  Ob- 
viously, this  arrangement  was  electrically  very  similar  to  the 
Thomson-Houston. 

The  principal  differences  were  in  details  of  the  mechanical  con- 
struction. The  fields  were  hinged  to  the  supporting  yoke  in  such  a 
way  that  they  could  swing  back  to  give  more  easy  access  to  the 
armature.  Also  the  gears  were  enclosed  in  gear  cases  which  were 
filled  with  lubricating  grease.  The  purpose  was  to  overcome  the 
very  objectionable  noises  of  the  double  reduction  gears.  Anyone 
who  is  familiar  only  with  the  present  gear  noises  from  traction 
motors  can  have  no  comprehension  of  the  fearful  racket  some  of 
the  double-reduction  equipments  made,  especially  after  the  gears 
had  worn  badly.  At  night,  when  other  noises  had  ceased  to  a 
great  extent,  the  electric  cars  could  be  heard,  in  some  cases,  at  a 
distance  of  one  to  two  miles* 

On  many  of  the  early  double-reduction  eqtdpments  cast  iroa 
gears  were  used  and,  as  a  consequence,  stripped  gears  were  riot 


728 


ELECTRICAL  ENGINEERING  PAPERS 


uncommon.  In  those  days  cars  were  operated  under  conditions 
which  no  one  would  dream  of  attempting  in  these  times.  In  one 
case,  in  the  writer's  experience,  a  track  was  being  repaired  in  a 
certain  part  of  Allegheny  City  and  the  only  way  to  get  around 
it  was  to  run  up  a  parallel  street  and  part  way  over  a  cross  street 
to  the  end  of  the  track,  which  was  about  thirty  feet  from  the 
original  track.  This  intervening  section,  paved  with  rough  cobble 
stones,  was  overcome  by  getting  the  car  up  to  considerable  speed 


FIG.  4—500  VOLT  WESTINGHOUSB  DOUBLE  REDUCTION  MOTOR— 1890 

and  running  across  the  space  by  means  of  inertia.  If  the  car  did 
not  get  across,  then  a  long  wire  was  carried  from  the  controller 
on  the  car  back  to  the  end  of  the  truck  and  thus  a  "ground"  was 
obtained  for  covering  the  rest  of  the  way.  In  one  instance,  a  car 
was  stalled  in  this  section  and  the  motorman  left  his  controller 
on  "full "  position  while  he  carried  his  conducting  wire  back  to  the 
end  of  the  track.  Upon  touching  the  rail,  the  car  did  not  move  out 
of  its  steps,  so  as  to  speak,  but  simply  gave  a  jerk  and  the  gears 
were  stripped. 

The  Westinghouse  double-reduction  motor  was  made  of  cast 
iron,  but  its  operating  characteristics  were  quite  comparable  with 


STREET  RAILWAY  MOTOR  IN  AMERICA  729 

the  other  systems,  except  the  Sprague.  However,  although  a  con- 
siderable number  of  these  motors  were  put  out  in  1890,  the  writer, 
along  with  certain  other  engineers  of  that  time,  did  not  believe 
that  any  one  of  the  then  existing  railway  systems  was  final, 
due  primarily  to  the  fact  that  the  motors  were  too  susceptible  to 
injury,  not  being  sufficiently  protected  in  view  of  their  location 
under  the  car.  It  was  believed  that  it  was  merely  a  question  of  time 
when  all  such  motors  would  have  to  be  rebuilt.  The  first  West- 
inghouse  motor  was  put  in  service  in  Allegheny,  Pa.,  on  July  3, 
1890.  This  date  is  given  to  indicate  the  short  time  which  elapsed 
before  the  writer,  who  had  been  instrumental  in  getting  out  this 
Westinghouse  system,  undertook  to  get  out  a  radically  different 
system  to  supersede  it. 

GENERAL  TREND  OP  DEVELOPMENT 

The  above  brings  us  up  to  the  period  when  the  single-reduction 
motor  was  developed.  The  double-reduction  motor  very  quickly 
disappeared  from  the  market  after  the  single-reduction  arrived, 
but  it  must  be  said  that  the  double-reduction  motor,  and  the  early 
system  as  a  whole,  left  its  impress  on  the  future  development. 
There  were  several  features  in  this  early  development  which  sur- 
vive even  to  the  present  time,  such  as  the  use  of  carbon  brushes, 
series-wound  motors,  motors  suspended  from  the  axles  and  geared 
to  them,  enclosing  gear  cases  with  grease  lubrication,  mummified 
field  coils,  under-running  trolley,  platform  controllers,  etc.  The 
fact  that  a  number  of  these  features  have  survived  in  very  much 
their  original  form  indicates  that  they  were  fundamental  in  their 
nature.  The  early  designers  of  such  systems  must  be  given  credit 
for  a  quite  comprehensive  knowledge  of  the  real  problem  of  electric 
traction*  Their  short-comings  were  more  in  their  inability  to  con- 
struct, than  in  their  lack  of  knowledge  of  the  correct  principles. 

Those  early  days  were  times  of  experimentation  by  the 
operators  as  well  as  by  the  manufacturers  and  it  was  not  an  unusual 
thing  for  a  small  electric  system  to  have  two  or  three  different 
types  of  equipment,  and  in  one  case  in  a  small  system  near  Pitts- 
burgh having  seven  cars  total,  there  were  five  different  kinds  of 
equipment  at  one  time.  Furthermore,  the  operator  was  rathet 
proud  of  the  situation.  In  this  early  work  there  were  a  number  of 
points  which  were  taken  v^ry  seriously  in  those  dayg,  but  which,* 
from  the  present  iriewpomt,  are  rather  amttsfog, 


730  ELECTRICAL  ENGINEERING  PAPERS 

For  example:  The  earth  was  considered  as  being  of  negative 
potential  and,  therefore,  many  Engineers  (or  so-called  engineers) 
held  the  opinion  that  the  positive  terminal  of  the  motor  could  not 
be  connected  with  safety  to  the  ground  side  as  there  was  danger 
of  a  short-circuit.  The  writer  spent  many  weary  hours  attempting 
to  show  some  people  the  absurdity  of  this  opinion,  but  generally 
without  success. 

Also  another  subject  on  which  there  was  considerable  contro- 
versy was  that  of  large-diameter  vs.  small-diameter  armatures. 
Many  people  contended  that  even  with  the  same  horse-power  and 
speed  a  large  diameter  armature  necessarily  gave  more  tractive 
effort  than  a  smaller  diameter. 

There  was  also  much  discussion  concerning  the  speed  and 
power  characteristics  of  the  various  motors.  Certain  makes  of 
motors  ran  faster  up  hill  than  others.  The  Sprague  motor,  for 
instance,  was  a  slow  hill  climber;  on  the  other  hand,  the  Thomson- 
Houston  double-reduction  motor  was  a  fast  hill  climber  and  the 
Westinghouse  was  in  between.  As  a  rule,  most  people  believed 
that  the  Thomson-Houston  motor  was,  therefore,  a  more  powerful 
one  than  either  of  its  competitors.  The  writer  had  quite  frequent 
contentions  that  the  Sprague  type  of  motor,  with  its  drooping 
speed  characteristics,  was  more  nearly  ideal  for  railway  work  than 
the  Thomson-Houston  with  its  flatter  speed  curve.  His  claim  was 
that  the  drooping  speed  characteristics  called  for  a  more  uniform 
and  a  lower  average  current  from  the  generating  system,  and  there- 
fore, required  less  generating  plant,  He  contended  that  the  place  to 
make  speed  was  on  the  level  and  rjot  on  the  hills.  Apparently  this 
argument  has  never  been  definitely  decided  in  favor  of  either  view- 
point, but  today  it  is  generally  recognized  that  the  steeper  speed 
characteristic  is  a  more  economical  one  as  far  as  the  generating 
or  transmission  system  is  concerned. 

In  comparing  the  merits  of  these  early  types  of  motors,  a  not 
unusual  test  was  to  couple  cars  with  two  different  makes  of  equip- 
ments, end  to  end  and  then  determine  which  could  outpull  the 
other,  starting  from  "rest."  Of  course,  a  good  deal  depended  upon 
the  skill  of  the  motorrnen,  but  in  many  cases  those  motors  with 
drooping  speed  characteristics  had  the  advantage,  and  therefore, 
according  to  this  test,  were  more  powerful,  although  when  it  caxne 
to  climbing  hills  they  were  supposed  to  be  less  powerful.  Here  was 
a  contradiction  which  puzzled  a  great  many  people. 


STREET  RAILWAY  MOTOR  IN  AMERICA  7,'U 

One  interesting  feature  in  connection  with  the  early  motors 
may  be  dwelt  on  more  extensively,  namely,  the  use  of  the  series 
motor.  Very  early  in  the  development,  shunt  motors  were  tried 
but  it  was  soon  recognized  that  they  did  not  meet  practical  condi- 
tions, and  the  scries  motor  was  adopted  exclusively.  However,  in 
the  use  of  the  series  motor  itself  there  were  certain  differences  in 
practice.  For  instance,  most  of  the  railway  systems  paralleled  the 
field  windings  and  the  armatures,  independently.  For  reversing  it 
was  necessary  to  bring  out  leads  between  each  armature  and  its 
field  windings,  and  the  field  windings  of  the  different  motors  were 
permanently  paralleled  with  each  other.  The  same  was  true  of  the 
armatures.  Then  by  means  of  one  reversing  switch  all  the  arma- 
tures, or  all  the  fields,  could  be  reversed.  In  the  Thomson-Houston 
system,  however,  the  different  field  coils  and  armatures  were  not 
paralleled  with  each  other,  but  separate  reversing  switches  were 
supplied  for  each  motor.  Obviously  this  required  more  wiring  and 
reversing  switches  than  the  other  systems  and  was  a  subject  of 
much  criticism.  But,  this  arrangement  was  fundamentally  correct, 
and  has  come  down  to  the  present  day.  The  other  methods,  with 
paralleled  field  coils,  were  subject  to  the  difficulty  that  there  could 
be  greatly  unbalanced  currents  in  the  armatures,  where  the  mag- 
netic fields  were  not  of  equal  strength ;  whereas,  with  the  Thomson* 
Houston  motors  there  could  only  be  unbalanced  currents  between 
the  motors  as  a  whole  and  not  between  individual  armatures,  and 
any  unbalance  in  the  current  in  the  field  coils  tended  automatically 
to  correct  the  difficulty. 

In  the  double-reduction  motors,  with  their  excesssively  large 
air-gaps  compared  with  later  practice,  differences  in  the  magnetic 
properties  of  the  materials  did  not  count  for  much  becaxise  such  a 
large  percentage  of  the  field  magnetizing  force  was  expended  in  the 
air-gaps.  However,  when  it  came  to  the  later  single-reduction 
motors,  with  their  smaller  air-gaps  and  higher  saturation  in  the 
cores,  the  fallacy  in  the  parallel  arrangement  of  the  field  coils  began 
to  show  up  quite  early. 

SINGLE  REDUCTION  MOTORS 

In  August,  1890,  the  writer  began  work  on  a  radically  new 
type  of  railway  motor  of  only  about  one-third  the  speed  of  the 
ordinary  motor,  with  a  view  to  using  only  one  gear  reduction 
between  the  armature  and  axle*  In  goittg  into  this  matter  mw 
the  electrical  and  magnetic  standpoint,  it  soon  developed  that  tito 


732  ELECTRICAL  ENGINEERING  PAPERS 

surface-wound  type  of  armature  was  impracticable.  Furthermore, 
it  became  evident  that  a  cylindrical  type  of  field  construction  with 
inwardly  projecting  poles,  such  as  was  common  in  alternators  in 
those  days,  would  furnish  magnetic  conditions  much  better  than 
any  previous  type,  provided  more  than  two  poles  were  used.  The 
writer  then  laid  out  a  four-pole  field  construction  with  radial  poles 
and  external  cylindrical  type  yoke,  and  with  a  slotted  type  of 
armature.  It  was  at  once  obvious  that  such  type  of  machine  was 
inherently  better  protected  than  the  ordinary  construction,  due 
to  the  external  yoke.  However,  in  this  general  construction  one 
serious  stumbling  block  appeared,  namely,  the  fact  that  for  acces- 
sibility only  two  sets  of  brushes  were  desirable  with  a  four-pole 
armature.  This  appeared  to  be  quite  a  problem,  for  apparently 
the  only  known  solution  was  in  cross-connecting  the  commutator 
at  every  bar,  which  was  at  that  time  a  fairly  well-known  construc- 
tion. This  construction,  however,  appeared  to  the  writer  to  be 
prohibitive  and  he,  therefore,  set  out  to  devise  some  other  arrange- 
ment, and  in  doing  so  developed  the  now  well  known  two-circuit  or 
series  type  of  winding  for  "drum"  armatures.  A  great  deal  of 
criticism  appeared  in  connection  with  this  winding,  but  the  writer 
was  nevertheless  sure  of  the  principle  and  felt  confident  that  it  was 
a  correct  solution  of  the  problem,  and  his  confidence  was  sufficient 
to  carry  it  through  to  a  test.  Two  trial  motors  were  built  of  this 
general  construction,  in  the  Fall  of  1890.  In  these  two  early  motors 
the  lower  half  of  the  field  yoke,  or  frame,  was  carried  out  and 
upward,  forming  housings  which  enclosed  the  lower  half  of  the 
field  winding  and  shielded  the  armature  from  injury  from  below. 
The  two  brush  arms  were  placed  on  the  upper  quadrants  of  the 
armature,  making  them  more  accessible. 

The  armatures  of  these  two  motors  were  of  the  slotted  type, 
with  ninety-five  slots  (one  less  than  a  multiple  of  the  number  of 
poles,  on  account  of  the  two-circuit  winding).  At  first,  attempts 
were  made  to  wind  these  armatures  by  hand,  but  it  was  quickly 
recognized  that  this  would  be  a  rather  doubtful  construction  and 
the  writer  proposed  machine-wound  coils  which  were  at  once  made 
up  and  tried  on  the  cores.  Various  modifications  were  tried  on 
these  first  sets  of  coils  and  one  attempt  was  made  to  shape  the  coils 
in  such  a  manner  that  they  would  all  be  exact  duplicates  and  could 
be  placed  symmetrically  on  the  armature,  in  two  layers  in  the  slots, 
one  half  of  each  coil  being  in  the  lower  layer,  the  other  half  on  top, 
just  as  in  modern  railway  armatures.  We  succeeded  in  getting 


STREET  RAILWAY  MOTOR  IN  AMERICA 


733 


about  two-thirds  of  the  winding  in  place  in  this  manner,  but  then 
the  end  parts  began  to  interfere  so  that  \ve  failed  in  getting  the 
other  one-third  in  place,  and  this  experiment  was  temporarily 
given  up.  It  developed  later  that  a  little  more  knowledge  of  the 
correct  shape  of  the  coil  would  have  allowed  a  successful  construc- 
tion of  this  type,  and  thus  one  of  the  big  steps  in  the  later  develop- 
ment would  have  been  anticipated.  However,  after  several  weeks 
of  experimenting,  it  was  decided  to  put  the  machine  wound  coils 


PIG.  S— DIAGRAM  OF  WENSTROM  MOTOR— 1890 

on  in  two  layers,  hammering  down  the  ends  of  the  first  layer  in 
order  to  obtain  end  space  for  the  second.  With  this  arrangement, 
machine-wound  coils  ^rere  used  successfully  and  the  first  two 
armatures  were  then  wound  in  this  manner,  the  writer  personally 
winding  one  of  them,  although  not  an  experienced  winder. 

The  first  completed  machine  was  put  on  test  and  at  the  first 
trial,  for  a  wonder,  it  started  off  and  performed  admirably  over  the 
whole  range  for  which  it  was  designed*  The  commutation  was  very 
good — unexpectedly  so — as  this  was  one  of  the  points  where 
trouble  was  feared.  The  two-circuit  type  of  winding  functioned  as 
expected.  By  good  fortune,  one  big  departure  from  previous 
practice  proved  to  be  a  stepping-stone  to  later  work,  namely,  in 
these  first  machines  the  mica  between  commutator  bars  had  been 
made  only  1-32  in.  thick;  whereas,  in  double-reduction  types  of 
motors  1-16  in*  mica  was  common  practice*  This  "thin**  tmca, 
however,  was  objected  to  so  seriously  by  almost  everybody  in- 
terested, that  on  the  following  motors  it  was  changed  to  practicaJly 
double  thickness,  but  with,  disastrous  results,  as  will  be  described 
later*  This  first  Westinghouse  siagleHt^uctiofr  motor  was  tested 


734  ELECTRICAL  ENGINEERING  PAPERS 

in  the  Fall  of  1890,  but  was  not  considered  quite  ready  for  the 
market  from  the  manufacturing  standpoint,  although  in  its  elec- 
trical characteristics  it  had  proven  entirely  satisfactory.  It  was 
decided  to  improve  the  motor  by  hinging  the  two  halves  of  the 
cylindrical  field  to  a  supporting  frame  which  carried  the  armature 
and  axle  bearings  It  was  also  decided  to  enclose  more  completely 
the  lower  half  of  the  frame  so  that  the  armature  and  field  would  be 
protected  from  below.  The  motor  was  considered  to  be  a  very 
radical  step,  and  it  was  thought  advisable  to  take  ample  time  in 
getting  it  ready  for  the  market. 

THE  WENSTROM  MOTOR 

Meanwhile,  during  this  development,  a  situation  arose  which 
materially  hurried  up  the  work.  The  Wenstrom  Company  came 
out  with  a  single-reduction  motor  which  was  heralded  as  being 
revolutionary  in  character.  This  motor  was  of  the  foxir-pole  type 
with  two  salient  and  two  consequent  poles  The  armature 
was  of  a  four-pole  type.  The  armature  winding  was  imbedded  in 
holes  or  tunnels  below  the  surface  of  the  core.  This  armature  was, 
therefore,  one  form  of  the  slotted  type.  This  machine  created  such 
interest  that  it  was  immediately  decided  to  rush  the  completion  of 
the  Westinghouse  motor  for  the  next  Spring  trade,  whereas,  the 
former  intention  had  been  to  continue  the  double-reduction  motor 
for  sometime  to  come.  Moreover,  the  appearance  of  this  Wenstrom 
motor  immediately  hurried  all  other  motor  manufacturers  in  their 
development  of  single-reduction  motors.  Apparently  a  number  of 
them  had  already  been  working  on  this  line,  for  their  new  single- 
reduction  motors  appeared  so  quickly  on  the  market,  that  there 
was  good  reason  to  believe  that  they  had  already  partly  developed 
the  machines  before  the  demand  came.  Some  of  these  motors  were 
put  on  the  market  before  they  were  properly  developed  and  they 
proved  to  be  merely  makeshifts  to  be  superseded  soon  by  radically 
different  types.  This  Wenstrom  motor  did  not  persist  as  it  ap- 
parently contained  certain  defects  which  put  it ' '  out  of  the  running  " 
before  it  had  gotten  very  far.  It,  however,  hurried  the  situation 
very  materially, 

WESTINGHOUSE  No.  3  MOTOR 

The  commercial  single-reduction  motor,  which  the  Westing- 
house  Company  put  out  in  the  Spring  of  1891  was  simply  a  further 
development  of  the  experimental  Westinghouse  four-pole  single- 
reduction  motor  already  described.  This  motor  immediately  "took" 


STREET  RAILWA  Y  MOTOR  IN  A  MRRICA  735 

and  a  very  large  number  (for  those  times/  was  sold1  the  first'season. 
In  fact  the  demand  for  this  motor  was  so  pronounced  that  the 
company  could  not  dispose  of  all  of  the  double-reduction  motors 
on  hand  partly  or  wholly  completed 

This  No.  3  motor  might  be  called  the  progenitor  of  the  present 
practically  universal  type  of  direct-current  railway  motor.  It  con- 
tained a  fairly  large  number  of  the  fundamental  features  found  in 
the  present  motors.  Some  of  these  may  be  classified  as  follows : — 

1 — Four-pole  field  construction  with  internal  radial  poles. 

2 — Symmetrical  flux  distribution,  thus  improving  commutation. 

3 — Four  coils  all  similar  in  size  and  shape. 

4 — Field  coils  without  bobbins  or  supports,  each  coil  being  wound  on  a 
form  and  afterwards  insulated. 

5 — Electrical  parts  naturally  protected  from  below  by  the  iron-clad  con- 
struction of  the  magnetic  circuit  and  frame. 

•6 — Four-pole  slotted  drum  typ-*  armature  \vith  open  slots. 

7 — Machine  wound  armature  coils,  insulated  before  being  placed  on  core, 

8 — Two-circuit  or  series  direct-current  armature  winding,  which  is  in 
almost  universal  use  at  present  for  railway  work. 

9 — Saturated  pole  tips. 

In  addition  the  first  motors  built  had  1-32  in,  mica,  which  is 
now  a  standard  for  such  work.  However,  on  later  No,  3  motors, 
the  mica  was  changed  to  practically  double  this  thickness,  on  ac- 
count of  the  general  insistence  that  1-32  in.  mica  was  utterly 
impracticable  from  the  commercial  standpoint.  In  the  early  days 
of  the  railway  motor,  thick  mica  was  supposed  to  be  the  "  cure-all" 
for  all  flashing  troubles.  If  the  mica  did  not  wear  fast  enough,  and 
lifted  off  the  brushes,  the  machine  sooner  or  later  would  spark  and 
flash  badly,  The  cry  would  be  for  more  mica  and,  in  some  cases, 
thicknesses  of  as  much  as  1-8  in.  were  used,  but  without  advantage 
as  far  as  the  writer  could  see,  but  the  claim  was  made  that  the 
trade  absolutely  required  such  mica. 

The  writer  and  his  associates  yielded  to  this  demand,  with 
unfortunate  results,  The  motors  with  thicker  mica  soon  developed 
blackening  and  burning  at  the  commutators,  and  the  only  direct 
remedy  found  for  this  was  undercutting  the  mica.  This  was  prac- 
ticed on  a  number  of  the  first  motors  put  out,  but  was  considered 
such  an  impossible  practice  that  it  was  evident  that  some  other 
remedy  was  necessary.  Meanwhile  the  first  two  experimental 
motors  had  been  running  in  regular  service  on  the  Second  Avenue 
Line  in  Pittsburgh,  and  had  developed  no  trouble  whatever  from 
commutator  blackening  or  burning*  After  axx  exhaustive  uavestt* 
gation  of  the  conditions,  the  writer  recommended  going  b&ok  to 


736  ELECTRICAL  ENGINEERING  PAPERS 

the  1-32  in.  mica,  regardless  of  any  demands  to  the  contrary.  A 
large  number  of  the  commutators  with  thick  mica  were  then 
replaced  with  this  thinner  mica  and  the  results  were  soon  apparent 
in  the  fact  that  undercutting  was  unnecessary.  This  was  con- 
clusive proof  that  the  thinner  mica  was  a  solution  of  the  problem. 
However,  it  must  be  borne  in  mind  that  even  the  1-32  in.  mica  of 
that  early  date  (1891),  was  inferior  to  modern  mica  in  wearing 
characteristics,  as  it  was  simply  punched  out  of  solid  mica,  the  only 
sub-division  being  the  splitting  up  of  the  mica  segments  into  thin 
sheets  and  then  assembling  again  in  exactly  the  same  form.  ' '  Mica- 
nite"  or  built-up  mica  did  not  appear  until  some  time  after  this. 

This  No.  3  motor  was  very  heavy  for  several  reasons.   It  had 
a  cast  iron  magnetic  circuit;  it  had  a  relatively  low  gear  ratio  com- 


FIG.  6— WESTINGHOUSE  No.  3  MO  TOR— 1891 

pared  with  later  practice,  as  it  used  an  eighteen-tooth  pinion  and  a 
sixty-four-tooth  gear.  In  service,  one  difficulty  soon  showed  itself, 
which  had  not  been  noticed  in  the  corresponding  double-reduction 
motors,  namely,  a  very  decided  tendency  to  unbalance,  in  the 
armature  currents  of  the  two  motors  on  a  car.  It  was  soon  found 
that  this  was  due  to  unequal  counter-e.  m.  f.'s  due  to  inequalities 
in  field  material,  slight  differences  in  manufacture,  etc.  On  account 
of  the  relatively  small  air-gap,  errors  in  manufacture  produced  an 
exaggerated  effect.  However,  this  difficulty  was  overcome  by  ad- 
justing the  air-gaps  of  the  motors.  It  happened  that  this  could  be 
easily  done  by  means  of  the  two  hinged  halves  of  the  fields.  This 
arrangement  permitted  the  motor  to  be  opened  slightly  at  the  two 


STREET  RAILWAY  MOTOR  IN  AMERICA  737 

opposite  joints,  so  that  sheet  iron  liners  of  suitable  thickness  could 
be  inserted  in  the  joints  of  one  motor  until  a  suitable  balance  in  the 
currents  was  obtained.  It  so  happened  that  the  Westinghouse 
Company  had  on  hand  a  large  stock  of  small  compact  ammeters 
built  for  the  Waterhouse  arc  system,  which  had  practically  become 
obsolete.  Little  testing  sets  were  made,  using  two  of  these  am- 
meters mounted  on  a  supporting  base.  These  were  furnished  to  the 
customers  for  use  in  balancing  their  car  motors.  Later  the 
straight  series  arrangement  of  armature  and  fields  was  adopted 
and  this  unbalancing  trouble  was  thereafter  negligible. 

When  the  single-reduction  motors  first  came  in,  one  of  the 
subjects  for  frequent  argument  was  in  regard  to  the  torque  which 
such  motors  could  develop.  Many  people  claimed  that  inherently 
the  single-reduction  motor  could  not  pull  a  car  as  well  as  the 


FIG.  7— THOMSON-HOUSTON  SINGLE  REDUCTION  MOTOR— 1891 

double-reduction,  even  at  the  same  horse-power  rating,  when  de- 
veloping the  same  car  speed.  This  even  went  so  far  as  to  result  in 
competitive  tests.  In  one  case  in  the  writers  experience,  a  com- 
petitive test  was  run,  about  1891,  on  the  Second  Avenue  Railway 
under  the  impression  that  such  a  test  would  prove  conclusively 
that  the  single-reduction  motors  did  not  have  the  required  torque, 
and,  therefore,  would  take  enormous  currents  compared  with  the 
double-reduction.  Local  representatives  of  the  Thomson*Houston 
Company  agreed  to,  and  took  part  in,  this  test,  but  apparently 
without  any  definite  opinions  as  to  which  equipment  would  make 
the  better  showing.  The  test  was  continued  during  the  greater  part 
of  one  day,  several  round  trips  being  tatoa  over  the  whole  tagtth 


738  ELECTRICAL  ENGINEERING  PAPERS 

of  the  system,  and  current  and  voltage  readings  were  taken  at  ten- 
second  intervals.  An  interesting  result,  noticeable  during  the 
progress  of  the  test,  was  that  the  Westinghouse  equipment  seldom 
took  less  than  25  to  30  amperes  when  running  light  and  seldom 
above  60  to  70  amperes  under  the  heaviest  conditions;  whereas, 
the  Thomson-Houston  equipment  at  times  took  as  low  as  10  am- 
peres and  at  other  times  up  to  100  amperes.  This  was  just  what 
the  writer  expected,  from  his  knowledge  of  the  speed  character- 
istics of  the  two  machines,  and  he  did  not  consider  that  the  tests 
proved  anything  more  than  the  general  characteristics  of  the  two 
machines  would  indicate.  However,  most  of  those  present  com- 
pared the  maximum  currents  taken  by  the  two  equipments  and 
drew  the  conclusion  at  once  that  the  single-reduction  was  more 
economical.  The  writer,  however,  did  not  consider  this  a  just 
comparison  and  took  the  trouble  to  carefully  analyze  the  whole  set 
of  readings  and  found  that  the  total  power  consumptions  for  the 
two  equipments  were  so  nearly  equal  that  differences  in  the  motor- 
men's  method  of  operation  could  easily  account  for  any  discrepan- 
cies. As  a  result  of  this  test  many  people  who  heard  of  it  revised 
their  opinions  of  the  pulling  characteristics  of  the  single-reduction 
equipment. 

THOMSON-HOUSTON  SINGLE-REDUCTION  MOTOR  (S,  R.  G.) 

This  was  one  of  the  motors  which  was  rushed  on  the  market 
shortly  after  the  Wenstrom  motor  appeared.  It  was  a  two-pole  ma- 
chine. The  magnetic  core  was  made  of  wrought  iron  in  order  to 
keep  down  the  weight.  The  armature  of  this  machine,  according 
to  the  writer's  memory,  was  of  the  ring  type.  From  the  electrical 
standpoint  this  motor  was  no  improvement  over  the  old  double- 
reduction,  and  the  ring  armature  in  reality  proved  to  be  much 
poorer  than  the  drum  type  used  on  the  Thomson-Houston  double- 
reduction  motors.  In  fact,  the  only  real  merit  of  this  machine  was 
in  its  lower  speed,  thus  allowing  single-reduction  gears.  An  at- 
tempt was  made  to  protect  this  machine  by  an  encasing  or  pro- 
tecting sheet  metal  pan  underneath.  This  pan  was  to  a  certain 
extent  effective,  but  unless  rigidly  supported  it  made  very  notice- 
able noise,  due  to  vibration. 


STREET*  RAILllrA  V  MO'WRJX  AMERICA  730 

W  P.  MOTOR 

It  was  soon  recognized  that  the  S,  R.  G.  motor  was  not  a 
permanent  one,  so  that  very  soon  a  new  type  was  gotten  out, 
namely,  the  "  W.  P."  (weather-proof).  This  was  an  enclosed  motor 
and  it  was,  from  the  electrical  and  magnetic  standpoint,  of  a  very 
peculiar  design.  There  was  but  one  field  coil,  placed  above  the 
armature.  The  armature  itself  was  of  very  large  diameter  and 
weight  and  of  the  slotted  type,  with  partially  closed  slots  and  the 
winding  was  of  the  ring  type.  The  winding  consisted  of  a  copper 
ribbon  threaded  through  the  openings  at  the  top  of  the  slots  and 
was  wound  in  place  by  hand.  On  account  of  the  magnetic  arrange- 
ment, a  non-magnetic  spider  was  necessary .  Alsoonaccountofthere 
being  only  one  magnetizing  coil  there  was  some  stray  field  out 
through  the  shaft  and  bearings.  This,  of  course,  was  minimized  to 
a  great  extent  by  the  non-magnetic  spider.  Possibly  one  of  the 
worst  features  in  this  W.  P.  motor  was  the  unsymmetrical  corn- 
mutating  zone.  Due  to  the  type  of  the  magnetic  circuit,  the  flux 
distributions  were  not  symmetrical  under  the  two  Doles.  Further- 
more the  armature  reaction  tended  to  distort  the  field  quite  seriously, 
thus  affecting  the  commutating  conditions.  Very  heavy  mica  was 
used  in  commutator  and  sparking  was  so  bad  that  the  life  of  com- 
mutator, in  many  cases,  was  only  a  few  months.  The  armature 
leads  had  "eye"  terminals  to  permit  easy  change  of  commutators, 

In  order  to  keep  down  the  weight,  this  W.  P.  motor  was  either 
xnade  of  steel  or  an  iron-aluminum  alloy  of  good  magnetic  proper- 
ties. This  motor  survived  for  a  number  of  years,  but  due  to  in- 
herent defects  in  its  characteristics  and  construction  it  was  doomed 
to  eventual  obsolescence.  The  ring  type  of  armature  and  the  un- 
symmetrical  flux  distributions  were  two  conditions  sufficient  to  con- 
demn this  machine,  from  the  present  viewpoint.  However,  it  must 
be  borne  in  mind  that  in  those  days  certain  features  were  considered 
very  meritorious  which  now  would  be  looked  upon  as  prohibitive, 
the  ring  type  of  armature  being  one  example.  This  W.  P.  motor 
had  its  place  in  the  ultimate  development  of  railway  apparatus, 
regardless  of  the  fact  that  it  did  not  survive, 

EDISON  SINGLE-REDUCTION  MOTOR 

The  Sprague  double-reductiott  motor  was  one  of  the  best  of 
its  type,  and 'persisted  longest  of  any  of  this  type.  However,,  the 
Edison  Company  who  had  taken  over  tbe  manufacture  of  the 
Sprague  xaotor,  finally  -recognised  that  the  day  of  the 


740  ELECTRICAL  ENGINEERING  PAPERS 

reduction  motor  was  past  and  a  single-reduction  motor  was  then 
gotten  out.  This  was  a  steel  frame  four-pole  motor.  The  armature 
was  of  comparatively  large  diameter  and,  according  to  the  writer's 
memory,  was  of  the  surface-wound  type.*  An  attempt  was  made 
to  retain  some  of  the  features  of  the  Sprague  double-reduction 
motor,  by  having  commutated  field  coils,  but  due  to  the  more 
highly  saturated  magnetic  circuits,  this  was  not  very  satisfactory. 


FIG   8— EDISON  SINGLE  REDUCTION  MOTOR— 1891 

This  motor  had  a  comparatively  short  commercial  life  and  it  was 
evidently  simply  rushed  into  the  market  to  meet  the  competition 
of  other  single-reduction  motors. 

THE  SHORT  SINGLE-REDUCTION  MOTOR 
Professor  Short,  early  recognizing  the  trend  of  development, 
got  out  a  single-reduction  railway  motor  along  lines  somewhat 
similar  to  his  former  double-reduction.  The  principal  difference 
was  that  this  new  motor  was  of  a  four-pole  instead  of  two-pole 
type.  The  disc  type  of  armature,  with  side  poles,  was  retained 
along  with  most  of  the  other  characteristic  features  of  the  older 
motor.  This  motor  attracted  much  attention,  but  as  it  possessed 
a  number  of  fundamentally  wrong  features,  such  as  a  ring  type  of 
armature,  danger  from  unbalanced  side  pull,  etc,,  it  was  a  type 
which  was  doomed  to  disappear  eventually. 

*Mr.  W.  E.  Moore,  Consulting  Engineer,  writes  the  author  as  follows: 

4  'The  old  Edison  S.R  G  motor  had  a  Gramme  ring  armature:  the  coils  beiagwound 
of  flat  copper  ribbon  in  a  toothed  core.  This  motor  was  built  in  two  sizes.  The  one 
which  you  illustrate  in  Figure  No  8  was  the  smaller  size,  known  as  the  Edison  No,  14, 
The  larger  size  was  known  as  the  Edison  No  16,  which  was  quite  similar,  except  that  the 
armature  bearings  were  earned  in  arms  projecting  from  the  axle  housing  forward,  instead 
of  the  vertical  yokes  and  the  consequent  pole  pieces  ware  split  vertically,  with  A  hinge  on 
top  and  bolted  together  at  the  bottom  for  removal  of  the  armature." 


STREET  RAILWAY  MOTOR  IN  AMERICA  741 

In  this  early  period  of  the  single-reduction  motor,  the  belief 
was  held,  rather  generally,  that  the  ring-type  railway  armature  was 
essentially  superior  to  the  drum- type.  As  the  Wcstinghouse  Com* 
pany  never  put  out  anything  but  the  drum-type  railway  armatures 
and  as,  at  different  stages  in  the  early  development,  several  of  the 
competing  companies  used  the  ring-type,  the  writer  was  "hard 
put "  at  times  to  defend  his  company's  practice.  Many  and  long 
were  the  arguments  which  he  had  on  this  score.  At  one  time  it 
looked,  to  an  outsider,  as  if  the  ring-type  was  capturing  the  field. 
This  was  when  the  Short  single-reduction  motor  and  the  Thomson* 
Houston  "WP"  were  the  principal  competitors  of  the  Westing- 
house  single-reduction.  Both  the  former  motors  had  ring-type 
armatures  against  the  Westinghouse  drum-type.  However,  prac- 
tical operation  gradually  developed  thfe  superiority  of  the  drum- 


FIG.  9— SHORT  SINGLE  REDUCTION  OR  WATER  TIGHT  MOTOR 

type  and  the  use  of  machine-wound  armature  coils  had  much  to  do 
with  deciding  the  problem,  for  unquestionably  the  machine- 
wound  armature  coil  was  much  more  applicable  to  the  drum-type 
than  to  the  ring-type.  Moreover,  there  were  inherent  weaknesses 
in  the  ring-type,  such  as  the  use  of  non-magnetic  spiders,  methods 
of  attaching  the  spider  to  the  core,  etc.  In  the  light  of  present 
experience,  it  is  surprising  that  the  ring-type  armature  made  as 
good  showing  as  it  did* 

GEARLESS  MOTORS 

Following  the  success  of  the  single-reduction  motors,  two  of 
the  companies,  txamely ,  the  Westioghouse  and  the  Short,  attempted 
to  make  gearless  motors  alon^  the  same  geneiul  lines  as  the  single 
type,  Thfc  Weetata^bouse  Cotaypariy  pat  out  two 


742  ELECTRICAL  ENGINEERING  PAPERS 

stractions,  one  having  four  poles  and  the  other  having  six,  the 
latter  being  considerably  lighter.  However,  both  of  these  motors 
were  too  heaw  and  it  soon  developed  that  the  gearless  principle 
was  not  a  satisfactory  one  for  ordinary  street  car  purposes,  due 
largely  to  undue  weight  directly  on  the  axle,  and  to  the  difficulty 
in  removing  an  armature  from  the  axle,  in  case  it  was  necessary  for 
repair  purposes. 

The  Short  Company  built  a  gearless  motor  along  the  same 
lines  as  its  single-reduction  and  tested  it  out  in  practice,  but  it  was 
soon  abandoned  for  the  same  general  reason  as  the  Westinghouse, 
namely,  that  the  gearless  principle  was  fundamentally  incorrect 
for  ordinary  street  railway  service. 

FURTHER  DEVELOPMENTS  OF  SINGLE-REDUCTION  MOTORS 

As  indicated,  the  Edison  motor  soon  dropped  out  of  the  run- 
ning. It  contained  nothing  lasting  in  its  type.  Also,  although  it 
persisted  longer  than  the  Edison,  the  Short  type  gradually  dropped 
out.  Meanwhile  the  Edison  and  the  Thomson-Houston  Compan- 
ies had  combined  and  formed  the  General  Electric  Company. 
This  company  continued  to  develop  its  railway  motors  in  the  at- 
tempt to  find  something  better  than  its  W.  P.,  already  described. 
The  Westinghouse  Company  also  persisted  in  its  development, 
principally  with  a  view  to  reducing  the  size  and  weight  of  the  No.  3 
motor.  The  future  development  of  the  railway  motors,  therefore, 
lies  almost  entirely  with  these  two  companies. 

The  Walker  Company,  about  1895  or  1896,  appeared  on  the 
market  with  a  railway  motor  and  did  a  very  considerable  amount 
of  business  until  absorbed  by  the  Westinghouse  Company.  The 
Lorain  motor  attracted  considerable  attention  for  a  time,  but  was 
also  taken  over  by  the  Westinghouse  Company*  Both  of  these 
were  so  nearly  along  the  general  lines  of  the  Westinghouse,  that 
they  need  not  be  considered  as  special  types, 

LATER  TYPES  OP  WESTINGHOUSE  MOTORS 

After  the  No.  3  Westinghouse  motor  had  proved  to  be  com- 
mercially a  very  successful  type,  the  writer  turned  his  attention 
toward  improvement  in  its  general  type  without  losing  any  of  the 
more  advantageous  features.  One  of  the  features  in  the  design  of 
the  No.  3  was  the  use  of  as  many  slots  in  the  armature  as  there  were 
armature  coils  and  commutator  bars.  This  was  supposed  to  give 
ideal  magnetic  symmetry  and,  therefore,  was  assumed  to  be  the 


STREET  RAILWAY  MOTOR  IN  AMERICA  743 

best  possible  arrangement.  However,  the  writer  in  going  over  the 
magnetic  principles  and  proportions  of  the  motor,  decided  that  by 
sacrificing  magnetic  symmetry  to  a  certain  extent,  considerable 
gains  could  be  made  in  reducing  the  dimensions  of  the  machine. 
For  instance,  calculations  indicated  that  by  cutting  the  number  of 
armature  slots  to  half  the  number  of  armature  coils  or  commutator 
bars,  there  would  be  an  appreciable  saving  in  slot  space  with  a  cor- 
responding gain  in  iron  section  in  the  armature  teeth  which  war> 
one  of  the  limiting  conditions  in  the  machine.  However,  this  in* 
volved  the  use  of  two  coils  side  by  side  per  slot  and,  with  a  four 
pole  machine  it  meant  an  unsyrnmetrical  armature  winding,  for 


FIG.  10— WBSTINGHOUSE  NO.  12  MOTOR 

with  the  two-circuit  winding  on  a  four-pole  machine,  an  odd  num- 
ber of  armature  coils  was  necessary.  This  meant  an  idle  coil,  or  idle 
coil  space,  on  the  armature.  This  was  considered  as  detrimental  in 
theory,  but,  on  the  other  hand,  it  was  believed  that  the  wider  arma- 
ture slots  with  their  lower  self-induction,  together  with  the  much 
shorter  armature  core  resulting  from  this  construction,  might  com- 
pensate for  some  dissymmetry  in  the  winding.  This  was  coaly  a 
theory,  but  it  was  tjxottght  worth  while  trying  out*  According  to  the 
calculations,  with  tibfe  constmctioja  together  "with  higher  spaed  daft 
to  increased  gear  ratio,  the  old  No,  3  itt&tor 


744  ELECTRICAL  ENGINEERING  PAPERS 

same  diameter)  could  be  shortened  about  40  per  cent  and  the  field 
could  be  modified  in  proportion.  This  meant  a  very  considerable 
reduction  in  size  and  weight  and  was  well  worth  going  after.  A 
trial  machine  was  built  and  tested  and,  instead  of  being  materially 
poorer  in  commutation,  it  developed  that  the  gain  due  to  the  wider 
slots  and  shorter  core,  more  than  offset  any  harmful  effects  of  the 
unsymmetrical  winding,  so  that  the  resultant  machine  was  a 
somewhat  better  commutating,  cooler,  more  efficient  and  much 
lighter  machine  than  the  No.  3.  This  was  a  somewhat  startling 
result,  but  the  tests  showed  conclusively  that  it  was  correct.  It 


PIG.  11— ARMATURE  OF  WESTINGHOUSE  NO.  12  MOTOR 

was  then  arranged  to  bring  out  a  new  Westinghouse  motor  to  take 
the  place  of  the  No.  3.  It  was  decided  to  go  as  far  as  possible  in 
reducing  the  dimensions  and  weight  of  this  machine  and,  therefore, 
the  supporting  or  surrounding  frame  of  the  No.  3  motor  was  aban- 
doned and  extensions  from  the  yoke  of  the  motor  itself,  forming 
the  end  housings,  were  designed  to  carry  the  armature  bearings, 
In  this  way  a  further  reduction  in  weight  resulted. 

THE  WESTINGHOUSE  No.  12  MOTOR 

This  new  motor  was  known  as  the  Westinghouse  No.  12.  In 
this  motor  the  lower  half  of  the  field  was  enclosed  by  means  of  the 
end  housings.  The  armature  winding  was  of  the  formed-coil  type, 
like  the  No.  3,  arranged  in  two  layers  and  with  the  end  windings 
hammered  down. 

Very  shortly  after  this  motor  was  put  out  an  improved  f orm, 
known  as  the  No.  12*A  was  brought  out.  This  was  quite  similar 
in  general  to  the  No.  12.  The  principal  improvement  in  the  No. 
12-A  was  in  the  armature  construction.  The  armature  core  was- 
ventilated,  to  secure  increased  continuous  capacity  and  the  arma- 


STREET  RAILWA  Y  MOTOR  IN  AMERICA  745 

ture  winding  was  of  the  modern  type  with  all  coils  of  the  same  size 
and  shape,  and  arranged  symmetrically.  The  armature  core  of  this 
machine  was  quite  highly  saturated  at  heavy  load  and  this  was 
found  to  materially  improve  the  commutation.  This  No.  12-A 
motor  was  found  to  be  quite  superior  to  any  preceding  motors  in 
its  general  characteristics,  especially  in  its  continuous  capacity. 

In  its  field  construction  it  resembled  the  old  No.  3,  in  the  fact 
that  it  had  cast  iron  yoke  and  poles  and  the  field  poles  were  cast 
integral  with  the  yoke  and  were  straight-sided  so  that  the  field  coils 
could  be  slipped  on  directly  over  the  pole  tips.  There  was  one 
feature  in  these  motors  and  their  variations  which  materially  af- 


1 1 1 

,r  -  I 


FIG.  12— WESTINGH00SE  No,  3»  MOTOR 

fected  their  operation,  but  which  was  not  fully  appreciated  at  the 
time  they  had  been  designed,  namely,  the  effect  of  the  cast-iron 
poles  in  improving  the  commutation.  The  pole  tips  of  these  motors, 
as  a  rule,  were  somewhat  smaller  in  cross-section  than  the  pole, 
bodies  or  cores  and,  theref  orre,  there  was  quite  high  saturation  in 
the  pole  tips,  particularly  at  heavy  load.  This  high  saturation  had 
very  much  the  effect  of  the  " cut-away"  pole  corners,  used  later 
on  laminated  pole  machines. 

THE  WBSTINGHOUSB  No.  38  MOTOR 
Recognizing  that  in  the  No.  12-A  motor  the  general  type  of 
construction  had  been  carried  as  far  as  possible,  due  to  the  Httuta- 
tiotis  in  the  cast-iron  field  structure,  it  was  decided  to  attempt  a 
different  field  construction,  in  which  the  limitations  in*  design 
cottld  be  pushed  up  very  considerably.  This  was  embodied  in  the  * 


746  ELECTRICAL  ENGINEERING  PAPERS 

Westinghouse  No.  38  motor.  This  motor,  in  general  type,  was 
similar  to  the  No.  12-A,  except  that  in  the  first  motor  built,  the 
field  was  made  of  solid  cast  steel,  both  poles  and  yoke,  with  the 
poles  cast  integral  with  the  yoke.  This  construction  allowed  very 
materially  higher  field  fluxes  than  in  the  former  motors,  so  much 
so  that  again  the  armature  teeth  became  the  limit  in  saturation. 
Therefore,  in  the  armature  three  coils  per  slot  were  used  instead  o£ 
two,  thus  gaining  in  armature  tooth  section.  This,  the  writer  be- 
lieves, was  the  first  use  of  the  three-coil-per-slot  arrangement  in 
railway  motors.  This  first  No.  38  solid-steel-pole  motor  showed 
unduly  high  losses  due  to  the  solid-pole  construction.  Immediately 
it  was  changed  to  laminated  pole  construction  with  the  poles  cast 
integral  with  the  yoke.  This  apparently  was  the  first  use  of  lamin- 
ated poles  with  steel  yokes,  in  street  railway  motors. 

This  No.  38  motor  represented,  with  minor  differences,  the 
present  type  of  railway  motor.  One  principal  difference  was  in  the 
cast-in  laminated  poles,  instead  of  the  present  practice  of  bolted-in 
laminated  poles  It  had  ventilated  armature  windings  and  relatively 
high  saturation  in  the  armature  core  to  help  commutation  at  heavy 
loads.  Also  with  the  three-coil-per-slot  arrangement,  with  four 
poles,  a  more  symmetrical  armature  winding  was  possible  than  in 
the  two-coil-per-slot  No.  12-A  motor,  there  being  no  idle  coils.  In 
the  former  motors  the  bearings  were  lubricated  with  grease,  as  was 
common  practice  in  all  motors  at  that  time.  However,  when 
heavier  and  more  difficult  service  was  encountered,  as  was  the  case 
with  the  No.  38,  which  was  of  higher  capacity  than  most  of  the 
former  motors,  it  was  found  that  the  grease  method  was  not  very 
effective.  This  resulted  in  a  modification  which  provided  a  felt 
wick  and  an  oil  well  under  both  the  armature  and  axle  bearings,  so 
that  the  motor  was  adapted  for  use  either  with  grease  or  oil.  This 
was  on  the  No.  38-B ,  which  was  a  modification  of  the  No.  38,  Like 
all  compromises  which  attempt  to  adopt  all  the  good  features  of  all 
methods,  it  was  only  moderately  successful,  although  it  served  to 
keep  the  motors  in  service  for  many  years. 

WESTINGHOUSE  No.  49  MOTOR 

This  motor  had  much  the  same  lines  as  the  No.  38-B.  It  had 
laminated  poles  cast  in.  The  fractional  pitch  or  "chorded"  type 
of  armature  winding  was  purposely  used  in  this  motor  to  improve 
commutation,  careful  shop  tests  being  made  with  an  approximately 


STREET  RAILWA  Y  MOTOR  IN  AMERICA  747 

full  pitch  and  with  various  chorded  windings  to  find  what  would 
give  the  best  result.  It  was  found  that  a  "  throw  "  of  the  armature 
coil,  one  and  one-quarter  slots  less  than  full  pitch,  gave  materially 
better  commutation  than  any  other  combination.  Chorded  wind- 
ings had  been  used  on  other  types  of  machinery,  to  a  limited 
extent,  before  this,but  it  is  believed  that  this  was  the  first  time  that 
it  was  used  on  a  railway  motor  purely  for  the  purpose  of  improving 
commutation. 

THE  G.  E.  No.  800  MOTOR 

This  was  a  new  motor  gotten  out  by  the  General  Electric 
Company  to  replace  the  W.  P.  It  was  a  four-pole  machine  with 
two  salient  and  two  consequent  poles.  This  was  a  more  symmetrical 
type  of  machine  than  the  W.  P.,  but  yet  was  not  a  purely  sym- 
metrical machine,  such  as  the  Westinghouse  motors  from  No.  3  on, 
and  the  later  types  of  G.  E.  motors.  Its  designation  of  No.  800, 
was  a  new  method  of  rating,  to  indicate  its  tractive  effort  instead 
of  its  horse-power.  Its  nominal  rating  was  about  27  h.p,  This 
tractive  effort  method  of  rating  was  carried  into  several  other  sizes 
such  as  the  G,  E.  1200  and  G.  E.  1000. 


PIG.  13— G.  E.  800  MOTOK  \VITH  COMMUTATOR  LID  OPEN 

This  G.  E.  No.  800  motor  was  a  very  considerable  improve- 
ment over  the  W.  P.,  but  possessed  certain  fundamental  defects. 
For  instance,  the  consequent  pole  arrangement  meant  very  con- 
siderable magnetic  fluxes,  through  the  shaft  and  bearings  with  con-* 
sequent  tendency  for  unipolar  action  in  the  bearings,  tha  bearing 
shells  and  stuiaoe  foi^ng  tOie  coUectiug  brushes.  Therefore,  th«re 


748  ELECTRICAL  ENGINEERING  PAPERS 

was  a  tendency  for  current  in  such  bearings,  as  in  all  consequent- 
pole  machines.  It  may  be  assumed  that  this  defect  was  encountered, 
for  in  some  of  these  motors  very  deep  bronze  shells  were  used,  ap- 
parently for  the  purpose  of  introducing  so  large  a  gap  in  the  shaft 
magnetic  path  that  the  flux  through  the  bearings  would  be  mini- 
mized to  a  non-injurious  point.  Moreover  as  in  consequent-pole 
machines  in  general,  the  commutating  zones  were  not  truly  sym- 
metrical and  thus  commutation  troubles  were,  to  a  certain  extent, 
existent. 

A  similar  motor  to  the  No.  800  was  the  No.  1 200.  Both  of  these 
motors  persisted  for  several  years,  but  were  later  dropped  in  favor 
of  the  radial  pole  type  with  salient  poles  of  which  the  G.  E,  1000 
was  an  example. 

From  this  point  on  the  general  design  of  the  direct-current 
railway  motors  of  all  manufacturers  has  been  practically  along  the 
same  lines.  In  other  words,  a  definite  type  has  become  universal. 
The  fundamental  features  of  this  universal  type  may  be  classified 
as  follows: — 

1 — Outside  cylindrical  or  approximately  cylindrical  yoke. 

2 — Extension  of  the  yoke  to  form  protecting  end  housings  and  to  carry 
the  bearings. 

3 — Radial  field  poles,  usually  four  in  number, 

4 — Laminated  field  poles. 

5— Bolted-in  field  poles. 

6 — Field  coils  without  bobbin  shells. 
(Mummified  coils) 

7 — Drum  wound  armature. 

8 — Slotted  armature  core. 

9 — Two-circuit  or  senes  direct-current  winding. 
10 — Two  or  more  armature  coils  per  slot. 

11 — Machine  wound  armature  coils,  insulated  before  placing  on  the  core, 
12 — Relatively  thin  mica  between  commutator  bars. 

It  is  of  interest  to  note  how  many  of  these  characteristics  ap- 
peared in  the  very  early  motors.  For  instance,  1 , 8, 6, 7, 8,9,11  and 
1 2  all  appeared  in  the  original  experimental  Westinghouse  single- 
reduction  motor  described,  which  later  was  developed  into  the 
No.  3.  Item  No.  2  appeared  in  the  Westinghouse  No,  12  and  in  the 
Thomson-Houston  "W.P."  motor.  Item  4  first  appeared  in  the 
Westinghouse  No.  38  motor.  Item  No,  5  appeared  in  one  of  the 
earliest  radial-pole  G.  E.  motors  and  also  in  the  Westinghouse  Nos. 
56,  68  and  69.  Item  10  appeared  first  in  the  Westinghouse  No.  12 
motor. 


STREET  RAILWAY  MOTOR  IN  AMERICA 


749 


Thus  it  is  obvious  that  the  Wcstinghouse  No.  3  motor,  in  its 
first  experimental  form  (back  in  1890),  contained  nearly  all  the 
fundamental  features  of  the  present  universal  type.  The  end 
housings  carrying  the  bearings,  and  the  laminated  bolted-in  poles, 
constitute  the  two  principal  additional  developments.  Moreover, 
the  experimental  No.  3  motor  did  partially  contain  the  element  of 
enclosing  end  housings.  Thus  it  may  safely  be  stated  that  the  No. 
3  motor  practically  fixed  the  type  of  the  modern  railway  motor. 


PIG,  14r-G.  E.  1000  MOTOR,  OPEN 


It  may  also  be  mentioned  that  there  was  much  argument  over 
the  various  types  of  armature  windings  used  by  different  manu- 
facturers. In  connection  with  the  machine-wound  coil,  as  used  on 
the  No.  3  motor,  many  weird  claims  were  made  for  it  In  one  case 
within  the  writer's  knowledge,  an  over-enthusiastic  representa- 
tive of  the  Company,  with  practically  no  knowledge  of  the  matter 
assured  a  customer  (and  he  was  doubtless  sincere  in  his  assurance) 
that  spare  coils  could  be  carried  along  with  the  car  and  in  case  of  a 
bttm-out,  the  trap-door  could  be  lifted  and  new  armature  coi'& 
dropped  in  place.  In  this  case,  fortunately,  the  customer  actually 
knew  both  what  oould  and  could  not  be  done  and  he  had  many  a, 
good  laugh  afterwwds  while  telling  the  incidaat. 


750  ELECTRICAL  ENGINEERING  PAPERS 

LATER  MOTOR  DEVELOPMENTS 

Following  the  Westinghouse  No.  49  and  the  G.  E.  No.  1000, 
the  developments  of  the  two  companies  might  be  said  to  be  so 
nearly  along  the  same  general  lines  that  the  differences  were 
largely  in  details,  although  some  of  the  improvements  in  details 
were  of  great  importance.  A  few  of  the  improvements  in  the  West- 
inghouse later  motors  might  be  mentioned.  The  General  Electric 
Company  had  already  adopted  bolted-in-poles,  following  the  West- 
inghouse No.  38  motor  with  cast-in  poles.  The  Westinghouse 
followed  in  its  No.  56  and  No.  68  motors  with  bolted-in  poles. 
Both  of  these  motors  had  ventilated  armature  windings  and 
curved  field  coils,  this  latter  practice  being  derived  from  the 
Walker  and  Lorain  motors,  both  of  which  companies  had  been 
taken  over  by  the  Westinghouse.  In  the  No.  68  motor  the  alternate 


FIG.  15— .WESTINGHOUSE  NO.  68  MOTOR 

corners  of  the  pole  tip  laminations  were  cut  away,  in  order  to  give  a 
higher  degree  of  saturation  with  heavy  load  and  thus  lessen  the 
field  distortion  and  reduce  loss  in  the  pole  face.  This  had  been  com- 
mon practice  for  some  time  in  the  railway  generators.  Various 
detail  improvements  were  also  incorporated,  notably,  brush 
holders  with  adjustable  spring  tension. 


STREET  RAILWAY  MOTOR  IN  AMERICA  751 

THE  WESTINGHOUSE  No,  101  MOTOR 

A  most  important  step  in  the  development  of  the  street  rail- 
way motor  came  in  1904  in  the  Westinghouse  No.  101  motor.  In 
this  motor  it  was  planned  to  incorporate  all  the  requirements  of 
service  as  indicated  up  to  that  time,  together  with  all  the  good 
results  found  in  previous  motors.  The  No.  101-B  motor,  which 
was  a  modification  of  the  original  No.  101,  has  had  a  most  enviable 
reputation.  It  contained  a  number  of  most  desirable  features, 
such  as  field  coils,  wound  in  a  straight  mould,  of  copper  strap  in- 
sulated with  asbestos  paper  between  turns;  a  symmetrical  arma- 
ture winding  with  three  co  Is  side  by  side  per  slot  and  no  idle  coils, 
armature  coils  banded  solidly  to  coil  supports,  and  completely 
enclosed;  armature  core  and  commutator  built  up  on  a  spider, 
thus  permitting  the  shaft  to  be  replaced  without  interfering  with 
the  armature  winding  or  core;  brushholdcrs  insulated  in  the  same 
way  as  more  modern  motors,  with  micarta  tubes  protected  by 
cartridge  shells,  and  clamped  firmly  in  position,  allowing  for  radial 
adjustment. 

Probably  the  most  noteworthy  improvement  in  the  No. 
101  B  motor  was  in  the  armature  bearings  and  lubrication.  The 
journals  were  made  larger,  the  shafts  of  a  higher  grade  of  material 
and  the  old  system  of  combined  oil  and  grease  was  discarded  and 
oil-soaked  woolen  waste  was  substituted.  This  motor  had  the 
armature  bearings  carried  in  housings  which  were  bolted  to  the 
top  half  of  the  field  and  clamped  between  the  two  halves  of  the 
field.  The  housings  had  large  reservoirs  for  the  oil  and  waste  and 
allowed  for  separate  gaging  of  the  oil,  This  motor  made  a  phe- 
nomenal record  in  respect  to  armature  lubrication.  Where  former 
motors  were  overhauled  each  two  or  three  months,  in  order  to 
change  the  bearings,  with  the  No.  101-B  it  was  unnecessary  to 
change  bearings  until  they  had  been  operated  several  years. 

The  above  improvements  were  not  added  without  a  substan- 
tial increase  in  weight,  which,  however,  was  considered  well  worth 
while.  This  motor  had  a  tremendous  sale  and  there  are  still 
operating  companies  who  prefer  the  No,  101-B  to  any  of  the  more 
modern  motors  which  have  been  developed.  Other  sizes  corres- 
ponding to  the  No.  101-B  were  the  No.  92  and  No,  93-A. 


752  ELECTRICAL  ENGINEERING  PAPERS 

COMMUTATING  POLES   IN  RAILWAY  MOTORS 

The  next  great  improvement  in  railway  motors  came  in  1907 
and  1908  in  the  use  of  commutating  poles.  In  stationary  motor 
practice,  a  number  of  electrical  manuf  acturing  companies  had  used 
commutating  poles  for  motor  work,  especially  for  variable  speed 
service  over  wide  ranges.  It  was  but  a  direct  step  from  this  to  the 
use  of  commutating  poles  in  railway  motors.  However,  the  General 
Electric  Company  was  the  first  to  put  such  motors  on  the  market, 
to  be  followed  soon  after  by  the  Westinghouse  Company  in  their 
No.  300  line  of  motors,  Nos.  305,  306  and  307,  being  motors  which 


PIG.  16— WESTINGHOUSE  NO,  101  B  MOTOR 

corresponded  to  non-commutating  motors  immediately  preceding 
them.  These  motors  had  all  the  mechanical  characteristics  and 
general  features  of  design  of  their  predecessors,  with  the  addition  of 
the  commutating  poles.  Since  that  time  commutating  poles  in  rail- 
way motors  have  been  so  thoroughly  established  that  no  new  rail- 
way motors  would  be  considered  without  them. 

LIGHT  WEIGHT  MOTORS 

Somewhat  later  than  this  an  agitation  was  started  against 
excessive  weight  in  cars,  trucks  and  electrical  equipments.  This 
agitation  bore  fruit,  and  a  weight  cutting  campaign  began  which 
has  resulted  in  the  adoption  of  extremely  light  weight  cars,  trucks 
and  motors.  The  question  of  car  and  truck  design  may  not  be  dis- 
cussed here,  although  it  looks  now  as  if  the  weight-cutting  cam- 
paign has  gone  past  the  best  limit.  However,  a  large  part  of  the 
reduction  in  the  weight  of  motors  has  been  entirely  logical  and  is 
largely  the  result  of  careful  design  and  improvements  in  ventilar 


STREET  RAILWA  Y  MOTOR  IN  AMERICA  753 

tion.  Motors  are  now  built  with  large  fans,  mounted  on  the  pinion 
end  of  the  armature  shaft,  which  pull  air  through  the  armature 
core  and  over  the  surface  of  the  armature  and  between  the  field 
windings,  which  has  made  an  increase  of  probably  50  per  cent  in 
the  continuous  rating  of  the  motors.  In  addition  to  this,  the  arma- 
ture speed  has  been  very  considerably  increased  and  the  gears  have, 
in  many  cases,  been  changed  from  3-pitch  to  3}^,  4  and  even  4^. 
Open  ventilation  of  the  motors  has  been  a  natural  consequence 
of  the  great  improvement  in  insulation  made  in  the  last  few  years. 
The  early  motors  were  made  open  to  the  weather  but  this  had  to  be 
abandoned  because  of  the  large  amount  of  insulation  trouble. 
After  a  good  many  years  with  the  enclosed  motor,  it  gradually 
became  the  practice  to  open  the  motor  up  somewhat  for  better 
ventilation,  and  finally  fans  were  installed  to  create  a  circulation  of 
air,  so  that  now  the  continuous  rating  of  railway  motors  is  higher 
per  pound  than  ever  before. 


TECHNICAL  TRAINING  FOR  ENGINEERS 

"FOREWORD — This  paper  was  compiled  from  two  addresses,  one 
given  by  special  request  before  the  Pittsburgh  Section  of  the 
American  Institute  of  Electrical  Engineers  in  1916,  and  the 
other,  covering  much  the  same  subject  matter,  was  given  before 
the  National  Association  of  Corporation  Schools  at  its  annual 
meeting  in  Pittsburgh,  1916.  On  account  of  the  favorable 
comments  on  the  two  addresses,  they  were  afterwards  combined 
and  printed  in  the  Electric  Journal.  The  author  has  had  a  wide 
experience  in  the  education  of  so-called  "educated"  men. 
Almost  since  his  entrance  into  the  employ  of  the  Westinghouse 
Electric  &  Mfg.  Company,  early  in  1888,  he  has  given  a  con- 
siderable part  of  his  time^to  the  development  of  the  more  prom- 
ising young  engineers  with  whom  he  came  in  daily  contact. 
Being  himself  extremely  fond  of  the  analytical  side  of  his  work, 
he  has  been  very  free  in  imparting  his  methods,  data  and  ex- 
perience to  his  associates  and  assistants,  thus  in  fact,  ^although 
not  in  name,  becoming  an  educator  along  advanced  lines.  He 
always  has  been  in  search  of  young  men  of  the  right  turn  of 
mind  whom  he  could  develop  into  "stars"  in  his  profession,  and 
many  men  prominent  in  the  electrical  industry  today  can  speak 
with  pride  of  the  training  they  received  while  associated  with 
him.  Recognizing  that  the  engineering  development  work  of 
the  manufacturing  companies  is  becoming  increasingly  difficult 
from  year  to  year,  he  has  given  special  attention,  during  the 
past  several  years,  to  the  selection  and  training  of  graduates  of 
the  technical  schools  who  show,  to  an  unusual  degree,  certain, 
characteristics  and  aptitudes  which  he  believes  to  be  necessary 
in  maintaining  the  high  standard  of  the  Westinghouse  Com- 
pany in  the  engineering  field.  In  other  words,  he  is  applying 
ids  analytical  methods  to  men  very  much  as  he  has  applied  them 
to  apparatus  and  principles  in  the  past  years. — (E&.) 


IN  the  earlier  days  of  the  Westinghouse  Electric  &  Mfg.  Com- 
pany many  young  technical  students  were  taken  directly  into 
the  various  departments  and  there  trained.  But  in  time  the  student 
problem  became  so  large  and  important  that  an  educational  de- 
partment was  developed  to  meet  in  a  systematic  manner  the  grow- 
ing needs  of  all  departments.  This  educational  department  works 
in  conjunction  with  the  other  departments  in  training  men  and  in 
placing  them  where  they  will  have  opportunities  in  accordance 
with  their  special  abilities* 

The  following  remarks  represent  the  writer's  own  personal 
opinions  based  largely  upon  a  comparatively  wide  experience  with 
the  young  engineers  who  have  entered  the  student's  course  during 
the  past  five  or  six  years.  In  that  time  this  company  has  taken  into 

756 


756  ELECTRICAL  ENGINEERING  PAPERS 

its  educational  department  over  one  thousand  graduates  of  tech- 
nical schools  from  all  over  the  United  States  and  Canada.  Of  these, 
several  hundred  have  wished  to  specialize  in  engineering,  while  the 
aim  of  the  others  has  been  toward  the  manufacturing  and  the 
commer  ial  lines,  both  of  which  require  good  technical  training. 
The  electrical  salesman  of  today  is  quite  technical,  regardless  of 
how  he  got  his  training.  Also  the  complexities  of  the  electrical 
business  of  today  require  many  high-class  technical  men  in  the 
manufacturing  departments.  As  to  engineering,  it  goes  without 
saying  that  those  who  follow  this  branch  of  the  electrical  business 
should  be  technical  men,  if  they  are  to  advance  very  far.  In  con- 
sequence the  Westinghouse  Company  takes  on  technica  graduates 
almost  exclusively  for  its  student's  course,  regardless  of  what 
branch  of  the  electrical  buiiness  they  expect  to  follow. 

The  writer's  personal  experience  has  been  very  largely  with 
those  students  who  expect  to  follow  the  engineering  branch  of 
electrical  manuf a  storing.   During  the  past  few  years  he  has  come 
in  contact  with  practically  all  those  who  leaned  toward  engineer- 
ing work.    One  of  the  most  important  considerations  in  the  engi- 
neering student  problem  has  been  that  of  fitting  the  men  to  the 
kinds  of  work  for  which  they  are  best  adapted.  In  former  years  this 
was  done  in  a  more  or  less  haphazard  manner  by  trying  the  men 
out  in  different  classes  of  work  to  see  whether  they  would  make 
good.    This  procedure  proved  so  unsatisfactory  that  it  became 
necessary  to  adopt  some  method  of  classifying  the  students  ac- 
cording to  their  aptitudes  and  abilities,  and  then  try  each  one  out 
on  that  line  of  work  for  which  he  seemed  to  be  best  fitted.   Obvious- 
ly, this  method  was  in  the  right  direction,  but  the  primary  diffi- 
culty lay  in  determining  the  characteristics  of  the  individual 
students.    The  writer  has  spent  quite  a  considerable  amount  of 
time  in  the  past  few  years  in  studying  the  characteristics  of  the 
students  to  see  whether  their  natural  and  their  acquired  abilities 
can  be  sufficiently  recognized,  during  the  preliminary  stages  of  the 
work,  to  allow  them  to  be  properly  directed  toward  that  field  in 
which  they  will  make  the  best  progress,    In  this  study,  in  which 
hundreds  of  young  men  were  analyzed  with  regard  to  their  char- 
acteristics, many  very  interesting  points  developed,  quite  a  number 
of  which  have  a  direct  bearing  on  the  subject  of  technical  training. 
In  the  first  years  of  this  study  the  results  were  very  discouraging, 
due  largely  to  the  fact  that  the  young  men  had  been  brought  to  us 
in  a  wholesale  way,  regardless  of  their  characteristics  or  their 


TECHNICAL  TRAINING  FOR  ENGINEERS  757 

suitability  for  our  engineering  work.  Many  of  them  had  no  ideas 
whatever  in  regard  to  the  kind  of  work  for  which  they  were  fitted. 
Apparently  the  man  who  had  not,  at  least  partly,  made  up  his 
mind  as  to  his  preferences  or  his  capabilities  for  some  given  line  of 
endeavor  by  the  time  he  had  gone  through  four  years  of  college  and 
then  entered  our  course,  had  much  difficulty  in  making  up  his  mind 
after  he  had  been  with  us  a  year  or  two.  It  developed,  in  many 
cases,  that  he  was  lacking  in  decision.  This  was  a  very  predomin- 
ant fact  in  the  first  few  years  after  the  writer  had  gotten  into  this 
work  more  actively.  After  a  careful  study  of  the  situation  it  was 
recommended  that  an  attempt  be  made  to  get  a  different  class  of 
college  men,  namely,  those  who  had  more  definite  ideas  as  to  what 
they  wanted  and  what  they  were  fitted  for.  This  policy  was  tried, 
and  with  great  improvement  in  the  grade  of  men  obtained.  It  is 
principally  from  the  study  of  these  later  men  that  the  writer  has 
been  able  to  draw  some  of  the  conclusions  which  are  here  given. 

One  of  the  most  prominent  features  which  has  developed  from 
the  study  of  these  young  men  is  that  in  practically  all  cases  the 
most  valuable  aptitudes  or  characteristics  which  they  have  shown 
were  possessed  by  them  long  before  they  entered  college.  In  fact, 
many  of  them  have  apparently  possessed  such  aptitudes,  more  or 
less  developed,  from  comparatively  early  childhood.  For  example, 
the  best  constructing  or  designing  engineers  all  had  a  strong  ten- 
dency toward  the  construction  of  mechanical  toys  and  apparatus  in 
cMldhood.  In  regard  to  such  characteristics,  the  schools  and  the 
colleges  have  merely  directed  and  developed  to  a  greater  extent 
what  is  already  there.  Prom  this  viewpoint,  therefore,  the  college 
simply  develops*  If  the  tendency  isn't  there,  it  would  seem  that 
there  is  but  little  use  to  try  to  develop  or  cultivate  it.  Viewed  from  this 
standpoint,  quite  a  large  percentage  of  the  young  men  who  take  up 
engineering  courses  in  college  are  quite  -unfitted  for  such  work. 
Therefore,  one  function  of  the  college  should  be  to  sort  out  and 
classify  the  young  men  according  to  their  characteristics,  to  dis- 
courage them  from  following  along  any  line  of  endeavor  for  which 
they  have  no  real  aptitudes,  and  to  direct  them  into  more  suitable 
lines.  This  applies  particularly  to  technical  schools.  It  might  be 
said  that  in  our  present  educational  system  the  usual  method  is  to 
educate  the  young  men  and  then  select  the  real  engineers,  this 
selection  being  made  afterwards  through  bitter  experience.  The 
ideal  method,  apparently,  would  be  first  to  select  the  real  engi- 
neers  and  then  to  educate  them.  In  other  words,  those  who  show  a 


758  I!!  ELECTRICAL  ENGINEERING  PAPERS 

natural  aptitude  for  engineering  should  be  educated  along  tech* 
nical  lines. 

In  the  technical  school  one  of  the  first  efforts  should  be  toward 
finding  the  student's  natural  aptitudes.  Some  boys  apparently 
have  no  leaning  toward  any  special  line  of  endeavor.  On  the  other 
hand,  many  boys  really  have  some  inherent  preference  which, 
however,  may  not  have  been  strongly  enough  developed  to  stand 
out  prominently.  Too  often  his  real  preference  has  been  entirely 
neglected  or  even  discouraged.  In  the  writer's  own  case,  as  a  boy, 
he  was  very  frequently  and  severely  criticised  for  his  inclination  to 
' '  waste  valuable  time ' '  in  trying  to  make  what  were  called  ' '  useless 
things."  However,  fortunately  for  himself,  no  real  pressure  was 
brought  upon  him  to  prevent  him  from  following  his  preferences 
or  tendencies,  and  eventually  the  "call"  was  so  strong  that  it  took 
him  into  the  very  work  which  he  wanted  above  all  else. 

On  the  other  hand,  the  boy  may  express  a  preference  for  a  line 
of  work  for  which  he  is  entirely  unfitted.  In  other  words,  this 
preference  may  not  be  based  upon  natural  aptitudes  or  character- 
istics and  is  not  a  real f '  call. "  It  is  these  boys,  who  are  unfitted  for 
the  lines  which  they  have  chosen,  who  are  a  real  handicap  on  their 
classmates.  The  class  never  moves  along  faster  than  its  average 
man,  and  very  often  at  the  speed  of  the  poorest  men.  If  these 
poorest  men  were  eliminated,  naturally  the  progress  would  be  much 
faster.  Apparently  the  present  methods  of  training  have  not  yet 
overcome  this  difficulty,  although  very  many  teachers  recognize 
the  evil,  and  are  attempting  to  correct  it.  This  will  be  referred  to 
again  later. 

Coming  to  the  technical  training  of  the  students,  experience 
indicates  that  too  much  specialization  is  a  mistake.  He  gets  enough 
of  that  in  after  years,  What  is  needed  is  a  good,  broad  training  in 
fundamental  principles.  In  engineering  matters,  a  thorough  grasp 
of  such  fundamentals  is  worth  more  than  anything  else,  By 
fundamentals  is  meant  basic  principles  or  facts.  These  should  not 
be  confused  with  theories  or  explanations  of  facts,  A  fact  is  basic, 
and  does  not  change,  although  the  theories  which  explain  it  may 
change  many  times.  A  thorough  knowledge  of  basic  principles  will 
enable  a  direct  answer  to  be  made  in  many  cases,  even  where  the; 
conditions  of  a  problem  may  appear  to  be  very  complex.  Take,  for 
example,  the  perpetual  motion  fallacy  in  its  various  forms*  A  per- 
petual motion  scheme  may  be  made  so  complex  and  involved  and 
may  include  so  many  principles  and  appurtenances  that  the  beet 


TECHNICAL  TRAINING  FJR  ENUMEERS  759 

analyst  may  be  more  or  less  puzzled  to  explain  the  various  rela- 
tions clearly.  But  by  applying  the  principle  of  conservation  of 
energy  no  further  explanation  is  necessary.  This  one  fundamental 
fact  covers  the  whole  case.  In  the  same  way  a  thorough  grasp  of 
some  basic  principle  will  often  clear  up  the  most  complex  problems 
or  situations  and  will  allow  a  conclusive  answer  to  be  made.  With 
such  a  grasp  of  fundamentals,  one  is  not  liable  to  believe  that  a 
"pinch"  of  some  wonderful  new  powder  or  chemical,  mixed  with  a 
gallon  of  water,  will  give  the  equivalent  of  a  gallon  of  gasoline,  and 
at  the  cost  of  few  cents.  And  yet  this  fallacy  * '  breaks  loose ' '  period- 
ically, and  is  given  wide  circulation  in  the  news  of  the  day.  What 
is  needed  in  such  cases  is  a  little  knowledge  of  fundamental  prin- 
ciples. 

This  very  grasp  of  fundamentals  accustoms  the  boy  to  think 
for  himself.  In  other  words,  it  develops  his  analytical  ability.  As 
one  educator  mentioned  to  the  writer  some  time  ago,  *'  If  a  boy  has 
analytical  ability,  there  is  hope  for  him;  if  he  has  none,  he  is 
'punk.'  "  By  analytical  ability  is  not  necessarily  meant  mathe- 
matical ability  with  which  some  people  are  inclined  to  confuse  it. 
By  analytical  ability  is  meant  the  ability  to  analyze  and  draw  cor- 
rect conclusions  from  the  data  and  facts  available*  This  faculty 
can  be  cultivated  to  a  considerable  extent,  although,  in  the  writer's 
opinion,  it  originates  rather  early  in  life.  This  is  considered  by 
many  as  the  first  and  foremost  characteristic  that  an  engineer 
must  have,  and  therefore  the  schools  should  expend  their  best 
energies  in  this  direction. 

Allied  with  a  grasp  of  basic  principles  is  the  requirement  of  a 
physical  conception  of  such  principles  as  distinguished  from  the 
purely  mathematical  This  can  be  cultivated,  as  the  writer's 
personal  experience  with  many  students  has  indicated.  As  a  con- 
crete example  of  the  value  of  a  physical  conception  the  following 
may  be  cited: — Three  electrical  engineers,  familiar  with  induction 
motor  design,  are  given  some  new  problem  regarding  the  action  of 
an  induction  motor.  One  of  them  immediately  thinks  of  a  "circle 
diagram1';  the  second  thinks  of  a  mathematical  formula;  the 
third  thinks  of  flux  distributions  and  conductors  cutting  them  at 
certain  speeds,  etc.  Assuming  equal  mathematical  skill  for  these 
three  men,  the  one  with  the  physical  conception  of  the  conductors 
cutting  fluxes  has  a  broader  means  for  attacking  the  problem  than 
either  of  the  others  can  be  said  to  have,  He  can  tackle  a  new  con- 
dition with  better  chance  of  success,  as  he  goes  back  to  the  funda- 


760  ELECTRICAL  ENGINEERING  PAPERS 

mental  principles  of  the  apparatus.  He  thus  may  create,  con- 
fidently, new  formulae  and  diagrams  to  meet  new  conditions  and 
problems. 

-  This  physical  conception  is  closely  related  to  the  development 
of  imaginative  powers,  and  without  such  powers  highly  developed 
no  engineer  can  expect  to  advance  far  in  his  profession.  The  man 
with  originality,  resourcefulness  or  with  the  constructive  faculties 
well  developed,  or  the  man  who  "can  see  through  things'1  readily, 
must  have  strong  imaginative  powers.  This  faculty  also  should  be 
developed  to  the  utmost,  but  should  also  be  directed.  It  begins 
early  in  some  children,  but,  unfortunately,  instead  of  being  direct- 
ed, it  is  too  often  discouraged,  both  at  home  and  in  the  school. 
If  the  boy  in  the  public  school  develops  a  new  method  of  solving 
&  problem,  or  reaches  any  conclusion  by  other  than  the  well- 
established  routine  way,  he  is  criticised  more  of ten  than  encouraged 
for  his  departure  from  the  beaten  track,  or  rather  his  instructor's 
particular  methods. 

As  stated  before,  the  student  should  be  well  trained  in  funda- 
mentals or  basic  principles.  In  many  branches  of  engineering  this 
means  that  he  should  have  a  good  training  in  mathematics.  Most 
of  the  graduates  of  the  technical  schools  are  woefully  weak  in 
mathematics.  Apparently  this  is  not  due  entirely  to  lack  of  mathe- 
matical ability  on  the  part  of  the  students,  but  largely  to  defective 
training  in  their  earlier  work.  One  great  defect  in  many  colleges 
is  due  to  passing  the  entrants,  in  algebra  and  trigonometry,  on  the 
basis  of  their  high  school  training.  In  most  cases  this  early  training 
in  algebra  is  very  defective,  as  sufficient  skill  is  not  developed  in  the 
student  and  the  practical  side  is  largely  neglected, ,  Algebra  and  its 
applications  to  geometry,  trigonometry,  etc.,  should  be  taught  in  a 
more  practical  manner  in  the  engineering  college  course,  as  a  found- 
ation for  the  higher  engineering  mathematics.  The  higher  the  struc- 
ture is  to  be,  the  stronger  must  be  the  foundation.  If  the  engineer- 
ing student  is  not  sufficiently  practiced  in  these  elementary  mathe- 
matics, then  he  should  be  drilled  specially  as  a  step  to  further 
engineering  work.  In  the  practical  engineering  work,  beyond  the 
college,  skill  in  the  use  of  algebra  and  trigonometry  is  of  relatively 
much  more  importance  than  practice  in  the  higher  mathematics, 
for  it  is  needed  one  hundred  times  where  the  other  is  used  once. 
In  the  writer's  experience  with  engineers  he  has  reached  the  con- 
clusion that  the  principal  reason  why  mathematics  axe  not  used 
mote  in  everyday  work  is  because  the  average  engineers  have  not 


TECHNICAL  TRAINING  FOR  ENGINEERS  761 

the  necessary  skill.  Most  of  them  claim  that  they  have  become 
"rusty"  in  such  mathematics  through  disuse.  However,  in  many 
cases,  this  excuse  is  worse  than  none  at  all,  for  the  occasion  for  such 
mathematics  exists  in  practical  engineering  work  and  has  been 
there  all  along. 

In  the  education  of  the  engineer,  higher  mathematics  forms  a 
very  valuable  part  of  the  training.  One  of  its  uses  is  to  show  how 
one  can  do  without  it.  In  other  words,  if  properly  taught,  it  gives 
a  broader  grasp  of  methods  of  analysis;  it  tends  to  fix  certain 
fundamental  principles.  However,  as  a  tool  in  actual  engineering 
work  it  is  seldom  required,  except  in  rather  special  lines.  The 
higher  mathematics  might  be  looked  upon  as  a  fine  laboratory  in- 
strument or  tool  to  be  used  on  exceptional  occasions,  while  the  or- 
dinary mathematics  should  be  considered  as  an  everyday  tool  in 
engineering  work,  and  should  be  ready  at  hand  at  all  times. 

There  has  been  quite  a  fad  for  specialization  in  engineering 
training.  The  writer's  personal  opinion  is  that  specialization  in 
college  training  is  not  advisable,  except  possibly  in  a  very  general 
way.  There  has  been  a  false  idea  in  many  schools  that  if  a  man 
specialized  along  some  individual  line  of  work  it  would  advance 
him  more  rapidly  when  he  leaves  school  for  active  work.  The 
writer  almost  never  asks  the  student  in  what  field  he  specialized. 
It  is  desired  to  know  whether  he  is  a  good  analyst,  if  he  is  fairly 
skillful  at  mathematics,  if  he  has  the  imaginative  faculty  and 
what  goes  with  it.  Has  he  initiative,  resourcefulness,  etc.?  Is  he 
a  man  with  a  broad  grasp  of  general  principles  rather  than  one 
who  has  made  a  special  study  of  one  individual  subject? 

In  college  training  the  time  spent  oil  commercially  practical 
details  is  usually  largely  wasted,  as  it  may  give  the  student  en- 
tirely wrong  ideas.  When  a  young  man  says  that  he  has  had  a 
course  in  practical  design  and  is  positive  that  he  can  design,  the 
chances  are  about  ninety-nine  out  of  one  hundred  that  he  knows 
nothing  about  the  really  fundamental  conditions  in  practical  de- 
sign. The  chances  are  that  he  doesn't  even  know  the  real  starting 
point  in  making  up  a  commercial  design.  Even  worse,  if  he  has 
taken  such  training  seriously,  he  tnay  have  to  "unlearn*'  many 
of  his  ideas,  if  the  use  of  this  term  is  allowable.  The  mental  train- 
ing and  the  aid  in  grasping  principles  which  he  may  have  obtained 
through  his  school  design  is,  of  course,  worth  something,  but  in 
many  cases  the  same  time  expended  in  other  channels  may  produce 
larger  results.  Teaching  of  design  should,  therefore,  be  for  the 


762        [•     IELECTRICAL  ENGINEERING  PAPERS 

purpose  of  exemplifying  principles  rather  than  practice.  There  are, 
of  course,  some  lines  of  specialization  in  colleges  which  lead 
directly  to  practical  results  afterwards.  Research  work  is  one  of 
these.  However,  it  is  probable  that  if  a  large  part  of  the  time  given 
to  research  work  by  the  student  in  college  were  expended  in  ac- 
quiring a  broader  foundation  in  fundamental  principles  the 
results  would  be  better  in  the  end. 

As  referred  to  before,  there  has  been  one  serious  defect,  in  our 
systems  of  technical  training  today,  namely,  it  holds  back  the 
leaders  and  pushes  the  laggards,  thus  tending  toward  mediocrity 
as  the  general  result.  There  should  be  some  system  in  colleges  for 
weeding  out  the  " negatives"  in  any  given  line  of  endeavor. 
Many  of  these  are  simply  "misapplications,"  to  use  a  manufactur- 
ing company  term.  In  some  other  lines  they  may  be  highly  success- 
ful. 

In  an  ideal  engineering  course  each  student  should  be  pushed 
to  the  utmost  of  his  capabilities.  One  solution  of  this  problem 
would  be  for  each  teacher  to  assign  a  certain  amount  of  work  to  his 
students  individually,  and  they  should  report  to  him  individually 
on  such  work,  explaining  to  him  fully  what  they  have  accomplished. 
Each  man  thus  could  be  pushed  along  independently  of  his  fellows. 
The  weaknesses  of  the  individual  men  would  soon  appear.  If,  for 
example,  it  develops  that  certain  of  the  students  are  behind  in  the 
necessary  mathematics,  then  steps  could  be  taken  to  correct  this 
defect.  Each  student  would  have  to  think  more  for  himself  and 
would  be  put  more  or  less  upon  his  own  resources.  His  various 
characteristics  could  be  studied  and  developed.  He  should  be  made 
to  work  out  and  apply  fundamental  principles*  He  would  thus 
practice  using  his  own  mind.  As  soon  as  it  develops  that  he  has  no 
mind  of  his  own,  then  he  could  be  dropped.  In  such  a  course  of 
teaching  the  advancement  of  each  man  would  be  dependent  upon 
himself,  to  a  large  extent.  At  this  point  a  principle  of  mechanics 
can  be  applied  rather  aptly.  In  machines  a  force  does  work  in 
overcoming  resistance.  In  man  the  same  principle  holds  true. 
No  matter  how  much  force  a  man  may  have,  if  no  resistance  is 
presented,  no  result  is  accomplished.  And  if  the  force  is  small, 
then  the  result  is  also  liable  to  b,e  small.  But  a  stnaller  force  over- 
coming a  larger  resistance  may  result  in  greater  accomplishment 
than  a  larger  force  with  but  little  resistance.  An  unusually  bril- 
liant boy  with  too  small  a  task  set  for  him  may  accomplish  little. 
His  task  must  be  enlarged  to  suit  bis  abilities;  for,  as  in  machines^ 


TECHNICAL  TRAINING  FOR  ENGINEERS  703 

to  obtain  the  greatest  result  the  resistance,  or  task,  must  be  com- 
mensurate with  the  force  acting.  Unfortunately,  many  good  men 
of  great  capabilities  accomplish  practically  nothing,  through  too 
little  resistance,  due  to  life  being  made  too  easy  for  them. 

Such  a  course  of  "forcing,"  as  indicated  above,  might  be  diffi- 
cult to  apply  in  many  of  the  schools  as  constituted  today.  But  thq 
writer's  personal  experience  indicates  that  the  better  class  of  men 
will  develop  rapidly  under  such  treatment,  while  the  laggards  are 
eliminated  more  quickly.  He  has  tried  this  system  in  general  on 
many  graduates  from  the  technical  schools  and  unusually  satis- 
factory results  have  been  obtained. 

All  of  the  foregoing  points  to  the  fact  that  the  mere  accumula- 
tion of  knowledge  is  not  a  training,  nor  an  education.  The  old 
saying  that  "knowledge  is  power"  is  not  technically  correct  any 
more  than  is  the  statement  that  torque  (or  force)  is  power,  to  use 
an  engineering  comparison.  Torque,  or  force,  is  not  power,  but 
torque  in  motion  is  power  and,  to  continue  this  comparison,  knowl- 
edge in  motion,  or  in  action,  is  power.  Activity  in  some  form  is  one 
of  the  essential  factors. 

To  sum  up,  the  colleges  should  aim  to  develop  the  student's 
characteristics,  as  far  as  practicable.  They  should  aim  to  develop 
analytical  ability,  imaginative  faculty,  ability  to  do  independent 
thinking.  They  should  teach  fundamental  principles,  and  the 
course  of  teaching  should  be  such  as  to  give  the  individual  student 
a  real  grasp  of  such  principles.  A  broad  general  training  is  most 
desirable  for  the  man  who  has  the  ability  to  do  something  in  the 
world. 


ENGINEERING  BY  ANALYSIS 

FOREWORD^— In  the  latter  part  of  1916,  the  engineering  students  at 
the  Ohio  State  University  decided  upon  the  publication  of  a 
college  engineering  paper,  and  the  author  was  asked  to  prepare 
an  article  for  the  first  issue.  In  answer  to  this  request,  a  paper 
entitled,  "The  Electrical  Engineer  of  Today"  was  submitted. 
This  article  appeared  in  the  first  issue  of  the  Ohio  State  En- 
gineer, in  January,  1918,  and  it  is  here  reproduced  in  practically 
the  same  form  as  the  original,  except  in  title. — (ED.) 

THE  early  engineering  in  any  field  is  usually  of  the  "cut-and- 
try"  kind,  followed  later  by  the  refinements  of  more  highly 
trained  specialists.  A  comparatively  recent  development  in  indus- 
trial and  manufacturing  engineering  is  the  analytical  engineer.  By 
this  is  meant  the  engineer  who  translates  facts  into  relationships, 
formulae  and  figures,  and  eventually  retranslates  them  into  other 
facts.  The  analytical  engineer  in  this  sense  does  not  mean  the 
mere  user  of  figures  and  formulae.  He  starts  with  fundamental 
principles  and  laws  from  which  he  then  draws  his  conclusions,  the 
applications  of  which  are  made  directly  to  the  final  product  with- 
out intermediate  experimentation.  The  analytical  engineer  has 
led  the  way  to  new  and  more  difficult  fields  of  endeavor  and  many 
of  our  most  rapid  advances  have  been  made  under  his  guidance. 

Electrical  engineering,  is  one  of  the  youngest  of  the  en- 
gineering lines  of  endeavor,  but  its  "cut-and-try"  period  was  of 
comparatively  short  duration.  The  coming  of  the  analytical  en- 
gineer was  almost  coincident  with  the  rise  of  electrical  engineering 
as  a  business.  This  branch  of  engineering  deals  with  more  or  less  ob- 
scure phenomena,  of  which  there  are  only  indirect  evidences  in 
many  cases.  Many  of  the  laws  primarily  are  only  mathematical 
relationships.  Many  of  them  can  only  be  grasped  or  handled  by 
those  who  have  considerable  analytical  and  mathematical  ability. 
In  consequence,  even  comparatively  early  in  the  work,  the  highly 
technical  engineer  was  a  necessity.  Probably  in  no  other  branch 
of  engineering,  since  its  first  development,  has  there  been  as  large 
percentage  of  men,  having  high  technical  training,  engaged  in  the 
work;  and  as  a  consequence,  in  no  other  lines  of  engineering  has 
there  been  as  rapid  growth  as  in  the  electrical, 

Coincidentally  with  the  growth  of  electrical  engineering,  there 
have  been  rapid  advances  in  the  older  and  better  established  lines 

765 


766  ELECTRICAL  ENGINEERING  PAPERS 

of  engineering,  especially  in  those  which  have  been  rather  inti-1 
mately  associated  with  the  electrical  industry.  The  steam"  tur- 
bine which  now  dominates  the  field  of  steam  prime  movers,  re- 
ceived its  greatest  impetus  in  connection  with  electrical  work,  and 
its  present  high  development  may  be  said  to  be  the  product  of 
the  analytical  engineer.  Water-wheel  development  has  also 
made  great  advances  under  much  the  same  conditions, 

One  characteristic  of  the  analytical  engineer  of  the  present 
time,  especially  in  electrical  work,  is  that  he  is  very  often  working 
far  ahead  of  his  available  data.  He  is  obliged  to  plot  his  existing 
data  and  experience  and  then  exterpolate  for  the  new  points  which 
he  finds  necessary  in  his  work.  He  is  thus  working  in  the  un- 
known to  a  greater  or  less  extent,  but  his  ability  to  analyze  and 
correlate  very  often  leads  him  to  be  fairly  certain  of  his  results. 
It  is  this  abibty  to  work  with  confidence  in  comparatively  un- 
known fields,  which  has  produced  such  astonishing  results  in 
electrical  engineering. 

The  analytical  engineer  of  today,  whether  electrical  or  other- 
wise, must  forsee,  through  his  analysis  of  data  and  practice,  what 
the  trend  of  future  practice  will  be.  If  his  analysis  shows  him 
that  certain  lines  of  development  are  scientifically  more  consistent 
than  other  lines,  he  will  naturally  tend  to  work  along  what  he 
considers  to  be  the  correct  direction.  If  he  sees  that  certain 
practices  are  fundamentally  wrong  and  represent  only  makeshift 
conditions,  or  merely  commercial  expediency,  he  will  naturally 
feel  that  such  practices  eventually  will  be  replaced.  He  must 
weigh  both  theoretical  and  practical  conditions  in  determining 
which  direction  to  work. 

With  the  true  analytical  engineer  there  will  be  no  standardiza- 
tion of  practice  unless  such  practice  has  good  fundamental  reasons 
back  of  it.  His  tendency  is  rather  toward  standardization  ac- 
cording to  certain  scientific  principles  and  limitations  than  by 
practices  which  have  insufficient  basis.  The  latest  standardiza- 
tion rules  of  the  American  Institute  of  Electrical  Engineers  repre- 
sent an  attempt  along  this  line,  and  it  is  a  pretty  safe  prediction 
that  the  basic  features  of  these  new  rules  will  be  retained  for  many 
years  to  come. 

Analytical  engineering,  of  a  very  advanced  kind  is  represented 
by  the  modern  research  and  testing  departments  and  laboratories 
of  the  big  engineering  concerns  who  do  electrical  and  other  manu- 
facturing. Much  of  the  technical  data,  which  the  designing, 


ENGINEERING  BY  ANALYSIS  767 

developing  and  manufacturing  departments  require,  is  a  direct 
product  of  such  departments.  No  progressive  industrial  estab- 
lishment of  the  present  time  can  get  along  without  extensive 
research  departments.  Recently  Congress  has  approved  of  a  large 
Naval  Laboratory  for  research  and  experimental  work,  in  line  with 
other  engineering  and  industrial  organizations. 

A  good  example  of  modern  electrical  design  work  of  a  highly 
analytical  character,  is  the  present  turbo  generator.  The  present 
huge  capacity  high  speed  machines  are  almost  beyond  the  dreams  of 
ten  years  ago.  These  machines  are  almost  entirely  the  product  of 
the  analytical  designing  engineer.  In  these  machines  nearly  all  pre- 
vious developments  and  experience  in  other  lines  of  apparatus  have 
ountcd  for  little.  New  methods,  new  materials,  new  practices  and 
new  limitations  have  been  established  in  these  machines,  and  for 
these  reasons,  the  turbo  generator  engineer  has  been  compelled  to 
work  ahead  of  his  data  and  experience  much  of  the  time.  For  ex- 
ample: the  twenty  thousand  kilowatt,  1800  r.p.m.,  60-cycle,  turbo 
generator  was  undertaken  when  the  ten  thousand  kilowatt  ma- 
chine of  the  same  speed  and  frequency  was  the  nearest  size  from 
which  to  obtain  data,  and  this  smaller  size  unit  had  already  been 
carried  up  to  what  were  considered  as  the  permissible  limits,  in 
many  ways.  ,In  such  case  the  designer  had  to  overstep  his  data  and 
limits,  and  depend  largely  upon  analysis. 

Another  good  example  of  analytical  engineering  is  the  in- 
duction motor.  While  such  motors  possibly  could  have  been 
developed  by  cut  and  try  methods,  at  great  expense  and  with 
many  failures,  yet  the  present  advanced  status  of  this  type  of 
apparatus  can  be  considered  only  as  the  product  of  the  analyst. 
The  production  of  cage-wound  induction  motors  with  good  start- 
ing torque,  suitable  for  general  purposes,  was  the  result  of  analysis, 
not  experiment. 

In  the  electrical  manufacturing  industry  the  analysts,  as  repre- 
sented by  the  designing  engineers,  hold  an  important  place.  The 
term  is  here  used  broadly  to  include  the  designers  of  systems, 
applications,  methods,  etc.,  as  well  as  apparatus.  They  form  a 
very  necessary  part  of  the  organization,  especially  so  in  connection 
with  those  departments  where  cut-and-try  methods  have  been 
largely  eliminated.  Many  of  the  largest  engineering  -under- 
takings axe  on  customers'  orders,  covering  apparatus  which 
has  never  been  built  before.  In  most  cases,  by  the  time  any  tests 
of  the  completed  apparatus  axe  obtainable,  the  work  as  a  whofe 


768  ELECTRICAL  ENGINEERING  PAPERS 

has  progressed  beyond  the  point  wnere  any  important  changes  can 
be  made.  Even  such  preliminary  tests  as  are  obtainable  in  the 
shop  are  liable  not  to  tell  the  whole  tale,  for  the  real  test  or 
proof  of  the  adequacy  of  the  design  comes  from  duration  tests 
furnished  by  ctual  service.  The  real  troubles  may  not  show  up 
until  six  months  or  a  year  after  the  apparatus  has  been  put  in 
service.  Here  is  one  of  the  difficulties  that  the  designing  engineer 
encounters;  and,  the  more  progressive  he  is,  the  more  liable  he  is 
to  run  into  this  very  difficulty,  simply  because  he  is  pushing 
further  into  unknown  ground.  A  serious  difficulty  possibly  de- 
velops a  year  or  so  after  the  apparatus  has  been  put  in  service. 
Then  he  is  criticised  both  for  not  having  forseen  and  for  not 
having  immediately  corrected  it.  Such  criticism  might  be  con- 
sidered, in  one  sense,  as  complimentary,  for  it  is  an  assumption 
that  he  knows  much  more  than  he  really  does.  However,  most 
engineers  are  not  particularly  pleased  over  such  criticism,  for  they 
usually  find  it  hard  enough  to  cure  an  unknown  and  unforseen 
trouble,  without  being  told  that  they  were  careless  and  did  not 
use  proper  foresight.  A  true  engineer  has  pride  in  his  work,  and 
a  defect  or  failure,  in  itself,  usually  hurts  him  even  more  than 
criticism.  He  also  feels  that  when  a  man  has  done  the  best  he 
can  and  has  attempted  something  never  accomplished  before,  he 
should  have  sympathy  in  his  trouble,  or  at  least  constructive 
criticism. 

It  may  be  added  here  that  in  addition  to  ability  to  undertake 
and  carry  through  a  given  design,  it  is  important  that  the  engineer 
be  able  to  "let  go"  of  it  at  the  proper  time.  Each  new  develop- 
ment or  test  shows  the  way  to  still  further  improvements  or  de- 
velopments, and  if  each  of  these  is  to  be  incorporated  in  the  de- 
sign, then  it  will  never  reach  completion  until  absolute  perfection 
is  attained  or  the  designer  has  reached  the  ultimate  limit  of  his 
ability.  Neither  of  these  conditions  is  practicable  in  a  live 
manufacturing  business,  and,  therefore,  the  engineer  should  be 
able  to  let  go  of  his  design  when  a  sufficiently  good  practical  result 
is  obtained.  Some  engineers  seem  to  know  just  when  to  stop. 
This  is  to  some  extent  dependent  upon  a  proper  appreciation  of 
commercial  requirements. 

To  be  a  successful  electrical  engineer  does  not  mean  one  is  fitted 
to  be  a  manufacturing  engineer;  further,  one  may  be  a  very  good 
electrical  manufacturing  engineer  and  yet  not  be  fitted  for  elec- 
trical design,  for  this  latter  is  a  branch  of  the  industry  which  re- 


ENGINEERING  BY  ANALYSIS  769 

quires  rather  special  characteristics.  Experience  shows  that  the 
designing  engineer  must  have  a  special  aptitude  for  such  work 
regardless  of  his  education  or  general  abilities,  if  he  is  to  be  thor- 
oughly successful.  In  design  work,  experience  has  also  shown  that 
combinations  of  the  requisite  natural  aptitude  and  the  necessary 
technical  training  are  comparatively  rare,  and  the  really  successful 
men  in  this  line  of  work  are  but  very  few  in  number. 

If  certain  aptitudes  and  characteristics  are  essential  for  the 
designing  engineer  it  might  be  asked — what  are  these  essentials? 
However,  it  is  almost  impossible  to  pick  out  any  characteristic 
which  could  be  considered  as  the  one  essential  in  the  electrical 
designing  engineer,  except,  possibly,  good  common  sense;  but  as 
this  is  at  the  bottom  of  all  true  success,  it  should  not  be  considered 
as  peculiarly  characteristic  of  the  engineering  profession. 

As  the  competent  electrical  designing  engineer  must  necessarily 
be  an  analyst,  obviously  analytical  ability,  in  the  broad  sense, 
must  be  one  of  his  foremost  characteristics.  He  should  also  have  a 
certain  amount  of  mathematical  ability  and  training.  In  general, 
skill  in  the  ordinary  mathematics,  such  as  in  algebra  and  analytical 
trigonometry  is  of  more  use  than  a  mere  working  knowledge  of  the 
higher  mathematics.  There  are  certain  lines  of  work  in  which 
the  higher  mathematics  are,  of  course,  very  valuable  and  necessary. 
These,  however,  represent  a  relatively  small  percent  of  the  total 
field.  The  young  engineer  should  not  become  unduly  impressed 
with  the  idea  that  ability  to  use  extremely  complicated  mathe- 
matics is  the  prime  requisite.  He  should,  however,  recognize 
that  without  mathematical  aptitude  of  any  sort,  he  is  very  greatly 
handicapped.  The  "handy  man"  with  mathematics  appears  to 
have  a  decided  advantage  over  others,  in  practical  work. 

The  engineer  who  can  develop  a  mental  picture  or  a  *'  physical 
conception"  of  what  is  going  on  in  a  machine,  in  distinction  from  a 
purely  mathematical  conception,  appears  to  have  a  very  consider- 
able advantage  over  his  fellows.  The  man  with  both  the  physical 
conception  and  with  good  mathematical  ability  will  probably  go 
further  in  analysis  than  any  of  the  others* 

Let  us  return  to  one  of  the  conditions  which  is  very  necessary 
in  all  engineering,  namely — a  good  knowledge  of  fundamental 
principles-  The  engineer  should  know  the  derivation  of  his  var- 
ious methods  and  formulae.  Many  of  these  which  are  now  used 
by  rapid  workers  are  really  short  cuts  or  empirical  methods  which 
are  primarily  based  upon  correct  but  more  camples 


770  ELECTRICAL  ENGINEERING  PAPERS 

Their  use,  without  a  proper  knowledge  of  their  derivations  and, 
therefore,  their  limitations,  is  dangerous  and  not  infrequently 
leads  to  serious  trouble.  Above  all  the  electrical  designing  engi- 
neer should  have  a  broad  conception  of  certain  fundamental  rela- 
tionships or  laws  entirely  apart  from  the  mathematics  of  the  case. 
With  a  clear  understanding  of  fundamental  principles  there  is 
much  less  liability  of  waste  of  time  and  effort  from  following  out 
impracticable  schemes. 

There  was  a  time,  and  not  so  many  years  ago,  when  an  elec- 
trical engineer  could  cover  almost  the  entire  field.  At  that  time  a 
fairly  complete  training  in  the  various  branches  of  electrical  engin- 
eering was  possible,  but  with  the  widening  of  the  field,  it  has  become 
too  great  for  the  single  individual  to  cover,  and  the  problems  have 
become  too  difficult  for  any  one  man  to  handle  all  of  them.  There- 
fore, it  has  become  necessary  for  individual  engineers  to  devote 
themselves  to  some  special  field  of  endeavor  and  to  leave  the  broad 
field  to  be  covered  by  the  co-operation  of  many  specialists.  Con- 
sequently, the  engineering  of  today  is  sub-divided  into  many 
groups,  each  more  or  less  distinct  in  itself,  but  each  overlapping 
and  interrelated  with  many  other  groups.  The  engineer  of  today 
is,  therefore,  always  some  kind  of  a  specialist,  for  it  is  impossible 
to  be  otherwise  if  he  is  to  lead  in  anything, 

It  is  on  account  of  this  specialization  that  it  is  so  important 
that  the  young  engineer  of  today  obtain  a  broad  knowledge  of  the 
fundamentals  of  his  chosen  line  of  engineering.  The  same  fun- 
damentals underlie  the  whole  electrical  field,  so  that  a  knowledge 
of  them  is  about  as  near  as  he  can  come  to  a  broad  knowledge  of 
the  whole.  Such  should  be  obtained  as  early  as  possible  in  his 
career,  for,  after  specialization  begins,  his  own  particular  field  of 
endeavor  is  liable  to  absorb  all  of  his  efforts. 

It  is  now  being  recognized  by  the  ablest  engineers  that  much 
specialization  in  the  schools  is  not  an  advantage  to  the  student, 
If  the  colleges  could  confine  themselves  to  a  broad  teaching  of 
fundamental  principles  they  would  turn  out  vastly  more  effective 
men  than  at  present.  Analytical  ability  (not  necessarily  mathe- 
matical) is  one  of  the  crying  needs  of  the  electrical  industry  of 
today,  as  regards  its  young  men.  And  this  need  exists  in  spite 
of  the  fact  that  this  industry  doubtless  gets  its  full  share  of  the 
analytical  men  turned  out  by  the  schools.  An  analytical  man  per 
se  is  oae  who  thinks  for  himself  and,  therefore,  the  problem  really 
narrows  down  to  the  thinking  man.  If  the  schools  could  turn  out 


ENGINEERING  BY  ANAL YSIS  77 1 

a  much  higher  percentage  of  thinking  men,  the  engineering  pro 
fession  would  be  vastly  benefited. 

There  is  another  quality  or  characteristic  which,  while  possibly 
not  as  valuable  as  analytical  ability,  goes  a  long  way  toward  suc- 
cess, namely — persistency.  A  brilliant  tnind  with  but  little  per- 
sistency back  of  it,  will  usually  accomplish  less  than  a  much  less 
brilliant  mind  backed  by  great  persistency.  This  latter  charac- 
teristic has  turned  many  an  apparent  failure  into  positive  success. 
A  brilliant  man  without  persistency  is  liable  to  pass  from  scheme 
to  scheme  and  perfect  none  of  them.  However,  persistency  alone 
usually  accomplishes  no  more  than  brilliancy  alone.  Men  have 
expended  years  of  patient  effort  along  lines  which  a  little  common 
sense  analysis  would  quickly  have  shown  to  be  impracticable. 
Here  is  persistency  gone  to  waste. 

The  emphasis  placed  upon  the  above  mentioned  characteristics 
is  not  intended  to  belittle  other  very  important  ones,  such  as 
initiative,  originality,  resourcefulness,  etc.  These  qualities  might 
be  classed  even  higher  than  analytical  ability  and  persistency  by 
some  persons,  and  possibly  rightly  in  some  lines  of  effort.  But  in 
the  higher  electrical  work  the  conditions  may  be  otherwise.  Here 
one  may  have  strong  initiative,  but  be  utterly  unable  to  make  any 
great  progress  due  to  lack  of  analytical  ability;  he  may  have  great 
originality,  but,  lacking  the  fundamentals,  be  unable  to  touch  on 
the  higher  work;  he  may  be  exceedingly  resourceful,  but  be  limited 
only  to  lesser  things  due  to  lack  of  knowledge  of  basic  principles, 
and,  thus,  inability  to  handle  advanced  work.  However,  a  leader 
must  have  all  of  these  qualities  to  a  certain  extent,  Now  and  then 
a  man  is  found  who  has  all  of  them  to  a  fairly  high  degree,  com- 
bined with  unusual  analytical  ability  and  perseverance.  Such  a 
man  eventually  is  liable  to  become  known  as  a  genius,  but  it  should 
be  remembered  that  genius  is  of  two  kinds, — creative,  in  the  sense 
of  being  able  to  think  in  new  fields,  and  constructive,  in  the  sense 
of  being  able  to  use  present  known  facts  and  principles  to  bring 
about  successful  results. 

Then  there  is  another  feature  which  may  be  referred  to,  name- 
ly, the  commercial  side  of  engineering.  An  electrical  manufacturing 
business  lives  by  tke  goods,  not  the  engineering,  which  it  sells. 
The  successful  designer  of  such  goods  must,  therefore,  have  con- 
siderable knowledge  of  commercial  conditions  or  he  cannot  design 
adequate  or  competitive  apparatus*  This  is  a  feature  of  the  bum- 
ness  about  which  the  young  engineer,  fresh  from  school, 


772  ELECTRICAL  ENGINEERING  PAPERS 

nothing.  This  appears  to  oe  a  very  difficult  thing  for  some  6Ek 
gineers  to  acquire,  while  certain  of  them  never  really  do  so.  On  the 
other  hand,  it  has  been  said  of  some  very  good  engineers  that  they 
ought  to  have  been  salesmen,  because  they  grasped  so  readily  the 
customer's  conditions  and  requirements.  The  broad  gauge  elec- 
trical designer  is  usually  quite  successful  in  aiding  the  salesman, 
because  he  sees  the  commercial  bearing  of  his  engineering  work. 

This  relation  of  the  engineer  to  the  commercial  side  of  the  busi- 
ness brings  up  another  point,  namely,  his  ability  to  talk  clearly  and 
logically  in  private  and  in  public.  It  was  once  supposed  that  an 
engineer  never  had  to  talk  in  public  and  that  all  he  had  to  do  was 
to  go  off  in  a  corner,  by  himself,  and  use  a  slide-rule.  But  that 
day  is  long  past,  for  now  the  man  who  knows  most  about  the  appa- 
ratus must  be  able  to  tell  others  what  he  knows.  Presumably  in 
all  large  concerns  there  are  men  who  are  seldom  or  never  sent  out- 
side on  account  of  their  inability  to  make  a  good  presentation  of  a 
subject.  Assuming  equal  ability  otherwise  such  men  are  of  less 
value  than  those  who  can  make  a  good  presentation  of  any  given 
matter.  In  general,  a  good  logical  thinker  can  develop  into  a 
fairly  good  logical  speaker  through  practice. 

The  foregoing  has  had  most  to  do  with  electrical  designing  en- 
gineers, but  while  they  are  a  very  important  part  of  the  industry, 
yet  they  are  not  the  only  engineers  in  the  electrical  manufacturing 
business.  In  fact,  the  electrical  industry  today  is  managed  almost 
entirely  by  men  who  should  be  classed  as  engineers.  A  large  per- 
centage of  the  electrical  salesmen  of  today  have  had  a  very  good 
engineering  training  of  one  kind  or  another.  In  fact,  in  many 
lines  they  must  have  such  training  in  order  to  be  successful.  In 
the  manufacturing  part  of  the  business,  many  of  the  leading  men 
are  also  good  engineers.  Also  many  of  the  high  executives 
in  the  industry  are  trained  engineers  of  high  grade. 

In  conclusion  it  may  be  said  that  this  is  an  age  of  engineering 
construction.  It  is,  or  rather  it  forshadows,  the  golden  age  of 
the  engineer.  His  successes  and  attainments  have  led  him  to 
view  hopefully  hitherto  totally  unattainable  things,  and  in  conse- 
quence his  problems  are  becoming  increasingly  difficult.  At  no 
time  has  such  boldness  been  shown  in  attacking  the  problems  of 
nature  for  the  benefit  of  mankind,  and  it  is  the  engineer  in  one 
guise  or  another  who  is  behind  the  attack,  and  his  aim  almost 
invariably  is  something  which  is  ultimately  for  the  advancement 
of  humanity.  Construction,  not  destruction,  is  his  preference, 


ENGINEERING  BY  ANAL YSIS  773 

He  is  an  optimist  and  not  a  pessimist.  In  research  work  he  is 
delving  into  the  unknown  in  search  for  properties,  principles  and 
laws  of  nature  and  of  material.  He  is  making  vast  strides  in  the 
conservation  of  natural  resources,  by  the  economical  generation 
and  utilization  of  power.  In  transportation  he  is  bringing  the 
whole  world  together.  He  is  making  steel  and  concrete  the  rule 
in  constructions,  doing  away  with  more  perishable  materials. 

Engineering  should  be  considered  of  highest  rank  among  the 
professions.  No  engineer  need  apologize  for  his  calling.  He 
should  feel  the  greatest  pride  in  it,  for  it  may  be  said  that  it  is  the 
very  heart  and  soul  of  material  progress. 


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