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Full text of "Armature Winding"

ARMATURE WINDING 



A PRACTICAL ANALYSIS OF ARMATURE WINDINGS *j 

FOR, DIRECT -CURRENT AND ALTKKNATING || 

CURRENT MACHINES, INCLUDING RULES jf 

AND DIAGRAMS FOR RECONNECT- f 

ING INDUCTION MOTOR f 
' ARMATURES ' . ,_ 



11 Y \ 

DAVID I*. MORMTON 



A0HOCIATW PttOFWHHOR OF KUflCTttKM NmIUNU, 
AHMOtTU IMHTITUTB V TWOHNOhOOY, rUK'A(K 

AB8OOIATB MKMBKtt, AMHIUO^N INTITt!T OF II*|fi<*ritft f Al HW 

AUTHOR OF "PRACTICAL AIPLISM> KI.MCTKIOITY"; "KLKCTU 

MBASURKMUNTB AN MKTKIt TKHTINii"; ** 

JBQUIPM8NT fC)R TWK MOTOtt OAR M 



AMERICAN TECHNICAL HtK'IIhTY 
CHICAGO 



COPYRIGHT, 1920, 1923, 1925, BY 

AMERICAN TECHNICAL SOCIETY 



COPYRIGHTED IN GREAT BRITAIN 




/I" 



INDEX 



A PAGE 

Advance of winding ......... , ......................... 5g 

Altcrnat ing-cur rent maehineM, armature windingn for. ..... Ill 

cltiHsoH of ariAaturo wiiwlingH fora.c. marhincH ........ 135 

comparison of concentrated and distributed windings. , 120 
concentrated and distributed armature winding ...... 125 

conclusion ....................................... j[g4 

delta connections . ...... . . , ....................... 122 

development of e.m.f. equal ion ............... t ..... 129 

electrical degrees ...... , ........................... 119 

generators and motors. ............................ m 

rating of alternators. . . . . . ......................... 133 

reconnecting induct ion-motor windings ......... ..... 154 

clarification of probable changos in connections of 
motor winding ............................ 156 

fundamental idean of e.leef.ric motor ....... . ..... * 154 

possibles rcMu>niu k ctionN . . ....................... 170 

shop and working diagrams .................... 157 

relation of e.mJ/H in sinpl<^ alternator ....... ........ Ill 

simple sjngl(*-phas<^ winding ........................ 1*12 

O winding .......................... 116 

winding ......................... 117 

factor ........................ 132 

f!tirrent relation in two-phase winding ..... 123 

Y connections .............. , ..................... 12JX " 

Alternating-numuit windingH^ examples of ................ . 1^ ^ 

Alternaiion, 'dafinition ........ ..... .................... 

Alternators " \ 

rating of , . . < . .................................... 

relation of jn.f. f In. ,,,... .................... l . . . , 

Armature 4 

'^" 

suggestions for clipping and baking ................... 

types of ..,.,,., .......... ..................... ,19, 

Armature binding wires .......... . ................. .. -.25, 

channels for* . * * ...... ... .................... .' . . , J 25 



If 




2 INDEX 

PAGE 

Armature construction 23 

binding-wire channels 25 

binding wires 28 

mounting of core-discs 25 

purpose of armature core 23 

shapes of armature teeth 24 

ventilating ducts 27 

wedges * * * * 33 

Armature core 

core bodies 23 

discs, mounting. 25 

purpose of < 23 

Armature inductors (see Armature winding) 

Armature teeth and slots, shapes of 24 

Armature winding ...,-.. , 1-185 

arrangement of inductors in slots 90 

for a.c. machines Ill, 135 

for d.c. machines 10 

introduction 1 

mounting armature windings * 80 

reconnecting induction-motor armature windings 154 

simple generator 5 

B 

Baking armatures, suggestions for 108 

Barrel windings 91 

Bastard windings 04 

Binding wires of armature 28 

Brush calculations 97 

Brush rigging 39 

Brushes , 39, 60 

required for lap winding 57 

resistance of circuits ,.....,...., 59 

C 

Carbon brushes 39 

Chain windings 145 

Circuits, number of f 70 



INDEX 3 

PAGE 

Classes of armature windings for a.c. machines ........... 135 

arrangement of coil sides in slots .................... 143 

form of armature ................................. 135 



'if 



form of inductors ................................. 140 

kind of current delivered ........................... 141 

method of advancing around armature in tracing 

through winding ........................ * . 135 

miscellaneous windings ........................... , 145 if ^ 

number of circuits through winding per phase ....... '. 144 , % \' 

number of coils per phase per pole .................. 140 \l J 

number of slots ................................... 133 1 1 

relation between number of poles and number of coils 136 j *, , 

shape of coil ends ................................. 141 j&| 

Closed-circuit winding ..................... .. . .. ......... 18 f |f 

Collecting rings ............ ; .............. , ........... 1 

Commutation, time of for lap and wave windings ......... 86 

Commutator, function and operation of .......... 10, 12, 15, 16 

Commutator and brush construction ................ 1, 33, 100 

brushes and brush-rigging .......................... 39 

commutator bars ........................ , ........ 33 

commutator construction .......... ' ................ 35 

commutator risers ................................. 38 

rocker and rocker arms ............................ 43 

size of commutator ................................ 100 

Commutator calculations .............................. 97 f 

Commutator pitch .................................... 67 I * 

relation to number of paths and winding pitch ........ 70 j 1 

Concentrated and distributed armature windings. . 125, 129, 131 f ' 

Conductor, straight, cutting magnetic field ............... 5 I ' 

Core of motor armature .............................. 23, 25 f 

Creeping windings .................................... 145 , 

Current and voltage relation in two-phase winding ........ 123 t r 

Cycle, definition ...................................... 10 } 



Delta connections ..................................... 122 ' } 

Delta diagram .................. .......... ....,' ....... 161 Jf 

Design of armature windings .................... ....... 49 j 

brushes required for lap winding ...... ,..,,,,,.,.,,, 57 , 



4 INDEX 

Design of armature windings (continued) PAGE 

brushes for wave winding ^ 

current and voltage relations for lap and wave windings 61 

distribution of lap and wave windings 60 

general design considerations 07 

advance or retreat of winding 08 

application of general equation to lap or parallel 

winding 72 

application of general equation to wave or series 

winding 73 

commutator pitch * 67 

equipotential connections 88 

examples of lap windings 78 

examples of wave windings 79 

field displacement *...., 08 

general relations * . . * . 70 

method of determining re-entrancy * . . 76 

numbering sides of element 71 

progressive and retrogressive windings (57 

reduction of total inductors to elements of wingle 

turn 82 

relation between number of paths, or circuits, and 

winding and commutator pitches 70 

slot pitch 67 

time of commutation for lap and wave windingH , , 86 

winding pitch , , 67 

winding tables for armature windings .,,> 83 

position of brushes , , 60 

ring and drum windings 49 

ring, lap, and wave windings 51 

simplex and multiplex windings 68 

wave-wound ring armature 61 

Dipping armatures, suggestions for , . , 108 

Direct-current machines, armature windings for 10 

armature construction. . : 23 

design of windings 49 

development of e.m.f. equation for d.c. generator 45 

mounting armature windings , , f 39 

types of armatures f , jj 



INDEX r> 

Disc winding ,...,.....,., 21 

Distributed windings , . I2f 1UO, 131 

Distribution factor, values of ....,...,. , , 132 * f I 

single-phase windings , ,,,,,,,,, 132 ' ^ 

three-phase windings , , , , 133 f 1| 

, two-phase windings ............,..,,..,..,,.,,.,, 133 ' $ - 

Don'ts to be observed in armature winding .,,,.,, KM! 

'.. Double re-entrant winding ,..,,,,,,..,, fir* 

Drum windings , , , , U0 40, fKJ 

advantages of ,,.,...,,,.,,,,,,,,, 533 f ? 

barrel windings , ,,,.,,,,,,,., Ill ? ' 

bastard windings ,,,,,,,,,,, 114 ^ \ 

evolute windings ,,.,,,.,.,,,,,. IX) *i'j 

form-wound drum windings .,,,.,..,,,,,. 9S '' 

hand windings ,.,..,...,,,, 00 fj , 

Dummy coils, use- of , , , ,,,,,, 74 f| 1 ' 

Duplex lap winding, time of commutation for, .,,,,,.,,.. SCI |*j 

Duplex winding, dnt<rmining nn^ntruncy of. . , * , . . 77 f ! 

Dynamo (s<c G(HKTator) 

R 

E.M.F. (see Electromotive, fonu) f 

Electrical degrees 1 jg r 

Electromotive force, 

effective , 113, 116 ^ ' 

fundanutntal equation for * , . ,45, 120 rf ^ 

producuMl by cutting lin< of forw , . . 2, 5 f ' 

in simple alternator , , Ill ^ 4 

variations in on<i revolution of loop 8 ,. , 

Element of armatun^ winding, , 40, 71 r ; 

numbering sides of 71 , ( , 

Equipotential (Connections . , *,.,...,... 88 ^ 

Evolute windings 90 f , 

F 

Flux ,,,...., 2, & . h 

Form factor >,. ' J?4 | , ] 

Form-wound drum windings , * " ^ f f 



6 INDEX 

PAGE 

Four-part commutator and four loops of wire, operation of 16 
Four-part commutator and two loops of wire, operation of 12 
Frequency, reconnecting for change in 176, 185 

G 
Generator 

i armature windings in Ill 

i equation for 45 

| essential parts of 1 

! simple type 5 

; analysis of operation 5 

t effect of more loops 10 

,****-.. function and operation of two-part commutator, . . 10 

function of slip rings , 10 

open- and closed-circuit armature windings IS 

operation of four-part commutator and four loops 

of wire 10 

operation of four-part commutator and two loopn 

of wire , 12 

operation of six-part commutator and throo loops 

of wire 15 

operation of two-part commutator and two loops 

of wire 1(1 

variations of e.m.f. in one revolution 8 

Gramme armature 1,1) 

H 

Hand windings , Q 

High voltage, effect of in motors 184 

Horsepower ^ 155 

Hysteresis loss (see Iron losses) 

I 

Induction-motor armature windings, reconnecting. ,,.,.,. 154 
classification of-, probable changes in connections of 

Binding , tt J5 6 

conclusion * . . , . 184 

fundamental ideas of electric motor ,,,,, 154 

possible changes w * -j^, 

shop and working diagrams ,......,-, 157 



INDEX 7 

Inductors, armature (see Armature winding) PAGE 

Insulating varnish, specific gravity of , K)8 

Insulation of commutator bars 34 

L 

Lap winding 51 

application of general equation to 72 

brushes required for 57 

current and volt/age, relations for 61 

distribution of 00 

examples of 78 

time of commutation for 80 

Least common multiple connection 167 

Lines of force 2, 5 

Loop (5, 10 

cutting magnetic, field 

effect, of adding morn turns to 10 

Low voltage, dTiv-t of in motors 185 

M 

Magnetic field 1 

Magnetic flux, cutting 2, 5 

Motor 

armature windings in a.c. type Ill 

fundamental ideas of 154 

motor acting as generator 156 

torque and horsepower. . , , 155 

Mounting armature windings 89 

arrangement of inductors in slots 96 

commutator and brush calculations 97 

don'ts to be observed 109 

drum windings 90 

number of segments 99 

precautions to be observed 110 

process of winding small armature 100 

size of commutator 100 

slot insulation , , 97 

suggestions for dipping and baking of armatures.. ...... 108 



8 INDEX 

PAOE 

Multiplex windings 63 

Mummified windings v 145 

N 

Number of paths, relation to winding and commutator 

pitches 70 

Number of poles, reconnecting for change in 1 80, 185 



Open-circuit winding lg 

Operation of simple generator , . 5 

P 

* Parallel star diagram ,.. 100 

Parallel winding, application of general equation to, , .... 71 

Paths, number of 70 

Phases.. 134, 172, IS5 

Poles, reconnecting for changes in number of 180, 185 

Precautions to be observed in armature winding 1 10 

Progressive windings..* 57 

Q 
Quadruplex windings , 77 

R 

Rate of cutting lines of force, e.m.f. depends on 2 

Re-entrancy of winding, determining , , t , 7$ 

duplex winding 77' 

quadruplex winding ] 77 

triplex winding 77 

Retreat of winding. * * * * /*o 

Retrogressive windings ',"*"* 67 

Right-hand rule g 

Ring-wound armatures V.V.VlV 22 49 

Rockers and rocker arms M ' ' A * 




INDEX 9 II I 

\\ 

Segments, number of 09 ' j 

Series winding, application of general equation to 73 1 1 

"Shop and working diagrams 157 f ? 

conventional method 157 

delta diagram , 101 

least common multiple connection 107 

parallel star diagram 1 00 

three-phase development 107 

three-phase motor diagram 108 

two-phase development 1 03 

two-phase two-speed diagram 100 

Short-pitch windings, e.m.f. for 131 

Shuttle windings 145 

Simplex lap winding, time of commutation for 86 

Simplex windings 03 

Single-phase a.c. winding 112, 132 

effective c.in.f 113 

form factor H4 

Singly re-entrant winding 04 ? 06 

Six-part commutator and throe loops of wire, operation of. . 15 

Skew-coil windings 145 

Slip rings, function of JQ 

Slot, arrangement of inductors in 90 

Slot insulation 97 

Slot pitch 67 

T 

Tables 

comparative performances of two-phase motor with 

various connections 106 

comparison of motor voltages with various connections. 16# 
distribution factors for singles-, two-, arid three-phase 

windings 134 

effectiveness of single-phase armature winding having 

six slots per phase per pole 141 

permissible temperature and temperature rises for 

insulation material 198 

thickness of commutator insulation 34 



10 INDEX 

Tables (continued) PAGE 

voltage and number of segments 90 

winding table for six-pole, duplex, doubly re-entrant, 

retrogressive, wave winding, shown in Fig. 103 84 
winding table for six-pole, duplex, singly re-entrant, 

progressive, lap winding, shown in Fig* 91,.., 83 
winding table for six-pole, triplex, trebly re-entrant, 

progressive, lap winding, shown in Fig. 107. . , 85 

Teeth in armature 24: 

Three-phase a.c. winding 117, 133 

Three-phase connection from two-phase winding *. . , 107 

Three-phase motor diagram 168 

Torque 155 

Tracing circuits . . . 55 

Trebly re-entrant winding 00 

Triplex lap winding, time of commutation for 87 

Triplex singly re-entrant winding .,,,.,, 0(5 

Triplex windings, determining ,, 77 

Two-part commutator , , 10, IS 

Two-phase a.c. winding 116, 123, 133 

effective e.m.f 110 

voltage and current relation in 123 

Two-phase development HJ3 

Two-phase two-speed diagram JO!) 



' Ventilating ducts in armature , . 27 

Voltage, reconnecting for change in 17 i t l$5 

Voltage and current relation in two-phase winding ..... 133 

Voltage equation , , , . ; 45 

W 

Wave winding g2 

application of general equation to 73 

current and voltage relations for $1 

distribution of 00 

examples of yn 

tune of commutation for QA 



INDEX 



lr 



Wave- wound ring armature ..................... 

Wedges ...................................... ..... ^ 

Winding element., . , ........................... ' ' ^ 

Winding pitch .................................. ^ 

relation to commutator pite.li and number of poles ..... 70 

Winding small armature, process of .................. JQQ 

Winding tables for armature windings ................ 3 

Working diagrams (see Shop and working diagrams) 



Y connections.. 



120 



Qi LIBRARY 





(I 1 



INTRODUCTION 

WITH the discovery of electro-magnetism as a source of 
energy, electrical science progressed from an interesting 

phenomenon to the beginning of modern industrial develop- ,' . 
ment. Picture the world today without the dynamo, the 
electric light, electric cans, the telephone or the wireless 
telegraph. In a few short years this much progress has been 
made, and with the present knowledge of electrical science, 
what may not be done during the lifetime of present scholars 
and experimenters? 

t 1 

flf As the armature, with its .windings of insulated wire, is the 

heart of the whole system of electrical energy, should not this 

phase receive special study? We think so, and it is with the t ' ; ' 

purpose of showing the practical and theoretical considera- , r 

tions due the subject of armature winding that this volume ^ 

has been prepared. \\ 

lj' 

<jf For many years only an inkling of the principles of electrical jj 

energy was known. From time to time discoveries, the result i 

of experiment rather than calculation, led inventors nearer and ;j 

nearer to one hundred per cent in electrical efficiency. Long (\ 

observations of electrical effects led to more or less empirical s 
formulas, which have been corrected from time to time 

as additional observations corrected original impressions. ; |/ 

Advances in mechanical and chemical processes have aided in ; *' 

making electricity the willing servant of mankind until today ^ j 

the weight of the water in the mountain stream traveling ; j., 
from the far-off hills to the distant sea becomes, through the , 5 

armature of the dynamo, the energy which lights our cities, , j 

turns the wheels of industry and carries our messages around > [[ 
the world, The development of the armature has thus had ; f 

much to do with bringing about this happy condition in our f; 

economic existence. * ' 

I ' k 

<f In preparing this volume the author has drawn heavily upon f < / 

his wide experience in the theoretical and practical design of t ; 



INTRODUCTION 

electrical apparatus. Practically all of the Hne drawings have 
been drawn especially for this work, while the photographic 
reproductions represent the best practice of modern whops. 
Among the illustrations will be found many blue-print repro- 
ductions, indicating in the most practical method the princi- 
ples of correct armature design and construction. 

<f Such advances have been made since present workmen were 

j in school that it has become necessary for many to supplement 

il school knowledge with information brought down to date, 

' This volume is therefore particularly adapted for purposes 

1 1 of home-study and self-instruction. The treatment of each 

I 1 subject will appeal not only to the technically trained export 

I but also to the beginner and the shop-taught practical man 

if* who wishes to keep abreast of modern progress. Without 

, sacrificing any of the essential requirements of thorough 

,{ l practical instruction the author has avoided many of the 

\ \ heavy technical terms and formulas of higher mathematics, 

I producing a book in clear and simple language, on this 

^ ) important branch of electrical work. 



|!^( L ! E R A H Y )z| 



"/-- L,O '"" l " U "' ' ' PAGE i ' 

Introduction 1 [ 

Essential parts of dynamo 1 

Producing c.m.f. by cutting magnetic lines of force 2 

Right-hand rule 3 f 

Simple generator 5 

Analysis of operation f> 

Function of slip rings 10 *' 

Function and operation of commutators 10 

Open- and closed-circuit armature windings 18 

ARMATURE WINDINGS FOR D.C. MACHINES 

Types of armatures 19 

Ring armatures 19 , J 

Drum armatures 20 [ 

Disc armatures 21 

Armature construction 23 i 

Purpose of armature core 23 ' 

Shapes of armature teeth 24 < ' | 

Mounting of core discs 25 f 

Ventilating ducts 27 ; 

Binding wires 28 

Commutator and brush construction 33 j 

Commutator bars , 33 fr ' 

Commutator risers 38 

Brushes and brush-rigging 39 

Rockers and rocker arms 43 , 

Development of e,m.f, equation 45 , * 

Design of windings , . 49 / * 

Ring and drum windings 49 * > 

Ring, lap, and wave windings 51 ' 

Brushes required 57 

Position of brushes 60 [ i 



CONTENTS 



Design of windings (continued) 

Distribution of lap and wave windings ........... .... 00 

Current and voltage relations for lap and wave wind- 

ings ...................................... * ;; 

Wave-wound ring armature ........................ "^ 

Simplex and multiplex windings .................... M 

General design considerations ......................... ^ 

Winding pitch ...... . ............................. ^ 

Commutator pitch .......................... * ..... * 

Slot pitch ................... ...... ' ............ (] l 

Progressive and retrogressive windings ............... 67 

Field displacement ................................ ^ 

Relation between number of paths and winding nod 

commutator pitches ........................... '0 

General relations .................................. 70 

Numbering sides of element ........................ 71 

Application of general equation to windings .......... 72 

Method of determining re-entrancy of winding ........ 76 

Examples of windings ............................. 78 

Reduction of total inductors to elements of single turn 83 

Winding tables ............................... . * 84 

'Time of commutation for lap and wave windings ...... 87 

Equipotential connections .......................... 80 

Mounting armature windings ....................... ... 90 

Drum windings ........ ........................... 01 

Arrangement of inductors in slots ................ ... 07 

Commutator and brush calculations ................. 98 

Size of commutator ............................... 10 1 

Process of winding small armature ....... , .......... 101 

Suggestions for dipping and baking of armatures. . , . , 109 

Don'ts and precautions to be observed ...... . ...... , , 1 10 

ARMATURE BINDINGS FOR A.C. MACHINES 

Theoretical considerations ........................ , , , , 111 

Generators and motors ......... . ............. ..... Ill 



. CONTENTS 

Theoretical considerations (continued) PAGE 

Simple three-phase winding 117 

Electrical degrees 119 

Y connections 120 

A connections 122 

Voltage and current relation in two-phase winding .... 123 

Concentrated and distributed armature windings 125 

Development of e.m.f. equation 129 

Values of distribution factor 132 

Rating of alternators 133 

Classes of armature windings 135 

Examples of a.c. windings 145 

Reconnecting induction-motor armature windings 154 

Possible changes 154 

Fundamental ideas of electric motor 154 

Classification of probable changes in connections 156 

Conventional method of representing winding diagrams 157 

Parallel star diagram 160 

Delta diagram 161 

Two-phase development 163 

Three-phase development 167 

Least common multiple connection. 167 

Three-phase motor diagram 168 

Two-phase two-speed diagram 169 

Changes in voltage only 171 

Changes in phase only, 172 

Changes in frequency, 176 

Changes in number of poles ,...,-, 180 

' Conclusion, , , 




ARMATURE WINDING 

PART I 



INTRODUCTION 

Essential Parts of a Dynamo. The dynamo is a machine for 
converting mechanical energy into electrical energy or electrical 
energy into mechanical energy, When it is used in transforming 
mechanical into electrical energy, it is called a generator; and 
when it transforms electrical into mechanical energy, it is called 
a motor. The great majority of dynamos have the following 
essential parts: the magnetic field; the armature winding'; the 
commutating and collecting devices (not required in all machines 
the squirrel-cage induction motor, for. example); and the neces- 
sary mechanical structure, such as bed plate, iron composing the 
magnetic circuit and its supporting structure, armature core, 
bearing supports, etc. 

Magnetic Field. The function of the magnetic field is to. 
provide a magnetic flux, which is cut by the inductors forming 
the armature winding. 

Armature Winding. The armature winding is composed of & 
large number of wires, called inductors, in which an electromotive 
force (e.m.fO or electrical pressure is induced when there is a 
relative movement of these inductors with reference to the mag- 
netic field of the machine, 

Commutator or Collecting Ring*. The function of the com- 
mutating and collecting devices is to bring about the necessary 
reversal of connections between the various ejements composing 
the armature winding and the external circuit, and at the same 
time to provide the necessary continuous electrical connection 
between the circuits on the moving .part of the machine and the 
outside circuits, 

Meohanical Parts. The function of the various mechanical 
parts. i$ obvious, and the iron composing the magnetic circuit 



2 . ARMATURE WINDING 

the armature core serves as a mechanical support for the arma- 
ture windings. 

In commercial continuous-current machines, the field magnet 
Is nothing more than a simple electromagnet which remains sta- 
tionary, but the armature is a great deal more complex and 
always rotates. In alternating-current machines either the arma- 
ture or field may be stationary. Continuous-current machines 
always require a commutator, which is mounted on tt\e same 
shaft as the armature, while the alternating-current machines 
are provided with slip rings when an electrical connection must 
be established between the rotating part of the machine and an 
outside circuit. 

The development of the various forms of armature windings 
for both continuous- and alternating-current machines will be 
discussed in the following sections. 

Producing an E.M.F. by Cutting Magnetic Lines of Force. 
When a conductor and a magnetic field are caused to move 
relative to each other, so that the imaginary lines of force that 
are supposed to compose the magnetic field are cut by the con- 
ductor, there will be an e.m.f. induced in the conductor. 

E.M.F. Depends on Rate Lines Are Cut The value of this 
induced e.m.f. at any instant will depend upon the rapidity with 
which the lines of force are being cut by the conductor at that 
particular instant. If the lines of force are being cut at a per- 
fectly uniform rate, that is, if the same number are cut in each 
succeeding fractional part of a second, say one hundredth part of 
a second, and there is a total of 100,000,000 lines cut in one 
second, then there will be an e.m.f. of one volt induced in the 
conductor. Thus if a horizontal conductor 50 centimeters long be 
moved downward across a horizontal magnetic field whose inten- 
sity is 20,000 gausses, as indicated in Fig. 1, at a uniform velocity 
of 10 centimeters each second, all the magnetic lines in the area 
10X50 centimeters will be cut in one second. Since there are 
20,000 magnetic lines passing through each unit of area, then the 
total number of magnetic lines cut by the conductor in one 
second will be equal to 10X50X20000, or 10,000,000. Dividing 
10,000,000 by 100,000,000 gives 0.1 volt, the value of the e.m.f, 



ARMATURE WINDING . -3 

If the conductor be moved at a greater velocity, say twice as 
fast, then the e.m.f. induced will be equal to twice the stated 
value, and if its velocity be decreased there will be a correspond- 
ing decrease in the induced e.m.f. If the strength of the magnetic 
field be increased or decreased in value, there will be a corre- 
sponding increase or decrease in the value of the induced e.m.f. 
Likewise, if the length of the conductor in the magnetic field, or 
that part of the conductor which is actually cutting lines of force, 
be increased or decreased, there will be a corresponding increase 
or decrease in the value of the induced e.m.f. 




Fig. 1, Horizontal Conductor Moving Downward across a Uniform Magnetic Field 
Motion Perpendicular to Field 

If this conductor be made to form part of a closed electrical 
circuit, there will be a current of electricity produced in the 
circuit due to the e.m.f. induced in the conductor. 

Right-Hand Rule. There is a definite relation between the 
direction of the magnetic field, the direction of motion of the 
conductor, and the direction of the induced e.m.f., which is as 
follows; If the first and second fingers and the thumb of the 
right hand be placed at right angles to each other and in such 
a position that the first .finger points in the direction of the 
magnetic field and the thumb points in the direction of motion, 
then the second finger will point along the conductor in the 
direction of the induced e.m.f. The direction of the induced e.m.f. 



4 ARMATURE WINDING 

the magnetic field be reversed. If the direction of the magnetic 
field and the motion both be reversed, then the direction of the 
induced e.m.f. will remain the same. 




Fig. 2. Horizontal Conductor Moving Downward across Uniform Magnetic Reid- 
Motion Not Perpendicular-to Field 

The motion of the conductor in Fig. 1 is perpendicular to 
the direction of the magnetic field, and,, as a result, more mag- 
netic Hues are cut by the conductor when it moves a certain 
distance along its path than would be cut if the motion of the 
conductor were along a path making an angle of less than 90 
degrees with the direction of the magnetic field, Fig. 2. In 
Fig. 2 it is the component of the velocity of the conductor per- 




to the direction of the magnetic field that determines 
tie rate of cutting of the magnetic lines. This component of 
tiie velocity of the conductor is equl f.o the -H-n I v^wi**' 



ARMATURE WINDING 



multiplied by the sine of the angle between the direction of 1 
magnetic field and the direction of motion of the conduct 
Thus in Fig. 3, if the angle is equal to 30 degrees, and 
other conditions are .the same as in the above problem, th 
when the conductor moves a distance of 10 centimeters it I 
moved a distance perpendicular to the magnetic field equal 
lOXsine 30, or 10X0.5, which equals 5 centimeters. The n 
at which the conductor is actually cutting the magnetic lines 
then, just one-half of what it was originally, and the e.m.f. indue 
in the conductor will be one-half as great, or 0,05 volt. 

SIMPLE OENERATOR 

Analysis of Operation. Straight Conductor. When the co 
ductor, Fig. 1, has moved downward a sufficient distance to 1 
out of the magnetic field, 
there will be no e.m.f. in- 
duced in it, as it continues 
to move on down, for there 
will be no magnetic lines of 
force cut by the conductor. 
Now, in order that the con- 
ductor may continue cutting 

the magnetic lines Of force, Fl. 4 Curve fepnwntinc Variation in Value < 
MI i f 1 1 Electromotive Force Irxluwd in a Conductor Tha 

it will be necessary for the i MOWI iwk and Forth * * uniform SmmS 

, - , . , MHd at A Cwwteni Velocity 

motion of the conductor to 

be reversed when it reaches the edge of the magnetic field in !fe 
downward travel; that is, the motion of the conductor must be 
alternately up and down across the- magnetic field. If the strength 
of the magnetic field is uniform in the region in which the conductor 
moves and the velocity of the conductor is constant and the 
direction of its motion is reversed instantly, then the variation fh 
the e.m.f. induced in the conductor may be represented graphi- 
cally as shown in Fig. 4 Assume that the conductor starts from 
its uppermost position in the magnetic field, as shown at A, 
Fig, 5, and moves at a constant velocity downward across the 
magnetic field to its lowermost position, as shown at B, Fig. 6 
During this time the conductor is cutting the magnetic Kit 
a constant * t*> f<vr ! 1 * s it' " 




5 ARMATURE WINDING 

induced In it is constant, as represented by the upper part of the 
line AB, Fig, 4. The height of the horizontal line AB above the 
zero line is a measure of the e.m.f. induced in the conductor. 
Now when the conductor reaches the lowermost position, it 
immediately starts 'to move upward across the magnetic -field at 
the same rate it was originally moving downward across the field, 
and, as a result, the value of the induced e.m.f. will be the same 
but its direction will be exactly opposite what it was originally 
This fact is shown diagrammatically in Fig. 4 by the line B'A' t 
which is parallel to the zero line and exactly the same distance 
below the zero line as the line AB is above the zero line. The 



MAGNETIC FIELD 



FIELD 



Fig. 5, Conductor Entering T 

Magnetic Field and Mov- - 

ing Downward across Fig 6 Conductor at Lower 
Same Edge of Magnetic Field 

lengths of the lines AB and B'A'&re drawn to represent time to any 
convenient scale; thus each inch may correspond to one second, etc. 
Action of Loop. The arrangement just described may be 
greatly improved upon by revolving a loop of wire in a magnetic 
field, as shown in Fig. 7 Four positions of the loop are shown 
in cross-section in Fig. 8, and the e.mJ induced in the loop for 
these different positions may be determined as follows; In posi- 
tion / the plane of the loop is perpendicular to the direction of 
the magnetic field, and if the loop be rotated a small angle about 
its axis, there will be no e.m.f induced in it because there are 
no magnetic lines cut by any part of the loop. The two sides o| 
tfee loof> will be moving parallel to the magnetic field, and hootee 
no lines of force; while the planes in which the two ends 
r r*lM try the dimtfon of the magnetic field, and 



ARMATURE WINDING 7 

hence the ends will never cut across any of the lines of force 
fanning the magnetic field, regardless of-flie angular position of 
the coil, so long as the axis of the loop is perpendicular to the 
direction of the magnetic field. 

In Paw*"* * the plane of the loop is parallel to the magnetic 
held and the two sides of the loop are moving perpendicular to 
the magnetic field for an instant while the loop is in this position 
Since the sides of the loop are moving perpendicular to the direc- 
tion of the magnetic field, when the loop is in position 3 they 
will be cutting the magnetic lines at'the greatest possible rate. 




Fig. 7. Closed Loop of Wire Revolving in a Uniform Magnetic Field 

In position 3 the plane of the loop is perpendicular to the 
direction of the magnetic field and the e.m.f. induced in the two 
sides is zero, for the same reasons as those given for position / 
In position 4 the plane of the loop is parallel to the direction of 
the magnetic field and the two sides are moving perpendicular to 
the direction of the magnetic field just as explained for position 2. 
In position $, however, the side E is moving downward across the 
magnetic field and the side ^F is moving upward across the mag- 
netic field, while in position 4 just the reverse is true; that is, 
side^ is moving upward across the magnetic field, and side F is* 
moving downward aoms* t.h* mo^n^tir fmlH Tli m t : i . i 



8 ARMATURE WINDING 

In the two sides will be in opposite directions for all positions of 
the loop as you look along the two sides, but it will be observed 
that they are acting together around the loop rather than oppos- 
ing each other, for all positions of the loop. 

From position 1 to position 3, the side E is moving downward 
across the magnetic field and the side F is moving upward across 
the field, while from position 3 back to position 1 the side E is 
moving upward across the magnetic field and the side F is moving 
downward. As a result of this relation between the direction of 
motion of the sides of the loop and the direction of the magnetic 




Fie 8. Four Different Positions of a Loop as It Revolves in a 
Magnetic Field 

field, there will be an electrical pressure induced in the loop 
which will act around the loop in a certain direction while the 
loop is rotating from position 1 to position S and around the loop 
in the opposite direction while rotating from position 3 to 4 & n d 
on back to position J, or the Starting point. 

Variations of E.M.F. in One Revolution. The value of the 
e.m.f. in the loop does not remain constant, but changes in value 
as the position, of the loop in the magnetic field changes, the 
reason being that the .velocity of the sides across the magnetic 
lines of force for a certain constant angular rotation of the loop 



9 



PIRtCTIOTI OF MAGNETIC FIELO 



PERPENDICULAR T< 
V SINE- 



ARMATURE/S^KJilNG 

is continuously varying in value.rfte 

two sides of the loop can have ac^Sne magnetic field occurs when 
the loop is parallel to the magneW^fieid x pr in position 2 or 
Fig. 8. The component of the veloc^ B ofal^-sides^jQLth<^. j ^ i/ 
which is actually perpendicular to thelfe^iOT S tt .magnetic 
field when the loop is in any position whatever,' may be expressed 
in terms of the maximum velocity which the sides may have and 
the angular position of the loop with reference to the plane per- 
pendicular to the magnetic field and corresponding to position 1 of 
the loop. Thus in Fig. 9 the side of the loop is shown between 
positions / and 2, and the 
plane of the loop is making 
the angle 6 with position 1. 
The component of the ve- 
locity of the side of the 
loop, which is perpendicu- 
lar to the direction of the 
magnetic field, is equal to 
the velocity of the side of 
the loop multiplied by the 
sine of the angle 6. Thus 
if the angle 6 is equal to 
60 degrees, then the veloc- 
ity of the side of the loop, 
perpendicular to the direc- 
tion of the magnetic field 
will be equal to V sine 60, 
or 1"X 0.866. Since the e,m.f. induced in the side of the loop 
depends upon the component of the velocity of the side of the 
loop perpendicular to the magnetic field, all other things being 
constant, and since this component of the velocity of the side of 
the loop varies as the sine of the angle 6 in Fig, 9, then it follows 
that the e.rn.f. induced in the sides of the loop for any position 
will be equal to the e.m.f. in the side of the loop for positions % 
and 4 multiplied Jay.,, the sine of the 'angle 6, The e.m.f. induced^ 
'in the sides of the loop is a maximum for positions 2 and 4 and 
may be represented by $ max . and for any other position it will be 
i equal to E mM , sine 0. The variation in the e.m.f. induced in the 



,! 




. 9. Method of Determining Velocity of Side of a 
oop Perpendicular to Direction of Magnetic Field 

at Any Instant as Loop Is Rotated at a Uniform 

Velocity 



4 



10 



ARMATURE WINDING 



loop for all positions of the loop is shown diagrammatically in 
Fig. 10. The distances along the zero line correspond to the 
values of the angle 6. Such a curve is called a sine curve. 

Effect of More Loops. The e.m.f. may be increased by add- 
ing more turns to the loop and connecting these turns' in series 
so that the e.m.f. induced in the different turns acts in the same 
direction around the loop. 

The complete set of positive and negative values represented 
in Fig. 10 constitutes what is called a cycle. A complete set of 
positive or negative values constitutes what is called an alterna- 
tion. Inhere are always twice as many alternations as there are 
cycles. The number of complete cycles that occur in one second 
is called the frequency. In Fig. 10 one revolution of the loop 

constitutes a cycle, or two 
alternations; and if the loop 
is made to revolve at the 
rate of 60 revolutions per 
second, the frequency of the 
induced e.m.f. will be 60 
cycles. 

Function of Slip Rings. 
In order to make use of the 
e.m.f. generated in the loop, 




Curve Representing Variation in Value of 

otnotive Fore? Induced in a Loop That Is 

Rotated at a Uniform Angular Velocity in a Uniform 

Magnetic JReJd 



Fig, 7, in producing a current 
in an electrical circuit, it is necessary to provide some means of con- 
necting the loop in series with the circuit in which the current is to 
be produced. Such an electrical connection may be provided by 
fopening up the loop and connecting the two ends thus formed to 
ffcwo continuous metal rings, mounted on the axia of the loop and 
insulated from each other. Upon these rings are two metal or 
carbon brushes, connected to the external circuit, as shown in 
Fig. 11. Such a device constitutes a simple alternating-current 
generator. A complete discussion of the armature windings for t 
alternating-current machines will be given in the section "Arma- 
ture Windings for Alternating-Current Machines." 

Function and Operation of Two-Part Commutator. As the 
loop of wire in Fig. 11 is made to revolve, an e.m.f. will be 
induced m it, and this e.m.f. will reverse in direction twice every 



ARMATURE WINDING 



II 



- 



revolution, as shown by the curve in Fig. 10. If the external 
circuit be closed, the alternating e.m.f. induced in the loop will 
produce an alternating current in the circuit. Such a current is 
not suitable for all purposes, as, for example charging storage 
batteries, and must be changed to a unidirectional or direct 
current. Itjs the function of the commutator to change, the 
alternating current in the loop into a direct current in the external 
circuit and at the same .time provide the necessary electrical 
connections between the loop and external circuit. 




Fig. 11. Simple Alternating-Current Generator 

The simplest form of commutator consists of a metal ring 
divided into two equal parts and mounted on a tube of insulating 
material, the two halves of the ring being insulated from each 
other. Each half of the ring should be connected to one of the 
ends of wire formed by opening up the loop, Fig. 12. The metal 
parts composing the commutator a;;e called segments. The two 
segments in the commutator are shown in Fig. 12. The electrical 
connection to the external circuit is made by means of suit- 
able brushes which make electrical contact with the segments 
of the commutator. Two brushes are required with a two-part 
commutator and single loop, as shown in Fig. 12, and these 



1 



!2 



ARMATURE WINDING 



the com- 



brushes should be equally spaced on opposite sides 
mutator, and In such a position that the msula ^" 
segments of the two-part commutator is exactly in the _ 

ihe brushes when the plane of the loop is perpend* ulai -to the 
direction of the magnetic field, or the induced e.m.f. m the loop 
is zero. A two-part commutator of this kind will reverse the 
connections of the loop of wire with respect to the external elec- 
trical circuit when the e.m.f. in the loop is zero and the e.m.t. 
acting on the external circuit always will be in the same direc- 
tion and may be represented graphically 'by a curve such as the 
one shown in Fig. 13: This kind of an e.m.f. is called a pulsat- 




Fig. 12. Simple Direct' Current Generator 



ing e.m.f. becauseji^ulsates in value at regular intervals; it is, 
however, continuous indirection." ln*"oro!er to produce an e.m.f. 
nearer constanFmlSlue more commutator segments and loops of 
wire must be used. 

Operation of Four-Part Commutator and Two Loops of Wire. 
The fluctuation in the value of the e.m.f. between the brushes 
with the arrangements shown in Fig. 12, can be reduced by using 
two more commutator segments and a second loop* In this case 
the metal ring is cut in four parts instead of two, thus forming 
a commutator composed of four segments instead of two* The 
two loops are placed at right angles to _each other and the termi- 



ARMATURE WINDING 13 

nals of each loop are connected to commutator segments that are 
'opposite to each other instead of adjacent to each other. The 
connections of the loops and segments are shown in Fig. 14. 




DEGRCE5 

Fig. 13. Curve Representing Variation .in Value of Electromotive 

Force between Brushes of Direct-Current Generator 

Shown in Fig. 12 

Two brushes are required, and they should be placed exactly 
opposite each other and in such a position around the commu- 
tator that they pass from one segment to the next when the 
s planes of the two loops are making angles of 45 degrees with a 
plane perpendicular to the direction of the magnetic field. The 
proper position of the brushes is shown in Fig. 14. Let us now 
consider the operation of this machine. Starting with loop A 




Fie. 14. Direct-Current Generator Composed of Two Loops of Wire and ft 
Commutator of Four Segments 

parallel to the magnetic field, loop #, which is at right angles to 
loop A, will b<? perpendicular to the direction of the magnetic 
field. When the loops are in this position the brushes should be 



14 ARMATURE WINDING 

in the center of the segments connected to loop A. Now as the 
combination of loops and commutator (called the armature)- 
'rotates, the e.m.f. induced in loop A decreases in value and the 
e m.f. induced in loop B increases in value (it is to be remembered 
at the start the e.m.f. in A is at its maximum value and the 
e.m.f. in B is zero). When the armature has turned through an 
angle of 45 degrees, the commutator segments connected to loop 
A move from under the brushes and the commutator segments 
connected to loop B move under the brushes. This results in- 
loop B now being connected in series with the external circuit 
instead of loop A. Loop B will remain in electrical connection 
with the external circuit for the next 90 degrees' rotation of the 
armature, or one quarter turn, % when the segments connected to 




Kg. 15. Curve Representing Variation in Value of Electromotive Force between 
Brushesof Direct-Current Generator Shown in Fig 14 

B move from under the brushes and those connected to A move 
under the brushes. From the above statements and a careful 
inspection of Fig. 14 it is apparent that the loops A and B are 
alternately connected to the external circuit, and each time either 
of them is connected it is for one quarter of a revolution of the 
armature. TJe e.m.f. between the brushes varies in value, but 
it will never o!rop to zero value as with the single loop. The 
connections of the loops are changed when they are making an 
angle of 45 degrees with a plane perpendicular to the direction of 
ffee magnetic field and the e.m.f.'s induced in the loops at this 
.iaslknt are equal in value and equal to 0.707 of the maximum 
e,ml. " induced in either loop when its plane is parallel to the 
direction of the magnetic field. This results in the e.m.f. between 
the brashes fluctuating in value between a maximum value and 



ARMATURE WINDING 



15 



0.707 of this maximum value. The fluctuation in e.m.f. for one 
"complete "revolution of the armature is shown in Fig. 15. 

Operation of Six-Part Commutator and Three Loops of Wire. 
The fluctuation in the value of the e.m.L between the brushes 
with the arrangement described in the preceding section may be 
decreased by using three loops of wire and a commutator com- 
posed of six segments. The terminals of each loop should be 
connected to two segments exactly opposite each other and the! 
brushes should be exactly opposite each other and in such aj 
position that they are in the center of the commutator segments 
connected to a loop when that loop is in a position parallel to 




Fig. 10. Direct-Current Generator Composed of Three Loops of Wire and a 
Commutator of riix Srgrnentd 

the direction of the magnetic field, or when the e.m.f. induced 
in the loop is at its maximum value. The arrangement of the 
loops, brushes, and commutator segments is shown in Fig, 16. 
Now as the armature rotates, the e.m.f. induced in loop A 
decreases in value, the e.m.f. induced in B decreases in value, and 
the e.m.f. induced in C increases in value. When the armature 
has turned through an angle of 30 degrees the segments con- 
nected to the loop A move from under the brushes, and the seg- 
ments connected to loop C come ioto contact with the brushes 
and remain in contact for a rotation of the armature of 60 
degrees, or one-sixth revolution. When the segments connected 



16 



ARMATURE WINDING 



to loop C leave. contact with the brushes, the segments connected 
to loop B make contact, and remain in contact for one-sixth 
revolution, then loop A comes into contact again for one-sixth 
revolution. Then loop C for one-sixth revolution, loop B for 
one-sixth revolution, and back to loop A for an angular movement 
of 30 degrees. This brings the armature back to the starting 
point. The fluctuation in e.m.f. for one complete revolution of 
the armature is shown in Fig. 17. 

Operation of Two-Part Commutator and Two Loops of Wire. 
Two loops of wire may be connected in parallel between two 
commutator segments as shown in Fig. 18. The e.m.f. between 
the brushes will be the same as though a single loop of wire 
were used, but the current the armature is capable of delivering 
will be doubled if the wire used in winding the loops is of the 




oM 60* 4- 60* 4* 60* 4- 60* *L 60* 



Fig. 17 Curve Representing Variation in Value of Electromotive Force between Bruah* 
of Direct-Current Generator Shown in Fig. 16 "*i> 

same size as that used in winding the single loop. The variation 
in the e.m.f. between the brushes for such a combination is 
shown in Fig. 13. 

Operation of Four-Part Commutator and Four Loops of Wire. 
An armature may be formed by interconnecting four loops of 
wire and four commutator segments. The connections are shown 
in Fig. 19. Each loop has its terminals connected to adjacent 
commutator segments. The brushes must be broad enough to 
bridge the insulation between adjacent segments, and they are 
mounted on the commutator in such a position that they short- 
circuit the loops when the .sides of the loops are moving parallel 
to the magnetic field. 



ARMATURE WINDING n 

An inspection of Fig. 19 will assist you in understanding the 

n ir t r ents ; w , hen J the armature is ;n the *** 'ho- 
rn F,g. 19, the e.m.f. mduced ,n the loops A and C is zero, and 

the e.m f. induced in the loops B and D is a maximum Of 
course, loops A and C are short-circuited by the brushes, but no 
damage results, as there is no e.m.f. induced' in these loops in 
th* position. Loops B and D are connected in parallel between 
the brushes, and the e.m.f. between the brushes is that induced 
m either loop B or D, which is supposedly the same. Now, as 
the armature rotates from the position shown in Fig. 19, the 
e.m.f. in loops B and D decreases in value and the e.m.f. in the 




Fig. 18 Din 



rt-Current Generator Composed of Two Loops of Wire and a 
itor of Two Segments 



Commutat< 



loops A and C increases in value, starting with zero. A small 
angular rotation of the armature results in the short-circuit of 
the loops A and C being removed, and the loop A is connected 
in series with the loop B and likewise the loop C is connected in 
series with the loop D. This connection remains while the arma- 
ture rotates for one-fourth revolution, when the loops B and D 
are short-circuited by the brushes arid the loops A and C are in 
parallel between the brushes. During this one-fourth revolution 
the e.m.f. induced in loops B and D decreased in value from a 
maximum to zero value, as shown by the curve bd in Fig. 20, 
and the e.m.f. induced in the two loops A and C has increased 
in value from zero to a maximum value, as shown by the curve 



18 



ARMATURE WINDING 



m in Fig. 20. The e.m.f. between the brushes is the sum of 
the e.m.f.'s and it is represented by the heavy curve in Fig. 20. 
The maximum value of this e.m.f. occurs when the loops are 
making an angle of 45 degrees with the position shown in Fig. 19, or 
they have turned one-eighth turn from the starting point. The 
e.m.f. in all the loops is the same for this position of the arma- 
ture and is equal to 0.707 of the maximum e.m.f. The total 
e.m.f. between the brushes is equal to twice this value, since two 
loops are in series, or it is equal to 1.414 times the maximum 
e.m.f. that can occur in any one of the loops. The e.m.f. between 
the brushes will fluctuate between the maximum value occurring 
in a single loop and 1.414 times this maximum value. With a 
four-loop armature there will be four of these pulsations for each 




Fig. 19, Direct-Current Generator Composed of Four Loops of Wire and a 
Commutator of Four Segments 

revolution. As the number of loops and segments is increased the 
amount of this fluctuation is decreased, but the number of fluctua- 
tions per revolutions is increased. 

Upen- and Closed-Circuit Armature Windings. In an open- 
winding the different loops do not as a whole form a 
1 closed circuit, but each loop is in circuit only when the commu- 
tator segments to which it is connected are in electrical contact 
with the brushes. The windings shown in Figs. 14 and 16 are of 
the open-circuit type. 

A^ closed-circuit winding is one in which the loops forming 
the winding are interconnected and form one or more closed 
circuits upon themselves, and each loop is always in circuit 
except ^when it is short-circuited by the brushes. The winding 
shown in Fig. 19 is of the closed-circuit type. 



ARMATURE WINDING 19 

Practically all modern armatures are of the closed-circuit 
.type. Open-circuit armatures have been used to some extent in 



E M F BETWEEN BRU5HE5 r 




t 



Fig. 20. Curve Representing Variation in Value of Electromotive Force 
between Brushes of Direct-Current Generator Shown in Fig. 19 

series arc-lighting generators and quite extensively in alternating- 
current machines. 



ARMATURE WINDINGS FOR DIRECT-CURRENT 
MACHINES 

TYPES OF ARMATURES 

Classification. Armatures, considered as a whole, may be 
divided into three classes according to the shape of the core upon 
which the winding is placed and the manner in which the winding 
is^pla^e^.Qjx.tb^.wre. These three classes are: 

Ring armatures 
Drum armatures 
Disc armatures 

Ring Armatures. Ring armatures were first used by Pacinotti 
in 1860, but they are commonly known by the name of Gramme, 
the French electrician, who reintroduce'd them in 1870. Gramme 
wound the wire around the entire surface of the annular core, 
which was made of varnished iron wire in order to reduce the 
losses due to eddy currents. Pacinotti wound the wire between 
projecting teeth upon an iron ring, as shown in Fig. 21. 

In ring windings the parts of the windings which pass through 
the inside of the ring do not cut any magnetic lines (assuming 



20 



ARMATURE WIN-DING 




-Wound 



'there is no magnetic flux passing across the opening inside of the 
iron ring), and, as a result, are inoperative, so far as the e.m.f. of 
the machine is concerned. 

An armature of the Gramme 
ring type is shown in Fig. 22. Some 
of the machines of this type were 
so designed as to have the outside 
of the ring act as a commutator, 
the current being collected directly 
from the winding by brushes which 
trailed on the periphery of the ring, 
while the inner part of the con- 
ductors cut the magnetic lines. A 
complete Gramme ring armature 
provided with a commutator of the 
usual form is shown in Fig. 23. 

Drum Armatures. Drum armatures were first introduced by 
Siemens, who wound coils of iron wire upon a frame of nonmagnetic 
material Armatures of this type in their complete form were 
first brought out in 1871 by Von Hefner Alteneck, and improved 
later by Weston and others. The principle of the drum winding 
is shown in Fig. 24, and it is apparent that it is much simpler 
tfean the ring winding. Each wire is placed on the outside of the 
dram, usually parallel to the axis of the armature core, and is 
connected to another wire by means of connecting wires called 
end-connections, which do -not pass through the core. The only 

reason for having any 
opening in the core at 
all, other than to save 
the material, is to im- 
prove the ventilation and 
cooling of the armature. 
In two-pole machines the 
end-connections run 
across the ends of the 
core and coftnect wires 

which are almost diametrically opposite. In multipolar machines 
the end-connections join wires which are separated by a distance 




Fie, 22. Simple Gramme Ring Winding 



ARMATURE WINDING 



21 



approximately equal to the distance between corresponding points 
on adjacent poles, so that the electrical pressures in the wires thus' 
connected will act in the same direction around the loop. 




Fig 23 Couple Gramme Ring Winding 

The drum armature may be thought of as derived from the| 
ring armature by moving the inner connections of the winding, ! 
or the part of the winding on the inside of the ring, to the outer | 
surface, at the same time stretching the coil so that the two sides will | ; 
occupy approximately corresponding positions under adjacent poles. | 

Disc Armatures. The disc armature differs from the other 
two, in that the wires in which the electrical pressure is induced 
instead of being on the outer cylindrical surface of the armature 




Fig, 24. Principle of Simple Drum Winding 



core, are placed radially on the flat sides of a disc. The prin- 
ciple of the disc armature is shown in Fig. 25. 

Discussion of Different Types. The drum armature is used 
more than either of the others in the construction of modern 



22 



ARMATURE WINDING 



machines. The ring armature is seldom used, and the disc 
armature is pracEcSEy 'obsolete. The principle of operation of 
the drum armature is practically the same as that of the ring 
armature, but since the ring armature is not nearly so common as 

w %f!T'i;l'"-"'"'~ ' ' ' " 

the drum armature, mp^LoLtha foEowjng , treatment on armatures 
for direct-current machines relates specifically to .druni armatures. 
Some Disadvantages of Ring Armatures. In developing or 
placing a ring winding in place on the armature core it usually 
is necessary to thread the wire through the hollow, cylindrical 
core; this necessitates bending the conductor back and forth. 
This winding operation must be carried on by hand. Since the 




Fig 25. Principle of Disc Armature 

wire forming the winding must be bent to conform to the arma- 
ture core, it is very difficult to wind an armature that requires 
a wire of large size. It is practically impossible to prevent the 
insulation on the wire being damaged during the process of wind- 
ing, and the extra amount of care that must be taken means that 
more time is required. 

Ring Windings Difficult to Hold in Position. Only one-half 
of each turn or loop has an e.m.L induced in it, and as a result 
more wire is required; this means additional expense. However, 
the resistance of the armature is greater, owing to the increased 
amount of wire in each of the circuits through' the armature 
winding. 



ARMATURE WINDING 23 

Some Advantages of Drum Armatures. Since all the loops or 
coils for a certain armature are of the same shape and size, they 
may be wound on forms. Such coils, called form- wound coils, 
in some cases may be made with automatic machinery. The 
economy resulting from this type of construction is very obvious. 
In form-wound coils it is possible to use wire of almost any size. 
The insulation on the wire is not subject to any great amount of 
abuse in winding a form-wound coil. The armature winding may 
be rigidly held in position by being placed in the armature slots 
provided in the surface of the armature core. Binding wedges 
or binding wires keep the winding from being thrown out of the 
slots by centrifugal force when the armature rotates. 

ARMATURE CONSTRUCTION 

Purpose of Armature Core. The function of the armature 
core is two-fold: it supports the armature winding and it carries | 
the flux from one pole core to the adjacent pole cores; that is, | 
it completes the magnetic circuit between the pole pieces. On 
account of its high permeability and great strength, iron is by 
far the best material for armature cores. Armature cores m#y be 
of annular, ring, cylindrical, or disc form. The cylindrical form 
is used on small machines, the annular on large machines, so that 
the cooling surface is larger and the weight of material is reduced. 
It has been seen, however, that when a mass of iron (or other 
conductor) is rotated in a magnetic field, wasteful eddy currents 
are set up in the mass; hence solid cores of metal should on no 
account be used in any armature. In order to reduce these cur- 
rents as much as possible, it has become the practice to build up 
armature cores of thin, soft iron or mild steel discs, insulated 
from one another by Varnish, rust, or paper. These discs are 
arranged to have their planes parallel to the direction of the flux 
and pjr$mdicu1ar to ..the.flovL.of. eddy... currents. An armature core 
composed of such sheets, forced together by hydraulic or screw 
pressure, is found to be from 85 to 95 per cent iron, the remainder 
of its volume being made up of insulation, air space, etc. 

Core Bodies, The cores of armatures are made of laminae 
(thin discs) of wrought iron or mild steel. These discs are 
stamped out of sheet metal, and range from 0.014 inch to 0.025 



ARMATURE WINDING 

in thickness, the formeptfthickness behjg that most often 
S * time. Cos, discs^p to*** 30 mches m 
deter_are P unchedJn.one piece, while larger dimeters are 





Kg. 27 Method of Building up Armature 
Core from Segments 



Fig. 26. Order of Jtamping Armature Core Segments 

stamped out in sections, Fig. 26, and the core built up as indi- 
cated in Fig. 27, alternating the joints. These stampings are now 
so accurately made that, after assembling the discs into a core, 
the slots need not be milled out, as was formerly necessary. 

Milling is most objectionable be- 
cause it burrs over the edges of 
the discs. The burrs thus pro- 
duced connect adjacent discs and 
facilitate the flow of eddy cur- 
rents, thereby defeating the pur- 
pose of lamination. Turning 
after assembling also tends to 
increase the iron losses. Hence, if it is found that the periphery 
of the core body is irregular, it should be ground true. 

The core discs are insulated from each other either by a thin 
coating of iron oxide on the discs or by a thin coating of japan 
varnish. Sometimes shellac or paper is used for insulating these 
laminae; but on account of the greater expense and the fact that 
the efficiency is only slightly bettered, 
these latter are applied only in special 
cases. 

Shapes of Armature Teeth. The 
armature cores used in practice are 
almost always provided with a toothed 
surface. Thus the armature winding is 
protected, the length of the air gap 
reduced, and the winding is prevented from slipping in tte core. 
The general efficiency of the machine is greater than when *' 
smooth core is used. The number of teeth must be relatively 




He, 28. Armature Teeth with 
Parallel Slots 



ARMATURE WINDING 



25 



large, about four per inch of armature diameter, to prevent noise 
and excessive eddy-current losses in the pole faces. A common 
form of armature tooth is slightly narrower at the root than at 





Fig. 29. Teeth with ffifcojecting 
Tops ,v ; 



.Fig. 30. Notched Teeth to Hold a 
Wedge 



the top, the resulting slot having parallel sides, Fig. 28. Fig. 29 
illustrates a form in which the tops are slightly extended to give 
a larger magnetic area at the top, thus decreasing the reluctance 
of the air gap and helping to retain the inductors in the slots 
by the insertion of a wedge of wood. The latter object is. also 
attained by notching the teeth as in Fig. 30, in case it is not 
desirable to increase the area of the top of the tooth. 

Binding-Wire Channels. In machines using binding wires to 
hold the armature inductors in the slots, it is usual to stamp 
some of the core discs of slightly reduced diameter so that the! 
binding wires may be flush with the surface of the .armature.| 
The reduction is seldom more than } inch on the diameter, 
giving a channel not more than. J inch deep. The width is 
determined by the number and the size of the binding wires. 





, Figa. 31 and 32. Formft of Armature Core Discs for Small Machines 

Mounting of Core Discs. Some mechanical means must be 
provided to hold the core discs together, and to connect them 
rigidly to the shaft. In the case of small cores not exceeding 



26 



ARMATURE WINDING 




Big. 33 Core Discs Bolted to 

Sfddaj^Bolts Pass through 

Disca 



15 inches in diameter, the core discs take either of the forms 
shown in Figs. 31 and 32, the latter being preferable on account 
of increased ventilation. The laminae are simply keyed to the 
shaft, being held together under heavy 
pressure by end-plates of cast steel or 
cast iron, which are in turn pressed 
inward either by nuts fitting in threads 
upon the shaft or by bolts passing 
through, but insulated from the arma- 
ture discs and end-plates," 

Large cores in which the discs are 
made in sections, or for which the mate- 
rial of the core near the shaft is not 
required, are built upon an auxiliary 
support called a spider, which has differ- 
ent forms, depending on the mode of attachment between it and 
the core discs. Fig. 33 shows the discs held together and to a 
skeleton pulley, or spider, by bolts passing through them, the 
spider being keyed to the shaft. The objection to this construc- 
tion is that the bolt-holes reduce the effective area of the core, 
thus strangling the magnetic flux. This difficulty may be over- 
come by placing the bolts internal to the core, as in Fig. 34, in 
which case they need not be so well insulated. Another and 
newer arrangement provides the discs 
with dovetail notches, or extensions 
which fit into extensions or notches on 
the spider arms, Fig, 35. The sectional 
view shows the method of holding the 
laminae together by means of bolts 
and end-plates, also the R extensions for 
supporting the end-connections of a bar- 
rel winding. 

The hubs of armature spiders are 
usually cleared out between their front 
and back bearing surfaces to facilitate 
fifing the shaft; and in larger sizes the seating on the shaft is often 
turned to two different sizes to admit of easier erecting, Fig* 30. 
Rgs. 37 Mid 38 show a spider and other features of construction of a 




Fig. 34. Core Discs Bolted to 
%wtei B^s^edl 



ARMATURE WINDING 



27/ 



large machine. The rim of the spider is cut in six pieces, each of 
which has four dovetail notches. If it is cast in one piece, .trouble 
may arise from unequal strains in the metal due to contraction. 

D D D 




Fig 35 Method of Mounting Large Armature Core on Armature Spider 

Ventilating apertures are provided, and on the side of each arm, 
Fig. 37, are- seen the seatings and bolt-holes for attaching the 
commutator hub and the rim which supports the winding. In 
Fig. 38, which shows a completed core, the supporting rim and 
narrow ventilating ducts are visible. Figs. 39 and 40 show two 
views of a completely assembled armature core and commutator 
ready for the winding; the arma- 
ture spider is shown in Fig 41 
An armature core (in the pro- 
cess of construction) for a revolv- 
ing armature is shown in Fig. 42, 
and a core (also in the process 
of construction) for a stationary 
armature, as used in alternating- 
current machines, is shown in 
Fig. 43. 

A completed armature cow for a small machine is shown 
in Fig. 44. 

Ventilating Ducts. Armature cores heat from three causes, 
namely, hysteresis, eddy-currents in the iron, and PR losses in 




Fig. 36. Construction of Armature Hub 






28 ARMATURE WINDING 

the copper inductors. In order that the temperature-rise of the 
armature shall not exceed a safe figure (60 C), it is necessary 
in the large and heavy-duty types to resort to means of ventila- 
tion, usually ducts which lead the air out between the core discs. 
To keep the core discs apart at these ducts, it is necessary to 
introduce distance pieces, or ventilators. Fig. 45 illustrates some 
f of these devices. At A are shown simple pieces of brass riveted 
radially at Intervals to a special core disc 0.04 to 0,05 inch thick. 




Fig. 37 Armature Spider for Large Generator 

This form fails to provide adequate support for the teeth, a diffi- 
culty obviated in the form shown at B, which has, behind each 
tooth, a strip of I^rass about 0.4 inch wide set edgewise. This 
Strip is cast with or brazed to a special casting of brass riveted 
ti> a stout core disc. In a recent construction, shown in Fig. 46, 
the core plate -next to the duct is ribbed, affording good support 
for both the core and teeth of the next plate. 

Binding Wires. With toothed-core armatures the inductors 
may be held in the slots by wedges of wood, as already stated, or 



.ARMATURE WINDING 



29 



by bands, of wire wound around the armature. These binding 
wires must be strong enough to resist -the centrifugal force which 
tends to throw the armature inductors out of the slots, and yet 
must occupy as little radial space as possible, in order not to 
interfere with the clearance between the armature and the pole 
pieces. The common; practice is to employ a tinned wire of hard- 
drawn brass, phosphor bronze, or steel, which, after the windings can 
be sweated together by solder into one contiauous band. 




Fig. 38. Armature Core and Commutator Mounted on Temporary Shaft 

Under each belt of binding wire a band of insulation is laid, 
usually consisting of two layers: first, a thin strip of vulcanized 
fiber or of hard red varnished paper slightly wider than the belt 
of wire, and then a strip of mica in short pieces of about equal 
width. Sometimes a small strip of thin brass, with tags which can be 
turned over and soldered down, is laid under each belt of binding 
wire to prevent the ends of the binding wires from flying out. 



', I 

t I 



II 

f I 



tt 

V 4 



30 



ARMATURE WINDING 



To estimate the proper size and number of binding wires 
required, assume that-d is the diameter (in inches) of the cir- 
cular path described by a mass of weight Wi pounds, and the 




Jig. 39. Completely Assembled Armature and Commutator 

Beady for Winding (Rear View) 
Courtesy of General Electric Company 



1 




k. 

A 



Kg 40 Completely Assembled Armature Core and Commutator 

Ready for Winding (Front View) 
Courtesy of General Electric Company 



\ 
* JP%:\ 



AEMATURE WINDING 



centrifugal force will be = 0.0000143 XdxJFiXr.p.m, pounds weight 
So that if we assume a value of 100,OQO pounds per square 
inch as the maximum allowable tensile stress in steel or phosphor* 




Fig. 41 Armature Spider for Armature Shown in Figs 39 and 40 
Courtesy of General Electric Company 



S^^HHHBBBBBHB^ 



* * 
! 



r 



fy 1 
, I 



< 

f. I 



42 Armature Core for Devolving Armature in Process of Construction 
Cpurlety of Attis-Chalmers Company 



32 ARMATURE WINDING 

bronze .wire, and allow a safety factor of, say 1 0, the total section of 
binding wire required will be equal to 



Q.OOOQ143X10XH 7 X NxdXr.p. 

irXlOO.OOO _ 2 



_ 
4.55XlO- l XJFX2VxdXr.p.m. sq. in., 




Kg. 4& Armature Core for Stationary Armature in Process of Construction 
Courtesy of Attia-Chdmera Company 




Fig 44 Complete Armature Core for Small Direct-Current Machine 
Courtesy nf Reliance Elestnc and Engineering Company 



ARMATURE WINDING- 



33 



in which W is the weight of one inductor, and N the number of 
inductors; consequently W\ will be equal to WN. From this 
total necessary section and an appropriate wire table, the number of 
wires is calculated, and they are then arranged in suitable belts. 

Example. Let F=0.39 lb.; AT1536; d = 62 in.; r.p.m.150. The 
total necessary section computed by the -above formula is 0.37f square inch. 
Referring to the wire gage tables, we find that 148 wires" of No. 15 B.& S. gage 
will fulfill the conditions. These may be arranged as follows: 5 belts of 16 
wires each over the core body, and 4 belts of 17 wires each over the extended 

ends of the winding (that is, 2 belts of 17 wires , 

each over each end). 

Wedges. In the cases where wedges 
are driven into grooves in the teeth to 
close up the slot, the usual material 

|lt\ TCI m ffl employed is a well-baked hardwood, such 

' c^^JOQIJ as. hornbeam or hard, white, vulcanized 

r^~lf/, 'fiber. 





,3FTg. 45. Different Types of Dia- 
, tance Pieces or Ventilators 



,' Fife. 46, Ribbed Core Plate Used in Forming 
Ventilating Ducts 



COMMUTATOR AND BRUSH CONSTRUCTION 

Commutator Bars. Commutator bars are almost always made 
of copper; other metals, such as brass, iron, or steel, are not satis- 
factory on account of pitting and burning. Rolled copper is 
preferable, because of its toughness and uniform texture; but in 
some cases, on account of the shapes necessitated by different 
methods of connection to the armature conductors and various 
clamping devices* the segments are either cast or drop-fqrged, the 
.latter being at present the commercial type. 

In order to secure a good fit, the cross-section of the bars 
should be properly tapered according to the number of segments 
that makes up the whole circumference. It is obvious .that if , the 
number of segments equals 360, each segment plus its insulation 
(on one side) should have a taper of 1 degree; while if the number 



AEMATURE WINDING 



TABLE f 
Thickness of Commutator Insulation* 



Voltage of Machine 


THICKNESS OF MICA 


Between Neighboring 
Segments 


Between Segments and 
Shell and between 
Segments and Clamping 
Device 


Less than 150 
Less than 300 
Less than 1000 


0.020 to 0.03 in. 
0.025 to 0.04 in. 
0.04 to 0.06 in. 


'0.06 to 0.10 in. 
0.08 to 0. 13 in. 
O.lO to 0.16 in. 




Ffe. 47. End Insulating Ring of 



of segments equals 36, the taper would be 10 degrees. It is not 
practicable, however, to use mica' insulation that has not parallel 
faces; hence the segment is tapered, and any defect in the taper of 
the latter cannot be made good with insulation. It is found, how- 
ever, that when the number of segments exceeds 150, bars of the 
same taper can be used in constructing a 
commutator', having either two more or 
two less than the xlesigned number. 

Insulation. It is important to have 
good insulation between each bar and its 
neighbor, and especially good inswlatiori 
between the bars and the" sleeve or hub around which they are 
mounted, as well as between the bars and the clamping devices that 
bold them in place, since the voltage between bars is not as great as 
that between the bars and the metal-work of the machine. It / 
is essential that the insulating material be such that it will not 
absorb oil or moisture; hence, asbestos, plaster, and vulcanized 
fiber are inadmissible. The end insulation rings may be of 
micanite, or, if for low voltage, of that preparation of paper pulp 
known as press-board or press-spahn. The conical rings, used to 
insulate the dovetails on the bottom of the bars from the hub, 
are usually built of micanite molded under pressure white hot. 
i%. 47 illustrates such an end-ring, cut away to show its section. 
^ Commutators using air gaps between the segments as insula- 
tion irnve been tried; but, excepting in the case of arc-lighting 
machines where the segments are few in number and the air gap 
large, they have not proved successful, owing to the difficulty of 
keeping the gaps free from metallic dust. 



ARMATUfeE WINDING 



35 



It Is of importance that the mica selected for. Insulating the 
bars from one another should be soft enough to wear away at the 
same rate as the copper bars, and not project above the segments. 
Amber mica, soft and of rather cloudy color, is preferred to the 
harder clear white or red Indian variety. The usual thicknesses 
are as given in Table I. 

Commutator Construction. For small machines 'two common 
constructions are shown in Fig. 48. The commutator segments 
are secured between a bushing or hub and a clamping ring, the 





Fig. 48. Common Method of Commutator Coiwtmction for Small Machines 

latter being mounted on the hub, and forced to grip the bars 
by means of a nut on the hub or by bolts passing through the 
ring and hub, as in Fig. 49. The ends of the bars are beveled 
so that the ring and bushing- draw the segments closer together on 
tightening. 

The hub in small machines is usually of cast iron keyed to the 
shaft; but in* large machines the commutator is built upon a strong 
flange-like support or shell, bolted to the armature spider, Fig. 50, 
or mounted on a separate spider secured to the shaft, Fi$. 51, 



36 ARMATURE WINDING 

-When drawn copper strip is used, the design- should be such 
that the available surface for the brushes takes up nearly the 
whole length of the bar, and the beveled ends should be as simple 
as possible. With drop-forged segments this is not so important. 




In .building commutators it is usual to assemble the bars to 
&e proper number, with the interposed pieces of mica, clamping 
them temporarily around the outside with a strong iron clamp, as 






I'l 



ARMATURE WINDING 



37 



shown in Figs. 52 and 53, or forcing them into an external steel 
ring under hydraulic pressure. They are then put into a lathe 
and the interior surface is bored out, after which the ends are 




Fig. 60. Commutator Construction for Large Machines. Commutator Spider is Bolted to 
Armature Spider 

turned up in such a way that the angular hollows will receive 
the clamping pieces. The whole is then mounted with proper 
insulation upon the sleeve, and the clamping end-pieces are 
screwed up. It is then heated and the clamps still further 




Fig. 51. Commutator Construction for Large Machines. 

Commutator Spider Is Mounted on Armature Shaft 

. Independent of Armature Spider 

tightened up, after which the temporary damp or ring is removed 
and the external surface turned up. The commutator shell or 
spider and clamping ring for a large commutator are shown in 



38 ARMATURE WINDING ** 

Fig. 54, and the mica insulating rings for same are shown in Fig. 55. f 
Two completed commutators are shown in Figs. 56 and 57. 




Fig, 52. Method of Constructing and Forming Small Commutator 
Cmutesy of Reliance Electric and Engineering Company 




Kg. 53. Method of Constructing and Forming large Commutator 
Coyrtesy of General Electric Company 

Commutator Risers. Connection is. made with the armature 
inductors by means ol radial Strips or, wires, sometimes called 



ARMATURE WINDING 39 

risers, which are inserted into .a cut at the corner of each bar and 
firmly held there by a screw clamp and solder. Figs. 58, 59, and 
60 illustrate various modes of making connection to the commu- 



if 




Fig. 64. Commutator Shell or Spider and Clamping Ring for Large Commutator SljowB in Fig. 53 
Courtesy of General Electric Company 

tator bar. The risers are connected to the armature winding in 
several different ways as indicated in Fig. 61. In some evolute 
windings no risers are needed, the ends of the evolute being 
fastened directly to the commutator bars. Similarly, in the case 




Fig. 55. Mica Insulating Rings for Commutator Shown in Fig. 53 
Courtesy of General Electric Company 

of barrel-wound armatures, no risers are needed if the commutator 
diameter is very nearly that of the armature. 

Brushes and Brush-Rigging. Carboy brushes are almost the 
only type that is now considered. Their shape depends upon the 



40 



ARMATURE WINDING 



type of brush-holder selected, and upon whether the brushes are 
applied to the commutator radially or at an angle. Fig. 62 




fig. 56, Completed Commutator for Small Machine 
Courtesy of Reliance Ekctric and Engineering Company 




Fig. 57 Completed Commutator for Very Large Machine 
Courtesy of Attis-Chalmers Manufacturing Company 

illustrates various shapes. The mechanism for holding the brushes 
must fulfill the following requirements: 






ARMATURE WINDING 



41 



(1) The brushes must be held firmly against the commutator, 
but allowed to follow any irregularity in the contour of the latter 
without jumping away. 




.Figa. 68 and 50, Methods of Connecting Commutator Risers to Commutator Bars 

(2) The mechanism must permit the rvr 
brushes to be withdrawn while the com- 
mutator is rotating, and must feed them 
forward as required. 

(3) Spring pressure must be adjust- 
able, and the spring must not carry cur- 
rent. 

(4) The springs must not have too 



Kg. 60. Armature Winding 

Connected Directly to 

Commutator Bar 




<DH> 
< 



Fig. 61. Methods of Connecting Armature Winding to 
Commutator Risers 

great inertia, or they will not readily fulfill the first condition in 
regard to following the commutator. 

(5) Insulation must be very thorough. 






42 ARMATURE WINDING 

(6) The mechanism must be so arranged that the position 
of the brushes may be shifted. 

(7) All parts must be firm 
and strong, so the brushes will not 
chatter as the result of vibration 
while the machine is running. 

i The commercial forms of 
holders for carbon brushes may 
be classified under three types: 
hinged structures, parallel spring 
holders, and reaction holders. 

Fig. 63 illustrates a hinged 
brush-holder, and an arm holding 
several. .The carbon moves in a 
light frame, being- held against 
the commutator by a spring whose 

tension may be adjusted. Connection is made between the brush 
and the arm by means of a flexible lead, tinned and laid in a 
slot in the upper part of the carbon. A metal cap placed over 
-the top and sweated in place makes a permanent contact. This 
is shown by the two illustrations of the brush, > 




Bg,62. Several Diffea-ent Forma of Carbon 
Brushes 




Kg. 63. Brush Rigging aad Hinged Brush-Holder 






ARMATURE WINDING 



Fig. 64 .illustrates a parallel-movement type. The brush is 
held firmly in the holder by a clamping screw, and the whole 
arrangement is pressed against the commutator by a pressure 



Clampiwj Screw." 



Commut&toi 




. 64, Parallel-Movement Type of Brush-Holder 

'spring, whose tension may be varied by means of the adjusting 
screw. Connection is made between the brush and the stationary 
part of the holder by means of two sets of rolled-copper leaves 
which at the same time act as flexible joints. 

In Fig. 65 is shown a reaction type of brush-holder. The 
brush C is pressed against the commu- 
tator by the adjustable spring L, the 
liolder B being secured firmly to the 
rocket arm P by means of the set-screw q. 
The brush is furnished with a dovetail- 
shaped groove along its entire inner ( 
edge, and into this groove is fitted a 
screw in the face of the holder B. 

Rockers and Rocker Arms. For ' ^ 
small machines the rocker arm Is usually \V 
clamped upon a shoulder turned upon Fi. es. Reason jype of Brush 
the bearing pedestal as indicated in 

Figs. 66 and 67. For large multipolar generators, the rocker arms, 
that is, the rods on which the brush-holders are held, are fixed at 
equidistant points around a cast-iron rocker ring, which is itself 




.: 



AEMATURE ^WINDING 




Pfp.tt.rt87. Bruah Rocker Arm fo^S^Ma^ne and Methcxi of Mooting Same on 
^ Courtesy of Reliance Electric and Engineering Company 




Kg. 68. Rocker Ring and Bruah Mounting 



ARMATURE WINDING 45 I 

supported on brackets projecting from the magnet yoke/ This f l 

construction is shown in Fig. 68. f 

DEVELOPMENT OF THE E.M.F. EQUATION FOR A J < 

DIRECT-CURRENT GENERATOR \ \ 

Analysis for One Inductor. Fundamentally the e.m.f. induced | 

'in an armature winding depends upon the rate at which the indue- j / 

tors composing the winding are cutting the magnetic lines, and the r t 

number of these inductors that are connected in series. Ixt us J ( 

first consider the value of the e.m.f. induced in a single inductor / * 

as it revolves in the magnetic field or the field revolves with t 

respect to it, as the case may be. If the magnetic lines that enter { 

or leave each of the magnetic poles be represented by the symbol <f>, I 

and the number of poles by the letter p, then each inductor will cut \ l 

pX<f> magnetic lines with each revolution. Now if the inductor ^ ^ 

and the magnetic field make r.p.s. revolutions per second, then } I 

each inductor will cut pX<i>X r.p.s. magnetic lines in one second. I I 

.Total E.M.F. The manner in which the total number of ' f 

inductors is connected will determine the number that are con- f 

nected in series.. The magnetic lines cut by one inductor multi- i 

plied by the number of inductors in series gives the value of j | 

the magnetic flux cut by one path or circuit, and this result I I 

divided by 10 8 will give the value of the average e.m.f. induced in | Jj 

the armature winding. If the total number of inductors on the J f 

armature be represented by N, t and these inductors are so con- j * 

nected that there are. several paths, or circuits, through the arma- 1 1 

ture in passing from the 1 negative terminal of the machine through J * 

the armature winding to the positive terminal, then the number -> 

of inductors in series in any one of these paths or circuits will be | 

equal to. the total number of inductors divided by the number of | -f 

paths or circuits. The number of paths or circuits through an J 

armature winding, usually represented by the letter a, depends | 
upon the kind of winding and the number of poles. This will 
be discussed in detail later. 

The above statements may be combined and all written in ,/ 

the form of an equation as follows: r J 

' I 

\l 



J 4$ ARMATURE WINDING" 

in which E.M.F. is electromotive force induced in the armature 
I winding; N is total number of inductors o'n the armature; a is 

I number of paths or circuits through the armature winding; p is 

i number of poles in field structure; .is magnetic lines per pole; 

/ } r.p.s. is revolutions per second; and 10 s is number of lines that must 

\ be cut per second in order to induce an e.m.f. of one volt* . ' 

'' Examples. 1. An armature winding for a four-pole generator has 188 

; inductors and these inductors are connected in such a manner that there are 

two circuits or paths through the winding from the negative to the positive 
! terminal of the machine. If the armature is rotated at 1050 revolutions per 

minute and there are 167,000 ^magnetic lines per pole, what e.m.f. will be 

induced in the winding? 
i Sdviwn. 

N = 188; a ~2fp^4;" $ = 167,000; and r.p.s. =.1050 * 60 175 

Substituting these values in the equation for the e.m.f. gives, 

. , 188X4X167000X175- 
E.M.F.- 



_21 077 OOP OOP 
2X10* 

109.88 volts 
-' v - - 

2. If the armature in the above example is rotated at a speed of 
1575 r.p.m., what change must be made in the number of magnetic, lines 
per pole in order that the e.m.f. generated in the armature winding will 
remain the same? 

Solution. It is obvious from an inspection of the equation giving the 
value of the e.m.f., that the induced e.m.f. will increase directly as the speed; 
that is, if the speed is doubled the e.m.f. will be doubled, and if the speed 
is reduced to one-half of its original value the e.m.f. will be reduced to one- 
half of its originaf value, assuming all the other quantities in the equation 
remain constant in value. Likewise if the magnetic lines per pole be increased 
or decreased in value there will be a corresponding increase or decrease in 
the value of the induced e.m.f., assuming all other quantities in the equation 
remain constant in value. Now if the speed of the armature of a machine 
is increased there must be a decrease in the value of the magnetic lines per 
pole in order that the e.m.f. may remain constant in value. Thus, if the 
speed is increased to twice its original value, then the magnetic lines per 
pole must be reduced to one-half their original value, etc. 

In this particular case the speed is increased to 1575-M050 or f of 
its original value, so that the magnetic lines must be decreased to f of 
their original value in order that the induced e.m.f. shall remain constant 
in value. 

3. If 'the winding in example 1 above be changed so that there are 
four paths or circuits rather than two, and all other conditions remain the 
same, what will be the value of the induced e.m,f,? 



I 



ARMATURE WINDING^ 47 

Solution. Changing the winding from a two-circuit to a four-circuit 
rwinding decreases the number of inductors in series in any one path to one- 
half of the previous value, assuming, of course, that the total number of 
inductors remains the same. A reduction in the value of the number of 
inductors in series results in a reduction in the value of the induced e.m.f. r 
and the reduction in the value of the induced e.m.f. will be in proportion 
to the reduction in the number of inductors in series. In this particular 
case the number of inductors in series is reduced to. one-half of its previous 
value. 

The above statements are clearly shown by the e.m.f. equation. Since 
'the value of the number of circuits appears in the denominator of the equa- 
tion, an increase in its value will result in a decrease in the induce e.m.f., 
and likewise a decrease in the number of circuits will Jesuit in an increase 
in the e.m.f., assuming all other quantities in the equation remain constant) 
in value. 



fr 

is 




" I 



ARMATURE .WINDING 

PART II ' 



ARMATURE WINDINGS FOR DIRECT CURRENT 

MACHINES Continued 

DESIGN OF WINDINGS 

y Ring and Drum Windings. In designing the armature wind- 
ing for a direct-current generator or motor, the number of arma- 
ture inductors is determined by means of the fundamental e.m.f. 
equation. The real problem is, then, the interconnecting of the 
various inductors in such a way that their individual e.muf.'s will 
add together to produce the required total e.m.f <, and in such a 
manner that the armature winding as a whole will be at all times 
symmetrical with respect to the brushes and magnetic poles. 
Three distinct types of closed-coil armature windings are shown in 
Figs. 69, 70, and 71. These windings are all for a four-pole 
machine. There are 24 inductors in the windings shown m 
Figs. 69 and 70, and 26 inductors in the winding shown in Fig. 71. 

The winding shown in Fig. 69 is a ring winding, while the 
windings shown in Figs. 70 and 71 are drum windings* The 
arrangement of the two drum windings may be made clearer by 
resorting to the use of what are called developed diagrams, as 
shown in Figs. 72 and 73. The developed diagrams might be 
thought of as being derived from Figs. 70 and 71 by rolling out 
the cylindrical surface of the armature core into a plane, and at 
the same time stretching the commutator segments so that they 
remain in the same relative position with respect to the inductors 
composing the winding. 

Winding Element Each of .the windings shown in Figs. 69, 
70, and 71 consist of a number of identical elements, and one of 
these elements is shown in heavy lines in each of these figures. 
An element of an armature winding is that portion of the winding, 
which, beginning at a commutator segment, ends at the next com- 
mutator segment encountered in tracing through the winding. ^ An; 



50 



ARMATURE WINDING 



., 
element in its simplest form consists of a single inductor in the 

case of a ring winding, as shown in Fig. 69, and in the case of a 
lap and wave winding it consists of , -two 'Inductors, as shown in 
Figs. 70 and 71. 

In the case of the ring winding, the number of inductors in an 
element is equal to the number of turns, while in a lap and wave 




Kg 69 Ring-Wound Armature for a Four-Pole Machine 

winding the number of inductors in an element is equal to twice 
the number of turns in the element. 

Small armatures that are designed to operate on relatively 
high voltages may have as many as five turns in an element; but 
in large machines there is, as a rule, only one turn per element so 
as to improve commutation. Keeping the turns in the elements 
low reduces the self-induction, and hence there is less opposition 




ARMATURE WINDING 



51 



'to the reversal of ;the current during commutation. The coeffi- 
cient of selfrinductipn .varies as the square of the number of turns. 
Ring, Lap, and Wave Windings. The three windings shown < 
in Figs. 69, 70, and 71 belong, respectively, to the ring,, lap, and 
wave types of closed-circuit windings. The origin of the terms 




Fig 70 Lap-Wound Drum Armature for a Four-Pole Machine 

lap and wave will be quite evident from an inspection of Figs. 72 
and 73. 

Lap Winding. In tracing through the winding shown in Fig. 72, 
you advance around the armature core in one direction at the 
back end of the armature, the end away from the commutator, 
and around the core in the opposite direction at the front end of 
the armature. In other words, the winding laps back on itself and 
for this very reason it is called a lap winding. 



I M 
)} 

1 



52 



ARMATURE WINDING 



Wave Winding, In tracing through the armature winding 
shown in Fig. 73, you advance around the armature core in the 
same direction at both the- back and front ends of the core in a 
wave fashion, and for this reason the winding is called a wave, 
winding. Lap and wave windings are frequently referred to as I 
parallel and series windings respectively. 




Fig, 71. Wave-Wound Drum Armature for a Four-Pole Machine 

Figs. 72 and 73 show that the two sides of an element are 
under the inductive influence of two adjacent magnetic poles of 
opposite polarity and the e.m.f.'s induced in the two sides of an 
element are in opposite directions across the surface of the arma- 
ture. The e.m.f.'s in the two sides of an element, however, act in 
the same direction around the element for practically all positions 
of the element and the e.m.f. between the terminals of an element 
is equal to the sum of the e.m.f.'s in series composing the element. 



ARMATURE WINDING 




ARMATURE WINDING 



'55 





Fig. 74. Element 
for Lap Winding 
Composed of a 
Single Turn 



In the simple lap winding shown in Fig. 72 the terminals of 
the element are connected to adjacent commutator segments, 
while in the case of the simple wave winding shown 
in Fig. 73 the terminals of an element are con- 
nected tp commutator segments which are approx- 
imately a double-pole pitch apart. The pole pitch 
is the distance between corresponding points on 
adjacent magnetic poles. 

An element for a lap winding, composed of 
a single turn is shown in Fig. 74, and one com- 
posed of two turns is shown in Fig. 75. Two 
elements for a wave winding are shown in Figs. 76 
and 77. 

Tracing Circuits. An examination of the wind- 
ings shown in Figs. 69 and 70 will show that there 
are four circuits through the winding from the 
negative brushes to the positive brushes. The 
two negative brushes are connected together and form one ter- 
minal of the armature winding as a whole, while the two positive 
brushes are connected together and form the other terminal. In 
the -case of a generator the direction of cur- 
rent through the armature winding is from 
the negative terminal to the positive terminal. 

f ' The position of the brushes on the commu- 

I tator is such that they short-circuit the ele- 

I ments, when the resultant e.m.f. induced in 

I the inductors forming the element is a min- 
imum This, however, is not always the case 
on account of commutation requirements, as 
will be explained later The inductors com- 
posing the winding are numbered consecu- 
tively from one up to the total number, in 
this case twenty-four 

The four circuits through the winding 
shown in Fig, 69 may be traced as follows: 
Starting with the negative brush marked 1 
and tracing through inductors 7, 2, 3, 4, 5, and 6 you arrive at the 
positive brush 2. A second circuit may be traced from the negative 





DDDD 



Fig. 75. Element for Lap 

Winding Composed of 

Two Turns 



* 



ARMATURE WINDING 






Hg. 76. Element for Wave Winding Composed of 
a Single Turn 



brush 1 through inductors 24, 23, 22, 21, 20, and 19 to the posi- 
tive brush marked 4- A third circuit may be traced from the 

negative brush marked 8, 
through the inductors 12, 11, 
10, 9, 8, and 7 to the positive 
brush marked 2, and a fourth 
circuit may be traced from 
the negative brush marked S 
through the inductors 13, 14, 
15, 16, 17, and 18 to the pos- 
itive brush marked 4. The 
width of the brushes shown 
in Fig. 69 is equal to the width 
of a commutator segment, and 
the position of the winding in 
the field, as shown in the 
figure, is such that no elements 
are short-circuited by the brushes. Just an instant after that rep- 
reseated in the figure four elements will be short-circuited by the 
brushes. Inpractice the brushes must be made wider than a cpm- 

mutator^segment. 

The four circuits of 
the winding shown in 
Figs. 70 and 72 may be 
traced in a manner similar 
to the above, and the in- 
' ductors composing each cir- 
'cuit determined. Starting 
with the negative brush 
marked 1 you may pass 
through inductors marked 
S, 10, 5. and 12 before you 
arrive at a positive brush, 
]D in this case the positive 
brush marked 2. A second 
circuit may be traced from 
the negative brush marked /, through the inductors 6, 28, 4, and 
21 to the positive brush marked 4. A third circuit may be traced 





Fig.. 77. Element for Wave Winding Composed of 
Two Turns 



.ARMATURE WINDING ? \ \ 

from the negative brush marked 3 through inductors marked" IB, I ' 

11, 16, and 9 to the positive brush marked 2. And a fourth cir- 1 ' 

cuit may be traced from the negative brush marked 3 through the 
inductors, 15, 22, 17, and 2J. to the positive brush marked 4. 

The position of. the winding in Figs. 70 and 72 is such that 
four elements are short-circuited by the brushes, since each brush 
is resting on two commutator segments. Brush 1 is short-circuiting 
an element composed of inductors 1 and B. Brush 2 is short- 
circuiting an element composed of inductors 7 and 14. Brush 3 is ',< s 

short-circuiting an element composed of inductors 18 and 20, and '< 

brush 4 is short-circuiting an element composed of inductors 19 ;/ j 

and 2. The inductors that are short-circuited by the brushes will i \ 

change as the armature^ rotates, as can be seen by an examination ' 4 ' 

of Figs. 70 and 72. ! \ \ 

The circuits through the winding shown in Figs. 71 and 73 j r f 

may be traced as follows: Starting with the negative brush marked 
Jf and tracing through inductors 1 and 8 you arrive at a commu- 
tator segment which is under a negative brush, so that the ele- ( * 
iment composed of inductors 1 and 8 is short-circuited by the two I ,< 
negative brushes marked 1. and 3, which are connected together. ( '} 
Tracing on through the winding from the negative brush marked ,^ I 
S, you pass through inductors 15 and 22, which are short-circuited ', I 
by brushes 1 and 3, and then through inductors 3, 10, 17, 24, 5, ft j 
12, 19, and 26, and you arrive at a segment under the positive 
brush marked 2, thus completing one circuit through the winding. 
Continuing from brush 2, you pass through inductors 7 and 14 
which are short-circuited by the two positive brushes, marked 2 
and 4, then through inductors 21 and 2, which are short-circuited 
by the two positive brushes marked 2 and 4, then through induc- 
tors 9, 16, 23, 4, 11, 18, 25, and 6 to the negative brush marked S, 
thus completing a second circuit through the winding. Tracing on 
through the winding from the negative brush marked 8 you pass 
through inductors 13 and 20, which are short-circuited by the two 
negative brushes, marked 1 and 3 back to the starting point, or 
segment number 1 to which inductor number 1 is connected. 

Brushes Required for Lap Winding. Since the current in an 
element must undergo commutation each time the element passes 
through the neutral zone of the magnetic field, that is .the position in 



58 ARMATURE WINDING 



1 which the resultant induced e.m";f . is zero, it follows that the element 

I may be short-circuited by a brush or brushes at each such position, 

j and not interfere to any great extent with the e.m.f. between the 

, terminals of the machine. The number of neutral zones each ele- 

ment passes through in one revolution is eQual to the number of 
magnetic poles in the field structure of the machine, hence the 
i number of permissible brush sets may in all cases be the same as 

I the number of poles. In all ring and lap windings, such as those 

shown in Figs. 69 and 70, it is imperative that as many brush sets 
be provided as there are magnetic poles. Let us assume that one 
of the brushes in Fig. 69 is removed, say the positive brush 
1 ,*r marked 2, and then trace the circuits through the armature 

1 winding. The two circuits on the left-hand side of the armature 

will not be disturbed, that is, the circuits from negative brush 1 to 
} positive brush 4 and from negative brush 8 to positive brush 4> 

Removing positive brush 2 results in two of the circuits being 
connected in series between the two negative brushes, and since 
the induced e.m.f. in the two circuits acts in a direction away 
' from the negative brushes and they are equal in value, it follows 

that the resultant induced e.m.f. in the circuit connecting the two 
, negative brushes on the right-hand side of the armature winding 

is zero. The removal of, the positive brush results in one-half of 
( the armature winding being inoperative, so far as the external cir- 

I euit to which the armatu're is connected } is concerned. The 

I ' removal of any one of the brushes will result in the same condi- 

I tions of affairs for a four-pole machine. 

1 The same results will be obtained if any one of the brushes in 

Figs* 70 and 72 be removed. In general, the removal of any one 
I of the brushes from the commutator of a lap- or ring-wound 
*j I armature results in all the inductors under one pair of poles 

f becoming inoperative so far as the external .circuit is concerned. 

f Thus, in a six-pole machine if one of the brashes is removed from 

tlie commutator, one-third of the inductors become inoperative, in 
an eight-pole machine one-fourth of the inductors become inopera- 
tive when one of the brushes is removed, etc. The removal of the 
single brush does not prevent the armature from operating; but it 
i, reduces the current capacity of the armature, because the number 
of circuits in parallel through the winding is reduced. 



ARMATURE WINDING 59 

If two of the brushes, say brushes marked 2 and 3 irr Fig. 69, 
be removed, the armature will still operate but not up to its full 
capacity, as explained above. In this case there will be one cir- 
cuit from the negative brush marked / to the positive brush 
marked 4> and a second circuit from the negative brush / around 
the right-hand side of the armature to the positive brush 4. The 
circuit around the right-hand side of the armature is composed of 
the inductors under three different poles while the circuit direct 
.from brush / to brush 4 is composed of the inductors under only 
one magnetic pole. The e.m.f. in the circuit on the right-hand 
side is a combination of three e.m.f. 's and each of these is the sum 
of the e.mJ.'s induced in the inductors under each of the three 
magnetic poles. Two of these e.m.f. 's oppose each other and the 
third acts in a direction from the negative brush / to the positive 
brush 4t so that the resultant e.m.f. in the right-hand circuit is 
numerically the same as the e.m.f. in the circuit direct from brush 

1 to brush 4- 

Resistance of Circuits. The resistances of the two circuits 
from brush / to brush 4 we not the same, as the one on the right- 
hand side is composed of three times as many inductors as the one 
on the left-hand side^ As a result of this difference in the resist- 
ance of the two circuits in parallel between brush / and 4, the 
two circuits will not carry the same current when the armature is 
delivering current to the external circuit; but the left-hand circuit 
will carry approximately three times as much current as the right- 
hand circuit, because its resistance is approximately one-third as 
great. 

Brushes for Wave Winding. In a wave winding, although as 
many brushes as there are poles may be used, two brushes will be 
sufficient irrespective of the number of poles. Thus in Fig. 71 
any one of the positive brushes may be removed if a negative 
brush is removed at the same time. Assuming that brushes 

2 and 3 are removed then brushes / and 4 will continue to operate 
and there will be no change in the number of circuits through the 
armature winding. A careful inspection of Fig. 73 will make it 
clear why two brushes instead of four are sufficient to collect the 
cwrent; for it will be observed that the^pesMve brushes / and 3 
are connected not only by the external conductor but also by 



60 ARMATURE WINDING 

elements which are in or near the neutral zone, and they are, 
therefore, equivalent to' additional conductors joining the positive 
brushed; hence the heavy external connection and one of the posi- 
tive brushes may be omitted. Likewise, the external connection 
between the negative brushes and also one of the negative brushes 
may be omitted. This relation holds for any number of poles, and all 
the brushes may be removed except one positive and one negative 
and the armature will continue to operate with the same number 
of circuits through the winding, 

When two of the brushes in Figs. 71 and 73 are removed, the 
remaining brushes short-circuit two of the elements in series. If 
the machine were six-pole and all the brushes were removed except 
two, one positive and one negative, each brush would short- 
circuit three elements in series. The short-circuiting of several 
elements in series results in poorer commutation than is obtained 
when each element is short-circuited by itself, which is the case 
when there are as many brushes as there are poles. 

Hie current-carrying capacity of two sets of brushes is not 
always sufficient to take care of the total capacity of the machine, 
and in such cases it is necessary to use more brushes. 

Position of the Brushes. It is interesting to note that the posi- 
tion of the brushes in the case of the ring winding is midway between 
the poles, and in the case of the lap- and wave-wound drum wind- 
ings the position of the brushes is in the center of the poles. 
With the brushes in these positions on the commutator, the 
length and form of the two end-connections of each element is 
approximately the same. The position of the brushes, however, 
can be changed by changing the relative position of the commuta- 
tor segments and the elements, when the armature is being wound, 
or by reconnecting the elements to the segments. Good commu- 
tation will require the brushes to be moved slightly in the direc- 
tion- of rotation in the case of a generator and in the opposite 
direction in the case of a motor from the position they occupy in 
Figs. 69, 70, and 71. 

Distribution of Lap and Wave Windings. In the case of the 
lap winding, all the inductors in each of the several circuits are 
under the influence of the same two magnetic poles, while in the 
case of the wave winding the inductors in the different circuits are 



II 



13 



Fi. 78, 



or a Four-Pole Ma&Mo 



60 

'i 

I , elements wh 

j. , therefore, ec 

J f i brushes; hen 

j tive brushes 

; ; , between the 

I may be omit 

i i the brushes 

f i and the am 

of circuits tl 

When t 

remaining b 

the machine 

two, one 

circuit thre 

elements in 

when each 

when there 

The ct 

>' always sufB 

j and in such 

Positio 

tion of the 1 

the poles, | - 

ings the p 

With the 

length and 

approximat 

can be cha 

j tor segmen 

i or by reca 

I tation will 

5 tioir of rot. 

direction h 

Figs, 69, 7* 

Distrii 

lap windin 

under the 

case of th 



i I III 



ARMATURE WINDING 



61 



distributed around the armature under all of the magnetic poles. 
Any variation in the magnetic flux under the different magnetic 
poles will result in the electromotive force in the different circuits 
in the case of a lap winding being unequal, while in the. wave 
winding all of the different circuits will have the same e.m.f. 
induced in them, since they are all affected alike by the unequal 
flux distribution. The unequal e.m.f.'s in the different circuits 
that are connected in parallel will result in there being a current 
through the winding, even though the armature may not be sup- 
plying any current to the external circuit, ^g-presence^ ol thes 4 e 
curj:ents_re^ults^j armature winding 

and hence a reduction in the efficiency of the machines. 

Current and Voltage Relations for Lap and Wave Windings. 
The number of inductors in series in each -of the circuits in the 
case of the wave winding, as shown in Figs. 71 and 73,, is greater 
than the number of indictors in series in each of the circuits in 
the case of the lap winding shown in Figs. 70 and 72, and as a 
result the e.m.f. induced in each circuit of the wave winding will 
be approximately twice as great as is induced in each circuit of the 
lap winding, since the number of inductors in series in each circuit 
of the wave winding in Fig. 73 is approximately twice as great as 
the number of inductors in series in each circuit of the lap wind- 
ing in Fig. 72. Wave windings^as J/ruJe jire used for niachines of 
relatively high yoftaginanTlap windings Jor machines of ^com- 
paratively lowj/oltage.. The current capacity of the lap winding 
sHown in Fig. 70 is just twice as great as : the. ..currfinf^capacliyi)! 
the wave winding shown in Fig. 71, assuming the same size wire 
is used in each of the windings. Since the power output of the 
armature in watts is equal to the product of the voltage and cur- 
rent, it follows that the power the two armature windings shown 
in Figs. 70 and 71 are capable of delivering will be approximately 
the same. 

Wave-Wound Ring Armature. A wave-wound ring armature 
for a four-pole machine is shown in Fig. 78, and a development of 
the winding is shown in Fig. 79. The winding is composed of 15 
elements and 15 commutator segments, and four brushes are shown. 
Elements 1 and 8 are short-circuited by the positive brushes, for 
the position of the armature as shown in the figure. The positive 



? 

* t 

:>, r 




ARMATURE WINDING 




II ! 



ARMATURE WINDING 63 

brushes are the ones on the right- and left-hand side of the com- 
mutator as shown in Fig. 78. Elements 5 and lg are short-circuited 
by the negative brushes just at the instant represented in the figures 
Simplex and Multiplex Windings. Two bipolar ring-wound 
armatures are shown connected in parallel in Fig. 80, and the 
combined current output of these two armatures, assuming they 
are identical, is equal to twice the capacity of either machine 
alone. Exactly the same results may be obtained by placing the 




Fig. .81. Duplex Armature Winding Composed of Two Independent Windings 



two windings on a single armature core, Fig. 81. Here the two 
windings are electrically independent of each other, except for the 
connection between the segments thajfe is made by the brushes. 
The commutator segments of the two windings are ''sandwiched/ 1 
or imbricated, and there are just twice as many segments in the 
commutator shown in this figure as there are in each of the ctfm- 
mutators shown in Fig. 80. A winding of the form shown in 
Fig. 81 is called a duplex winding as distinguished from each of 



fcf 

f? 

[i V 



I < 



64 ARMATURE WINDING 

the simplex windings shown in Fig. 80. This multiplication of 
simplex windings may be used in forming what are called triplex, 
quadruplex, etc., windings. In general, windings of this kind are 
spoken of as multiplex windings. 

Drum windings of both the lap and wave types may be treated 
in exactly the same manner as the ring windings described above. 
It must be borne in mind that the brushes in the case of a mul- 
, tiplex winding must be broad enough to bridge sufficient seg- 




Kg. 82. Duplex, Singly Re-entrant Ring Armature for a Two-Pole Machine 

ments so that the brushes are always' in electrical contact with 
each, of the several windings. 

Singly Re-entrant Winding. Suppose that one of the elements 
and one of the commutator segments shown in Fig. 81 is omitted 
and that the retraining* 23 elements are equally spaced around the 
armature and connected to- the commutator segments as shown in 
Fig. 82. In Fig. 81 there are two independent windings, but in 
Fig. 82 all the various elements- are interconnected. An inspection 



66 



ARMATURE WINDING 



with inductor 2. A winding of this kind, in which there are two 
independent closures, is said to be doubly re-entrant. 

Trebly Re-entrant Winding. A triplex ring armature for a 
two-pole machine is shown in Fig. 83. The twenty-four inductors 
are connected in such a manner that there are six circuits through 
the armature winding between the brushes. The winding is trebly 
re-entrant because there are three independent closures. 




Fig. 84. Triplex, Singly Re-entrant Ring Armature for s, Two-Pole Machine 

Triplex Winding, Singly Re-entrant. The winding shown in 

Fig. 84 is'of the triplex singly re-entrant type. It has one element 

-less and one commutator segment less. than the winding shown 

? in Fig. 83. It is triplex because there are three times as many 

I circuits through it as there are in the case of a simplex winding, 

I and it is singly re-entrant because there is only one closure. 

It is thus seen that the multiplicity and re-entrancy of a wind- 
ing can be carried to any extent that is desired, but it is very 
seldom that the multiplicity or re-entrancy exceeds three. 



ARMATURE WINDING- 



67 



In the above discussion, a ring type of winding was used on 
account of its simplicity, and it should be borne in mind that -all 
the conclusions apply equally well to lap and wave windings, as 
will be pointed out in the various examples of armature windings. 

* General Design Considerations 

Winding Pitch. It will be observed in Fig. 72 that the back 
end of half-element number 1 is connected to the back end of half- 
element number 8, and the front end of half-element number 8 is 
connected to the front end of half-element number 3. The dif- 
ference in the numbers of the half-elements is called the winding 
pitch. When this difference is taken at the back end of the arma- 
ture it is called the back winding pitch; it is represented by Yi. 
When the difference is taken at the "front end of the armature it 
is called the front winding pitch; it is represented by Fa. In 
tracing through any armature winding diagram it is customary to 
consider the clockwise direction around a circuit or element as 
positive. In Fig. 72 the back pitch is 81 or 4- 7, and the front 
pitch is 38 or 5. In Fig. 73 the back winding pitch is 8 1 
or 4- 7, and the front winding pitch is 158 or 4- 7. 

Commutator .Pitch. The numerical difference in the numbers 
of the commutator segments to which an element is connected is 
called the commutator pitch and is represented by the letter Y. 
Thus in Fig. 72 the terminals of the elements are. connected to 
adjacent segments and hence the commutator pitch is 1; while in 
Figs. 82 and 84, the commutator pitches are 2 and 3 respectively. 
In Fig. 73 the terminals of an element are under different brushes 
and the commutator pitch is equal to 7. 

Slot Pitch. In the case of slotted armature cores, the dif- w 
ference in the numbers of the slots, assuming the slots are numbered 
consecutively, in which the two sides of an element are placed isj 
called the slot pitch of the winding or the throw of the coil inj 
terms of slots. 

Progressive and Retrogressive Windings. Right-handed wind- 
ings are called progressive windings and left-handed windings are 
called retrogressive windings. A lap winding is right-handed if 
Yi is greater than Fa, and left-handed if Y^ is greater than Fi. 
In other words, if you face the commutator end of an armature, 



' i 

'' I 

r 

K ii 

6.' * 




'68 



ARMATURE WINDING 



the winding is right-handed if you advance around the commuta- 
tor in a clockwise direction as you trace through the elements in a 
clockwise direction. The winding shown in Fig. 72 is right- 
handed. In the case of a wave winding, it is right- or left- 
handed according to whether you arrive at a segment to the right 
or left of the one from which you started after having traced 
through p~2 elements of the winding in a clockwise direction. 
The winding shown in Fig. 73 is right-handed. 

Resultant Advance or Retreat of a Winding. The algebraic 
sum of the front and back pitches of a winding is a measure of 
the resultant advance or retreat per element in tracing through the 




Fig. 85. Element Having Four Active Sides 



winding, and it is usually expressed in terms of the commutator 
pitch 7. In the case of a lap winding, 



and in a wave winding 

YI+ Y =2 F 

In the above equations the factor 2 is introduced because each 
element i. supposed to have two sides. In certain cases there are 
more than two sides to an element, as shown in Figs. 85 and 86 
and in such cases the algebraic sum of the front and back pitches 
for each element will have to be taken. 

- HeM Displacement By field displacement is meant the 
amount that the two segments which form the terminals of an 



* ARMATURE WINDING 69 

^element lack m occupying exactly the same position with respect 
:to the polar axes. Field displacement is measured in commutator 
segments and represented by the letter m. In all lap windings: 
m= y, thus in Fig. 72, m F=l, and again, in Fig. 81, ?&= F = 2. 
In wave windings, such as the one shown in Fig. 73, the terminals 
of an element are separated by a space approximately equal to a 
double-pole pitch. The double-pole pitch that is, the distance 
between corresponding positions on adjacent poles of like polarity, 
or points around the field structure that are 360 electrical degrees 
apart may be expressed in terms of commutator segments b|f 
dividing the total number of segments S by the pairs of poles. 
Thus 

double-pole pitch 




Fig. 86> Element Having Four Active Sides 

The distance between the terminals of an element measured in, 

commutator segments is equal to the commutator pitch F, so that 

25 tr 



or 



JMJ. 

P 



(3) 



(4) 



The sign of m determines whether the winding is right- or 
sft-handed. If m is positive, the commutator pitch is greater than 
he double-pole pitch and the winding is right-handed, while if 
i is negative the commutator pitch is less than the double-pole 
itch and the winding is left-handed. 







70 ARMATURE WINDING 

In lap and ring windings, in is always a whole number, while 
in wave windings m may be a fraction. Thus in Fig. 73 m is 
equal to J. 

Relation Between Number of Paths, or Circuits, and Wind- 
ing and Commutator Pitches. One complete circuit will have been 
traced through in any winding when the sum of the field displace- 
ments of the elements traced through from any arbitrarily chosen 
commutator segment is equal to a pole pitch measured in com- 
mutator segments. In the process of tracing through one of the 
circuits a certain number of commutator segments, S' f (not neces- 
sarily an integral number) will be encountered, to each of which 
there corresponds a field displacement m, and the total displace- 
ment is mS'. 

mS'= . (5) 



jp^mp (6) 

Since there are S' segments encountered per circuit through the 
winding, the total number of circuits a must be 



which must be a whole number, therefore 



(7) 



(8) 



General Relations. In the case of a lap winding, equation (1) 
may be rewritten so as to obtain the value of the commutator 

pitch 7, which is equal to ^m and also ==- from equation (9), 

nrn V~ l~i \ """ *2,J . .^ ft 

2 p 

In the case of a wave winding, the value of 7 may be determined 



' ARMATURE WINDING' 71 

by rewriting equation (2), which is equal to the value of Y as 
given in equation (4). 



Thus 



Since m=- from equation (9), then equation (10) may be rewritten 

p 

as follows: 

f-SSs 

The only difference in the two expressions for the value of Y as 

* oo 

given in equations (10) and (12) is which corresponds to a 

double-pole pitch and expresses the fact that the terminals of an 
element having two sides are separated by that amount. In the 
case of a winding similar to the one shown in Fig. 86, the term 

9 Q A.Q Q 

would be replaced by . In general the coefficient of rep- 

resents what is called the field step (/)', and it is numerically equal 
to the number of single-pole pitches included between the termi- 
nals of an element. In general, therefore, 



In the case of the lap winding, the terminals of an element occupy 1 
positions in the same field 2one, so that/-o, and in the case of a 
'wave winding/ is usually 2. 

Numbering the Sides of an Element. In the case of a drum 
winding, t^num^ 

-half-elements are numbered consecutively, one-half of them will 
bear even numbers and the remaining half will bear odd numbers. 
In leaving a commutator segment and passing to the back end of 
the armature there must be a return path from the back end of the 
armature to the front end; and the numbering of the sides of 
:the elements may be 'so arranged that the even numbers will con- 
stitute the sides of the elements leading to the back end of the 
armature and the odd numbers will comprise all the sides of the 
elements leading to, the front of the armature. This will result in 



., 

f 



72 



ARMATURE WINDING 



even-numbered sides being connected to odd-numbered sides at 
both ends of the armature, and, therefore, the front and back 
pitches must be odd. The method of carrying out this system of 
numbering is shown in Fig. 87. 

Application of General Equation to Lap or Parallel Windings. 
In the case of a lap winding there are no restrictions upon the 
number of elements, which may be even or odd. Practically all 
commercial windings have only two sides to an element, so that 

1 2 ~ ""^T""* m 

From the above equation, it is readily seen that the front and back 

Q (2) 6) Q Cs> M Cfr < 





LSI] 












i 

6 






Fig. 87. Methods of Numbering the Sides of an Element 

pitches must differ by twice the multiplicity of the winding and, 
in addition, they must be odd, as already explained. 

The values of Yi and 7 2 should not differ too much from the 

value of the pole pitch *-, as otherwise the electromotive forces 

P 

\ induced in the two sides of the elements- will not act together 
I around" the element as effectively as they should. In certain types 
| of windings, called chord or ffactional-pitch windings, the value 
j of the front and back pitches are purposely made larger or smaller 
I than the pole pitch. 



!i 

ARMATURE WINDING 7 * ' l 

f f I 
I 4 

I 



= =1 y I+( -r !)=2r=2 



to the . 



- T - y. it foHows that in an TO -p,ex lap winding the wmmu . 



Y - ^ ** 

~T- (is) 

reduces to y **"*' ^ 2 _ jj^g * , 

2 -7- for the majority of commercial 
wave windings. 



26*2 
_ _. 



The value of Ft anrf V .i, u i. 

2S_26_ * should be app rox i mate l y equal to 

p = T -6|. It is impossible to have a fraction! pitch, and 



74 



ARMATURE WINDING 



" Use of Dummy Coils In complying with the requirements 
' of equation (IS), use must be made quite often of what are called 
dummy elements, or coils. For example, suppose you want to 
place a simplex wave winding on an armature core having 63 slots 
and you want to place four half-elements in each of the slots. 
This means that the total number of half-elements is equal to 
4X63, or 252. Substituting in the general equation for 7: 

y== 2xl26*2 =63i of 

4 

Neither of these values of 7 is possible, since the value must be 
an integer. The nearest values of 2S that will make 7 an 
integer are 250 or 254. The value of 25 = 254 is impossible, since 
the maximum number of half-elements that can be placed on the 
: armature core is 252. Taking 2S-250, it follows that there are 
two half-elements that are not a part of the winding, and these 



5| 7 

el U] 



ftUJT 1 SLOT I -&U0T Id SLOT 17 

Fig. 88. Location of Half-Elements in Slpta 

two half-elements are merely put in two of the slots in which there 
are only three active half-elements to fill up the space and help 
keep the armature in mechanical balance. Therefore, 



'Since Yi and F 2 must be odd, and since the average pitch Y 
.must be approximately equal to 2S*p, then the following pitch 
values are possible: 

^63 (7i=61 (7 lS =63 /Ti-65 
65 

Location of Half^Ekments in Slots. Many other combinations 
in addition to the above may be used. Now before selecting 
-the pitches, let us investigate the location of the different half- 
dements in the slots as shown in section in Fig. 88. The half- 
cdements 1 and 3 in the top of slot 1 should be connected to two 



(71=63(^=61 (7^65 (7 
\7*=63\7 2 =65 (7 2 =61 17 2 



ARMATURE WINDING 75: 

half-elements in the bottom of the same slot, such as half-elements 
'62 and 64i respectively, in slot 16, or half-elements 66 and 68,. 
respectively, in slot 17. In connecting to half-elements 62 and 64 a 
back pitch of 61 will be required, and in connecting to half-elements 
66 and 68 a back pitch of 65 will be required. Connecting the half- 1 
.elements in this manner will permit two of the elements being; 
taped and insulated complete as a unit and then installed on the 
armature core. If a back pitch of 61 is used, the front pitch 
should be 65; and if a back pitch of 65 is used, the front pitch 
should be 61. 

Field Displacement. The field displacement in a wave wind- 
ing is equal to , so that after tracing through - elements, which' 

corresponds to going around the surface of the armature core once,, 
the total displacement is 

-TpX =~- commutator segments 

Hence for a simplex wave winding, a=*2, the end of "the - element 

2 

from the starting point connects to a segment adjacent to the 
starting segment. The final segment may be to the right or left 
of the starting segment according to whether the winding is right-* 
or left-handed. In tracing through a duplex wave winding you 
terminate at a segment two removed from the starting segment, 

to the right or left, after passing through -|- elements of the 

winding. 

Pitch Values. Wave windings are frequently wound .so that 
there are more than two circuits between brushes. For, example, 
the armature core having 63 slots and 4 half-elements per slot may 
be wound with a four-circuit wave winding. Substituting in the \ | 

general equation for Y . I ,t 



Since yi arid Y* must be odd, the following combinations may' 
be used: 

y!=65 



{ 



7G ARMATURE WINDING 

Other combinations for Fi and F 2 may of course be used, and the 
selection of the pitches will depend upon the arrangement of the 
inductors in the slots, as explained above. y^ 

Method of Determining the Reentrancy of a Winding. If. 
both sides of the general equation 



P 
are divisible by a number q, we have 



, 
, P 

This last equation means that the original winding is in reality 
made up of q independent windings, and that there are S' elements 
in each of these windings. The commutator pitch of any one of 
these independent windings is equal to Y f when it is counted with 
respect to the S' segments connected to each of the independent 
windings. 

In general the winding will be multiplex and multiply re- 
* entrant of the gth degree, provided the average of the front and 
back pitches Y and the total number of commutator segments S 
have a common factor q. If the values of Y and S are prime to 
each other, that is, they have no common factor, then the winding 
is singly re-entrant. Thus in a duplex lap winding Y is equal to 
2; and if the value of S is divisible by 2, the winding is doubly 
re-entrant, otherwise it is singly re-entrant. In a triplex lap 
winding Y is equal to 3; and if the value of S is divisible by 3, 
the winding is trebly re-entrant, otherwise it is singly re-entrant. 
In a quadruplex lap winding Y is equal to 4; and if the value of 
5 is divisible by 4, the re-entrancy of the winding is 4. If the 
values of Y and S for a quadruplex winding are not divisible by 
4 but are both divisible by 2; then the winding is doubly re- 
entrant. If the values of Y and S are not divisible by 2 or 4, 
then the winding is singly re-entrant. 



ARMATURE WINDING 



77 



Duplex Windings. In all ordinary duplex wave windings the 
value of the field displacement / is 2, and 

^25*4^2(5*2) 

P P 

In the. above -equation if 7 is an even number, that is, if it is 

Q .fc. Q 

divisible by 2, the value of S must also be even because - must 

P 

be a whole number and p is always an even ' number. Hence a 
duplex wave winding is doubly re-entrant if 7 is even, and singly 
re-entrant if 7 is odd. 

Triplex Windings. In all ordinary triplex wave windings the 
value of the field displacement / is 2 and 

y,,?JL2(5*3) {18) 

P P 

Let us assume that 7 is 'divisible by 3 and it contains 3 a- times, 
then 7=3x. Substituting this value of 7 in equation (18) gives 

3^2(5*3) 

P 
Multiplying by p and dividing by 2 gives 



Dividing by 3 gives 



(19) 



Since the value of the left-hand side .of the above equation is a 
whole number, then the right-hand side must be a whole number, 
or 5 must be divisible by 3, and the winding will be trebly 
re-entrant. Hence a triplex wave winding is trebly re-entrant when 
the value of 7 is divisible by 3, and the winding will be singly 
re-entrant if 7 is not divisible by 3. 

Quadruplex Windings. In the quadruplex wave windings the 
-e-entrancy may be one, two, or four, according to the following 
elations: If 7 is divisible by four, the winding is quadruply 
e-entrant; if the value of 7 is divisible by 2 but not by 4, the 
rinding is doubly re-entrant. If the value of 7 is not divisible 
y 2 or 4, the winding is singly re-entrant. 



78 ARMATURE WINDING 

The best way to obtain a clear understanding of the applica- 
tion of the general equations and relations is to study a number of 
different windings and apply the equations and relations to each of 
them. This will be done for lap and wave windings m the follow- 
ing sections. 




Kg 89 Simplex. Singly Re-entrant, Progressive Lap Winding for a Six-Pole Machine 

N=60 S-30 1 \ = +i 1 Y >*" g 

F-+1 a- 6 f 

Examples of Lap Windings. A six-pole, drum, simplex, singly 
re-entrant, progressive lap winding is shown in Fig. 89, and a 
development of the winding is given in Fig. 90. This winding is 
of the lap type because the front and back pitches lead you around 
the armature in different directions as you trace through the wind- 
ing. Inductor / is connected to inductor 12 at the back end of the 
armature, so that Fi=+H, and inductor 12 is connected to indue- 



ABMATURE WINDING' 



: 



Y, if 



79 



commutator end, so the value of the f ront 




SO ARMATURE WINDING 

winding is progressive because Y, is greater than F* or you 
advance around the armature in a clockwise direction as you 
trace through the elements in a clockwise direction. It will be 
found that there are six circuits through the winding, and six 
brush 'sets will be required, unless the winding or commutator is 
cross-connected. . 

A six-pole, drum, duplex, singly re-entrant, progressive lap 
winding is shown in Fig. 91, and a development of the winding 
is given in Fig. 92. This winding could be changed to a retro- 
gressive type by interchanging the values of the front and' back 
winding pitches. In changing from the progressive to the retro- 
gressive type, the connections of the ends of the elements are 
crossed and the polarity of the armature as a generator and its 
'direction of rotation as a motor, all other -things remaining un- 
changed will be reversed. The winding is duplex because Y = 2 
and it is singly re-entrant because S is not divisible by 2. The 
front and the back .winding pitches differ by two times the 
multiplicity, or 4. . 

A six-pole, drum, duplex, doubly re-entrant, progressive lap 
winding is shown in' Fig. 93, and a development of the winding is 
riven in Fig. 94. Checking by our rule it will be seen that the 
winding is duplex because Y=2 and it is doubly re-entrant because 
S is divisible by 2. It may be changed to a retrogressive type 
by interchanging the values of the front and the back winding 

pitches. . 

A six-pole, drum, triplex, singly re-entrant, progressive lap 
winding is shown in Fig. 95, and a development of the winding is 
given in Fig. 96. The winding is triplex because K = 3. The 
value of 5 is not divisible by 3 .and consequently the winding is 
singly re-entrant. 

A six-pole, drum, triplex, trebly re-entrant; progressive lap 
winding is shown in Fig* 97, .and a development of the winding is 
given in Fig. 98. Again checking by the rile we find that in this 
case the value of S is divisible by 3, hence the winding is trebly 
reentrant. 

Examples of Wave Windings. A six-pole, drum, simplex, 
singly re-entrant, progressive wave winding is shown in Fig. 99, 
and a development of the winding is given in Fig. 10D. The 



" I 

! 



ARMATURE WINDING 8 1 

winding is snnp,ex, because, in starting with any eo mmutator 
segment and after tracing though J elements, you arrive at a 
segment adjacent to the one from which you started. The seg- 
ment you end on, after tracing through f elements in a clockwise 



,_, fnfailtn w. 
a. 2 H 



far . Six .p ole 
K- -HI 



s enc' C gt f the ne ' f ^ioh you 

started, hence the winding is progressive. Had you ended on a 
segment to the left of the one from which you staTt^ aLr 
tracmg through | elements, the winding would have been retro- 
gressive. The winding is singly re-entrant as there is only 
closure and the multiplicity cannot in any case exceed the 




ARMATURE WINDING 




of re-entrancy. There are two 
circuits through the winding, as 
is always the case in a simplex 
wave winding. The value of 
the field step / is 2, since the 
terminals of an element are sep- 
arated by approximately two 
single-pole pitches. 

A six-pole, drum, duplex, 
singly re-entrant, progressive 
wave winding is shown in 
Fig. 101, and a development of 
the winding is given in Fig. 102 
The winding is duplex because 
in starting with any commutator 
segment and tracing through 

~ elements you arrive at a seg- 
ment two removed from the one 
from which you started. The 
terminating segment after trac- 
ing through -y elements is to 

the right of the one from which 
you started, hence the winding 
is progressive. The winding is 
singly re-entrant because Fancl 
S are not divisible by 2 The 
number of circuits through the 
winding is twice the multiplic- 
ity, or 4. The value of the 
field displacement / is 2. 

A six-pole, drum, duplex, 
doubly re-entrant, retrogressive 
wave winding is shown in 
Fig. 103, and a development of 
the winding is given in Fig. 
104. The winding is 



'1 



ARMATURE WINDING 83 

i , ~ f V 

Fe*nteant because after tracing through - elements in a clockwise 
direction you arrive at a segment two removed from the one from 
which you started. It is retrogressive because you terminate at a 
segment to the left of the one from which you started. The wind- 
ing is doubly re-entrant because the values of Y and S are divis- 
ible by 2. There are four circuits through the winding. The 
front and back pitches are both odd, but differ by 2. The values 
of the pitches may be interchanged without changing the type of 
the winding, as in the case of the lap winding, which changes 
from a progressive to a retrogressive, or from a retrogressive to 
a progressive, when the values of the front and back pitches are 
-interchanged, .as explained in the preceding section. 

A six-pole, drum, triplex, singly re-entrant, retrogressive wave 
-winding is shown in Fig. 105, and a development of the winding 
is given in Fig. 106. The winding is triplex because after tracing 
through - elements you arrive at a segment three removed from 

the one from which you started, and it is retrogressive because 
you terminate at a segment to the left of the one from which you 
started. It is singly re-entrant because the values of Y and S 
are not divisible by 3. The number of circuits through the wind- 
ing is equal to twice the multiplicity, or 6. 

A six-pole, drum, triplex, trebly re-entrant, progressive, wave 
winding is shown in Fig. 107, and a development of the winding 
is given in Fig. 108. This winding is trebly re-entrant because 
the values of Y and S are both divisible by 3. 

Reduction of Total Inductors to Elements of a Single Turn. 

In each of the windings shown in Figs. 89 to 108 inclusive, each 

element is represented as being composed of only two inductors 

This is not always the case. So in applying the general equations 

it is necessary first to reduce the total number of inductors to the 

number that would exist on the armature if each element were 

composed of only two inductors, by dividing the total number of 

inductors in the winding by the number of turns in one of the 

'elements. For example, if a six-pole, simplex, singly re-entrant 

jltp winding is composed of 120 inductors and there are only 30 

commutator segments, then each element is compost! of 4 



ARMATURE WINDING 

TABLE II 

Winding Table for Six-Pole, Duplex, Singly Re-entrant 
Progressive Lap Winding, Shown in Fig. 91 



Yt K t Yi Yi Yi K, Yt 




B 


F 


B 


F 


B 


F 






1 


22 


5 


26 


9* 


30 






13 


34 


17 


38 


21 


42 






25 


46 


29 


50 


33 


54 






37 


58 


41 


62 


45 


66 






49 


70 


53 


74 


57 


78 






61 


82 


65 


86 


69 


90 






73 


94 


77 


98 


81 


102 






85 


106 


89 


110 


93 


114 






97 


118 


101 


4 


105 


8 






109 


12 


113 


16 


117 


20 






3 


24 


7 


28 


11 


32 






15 


36 


19 


40 


23 


44 






27 


48 


31 


52 


35 


56 






39 


60 


43 


64 


47 


68 






51 


72 


55 


76 


59 


80 






63 


84 


67 


88 


71 


92 






75 


96 


79 


100 


83 


104 






87 


108 


91 


112 


Oo 


116 






99 


2 


103 


6 


107 


10 






111 ' 


14 


115 


18 


1 







inductors, or two turns. In this case the number of inductors 
must be divided by 2 in order to get the value of the number of 
half-elements. 

Winding Tables for Armature Windings. All the electrical con- 
nections in an armature winding may be readily indicated by means 
af a winding table as shown on pages 84, 85, and 86. Thus in 



ARMATURE WINDING 

TABLE II! 

Winding Table for Six-Pole, Duplex, Doubly Re-entrant 
Retrogressive Wave Winding, Shown in Fig. 103 



85 



y* FI YI FI r* y, r 


f 


B 


F 


B 


F 


B 


F 






1 


22 


41 


62 


87 


102 






121 


18 


37 


58 


77 


98 






117 


14 


33 


54 


73 


94 






113 


10 


29 


50 


69 


90 






109 


6 


25 


46 


65 


86 






105 


2 


21 


42 


61 


82 






101 


122 


17 


38 


57 


78 






97 


118 


13 


34 


53 


74 






93 


114 


9 


30 


49 


70 






89 


110 


5 


26 


45 


66 






85 


106 


1 
















3 


24 


43 


64 






83 


104 


123 


20 


39 


60 






77 


100 


119 


16 


35 


56 






73 


96 


115 


12 


31 


52 






69 


92 


111 


8 


27 


48 






65 


88 


107 


4 . 


23 


44 






61 


84 


103 


124 


19 


40 






57 


80 


99 


120 


15 


36 






53 


76 


95 


116 


11 


32 






49 


72 


91 


112 


7 


28 






45 


68 


87 


108 


3 







Table II, which corresponds to the winding shown in Fig. 91, we 
find the following connections. Starting with inductor Jf you pass 
to the back of the armature, which is indicated in the table by 



ARMATURE WINDING 

TABLE IV 

Winding Table for Six-Pole. Triplex, Trebly Re-entrant 
Progressive Wave Winding, Shown in Fig. 107 



Y 


, y 


i y 


* y 


, y 


, y 


i 1 






B 


F 


B 


F 


B 


F 






i 


22 


7 


28 


13 


34 






19 


40 


25 


46 


31 


52 






37 


58 


43 


64 


49 


70 






55 


76 


61 


82 


67 


88 






73 


9-1 


79 


100 


85 


106 






91 


112 


97 


118 


103 


4 






109 


10 


115 


16 


1 
















3 


24 






9 


30 


15 


36 


21 


42 






27 


48 


33 


54 


39 


60 






45 


.66 


51 


72 


57 


78 






63 


84 


. 69 


90 


75 


96 






81 


102 


87 


108 


93 


114 






199 


120 


105 


6 


111 


12 






117 


18 


3 
















5 


26 


11 


32 






17 


38 


23 


44 


29 


50 






35 


56 


41 


62 


47 


68 






53 


74 


59 


80 


65 


86 






71 


92 


77 


98 I 


83 


104 






89 


110 


95 


116 


101 


2 






107 


8 


113 


24 


119 


20 






5 









- 


_. 





the letter B, at the head of the column; then you advance by an 
lamount equal to the back pitch Y it to inductor & and then to 



ARMATURE WINDING 



87 



the front end of the armature. From the front end of inductor 
.22 you drop back by an amount equal to Y z to inductor 5, etc. 
I All the inductors composing the winding in Fig. 91 are passed 
through before you return to the starting point. 

The winding shown in Fig. 103 closes twice, see Table III. 

The winding shown in Fig. 107 closes three times, see Table IV. 

Time of Commutation for Lap and Wave Windings. The 
time of commutation of an element of an armature winding is 
equal to the time in seconds that the element is short-circuited by 
the brushes, the latter forming an electrical connection between 
the segments of the commutator to which 
the element is connected. 

Simplex Lap Winding. In the case 
of a simplex lap winding the terminals 
of an element are connected to adjacent 
commutator segments as shown diagram- 
matically in Fig. 109. The two segments 
/ and 2 both will be in contact with the 
brush while the surface of the commu- 
tator is moving a distance equal to the 
width of the brush minus the thickness 
of the insulation between the segments. 
The time of commutation is given by 
the* following equation : 




(20) 



Fig. 100. Element of Simplex Lap 

Winding Short-Circuited by 

Brush 



in which t c is time of commutation in seconds; W b is width of 
brush in inches; W, is width of insulation in inches; and K c is 
velocity of the commutator surface in inches per second. 

Duplex Lap Winding. In the case of a duplex lap winding 
the terminals of an clem&it are connected to segments which are* 
not adjacent but are separated by one intervening segment, as 
shown diagrammatically in Fig. 110. Segments / and 3 are both 
in contact with the brush while the commutator is traveling a 
distance equal to the width of the brush minus the width of a 
segment plus twice the thickness of the insulation between the 
segments. The time of commutation will be given by the follow- 
ing equation: 



ARMATURE WINDING 



I 
> 



V* 



(21) 



in which W 8 stands for the width of a segment and the remaining 
symbols have the same meaning as given above. 

Triplex Lap Winding. The time of commutation for a triplex 
lap winding is given by the following equation: 



(22) 



ir Wi ^/ 



HPow? Windings. In the case of a wave winding the terminals 

of an element are connected to commutator segments which are 

/ 7 approximately 300 electrical degrees 

/ hRU^H / * 

apart. I he amount by which the 
distance between the terminals of an 
-J clement, in -the case of a wave wind- 
ing, differs from 3(iO electrical degrees, 
measured in commutator segments, is 
called the field displacement. Field 
displacement is represented by the 
letter m, and its numerical value foi 
the different kinds of wave winding; 
may be determined as follows: In { 
simplex wave winding JH is equal t< 
1 divided by the number of pairs o 
poles; in a duplex wave winding m i 
equal to 2 divided by the number of pairs of poles, etc. In genera 
7u is equal to the multiplicity of the winding divided by the pair 
of poles. The value of m is in reality measured in a unit whic 
represents the width of one segment plus the thickness of th 
insulation between segments. In general the time of commutatio 
for a wave winding is given by the following equation: 




Fig. 110. Element of Duplex Lap 

Winding Short-Circuited by 

Brush 



/ = 



I'c 



(23) 



The above equation assumes there are as many brushes t 
there are poles.. If some of the brushes have fyeen removed, th 
m in the above equation must be multiplied by the number < 
double-pole pitches in the particular region from which the brush* 



V 



ARMATURE WINDING 



89 



have been removed. An element of a wave winding is shown in 
Fig. Ill and it is short-circuited by two brushes and the outside 
connection between the brushes. 

Equipotential Connections. It sometimes happens that the 
e.m.f.'s induced in the different paths of an armature winding are 
not all equal and these unequal e.m.f.'s, due to the low resistance 
of the armature, may give rise to large internal equalizing currents, 
and excessive heating of the winding and sparking at the brushes 
may result unless some preventive measures are employed. The 




Kg. 111. Element of a Wave Winding Short-Circuited by Two 
Brushes and Outside Connection 

following ,are some of the causes of unequal e.m.f.'s in the 
different paths: 

1. The armature may not be exactly centered with respect to the pole 
shoes owing to irregularities in construction or wear of the bearings. The 
above condition results in lack of uniformity in the air gap, causing some of 
the poles to carry more magnetic flux than others; thus the inductors under 
their influence have a greater e.m.f. induced in them than i induced in the 
inductors under the weaker poles. The effect of unequal e.m.f.'s is most 
noticeable in lap and ring armatures, as all of the inductors in any one path 
are under one particular pole. In a wave winding the inductors in the 
different circuits are distributed under the different poles, and all circuits 
are affected alike. 

2. The poles may not be identical in construction, even though the 
air gaps are uniform. Thus the joints between the cores and the yoke, or 
between the shoes and the cores, may not all be equally good, or the mag- 
netizing effect of the field coils may differ owing to a difference in turns or 
current, as when the coils are connected in parallel. 



90 



ARMATURE WINDING 



3. The armature circuits may be unsymmetrical because the number 
of inductors is not a multiple of the number of paths. 

The equalizing currents are- a source of loss; to minimize them the 
greatest possible degree of magnetic and electrical symmetry should be 
secured. To overcome the remaining difficulties, such as sparking at the 
brushes, use is made of equipotential connections (low-resistance conductors 
joining 'points iri the winding), which, under ideal conditions, would at all. 
times have the same potential. When an unbalance occurs, currents will 




Fig. 112. Equalizer Connections on a Large Generator 
Courtesy of the General Electric Company 



flow through, these connections, relieving the brushes of the extra current 
and thus reducing sparking. The equalizer connections on a large generator 
are shown in' Fig. 112. These connections are usually made at the bade; 
end of the armature. 



MOUNTING ARMATURE WINDINGS 

The different methods of armature winding have already beeri 
treated theoretically; it now remains to consider the mechanical 
arrangements or means employed to carry out the scheme of 
winding adopted. 



ARMATURE WINDING 01 

Drum Windings. Drum windings may be subdivided into 
hand windings, evolute windings, barrel windings, bastard drum 
windings, and form windings, according to the manner in which 
the end-connections are made. It is essential that these latter be 
good conductors, well insulated from each other to facilitate repairs 
and ventilation, and mechanically sound. 

Hand Windings. Hand windings, historically the first, are. 
now seldom used, except for special machines. They involve a 
clumsy overlapping of wires at the ends of the armature, and this 
stops ventilation and hinders repairs; moreover, the outer layers, 
overlying those first wound, bring into close proximity inductors 
of widely different voltages. 

Ewlute Windings. Evolute 
windings, so named from the 
method of uniting the conduc- 
tors by means of spiral end- 
connectors, were quite early de- 
vised to overcome the objections 
to the hand windings. In Fig. 
113, which illustrates this con- 
struction, each inductor in the 
form of a bar is connected to the 
next by means of two evolute 

spiral copper strips, one bending ^ 113 . Evolute Armature winding 
inwardly, the other outwardly, 

their junction being in some cases secured to a block of wood upon 
the shaft. Their outer ends are attached to the bars by rivets or 
silver solder. 

A common form of such end-connector is shown in Fig. 114, 
another form is made of copper strip, folded. In place of the 
wooden block referred to above, the middle part of the evolute 
connector may be anchored to an insulated clamping device built 
up like a commutator and called from this resemblance a false 
commutator. 

In evolute windings the spiral connectors lie in two planes, 
back of one another; hence it is necessary that the armature bars 
should project to different distances beyond the core body, the 
shorter ones being joined to the inner layer of evolutes, the longer 




92 ARMATURE WINDING 

ones to the outer layer. For this purpose it is convenient to 
Lange one short and one long bar in a slot side by de, as m 
Fig. 115, or in the finished armature of Fig. 110- 



\~ 






LI 

Fig. 114. Spiral End Connectors for Evolute Winding 

Barrel Windings. Barrel windings were devised by C. E, L. 
Brown in 1892, as an improvement upon the evolute wmdmgs just 
n described, and are the ones most exten- 
sively used today. The method consists 
in arranging the inductors in two layers, 
so that their ends, instead of being car- 
ried down as evolutes, are continued out 
upon an extension of the cylindrical sur- 
- . , face of the armature and are bent, to meet 
** 115 ' tnt Conduct r8 ob i iqu ely in two overlapping layers. This 
scheme, like the evolute winding, is adapted to wave as well 
as to- lap windings. Its only disadvantage lies in the fact that it 




Fig. 116. Bar-Wound Armature 

requires the length of the armature parallel to the shaft to b< 
greater than in the preceding case. Its great advantage lies 11 
the excellent ventilation made possible by the larger cooling surfao 



ARMATURE WINDING 



93 



and the provision for air to enter the interior of the armature at 
the ends. 




Fig. 117. Armature Core and Commutator for 3000-k.w., 300-Volt ' 

Allis-Chalmers Generator 
Courtesy of Allis-Chalmers Company 

A usual method of supporting the extended end-connections 
is to attach to the end of the armature body ventilated brackets, 




& 

Fig. IIS. Element of Lap Winding Formed from a Strip of Copper 



Fig. 119. Element of Wave Winding Formed from a 

as indicated in Fig. 117. A simple way to construct such a wind- 
ing is to take a long bar of copper, and bend it as shown at A, 



94 



ARMATURE WINDING 



Fig. 118. The bar may be opened out as in B, Fig. 118, if the 
winding is to be lap-wound, or as in Fig. 119, if the -winding is 




Fig. 120. Allis-Chalmcra Lap-Wound Armature for 150-k.w., 240-Volt, 

240-r.p.m., Three-Wire Type I Generator 

Courtesy of Allis-Chalmera Company 

to be wave-wound. In Figs. 120 and 121 finished armatures of 
this type are represented. 




Fig. 121. General Electric Wave-Wound Armature for 125-Volt Form 

L.D. Generator 
Courtesy of General Electric Company 

Thus far the windings have been described as formed of cop- 
per bars; but it is also possible to wind either of these types with- 
wire, shaping the coils before placing the wire in the slots. 



AEMATURE WINDING 



95 



Cases also occur where more than two layers of wire are neces- 
.sary, either on account of the high voltage required, or to avoid 
harmful induction. 

Bastard Windings. Bastard drum-windings is the name given 
to that class of armature windings whose end-connections, instead, 




Fig. 122. Armature of Westinghouse Generator, Combination of Bastard 
and Barrel Winding 

of being carried in toward the shaft in evolutes or elongated 
cylindrically, are partly inward and partly cylindrical. This has 
the effect of making shorter that part of the armature parallel to 
the shaft than is the case with the barrel winding. It requires, 




Fig. 123. Evolute Wound 
Armature 



Fig. 124. Barrel-Wound' 
Armature 



Fig. 125. Bastard-Wound 
Armature 



however, special formers, and is applicable only to bar-woun'd 
armatures. To provide adequate ventilation, it is customary to 
use a barrel winding at the commutator end of the armature 
and a bastard winding at the other end, as shown in Fig. 123* 



ku. 



94 ARMATURE WINDING 

Fig. 118. The bar may be opened out as in B, Fig. 118, if the 
winding is to be lap-wound, or as in Fig. 119, if the -winding is 




Fig. 120. Allis-Chalmers Lap-Wound Armature for 150-k.w., 240-Volt, 

240-r.p.m., Three-Wire Type I Generator 

Courtesy of Allia-Chalmera Company 

to be wave-wound. In Figs. 120 and 121 finished armatures of 
this type are represented. 




Fig. 121. General Electrie Wave-Wound Armature for 125-Volt Form 

L.D. Generator 
Courtesy of General Electric Company 

Thus far the windings have been described as formed of cop- 
per bars; but it is also possible to wind either of these types with, 
wire, shaping the coils before placing the wire in the slots. 



ARMATURE WINDING 



95 



Cases also occur where more than two layers of wire are neces- 
,sary, either on account of the high voltage required, or to avoid 
harmful induction. 

Bastard Windings. Bastard drum-windings is the name given 
to that class of armature windings whose end-connections, instead, 




Pig. 122. Armature of Westinghouse Generator, Combination of Bastard 
and Barrel Winding 

of being carried in toward, the shaft in evolutes or elongated 
cylindrically, are partly inward and partly cylindrical. This has 
the effect of making shorter that part of the armature parallel to 
the shaft than is the case with the barrel winding. It requires, 




Kg. 123. Evolute Wound 
Armature 



Fig. 124. Barrel-Wound- 
Armature 



Fig. 125. Bastard-Wound 
Armature 



however, special formers,- and is applicable only to bar-wound 
armatures. To provide adequate ventilation, it is customary to 
use a barrel winding at the commutator end of the armature 
and a bastard winding at the other end, as shown in Fig. -122* 



96 ARMATURE WINDING 

Figs. 123, 124, and 125 show the relation of this scheme to the 
two types previously mentioned. 

Form-Wound Drum Windings. It was early found that hand- 
wound drums were both expensive in labor and unsymmetrical 




Fig. 126. Eickemeyer Form-Wound Coil,.and Same Bent Up 



electrically* Therefore a scheme was developed for arranging the 
winding in coils or formers, and then laying these formed coils in 
their respective places upon the core body. The individual sec- 
tions of the winding are first wound and shaped upon a frame, or 
former (the wire being plain cotton-covered). Each section is 
then separately insulated by a winding of tape, usually half-lapped, 
and is then baked, varnished, and baked again. 

Alioth, according to the patent records, was the first to devise 
this method. He was followed by Eickemeyer, who, in 1888, 




Fig. 127. Eickemeyer Coil on form and Opened Out 

patented a method of winding 'formed coils for evolute windings. 
This method attained almost universal use during the vogue of 
the evolute winding; and the first two stages in the construction 




ARMATURE WINDING 97 

of such a section are illustrated" in Fig. 126; Fig. 127 illustrates 
later type of the former, and Fig. 128 a completed armature wind- 
ing built up of such coils. 
What the Eicke- 
meyer coil accomplishes 
for the evolute winding, 
may be accomplished for 
the barrel winding by use 

p t, , . , J ,, Kg. 128. Eickemeyer Armature Complete 

of "straight-out" form- 
ers. Fig. 129 illustrates a simple former of this type, upon which 
a coil for a wave winding has been wound and then opened out. 
Figs. 123, 124, and 125 illustrate the three principal types of formed 
windings, while Fig. 130 illustrates successive stages in the construc- 
tion of a barrel-wound armature using formed coils. 

Arrangement of Inductors in Slots. Various methods of 
arranging the inductors in the slots have been mentioned. The 
most -frequent plan in large continuous-current generators is that 
of putting them in two layers, either two or more in a slot. Form- 
wound coils lend themselves to the two-layer arrangement, which, 
however, is adaptable for use only 
with parallel-sided slots. Yet by 
grouping the conductors six in a 
slot, or eight in a slot, as in Fig. 131, 
T-shaped teeth- can be employed. 
[t must' be remembered that, owing 
;o the magnetic shielding of the 
;eeth, the conductors are subjected 
tactically to centrifugal force only. 
Jnless the pole-faces are laminated, 
he top breadth of the slots must 
>e kept very narrow, i. e., not wider 
ban 2J times the length of the 
ir gap, because otherwise eddy 

urrents would be generated in the Kg. 129. wcraignwur- rorm-wouaa < ^ * fit 

ole-faces; also,' if straight teeth if * * 

re employed, they must be kept very narrow, otherwise the high 
ux-density required in the teeth for sparkless collection of cur- 
?nt will not be attained. 




98 



ARMATURE WINDING 



All electric and magnetic considerations point to having the 
slots and teeth narrow and numerous; while mechanical con- 
siderations impose a limit upon the minimum width of, teeth. 
Standard practice for parallel-wound armatures has had to choose 
a mean, and it is usual to put two, four, or six inductors or coil 
sides per slot. 

Slot Insulation. The coils must be protected from injury by 
the walls of the slot; hence slot insulation is employed. This may 
consist of empire cloth 7 to 10 mils thick, with fish paper of 5 to 




Kg. 130. -Barrel-Wountf Armature with Winding Partly Completed 
Co'urtesy of AUiit-Chalmers Company 

7 mils either side of it. In the case of high-temperature machines, 
mica tubes are frequently used for slot insulation. In general, 
the classes of armature winding and slot insulation are defined by 
the American Institute of Electrical Engineers as class A, B, or C 
insulation, and the temperature limits of the same are as given 
in Table V. 

Commutator an4 Brush Calculations. Commutators for con T 
tinuous-current machines may be divided into two classes, depend- 



ARMATURE 



.VINE&NG 

t ?Q; Ll 



Permissible Temperature an! 
for Insulation 




*sN 

99 ^ * 

RARY 




Class 


Description of Material 


b 'V' ' ^J^ (3: 

Temperature 
to which the 
Material May 
Be Subjected 


Maximum 
Tejnperature 
Rise 


A 


Cotton, silk, paper, and similar materials,, 
when so treated or impregnated as to in- 
crease the thermal limit, or when perma- 
nently immersed in oilj also enameled 
wire. 1 


105 C. 


65' C. 


B 


Mica, asbestos and other materials capable 
of resisting high temperatures, in which 
any Class A material or binder is used for 
structural purposes only, and may be 
destroyed without impairing 2 the insu- 
lating or mechanical qualities of the in- 
sulation. 


125C. 


85 C. 


C 


Fireproof and refractory materials, such as 
pure mica, porcelain, 'quartz, etc. 


No limits specified 



1 For cotton, silk, paper, and similar materials when neither treated, impregnated, nor 
mmersed in oil, the highest temperature rises shall be 10 C. below the limits fixed for Class A 
n Table V. 

2 The word impairing is here used in the sense of causing any change which would dis- 
[ualify the insulation for continuous service. 

ng upon whether they are for open- or- closed-coil armature wind- 

ngs. In the former, a special case used for arc-lighting generators, 

he commutator has a small number of segments separated by air 

japs, and each covering a con- 

iderable angle. With closed- 

:oil windings ordinarily used 

or direct-current lighting and 

>ower, in which case the terminal 

r oltage is kept comparatively 

onstant, in contradistinction to 

eries arc-lighting machines for 

phich the current is constant 

he commutator is of the original 

'acinotti type, that is, it consists of a considerable number of par- 

llel bars or segments separated by strips of insulation, usually 




Fig. 131. Eight Inductors Grouped in 
a Slot 



A 

r 



100 



ARMATURE WINDING 



TABLE VI 
Voltage and Number of Segments 



For Machines 
Working at 


Average' Volts per 
Segment f 


Average Segments per 
Pole or Circuit 


500 to 650 volts 
200 to 250 volts 
100 to 130 volts 


5 to 12 
3 to 8 
2 to 4 


40 to 150 or more 
25 to 75 
20 to 50 



mica. In both cases the completed commutator presents a cylin- 
drical surface against which the brushes press. 

Number of Segments. The number of segments depends 
upon the number of sections of the winding. Increasing the 
number of commutator segments reduces the tendency to spark 
at the brushes. This increase is limited, however, by the matter 
of cost, and by the fact that the number of sections in a drum- 
wound armature can never exceed one-half the number of inductors, 
while, in a ring-wound armature, the number of sections can never 
be greater than the number of inductors. 

The proper number of segments is, therefore, determined by 
the winding of the armature, which depends upon the voltage and 
output of the machine. If by experience the suitable number of 
average volts per segment e a of the commutator be known, then 
S, the number of segments, may be readily computed from the 
following formula 

S = E+e, 

Experience shows that the values of e a indicated in Table VI may 
be chosen, although the matter is influenced by the current to be 
collected. If the latter be less than 100 amperes, then the value 
of e a may be increased, but in no case should it exceed 15 volts, 

Arnold has given the rule that the number of commutator 
segments must never be less than 0.037 to 0.04 times the product 
of the number of armature inductors into the square root of the 
current carried by one circuit of the armature. This rule is an 
empirical one based on observations with regard to sparking; 
nevertheless it has been found that good machines were built in 
which the constant was slightly less than 0.037. 

The number of segments in the last consideration, is, as a 
matter of factj limited by the reactance voltage or voltage of 



ARMATURE WINDING -101 

self-inductance of the armature coils during commutation, and the 
turns per coil must be such that the reactance voltage in a non- 
interpole machine does not exceed 1.5 volts as a maximum. 

Example. A 1000-kiIowatt generator having 16 paths in 
parallel through its armature produced 500 volts at its terminals. 
The number of armature conductors was 2304. Hence, according 
to Arnold's rule, 5 must not be less than 0.037 X2304\/2000 4-16 = 
956. As a matter of fact, 1152 segments were taken for this 
machine, making the number of segments equal to one-half the 
number of inductors. 

Size of Commutator. The size of the commutator depends 
upon the number of segments, their thickness and the thickness of 
the insulation between them, and the length of the segments 
parallel to the shaft. The diameter is limited by the peripheral 
speed allowable. The length depends upon the amount of current 
to be collected, a density of 40 amperes per square inch being 
as much as should be allowed for the contact area between a 
carbon brush and the bar. Bars are rarely thinner than 0.2 inch 
or with insulation, say 0.25 inch, and the peripheral speed of the 
commutator seldom exceeds 3000 feet per minute; so that by 
keeping within these limits good results may be expected. A 
favorite size for commutator diameters is three-fourths tkat of the 
armature diameter. 

Process of Winding a Small Armature. The complete process 
of winding a small armature is shown in Figs. 132 to 142 inclusive. 
The armature core mounted on the shaft and ready for winding 
is shown in Fig. 132. The support for the winding at the back 
end of the armature is being insulated in Fig. 133 by winding 
tape about it. A number of coils ready to be taped are shown in 
Fig. 134. The coils are being taped by hand in Fig. 135 and by 
means of a machine in Fig. 136. Field coils are being taped by 
hand in Fig. 137. After the coils are taped they are impregnated 
with the insulating compound by dipping them in the compound 
as shown in Fig. 138. The coils are then baked or allowed to 
dry, according to the kind of insulating compound that is used. 
After the coils are completed they are placed on the armature 
core as shown in Fig, 139. The commutator is then placed on 
the shaft and the lower ends of all the coils are bent to the right 




r , . ,> 0^11 Ature Core Ready founding 
J '"'. . / , ring Vo/npany 

<"w-r', ,, ,: '!<. .:-"' 




Rg. 134, Form-Wound Coils Ready to be Taoed 
Courtesy of the Reliance Ehflnc and Engineering Company 



T 



ARMATURE WINDING 



V, 
^ 




JX 

"" 











ff 140 nr,ri +1 , ^v* <*< uunnections. a 

g- HO, and the upper l ayero f connections is placed i 



104 



ARMATURE WINDING 




LL 



Fig. 137 Taping Field Coils 
Courtesy of the Reliance Electric and Engineering Company 




Fig. 138. Dipping Coils in Insulating Compound 
Courtesy of the fte/iance Eltclnc ami Ent)\neenng Company 



ARMATURE WINDING 



105 




Fig. 139. Placing Coils on the Armature Core 
Courtesy of (he Reliance Electric and Engineering Company 




Fig. 140 Placing Insulation over the Lower Layer of End Connection* 
Courtesy of the Reliance Electric and Engineering Company 



ARMATURE WINDING 




Fig 141 Placing "Upper Layer of Connections in Position 
Onnfett of the Reliance Electric and Engineering Company 




Fig. 142. Dipping Completed Winding in Insulating Compound 
Cwrtwv of the Reliance Electric and Engineering Company 



AEMATURE; WINDING 



position, as shown in Fig. 141. The ends of the coils are then all! 
soldered to the commutator risers. Wedges are then driven |n, ! 
the tops of the slots over the winding and binding wires 




fig. 143, Adjustable Press for Forcing the Shaft in and out of the Arma(u*e 
Courtesy of the Reliance Electric and Engineering Company 




Fig. 144. Completed Armature Core with Shaft Removed 
Courtesy of the Reliance Electric, and Engineering Company 

around the ends of the winding. The completed winding is 
dipped in insulating compound, as shown in Fig. 142, and baked 
or allowed to dry. 



108 



4KMATUBE WINDING 




1. Leads and Connectors; 2. Terminal Board; 3. Front Bearing Bracket: 4. Brash Holder 
5 Brush-Holder Stud; 6. Brush-Stud Insulation; 7. Bruah Yoke; 8. Brush-Yoke Dowel; 9 
BaSSte Insulating Washers; 10. Oil Ring; 11. Oil-Hole Cover; 12. Bearing End Cover;, 13 
Bearing; 14. Bearing Dowel; 15. Commutator Metal V Ring; 16. Commutator Insulating Rings 
ITVCommutator Segments; 18. Commutator Sleeve; 19. Commutator Key; 20. Field Coil: 21 
*-- Cofl InsulatiS >asher; 22._Frame;. A JEye Bolt: *. A^t^^Cod.:.^ A.J.r. 



Ot MO. 27); 29. Armature j^na-jriaM) n.ey; o 
33. OU Overflow; 34. Back Bearing Bracket 



ARMATURE WINDING 100 

An adjustable press for removing and forcing the shaft In 
place is shown in Fig. 143. A completed armature with the shaft 
removed is shown in Fig. 144. 

A partial cross-section of a small dynamo is shown In, Fig. 145, 
and the names of the principal parts are given below the figure. 

Suggestions for Dipping and Baking of Armatures. Dipping 
the armatures in varnish and then thoroughly baking fills all 
cracks and pores in the insulation. This greatly reduces the pos^ 
sibility of break-downs which might occur if moisture or other 
conducting material should get into the cracks. Further, it acts 
as an effective bond to prevent vibration of the different parts. 
Dipping and baking of comparatively new armatures is an insur- 
ance against maintenance charges for rewinding, etc. It improves 
the insulation, fills up the pores, keeps a smooth surface on the 
coils and prevents vibration of armature parts. 

Equipment. The following equipment is required: a tank to 
contain the dipping solution; an oven in which to bake; and means 
of handling the apparatus. 

Cleaning. Remove oil and dirt thoroughly with clean, com- 
pressed air. Where oil is excessive use a cloth dampened with 
benzine. To protect the polished surfaces, such as the journal 
and the face of the commutator, tape with friction tape. The journal, 
after dipping, can also be rubbed with a cloth wet with benzine. 

Drying. Heat the apparatus in an oven to 100 C. so that, 
with the delay involved in getting it to -the dipping tank, it will 
be at a temperature of 40 to 60 C. at the time of dipping. 

Dipping. Dip in an oil-proof and moisture-proof baking and 
insulating varnish at approximately the specific gravity listed 
below. If the varnish is too heavy, thin with benzine. 

Specific Gravity of Insulating Varnish. The specific gravities of 
Insulating varnish at the temperature of solution are 0.850 at 15 C.; 
0,846 at 20* C.; 0.843 at 25 C., and 0.840 at 30 C. 

Dip armatures in the varnish in a vertical position, so that 
all windings are totally immersed. Allow them to soak until all 
signs of bubbling cease (20 to 30 minutes). 

If a tank is not available, immerse the armature in a pan of 
varnish, turning the armature at intervals of 20 to 30 minutes. 
The varnish should be deep enough to cover the slots. Turn 



110 ARMATURE WINDING 

until all the coils have been thoroughly soaked. The insulated 
creepage .surface at the end of the commutator should have 
repeated paintings of the varnish. 

For direct-current machines, the field coils should be removed, 
dipped, and baked in the same way as the armatures. 

For alternating-current machines, dip the frame (the brush- 
holders having been removed) in a vertical position, rear end 
down. All windings and connections should be covered. Allow 
it to remain in the varnish until all bubbling ceases. Do not 
'immerse brush-holder, pads, nor arms.. 

Draining. Drain at room temperature until all dripping 
ceases. The apparatus should be placed in such a position that 
pocketing will not occur. 

Baking. Place armatures in vertical position in 'an oven and 
bake at 125 C. for the following time: armatures below a 12-inch 
diameter, 24 hours; armatures of 12- to 30-inch diameter, 36 hours; 
And armatures over a 30-inch diameter, 48 hours. 

If the rotor is baked in a horizontal position, it should be 
given a half turn every 15 to 30 minutes during the first half of 
the baking period, otherwise the varnish will drain toward the 
lower side and throw the armature out of balance. 

Place the frames in vertical position in the oven, pinion end 
down, and bake at 125 C. for the same time as the corresponding 
size armature. 

SOME DON'TS TO BE OBSERVED 

Do not use matches; do not smoke; do not use lighted torches, electric 
hoists or any other device that may produce sparks around the dipping 
tank, as the varnish is inflammable. 

If steam is used for heating, do not permit it to enter the oven, thereby 
giving the apparatus a vapor bath. This is worse than .no dipping. 

Do not permit the temperature to exceed 130 C. 

Do not rush the baking .period. A wet motor is worse than one that 
has not been treated. 

SOME PRECAUTIONS TO BE OBSERVED 

Provide for ventilation of the oven, no matter how small it is made; 
holes near the top and bottom wiU usually provide natural ventilation. 
There should be a complete change of air in the oven once every hour. 

Provide uniform temperature of air in the oven. Place thermometers at 
various heights hi the oven to determine the temperature. 

Turn armatures frequently, if baked horizontally, to prevent unbalancing. 



ARMATURE WINDING 

PART III 



ARMATURE WINDINGS FOR ALTERNATING- 
CURRENT MACHINES 

THEORETICAL CONSIDERATIONS 

Generators and Motors. The armature windings for alternating- 
current generators and motors are essentially alike. For different 
types of machines there may be a difference in the form of the 
slots in the armature core, in which the windings are placed, but 
the same form and arrangement of inductors may be used in every 
case. In the following discussion particular reference will be 'made : 

to alternating-current generators in order to make as clear as. 
possible the e.ml. relations that are involved. 

In the case of alternating-current machines either the arma- 
ture or the magnetic field may be the rotating part of the machine, !' 
but it is usually more convenient to make and to understand .a 
winding diagram representing a winding if it represents a revolving ' > 
armature. The form of the winding may be exactly the same, no - ' 
matter whether the armature is the rotating or stationary part. 
In the case of a revolving armature the inductors are placed in j, 
slots cut in the outside cylindrical surface of the- armature core, \ 
while in the case of a stationary armature the inductors are placed 
in slots cut in the inside cylindrical surface of the stationary 
structure. The rotating part of the machine is called the rotor 
and the stationary part is called the stator. \ 

Relation of E.M.K's in Simple Alternator. A loop of wire, ; 

revolving in a magnetic field, as shown in Fig. 11, constitutes a ; 

simple alternating-current generator, and if the magnetic field is s 

uniform in strength the e.m.f. induced in the loop may be repre- \ 

sented by a sine curve as shown in Fig. 10. If an alternating- \ 

current voltmeter be connected to the terminals of this loop, it j 

will indicate the value of the effective e.m.f. induced in the loop. ; 

Xhe value of the effective e.m.l is numerically equal to the square I 

? ' I 



112 ARMATURE WINDING 

jcoot of the average of the squares of the successive instantaneous 
values during one alternation. For a sine-wave em.f the value 
of the effective e.m,f. is equal to 1* V* or 0.707 of the maxi- 
inum e.m.f. induced in the loop. The average e.m.f . is numerically 
equal to the average of the successive instantaneous values of the 
e.m.f. during one complete alternation. For a sine-wave e mJ 
the value of the average e.mJ. is equal to 2+v or 0.636 of the) 
maximum e.m.f. induced in the loop. 




E*. ue. 



Composed 



The effective value of an e.m.f. divided by the average value 
of the e.m.f. gives the value of what is called the form factor of 
the e.m.f. wave. The form factor for a sine-wave e.m.f. is numer- 
ically equal to 0.7074-0.636 = 1.11 

Simple Single-Phase Winding. A single-phase concentrated 
armature winding composed of a single inductor per pole is shown 
diagrammatically in Fig. 146, and a complete development of the 
winding is given in Fig. 147. A four-pole magnetic field has 
been chosen merely on account of simplicity; the principles and 
operation could be applied and explained equally well for a twOr> 



ARMATURE WINDING 



113 



six-, or eight-pole- field, or, in fact, for any even number of poles, 
but the diagrams would not be so easy to trace. The inductors 
are shown rotating in a clockwise direction and when they are 
moving under the north magnetic poles there will be an e.m.f. 
induced in them whose direction: ia away from the observer; and 
while the inductors are moving under the south magnetic poles, 
there will be an e.m.f. induced in them whose direction is toward 
the observer. The plus (+) signs indicate an e.m.f. away from the 
observer, and the minus ( ) signs an e.m,f. toward the observer. 

Effective E.M.F. of Winding. The mechanical construction of 
'the different magnetic poles is assumed to be the same and the 



N 




S 


OTlph 


N 




s 




e 








Fig. 147. Development of Winding Shown in Fig. 146 

armature is assumed to be centrally located. This will result in 
the same magnetic flux distribution under all the different poles. 
The inductors are all spaced an equal distance apart so that they 
occupy exactly corresponding positions under the different poles at 
any given instant. Hence the ejn.f.'s induced in all the inductors 
reach their maximum values at the same time, then pass through 
zero value at the same time, that is, they are all passing through 
corresponding phases of their respective cycles at the same time, 
or they are in phase with each other; 

In the developed diagram shown in Fig. 147, the four induc- 
tors are shown connected in series in such a manner that the 



114 



ARMATURE WINDING 



e.m.f/s add together to produce the total e.m.f. that exists 
between the slip rings. A continuous electrical connection is 
established between the winding and the outside circuit by means 
of brushes which rest upon the slip rings. The form of the result- 
ant e.m.f. wave will be exactly the same as the form of the e.m,f. 
wave for any one of the inductors, as they are all identical and 
exactly in phase with one another. The maximum value of the 
resultant e.m.f. wave will be equal to the sum of the maximum 
values of the four component waves. The effective e.m.f. of the 
entire winding will be equal to the arithmetical sum of the 
effective e.m.f.'s in the four inductors, and the average e.m.f, of 





Pig. 148. 



Representing Variation in Electromotive Force Induced in Each Inductor tit 
Shown in Fig. 146 and Total Electromotive Force between Slip Rings 



'the entire winding will be equal to the sum of the average e.m.f.% 
in the four inductors. 

Form Factor. The form factor of the e.m.f. wave will depend 
upon the distribution of the magnetic flux under the different 
poles. The ideal distribution of magnetic flux is one which results 
in what is called a sine-wave e.m.f. being induced in the inductors 
as they are made to move in the magnetic field. 

Assuming there is an e.m.f. induced in each of the inductors 
as represented by the curve a in Fig. 148, then the e.m.f, between 
the collector rings may be represented by a curve A, whose 
ordinate at any instant is four times the ordinate of the curve a 
at the same instant. In general the* ordinate of the resultant 
curve A will be as many times the ordinate of the curve a as 
there are inductors in series. 



ARMATURE WINDING 



115 




Fig. 160. Development of Winding Shown in Fig. 149 



H6 ARMATURE WINDING" 

Simple Two-Phase Winding. A two-phase concentrated 
armature winding composed of a single inductor per phase per 
pole is shown diagrammatically in Fig. 149, and a complete devel- 
opment of the winding is given in Fig. 150. Four inductors are 
connected in series between the slip rings /and 2 and these 
inductors are equally spaced around the surface of the armature 
and connected in such a manner that the e.m.f.'s induced in them 
are all in phase and acting in the same direction with respect to 
the slip rings. The four remaining inductors are, likewise, con- 
nected in series between the slip rings 3 and 4 and they are 
equally spaced around the surface of the armature and connected 
in such a manner that the e.m.f.'s induced in them are all in 




^ 

phase and acting in the same direction with respect to the slip 
rings. The e.m.f. between slip rings 1 and 2, and the' e.m.f. 
between slip rings 5 and will be displaced in phase with respect 
to each other by 90 degrees. 

Effective EMI. of Winding. The e.m.f. induced in each of 
the inductors connected between slip rings 1 and % may be repre- 
sented by the curve a in Fig. 151, and, since there are four induc- 
tors in series, the total e.m.f. between the slip rings 1 and 2 may 
be represented by curve A. The ordinates of curve A are four 
times the ordinates of curve o, since the e.m.f . induced in each of 
the four inductors has the same value and they are all in phase 
with each other. Likewise, the e,m.f. induced in each of the 



ARMATURE WINDING 



ilf 



inductors connected between slip rings 3 and 4 may be repre- 
sented by a curve I, Fig. 151, and the total e.m.f. between slip 
rings S and 4 by curve B. The e.m.f. induced in the inductors 
between slip rings 1 and 2 will be displaced in phase by 90 
degrees from the e.m.f. induced in the inductors between slip 
rings 3 and 4) owing to the relative location of the inductors on 
the surface of the armature; this fact is represented graphically 
in Fig. 151, by drawing curves A and B 90 degrees apart.. 




fig. 152. Three-Phase 



z for Four-Pole Machine, Composed of Single 
r per Phase per Pole 



Simple Three-Phase Winding. A three-phase concentrated 
armature winding composed of a single inductor per phase per 
pole is shown diagrammatically in Fig. 152, and a complete devel- 
opment of the winding is given in Fig. 153. Four inductors are 
connected in series in each of the three electrically independent 
circuits* The four inductors in each circuit are equally spaced 
around the armature surface and so connected that the e.m.f.'s 
induced in them all act in the same direction with respect to the 



118 



ARMATURE WINDING 



slip rings forming the terminals of the circuit. The inductors 
forming the three electrically independent circuits are distributed 




Fig. 153. Development of Winding Shown in Fig. 152 




Fig. 154. Curves Representing Variation in Electromotive Force in Each Inductor of 
Winding Shown in Fig. 152 and Total Electromotive Force between Slip Rings 

over the armature surface in such a manner that the e.m.f/s 
induced in the three circuits are displaced in phase with respect 



ARMATURE WINDING 119 

to each other by 120 degrees. The relation of these e.m.f/s is 
shown in Fig. 154, by the curves marked A, B, and C, which are 
displaced from each other by 120 degrees. The ordinate of each 
of these curves is four times the ordinate of the smaller curves 
a, b, and c. 

Electrical Degrees. The- e.m.f. curve -of an alternator is 
usually represented by a- sine curve and completes one cycle in 
360 degrees, as shown in Figs. 148 and. 151. This cycle is com- 



Fig. 155. Three Vectors Representing Three Equal Electromotive 

Forces or Currents Displaced in Phase with Respect to 

Each Other by 120 Degrees 

pleted while the armature moves, relative to the p.oles, through 
a distance equal to- twice the pole pitch, and it is customary and 
very convenient to call this distance 360 electrical degrees. In 
a two-pole . machine each revolution corresponds to 360 electrical 
degrees; in a four-pole machine each revolution corresponds to 
720 electrical degrees; in a six-pole machine each revolution corre- 
sponds to 1080 electrical degrees, etc. In genera!* the number of 



120 



ARMATURE WINDING 



electrical degrees will be equal to the product of the pairs of' 
poles by 360. 

Y Connections. It will be seen from Figs. 152 and 153 that 
a three-phase winding requires six leads, two for each phase. It 
is usual, however, to connect certain of these leads together so 
that only three leads have to be brought out and connected to 
the load. 

The e.m.f?s- induced in the -three windings are displaced in 
phase from each other by 120 degrees and may be represented by 




Fig. 156. Vector Diagram for Three-Phase Y Connections 

three vectors a, 6, and c, as shown in Figs. 155 and 156, The 
three windings in which the e.m.f.'s are induced may be connected 
as indicated 'in Fig. 157. In this type of connection, called a Y 
connection, one end of each of the three windings is connected to 
the common junction point, which is called the- neutral, and the 
remaining three ends are connected to slip rings, which in turn 
are connected to the external circuit by means of the brushes. 
In some cases an additional slip ring is provided and tHe neutral 
is connected to it so that an outside connection may be made to 
the neutral. The positive direction of the e.m.f/s is taken as being 



ARMATURE WINDING 121 

away from the neutral point as represented by the arrows along 
the windings. Each of the three windings will carry the same 
current at any instant as is carried by the line wire connected to 
the end of that winding by means of the brush and slip ring. 
Thus the current in line S at any instant will be the same as the 
current, in winding c at that same instant, etc. 

The voltage between any two of the lines is the vector sum 
of the voltages induced in the two windings connected between 
the two lines. Thus the voltage acting around the circuit from 
line 1 through the load connected between lines 1 and 2, through 



LINE 3 




Fig, 157 Y-Conneoted Armature Winding 

winding b to the neutral, through winding a and back to line 1 
will be equal to the e.m.f. in winding a minus the e.m.f. in winding 
6, since the e.m.f. in winding b is acting around the circuit in a 
direction opposite to the direction traced through above. In sub- 
tracting one vector from another, merely reverse the direction of I 
the vector you wish to subtract and then add them. Thus in *J 
subtracting vector b from vector a in Fig. 156 you draw the 
vector b in an exactly opposite direction and call it b and then < 
adding vectors a and b gives the vector a b. Vector bc 
represents the e.m.f. acting on the load connected between lines i 
$ and 3 and in a direction from line 2 to line 8. Likewise, ? 



122 



ARMATURE WINDING 



vector ca represents the e.ni.f. acting on the. load connected 
between lines 3 and 1 and in a direction from line 8 to line 7. 

Assuming there are equal e,m,f.'s induced in each of the 
three windings, then the value of the e.m.f. between any two 
lines will be equal to the \/3 or 1.73,2 times the e.m.f. in any one 
of the windings. 

A Connections. The three windings in which the e.m.f. 's are 
induced may be connected as shown in Fig. 158. In this type 
of connection, which is called a A (delta) connection, the three 
windings are connected end to end and form a closed circuit. 
The junction points of the windings are connected to slip rings, 
which in turn are connected to the three lines by means of 
brushes. 

LINE 1 




f HA5E 2. 



LINE 



PHX5E3 



LIME 3 



Fig. 158. Delta-Connected Armature Winding 

The positive direction of the e.m.f.'s is taken as being clock- 
wise, as indicated by the three arrows along the windings in 
Fig. 158. The e.m.f. between any two lines will be equal to the 
ejnJ. induced in the winding connected directly between the two 
lines. Thus, the e.m.f. between lines 1 and 3 is equal to the 
jn.f. induced in winding a, the e.m.f. between lines 2 and I is 
equal to the e.m.f. induced in winding b, etc. 

The current in any one of the, lines is a combination of the 
currents in two of the windings. Thus, the current in line 1 is 
a combination of the currents in windings a and b. The current 
in winding. a is toward line 1 and the current in winding 6 is 
away from line 1, so the current in winding b must be subtracted 
from the current in winding a in order to obtain the value of 
the current_jn line 1. The three currents in the three windings, 



I f 



ARMATURE WINDING 



123 



assuming they are all equal, may be represented by three equal 
vectors, such as a, b t and c, as shown in Fig. 159. The current in 
line / will be equal to a 6, the current in line # will be equal to 
b c and the current in line 3 will be equal to c a, as shown in 
the figure. The numerical value of the current in -any line, assum- 
ing the currents in the three windings are equal, will be equal to 
VlL or 1,732 times the current in any one of the windings. 

Voltage and Current Relation in a TwoPhase Winding. The 
simple two-phase winding shown in Fig. 149, requires four slip 




Fig. 159- Vector Diagram for Three-Phase Delta-Connection 

rings, and each of the windings has independent electrical con- 
nections to the outside or load circuits. One of these four slip 
rings may be omitted by joining one end of each of the two 
windings and then connecting this junction to a slip ring which 
will serve for both windings, as shown in Fig. 160. 

The e.m.f. between line 1 and the neutral will' be equal to 
the e.m.f. induced in the winding a, and the e.m.f. between line 
2 and the neutral will be equal to the e.m.f. induced in winding 
6. The e.m.f. acting from line 1 to line 2 through windings 6 
and a back to line 1 is equal to the e.m.f. in winding a minus 



124 



ARMATURE WINDING 



the e^ox-f. in winding 6, because in tracing around the above 
circuit you pass through the winding a in the direction of the 
e.m.f. induced in it and through the winding 6 in the opposite 



UNE 1 




Fig. 160. Three-Wire Two-Phase Winding 

direction to the e.m.f. induced in it. The subtraction of the 
e.m.f. in winding 6" from the e.m.f. in winding, a is shown in 
Fig. 161, and the result is represented by the vector marked a b. 
The current in line 1 is exactly the same as the current in 
winding a, and likewise, the current, in line 2 is exactly the same 
as the current in winding 6. The current in the neutral wire is 




Fig. 161. Vector Diagram of Electromotive Forces for Two-Phase 
Connection Shown in Fig. 160 

equal to the vector sum of the currents, in the two windings Id 
and 16, as shown in Fig. 162, in which. la+Ib represents the 
neutral current. In Fig. 162 the currents in lines 1 and 2 are 



m. 



ARMATURE WINDING 



125 



la 




Ib 

Fig. 162." Vector Diagram of Cur- 
rents for Two-Phase Connection 
Shown in Fig. 160. Balanced 
Power 



Load, Unity 



. 
r Factor 



assumed to be equal and in phase with their respective e.m.f.'s. 
A vector diagram is given in Fig. 163 in which the currents in 
lines 1 and 2 are unequal, and the cur- 
rents in the two windings are displaced, 
in phase from the e.m.f.'s by angles 61 
and 62 respectively. A counterclockwise 
direction of rotation is considered as 
being positive. The neutral current is 
equal to la+Ib, as shown in the figure. 
A two-phase "star" connection is shown, 
in Fig. 164. In this type of connec- 
tion^ the middle points of each of the 
two phases are joined, and the four 
sides are brought out to four slip rings. This connection is equiva- 
lent to what might be called .a four-phase system. A two-phase 
mesh connection is shown-in 
Fig. 165. This is' a form of 
closed-circuit winding, as will 
be explained in one of the fol- 
lowing sections, 

Concentrated and Dis- 
tributed Armature Windings. 
Distinction Between. A -con- 
centrated armature winding, 
as its name indicates, is one 
in which .all the . inductors per 
phase per pole are grouped 
in a single slot or in a single 
bundle on the surface of the 
armature. The windings 
whose developments are given 
in Figs. 147, 150, and 153 are 
all of the concentrated type 
and in each of these there is 
a single inductor per phase 
per pole. 

Parts of the developed diagrams of two different forms of. 
single-phase distributed windings having six inductors per phase 




Fig. 163. Vector _. 
motive Forces for 



. of Currents and Electro- 
Phase Connection Shown 



in Fig. 160. Unbalanced Load, Both Currenta 
Lagging by Different Angles 






126 ARMATURE WINDING 

per pole are given in Figs. 166 and 167. In each of these wind- 
ings, the e.m.f.'s induced in adjacent inductors are displaced m 
phase from each other by 30 electrical degrees. 




Fig. 164. Star-Connected Two-Phase System 
LINE 1 



.INC. I 




LIMES... 



UME4 
Fig. 165. Mesh-Connec,,ed Two-Phase System 

In Fig. 166 the e.m.f. induced in the band of inductors 
composed of numbers 1, 2, 3, 4, 5, and 6, which will be deferred 



ARMATURE WINDING 



127 



to as band 1 3 acts in series with the e.m.f. induced in the band 
of inductors composed of numbers 7, 8, 9, 10, 11, and 1%, which 




Fig. 166. Portion of Uniformly Distributed Single-Phase Armature Winding. 
Six Slots per Pole 

will be referred to as band 2. The e.m.f. in each of these bands 
of inductors is equal to the vector sum of the e.m.f.'s in the induc- 
tors composing the band. Thus in band /, the total ejn.f. can he 




Fig. 167 Portion of Uniformly Distributed Single-Phase Arma- 
ture Winding, Six Slots per Pole 

obtained as shown in Fig. 168. The e.m.f induced in each of 
the inductors is represented by the letter 0, and there are six of 



128 



ARMATURE WINDING 



them, which, when combined, give the value of the resultant 
e.m.f. Ei. This resultant e.m.f. is equal to 0.64 of 60; that is, 
the total e.m.f. produced by the six inductors per pole when 




Fig. 168. Vector Addition of "Equal Electromotive Forces Displaced' in 
Phase with Respect to .Each Other by 3ft Electrical Degrees 

uniformly distributed is equal to 0.64 of the e.m.f. that the same 
number of inductors would, give if they were all located at the 
same point on the surface of the armature. The e.m.f. induced 
in the second band can be determined in exactly the same manner 
as that used for the first band. Now if the centers of these two 
bands are exactly 180 electrical degrees apart the resultant e.m.f/s 
for the different bands may be added to obtain the- total e.m.f. 




F^g, 169. Vector Addition of Electromotive Forces Displaced in Phase with 
Respect to Each Other by 30 Electrical Degrees 

If the- bands are not 180 electrical degrees apart then the e.m.f/s 
induced in them must be added by means of vectors which will 
take caje of the difference in .phase. 



ARMATURE WINDING 129 

In Fig, 167 band 1 is composed of inductors /,. .2, and 3; 
band 2 is composed of inductors 4> &> and 6. The e.m.f. induced 
in -band 1 is equal to EI in Fig. 169, and the e.m.f. induced in 
band % is equal to E$. These two bands are displaced in- phase 
by 90 electrical degrees, so that EI and E* must be added vec- 
torially and the resultant E 3 is equal- to 0.64 of 6e which is exactly 
the same as the resultant E f or one of the bands in Fig. 166, An 
armature wound as shown in Fig. 166 will have as many bands as 
there are poles and one wound as shown in Fig. 167 will have 
twice as many bands as poles. The resultant e.m.f. for the entire 
winding will be numerically the same in both cases. 

Comparison of Concentrated and Distributed Windings. The 
e.m.f. induced in an armature winding when composed of a num- 
ber of inductors, distributed over the surface of the armature is" 
less than the e.m.f. induced in an armature winding composed of 
the same number of inductors but concentrated in one slot "or 
bundle per phase per pole. The distributed winding, however, 
possesses the great .advantage of affording a means of controlling 
the resultant wave form, while in the concentrated winding the 
wave form is dependent upon the 'manner in which the magnetic 
flux is distributed under the magnetic poles. 

When the winding is concentrated, the number of turns per 
joil is increased; and since the inductance of a coil increases as 
,he square of the number of turns, the reactance of a concentrated 
vinding is much greater than the reactance of a distributed 
vinding. The result is that the concentrated winding gives 
>oorer voltage regulation. 

Development of Electromotive-Force Equation. In the devel- 
ped windings given in Figs. 147, 150, and 153, the inductors 
hat are connected together are displaced from each other by an 
xact pole pitch, and such windings are called full-pitch windings, 
n a full-pitch winding, all the inductors that are connected ia 
iries have e.m.f . J s induced in them which are in phase with each 
ther, and the total e.m.f. between collector rings is equal to JTe. 
r stands for the number of inductors in series and e for the 
m.f. induced in each inductor. 

Average Electromotive Force. The average e.m.f. induced in 
ich inductor will be equal to the magnetic lines cut by the 



130 



ARMATURE WINDING 



4 |- 

I I 



inductor in one second divided by 10 s . The flux per pole <f> 
multiplied by the number of poles ;), and this product multiplied 
by the number of revolutions per second that the inductor makes, 
will give the value of the flux cut by each inductor per second. 
The above statements may be written in the form of an equation 
as follows: 



in which e av stands for the average e.m.f. If the e.m.f. induced 
in the inductors follows a sine law, then the effective e,m.f. will 
be equal to 1. 11 times the average e.m.f., and the following 
equation may be written giving the value of the effective e.m.f. e. 



Total Electromotive Force. If there are N inductors in series 

T] 




Fig. 170. Short-Pitch, Concentrated, Single-Phase Armature Winding 

in each circuit and the e.m.f/s are in phase, the total e.m.f. E 
will be given by the following equation: 



Since 



then 

r.p.s.Xp2/; 

and the above equation may be written as follows: 
JVX2.22X0X/X10- 8 



ARMATURE WINDING 



This equation gives the value of the e.m.f. per phase for a full- 
pitch concentrated winding. Each of the small lines a in Fig. 148 
represents the e.m.f. induced in each of the inductors, and since 
these e.m.t's are all in phase, the line A, which represents the 
total e.m.f,, will be equal to four times the length of any one of 
the shorter lines a. 

E.M.F. for Short-Pitch Windings. Now supposing the induc- 
tors composing the winding -be arranged in 
such a way that they are no longer displaced 
from each other by an exact pole pitch, as 
shown in Fig. 170. In this case the induc- 
tors in each coil are 150 electrical degrees 
apart instead of 180 degrees, as shown in 
Fig. 147. A winding of this kind is called a 
short-pitch winding. The e.m.f. induced in 
each of the four inductors will have the same 
value as in Fig. 147, but they will not all 
be in phase with one another. The four 
e.m.f. 's are shown in their proper phase rela- 
tion to each other in Fig. 171 by the four 
small vectors marked i, e z , 63, and e*. These 
four e.m.f. 's combined give the value of the 
resultant e.m.f. E. The e.m.f. induced in 
each coil is equal to 2 times the e.m.f. in 
one of the half coils multiplied by the cosine 
of the angle 6 that the coil "lacks of being 
full pitch. The e.m.f. equation for a short- 
pitch concentrated winding is as follows: 




= A T X(2.22X0X/XlO- 8 )Xcos 



Fig. 171. Vector Addition of 
Electromotive Forces for 
Short-Pitch Armature 
Winding 



The part of this equation inside the parenthesis represents the 
value of the e.m.f. induced in each of the half coils. 

E.M.F. for Distributed Windings. In distributed windings 
such as the ones shown in Figs. 166 and 167, it is necessary first 
to determine the resultant e.m.f. induced in each of the bands of 
inductors and then add these results vectorially. Each band in 
Fig. 166 is composed of six half coils, 30 degrees apart, and the 
Centers of these bands are 180 electrical degrees apart, so that the 



i 
t * 



132 ARMATURE WINDING 

e.m.f. f s in the different bands are in phase, when properly con- 
nected as shown in the figure. The e.m.f. in a single band is 
given by the vector diagram shown in Fig. 108, and is equal to .64 
of the arithmetical sum of the six equal component e.rn.f.'s. The 
factor by which the arithmetical sum of the e.rn.f.'s in any band 
must be multiplied in order to get the value of the effective e.m.f. 
is called the distribution factor. Thus in Fig. 166 the distribution 
factor is .64 and in Fig. 167 it is .911. In a four-pole machine 
with a winding like the one shown in Fig. 166 there will be four 
bands, and in the case of a four-pole machine with a winding like 
the one shown, in Fig. 167 there will be 8 bands. The same e.m.f. 
will be induced in both these windings under identical conditions, 
as shown by the following equations: 

=4X(6eX.64)Xl (for winding in Fig. 166) 
=*24X.64Xe 
= 15.360 

90 
=8X (&X.911) Xcos (for winding in Fig. 167) 

=24X.911X.707Xe 
= 15.36e 

This same process of reasoning can be followed in calculating the 
e.m.f. induced in each phase of any type of winding. 

Values of Distribution Factor. The value of the distribution 
factor for any winding will depend upon the number of slots per 
phase per pole and the number of these slots that are actually 
used. The distribution factor will also depend upon the manner 
in which the inductors are distributed in the slots that are actually 
used. Thus in a concentrated winding the value of the distribu- 
tion factor is equal to unity, as all of the inductors composing 
each phase are located in a single slot per phase per pole. 

Single-Phase Windings. In a single-phase winding having 
two slots .per phase per pole the e.m.f, induced in the inductors in 
the two slots will be displaced in phase from each other by 90 
electrical degrees, assuming the slots are equally spaced around 
the surface of the armature. If each slot contains the same num^ 
ber of inductors, then the same resultant e.m.f. will be induced in 
each slot^ and these two^ e.m.f/s combined yectprially will give 



ARMATURE WINDING 133 

resultant e.m.f. equal to 707 times the sum of the component 
e.m.f.'s. 

If three equally spaced slots be used per phase per pole, and 
the winding is uniformly distributed in the three slots, then the 
resultant e.m.f. will be equal to the vector sum of three equal 
e.m.f.'s which are displaced in phase with respect to each other by 
60 electrical degrees. The numerical value of the resultant is 
equal to .662 times the sum of the component e.m.f/s. 

If there are four slots per phase per pole and the winding is 
uniformly distributed in these slots, the resultant e.m.f. will be 
equal to .653 times the sum of the e.m.f.'s induced in the induc- 
tors in the different slots. 

If there are six slots per phase per pole and the winding is 
uniformly distributed, the resultant e.m.f. will be equal to .64 
times the sum of the e.m.f.'s induced in the six slots. 

Two-Phase Windings, In a two-phase winding having two 
slots per phase per pole and the winding distributed uniformly in 
the slots, there will be equal e.m.f.'s induced in adjacent slots, 
which belong to one of the phases, and these e.m.f.'s will be dis- 
placed in phase by 45 electrical degrees. The value of the result- 
ant of these two e.m.f.'s will be equal to .924 times their sum. In 
a two-phase winding having three slots per phase per pole, there 
will be three e.m.f.'s per phase per pole that are in series and dis- 
placed in phase by 30 electrical degrees, and the numerical value 
of the resultant is equal to .911 times the sum of the three 
components. 

Three-Phase Windings. In a three-phase winding having two 
slots per phase per pole, there will be two e:m.f.'s per pole that 
are in series and displaced in phase by 30 electrical degrees. The 
resultant of these two e.m.f.'s is equal to .966 times their sum. 

The values of the distribution factor as given in Table VII 
are based upon the same number of inductors being placed in each 
of the slots and all of the slots being, used. 

Rating of Alternators. The maximum voltage that an alter- 
lator can develop continuously depends upon the permissible 
/alue of the flux per pole, and the maximum current is limited by 
;he armature copper loss which, along with the core loss, heats 
:he machine. With the value of the voltage and current fixed, 



134 



ARMATURE WINDING 

TABLE VII 

Distribution Factors for Single-, Two-, and 
Three-Phase Windings 



., 

Slots per 
Phase 
per Pole 


DISTRIBUTION FACTOR 


Single-Phase 


Two-Phaae 


Three-Phase 


1 
2 
3 
4 
6 


1.000 
0.707 
0.663 
0.653 
0.644 


1.000 
0.924 
0.911 
0.906 
0.903 

- 


1.000 
0.966 
0.960 
0.958 
0.956 



the kilowatt rating depends upon the power factor of the load 
The power factor is a variable quantity, and beyond the control 
of the builder of the machine, so that an alternator is usually 
rated by giving the product of the volts and amperes which is 
called the volt-ampere rating, and this quantity divided by 1000 
gives the rating in kilovolt amperes. 

Effect of Number of Phases on Rating of Alternator. Consider 
an armature having six slots per pole and imagine a fixed number 
of inductors placed in each of the six slots and connected tor 
single-, two-, and three-phase operation, then the following voltages 
per phase will be obtained: 

Number of Phases Voltage per Phase 

Single (all slots used) Constant X6eX 0.64 

Single (! of slots used) Constant X4eX 0.84 

T W * Const,antX3eX0.91 

^^ Constant X2eX0.96 

In the above expressions for voltage, e represents the total 
e.ni.f. induced in the inductors in any one of the slots. 

Since there are the same number of inductors in each of the 
slots, the inductors will all have the same cross-section and there- 
fore carry the same current which we will represent by J c . The 
volt-ampere rating, which is equal to the volts per phase times the 
current per phase times the number of phases, is given as follows; 

Number ofPhases Volt-Ampere Hating 

Single (all slots used} Constant XfoX0.64XlXl c =a constant X0,64 

Single (I of slots used) Constant X4eXO,84XlXlc=a constantXO,56 

Tw o ConstantX3eX0.91X2Xl c =aconstantX0.91 

Three 



ARMATURE WINDING 135' 

From the above relations, it is seen that the two- and three- 
fphase ratings are practically the same, although the three-phase 
rating is the better. The single-phase machine is usually given 65 
per cent of the rating of a three-phase machine when all the slots 
are used. 

Classes of Armature Windings for A.C. Machines. Alternating- 
current machines are built in many different forms, and many 
different kinds of armature windings are required to meet the 
different conditions of operation. The different forms of armature- 
windings may be classified as follows: 

(1) With respect to form of armature 

(a) Revolving armature 

(b) Stationary armature 

The armature of a generator or motor is the part of the 
machine in which an e.m.f. is induced owing to a relative move-, 
ment of a magnetic field and the inductors composing the arma-! 
ture winding. When the inductors are mounted on the revolving 
part of the machine the armature is known as a revolving arma- 
ture, and when the inductors are mounted on the stationary part 
of the machine the armature is known as a stationary armature. 

(2) With respect to method of advancing around armature in 
tracing through winding 

(a) Lap winding (c) Progressive winding 

(b) Wave winding (d) Retrogressive winding 

A lap winding is one in which you advance around the arma- 
ture in opposite directions at the front and back ends of the 
armature as you trace through the winding. A three-phase, lap- 
wound armature is shown in Fig. 172. 

A wave winding is one in which you advance around the 
armature in the same direction at the front and back ends of the 
armature as you trace through the winding. A three-phase, wave- 
wound armature is shown in Fig. 173. 

A progressive type of winding is shown in Fig. 174 It mil 
be observed that after tracing through inductors 1, 8, 5, and 7 you 
advance more than a full pole pitch to inductor 2 then around the 
armature again to slip ring & 

A retrogressive type of winding is shown in Fig. 175. It will 
oe observed that after tracing through inductors 2, 4* @* $ ^ 




136 



ARMATURE WINDING* 



vou advance less than a full pole pitch to inductor 1 then around 
the armature again to slip ring 2. 



Fig. m. Developed Diagram of Sinde-Phaae Progressive Winding 




4 



(3) With respect to region between number of poles and num, 

ber of coils 

(a) Half-coil windings 

(b) Whole-coil windings 

A half-coil, or hemitropic, winding is one m winch there B on. 
coil group P- Phase per pair of pole, A smgle-phase, half-cod 




Fig. 175. 



Developed Diagram of Single-Phase Retrogressive Winding 



^ for a four-pole machine is shown in Kg. 
opment of the winding is given in Fig. 17<. 



ARMATURE WINDING 



137 



groups in Figs. 176 and 177 is composed of two coils, one inside 
of the other and connected in series. 




Fig. 176. Eadial Diagram of Sinsle-Phase Half-Coil Winding for 
Four-Pole Machine 

, ,_-, 




Fig. 177. Developed Diagram of Single-Phase Half-Coil Winding for 

A whole-coil winding is one in which there is a whole-coil 
rotip per phase per pole. A single-phase, whole-coil winding for a 




13 8 ARMATURE WINDING 

four-pole machine is shown in Fig. 178, and a development of the 

winding is given in Fig. 179. Each-of the coil groups m Figs 178 

and 179 is composed of two coils in series, one inside the other. 

(4) With respect to number of slots 

(a> Concentrated or unicoil windings f . 
(b) Distributed or multicoil windin 



'("Partially distributed 
iFully distributed 



A concentrated winding is one in which all the inductors per 
phase per pole are grouped in a single slot per pole. 




Fig. 178. 



Whole-Coil Winding for 



A distributed or multi-coil winding is one in which the induc- 
tors per phase per pole are distributed in several slots per pole. 
Partially distributed single-phase armature windings, placed in 
two slots per pole, are shown in Figs. 176, 177, 178, and 179. A 
fully distributed armature winding which is distributed m six 
slots per phase per pole is shown in Fig. 180, and a developed 



ARMATURE WINDING 





140 



ARMATURE WINDING 



diagram of the winding is given in Fig. 181. In a winding of this 
kind the total e.m.f. per half-coil group is made up of three e.m.f.'s 
equally spaced from each other by 30 electrical degrees. Each of 
these e.m.f.'s will have the same numerical value provided each 
slot contains the same number of inductors. 
(4) With respect to form of inductors 

(a) Wire winding 

(b) Strap winding 

(c) Bar winding 

In the construction of alternating-current armatures either 
wire, strap, or bar inductors may be used, depending upon which 
is best suited for the particular requirements. The size and shape 




Fig. 181 Developed Diagram of Fully Distributed Single-Phase Armature 
Winding for Four-Pole Machine 

of the inductor is governed by the current the inductor must carry 
and the space in which the inductor is to be-placed. Thus in a 
high-voltage machine small well-insulated wire will be required, 
while in a low-voltage machine large inductors not so well insulated 
will be required. In order that the coils may be flexible several 
small wires in parallel are often used instead of a single large wire. 
(6) With respect to number of coils per phase per pole 

(a) Single-slot winding 

(b) Multi-slot winding 

A single-slot winding is one in which all the inductors per 
phase per pole are placed in a single slot, while a multi-slot wind- 
ing is one in which the inductors per phase per pole are distributed 



ARMATURE WINDING 

TABLE VIII 

Effectiveness of SingIePhase Armature Winding 
Having Six Slots per Phase per Pole 



Slots in Use 


Voltage 
Across 
Coils 


Distribution 
Factor 


Quantity of 
Copper to Produce 
Same Voltage 


1 


1.00 


1.00 


100 


2 


1.93 


0.97 


1.03 


3 


2.73 


0.91 


1.10 


4 


3,34 


0.84 


U9 


5 


372 


0.74 


1,35 


6 


3,86 


0.64 


1.56 



in several slots. In the majority of cases only two-thirds of the 
total number of slots, assuming they are all equally spaced, are 
used for a single-phase armature winding. The reason for this is 
that more copper is required for a given generated pressure as the 
distribution of the winding is increased. If much less than two- 
thirds of the surface of the armature be wound, it is often quite 
difficult to provide a sine wave of pressure. Table VIII shows 
what might be called the effectiveness of the winding for an 
armature having six slots per pole. 

(7) With respect to kind of current delivered 

(a) Single-phase winding 

(b) Two-phase winding 

(c) Three-phase winding 

A single-phase winding is one in which there is but a single 
e.m.f. induced in the winding. 

A two-phase winding is one in which there are two e.m.f. 's 
induced in the winding and these e.m.f /s are displaced in phase 
with respect to each other by 90 electrical degrees. 

A three-phase winding is one in which there are three e.m.f.'s. 
induced in the winding and these e.m.f.'s are displaced in phase 
with respect to each other by 120 electrical degrees. 

(8) With respect to shape of coil ends 

(a) Single range 

(b) Two range, etc. 

A single-range armature winding is one in which all the coil 
*nds are of the same shape. 



n 



142 



ARMATURE WINDING 



>, 



A two-phase, two-range armature winding is shown in Fig. 182 
and it will be observed that the coils composing phase A have 




A 



~\ 



Fig. 182. Diagram Showing End-Connections of Two-Phase, 
Two-Range Armature Winding 

their ends coming straight out from the slots, while the coils com- 
posing phase B have their ends bent down. Another two-phase, 
two*range winding is shown in Fig. 183. 




Fig. 183. Diagram Showing End-Connections of Two-Phase Two-Ranac 
Armature Winding ' 

Two three-phase, three-range windings are shown in Figs. 184! 
and 185. The arrangement shown in Fig. 185 will permit the 
armature being separated without unwinding any of the coils. 



144 ARMATURE WINDING 

A double-layer winding is one in 



vi 




of a two-layer winding is shown 
in Fig, 187. 

(10) With respect to number 
of circuits through winding per 
phase 

(a) Single-circuit winding 

(b) Two-circuit winding, etc. 

A single-circuit winding is 
one in which all the inductors 
per phase are in series and there. 
is only one path for the cur- 
rent from pne collecting ring to the other for each particular 



Fk. 186. Form of 
^ Layer 



ie Coil for Two- 
ing 



A two-circuit armature winding is one in which the inductors 
per tthase are connected in such a manner that there are two 
Sh? for the current from one collecting ring to the other for 
Ld, particular phase, etc. For a constant number of inductors, 
the number of inductors in series in each path for a two-drcuit 
winding is just one-half of what it is for a single-circuit winding, 
to Z current capacity of the two-circuit windmg .s twice that 
of the single-circuit windmg. A single-circuit, three-phase Y- 
Meted winding is shown in Fig. 188, and the same wmdmg 
reconnected as a two-circuit winding is shown , 

in Fig. 189. 

A single-circuit, three-phase A-connected 

_ -y-w-mm--- winding is shown in Fig. 190, and the same 
***" HBP . winding reconnected as a. two-circuit winding 
is shown in Fig. 191. 

A single- and a two-circuit, two-phase 

^j*foi?otlt i r T%? armature winding are shown in Figs. 192 and 
usSrwiKBi* 193 respectively. 

Four different types of connections for a three-phase, double- 
layer armature winding are shown ift Figs. 194* 195, 




ARMATURE WINDING 



145 



(II) Miscellaneous windings 

(a) Chain windings (d) 

(b) Skew-coil, windings (e) 

(c) Fed-in windings (f) 



Mummified windings 
Shuttle windings 
Creeping windings, etc. 



A chain winding is one in which the different coils link one another 
as the links in- a chain, which accounts for the name. A chain 
winding is shown in Fig. 198. 

A. skew-coil is one in which the ends of the coils are all made 
the same shape, which results in, only a single form being required 
for the coils. 

A fed-in winding is one in 
which the inductors are fed into 
the slots from the top, or ends, 
the slot being provided with a 
lining of horn fiber or other 
suitable .insulating material, 
which is usually folded over and 
secured by means of a wedge, 
or by some other suitable means. 

A mummified winding is 
one composed of coils which 
were saturated and baked before 
they were placed on the arma- 
ture. 

A shuttle winding is one 
consisting of a single coil having 




Fig 198. Three-Phase Chain Winding. Ha 

ing Three Half Coils per Phase per Pole 

Courtesy of General Electric Company, 

Schenectady. New York 



a large number of turns wound in two slots on the armature 180 
electrical degrees apart. This type of winding is used a great 
deal- in the construction of magnetos. 

A creeping winding is one composed of coils having a fractional 
pitch and the coils composing each phase are so arranged as to 
gain or lose one or more complete pole pitches as you trace 
around the armature. 

Principal Parts of Alternating-Current Machine. A sectional 
view of an alternating-current motor is shown in Fig. 199, and the 
names of the principal parts are indicated below the figure. 

Examples of Alternating-Current Windings. Bars for lap 
and for wave windings are shown in Fig. 200, The upper formed 



146 



ARMATURE WINDING 



bar is for a lap winding and the lower formed bar is for a wave 
winding. .A number of armature, coils are shown in Fig. 201 




Fig. 199. .-Sectional View Showing Principal Parts of Alternating-Current Motor 
Courtesy of Reliance Electric and Engineering Company, Cleveland, Ohio 

I. 'End Bracket; 2. Shaft; 3, Rotor Short-Circuiting Rings; 4. Oil Rinpe; 5. 8elf-Al'gnint 
Bearing Bushing; 6. Spider; 7. Rotor Bars; 8 Stator Coils; fl. Stator Lamination End-PIa ; 
10. Stator Laminations; 11. Eyebdta; 12. Stator Locking Key; 13. Rotor Laminations; 11. 
Rotor Lamination End-Plate; 15. Rotor Locking Key; 16: Duet Cap; 17, Oil -Well Cover; 18. 
Oil Throws;' 19. Stator Frame; 20, Rotor 



A 




Fig. 200. Bare for Lap and Wave Windings 
Courtesy of General Electric Company, Schenfctady, New York 



ARMATURE WINDING 



147 



for an 11,000-volt generator. The different steps in the insulation 
of the coils are shown from the bare coil to the completed coil. 




Fig. 201. Armature Coils in Different Steps in Process of Being Insulated 
Courtesy of General Electric Company, Schenectady. New York 





Fig. 202. Completed Armature Coils 
Courtesy of Allis-Cholmers Manufacturing Company, Milwaukee, Wisconsin 

A number of completed armature coils are shown in Fig. 202. 
^These coils have a uniform insulation on the slot. portion of the 
coils and on the ends. Oftentimes the .slot part of the coil is 
better insulated than the ends. 



148 



ARMATURE WINDING 




Fig. 203. Bar Winding in Process of Construction 
Courtesy of General Electric Company, Schenectadv, New York 




i 



Fig. 204. Method of Connecting Ends of 

Bars in Bar Winding 
Courtesy oj General Electric Company 



ARMATURE WINDING 



149 



A bar winding is- shown in Fig. 203, and the method of mak- 
ing the end-connections is shown in Fig. 204. This is a wave 
winding as the ends of the bars are bent in opposite directions at 
the front and back of the armature. 

The stator for a 600-volt General Electric generator is shown 
in Fig* 205, and the stator for a 2300-volt General Electric 




Fig. 205. Partially Completed Stator for General Electric 600-Volt Generator 

synchronous motor is shown in Fig. 206. Both these windings are 
at the double-layer type. 

A double stator winding is shown in Fig. 207, as used by the 
Allis-Chalmers Company, in a 2200-volt, 25-cycle, 3-phase, 2-speed 
'induction motor. 

The stator for a 1500-horsepower, 6600-volt, 3-phase> wound- 
-otor, induction motor, as manufactured by the Allis-Chalmers 
ompany, is shown in Fig. 208. 




Fig 206 Completed Stator for 2300- Volt Synchronous Motor 
Courtesy of General Elf.ctnc Company, Scheneclady, New York 




Tig 207 Double-Stator Winding for 600-Korsepower, 2200-Volt, 3-Phafee, 25-Cycle, 
Two-Speed, Allis-Chalmers Induction Motor 




Fig. 208. Stator for 1500-Horsepower. 150 R.P M.. 6600-Volt. Wound-Rot&r 

Induction Meter 
Courtesy of Allis-Chalmers Manufarturinj Company, Milwaukee. Wisconsin 




Fig, 209 1 . Bracket Type of Coil Bracing for Large Turbo-Generator 
rtesy of AUfcC.HO'lmtrs Manufacturing Company. Milwaukee, Wisconsin 



152 



ARMATURE WINDING 




Fig. 210. Method of Bracing Coila for Large Turbo-Generator 
Covfriesy of Alli9~Chalmera Maunfocturing Company, Milwavkt.e, Wisconsin 




Tie. 21 1. End-Connections for Large Turbo-Generator 
.Courtesy of Affw-CAoJroerj Manufacturing Company, Milwaukee, Wittcontin 



ARMATURE WINDING 



153 




Fig. 212. Rotor for Large Squirrel-Cage Induction Motor 

Courtesy of Weatinghouae Electric and Manufacturing Company, 

East Pittsburgh, Pennsylvania, 




Fig. 213. Rotor for Small Squirrel-Cage Induction Motor 
Courtesy of Reliance Electric and -Engineering Company. Cleveland, Ohto 




Fig. 214. Special Squirrel-Cage Winding on Rotor of Synchronous Motor 
'Courtesy of Aftis-Chalmers' Manufacturing Company, Milwaukee. Wisconsin 




m ARMATURE WINDING 

The method of bracing the coils on two large turbo generators 
is shown -in Figs. 209 and 210. The end-connections for a large 
turbogenerator are shown in Fig. 211. ^ 

The rotor for a Westinghouse squirrel-cage induction motor is 

shown in Fig. 212. The winding consists, of a number of bars 

which have their ends electrically connected to two metal rings, 

one at each end of the rotor. The bars are usually insulated 

from the iron core, but riot always. 

The rotor for a small induction motor is shown in tig. ^l5- 
The slots are given a slight twist, as shown in the figure. This 
assists in the operation of the machine, as the inductors do not 
move into and out of the magnetic field as suddenly as they 
would otherwise, 

,A special squirrel-cage winding is shown in Fig. 214. Induc- 
tors are embedded in the surface of the pole shoes, and their ends 
are connected to metal rings at opposite ends of the rotor, Such 
windings are used on synchronous motors in starting them as they 
may then be started as induction motors. 

RECONNECTING INDUCTION-MOTOR ARMATURE 
WINDINGS 

Possible Changes. Occasionally it is necessary to make 
changes in the motor equipment of a plant, due to changes in the 
operating voltage, frequency, or number of phases of the circuit 
supplying energy. In such cases it is frequently possible to use 
the old motors by making certain changes in their connections or 
construction, and in some cases no changes in the internal con- 
nections of the motor may be required. Considered in the order 
of their desirability, the possibilities in such cases are as follows: 

(1) Motor operated under new conditions without change in internal 

connections 

(2) Reconnection of the old windings to meet the changed conditions 

(3) Supplying a complete new set of coils 

(4) Supplying new laminations and also new coils 

Fundamental Ideas of the Electric Motor. An electric motor" 
is a means of transforming electrical energy into mechanical 
energy in the form of a rotative force. This rotative force, or 



ARMATURE WINDING 155 

torque, is produced by the force exerted on a current flowing in a 
conductor which is located in a magnetic field. Hence, it follows ?* 

at once that" the capacity of a motor for producing torque is j 

limited by the capacity of the electrical circuit to carry current 1 

and also by the capacity of the magnetic circuit to carry magnetic ^ 

lines of force, or flux. The heating of a motor depends upon both <* 

the current density in the copper and the flux density in the iron. \ 

These densities are usually fixed by the designing engineer so that 
the temperature of the motor will not become excessive- under its J 

rated or guaranteed load. It follows, that if changes are to be ( 

made in the voltage, frequency, phase, or speed of the motor, the 
number of turns of "copper must be changed or a reconnection ( 

made so as to preserve approximately the same current density in \ 

the copper, and the' same' flux density in the iron that existed < 

before the changes were made* The above statement is true over j 

a very wide range of conditions, and would be true universally 
were it not for the fact that the ventilation of -a machine is , 

poorer at low speed than at high, and hence the same heat losses 
cannot be dissipated. For this reason it is generally true that the 
capacity of a motor may increase directly as the speed when the 
speed is being increased, but may decrease somewhat faster than * f 

the speed when the speed is being decreased. As an example, a ) 

50-horsepower motor at 600 r.p.m. may 'be made to develop 100 j 

horsepower at 1200 r.p.m., assuming that the mechanical design I 

would stand the increased speed, but conversely, a motor originally \ 

designed for 100 horsepower at 1200 r.p.m. when reduced to 600 I 

r.p.m. might not develop more than 40 horsepower on account of | 

reduced ventilation. ; 

It is the object of the following sections to describe briefly ^ 

what questions must be considered to determine whether the t ^ 

characteristics of the motor may be changed in the manner * " 

desired; second, what the effect will be on the windings of the 

motor with respect to the number of turns in the coils and the | 

mechanical form of the coils; and third, by what simple mechanical 
means, such as reconnection, if possible, the desired change may be | 

* accomplished. ?/ 

Torqite and Horsepower. In considering the operation of any /| .> 

motor it is essential that you get a clear conception of the , 



156 ARMATURE WINDING 

distinction between torque and horsepower. It is the primary func- 
tion of a motor to produce torque, or rotative force, and it is 
incidental that when this same torque is allowed to rotate at one 
speed or another a different horsepower is produced. For this 
reason it is not correct in speaking of a motor to say "It required 
30 horsepower to start the load," because, when starting, the 
motor was at a standstill that is, there was no rotation, and 
hence it was developing no horsepower. The motor, however, 
was taking current and developing torque, and the correct expres- 
sion would be: The current taken at the start was equivalent to 
the current when developing 30 horsepower after the motor is up 
to speed." 

Motor Acting as Generator. In addition to the above it is 
essential that you always bear in mind that a motor is acting as a 
generator, aside from and in addition to its motor action. To 
think of this action it is necessary to forget for the time the 
torque action produced by the conductor in the magnetic field arid 
to think of the same conductor moving across the magnetic field 
and having an e.m.f. induced in it. The cycle of operations in a 
motor are briefly as follows: first, the magnetic field is produced; 
.second, current flows in the conductors producing torque; and third, 
the torque moves the conductors across the magnetic field and an 
e.m.f. is induced in them. This e.m.f. is called the counter-e.m.f., 
as its direction is practically opposite to the applied voltage, and 
it is practically equal to the applied voltage except for the small 
loss in the motor. It necessarily follows, that designing a motor 
is primarily designing a generator for the line voltage. With this 
conception of the operation of the motor and the fundamental 
formula for the e.m.f., it is a simple matter to write expressions 
showing how the turns in a motor should vary with different line 
voltages and for different speeds, etc. 

The above relations are sufficiently simple to be borne in 
mind at all times and they offer the readiest first-hand answer to 
the probable results of operating a motor under changed conditions. 

Classification of Probable Changes in Connections of Motor 
Winding. Before taking up some of the more common changes in * 
detail it will be best to make a classification of the changes which 
may be made. 



ARMATURE WINDING 157 

Some changes are such that the operation of the motor remains 
practically the samp as before reconnection. Such changes, for 
example, are represented by connecting the polar groups of a 
winding in series for 440 volts and in parallel for 220 volts. These 
changes will be referred to as Class A changes. 4 j 

A second class of changes leaves the performance of the motor [ 

in some respects unchanged and alters it in others. For example, a ' 

motor may be operated in star on 440 volts, and in delta on 220 j 

volts. In such a case there is a little change in the efficiency or 
power factor, but the starting and maximum torques are only 4 

about 75 per cent of their original value. The advisability of such " [ 

a change will depend upon the nature of the work this motor is 
doing. If the altered values of the torques are sufficient to start f 

and carry the load, there is no objection to operating the motor / 

when reconnected, .as the motor will not run any warmer. Such ] 

changes will be referred to as Class B changes. * 

A third class of changes leaves the performance of the motor '/ 

practically unchanged so far as the torque is concerned, but so 
alters its performance as to heating, or efficiency, or power-factor, 
or insulation, that it is undesirable to leave the motor operating 
indefinitely in such a condition. For example, such changes are 
represented by taking a three-phase motor and reconnecting the 
coils as they stand for two-phase. A change of this kind is equiva- 
lent to operating a three-phase motor at approximately 125 per 
cent normal voltage. In addition, the phase insulation between 
the polar groups will not be correct. There will be a large increase 
in the iron and heating losses and the power factor will be 

decreased. Such changes should be considered as emergency */* 

changes and the permanent changes made as soon as possible. * 

For convenience such changes will be referred to as Class C , { 

changes. { - 

Shop and Working Diagrams. Conventional Method. It is 
quite difficult and tedious to represent winding diagrams as shown 
in Figs. 194, 195, 196, and 197, and for this reason a conventional 
, method of giving the same information has been adopted. This 
method is shown in Fig. 215. In this scheme the various pole 
groups are represented by short arcs. The arrows on the arcs are 
shown simply to indicate a method of checking up to ensure the 





B C A 

Fig. 215. Conventional Diagram of Armature Winding 




Fie. 216. Schematic Diagram of Series-Star, 
Four-Pole Winding. Showing Same Connec- 
tions as in Tig. 215. The Numerals Indi- 
cate Corresponding Pole Groups 



ARMATURE WINDING 



159 



proper phase relations. There is considerable danger of getting a 
60-degree relation between the phases in a three-phase winding 
instead of 120 degrees; or, the wrong end of one of the phases 
may be connected to the star point. Arrows are put on all the 
pole-phase groups, and when all three phases are traced through, 
the winding is correct if the arrows on consecutive groups run 




C A 



Fie 217 Conventional Three-Phase, Four-Pole, Parallel-Star Diagram, and 
6 " . Schematic Equivalent 

alternately clockwise and counterclockwise. There is but one 
exception to the correctness of the check as shown in Figs. 215 
and 216, and the succeeding figures where the current is assumed 
as flowing toward the star in all three phases and the arrows 
alternate in direction. This one exception to the rule is the case 
where the winding forms consequent poles or. passes through all 
the phase-pole groups in a north direction instead of alternately 



160 ARMATURE WINDING 

north and south. Such connections are rarely used, and then 
usually on special motors wound for multi-speeds 

Parallel Star Diagram. A combined conventional and 
schematic representation of a so-called "parallel star" diagram, 
where the two halves of each phase are in parallel, is shown in 
Fig. 217. If a machine be connected originally as shown in Fig. 215 
and its voltage rating be 440 volts, it could readily be connected 




C A 

Fig. 218. Conventional Three-Phase, Four-Pole, Four-Parallel, Star 
Diagram, and Schematic Equivalent 

as shown in Fig. 217 and it would operate on a 220- volt circuit 
satisfactorily. The performance would be the same in all respects 
except that it would draw from the 220-volt line twice as many 
amperes for a given load as it originally drew from the 440-volt 
line. 

If the machine had four poles or a multiple of four poles, it 
could still be paralleled again, or connected 4-parallel star, as 



ARMATURE WINDING 161 

shown In Fig. 218, and operated on a 110-voIt circuit, and would 
still have the same performance but a correspondingly increase/! 
current at the same load. 

>' Delta Diagram. Fig. 219 represents a so-called delta, or 
mesh, connection. If a machine connected as in Fig. 215 for 440 
volts be reconnected as in Fig. 219, it will be suitable for opera- 




Fig. 219. Conventional Three-Phasr. Four-Pole. Series-Delta Diagram, and 
Schematic Equivalent 



,tion on a circuit having a voltage of 440-5-1.73 or 254 volts. 
Reconnections or conversions of this kind are shown in Table IX, 
where the preceding problem may be worked out by selecting 
3-phase series star in the horizontal column first line and reading 
across to the vertical column headed "3-Ph. Series Delta" where 
the figure 58 appears. 



'162 



ARMATURE WINDING 



* 

3 a 

CO 



I! 



sis 



1 

5 

1 
a 

t,1 
Si 



1] 



S PL< S pk PH PH PH &H PH pu pi, PH 



ARMATURE WINDING 



163 



This means that if 100 volts was normal on the series-star con- 
nection and a change is made to series delta, the corresponding 
voltage is 58 volts. If the series-star voltage was 440, then the 
series delta voltage would be 4.4 times 58, or 254 volts. Single- 
parallel and four-parallel delta connections are shown in Figs. 220 
and 221 respectively. 




J5' C A* 
(A-B)(B-CJ(A-C) 

Pitt 220. Conventional Three-Phase, Four-Pole, Parallel-Delta Diagram, 
* and Schematic Equivalent 

Two-Phase Dewkpment. A development of a two-phase 
binding is shown in Fig. 222. An inspection of the coils repre- 
sented in heavier lines indicates what is meant by phase-insulated 
coils or "phase coils." In changing from a two- to a three-phase 
connection, or vice versa, the position of these coils will change; 



164 



ARMATURE WINDING 



"hence the phase insulation is not sufficient in the case of the 
reconnected winding. 

The conventional and schematic equivalent oJ Fig. 222 is 
given in Fig. 223. The arrows shown in the three-phase diagrams 
are omitted here, for the reason that the two phases are not inter- 
connected, and the only effect of reversing one phase is to reverse : 




TV r A 1 

(B-C) (A-C) (A-B) 

Fig. 221. Conventional Three-Phase, Four-Pole, Four-Parallel, Delta 
Diagram, and Schematic Equivalent 

the direction of rotation of the motor. Either phase can readily 
be reversed by reversing the two leads of that particular phase at 
the motor terminals. Single- and four-parallel, two-phase connec- 
tions are given in Figs. 224 and 225, respectively. Where the 
number of poles is a multiple of three, as 6, 12, 18, etc., there is a 
possible 3-parallel connection; where the number of poles is a 



ARMATURE WINDING 



165 




166 



AKMATURE WINDING 

TABLE X 
Comparative Performances 




Full-load Efficiency, 
Full-load Power Factor, 
Starting Torque. . - 
Maximum Torque. 



Degrees C. Rise at Full Load for 

Stator Copper 

Stator Iron. . . . 
Rotor Copper. 




A, A^ B, B* 



Fig. 223. 



Series Diagram, 



ARMATURE WINDING 167 

.multiple of five, such as 10, 20, etc., there is a possible 5-paralIei 
connection. 

Three-Phase Development. A possible three-phase connection 
which may be made from a two-phase winding by a method simi- 
lar to the Scott transformer connection is shown in Fig. 226. The 
effect of this connection is shown in Table X. It should be used 
only as a temporary arrangement. 




Fig. 224. Conventional Two-Phase, Four-Pole, Single-Parallel Diagram, and 
Schematic Equivalent 

Least Common Multiple Connection. An interesting connection 
is shown in Fig. 227, and it is called the "least common multiple" 
connection. In this winding the number of slots in the machine 
is not a multiple of the phases times the poles, and as a result 
there are more coils in some groups than in others, which intro- 
duces a slight displacement at these points. The coils 



ARMATURE WINDING 



16g 

however, displaced around the .acbine, so -topjod- 

poles. , -,. 99 o an( j 230 show the 




to give two sets of poles or two speeds in the rate of two to c^ 
In Fie 228 the high speed is parallel star and the low speed 
t a ; The JLtion of the poles for 4, two-speed comvec- 
given in Fig. 228 is given in Fig. 229, wluch .s an explana- 
m showing schematically how the two sets of poles are 
In Fig. 230 the high speed is parallel star and the low 



ARMATURE WINDING 



169 



speed series delta. The connections shown in Fig. 228 give better 
results where a constant torque is required, and it gives twice the 
horsepower on the high speed that it develops on the low speed. 
The connections shown in Fig. 230 give somewhat better results 
where a constant horsepower is desired at both the low and high 
speeds, as in the case with most machine-tool applications. 

Two-Phase, Two-Speed Diagram. A similar diagram for a 
two-phase, two-speed connection where the winding is in parallel 

B, 

xjy^ayim/ -y yfflfflff^r "- 1 Wj/UWVJ f u i..u- _ rnnnjjnj| __ . 

7( 




Fig. 226. Diagram of Scott or T Connections 

on the high speed and in series on the low speed is shown in 
Fig. 231. A very interesting point in connection with this wind- 
ing is that it overcomes one of the disadvantages of the corres- 
ponding three-phase windings shown in Figs. 228 and 230, by 
putting half of the winding in one phase for the low-speed connec- 
tion and half in the other phase for the high-speed connection. 
This is of particular advantage as the distribution factor remains 
the same for both speeds, as in a normal two-phase machine. In 
the three-phase windings shown in Figs. 228 and 230, the distri- 
bution factor is only 86.1 per cent as good on the low-speed 



170 



ARMATURE WINDING 



connection as on the high-speed connection. This results in a 
loss in horsepower on the low-speed connection of approximately 
30 per cent. 

In all the previous diagrams the phase pole group has been 
treated as a single unit. That is, if 'there were four coils per phase 
per pole these -'four coils were connected in series in a group and 




A C 



Fig. 227. Conventional Three-Phase, Eight-Pole, Parallel-Star Diagram and 
Schematic Equivalent. (With Balanced Phases on Machine Having 00 
Slots. The Number of Coils in Each Pole Phase Is Shown by Roman 
Numerals.) 

handled as a unit. A type of connection which breaks up these 
groups is shown in Fig. 232. This type of connection is not very 
satisfactory and should not be resorted to except in an extreme 
emergency. 

Possible Reconnections. The following changes are those 
which are most frequently encountered in ordinary commercial 



AEMATURE WINDING 



171 



work, and the manner in which they can best be taken care of 
will be discussed briefly in the following paragraphs: 

Changes in voltage and phase of the supply circuit, which may occur singly 
or in combination. 

Changes in the frequency of the supply circuit. 

Change in the number of toles of the motor, which may be independent of 
all other changes because a faster or slower speed may be desired, or it may 
follow as a result of a change in frequency in order to keep the same speed on 
the driven machine when the motor is operated on the new fmflimigy. 




Fig. 228. Conventional Three-Phase, Two-Speed, Four- and Eight-Pole 
Diagram. Parallel-Star Connection for Four Poles, Series-Star Connection 
for Eight Poles. For Four Poles At, Bi, and Ci Are Leads and A, B. and C 
Are Connected Together. For Eight Poles A, B, and C Are Leads and At, 
Bi, and Cj Are Open 

Changes in Voltage Only, All Other Conditions Remaining the 
Same. This is the simplest change that can be made in an 
induction-motor winding. Most of these changes are covered in 
Table IX. You must constantly bear in mind that a motor may 



172 ARMATURE WINDING 

always be reconnected for a lower voltage so far as the insulation 
is concerned, but it should not be reconnected for a voltage much 
in excess, of that for which it was designed and insulated. 

Changes in Phase Only. The most frequent problem m this 
connection is the change from two- to three-phase, and vice versa. 
Theoretically, for the same voltage there should be about 25 per 
cent more total turns in a two-phase winding than in a three- 




phase winding. If a three-phase motor be reconnected for two- 
phase at the same voltage and the same number of coils, it wi'l 
exhibit all the symptoms of a motor operating at approximately 25 
per cent over-voltage, and would overheat to a dangerous degree 
after a short period of operation. On the other band, a two-phase 
motor reconnected as a three-phase motor at the same voltage and 
the same number of coils will exhibit all the signs of a motor 



ARMATURE WINDING 



173 



operating at 20 per cent under-voltage. In this case there are too 
many turns in series, and one-fifth of the total coils mfght be 
deadened so as to .secure the proper voltage on the remaining 80 
per cent. These dead coils should be distributed as symmetrically 
as possible around the machine so as to balance the voltage. It is 
not advisable to connect coils in parallel, as this gives a chance for 




and F Are Open 



unbalanced winding and circulating local currents, which may 
cause excessive heating. 

The current taken by a three-phase motor at full-load arid at 
any given voltage is about 12.5 per cent greater than the current 
taken by a two-phase motor under the same conditions. Hence, 
in order to keep the current density the same the three-phase 



174: 



ARMATURE WINDING 



horsepower will have to be cut down about 12.5 per cent from 
what it was in the two-phase connection. 

The so-called Scott or T connection may be used in operating 
a two-phase motor on a three-phase circuit. When this connec- 
tion is used 14 per cent of the coils in one phase of the two-phase 




Fig. 231. 



\Q, V \f AzQiA.B^B, 

1. Conventional Two-Phase, Two-Speed, Four- and Eight-Pole Dia- 
i. (For Eight Poles Connect y and y'. Use Ai, A, and BI, B for Loads 
Leave AI. ai and bi, bst Open. For Four Poles Connect Ai,.Bi Together 



and At, B* Together; Leave y and y' Open and Uae AI, a and bt. b for Leads 

machine are omitted as symmetrically as possible around the 
machine. The connections are shown diagrammatically in Fig. 226. 
This connection would give fairly good results if the coils 
between Ai and BI were so situated on the machine that they 
would be acted upon by the magnetic field in exactly the same man- 
ner as the coils between BI and At. Practically, as motors are 
wound nowadays, this is rarely possible, and if the usual winding 



i I'M i 



ARMATURE WINDING 



175 



is connected in T there are practically always unbalanced currents 
in the three phases. The current in the high phase will be about 
20 per cent greater than the current in the low phase. This results 
in a poorer performance in torque, power-factor, efficiency, and 
heating, as illustrated by the actual test data in Table X, which 
shows in three parallel columns the performance of a standard 




ABC 

Fig. 232. Conventional Three-Phase, Six-Pole, Four-Parallel Star Diagram 

motor wound with normal two-phase coils, with two-phase coils 
connected in T and run on three-phase, and with normal three- 
phase coils. The efficiency on the T connection is 1.6 per cent 
lower, the power-factor 5.2 per cent lower, the starting torque 38 
per cent lower, the maximum torque 4 per cent lower and the 
temperatures from 8 to 13.5 degrees higher than* on the normal 
three-phase winding. This showing certainly puts this connection 



176 AKMATURE WINDING 

in the C class. The motor operates (if it can start the load), but 
it should not be considered where a large number of motors are 
concerned or where, the cost of power is given any weight. This 
summary shows that changing from two-phase to three-phase, and 
vice versa, is at best very unsatisfactory, and the advice given in 
practically all cases of this nature is "Don't!" Better rewind with 
normal three-phase coils and avoid the host of troubles which fol- 
low in the train of an indifferently operating motor. 

Of course, one essential in any phase reconnection is to go 
over the winding and rearrange the "phase coils," or coils having 
heavier insulation, so that they will come properly at the ends of 
the groups where the voltage is highest. This is illustrated in 
Fig. 222. 

One case of voltage and phase change which works out very 
well is the change from three-phase, 550 volts to two-phase, 440 
volts, or vice versa. This uses all the turns in the winding for 
either connection, since the two-phase voltage should be about 80 
per cent of the three-phase and since the higher voltage on the 
three-phase cuts down the current, which would otherwise be 
higher than the two-phase circuit. If the phase coils are rear- 
ranged there is practically no objection to such a reconnection, 
and the motor will' give essentially the same performance . on 
either connection. 

Table IX shows the possibilities of these interphase connec- 
tions, as well as the different voltage changes. For example, "take 
the case just cited. Follow the horizontal line marked "2-Ph. 
Series" to the first vertical column headed "3-Ph. Series Star." 
The figure is 125. This means that a motor originally connected 
two-phase series, if reconnected three-phase series, should be 
operated on 125 per cent of the original voltage. Or, if the two- 
phase voltage was 440, the three-phase would be 1.25X440 = 550 
volts. The, convenience of Table IX is demonstrated for phase 
changes, as well as voltage changes, or for combinations of both. 

Changes in Frequency. The occasion often arises for changing 
25-cycte motors to 60-cycle, and 60 to 25. There is also some 
changing done from 60 cycles to 50, and from 50 to 60. Occasion- 
ally 40-cycle motors are changed to 60, but these changes are too 
infrequent to be of very general interest. 



ARMATURE WINDING 177 

In all cases of changed frequency the question that first 
arises is: How is the resulting change in speed to be taken care 
,of? The synchronous speed of any motor (which is only a slight ' 

per cent higher than the M-load speed) is given by the general 

alternations per minute , . 3000 

expression , - r- Inis would be ' .. 

number of poles number of poles j 

for 25 cycles, ~-^~~- for 60 cycles, etc. If, then, the 

number of poles 

frequency is changed and the number of poles left the same, the | 

resulting r.p.m. will vary directly as the frequency. This immedi- ! 

ately brings up two questions: First, is the mechanical design of \ 

the rotating part adequate to allow such a change in speed? | 

Second, can the speed of the driven machine be adjusted to suit I 

the new speed on the motor? j 

Consider first the case where the frequency is changed and j 

the number of poles remains the same. The resulting change in j 

speed in this case is taken care of either by applying the motor { 

to a new load or by changing the pulleys on the old load so as to 
keep the same r.p.m. on the driven machine. The next thing that 5 

must be considered is the necessary change in the voltage applied ' 

to correspond to the change in frequency, or the other way about, 
if the new circuit at the new frequency has the same voltage as 
was used with the original frequency, how can the coils in the 
motor be reconnected so as to get the proper voltage on each 
coil? 

The easiest rule to remember is: apply voltage on the motor * 

in exactly the same way as the frequency is varied. If this be 
done the magnetic field in the iron will remain the same and the t 

current in the stator and rotor coils will remain the same, if the ! 

motor is working against the same torque. This is another way * 

of saying that if the frequency and voltage are varied together, 
the motor will develop the same torque at all times and have 
flowing in it approximately the same current. As noted before, ( ! 

if the torque remains the same, the horsepower developed wiU j 

vary directly as the applied frequency. For example, a dO^jycIe, '\ 

50-horsepower motor operated on 25 cycles at 41.6 per cent of its j 

original voltage would develop the same normal full-load torque, [ 

Which would inean 20.8 horsepower. j 



178 



ARMATURE WINDING 



The case most commonly met with, which is changing from 
25 cycles to 60 cycles, can often be taken care of by impressing 
twice the voltage on the coils on 60 cycles as on 25 cycles, or in a 
concrete case operating a 220-volt, 25-cycle motor on ^440 volts, 
60 cycles, at about double the horsepower Theoretically, this 
should be. 60-^25=2.4 times the voltage, instead of twice, and 
the resulting horsepower would be 2.4 times. However, 2.4 times 
is usually hard to get, and two times comparatively easy In 
this case suppose the motor is connected in series star for 440 
volts on 25 cycles and it is desired to run it on 440 volts, 60 
cycles. It should then be connected in parallel star and run on 
440 volts, which would have the same effect as impressing 880 
volts on the original series connection. On 60 cycles the motor 
would then run 2.4 times as fast and develop about twice the 
horsepower. 

Sixty-cycle motors are often ^tun on 50 cycles without change. 
From the rule above-M;hat the voltage must vary with the 
frequency to keep the same magnetic densities it will be noted 
that the densities on 50 cycles at the same voltage will be six- 
fifths of the 60-cycle densities. The motor will then operate as 
if it had 120 per cent of normal voltage impressed. This will 
result in increased iron losses, which make the motor hotter; the 
decreased speed on 50 cycles with same number of poles makes 
the ventilation poorer; so the output of the motor in horsepower 
should be reduced to keep down the copper losses. This is logical 
in ansther way, because the horsepower at five-sixths speed should 
not be expected to be more than five-sixths of its full speed value. 
Another point that should be watched in changing frequency 
if the motor has a squirrel-cage rotor is that the rotor winding 
has enough resistance to give the proper starting torque. As the 
frequency is raised the resistance of the short-circuiting rings at 
the ends of the rotor winding should be increased to keep the 
same relative value of starting torque to full-load torque. As long 
as the motor starts its load satisfactorily no change is necessary, 
but if trouble is experienced the short-circuiting rings may have t6 
be changed for rings of higher resistance. Conversely, wheft 
decreasing the frequency the resistance can be reduced to advan- 
tage, thereby cutting down the rotor copper loss and the heating. 



ARMATURE WINDING 179 

/Where the frequency is to be changed but" it is desired. to 
Keep the same speed, the number of poles must be changed in 
the samej ratio as the frequency, or as nearly so as possible. 
For example, if a motor has four poles -and is operated on 25 
cycles it will have a synchronous speed of 3000*4=750 r.p.m. 
If the motor is to have the same speed on 60 cycles, the nearest 
possible pole number is 10 and the synchronous speed will be 
7200-*- 10 = 720. It, is apparent that in very few cases of this 
kind is it possible to reconnect the same winding. The main 
reason for this is in the throw or pitch of the coil. In the four- 
pole winding the individual coil spans approximately one-fourth of 
the stator bore, and in the ten-pole winding normal coils should 
span about one-tenth of the stator bore. 

In discussing the fundamental e;m.f. equation it was shown 
that the throw of the coil has an effect on the generated e.m.f. 
"This makes hardly possible such a condition as connecting 
a winding for ten poles when the individual coils 'have a four- 
pole throw. When reducing the frequency the number of poles 
should become smaller to keep the same speed, and this intro- 
duces Another 'difficulty in the magnetic circuit. In reconnecting 
the winding the object is to keep the total magnetic flux in the 
, machine the same as it was originally. This keeps the magnetic 
density in the teeth constant. This total magnetic flux is divided 
up into as joany equal parts or circuits as there are poles. The 
iron in the stator core between the bottoms of the slots and the 
outside of the core has to carry the flux for each magnetic circuit. 
Consequently, if there are ten poles and ten magnetic circuits 
the core iron below the slots has to carry at a given cross-section 
one-tenth of the total magnetic flux. With the. same total mag- 
netic flux, if there are only four poles and four magnetic circuits, 
the same cross-section of core has to carry one-fourth of the total 
magnetic flux, and this it is probably unable to do. This is the 
reason why the rotor diameter and stator bore of a 25-cycle 
machine are smaller than those of a 60-cycle machine of the same 
horsepower, and speed, although the outside diameter may be 
nearly the same. It is to get a larger cross-section behind the 
slots for the passage of the magnetic flux, since the total flux is 
divided into fewer parts, owing to the smaller number of poles, * 



180 ARMATURE WINDING 

From this it follows that a" machine may in general be rewound; 
or reconnected for a larger number of poles, but that great caution 
is required in reconnecting for a smaller number of poles. This 
leads up to the statement that it is easier to rewind or reconnect 
25-cycle machines for 60 cycles than it is to reconnect 60-cycle 
machines for 25 cycles. This follows logically from the physical 
fact that there is more copper and more iron in 25-cycle machines 
for the same horsepower, voltage, and r.p.m. than in 60-cycle 
machines. It is always easier to make changes where there is a 
larger supply of material available. Another condition that is , 
against changing the number of poles on a squirrel-cage motor is 
the current in the short-circuiting rings of the rotor winding. 
These rings are in nearly the same' case as regards current that 
the primary core is as regards magnetic flux. That is to say, the 
total secondary 'amperes, which remain nearly the same if the 
reconnection is done properly, are divided into as many circuits 
as there are poles, and it follows at once that the smaller the 
number, of poles the larger must be the cross-section of the short- 
circuiting rings, although the total secondary amperes remain 
nearly the same. Altogether, the possibility of reconnecting for 
different numbers of poles when changing frequency is usually a 
matter for the designing engineer to investigate. 

Changes in ike Number of Poles, All Other Conditions Remain- 
ing the .Same. The need for such changes comes from the desire 
to speed up or slow down the driven machine to meet new require- i 
ments. It might be broadly stated that there are many cases; 
where a change of two poles is permissible, as, for example/ 
changing from four poles to six, or from ten to eight, and the 
like. The change would consist in rearranging the phase coils to 
agree with the new grouping and checking the distribution factor, 
as mentioned above, to note its effect on the voltage. The proper 
diagram for the new speed can then be made up by comparison 
with the corresponding typical diagram. It is often possible to 
get a fair operating half speed by connecting for twice the number 
of poles, as shown in Figs. 228, 230, and 231. Practically all 
reconnections involving pole changes are Class B changes, in that 
they give only a fab operating performance. 

The procedure in checking up a machine to see if it can be 



ARMATURE WINDING 181 

reconnected is first to ascertain the existing connection and the 
throw of the coils in order to know what the possibilities are in 
the way of number of turns and throw. Second, if it is a phase 
or voltage change, find directly from Table IX what connections 
will give approximately the proper new voltage and new phase. 
If any one of these connections is possible with the number of 
poles in. the machine, select it as the new connection and arrange 
the phase coils properly at the beginning or ending of the groups, 
or at each end of the groups if there are enough of them in the 
old winding. Since the speed has not changed the horsepower 
should remain approximately the same, and the current in the 
coils themselves will remain somewhere near the original. If the 
frequency is to be changed either independently or in conjunction 
with a phase or a voltage change, the applied voltage should 
be changed in the same direction and by the same amount as 
the frequency is changed; if the voltage is to remain unchanged the 
number of turns in series in the coils should be changed in the 
opposite direction to the frequency and by the same amount. 
For example, if a 25-cycle motor is to be run on 30 cycles, it 
should have the voltage increased 20 per cent,, or else it should 
have the groups reconnected so that there will be 20 per cent less 
Jurns in series, and be run on the same voltage. 

If the number of poles is to be changed, and consequently the 
sp^ed, check first the effect of the coil throw with the new number 
of poles. Then think of the motor winding as generating eon.f. 
and bear in mind that with a constant field a higher speed wil 
generate more e.m.f. and a slower speed less e.m.f. Converted into 
voltage this means that with a higher speed a higher voltage should 
be applied in direct proportion, and that with a lower speed a lower 
i voltage should be applied. If the voltage cannot be changed try 
ito change the diagram of group connections so as to vary the 
'number of turns in series in the right way; i.e., if voltage should 
*be increased the same effect can be obtained by decreasing the 
[number of turns a like amount. In all these cases it is the voltage 
per turn or per inductor which counts, just as hi a transformer, 
'and a careful consideration of the effect of different connections 
will show whether the desired change in voltage per inductor is 
being accomplished. 



182 ARMATURE WINDING 

As a concrete .example of the foregoing, assume a 25-horse- 
power, four-pole motor operating on 40 cycles, two phases, 220 
volts. It is desired to know whether it can be reconnected to 
operate on 60 cycles, three phases, 550 volts at the same speed 
and horsepower. 

An inspection of the machine shows that it has 72 slots and 
72 coils and that any individual coil lies in slots I and 15, also 
that the groups are connected in parallel. Since there are 724-4 
18 slots per pole, each slot is 180*18 = 10 electrical degrees and 14 
slots =140 electrical degrees. (The throw of 1 to 15 means span- 
ning 14 slots.) The sine of one-half of 140 degrees (or 70 degrees 
=0.94) = chord factor; or, figured by the formula without trigo- 
nometry, since there are 18 slots per jpole and a throw of 1 to 15 
means dropping 4 slots from exact pitch, the chord factor** 

/18 2 -2 X4 2 
-^ T^p =0.948. The synchronous speed of the motor on 40 

cycles, as it stands is 4800* 4 1200 r.p.m. To get this same 
speed on 60 cycles it is evident the motor will have to be con- 
nected for 7200-*- 1200-6 poles. If the throw of the coils be left 
1-15 they will throw two slots further than full pitch, since 72*6 
= 12 slots per pole, and 1 to 13 would be exact pitch. Throwing 
the coil over pitch has the 'same effect asjthrowing it under pitch 

Ij2 2 i?x 2 

so the new chord factor on six poles s=-%/-^ - ~ 0,97^ r 8m ^ 

of one-half of 150 degrees =0.98. Taking into account the changes 
in phase, poles, frequency and chording, the new applied voltage 

per phase should be ^XjX^Xg^-305 volts. 

The explanation of this expression by terms is: The first term, 
880*3, comes from the change in phase from 2 to 3. Since 
'the original connection was in parallel and was for two-phase, the 
voltage across one phase in series would be 2X220 = 440, and the 
voltage across both phases in series would be 2X440 = 880 volts. 
If the winding is divided into three separate phases not inter- 
connected, the applied voltage on each phase would be 880-5-3. 
The next term, 4*6, represents the change in poles. A motor 
with six poles would run slower on the same frequency than a 
motor with four poles and would generate less counter-e.m.f. 



ARMATURE WINDING 183. 

Consequently, the applied voltage should be decreased in the same, 
proportion. .This should not be confused with the fact that the 
frequency is being changed in this case and the speed kept the 
same, for a separate factor .is introduced to take care of the fre- 
quency. The pole' .change should be considered as an item, 
separate from the frequency change. The next term, 60-5-40, is 
due to the change in frequency and is the application of the rule, 
to change the applied voltage directly as the frequency is changed. 
The last term, 0.98*0.94, is due to the difference in chord factor. 
With a throw of 1-15 the. coils are more effective to generate 
counter-e.m.f. on the six-pole than on the four-pole connection by 
the ratio of the chord factors. 0.98 to 0.94; hence the applied 
voltage should be raised with the counter-e.m.f. 

As just stated, this figure of 305 volts means that if the, 

winding is divided into three separate phases not interconnected in, 

any way the voltage should -be 305 volts across each phase. If 

the three phases are connected in star, as in Fig. 216, the applied 

voltage should be 1.73X305 = 528 volts. Since this is. only about 

i 5 per cent off from the 550 volts which is to be used, this motor 

will operate satisfactorily/ This calculation for voltage so far 

neglects the difference in the so-called "distribution factor 

between three phase and two phase, but this is immaterial. Tks 

factor acts the same way as the chord factor, and is about 0.956 

for any normal three-phase windings and 0.903 for **y normal 

two-phase winding/ so that the applied voltage should really be 

528x94^=558 volts, which is almost exactly what is required. 

This motor could then have its phase coils rearranged for six. 

poles and be connected series star and would be proper for tte 

new conditions. The changes involved do not materially affect tiie 

slip, so that, no change is required in the rotor winding. This 

example is not intended as an exact method of design, but sunply 

illustrates a rough, calculation to show what are the possibilities. 

After a motor is reconnected or after any change is made in 
the winding, start it up slowly, throwing the load on gradual 
and observing carefully to see if there are any signs of distress 
such as sudden heating, noise, or mechanical vibration. If tfre 
motor seems to operate normally, read the amperes m each phase 



184 ARMATURE WINDING 

and the voltage across each phase to see that they are balanced 
and are of reasonable amount. The full-load current for three- 
phase, 550 volts is somewhere near one ampere per horsepower 
for normal motors of moderate speeds between 5 and 200 horse* 
power.. At other voltages the current will be inversely as the voltage," 
that is, at 440 volts, three phases, about 1,25 amperes per horse-' 
power. On two phases the current per phase is about 87 per cent 
of the corresponding three-phase value. If the readings as above 
look reasonable, place a thermometer on the stator iron and 
another on the stator coils and note at 15-minute intervals for an 
hour, and at half-hour intervals thereafter, till the temperature is 
constant. The speed should be checked at intervals. If the r.p.ra. , 
show a tendency to decrease rapidly or fall below 90. per cent of' 
synchronous speed it* may be suspected that the rotor has tool 
much resistance and is getting hot. By making all these checks/ 
reasonable assurance may be had that the recormection is satisfac- 
tory and that damage to the machine is avoided. 

Conclusion. From the foregoing it can be seen that all 
changes, whether of phase, voltage, poles or frequency, may be 
considered as voltage changes and reduced to such terms. In 
making such calculations and comparing the results it is best not 
to apply a voltage that differs from the iigured proper voltage by 
more than plus or minus ten per cent. The general effect of ,high 
and low voltage may be expressed briefly, thus: 

High Voltage 

(a) Increases magnetic density 

(b) Increases magnetizing current 

(c) Decreases "leakage current' 1 (leakage reactive component) 

(d) Increases starting torque and maximum torque 

(e) Decreases slip or change in speed from no load to full load 

(f) Decreases secondary copper loss 

(g) -Increases iron loss 

(h) Usually decreases power factor 

(i) May increase or decrease efficiency and heating, depending upon 
the proportions, of primary copper loss and if on loss in the normal 
machine and also the degree of saturation in the iron 

'Low Voltage 

(a) Decreases magnetic density 

(b) Decreases magnetizing current 

(c) Increases leakage current 



ARMATURE WINDING 185 

(d) Decreases starting and maximum torque 

(e) Increases slip 

(f) Increases secondary copper loss 

(g) Decreases iron loss 

<h> Usually increases power factor ' 

(i) May increase or decrease efficiency and heating, depending upon' 
the proportions of primary copper loss and iron loss in the normal 
machine and also the degree of saturation in the iron 

Finally, it may be stated that: 

(1) Changes in voltage alone are the easiest class of changes and can 
usually be made. , . 

(2) Changes in number of phases alone can rarely be made satisfac- 
torily and are usually only makeshifts. 

(3) Changes in number of poles are limited, due to the mechanical form 
of the coils. 

(4) Changes of frequency alone or in combrnat||n with voltage or phase 
,can sometimes be made if changes in speed are not objectionable. 

(5) Complicated changes should not be attempted except by persons of 
some experience and should be handled with cautioH. 

(6) If the peripheral speed of the rotor (which equals rotor diameter 
in feetX3.14Xr.p.m.) exceeds 7000 feet per minute on any proposed change, 
the maker of the motor should be consulted before making the change. 

(7) In case of any doubt on any point refer 'to the manufacturer of the' 
machine.