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Full text of "Armature winding and motor repair; practical information and data covering winding and reconnectig procedure for direct and alternating current machines, compiled for electrical men responsible for the operation and repair of motors and generators in industrial plants and for repairmen and armature winders in electrical repair shops"

ENGINEERING LIBRARY 



ARMATURE WINDING 

AND 

MOTOR REPAIR 



ARMATURE WINDING 

AND 

MOTOR REPAIR 



Practical Information and Data Covering Winding and Reconnecting 

Procedure for Direct and Alternating Current Machines, Compiled 

for Electrical Men Responsible for the Operation and Repair 

of Motors and Generators in Industrial Plants and for 

Repairmen and Armature Winders in 

Electrical Repair Shops 



BY 
DANIEL H. BRAYMER, A. B., E. E. 

AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS MANAGING EDITOR 
OF ELECTRICAL WORLD FORMERLY EDITOR OF ELECTRICAL 
ENGINEERING AND OF ELECTRICAL RECORD 



FIRST EDITION 
TWENTY-SEVENTH IMPRESSION 



McGRAW-HILL BOOK COMPANY, INC. 

NEW YORK AND LONDON 



HfiiJl 

UttW 



ENGINEERING LIBRARY 

COPYRIGHT, 1920, BY THE 
MCGRAW-HILL BOOK COMPANY, INC. 



PRINTED IN THE UNITED STATES OP AMERICA 



THE MAPLE PRESS - VORK PA 



PREFACE 

In this book no attempt has been made to discuss the sub- 
ject of armature winding from theoretical or design stand- 
points. On the contrary, it is a compilation of practical 
methods tha't are used by repairmen and armature winders. 
In selecting the material a special effort has been made to 
include as far as possible details of those methods which have 
been found by actual experience to represent best practice in 
a repair shop of average size. In this work the writer has 
drawn from his own experience in repair work, from the ex- 
periences of repairmen and armature winders in large and small 
repair shops and manufacturing plants which have been 
visited, from descriptions of practical methods and the pro- 
cedure followed in the solution of special problems as presented 
by practical men in technical journals. 

The title of repairman as used throughout this book is one 
that a good engineer can bear with pride when he measures 
up to all its qualifications. Such an engineer is one who in 
in the majority of cases not only knows what to do in the case 
of an electrical trouble but just how to proceed to do that 
particular thing and who seldom guesses without a good per- 
centage of the probabilities of being right in his favor. The 
main difference between the designer and the repairman is 
that the former must know what to do while the latter must 
know how to do it. A capable repairman combines both 
qualifications through years of experience. 

When called upon to locate troubles in motors and genera- 
tors, electricians and repairmen whose experience in this kind 
of work has been limited often find themselves wondering just 
what to do first. It is from just this viewpoint that the infor- 
mation on winding procedure and the hunting and correcting 
of troubles has been presented. That is, instead of discussing 
the fundamentals involved in any method of working out a 
repair problem, the actual problem or job as the case may be 
is discussed from the " how-to-do-it" standpoint. Then for 
each individual operation or procedure the applications of 



M184967 



vi PREFACE 

fundamental laws and rules are worked out. Considerable 
repetition of some details of similar methods will therefore be 
discovered in connection with information covering such pro- 
cedure as the rewinding of machines of the same class but 
of different types. This has been considered advisable since 
a repairman should not be required to study a complete 
volume when details and information are desired at any one 
time on the procedure for a particular type of winding for a 
particular design of machine. 

Liberal use has been made of practical data and practices 
in repair shops so as to combine the good features of a book of 
methods with handbook information covering these methods. 
If this book shall help young repairmen to absorb information 
that can be secured otherwise only through years of experience 
in handling one job after another, and if the older and more 
experienced repairmen find it a handy source of reference as 
a supplement to their own stock of information, then the aim 
of the author will be accomplished. 

When material has been taken from the experiences of 
engineers and their recommendations on repair methods as 
published in the technical journals, it has been the aim to 
give credit to both the author and the journal in the para- 
graph, or section where the material is used. Special acknowl- 
edgment is made to A. H. Mclntire, editor of the Electric 
Journal, for permission to make liberal use of information con- 
tained in several articles compiled at his suggestion and pub- 
lished in the Journal. This material has been incorporated 
in Chapters 3, 8, 9 and 11. To A. M. Dudley, engineer of the 
industrial division, Westinghouse Electric & Manufacturing 
Company, the author is also especially grateful for sugges- 
tions and for permission to use details of methods which he 
has developed for reconnecting and testing induction motor 
windings. This information appears in Chapters 9 and 11. 
The diagrams at the end of Chapter 11 have been selected 
from a series of eighty-one devised by Mr. Dudley and 
shortly to be published in a valuable treatise on "Connecting 
Induction Motors." 

The author also desires to acknowledge the assistance ren- 
dered by H. S. Rich and Alex R. Knapp in the form of data 



PREFACE Vll 

:''' !"''.- 

and information compiled from their own experiences in solving 
a variety of motor troubles met with in industrial plants and 
in making repairs. To Henry Scheril, formerly a member of 
the engineering department of the Crocker- Wheeler Company, 
acknowledgment is made for helpful suggestions in arranging 
the material and for assistance in checking and reading the 
proof. Credit is also due and is hereby accorded to the elec- 
trical manufacturers who furnished the photographs from 
which many of the halftone illustrations were made. 

DANIEL H. BRAYMER. 
NEW YORK CITY, 
December, 1919. 



INTRODUCTION 

Through the courtesy of the author of this book the writer 
has had the privilege of reading the proofs. I have found, 
with great delight, that the treatment of the subjects discussed 
is not only clear and easily understood but always from the 
practical man's standpoint. While the book will appeal 
strongly to practical men engaged in repair shop work, power 
station work and the maintenance of motors in industrial 
plants, it will also appeal, in the opinion of the writer, to 
students of electricity. Since the material presented in this 
book, which I dare say is unique in its field, has been ob- 
tained from actual practical experiences and outlines the 
practical remedies that have been applied by repairmen in the 
solving of puzzling problems, it will be of decided assistance 
to men who are in need of such practical help. 

It is an " electrical book of knowledge," for in its pages 
readers will find answers to practically all armature winding 
questions and solutions of many of the repair problems that 
they will meet in practical work. The diagrams are clear and 
easily followed by the shop man and run in synchronism with 
the text. Theory with mathematical considerations have been 
resorted to only in a very few cases so that the reader of the 
book can make use of the information and understand the 
discussions of all phases of armature winding even though he 
may have only a limited knowledge of mathematics. 

A book of this kind, in spite of the errors that are bound to 
creep in, is a very valuable asset to any practical man who de- 
sires to enlarge his own stock of knowledge by learning how 
other men in similar positions have solved the many electrical 
problems that come to the repairman. 

HENRY SCHEBIL. 

NEW YORK CITY, 

December, 1919. 



IX 



CONTENTS 

PAGE 

PREFACE ........ f . ,. v 

INTRODUCTION. . \ .... ..... ix 



CHAPTER I 



DIRECT-CURRENT WINDINGS 

Action of a Commutator .'..'. . 1 

Types of D-C Armature Windings i . . ^ . . . 2 

Winding Parts and Terms 1 ....'.. 2 

Armature Conductor or Inductor '...?. . 3 

Winding Element or Section . .-. V.. :; .'H'. , V . . ". ... 3 

Armature Coils. . ^i~. '..?-. , ; i . . N . . ; ' . "". - . '. , . . . 4 

Winding Pitch or Coil Pitch . 6 

Front and Back Pitch. . ; vV v . . , : . . . . . . . . . ; '%. 

Full Pitch and Fractional Pitch Coils . . . -. . . \ . . '. . . '^ - ' t 

Symbols Used in Winding Formulas . . . . .":..,/ . . / 8 

Numbering Coil Sides in Armature Slots . .VV.Y . ... 8 

Lap Multiple or Parallel Windings . . . . ......... 9 

Formulas for Lap Windings Multiple; Single, Double and 
Triple Windings Meaning of the Term Reentrant Multi- 
plex Lap Windings. 

Wave Series or Two-circuit Windings 16 

Formulas for Wave Windings Multiplex Wave or Series- 
Parallel Windings Formulas for Series- Parallel Windings 
Symmetrical Windings. 
Possible Symmetrical Windings for D-C Machines of Different 

Numbers of Poles 21 

Equipotential Connectors T . .' ; . . : . . . . 21 

Best D-C Windings for a Repair Shop to Use . .,;;..'.. . 23 

Number of Armature Slots .....';. . 23 

Voltage between Commutator Segments . . .... ...... 24 

Number of Commutator Bars 24 

Usual Speeds and Poles of Different Sizes of Generators. ... 25 

Safe Armature Speeds . . . ... r .;.v v .......';. . 25 

CHAPTER II 

ALTERNATING-CURRENT WINDINGS 

Types of A-C Windings , i . 27 

Distributed Windings . . 27 

xi 



xii CONTENTS 

PAGE 

Concentrated Windings . .7. .... ..'..' 27. 

Spiral or Chain Windings . . - . . . . 27 

Lap and Wave Windings .'. : . . ; ... 28 

Whole-coiled and Half-coiled Windings . . * v V^' v *' ^L'V . . 30 
Single-phase and Polyphase Windings .......,;.... 30 

Coil Pitch ../I' 32 

Phase Spread of Winding 32 

Two-phase from Four-phase Windings . .../..... . . . 33 

Three-phase from Six-phase Windings . . . ... ....... 33 

Wire, Strap and Bar Wound Coils 34 

Methods of Laying out and Connecting A-C Windings .... 35 
Group Windings Full and Fractional Pitch Windings 
Simple Winding Diagram Reconnecting a Winding Simple 
Method for Indicating Polarity of Coil Groups Changing Star 
to Delta Connection A-C Wave Windings -Progressive and 
Retrogressive A-C Wave Windings Connections for Coils 
of Polyphase Windings Double-layer Winding, Lap Con- 
nected Connecting a Chain Winding Other Common 
Windings. 

Easily Remembered Rules for Arrangement of Coils in an Induc- 
tion Motor . . ; , , 51 

Simple Rules for Checking Proper Phase Relationship in Two- 
or Three-phase Windings . . ,. -., .... 52 

CHAPTER III 

REPAIR SHOP METHODS FOR REWINDING D-C ARMATURES 

Dismantling a D-C Armature 56 

Winding Data Needed for a Dismantled Armature 57 

Removing Old Coils 57 

Winding D-C Armatures Having Partially Closed Slots .... 60 
Winding a Threaded-in Coil Insulating Lining for Slots 
Inserting Coils in the Slots Insulating Overlapping End Con- 
nections of Coils Connecting Finish Ends of Coils to Com- 
mutator Loop Windings for Small Motors. 

Winding D-C Armatures Having Open Slots 66 

Winding and Insulating Coils Insulating Open Slots In- 
serting Coils in Open Slots Shaping End Connections- 
Truing up the Heads of the Winding Insulation Between 
Commutator End Connections. 

Winding Large D-C Armatures 77 

Coils for Large D-C Armatures Lap and Wave Windings for 
Large Armatures Insulating the Core Inserting the Coils 
Banding Wire Balancing Large Armatures Rotary Con- 
verters Three-wire Generators. 



CONTENTS xiii 

PAGE 

Winding Railway, Mill and Crane Types of Armatures .... 90 
Railway Type Armature Coils Coil Insulation Insulating 
the Core of Railway Armatures Inserting the Coils Con- 
nections with Dead Coils Hooding and Banding. 



CHAPTER IV 

MAKING CONNECTIONS TO THE COMMUTATOR 

Locating First Connection to Commutator 101 

Testing Out Coil Terminals 102 

Commutator Connections for a Lap Winding. . . . . . . ' . . 102 

Requirements of a Lap Winding 103 

Commutator Connections for a Wave Winding 104 

Locating First Connection to Commutator for a \Vave Winding . 105 

Requirements for a Wave Winding 107 

Progressive and Retrogressive Wave Windings . . . .'". . . . 107 

Wave Winding with Dead Coils . V. : 108 

Cutting out Coils of a Retrogressive and Progressive Wave 

Winding 109 

Tables for Placing Coils and Connecting Them in a D-C Winding 110 

Wave vs. Lap Windings '....,' l! . ..'... . . 112 

Lap Windings for D-C Armatures . . . \ ''.*/." '". . . '.' . . . 115 

Lap Windings for A-C Machines '.,-. . . . 116 

Wave Windings for D-C Armatures ,; V/'V . . . 117 

Wave Windings for A-C Machines ;,v. .- . . . .118 

Single vs. a Number of Independent Windings . ; . . . . . .118 

Lap Windings vs. Multiple Wave Windings 119 

Use of Equalizer Rings .-. T ..../:...'.;:' ^ .... 120 



CHAPTER V 

TESTING DIRECT-CURRENT ARMATURE WINDINGS 

Causes of Short Circuits in an Armature. . . ... ... .. ,. . . . 122 

Test for Short Circuits in an Armature . . . ...... . .' . v . . 123 

Testing for Short Circuits and Open Circuits with a Small Trans- 
former ...-;. / v ... : ,....,. . . 125 

Causes of Open Circuits in an Armature 127 

Tests for an Open Circuit in an Armature 128 

Cutting Out Injured Coils 129 

Causes of Grounds in an Armature 130 

Tests for Grounds in an Armature 131 

Use of a Bar Magnet and Millivoltrneter to Locate a Reversed 

Armature Coil 

Use of a Compass to Locate a Reversed Armature Coil . . . .132 

Locating Low Resistance or Dead Grounds 133 



xiv CONTENTS 

PAGE 
Use of a Telephone Receiver in Testing for Short Circuits, Open 

Circuits and Grounds 135 

Testing for Reversed and Dead Field Coils 136 

The Commutator : ,.. . s . . 136 

Testing Equipment for a Repair Shop . .,,.-. ... . . . . . 138 

CHAPTER VI 

OPERATIONS BEFORE AND AFTER WINDING D-C ARMATURES 

Stripping Off an Old Winding 139 

Cleaning and Filing Slots 140 

Testing Commutator -, , y- .... 140 

Making New Coils 140 

Forms for Winding Coils f ..... 141 

Insulation of Core and Slots 144 

Testing Out the Winding V:* j y ;v.^ A . . . 144 

Soldering Coil Leads to the Commutator . , . .,-.,,. . . . . . 144 

Hoods for Armatures . . ...... ... . . .,.-.. . . . . 145 

Banding Armatures ,. . ..... . 146 

Seasoning and Grinding a Commutator . . ,- ..... ... . . 148 

Undercutting Mica of a Commutator . .... . '*. 150 

Balancing an Armature ., . ... '...-.-. .. ^ . ... * - , 150 

Painting the Winding . . . . N . ..;.* ... .,.<.. ... 151 

Relining Split Bearings . . . ... , . :. . , '. . , . .... . 151 

CHAPTER VII 

INSULATING COILS AND SLOTS FOR D-C AND A-C WINDINGS 

Insulation for Armature Coils and Slots . . . . . 153 

For Mechanical Protection and Electrical Insulation For 
High Temperatures and Electrical Insulation For Electrical 
Insulation Only. 

Descriptions and Uses of Insulating Materials ..... . . . 156 

Treated Cloths Pressboards, Fibres and Papers Coil and 
Slot Insulation Used in One Large Repair Shop Micarta- 
folium. 

Thickness of Insulation Required in Slots 163 

Insulation of Formed Coils -. ". . . . . 163 

Insulation for Coils Used in 240-Volt and 500-volt D-C Machines. 165 

Coil Insulation for Induction Motor Windings . . . / ; . . . 167 

Coil and Slot Insulation Employed by a Large Manufacturer. . 168 

Insulation of End Connections of Coils .... . ' / :r. .' . . 172 

Phase Insulation When Reconnecting from 2-phase to 3-phase . 173 

Mica Insulation for Armature Coils .!.;-.. . . 173 

Repairing Coils Damaged in Winding Process . -. . .'"'. . . . 174 

Voltage to Use When Testing Coil and Commutator Insulation. 175 



CONTENTS xv 

PAGE 

Field Coil Insulation . 179 

Varnishes and Impregnating Compounds for Coils 176 

Characteristics of Insulating Varnishes 178 

Solvent Chart for Insulating Varnishes 178 

Method for Making Tape from Cotton Cloth . 181 

Drying Out Insulation of D-C Generators 181 

Drying Out Insulation of Synchronous Motors 182 

Drying Out Induction Motors 183 

Measuring Insulation Resistance >;;';>-. . 185 



CHAPTER VIII 

REPAIR SHOP METHODS FOR REWINDING A-C MACHINES 

Winding Small Single-phase Motors. . ...'.. . i- ;''';".'".''.. . 186 
Insulating Lining for Slots Winding the Skein Coil Insert- 
ing Skein Coil in Slots Winding for a Repulsion-start Motor 
Winding Small Motors by Hand Windings for Odd Fre- 
quencies Connections for Main and Starting Windings 
Testing Small Induction Motor Windings Windings for 
Small Polyphase Induction Motors. 

Winding Small Induction Motors with Formed Coils in Partially 

Closed Slots ; *'..;.. . . 194 

Insulation for Slots Basket Coils Winding a 3-phase 
Stator with Basket Coils Threaded Diamond Coil Winding 
a 3-phase Stator with Diamond Coils. 

Winding Induction Motors Having Open Slots. . . . ; . . . 201 
Winding a 2-phase Stator Having Open Slots Testing the 
Windings Inserting a New Coil in a Winding Connecting 
the Coils Points to Consider when Connecting Coils Cleats 
and Terminals Painting Winding. 

Induction Motor Secondaries . 207 

Squirrel-cage Secondaries Phase- wound Secondaries. 

Winding Large Alternating Current Stators . . . ' . v . . . . . 210 
Coils for Partially Closed Slots Coils for Open Slots Lap 
and Wave Connections Insulation of Coils Inserting 
Shoved Through Concentric Coils Bar and Connector Wind- 
ing Diamond Coils Double Windings Testing Windings 
of Large Machines Connecting the Coils Bracing Needed 
for Heavy Windings. 

Winding the Stator of Alternating Current Turbo-generators . . 223 
Coils for A-C Turbo-generators Forming the Coils Insula- 
tion for Turbo-generator Coils Testing Turbo-generator 
Windings Inserting the Coils in a Turbo-generator Bracing 
for Windings Connecting the Winding Break-down Test. 



xvi CONTENTS 

CHAPTER IX 

PAGE 

TESTING INDUCTION MOTOR WINDINGS FOR MISTAKES AND FAULTS 

Testing for Grounds and Short-circuits . . . . .^ .,'.. . 231 

Reversal of One or More Coils or Groups . . . . .> , . . . . 233 

Open Circuits v . ..> . . . 234 

Placing Wrong Number of Coils .,:.,.*.. . N . . 234 

Using an Improper Group Connection . . . 234 

Order in Which Tests Should be Made f; - . . . 235 

Connecting for the Wrong Number of Poles 235 

Applying Direct Current and Exploring with a Compass. . . . 236 

CHAPTER X 

ADAPTING DIRECT-CURRENT MOTORS TO CHANGED OPERATING CON- 
DITIONS 
Changes in Speed. . . .^ ...... ..... r ,.'...-.-.. . . 237 

Changes in Operating Voltage . . . fe , .>,:.: . . , . ." . 238 
Operating a Motor on One-half or Double Voltage . . . , . . 239 
Size of 'Wire for D-C Armature Coils . > . .. / . ^ . . . 240 
Operating a Generator as a Motor and Vice Versa . ..... . . 241 

Motor Speed when Reconnecting a D-C Motor Winding Wave 

to Lap 242 

Adjusting the Air Gap on Direct-Current Machines. ;. N . v '.,. . . 242 
jChange in Brushes when Reconnecting D-C Motor from a Higher 

to a Lower Voltage 243 

Rewinding and Reconnecting D-C Armature Windings for a 

Change in Voltage 243 

Reconnecting a Lap Winding Reconnecting a Wave Winding 

Reconnecting Duplex Windings. 

CHAPTER XI 

PRACTICAL WAYS FOR RECONNECTING INDUCTION MOTORS 

Points to Consider before Making Reconnections ....... 262 

Diagrams for Different Changes of Connections . .... 262 

Diagrams for Three-phase Motors 265 

Use of Table of Connections V?^> ; . . 267 

Two-phase Diagrams . .. ^ V , >-...- v .-. . 267 

Meaning of the Term Chord Factor. . .. . ... v .-, \ : * . . . . 269 

Phase Insulation V. , I '-.- . . . 271 

Reconnecting Motors to Meet New Conditions. / 272 

Procedure when Considering a Reconnection of Windings 
Practical Example for Reconnection Changes in Voltage 
only with all Other Conditions Remaining the Same 
Changes of Phase Only Changes in Frequency Changes in 
Number of Poles Testing a Reconnected Winding. 



CONTENTS xvii 

PAGE 

Effects of High and Low Voltage on Motor Operation 283 

Operating Standard A-C Motors on Different Voltages and 

Frequencies. ,,-.... 284 

Factors which Limit a Change in Number of Poles of an Induction 

Motor . . ". ., 286 

Single-circuit Delta and Double-circuit Star Connections . . . 287 

Cutting Out Coils of an Induction Motor 288 

Procedure When Connecting Coils of an Induction Motor 

Winding 288 

Connecting Pole-phase-groups of a Winding General 
Theory on Which Connection Diagrams are Constructed 
Determining Number of Poles from Slot Throw of Coils 
Typical Circle Diagrams for Connecting Induction Motors. 

CHAPTER XII 
COMMUTATOR REPAIRS 

Causes of Commutator Troubles ...'..... . . 301 

Troubles Resulting from High Mica 301 

Remedy for High and Low Bars . . '" . 302 

Burn-out Between Bars . . . . . . . . . . ..... . , . 303 

Plugging a Commutator. .V. ; V': :. ..,...; . . . . . 304 

Removing Bars and Mica Segments for Repairs . . . . . . 304 

Repairing a Burned Commutator Bar . . ..'..: ... . . 305 

Replacing a Repaired Commutator Bar . . . . -, : . . >. . . . 306 

Tightening up a Repaired Commutator . . ... .... ... 306 

Baking Commutator with Electric Heat . .. . ,. ; ; ;-> 308 

Removing and Repairing Grounds in a Commutator 308 

Turning Down a Commutator without Removing Armature . . 309 

Temporary Cover for Use When Turning Down a Commutator. 310 

Refilling a Commutator 311 

Boring Out the End of a Commutator , \ ^ . . . 313 

Mica Used in Commutators ... > .- -315 

Shaping Mica End Rings . . 316 

Templet for Making Mica End Rings '. . . . /:./,. 316 

Micanite as a Commutator Insulation . . . ; . '. . 317 

Precautions when Tightening a Commutator. ... , . ... '. . . 317 

Making Micanite End Rings ' . . . 

Causes of Excessive Commutator Wear . . . . . ' . . . . 

Copper Used for Making Commutator Bars ..... . . 319 

Test for Oil-saturated Mica in a Commutator . 
Blackening of a Commutator at Equally Spaced Points . 

Undercutting Mica of Commutators 320 

Tools for Undercutting Mica Size of Circular Saw Required 
Finishing Slots and Commutator Surface after Undercut- 
ting Brushes for Use on Undercut . Commutators Hand 
Tools for Undercutting Mica of Commutators. 



xviii CONTENTS 

CHAPTER XIII 

PAGE 

ADJUSTING BRUSHES AND CORRECTING BRUSH TROUBLES 

Fitting or Grinding-in Brushes . . . v 326 

Adjustment of Brushholders 329 

Causes of Rapid Brush Wear , . % , . .331 

Methods for Locating the Electrical Neutral in Setting Brushes . 332 

Angle at Which Brush is Set. . . v 333 

Checking Brush Setting 334 

Brush Pressure /. ', .,!.= .'. . . 334 

Common Brush Terms . v vs. : , . . 334 

Procedure for Locating Causes of Brush Troubles. . . ... . 337 

Too Low Brush Pressure Incorrect Spacing of Brushes 
Brushes Not Operating on Electrical Neutral Incorrect 
Thickness of Brushes Using Brushes of Wrong Character- 
istics. 
Causes and Remedies for Sparking at Brushes . .... . ... . . 341 

Causes and Remedies for Flat Spots on Commutator ..... 342 

Causes and Remedies for Blackening of Commutator . . . . . 343 

Causes of Heating in a Motor or Generator . . .- \ . ... . 343 

Causes and Remedies for Honey-combing of Brush Faces . . . 343 

Causes and Remedies for Brushes Picking up Copper 343 

Causes and Remedies for Brushes Chattering .... N ^ 344 
Causes and Remedies for Loosening of Brush Shunts .... . 344 

CHAPTER XIV 

INSPECTION AND REPAIR OP MOTOR STARTERS, MOTORS AND GEN- 
ERATORS. 

Cost of Repairs for Polyphase Motors 346 

Points to Consider When Estimating Cost of Motor Repairs . . 347 
Inspection and Overhauling of 

Direct-Current Motor Starters. . >. . X 349 

Auto-starters for A-C Motors . .... 351 

Drum Type Controllers . ... . . % . . . 355 

Large Compound D-C Motor 358 

50-Hp. Induction Motor . ... . . . , 363 

25-Hp. Slip-ring Motor ,. . ... . . . . . ;.*'. 366 

Single-phase Commutator Motor . . . . . :.'. . . '-. . . . 368 
Direct-Current Engine Type Generator. . . . < . .... . 371 

CHAPTER XV 

DIAGNOSIS OF MOTOR AND GENERATOR TROUBLES 

Lack of Proper Cleaning >..* 376 

Operation in Damp Places . . . . 7 . *^,. . ;..,.*.:> 377 

Exposure to Acid Fumes and Gases 377 

Lack of Frequent Inspection and Replacement of Worn Parts . . 378 



CONTENTS xix 

PAGE 

Operating Temperatures too High 378 

Electrical Defects 379 

Causes and Remedies for Troubles in A-C Machines 386 

Induction Motor Troubles Locating Troubles in Windings 
of Induction Motors Mechanical Adjustments Troubles 
Due to Electrical Faults Troubles with Synchronous Motors 
Causes of A-C Motor Fuses Blowing Inspection of 
Motor Starting Devices Testing Motor for Grounds Hot 
Stator Coils Tension of Belts Troubles in Rotor Wind- 
ings Examination of Stator Winding Sparking at Slip 
Rings. 



CHAPTER XVI 

METHODS USED BY ELECTRICAL REPAIRMEN TO SOLVE SPECIAL 
TROUBLES 

Sparking at Commutator Caused by Poor Belt Joints 394 

Plugging a Commutator Uv;.'. , . v v . .' * . . 395 

Knock in an Armature Due to Band Wires Being too High . . 395 
Heating of Armature Traced to Poor Soldering of Commutator 

Connections 396 

How a Commutator was Repaired under Difficulties 397 

Holder for Sandpapering Commutator ; . . . 400 

Use of Portable Electric Drill to Undercut Mica 400 

Jerky Operation of New Commutator Traced to Burred Commu- 
tator Bars . . . . .'-..' ..... 402 

Why Brush Studs Heated on an 8-pole Machine . . . ..- , . . . 403 

An Accident Due to Incorrectly Set Brushes 404 

Wrong Setting of Brushes for Direction of Rotation Caused 1 lotor 

to Flash .V. .-,'. 405 

Proper Adjustment of a Reaction-type Brush-holder 406 

Heating of Brush-holders Traced to Defective Contact Springs . 407 

Simple Scheme for Banding Armatures 408 

Use of a Crane to Band an Armature . . . '.' . . . ". . . . . 409 
Method Used to Band a 2000-Hp. Rotor . '., . V : . .... . 410 

Improvised ft ethod Used to Turn a Commutator 411 

Cause of a Motor Reversing its Direction of Rotation on High 

Speed _.;': 

Checking Connections of an Interpole Motor 

Heating of Field Coils Traced to Wrong Type of Starting Box . .414 

Safe Operating Temperature of Portable Desk Fans 415 

An Adjustable Shunt for Series Fields of Exciters. . .416 

A Peculiar High-speed Motor Trouble. . . ...'.... . 417 

Ways that End Play Variations Show Up .418 

Connections for Two 220-volt Motors When Operated on 440 
Volts. .\. 419 



XX CONTENTS 

PAGE 

Cleaning Motors with Compressed Air 420 

Testing out Phase Rotation 420 

An Induction Motor Trouble Due to Wrong Stator Connections . 421 
Stalling of Wound Rotor Induction Motor Explained . .... 422 
Loose Bearing Caused Induction Motor to Fail to Start .... 423 

Three-Phase Motors Used on Single-phase Lines 425 

An Apparent Overload Trouble That was Traced to a Defective 

Fuse Block 427 

Cause of Noise in Three-phase Motor Driving Exhaust Fan. . 428 
Cause of Burned out Starting Winding in a Single-phase Motor. 429 
Cause of One Motor Failing to Start While Another was Running 

on Same Circuit 430 

Cause of Synchronous Motor Failing to Start . .... . . . 432 

Effect of Decreased Frequency on Induction Motor- Generator Set 433 
Simple Rules for Reconnecting A-C Motors . . . . . ... . 434 

Changing 440-volt Motor for Operation on 220 Volts . ; -' 435 
Multiple Connection Diagram for A-C Motor Windings .... 436 

Brush and Slip-ring Sparking Traced to Absence of Rotor Balanc- 
ing Weights 437 

Overheating of an Induction Motor Traced to Variation in Fre- 
quency. . . . . V . . * , ". . . . . . . . V , .. . 438 

Relief for a Hot Bearing. . . : . . . . . . ',. ,, ... . . . , . . 440 

Static Sparks from Belts . . . . _. -. ... o ... . V. ,- ... . 440 

Ratings of A-C Generators. . . : . . ...*,.*.*. . . 440 

Alternating Current Motor Phase Rotation . . '.',, ;, ,- .... 440 

End Bells or Heads . ....;..... ,'..,.,... . 441 

Brushes and Brush Holders . ',. ; .;.. . . . . .,;.,..... . 441 

The Rotor ,= ,>.!..:. -.'...-. . ... > s A ..\ . 441 

The Stator , . >.,-,,;.;. . 441 

Sizes of Fuses for A-C Motors ...... .";..-.. . . . 442 

CHAPTER XVII 

MACHINE EQUIPMENT AND TOOLS NEEDED IN A REPAIR SHOP 

Armature Winder's Tools 445 

Device for Shaping Insulating Cells of Armature Slots 447 

Tool for Cutting Cell Lining at Top of Slot . : . . . . ...,.'. . 447 

Special Winding Tools 449 

Repair Tools that can be Made from Old Hack-saw Blades . . 451 

Special Coil- winding Device . . . . . . . ,, rt^-> . . 452 

Steadying Brace for Repairing Small Motors. . , . ... . . 454 

Tension Block for Use when Banding Armatures ... . . 454 

Armature Sling . . , . . . . . V . . 455 

Pinion Puller ...'... .456 

Coil Winding Machines ! . . .. . ... . . . . 456 

Qoil- taping Machines . ....,..,.,,,;,*.>'< 4^7 



CONTENTS xxi 

r ( k 

PAGE 
Commutator-slotting and Grinding Machines . ; .!. '-. . . . . 459 

Armature Banding Machine . V . . . ' . .-. . v . . . . . . 462 

Combination Machines ...... .; '.-.', . . 462 

APPENDIX 

DATA AND REFERENCE TABLES 

How to Remember the Wire Table . . . - . - - \'', . . ,-. . . . 465 

Copper for Various Systems of Distribution . . . . V .... . 465 

Classification of Wire Gauges > 1, . .... 466 

General Wiring Formulas for A-C and D-C Circuits . . . . . ' 468 

Data for Connecting Motors to Supply Circuits .... . . . 473 

Voltage, Horsepower and Speeds of Motors . . > ^- , . . 478 

Transformer Rating for A-C Motors ... ........ 478 

Sizes of Fuses. Switches and Lead Wires for Motors of Different 

Sizes on Different Voltages ...-.. . . . 484 

Circuit Breakers for Overload Protection of Motors 489 

Belting .''..... . . .490 

Rules for Pulley Sizes . v . . . . . 492 

Speed of Pulleys ... , . . v . . . ... 492 

Chain Drives _ . . 493 

Points to Consider when Calculating Size of Chain 493 

Horsepower Transmitted by Steel Shafting . . A . v 495 

Horsepower Transmitted by Single Ropes . -. . ; . . . . . . 495 

Gear Table .'..-. 496 

Some Handy Rules . . . , , .% . /.'..;.... 498 

INDEX . . 500 



ARMATURE WINDING S 

AND 

MOTOR REPAIR 

CHAPTER I 
DIRECT-CURRENT WINDINGS 

The essential physical differences between a complete direct- 
current and a complete alternating-current armature winding 
is that the former is wound on the rotating member of the 
machine while the latter is wound on the stationary member 
and that the direct-current winding requires a commutator 
while the alternating-current winding does not. However, 
since the practical make-up and construction of windings 
will be discussed later for particular types of direct and 
alternating-current machines, the general theory of arma- 
ture windings will likewise be taken up first for direct-current 
and then for alternating-current machines (see Chapter II). 

Action of a Commutator. The emf and current produced 
in each armature conductor of a direct-current generator is 
alternating in character. It is the function of the commutator 
to deliver from the armature winding an electromotive force 
and current that is unidirectional, that is, such that one termi- 
nal will be always of positive polarity and the other of nega- 
tive polarity. The commutator and its brushes accomplish 
this by being connected in series between the generator leads 
and the armature windings so as to reverse (in effect) the con- 
n3ctions of the armature coils (connected to the commutator 
bars) with respect to the machine leads every time the emf 
and current induced in these coils reverse upon moving out of 
the influence of one pole into the field of the next adjacent 

1 



2 ARMATURE WINDING AND MOTOR REPAIR 

pole. The alternating emf and current generated in the 
armature winding is thus rectified or commutated into a uni- 
directional emf and current. 

In the case of an alternating-current generator no such 
rectifying of induced emf and current is necessary so that the 
coils or elements making up the armature winding can be 
connected directly together with the resulting terminals of 
the winding becoming the terminals of the machine. 

Types of D.-C. Armature Windings. In general, armature 
windings are either of the open circuit or the closed circuit 
type. The latter is used in all modern direct-current ma- 
chines, while alternating-current machines may have either 
open or closed windings. In the closed circuit winding of 
the direct-current machine the end joins up with the begin- 
ning or re-enters itself with the commutator tapped to the wind- 
ing at equally distant points. In the case of open circuit 
winding of an alternating-current generator wound on the 
revolving member, the ends terminate in collector rings and 
the winding is thus open until closed by the brushes of the 
external circuit. When the winding is on the stator of an 
alternating-current machine the ends are joined through the 
load circuit. The following classification of closed circuit or 
direct-current windings may be made: 

Direct-current windings (closed circuit). 

1. Lap multiple or parallel. 

(a) Single lap. 
(6) Multiplex lap. 

2. Wave series or two-circuit. 

(a) Single wave. 

(6) Multiplex wave or series-parallel windings. 

Winding Parts and Terms. In formulas for armature 
windings and in laying out a repair job, certain terms are used 
which refer to parts of the armature winding, the armature 
core and details of the arrangement of the former in the slots 
on the surface of the latter. In what follows these terms are 
explained. In most cases they are used alike both in windings 
for direct-current and for alternating-current machines. 



DIRECT-CURRENT WINDINGS 3 

Armature Conductor or Inductor. That part of a wire 
which lies in an armature slot and cuts the magnetic lines of 
force or field flux as the armature rotates, is called an armature 
conductor or an inductor. 




FIG. 1. Different types of coils used in armature windings. 
(A) One-piece series diamond strap coil. Leads at end of straight part. (B) One-piece 
series diamond coil. Leads at end of straight part. (C) Two-piece series diamond coil. 
(JD) One-piece multiple diamond coil. Leads at point of diamond. (E) Two-piece 
multiple diamond coil. (F) Concentric coil bent down at both ends. (G) Concentric 
coil, straight. (HI One-piece wire wound involute coil. Leads at point of involute. 
/) Two-piece involute coil. Leads at point of involute. (J) Threaded-in type diamond 
coil. Leads at point of diamond before and after pulling. (K) Basket coil. (LI Same 
as B of threaded-in type. (M) Same as D of threaded-in type. (AO Bar and involute 
end connector. (0) Group of concentric end connectors. (P) Concentric shoved 
through type coil bent down on one end. 

Winding Element or Section. That part of an armature 
winding which is connected between two commutator bars is 



4 ARMATURE WINDING AND MOTOR REPAIR 

called a winding element. In its simplest form a winding 
element consists of a coil of one turn of wire or two conduc- 
tors. An element therefore must have at least two conduc- 
tors but may consist of more than one turn of wire or even 
number of conductors. 

Armature Coil. When a winding element consists of more 
than one turn of wire or two conductors, it is usually known 
as a coil and the winding is a coil winding as distinguished 
from a bar winding, where the conductors in the armature 
slots are copper bars. 

From a mechanical standpoint an armature winding con- 
sists of a number of coils connected to a commutator in the 
case of a direct-current machine or connected together in the 
case of an alternating-current machine to form a series or 
group. Each coil may be made up of one turn of wire with 
each side forming one armature conductor or inductor, or a 
coil may be made up of several turns of wire or of copper 
strips. A classification of the different types and uses of 
armature coils which has been made by R. A. Smart 
(Electric Journal, Vol. VII, No. 6) is given in the accom- 
panying table. 

The so-called form-wound, diamond coils mentioned in the 
table are formed and completely insulated before being as- 
sembled on the armature. They can only be used in open 
slots Concerning the advantages of diamond coils, involute 
coils and concentric coils, Mr. Smart has the following to 
say: "The great advantage of diamond coils is the easy and 
simple manner in which they can be manufactured, especially 
in large quantities, which makes them well adapted for stand- 
ard machines. Since all the coils used on one machine are 
of the same size and shape, only one winding mould over 
which to form them is necessary. Moreover, the number of 
spare parts which must be kept on hand for repairing is re- 
duced and repairs can be made easily and quickly. From 
the electrical point of view, the diamond type of winding 
possesses the advantage of being absolutely symmetrical. 
Hence there is no tendency for unbalancing of voltages due 
to differences of self-induction; and in closed windings there 
is no tendency to produce internal circulating currents." 



DIRECT-CURRENT WINDINGS 



CLASSIFICATION OF ARMATURE COILS ACCORDING TO SLOTS IN WHICH 

THEY ARE USED, THEIR FORM AND TRADE NAMES AS EMPLOYED 

IN BOTH DIRECT- AND ALTERNATING-CURRENT-WINDINGS 









( Leads at ends of straight part 








1 Leads at point of diamond 






Involute 


j Leads, at ends of straight part 




Open slots 




\ Leads at point of involute ' 






Short type 


involute 


Leads at point of involute 






Concentric 




Straight 










Bent at both ends 


Mould wound 
coil of insulated 




Shoved through con- 


Straight 
Bent at one end 


wire or ribbon 




centric 




Bent at both ends 




Partially 


,' 
Threaded 


. ' , J Leads at end of straight part 
.Diamond < . 
|^ Leads at point of diamond 




closed slots 




Shuttle 


Leads at ends of straight part 








Basket 


Leads at point of diamond 






Hand-wound, pulled j S trai S ht 






,, , i Bent at one end 
through concentric ._ . . . 
1 Bent at both ends 







Diamond < 




Open slots < 


Involute 


Form wound 






coil of bare 




Concentric 


wire or strap 




Shoved through con- 




Partially 


centric. 




closed slots 


Threaded, diamond < 



Leads at end of straight part 

Leads at point of diamond 

Leads at end of straight part 

Leads at point of involute. 

Straight 

Bent at both ends. 

Straight 

Bent at one end 

Leads at ends of straight part 

Leads at point of diamond 



Bars and con- 
nectors of 
bare strap 



Partially 
closed slots 



Involute end connectors 
Concentric end connectors 



Involute Coils. " Involute coils share the advantages of 
the diamond coils in that all are of a standard size and shape. 
They also require less space for end connection than any other 
form of coil. They are, however, difficult to insulate properly 
on account of the larger number of bends and are difficult to 
assemble in position in the armature. For this reason their 
use is restricted. The bar type of coil with involute end 
connectors is easy to insulate and assemble and can be readily 
repaired. Their principal use is for direct-current and indus- 
trial motors where end space must be reduced to a minimum. " 

Concentric Coils. "Concentric coils can be used on any 
kind of slot. They can be hand-wound, machine-wound, or 
'shoved through' (a combination of the other two methods), 
as best suited. The shape of coils is simple, hence they are 



6 ARMATURE WINDING AND MOTOR REPAIR 

easy to wind either on a mould or by hand. They can be ade- 
quately insulated, and can be securely braced with simple and 
reliable coil support. However, the coils belonging to the 
same group are of different size, and the coils in different 
groups, except on single-phase machines, are bent in at least 
two and often in three different shapes. This is a disadvantage 
from the electrical point of view, since there will always be a 
tendency toward unbalancing due to differences in self- 
induction, and toward the production of circulating currents 
in closed windings. And it is also a disadvantage from the 
mechanical point of view, since for one machine, a large 
number of different moulds will be required and coils cannot 
be interchanged. Hence, the number of spare parts necessary 
for repairs is greatly increased, and both the manufacturing 
and repairing of the winding will require more time and be 
more expensive." 

Winding Pitch or Coil Pitch. The distance between the 
beginning of one winding element or coil side to the beginning 




Commutator Bars 



CommutatprJJars 



FIGS. 2 and 3. Comparison of wave and lap windings. 

At the left a wave winding showing front pitch of coils as yi; back pitch as j/a; total 
pitch as y and commutator pitch as yk. At right, a lap winding with the same notations 
for the different pitches of coils. 

of the next element or coil side connected to the first one is 
called the total winding pitch. This is shown as y for both lap 
and wave windings in Figs. 2 and 3. Winding pitch is meas- 
ured in number of slots or by number of elements or coil 
sides spanned by a single coil or by the number of commutator 
bars between the connections to the commutator of the two 
sides of a coil. In the third instance the measurement is 
known as commutator pitch and indicated as yk in Fig. 2 



DIRECT-CURRENT WINDINGS 



and Fig. 3. In this chapter unless otherwise stated coil 
pitch will be designated by the number of winding spaces 
or coil sides spanned. There are two winding spaces per 
slot in a double layer winding. 

Front Pitch. The distance between the two coil sides con- 
nected to the same commutator bar, measured in coil sides 
at the front or commutator end of the armature, is called the 
front pitch. It is indicated as y\ in Figs. 2 and 3. 

Back Pitch. The distance between two sides of a coil, 
measured in coil sides, at the back end of the armature is 
known as the back pitch. It is shown as y 2 in Figs. 2 and 
3. The total winding pitch y is equal to the algebraic sum 



I I 





TT 



Fro. 4. Lap and wave windings showing connections of formed coils to the 
commutator in double layer windings. 

of the front and back pitches. That is, since in a lap winding 
the front and back pitches are of opposite sign, being laid 
off in opposite directions on the armature, the total pitch y 
will be the difference between them. In a wave winding where 
the front and back pitches are laid off in the same direction, the 
total pitch y will be their sum. 

Full Pitch and Fractional Pitch Coils. When a coil spans 
exactly the distance between the centers of adjacent field poles 
(known as pole-pitch) it is said to be a full-pitch coil. In 
cases where a coil is less than full pitch, it is said to have a 
fractional pitch or to be a short-pitch coil. Such a winding is 
often referred to as a short- chord winding. 

Fractional pitch windings are much used because of the 
following advantages: (1) They make possible shorter end- 
connections and therefore call for less copper. (2) Armature 



8 



ARMATURE WINDING AND MOTOR REPAIR 



reaction is reduced since the currents in the neutral zone flow 
partly in opposite directions and neutralize each other. (3) 
In alternating current generators a smoothing out of the wave 
of induced emf is produced so as to more nearly approach a 
sine form. 

Symbols Used in Winding Formulas. The symbols which 
are much used in winding formulas for the parts of windings 
defined in the preceding paragraphs are as follows: 

Z = Total number of armature conductors, (2gC) or (2gK). 

C Number of winding elements or coils usually equal to K. 

g = Number of turns per winding element or coil. 

2g = Number of conductors per winding element or coil. 

N Number of coil sides of winding. 

y = Winding pitch or coil pitch. 

2/i = Front pitch of winding element or coil. 

2/2 = Back pitch of winding element or coil. 

2/fc = Commutator pitch. 

K = Number of commutator bars. 

p = Pairs of field magnet poles. 

2p = Number of field magnet poles. 

2a = Number of sections of armature winding in parallel. 

s = Number of slots on the armature. 

Numbering of Coil Sides in Armature Slots. Form wound 
coils are usually arranged in two layers with one side of each 





D 



5|D 
6| D 




FIG. 5. Winding with two 
coil sides per slot. 



FIG. 6. Winding with four coil sides 
per slot. The coil pitch in this case is 
7 winding spaces or coil sides. 



coil placed in the bottom of a slot and the other side in the 
top. For convenience in laying out windings, the coil sides 
forming the top layers in the slot are given odd numbers and 
those forming the bottom layers even numbers. This scheme 
of numbering is shown in Figs. 5 and 6. In those cases 
where the winding element consists of a coil of a number of 
turns of wire instead of two conductors or bars, numbers are 



DIRECT-CURRENT WINDINGS 9 

assigned to the consecutive bundle of conductors or wires of 
which the coil is made instead of assigning numbers -to the 
individual conductors or inductors as they are sometimes 
called. When such coils are used, the front and back pitches 
are determined by counting consecutive half coils between the 
the two coil sides of any coil beginning from one coil side and 
counting forward as the coil is laid off and considering the 
first side of the coil as one. That is, in case of a double layer 
lap winding, with one coil side in the bottom of slot 3, and 
the other side in the top of slot 7, the back pitch would be 7 
counted as follows, there being two coil sides in each slot: 
First coil side 6, next coil sides spanned are 7, 8, 9, 10, 11 and 
12. The number of slots spanned by the coil is called the 
slot pitch or throw of the coil. The throw of the coil, more 
often called coil throw, in the case mentioned, where one side 
of a coil is in slot 3 and the other in slot 7, is 4 slots. In the 
case of a four-pole machine and an armature having 16 slots, 
this coil throw is a full pole pitch, that is, from the center of 
one field pole to the center of the next, and the winding is a 
full pitch winding. 

LAP MULTIPLE OR PARALLEL WINDINGS 

-i 
-> 

A lap winding is so called from the lapping back of the coils 
or winding elements as they are wound on the armature core. 
This is shown in Figs. 7 and 8. A better name is a multi- 




FIG. 7. Lap winding showing the relative position of one winding element 
or coil of two turns on the armature of a 4-pole machine. 

pie or parallel winding because of the fact that such a winding 
consists of as many circuits as there are field poles and these 
circuits are connected in parallel between the brushes. The 
number of brushes required therefore equals the number of 



10 



ARMATURE WINDING AND MOTOR REPAIR 



armature sections in parallel and also equals the number of 
poles. That is a = p. A lap winding can be distinguished 



--fh 




9 



1 



6 



FIG. 8. Winding diagram for the lap winding of Fig. 7 showing how 
the finish end of one winding element or coil of two turns is joined to 
the commutator and to the start end of the next coil under the same pair of 
poles. 




FIG. 9. (a) Lap winding showing both end connections of each coil side 
bent toward center of the coil. (6) Wave winding showing end connections 
of each coil side bent in opposite directions. 

from a so-called wave winding from the appearance of the 
end connections of coils. In the wave winding the front and 



DIRECT-CURRENT WINDINGS 11 

rear ends of the coils lead in opposite directions while in the 
lap winding they continue in the same direction around the 
armature, as shown in Fig. 9. 

Formulas for Lap Winding. The following rules and restric- 
tions govern the assembling and use of a lap winding: 

1. The front and back pitches (in winding spaces or coil sides) 
must both be odd numbers and differ by two or some multiple 
thereof. 

2. The front and back pitches are of opposite sign (one positive 
and the other negative) since they are laid off in opposite direc- 
tions on the armature. 

3. The winding pitch is equal to the algebraic sum of the front and 
back pitches. That is, in case the front pitch is + 9 and the 
back pitch 7, the algebraic sum is (9 7) or 2. 

4. The total number of armature conductors or inductors must be 
a multiple of the number of slots on the armature. 

5. The number of armature slots may be odd or even. 

6. The number of current collecting points or brushes on the com- 
mutator must equal the number of poles. 

7. The maximum emf between two consecutive coil sides (top 
and bottom) in the same slot, is equal to or a little less than 
the terminal voltage. 

8. The end of one coil is joined to the commutator and to the 
start of another coil (usually the next) under the same pair of 
poles. 



The following formulas apply in lap windings : 

N b 
2p 

N b 



Front pitch = y\ = ^ - 2 

zp 



Back pitch = y k 

Winding pitch = y = algebraic sum of yi and y% = 2 
Commutator pitch = y k = 1 

In the formulas for front and back pitches, N (as given on 
page 8) is the number of coil sides in the winding; p the 
number of pairs of poles and b any even number which will 
make both t/i and y 2 odd whole numbers and about equal to 
the pole pitch. The value of b should be taken as large as 
allowable but not too large for then there is liability of the 
two sides of a coil coming under the influence of poles of the 
same polarity so that the induced emfs would oppose each 



12 



ARMATURE WINDING AND MOTOR REPAIR 



other. For 6 = 0, the back pitch y 2 becomes equal to the pole 
pitch. When b is positive t/ 2 becomes greater than the 
pole pitch, while with b negative, 1/2 becomes less than 
the pole pitch. In most cases b is taken negative so that 2/ 2 
is equal to or less than the pole pitch. 




FIG. 10. Lap winding for a 4-pole machine showing the direction of current 
flow in one of the four current paths for the brush positions as illustrated. 

The current (i) which flows in each conductor of a lap 
winding is, 



Where 7 is the total armature current or that flowing in the 
external circuit. The value of (i) should be considered when 
selecting the proper size of wire in making up or ordering coils. 

The only practical lap winding from the standpoint of 
symmetry is the one in which the number of pairs of circuits 
(current paths) in parallel equals the number of pairs of poles 
or a = p. 

A lap winding is symmetrical when s -f- a and s -r- p is a 



DIRECT-CURRENT WINDINGS 13 

whole number. Here s equals the number of slots, a the 
pairs of circuits in parallel and p, the pairs of poles. With 
four-pole motors it is not always found that s -f- a and s -f- p 
are whole numbers, since manufacturers sometimes make the 
number of commutator bars and slots odd so as to make the 
armature suitable for a two-circuit wave winding as well as a 
lap winding. 

In lap windings where the commutator pitch (y k ) equals 
one, when a = p, the common factor of the number of commu- 
tator bars, and yk will always be unity and there will be a single 
winding. 

In a double layer winding with two coil sides per slot, the 
number of slots equals the number .of coils in the winding. 
When there are more than two coil sides in a slot, the number 
of coils, C = sN 8 -T- 2 with no idle coils; where s equals the 
number of slots and N 8 equals the number of coil sides per slot. 
More than two coil sides per slot greatly reduces the number 
of slots required. Such a winding is symmetrical only when 
s -r- a is a whole number. 

For a lap winding the potential pitch, y p = K -~ p, where K 
equals the number of commutator bars and p is the number 
of pairs of poles. 

When equalized rings are needed with lap winding, it is 
good practice to use one ring for every 6 or 12 commutator 
bars. 

Multiplex, Single, Double and Triple Windings. In cases 
where it is necessary that the armature shall carry a very 
heavy current more than one winding may be used on the 
armature with an equal number of commutator bars for each. 
Both lap and wave windings may be so made up with one, 
two or three entirely separate windings. In the case of a 
double lap winding, the sides of a winding element or coil 
of one winding are sandwiched between the coil sides of the 
other and likewise the commutator bars of ona winding are 
sandwiched between those of the other. Each brush must 
be thick enough to always touch two commutator bars so that 
both windings will always be connected to the brushes and both 
deliver or receive current evenly. Three windings so sand- 
wiched make up a triple winding. 



14 ARMATURE WINDING AND MOTOR REPAIR 

A single lap winding always has the same number of current 
paths between brush sets as there are field poles while a double 
lap winding has twice the number of current paths as there are 
field poles. The triple lap winding has three times the number 
of current paths as there are field poles. A single wave wind- 
ing always has two current paths between brush sets while the 
double wave winding has four current paths between brush 
sets and the triple wave winding has six such paths between 
brush sets. 

An armature has one winding (single) when the number of 
commutator bars K and commutator pitch y k have no common 
factor; it is double when their common factor is two, and it 
is triple when their common factor is three. 

Meaning of the Term Reentrant. A winding is often said 
to be single or double reentrant. In the case of a single 
winding, this means that the winding closes on itself or re- 
turns to the beginning point after being traced through all the 
coils upon passing once around the armature core. A wind- 
ing is doubly reentrant if it only re-enters itself after making 
two passages around the coils of the armature. A single 
winding may be either single reentrant or doubly reentrant. 

In the case of double and triple windings, the term reentrant 
is sometimes used in an improper sense. For this reason it is 
advisable to specify types of armature windings by the num- 
ber of separate windings, used. Thus a single winding as a 
single-closed; two windings as double-closed and three wind- 
ings as triple-closed etc. r A winding made up of two single 
windings, each of which re-enters itself, will therefore be a 
double-closed winding not a "double-reentrant" one. 

Multiplex Lap Windings. As explained under the heading 
of "Multiplex Windings," a lap winding may be made up of 
one, two or three separate windings in order to handle heavy 
armature currents. In the case of a double lap winding, two 
similar windings insulated from each other are placed in the 
armature slots with the even numbered commutator bars 
connected to one winding and the odd numbered bars con- 
nected to the other winding. In the same way for a triple 
lap winding one-third of the commutator bars provided would 
be connected to each winding. 



DIRECT-CURRENT WINDINGS 15 

The formulas which apply in multiplex lap windings are as 
follows : 

Front pitch = y l = ^= 2m 
zp 

Nb 

Back pitch = y 2 = 

Zp 

Winding pitch = y = y 2 y = 2m 
Commutator pitch = y k = - 2 = m 

In these formulas w is 2 for a double winding and 3 for a 
triple winding, etc. When the number of commutator bars 
is exactly divisible by m, the windings will be entirely separate 
from each other. 

A double lap winding will have 2 X (2p) current paths be- 
tween brushes, where 2p is the number of poles. That is, each 
winding for a 4-pole machine will have four current paths so 
that a double lap winding on a 4-pole machine will have eight 
current paths between brushes. 

In order that the two sides of a winding element or a coil 
may move simultaneously under field poles of opposite 
polarity, the total number of coil sides that make up the com- 
plete winding divided by the number of the field poles, that is 
N -T- 2p, is the approximate value for both the front and back 
pitches as in the case of the single lap winding. Under such 
conditions the electromotive force induced in the two sides of 
a winding element add up. The smallest front or back pitch 
to satisfy this condition, is the distance across a single pole 
face and the largest front or back pitch is the distance from 
one pole tip to the nearest pole tip of the same polarity. 
If the front and back pitches are much less than N -5- 2p, 
then a chorded winding results. 

Since y\ and ?/ 2 must be odd numbers and approximately 
equal to N -f- 2p, where 2p is the number of field poles, this 
value will help in determining the value of b to be used in the 
formula for front pitch (7/1) and back pitch (t/ 2 ). 

The multiplex lap winding is largely confined to windings 
for small and medium sized machines carrying large currents 
and using coils made up of wire rather than copper strips or 



16 ' ARMATURE WINDING AND MOTOR REPAIR 

bars. This is for the reason that in such a winding for a 
large multipolar machine the bars would be too thin and a 
mechanical construction result which would make another 
type of winding advisable, probably a series-parallel design. 

WAVE SERIES OR TWO-CIRCUIT WINDING 

The wave winding is so called from the zig zag or wave path 
that the winding takes through the slots of the armature, as 
shown in Figs. 11 and 12. This type of winding is more 




FIG. 11. Wave winding showing relative position of one winding element or 
coil of two turns on the armature of a 4-pole machine. 

definitely described as a series or two-circuit winding because 
of the fact that half of the armature coils or sections are con- 
nected in series and the two halves are connected in parallel. 
This winding therefore has only two current paths in parallel 




FIG. 12. Winding diagram, for wave winding of Fig. 11 showing how the 
finish end of one coil is joined to the commutator and connected to the start 
of the next coil under the next pair of poles. &' is a continuation of 6. 

between brushes regardless of the number of poles. Only two 
sets of brushes are required for a machine of any number of 
poles but improved commutation is brought about when the 
number of brushes equals the number of field poles. The wave 
winding is used in small and medium sized machines where it 
is desired to keep the number of coils as small as possible. 



DIRECT-CURRENT WINDINGS 17 

Formulas for Wave Winding. The following rules and 
restrictions govern the assembling and use of a wave winding: 

1 . The front and back pitches (in winding spaces or coil sides) must 
both be odd. 

2. The front and back pitches may be equal or may differ by two 
or some multiple thereof. The former condition is usually the 
case. 

3. The front and back pitches are of the same sign since they are 
laid off in the same direction. 

4. The winding pitch is equal to the sum of the front and back 
pitches. 

5. The commutator pitch and number of commutator bars must 
not have a common factor. 

6. The number of current collecting points or brushes on the 
commutator is always two for any number of poles but the 
number of sets of brushes may equal the number of poles. 

7. The maximum emf between two consecutive coil sides (top 
and bottom) in the same slot, is equal or nearly equal to the 
terminal voltage of the machine. 

8. The finish of one coil is joined to a commutator bar and to the 
start of another coil which lies under the next pair of poles. 

The following formulas apply in wave winding: 

N 2 
Winding pitch = y = yi + y 2 = 

Front pitch = y\ 
Back pitch = y z 
Number pairs of poles = p 

^ u 2/i + 2/2 Kl 

Commutator pitch = y k = ~ 

4 P 

N 
Number commutator bars = K = -^=pXyk^-' 

N 2 
The number of coil sides (N) must be such that 

will be an even number with y\ and y% both odd -numbers. 
yi and y 2 are usually taken equal. Under these conditions 
the back pitch (y 2 ) is nearly equal to the pole pitch and the 
sum of the front and back pitches is nearly equal to double 
the pole pitch. If y 2 is reduced then yi must be increased so 
that 7/1 + 2/2 will be constant. This is what happens in a 
chorded or fractional pitch winding. 



18 



ARMATURE WINDING AND MOTOR REPAIR 



The current (i) which flows in each conductor of a wave 
winding is always one-half the total armature current, i = 
I -f- 2. This value of (i) should be considered when select- 
ing the proper size of conductors for coils of wave windings. 
The closed winding formula for a wave winding is yk = 
(K a) -f- p. Where yk is the commutator pitch, K the num- 




FIG. 13. Wave winding for a 4-pole machine showing the direction of 
current flow in one of the two current paths for the brush position as illus- 
trated. 

ber of commutator bars, a the pairs of parallel circuits in the 
winding and p the number of pairs of poles. When a = 1, it 
is only possible to make a single winding. The highest com- 
mon factor of yk and K gives the type of winding, whether 
single, double, etc. The number of coil sides in a wave wind- 
ing equals twice the number of commutator bars. 

The number of slots without idle coils must satisfy the 
formula, s = (2K) -i- N 8 . Where K is the number commu- 
tator bars and N a the number of coil sides per slot. 

A wave winding is symmetrical when K -f- a, s -f- a and 
p -5- a, are whole numbers. Where K is the number of com- 



DIRECT-CURRENT WINDINGS 19 

mutator bars, a, the number of pairs of parallel circuits in 
the winding, s the number of slots, and p the number of pairs 
of poles. 

When equalizer rings arc needed with a wave winding, the 
use of one ring to every 15 to 20 commutator bars is good 
practice. 

Multiplex Wave or Series-Parallel Winding. In a single 
wave or series winding the number of armature circuits in 
parallel is always equal to two, while in a lap or multiple wind- 
ing the number of circuits in parallel is equal to the number of 
field poles. The series-parallel winding is a design of wave 
winding in which the number of armature circuits in parallel 
may be larger than two and yet smaller than the number of 
field poles. It is especially suitable for large multipolar arma- 
tures which use winding elements made up of copper bars. 

Formulas for Series-Parallel Winding. The following 
rules govern the assembling and use of this winding: 

1. The front and back pitches (in winding spaces or coil sides) 
must be odd numbers. They may be equal to or different from 
each other although they are usually equal. 

2. The winding should be symmetrical as shown by the number 
of field poles divided by the number of armature circuits in 
parallel being a whole number. If this is not the case the final 
winding should be made up of a sufficient number of independent 
windings to make the number of field poles divided by the number 
of armature circuits in parallel a whole number for each of the 
independent windings. 

3. It is necessary that the number of commutator bars divided by 
one-half the number of armature circuits in parallel shall be 
a whole number. If the conditions of (2) are fulfilled, the 
condition named here is likewise satisfied at the same time. 

4. If the number of commutator bars and the commutator pitch 
have no common factor, the winding is a single one; if the com- 
mon factor is 2, the winding is double or if it is equal to m, the 
winding is multiplex of m separate windings. The common 
factor must, however, be smaller than half the number of circuits. 
An illustration will bring out these conditions. In the case of 
an 8-pole machine with 27 slots, 54 commutator bars, 4 Con- 
ductors per slot, and a commutator pitch of 13, the two series 
windings are singly closed forming one winding, since 54 and 1 3 



20 ARMATURE WINDING AND MOTOR REPAIR 

have no common factor. For a machine of 4-poles, with 20 slots, 
40 commutator bars, 4 conductors per slot, commutator pitch of 
18, the winding may be made up of two series windings, the 
values 40, 18 and 4 have a common factor of 2. This makes the 
winding doubly closed and consists of two series windings with 
each in turn consisting of two other series windings, each of the 
latter forming one continuous winding. 

5. For large machines using series-parallel windings, it has been 
found advisable in most cases to use equipotential connect- 
ions and connect about every fourth or eighth commutator 
section. (See heading " Equipotential Connectors" on page 
21.) 

6. The number of current collecting points or brushes on the com- 
mutator is equal to the number of armature sections in parallel. 

The following formulas apply in the case of a series-parallel 
winding : 

Front pitch = yi 
Back pitch = y 2 

Winding pitch = y = y l + y 2 = 

Commutator pitch = y k = - 

N 
Number commutator bars = K = -^ 

z 

Where p is number of pairs of poles and N is the number 
of coil sides in the winding. N must be such a value that 

N 2a. 

- is a whole number. 
P 

Symmetrical Windings. All armature windings should 
be made symmetrical if possible. A winding is symmetrical 
when tLe total number of slots divided by one-half the number 
of armature sections in parallel is a whole number. If this 
is not the case the emfs induced in different circuits will pro- 
duce circulating currents. A winding is also symmetrical 
if the number of field poles divided by the number of armature 
sections in parallel is a whole number or the number of com- 
mutator bars is divisible by one-half the number of armature 
sections in parallel. To be sure that any winding is symmetri- 
cal all three of these conditions should be fulfilled. 



DIRECT-CURRENT WINDINGS 21 

Possible Symmetrical Windings for D.-C. Machines of 
Different Number of Poles. In a discussion on armature 
windings, Stanley Parker Smith (London Electrician, June 16, 
1916) has selected the following as the most suitable windings 
for machines of different number of poles: 

1. For a two-pole machine either a two-circuit lap or wave winding 
can be used. The former is usually preferred. 

2. For a four-pole machine either a two-circuit wave or a four- 
circuit lap winding can be used. 

3. For a six-pole machine we are limited to the two-circuit wave 
and six-circuit lap windings, since the four-circuit wave wind- 
ing (a = 2, p = 3) is unsymmetrical. This is probably the 
most objectionable restriction of all, because so many cases 
arise where four circuits are desirable in a six-pole machine. 

4. For an eight-pole machine wave windings with two or four 
circuits and an eight-circuit lap winding can be used. This 
is the first instance of a symmetrical wave winding with 
a > 1< p, and can be used with great advantage in many cases. 

5. For a 10-pole machine there are again only two symmetrical 
windings the two-circuit wave and 10-circuit lap windings. 

6. For a 12-pole machine the possibilities are much greater, for 
wave windings with two, four or six circuits and a lap winding 
with 12 circuits can be used. 

7. The list can be continued further, if desired, but it is plainly 
seen that certain very important advantages are open to the 
designer by making use of wave windings with more than two 
circuits in 8, 12 and 16-pole machines. It is necessary, 
however, to observe the other conditions of symmetry, namely 
s/a must be a whole number and idle coils must be avoided. 

Equipotential Connectors (Equalizing Rings and Phase 
Tappings). In a symmetrical winding, that is, a winding 
with identical K/a phase systems, there are always a coils 
at the same potential, and these can be joined together if 
desired. If no dissymmetry whatever were present, however, 
there would be little object in making such connections, unless 
they are needed as phase tappings to obtain an alternating 
pressure. Actually there are many cases of dissymmetry in 
a machine, apart from those due to the arrangement of the 
winding already mentioned. Thus, the magnetic material 
may not be uniform, the pole-shoes may not be properly 



22 ARMATURE WINDING AND MOTOR REPAIR 

spaced, the gap may not be uniform, and so on. In general, 
perfect symmetry must be regarded as an unattainable ideal 
in practice according to Mr. Smith, who calls attention to the 
following conditions: 

Owing to these dissymmetries, the pressure induced in the 
several armature circuits varies, and causes equalizing currents 
to flow through the brushes. These equalizing currents, if of 
sufficient strength, produce sparking and in any case, load the 
brushes in an undesirable manner. It is interesting to note 
that, even when no equalizing rings are present, the positive 
and negative collecting rings act as equalizers, and the tend- 
ency of these equalizing currents is to neutralize the inequali- 
ties in the magnetic field to which they are due. When 
equalizing rings are used, however, large equalizing currents 
will flow along these and strongly damp out any inequalities 
in the field, and so reduce the difference of potential between 
corresponding points in the winding. Consequently the 
brushes are relieved, and are so much better able to perform 
their proper function of collecting the current. 

Equalizing rings must not be regarded as in any way es- 
sential, and many machines work quite well without them. 
Nevertheless, they add a certain factor of safety which the 
manufacturer is often glad to purchase at so small a cost, for 
he is not only surer of his machine passing the test satis- 
factorily but also knows that after-effects, like wear of the 
bearings, cannot give rise to such serious trouble as when equal- 
izers are absent. Consequently, equalizers are seen on many 
large machines with lap-wound armatures, or with wave 
windings with more than two circuits. Equalizing rings 
should not have an extremely low resistance. Such practice 
requires not only an excessive amount of copper, but leads to 
considerable loss and heating in the winding. All that is 
really necessary is to provide an alternative path of negligible 
resistance compared with that of the brushes, and for this 
purpose it is usually sufficient to make the section of the rings 
about half that of the conductors. 

Regarding the number of equalizing rings, much depends 
on the opinion of the designer. With lap windings, one ring 
for every 6 to 12 segments is common practice, but this is 



DIRECT-CURRENT WINDINGS 23 

scarcely feasible when p > a, for here the potential pitch, 1 
y p = K/a, may be fairly large and the number of rings be- 
comes prohibitive. In such cases (wave windings with more 
than two circuits), one ring for every 15 to 20 segments may 
suffice. There is no need to make the pitch between all the 
rings the same, but designers generally prefer to split up y p 
into a whole number of parts. In this case the tappings form 
a symmetrical polyphase system of pressures. 

Best D.-C. Windings for a Repair Shop to Use. To secure 
good commutation and eliminate heating troubles, all windings 
should be made symmetrical. This among other things 
means that there must not be more armature circuits in paral- 
lel than there are field poles. Where the number of armature 
circuits equals the number of poles either the lap or the wave 
winding may be used. The lap winding is however mostly 
used in such a case. The wave winding then need only be 
considered in those cases where the number of armature circuits 
is to be less than the number of field poles. A single winding 
is more simple and should be used in preference to a compli- 
cated one where conditions permit either a single winding 
or two or more separate windings except in those cases where 
a very heavy current is to be carried in the armature. In 
general then, where changes in armature windings are neces- 
sary that call for a choice of types of windings, it is advisable 
to employ either a lap winding with as many circuits as there 
are poles or a wave winding with two circuits. Usually a lap 
winding is possible if the number of coils is a common multiple 
of the number of poles and the number of slots. The number 
of coils is equal to the number of commutator bars. 

Number of Armature Slots. The open slot with parallel 
sides is most used for large armatures so that form wound 
coils can be easily assembled. These coils are held in place 
by wire bands or by wood or fiber wedges driven into grooves 
at the tips of the teeth as shown in Chapter III. The number 
of slots per pole is usually not less than 10. For multipolar 
armatures there are at least from three to four slots in the space 

1 The potential pitch is the number of commutator bars between suc- 
cessive equipotential points in the winding which can be joined together 
when equalizing rings are needed. 



24 ARMATURE WINDING AND MOTOR REPAIR 

between pole tips. In the case of high speed machines having 
large pole pitch there may be from 14 to 18 slots per pole. 
For machines above say 5 hp. the area of the slot will approx- 
imate one square inch. A rough rule for the capacity of 
a slot of this area is about 1,000 amp. turns for machines 
under 500 volts. 

Voltage between Commutator Segments. For direct-cur- 
rent machines without interpoles, about 15 volts between 
segments of the commutator is the maximum allowed while 
double this value can be considered as the maximum for ma- 
chines employing commutating interpoles. This value of 
voltage between segments can be obtained for an existing 
machine by measuring this voltage between plus and minus 
brushes and dividing by the number of commutator segments 
between these brushes. The voltage between a positive and 
nagative brush will depend upon the number of inductors (wires 
making up one side of a coil) connected in series between the 
brushes. For calculating windings for machines, Prof. Still 
gives the following relationships between machine voltage 
and volts between commutator segments for good operation: 

VOLTAGE BETWEEN SEGMENTS OF COMMUTATORS 



Machine voltage 


Volts between commutator segments 


110 


Ito6 




220 


2.5 to 10 




600 


5 to 18 




1200 


9 to 25 





Number of Commutator Bars.^-The proper number of 
commutator bars for a particular winding depends on the volt- 
age between commutator bars. The number of bars may be v 
a multiple of the number of slots. For low voltage machine 
there may be one, two or three bars per slot while in higher 
voltage slow speed machines, there may be as many as four 
or five bars per slot. While improved commutation results 
from a large number of commutator bars the difficulty in 
repair and assembly from a mechanical standpoint offset 
their advantage over fewer bars for the same conditions of 
winding and machine operation. 



DIRECT-CURRENT WINDINGS 



25 



Usual Speeds and Poles for Different Sizes of Generators. 

For direct current generators Prof. Still 1 gives the following 
relationship between number of poles and speed for different 
ratings of machines. This table represents usual practice 
by designers. 

NUMBER OF POLES AND USUAL SPEED LIMITS OF D.-C. GENERATORS 



Output in kw. 


Number of poles 


Speed in rpm, 
maximum and minimum 


Oto 10 


2 


2400 to 600 


10 to 50 


4 


1300 to 350 


50 to 100 


4 or 6 


1100 to 230 


100 to 300 


6 or 8 


700 to 160 


300 to 600 


6 or 10 


500 to 120 


600 to 1000 


8 or 12 


400 to 100 


1000 to 3000 


10 or 20 


200 to 70 



Safe Armature Speeds. The safe speed of an armature 
varies with the armature construction. Direct-current ma- 
chines are built with a peripheral speed of 2,500 to 3,500 feet 
per minute. It is not advisable to exceed an armature periph- 
eral speed of 6,000 feet per minute in machines which are 
not designed to take care of the mechanical stresses incident 
to the higher speeds. 

1 Principles of Electrical Design, page 81. 



CHAPTER II 
ALTERNATING -CURRENT WINDINGS 

In the main, the windings for alternating-current motors 
and generators are alike. For this reason details of the wind- 
ings that can be used for either machines will be given. It 
should be noted at this point that direct-current and alter- 
nating-current windings differ essentially by the former being 
of the closed-circuit type while most alternating-current wind- 
ings are of the open-circuit type. Either open- or closed- 
circuit types of windings may however be employed in alter- 
nating-current machines, but the open type is in most common 
use. By a closed winding is meant one which has a continuous 
path through the armature and re-enters itself to form a 
closed circuit. Such a direct-current winding always has at 
least two current paths between brushes. In the case of an 
open-circuit alternating -circuit winding, there is a continuous 
path through the conductors of the coils of each phase of the 
winding, with the ends of this path forming two free ends. 
Such a winding does not close on itself. 

In the closed-circuit windings of direct-current machines, 
the bars of the commutator are simply connected at equally 
distant points around the winding. In the open-circuit wind- 
ing of an alternating-current generator with revolving arma- 
ture, the terminals of the completed winding are connected to 
collector rings and the winding is open-circuited until closed 
by the connections between the brushes. Closed-circuit wind- 
ings are only used for special alternating-current machines. 
The conditions which usually call for a closed-circuit wind- 
ing are the following: 

1. An alternating-current machine which must deliver a large 
current at low voltage. In such a case the winding usually 
consists of several similar paths connected in parallel to the 

26 



ALTERNATING-CURRENT WINDINGS 27 

terminals of the armature, thus forming one or more closed cir- 
cuits within the armature winding. 

2. In designs of machines that must handle direct and alternating 
current, as in double current generators and in rotary converters. 

In general it may be said that a direct -current winding may 
be changed so that the machine can be used as an alternator, 
but an alternating-current winding cannot be used for a direct 
current generator since it is not reentrant. 

Types of A.-C. Windings. With reference to the arrange- 
ments of coils used in an alternating-current armature, wind- 
ings may be divided into two general classes as follows: 

I. Distributed Windings. 

1. Spiral or chain. 

2. Lap. 

3. Wave. 

II. Concentrated Windings. 

1. Lap. 

2. Wave. 

Distributed Windings. An armature winding which has 
its inductors of any one phase undigr a single pole placed in 
several slots, is said to be distributed. When these inductors 
are Dunched together in one slot per pole, per phase, the 
winding is called concentrated. It is usual in a distributed 
winding to distribute the series inductors in any phase of the 
winding among two or more slots under each pole. This 
tends to diminish armature reactance and gives a better emf . 
wave, besides offering a better distribution of the heating due 
to armature copper loss than in concentrated windings. 

Concentrated Windings. The uni-slot or concentrated 
winding gives the largest possible emf from a given number of 
inductors in the winding. That is for a definite fixed speed 
and field strength in an alternator, the concentrated winding 
requires a less number of inductors than a distributed winding, 
but increases the number of turns per coil. 

Spiral or Chain Winding. In this winding as shown in 
Fig. 14 there is only one coil side in a slot. An odd or even 
number of inductors per slot may be used but several shapes 
of coils are required since the coils enclose each other and must 



28 



ARMATURE WINDING AND MOTOR REPAIR 



have special end shapes to clear each other. This arrange- 
ment, however, makes possible good insulation of end con- 
nections through adequate separating air spaces in high vol- 
tage machines. The number of coils required in this winding 



D - 
































































| 


1 






f j 










> 




H 












y,, 




x 


/ 




| 






> 


/ 






\ 








/ 


y 






( 








f 


7 
























* 












f 
















1 - 


































' 


















FIG. 14. A spiral or chain winding for a 2-phase, 4-pole machine with 
the coils of one phase in place and connected. The coils of the other phase 
go in the slots shown by the full lines inside the coils. 

is also small compared with other windings. This type of 
winding is mainly used in alternating-current generators. 

Lap and Wave Windings.- Both distributed and concen- 
trated windings make use of lap and wave connections. These 




FIG. 15. Single-phase lap winding for the same conditions as the wave 
winding in Fig. 16. 




FTG. 16. Single-phase distributed wave winding with two slots per pole per 
phase and one coil side per slot. 

arrangements are in principle the same as used in direct-current 
windings. (See Chapter I, pages 9 and 16.) The diagrams of 
Figs. 15 and 16 show single-phase distributed lap and wave 



ALTERNATING-CURRENT WINDINGS 



29 



windings for a four-pole armature, having two slots per pole 
with one inductor per slot. 

In a double-layer lap or wave winding for an alternating- 
current armature, the coils are usually of the same general 
shape as used in direct-current windings. Instead of con- 
necting the terminals of the coils to a commutator, they are 
connected together in a definite order (see Chapter XI, page 
288) for each phase. The phase windings are then connected 
together in star or delta as shown in Fig. 22. In that diagram 
a three-phase wave winding is shown for a four-pole machine. 
In the double-layer winding, the number of conductors per 




FIG. 17. At the left is shown the appearance of one formed coil in a 2- 
layer winding. One side of the coil lies in the top of the slot and the other 
side in the bottom of the slot. The top side bears an odd number and the 
bottom side an even number. The coil pitch in this case is 6 slots or 13 
winding spaces. At the right the method of connecting coils for a 2-layer 
winding using form coils is shown. The finish (F) of one coil is joined 
to the start (S) of the next. 

slot must be a multiple of two. It lends itself to a variety 
of connections, particularly to a fractional pitch lap winding 
where the two sides of a coil are not similarly placed in respect 
to the center lines of the poles. The double-layer winding 
is not, however, as well suited for high voltages as the single- 
layer winding. For this reason many water-wheel types of 
generators have been built with a single-layer winding, which 
in its most common form, is known as the spiral or chain 
winding. 

When laying out or changing a double-layer winding, it 
is usual to assign odd numbers to the sides of coils in the top 
of the slots and even numbers to the sides in the bottom of 
the slots. This is important when the pitch of the armature. 



30 ARMATURE WINDING AND MOTOR REPAIR 

coils is expressed in terms of coil sides (winding spaces) in- 
stead of slots. 

Whole-coiled and Half-coiled Windings. When the coils 
of an alternating current winding are connected so that there 
are as many coils per phase as there are poles, the winding is 
called "whole-coiled." When the coils are connected so that 
there is only one coil per phase per pair of poles, the winding is 
called " half-coiled." The main difference between these two 
connections is in the method of making the end connections 




A Whole-coiled winding B Half-coiled winding 

FIG. 18. A 6-pole stator with whole-coiled and half-coiled windings. 
The whole-coiled winding, A, has as many coils per phase as there are poles. 
The half-coiled winding B has only one coil per phase per pair of poles. 

for the coils. In the " whole-coiled " winding each slot con- 
tains two coil sides. It is not, however, strictly a double- 
layer winding, as the coil sides are placed side by side and 
not one above the other. In the " half -coiled" winding, how- 
ever, each coil may have twice the number of turns of a " whole- 
coiled" winding or the two coils under a north or south pole 
of the latter type may be connected in series and taped to- 
gether to form one coil in case of a change in connections. 

The " half -coiled " winding has the advantage that, when 
used with large generators the armature frame may be split 
into two sections for shipment or repair, without disturbing 
many of the end connections. 

Single-phase and Polyphase Windings. The winding of 
a single-phase motor or generator has only one group of induc- 
tors per pole, placed in one slot or several slots depending 
upon whether or not the winding is concentrated or distrib- 



ALTERNATING-CURRENT WINDINGS 



31 




FIG. 19. A simple single-phase winding. 




FIG. 20. Simple 2-phase winding. 







FIG. 21. Simple 3-phase winding. 




FIG. 22. A 3-phase winding showing how it may be connected in delta 

or star. 



32 ARMATURE WINDING AND MOTOR REPAIR 

uted. Such a single-phase concentrated wave winding for 
a four-pole armature is shown in Fig. 19. 

Two-phase and three-phase windings may be considered 
as made up of single-phase windings properly placed on the 
same armature. For the two-phase windings two separate 
single-phase windings are used spaced 90 electrical degrees 
apart. This is shown in Fig. 20. For the three-phase wind- 
ing, three single-phase windings are used, spaced 120 degrees 
apart, as illustrated in Fig. 21. Although the single-phase 
windings are independent of each other, their terminals are 
connected in star or delta as shown in Fig. 22. 

Coil Pitch. In the case of a two-phase winding, the total 
number of slots should be just divisible by two so that each 
phase will have the same number of winding elements or 
coils per pole. In the same way, for a three-phase winding 
the total number of coil sides or the total number of slots 
should be just divisible by three (the number of phases) 
and sometimes by the number of poles. This will result in 
a full pitch winding, that is, a winding in which a coil spans 
exactly the distance between the centers of adjacent poles. 
If the coil spans less than this distance, so that its two sides 
are not exactly under the centers of adjacent poles at the same 
time, it is said to have a fractional-pitch. When a fractional- 
pitch is used in alternators on account of the electrical factors 
of the design, such as to secure as nearly as possible a sine 
wave shape of emf, the total number of slots per phase must 
be a whole number. A fractional-pitch is also widely used 
in induction motors. 

Coil pitch is expressed as a fraction of the pole pitch, in 
slots, in electrical degrees or in winding spaces (coil sides). 
In the case of a six-pole machine having 72 stator slots, and 
a double-layer winding, the pole pitch would be 12 slots. If 
the coil pitch were given as %, this would be 120 degrees 
or eight slots or 13 winding spaces (coil sides). A full coil 
pitch for this winding would be 180 degrees, 12 slots or 21 
winding spaces. 

Phase Spread of Windings. The spread or space occupied 
by each single-phase winding is known as the phase spread 
of the winding. For a two-phase winding the phase spread 



ALTERNATING-CURRENT WINDINGS 33 

is (180 -r- 2) or 90 degrees. For a three-phase winding, it is 
(180 -T- 3) or 60 degrees. In a single-phase winding, the phase 
spread is theoretically 180 degrees. Prof. Alfred Still points 
out, however, in his book on " Principles of Electrical Design, " 
that nothing is gained by winding all the slots on the armature 
surface of a single-phase machine. After a certain width of 
winding has been reached the filling of additional slots merely 
increases the resistance and inductance of the winding with- 
out any appreciable gain in the developed voltage. In prac- 
tice only about 75 per cent, of the available slot space is 
utilized making the phase spread for a single-phase winding 
about 135 electrical degrees. 

The fact that, in polyphase machines, the whole of the arma- 
ture surface is available for the winding, while only a portion 
is utilized in a single-phase alternator, accounts for the fact 
that the output of the latter is less than that of the polyphase 
machine using the same size of frame. In a three-phase 
machine it is only necessary to omit one of the phase windings 
entirely and connect the two remaining phases in series to 
obtain a single-phase generator. Such a modified generator 
will give about two-thirds of the output of the polyphase 
connection. A three-phase star connected induction motor 
can also be used as a single-phase motor by properly con- 
necting two phases of it (see Chapter XI). 

Two-phase from Four -phase Windings. In many cases the 
two-phase induction motor is designed as a four-phase machine 
with the connections between conductors of the winding 
arranged so as to permit operation on a two-phase supply 
circuit. As shown in Fig. 23, a two-phase winding maybe 
secured from a four-phase grouping of coils by connecting the 
first and third groups in series and the second and fourth 
groups in series. 

Three-phase from Six-phase Windings. Few strictly 
three-phase induction motors are built. The design may be 
more properly called a six-phase winding with the three phases 
' spaced 120 electrical degrees and the connections of coils 
such as to permit the motor to be operated on a three-phase 
circuit. As shown in Fig. 24 in a six-phase winding the coils 
of the six-phases are spaced 60 electrical degrees. For three- 



34 



ARMATURE WINDING AND MOTOR REPAIR 



phase operation the coils of phases one and four, three and 
six, and five and two are connected in series. The terminals 
of phases two, four and six are connected to a common point. 




FIG. 23. Connections for four windings spaced 90 electrical degrees apart 
to secure a 2-phase motor. There are really four phases shown, the first and 
third and the second and fourth are connected in series. The full lines are 
front connections of coils and dotted lines the back connections. $ and F 
indicate the start and finish of the different groups of coils. 





FIG. 24. Connections for a 6-phase winding design so that a 3-phase 
winding is secured. The six windings are spaced 60 electrical degrees 
apart. In this diagram phases one and four, three and six, and five and two 
are connected in series and the terminals F 2 , F4 and Fe jointed to a common 
point. The 3-phase leads are Si, S 3 , and SB. The full lines indicate 
front connections and dotted lines back connections. S and F indicate the 
start and finish of the different groups of coils. 

Wire, Strap and Bar Wound Coils. For the coils used in 
small motors round insulated wire is most employed. These 



ALTERNATING-CURRENT WINDINGS 35 

coils are either wound in the slots by hand or assembled by 
use of specially formed coils wound in forms and insulated 
before being placed in the slots. Such formed coils are usu- 
ally used except in cases where the slots are closed or nearly 
closed. For further information on formed coils see page 4, 
Chapter I, and page 141, Chapter VI. 

For large motors and generators where the amperes to be 
carried in each armature circuit is a large value, copper straps 
are frequently employed for making up the armature coils. 
In very large machines a copper bar is used instead of the 
copper straps. In such a case one bar serves as the inductor 
of a coil having one turn per slot. A two-layer bar winding 
made up of two bars per coil and four bars per slot is also 
used. The copper bars are connected to the end connections 
of the coils by brazing, welding or bolting. In all cases, 
whatever the construction of the coil used, the slots must be 
properly insulated with fullerboard, mica, fish paper or other 
suitable insulating material. For data on slot insulation see 
Chapter VII. 

The current density in alternating-current windings is 
about 2500 amp. per square inch of armature conductor in 
small machines, 2000 amp. in medium sizes and 1500 amp. in 
high-voltage designs. Except in high-speed machines it is 
not safe to use the maximum limit owing to damage to wind- 
ings from over heating. 

METHODS FOR LAYING OUT AND CONNECTING ALTERNATING- 
CURRENT WINDINGS 

In rewinding an alternating-current machine, the number of 
slots on the stator, the operating voltage, speed, phase and 
frequency of the supply circuit are points that must be con- 
sidered in laying out a new winding or reconnecting an existing 
one. The fundamental requirements of windings and the ways 
in which they can be fulfilled as outlined by M. W. Bartmess 
(Electric Journal, Vol. VIII, No. 5) are given in what follows. 

Group Windings. Group winding may be defined as that 
class wherein the total winding is divided into separate parts, 
composed of adjacent coils or conductors. The grouping is, 
in the case of lap and wave windings, an arbitrary one, the 



36 



ARMATURE WINDING AND MOTOR REPAIR 



coils being all similar and divided into groups solely by their 
connections. The number of coils per group may equal the 
number per pair of poles divided by the number of phases, or 
the number per pole divided by the number phases. The latter 
method of grouping is generally used on modern machines. 
In the case of a six-pole, three-phase winding of 36 slots, the 
number of coils per group is 36 -j- (6 X 3) or 2. When the 
number of slots is not evenly divisible by the product of poles 




FIG. 25. Methods of connecting pole-phase groups shown in (a), (6) and (c). 
(a) Four-pole winding with alternately positive and negative pole-phase groups, 
(fc) Four-pole winding of the consequent pole type, (c) Two-pole winding obtained 
from (b) by reconnecting the pole-phase groups alternately positive and negative. 

and phases, dissimilar groups must be employed. In such 
cases it is advisable to arrange the grouping so that all the 
phases have an equal number of coils, and if possible the group- 
ing should be arranged symmetrically with respect to the core 
itself. To prevent local currents, which may prove injurious, 
all circuits which are in parallel must have an equal number of 
coils and should be symmetrically arranged with respect to 
each other and to the other phases. 

Although one turn coils only are shown in Figs. 27 to 29, 
the same connections are applicable to windings having 



ALTERNATING-CURRENT WINDINGS 37 

any number of conductors per coil. These conductors may 
all be in series, in which case there is one lead at each end of the 
coil or the conductors may be divided into any number of equal 
parallels, in which case there are as many leads at the ends of 
the coils as there are parallel circuits. The leads at the 
beginning and end of the coils are connected in the same man- 
ner as indicated for the one turn per coil winding. For the 
sake of simplicity the number of coils per group and hence the 
total number of coils in the diagrams has been kept lower than 
is generally found in commercial machines. 

Full and Fractional Pitch Windings. The number of 
slots in the core, divided by the number of poles gives a value 
of the pole arc expressed in terms of the slots. A full pitch 
winding is one in which the effective span of the coils is equal 
to the pole arc, and a fractional pitch winding is one in which 
the effective span of the coils is not equal to the pole arc. For 
a two coil per slot, lap or wave winding, the effective span of the 
coil is equal to the actual span of the coil. In this case the 
full pitch winding is one where the coil throw is equal to 

/total number of slots , A ^ 

I r e~ r~ ~ plus 1 ) . For a one coil per slot lap 

\ number of poles / 

winding the effective span of the coil may be greater or less than 
its actual span. In Fig. 26 (a) and (6) show two different coils, 
in each of which the effective span is the full pitch of 12 slots 
while the actual span in (a) is only 11 slots and that in (6) 
is 13 slots. Needless to say, (a) is more generally used on 
account of the saving in copper and space for end connections. 
A coil with a span either less or greater than that shown 
would result in a fractional pitch, as in (c) and (d) . 

Representative cases of concentric group windings are 
shown in Fig. 26, (e) and (f), (e) representing a three-bank 
winding, in which the number of coils per group equals the 
total number of coils per phase divided by the number of poles, 
while (f) represents a two-bank winding of the consequent 
pole type in which the number of coils per group equals the 
total number of coils per phase divided by the number of pairs 
of poles. Neither of these types can be conveniently wound 
with a fractional pitch, especially with formed coils. Where 
dissimilar groups are employed, that is where the number of 



38 



ARMATURE WINDING AND MOTOR REPAIR 



slots is not evenly divisible by the product of phases and poles, 
the full pitch is frequently not a unit and hence a fractional 
pitch is necessary. In the case of a three-phase, four pole 
winding with 30 slots, 30 coils, two coils per slot, full pitch 
covers a span of 7.5 slots and the nearest lower even pitch gives 
a throw of 1-8. 

In general, fractional pitch affects the performance of the 
apparatus similarly to a reduced number of turns in the wind- 



+ + + +O OOO 4 -1-4.4. 




(X) (/) 

FIG. 26. Possible pitch for one coil per slot windings. 

(a) Full pitch, effective space 12, actual space 11, throw 1 to 12. (6) Full pitch, 
effective space 12, actual space 13, throw 1 to 14. (c) Fractional pitch, effective 
space 10, actual space 9, throw 1 to 10. (d) Fractional pitch, effective space 10, 
actual space 15, throw 1 to 16, (e) Concentric group, full pitch, effective space 12, 
actual space 9 and 11, throw 1 to 11. (/) Concentric group, full pitch, consequent 
poles, effective space 12, actual space 9, 11, 13, and 15, throw 1 to 13. 



ing, but not in the same proportion. In a generator this re- 
duces the voltage of the machine. In an induction motor, the 
maximum available torque is increased but the densities in the 
magnetic circuit are also increased with a resulting reduction of 
power-factor. For either motor or generator, considerable 
copper may thus be saved in the coil ends and a standard 
frame may frequently be used for special purposes. 

Simple Winding Diagram. It is evident that for all com- 
binations the number of diagrams necessary would be unlimited. 
A simplified diagram may be employed which will not only 
reduce the required number of such diagrams but will also 



ALTERNATING-CURRENT WINDINGS 



39 



minimize the labor in tracing out the connections. Thus it will 
be seen that the diagram in Fig. 27, will satisfy many require- 
ments for connections of groups. In addition to this it will 
apply for any similarly connected three-phase, four-pole, series 
star lap-winding, irrespective of the number of coils per 
group (provided the groups are regular) or of the throw of the 
coils, that is, whether the winding is full or fractional pitch. 
This information for the throw of the coils and the number of 
coils per group may be carried 
on the same specification with 
the remaining winding con- 
stants. The groups are formed 
by connecting the required num- 
ber of coils together, the end of c 
the first coil to the beginning of 
the second, etc., the beginning 
of the first coil and the end of 
the last coil in the group forming 
the beginning and end of the 
group. Such diagrams may be 
made for any number of phases, 
poles or possible parallel circuits, 
and for any desired method of 
connection of the groups. In 

case the coils per group are irregular or unbalanced, it is advisa- 
ble to have a special diagram giving the number of coils in each 
group, their location and any other information necessary. 

Reconnecting a Winding. It is obvious that if a winding 
gives satisfactory operation on a certain voltage, a similar 
winding of one-half the number of series conductors, but of 
double the current carrying capacity, will give satisfactory 
operation on one-half the voltage. This latter condition may 
be obtained by paralleling the groups, as in Figs. 28 and 29, or 
where this is impossible by paralleling the series conductors in 
the slots. For example, if the full voltage connection of a 
14-pole motor corresponds to the parallel connection the only 
method to change to half voltage would be to change the wind- 
ing itself since 14 poles does not permit of a four-parallel 
connection. Again an irregularity of coils per group will at 




FIG. 27. General winding dia- 
gram for a 3-phase, 4-pole motor 
with pole-phase groups connected 
in series star. 



40 



ARMATURE WINDING AND MOTOR REPAIR 



times prevent doubling the parallel circuits where otherwise 
this might be possible. 

When it is desired to use one winding for either full or half 
voltage, the winding, if possible, is laid out for equally satis- 
factory operation on either connection, and for the minimum 
amount of labor required to connect from one to the other. 
This is exemplified by Figs. 28 and 29. It is evident that any 
eccentricity of the rotor with respect to the stator will affect 
equally the circuits which are in parallel. 

Simple Method for Indicating Polarity of Coil Groups. 
A simple method for obtaining the proper polarity of the groups 
is indicated in Figs. 27 and 28 for three-phase and Fig. 29 







FIG. 28. Winding diagram for a 
3-phase, 4-pole motor with pole- 
phase groups connected in 2-parallel 
star. This is the winding of Fig. 27 
connected in parallel. 




FIG. 29. Winding diagram for a 
2-phase, 14-pole motor with pole- 
phase groups connected in 2 parallels. 



for two-phase winding. In a three-phase star winding by 
traveling from each of the three leads to the star points, the 
direction of travel is reversed in adjacent groups. In a two- 
phase diagram the only necessary precaution in determining 
the proper polarity is to remember that adjacent groups of the 
same phase are reversed. By marking the groups A, B, C, 
etc., and indicating the direction of travel, it is a simple matter 
to connect them in the proper direction. Additional index 
marks may be given on the diagram to aid in connecting the 
winding, by marking those group ends which are joined by the 



ALTERNATING-CURRENT WINDINGS 41 

same connector, with the same numeral. For further details 
in using this method see Chapter XI. 

Changing Star to Delta Connection. Any star diagram can 
be readily changed into a corresponding delta diagram by 
opening up the star points and connecting the inner end of 
phase A to the outer end of phase B or C, the inner end 
of phase B to the outer end of phase C or A, and the inner end 
of phase C to the outer end of phase A or B. If the star 
diagram is not symmetrical with respect to the three phases 
it is never advisable to change over to delta. 

A.-C. Wave Windings. In a wave winding, correspond- 
ingly placed conductors under adjacent poles are connected 
in series, the circuit proceeding from pole to pole one or more 
times around the core, and not forward and back upon itself 
as in a lap winding. The circuits are then interconnected in 
such a manner as to give the requisite phase relations. The 
total number of these circuits must be a multiple of the number 
of phases and is ordinarily twice the number of phases. Due to 
certain limitations, this type of winding is not used to as great 
an extent as the lap or the concentric windings. Its use on 
small motors is limited to phase-wound secondaries. Since a 
two-phase secondary would require four collector rings, or if 
connected for a three-wire system, would overload one of the 
rings, while a three-phase winding requires but three rings, the 
latter only is general for such applications. 

Progressive and Retrogressive A.-C. Wave Windings. The 
number of slots for a wave winding (plus or minus one) is 
so chosen as to be divisible by the number of pairs of poles 
or preferably by the number of poles. If plus one, it is said 
to be a progressive winding, since after traveling once around 
the circuit it returns to the starting slot plus one. If minus 
one, it is said to be retrogressive since the circuit returns the 
winding to the starting slot minus one. With this arrange- 
ment it is impossible to balance the phases exactly, but the 
effective unbalancing is small with a large number of slots. 
Since an unbalanced three-phase winding is less objectionable 
than a two-phase winding, the scheme is used chiefly for the 
former. Again, since an unbalanced winding is less detri- 
mental in a secondary circuit than in a primary, the principal 



42 



ARMATURE WINDING AND MOTOR REPAIR 



application of this type of winding has been in secondary 
circuits. 

The diagram of Fig. 30 represents a two conductor per slot 
winding, such as a bar and end conductor type but is equally 
applicable to strap or wire wound coils of two or more series 
turns per coil, in which case the connector on the rear end 
of the coil takes care of itself and the front end is connected 

in a manner similar to the 
sketch. There are several 
methods of connecting up the 
three-phases depending on the 
desired voltage. The princi- 
pal connection is indicated in 
Fig. 30. By connecting the 
end of each series circuit to 
the beginning of the next and 
taking off leads to the point 
of connection of series 12, 

FIG. 30. Wave winding for a 3 ~4, 5-6, instead of the COn- 

3-phase, 4-poie motor, having 19 slots, nections between the series 

19 coils, two coils per slot, throw 1 to , -n- on 

6, with unbalanced phases connected shown in Fig. 30, a COnnec- 

in series star. tion is obtained for one-half 

First series begins in bottom slot 6, ends vo l tage . An 86 per cent. 
in top slot 12. Second series begins bot- 
tom slot 17, ends top slot 4. Third series Voltage tap in terms of the 




connections shown in Fig. 30 
is secured by connecting the 



begins bottom slot 9, ends top slot 15, 

Fourth series begins bottom slot 1, ends 

top slot 7. Fifth series begins bottom 

slot 12 ends top slot 18. Sixth series end Q f geries 2 to the begin- 

begins bottom slot 4, ends top slot 1. p v' 

ning of series 3, the end cf 

series 4 to the beginning of series 5, the end of series 6 to 
the beginning of series 1. To connect in star join the ends 
of series 1, series 3, and series 5, and take off leads at the 
beginning series 6, series 2, and series 4. Since this connec- 
tion reduces the voltage without increasing the cross section 
of the copper the winding will be less efficient on account of 
higher copper loss for the same output. This is also true of 
the 50 per cent, voltage connection, since for the output the 
current density in the windings is 15 per cent, greater than 
for the full voltage connection. 

It is possible, by choosing a number of slots which is divisible 



ALTERNATING-CURRENT WINDINGS 



43 



by the product of the number of phases by the number of 
poles, to lay out a winding which is balanced. For such a 
winding the circuit after passing once around the armature, 
returns to the starting slot. It is then only necessary to 
supply a special connector to join it to the conductor in the 
starting slot plus or minus one. This winding thus embodies 
the best features of both types as, for example, an electrical 
balance, and a minimum number of special connections, which 
means a very compact and easily assembled winding. The 
number of special connections is comparatively small with 
respect to the number of coils, and this feature is more pro- 
nounced as the number of poles is increased. Hence for a 
winding for a large number of poles, 12 to 40, the number of 
special connections becomes insignificant. 

Connections for Coils of Polyphase Windings. 1 A two- 
phase winding for a four-pole alternator is shown in the sim- 
plest possible form, as a 
radial diagram, in Fig. 31. 
It is a concentrated wave 
winding, having one slot per 
pole per phase, full pitch. 
Each phase has one element 
of winding, or one slot, under 
each pole, and the elements 
of phase B are distant from 
the corresponding elements 
of phase A by exactly 90 
electrical degrees. The 




FIG. 31. Radial diagram of a 2-phase 
winding for a 4-pole machine. 



whole winding is thus di- 
vided into two exactly sim- 
ilar halves, electrically dis- 
tinct from each other, just like two single phases, their positions 
being relatively fixed so that S a passes under middle Ni just 
one-quarter period after (or before, depending on direction 
of rotation) Sb passes the same point. They are thus tied 

1 The following descriptions and illustrations of windings, pages 43 to 
51, have been taken by permission from Alternating-Current Electricity 
by W. H. Timble and H. H. Higbie (John Wiley & Sons, Inc., New York, 
N. Y.). 



44 



ARMATURE WINDING AND MOTOR REPAIR 



UJ 



together in phase relation through the magnetic field and 
the mechanical distribution of the winding. The slip rings 
of phase A would be connected to S a and F a , and the rings 
of phase B to Sb and F b . 

A three-phase winding for a four-pole machine is shown in 

Figs. 32 to 34. The winding 
is the simplest practicable one 
to make, occupying only one 
slot per pole per phase. This 
winding is laid out as follows: 
First, make sure that the total 
number of slots is divisible by 
3 (phases) and 4 (poles). Then, 
holding the rotor stationary, 
mark One element (or slot) 
under the middle of each pole 

FIG. 32. Radial diagram of a as belonging to phase A, and 

connect them properly to- 
gether in series so that their 
emfs add together when they are in the position where the 
maximum instantaneous emf for the whole group is induced 
(as shown in Fig. 32). Mark one end of this series S a as in 




3-phase winding for a 4-pole ma- 
chine, star-connected. 




FIG. 33. Developed diagram of the 3-phase winding of Fig. 32, delta- 
connected. 

Fig. 32, and the other end F a . With the rotor still fixed in 
the same position and starting from S a , proceed to count the 
slots or coils in one direction around the armature until two- 
thirds of those which lie between the middle of adjacent 



ALTERNATING-CURRENT WINDINGS 45 

poles have been passed. Since the distance between the 
middle of adjacent poles is 180 electrical degrees, the dis- 
tance passed over is 120 electrical degrees. Label this slot, ele- 
ment or coil S b and locate the other three B elements or coils with 
respect to each other, exactly as the A elements are related 
to each other. Connect the B group in additive series exactly 
as the A group was connected, and mark F b on the finishing 
end of the B series. Now from $5 continue to count slots or 
coils around in the same direction until you have passed as 
far ahead of Sb as Sb is ahead of S a . This will be 120 electri- 
cal degrees from S& or 240 electrical degrees from S a . Mark 
this slot or coil S c? and locate the other C slots or coils in simi- 
lar positions with respect to all other poles. Connect the C 
slots together in additive series just as the A slots were con- 
nected, and put the F c label on the finishing end of the series. 
If the emf . in phase A is at its maximum value from S a to F a 
at the position shown in Figs. 32 and 33, it is apparent that 
the emf S a to F a must pass through one-third cycle or 120 
electrical degrees before the emf. from Sb to Fb reaches its 
maximum value (or is brought into the same position in the 
magnetic field by a counter-clockwise rotation of the rotor). 
Also, the emf from S a to F a must pass through % cycle or 
240 electrical degrees before the emf from S., to F c reaches its 
maximum value. Since these three emfs reach their maxi- 
mum value in a direction away from S just 120 electrical de- 
grees apart, consecutively, the S ends must be connected 
together to neutral, to get a star-connection. The terminals of 
the three-phase armature thus connected in star are F a , Fb, 
F c , as shown in Fig. 32.' 

A developed view of this same winding (three-phase, four- 
pole, one slot per pole per phase) is shown in Fig. 33, connected 
in delta or mesh. Notice that if only that end of each phase 
is marked which is separated by 120 electrical degrees from 
the similar end of the preceding phase, the connection be- 
comes very simple, because we merely connect the finishing 
end of one phase to the starting end of the next phase 120 de- 
grees ahead, and so on. Thus connect F a to S b , F b to S c and 
F c to S a - These junction points are then the terminals of 
the delta winding. The equal emfs (S a to F a ), (S b to Fb), 



46 



ARMATURE WINDING AND MOTOR REPAIR 




FIG. 34. Three-phase winding 
having two slots per pole per phase. 
Two phases only are shown. 



(S c to F c ) are 120 electrical degrees apart successively in 

the same direction through the closed mesh or series. 

This relation gives a resultant emf of zero volts around 

the mesh. 

If the winding is to occupy two slots per pole per phase, 

requiring 3 X 4 X 2 = 24 slots altogether, the connections 

for a two-layer lap winding are 
shown in Fig. 34. There are as 
many coils as slots, and all 
coils are exactly alike. Each 
slot contains two coil-sides (Fig. 
35). The one at the bottom is 
the right-hand side of a coil 
lying to the left of the slot, and 
the one at the top is the left- 
hand side of a coil lying to the 

ri ht of the slot ' The elements 
at the bottom of the slot are 

shown in dotted lines. 
Such an arrangement of coils is typical 'of all lap-wound 
direct-current machines, and synchronous converters; but 
it may be used for any sort of an alternating-current winding, 
closed or open. If the similar end of every coil at its starting 
end is labeled S, and the other end F (as it is wound up on a 
form, for instance), then a closed winding is obtained by simply 
soldering the F of one coil to the S of the one lying in the next 
slot, and so on all around the armature until the last F is 
soldered to the first S. This closed winding could be tapped 
at equidistant points, depending on the number of poles. 
Or this closed winding could be opened up at two or more 
points and the parts connected in series as an open winding for 
any number of phases. The winding of Fig. 35 may be 
recognized for a four-pole stator, because each coil spans one- 
quarter of the circumference. To get the greater emf in a 
coil, its opposite sides should both come as nearly as possible 
simultaneously under the middle of adjacent poles. There are 
24 coils altogether, for three phases and four poles, which allows 
two coils per pole per phase. These two will, of course, be adja- 
cent coils, in order that their emfs shall be as nearly as possible 



ALTERNATING-CURRENT WINDINGS 



47 



in phase with each other so as to get the greatest possible 
resultant emf . from the series. 

Double-layer Winding, Lap Connected. In connecting a 
winding such as shown in Fig. 35, choose a coil located exactly 
under the middle of NI and Si, and label it AI. Mark its 
starting end S a . Connect the finish end of coil AI to the 
starting end of coil A 2 which is adjacent to coil AI. Now 




FIG. 35. TWO layer winding for a 3-phase, 4-pole machine. 



F B S * F c 

L_^ 



S 8 




FIG. 36. Winding of Fig. 35, delta-connected. 



F B S A F c S a S c 

999 ? Neutral 9 



FIG. 37. Winding of Fig. 35, star-connected. 

locate coils A z and A 4> also belonging properly to phase A, 
because they are located with relation to pole Si and N 2 jus. 
exactly as coils A\ and A 2 are located with relation to pole 
NI and Si. Similarly, locate A 5 and A$ under poles N 2 and 
2 , and A 7 and A 8 under poles S 2 and NI. Then group A 3 
and A 4 in additive series by soldering the finish of As to the 
start of A 4 . Similarly, connect A 5 and A G together, and A^ 
and AS together. Now, since the emf is clockwise around 
coils AI and A 2 , counter-clockwise around coils A 3 and A 4, 



48 ARMATURE WINDING AND MOTOR REPAIR 

clockwise around A 5 and A 6 , and counter-clockwise around 
A 7 and A 8 , it will be seen, that in order to get these groups of 
coils together into additive series AI, A 2 , A 5 and A 6 must be 
connected together similarly but A 3 , A 4 , A 7 and A 8 oppositely. 
If the series of coils composing phase A are carefully traced 
starting at S a and going right through to F a , it will be seen 
that the instantaneous emfs are all in the same direction at 
about the same time when the emfs induced in the coils of 
phase A are greatest (which is about the position shown in the 
diagram). 

Phase B has been started at one end, Sb (similar to the end 
/S ), of a coil located 120 electrical degrees from coil AI, and 
from this point through to F b the connections and arrangement 
of coils are an exact duplicate of phase A, except as to actual 
position in the magnetic field. Likewise phase C is a duplicate 
of phase AI, but S c is located 120 degrees further along in the 
same direction from Sb, or 240 degrees from S a - This gives 
the six terminals of the three phases all properly labeled. In 
Fig. 36 are shown the proper connections between these six 
terminals to give a three-phase delta. In Fig. 37 are shown 
the connections between the same six terminals to give a 
three-phase star. 

Connecting a Chain Winding. The chain winding has 
been sometimes used by makers for alternating-current genera- 
tors, in addition to the two-layer windings. Fig. 38 repre- 
sents a three-phase four-pole chain winding, using two slots 
per pole per phase on the same 24-slot armature which we have 
been using throughout for illustration. In order not to confuse 
the diagram, phases A, B and C have been drawn out sepa- 
rately, in Figs. 38, 39 and 40. Notice that they are exactly 
alike, except as to relative position on the armature. Phase 
B is 120 electrical degrees from phase A, and phase C is 120 
electrical degrees further in the same direction, from phase B, 
or 240 electrical degrees from phase A, the positive direction 
of emf in each phase being from the S end to the F end. 
When the three phases are assembled altogether as in Fig. 41, 
it is seen that there must be a different shape or length of coil 
for each phase in order that the ends of the coils shall not 
interfere with each other. This is expressed usually by saying 



ALTERNATING-CURRENT WINDINGS 



49 



that the end-bends of the coils are in three ranges. This is due 
to the fact that the winding is a single- layer winding (the 



22 23 



19 20 21 22 




FIG. 38. Phase A of a 3-phase chain winding. Two slots per pole per 




FIG. 39. Phase B of the 3-phase chain winding of Fig. 38. 




21 22 



FIG. 40. Phase C of the 3-phase chain winding of Fig. 38. 




FIG. 41. -Three-phase chain winding for which the separate phases are 
shown in Figs. 38, 39 and 40. End connections of phase A only are shown. 
Three forms of coils are necessary. 

coils of a two-layer winding are all exactly alike), and also 
because all coils in each phase have been made the same shape. 



50 



ARMATURE WINDING AND MOTOR REPAIR 



In Fig. 41 the end-connections between coils are shown only 
for phase A, to avoid confusion. The system of connections 
would be exactly the same as in other figures which are 
complete 

Other Common Windings. A few other typical forms of 
windings are illustrated in Figs. 42 to 46. Fig. 42 is a 




FIG. 42. Three-phase bar winding (wave) using one slot per pole per phase, 

star-connected. 




FIG. 43. Skew winding with all coils alike, each having one side shorter 

than the other. 




FIG. 44. Short-coil winding with each coil only two-thirds of the pole 
pitch. The emf's of the two sides do not add to such good advantage as 
in other types. 

three-phase bar winding (wave) using one slot per pole per 
phase. It may be extended as shown for any number of 
pairs of poles and is drawn star-connected. Fig. 43 is known 
as a " skew-coil" winding; although there is only one coil- 
side in each slot, all coils are of the same shape for all phases. 
Conflicts of the coil-ends are avoided by making one side of 



ALTERNATING-CURRENT WINDINGS 



51 



each coil longer than the other side. Fig. 44 illustrates 
what is called a " short-coil winding" for a three-phase ma- 
chine using two slots per pole per phase. By making the 
breadth of each coil only % of the pole pitch, overlapping of 
coils is altogether avoided, and all coils in the entire winding 
are exactly alike. The series emfs composing each phase are 
not added to as good advantage as in other types of winding 
using coils nearer full-pitch, and, therefore, more copper would 




FIQ. 45. Creeping winding in which coils have a fractional pitch. Three 
coils cover four poles. The small dash lines represent slots left vacant 
for clearness. 



Mil 



1 Mil 


HIM 

^<^^r^L* 


Mill 

' ^ < K 

L ^-^- _* 


INN 


M iml 

Cl^2S^-> 


Mill 
<^~^^ 


JIIM 



FIG. 46.- 



-Single-phase, whole-coiled winding for 8 poles using 3 slots 
per pole. Armature has 64 slots. 



be needed for the same capacity. The wave-form is also 
likely to be more peaked. Fig. 45 shows a " creeping wind- 
ing" in which the coils are of fractional pitch and the series of 
coils in each phase are arranged so as to gain or lose one or more 
poles around the armature. In Fig. 45 three adjacent coils 
each spanning 240 electrical degrees, together cover 720 degrees 
or four poles. Fig. 46 shows a single-phase whole-coiled 
winding for eight poles, using three slots per pole, for an 
armature having altogether 64 slots. 

Easily Remembered Rules for Arrangement of Coils in an 
Induction Motor. The following are rules that can be 
easily remembered and cover conditions frequently encoun- 
tered in laying out alternating-current windings. 

1. Number of coils per pole-phase-group = No. slots -f- (No, 
poles X No. phases). 



52 ARMATURE WINDING AND. MOTOR REPAIR 

2. When the number of slots is not evenly divisible by the number 
of poles times the number of phases, dissimilar groups must be 
employed. These groups should be arranged so that all the phases 
have an equal number of coils. The grouping should also be sym- 
metrical, with respect to the core. 

3. A full pitch winding is one in which the span of a coil equals 
the number of slots divided by the number of poles. That is in a 
36 slot, six-pole, three-phase winding, 36 -f- 6 = 6 slots or the arc of the 
stator covered by one pole. A coil with a span of 6 slots makes the 
winding a full pitch winding. When the coil span is less than this, 
the winding is known as a fractional pitch winding. 

4. If a particular winding has given satisfaction on any particular 
voltage, a similar winding of one-half the number of series conductors 
but of double the current-carrying capacity will also give satisfaction 
on one-half the voltage. This latter condition can usually be ob- 
tained by paralleling the groups of coils. Or when a machine has 
two windings connected in series, say for 440 volts, they can be con- 
nected in parallel for 220 volts. 

5. In a wave winding, correspondingly placed conductors under 
adjacent poles are connected in series, and the circuit proceeds from 
pole to pole one or more times around the core. The circuits are 
then interconnected to give the requisite phase relations. The 
total number of these circuits must be a multiple of the number of 
phases and is ordinarily twice the number of phases. The number 
of slots for this winding (plus or minus one) should be divisible 
by the number of pairs of poles or preferably by the number of poles. 

"If plus one, the winding is progressive, since after traveling once 
around the circuit it return to the starting slot plus one. If minus 
one, it is said to be retrogressive since the circuit returns the winding 
to the starting slot minus one. To have a balanced wave winding, 
the number of slots must be divisible by the product of the number 
of poles and phases. This winding returns to the starting slot after 
going once around the core arid special connectors must be used to 
connect the finish end to the conductor in the starting slot plus or 
minus one. 

Simple Rule for Checking Proper Phase Relationship in 
a Two- or Three-phase Winding. A fundamental considera- 
tion when checking the instantaneous flow of current in a three- 
phase circuit, is to imagine that when the current flows in 
the same direction in two legs of the circuit, it flows in the 
opposite direction in the third leg. This principle can be 



ALTERNATING-CURRENT WINDINGS 



53 



applied to both motors and generators. When this scheme 
is applied to an alternating-current winding in checking the 
connections of the coils a great deal of experience is required 




A 

FIG. 47. Simple scheme of alternately reversing arrows of pole-phase 
groups to check correct phase polarity of a 3-phase winding. It is supposed 
in this case that current flows in the three leads toward the star points of the 
winding which are indicated thus ( *) . 




FIG. 48. Developed winding diagram for a 4-pole induction motor showing 

series-star connected end connections. 

This winding shows the groups of phase coils marked A, B, C with arrows pointing 
in opposite directions on adjacent pole-phase groups. It is another way of showing the 
connections illustrated in Fig. 47. 

in order to make sure that the leads of the three-phases of the 
motor are brought out at an electrical angle of 120 degrees 
apart. This is usually a separation of two poles. 



54 



ARMATURE WINDING AND MOTOR REPAIR 





FIG. 50. A 3-phase, 6-pole wind- 
ing having three coils per pole-phase 
group on a stator of 54 slots. This 
winding can be connected for 2-phase 
operation with an odd grouping of 
coils as shown in Fig. 51. 



yf 




FIG. 51. A 2-phase, 6-pole wind- 
ing on a stator of 54 slots. The 
3-phase, 6-pole winding is shown in 
Fig. 50. 




FIG. 52. A 3-phase, 6-pole wind- 
ing showing the arrangement of odd 
groups of coils. The groups of two 
coils are located diagonally opposite 
each other. The small arcs num- 
bered I to VI each span one pole 
containing 8 coils. The stator in 
this case has 48 slots. 



ALTERNATING-CURRENT WINDINGS 55 

A simple and more reliable method for the repairman to 
use is shown in Fig. 47. In this scheme it must be supposed 
that current flows in all three leads of the star connection to- 
ward the point of the star connection. And that in the case 
of a delta connection the current flows around the three sides 
of the delta in the same direction. Then in either case for 
a three-phase winding, the polarity of each of the pole-phase- 
groups will alternate regularly around the winding and can 
be indicated by arrows as in Fig. 47. As shown, for a three- 
phase winding there will be three times as many pole-phase- 
groups as there are poles. By the use of this scheme there is 
no chance for a reversal of a phase to be passed by not noticed 
when checking the winding. 

In adopting this scheme for a two-phase winding it need 
only be remembered that both groups of coils in each phase 
must reverse alternately, that is, they should be so indicated 
on the diagram by arrows, as shown in Fig. 29, page 40. 

By marking each of the pole-phase groups on a diagram, 
A, B, C, to indicate a complete phase-group, and placing the 
arrows on each single group as shown in Fig. 47, the armature 
winder will have little trouble in understanding the diagram 
and making the proper connections. To make it still easier, 
the group ends that are to be joined by the same connector 
can be marked on the diagram with the same number. 

The use of this method for regrouping coils of a three- 
phase winding for operation on a two-phase circuit and for 
any odd grouping of coils is shown in Figs. 50 to 52. 



CHAPTER III 

REPAIR SHOP METHODS FOR REWINDING 
DIRECT-CURRENT ARMATURES 

Dismantling a D.-C. Armature. When an armature is to 
be entirely rewound, it must be stripped and reinsulated 
throughout. First remove the wedges in the slots when 
banding wires are not used. If there are banding wires, 
remove them by filing in two parts. When using a hammer 
and chisel in cutting banding wires be careful not to mash the 
armature teeth out of shape. Now look over the connec- 
tions to the commutator and the end connections of the coils 
to determine whether a lap or a wave winding was used. In 
a lap winding with formed coils, the start and finish terminals 
are connected to adjacent commutator bars while in the wave 
winding the terminals of any coil are a considerable distance 
apart around the commutator. This distance is approxi- 
mately equal to the number of commutator bars divided by 
one-half the number of poles. Also in a lap winding the end 
connections of the coils at both ends of the armature bend 
toward the center of the coil or in the same direction, while 
in a wave winding they bend in opposite directions. 

The wave winding usually has either two or four sets of 
brushes. Two sets of brushes only are needed regardless of 
the number of poles in the machine since there are only two 
current paths in parallel through the armature winding 
(see Fig. 13). In the lap winding there are as many paths 
for the current as there are poles in the machine (see Fig. 10) 
and there are usually as many sets of brushes as there are 
poles. Simply because there are only two brush holders on a 
multi-polar machine, however, does not always mean that a 
wave winding is used. The commutators of some few lap- 
wound machines have internal cross-connections so that the 
commutator bars of the same potential are joined together. 

56 



REWINDING DIRECT-CURRENT ARMATURES 57 

This permits the use of only two sets of brushes. In such a 
case an extra long commutator is used which will serve as an 
indication of such connections. 

Winding Data Needed for the Dismantled Armature. 
After having made an inspection of the winding, enter the 
following data in a note book: 

1. Number of armature slots. 

2. Number of coil sides per slot. 

3. Number of commutator bars when there are two coil 
sides per slot, the number of bars will equal the number 
of armature slots in both lap and wave windings. 

4. Coil throw in slots. 

5. Commutator pitch. 

6. Number of turns per coil and size of wire used. 

A convenient chart and diagram for recording these data are 
shown in Figs. 53 and 54 as used by a large manufacturer. In 
case new coils must be ordered, this is the information that the 
manufacturer needs. It is a good plan to have a note l^ook 
with duplicate printed pages so that the record made on the 
first page can be transferred to the next by apiece of typewriter 
carbon paper. The first sheet can then be given to the 
armature winder and the copy kept in the book as an office 
record for use in case the winder destroys his copy, for use in 
making up a bill for the job and for reference in case further 
repairs should be later needed on the same machine. Such a 
record of repairs often helps in the location of new trouble in a 
repaired machine. 

Removing the Old Coils. The next operation is to unsolder 
the leads from the commutator, and proceed to remove the 
coils. This can be done usually by raising the top sides of 
the coils for a distance of the coil throw, when the bottom side 
of a coil can be reached and the others taken out one after the 
other. In removing the coils, try and preserve one in its 
original shape, to use as a guide for winding new ones. Enter 
in the note book the number of turns per coil, and with a wire 
gauge determine the size of wire. Also note whether the wire 
is single or double cotton covered. If the commutator contains 
twice as many bars as there are slots on the armature, then 



58 



ARMATURE WINDING AND MOTOR REPAIR 



each armature coil will contain two coils taped together. 
Some armatures, especially those designed for low speed, may 
contain three or even four times as many bars as slots. By 



ARMATURE WINDING AND CONNECTING DIAGRAMS 




Eig.A 



K. W. 
H. P.~~ 

Serial No. 



Volts 



Xo. Armature Slots 



No. Commutator Bars 



Refer to Figure 



Throw of Coils, A-B 



Throw 
of Leads 



Left, E-F. 



Total, C-F. 
Right, C-D 



Remarks. 




FIG. 53. Convenient form for use in recording winding data when new coils 
must be ordered from the manufacturer. 

this construction, the inductance and the current per coil is 
kept low, and the voltage between bars reduced. 

When the armature has been stripped, all old insulation 
should be removed from the slots by scraping and burning with 



REWINDING DIRECT-CURRENT ARMATURES 



59 



Electrical Specification 
For D.C.Xachinei 

Armature- CoU same asLNo 

She of strand Material Arrangement 
D.UCXRlbbon Wide Deep 
D.C.U.Ku.\\ire 



Method of Coil Layout 




B.C. C. Ribbon 

D.C.C.Kd.Wlre 
D.C.C.8q.Wlre X- 

Bare Strap 
__i Turns per single coil 

_J Single coils per cofl 

12 Total Strands per Coil"! 

_!___ Wide X ___L__deiJ 
-Silots-Two Ooih per Slot 
-Com. Bars 




i ouptd Ends-Strap Commutation Coil Drilled Ends 
Frame side JramejiJe_ 




Numbering of Slots and liars to bo frju, right to left 




Coil In bottom of dot 
Ko.l 



Connect this single 
Coil to bar No.1 



M. Btry,,. j. 

tfr "H 



Wave winding 
progressive 
retrogressive 
with dead coil 
rlthout dead coll 



bottom of slot No.l 

Connect firet act he 
single coil on rurht 
of slot No 1 to bar 




- 1 Center punch marks on \ 
ends of com bars >' 



bottom of slot No.1 




Bar No. I Bar No. 
Bar No. i. | . Bar No. Bar No 

Bar No. 

.....Number of special coils with right hand leads stretched one bar 
batween half idle bmr No and bar No 



Fia. 54. Diagrams for recording coil throw and commutator throw when 

stripping an armature or when ordering new coils. 

These diagrams are useful for checking up a wave winding that must duplicate the 
original one. In this case center punch marks are made on the commutator bars and 
slots to indicate the coil throw and commutator connections of one of the orignal 
coils as indicated in section (E) of the illustration. Here two punch marks indicate 
terminal for coil side in bottom of slot and one punch mark for the terminal of the 
coil side in the top of the slot. Two crosses (XX) indicates the slot in which the coil 
side is at the bottom and one cross (X) the slot in which it is at the top. When the 
center line of the coil is marked on the drawing to show whether it falls on a commutator 
bar or a mica, the commutator can be removed and replaced to line up with the original 
markings. For other details of determining the commutator throw, especially with 
dead coils, see "Practical Method for Locating First Connection to Commutator for a 
Wave Winding" on page 105. Section (D) of the above illustration shows the use of the 
diagram for marking the coil throw of a lap winding. Section (A) is filled out to in- 
dicate its use in specifying new coils for a wave winding. 



60 ARMATURE WINDING AND MOTOR REPAIR 

a torch and each slot filed to remove any burrs or rough places. 
Then clean the core thoroughly with a blast of compressed air. 
The core is now ready for its insulation and a new winding. 
Details are given in the following paragraphs for insulating 
and winding the different types of armatures according to the 
types of slots used and the requirements of different sizes and 
types of machines. 

I. WINDING D.-C. ARMATURES HAVING PARTIALLY CLOSED 

SLOTS 

Direct-current motors of the industrial type in sizes of 
from one to five horsepower are often built with partially 
closed slots. When winding the armature of such a motor, a 
wire-wound, threaded-in coil is used. Practical details for 
rewinding an armature of this kind with recommendations 
for the insulation of slots and testing as outlined by G. I. 
Stadeker in the Electric Journal, Vol. VII, No. 7, are given in 
what follows: 

Winding a Threaded-in Coil. Each complete coil is usually 
wound with from one to four separate wires double cotton 
covered, on a special form or mould. When winding the coil 
the two or more wires are held together in the hand and wound 
as one wire. Such a coil is then made up of two or more 
separate coils, and there will be as many beginning ends or 
terminals and finish ends to the coil as there are wires-in- 
hand when winding. Each of these ends or terminals should 
be provided with an extra insulation in the form of woven 
cotton sleeves. As an aid when connecting the terminals to 
the commutator, sleeves of different colors should be used. 
For a coil wound with three wires, black, white and red sleeves 
can be used. The same color of sleeve must be used on the 
start and finish ends of each of the wires used in winding the 
coil. To make sure of this, two sleeves of the same color 
should be slipped over each wire before beginning to wind the 
coil. When starting to wind the coil, one set of the cotton 
sleeves should be slipped down to the end of each wire and 
adjusted to reach about three-quarters of an inch along the 
body or slot side of the coil. The other set of cotton sleeves 
can be slipped back along the wires as the coil is being wound. 



REWINDING DIRECT-CURRENT ARMATURES 



61 



Then fasten the ends of the wires on the form or mould. As 
the spindle on which the winding form is mounted is revolved 
slowly, the winder can guide the wires into the mould, placing 
strips of tape under them at the corners when the first turn is 
applied. The wires should be kept under some tension 
to make them conform closely to the shape of the mould or 
form. On the last turn another sleeve should be slipped down 
each wire and allowed to extend along the body of the coil 
a sufficient distance to be bound in place with the tape which 
was previously inserted. The leads can then be cut off to the 
proper length. 

In case a coil of a large number of turns is to be wound and 
the cotton sleeves used do not slip easily over the wires, it may 
l>e quicker to use a test lamp to find the correct finish end on 
which to use the proper color of sleeve. In such a case one of 
the start ends of the coil can be placed on one terminal of the 
test lamp and each of the other finish ends tried until the lamp 
will light. On the two ends thus located the same color of 
sleeve should be used. The second pair of ends can be located 
in the same way. 

Insulating Lining for Slots. The slots should be insulated 
with an outer protective layer of fish paper (for thickness see 
pages 163 to 172, on ''Insulation for 
Slots") about three-quarters of an inch 
longer than the slot and two inner cells 
of treated cloth. One of these cells 
should enclose the lower and the other 
the upper coil as shown in Fig. 55. 
For machines having a terminal vol- 

r/\f\ ij. "j. j 

tage of 500 volts or over it is good 
practice to use a third cell of treated 
cloth placed next to the fish paper cell and enclosing both of 
the coils. This provides a better insulation of the winding 
from the core. These cells should be cut so that they will 
project about an inch beyond the slot opening to serve as a 
guide when the strands of the coil are being inserted in the slot. 
Inserting Coils in the Slots. Since the slots are partially 
closed, all the bottom sides of the coils can be inserted in the 
slots before the top sides are inserted. In each case the indi- 




FIG. 55. Slot insulation 

for a double layer winding 

in partially cl J ed slots . 



62 ARMATURE WINDING AND MOTOR REPAIR 

vidual strands may be forced into the slot with a flat fiber 
drift. As each coil is put in place the lower leads should be 
inserted into the slits of the proper commutator bars, care 
being taken that the different colored leads are connected 
always in the same order. (For details of connections for 
lap and wave windings, see Chapter " Making Connections 
to the Commutator," page 101). 

After all the coils are in position in the bottoms of the slots, 
the protecting edges of the inner cell enclosing the lower 
coil side should be drawn up as far as the coil side will allow, 
then cut off close to the slot and folded in. With a fiber drift 
and mallet force the wires and cell into proper position in 
the lower half of the slot in order to make room for the 
upper coil side. The unprotected wires of the coil which cross 
the end of the core should now be taped up with cotton tape 
for about two-thirds of their length, starting up close to the 
core and enclosing the ends of the lower cells, which project 
from the slot. 




Steel driving 
1 Slot wedge slide 

FIG. 56. Device for driving wood and fiber wedges in slots. 



When ready to insert the top sides of the coils, the upper 
cells of treated cloth should be inserted in the slots preferably 
one at a time. Now face the commutator and count the 
throw already calculated for the winding in a counter-clockwise 
direction from the first slot prepared with its insulating cell, 
in order to determine which coil should go into this slot. The 
proper coil should then be bent into shape, its strands waxed 
and inserted into the slot. The edges of the projecting cell 
can now be clipped and folded in and hammered tightly into 
place with the mallet and drift. Then drive in a fiber wedge 
over the coil by using a wedge driver such as shown in Fig. 56. 
This consists of a hollow rectangular piece of steel fitted with a 
sliding steel strip about the same size as the wedge. The wedge 



REWINDING DIRECT-CURRENT ARMATURES 63 

is inserted into the driver, its end beveled and forced into the 
slot by tapping on the steel strip of the driver with a mallet. 

Insulating Overlapping End Connections of Coils. After 
the wedge has been driven in, take a piece of cotton tape and 
wrap firmly around the coil and the projecting tip of the wedge 
close up against the core and continue to tape up the end con- 
nection of the coil to the point where it was insulated from 
the bottom upward when the other side was placed in the 
bottom of the slot. Glue the ends of the tape where they 
meet. 

When two or three of the top sides of coils have been placed 
in position, insert one or more strips of treated duck 
between the end connections of the upper and lower coils 
where they cross each other at each end of the armature. 
As the remaining upper sides of the coils are placed in the 
slots, these strips should be wound around the armature 
so that they finally form a complete band of insulation between 
the upper and lower coils. Each coil as it is finally placed 
should be shaped at its ends with a mallet and fiber drift so 
that one coil fits .snugly against the other and there is a 
rigid construction when the armature is completed. 

Connecting Finish Ends of Coils to Commutator. The 
ends of the upper sides of the coils can now be connected to 
the commutator using a test lamp. Connect the colored leads 
in proper order as explained on page 101 under " Making 
Connections to the Commutator." Before soldering the 
coil ends, the entire winding must be tested for grounds 
with proper voltage and for short circuits and open circuits. 
(See page 122 under the heading of " Testing Armature Wind- 
ings.") If no faults are discovered the ends of the coils can be 
soldered to the commutator and the latter turned and finished 
with sandpaper. The armature is now ready for banding. 

Loop Windings for Small Motors. Small direct-current 
armatures for fan motors and general utility use, are often 
wound by hand using what is sometimes called a loop winding. 
In the case of a 2-pole armature having 15 slots and 15 com- 
mutator bars, such a winding might be made up as follows: 
Wind slots Nos. 1 and 8 half full, using double cotton-covered 
wire, leaving the ends a little longer then needed to connect to 



64 ARMATURE WINDING AND MOTOR REPAIR 




FIG. 57. Winding the first coil in the slots of a small direct-current motor. 




FIG. 58. The operator is here shown completing the last turn of the winding. 




FIG. 59. In this illustration the operator iri driving down the coils before 
inserting fiber wedges in the slots of the completely wound armature. 
In Figs. 57 to 59 three steps are shown in \vinding a small direct-current armature. 
Before starting to wind the coils, the cores are insulated on the ends as shown in the 
lower right-hand corner of Fig. 57. The armature illustrated is for a 32-volt motor and 
has 19 slots with a coil pitch of 1 to 8 slots. Ten turns of No. 11 wire per coil are used 
with two coils per slot. There are a total of 38 coils and 38 commutator bars (Robbing & 
Myers Company). 



REWINDING DIRECT-CURRENT ARMATURES 



65 



the commutator and without cutting the wire wind a similar 
coil in slots 2 and 9 and bring out a second loop. Proceed 
in this way throughout all the pairs of nearly opposite slots 
until on the second round the slots are completely filled when 
the beginning and the end of the wire can be twisted together 
for the 15th loop. Cotton sleeves can now be inserted over 
the loops and connections made to the commutator. Figs. 
57 to 59 show an armature being wound by this method. 
In the illustrations the necessary insulation on the shaft close 
to the core and on the core at the ends of the slots is shown. 
The slots should be insulated in this case with heavy fish 
paper about 10 mils thick. 

Banding a Small D.-C. Armature. When applying the 
banding wire, the armature should be inserted in a lathe. 







FIG. 60. Dissembled view of a small direct-current motor showing banded 
armature (Fidelity Electric Company). 

The first operation is to hammer down the ends of the coils 
until their diameter at the armature ends is no greater than 
that of the core. Extreme care must be taken not to injure 
the insulation with the mallet. As a base for the banding 
wires, two bands of cotton tape separated by a band of var- 
nished paper should be wound over the end connections near 
the core and the whole tied down with several layers of twine. 
Short strips of tinned copper about 0.02 by 0.25 inch in cross- 



66 ARMATURE WINDING AND MOTOR REPAIR 

section should be slipped under the temporary banding twine 
at intervals of two or three inches with two extra ones used 
where the banding wire is started and ended. For the banding 
wire of small armatures No. 14 to No. 17 B. & S. gauge tinned 
steel wire can be used. The start should be fastened to a peg 
slipped into an air duct or fastened to the end of the banding 
twine and the first layer guided so that it crosses itself to 
relieve the strain on the end fastening. After two or three 
revolutions of the band wire the temporary banding twine 
can be cut off and the banding wire wound on tightly across 
the protecting tape. After the required width has been 
wound, the copper strips at the start end should be turned 
up and clipped off so that about one-quarter inch projects 
from under the banding wire. Then bend the strips over to 
hold the banding wire in position. 

Without cutting the banding wire, it should be guided across 
the core to the opposite end of the armature and wound in the 
same manner as before to within about a quarter of an inch 
of the edges of the end connections. The clips at beginning 
and end can now be bent over and soldered. The banding 
wires on both ends of the armature should now be driven up 
close together, all the clips bent over and the wires soldered 
together. In the soldering of band wires no acid should be 
used. A solution of rosin in alcohol is recommended. After 
the soldering operation is completed the surplus turns and 
crossovers of the banding wire can be cut off. The armature 
is now completed except for the balancing. (See page 150 
under the heading of "Balancing an Armature. 7 ') 

II. WINDING D.-C. ARMATURES HAVING OPEN SLOTS 

Motors in sizes above five horsepower that nave armature 
cores with open slots may be wound with coils made up of wire 
or of copper strap. The coils may be of the form wound or 
pull in types but are fully insulated before being inserted into 
the slots. In general two types of open slots are found, one 
with wedge grooves in the top of the slot in which wedges can 
be driven to hold the coils in position; while the other is a slot 
with smooth sides requiring banding wire to hold the coils in 



REWINDING DIRECT-CURRENT ARMATURES 



67 



place. Grooves are usually provided on the armature for 
bands in addition to those over the end ' connections. Very 
often the same armature core with open slots is used for differ- 
ent types and sizes of machines. The coils may therefore be 
smaller in some cases than the original ones and do not fill the 
slots. In these cases fillers must be used to fill out the sides 
and below the coils in the slot so that the wedges or band wire 




FIG. 61. Direct-current armature showing duck or canvas pad held by 

cordage on ring which supports coils 

This insulation is used to prevent coils from rubbing on the frame and gives a larger 
margin of safety against grounds. Slot insulation made of treated pressbpard is shown 
in place. Separators between coils in each slot are made of the same material but a little 
thicker (Roth Brothers & Company). 

will press firmly on the top of the coils and prevent any pos- 
sible motion of the coils in the slots that will result in chafing 
and damage to the insulation. Fillers most used consist of 
strips of treated fullerboard or treated wood. If a coil is 
only slightly loose it is better to add insulation to it. 

Winding and Insulating Coils. In case coils must be wound 
for a particular job proceed as outlined under the heading 



68 ARMATURE WINDING AND MOTOR REPAIR 

of " Forms for Winding Coils," on page 141. In rewinding 
a bar wound armatAire, it will seldom be necessary to form new 
coils, because each coil consists of a single turn of heavy copper 
strip, which is not as easily damaged as are wire wound coils. 
In this case, all that will generally be necessary is to reinsulate 
the coils and the armature core. When all the coils have been 
wound, they should be covered with cotton tape about three- 




FIQ. 62. Armature shown in Fig. 61 with a partial lap winding in place. 

The insulation on ends of coils and method of interlacing coils so that they occupy 
a minimum amount of space are shown. Coil ends are later to be fastened to supporting 
spider ring with treated canvas or duck and cordage. Band wires are wound on core 
with stripes or pads of thin canvas underneath, after winding is completed (Roth Brothers 
& Company). 

quarter inch in width. The end of the tape winding can be 
made fast with a piece of thread, care being taken to have the 
taped end on a part of the coil that does not go into the slots, 
as the thickness of the thread on each side may be sufficient 
to cause the coil to stick while being placed in position on the 
core. The finished coils should go into the insulated slots 
without undue driving. The coils should now be given a coat 



REWINDING DIRECT-CURRENT ARMATURES 69 

of moisture repelling varnish and allowed to dry. When thor- 
oughly dry the armature can be rewound. 

Insulating Open Slots. When the armature is not too large 
it can be wound in a lathe or bench stand. For large armatures 
a suitable floor stand can be easily made. Place the armature 
in the lathe or on the stand with the commutator at the 
winder's right. Before inserting the insulation for the slots, 
they should be thoroughly cleaned and all burrs and sharp edges 
removed with a file. The same arrangement of slot insulation 
described for partially closed slots on page 61 can be employed. 
However, for low-voltage machines (not over 250 volts) to 
be used in dry places a satisfactory slot insulation consists of 
two layers of fish paper each 0.005 inch thick between which is 
placed a layer of empire cloth 0.010 inch thick. This insula- 
tion should be cut so that it will extend about J^j inch past the 
end of the slot and project about one inch above the entrance 
bo the slots. This will form a mechanical protection for the 
coils and serve as guides through which the coils can be slid 
into place. The corners of the projecting edges should 
be clipped to keep them from interfering with the insertion of 
the coils. If end-ring insulation is to be used, hold it in place 
by winding thread over the ends of the core through the slots. 

Inserting Coils in Open Slots. After the slots have been 
insulated, place one side of a coil in a slot and force it to the 
bottom with a fiber drift a little narrower than the width of 
the slot. The other half of the coil (or top side) should be left 
out of the armature for the present. Insert the bottom half 
of coil No. 2 in the next slot. turning the armature in a counter- 
clockwise direction (looking toward the commutator end). 
Proceed in this manner until the top side of one coil is to be 
placed into the slot containing the bottom side of the first 
coil placed on the armature. This will be the first coil in 
which the top side can be placed in a slot over the bottom side 
of coil No. 1. Before doing this, reference should be made to 
the note book data in case an armature is being rewound 
exactly as before, in order to make sure that the proper number 
of slots have been spanned. 

To make this method of winding clear, refer to Fig. 63. 
Here the coils span five slots, that is, the top side of coil No. 1 



70 ARMATURE WINDING AND MOTOR REPAIR 

is removed five slots from the bottom side of the same coil. 
The top half of coils Nos. 1, 2, 3, 4, and 5 (called the throw 
coils because they cover a part of the armature equal to the 
throw of a coil) are left out of the slots as shown, as the bottom 
half of other coils will have to be placed in these slots before 
these top halves can be put in. When the bottom half of coil 
No. 6 is placed in slot 11, its top side may be placed in slot 6, 
because the bottom side of coil 1 is located in this slot. Con- 
tinue to place the coils on the core, traveling in the direction 
indicated by the arrow. The bottom half of coil No. 7 is 




FIG. 63. Method of placing throw coils on the armature of a lap winding. 

placed in slot 12, and the top half in slot 7. Before inserting 
the coils in the slots it is a good plan to rub the sides with 
paraffin wax. This helps in inserting the coils and prevents 
damaging the insulation. 

As the armature is being wound, a strip of heavy pressboard 
(about 0.050 inch thick) should be placed in the slots between 
coils to thoroughly insulate them from each other. When all 
the coils have been inserted, the top sides of coils Nos. 1, 2, 
3, 4, and 5 can be placed in the slots. As each coil is put in 
position, the end connections should be carefully shaped to 
the core. 

Shaping End Connections. The end connections are shaped 
by means of a winding drift. This consists of a steel bar about 
12 inches long, one inch wide and tapered in thickness from 
one-half to one-eighth inch, having all the corners rounded 



REWINDING DIRECT-CURRENT ARMATURES 



71 



and smooth. The tip of this drift should be placed against the 
inner side of the end connection of the coil and tapped with a 
mallet, forcing the upper part of the end connection out from 
the armature and away from the lower half. Each coil as it 





FIG. 64. Rear and commutator ends of a wave-wound armature using 
strap coils. Note the insulation of slots and the insulated support of rear 
end connections (General Electric Company). 

is put in place can be similarly shaped so that when the arma- 
ture is completely wound a circular air chamber is formed 
between the upper and lower halves of the end connections at 
both front and rear. This process should be continued until 
the first slot is again reached. 



72 ARMATURE WINDING AND MOTOR REPAIR 

Six Steps in Winding a Small Direct-current Armature 

No. 1 




FIG. 65. This illustration and those of Figs. 66 to 71 show successive 
steps in winding a small direct-current armature (Crocker-Wheeler Company). 
The slot and shaft insulation and first two coils are shown in place here. 



No. 2 




FIG. 66. Appearance of the winding before the last three coils are inserted in 
bottoms of the slots. 



REWINDING DIRECT-CURRENT ARMATURES 73 



No. 3 




iG. 67. All bottom sides of coils in place and strips of insulation inserted 
between bottom and top coil sides ready for the top layer of the winding. 



No. 4 




FIG. 68. Inserting the last few top sides of coils. Note the shaping of the 
end connections of this winding alternately in and out to give the compact 
appearance shown in Fig. 71. 



74 ARMATURE WINDING AND MOTOR REPAIR 



No. 5 




Fiu. G9. Connecting the first two leads of bottom coil sides to the commu- 
tator. 



No. 6 




FIG. 70. Connecting the leads of top coil sides to the commutator, 
the tape insulation between coil terminals. 



Note 



REWINDING DIRECT-CURRENT ARMATURES 



75 




FIG. 71. Completed armature banded and treated with insulating com- 
pound. Note winding insulation over coil terminals at commutator end. 




FIG. 72. Portable floor stand for winding small direct-current armatures- 
The operator is shown shaping the end connections with a fiber drift. 



76 



ARMATURE WINDING AND MOTOR REPAIR 



Truing Up the Heads of the Winding. When the winding 
is completed, bend the leads back over the surface of the core 
so as to expose the head of the winding. This must be trued 
by revolving the armature and marking the high places with 
chalk. These high spots can be driven down with a rawhide 
mallet, and the low places raised even with the others. The 
back head should also be carefully trued up so as to present a 
good appearance when running. Trim off the projecting 
slot insulation even with the surface of the armature if banding 
wires are to be used. In case the coils are to be held in place 
by . wood or fiber retaining wedges, lap this insulation down 
over the coils and drive in the wedges. If the armature leads 
are not covered with tape, they must be protected in some 
manner and for this cotton sleeving can be used. 




FIG. 73. Insulation of end connections showing friction cloth blanket i 
place at the commutator end as a protection for the leads. 

Insulation Between Commutator End Connections. A 

friction cloth blanket should be placed over the end connec- 
tions on the commutator end, as shown in Fig. 73, as a 
protection for the terminals of the coils. It is good practice 
to shape the ends or terminals of the side of the coils in the bot- 
toms of the slots as shown in this illustration and to bend 
back over the core the terminals of the top sides of the coils. 
This helps in arranging the connections from the coils in the 
bottoms of the slots, which can now be made. 



REWINDING DIRECT-CURRENT ARMATURES 77 

Now wrap several thicknesses of treated cloth in a belt 
over the bottom leads and bind with thread. The top lead, 
located by a lamp tester of magneto can now be connected 
to the commutator. In the same manner connect the remain- 
ing leads. Cut off the ends of the leads that project past 
the neck of the commutator, and if there is space after the lead 
have been driven down, these pieces can be used as " dummies" 
to fill such spaces. When this stage of the winding process 
has been reached, a short-circuit and open-circuit test must 
be made. (See page 122 under heading of " Testing D.-C. 
Armature Windings.") If the armature tests clear, the leads 
can be soldered in. 

When the wedges are not used to hold the coils in place some 
means must be provided to prevent them from becoming 
loosened from the slots, during the subsequent operations 
before banding. To accomplish this a single band of wire 
can be tightly fastened around the coils at each end of the 
armature. 

After these operations the commutator can be turned and 
polished and the armature banded if this is required. When 
balanced and painted with insulating varnish, the armature 
is ready for use. 

m. WINDING LARGE D.-C. ARMATURES 

In contrast with the winding of small armatures, the winding 
of large direct-current armatures is not a particularly compli- 
cated operation. Although the insulation throughout must be 
moisture and oil proof, no such elaborate precautions as in 
the case of industrial motors is necessary in the larger sizes 
of machines for they are in most cases installed in dry clean 
places. The centrifugal strains, however, may be higher in 
the larger machines and the windings must also be braced 
against the magnetic strains produced by heavy short circuits 
which may occur on account of the very low resistance of 
circuits using wire of large cross-section. The following 
recommendations are given for the winding of large armatures 
by a writer in the Electric Journal, Vol. VII, No. 11. 

Coils for Large D.-C. Armatures. For the large sizes 
of direct-current machines the armature coils are usually 



78 ARMATURE WINDING AND MOTOR REPAIR 

formed of bare copper strap. They are usually of the one 
piece or two piece, one turn, diamond type with the number 
of coils equal to the number of commutator bars. To secure 
best possible space factor, and for other mechanical reasons, 
single coils are often bound together into a larger coil, each 
single coil being insulated from the other and electrically 
separated. In such a case the number of slots is only a 
fraction of the number of commutator bars. 




FIG. 74. Lap-wound armature using two-part strap coils. Note the 
four layers of insulation between end connections at commutator end (General 
Electric Company) . 

The method of insulating this type of coil depends upon 
the size, voltage and operating conditions of the machine and 
on the number of single coils composing a complete coil. 
When there are less than four single coils per complete coil, 
the ends of each single coil are taped with one layer of cotton 
tape, half overlapped. This taping extends a sufficient dis- 
tance along the straight part to assure that the joint between 
it and the rest of the insulation will be well protected. The 
straight parts are then wrapped with a fish paper and mica 
wrapper, interwoven between the straps in such a manner as to 
furnish insulation between the single coils, and then wrapped 
several times around the complete coil, the exact number of 



REWINDING DIRECT-CURRENT ARMATURES 79 

turns depending on the size, voltage and operating conditions 
of the machine. (See "Coil Insulation," pages 163 to 172.) 

When there are four or more single coils per complete 
coil, alternate single coils should be wrapped with one turn 
of fish paper and mica, held in place by a non-overlapping layer 
of cotton tape. The single coils are then assembled and a 
cell of fish paper and mica wrapped over the whole. The coil 
should then be taped with a layer of cotton tape, non-over- 
lapping over the wrapper and half lapped over the ends. 
Then brush with or dip in a black finishing varnish and air 
dry. After this dip twice in insulating varnish and dry in 
an oven for twelve hours after each immersion. Before 
the coil is used in the armature, the leads should be cleaned 
of insulation and varnished and thoroughly tinned. 

Lap and Wave Windings for Large Armatures. In the 
main the same conditions outlined on page 112 under the 
heading "Wave Lap vs. Direct-current Windings" apply in 
the case of largo armatures. The wave winding has the 
decided advantage, however, that no cross-connections are 
required. It has, therefore, a wide use in machines where the 
size of coil required does not become excessive nor the voltage 
between commutator segments too great to permit of good 
commutation. Under ordinary conditions this consideration 
limits the wave winding to four- or six-pole machines. Where 
the number of poles is greater the lap winding seems to suit 
the conditions of good operation best. With this winding 
the voltage between segments is kept down and high conduc- 
tivity through the armature is made possible without using 
coils of large cross-sections. For both lap and wave windings, 
one-piece coils can be used, but a two piece or half coil has 
some advantages in that less skill is needed to wind the 
armature and the coils are easily repaired if the damage is only 
to the top half. The half coil however calls for a soldered 
joint at the rear of the armature which must be made with 
great care. 

Insulating the Core. Before starting the winding operation 
the core should be thoroughly cleaned with an air blast, thus 
removing any iron filings or other foreign matter from the 
slots. The commutator necks must then be carefully ex- 



80 ARMATURE WINDING AND MOTOR REPAIR 

amined to see that all are straight and that the openings at 
the top are wide enough to admit the coil leads easily. Test 
the commutator for breakdown to ground, and for short- 
circuit between segments, with the standard test voltage for 
the machine. (See Chapter V, also page 175.) All parts of 
the spider which come in contact with the coils, such as coil 
supports, etc., should be carefully insulated with either tape, 




FIG. 75. Medium-sized heavy duty armature showing use of strap coils 
for a wave winding (Westinghouse Electric & Mfg. Company). 

fullerboard channels, or two or three thicknesses of fish paper 
strips. When tape is used it is wrapped in overlapping layers 
over the entire support. At the point where the spurs which 
hold the coil support in position prevent winding on the tape, 
the iron should be covered with insulating cloth, and held in 
place by the tape on each side. Each layer of the tape should 
be shellaced as it is wound. When fullerboard strips are 
used, the first layer is frequently screwed to the iron to pre- 
vent lateral motion. Other layers are shellaced over this, 



REWINDING DIRECT-CURRENT ARMATURES 



81 



and the whole is usually bound with twine. Special care 
should be taken to stagger all joints. 

Inserting the Coils. The assembly of the different types of 
coils is essentially similar. Mark two slots with chalk to 
receive the first coil, and count off and mark the commutator 
necks into which its leads will be connected. Fish paper cells 
can then be inserted into the slots, and the coils driven into 
position one after another with a mallet and a fiber drift. If 
a two-piece coil is used, the lower half coils should be inserted 
first all the way around the armature and then the upper half 




FIG. 76. Wave-wound armature partly completed showing insulation used 
between end connections of coils. 

coils. If one piece coils are used the coils should be inserted 
in regular succession, the bottom half of the coil being driven 
into the bottom of its slot first. The other half is driven into 
close contact with the coil which is already in the bottom of 
the slot. If there is no coil in the bottom of the slot, as hap- 
pens with the throw coils, this top half is inserted only tem- 
porarily until the winding has been carried entirely around the 
armature. Then the throw coils must be removed so that 
the coil sides can be placed in the bottom of the slots. 

When a one-piece coil is used in a wave winding, the throw 
coils span so large a part of the armature that it is not usually 
advisable to insert the upper part of the coils and then remove 
them but allow them to hang free as shown in Fig. 76 until 



82 ARMATURE WINDING AND MOTOR REPAIR 

all the coils have one side in place in the slots. The upper 
sides can then be driven into place in regular order. As in 
the smaller windings, the upper and lower coil ends or termi- 
nals should be separated with bands of oiled duck or drilling. 
With one piece coils, this should be threaded into place as the 
coils are inserted in the upper part of the slots. With two- 
piece coils it is simply wound over the lower set of coils before 
the others are placed in the slots. 




FIG. 77. Core of a large armature showing construction for good ventilation 
(General Electric Company}. 

The coils must be a close fit in the slots, in order to prevent 
any possibility of chafing. If necessary, strips of fullerboard 
or treated wood should be inserted at the sides or bottom of the 
slot, to make the coils a tight fit: As each top coil is put in 
place it should be driven into the slot, the protecting cells then 
cut off, and folded over it, and fiber wedges driven into the 
wedge grooves. The slots on a large-sized machine are too 
long to allow one wedge to be used, so that one or more must 
be driven in from each side of the slot to furnish complete 
protection for the face of the coil. The armature should then 
be tested for grounds, before the connections are soldered. 

After the winding is completed, the armature may be banded 
temporarily at both ends. Then drive wooden wedges loosely 



REWINDING DIRECT-CURRENT ARMATURES 



83 



in between the commutator necks, all around the armature to 
insure even spacing. After this drive them in tightly, to 
force the necks and coil ends into tight contact and hold them 
rigidly in place. With two-piece coils, connecting clips are 
placed over the leads at the rear end, and wedges driven in 
between these in the same manner. The connections to the 
necks, and the rear end connections, if any are used, can be 
soldered. This soldering should be done on the side of the 




FIG. 78. Large direct-current engine type armature showing method of 
ventilation (Westinghouse Electric & Mfg. Company). 

'machine instead of the top, as a better joint can be made in 
this manner, and there is less liability of the melted solder 
running along the necks and short-circuiting the commutator 
segments. 

Before removing the wedges the armature should be mounted 
in a lathe, or if no suitable lathe is available, in its bearings 
with the field frame removed, and the soldered connections 
turned down. If the armature is mounted in its bearings, a 
suitable tool holder 'must be fastened to the frame, or some 
rigid support. The commutator may then be turned down and 
given its final polishing at the same time. Now knock out the 



84 



ARMATURE WINDING AND MOTOR REPAIR 



wedges from between the leads, and round off the sharp cor- 
ners with a file. When the bare copper is not covered with 
insulation, insulating material can be inserted between the 
adjacent leads to prevent accidental contact. At the rear 
end, this usually takes the form of asbestos tape which can be 
interwoven between the leads or of canvas hoods, sewed in 
place. At the front end, the necks may be separated at 
the tops by strips of heavy fish paper, bent over the tops of the 
leads so as to be held in place by the band wires. Where the 




FIQ. 79. Large direct-current engine type armature partly wound with* 
strap coils (Westinghouse Electric & Mfg. Company). 

necks are quite long, additional separators may be inserted 
half way up from the commutator. These may be in the form 
of fiber buttons or may merely consist of heavy twine, inter- 
woven between the long commutator necks. 

Banding Wire. Bands of steel wire are ordinarily placed 
over both ends of the armature, and frequently another over 
the connections to the commutator necks. No bands need be 
used over the surface of the core, as the wedges are sufficient to 
retain the body of the coils in place. The coils can be pro- 



REWINDING DIRECT-CURRENT ARMATURES 



85 



tected from the mechanical pressure of the banding by layers of 
surgical tape separated by strips of cement paper, over which 
the bands are wound. The wire should be wound on under 
heavy tension, secured by clamping it between blocks of wood. 
These blocks can be held from moving by heavy straps of wire, 
fastened to some rigid object usually the machine frame. If 
desired, a spring balance may be inserted, which will give the 




FIG. 80. Armature of a 350 kw., 250-volt, engine type generator showing 
the core construction for good ventilation and the use of copper strap coils 
(Fairbanks-Morse & Company). 

exact tension that is being applied. This should run between 
300 and 400 Ib. The bands should be firmly soldered in place. 
When it is desired to secure extra mechanical strength, 
the wire is sometimes wound on two or three layers 
deep, each layer being soldered separately. The proper 
banding of a large armature by this method is often a serious 
problem on a repair job. A sectional band wire such as shown 
in Fig. 82, is therefore convenient. When using this type of 
banding, the two ends of any section are keyed together into an 



86 



ARMATURE WINDING AND MOTOR REPAIR 




FIG. 81. Completed armature of a 350 kw., 250-volt, direct-current engine 
type generator, lapwound for 8 poles with equipotential connections. Four 
coils per slot are used (Fairbanks-Morse & Company). 







FIG. 82. Tool for use in applying section bands on large armatures. 



REWINDING DIRECT-CURRENT ARMATURES 87 

open loop and then applied to the armature. In making the 
final connection the special clamp shown in Fig. 82 is needed. 
The two jaws of this clamp grip the ends of the band and by 
means of the handle whose lower end is formed into a cam, the 
jaws can be forced together so as to interweave the loops of the 
band wire and permit the steel key B to be inserted. In this 
operation the clamp can be held in any position by inserting 




FIG. 83. Large direct-current armature partly wound with strap coils. 
Note the insulating strips between end connections and double insulation in 
slots (Crocker-Wheeler Company). 

the pin A through the movable jaw and beam. For other 
details of banding see Chapter VI, page 146. 

Balancing Large Armatures. After the armature is banded 
it is ready for balancing. Suitable balancing ways may con- 
sist of heavy steel beams with polished steel plates mounted 
on their upper edges. The surface of the polished plates 
must be accurately leveled. The shaft should rest on the 
inner surface of a polished steel ring, which in turn rests on 
the polished plates. In this way, an almost frictionless 



88 



ARMATURE WINDING AND MOTOR REPAIR 



bearing surface is obtained, and the armature tends to roll 
until the heaviest part is at the bottom. Melted lead can be 
poured into recesses in the spider, or cast-iron weights bolted 
to the spider arms to correct any unbalanced condition, until 
the armature will lie with any part uppermost. The armature 
should then be thoroughly cleaned with an air blast and 




FIG. 84. Method of cross connecting a large direct-current armature having 
a lap winding ( Westinghouse Electric & Mfg. Company). 

sprayed inside and out with black finishing varnish. Special 
care should be taken to reach all exposed parts of the core, 
to prevent rusting. 

Rotary Converters. There is no essential difference in the 
winding operations as described between rotary converters, 
and other direct-current machines. At the rear of the arma- 
ture, however, taps are brought out from the coils at regular 



REWINDING DIRECT-CURRENT ARMATURES 



89 



intervals. On a three-phase machine, this will be two-thirds 
of the pole pitch; on a two-phase machine, one-half the pole 
pitch, and on a six-phase machine one-third the pole pitch. 
These taps are connected to the collector rings. A two-phase 
or a six-phase rotary converter cannot be wound with a wave 
winding on account of the necessity of having an equal num- 
ber of coils between taps on the armature. 

Three -wire Generators. Practically any standard genera- 
tor can be adapted for use as a three-wire machine by the 




FIG. 85. Completed armature for a large 3-wire direct-current generator 
(Crocker- Wheeler Company). 



addition of suitable collector rings and balancing coils. These 
coils are entirely self-contained and may be installed apart 
from the generator. The collector slip rings are usually much 
smaller than those of a rotary converter, as each one carries 
only a fraction of the unbalanced current. The current 
which they carry is largely unidirectional, only enough alter- 
nating current flowing to excite the core of the balancing coils. 
They are accordingly made of iron to avoid the blackening 
from electrolysis which takes place when the direct current 
flows from copper to carbon. They may be placed at either 
end of the armature, but are usually placed at the end of the 
commutator, for greater convenience (see Fig. 85) . 



90 



ARMATURE WINDING AND MOTOR REPAIR 



IV. WINDING RAILWAY, MILL AND CRANE TYPES OF 
ARMATURES 

The methods of winding and insulating armatures for rail- 
way, mill, mining and crane types of armatures are practi- 
cally the same, since the service is somewhat similar and the 
windings must stand up against much hard usage and abuse 
of the motor. Thorough insulation and protection against 
vibration and chafing of the insulation are essential points 
in the winding of these armatures and must be given more 
attention than in any other. 

Railway Type Armature Coils. This type of armature coil 
is completely formed and impregnated with insulating com- 
pounds. It needs no shaping when placed on the armature. 



Fullerboard Protecting Strip 
.-'Coil in Top of Slot Only 

Plaetlo Varnish 




Cotton Tape 



Fuller Board Separators 




ullerboard Protecting Strip" 
'Coilln Top of Slot Only 
,D.C.C. Wire 

Treated Cloth 



Fuller Board Separators 



Black Plastic Varnish 



Fro. 86. Wire wound coils show- 
ing single coils arranged in vertical 
layers. 



| -^ Cotton Tape 



FIG. 87. Wire wound coils 
showing single coils arranged in 
horizontal layers. 



It is usually advisable for the repairman to purchase these 
coils from the manufacturer of the motor which is to be re- 
paired. Usually the coils are made up of two or more single 
coils wound with round wire or copper strap. These single coils 
are separately wound but grouped together for mechanical 
reasons in handling and insulation. The windings of these 
motors are of the wave or two-circuit type, except for the 
largest sizes of mill or locomotive motors. The insulation 
for coils and core and the details of the winding of railway 
armatures as given here, are the recommendations outlined by a 
writer in the Electric Journal, Vol. VII, No. 10. 

Wire Coils. The coils for smaller machines are made of 
double cotton-covered wire. The single coils which form a 



REWINDING DIRECT-CURRENT ARMATURES 



91 



complete coil are insulated from each other by fish paper or 
fullerboard separators and may be arranged radially or cir- 
cumferentially in the slot as shown in Figs. 86 and 87. The 
leads from wire wound coils are secured along the diamond end 
and leave the coil one after the other, each being firmly tied 
and taped in position so that there is no possibility for them 
to chafe against one another. They are reinforced with 
cotton sleeves. 

Strap Coils. Most of the larger railway motors have the 
armature coils made from rectangular conductors instead 
of round wire, as with this form of conductor a greater pro- 
portion of the slot space may be filled with copper, without 



Fish Paper Protecting Strip 
'"Coil in Top of Slot Only 



'ullei board Protecting Strip 
Coil in Top of Slot Only 




FIG. 88. Section of slot showing 
strap wound coil of two turns. - 



FIG. 89. Section of slot showing 
strap wound coil of one turn. 



sacrificing the insulation requirements. The pressure on 
the insulating surfaces is also more evenly distributed, as 
with the round wire the pressure bears on a line, while with 
the rectangular conductors it is distributed over a flat surface 
and is much less liable to injure the insulation. Figs. 88 
and 89 show a cross-section through two-turn and one-turn 
coils, respectively. 

The one turn coil readily lends itself to bringing out the 
leads in position to enter the top and bottom of the com- 
mutator necks respectively, by the use of the standard form 
of diamond end. A two turn coil requires a special turn at 
the rear end of the coil, in order to bring the leads out in 
the proper position. By the use of this form of coil, all the 



92 ARMATURE WINDING AND MOTOR REPAIR 

advantages of the strip windi ig can be secured for the smaller 
machines on which more than one turn per slot is usually 
required. 

In some large motors, coils of the rectangular types are 
made in two pieces, and are known as two-piece coils. Their 
advantage lies in the fact that if a coil becomes damaged, only 
one-half of the complete coil need be removed to overcome 
the defect. As damage to the coils nearly always occurs on 
the outside of the armature, this type of coil is peculiarly 
adapted to railway type armatures. It requires, however, 
a soldered connection at the back, in addition to the usual 
soldered connection at the commutator, and hence is used 
only on the large motors where the saving of copper for repair 
parts would be great. 

Coil Insulation. The insulation used on motors of the type 
under consideration depends largely on the type of coil used. 
Where double-cotton-covered coils are used there is little 
advantage in using materials for the remainder of the slot 
insulation which have higher heat resisting ability than the 
cotton strands which are in immediate contact with the con- 
ductors, since it is necessary under the circumstances to limit 
the temperature to values consistent with the cotton insulation. 
For this reason on certain types of mill motors, where the 
temperature conditions are exceptionally severe, asbestos 
covering is used instead of the cotton, with mica insulation 
around the complete coil, cotton being used only in the protec- 
tive taping over the outside. 

With strap wound coils it is possible to use built up mica 
in immediate contact with the conductors and cotton only 
on the outside protective coverings, where it is in contact with 
the air or the relatively cooler iron. Hence the copper can 
be safely worked to a higher value and continued overloads 
and abuse will not be so liable to cause breakdown. The coils 
should be vacuum impregnated before insertion in the arma- 
ture. This process renders them thoroughly moisture and 
oil proof and prolongs the life of the coils over that of un- 
impregnated ones, especially where they are subject to mois- 
ture, acid fumes and deleterious gases. It is applied to all 
railway type coils except those for mine locomotives, whose 



REWINDING DIRECT-CURRENT ARMATURES 



93 



armatures are usually impregnated as a whole after the wind- 
ing is completed. 

Insulating the Core of Railway Armatures. In this type of 
armature it is advisable to supply extra protection at every 
point of special electrical or mechanical stress such as where 
the coils leave the slots, where the leads leave the coils and 
where the leads cross one another or cross the ends of the coils. 
Before applying the core insulation, the core should be 
thoroughly cleaned with an air blast and burrs taken off 
with a file. When wire wound coils are used the coil supports 
have curved surfaces and should be insulated with treated 
cloth in strips or with layers of rope paper and treated cloth. 



Comtnuta'tor 




Coil Support 

FIG. 90. Insulation of support for front coils of railway armature. 



Slits should be made in the strips where necessary to make 
them lie smooth as over the end bell, care being taken that 
the slits in the successive layers are staggered. These strips 
should be bound together with shellac and ironed smoothly 
into place. They should be built up to a thickness of about one- 
eighth inch (six layers of rope paper and five layers of treated 
cloth are sometimes used) over the entire support and to the 
level of the bottom of the slots and the commutator necks 
at each edge. Where this would require an excessive amount 
of insulating material, as occurs in the rear of the commutator 
on certain types, a bed of rope can be built up, as shown in 
Fig. 90, and bound in place with an insulating cement. A final 
layer of friction tape can be applied over all the insulation, 
great care being observed to make the layers lie smooth, and 
to build up a firm support for the coils where they leave the 
slots. 



94 ARMATURE WINDING AND MOTOR REPAIR 

On the cores for strap wound coils, the coil supports are 
usually straight and insulated with built-up mica bushings or 
with heavy bands of treated cement paper. No tape is used 
in this case, but the bushings are arranged to come up level 
with the bottom of the slots. 

On both wire and strap wound armatures, the slots for 
about an inch at the ends may be slightly wider than the coils. 
In such cases narrow strips of heavy fish paper, projecting 
slightly from the slots, should be inserted for additional pro- 
tection to the coils at this point. The slots should be further 
insulated with regular fish-paper cells for the mechanical 
protection of the coils. 

Inserting the Coils. Before starting to wind the armature, 
the commutator should be tested for breakdown with 5000 
volts to ground and 200 volts between segments. Mark two 
slots, separated by the proper throw, with chalk for the first 
coil. Then count off from the bar opposite the center of the 
first slot, the commutator bars into which the leads from these 
slots must fit. In a lap winding these bars should lie adjacent. 
In a wave winding, the number of bars between them must be 
determined. (See page 105, Chapter IV.) The first coil is 
then placed in these two slots, the bottom half being driven 
into the lower half of the slot, and the top half being merely 
caught in its proper slot, as it will have to be removed later, 
to allow a coil to be inserted beneath it. 

Wire Coils. In wire wound armatures the lower leads 
should be taped with friction tape when necessary to make 
the insulation continuous from the coil to the commutator. 
These leads should then be laid along the coil supports of the 
armature core in smoothly fitting rows and the bare ends 
driven into the proper commutator necks. Heavy insulation 
is needed between the coil ends and the upper and lower 
leads and between the upper and lower coil ends. This may 
be of different form in different types of armatures. In one 
type treated canvas strips may be inserted so as to furnish 
extra insulation between the ends of adjacent coils and be- 
tween the coil ends and the lower leads In addition a friction 
cloth strip, doubled oVer a piece of rope, may be inserted at 
each end between the upper and lower coil as the coils are 



REWINDING DIRECT-CURRENT ARMATURES 



95 



inserted, the rope fitting in the point of the diamond. The 
coils should be shaped with a fibre drift and rawhide mallet 
so as to fit snugly against one another at both ends. It is 




FIG. 91. Inserting the top sides of coils in an armature. The leads of 
the bottom coil sides are shown connected to the commutator. Note the 
band of insulation between end connections. 



essential that they be made to fit closely together when first 
inserted, otherwise the armature will bulge at the ends. Any 
attempt to shape the coils in a completed armature is liable 
to injure the insulation. 



96 ARMATURE WINDING AND MOTOR REPAIR 

After all the coils have been inserted and the top parts of 
the throw coils have been replaced, the ends of the canvas 
strips which project up between the coil ends should be trim- 
med off level with the top of the coils at both ends of the arma- 
ture. Those strips which project out from beneath the 
coils, should be turned up over the coil ends and bound in 
place with friction tape. This tape when wound completely 
across the upper surface of the coil ends, serves as a protecting 
and insulating layer between the coils and the upper leads. 

Another method of providing extra insulation on some 
armatures is to slit two strips of treated canvas and insert 
them between the lower leads and the coil ends with the slits 
staggered. No strips are inserted at the rear end and no 
insulation is required between the ends of adjacent coils, 
beyond that on the coils themselves. Strips of friction cloth 
and rope may be inserted between the upper and the lower 
coil ends, and the canvas strips folded over the ends of the 
coils and covered with friction tape as just described. In 
this case, however, strips of fish paper should be slipped over 
each coil and wedged between the coil ends close to the core, 
for further protection to the upper leads. 

Before connecting the upper leads to the armature they 
should all be tied together with bare copper wire and the coils 
subjected to a break-down test of 3600 volts. Any defective 
coil must be replaced. The armature can then be trued up. 
This can be done by tapping down with a mallet all the high 
coils as located by holding a piece of chalk so that it will rub 
against the high parts when the armature is revolved. A some- 
what better method is to squeeze the coils into place by means 
of a flexible metallic strap and turn-buckle. The latter 
method is less liable to damage the insulation and all the coils 
receive uniform treatment so that a better balance of the 
armature results. Both ends must form a compact mass. 
Where end room is especially short, as in mine motor?, a special 
form can be used on the rear end of the armature so as to 
press all coils against this when being inserted to give uni- 
formity to the arrangement. 

Where the top of the coil is above the top of the commutator, 
there is sometimes difficulty in keeping the leads properly 



REWINDING DIRECT-CURRENT ARMATURES 97 

separated in bringing them down to the commutator. In 
such cases a canvas strip may be interwoven over every other 
one, making it possible to have two layers of leads on the 
vertical part. The leads are then inserted into the slits in the 
proper commutator necks and copper wire dummies driven 
over them, to prevent any possibility of a portion of the leads 
being removed when the necks are turned down. Both leads 
and dummies should be tinned and make a driving fit in the 
necks. 

Strap Coils. The leads of strap wound armatures are 
formed to shape and, therefore, require little bending during 
their installation. The coil supports of the two turn strap 
coils are shaped and insulated in a manner very similar to 
that for wire wound coils. In addition a bed of the insulating 
cement is made over the insulation at the rear of the commuta- 
tor into which the lower leads are forced as they are inserted 
into the commutator necks. They are thus held rigidly in 
place after the paste hardens. Strips of treated canvas or 
of friction cloth folded over fish papsr and mica should be 
threaded between the upper and lower coil ends, at each side 
of the machine, as the coils are inserted. A length of rope 
should also be threaded through the diamond point at each 
end. Between the coil ends and the upper and lower leads, 
strips of treated canvas with slit edges should be inserted 
so that the openings will be staggered. The edge toward 
the core must be shaped to fit up between the coils and furnish 
added protection to the leads. After all the coils have been 
inserted, the upper leads* may be bent up slightly, and the 
edges of the various insulation strips cut off even with the 
commutator. Friction tape should then be wound smoothly 
over the treated canvas, holding it in place, and forming a bed 
for the upper leads. These leads can then be bent down and 
inserted into the proper commutator necks. 

Coil supports for one-turn strap coils, whether two piece or 
one piece, are straight, and can be insulated with built up mica 
forms, or strips of fish paper or treated cement paper, shellaced 
and tied in place. Insulating cement should be plastered 
over the insulation back of the commutator, to hold the 
leads from the individual coils in place. Separate the ends 

7 



98 ARMATURE WINDING AND MOTOR REPAIR 

of the coils by two thicknesses of treated canvas, threaded in 
place as the coils are inserted. 

The coil supports for two-piece coils may be insulated in the 
same manner as the one-piece coils. The end bells for this 
type of armature, however, are usually separate from the core 
and are not put in place until the winding is complete, so that 
the winder has plenty of room to work on the rear end of the 
coils. The winding operation can be greatly facilitated by 
the use of a steel winding jig, consisting of a slotted disk with 
a hub bored to fit the armature shaft. The number of slots 
should be equal to the number of single coils in the armature, 
and the thickness of the disk equal to the width of the connect- 
ing clips. As each coil is placed in the armature slot, the 
straps composing it should be placed in the proper slots in the 
jig and in the commutator necks until all the lower half of the 
winding is in place. The leads on the upper and the lower 
half should be separated by a couple of thicknesses of treated 
cement paper, or by a layer of fish paper and mica. After all 
the coils are in place, the straps which are to be connected 
together at the rear end lie one above the other in the slots 
of the jig. These can be cut off even with the surface of the 
jig, a temporary band wrapped around the coil ends, and the 
jig removed. Copper connector sleeves should then be slipped 
over the coil ends and wooden wedges driven in between them. 
The connectors may then be soldered and the coil ends turned 
down at the top and side. Next knock out the wedges and 
interweave asbestos braid between the connectors to prevent 
accidental contact. The end bell, properly insulated with 
moulded mica or moulded paper, can then be bolted into 
position. 

Connections with Dead Coils. If in a four-pole motor with 
a two-circuit winding there is an even number of single coils 
in a complete coil, one single coil in the armature must be cut 
out in order that the winding may be made continuous. This 
coil is called a dead coil, because it is not connected to the 
circuit. It is necessary first to determine the number of single 
coils in a complete coil, by dividing the number of complete 
coils or slots by the number of commutator bars. If there are 
more leads from each side of the coil than there are single 



REWINDING DIRECT-CURRENT ARMATURES 99 

coils, each single coil is composed of two or more wires in 
parallel and these must be treated as a single lead. In strap 
coils each strap corresponds -to a single coil. The coil is cut 
out by cutting off the leads on both sides of a single coil about 
an inch from where they separate from the coil. Then care- 
fully tape them up. The body of the coil of course, must be 
left in the slot for uniformity of the winding. 

Hooding and Banding. As a final protection to the arma- 
ture coils, heavy hoods may be put on over the ends of the 
coils, covering the armature from the commutator to the core 
and from the core to the end bell. At the commutator end 
the hood may be of woven asbestos sewed to a conical shape, 
and impregnated in a moisture and oil repelling compound. 
It should be put in place while wet. The small end should be 
drawn up over the commutator, turned inside out, and firmly 
tied over the leads and commutator necks with heavy twine. 
The body of the hood must then be turned back over the arma- 
ture. If the commutator necks are lower than the level of the 
core, another layer of twine should be wound over the hood 
near the commutator and a band of canvas sewed over the 
whole. Then stretch the hood tightly back over the armature 
and tie with twine. 

Around the rear end of the armature, a band of canvas may 
be wrapped so that the greater part of the strip extends out 
over the shaft, only enough being wound over the armature to 
permit a secure fastening. This may be bound in place with 
a band of twine wound tightly in the groove between the coil 
ends and the end bell. The canvas should then be turned 
back over the armature and bound smoothly in place. 

The number and size of bands depends upon the size and 
speed of the armature. All armatures should have bands on 
each end, placed as far out as possible, so as to cover the 
greater part of the coil ends. When such a band would be 
quite wide, two separate narrower bands may be wound on 
each end. These bands should be insulated from the coils 
by three turns of canvas tape separated by treated paper 
which extends at least one-eighth inch beyond the band on 
each side. The bands around the body of the core in the band 
grooves should be insulated from the core and coils by strips 



100 ARMATURE WINDING AND MOTOR REPAIR 

of fish paper. They should also be put on with a tension 
of about 350 pounds sufficient to make a good firm band and to 
bring the coils down so that they will not project above the 
surface of the core at any point. The individual turns of the 
banding should be well soldered together and held at several 
places by thin copper clips- placed before the banding was 
started. For other details of banding see Chapter V, page 
146. 

After the banding is completed, the mica insulation between 
the commutator bars should be undercut to a depth of about 
one-sixteenth inch, with a special milling cutter. The entire 
armature except the commutator, can now be sprayed with an 
air-drying finishing varnish. 



CHAPTER IV 
MAKING CONNECTIONS TO THE COMMUTATOR 

Before starting to insert coils in the slots of the armature, 
the commutator should be tested for grounds. This can be 
done by touching one lead of a high-voltage transformer 
(1200 to 2000 volts) to the shaft and moving the other over the 
surface of the commutator and at the edges. If there is no 
arcing, the commutator is properly insulated. The winder 
can now insert the coils. As each coil is put in place the 
sleeving on the ends of the lower leads should be fastened to the 
wire by a few turns of friction tape and these leads inserted 
into the slits of the proper commutator bars. In case the 
coil has two or more start and finish ends, care must be taken 
that the different-colored sleeves are connected to the com- 
mutator always in the same order. 

Locating First Connection to Commutator. Before con- 
necting the first coil to the commutator, the winder must 
examine the setting of the brushes, to see whether they are 
centered between pole tips or opposite the center of the pole. 
When the brushes are centered between the pole tips, the 
start end of each coil must be connected straight out to the 
bar opposite the slot in which the beginning of the coil is 
located. When the brushes are opposite the center of the poles, 
the start end of the coil must be swung a certain number of 
bars (equal to 90 electrical degrees) to the right or left of the 
bar opposite the slot in which the beginning or bottom side 
of the coil is located. The number of bars right or left (equal 
to 90 electrical degrees) can be determined by the following 
formula: Total number of commutator bars -r- (Number of 
poles X 2). If this number is mixed such as 6.5, use the next 
higher whole number as 7. The reason for these connections 
of leads is that the coils must be commutated or short -cir- 

101 



102 ARMATURE WINDING AND MOTOR REPAIR 

cuited by the brushes while the coil sides are in a neutral 
position or outside the pole flux. 

The spacing of the brushes from the heel of one brush to the 
heel of the next for a lap winding will be equal to the num- 
ber of commutator bars divided by the number of poles. 

Testing Out Coil Terminals. After the commutator pitch 
has been determined, take the bottom lead of one coil and 
connect it to a commutator riser, using the same throw as 
employed on the old winding in case it is being duplicated. 
After the position of the first lead has been determined, the 
remainder of the bottom leads can be connected in rotation. 
When all the bottom leads are in place, a lighting out test 
should be made to see if the leads are connected to the proper 
bars. A short circuit and ground test must also be made at 
this time. 

For a lighting-out test, place one terminal of the lamp tester 
on a commutator bar, and with the other touch the top leads 
of several coils until the lamp lights. This will locate the top 
side of the coil corresponding to the bottom side connected 
to the test lamp. If the lamp lights on more than one lead, 
it indicates a short circuit between coils. In an armature 
containing twice the number of commutators bars as there 
are armature slots, the same procedure is followed in locating 
the two leads of the same coil. 

Commutator Connections for a Lap Winding. After all 
the beginning ends of the coils or the ends for the coil sides in 
the bottom of the slot have been connected to the commutator 
and all the coils tested out for open circuit, short circuit 
and grounds, the finish ends or the ends of the coil sides in 
the top of the slot, can be connected to the commutator In 
the case of a coil wound with one wire or two wires in parallel, 
the finish end of coil No. 1 is connected to the commutator 
bar next to that to which the start end is connected. Thus, 
if for coil No. 1 the beginning end is connected to bar No. 1 
the finish end will be connected to bar No. 2. The beginning 
end of coil No. 2 will also be connected to bar No. 2 
and its finish end to bar No. 3 and so on until for the last 
coil the finish end will connect to bar No. 1 and close the wind- 
ing (see Fig. 97). 



MAKING CONNECTIONS TO THE COMMUTATOR 103 

On the armature of some large direct-current machines 
a commutator is provided which has twice or three times as 
many commutator bars as slots or winding coils. The object 
of this is to improve commutation and prevent sparking by 
commutating only a part of the current at a time. In a case 
where the commutator has twice as many bars as slots the 
coils are made up with two wires-in-hand when winding. 
For the case where there are three times as many bars as slots, 
coils made up with three wires-in-hand when winding are used. 
The coils in either case are placed in the slots the same as 
those wound with one wire but connected to the commutator 
differently. 

In the case where the number of bars is double the number of 
slots and coils, the start and finish ends of the coils will have 
two wires. For coil No. 1 connect the start ends as follows: 
One wire to bar No. 1 and the other end to bar No. 2; for 
coil No. 2, one end to bar No. 3, and the other to bar No. 4; 
and so on. When connecting the finish ends proceed as 
follows: For coil No. 1, connect one end to bar No. 3 and the 
other to bar No. 4; for coil No. 2, one end to bar No. 5 and the 
other end to bar No. 6 and so on until for the last coil one of 
the finish ends will connect to bar No. 1 and the other to bar 
No. 2. In this case, each brush will cover two bars. 

When there are three start and three finish ends to each 
coil and three times as many commutator bars as slots, 
proceed in the same way by connecting the start ends to 
adjacent bars and the end of one coil to the start of the next. 
In this case each brush will cover three bars. 

Requirements of a Lap Winding. A lap winding can be 
wound on an armature having any number of slots provided 
each slot will accommodate two coil sides for each commutator 
bar and the number of bars is an even multiple of the number 
of slots. When the total number of coils is not exactly di- 
visible by the number of pairs of poles the winding pitch 
can not be equal to a pole pitch. This, however, is not an 
obstacle except where it is desirable to use equipotential 
connectors. When the winding pitch can not be equal to a 
pole pitch it is made so as nearly as possible. When the front 
and back pitches are specified to be odd for a lap winding, 



104 



ARMATURE WINDING AND MOTOR REPAIR 



the coil pitch in winding spaces is meant and not the slot 
pitch. It will be found that with the usual type of form 
wound coils, the pitches for back and front are necessarily 
odd, because one side of the coil is in the bottom of a slot 
and the other side in the top of the slot. The terminals of the 
coil side in the bottom of the slot are usually considered the 
start ends and the terminals of the side in the top of the slot 
the finish ends. 

Commutator Connections for a Wave -Winding. For a 
wave winding, the start and finish ends of coils are connected 
to the commutator differently than for a lap winding. In 
the case of a coil wound with one wire, having one start and 
one finish end, these are not connected to adj acent commutator 
bars as in a lap winding, but a number of bars apart. When 
the coils are inserted in the slots the start ends of the coil 

sides in the bottom of the 
slots should be connected 
to the commutator as in 
the lap winding when each 
coil is placed. When ready 
to connect the ends, the 
commutator pitch must be 
determined. The formula 
for commutation pitch in 
numbers of bars is (y k = 
2/i + 2/2) -5- 2. 

Where 2/1 is the front 

FIG. 92. A 4-pole, wave winding for pitch and 2/2 is the back 

pitch of the coil, each 
counted in winding spaces. 
With a double layer winding or two coils per slot, there will 
be two winding spaces per slot. 

In the case of a 4-pole, double layer with 13 slots and 13 
commutator bars, 2/2 equals 7 and 2/1 equals 5. Then y k = 
(5 + 7) -T- 2 or six bars. With a back pitch of 7 winding 
spaces and a front pitch of 5 winding spaces coil No. 1 would 
lie in slot No. 1 and slot No. 4. Its start end would be con- 
nected to commutator bar No. 1 and its finish end to bar 
No. 7. Coil No. 2 would he in slots No. 2 and 5. Its start 




an armature having 13 slots and 13 com- 
mutator bars. 



MAKING CONNECTIONS TO THE COMMUTATOR 105 



end would be connected to bar No. 2 and its finish end to bar 
No. 8 and so on until coil No. 13 in slots NGS. 13 and 3 would 
have its start end connected to bar No. 13 and its finish end 
to bar No. 6. 

It will thus be seen that in a 4-pole machine the finish of 
one coil is joined to the commutator and to the start of another 
which lies under another pair of poles. (See Fig. 92.) The 
finish end of the latter coil is connected to the commutator 
bar adjacent to the one to which the start of the former coil is 
connected. In Fig. 92 it can be seen that the coils referred 
to are the ones connected to commutator bars, No. 1, 7 and 13. 
In other words the two coils are connected in series with 
their start ends under different north poles and their finish 
ends under different south poles. This explains why the 
wave winding is called a series or two circuit winding. 



Top. 




25 Slot. 
^123 Bars 
"Ooil Slots 1 and 7 

Lead Bart 1 and W 



FIG. 93. Wave winding with an odd coil throw and an odd throw of coil 

leads. 

Practical Method for Locating First Connection to Com- 
mutator for a Wave Winding. When there are an odd num- 
ber of leads per coil, or in case of a dead coil an odd number of 
coils remain, the following procedure will locate the first 



106 



ARMATURE WINDING AND MOTOR REPAIR 



connection to the commutator. Locate the center line be- 
tween the slots of the coil throw. If the throw of the leads of 
the coils is an odd number of bars, this center line will fall on 
a commutator bar. If it is an even number, the center line 
will fall on the mica between bars. In either case the bar or 
mica so located is the starting point for laying off the connec- 
tions to the commutator. 

If there are an odd number of bars in the throw of the leads, 
take one less than the number of bars and count off half of this 




FIG. 94. Wave winding with an even coil throw and an even throw of coil 

leads. 

number in each direction from the starting bar, and this will 
give the first and last bar of the commutator throw. If 
there is an even number of bars in the throw, count off half 
the number in each direction from the starting mica. A 
check is to count from the first to the last bar, and see if it 
agrees with the information given. As the first coil put down 
will have an odd number of leads, the center one of the top and 
bottom leads should be placed in the first and last bar of the 
throw as determined. 



MAKING CONNECTIONS TO THE COMMUTATOR 107 

When there are an odd number of coils per slot and an even 
number of slots, a somewhat different method is required. 
This, however, seldom occurs. In such a case, if the lead 
throw is an odd number of bars, the center, as indicated by the 
coil throw, will line up on the mica and, if an even number of 
bars, it will line up on a bar. If there is an odd number of 
bars in the throw, take one less than the number of bars and 
count off half this number to the left and one more than half 
to the right, and this will give first and last bar of the commuta- 
tor throw. If there is an even number of bars in the throw 
count off half the throw to the right and one less than half to 
the left. If there are two leads in the first coil, No. 1 lead 
should lie in No. 1 bar, and if there are four leads in the first 
coil, No. 2 lead should lie in No. 1 bar. 



Winding conditions with odd number 
of commutator bars 


Center line of coil lines up 


with 


Coil throw and lead throw even 


Mica and tooth 




Coil throw odd, lead throw odd 


Bar and slot 




Coil throw even, lead throw odd .... 


Bar and tooth 




Coil throw odd, lead throw even 


Mica and slot 











Requirements for a Wave Winding. For the wave or two- 
circuit winding there should always be an odd number of 
commutator bars. With an odd number of slots on the arma- 
ture core, and an odd number of coil sides per slot, a balanced 
wave winding with no dead coils is possible. The actual 
number of coil sides per slot can be determined by counting 
the terminals of each taped up coil in the slot. There may be 
only two taped-up coil sides in the slot but each coil may be 
wound with three wires in-hand while winding. In such a 
case there will be six coil sides per slot for a two-layer winding. 

Progressive and Retrogressive Wave Windings. When a 
wave winding passes once around the complete armature and 
has its start and finish ends connected to the commutator as 
shown in Fig. 95 (at left), it is said to be a progressive winding. 
When after passing once around the armature the start and 
finish ends are connected as shown in Fig. 95 (at right) it is 
said to be retrogressive. 



108 ARMATURE WINDING AND MOTOR REPAI"R 

The conditions under which a wave or series winding can be 

kp 
used are shown by the formula: K = -^ 1. 

Where K is the number of commutator bars, p the number of 
poles and k a whole number. When the minus sign is used 
the winding will be progressive and when the plus sign is used 
it will be retrogressive in a case where the number of com- 
mutator bars is odd. 




FIG. 95. The illustration at the left shows one turn of a progressive wave 
winding; the one at the right a retrogressive wave winding. 

Wave Winding with Dead Coils. For a wave winding, as 
already mentioned, the number of commutator bars must 
not be a multiple of the number of poles, otherwise the wind- 
ing would close after it has passed once around the armature, 
instead of advancing or falling behind one commutator bar 
as required by the wave winding. In other words, there must 
be as many coils in series between adjacent commutator bars 
as there are pairs of poles. Also with two or more coil sides 
per slot, the total winding pitch of the end connections (front 
pitch + back pitch) should be a whole number. It frequently 
happens that is found to be a fraction. By dropping the 
fraction when the number is odd, a wave winding can usually 
be connected with a dead coil. A rule which is often used by 
engineers to determine when a wave winding is possible with- 
out a dead coil is that given in another paragraph, namely: 

kp 

Number commutator bars = -^ 1 ; where p is the num- 
ber of poles and k must be a whole number. When an even 
number of coil sides per slot are used and an odd number of 



MAKING CONNECTIONS TO THE COMMUTATOR 109 

commutator bars, there will always be a dead coil. Also 
when there are an even number of slots on the armature 
core, an odd number of commutator bars, and an odd 
number of coil sides per slot are used, there will always be a 
dead coil. 

Another rule for a possible wave winding is that the total 
number of slots times the number of coil sides per slot may 
be any number divisible by the number of poles. 

When it is found that a dead coil must be used, one coil, 
(any one) on the armature should be cut back a short distance 
from the commutator and taped up with friction tape. The 
other coils can then be connected as in any other waye winding. 
That is, connect all the leads of the bottom sides of the coils 
to the commutator first, then connect the leads of the top 
sides of the coils the proper commutator pitch away from the 
bottom leads. 

When there are twice as many commutator bars as coils in a 
wave winding, each coil is wound with two wires and there are 
two start ends and two finish ends. In making connections 
to the commutator the two start ends should be connected to 
adjacent bars and the two finish ends to adjacent bars, spaced 
the proper commutator pitch apart. 

Cutting Out Coils of a Retrogressive and Progressive Wave 
Winding. In Fig. 96a is shown a portion of the winding diagram 
of the armature, for the case of a retrogressive winding with a 
connecting pitch of 1 to 49. If the burned out coil is between 
commutator bars 47 and 95, disconnect it from the commutator 
at a and b and cut it at c if it. has more than one turn. Then 
connect commutator bars 46 to 47, and 94 to 95 and disconnect 
the coil between bars 95 and 46 at a' and b'. Or connect bar 

95 to 96 and 47 to 48 and disconnect the coil between bars 

96 and 47 at a" and b" '. 

For the case of a progressive winding with a connecting pitch 
of 1 to 50, the winding diagram is shown in Fig. 966. If the 
burned out coil is between commutator bars 3 and 52, dis- 
connect it from the commutator at a and 6 and cut it at c, 
if it has more than one turn. Then connect bars 3 to 4 and 
52 to 53 and disconnect the coil between commutator bars 
52 and 4 at a' and b'. Or connect bars 2 to 3 and 51 to 52 



110 



ARMATURE WINDING AND MOTOR REPAIR 



and disconnect the coil between bars 51 and 3 at a" and 6". 
For definition of retrogressive and progressive wave winding, 
see page 107, Chapter IV. 




94 95 9C 97 



45 4G 47 48 49 



93 94 95 9C 97 



FIG. 96a. Method of cutting out a damaged coil in a retrogressive wave 

winding. 




t 2 



FIG. 966. Method of cutting out a damaged coil in a progressive wave 

winding. 



345 




FIG. 97. Double layer winding for a 4-pole armature having 24 slots and 
24 commutator bars. The connections for this diagram are given in tabu- 
lated form on page 111. 

Tables for Placing Coils and Connecting Them in a D.-C. 
Armature Winding. To avoid mistakes in reading a complete 
winding diagram and to avoid the necessity of making such a 
diagram, Fig. 97, it is advisable to furnish an inexperienced 
armature winder with a table for laying the coils in the slots 
and also a table for connecting the proper leads to the com- 
mutator. These tables can be made up as follows: 



MAKING CONNECTIONS TO THE COMMUTATOR 111 
TABLE FOR PLACING COILS IN ARMATURE SLOTS LAP WINDING 



Coil number 


Place sides of coil in 


Winding space numbers 


In slot numbers 


1 


1 and 12 


1 and 6 


2 


3 and 14 


2 and 7 


3 


5 and 16 


3 and 8 


4 


7 and 18 


4 and 9 


5 


9 and 20 


5 and 10 


6 


11 and 22 


6 and 11 


7 


13 and 24 


7 and 12 


8 


15 and 26 


8 and 13 


9 


17 and 28 


9 and 14 


10 


19 and 30 


10 and 15 


etc. 


etc. 


etc. 


24 


47 and 10 


24 and 5 



This table is for a four-pole, double layer, lap winding on an 
armature having 24 slots and 24 commutator bars. By giving 
the throw of the coils both in winding spaces and in number of 
slots the winder will not become confused when either is used 
in referring to the winding. A table of connections to the 
commutator for this winding is made up as follows: 

TABLE FOR CONNECTING COIL LEADS TO THE COMMUTATOR LAP 

WINDING 



Connect terminals of coil 



tJoil number 


Start end to bar number 


Finish end to bar number 


1 


1 


2 


2 


2 


3 


3 


3 


4 


4 


4 


5 


5 


5 


6 


6 


6 


7 


7 


7 


8 


8 


8 


9 


9 


9 


10. 


10 


10 


11 


etc. 


etc. 


etc. 


24 


24 


1 



112 ARMATURE WINDING AND MOTOR REPAIR 

Such tables are particularly useful in the case of a double 
layer wave winding where each coil is wound with two or more 
wires in-hand. The following tables for a four-pole, double 
layer, wave winding with two terminals per coil, 15 slots and 
30 commutator bars, will illustrate their usefulness in such a 
winding: 



TABLE FOR PLACING COILS IN A.RMATURE SLOTS DOUBLE WAVE 

WINDING 



Place sides of coil in 



0011 numoer 


Winding spaces numbers 


In slots numbers 


1 


1 and 8 


1 and 4 


2 


3 and 10 


2 and 5 


3 


5 and 12 


3 and 6 


4 


7 and 14 


4 and 7 


5 


9 and 16 


5 and 8 


etc. 


etc. 


etc. 


16 


29 and 6 


15 and 3 



TABLE FOR CONNECTING COIL LEADS TO THE COMMUTATOR DOUBLE 
WAVE WINDING 



Connect terminals of coil 





Start ends to bars numbers 


Finish ends to bars numbers 


1 


1 and 2 


15 and 16 


2 


3 and 4 


17 and 18 


3 


5 and 6 


19 and 20 


4 


7 and 8 


21 and 22 


5 


9 and 10 


23 and 24 


etc. 


etc. 


etc. 


15 


29 and 30 


13 and 14 



Wave vs. Lap Windings. The wave winding is mostly 
used on the smaller types of direct current machines of multi- 
polar construction. This winding has but two paths through 
the armature, regardless of the number of poles, and half the 
coils in the armature are connected in series in each path, 



MAKING CONNECTIONS TO THE COMMUTATOR 113 

whereas the lap winding has as many paths as there are poles, 
and a correspondingly smaller number of coils in series. For 
instance, in the case of a four-pole armature, wound with 
coils of the same number of turns and same size conductors; 
if all are connected in a wave winding, the armature will be 
suitable for double the voltage at half the current that would 
be used if the same armature were connected as a lap winding. 
Thus on small machines, where the number of turns is neces- 
sarily limited, the wave winding is usually much cheaper and 
easier to use. The wave winding is widely used in armatures 
for railway, hoist and crane motors. 

The limitation in the use of the wave or series winding is in 
the main the amount of current which can be carried by one 
armature circuit. Since the wave winding consists of two 
circuits, the current per circuit is equal to one-half the total 
armature circuit of the machine. Where the value of this 
current exceeds that which has been found to be consistent 
with good practice, (up to 250 amperes per circuit in non- 
interpole machines and up to 550 amperes in interpole ma- 
chines) then it becomes necessary to arrange more circuits on 
the armature, each circuit carrying a part of the total current. 
In this case the lap or multiple winding is usually employed, 
which as explained has a number of circuits equal to the number 
of poles of the machine. However, the choice of wave or lap 
windings is usually not only determined from the amount of 
current to be handled but is greatly influenced by the require- 
ments for good commutation and by the size and design of the 
armature to be wound. 

It is customary with some motor manufacturers to use wave 
windings wherever the current to be handled permits the use 
of a conductor of a sufficient size to form it into a wave shaped 
coil and where the number of coils in the slot does not become 
too large. The latter condition is objectionable on account of 
the space required for insulating the coils. Other manufac- 
turers employ the lap winding wherever possible, that is, in 
cases where the number of turns per slot does not become 
too small. As a general rule, it can be said that the wave 
winding often works out to advantage in cases of compara- 
tively large machines for low voltage whereas the lap winding 



114 ARMATURE WINDING AND MOTOR REPAIR 

is preferable in cases of large or small machines for high vol- 
tage as well as for small machines for low voltage. 

Except under the special conditions named above, such as 
heavy current to be handled by the armature, machines of 
high voltage, and small machines of low voltage where the lap 
winding has advantages over the wave winding, there is little 
choice between the wave or lap winding for those machines 
where the number of poles may equal the number of circuits 
of which the winding may be made up. It may be said that 
in this case, American practice favors the lap winding and 
European practice the wave winding. 

It might be pointed out further that in a wave or two-circuit 
winding all of the coils between two diametrically opposite 
points on the winding are in series with each other while in a 
lap winding only the coils between adjacent pole centers are 
in series. Therefore, under similar conditions, the voltage 
in a wave winding will equal the voltage of a lap winding 
multiplied by the number of pairs of poles. The voltage 
between the commutator segments will also vary in the same 
ratio. In order to obtain the same voltage of the machine 
the relative number of coils in the two types must vary in- 
versely with the number of poles and the size of the coils 
in the wave winding must increase in proportion. Where the 
size of armature coil does not become excessive nor the voltage 
between segments too great to permit good commutation, 
the wave winding can be used. These conditions, however, 
limit this type of winding under ordinary conditions to four 
or six pole machines. In cases where the number of poles 
is larger the lap winding has a wider application for direct- 
current armatures. 

In regard to the selection of the proper type of winding for 
direct-current and alternating-current machines, Henry Scheril, 
formerly a member of the engineering department of the 
Crocker- Wheeler Company, has made the following comment 
(Electrical Record, January, 1919) : 

The selection of the proper kind of winding depends upon 
the capacity of the machine, the voltage and speed. Wave 
windings are used on machines of small capacity. They are 
also being used for low speed and medium ratings. On small 



MAKING CONNECTIONS TO THE COMMUTATOR 115 

high voltage machines, like the railway motor, the wave wind- 
ing is being used to advantage because it has only two sets of 
brushes and it can be inspected very easily. Wave windings 
are also used for the rotors employed in induction motors. In 
all other cases, lap windings are to be preferred. 

Lap Windings for Direct -current Armatures. In general, 
the ends of a coil of lap winding in a direct-current machine 
are connected to the adjacent commutator bars. The sides 
of the coil arc spread over and placed in the slots of the arma- 
ture corresponding to the distance equivalent to 180 electrical 




FIG. 98. Lap or parallel winding for a 6-pole machine. 



degrees. That is to say, if one side of the coil is under the 
center of a north pole, then the other side of the coil will be 
placed in a slot having a similar position under the adjacent 
south pole. This brings one to a consideration of the wind- 
ing pitch. The pitch is the number of slots spread over the 
periphery of the armature corresponding to the arc equal to the 
distance between two similar points of two consecutive poles. 



116 ARMATURE WINDING AND MOTOR REPAIR 

For instance, if a machine has 72 slots and eight poles, the 
full pitch will be 72 -5- 8 = 9, and the coil will lie in slots of 
one and ten. 

In a direct-current machine the lap or multiple winding has a 
number of paths for the current to travel through the winding 
from positive to negative, equal to the number of poles. It 
becomes necessary, sometimes, to wind a machine with what 
is known as the "short chord" or fractional pitch winding. 
In this kind of winding, using a fractional pitch, the coil is 
spread over the distance which takes in less than the number 
of slots between two similar points on two consecutive poles. 
The use of this kind of winding on a direct-current machine 
is made when there is a desire to improve the commutation. 
That is, because with a fractional pitch winding the emf., 
due to inductance of the armature, is lessened at the time of 
commutation. In direct-current machines it is not advisable 
to use a shorter pitch than about 90 per cent, of the full pitch. 
If this figure is exceeded then the advantages which this wind- 
ing would give are practically eliminated and liable to bring 
about worse results. 

All interpole machines are built with lap windings and 
equalizer bars are used. The setting of the brushes on inter- 
pole machines must be done very accurately because their 
position is fixed once and for all, but due to irregularities in 
manufacture, in order to eliminate any possibility of poor com- 
mutation equalizer connectors are being used. 

Lap Windings for Alternating-current Machines. Practi- 
cally all alternating-current machines use windings having a 
short pitch. The reasons for using the short pitch winding, 
especially on alternating-current machinery, are many. To 
begin with, most manufacturers standardize their frames, 
standardize their shields, standardize the clearances between 
the rotary elements and the stationary elements. In some 
machines, the clearances between the windings and the shields, 
especially in machines carrying heavy currents, and also in 
high voltage machines, become very small due to the form of 
end connections which are being used in lap windings. It 
has been found that by using the short pitch winding, the 
end connections can be shortened considerably and thus ob- 



MAKING CONNECTIONS TO THE COMMUTATOR 117 

tain more clearance. Another reason for using the short 
pitch winding in alternating-current machinery is because if 
the machine is to be designed with existing punchings, there 
is a possibility that in designing the winding, one may get 
too many turns by using the full pitch due to a greater number 
of slots than required. By reducing the pitch from full to 
fractional, the same results are brought about as when the 
number of slots is reduced. In alternating-current machinery, 
short pitches are used as low as % of the full pitch. Practic- 
ally all two-speed induction motors have % pitch. 

Wave Windings for Direct -current Armatures. The 
series or wave winding consists of coils spread over the per- 




P N 1 

FIG. 99. Wave or two circuit winding for a 6-pole machine. 



iphery of the armature in the same way as the multiple winding 
but with the ends of the coil in the direct-current machines 
connected to the commutator bars whose relative positions 
correspond to about double pitch. From the nature of this 
winding, there are only two paths for the current to flow from 



118 ARMATURE WINDING AND MOTOR REPAIR 

positive to negative brush independent of the number of poles. 
Now since the winding goes around the armature several 
times, depending upon the number of slots, in order that the 
winding may not close upon itself, the number of coils in 
series must be one more than, or one less than the number of 
poles. Since this winding has only two circuits, regardless of 
the number of poles, only two sets of brushes are needed. 
However, when this winding is being used on machines of 
large capacity, there will usually be as many sets of brushes 
as there are poles. The purpose of this is twofold. One is 
to reduce the size of the commutator and the other is to reduce 
the brush density. 

Wave Windings for Alternating -current Machines. In 
alternating-current machines wave windings are mostly used 
in rotors of slip ring induction motors. They are either 
single or double circuit and their use for that purpose is two- 
fold. One reason is because the end connections are much 
easier to make than in the case of the lap winding and the 
second reason is because of certain definite ratios to be ob- 
tained. Double or triple wave windings are also used on 
alternating-current machines of very low voltage and high 
currents where lap windings are found to be not practical 
because of the form of end connections. All wave windings 
should be symmetrical. 

For a wave winding in an alternating-current motor the 
number of slots (plus or minus one) is chosen so as to be divis- 
ible by the number of poles. If the number of slots is plus 
one, the winding is called progressive, since after traveling once 
around the stator it returns to the starting slot plus one. If 
it is minus one, it is called retrogressive since the circuit 
returns the winding to the starting slot minus one. In this 
winding the correspondingly placed conductors under adja- 
cent poles are connected in series with the circuit proceeding 
from pole to pole several times around the stator. The circuits 
are then interconnected to give the required phase relations. 

Single vs a Number of Independent Windings. Most 
machines are built with a single winding with the exception of 
some three-wire generators, having double windings, the two 
windings being independent of each other. All alternating- 



MAKING CONNECTIONS TO THE COMMUTATOR 119 

current machines are built with windings so that they can be 
easily changed from multiple circuit to single circuit. That is 
to say, the design of a winding for an alternating-current ma- 
chine is made such that if it is built in two circuits, for say 
220 volts, that same machine can have the windings recon- 
nected for 440 volts. 

Double or triple windings, either lap or wave, have been 
found to give much trouble in direct-current machines, espe- 
cially in commutation. Sparking at the commutator be- 
comes very pronounced and the commutator wears down very 
rapidly. For this reason this kind of winding is not widely 
used on direct-current machines. 

Lap Windings vs Multiple Wave Windings. Although they 
have not been much used in the past, multiple wave windings, 
sometimes called series-parallel windings since they are wave 
windings with more than two circuits, have some advantages. 
The points in favor of their use are outlined as follows by 
Albert A. Nims of the engineering department of the Crocker- 
Wheeler Company (Electrical Record, February, 1919) : 

There seems to be no reason for avoiding wave windings 
with more than two circuits on ratings where they would be 
desirable, provided they be made perfectly symmetrical. 
This condition would bar out a four circuit winding on a six- 
pole machine. Arnold, who is generally regarded as the origi- 
nator of this type of winding, did not always insist on complete 
symmetry, but the writer believes that the unsuccessful 
experiences with series-parallel windings in the past has been 
largely due to an incomplete understanding or observance of 
the laws of symmetry of armature windings. 

In windings with circuits equal in number to the poles mul- 
tiple or lap windings are ordinarily used. They possess the 
advantage of having each coil commutated at only one brush, 
and also the disadvantage of having each circuit under a dif- 
ferent pole, or pair of poles. If the field strengths under the 
various poles are not identical, the induced emf s in the various 
circuits are not the same. Undesirable currents, which may 
be of considerable magnitude because of the low resistance of 
the winding, then circulate between the different circuits, 
overloading some brushes and causing them to spark, and 



120 ARMATURE WINDING AND MOTOR REPAIR 

uselessly heating the armature. The brushes may be pro- 
tected by diverting these currents through internal equi- 
potential connections, but the useless heating of the armature 
still remains. This, however, is usually not excessive com- 
pared with the heating of other losses, so that this winding may 
be called standard today. 

Series-parallel wave windings with circuits equal in number 
to the poles possess the disadvantage, common to all series 
wave windings, that the terminals of each coil lie under two 
different brushes, which are of the same polarity and, therefore, 
connected by a low resistance conductor. That is, a coil is 
commutated by two brushes and the connector between them 
and there is a tendency for selective commutation or unequal 
division of the current among the brushes to occur, especially 
in large non-interpole machines. This generally results in 
sparking at some of the brushes. They are eaten away so 
that the effective position of their contact surfaces are changed. 
Other brushes then get more current than they should and 
eventually all the brushes are damaged. Series-parallel 
wave windings do have the advantage that each circuit is 
influenced by all the poles, so that the induced emfs in all 
circuits are equal and there is little tendency for circulating 
currents to occur due to unbalanced induced emfs. To 
eliminate the disadvantages and combine the advantages of 
these two classes of windings, multiple wave windings have 
been used, particularly on four-pole machines. Although they 
are " special" and, therefore, cost more than standard windings, 
they accomplish their purpose admirably. There seems to be 
no reason why they would not be equally successful on ma- 
chines with six or more poles. 

Use of Equalizer Rings. Equalizer rings are being used on 
lap windings only on large multipolar machines, where un- 
balanced conditions in the magnetic circuit are liable to cause 
circulating currents between the several paths through the 
armature. This condition will occur for instance in the 
course of operation where the air gap will not be uniform 
so that the emf induced in a coil opposite a small gap will 
be larger than that induced in the coil which is opposite the 
large gap and this difference in the emfs will bring about 



MAKING CONNECTIONS TO THE COMMUTATOR 121 

a circulating current which will flow between the windings and 
thus interfere with the performance of the machine. This 
condition will also occur if the machine has been in operation 
any length of time, the inequality of the air gap being brought 
about by the wear in the bearings. The same condition will 
occur if the brushes are not spaced properly. In a wave 
winding, each circuit has its conductors pass under all poles 
and, therefore, there is no necessity of using equalizer rings. 



CHAPTER V 
TESTING D.-C. ARMATURE WINDINGS 

The common causes of trouble in armatures are practically 
the same as in any electrical circuit, namely, a short circuit 
in or between coils, an open circuit, reversed coils and grounds. 
Of all the faults inherent to armatures, probably the most 
dangerous is the short circuit between coils. If it is not 
detected and remedied as speedily as possible, the result in 
most cases is the burning out of the coils affected, and possibly 
the whole armature. 

Causes of Short Circuit in an Armature. There are numer- 
ous ways in which a short circuit of coils may occur. In the 
case of wire-wound coils, it sometimes happens that one of the 
turns forming the coil becomes twisted during the process 
of inserting the coil in the slot, and in order to force the wind- 
ing down to an even depth, the turn of wire was driven down 
upon other turns, cutting through the insulation and causing 
a short circuit between turns of the same coil. When this 
occurs, the resistance of the coil is reduced, allowing more 
current to flow, increasing the temperature of the coil and 
eventually causing a deterioration of insulation on other wires 
at that point. In the majority of cases this results in short- 
circuiting the entire coil upon itself. 

Also a frequent cause of trouble is the short circuit between 
coils This is often caused on the back end of the armature 
by oil soaking into the coils by leakage from the out-board 
bearing, which, together with the dust that will invariably 
work in between the windings, break down the insulation and 
cause an electrical leak between coils. A short circuit may 
also result from the top and bottom armature leads coming 
in contact with each other. A short circuit between commuta- 
tor bars is often the cause of burned out coils. In soldering 
leads to the commutator, great care must be exercised not to 

122 



TESTING D.-C. ARMATURE WINDINGS 123 

allow any of the molten solder to run down behind the bars, 
as this very frequently short circuits the bars. A good way 
to avoid this is to raise the back end of the armature a trifle 
so as to allow the solder to run to the front where it can be 
easily removed. 

Tests for a Short Circuit in an Armature. Probably the 
best way of detecting the presence of a short circuit in the 
armature of a motor in operation, is to carefully watch it 
when starting as soon as faulty operation is noticed. Some- 
times the armature will not start upon the first few points 
of the rheostat, and will then take an excessive current. This 
will cause it to run with a slow and unsteady motion (especially 
at low speeds), due to the fact that every time the short-cir- 
cuited coil comes under the influence of a pole, it will have a 
tendency to retard the motion of the armature. By running 
the motor for a short time, the bad coil will heat much more 
than the others, and its location can usually be detected by 
passing the hand over the end windings. 

When a short circuit is suspected in an armature, the ma- 
chine should be shut down at once and the necessary repairs 
made. One sure method of locating a short-circuited coil 
is to disconnect all the leads from the commutator and test 
out the coils with a test lamp. A test lamp consists of two 
wires about 10 feet long, connected to a 110-volt circuit with 
an incandescent lamp connected in series in the circuit. This 
method requires a great deal of work in unsoldering all the 
leads, which may not be necessary since in the majority of 
cases, the seat of the trouble will be found in the commutator 
itself, due to short circuits between bars. 

A rapid test often used to locate the trouble without dis- 
connecting any of the wires on the commutator during the 
test, is the bar to bar test. This test can be applied to arma- 
tures with any style of winding connections, for there will be 
exactly the same drop of potential between any two adjacent 
commutator segments no matter which scheme of connection 
is used. Fig. 100 shows the connections necessary for a test of 
this kind. A steady current, taken from a 110-volt circuit 
should be sent through the armature at opposite sides of the 
commutator. The brushes B should only be wide enough to 



124 



ARMATURE WINDING AND MOTOR REPAIR 



cover one bar. C is a fiber block holding the copper contact 
points, so spaced as to rest on adjoining segments as shown. 
Adjust the lamp bank until the voltmeter gives a readable 
deflection when C is in contact with what are supposed to be 
good coils- The deflection of the voltmeter will depend upon 
the difference of potential between the bars. If everything 
is all right, practically the same deflection will be obtained 
all around the commutator regardless of what pair of bars C 



Mail 



1 




FIG. 100. Connections for testing out armature coils with a millivoltmeter. 

may rest upon. Pass the contact points over each pair of 
bars and note the deflection on the voltmeter. When the 
short-circuited coil, to which the bars are connected, comes 
under the contacts there will be very little if any movement of 
the needle, because there will be little or no drop through the 
coil A more satisfactory test for use on a removed armature 
is the transformer test discribed on page 125. 

When the coil at fault has been found in this manner, its 
leads should be disconnected from the commutator, together 
with the leads adjoining it on either side. The commutator 
should now be tested by use of the test lamp to determine if 
the bars to which the coil was connected are short-circuited. 



TESTING D.-C. ARMATURE WINDINGS 125 

The banding wires should be removed from the armature 
next, and the defective coil taken out by raising the top sides 
of other coils clear of the armature as far around as the bottom 
side of the damaged coil, when it can be lifted out. In many 
cases the insulation on the wire of the coil has reached such a 
stage of deterioration that a new coil will be necessary, in 
which event a new one should be formed with exactly the 
same number of turns and size of wire as the old one. Care 
should be taken not to wrap a thicker layer of tape on the 
new coil than was on the old one, for if this is done trouble 
will be experienced in forcing the coil back into the slot. The 
finished coil should be given a coat of insulating varnish. It is 
a good plan to re-insulate the armature slots also, before re- 
turning the coil. If the commutator is free from short-cir- 
cuits, the coil may be replaced, the raised coils returned to 
their slots, and the leads soldered to the commutator again. 

Testing for Short Circuits and Open Circuits with a Small 
Transformer. While the method of testing between adjacent 
commutator bars with a millivoltmeter will indicate short-cir- 
cuited or poorly soldered leads by a low reading and open- 
circuited or poorly soldered leads by a high one, it often occurs 
that an armature is rewound and reinstalled with considerable 
time and labor and found to be defective after all. The 
millivoltmeter or drop of voltage method merely measures 
the resistance of each coil but when an armature is subjected 
to magnetic induction, an emf is induced in its windings 
which will cause current to flow in the turns that are short- 
circuited. When one turn in a coil has been forced so hard 
against another that the insulation is broken and the turns 
become short circuited, the millivoltmeter test may not serve 
to detect the short circuit and as a consequence the turns, and 
probably the coil, would be destroyed by the immense current 
which would flow in the short-circuited turns when running 
in the magnetic field of the machine. 

A simple way of detecting such defects is by the use of a 
small transformer often called a "mill" or "bug" and con- 
structed as shown in Fig. 101. When this transformer is 
applied to the armature core as shown, the alternating flux 
produced by it flows through the core and produces an emf. in 



126 ARMATURE WINDING AND MOTOR REPAIR 

the coils. If the winding is correct no current will flow as 
the voltages will balance each other. If, however, a coil is 
short-circuited, a current will flow in the turns that are short- 
circuited. To locate the defective coils take a sharp piece of 
steel or a knife blade and pass it around the commutator so as 
to short circuit in succession, the coils which have one side 
under the transformer. A decided sparking, indicating a 
potential difference between the bars, shows that the coil is 
in good condition. Absence of sparking indicates either 




LI 



FIG. 101. Method of testing an armatuie for short circuits and open circuits 
in coils by use of the special transformer shown. 



an open circuit or a short circuit. The latter can be readily 
determined by running a light piece of sheet iron over the 
surface of the armature core so as to bridge the slots in suc- 
cession. If there is a short circuit in one of the coils which 
has one side under the transformer, a local current will flow 
through this coil generating a magnetic flux which will attract 
the piece of sheet iron. If it is held away slightly it will be 
made to vibrate very rapidly. The coil will also heat very 
rapidly and if the transformer is large enough for the armature 
being tested, the coil will be burned out completely. 

In case there is no sparking at the commutator when the 
coils are short-circuited as described above and there is no 



TESTING D.-C. ARMATURE WINDINGS 127 

local magnetic flux when moving the piece of sheet iron over 
the slots of the core, an open circuit is indicated. 

A transformer cf the dimensions shown in the accompanying 
illustration can be operated on a 110- volt, 60-cycle circuit and 
will serve for testing many sizes of armatures. The one 
illustrated was made up by E. W. Copeland (Electrical World) 
using 60 turns of No. 6 magnet wire. When using such a 
transformer it should be fastened under the armature so that 
there is just enough clearance between the transformer and 
armature core that neither will touch. Current should always 
be off while placing the transformer and also when it is not 
in use. 

When testing an armature with a small exploring trans- 
former, the number of poles in the machine must be taken 
into consideration. In a two-pole armature, a single short 
circuit may heat up two coils, in a four-pole armature, four 
coils and so on. A good way to locate the defective coil is 
suggested by Maurice S. Clement (Electrical Record, Novem- 
ber, 1918) by applying the telephone receiver test. In apply- 
ing this test the terminals of a telephone receiver are placed 
on adjacent commutator bars which are connected to one of 
the affected coils, and the volume of sound transmitted to the 
receiver noted. The same should be done to all the other 
affected coils. A short circuit will have a greater volume of 
sound than a perfect coil. 

Causes of Open Circuits in an Armature. An open circuit 
may result from a number of causes. In the first place, when 
the armature was wound, the coil may have been driven into 
position in such a manner that one of the wires was strained 
or partly cut in two. The momentum of the armature, and 
constant vibration of the machine will finally break the wire, 
and in this way form an open circuit. Sometimes an open 
circuit of this kind will only show up when the armature is up 
to speed, the centrifugal force causing the wires to separate, 
thus opening the circuit. When the motor is at rest, the wires 
will come together again, and a test will reveal nothing. This 
condition is known as a " flying" open circuit, and occurs 
quite frequently. The same state of affairs may result with 
a short circuit between overlapping coils. An open circuit 



128 ARMATURE WINDING AND MOTOR REPAIR 

may also be caused by the armature leads being drawn too 
tightly when they are soldered to the commutator. This will 
cause a break due to expansion and contraction of the wire 
from the constant heating and cooling. 

Another common cause of open circuits is poor workman- 
ship when the leads are soldered to the commutator. If the 
lugs or risers are not perfectly tinned before attempting to 
solder the leads into them, the solder will not take hold over 
the entire area, and a lead may be held in place only by a 
thin film of solder on the outside surface. When the current 
through the armature is heavy, the contact area between the 
riser and the leads may not be sufficient to carry the necessary 
current without excessive heating. This will melt out what 
little solder there is, and an open circuit will result. Sometimes 
a commutator will become so hot from excessive brush friction, 
resistance drop, overloads, or the like, that it may throw 
solder, and cause an open circuit. 

Tests for an Open Circuit in an Armature. The symptoms 
of an open circuit are often very prominent. A vicious green- 
ish-purple spark will usually appear at each brush as the 
open-circuited coil passes from one pole to the next. This 
spark has a tendency to leap out from the brush and follow 
around the commutator for quite a distance. The bars to 
which this coil is connected will be found to be burned and 
roughened, and the mica insulation between eaten out to a 
considerable depth. 

In a lap wound armature the position of an open-circuited 
coil is easily located, because each end of the coil is connected 
to adjoining bars. In a wave winding this is not the case. 
Each end of a coil is connected to a bar removed a certain 
distance around the commutator from the other, depending of 
course, upon the number of poles and the winding pitch 
employed. 

If an open circuit exists in an armature for any length of 
time, the burned condition of the commutator bars will usually 
indicate where the trouble is located. Both lap and wave 
wound armatures may be tested for open circuit by a testing 
transformer, by use of the ordinary test lamp, and a bar to bar 
test, or by ringing out between adjacent bars with a magneto. 



TESTING D.-C. ARMATURE WINDINGS 



129 



If the test lamp is used, test the commutator from bar to 
bar and note the brightness of the lamp on each pair of bars. 
When the bars are reached to which the open-circuited coil is 
connected, the light will dim considerably, and may go out, 
depending upon the resistance of the winding. When more 
than one coil is open-circuited, the winding will be divided into 
two or more sections, and the test lamp will only light when 
the test leads are in connection with the bars in each section. 

The testing set described for locating short circuits (page 
124) may also be used for open circuits. Proceed in the same 
manner as when testing for short circuits. When the con- 
tact points C (Fig. 100) are connected to the open-circuited 
coil (indicated at D), there will be a violent throw of the needle, 
because the voltmeter will then be connected to brushes B 
through the intervening coils. When C is moved to the next 
segments, there will again be no deflection, thus locating the 
break definitely. 

If an open circuit results from a lead breaking off at the 
commutator, it is an easy matter to solder it back again. 
When the break occurs within the coil itself, a new one must be 
substituted, as described for the short-circuit test. 



y. y 



'I'' 



i i ' ' ' 



' i i 



i i M 



1 234 S 



B B' 

FIG. 102. The illustration at the left shows method of bridging a coil of a 
wave winding. That at the right for cutting out a coil of a lap winding. 

Cutting out Injured Coils. In case of emergency, the bad 
coil can be cut out of circuit and the commutator bars con- 
nected together by a wire large enough to safely carry the 
current. This wire should be well insulated from the other 
leads, as any connection with them would constitute a short- 
circuit. In Fig 102 the method of bridging out a coil in a 



130 ARMATURE WINDING AND MOTOR REPAIR 

wave winding is shown. When one of the coils is short-cir- 
cuited as shown at A, the top side of the coil is disconnected 
from bar B, and the bottom side from bar B' Jumpers 
should be soldered in as shown by the dotted line. The ends 
of the coil leads can then be cut off close to the armature core 
and taped. The coil should be cut completely in two at X 
and X', and the ends taped. This will prevent self-induced 
currents from being generated within the coil, which might 
cause heating and injure the insulation on other good coils. In 
Fig. 102 (at right) the method of cutting out a coil in a lap 
winding is shown. The coil is open-circuited at A. The top 
side of this coil should be disconnected from bar 3, and the 
bottom side from bar 4. In this case the jumper must be run 
from bar 3 to bar 4. The dead coil can be taped up the same 
way as the series or wave coil mentioned above. 

One coil cut out of an armature will not perceptibly affect 
the running of a motor, and several of them can usually be 
cut out with safety, providing they are not bunched together. 
It is not wise to cut out too many coils, as this increases the 
heating and speed of the armature and lowers the efficiency of 
the machine. 

Causes of Grounds in an Armature. A ground occurs when 
current leaks from the current carrying parts of the armature 
into those parts that are not intended to carry current. A 
single ground will have little effect on the operation of a motor, 
but it should be removed as soon as possible, as there is always 
danger of a second ground coming on at some other point, 
which would produce the same effect as a short circuit. When 
a ground occurs, a small hole will be found burned through the 
insulation and into the iron parts of the armature. Across 
this carbonized insulation, current will pass. Grounds occur 
very frequently on the ends of the armature core at the points 
where the coils leave the core. If the bend has been too 
sharply made, or has been hammered too hard, the sharp 
edge of the core will cut through the insulation. To avoid 
this, the slot insulation should extend at least one-quarter 
to one-half inch past the end of the core on each end. 

Grounds also frequently occur in the commutator, caused 
by oil creeping up on the mica ring. Combined with the 



TESTING D.-C. ARMATURE WINDINGS 131 

copper and carbon dust from the commutator, this forms a 
good path for leakage of current. 

Tests for Grounds in an Armature. A ground can usually 
be located by using the test lamp. Disconnect the leads from 
four of the commutator bars on one-quarter of the circumfer- 
ence. This will determine the section of the winding in 
which the ground is located. Raise the leads of the defective 
section out of the commutator. Place one wire of the test 
lamp on the shaft, and with the other, test each coil separately 
to locate the ground. Sometimes the trouble may not b& in 
the armature, but may be caused by a grounded commutator. 
If the coil is at fault, it should be removed and reinsulated. 

Reversed Coils. A reversed coil, that is, one with the leads 
to the commutator reversed, frequently occurs. A practical 
way of locating a reversed coil is to pass a current through 
the armature at opposite points. The lamp bank and con- 
nections of Fig. 100 can be used for this test. Then with a 
compass or small bar of magnetized steel explore around the 
armature to determine the direction of magnetism from slot 
to slot. When the compass is over the reversed coil, the needle 
will reverse, giving a very definite indication of the coil which 
is connected wrong. The leads of this coil should simply be 
reversed. 

Use of a Bar Magnet and Millivoltmeter to Locate a 
Reversed Armature Coil. In armatures where both leads of a 
coil are taped together, and led out from an identical point, 
there is considerable danger of getting the leads crossed while 
connecting, thus reversing the direction of flow of the current 
in that coil. Such a reversal will in all cases "light out" 
as though perfect when tested with a lamp, but will cause 
bucking when the machine is run. A reversed coil of this 
sort is unusually difficult to locate and in many cases a whole 
machine has been stripped because ordinary methods of testing 
failed to locate the trouble. 

An efficient test which has been used by J. G. Yoerns 
(Electrical Record, September, 1918) is illustrated in Fig. 103. 
It will be noticed that a millivoltmeter is used rather than a 
voltmeter because of the greater sensitiveness of the former. 
Both terminals of the meter are connected to adjacent 



132 



ARMATURE WINDING AND MOTOR REPAIR 



commutator bars. Next, take a piece of metal which has 
been magnetized and move it in a direction corresponding 
to the revolving of the armature, directly above the coil 
to which the meter is connected. It is well to keep in mind 
the fact that if a clockwise motion is used on the first coil, the 
same direction must be retained on all remaining coils, 
otherwise the meter reading will be backward. On the 
downward stroke of the magnetized bar and as it approaches 
the coil to be tested, magnetic lines of force will travel from 
the 'bar to the coil, thence to the meter, causing the needle 
to fluctuate slightly. If the needle jumps ahead on the 
downward stroke, it will jump backward on the upward 
stroke, and vice versa. The reversed coil will read opposite 
from the others. To change the direction of the fluctuations 
of the needle, either reverse the meter terminals or reverse 
the motion of the magnet. 

iwrfvorfrnefer Compass 





FIG. 103. Method of using a 
magnetized bar and millivoltmeter 
to locate a reversed armature coil. 



FIG. 104. A convenient method 
for using a compass when testing 
for a reversed coil. 



Use of a Compass to Locate a Reversed Armature Coil. 

In connecting up small armatures with lap windings wound on 
by hand with four or more leads coming out of each slot, 
the leads may be easily confused as already mentioned so that 
some of the individual armature coils will be reversed. E. C. 
Parham (Electrical Record, August, 1918) has, therefore, 
suggested the use of the testing device shown in Fig. 104. 
While several reversed coils distributed around the armature 



TESTING D.-C. ARMATURE WINDINGS 133 

may not cause sufficient effect to excite suspicion, they are 
likely to give trouble in time. It is conceivable that if alter- 
nate coils were reversed, a highly improbable condition, the 
armature would be inoperable because then there would 
be an equal number of coils tending to turn the armature 
in opposite directions. 

The accompanying diagram (Fig. 104) shows a simple cheap 
method of readily locating any reversed coils that may exist. 
The armature rests in a support A that permit of rotating the 
armature as the test progresses. A strip of copper B is bent 
over the armature, as indicated, and a compass placed upon it. 
Current from an incandescent lamp test circuit is then applied 
to adjacent commutator bars that are connected to the coils 
that lie in the slot that is immediately under the compass. 
Suppose that the compass needle is deflected to the right on 
touching bars 1 and 2 and bars 2 and 3, there being two coils 
per slot. Rotate the armature until the next slot comes under 
the compass and touch the test points to bars 3 and 4 and then 
to bars 4 and 5, and so on all round the commutator. 

The compass deflections obtained should be always in the 
same direction. Any pair of adjacent bars touched by the 
test points causing a reversed deflection, includes a coil the 
leads of which have been brought down to the commutator 
in reversed order. In order to test the effectiveness of the 
method, it is necessary to only apply the test points to adjacent 
commutator bars in reversed order and observe that the com- 
pass deflection is thereby reversed. 

Locating Low Resistance or Dead Grounds. It is often 
difficult to locate a low resistance or "dead ground" in a low 
resistance armature owing to the very low resistance of the 
windings themselves. In such cases the following method 
can be used: 

First, short circuit all commutator bars by winding several 
turns of bare copper wire around them; then apply a source 
of energy, direct current being preferable, to the commutator 
and shaft. The voltage to be used depends upon the resistance 
of the "ground. " This produces a circuit from the commuta- 
tor through the grounded coil to the ground and out through 
the shaft, thus setting up a field around the conductors in 



134 



ARMATURE WINDING AND MOTOR REPAIR 



this coil. By applying a small piece of iron to the surface of 
the armature core and gradually moving it around, the 
grounded coil can be located by means of its field, which will 
attract the iron. 

The same method can also be applied to alternating current 
windings although not quite so readily. For example, in 
the case of a three-phase, single-circuit, F-connected armature, 
first disconnect the F, splitting the winding up into three 
separate circuits. Then test out each circuit with a magneto 




















\ 


1 




i? 




















1 




i i* 




















1 




i -. 


















\ 


1 


r 


y j/ 

1 




















1 


i 


TJ 




















1 




d 
























3 




















I 
1 




2 

O 1 
















J 


j 


/ 


/ 






W Ground to Armature Shaft 
FIG. 105. Method of locating grounds in an armature winding. 

or test lamp. Next apply a current to one end of the grounded 
circuit and to the shaft. Assume that there are 12 coils as 
shown in Fig. 105, coil No. 7 being the grounded coil while 
coil No. 1 is connected to the line as shown. There will then 
be a circuit through coils Nos. 1, 2, 3, 4, 5, 6, and 7 which can 
be readily detected with a piece of iron as already explained, 
while coils Nos. 8, 9, 10, 11, and 12 are dead. It is then, of 
course, obvious that if coils Nos. 1, 2, 3, 4, 5, 6, and 7 carry a 
current while coils Nos. 8, 9, 10, 11, and 12 carry no current, 
the ground must be in some section of coil No. 7, the circuit 
being completed at that point. 



TESTING D.-C. ARMATURE WINDINGS 



135 



Use of a Telephone Receiver in Testing for Short Circuits, 
Open Circuits and Grounds in an Armature. The telephone 
receiver on account of being very sensitive to sound, makes 
a convenient testing instrument in a repair shop. The ways 
it can be employed to discover a short circuit, open circuit, 
or ground are given here as used by Maurice S. Clement 
(Electrical Record, December, 1918). 

This testing device can be used either with alternating cur- 
rent or direct current. If alternating current is to be used, a 
test lamp and a pair of 
leads from a 110- volt cir- 
cuit are connected to the Source of Energy 

commutator as in Fig. 1C6, 
from one-fourth to one-half 
of the circumference apart. 
Next, take the receiver 
which has about two feet 
of two-wire telephone cord 

attached and hold it to the FlG i 06 .-Connections for using a 
With the Other hand telephone receiver for locating short- 
i i circuits and open circuits in an armature 




ear. 
press 



the 



recever 



winding. 

firmly to the commutator, 

taking care to touch adjacent segments. Move from one lead 
of the test lamp to the other segment by segment and repeat 
the operation until the commutator has been circled. 

If the wire in the coils is of too low a resistance to make a 
buzz in the receiver, put a rosette fuse in place of the test 
lamp and cut a rheostat in series with the fuse. This will 
bring the resistance up sufficiently to make a buzz in the 
receiver. A low buzz indicates a good flow of current; if 
no sound whatever can be heard from two commutator bars, 
there is a dead short circuit. An open circuit is indicated by a 
very loud buzz. A "cross connection will produce a defective 
sound on three segments. These three leads should be taken 
off and reconnected immediately and the receiver test once 
more applied. 

For a dead ground which persists in remaining invisible, 
place one side of the receiver line to the shaft, as in Fig. 107, 
and the other side of the test line to the commutator, then, 



136 



ARMATURE WINDING AND MOTOR REPAIR 



Test Lamp 
Source of Energy j '^ 




with the receiver to the ear, buzz each segment. The 

grounded coil will buzz louder than the rest. 

If direct current is to be used, the source of energy should be 

a battery. A buzzer connected in series on one side of the 

battery completes arrangements. 

Sometimes, when testing an armature with a transformer, 

a single short circuit will heat up coils in two, four, six or eight 

places, according to the 
polarity of the winding. 
By applying the receiver 
leads to the bars con- 
nected to each coil thus 
affected, a short circuit 
can be located quickly. 
Testing for Reversed 

Ground on -^ and Dead Field Coils. 

Machine Frame In multipolar machines 

FIG. 107 Connections for using a tele- tne po l a rity of field coils 

phone receiver for locating grounded coils J . 

in an armature winding. can be tested with a 

carbon filament lamp. 

By placing the lamp while lighted between the pole tips, the 
loops of the filaments will draw close together or separate far 
apart depending upon the direction of the magnetic flux. A 
dead pole can be quickly found in this way. 

Another very simple and positive method for testing the 
polarity of field coils is by means of two ordinary iron nails. 
To do this, pass a current through the field coil winding and 
place the nails on two adjacent poles. If the polarity is right, 
they will attract each other and if wrong they will repel each 
other. This method can be used also for detecting a dead coil. 

The Commutator. Trouble in a commutator may be traced 
to either a short circuit or a ground. In a commutator a 
short circuit may be caused by any of the following: Particles 
of metal touching two adjacent bars, or mica burned away 
allowing current to arc across. In an undercut commutator 
be sure the slots between bars are thoroughly free from all 
dirt before testing. A grounded commutator must invariably 
be removed from the shaft, and the damaged part well cleaned 
and reinsulated. In a case where a motor has been through a 



TESTING D.-C. ARMATURE WINDINGS 



137 




138 ARMATURE WINDING AND MOTOR REPAIR 

fire, the segments will in nearly all cases be undamaged, but 
the mica will be burned beyond future use, and in such a case 
refilling will be necessary. 

Testing Equipment for a Repair Shop. In addition to the 
facilities necessary for testing out armatures and machines 
for short circuits, open circuits, grounds and the like, every 
repair shop must be equipped to test and load motors of differ- 
ent sizes requiring voltages from 110 to 2300. A voltage of 
from 1200 to 1500 volts is needed for testing an armature for 
grounds. A transformer is therefore required with taps and 
connections so that the voltage can be varied from about 500 
volts up to 2500 volts. 

For testing for short circuits and open circuits a small 
transformer with specially shaped laminations about which a 
coil of wire is wound should be available (see Fig. 101 on page 
126). When applied to an armature with a short-circuited 
coil, the defective coil will show up by getting hot. 

For testing out circuits for opens, and to match up ends of 
coils when inserting them in the armature core, a test lamp and 
terminals is convenient. A magneto is also used for this 
purpose. In the case of testing for open circuits, two leads 
connected to a 110- volt circuit with a lamp connected in series 
in one of the leads is sufficient. For selecting coil terminals 
or " lighting-out " an armature winding while in the machine 
to discover trouble, a 6- or 12- volt automobile storage battery 
provided with two leads having snap testing clips and a lamp 
in series with one lead, is a convenient outfit that can be taken 
about the shop and to any job. 

At least one portable ammeter and one voltmeter are needed 
and one ringing out magneto and a pair of head telephone 
receivers. 

For testing motors a small switchboard wired so that con- 
nections can be made on the front by plugs and the voltage 
increased by using jumpers, is a convenient outfit that saves 
much time in making a test. This switchboard should have 
mounted on it, a voltmeter, ammeter, frequency meter and 
the necessary switches to connect and operate both motors and 
generators on either direct or alternating current. 



CHAPTER VI 



OPERATIONS BEFORE AND AFTER WINDING 
D.-C. ARMATURES 

Before an armature goes to an experienced armature winder 
and after it leaves his hands, there are certain steps in the 
process which can be properly termed "before" and "after" 
operations. In a case where a damaged armature must be 
Completely rewound, these operations may be outlined as 
follows : 



Operations before winding 



Operations after winding 



Stripping off old winding. 

Cleaning slots and ends of core. 

Filing burrs off slots. 

Testing commutator. 

Repairing commutator. 

Making new coils. 

Insulating ends of core and slots. 



Testing out winding. 
Soldering leads to commutator. 
Hooding armature. 
Banding armature. 
Turning commutator. 
Undercutting mica of commutator. 
Balancing armature. 
Painting armature. 
Relining bearings. 



Reference has already been made to a number of these 
operations in connection with the procedure in rewinding 
machines as outlined in Chapters III and VIII. The details 
that are given here refer to the requirements in all cases, with 
references to other Chapters where the operation has been 
taken up as a special subject. 

Stripping Off an Old Winding. In this operation when 
band wires must be removed, they should be cut or filed in 
several places. When a chisel and hammer are used care must 
be exercised not to mash down the armature teeth. The next 
step is to unsolder the leads to the commutator and clean out 
the slits hi the commutator necks carefully. By pulling out 

139 



140 ARMATURE WINDING AND MOTOR REPAIR 



the top sides of the proper number of coils according to the 
throw, the bottom sides can be reached that will allow the 
coils to be removed easily. (See Chapter III.) 

Cleaning and Filing Slots. After the coils have been re- 
moved, all old insulation must be thoroughly taken off the 
core and the slots. A solution composed of 25 per cent, alco- 
hol and 75 per cent, benzole is good for loosening the varnish 
of old insulation so that it can be scraped from the slots in 
the armature. This will produce no bad effects on the lami- 
nations or on the winding when the armature is rewound. 
Alkali solutions such as caustic soda will also loosen the insula- 
tion without injury to the laminations, but will creep be- 
tween the laminations and after the armature is rewound the 
alkali fumes are liable to damage the insulation. A tool 
made from bar steel one by one-sixteenth inch, of suitable 
length and drawn down to a long thin point like a chisel 
answers very well to remove old insulation after it has been 
softened, and will also give very satisfactory results with- 
out the use of any chemicals whatever. After removing 
the insulation in this way a file drawn through the slots will 

remove small pieces of insulation 
and smooth off all roughness. The 
edges of the slots should be filed to 
X siit 8 for remove sharp edges and burrs that 
thread to tie would injure the new coils while 

coiltogether ^.^ ^^ ^ ^ ^^ ^ 

entire core should then be thor- 
with a blast of 



-KJ 



oughlv cleaned 

FIG. 109. Shuttle form for 
winding a coil that can be pulled compressed air. 
into a diamond shape in a spe- 
cial pulling machine (Fig. 291). 



Testing Commutator. Details 
of this operation are given in 
Chapter XII as well as the steps in repairing a commutator, 
testing it out after reassembly, baking the insulation and 
tightening end rings. 

Making New Coils. This operation calls for an inspection 
of the old coils and winding data as outlined on page 57, 
Chapter III. In case of changes in speed or voltage it requires 
certain calculations which involve the size of wire to use. These 
calculations are given in Chapter X, page 240. When the 



OPERATIONS BEFORE AND AFTER WINDING 141 

* 

size of wire is known the winding of the coils can be done on 

dne of the forms shown in Figs. 109 to 114. 

In making coils in large repair shops three methods are 
used, namely winding on a mould, on a former or on a shuttle. 
Mould coils are usually those made -on a form rotated in a 
lathe with all necessary shaping done with very little 
pounding on the conductors. By formed coils is usually 
meant, those coils made over a stationary form with the bends 




FIG. 110. Three steps in the construction of shuttle wound coils (Fairbanks- 
Morse & Company). 

(a) The coil at the bottom is shown as wound on a shuttle form using square copper 
wire. (6) At the left it is shown after being pulled into final shape, dipped in insulating 
varnish, thoroughly baked, and then taped, (c) The finished coil is shown at the right 
after four to six alternate dipping and baking treatments. 

made by the use of levers and mallets to force the coil to the 
proper shape. Shuttle or "pulled" coils are first wound on 
a simple shuttle such as shown in Fig. 109 which is fastened 
to a lathe and then pulled on a coil puller to the shape 
required for the particular throw of the coil as shown in 
Fig. 110. 

Forms for Winding Coils Like Those Previously Used. 
A simple form that can be made from a pine board for dupli- 



142 ARMATURE WINDING AND MOTOR REPAIR 



eating the shape of coils previously used in an armature that 
is being rewound is shown in Fig. 111. The dimensions and 
general shape can be secured from a sample coil preserved 
from the old winding. In the illustration A is a small hole 
for mounting on a spindle. Small pins B are driven into the 
form to hold the coil in place while winding. C and C 1 are 
slots cut into the sides so that the several turns of the coil 
may be tied together with thread. This keeps the wires 
together after the coil is removed from the form. At the ends 
D and D f two pins are used in order to make the turn shown 
on the end section at the right in Fig. 111. 




c' 

c 




I 






FIG. 111. Form for shaping diamond armature coils to match old ones used. 

Now mount the form on a spindle so that it can be revolved 
and wind on the required number of turns, using the pins as a 
guide. If the completed coil is to consist of two coils, take 
two wires and wind them together. When the proper number 
of turns have been made, cut off the wire and tie the coil with 
thread at C and C'. Remove the coil from the form and com- 
pare it with a sample if such a sample was preserved from the 
old windings. It is also well to try this first coil in the arma- 
ture, and see that it has the proper span, and that the leads to 
the armature are long enough. 

The following method for winding coils for small motors 
has been found convenient by Maurice S. Clement (Electrical 
Record, November, 1918) in those cases where a repair must 



OPERATIONS BEFORE AND AFTER WINDING 143 



be made on the job or coils made up where a coil winding 
machine is not available. The form is made up as follows: 
First take a flat piece of wood about three-fourths of an inch 
thick and plot out the shape of the coil. Then place right 
angle screw hooks, at all angle points. The screw hooks 
should point away from the center of the form. Mark out the 
center and run a breast drill through as far as it will go. Place 
the breast drill in a vise in a horizontal position with the coil 
form toward the left, taking care to place it so as to allow free 





Bolts / 
extending 
through 
form 



FIG. 112. FIG. 113. 

FIG. 112. A six gang form for winding coils in series. 

Six coils can be wound in series and removed in a group so they can be inserted in the 
slots of an alternating current motor stator without the necessity of makihg the series 
connections. This form can also be used to wind a single or double coil. It is built up in 
sections which are held together by the bolts at A. The slots in the divisions between 
sections enable the winding and removal of the coils in series. 

FIG. 113. Continuous jointless phase-group coils used on rotors of 
Fairbanks- Morse phase wound motors. These coils are wound on a form 
similar to that shown in Fig. 112. 

movement of the handle. Turn the wire of which the coil is 
to be made, once or twice around a nail on the back of coil 
form, and lead the wire over the edge to the face of the form 
and turn the handle. The form can be revolved at any de- 
sired rate of speed and the wire run over all the screw hooks. 
Keep sufficient tension on the wire to permit each turn to lie 
snugly beside its predecessor. 

This is a convenient way to wind coils, for the reason that 
unnecessary crossing of wires can be prevented. When the 
specified number of turns have been wound, twist a short 
piece of wire around each end of coil to hold it in proper shape. 



144 



ARMATURE WINDING AND MOTOR REPAIR 



To finish the operation, turn all screw hooks toward the center 
of the form and slip the coil off. 

Another hand-made form also recommended by Mr. Cle- 
ment for use in winding larger coils than the one described 
above is shown in Fig. 114. 

Insulation of Core and Slots. The insulation needed with 
coils used in partially closed slots and in open slots of alternat - 
ing current machines is given in Chapter VIII and in connection 
with the winding of direct-current machines in Chapter III. 




Slit through whicli 
First Lead of Coil 
Passes 

Slits through which 
Strings are Laid 
to Bind Coil 



-Handle 




-Clamp 



Plate 



FIG. 114. Construction of a convenient bench winder for forming armature 

coils. 

Testing Out the Winding. Details of this operation both 
before the leads to the commutator are connected and when 
trying to locate troubles in the winding are given in Chapter V. 

Soldering Coil Leads to the Commutator. After the wind- 
ing of an armature has been thoroughly tested out, the coil 
terminals can be soldered to the commutator. In the solder- 
ing operation great care should be taken that pieces of solder 
do not fall or run down back of the commutator to later pro- 
duce a short-circuit and cause the armature to be returned 
for further repairs. To prevent damage to windings, acid 
fluxes should never be used in soldering a commutator. A 
solution of rosin in alcohol is recommended instead. A tin 
and lead solder is considered best in soldering leads to the 
commutator but a pure tin solder is used in making all other 



OPERATIONS BEFORE AND AFTER WINDING 145 

joints on the coils as the insulation is less liable to be damaged 
with this solder on account of its lower melting point. When 
tin is used the best results are obtained by working upon 
the side of the armature, so that the joint is level. After 
soldering, the armature should be mounted in a lathe and the 
rough solder on the necks of the commutator turned down, 
the commutator polished and the wiper rings turned to give 
the exact distance between bearings. In some armatures 
wedges are inserted in the slots above the coils. These extend 
above the surface of the banding grooves and should be turned 
down while in the lathe if they are used, so the banding grooves 
will present a smooth bed for the band wires. 

Hoods for Armatures. In order to protect armature end 
connections of railway motors and of mill motors that must 
be used in places where dirt and dust is liable to accumulate 
on the armature, a heavy hood is often put over the ends of 
the coils. For the commutator end a hood of woven asbestos 
is suitable. This hood is usually sewed in a conical shape and 
impregnated with a moisture and oil-repelling compound 
and fastened in place while wet. The small end should be 
drawn up over the commutator and turned inside out and 
firmly tied over the coil leads and commutator necks with 
h?avy twine. The body of the hood should then be turned 
back over the armature. If the commutator necks are lower 
than the level of the core, another layer of twine should be 
wound over the hood near the commutator and a band of 
canvas sewed over the whole. The hood should then be 
stretched tightly back over the armature and tied with twine. 

Around the rear end of the armature a band of canvas should 
be wrapped so that the greater part of the strip extends out 
over the shaft only enough being wound over the armature to 
permit a secure fastening. This should be bound in place 
with a band of twine wound tightly in the groove between the 
coil ends and the end bell. The canvas should then be turned 
back over the armature and bound smoothly in place. 

In case the armature is to be banded with steel wire, two 
strips of cotton tape separated by a band of varnished paper 
should be wound over the hood and end connections near the 

core of the armature, as a base for the banding wires. 
10 



146 ARMATURE WINDING AND MOTOR REPAIR 

Banding Armatures. In repair shops where banding 
machines are not available, the banding of armatures is done 
in a more or less indifferent way with the result that the 
banding may only perform a part of its function, namely, 
to prevent the coils from being thrown off the armature core. 
Unless the banding is placed on the armature with great care, 
there may be sufficient movement of the coils in the slots to 
wear the insulation and cause grounds. Such movement is 
also liable to cause breaks in the copper leads where they are 
soldered rigidly into the commutator. The details for banding 
an armature given in what follows, are taken from an article 
in the Electric Journal and represent good shop practice that 
can be followed in both large and small repair shops. 

Shrinking Coil Insulation. Since the coil insulation shrinks 
upon being heated, it is necessary to shrink it as much as 
possible before the final banding wire is applied. This is 
done by heating the whole armature to about 75C. (167F.), 
when the insulation becomes pliable and can be pressed down 
into permanent shape. 

Temporary Bands. The hot armature should then be 
mounted in a lathe and a protecting strip of cloth placed over 
the end windings. Wind a temporary banding wire over the 
coils with enough tension to draw them down into place, and 
fasten the ends by soldering tin clips over the wire. The 
armature should then be allowed to cool. After the tem- 
porary wires are removed the armature is ready for the 
permanent banding. 

Banding Machine. When a banding machine is used, the 
tension in the wire is regulated by passing it over a train of 
friction pulleys, mounted on the carriage. The friction of the 
pulleys can be adjusted to any desired value by the regulating 
screws. In the absence of such a device, fair results can be 
obtained by passing the wire two or three times around a 
round wooden banding stick approximately two inches in 
diameter and adjusting the tension by hand. 

Core Bands. When core bands are used, the grooves should 
be fitted with thin strips of tin, which protect the coils from 
the cutting action of the bands. In starting the permanent 
banding, wind a few turns at one end fco secure the necessary 



OPERATIONS BEFORE AND AFTER WINDING 147 

Then wind all the banding groups continuously, to 
eliminate the necessity of fastening the ends of each group as 
they are wound. The bands should be held together and the 
ends fastened by means of narrow tin strips (about 0.012 to 
0.02 inch thick and 0.25 inch wide) placed under the wires 
and bent back over the top and held by pure tin solder. These 
strips should be inserted while the wire is being fed on and 
located about every three inches around the armature, with 
closer spacing at the beginning and end of each band. For 
the core bands, these strips should be placed in the slots and, 
being wider than the groove, they prevent any tendency for 
the bands to slide around the armature. The ends of the 
groups should then be cut and secured by being bent back 
outside one clip and inside the next one. Pure tin solder 
should be applied to the whole surface of the bands to form 
a solid web. ; 

End Bands. The end windings should be secured by 
groups of wire wound on insulating hoods to protect the coils. 
On the commutator end, strips of thin mica with overlapping 
ends are usually placed on the commutator neck and held in 
place with a few turns of twine. If a hood or head of canvas 
is to be used it should be wrapped around the neck, extending 
about an inch from the edge and turned inside out. After 
this end is secured with twine, the free end of the hood should 
be pulled back over the windings, bringing the outside of the 
hood at the surface and making a neat folded-under edge. 
This hood can be held temporarily with twine until the wire 
is applied. The other end of the armature should be similarly 
covered with a hood and banded. 

Tension to be Applied to Band Wire. The proper tension 
for banding wire when being applied varies with the size of 
wire and the construction of the end windings. When 
the end coils have no rigid support and extend out a con- 
siderable distance from the core, the tension should be gradu- 
ally reduced, as shown in the accompanying table. 

Wire. The best material is a high-grade steel piano wire, 
having a final breaking strength of 200,000 Ib. per sq. in. For 
temporary bands a cheaper grade can be used. The band 
wire should be tinned. 



148 ARMATURE WINDING AND MOTOR REPAIR 
POUNDS TENSION FOR BANDING WIRES 







End bands 


Diameter of wire, 
in. 


Core bands 












At core 


At end of wdg. 


0.045 


200 


175 


160 


. 0641 


300 


250 


225 


0.0803 


400 


300 


260 



Solder. Pure tin should be used, as this gives a band that 
will hold together for a longer time than half-and-half solder. 

Flux. About 1.5 lb. of powdered rosin, dissolved in one 
quart of denatured or wood alcohol makes a good flux. 

Tin Clips and Strips should be of commercial sheet tin 
about 0.012 to 0.02 inch thick. 

Precautions. Use a band wire that is strong enough to 
prevent movement due to high speed and vibration. 

Secure the ends of all band wires under the clips. 

See that the bands are below the surface of the core to 
keep them from rubbing on the poles. 

Before applying core bands, see that the tops of the coils 
are about ^ 2 inch above the band groove, so that the bands 
pull the coils down even with the core. If the coils are too 
high and the bands do not rest on the cores, loose bands will 
result when the insulation dries out. 

Wind all core bands in one operation. 

In soldering, use a 4-lb. clean iron, well tinned. 

Bands to be effective should be kept tight. If they are 
allowed to become loose, grounded armatures and broken leads 
may result. It is considered good practice to reband new or 
newly rewound railway or hoist armatures after about 12 to 
18 months as a safety-first measure; and to make renewals on 
old ones whenever the bands start to loosen. The service 
conditions temperature and speed largely determine the 
length of time that bands will hold tight. One large operating 
company rebands all armatures every two years. 

Seasoning and Grinding a Commutator. A new or reas- 
sembled commutator becomes "seasoned" that is the 
insulation baked out and all parts in their final set position, 



OPERATIONS BEFORE AND AFTER WINDING 149 

only after being in operation for a time with the necessary 
tightening and grinding. This is particularly true of a large 
commutator. Need of attention to a commutator will be 
indicated by roughness, high or low bars, flat sections result- 
ing in poor commutation. If the commutator is in very bad 
condition it may be necessary to turn it down, but for ordinary 
cases a grinding: tool is preferable and recommended such as 
shown in Fig. 115 (Instruction Book, Westinghouse Electric & 
Mfg. Co.). 

Commutators should always be ground at from 100 to 120 
per cent, normal speed. Turning requires a much lower 
speed; it should not be higher than 150 feet per minute. 




FIG. 115. Grinding device for truing commutators when they do not require 

turning down. 

Before grinding a commutator, the machine should have 
been in service a sufficient length of time to bring the tem- 
peratures up to a constant value. Before grinding, the 
brushes should be lifted off the commutator as the copper and 
stone dust will rapidly wear them off. The dust will also 
become imbedded in the brush contact surface and later 
damage the commutator or cause poor commutation. The 
armature winding should also be thoroughly protected during 
this operation to prevent an accumulation of dirt and metal 
chips which may result in an insulation failure when the 
machine is again put in service. This protection can usually 
be obtained by using a circular shield of fullerboard, or similar 
material, around the commutator at the end next to the 
armature. This shield can be easily supported from the 



150 ARMATURE WINDING AND MOTOR REPAIR 

brush-holder arms and should extend from the commutator 
surface to an inch or two above the surface of the armature. 
It may also be desirable to put a temporary canvas hood 
over the armature winding. This protection can be best 
provided by carrying the copper dust away by means of a 
vacuum system. Even when this is done the armature should 
be protected as described. After grinding, the complete 
machine should be thoroughly cleaned by the methods already 
described. It may be nec.essary to repeat the heating, tight- 
ening and grinding one or more times before the commutator 
is in first-class condition. Emery cloth or paper should never 
be used for this purpose on account of the continued abrasive 
action of the emery which becomes embedded in the copper 
bars and brushes. Even when sandpaper is used the brushes 
should be raised and the commutator wiped clean with a piece 
of canvas lubricated with a very small quantity of vaseline or 
oil. Cotton waste should never be used and an excess of 
lubricant should be avoided. 

The grinding device shown in Fig. 115 can be mounted in one 
of the brush-holder arms or brackets of a large machine. 
The grinding stones should be adjusted against the rotating 
commutator until a clean cutting effect is secured, but should 
be carefully shaped to the commutator surface before being 
placed in the grinding device. The stones can be moved 
across the surface of the commutator while it is running, by 
means of the handle shown at the right in the illustration. 

Undercutting Mica of Commutator. Several devices and 
hand tools are used for this operation. Details of their use 
and descriptions of the devices are given in Chapter XII, 
page 320. 

Balancing an Armature. After an armature has been wound, 
banded and its commutator trued it must be balanced. In 
the case of small medium speed armatures this operation can 
be successfully done on a pair of steel knife edges mounted 
parallel to each other and perfectly level. The ends of the 
armature shaft are placed on these knife edges so that the 
armature is free to roll. It is then given a slight roll with 
the hand and when it comes to rest, the bottom marked with a 
piece of chalk. This rolling and marking should be repeated 



OPERATIONS BEFORE AND AFTER WINDING 151 



several times. If the marks fall well distributed around the 
armature core the armature is in sufficient balance for service 
and can be placed in the motor. In case the marks fall close 
together or all on one side of the armature, it must be balanced 
by adding weight to the opposite side or removing weight 
from the heavy side. This is done in different ways depend- 
ing upon how much the armature is out of balance. Before 
the weight is permanently fixed it must be determined exactly. 
This can be done by the use of slugs of lead properly attached 
and shaved down with a knife until the proper balance is 
secured. Frequently a nut or bolt head can be filed on the 
heavy side when the amount to be removed is small. Other- 
wise some method of attaching a piece of metal or solder must 
be devised equal in weight to the lead slug determined by 
the test. 

For high-speed armatures and those machines where a 
perfect balance must be secured, the armature should be 
tested in a special balancing machine. 

Painting the Winding. The kinds of insulating varnish 
and impregnating compounds that should be used on an arma- 
ture winding are given on page 176 of Chapter VII. Only 
good grades of insulation paints should be used on windings. 
When the winding is subjected to acid fumes, or the machine 
located in damp places the manu- 
facturer should be consulted on the 
treatment that should be given the 
winding. 

Relining Split Bearings. The 
following suggestions (Lnstruction 
Book, Westinghouse Electric & 
Mfg. Co.) can be followed in re- 
newing split bearings of the oil ring 
type. Melt the old bearing out 
of its shell and prepare an iron 
mandrel such as shown in Fig. 
116 having the same diameter / as the shaft and dimensions 
c, d and e taken from the bearing to be renewed. Iron pieces 
BB and C should be attached by screws to the mandrel to 
form the oil ring slots and the horizontal inspection opening 




FIQ. 116. Mandrel for 
use when relining split bear- 
ings. 



152 ARMATURE WINDING AND MOTOR REPAIR 

in the top half of the bearing. The pieces BB should be 
tapered so that they will withdraw easily from the cast metal. 
A shpulder D, so placed as to fit against the end of the bear- 
ing shell serves as a guide. 

For the Lower Half. Warm the madrel and bearing shell 
and while both are still warm so as not to cool the metal too 
rapidly when pouring, place the mandrel in the lower half of 
the shell with the shoulder D tight against the end of the shell 
and the straight bottom portions of the pieces BB resting on 
the split plane of the bearing. Close the joints x with putty 
and fill the openings between the shell and the housing lead- 
ing to the oil well with wet waste. Pour the molten metal, 
heated just enough to flow readily, into the space between the 
shell and the mandrel until the metal is flush with the split 
surface of the housing. The metal will harden very quickly 
and the mandrel can then be removed. 

For the Upper Half. Fill the openings in the upper half 
shell with putty and lay the mandrel in the shell with the 
split side up. Block the openings leading to the oil well with 
waste, and pour as described for the lower half. Remove the 
mandrel, smooth all rough edges in the bearing by chipping 
or filing, and chip the oil grooves in the lining of the upper 
half. The bearing surface of both halves should be eased off 
by scraping and the edges along the split surface should be 
filed flush with the shell. 



CHAPTER VII 

INSULATING COILS AND SLOTS FOR DIRECT -CUR- 
RENT AND ALTERNATING -CURRENT WINDINGS 

On account of the fact that the voltage between commutator 
segments in a 1 10- volt direct-current machine is not often more 
than six volts, in a 600- volt machine not over 18 volts and in 
a 1200- volt machine a maximum of about 25 volts, the voltage 
between the conductors of an armature coil is relatively low. 
These conductors are usually provided with either a single, 
double or triple cotton covering, the triple covering being used 
when the voltage between the conductors is near the upper 
limit of 25 volts. Where the winding space is small a silk 
covering is sometimes used in small machines. After the coils 
have been formed and bound together to hold the strands in 
place, the entire coil receives a special insulation as a protection 
against breakdown between the copper of the coil and the iron 
of the slot in which it is laid. The insulation for small low 
voltage machines may consist of a wrapper of treated cloth 
or mica held in place by a layer of cotton tape wound so that 
it does not overlap. The end connections of the coil are then 
protected by an overlapping layer of cotton tape with cotton 
sleeves used on the leads as further protection. The entire 
coil is then dipped in an insulating varnish and baked. 

Insulation for Armature Coils and Slots. In addition to the 
insulation provided on the coils, it has been found necessary 
to pay particular attention to the insulation of the winding as a 
whole from the armature core. The insulation used performs 
two functions, namely, to provide mechanical protection and 
to serve as electrical insulation. The following classification 
can be made of the materials that are much used as slot and 
coil insulation: 

For Mechanical Protection. Pressboard, presspahn, vul- 
canized fiber, hard fiber, fish paper, rope paper, Japanese 
and Manila paper. 

153 



154 ARMATURE WINDING AND MOTOR REPAIR 

For High Temperatures and Electrical Insulation. Mica, 
micanite, mica paper and mica cloth. 

For Electrical Insulation Only. Cotton tape and oiled or 
treated cloth which includes cotton or linen muslin, varnished 
cambric, varnished muslin, and empire cloth. 




FIG. 117. Type of wire wound coils used in stator of Fairbanks-Morse 
alternators (Fig. 155). These coils are thoroughly insulated and require 
only a non-abrasive material in the slot to give mechanical protection to the 
coils. 

For Mechanical Protection and Electrical Insulation. 

Pressboard or fullerboard is a material resembling cardboard 
and made from cotton rags and paper clippings. It varies 
from seven to 125 mils in thickness and when properly treated 
and varnished has a dielectric strength of about 500 volts per 
mil in thickness up to 25 mils and then reducing to about 200 
volts per mil in the thicker sheets. 

Presspahn is the name given to the pressboard which is made 
in Germany. 

Vulcanized fiber also known as hard fiber is a dense hard 
material with a dielectic strength of about 200 volts per mil 
at thicknesses of from 50 to 150 mils. It is used wherever 
an insulating material of exceptional mechanical strength is 
needed such as wedges in armature slots and coil braces. 

Horn fiber is a material made in different colors that has 
a high tensile strength and also a good dielectric strength 
(250 volts per mil when 10 mils thick) which can be increased 
by impregnation with oil or varnish. 

Fish paper is made from rag stock and through a treating 
process becomes a hard fiber-like paper which is very strong. 
This material is not affected by heat and on account of this 



INSULATING COILS AND SLOTS 



155 



and its mechanical strength is much used as cell lining in 
armature slots. 

Manila paper is made from linen or Manila fiber producing a 
tough strong paper which when dry has a dielectric strength 
varying from 100 to 230 volts per mil in thicknesses of from 
1.8 to 28 mils. 

For High Temperatures and Electrical Insulation. Mica 
is one of the very few materials which maintains a high dielec- 
tric strength at high temperatures. It is not, however, me- 
chanically strong. Flexible sheets of mica are made up by 
sticking thin splittings of mica on one side of a sheet of paper 
or cloth with a suitable varnish in such a way that the joints 




FIG. 118. Different types of coils used in rewinding motors. 

(1), (2), and (3) are wire wound direct-current coils. Two sections are shown in (l) 
before assembling the complete coil in (2). The coil in (3) is ready to be spread into a 
diamond shape in a pulling machine. (4) is a so-called mush or basket coil for partially 
closed slots. The coil as shown is ready for use in A. C. motors from about H to 15 hp. 
(5) is a completed strap coil ready for final treatment or dipping. (6) is a shuttle 
wound coil, three wires wide by eight wires deep in series for A. C. motors with open 
slots. This coil is spread into shape in a pulling machine. It is insulated with 
varnished cambric at the end where wires cross in winding. The coil at (7) is the same 
as (6) after being spread and insulated with tape. 

are staggered. Such built-up sheets are known under different 
names, such as " Japanese Paper and Mica," "Fish Paper and 
Mica" and " Treated Cloth and Mica." These are also 
referred to as Mica paper and Mica cloth. 

Micanite is a form of reconstructed mica made both in plate 
and flexible forms. The latter is used for armature slots and 
the former for commutator segment insulation. Its dielectric 
strength is very high ranging in the flexible form from about 
600 volts per mil for five mil thickness to 500 volts per mil in 



156 ARMATURE WINDING AND MOTOR REPAIR 

125-mil thicknesses. Micanite paper and Micanite cloth is 
also made with Japanese paper and with muslin. 

For Electrical Insulation Only. Cotton is used in the form 
of tape or cloth. When used primarily for its dielectric 
strength, it is treated with an insulating compound. After 
such treatment cotton cloth will withstand about 1000 volts 
per mil thickness. Because this insulation is flexible and 
tough it is 'much used as an insulation in the insulating of coils 
and other parts of electrical machinery. Since ifc is quite 
susceptible to damage and abrasion it is mostly used with a 
protective covering such as untreated cotton tape or friction 
tape or a tough paper such as fish paper. Different weights 
and thicknesses of cloth are used for insulating purposes such 
as cambric five mils thick; muslin eight mils thick; heavy cotton 
11 mils thick; drilling about 17 mils thick; and duck about 30 
mils thick. When duck is treated with linseed oil or varnish 
it is frequently used as a protective covering over coil supports, 
between coil ends and over the ends of armatures. Cotton 
insulating material when treated with an insulating compound, 
varnish or linseed oil, is known under several names such as var- 
nished cambric, varnished muslin, Empire cloth, Kabak cloth, etc. 

Any of the cotton materials can be cut into narrow widths 
for use as tape when insulation is more important than me- 
chanical strength. Tape cut on the bias is used to tape-up 
coils of irregular shapes. 

Descriptions and Uses of Insulating Materials. In the 
following paragraphs the composition of the insulating ma- 
terials already mentioned together with many others that are 
available are given with their various uses. This information 
is taken from a comprehensive classification of treated cloths, 
pressboards, fibers and papers by Hugh E. Weightman, 
Chief Engineer, Engineering Service Company, Chicago, 111. 
(Electrical Record, July, 1919). 

Treated Cloths. The different kinds of treated cloths 
which are available are as follows: 

Black varnished cambric Yellow varnished cambric 

Japanned muslin Oiled muslin 

Japanned duck Yellow oiled canvas 

Varnished silk Yellow oiled cotton drill 



INSULATING COILS AND SLOTS 157 

Uses and Properties of Treated Cloths. Each of the materials 
in the foregoing list has somewhat different insulating pro- 
perties. Black varnish cambric is a varnish coated cloth 
used in the form of straight-cut tape for wrapping wire and 
cables and as bias cut tape for armature coils. It is usually 
supplied in 0.010-in. and 0.012-in. thicknesses in rolls about 
36 in. wide. It is also supplied in ready cut tapes. The 
material is obtainable in two or more grades, the cheaper 
grades being used for phase insulation. 

Japan muslin is unbleached muslin cloth treated with black 
japan and baked to produce a waterproof material of good 
insulating properties. It is used for wrapping where a coarse 
cloth is permissible and is usually supplied by the manufacturer 
in 0.017-in. thick by 30-in. wide rolls. 

Japan duck is a high grade 8-oz. duck approximately 
0.025 in. thick and of close weave. It is treated with japan 
and oven cured. It is chiefly used under the binding bands 
of railway motors as a protecting and moisture excluding 
fabric. This material is usually supplied in rolls 36 in. 
wide. 

Varnished silk is made of Japanese silk treated with a 
high grade insulating varnish and oven cured. This makes 
a thin, tough insulating material of high dielectric strength 
which is used where light weight and a minimum of thickness 
is required. Varnished silk is also employed on meter coils 
and as insulation in airplane apparatus. The material in 
addition to being light does not become brittle in extreme cold 
nor gummy in heat. It is quite expensive and for that 
reason is not generally applicable. The usual thicknesses 
are 0.003 in. and 0.005 in. Sheets are 27 in. wide. 

Yellow varnished cambric is a strong, closely woven cotton 
cloth having an especially soft finish and treated with high 
grade varnish. The varnish is baked in place, producing 
a material having a very high dielectric strength and a hard 
smooth surface. Its insulation resistance is usually not as 
high as black varnished cambric, especially at high tempera- 
tures. It is more easily handled than the black varnished 
cambric. It is usually supplied in rolls and sheets 36 in. wide 
and in tapes straight-cut or bias-cut, 0.010 in. and 0.012 in, 



158 ARMATURE WINDING AND MOTOR REPAIR 

thick. This material is used for much the same purposes as 
black varnished cambric. 

Oiled muslin is a linen finished cloth coated with oil and 
oven cured to set the film to a hard smooth surface. The 
product is a flexible cloth having high insulating properties 
and good resistance to deterioration through vibration or 
aging. It is used for a large variety of purposes, especially 
for wrapping armature coils and, in tape form, for taping coils 
and leads. It is supplied 0.007 in. thick in 36-in. rolls or in 
standard width tapes. 

Yellow oiled canvas is a high grade duck treated with 
oil to produce a flexible waterproof material. It is used 
for pads under railway motor field coils and in similar places. 
This material is commonly obtained in one thickness of 0.045 
in. in rolls 36 in. wide. It is also used for tarpaulins for genera- 
tors, switches and such apparatus. Yellow oiled cotton drill 
is a light unbleached cotton drill treated with oil and oven 
cured to set the film to a firm surface. It is made 0.017 in. 
thick and in rolls 30 in. wide. It is often used for coil separa- 
tors in transformers and regulators. 

Pressboards, Fibres and Papers. Fibers, leatheroids and 
such materials, are especially selected for their high insulating 
properties, and they are treated to render them more pliable 
and easily worked. The treatment does not appreciably 
affect the thickness except in the case of the shellacked 
materials. This treatment adds approximately 0.005 in. 
in thickness to the size of each sheet. When ordering treated 
material from the manufacturer the thickness always refers 
to the untreated material. The following kinds of this class 
of treated materials are available: 

Pressboard Varnished rawhide fiber 

Japanned pressboard Leatheroid 

Oiled pressboard Express parchment paper 

Shellacked pressboard Shellacked express paper 

Varnished pressboard Varnished express paper 

Horn fiber Red rope paper 

Japanned horn fiber Oiled red rope paper 

Oiled horn fiber Shellacked red rope paper 

Shellacked horn fiber Varnished red rope paper 

Varnished horn fiber Shellacked bond paper 



INSULATING COILS AND SLOTS 159 

Rawhide fiber Asbestos paper 

Japanned rawhide fiber Oiled asbestos paper 

Oiled rawhide fiber Varnished asbestos paper 
Shellacked rawhide fiber 

Pressboards. Pressboard is a specially prepared paper which 
is dense, flexible and easily worked. It readily absorbs oils 
and varnishes which render it less hygroscopic than in its 
untreated state. It is used very extensively as layer insulation 
and spacers in various types of generator, motor, transformer 
and regulator coils. Collars and shields on high-voltage 
transformers are also made of this form of insulation. One 
manufacturer carries this material in stock in the following 
sizes : 

Thickness in inches Size of sheets in inches 

0.009 30 X 40 

0.020 33 X64 
0.030 " v 34X40 

0.030 36X84 

0.60 34X40 

0.60 36X84 

^2 24 X 60 

H 40 X 60 

Japanned pressboard is used for separators and fillers 
in armature and field coils. It is made in the same sizes 
listed in the preceding paragraph. Oiled pressboard is used 
in the same way as japanned pressboard, and it is sometimes 
preferred since it is more flexible. It is supplied in the same 
sizes as given for pressboard. Shellacked pressboard finds 
its principal application as separators in bonding railway 
armature coils. The shellac upon the application of heat, 
melts, forming a close union between the separator and the coil. 
The principle thickness used is 10 mils or 0.010 in. although 
other sizes are available. This material is often made up 
locally by manufacturers. Any sizes previously listed are 
available. 

Pressboard varnished and oven cured is used for fillers and 
separators. The usual sizes are available as listed above. 

Horn Fibers. Horn fiber is a tough flexible insulating 
material of high mechanical and dielectric strength. It is 
used for separators, slot channels, angles and joint insulation. 



160 ARMATURE WINDING AND MOTOR REPAIR 

and wherever a high degree of flexibility is essential. This 
material can be bought in both sheets and rolls. All horn 
fibers are obtainable in the same thicknesses as listed for 
pressboard. Japanning greatly increases the flexibility and 
dielectric strength of horn fiber. One special application of 
japanned horn fiber is as fillers for bracket insulation and 
around rocker arms. 

Treated with oil and oven cured horn fiber makes a superior 
dielectric material for general work. Shellacked horn fiber 
finds its greatest application as slot armors, it being customary 
to form the armors before shellacking. Varnished horn fiber 
is used for fillers and separators in preference to pressboard 
on better machines and especially on high speed units because 
it is more flexible and does not disintegrate as rapidly when 
subjected to vibration. 

Rawhide Fibers. Rawhide fibre is a harder pressed material 
than horn fiber and consequently lacks some of the flexibility 
of the latter. It is very tough and can be rendered more 
flexible by treatment with japan, oil shellac or varnish. 
Untreated it is used where toughness is essential and where 
great flexibility is not required. The thicknesses of the stock 
sizes of the different forms of rawhide fiber usually found on the 
market are: 0.005 in., 0.010 in., 0.015 in. and 0.020 in. Sheets 
or rolls are 40 in. to 48 in. wide. Sheets are usually 72 in. 
to 120 in. long. Japanned rawhide fiber has increased diel- 
ectric strength and is more flexible than the untreated fiber. 
It is used for the more simple shapes of slot armor and for 
fillers and separators. Rawhide fiber treated with a high 
grade oil and oven cured to produce a firm surface is used for 
the same purposes. 

Shellacked rawhide fiber is used for the same purposes as 
shellacked horn fiber, where great flexibility is not required. 
Varnished rawhide fiber is the most flexible of the rawhide 
fibers and is used for fillers and separators. 

Leatheroids. In appearance leatheroid is quite similar to 
rawhide fiber. It is used for the same general purposes as 
the rawhide and horn fiber. It is, however, more resistant 
to heat, is more dense and molds more readily than either of 
the others. The usual thicknesses available in sheets or rolls 



INSULATING COILS AND SLOTS 161 

42 in. wide are 0.010 in., 0.015 in., 0.020 in., 0.030 in., 0.060 in., 
and % m - The sizes apply to all forms of leatheroids. The 
japanned leatheroid is used similarly to japanned horn or 
rawhide fiber where these materials are unable to stand the 
heat. Oiled leatheroid is used in place of oiled horn or raw- 
hide fiber where a greater resistance to heat is desirable. 

Paper Insulation. Express parchment paper is a strong 
high-grade wood fiber paper selected to insure freedom from 
pin holes and metallic particles. It is used for layer insulation 
and in making up pasted mica sheets. The commercial 
sizes are as follows: 

Rolls 0.003 in. thick, 30 in. wide 

Rolls . 005 in. thick, 32 in. wide 

Rolls 0. 009 in. thick, 32 in. wide 

Sheets 0.005 in. thick, 30 in. X 35 in. wide 

Sheets 0.009 in. thick, 30 in. X 35 in. wide. 

When it is shellacked, express parchment paper is used 
for armature and field coil bonding. The 5- and 9-mil 
sizes are most frequently used for this purpose. Varnished 
express parchment paper is used chiefly for slot insulation, 
its best recommendation being its glossy surface to which coils 
will not readily adhere when putting them in place. The 
same commercial sizes are available. 

Red rope paper of a good hemp rope stock, selected to 
avoid pin holes and metallic particles is used in conjunction 
with parchment paper and other insulations in building 
up layer insulations. Treated with hot oil red rope paper is 
excellent for troughs, slot insulation, etc. Where heavy bar 
wound armature coils are used which are subjected to steam 
mold bonding shellacked red rope paper is usually used. The 
sizes listed for express parchment apply equally to these red 
rope papers. The 5- and 9-mil sizes of shellacked red rope 
paper are the most used. Red rope paper varnished and 
baked is used principally for slot insulation. As a rule it is 
made only in 5- and 9-mil thicknesses in sheets 30 in. by 55 in. 

Shellacked bond paper is bond paper treated with a medium 

bodied shellac solution. It is used for separators in steam 

molds and is usually obtainable in rolls of any desired width. 

The thickness is 0.010 in. or 10 mils. Asbestos paper is used 

u 



162 ARMATURE WINDING AND MOTOR REPAIR 

in making layer insulations where heat resistance is needed. 
This material must be purchased under specifications having a 
test or inspection clause in order to guard against the presence 
of conducting particles of iron oxide. It is furnished usually 
in rolls 36 in. wide and in the following thicknesses: 6 mil, 
15 mil and 20 mil, J 2 m -> KG in. and ^ in. Asbestos paper 
treated with hot oil and oven cured is used in railway motor, 
field and armature coils and in machinery operating at high 
temperatures. 

When the asbestos paper is treated with a special black 
plastic varnish and oven cured it becomes a great moisture 
proof insulation and is used in forms for cores or spools. It 
is used as a moisture-proof, heat-resisting insulation for motors 
intended for naval, mine or similar services. 

Coil and Slot Insulation Used in One Large Repair Shop. 
The following grades and thicknesses of coil and slot insulation 
is carried in stock and used by one large repair shop where all 
types, makes and sizes of motors are rewound. 

Fish Paper: In sheets 4, 7, 10, 15 and 23 mils thick. 

Fish Paper with Mica Splittings: In sheets 13 mils thick. 

Fullerboard: In sheets 7, 10, and 15 mils thick. 

Shellacked Fullerboard: In sheets 7, 10 and 15 mils thick. 

Treated Cement Paper and Mica Splittings: In sheets 14 mils 
thick. 

Barren Paper: In sheets 3 and 5 mils thick. 

Empire Cloth: In sheets 9 mils thick. 

Cotton Duck: In sheets 12 mils thick. 

Cotton Tape: In widths % and 1}^ in., 7 mils thick. 

Treated Cloth Tape: In width % in., 8 mils thick. 

Under the different headings of Chapter III dealing with the 
windings of particular machines, details are given concerning 
the uses of the different insulating materials described here. 

Micartafolium. A special insulating material has been 
developed by the Westinghouse Electric & Manufacturing 
Company which is known as micarta. In the form of 
micartafolium it can be used as a wrapping. This insulation 
is used on the larger alternating-current armature windings, 
particularly those of alternating-current turbo-generators. 
It is also used for direct-current armature windings. For 



INSULATING COILS AND SLOTS 163 

commutators it is used between the bottom of the segments 
and the sleeve on the driving spindle. It is also used in 
direct-current machines for flash guards between the com- 
mutators and the armature winding of large units. For 
insulating studs and the capping of nuts and bolt heads it 
has also been found satisfactory. This type of insulation 
is made in sheets, blocks and tubes. 

Thickness of Insulation Required in Slots. Prof. Alfred Still 
in his book on Electrical Design, under the heading of slot 
lining points out that the insulation may be placed around 
the individual coils or in the slot before the coils are inserted. 
Part of the insulation may also be placed around the coils 
and the remainder in the form of slot lining. The essential 
thing is to have sufficient thickness of insulation between the 
cotton-covered conductors and the sides of the slot. In this 
connection the following figures are given by Prof. Still for 
the thickness of slot lining required for machines of different 
voltages. The thickness in inches is for one side only of the 
slot, and is as well the thickness that should be provided be- 
tween the upper and lower coil sides in the slot. 

Oper i ting voltage Slot insulation one side 

for direct-current machines 

AI B 

Up to 250 0.035 inch 

Up to 500 . 045 inch . 035 inch 

Up to 1000 0.060 inch 0.055 inch 

Up to 1500 0.075 inch 

For alternating-current machines 

500 to 1,500, same as for direct-current machines 
Up to 2,000 0.080 inch 0.090 inch 

Up to 4,000 . 120 inch . 130 inch 

Up to 8,000 0.190 inch 0.185 inch 

Up to 12,000 . 270 inch . 220 inch 

Insulation of Formed Coils. The practice of most motor 
manufacturing companies is to insulate coils for two or three 
ranges of voltage. In making changes in the connections of 

1 Prof. Alfred Still Principles of Electrical Design. 

2 H. M. Hobart Design of Polyphase Generators and Motors. 



164 ARMATURE WINDING AND MOTOR REPAIR 

induction motors, therefore, where the changes involves an 
increase in voltage, it is important to know that the changes 
do not subject the coils to a voltage outside the range for 
which they were insulated. This precaution does not, of 
course, apply when reconnecting for a lower voltage. 




FIG. 119. Diamond shaped coil insulated and impregnated with compound 
ready for use (Westinghouse Electric & Mfg. Company). 

The practice of one large manufacturer is to insulate coils 
for three ranges of voltage as follows: 

/. Direct-current and Alternating-current Group Coils for Vol- 
tages up to 220. 

For plain coils other than phase coils : 

1. Wound with double cotton-covered wire. 

2. Binding paper over slot side, 0.003 inch thick. 

3. 1^ turns treated cloth on slot side, 0.008 inch thick. 

4. Cotton tape, one-half lapped all around, 0.004 to 0.007 inch 
thick. 

5. Entire coil dipped in insulating compound and baked. 

For phase coils the same insulation is provided and in addition a 
layer of empire cloth and tape over each end of the coil up to the 
straight sides. The diamon'd points are painted red to dis- 
tinguish these coils from plain coils. Phase coils are used at 
beginning and end of each phase group. 

II. Direct-current and Alternating-current Group Coils for Vol- 
tages up to 500. 

For plain coils other than phase coils: 

1. Wound with double cotton-covered wire. 

2. Binding paper over slot side, 0.003 inch thick. 

3. 1^ turns mica and fish paper on slot side, 0.012 inch thick. 
This is made up of fish paper, 0.004 inch; three layers mica 
splittings, each 0.001 to 0.003 inch thick ; Japanese paper, 0.001 
inch thick. All shellacked together. 



INSULATING COILS AND SLOTS 165 

4. Cotton tape, one-half lapped all around, 0.007 inch thick. 

5. Entire coil dipped in insulating compound and baked. 

For phase coils the insulation of ends is the same as for 220 volts of 
the first group. 

Ill . Alternating-current Group Coils for Voltages from 500 to 
2300. 

For plain coils other than phase coils : 

These coils are insulated the same as group II except 3^ turns 
instead of 1^ turns of fish paper and mica are used. A layer 
of treated cloth on the ends of the coils is also used. 

For phase coils two layers of treated cloth and a layer of cotton tape 
are added. 

Insulation for Coils used in 240-Volt and 500-Volt Direct- 
current Machines. The following recommendations are given 
by Prof. Alexander Gray (Electrical Machine Design, Chapter 
IV on Insulation) for the windings of 240- and 500- volt direct- 
current machines. 

For a 240-volt, double-layer winding with two turns per coil 
and four coil sides (eight conductors) per slot, the conductors 
being of strip copper wound on edge: 

(a) After the copper has been bent to shape, tape it all over with one 
layer of half-lap cotton tape 6 mils thick. This forms the insulation 
between the adjacent conductors in the same slot. 

(6) Tape together the two coil sides when they form one group (as in 
this case) with one layer of half-lap cotton tape 6 mils thick all around 
the coils. This forms the end-connection insulation and also part of the 
slot insulation. 

(c) Bake the coil in a vacuum tank at 100C. (212F.) so as to expel 
all moisture, then dip it into a tank of impregnating compound at 120C. 
(248F.) and leave it there long enough to become saturated with the 
compound. 

(d) Put one turn of empire cloth 10 mils thick on the slot part of the 
coil and lap the ends. This insulation should extend % inch past the 
ends of the slot. 

(e) Put one turn of insulating paper 10 mils thick on the slot part of 
the coil and lap it over at the ends. This paper should be long enough 
to also extend past the edge of the slot % inch on each end. It is not 
put on for insulating purposes but to protect the other insulation which 
is liable to become damaged when the coils are being placed in the slots. 

(/) Heat the coil to 100C. (212F.) and then press the slot part to 
shape while hot. The heat softens the compound and the pressing forces 
out all excess of compound. The coil can then be allowed to cool while 



166 



ARMATURE WINDING AND MOTOR REPAIR 




INSULATING COILS AND SLOTS 167 

under pressure and will come out of the press with such a shape and size 
that it will slip easily into the slot. 

(gr) Dip the ends of the coil in elastic insulating varnish. 

For a 500-volt, double-layer winding having five turns per 
coil, and thirty conductors per slot, with the conductors covered 
with double-cotton coverings: 

(a) Put one turn of insulating paper 5 mils thick around the slot 
part of the two conductors that form the individual coils. This paper 
should extend % inch beyond the edge of the slot at each end. It forms 
part of the insulation between the individual coils in the same slot and 
also part of the insulation from winding to core. 

(6) Put one turn of empire cloth 10 mils thick around the three coils 
that form one group and lap it over at the ends. Allow this insulation 
to extend as before % inch at each end of the slot. 

(c) Put one turn of paper 5 mils thick on the slot part of the coil and 
lap it on the top. As in the other cases allow the paper to extend over 
the slot length % inch. 

(d) Tape the ends of the groups of three coils with one layer of half- 
lap cotton tape 6 mils thick and carry this tape on to the paper for ^ 
inch to seal the coil. 

(e) Wind the machine with these coil groups, putting a lining of paper 
10 mils thick in the slot and then hold the coils down with band wires. 

(/) Place the armature in a vacuum tank and bake it at 100C. (212F.) 
to expel moisture, then force impregnating compound into the tank at a 
pressure of 60 Ib. per square inch and maintain this pressure for several 
hours until the winding has been thoroughly impregnated. 

(g) Rotate the armature while it is still hot at a high speed so as to 
get rid of the excess of compound which will otherwise come out when the 
machine is carrying a heavy load. 

(h) Paint the end connections with elastic finishing varnish, taking 
care to get into all the corners. 

Coil Insulation for Induction Motor Windings. The fol- 
lowing recommendations are given by Prof. Alexander Gray 
(Electrical Machine Design, Chapter XX on A.-C. Insulation) 
for the coil and slot insulation of induction motors : 

For 440-volt, wire-wound coils of double-layer windings: 

(a) Double cotton covering on the conductors. 

(6) A layer of empire cloth six mils thick between horizontal layers of 
conductors. 

(c) One turn of paper 10 mils thick on the slot part of the coil to hold 
the conductors in layers. 

(d) One layer of half-lapped empire cloth tape six mils thick all around 
the coil. 



168 ARMATURE WINDING AND MOTOR REPAIR 

(e) One turn of paper 10 mils thick on the slot part of the coil to protect 
the empire cloth. 

(/) One layer of half-lapped cotton tape six mils thick on the end con- 
nections to protect the empire cloth. 

(g) The coil should be baked and impregnated before the paper and 
cotton tape are put on and dipped in finishing varnish after they are all 
put on to make it water and oil proof. 

For a 2200-volt induction motor with strip copper coils and 
a double-layer winding: 

(a) One layer of half -lapped cotton tape six mils thick on each conduc- 
tor to form the insulation between the conductors. 

(6) One layer of half-lapped cotton tape six mils thick all around the 
coil to bind the conductors together. 

(c) One turn of micanite 20 mils thick on the slot part of the coil. 

(d) Two layers of half -lapped empire cloth six mils thick all around the 
coil. 

(e) One turn of paper 10 mils thick on the slot part of the coil to protect 
the empire cloth. 

(/) One layer of half -lapped cotton tape six mils thick on the end con- 
nections to protect the empire cloth. 

(g) Bake and impregnate the coil before the paper and the last taping 
of cotton tape are put on. After that, the slot part of the coil should be 
hot pressed and allowed to cool under pressure. Then the coil can be 
dipped in finishing varnish to make it water and oil proof. 

Coil and Slot Insulation Employed by a Large Manu- 
facturer. The following insulation for coils and slots repre- 
sents the practice of a large manufacturer specializing in the 
construction of motors and generators in a wide range of 
sbes and for all commercial voltages. 

I. For Small Direct-current Machines Using Wire-wound Coils: 

1. Coils are wound with double cotton-covered wire. 

2. Dipped in varnish and baked to about 100C. (212F.) until coils 
are free from stickiness. 

3. Taped with 6-mil linen tape. 

4. Coils are then placed in armature and soldered to commutator 
risers. After cleaning, the armature is dipped or sprayed with 
varnish and baked. 

5. The final operation is to spray with shellac. 

6. For slot insulation 10-mil pressboard is used. 

II. For Engine Type Direct-current Armatures Using Strip Copper Coils: 

1. Copper strips are taped with 6-mil linen tape, % lapped. Width 
of linen tape is %, 1 or 1 % inch, depending upon size of the coil. 

2. Coil is then dipped in good insulating varnish and baked until 
free from stickiness. 



INSULATING COILS AND SLOTS 



169 



3. When coils are used in groups, the groups are taped with 6-mil 
linen tape % lapped. 

4. Operation two is then repeated. 

5. Coil is now ready to be inserted in armature slots. When arma- 
ture is completed, and bands are on but before assembling in 
frame, the windings are saturated with air-drying varnish or it 
is baked until free from moisture. As a rule, machine is well 
baked by running under full load. It can then be sprayed with 
air drying varnish. 

6. For slot cells, 10-mil pressboard is used not so much for insulation 
as for mechanical protection. 





FIG. 121. At the left, armature coil insulated for a 250-volt direct-current 
motor. At the right, stator coil insulated for a 220-volt induction motor 
(Crocker- Wheeler Company}. 

Variation from above practice depends upon the voltage. For high 
voltages, say 550 volts, the coil will perhaps be taped with 9-mil oil 
muslin half lapped and an additional varnish and baking treatment. 

For mill motors the coils are usually insulated with flexible mica. The 
coil is heated in a form and connected in the secondary of a transformer. 
While the current passes through the coil, it is tightened so as to give 
the proper thickness, alter which it is insulated with asbestos tape. 
Asbestos tape is used because of the severe duty to which these machines 
are subjected and because they are completely enclosed. 

III. General Method for Insulating Direct-current Field Coils: 

1. Coils are wound with double cotton-covered wire. 

2. The coil is then taped with 6-mil linen tape about % lapped on 
the outside of coil. Width of tape one inch. 

3. Coils are next placed in vacuum tank until free from moisture. 



170 ARMATURE WINDING AND MOTOR REPAIR 

4. Dipped in good insulating varnish and baked. Or if the varnish 
has air drying qualities, the coils are dried in air until tree from 
stickiness. 

IV. Insulation for Stator Coils of Alternating-current Machines for 600 
Volts and Under Using Wire-wound Coils: 

1. Coils are wound with double cotton-covered wire. 

2. Heated and dipped in insulating varnish and baked. 

3. Operation two repeated. 

4. Taped with 6-mil linen tape half lapped. 

5. Coil is then shaped. 

6. Dipped in varnish and baked. 

7. Operation six repeated. 

8. Operation four repeated. 

9. Dipped in good moisture-proof varnish and baked until all 
crevices are well filled. 

10. For slot insulation 10-mil pressboard is used. 

V. Insulation for Stator Coils of Alternating-current Machines, Made up 
of Strip Copper: 

1. One-turn coils are taped with 6-mil linen tape one inch wide, 
half lapped. Two turn coils are taped with one turn as above. 
Three turn coils have middle turn taped. Four turn coils have 
first and third turns taped. 

2. Coil is then taped with 6-mil linen tape half lapped. 

3. Dipped in insulating varnish and baked. 

4. Operations seven, eight and nine as for wire coils are then applied. 

5. For slot insulation 10-mil pressboard is used. 

VI. Insulation for Coils Used in 2200-volt Machines: 

1. Coil is wound with double cotton-covered wire. 

2. Wrapped with 6-mil linen tape half lapped. 

3. Heated and dipped in insulating varnish and baked. 

4. Operation three repeated. 

5. Operation two repeated. 

6. Coil is then formed. 

7. Last tape is now removed from coil, which was put on in order 
to protect the first linen tape while the coil was being shaped. 

8. Wrapped with 9-mil oil muslin half lapped and brushed with best 
quality insulating varnish. 

9. Operation eight repeated. 

10. Wrapped with 6-mil linen tape half lapped. 

11. Coil is then dipped in insulating varnish and baked. 

12. Operation ten repeated. 

13. Operation eleven repeated. 

14. For slot insulation, 10 to 20-mil pressboard is used and for insu- 
lation between layers 10-mil pressboard. 

In making up coils for any kind of electrical machinery, the main 
thing to bear in mind is to apply such processes as to eliminate moisture 
from the coil. Even for 2200-volt machines, except in special cases, the 



INSULATING COILS AND SLOTS 



171 




S .S , 
CH _^ C 



u S 8 

S -o 8? 

I M 

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c as .a 

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111 



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172 ARMATURE WINDING AND MOTOR REPAIR 

practice of the company employing the insulating methods described is 
to use oil muslin and depend to a great extent upon the proper varnish 
treatment. After much experimenting with varnishes it has been found 
possible to wind coils that will withstand high voltages equally as well as 
insulated coils wound for the same voltages. 

Insulation of End Connections of Coils. In a double-layer 
winding, the voltage between the end connections of two 
coils where they cross each other at the ends of the slot may 
be about equal to the terminal voltage. Suitable insulation 
should be used at this point. A belt of cotton duct is usually 
used between the end connections for this protection, and also 
to protect the coils from mechanical injury by rubbing against 
each other. 

Also the slot insulation should be allowed to extend out each 
end of the slot a certain distance depending upon the machine 
voltage. Prof. Alexander Gray gives the following values as 
typical of good practice. 

Terminal voltage of machine Length of insulation out of slot 

Not over 800 . 75 inch 

800 to 2500 1 . 25 inches 

2500 to 5000 2.00 inches 

5000 to 7500 : 3 . 00 inches 

7500 to 11,000 4 . 50 inches 

Coils for alternating-current windings must be wound to 
stand the line voltage to ground. In a Y-connecJied machine 
the total insulation from copper to copper between two coils 
should stand about 1.7 times this value. The first coil of 
each phase in a Y-connected machine has a voltage equal to 
the line voltage to ground (volts between lines -s- \/3) against 
which to insulate and should be given a protection equal to 
the line voltage. The first and last coils of each phase group 
must also have extra insulation for protection at the points 
where they cross. These are called the phase coils. One 
layer of six-mil empire cloth on the end connections covered 
with six-mil cotton tape half lapped in addition to the coil 
insulation will give sufficient insulation to the phase coils 
except in cases of voltages past 2300. (See page 165 for 
phase coil insulation for 2300 volts.) 



INSULATING COILS AND\8JbQTS 173 

Phase Insulation when Reconnecting fro.nl Two-phase to 
Three-phase and Vice Versa. In reconnecting the winding 
of an induction motor from two-phase to - three-phase or 
vice versa and in reconnecting a winding for a different number 
of poles to change the speed, it is necessary to. rearrange the 
phase insulation because the spread of the coils per-phase, 
per-pole is being changed. 

Mica Insulation for Armature Coils. Mica has been found 
to be a first class insulating material. Its insulation re- 
sistance increases with temperature, a valuable characteristic 
for machines operating at high temperatures and in direct 
contrast with the properties of treated tapes, in which the 
insulation resistance and loss increases rapidly at temperatures 
above 100C. or 212F. It is unaffected by temperatures 
far in excess of those encountered in the modern, well-ven- 
tilated alternator. It is also impervious to the static dis- 
charges present in all high voltage machines. Furthermore, 
it is resilient and retains its resiliency indefinitely thus 
helping to hold the coil tight in its slot. 

Mica is a mineral obtained in the form of large crystals. 
These split readily into thin, parallel-sided laminae, or flakes. 
The flakes can be pasted uniformly on cloth or paper to 
facilitate handling and to provide a mechanical support dur- 
ing application. In the form of a " wrapper/' that is, pasted 
on large sheets of specially treated paper, mica is mostly used 
on the straight sides of each armature coil, to provide in- 
sulation between conductor and iron, the operating voltage 
of the machine determining the number of turns, or the thick- 
ness of this ineulation wall. 

All known insulating materials are relatively poor heat 
conductors. This is equally true of mica and treated tapes. 
Therefore, the tighter and the thinner the wall, the better the 
heat radiating characteristics of the coil. For the lower 
voltage machines the mica wrapper is applied as tightly as is 
possible by hand. For the higher voltage windings, 6600 
and above, where the insulation wall must be relatively thick, 
special, patented machines are used which apply the wrapper 
under heat and pressure, and finish it to a solid, compact wall. 

In general, all of the larger capacity generators have 



174 



ARMATURE WINDING AND MOTOR REPAIR 



relatively wide cores. Internal "hot spot" temperatures, 
considerably higher than those measurable by thermometer, 
exist. On all such machines, each conductor of the coil is also 
insulated with mica tape. 

Repairing Coils Damaged in Winding Process. It often 
occurs that the insulation of a coil becomes damaged from 
charing or from carelessness in the use of tools when laying 
the coils in the .armature slots. Such damage should be re- 
paired at once to prevent possible trouble later which will 
make the repair more difficult and expensive. To repair 
a damaged coil where the injury to it is only slight, the coil 
should be removed from the slot and all the insulation removed 
from the injured section. Then apply an overlapping wrapper 




FIG. 123. At top, one whole and two damaged Eickemeyer coils taken 
from an elevator motor. At bottom, fiber drifts, slot insulation, hammer, 
parallel jaw pliers and coil lifter. 

of treated cloth and around this a protecting covering of cot- 
ton tape. Glue down the ends of the tape securely and apply 
a good heavy coat of shellac. The shellac can be dried quickly 
by touching a lighted match to it when the alcohol in which 
it is dissolved will burn with a blue flame. Care must be 
taken that the tape is not burned in doing this. In such a 
case the flame will turn yellow and should be smothered at 
once. The heat of the burning alcohol is not usually sufficient 
to burn the tape. After this the coil is dry and can be put 
back in the slot. While this method of repair can be safely 
used in small armatures, it is not good enough for large arma- 
tures. In the latter case a new coil should be used or the old 
one stripped and completely reinsulated, dipped and baked. 



INSULATING COILS AND SLOTS 



175 



On account of the stiffness of the insulation on the terminals 
of formed coils, before connections are made to the commutator 
it is a good plan to soften the leads with ^armalac" or a similar 
armature compound at the points where the leads leave the 
bottom coils in the slots. This will prevent breaks while 
handling the leads. 

Voltage to use when Testing Coil and Commutator Insula- 
tion. Under certain conditions the difference of potential 
between coils and the iron core of a machine may be equal to 
the terminal voltage. Under abnormal operating conditions 
it may even be greater. It is important, therefore, that the 
insulation of coils and commutator shall be sufficient to stand 
a voltage considerable larger than the terminal voltage of the 
machine without developing grounds. For this reason a 
high-voltage test is made on armature windings of both direct- 
current and alternating-current machines and the commutator 
of the former before the winding is finally completed. 

The following values of test voltages that should be applied 
are given by Prof. Alexander Gray (Electrical Machine Design, 
page 34) based upon the standardization rules of the American 
Institute of Electrical Engineers. 



Rated terminal voltage 
of machine 


Rated output of machine 


Testing voltage 


Up to 400 


Under 10 kw. 


1000 


Up to 400 


10 kw. and over 


1500 


400 to 800 


Under 10 kw. 


1500 


400 to 800 


10 kw. and over 


2000 


800 to 1200 


Any 


3500 


1200 to 2500 


Any 


5000 


2500 and over. 


Any 


Double-rated voltage. 



Some insulating materials will withstand very high voltages 
before used on coils. For instance, a good quality of oiled 
muslin will withstand as high a test voltage as 1500 to 2000 
volts per mil for 9-mil thickness, however, when applied to 
coils, its insulating properties will diminish because of handling. 
This is, of course, the reason why so much insulation is used 
on coils, to protect them from becoming grounded. For 



176 ARMATURE WINDING AND MOTOR REPAIR 

coils that are to be used for machines below 2200 volts, no 
steps are usually taken to test coils before placing them in 
the machine. However, for voltages over 2200, the coils are 
tested before they are placed in the slots. For instance, coils 
that are to be used on machines of 6600 volts, are tested for 
ground at 20,000 volts by wrapping them with tin foil before 
placing them in the slots. Then they are tested with 15,000 
to 16,000 volts after being placed in the slots. Finally a test 
of at least twice normal voltage is made for one minute. 

Field Coil Insulation. For the insulation of field coils, 
10 mil paper, J^g inch cardboard and 6 mil tape can be used 
where the field coil has a cardboard spool. The insulation 
can be applied in the order named using 2 layers of tape and 
paper with the paper next to the wires on the spool. The 
coils should then be baked in a vacuum tank and impregnated. 

Varnishes and Impregnating Compounds for Coils. The 
varnish used over the outside of insulated coils should be 
water, oil and acid proof and dry quickly in air forming a hard 
smooth surface. Such varnishes are made by a number of 
manufacturers. 

Compounds for use in impregnating coils are also available 
in the open market and are usually an asphaltum or a paraffin 
base dissolved in a suitable thinning solution. It is important 
that the material will not attack copper, iron or insulating 
materials used and form a solid at all temperatures below 
212F. without contraction when changing from the fluid to 
the solid state. This material should not be applied at 
temperatures above the breakdown of cotton materials, 
that is, above a temperature of 248F. 

Because the desirable characteristics for a perfect varnish 
cannot be combined into one compound, a number of varnishes 
have been developed, each having its own characteristic. 
The purposes of some insulating varnishes of the Sherwin- 
Williams company are indicated in the accompanying table. 
Where more than one varnish can be successfully used the 
different types are indicated as first, second and third choices. 

All of the varnishes mentioned in the table are of the 
baking type. However, insulating varnishes in general 
may be divided into several general classes, such as clear var- 



INSULATING COILS AND SLOTS 



177 



nishes, black varnishes, baking varnishes and air-drying 
varnishes. The most marked difference between the clear 
and the black varnishes is the color, but owing to fundamental 
differences in the characteristics of the ingredients entering 
into their composition, there are also some differences in the 
physical properties of the varnishes themselves. As a general 
rule clear varnishes possess greater mechanical strength and 
resist oil better than black varnishes. An exception to this is 
the black elastic baking varnish shown in the table. Where 
extreme mechanical strength is required as on small high-speed 
armatures clear varnishes are almost always used. 

TABLE SHOWING SUITABILITY OF INSULATING VARNISHES 



Characteristics of clear and black 
baking insulating varnishes and 
uses for which they are 
recommended * 


Clear varnishes 


Black varnishes 


Clear 
quick 
baking 


Clear 
quick 
elastic 
baking 


Clear 
elastic 
baking 


Black 
quick 
baking 


High 
Heat 
resist- 
ing 
baking 


Black 

plastic 
baking 


Black 
elastic 
baking 


Dielectric strength 
Mechanical strength 


3 
3 
3 


1 
2 

1 
2 
3 

2 

1 
1 
1 
2 
2 
1 
1 
1 

1 
1 
1 

1 
2 


1 
2 
1 

1 
1 
2 

2 
2 
2 
1 

1 

1 

2 
2 
2 
2 
2 


3 
3 
3 

2 
2 
4 

3 

2 
3 
3 
3 
3 

1 
3 


2 
4 
3 

3 
2 

2 

3 
3 

3 
2 
2 

1 
2 

2 


2 
5 
2 
1 
4 
1 
1 

2 
3 

3 
2 
1 

1 
1 


1 
2 

1 

1 
2 
1 

2 
2 
2 
1 
1 

1 

2 
2 
2 
2 
2 


Flexibility 


Plasticity 


Oil resistance 


2 
2 

4 

3 

2 
3 
3 
3 
3 

1 


Water resistance 




Treating cloth, paper and thin fib- 
rous materials 
Treating fullerboard and heavy 
fibrous materials 


Small high-speed armatures 
Intermediate-speed armatures 
Large low-speed armatures 
Field and stator coils 


Automobile-starting motors 
Vacuum-cleaner motors 


Washing-machine motors 


Street railway and electric locomo- 
tive motors 




Fan motors 


3 


Magnetos and induction coils 
High-potential apparatus 


Transformers 


Average repair shop conditions. . . 



* Numbers indicate order of suitability. 

Black varnishes are not quite so strong mechanically 
as clear varnishes but are sufficiently strong for most purposes. 



178 ARMATURE WINDING AND MOTOR REPAIR 

On stationary windings, as on alternating-current stator 
windings, the varnish is not subjected to centrifugal stresses 
and there is no advantage in using a clear varnish. Certain 
black varnishes are made from plastic materials and have the 
ability to withstand long-continued heating without hardening. 
Black varnishes as a rule are cheaper than clear varnishes and 
are more commonly used for that reason. 

The chief difference between baking varnishes and air- 
drying varnishes is in the proportion of oxidizing ingredients 
contained. The baking varnishes are tougher, more elastic, 
more resistant to oil and water, and have longer life under 
heat. Speed in drying is always accomplished at the expense 
of these characteristics, and the air-drying varnishes are less 
durable and elastic than the baking varnishes. The air- 
drying varnishes find their principal field of usefulness on 
apparatus where severe conditions of usage are not encountered 
and for quick repair work. 

Characteristics of Insulating Varnishes. An important 
factor concerning insulating varnishes is that the dielectric 
resistance increases directly with the length of baking or dry- 
ing period, the slow varnishes imparting the highest degree of 
insulation and flexibility and producing a tough, flexible film 
which may also be depended upon for mechanical strength and 
extreme durability. Black varnishes are claimed to be 
better for work where a transparent coating is not absolutely 
essential because they produce a more flexible and highly 
insulating film than clear varnishes of the same class. Var- 
nishes should not crystallize under prolonged vibration, and 
their mechanical structure should be elastic and homogeneous. 
The accompanying table (page 179) gives the characteristics, 
uses, drying time and solvents of several varnishes. 

Solvent Chart for Insulating Varnishes. Since varnish is 
usually sold in concentrated form care must be taken in 
dissolving to avoid wrinkles or stringy drip forming on coils 
when varnish is too heavy. Benzine is preferable to gasoline 
as a solvent but gasoline can be used. The solvent and varnish 
should be approximately the same temperature and neither 
should be under 60F. (15.5 C C.). The chart on page 180 
shows the percentage of 58 degrees benzine to be added to every 



INSULATING COILS AND SLOTS 



170 













3 


a 


a 


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CP 


CD 

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Q 


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bi 


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MU3 


bio 


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IB 


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M 


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s^ 














^, ^ 


Jj rH 


H 














^~-s 


^^^-> 





CO "t^ 


11 


resisting 


resisting 


b 

a 


durable . 


1 
"I 


resisting 




11 


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"2 


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11 


t? i 

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"5 ft ! 


Prevents ab 
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Water, oil a 
Heat cond 


Waterproof 




ft : 


a 










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180 



ARMATURE WINDING AND MOTOR REPAIR 







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^TABJO ogioadg pajisaQ 



INSULATING COILS AND SLOTS 181 

100 gallons of insulating varnish to bring it to the correct 
specific gravity. The most suitable gravity should be estab- 
lished by trial and the specific gravity kept constant at this 
value. With 63 degrees gasoline 15 per cent, less should be used 
than that shown and with 54 degrees benzine 10 per cent, more 
should be added. 

Method for Making Tape from Cotton Cloth. Maurice 
S. Clement has described the following method (Electrical 
Record, October, 1918) for making tape as used by a middle 
western electrical repair shop owing to a shipment of cotton 
tape being held up on account of war conditions. The 
repair job was urgent and the repairman was forced to 
make up an amount of cotton tape of different widths rang- 
ing from J-^ inch to 2 inches, sufficient to keep its main- 
tenance men going until the order arrived. After considerable 
thought, the following was decided upon as a temporary 
remedy. 

A bolt of white cotton cloth was purchased in a nearby dry 
goods store. The cloth was rolled tightly on a half -inch dowel 
and then marked off to the various widths on the outside. A 
band-saw was used to good advantage cutting the cotton cloth 
into tape; as this tape had no salvage edge, steps had to be 
taken to prevent it from unravelling. Both edges of each 
roll of tape were given a heavy coat of shellac and before it 
dried the alcohol was burned off. This has a sort of semi- 
baking effect which tends to strengthen the tape. It also 
prevents the tape from curling. 

Drying Out Insulation of Direct-current Generators (In- 
struction Book, Westinghouse Electric & Mfg. Co.). Drive 
the generator by a motor connected by a belt and short- 
circuit the armature beyond the ammeter using a very weak 
field excitation. If the generator is shunt wound, low voltage 
separate excitation must be employed; if compound wound 
the armature may be short-circuited through the series field 
coifs. Direct - current generators are very sensitive when 
operated as series machine and there is danger of generating 
an excessive current. Consequently this method should be 
undertaken only by experienced operators. 

The field coils may be dried by applying from some separate 



182 ARMATURE WINDING AND MOTOR REPAIR 

source of excitation approximately two-thirds of the normal 
direct-current voltage. 

There is always danger of serious injury to the windings 
when drying out with current since the heat generated in the 
inner parts is not readily dissipated; furthermore, coils con- 
taining moisture are much more susceptible to injury from 
overheating than when thoroughly dry. The temperature 
of all accessible parts should be carefully observed during 
the drying out process and never allowed to exceed 80C. 
(176F.), total temperature. Several hours or even days 
may be required for thoroughly drying out large machines. 
During the drying out process the temperature should 
not be allowed to drop below that of the surrounding 
air as moisture then condenses on the coil surfaces and the 
effect of the previous drying would be largely lost. At 
regular intervals during the drying out run, readings of the 
insulation resistance (see page 185) may be taken at regular 
intervals and plotted as a curve, using time for the horizontal 
scale and resistance for the vertical scale. The drying should 
continue until the resistance has begun to increase. If the 
insulation contains appreciable moisture the resistance will 
decrease during the first part of the drying out process. 

Heating windings by current is more effective than any 
process of heating from the outside, such as enclosing the 
machine and heating the air by resistance or fires, because in 
the former method the inside of the coils becomes hotter than 
the outside and .moisture is driven outward. With external 
heating the reverse is true. 

Drying Out Synchronous Motors and Generators. Syn- 
chronous motors and generators can be dried out by rotat- 
ing the motor or generator at any convenient speed and short- 
circuiting the armature beyond the ammeters. The field 
should be excited so that the desired heating current will flow 
in the armature winding. For windings of 2400 volts or lower, 
the temperature, as measured by thermometers properly 
applied to the hottest accessible part of the winding should not 
be higher than 80C. (176F.). For 6600-volt windings the 
temperature should not be higher than 75C. (167F.) and for 
11,000 to 13,200-volt windings not higher than 65C. (149F.). 



INSULATING COILS AND SLOTS 183 

The reason for specifying the lower temperature for the higher 
voltage windings is the greater difference in temperature be- 
tween the inside of the coil and the outside (when the tem- 
perature is measured with a thermometer) in the coils having 
the thicker insulation. 

If a low voltage (5 to 15 per cent, of normal) can be obtained 
from the taps on a transformer for example, the armature 
winding can be dried out by applying this low voltage to the 
armature terminals, the rotor remaining stationary. The 
field winding should be short-circuited and the temperature 
of the cage winding on the rotor should be watched. Less 
than normal current will be necessary on account of the 
absence of ventilation. 

For medium sized alternators and synchronous motors a 
satisfactory way of drying out both field and stator windings 
is to connect the machine to an alternating-current circuit 
and run as a motor with fields overexcited at zero power factor. 
This method is both cheap and effective. 

Drying Out Induction Motors. Small motors can be baked 
in ovens. The temperature should be raised gradually taking 
several hours to bring it to the maximum value which should 
not be more than 80C. (176F.) at the hottest point. The 
temperature should be maintained constant for from one day 
to a week depending on the size and voltage of the machine 
and the history of its exposure to moisture. Induction motors 
can also be dried by operation at no load on low voltage (the 
primary current and heating increases as the voltage is re- 
duced) or by a still lower voltage that will circulate a suffi- 
ciently heavy current with the rotor blocked. If a sufficiently 
low alternating-current voltage is not available, direct current 
may be used. There is always more or less danger of over- 
heating the windings of a machine when drying them with 
current as the inner parts which cannot quickly dissipate 
the heat generated in them and which cannot be examined, 
may get dangerously hot while the exposed and more easily 
cooled portions are still at a comparatively moderate tempera- 
ture. The temperature of the hottest part accessible should 
be measured during the drying out process and not allowed 
to exceed 80C. (176F.). 



184 



ARMATURE WINDING AND MOTOR REPAIR 



Insulation Test. During the drying out of a machine, 
insulation resistance tests should be made at regular intervals 
and plotted in the form of a curve using time on the horizontal 
scale and values of insulation resistance on the vertical scale. 
The drying out should be continued until the resistance 
reaches it proper value. The insulation resistance is at best 
only a rough guide in determining the condition of the ma- 
chine as to moisture and relative values in the same winding 
during a drying out run are of more value than the relative 
values of windings in other machines. 





500 to 600 Volt D.C. Circuit 




[ 






^ 




1 2 






L 


3 4 







Double throw, 
double pole switch 



To resistance to 
be measured 



Voltmeter 

FIG. 124. Connections of double-pole, double throw switch and 500-volt 
v voltmeter for measuring insulation resistance. 

A megger is also sometimes used for testing the condition 
of the windings during the drying out prcoess. 

The insulation resistance of a machine in good condition 
and at its operating temperature will usually not be less than 
the value given by the following formula: 

T , . . . Machine voltage 

Insulation resistance in megohms = ^ . , T ^ 

Rated Kva. + 1000 

For example a 1000-Kva., 11,000-volt motor should have an 
insulation resistance, if clean and dry, of 5.5 megohms. 
The insulation resistance of field windings will, in general, be 
much higher in proportion to the operating voltage than that 
of the armature. Since large armatures have much greater 
areas of insulation, their insulation resistance will be propor- 
tionately lower than that for small machines. The insulation 



INSULATING COILS AND SLOTS 185 

resistance of any machine will also be much lower when hot 
than when cold, especially when the machine is heated rapidly. 
Measuring Insulation Resistance. Insulation resistance 
may be measured with a megger or by the use of a 500-volt 
direct-current voltmeter and a 500 volt-direct-current circuit. 
Connect the voltmeter as shown in Fig. 124 and read first, the 
voltage of the line; then connect the resistance to be measured 
by throwing the double throw switch and read the voltmeter a 
second time. The insulation resistance is then calculated by 
the following formula: 

Insulation resistance (R) = , ^ MOfrOOO ' '; 

Where V is voltage of the line; v the voltage reading with 
insulation in series with the voltmeter; r the resistance of 
the voltmeter in ohms which is generally marked inside the 
instrument cover, and R the resistance in megohms. A meg- 
ohm is equal to one million ohms. 

If a grounded circuit is used in making the measurement, 
care*must be taken to connect the grounded side of the line to 
the frame of the machine to be measured and the voltmeter 
between the windings and the other side of the circuit. 



CHAPTER VIII 

REPAIR SHOP METHODS FOR REWINDING ALTER- 
NATING-CURRENT MACHINES 

I. WINDING SMALL SINGLE-PHASE MOTORS 

The complete stator winding of most small single-phase 
induction motors is made up of two windings; the main wind- 
ing of many turns of heavy wire and what is known as the 
"teaser" or starting winding. The latter is necessary because 
a single-phase motor is not self -starting and, therefore, requires 
some means of producing a rotating field to overcome this 
deficiency. This the starting winding does. It is of smaller 
wire than the main winding and of high resistance. 

The method of winding single-phase motors differs from^that 
used for other alternating-current motors in that a skein 
winding is often used. That is, the winding coil is in the 
form of a skein of wire which is looped many times through 
several slots to form a pole of the winding. The details of 
this method of winding as given in what follows, are based 
on articles that have appeared in the Electric Journal by 
G. I. Stadeker and C. A. M. Weber. 

Insulating Lining for Slots. The slots should be lined with a 
protecting cell of fish paper cut to fit the slot. Inside this a 
cell of treated cloth should be placed cut so that its edges will 
project about % inch beyond the entrance to the slot. In 
those slots which will contain both the main and the starting 
winding, an extra treated cloth cell should be inserted over 
the main winding to enclose the starting coils. End plates of 
fullerboard or fiber are used to insulate the core from the 
windings. The end connections of the main and starting 
windings should be separated by friction cloth. 

Winding the Skein Coil. In repairing a motor, the number 
of times the skein is to be looped through the slots and .the 
length of the skein can best be obtained from a skein taken 

186 



REWINDING ALTERNATING-CURRENT MACHINES 187 

from the burned-out machine. In removing the old winding 
care should be taken to preserve one entire skein if possible. 
If this is impossible a satisfactory scheme for the repairman 
is by trial with a single wire. This wire should be laid in the 
slots exactly as the skeins of wire will be laid, proper allowance 
being made for the building up of the skein ends from slot to 
slot. The wire should then be removed and measured. Make 
up a trial skein of this length and wind it in the slots. Cor- 
rections if necessary can be made on the next skeins made up. 
Inserting the Skein Coil in the Slots. After the skein length 
and distribution have been obtained from the old motor, the 




abode f g 

FIG. 125. Successive steps in applying a skein coil for main winding of a 
split-phase, GO-cycle, 4-pole, 24-slot induction motor. 

exact procedure in winding is as shown in Fig. 125 (a) to (g). 
The distribution of a 24-slot primary winding, indicating the 
number of times the main winding skein is wound into each slot, 
is shown in Figs. 126 to 129. The distribution in Fig. 126 is 
one commonly used. Other distributions* may be used, but in 
all skein windings the wires in any slot must be a multiple of 
the wires in the skein. 

A developed view of the primary or stator winding is shown 
in Fig. (125a), looking at the teeth with the first operation of 
putting the skein winding in slots 3 and 5 completed. The 
end of the coil thus formed should be firmly pressed against 
the core, using a rawhide or fiber mallet or a piece of smooth 
wood. A half twist is next made in the skein, as shown in 



188 



ARMATURE WINDING AND MOTOR REPAIR 



Fig. 125 (6), and the loop laid back over the winding and 
threaded into slots 2 and 6, as in Fig. 125 (c). The half twist 
is repeated, as in Fig. 125 (d), and the loop laid back in slots 2 
and 6 for the second time, as in Fig. 125 (e). This second 
half twist, in Fig. 125 (d), must be in the opposite direction to 
the first one (Fig. 125 (6)), to bring the same side of the loop on 
top. Otherwise a twist will be put in the skeins, which will 



Slot Number 
Main Winding 


1 


2 


3 


4 


fi 


8 


7 


8 


9 


10 


11 


12 


13 


U 


15 


16 


17 


18 


19 


20 


21 


22 


23 24 


Distribution 


1 


1 






-~ 


i 


i 


1 






1 


! 


i 


1 






i 


! 


1 


! 






1 1 


Starting Winding 
Distribution . 






1 


1 










] 


1 










1 


1 










1 


1 








J 






































L 





FIG. 126. Distribution of main and starting winding coils of a 4-pole motor 
showing skeins overlapped. 



Slot Number 1 23 i 5 6 78 9 10 11 12 13 14 16 16 17 18 19 20 21 22 23 24 



Main Winding 
. Distribution 



Starting Winding 
Distribution 



FIG. 127. Distribution to avoid overlapping of skeins. 



Slot Number 


1 


2 


9 


t 


r> 


6 


7 


i 


9 


10 


11 


12 


13 


11 


15 


16 


17 


18 


19 


20 


21J22 


23 24 


Distribution 


1.1 


1 


i 




i 


! 


i. 


_ 


1 




1 


2 


,. 


| 


1 




1 


> 


1.1 


1 


T. 


1 2 


Starting Winding 
Distribution 






i 


1.1 


i 










1.1 


1 








I 


1.1 


& 








i 


2 



FIG. 128. Distribution of an 8-pole machine. 



Slot Number 1234 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 



Main Winding 
Distribution 



Starting Winding 
Distribution 



FIG. 129. Distribution of an 8-pole consequent wound machine. 

make it hard to winfl smoothly, especially if it is looped back 
and forth many times. The half twist in Fig. 125 (/) is made 
in the same direction as that in Fig. 125 (6) By looping the 
turns into slots 1 and 7 the winding of the skein is completed. 
The winding of the second and subsequent coils is exactly the 
same as the first. The completed winding for a four-pole, 
24-slot machine is shown in Fig. 130 (at top). 

In split-phase starting, squirrel-cage motors the starting 
winding is connected across the line until approximately two- 



REWINDING ALTERNATING-CURRENT MACHINES 189 



thirds synchronous speed is reached, when its circuit is auto- 
matically opened by a centrifugally-operated switch. The 
center of the starting winding is between the pole centers of 




FIG. 130. At top, complete main winding for a 4-pole, 60-cycle, split- 
phase induction motor. At bottom, complete main and starting winding for 
the same motor. 

the main winding. Its distribution and the length of the 
skeins must be determined by trial as explained for the main 
winding. The starting winding is a resistance winding, con- 
sequently it is very important that its re?istance in the 




FIG. 130 (a). Dissembled view of a single-phase repulsion induction motor. 

motor be the same as it was originally, which makes the length 
of the skein important. The distribution of both the main 
and starting windings and the number of times the skein is to 



190 ARMATURE WINDING AND MOTOR REPAIR 



be wound into each slot are shown in Fig. 130 (lower diagram). 

After all the slots have been filled the coils should be forced 
into position with a fiber drift, the insulating cells folded in 
and fiber wedges driven in the slots. 

Winding for a Repulsion- start Motor. For repulsion- 
starting, induction-running motors the primary winding is 




1234 

FIG. 131 (a), (6), (c). At left, primary connections for a single-phase, 
repulsion-starting induction motor. In center, connections for a 4-pole, 
split-phase motor. At right, same as in center but for a series-parallel 
connection of coils. 

In the diagram at the left, N denotes neutral points. For a 220- volt circuit leads 2 and 
3 should be connected together and leads 1 and 4 to the line. For a 110-volt circuit 
leads 1 and 2 should be connected in parallel to one line and leads 3 and 4 to the other. 
In the center diagram the coils are connected in series. To obtain clockwise rotation, 
leads 1 and 2 should be connected to one line and leads 3 and 4 to the other. For counter 
clockwise rotation, leads 1 and 3 should be connected to one line and leads 2 and 4 to the 
other. 

complete, as shown in Fig. 130, and the coils are connected 
together, as indicated in Fig. 131 (a). Four leads arc brought 
out, so that these motors may be connected externally for 
either 220 or 110 volts. 

Winding Small Motors 
by Hand . A method 
sometimes employed in 
winding small motors of 
the induction type is 
known as hand-winding. 

FIG. 132. Hand-winding method for main The Conductors are 
coils of primary shown in Fig. 130. 




at a time, beginning at the center of a pole, as shown in 
Fig. 132. This method is mostly used when the number of 
turns in the slots have no relatively large common divisor, 
thereby eliminating the skein-winding method. The hand- 
winding method should not be employed for the starting 



REWINDING ALTERNATING-CURRENT MACHINES 191 

winding of a split-phase motor, because the resistance of the 
starting winding would not always be that required. A 
skein winding should be used. 

Windings for Odd Frequencies. It frequently happens 
that small motors must be wound for odd frequencies, such as 
125, 133, and 140 cycles on standard 60-cycle motor cores. 
The number of poles of such motors is usually large, resulting 
in a small number of slots per pole. Using a 24-slot primary, 
wound eight poles, the slots per pole would be three and the dis- 
tribution of the winding would be as shown in Fig. 128. Such 
a winding is very difficult to wind and in a case of this kind, 




1231 



FIG. 133 (a) and (6). Connections for a single-phase, 8-pole, 24-slot, serie* 
connected, consequent-pole, induction motor shown at left. At right, same 
for series-parallel connection . 

a consequent-pole winding would be used with a distribution 
as shown in Fig. 129. This motor would be wound in the 
same manner as shown in Fig. 125 (a) to (g), but all the coils 
would be connected with the same polarity as indicated in 
Fig. 133. 

Connections for Main and Starting Windings. The coils 
of each winding are connected in series, as shown in Fig. 134. 
The leads of the starting winding are interrupted by a cir- 
cuit opening device. Two circular stationary segments are 
used in one design of motor insulated from each other and 
from the frame and are mounted side by side on the bearing 
housing. At the start they are short-circuited by the rotat- 
ing shoes which close the circuit of the starting winding until 
the speed reaches the point at which centrifugal force throws 
them off. This is ordinarily at about half speed. The main 



192 ARMATURE WINDING AND MOTOR REPAIR 



Terminal Leads==i? 



and starting windings are usually connected in parallel outside 
the machine. The direction of rotation is determined by the 
way the connections are made. To reverse the direction 
of rotation, interchange either the two main winding leads or 
the two starting leads; adopting the most convenient method. 

Testing Out Small Induction 
Motor Windings. The wind- 
ings should be tested for 
grounds with 1000 volts for 
one minute. Each winding 
should also be tested for short 
and open circuits by applying 
alternating current through a 
wattmeter. An excessive read- 
ing indicates a short circuit, 
while no readings show an open 
circuit. Polarity can be tested 
by applying direct current to 
the windings and testing with 
a compass. % Adjacent poles 
should attract opposite ends 
of the needle. If the wind- 

FIG. 134. Connections * for single- j are correct fog e nd COn: 

phase motor windings. . 

nections should be dipped in 

a quick drying plastic varnish, which serves not only as an 
insulating medium, but excludes all dust and serves to stiffen 
the windings and make them moisture proof. After dipping, 
the windings should be thoroughly dried in an oven. 

Windings for Small Polyphase Induction Motors. The 
method of winding a small polyphase motor is similar to the 
winding of a single-phase motor. The slots are insulated with 
fish paper and treated cloth cells, and the skeins, which are 
also similar, are inserted into the slots as shown in the winding 
diagrams of Figs. 135 and 136. Each skein or group of coils 
overlaps the preceding one at the ends, as shown in Fig. 137, 
the ends of the coils being separated from one another by a 
layer of treated cloth. To produce a symmetrical winding, it 
may be necessary to remove half of the first skein before the 
last one can be put in place, in order to make them overlap 




Line ' , 



REWINDING ALTERNATING-CURRENT MACHINES 193 

in regular order. As the electrical characteristics of the ma- 
chine would not be changed by so doing, the last skein can be 
allowed to overlap two skeins, while the first skein is over- 
lapped by two others. 



Phase A 




FIG. 135. Winding diagram and connections for stator of small 3-phasc 

motor. 

The skeins are connected according to the connection dia- 
gram shown at the right in Figs. 135 and 136. The connections 
determine the phase and polarity of the machine. For in- 
stance, an armature having twelve skeins or groups of coils, 



Phase A 




Fia. 136. Winding diagram and connections for stator of a small 2-phase 

motor. 

can be connected for six poles, two-phase, or eight poles, 
three-phase. 

No starting winding is required for polyphase machines, and 
since they have a good starting torque there is no need for a 

13 



194 ARMATURE WINDING AND MOTOR REPAIR 



friction clutch on these machines. They should be tested 
for breakdown between phases, and from each phase to ground, 
and for short and open circuits, as described for single-phase 
machines. 




FIG. 137. Stator of a small 3-phase motor completely wound with skein coils. 



II. WINDING SMALL INDUCTION MOTORS WITH 
COILS IN PARTIALLY CLOSED SLOTS 



FORMED 



Like the armatures of direct-current machines, the stators 
of small induction motors are made with partially closed and 
with open slots. The coils used with partially closed slots 
are usually either of a diamond shape or of a square shape 
such as shown in Fig. 138. These 
coils are wound on forms as in the 
case of direct-current coils using wire 
having a double-cotton covering. The 
coils when made up are not insulated 
as a whole, since the turns of the coil 
must be inserted in the slot one at a 
time. The insulation between coils 
and the core must therefore be placed 
in the slots before the coils are in- 
serted. The character of insulation 
required and the methods of placing 
the coils in the stator of motors 
having partially closed slots and for those having open slots 
are outlined in what follows according to recommendations 
by a writer in the Electric Journal, Vol. VII, No. 9. 




FIG. 138. At left, a 
basket or mush type coil. 
At right a diamond type coil. 



REWINDING ALTERNATING-CURRENT MACHINES 195 




Fleh Paper Cell 



I- Fttx* Strip 



FIG. 139. Slot insula- 
tion for a small induction 
motor having partially 
closed slots. 



Insulation for Slots. Clean the slots with a blast of air to 
remove all burrs and rough edges. Then cut a fish-paper cell 
large enough to just line the slot up to the opening and so as 
to extend at each end of the slot about three-quarters of an 
inch. This fish-paper cell possesses sufficient mechanical 
strength to protect the inner layer or cell of insulation which 
should be of treated cloth that have high insulating strength 
such as empire cloth or micanite. The inner cell should be 
cut large enough to extend beyond the protecting cell and the 
ends of the slot about one-quarter of an 
inch and up through the opening in the 
slot about an inch. The projections of 
this cell serve as a guide when inserting 
the coils and protect the wires from 
any possible abrasion when being in- 
serted. The winding and the core 
should be insulated at the ends of the 
slots by a fullerboard ring and a fiber 
strip at each end. This will protect 
the overhanging ends of the coils from coming into contact 
with the core. 

Basket Coils One Coil per Slot. Basket coils are wound 
to a shape corresponding only approximately to their final 
shape, as shown at the left, Fig. 138 It is important only 
that the total length of the loop be correctly proportioned so 
that the ends may be formed to the proper shape after the 
wires are in the slot. When a suitable form is not available, 
the coil may be wound around two pegs spaced the proper dis- 
tance apart. Small pieces of tape should be fastened around 
the coil at convenient points so that the wires may be 
held together while they are being threaded into the slot. 
Basket coils are largely used with a one-coil per slot wind- 
ing. This means that each side completely fills one slot, 
so that for a 48-slot core only 24 coils are required. In 
this class of winding, one end of the coil has to drop below 
the bottom of the slot in order to permit the adjacent coil 
to pass over it, while the other side of the same coil remams 
on a level with or only slightly below the top bore of the 
slot. 



196 



ARMATURE WINDING AND MOTOR REPAIR 




Winding a Three-phase Stator with Basket Coils. Assume, 
for example, that a 48-slot core is to be wound for a four-pole, 
three-phase machine with a coil throw (in slots) of 1 and 12. 
Mark two slots to serve as a guide in placing the coils with 
respect to their proper span or throw. Any slot may be 
considered slot 1 and the other slot located by counting 
the throw in a clockwise direction. In this case (Fig. 140) the 
lower side of the first coil will go in slot 12. This side of the 
coil must now be laid wire by wire, inside the insulating cell 

in slot 12. The pro- 
jecting edges of this 
cell can then be cut 
off close to the lami- 
nations, the ends 
folded over one an- 
other and the slot 

FIG. 140. Winding diagram for motor using closed by driving a 
basket or loosely formed coils. tight-fitting fi b 6 r 

wedge between the outer cell and the tips of the teeth. This 
retaining wedge should not extend more than 1/2 i ncn beyond the 
core on either side, as it is liable to curl up and rub against the 
rotor. That part of the coil projecting beyond the slot should 
now be taped with a layer of treated tape and a covering of 
cotton tape for about half its length on each side of the core, 
and formed to drop below the bottom of the slot. This is 
known as the bottom part of the coil, as distinguished from 
the other side which will remain on a level with the slots 
known as the top part. In continuing the winding, the top 
part should be left out for the present, of slots 1, 3, 5, 7 and 9, 
since in completing the winding the coils in slots 2, 4, 6, 8 
and 10, which drop below the slots, must be in place before 
these top parts can be inserted. These coils which have one 
side left out of the slot until the rest of the winding is com- 
pleted, are known as throw coils. In the present case they 
are coils, 1-12, 3-14, 5-16, 7-18, 9-20. 

The first coil that can be wound into two slots as a top and 
bottom coil is 11-22. The coil in slot 22, being a bottom coil, 
is inserted in the same manner as the coil in slot 12 already 
described, but is not taped or shaped until its other side is 



REWINDING ALTERNATING-CURRENT MACHINES 197 

threaded into slot 11. The ends can then be taped from iron 
to iron with treated tape, in such a way as to overlap and seal 
the projecting end of the insulating cell. This in turn should 
be covered with a layer of cotton tape. The lower part of 
the coil should then be shaped with a rubber or rawhide mallet 
and fiber drift and treated with an insulating compound before 




FIG. 141. In this illustration the armature winder is shown inserting the 
turns of a basket coil into partially closed slots of a small induction motor 
stator (Westinghuse Electric & Mfg. Company). 

the next coil is put in place. The drift and mallet used in 
shaping the coils should have their sharp corners smoothed 
off, and in no case should the coils be struck directly with an 
iron tool. 

Fig. 142 is a partially wound stator, showing the finished 



198 



ARMATURE WINDING AND MOTOR REPAIR 



shape of the coil ends. It is particularly necessary that both 
tapings should be applied as tightly as possible, as a failure 
to watch this point will result in the taping becoming loose 
and baggy when the coil is shaped. This is especially true 
if the slot is very deep as the process of dipping or shaping 
the bottom coil below the bottom of the slot has a tendency 
to pull the tape away from the laminations. To avoid this 
tendency it is desirable to use glue on those turns of tape 
that cover the projecting ends of the fish-paper cell. 




FIG. 142. A three-phase stator partly wound with basket coils. 

During the operation of taping, the beginning and end of 
each coil should be brought out in such a manner as to lie on the 
side of the winding farthest from the bore of the stator and so 
placed that the beginning of one coil faces the end of the 
adjacent coil in a manner convenient for connecting. The 
method of winding just described should be continued until 
all the coils are in place. The winding is completed by placing 
the top parts of the throw coils in their respective slots. 

Threaded Diamond Coil Two Coils Per Slot. The 
threaded type of diamond coil is wound with insulated wire, 
either on a form which finishes the coil to shape, or on a shuttle 
which winds the coil first in the shape of a loop, after which 
the loop is placed in a universal former or " puller." In 
this device (see page 457, Chapter XVII), the two straight 



REWINDING ALTERNATING-CURRENT MACHINES 199 

sides are clamped between suitable jaws and are pulled 
apart the required distance, the coil assuming the shape 
shown on the right in Fig. 138. This type of winding is similar 
to the basket winding, in so far as the slot is insulated instead 
of the coil. On the other hand, the structure and the shape 
of the finished coils is identical with that of an open slot 
insulated coil. It is, therefore, not necessary to shape the 
coil after it has been placed in the slot as in the case of the 
basket winding. The slot insulation is practically identical 
with that of the basket winding with the addition of a fiber 
center strip to separate the upper and lower coils in the same 
slot. In this case, however, the slots should be laid out in 
groups according to a winding diagram, the number of groups 
being equal to the product of the number of poles by the phase 
of the motor. This grouping may be either uniform, alternate 
or irregular, according to the design of the winding. When 
the grouping is uniform, all groups consist of an equal number 
of slots, and contain an equal number of coils. In alternate 
grouping, every other group contains an equal number of 
slots and coils. In irregular grouping there is no apparent 
uniformity in the number of slots and coils per group. 

Winding a Three-phase Stator with Diamond Coils. In 
preparing to wind the machine, the slots forming the beginning 
of each group should be marked to indicate that the coils 
to be placed in them must be furnished with additional in- 
sulation at the ends. This is necessary as these coils form a 
boundary between phases, and consequently are subject 
to phase potential, whereas the potential between any other 
two adjacent coils is much less. Such coils are called the 
" phase coils" of the winding. 

Taking as an example a 72-slot, six-pole, three-phase motor 
with a throw of 1 and 11. There will be 6 X 3 = 18 groups 
of four coils each, with 18-taped "phase coils" in the winding. 
The winding of the coils into the slots can be started by thread- 
ing the bottom of the first coil into slot 11, the upper half 
of the coil being left up as a throw coil. The bottom parts 
of the next nine coils should be inserted in rotation, thus mak- 
ing ten coils left up as throw coils. The succeeding coil should 
be placed in two slots, 11 and 21. The rule to follow is, 



200 ARMATURE WINDING AND MOTOR REPAIR 

that no upper part of a coil can be placed into a slot, the lower 
part of which is empty. After each bottom coil has been 
threaded into its cell, both the insulating cell and the coil 
should be raised to the top of the slot and the projecting 
sides of the cell cut off as close to the laminations as possible. 
The coil must then be forced back to the bottom of the slot. 
The edges of the insulating cell should be folded over each 
other and held in place by the fiber center strip. The latter 
should be of a width to make a driving fit in the slot so as to 
hold the lower coil and cell firmly in place, and should be long 
enough to project out of the slot beyond the straight part 
of the coil. 

Every fourth coil, in the present case, is one of the special 
" phase coils," and should be taped at the end from iron to 
iron with an overlapping layer of insulating tape and a protec- 
tive layer of cotton tape. The remaining coils may be left 
without any special insulation at the ends. 

The placing of the upper part of the coil in the slot requires 
more care and skill, since the remaining space in the slot is 
just large enough to receive the coil with its wires lying parallel 
to each other. Fig. 139 shows the cross-section of a coil 
three wires wide by four deep, the wire being threaded through 
in such a way that wire 1 is first passed through the slot, 
then 3 and finally 2. This should be continued with each 
succeeding layer until the whole coil is in place. The cell 
can then be folded in and the slot closed with a fiber wedge. 
The two coils must completely fill the slot. If the insulation 
already mentioned is not sufficient to do this, additional 
filling strips of fullerboard, or treated wood, should be packed 
into the bottom or sides of the slot as may be required, until 
the coil is tight enough to prevent movement in the slot. 
The upper and lower parts of the winding outside of the slots 
must be carefully insulated from each other by a strip of 
treated duck cloth. This strip should be wide enough to 
extend from the fish-paper cell to the portion of the coil 
farthest away from the slot, and should be threaded between 
the ends of the coils as they are inserted in the slots. 



REWINDING ALTERNA TING-CURRENT MACHINES 201 



III. WINDING INDUCTION MOTORS HAVING OPEN SLOTS 

The main difference between the coils used for a partially 
closed-slot and an open-slot winding, is that the latter are com- 
pletely insulated before being inserted in the slots. They 
may, therefore, be wire or bar formed, and can readily be 
insulated for any commercial voltage. Consequently, motors 
of large size, or for voltages exceeding 550 volts are nearly 
always of the open-slot type. The insulation over the coils 




FIG. 143. Stator winding of a Fairbanks- Morse induction motor showing 
the type of coil used. 

consists of a cell of treated cloth or mica over the straight part, 
and an overlapping layer of cotton tape over the whole coil. 
Extra insulation for high voltages (see pages 163 to 172) is 
made up by extra thickness or extra turns of the insulating cell. 
The phase coils receive an extra wrapping of treated cloth tape 
over the diamond ends in addition to the cotton tape, which, 
as a distinguishing mark, may be of a special color on these 
coils. After the final wrapping is completed, the coils should 



202 ARMATURE WINDING AND MOTOR REPAIR 

be dried in a moderate temperature in order to expel all mois- 
ture, then, while still hot, they should be dipped in an insula- 
ting compound and again subjected to moderate heat until the 
compound is thoroughly dried. This compound serves to fill 
up all the pores in the insulating materials and make the coils 
dust and moisture proof. Before inserting the coils in the 
slots the latter should be lined with cells of paraffined fish 
paper cut wide enough to project out of the top of the slot when 
folded, and to serve as a guide to the coils. These winding 
cells sometimes called "slippers" furnish mechanical protec- 
tion only to the coils. 

Winding a Two -phase Stator Having Open Slots. Assume 
a stator designed for six poles, 72 slots, two-phase, with a coil 
throw (in slots) of 1 and 12. The total number of groups with 
uniform grouping is equal to the product of the number of 
poles by the phase of the motor, in this case will be 12, with six 
slots each. The groups should be laid off in a clockwise 
direction, the outside slots of each group being marked to 
receive phase coils, of which there are two per group. The 
coils can be inserted in regular order, beginning with the 
bottom part of phase coil in slot 12, and inserting phase coils 
wherever indicated. They should be driven into place by 
means of a fiber drift and mallet. Paraffin may be used as a 
lubricator, if necessary, as the coil should be a good driving fit 
in the protecting cell. The top parts of the first eleven coils, 
which are to be the throw coils will be inserted only temporarily 
until the bottom coils have been inserted into these slots. 
The remaining coils should be driven tightly into place, the 
projecting edges of the winding cell trimmed off close to the 
core and folded in, and the slot closed by driving a fiber retain- 
ing wedge into the grooves at the top of the slot. In some 
machines this wedge covers the entire face of the coil, while 
in others short wedges are driven in at each side, leaving the 
coil exposed at the middle of the core. 

Testing the Windings. After all the coils are in place on 
either of the types described, the winding must be carefully 
inspected for mechanical defects. All coils must clear the 
bore of the stator by at least one-sixteenth of an inch, and any 
coil which obstructs the bore should be tapped down with a 



REWINDING ALTERNATING-CURRENT MACHINES 203 

mallet and fiber drift to give the allowed clearance. All cells 
must be intact and sound, especially at the bottom of the 
slots. If the punchings have been spread apart by driving in 
the fiber wedges, they must be closed up again. The punch- 
ings should also be inspected to see that no fragments project 
into the bore of the stator where the rotor will rub against 
them. 

The winding should then be subjected to a break-down test 
to make sure that the insulation is sound at those points which 
in service will receive the most strain. These points are, from 
coils to iron, and from phase to phase. Connect all coils of 
the same phase together by a piece of copper wire. Then 
connect the terminals of a testingtransformer between the dif- 
ferent phases and from each phase to iron and apply the re- 
quired voltage for a length of time depending upon the charac- 
teristics of the machines. (See page 175, Chapter VII.) In 
case this test punctures the insulaton of any coil or the insula- 
ting cell in any of the slots, this insulation must be removed 
and replaced. If the puncture occurs in a top coil, it may 
sometimes be repaired by raising the coil out of the slot, 
removing the punctured insulation, and carefully replacing 
it with new material. If, however, a bottom coil or cell is 
punctured, it must be removed by raising the tops of the over- 
lapping coils. The same procedure must, be followed if a 
coil has to be replaced, as in the case of a burn-out due to a 
short-circuit. Such a process amounts practically to retracing 
the operation performed in the original winding until the 
injured coil is exposed. 

Inserting a New Coil in a Winding. In repair work where 
only one coil of a basket winding is burned out or damaged, 
and in general if the coils have been painted and are stiffened, 
rather than to remove all the throw coils, it is frequently 
easier to thread in a new coil. To do this cut the damaged 
coil at each side of the core and pull the wires out. New fish 
paper and treated cloth cells must be inserted. In emergency, 
f ullerboard or rope cement paper may be used in place of fish 
paper. The treated cloth is variously known as treated 
cloth, oiled linen, empire cloth, etc. (See Chapter VII.) 
In this operation coils can be bent slightly to get them out of 



204 



ARMATURE WINDING AND MOTOR REPAIR 



the way. Double cotton-covered wire of correct size and suit- 
able length for rewinding must be used. The length and size 
can be obtained from the old coil by counting the turns and 
measuring the length of one. To make the wire thread in 
easily it can be rubbed with paraffine. If the coil consisted 
of two or more wires in parallel, that number should be used 
in rewinding. The number of wires in parallel is the number 
entering a given coil from any junction point of coil terminals. 
Great care must be exercised during the threading process to 




FIG. 144. Method of threading in a new coil to replace a damaged one. 

see that all the wires lie parallel in the slots and are free from 
kinks. Otherwise it will be impossible to get the full number of 
turns into the slot. In case a large number of turns are needed 
(more than 20 per slot) two or more lengths of wire may be 
wound at the same time and connected in series after winding, 
the joint being made outside of the slot. When the required 
number of turns have been inserted, the edges of the cells 
should be trimmed and folded in, the coil wedged in place and 
taped at each end. It should then be painted with an insula- 
ting paint. 



REWINDING ALTERNATING-CURRENT MACHINES 205 

Connecting the Coils. After the ground test, the coils 
can be connected into groups, by joining together the begin- 
nings and ends of adjacent coils until the group has but one 
lead at each end unconnected, which form the leads of that 




FIG. 145. The stator of an inductor motor with coils in place ready for 
connecting the pole-phase-groups and making end connections of such 
groups. 

group. In doing this, the wires should be scraped clean 
and a sleeve connector of tinned copper slipped over them. 
They can then be soldered and taped. If suitable connectors 
can not be obtained (as may occur in making repairs), the 



206 ARMATURE WINDING AND MOTOR REPAIR 

stubs may be wrapped with fine bare copper wire and then 
soldered. 

The terminals of the various groups should next be connected 
into proper phase relations (see Chapter XI) with double- 
braided rubber-covered wire or cable. The size of wire and 
the grade of insulation will depend upon the current and vol- 
tage. Joints in cables or wire larger than No. 6 should be 
made by wrapping with fine bare copper wire. Great care 
must be taken while connecting and soldering joints to pro- 
tect the winding from the molten metal. After soldering, 
all joints and splices should be rubbed smooth with emery 
cloth and insulated with treated cloth tape, covered with 
one or more layers of cotton tape, and saturated with an 
insulating varnish or shellac. The connecting cables should 
then be arranged over the end of the windings in such a manner 
as to occupy the least possible space and yet keep them clear 
from the frame or end brackets. 

Points to Consider when Connecting Coils. The fol- 
lowing points should be carefully watched in order to make a 
good job in connecting induction motor windings. 

1. All wires should, if possible, be tinned before the connectors are 
put on. If this cannot be done they must be thoroughly cleaned by 
scraping. 

2. All soldering must be thoroughly done, making a smooth and 
solid joint. 

3. The wires of the joints should lap if space permits. 

4. No acid flux should be used. 

5. The wires or cables must be arranged in such a way as to occupy 
the least space and not obstruct ventilation. 

6. Wires or cables must be clamped or tied down to avoid vibration. 

7. Joints must be carefully insulated. 

8. After connecting, the winding should be tested for electrical 
balance by causing a current to flow through it and comparing the 
voltage and current readings of each phase. 

The windings on the side on which the connections are made, 
usually called the front side, are as a rule more rigid than those 
on the rear side, due to the fact that the connections exert a 
bracing effect. Since the windings in the rear lack this brac- 
ing effect, it is usually necessary with diamond windings to 



REWINDING ALTERNATING-CURRENT MACHINES 207 

supply a supporting ring of insulated steel to which all the 
coils are laced with rope made of six- or eight-ply waxed ends. 
The inside diameter of this ring should just be large enough 
to make a good fit over the winding. If it is much larger than 
that of the winding, the coils will be under tension which 
will in time tend to loosen them from the supporting ring. 
Care should be exercised in lacing the coils to the ring, when 
the rope is led through ends of the coils with a steel needle, 
to see that no damage is done to the insulation. 

Cleats and Terminals. The wires that tap into the winding 
and extend to the outside of the frame are called the leads. 
To avoid the possibility of these leads transmitting jerks 
and vibrations to the soldered joints of the winding, they 
should be fastened to the frame by iron or porcelain cleats. 
Such cleats should also be used to support the leads from sag- 
ging wherever they pass close to the iron or moving parts. 

Painting. All of the windings, and especially the taped 
joints, should be thoroughly brushed with shellac or a finish- 
ing varnish. This material seals up any porous places in the 
insulation, and excludes dirt and moisture (see page 176). 

IV. INDUCTION MOTOR SECONDARIES 

The squirrel-cage secondary is the simplest mechanically, 
and at the same time is the most rugged and compact form 
of moving element to be found in any electric motor. The 
operating characteristics of a squirrel-cage rotor are dependent 
on its resistance. A winding of low resistance will have 
good efficiency and small slip, but will have poor starting 
torque for a given maximum current. A winding of high 
resistance, on the other hand, will have lower running effi- 
ciency and large slip, but will give a high starting torque with 
minimum current, and is suitable for mill or crane work 
where starting under heavy load is frequent and operation 
is for short intervals only. Where it is necessary, however, 
to start a heavy load with small starting current, and operate 
for long intervals with good efficiency, or wherever it is neces- 
sary to vary the speed or operating characteristics of the motor 
from time to time, a wound secondary should be used. The 
windings have a low resistance, and are connected in star 



208 



ARMATURE WINDING AND MOTOR REPAIR 



with the open ends connected to slip rings. Adjustable ex- 
ternal resistance can then be connected in series when start- 
ing, after which the rings can be short-circuited. 




FIG. 146. Squirrel-cage rotor for a 350-hp., 3-phase, 440-volt, 1465 r.p.m. 
induction motor (Crocker- Wheeler Company). 

Squirrel -cage Secondaries. The core of a squirrel-cage 
rotor is built up of laminated steel on a spider, and keyed 
in place with a feather key on the spider and ring keys at the ends. 




FIG. 147. Solid metal rotor "cage" winding of squirrel cage induction motor 
(Fairbanks-Morse & Company). 

This rotor cage is made by casting end rings of copper or brass around the ends of the 
rotor bars which have previously been treated so that the metal of the bars is melted and 
fused together with the metal of the ring into a solid mass. This makes a rotor cage 
which is of one piece without joints. The iron core of the rotor was removed in order to 
obtain this photograph of the winding only, to show the characteristic structure from 
which the name "squirrel cage" is derived. 

Ventilators are used on the rotors with wide cores. Rectangular 
copper bars are usually used, cut from soft drawn bar stock. 



REWINDING ALTERNATING-CURRENT MACHINES 209 

The end rings are made of copper, brass, or various grades 
of resistance alloys. The resistance of the ring, and conse- 
quently the characteristics of the motor, depend upon both the 
width and thickness of the ring, as well as its composition, and 
may be varied over a wide range by changing these dimensions. 

Phase-wound Secondaries. With wound secondaries 
either diamond or basket coils can be used. Partially closed 
slots are usually used and the slots are skewed, to prevent 
humming in single-phase designs. Wire-wound coils can be 
inserted in a manner as wire-wound, threaded-in coils for 




FIG. 148. Large induction motor rotor partly wound with two-part strap 
coils (Crocker-Wheeler Company), 

direct-current armatures. The slots should be insulated with 
fish paper and treated cloth cells. The coil throw is determined 
by the number of slots per pole. The coils in each group 
should be connected in series, and the groups of each phase 
usually connected in series. The phases may be connected 
in star. Wedges are required to hold the coils in the slots, 
and the rotor should be banded and balanced in the same 
manner as a direct-current armature. 

In rotors of large machines, the coils are generally form 
wound, of strip copper, and should be completely insulated 
before insertion in the slots. For partly closed slots the com- 

14 



210 ARMATURE WINDING AND MOTOR REPAIR 

plete coil may be composed of several strips, each completely 
insulated with a wrapper of treated cloth and a winding of 
cotton tape. In some types of machines, designed for espe- 
cially heavy service, the insulation is composed of sheet mican- 
ite, wrapped with cotton tape. A cell of fish paper should be 
inserted in the slot for mechanical protection and the straps 
threaded in one by one. The cell may then be folded in and 
a wedge inserted. Wave windings are used for form-wound 
rotors. 

Collector rings are made of copper or brass. If mounted in- 
side the bearings, they are usually provided with lugs which 
are bolted through insulating washers to a ring in a small 
spider. If mounted outside the bearings, the leads are 
brought out through the hollow shaft and bolted to the rings. 

V. WINDING LARGE ALTERNATING-CURRENT STATORS 

The machines which fall under this heading include large 
induction motors used in industrial plants, engine and water- 
wheel driven generators, motor-generator sets, synchronous 
motors and frequency changers. While in many cases differ- 
ent types of windings are used, the method of winding the 
stators is essentially the same in all cases. Particular refer- 
ence will be made, however, in the winding details to the usual 
construction and requirements of induction motors. The 
recommendations for insulation of slots, make up of coils and 
their insertion are those of a writer in the Electric Journal, 
Vol. VII, No. 12, and represent good practice both for closed 
and open slot construction. 

Coils for Partially Closed Slots. When it is necessary to 
replace the coils in large machines, they should be purchased 
from the manufacturer since it is very difficult to properly 
form and insulate a number of large heavy coils with the facili- 
ties available in an ordinary repair shop or at the location 
where the machine is operated, such as a generating station 
or industrial plant. 

The partially closed slot requires a form of coil which can 
be either threaded-in through the slot opening or inserted 
from the end. As it is ordinarily impracticable to insulate 



REWINDING ALTERNATING-CURRENT MACHINES 21 1 

the slots for the voltages commonly used on large machines 
by the methods used for threaded-in coils, it is necessary 
either to insulate each strand for the full voltage or to use 
some form of winding in which the complete coil can be in- 
sulated from ground and impregnated and then inserted into 
the slot from the end. 

The former method may be used where strap diamond coils 
of a limited number of turns are to be used, and the voltages 




FIG. 149. Large induction motor stator partly wound showing type of coil 
used (General Electric Company). 

are moderate. A standard application of this method which 
is much used on large induction motors consists of a four coil 
per slot winding. Two strap diamond coils for each slot are 
completely insulated and impregnated. The width and depth 
of the insulated coil are each equal to half the width and depth 
of the slot. The width of the opening at the top is one-half 
the width of the slot, and the winding process is the same as 
for an open slot winding, except that there are twice as many 
coils for the number of slots as in an ordinary winding. 

Where a number of turns per slot are required with a partly 
closed slot a concentric shoved through coil can be used. 



212 



ARMATURE WINDING AND MOTOR REPAIR 



This may have as many turns as desired. The coils may be 
formed from double cotton covered wires, round in the smaller 
sizes and square in the larger. The wires should be cut off in 
lengths equal to the total length of the coil plus enough to 
allow for joints, and bound together in a long straight bar, 
having the correct cross-section for the coil. This bar is 




FIG. 150. Section of stator of water-wheel driven generator showing chain 

winding. 

then clamped by the middle in a forming machine, and the 
ends are bent over suitable wooden forms to give the correct 
shape to the finished end of the coil. The two free ends should 
be left straight, so that they may be shoved through the slots. 
Coils for Open Slots. Although the closed slot has some 
advantages in the case of induction motors, the form of winding 
necessary for more than four or at most six turns per slot, is 



REWINDING ALTERNATING-CURRENT MACHINES 213 

complicated and expensive to wind. The coils for an open 
slot winding are easy and cheap to form, to insulate and to 
install. A diamond winding can be readily chorded, and thus 
a standard frame and core can be used for different windings. 
In addition, a coil which can be completely insulated and im- 
pregnated before insertion in the slots is more reliable. Open 




FIG. 151. Partly wound stator of a 30-hp., 3-phase, 60-cycle, 685-r.p.m. 
induction motor (Crocker- Wheeler Company). 

slot windings are used, therefore, for generators and synchro- 
nous motors, and for the large sizes of induction motors. 

Both concentric (also called spiral or chain) and diamond 
windings can be used with open slots Either type of coil 
(see page 3, Chapter I) is formed at both ends, and can be 
completely insulated and impregnated before assembling in 
the core. The concentric winding takes up less room where 



214 ARMATURE WINDING AND MOTOR REPAIR 

the throw of the coils is great. All the coils in a group are of 
different size and shape, however, and the adjacent groups 
must be of different length. On account of the number of 
different coils used, repairs are difficult to make and a larger 
supply of extra coils must be kept in stock. For this reason 
the diamond winding is quite generally used. A diamond 
coil is of a simple form, easy to build and insulate, and one 
form of coil is used throughout. Repair parts can thus be 
reduced to a minimum. 

Lap and Wave Connections. Either the lap or wave form 
of diamond winding may be used. The end connections, by 
which the coil ends are connected into groups and the groups 
into phases, are very much more simple with the wave wind- 
ing. This form of winding is, therefore, used whenever appli- 
cable. The voltage between adjacent coil ends is much 
greater, but as the coil must be insulated for full line voltage 
under any conditions, this does not make any particular differ- 
ence. Where more than one turn per coil is required, however, 
or where a series-parallel combination is necessary, the wave 
winding is more cumbersome than the lap winding. It is 
accordingly used only with a one turn per slot coil, where all 
the groups in a phase are connected in series. The lap winding 
is thus much more common as most high voltage machines 
require more than one turn per slot. The coils may be wound 
from round or square wire or copper strap, depending on the 
number of turns and the size of the conductor. 

Insulation of Coils. The insulation to ground is practically 
the same for a given voltage for all types of coils. The wire 
in wire-wound coils should be cotton covered, and no other 
insulation is needed between turns. When the coils are made 
up of two or more layers of conductors, however, the layers 
should be separated by drilling, cotton tape or treated paper, 
according to the type of coil used. Strap conductors may be 
insulated between turns with overlapping cotton or mica 
tape. In most cases, however, the tape is used over the ends 
of the coils only, the straight parts being insulated by inter- 
weaving an insulating cell of cement paper and mica between 
them. Where several straps are connected in parallel for 
greater conductivity, they are ordinarily enameled, cotton 



REWINDING ALTERNATING-CURRENT MACHINES 215 

covered or taped to prevent eddy current loss. The several 
turns, whether connected in parallel or series, may be bound 
together with non-overlapping cotton tape and impregnated 
before being insulated from ground. While drying, the straight 
parts should be clamped in a press so that they will dry per- 
fectly straight and without any interstices between the turns. 
The hardened impregnating gums then bind the dried coil 
into a compact unit. 

The insulation to ground may consist of treated taping over 
the whole coil, with a protective covering of cotton tape. This 
material is widely used on the smaller machines, and for com- 
paratively low voltages. For larger machines, however, and 
for high voltages, the customary insulation consists of a wrap- 
per of cement paper and mica on the straight parts and treated 
tape or mica tape on the ends, with a protective layer of un- 
treated cotton tape or mica tape on the whole coil, overlapping 
on the ends and non-overlapping on the straight parts. The 
entire coil may then be dipped twice in an insulating varnish, 
and dried thoroughly in an oven after each dipping. The 
requisite insulation for the high voltage machines can be se- 
cured by extra turns of cement paper and mica wrapper. 
Not over three and one-half turns is ordinarily used, however, 
on account of the difficulty of properly impregnating such a 
coil. Where this does not give sufficient insulation, two or 
more separate wrappers may be used, and the coil dipped 
twice in varnish and dried after the application of each 
wrapper (see pages 163 to 172). 

Inserting Shoved Through Concentric Coils. Windings 
using coils of this type may be divided into two classes, de- 
pending on whether the ends are bent down at one end or bent 
down at both ends. The winding processes are practically 
similar for both. Coils bent down at one end are used on 
both two-phase and three-phase machines, the two windings 
being practically identical with the exception of the end con- 
nectors. In a two-phase machine the alternate groups are of 
the same phase, that is, all the coils in each bank of coil ends 
belong to the same phase. 

In a three-phase machine, with a winding of this type, every 
third group belongs to the same phase, and the groups of each 



216 ARMATURE WINDING AND MOTOR REPAIR 

phase alternate from one bank to the other. It should be 
noted in this case that the groups of the same phase do not lie 
adjacent to one another, but are separated by a distance 
equal to the pole pitch and that there are only three groups of 
coils per pair of poles. Where the groups of each phase have 
one side adjacent to another group of the same phase, under 
the same pole, with six groups of coils per pair of poles, it is 
necessary to have three banks, to allow the end connections to 
cross one another. In this case the coils of two of the banks 
must be bent down on both ends. The ends of the third 
bank are, therefore, bent down also, to secure uniformity of 
the windings. 

Considerable care is required in inserting coils of the shoved- 
through type into the slots. The slots must be first cleaned 
from all foreign matter. The coils should then be rubbed 
with paramne, and the slots lined with paraffined fish paper, 
cut and bent to an exact fit. The coil is thus made to slip 
easily into position, and at the same time is protected from 
damage by sharp edges of the iron. A tight driving fit is 
absolutely essential with this type of winding. If, on trial, 
the fit is too loose, strips of treated fullerboard or wood can 
be placed in the slot, .or taped to the coils. 

The smallest coil in each group should be placed in the 
slots first, and worked into them both sides at once, until 
the formed end comes within a short distance of the iron. 
Wooden distance blocks of appropriate dimensions can be 
used to secure uniformity. The other coils may then be 
inserted in order. All the coils of one bank should be inserted 
before the other bank is started. Where a two-bank winding 
is used, and the coils are bent down at one end only, it is 
necessary to insert the coils of the different banks from op- 
posite sides of the core. By this method the winder forms 
only the straight end of the coils, and his work is very much 
simplified. When a three-bank winding is used, however, 
the winder must bend down the ends of all the coils as he 
connects them. In this case it is easier to insert all the coils 
from one end. After all the coils are in place a fiber retaining 
wedge should be driven in over the top of each coil to hold 
it tightly in place, the fit being as close as possible without 



REWINDING ALTERNATING-CURRENT MACHINES 217 



Second Layer 




Lead' 



Connections when concentric coils 
are used. 



damaging the coil or spreading the lamination at the ventilat- 
ing slots. 

In forming the ends of the coils of both straight and bent 
down type, wooden blocks may be used over which to bend and 
shape the conductors. The inner layer should be connected 
first, the scheme of connections shown in Fig. 152 giving the 
best results. The connector consists of a copper sleeve, 
which is soldered over the ends, the joints being staggered, 
so that the coil ends 
will not be unnecessar- 
ily bulky. While making 
the joint it is advisable 
to protect adjacent con- 
ductors from the heat 

and Solder bv placing a 
\ 

layer of cloth or mica 
between them. The finished joint should be smoothed off 
with a file or with emery cloth, and then insulated with 
treated cloth and friction tape. 

Each coil, during assembly should be bent to correspond 
with the curve of the stator, so that no part of the finished 
coil will extend above the bore. A piece of insulating material, 
usually treated fullerboard, should be placed between the 
respective layers, which are then bound together with treated 
cloth and cotton tape. This serves both as insulation from 
ground and as mechanical protection for the coils. 

In the process of connecting, care must be exercised that the 
ends of the same conductor are not joined together, thus produc- 
ing a short-circuited turn. This can be prevented by testing 
out with a test lamp. As a precautionary measure, however, 
after the coils have been all connected, each one should be 
tested out with a testing transformer. This device (see page 
126, Chapter V for construction) should be placed over one 
side of a coil and a thin piece of sheet steel over the other. If 
there is a closed circuit any place in the coil, a heavy current 
will flow, and the steel feeler will be strongly attracted to 
the iron of the core. 

Bar and Connector Winding. This type of winding consists 
of solid copper bars which are completely insulated and shoved 



218 



ARMATURE WINDING AND MOTOR REPAIR 



through the slots. One or two bars may be used per slot. 
Within an inch or so of the end, the bar is uninsulated and the 
bare ends are tinned. End connections of diamond or in- 
volute form, insulated and provided with tinned ends, are 
used to complete the coil. In order to facilitate the operation 
of soldering, the ends of the connectors are slotted or drilled, 




FIG. 153.- Section of the stator of a water-wheel driven generator showing 
two-part bar coils, horn fiber slot insulation, wood wedges and ventilating 
spaces in the core. 

for without these openings it is difficult to force the solder 
into the center of the joint. 

In the case of a two-bar per slot winding, it Is necessary 
that the bar next to the rotor be shorter so that the joints 
at both ends of the connectors will be accessible to a soldering 
iron. In this case, sufficient filling must be placed between 
the two bars in the slot to allow the end connector to slip 
over the top bar with sufficient clearance for taping. In case 




REWINDING ALTERNATING-CURRENT MACHINES ' 219 

there is not sufficient room in the slot to allow of this filling, 
the top bars are cut away. The connector is shaped, as 
shown in Fig. 154, so as to allow it to slip between the lower 
bars and over the ends of the upper ones. All connectors 
must be soldered in place and the joints taped. With this type 
of winding repairs are very easily made, as any bar can be 
removed without disturbing the bars in any other slot, It 
is not adaptable to high volt- 
ages, however, on account of 
the limited number of possible 
turns. Furthermore, the end 
connections are very difficult to 
brace adequately. 

Diamond Coils. On large 
machines a split frame con- FlG ' 
struction is necessary. After 
the machine is completed and tested, the windings are re- 
moved by the maker at two points, in order to allow the 
frame to be disconnected for shipping. Stfme form of wind- 
ing must, therefore, be used that can readily be disconnected 
and reconnected on arrival at its destination. This condition 
is most readily met by the open slot diamond coil. The 
diamond winding is by far the easiest to put in place, to 
remove when necessary without damage to the coil, to con- 
nect into groups and phases, to brace at the ends, etc. It is, 
therefore, much used on large or high voltage machines. 

The assembly of wave and lap windings with these coils is 
practically the same, and the processes on the larger machines 
are similar to those employed on the smaller ones. The slots 
should be cleaned and lined with paper protective cells. 
The coils may then be inserted one after the other in order, 
no attention being paid to grouping, as the coils are all alike. 
On a large machine, the careful wedging of the coils in the 
slots is especially necessary, as the mechanical stresses that 
are brought to bear on account of the heavy currents in case 
of short-circuit, are very large. Hence the coil must fit very 
tightly in its place. If necessary, strips of treated cement 
paper, fullerboard or wood should be placed in the sides and 
bottoms of the slots to ensure a tight fit. 



220 



ARMATURE WINDING AND MOTOR REPAIR 



When inserting the coils, the first half of each coil should 
be driven tightly into the bottom of the slot with a fiber 
drift and mallet. The coil which goes over it must also 
be driven snugly into place. The fish-paper cell can then be 
cut off even with the top of the slot and folded over the coil. 
In order that this cell may not be torn by the wedge, a strip 
of treated fullerboard, the full width of the coil, may be laid 




FIG. 155. Stator of a 3-phase Fairbanks-Morse belted type alternator 
wound with the coils shown in Fig. 117. 

into the slot, so that the wedge will slide over it readily, and 
at the same time compress the coil tightly into the slots. 
The wedges may, for convenience, be divided into sections 
6 to 8 inches long and driven into position by means of a 
standard wedge driver or a blunt chisel. If the wedge is 
a very tight fit, the coil should be driven down with a drift 
just ahead of the wedge. 



REWINDING ALTERNATING-CURRENT MACHINES 221 

A four-coil per slot diamond winding used with partly 
closed slot cores, is assembled the same as any other diamond 
winding, with the exception that two coils lie side by side in 
both the bottom and top of the slot. The shape of the slot 
is such that no difficulty is encountered in inserting the second 
coil. This is essentially an induction motor winding, being 
used quite generally on both stator and rotor of large machines. 

Double Windings. On large induction motors, two speeds 
are sometimes secured by the use of two separate windings in 
the same slots, connected for a different number of poles. 
In this case the windings may be so designed that the coils 
for one speed lie inside the coils for the second speed. The 
two windings can be assembled at the same time, each pair 
of coils being treated as if it were a single coil of a one speed 
winding. Each slot thus contains four coils one above the 
other. In such cases it is quite common to have the two wind- 
ings of widely different capacities, depending, of course, on 
the load to be carried at the separate speeds. 

Testing Windings of Large Machines. After the coils 
of the winding are all in place, their free terminals should be 
temporarily connected together, and a test made for break 
down to ground, with the standard voltage for the size and 
type of machine (see page 175, Chapter VII). After the coils 
have been connected into groups, and the groups connected 
into phases they should be tested for break down between 
phases, and for short-circuits between turns. 

The break-down test consists of the application of the stand- 
ard break-down voltage from the conductor to ground and 
from phase to phase. The test for short-circuit consists in 
placing an alternating-current magnet of the type shown in 
Fig. 101, page 126 over one side of the coil or group, and hold- 
ing a light piece of steel over the other side of coil. If current 
flows in the coil, the piece of steel will be attracted. As no 
current can flow under normal conditions, this test is a sure 
indication of short-circuits. It cannot be applied where the 
groups are connected in parallel but must be made, in this 
case, before they are connected. 

Connecting the Coils. The number of groups in any ma- 
chine may be either one or two per phase per pair of poles, 



222 ARMATURE WINDING AND MOTOR REPAIR 

depending on the arrangement of the coils and groups. The 
coils of each group are connected in series but the groups in 
a phase may be connected in series, parallel or series-parallel, 
although for voltages of 2200 or higher they are nearly al- 
ways connected in series. The phases on a two-phase machine 
are never connected together inside the machine. 

Stator windings of alternating-current machines may be 
connected either star or delta, the choice depending upon many 
conditions. For machines of 2200 volts and over it is ad- 
visable to connect the winding in star because the voltage 
from any one terminal to ground is only (2200 -T- 1.73) of the 
voltage between terminals and therefore the winding has a less 
chance to break down to ground. However, this is not 
a strict rule for sometimes it is more convenient to have the 
stator delta connected for the reason that a smaller size of 
wire can be used and a larger number of turns. 

On a concentric winding the groups are readily distinguish- 
able. In a diamond or involute winding, the number of coils 
per group must be counted off, and the groups temporarily 
connected by bending together the leads from a wire coil or 
by slipping a copper connector over the stubs from a strap 
coil. They should be permanently connected by soldering 
the joints so made. On most machines, the connectors be- 
tween groups consist of insulated copper strap, with suitable 
openings at each end to slip over the group leads. 
All joints after being carefully soldered, should be smoothed 
up with emery cloth, so that no sharp-pointed edges are left 
to damage the insulation. 

The connectors between coils, ordinarily called stubs, should 
be insulated with treated cloth tape, and a protective cover- 
ing of untreated cotton tape. Large stubs are sometimes 
covered with drilling caps, sewed to shape, and painted with 
insulating varnish after they have been fastened in place. 
Treated tape should be wound over the joints in the insulation. 
The thickness of the taping and other insulation, depends on 
the voltage of the machine. The connectors between groups 
are ordinarily insulated and impregnated before they are put 
on the machine. The joints between the connectors and the 
group leads should be insulated in the same way as the stubs. 



REWINDING ALTERNATING-CURRENT MACHINES 223 

Where double windings are used they are usually arranged 
for the connections to be made on opposite sides of the 
machine. Each winding may be connected as if it were 
entirely independent. 

Bracing Needed for Heavy Windings. The stresses occur- 
ring in the end connections of a large machine, due to magnetic 
reactions between current carrying conductors, are quite 
large. In addition, these effects are greatly magnified in 
case of a short-circuit on the generator. Some method of 
bracing these end connections is therefore necessary. This 
ordinarily takes the form of an insulated steel ring, to which the 
coils are tied with heavy twine. On a concentric winding 
a separate ring is used for each bank of end connections. On 
a diamond winding one ring at each end is sufficient. On a 
large machine, however, it is usually necessary to brace this 
ring by additional steel supports bolted to the frame. 

VI. WINDING THE STATOR OF ALTERNATING-CURRENT 
TURBO-GENERATORS 

Although the alternating-current turbo-generator falls in 
the previous group of large machines, on account of its winding 
being more or less special, it will be dealt with separately. 
There are special features of windings for this class of machines, 
owing to the high speed at which they are operated and the 
forced requirement that the inside diameter must be small on 
account of the centrifugal strains of the revolving field rotor. 
The generator core is therefore long requiring large coils fre- 
quently more than 10 feet long with a span from 30 to 40 
inches and weighing as much as 100 pounds. One of the best 
discussions of the methods for winding an alternating-current 
turbo-generator has appeared in the Electric Journal, Vol. 
VIII, No 3. The winding details that follow have been taken 
from that source. 

Coils for A.-C. Turbo-generators. These machines have 
open slots and usually employ a diamond coil built up in two 
pieces for one or several turns per slot. The two-piece coil is 
used where the throw of the coils is very great, as in large size 
bipolar machines for 25 cycles. The one-piece coil is used 



224 ARMATURE WINDING AND MOTOR REPAIR 

on smaller machines, and in fact, wherever such a coil is not too 
cumbersome to handle, provided the complete coil can be 
passed through the bore. The coils of either type may be 
formed from cotton-covered strap or cotton-covered wire. 
Where a conductor of large cross-section is required, it is ordi- 




FIG. 156. End of a large turbo-generator slowing method of bracing end 
connections to resist short-circuit stresses. 

narily built up of a number of square copper wires in parallel, to 
facilitate bending and to obtain proper lamination. The 
individual strands are usually cotton covered. Most turbo- 
generators require more than one turn per slot in order to 
obtain the desired voltage. These several turns are bound 



REWINDING ALTERNATING-CURRENT MACHINES 225 

together in a mechanical unit, insulated from each other and 
insulated as a group from the frame of the machine. 

The coils may have their end connections bent down at any 
angle from zero to 90 degrees. The involute coils, of course, 
lie flat against the face of the frame. Diamond coils are al- 
ways bent down at the ends from 30 to 60 degrees, both to 
provide ample clearance and ready access to the rotor, and in 
order that they may be more suitably braced, as shown in 
Fig. 156. Especial care should be taken to so shape the coil 
ends that cool air can circulate freely through them. Coilp 
of several turns sometimes have the individual turns insulated 
separately after they leave the slots to give greater cooling 
surface to the ends. 

Forming the Coils. Except for conductors of large capacity, 
the conductors are formed from a single copper strap. The 
process of forming the coil is the same whether each conductor 
consists of one bar or several bars or straps in parallel. Each 
group of wires or straps which make up a conductor in the 
latter case may be formed as a single strap and then bound 
together with tape and treated as a unit. For a one- conductor 
coil, designated as an open coil, a strap of suitable length may 
be bent at the middle around a pin, forming a U with the sides 
separated very slightly. This loop should then be mounted 
directly in a mould. A steel pin through the bend of the U 
will serve to hold the point of the diamond vertical while the 
sides are bent to conform to the shape of the mould. Coils 
of several conductors are sometimes built in this same way, by 
forming the individual conductors in a group with a copper 
strip of the same thickness as the insulation inserted between 
them. The conductors should be separated and insulated 
from one another, and then insulated from ground the same 
as a one-conductor coil. The several conductors can be con- 
nected in series when placed in the machine. 

A coil of more than one conductor formed from a single 
strap is known as a closed coil and requires several operations. 
The strap should be first wound around pins set in a flat 
table, to the required number of turns, and then given its 
final shape by forming over a mould. Throughout the entire 
operation the conductors must be kept apart by straps of 



226 ARMATURE WINDING AND MOTOR REPAIR 

metal to the same distance that they will be separated when 
insulated and placed in the machine. Two-piece coils can 
be formed from lengths of straight copper strap. The ends 
should be bent at a suitable angle to the straight part of the 
coil around two pins spaced a distance equal to the length of 
the straight part. The conductor may then be clamped in a 
mould and the ends bent into shape. The several conduc- 
tors in a coil can be formed in the same mould, and separated 
by strips of copper strap. 

Insulation for Turbogenerator Coils. Since a turbogen- 
erator may at times be subjected to very heavy overloads, 
the insulation should be as nearly heat proof as possible. 
For this reason, mica is much used. Where several con- 
ductors per coil are required, the insulation between conduc- 
tors usually consists of flexible mica tape and mica cells. The 
individual conductors may then be suitably bound together. 
All coils should be impregnated with an insulating compound 
before the insulation from ground is supplied. The straight 
parts may be clamped tightly in metal clamps and impreg- 
nated, or may be impregnated without the clamps and placed 
in clamps to dry. Either method results in a compact, uni- 
form construction so that the coil may be fitted tightly into 
the slot with minimum risk of damaging the insulation. 

The insulation from ground usually consists of wrappers of 
paper and mica over the straight part. The coil ends should 
be insulated by layers of treated tape, with a covering of 
untreated tape, and the whole coil treated with insulating 
varnish. As in the case of engine type generators, if extra 
insulation is required additional wrappers of the paper and 
mica are frequently used, the coil being dipped and dried 
after each wrapper is applied. This makes the building of a 
high-voltage coil a long process, two weeks or more frequently 
being required for the completion of a single coil. 

Testing Turbogenerator Windings. Open coils may be 
tested for short-circuits by applying a suitable voltage between 
conductors. No short-circuit tests are usually made on the 
individual closed coils, but an over-voltage test should be 
applied after they are on the machine. All coils should also 
be tested for insulation by wrapping the outside with tin 



REWINDING ALTERNA TING-CURRENT MACHINES 227 

foil, and applying from two to two and a half times normal 
operating voltage across the insulation. 

Inserting the Coils in a Turbogenerator. Except for the 
great weight and large size of the coils which makes them more 
difficult to handle, the winding of a turbogenerator is in all 
essentials similar to the winding of an engine-type generator. 
The straight part of the coil should be waxed and laid over 
the slot opening, inside a paper cell with which the latter is 




FIG. 157. Turbo-generator stator partly wound using one-piece coils. 
The operator is binding the coils in position. 

lined. In order to avoid bending the coil, a wooden drift 
as long as the straight part may be used to drive it into the 
bottom of the slot. The bottom halves of two-part coils 
should be inserted first in all slots in the span of one coil, 
the top halves being inserted later. One-piece co Is should be 
inserted in regular order. After each top coil has been fitted 
into place, the cell which lines the slot may be folded over, 



228 ARMATURE WINDING AND MOTOR REPAIR 

and retaining wedges in short sections driven in place over the 
full length of the coil. Enough strips of extra material must 
be used in the slot to make the wedges fit tightly over the coil. 
The wedges may be driven in place by a tool of special shape, 
operated by a compressed-air hammer. 

In order to get the bottom half of the last few one-piece 
coils into the slot, it is necessary to lift the top half of the 
coils which go in the same slots. With a full pitch winding 
this would mean one-half of the coils on a two-pole machine, 
and one-fourth the coils on a four-pole machine. With a 
"chorded" or fractional pitch winding, however, the number 
of throw coils is less. The throw of the coils can be readily 
seen in Fig. 157 which shows a generator partly wound with 
one-piece coils. 

In some cases, the improved space factors that can be se- 
cured makes it worth while to use a one-coil-per-slot winding. 
This winding can be inserted in the same way as the two-coil- 
per-slot winding, except that alternate slots contain, respec- 
tivelv, front and rear ends of coils. 

Bracing for Windings. The instantaneous current which 
flows in case of a short-circuit to a turbo-generator op, in fact, 
any other generator, is very large and the magnetic stresses, 
which vary as the square of the current and the sp^n of the 
end windings are consequently enormous in the turbo-genera- 
tor. For this reason adequate bracing of the coil ends is of 
supreme importance. Greatest reliance is placed on metallic 
braces securely bolted to the generator frame. For involute 
coils, braces may be used that consist of U shaped clevises 
which are thoroughly insulated with treated tape and bolted 
to the machine frame at the ends through spacers. For dia- 
mond coils, the braces take the form of malleable iron brackets 
bolted to the frame. The coils should be rigidly secured 
to these braces by nonmetallic clamps, reinforced by brass 
plates, as shown in Fig. 156 and fastened to the braces by 
insulated bolts. 

Connecting the Winding. Turbine windings are almost 
always connected with one group of coils per pole per phase. 
The coils of each group are connected in series. In some 
cases these connectors are riveted as well as soldered in place. 



REWINDING ALTERNATING-CURRENT MACHINES 229 

The groups per phase may be connected in parallel or series, 
depending on the requirements of the machine. Ordinarily 
the series connection is used on machines of 2200 volts or 
higher. The end connections should be fully insulated and 
supported on the rear of the braces. 

Break-down Test. After all the coils have been placed 
in the core, their free ends should be connected together and 
the standard break-down voltage (see page 175, Chapter VII) 
applied from the copper to the machine. A similar break- 
down test should be applied between phases after the coils 
have been connected into groups. No further tests need be 
applied until the final load tests are run. 



CHAPTER IX 

TESTING INDUCTION MOTOR WINDINGS FOR MIS- 
TAKES AND FAULTS 

After the coils have been placed in the stator of an induction 
motor and connected by the winder to give the required com- 
bination' of phases, volts and poles, the next step is to connect 
the motor to a test circuit and note whether or not the current 
taken is the same in all the leads. If it is not, or the motor 
does not operate properly in other respects, the winding must 
be checked and tested for possible errors in connection of 
coils and for other defects. For an experienced induction 
motor winder, this is a simple job and one that is done very 
quickly. For the repaiman who has had less experience in 
connecting induction motors, the job is not so quickly per- 
formed. Yet it is not a difficult one when the procedure is 
correct. One of the most easily understood methods of pro- 
cedure for checking induction-motor windings has been formu- 
lated by A. M. Dudley and published in the Electrical Journal. 
It is so simple yet complete that any repairman can with a 
little patience discover and correct even the most puzzling of 
errors and defects. It is given complete in what follows. 

The errors which may occur in induction-motor windings 
have been arranged by Mr. Dudley in the approximate order 
of their occurrence as follows: 

I. Short Circuit and Grounds. 

1. Short circuits or grounds of one or more turns in an individual 

coil caused by a break in insulation. 

2. Short circuit of a complete coil caused by connecting together the 

two ends of the coil. This is known by winders as " stubbing a 
coil dead." 

3. Short-circuiting a complete pole-phase-group. 

4. Short-circuiting one complete phase of the winding. 

II. Reversal of Part of the Winding. 

1. This may be confined to a single coil or it may be a. pole-phase- 
group or a complete phase. 

230 



TESTING INDUCTION-MOTOR WINDINGS 



231 



III. Open Circuits. 

1. Caused by actually leaving the winding disconnected or open 

at some point. 
IV Placing One or More Coils too Many or too Few in a Pole-phase-group. 

V. Using an Improper Group Connection. 

1. For example, connecting in series when the winding should have 
been in parallel or vice versa. This is called "connecting for 
double or half voltage, respectively." A similar error would be 
connecting delta instead of star. 

VI. Connecting for the Wrong Number of Poles. 

1. While this error comes under No. V, it is usually regarded as dif- 
ferent and is sometimes difficult to locate unless it hasbeen noticed 
that the speed of the motor when running light is not what it 
should be. 

Testing for Grounds and Short Circuits. Grounds of any 
description are located by "ringing out" between the copper 



Support at End 
of Laminations 





FIG. 158. Section and plan views of a short-circuit detecting device for 
testing stator windings. 

and the frame with a magneto or "lighting out" between the 
same points with an ordinary ItO-volt lighting circuit test 
lamp. They are comparatively simple to locate and easy tc 
repair by the use of insulating material used with the job. 
Short circuits of a few turns or a single coil become hot in a 
short time if the motor is run light on normal voltage. Their 
presence can be detected by feeling around the winding with 
the hand immediately after starting the machine and noting 
if any coils are much warmer than others. 



232 



ARMATURE WINDING AND MOTOR REPAIR 



Short-circuit Detecting Device. A device that can be used 
for detecting short circuits before the rotor is placed in the 
stator and without applying voltage to the winding itself, is 
shown in Fig. 158. It it somewhat similar to a large horse- 
shoe magnet except that the iron part is built up of lamina- 
tions. It may also be considered a core-type transformer 
having the primary coil only and having one side of the iron 
core missing. The coil is excited with alternating current of 

a suitable voltage and then 
the complete device is 
passed slowly around the 
bore of the stator as shown 
in Figs. 159 and 160. The 
laminations of the stator 
core complete the magnetic 
circuit of the testing de- 
vice and an alternating 
magnetic field flows in the 
stator core as shown by the 
dotted lines in Fig. 159. 
When moving the device 
around the stator bore, and 
it passes over a short-cir- 
cuited turn or coil, such 
short-circuited turn or coil 

immediately acts as a short-circuited secondary coil on a 
transformer of which the exciting coil on the testing device is 
the primary. As in -any short-circuited transformer an in- 
creased current flows both in the primary and in the secondary 
coil and can be detected by an ammeter in series with the 
device or by the heating which immediately takes place in the 
defective coil in the winding or by the attraction which the 
other side of the short-circuited coil has for a strip of sheet 
iron. By passing the device slowly around the core and ob- 
serving its behavior from point to point, short circuits can 
readily be detected. This refers particularly to short circuits 
in individual turns or in one complete coil. A short circuit of 
a complete pole-phase-group is more readily located by a com- 
pass test, and a short circuit of an entire phase can be found 




FIG. 159. Method of using the short- 
circuit detecting device shown in Fig. 
158. 



TESTING INDUCTION-MOTOR WINDINGS 233 

by a balance test. The balance test is made after the winding 
has been checked for grounds and short-circuited turns and 
has had the resistance of all the phases measured. If these 
checks indicate the proper number of turns in series, a com- 
paratively low alternating voltage is applied to the winding 
of the stator without the rotor being in place. The current is 
then read in all the phases and if it checks up the same or 
balances as it is called, the machine is considered free of 
grounds and short circuits. 




FIG. 160. Testing stator winding for short-circuited coils by use of a small 
alternating-current magnet. 

Reversal of One or More Coils or Groups. It happens that 
individual coils or sometimes entire groups are connected in 
backward. If the error is confined to one coil it does not usu- 
ally show up on the " balance " test and, of course, would not 
be found on the resistance test since the resistance is the same 
no matter whether the ends of the coils are interchanged or 
not. Such reversed coils or groups can be located by means of 
a polarity test with a compass. In this test the motor wind- 



234 ARMATURE WINDING AND MOTOR REPAIR 

ings are excited by a comparatively low direct-current voltage 
so chosen that the current is limited to a reasonable value. 
If the windings are so excited and a compass is placed inside 
the bore and passed around slowly following the inner periph- 
ery, the needle of the compass will reverse in passing from 
a north pole to a south pole group and vice versa. If an indi- 
vidual coil is reversed it will show a tendency to reverse the 
compass needle when the needle is directly over that coil. If 
an entire pole-phase-group is reversed the compass needle will 
indicate the same direction of field on two successive groups. 
Also if a coil is left out of circuit or is "dead," as already men- 
tioned, it will indicate an irregularity at the instant the com- 
pass passes directly above it. By checking the three phases 
of a three-phase winding separately, in this manner and 
marking the result inside the bore with chalk of different 
colors, it is possible to check for the reversal of an entire phase. 

Open circuits are best checked by " lighting out." This 
test it made by connecting a 110-volt lighting circuit and an 
incandescent lamp in series with the winding. If there is an 
open circuit the lamp will not light. 

Placing the wrong number of coils in two or more phase 
groups can hardly be deteced otherwise than by actually 
making a physical count. Since this is a simple matter, the 
check is best made in that way. 

Using an improper group connection has in general the 
same effect as raising or lowering the voltage on a machine 
and can best be checked by operating the motor on what 
should be the correct voltage. If this voltage is considerably 
too high as would be the case if the winding was connected 
parallel when it should be series, or delta when it should be 
star, the motor will emit more than the ordinary amount of 
magnetic hum and will probably overheat in a short time even 
when not mechanically loaded. If the voltage is much too 
low, as would be the case if the winding was connected series, 
when it should be parallel, or star when it should be delta, 
the fact can be determined by trying a load on the motor. 
When the load comes on the drop in speed will be too great 
and the motor will pull out and come to a standstill on a load 
much less than normal. 



TESTING INDUCTION-MOTOR WINDINGS 235 

Connecting for the wrong number of poles can most readily 
be detected by checking the no-load speed. (If it has a wound 
rotor, this must be short-circuited.) The no-load speed, being 
approximately synchronous, is very nearly equal to cycles 
X 120 -r- number of poles. If this gives a result differing 
from that expected, the winding is connected for the wrong 
number of poles. 

Order in Which Tests Should be Made. The order in which 
these various checks should be performed is usually as follows : 
After the winder has completed the connection, the windings 
are checked against the winding diagram. The coils per 
group are counted and a visual inspection made for short 
circuits, open circuits and reversed coils. A balance test is 
made with low voltage to see if the separate phases show the 
same result. A high voltage test is then made on the insula- 
tion to ensure that the coils are not grounded between phases 
nor on the iron core. The machine is then assembled and the 
resistance of the completed winding is measured on all phases. 
If these checks are satisfactory the machine can be passed for a 
running light test without load. Sufficient voltage is put on 
the windings to start the motor up. If it comes up to speed 
without apparent distress or irregularity of any kind, the speed 
is checked and the temperature of the winding is tested with 
the hand all the way around the machine. If this is normal 
the voltage is raised to its normal value and the no-load cur- 
rent in all phases and the watts are read. If these values 
check with previous calculations or tests on duplicate machines 
the windings are considered to be correctly connected. If no 
data is available on the no-load current it may be considered 
reasonable if it does not exceed 40 per cent, and is at least 20 
per cent, of full load current. The no-load watts running 
light may be considered reasonable if they are roughly in 
the neighborhood of seven or eight per cent, of the normal 
rating of the motor. If the motor does not readily come up 
to speed or the phases do not balance or there are signs of 
unequal heating in the winding or other distress, the rotor 
should be removed and the connections checked. If the error 
is not apparent and a source of direct current is available the 
compass test may be applied. 



236 ARMATURE WINDING AND MOTOR REPAIR 

Connections for Applying Direct Current when Exploring 
A.-C. Windings with a Compass. The accompanying dia- 
grams show the methods for making connections for the use 
of direct current to excite the windings of an alternating-current 
motor so as to explore the windings by the use of a compass. 
When the winding is star connected, one lead of the direct- 
current circuit is joined to A-B-C connected together and 



D.C.Supgly> 





FIG. 161. Method for checking 3-phase, star and delta connected windings 
with direct current. 

the other lead to the neutral point. When the compass is 
held over the coils in the slots and moved around the stator, 
there should be three times as many poles as there are poles 
in the machines reversing alternately and spaced equally when 
the winding is balanced. In case of a delta-connected winding, 
it is only necessary to open the delta connection as shown in 
Fig. 161 at the right and connect the two ends to the direct- 
current leads. 



CHAPTER X 

ADAPTING DIRECT-CURRENT MOTORS TO CHANGED 
OPERATING CONDITIONS 

The changes in direct-current motors which the repairman 
is most frequently called upon to make are: 1. Changes in 
speed. 2. Changes in operating voltage. 3. Changes to 
operate a motor as a generator and vice versa. 

Changes in Speed. A change in speed of a motor from 10 
to 15 per cent, can usually be made by increasing or decreas- 
ing the air gap between the armature and fields. In general 
the per cent, changes in air gap required will be three or four 
times the per cent, change in speed. This will serve only as 
a rough check for use when measuring the air gap and consider- 
ing the change of speed in this manner. To make this change 
with accuracy, it is necessary to have a magnetization curve 
for the design of the particular machine dealt with. How- 
ever if the change in speed desired is not too great, the change 
in air gap will in most cases give desired results. The excep- 
tions are those cases where the motor has been built to work 
high up on the saturation curve where the iron is practically 
saturated or very low down on the saturation curve where the 
excitation is practically all used in the air gap. The speed is 
increased by increasing the air gap and reduced by reducing 
the air gap. 

It is usually easier to reduce the speed by change in air gap 
than to increase the speed. In the former case "sheet steel 
liners or shims can be inserted next to the frame and different 
lengths of air gaps secured over a considerable range depending 
upon the length of the original air gap of the machine. If 
the air gap cannot be increased by removing such shims, it 
may be necessary to grind off the required amount from the 
pole faces. Such a grinding should not be done more than 
once if the operation will narrow the width of pole face to an 
appreciable extent. When the increase in speed is more than 

237 



238 ARMATURE WINDING AND MOTOR REPAIR 

can be secured by increasing the air gap, it may be possible to 
add a resistance to the shunt-field circuit. In such a case it 
will be necessary to check carefully the no-load and full-load 
speeds. 

In the case of a shunt motor, a compound characteristic 
can be secured by increasing the air gap and then adding series 
turns to the field poles until the full-load speed is the same as 
it was before the change. For this purpose flexible copper 
cable can be wound around the shunt coil and bound in place. 
In reversing this change, that is, in changing a compound 
motor to a shunt characteristic, the air gap should be reduced 
and current shunted out of the series coils of the field until a 
constant speed range is secured. 

An inefficient and emergency method of reducing the speed 
of a motor for a short time only is by the use of a resistance 
in the armature circuit. When a variable resistance is 
connected in series with the armature leads, it simply reduces 
the impressed voltage on the armature winding by using up a 
part of the voltage in the resistance. Since the speed of the 
motor is proportional to the operating voltage, the desired 
speed reduction can be thus secured. 

In all cases where the speed of a motor is increased, atten- 
tion must be paid to the armature construction so that the 
increased centrifugal strains on the coils and the banding due 
to the increase in speed will develop no serious defects, such 
as rubbing of coils on the fields, high commutator bars and poor 
commutation. When the increase in speed is 50 per cent, or 
more, the manufacturer should be consulted for advice as 
to the safety of the high speed for continuous operation. 

Changes in Operating Voltage. The speed of a direct- 
current motor varies directly with the operating voltage and 
(theoretically) inversely with the flux of the fields. When 
the operating voltage is increased on a motor, the excitation 
of the fields is also affected but on account of the saturation 
of the iron, the field flux is not affected in direct proportion 
so that the speed of the motor on an increase of voltage follows 
about in proportion to the increase. When the voltage is 
not increased or decreased more than 25 per cent, over the 
rated value for the motor, the correct speed can be obtained by 



CHANGES IN DIRECT-CURRENT MOTORS 239 

a change of the air gap. In such a change the operating 
temperature of the field coils must be watched, since the 
temperature will vary about as the square of the voltage. 
A temperature higher than 160F. will injure the insula- 
tion. To prevent this when the temperature of the fields 
is found to be around this figure, it will be advisable 
to rewind the field coils. In case the machine is operated 
on under voltage with the air gap as large as possible, a re- 
sistance can be used in the shunt field until the required speed 
is secured. On a compound motor, no-load and full-load 
speeds can be adjusted by changing the series field coils in 
either of the cases mentioned. 

Operating a Motor on One-half or Double Voltage. 
It frequently happens that a 220-volt motor must be changed 
to operate on 110 volts or a 110-volt motor on 220 volts. 
In the case of a 220-volt motor, it is usually possible to connect 
the shunt-field coils in two groups and then connect these 
groups in parallel. In such a case when the motor is connected 
to a 110-volt circuit, the voltage per coil will be the same so 
that the field flux will be the same, but the speed will be only 
one-half that on 220 volts. The speed can be brought up 
to normal by increasing the air gap as much as possible and 
using resistance in the shunt field as explained in a preceding 
paragraph. The rating in horsepower of a motor so changed 
will only be one-half of what it was before. 

When a 110-volt motor must be changed to operate on 220 
volts, the conditions are not so easy as in the previous case 
for if the smallest air gap with the shunt fields in series cannot 
be used, the field coils must be rewound. When operated 
in this way the motor will have double the horsepower rating 
on 220 volts that it did on 110 volts. The changes in the 
motor which have been mentioned do not require a change 
in the armature winding. 

Changes in Armature Winding for Operating Motors on 
One-half or Double Voltage. The most satisfactory way 
of changing a motor to operate on a voltage which is one-half 
or double the rated voltage is to rewind the armature to suit 
the new conditions. This is also the most expensive way 
and need not be resorted to when the change in voltage is 



240 ARMATURE WINDING AND MOTOR REPAIR 

not over 25 per cent. A direct-current armature winding can 
usually be changed for operation on a lower voltage in two 
ways. 1. By using new coils. 2. By reconnecting the old 
armature winding. When the armature is to be rewound 
for a different voltage, the number of turns in series between 
brushes will vary directly as the old voltage to the new. The 
cross-section of the wire for the coils will also vary inversely 
as the old voltage to the new. For instance if a 110-volt 
motor is to be operated on 220 volts, the armature conductors 
will be doubled in number and their cross-section made one- 
half, when the speed and horsepower rating will remain the 
same as before. When the size of wire previously used is 
known, one of one-half the cross-section will be three B. & S, 
gauge numbers higher. 

A practical way of making these changes in armature 
windings in a repair shop is given on pages 243 to 260 as out- 
lined by T. Schutter (Electrical Engineering, July and August, 
1918) for the changes of one -half and double- volt age operation. 

Size of Wire for D.-C. Armature Coils. Before winding 
the coils needed in the repair of an armature it must be decided 
whether a lap or a wave winding will be used. In the lap 
winding there are as many current paths or circuits through 
the armature winding as there are poles with the coils con- 
nected in series in each of these paths or circuits. The wave 
winding has only two current paths through the armature 
winding regardless of the number of poles the machine has. 
In each of these two paths the coils are connected in series. 
Considering / the total armature current as given on the name 
plate of the machine and i the current in each circuit of the 
armature winding, the following formulas can be used to find 
the maximum current that can be carried by the coils of 
each circuit connected in series. 

For a lap winding in which the number of circuits equals 
the number of poles or p, 



For a multiple lap winding, having two or more single lap 
windings in parallel, 

i = I -T- mp 



CHANGES IN DIRECT-CURRENT MOTORS 241 

In this case m is the number of single lap windings in parallel. 

For a wave winding which has two circuits 
i = I + 2 

For wave windings having more than two circuits, usually 
called multiple-wave or series-parallel windings 

i = I -f- 2m 

In this case m is the number of wave windings used in the 
armature. 

The permissible current density in armature conductors 
varies from 1500 to 3000 amperes per square inch. The 
cross-section or diameter of the wire for the armature coils 
can then be found by dividing the current in each section 
of the winding (i) by the allowable amperes per square inch. 
The value to use in the case of small machines should not 
exceed 3000; in intermediate sizes 2000; and in large sizes 
1500. 

When winding new coils in the repair of an armature it is 
necessary to determine the available winding space in the 
slot before deciding upon the number of turns and form of 
coil to use. In case the armature is to be rewound for changed 
conditions of speed or voltage reference should be made to 
Chapters X and XI where details to be observed are given 
for different changes in operating conditions. 

Operating a Generator as a Motor and Vice Versa. It is 
usually possible to operate a generator as a motor by setting 




FIG. 162. Direct-current generator with armature rotating clockwise. 

the brushes for the correct rotation of the armature and with 
a backward lead instead of a forward lead when operating as 
a generator. To successfully operate a motor as a generator 
the air gap should be reduced to a minimum and the speed 

16 



242 ARMATURE WINDING AND MOTOR REPAIR 

increased where the voltage as a generator is to be equal or 
higher than the voltage as a motor. When the voltage is to 
be lower, the same speed and air gap can usually be used. 
When a direct-current machine is changed from a generator 
to a motor, the current in both the armature and series field 
reverses. If, therefore, the machine is operated as a cumu- 
latively compounded generator, it will also operate as a differ- 
entially compounded motor. In order to use a compound 
generator as a motor, it is usually necessary to reverse the 
connections of its series fields. The diagrams of Figs. 162 
and 163 are self-explanatory. 




Fio. 163. Differential motor with armature rotating clockwise and series 
field opposing shunt field. 

Adjusting the Air Gap on D.-C. Machines. When insert- 
ing the armature in any machine in which the bearings are 
independent of the frame, the air gap between the armature 
core and pole faces should be checked up. Any inequality 
of air gap will cause unnecessary friction and heating of the 
bearings and an unequal heating of the armature iron. The 
air gap may be adjusted horizontally in many cases by cross 
beams and jack screws on the bedplate and vertically by 
thin sheet liners inserted between the bedplate and the 
yoke. The air gap can be gauged during the operation by 
inserting a hardwood wedge on the front and back ends of 
the machine at different points, noting the distance to which 
the wedge enters each time. The adjustment should be con- 
tinued until the air gap is the same around the entire circum- 
ference of the machine. 

Motor Speed when Reconnecting a D.-C. Motor Winding 
Wave to Lap. When reconnecting a wave winding to form 
a lap winding without changing the coils, or voltage, the speed 



CHANGES IN DIRECT-CURRENT MOTORS 243 

of a four-pole motor will be increased twice, and the speed 
of an eight-pole motor increased four times. This is because 
the wave winding is a two-circuit winding while the lap 
winding has as many circuits as there are poles. The 
speed, therefore, varies directly as the number of circuits in 
the winding. 

Change in Brushes when Reconnecting a D.-C. Motor 
from a Higher to a Lower Voltage. When reconnecting a 
direct-current armature from a higher to a lower voltage, 
say 220 volts to 110 volts, the size of conductors must be 
doubled as the current will be about double its original value. 
The original brushes on the 220-volt machine must therefore 
be changed to handle this increase in current without heating 
and sparking. If twice as many brushes of the same size as 
before cannot be used, the size of brush must be changed to 
give twice the bearing surface. An increase in width of the 
brush should be avoided, the length being increased to get the 
proper bearing surface. A current density of more than 50 
amperes per square inch will give trouble sooner or later in 
a reconnected armature. 

For a four-pole motor originally 220 volts, changed to 
operate on 110 volts, the field coils should be connected two 
in series and these groups connected in parallel in order to 
get the same field current. 

REWINDING AND RECONNECTING DIRECT- CURRENT ARMA- 
TURE WINDINGS FOR A CHANGE OF VOLTAGE* 

When a direct-current armature is to be rewound or re- 
connected for a change from one voltage to another, the 
number of turns in series between brushes as explained on page 
239 will vary directly as one voltage to the other and the 
cross-sectional area of the wire will vary inversely as the vol- 
tages. To illustrate, take the case of a 2-pole armature con- 
sisting of 24 coils, 20 turns per coil wound one wire-in-hand 
using number 19 B. & S. gauge wire, which has a cross-sec- 
tional area of 1290 circular mils. The armature has a 240-volt 
winding and is to be changed so as to operate on a 120- volt 

* T, Schutter, Electrical Engineering, July, 1918, 



244 ARMATURE WINDING AND MOTOR REPAIR 

circuit. This can be accomplished in two ways. First, by 
rewinding the armature; second, by reconnecting the present 
winding. 

By comparing the original voltage with the new operating 
voltage, it will be seen that the new voltage is just half of the 
original voltage. If it took 20 turns per coil for 240 volts 
then it will take half of 20 or 10 turns per coil for 120 volts. 
As the machine is to do the same amount of work, it will 
carry twice the current at 120 volts that it did at 240 volts. 
For this reason the wire must have twice the cross-sectional area, 
or 1290 X 2 = 2580 circular mil area. This is equal to a num- 
ber 16 B. & S. gauge wire, still using the same number of coils 
as before. 




Original 240 volt connection 

FIG. 164. A 240- volt lap winding which is reconnected for 120 volts as 
shown in Fig. 165. 

The other method is to reconnect the winding so that two 
coils are in parallel, and bridge two commutator bars. This 
will result in a winding of 12 coils, two in parallel, with 20 
turns per coil. In Fig. 164 the original winding is shown, as 
it was wound and connected to the commutator. The arma- 
ture core contained 24 slots and there were two coil sides per 
slot. The part of the slot which is occupied by a coil side is 
called a winding space. The odd numbered winding spaces 
are considered as being in the bottom of the slot and the even 
numbered winding spaces being in the top of the slot. The 
table for winding coils in Fig. 164 is given on page 245 and 
the connecting table for the original winding as illustrated in 
Fig. 164 on page 247. 



CHANGES IN DIRECT-CURRENT MOTORS 



245 



By tracing the direction of flow of current through the wind- 
ings from the positive brush to the negative brush, it will be 
seen there are two paths or circuits, each consisting of 12 coils 
in series, the two paths or circuits being in parallel with each 
other. This winding operates on 240 volts. 

TABLE FOR WINDING COILS ON ARMATURE OF FIG. 164 



Coil number 


Coils are wound 


In spaces number 


In slots number 


1 


1 and 24 


1 and 12 


2 


3 and 26 


2 and 13 


3 


5 and 28 


3 and 14 


4 


7 and 30 . 


4 and 15 


5 


9 and 32 


5 and 16 


6 


11 and 34 


6 and 17 


7 


13 and 36 


7 and 18 


8 


15 and 38 


8 and 19 


9 


17 and 40 


9 and 20 


10 


19 and 42 


10 and 21 


11 


21 and 44 


11 and 22 


12 


23 and 46 


12 and 23 


13 


25 and 48 


13 and 24 


14 


27 and 2 


Hand 1 


15 


29 and 4 


15 and 2 


16 


31 and 6 


16 and 3 


17 


33 and 8 


17 and 4 


18 


35 and 10 


18 and 5 


19 


37 and 12 


19 and 6 


20 


39 and 14 


20 and 7 


21 


41 and 16 


21 and 8 


22 


43 and 18 


22 and 9 


23 


45 and 20 


23 and 10 


24 


47 and 22 


24 and 11 



246 ARMATURE WINDING AND MOTOR REPAIR 

TABLE FOR CONNECTING COILS OP FIG. 164 TO COMMUTATOR 



Beginning of coil 
number 


To bar number 


End of coil number 


To bar number 


1 


6 


1 


7 


2 


7 


2 


8 


3 


8 


3 


9 


4 


9 


4 


10 


5 


10 


5 


11 


6 


11 


6 


12 


7 


12 


7 


13 


8 


13 


8 


14 


9 


14 


9 


15 


10 


15 


10 


16 


11 


16 


11 


17 


12 


17 


12 


18 


13 


18 


13 


19 


14 


19 


14 


20 


15 


20 


15 


21 


16 


21 


16 


22 


17 


22 >' 


17 


23 


18 


23 


18 


24 


19 


24 


19 


1 


20 


1 


20 


2 


21 


2 


21 


3 


22 


3 


22 


4 


23 


4 


23 


5 


24 


5 


24 


6 




PIG. 165. The 240-volt winding of Fig. 164 reconnected for a 120-volt 
circuit by connecting two coils in parallel and using wider brushes. 

Reconnecting a Lap Winding. In Fig. 165 the winding 
shown in Fig. 164 has been reconnected so as to operate on 
a 120-volt circuit. 



CHANGES IN DIRECT-CURRENT MOTORS 



247 



As explained it will require one-half as much winding on 120 
volts as it did on 240 volts. This can be accomplished by 
connecting two coils in parallel, and using wider brushes; 
that is, the brushes should be at least as wide as 1J/2 commu- 
tator bars and not more than two commutator bars. If it 
is possible to arrange for the use of the wider brushes, the 
commutator can be bridged, as shown by the jumpers A, B, C, 
etc., Fig. 165. 

v 'I'' 

TABLE FOB CONNECTING COILS OF FIG. 165 TO COMMUTATOR 



Beginning of coil 
number 


To bar number 


End of coil number 


To bar number 


1 


5 


1 


7 


2 


6 


2 


8 


3 


7 


3 


9 


4 


8 


4 


10 


5 


9 


5 


11 


6 


10 


6 


12 


7 


11 


7 


13 


8 


12 


8 


14 


9 


13 


9 


15 


10 


14 


10 


16 


11 


15 


11 


17 


12 


16 


12 


18 


13 


17 


13 


19 


14 


18 


14 


20 


15 


19 


15 


21 


16 


20 


16 


22 


17 


21 


17 


23 


18 


22 


18 


24 


19 


23 


19 


1 


20 


24 


20 


2 


21 


1 


21 


3 


22 


2 


22 


4 


23 


3 


23 


5 


24 


4 


24 


6 



By tracing the direction of flow of current through the wind- 
ing it will be seen that the current flows in four circuits from 
the positive brush to the negative brush. In each path or cir- 
cuit there are six coils in series, and the four paths or circuits 
are in parallel. This reconnection is equivalent to rewinding 



248 ARMATURE WINDING AND MOTOR REPAIR 



the armature with twice the size of wire and one-half the 
number of turns with which it was wound for 240 volts. Both 
Figs. 164 and 165 show lap or parallel windings. 

Reconnecting a Wave Winding. Fig. 166 represents a 
four-pole wave or series winding, consisting of 31 .coils. The 




Slot No. 



Original240 v olt Connection 

FIG. 166. Wave winding for a 240-volt, 4-pole armature. 

armature core has 31 slots, and there are two coil sides per 
slot. It is wound so as to be operated on a 240-volt circuit. 
The coils in this Fig. 166 are placed so that the beginning of 
each coil is placed in the bottom of the slot, and is represented 
by the odd numbered coil sides. 



Winding 
Space No. 




Slot No. 



Reconnected for 120 Volts 

- FIG. 167. The wave winding of Fig. 166 reconnected so that there are 
four paths for current instead of two with four brushes instead of two. This 
winding can now be used on a 120-volt circuit. 

By tracing the current from the positive to the negative 
brush, it will be seen that the winding is divided into two paths 
or circuits. To change this winding so that it can be operated 
on a 120-volt circuit, two methods can be used : First, reconnect 
the winding so that it will consist of four paths or circuits, 
and add another set of brushes as shown in Fig. 167. The 



CHANGES IN DIRECT-CURRENT MOTORS 
TABLE FOR WINDING COILS IN SLOTS OF FIG. 166 



249 



Coil number 


Coils are wound 


In spaces number 


In slots number 


1 


1 and 16 


1 and 8 


2 


3 and 18 


2 and 9 


3 


5 and 20 


3 and 10 


4 


7 and 22 


4 and 11 


5 


9 and 24 


5 and 12 


6 


11 and 26 


6 and 13 


7 


13 and 28 


7 and 14 


8 


15 and 30 


8 and 15 


9 


17 and 32 


9 and 16 


10 


19 and 34 


' 10 and 17 


11 


21 and 36 


11 and 18 


12 


23 and 38 


12 and 19 


13 


25 and 40 


13 and 20 


14 


27 and 42 


14 and 21 


15 


29 and 44 


15 and 22 


16 


31 and 46 


16 and 23 


17 


33 and 48 


17 and 24 


18 


35 and 50 


18 and 25 


19 


37 and 52 


19 and 26 


20 


39 and 54 


20 and 27 


21 


41 and 56 


21 and 28 


22 


43 and 58 


22 and 29 


23 


45 and 60 


23 and 30 


24 


47 and 62 


24 and 31 


25 


49 and 2 


25 and 1 


26 


51 and 4 


26 and 2 


27 


53 and 6 


27 and 3 


28 


55 and 8 


28 and 4 


29 


57 and 10 


29 and 5 


30 


59 and 12 


30 and 6 


31 


61 and 14 


31 and 7 



additional brushes are marked AI and B\. Second, connect 
two coils in parallel and still have two circuits in the winding. 
This, however, will necessitate the dropping of one coil, as 
shown by the heavy lines in Fig. 168. Only two brushes 
will be required as shown, if they can be set so that they 
cover at least 1J^ bars, the jumpers A, B, C, etc., can be 
omitted. 



250 ARMATURE WINDING AND MOTOR REPAIR 

TABLE FOR CONNECTING COILS OF FIG. 166 TO THE COMMUTATOR 



Beginning of coil 
number 


To bar number 


End of coil number 


To bar number 


1 


28 


1 


12 


2 


29 


2 


13 


3 


30 


3 


14 


4 


31 


4 


15 


5 


1 


5 


16 


6 


2 


6 


17 


7 


3 


7 


18 


8 


4 


8 


19 


9 


5 


9 


20 


10 


6 


10 


21 


11 


7 


11 


22 


12 


8 


12 


23 


13 


9 


13 


24 


14 


10 


14 


25 


15 


11 


15 


26 


16 


12 


16 


27 


17 


13 


17 


28 


18 


14 


18 


29 


19 


15 


19 


30 


20 


16 


20 


31 


21 


17 


21 


1 


22 


18 


22 


2 


23 


19 


23 


3 


24 


20 


24 


4 


25 


21 


25 


5 


26 


22 


26 


6 


27 


23 


27 


7 


28 


24 


28 


8 


29 


25 


29 


9 


30 


26 


30 


10 


31 


27 


31 


11 



The winding as shown in Fig. 167, is a four-pole lap or 
parallel winding, with four circuits or paths through the 
winding. The second method of reconnecting Fig. 166 so as 
to operate it on 120 volts is shown in Fig. 168. As previously 
explained, each two adjacent coils will be connected in parallel, 
and since there are 31 coils on the entire winding, one coil 
must be dropped. In this winding (Fig. 168) coil No. 2, shown 
with the heavy lines has no connection with the commutator 



CHANGES IN DIRECT-CURRENT MOTORS 251 

TABLE FOR CONNECTING COILS OF FIG. 167 TO THE COMMUTATOR 



Beginning of coil 
number 


To bar number 


End of coil number 


To bar number 


1 


4 


1 


5 


2 


5 


2 


6 


3 


6 


3 


7 


4 


7 


4 


8 


5 


8 


5 


9 


6 


9 


6 


10 


7 


10 


7 


11 


8 


11 


8 


12 


9 


12 


9 


13 


10 


13 


10 


14 


11 


14 


11 


15 


12 


15 


12 


16 


13 


16 


13 


17 


14 


17 


14 


18 


15 


18 


15 


19 


16 


19 


16 


20 


17 


20 


17 


21 


18 


21 


18 


22 


19 


22 


19 


23 


20 


23 


20 


24 


21 


24 


21 


25 


22 


25 


22 


26 


23 


26 


23 


27 


24 


27 


24 


28 


25 


28 


25 


29 


26 


29 


26 


30 


27 


30 


27 


31 


28 


31 


28 


1 


29 


1 


29 


2 


30 


2 


30 


3 


31 


3 


31 


4 




FIG. 168. The wave winding of Fig. 166 reconnected for a 120-volt circuit 
by connecting two coils in parallel and using wider brushes. In this case one' 
toil is dead as shown by the heavy lines. 



252 



ARMATURE WINDING AND MOTOR REPAIR 



There are 31 bars in the original commutator, so that by taking 
any two bars and putting a jumper across and considering 
them as one bar, it will be reduced to 30 bars. The connections 
will be made as shown by the following table: 

TABLE FOR CONNECTING COILS OF FIG. 168 TO THE COMMUTATOR 



Beginning of coil 
number 


To bar number 


End of coil number 


To bar number 


Coil 1 is not con- 








nected. 








2 


28 


2 


12 


3 


29 


3 


13 


4 


30 


4 


14 


5 


1 


5 


15 


6 


2 


6 


16 


7 


3 


7 


17 


8 


4 


8 


18 


9 


5 


9 


19 


10 


6 


10 


20 


11 


7 


11 


21 


12 


8 


12 


22 


13 


9 


13 


23 


14 


10 


14 


24 


15 


11 


15 


25 


16 


12 


16 


26 


17 


13 


17 


27 


' 18 


14 


18 


28 


19 


15 


19 


29 


20 


16 


20 


30 


21 


17 


21 


1 


22 


18 


22 


2 


23 


19 


23 


3 


24 


20 


24 


4 


25 


21 


25 


5 


26 

27 


22 
23 


26 
' 27 


6 

7 


28 


24 


28 


8 


29 


25 


29 


9 


30 


26 


30 


10 


31 


27 


31 


11 



Reconnecting Duplex Windings.* The windings discussed 
in what follows are wound with two wires in hand. The 
* T. Schutter, Electrical Engineering, August, 1918. 



CHANGES IN DIRECT-CURRENT MOTORS 



253 



same results can be accomplished by two windings, using one 
strand (two simplex windings) at a time. In a winding of the 
duplex type there are usually twice as many commutator 
bars as there are slots and each of the two wires is connected to 
separate bars. The brush, however, will cover at least 1J/2 
to 2 commutator bars, as shown in Fig. 169, which is the wind- 
ing and connections for a 120-volt lap-wound armature. 



12 34 1 la 2 2a 33a 




Slot No. 



Original 120 Volt Connection 

FIG. 169. Duplex lap winding for a 120-volt armature. 

Each coil consists of two parts, which will be called Sec- 
tion 1, and la, Section 2 and 2a, Section 3 and 3a, etc. The 
following is the winding table for Fig. 169. 

TABLE FOR WINDING COILS IN SLOTS OF FIG. 169 



Coil number 


Coils are wound 


In spaces number 


In slots number 


1-la 


1 and 6 


1 and 3 


2-2a 


3 and 8 


2 and 4 


3-3a 


5 and 10 


3 and 5 


4-4a 


7 and 12 


4 and 6 


5-5a 


9 and 14 


5 and 7 


6-6a 


11 and 16 


6 and 8 


7-7a 


13 and 18 


7 and 9 


8-8a 


15 and 20 


8 and 10 


9-9a 


17 and 22 


9 and 11 


10-10a 


19 and 24 


10 and 12 


11-lla 


21 and 2 


11 and 1 


12-12a 


23 and 4 


12 and 2 



This 120-volt winding is to be changed so that it can be 
operated on a 240- volt circuit. As shown in Fig. 169 Section 



254 ARMATURE WINDING AND MOTOR REPAIR 
TABLE FOR CONNECTING COILS TO THE COMMUTATOR OP FIG. 169 



Start of coil number 


To bar number 


End of coil number 


To bar number 


1 


3 


1 


5 


la 


4 


la 


6 


2 


5 


2 


7 


2a 


6 


2a 


8 


3 


7 


3 


9 


3a 


8 


3a 


10 


4 


9 


4 


11 


4a 


10 


4a 


12 


5 


11 


5 


13 


5a 


12 


5a 


14 


6 


13 


6 


15 


6a 


14 


6a 


16 


7 


15 


7 


17 


7a 


16 


7a 


18 


8 


17 


8 


19 


8a 


18 


8a 


20 


9 


19 


9 


21 


9a 


20 


9a 


22 


10 


21 


10 


23 


lOa 


22 


*10a 


24 


11 


23 


11 


1 


lla 


24 


lla 


2 


12 


1 


12 


3 


12a 


2 


12a 


4 



1 and la of coil 1 are connected in parallel through the brush 
connections, and by changing the connections so that Sec- 




Slot No. 



Reconnected for 2iO Volts 

FIG. 170. The 120-volt lap winding of Fig. 169 reconnected for 240 volts by 
connecting the turns of each coil in series and using smaller brushes. 

tions 1 and la will be in series instead of in parallel, it will then 
be possible to operate the winding on a 240-volt circuit. The 



CHANGES IN DIRECT-CURRENT MOTORS 



255 



winding table for Fig. 170 will be the same as for Fig. 169 
but the connecting table will be as follows: 

TABLE FOR CONNECTING COILS OF FIG. 170 TO COMMUTATOR 



Start of coil number 


To bar number 


End of coil number 


To bar number 


1 


1 


1 


2 


la 


2 


1 


3 


2 


3 


2 


4 


2a 


4 


2 


5 


3 


5 


3 


6 


3a 


6 


3a 


7 


4 


7 


4 


8 


4a 


8 


4a 


9 


5 


9 


5 


10 


5a 


10 


5a 


11 


6 


11 


6 


12 


6a 


12 


6a 


13 


7 


13 


7 


14 


7a 


14 


la 


15 


8 


15 


8 


16 


8a 


16 


8a 


17 


9 


17 


9 


18 


9a 


18 


9a 


19 


10 


19 


10 


20 


lOa 


20 


107 


21 


11 


21 


11 


22 


lla 


22 


lla 


23 


12 


23 


12 


24 


12a 


24 


12a 


1 



The brushes used on Fig. 170 should only be as wide as one 
commutator bar. By tracing the current through the winding 
in Fig. 169 it will be seen that there are three coils in series 
in each of the paths or circuits from the positive brush to the 
negative brush. In Fig. 169 there are six coils in each path or 
circuit, or twice as many as before the reconnection. 

If the winding in Fig. 169 had been connected to a 12-bar 
commutator instead of a 24-bar commutator, the reconnecting 
from 120 volts would have been somewhat different. In 
Fig. 169 the numerals from 1 to 12, which are placed above 
and between each two bars, will give an idea of how the con- 
nections would look. For instance, Sections 1 and la of 



256 ARMATURE WINDING AND MOTOR REPAIR 

coil No. 1 would be connected across bars Nos. 2 and 3, instead 
of Section 1 to bars 3 and 5, and la to bars 4 and 6. 

Then to reconnect the winding from 120 volts to 240 volts 
when only 12 commutator bars are used, Sections 1 and la 
of Coil No. 1 would have to be connected in series, but not 
through commutator connections as in Fig. 170. By omitting 
the connections of commutator bars, No. 2, 4, 6, 8 and 10, 
etc., and simply splicing the end of Section 1 to the beginning 
of Section la of coil No. 1, the same results will be obtained 
by using only 12 commutator bars. 

Fig. 171 is a " duplex wave" winding for operation on a 
120-volt circuit. This winding consists of 13 coils wound with 
two strands of wire and connected to 26 commutators bars. 
This winding could also be connected to a 13-bar commu- 
tator. The winding table for Fig. 171 is as follows, each coil 
being considered as two Sections, 1 and la. 



TABLE FOR WINDING COILS IN SLOTS FOE FIG. 171 



Coil number 


Coils are wound 


In spaces number 


In slots number 


1 and la 


1 and 6 


1 and 3 


2 and 2a 


3 and 8 


2 and 4 


3 and 3a 


5 and 10 


3 and 5 


4 and 4a 


7 and 12 


4 and 6 


5 and 5a 


9 and 14 


5 and 7 


6 and 6a 


11 and 16 


6 and 8 


7 and 7a 


13 and 18 


7 and 9 


8 and 8a 


15 and 20 


8 and 10 


9 and 9a 


17 and 22 


9 and 11 


10 and 10a 


19 and 24 


10 and 12 


11 and lla 


21 and 26 


11 and 13 


12 and 12o 


23 and 2 


12 and 1 


13 and 13a 


25 and 4 


13 and 2 



The winding as it is now placed and connected will operate 
on a 120-volt circuit. It is desired to change it so that it 
will operate on a 240-volt circuit. There are two methods 
by which this can be accomplished. The first is shown in 



CHANGES IN DIRECT-CURRENT MOTORS 257 

TABLE FOR CONNECTING COILS OF FIG. 171 TO COMMUTATOR 



Start of coil number 


To bar number 


End of coil number 


To bar number 


1 


23 


1 


9 


la 


24 


la 


10 


2 


25 


2 


11 


2a 


26 


2a 


12 


3 


1 


3 


13 


3a 


2 


3a 


14 


4 


3 


4 


15 


4a 


4 


4a 


16 


5 


5 


5 


17 


5a 


6 


5a 


18 


6 


7 


6 


19 


6a 


8 


6a 


20 


7 


9 


7 


21 


la 


10 


7a 


22 


8 


11 


8 


23 


8a 


12 


8a 


24 


9 


13 


9 


25 


9a 


14 


9a 


26 


10 


15 


10 


1 


lOa 


16 


lOa 


2 


11 


17 


11 


3 


lla 


18 


lla 


4 


12 


19 


12 


5 


12a 


20 


12a 


6 


13 


21 


13 


7 


13a 


22 


13a 


8 



12 34 llo 22 a 3 3a 




Original 120 Volt Connection 

FIG. 171. A duplex wave winding for a 120- volt armature. 

Fig. 172. In this case it will be seen that Section 1 of coil 
1 is dropped, that is, it is not connected to the commutator. 
This may be done so that a wave winding is possible. In- 
stead of connecting the different sections of a coil in parallel 

17 



258 ARMATURE WINDING AND MOTOR REPAIR 



through the commutator and brush connections, they are 
now connected in series in the same way. The connecting 
table is as follows : 

TABLE FOR CONNECTING COILS OF FIG. 172 TO COMMUTATOR 



Start of coil number 


To bar number 


End of coil number 


To bar number 


la 


21 


la 


9 


2 


22 


2 


10 


2a 


23 


2a 


11 


3 


24 


3 


12 


3a 


25 


3a 


13 


4 


1 


4 


14 


4a 


2 


4a 


15 


5 


3 


5 


16 


5a 


4 


5a 


17 


6 


5 


6 


18 


6a 


6 


6a 


19 


7 


7 


7 


20 


7a 


8 


7a 


21 


8 


9 


8 


22 


8a 


10 


8a 


23 


9 


11 


9 


24 


9a 


12 


9a 


25 


10 


13 


10 


1 


lOa 


14 


lOa 


2 


11 


15 


11 


3 


lla 


16 


lla 


4 


12 


17 


12 


5 


12a 


18 


12a 


6 


13 


19 


13 


7 


13a 


20 


13a 


8 



It will be seen that bars Nos. 26 and 1 are bridged and are 
acting as one bar. This is due to the dropping of Section 1 
in coil No. 1 for reasons given above. By tracing the flow of 
the current from the positive to the negative brush in both 
Figs. 171 and 172, is will be found that there are twice as 
many coils in series in Fig. 172 as there are in Fig. 171. 
It is, therefore, possible to operate Fig. 172 on a 240- volt 
circuit. 

The other method of reconnecting Fig. 171 so as to operate 



CHANGES IN DIRECT-CURRENT MOTORS 



259 



it on a 240- volt circuit is shown in Fig. 173. The coils in 
this case have been placed the same as in Figs. 171 and 172, 



1 la 2 2a 3 3 




FIG. 172. The duplex wave winding of Fig. 171 reconnected for 240 volts by 
connecting the coils in series through the commutator and the brushes. 



12 3* 







FIG. 173. The duplex wave winding of Fig. 171 reconnected by connecting 
the turns of each coil in series and using one-half the number of commutator 
bars. In this case two adjacent bars must be connected together and brushes 
wide enough to cover two bars. 

therefore the winding table will be the same. The connect- 
ing table is as follows: 

TABLE FOR CONNECTING COILS OP FIG. 173 TO COMMUTATOR 



Start of coil number 


To bar number 


End of coil number 


To bar number 


1 


23 


la 


9 


2 


25 


2a 


11 


3 


1 


3a 


13 


4 


3 


4a 


15 


5 


5 


5a 


17 


6 


7 


6a 


19 


7 


9 


7a 


21 


8 


11 


8a 


23 


9 


13 


9a 


25 


10 


, 15 


lOa 


1 


11 


17 


lla 


3 


12 


19 


12a 


5 


13 


21 


13a 


7 



260 ARMATURE WINDING AND MOTOR REPAIR 

From the above connecting table it will be seen that the 
end of Section 1 is connected to the beginning of Section la 
or, in other words, the two sections of a coil, which were in 
parallel with one another in Fig. 171 are now connected in 
series. 

As already explained, the number of turns per coil will be 
directly proportional to the voltage. In Fig. 171 assume 
that there are 20 turns per coil with two wires in parallel. By 
reconnecting as in Fig. 173, there will be 40 turns per coil, 
using one wire. From this it will be seen that while there are 
13 coils in each winding, Figs. 171 and 173, one has twice as 
many turns per coil as the other. This fact makes it pos- 
sible to operate Fig. 173 on a 240-volt circuit. 



CHAPTER XI 

PRACTICAL WAYS FOR RE- CONNECTING INDUCTION 

MOTORS 

There are certain changes in the windings of an induction 
motor that can be made by the repairman to adapt a motor to 
changed operating conditions. There are also certain changes 
that should not be attempted. Details of the most practical 
re-connections and their effects on motor operation have been 
carefully outlined accompanied by diagrams, in the Electrical 
Journal by A. M. Dudley. These details are summarized in 
what follows. 

The changes that can be made in the connections of the 
induction motor may be divided into three classes. The first 
class includes those which leave the motor entirely normal and 
the performance in all essential respects the same as before 
re-connection. 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. The second class of changes 
leaves the performance in some respects unchanged and alters 
it in others. These may be represented by operating a motor 
in star on 440 volts and in delta for 220 volts. In this case 
there is little change in efficiency or power factor; the starting 
and maximum torque, however, are only 75 per cent, of their 
original values. In such a case the advisability of the change 
depends upon the work that the motor must do. If the torque 
at the altered values is sufficient to start and carry the driven 
load easily, there is no objection to operating the motor indefi- 
nitely so reconnected, since the motor will not run any 
warmer than before and its efficiency and power factor may be 
better. 

The third class of changes leaves the motor operative in the 
sense of producing torque enough to do the work required, 
but so alters its performance as to heating, or efficiency or 
power factor or insulation that it is undesirable to leave the 

261 



262 ARMATURE WINDING AND MOTOR REPAIR 

motor operating indefinitely in such condition. Such changes 
are represented by re-connecting a three-phase motor without 
changing the coils for two-phase operation. This is equiva- 
lent to operating the three-phase motor at 125 per cent, normal 
voltage. In addition, the coils which should have extra in- 
sulation where the phases change have only group insulation. 
The iron loss and heating may be increased to a dangerous 
degree and the power factor greatly decreased. Such changes 
should only be used in an emergency and the proper permanent 
changes made as soon as possible. 

Points to Consider before Making Re -connections. Before 
a repairman attempts to make a radical change in the connec- 
tions of an induction-motor winding he should consider care- 
fully the limitations of the design and the effects of the changes 
The following points will serve as a guide. 

1. Changes in voltage alone are the easiest and can usually be made. 

2. Changes in number of phases alone can rarely be made satisfactorily 
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 combination 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 caution. 

6. If the peripheral speed of the rotor (which equals rotor diameter 
in feet X 3.14 X rpm.) 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. 

Diagrams for Different Changes of Connections. Two 

kinds of diagrams are most used to indicate the connections 
to be made in induction motors. These are shown in Figs. 
174 and 175. The diagram of Fig. 174 is a three-phase, four- 
pole winding connected in star on a 36-slot core. It represents 
the coils as they would look if removed from the machine and 
laid on a table with the actual connections made. The dia- 
gram of Fig. 175 is a conventional sketch for the winding of 
Fig. 174 showing the polar groups. The latter diagram is 
much used, 



RE-CONNECTING INDUCTION MOTORS 



263 



In the winding illustrated, since there are three times four or 
12 polar groups and 36 coils, there are three coils connected 
in series to form a polar-phase-group. It should be borne in 




Full lines represent coils in tops of slots, 
broken lines coils in bottoms of slots 

FIG. 174. Complete winding diagram for a 3-phase, 4-pole motor having 
36 slots, and connected in series-star. 

mind that there are not actually 12 magnetic poles in the 
machine, for the reason that three consecutive polar-phase- 
groups unite to form orje magnetic pole by virtue of the phase 
difference of the currents in the three phases. There are two 
magnetic north and two magnetic south poles formed by this 
winding at any instant, and these poles are equally spaced 
around the air-gap like four mechanically projecting pole 
pieces excited by four coils carrying direct current. Some- 
thing of this conception is gained 
if one imagines the armature of 
a direct-current generator held 
stationary and the field poles 
rotated around it. At any given 
instant the magnetic field can 
be conceived to be the same as 
the field which is formed by 
the winding shown in Fig. 174. 
The coils in slots 3, 6, 9, 12, 
etc., are shown by heavy lines 
to indicate that the insulation 

on these Coils is heavier to FlG - 175 - Circle diagram for the 
j.u j XT. connections shown in Fig. 174. 

withstand the greater strain at 

the points where the winding crosses or lies adjacent to coils 
differing greatly in potential. This is called "phase insula- 
tion," and may be put on the first coil in each group or it 




264 ARMATURE WINDING AND MOTOR REPAIR 

may be put on the first and last coil of each group where there 
are a large number of coils in the group. This is one of the 
reasons why a machine may not at times be reconnected 
for another number of poles or phases. If such reconnections 
were made the maximum differences in potential might occur 
between two adjacent coils unprotected by this extra insula- 
tion and a break-down result. 

The conventional diagram of Fig. 175 represents the same 
connections as Fig. 174, except that each pole-phase-group 
as numbered 1, 2, 3, etc., in Fig. 174, is shown by a short arc 
in Fig. 175. The numbers on the groups are identical with 
Fig. 174, and the group connections. The arrows are shown 
simply to indicate a method of checking up to insure the proper 
phase relations. There is considerable danger, in a three- 
phase connection, of getting a 60-degree relation between the 
phases instead of a 120-degree relation or, as it might be ex- 
pressed on the diagram, there is danger that the wrong end of 
the B phase, for example, may be connected to the star point. 
As a check against this, when the diagram is completed, the 
current is assumed as going in at all three leads toward the 
Y point. Arrows are put on each pole-phase-group as shown, 
and when all three phases are traced through, the winding is 
correct if the arrows on consecutive groups run alternately 
clockwise and counter-clockwise. It may be argued that this 
is an artificial assumption and that at no instant is the current 
flowing toward the star in all three phases. It may also be 
argued that in a correct winding, if the current be assumed as 
flowing toward the star in two phases and always from it in 
the third, the arrows will fall in successive sets of three in the 
same direction and then three in the reverse direction. These 
statements are true, but a little experimenting will show that 
an incorrectly connected or 60-degree winding can in this way 
be shown to give successive sets of three arrows and still be 
wrong. There is but one exception to the correctness of the 
check as shown in Fig. 175 where the current is assumed as 
flowing toward the star in all three phases and the arrows 
alternate in direction. This exception to the rule is the case 
where the winding forms consequent poles or passes through 
all the pole-phase-groups in a north direction instead of 



RE-CONNECTING INDUCTION MOTORS 



265 



alternately north and south. Such connections are rarely used , 
and then usually on special motors wound for multi-speeds. 
Diagrams for Three-phase Motors. Fig. 176 gives a com- 




B C A 



FIG. 176. Diagram for a 3-phase, 
4%pole winding with parallel-star end 
connections. Schematic equivalent 
in center. 



FIG. 177. Diagram for a 3-phase, 
4-pole winding with 4-parallel-star 
end connections. Schematic equiva- 
lent in center. 




(A-BXB-CXA-CV 

Fio. 178. Diagram for a 3- 
phase, 4-pole winding with series- 
delta end connections. Schematic 
equivalent in center. 



FIG. 179. Connecting diagram for 
a 3-phase, 4-pole, parallel-delta wind- 
ing. A schematic equivalent is shown 
in the center of the diagram. 



bined conventional and schematic representation of a so-called 
parallel star diagram, where the two halves of each phase are 



266 ARMATURE WINDING AND MOTOR REPAIR 

\ 

in parallel. If a given machine were connected, as shown in 
Fig. 175, for a normal voltage of 440, it could readily be recon- 
nected according to Fig. 176, and would then be suitable for 
operation on 220 volts having the same performance in all re- 
spects except that it would draw from the 220-volt line twice 
as many amperes under a given load as it previously drew from 
the 440-volt line. Similarly, if it had four poles, or a multiple 
of four poles, it could still be paralleled again, or put in 4- 
parallel star, as shown in Fig. 177, and operated on 110 volts, 
and would still have the same performance at a correspond- 
ingly increased current at the same load. ' 

Fig. 178 represents a variation in connection from the fore- 
going, which is possible only with three-phase machines. 
This is the so-called delta or mesh connection. If a machine 
connected as in Fig. 174 for 440 vofts be reconnected as in 
Fig. 178 it would be suitable for operation on a circuit having 
a voltage of 440 -f- 1.73 or 254 volts. 



COMPARISON OF MOTOR VOLTAGES WITH VARIOUS CONNECTIONS 

If a motor connected originally as shown in any horizontal column had 
a normal voltage of 100 its voltage when reconnected as indicated in any 
vertical column is shown at the intersection of the two columns. 





02 


QQ 


02 


O2 


1 


V 

Q 



P 





a 

13 




1 














J 
1 


o3 

N 


1 
CO 


1 
4< 


1 

o 


J 
1 


i 

e!, 


- 

CO 


- 
* 


1 

U5 


1 


1 


t 

CO 


PH 
^ 


1 

o 




% 


fi 

co 


_c 
b 
co 


6 




& 

A 




CO 




CO 


x 

- 

CO 




CO 




CO 



et 




e!, 


fi 
<A 


4 
<N 



.A 


3-Phase Series Star 



100 


50 


33 


25 


20 


58 


29 


19 


15 


12 


81 


41 


27 


20 


16 


3-Phase 2-Parallel Star. . 


200 


100 


67 


50 


40 


116 


58 


38 


29 


23 


162 


81 


54 


40 


32 


3-Phase 3-Parallel Star.. 


300 


150 


100 


75 


60 


174 


87 


57 


44 


35 


243 


122 


81 


60 


48 


3- Phase 4-Parallel Star. . 


400 


200 


133 


100 


80 


232 


116 


76 


58 


46 


324 


163 


108 


80 


64 


3-Phase 5-Parallel Star.. 


500 


250 


165 


125 


100 


290 


145 


95 


73 


58 


405 


203 


135 


100 


80 


3-Phase Series Delta 


173 


86 


58 


43 


35 


100 


50 


33 


25 


20 


140 


70 


47 


35 


28 


3-Phase 2-Par. Delta. . . . 


346 


172 


116 


86 


70 


200 


100 


66 


50 


40 


280 


140 


94 


70 


56 


3-Phase 3-Par. Delta.... 


519 


258 


174 


129 


105 


300 


150 


100 


75 


60 


420 


210 


141 


105 


84 


3-Phase 4-Par. Delta.... 


692 


344 


232 


172 


140 


400 


200 


133 


100 


80 


560 


280 


188 


140 


112 


3-Phase 5-Par. Delta 


865 


4-30 


290 


215 


175 


500 


250 


165 


125 


100 


700 


350 


235 


175 


140 


2-Phase Series 


125 


63 


42 


31 


25 


73 


37 


24 


18 


15 


100 


50 


33 


25 


20 


2- Phase 2-Parallels 


250 


125 


84 


63 


50 


146 


73 


49 


37 


29 


200 


100 


67 


50 


40 


2-Phase 3-Parallels 


375 


188 


125 


94 


75 


219 


110 


73 


55 


44 


300 


150 


100 


75 


60 


2- Phase 4-Parallels 


500 


250 


167 


125 


100 


292 


146 


97 


73 


58 


400 


200 


133 


100 


80 


2-Phase 5-Parallels 


625 


313 


208 


156 


125 


365 


183 


122 


91 


73 


500 


250 


167 


125 


100 



RE-CONNECTING INDUCTION MOTORS 



267 



Use of Table of Connections. Different reconnections or 
conversions are shown in the Table on page 266 where the 
problem just shown may be worked out by selecting Three-phase 
Series Star in the horizontal 
column (first line) and reading 
across to the vertical column 
headed Series Delta where the 
figure 58 appears. This means 
that if 100 volts was normal on 
the series star connection and a 
change is made to series delta 
the corresponding voltage is 58. 
By multiplication, if 440 was 
the series star voltage, the series 
delta voltage would be 4.4 X 58 
= 254, as noted above. Figs. 
179 and 180 show a parallel 
and 4-parallel delta connec- 
tion, respectively, and bear 
the same relation to Fig. 178 that Figs. 176 and 177 do to Fig. 
175. 

Two-phase Diagrams. In Fig. 181 a development of a two- 
phase winding is shown similar to Fig. 174, except for the differ- 
ence in the number of phases. An inspection of the coils 
represented in heavy lines and a comparison with the coils in 




B c A 

(,B-C)(A-C)IA;B) 



FIG. 180. Connecting diagram 
for a 3-phase, 4-pole, 4-parallel- 
delta winding. 




Full lines represent Coils in tops' of slots, broken lines coils in bottoms of slots 

FIG. 181. Complete winding diagram for a 2-phase, 4-pole motor series 
connected. The coils from x to y form one pole-phase-group. 

Fig. 174 indicates at once what is meant by the " phase coils" 
or phase insulated coils being differently situated. This also 
explains one of the good reasons why two-phase motors should 



268 ARMATURE WINDING AND MOTOR REPAIR 

not be reconnected for three-phase, or vice versa, without 
changing the position of these " phase coils. 
Fig. 182 gives the conventional and schematic equivalent cf 





FIG. 182. Connecting diagram for 
a 2-phase, 4-pole winding with series 
connections. 



FIG. 183. Connecting diagram for 
a 2-phase, 4-pole winding with parallel 
connections. 



Fig. 181. 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 the direction of rotation 
of the motor. This is readily 
corrected by reversing the two 
leads of one phase at the motor 
terminals. Figs. 183 and 184 
give parallel and 4-parallel, two- 
phase connections and bear the 
same relation to the series con- 
nection as the three-phase star 
and delta diagrams. From these 

n l n 2 l z 

FIG. 184. Connecting diagram 2-parallel and 4-parallel connec- 
for a 2-phase, 4-pole winding with tions it may be readily seen that 

4-parallel connections. . , , r i 

where the number of poles is a 

multiple of three, as 6, 12, 18, etc., there is a possible analogous 
3-parallel connection; also where the number of poles is a 
multiple of five, such as 10, 20, etc., there is a corresponding 




RE-CONNECTING INDUCTION MOTORS 269 

possible 5-parallel diagram. These are the connections which 
are indicated in the Table on page 266 as "3-parallel" and 
" 5-parallel." 

Fig. 185 shows a possible three-phase connection which 
may be made from a two-phase winding by a method similar 
to the Scott transformer connection. It is a connection which 
should be used only as a temporary expedient until better 
arrangements can be made. 



-Coils removed 
, from circuit 

;7 




IBs 

FIG. 185. The T-connection by which a 2-phase motor may be operated on 
a 3-phase circuit. 

Meaning of the Term Chord Factor. It is well known that 
the span of the coil must in general be somewhere near the 
quotient of the bore periphery divided by the number of poles. 
It is not so generally understood that changing the span of the 
coil within limits has an effect similar to increasing or decreas- 
ing the number of wires in the coil. If the coil is exactly pitch, 
i.e., spans exactly from the center of one pole to the center of 
the next, the turns of wire in that coil are producing their 
maximum effect upon the magnetic field. The coil is then 
considered to span 180 electrical degrees. It is customary to 
wind the coil in slots so that it spans something less than a 
full pole pitch. The effect of the turns in the coil is then some- 
what less than the maximum. The effect of the turns in the 
coil varies as the sine of half of the angle in electrical degrees 
which the coil spans. 

To illustrate, if there are 72 slots in an eight-pole machine, 
the coils would be exactly " pitch" if they lay in slots 1 and 10, 



270 ARMATURE WINDING AND MOTOR REPAIR 

or in other words, if there were eight slots between the two 
slots in which the two sides of any coil were located. Such a 
coil would span 180 electrical degrees. Half of 180 degrees is 
90 degrees and the sine of 90 degrees is 1 ; therefore the effect 
of the turns in such a coil is one. Suppose instead the coil lies 
in slots 1 and 8. It would then t span 140 degrees elec- 
trically, since 72 -f- 8 = 9 slots represents 180 degrees, and 
one slot therefore represents 20 degrees. The sine of half of 
140 degrees, or 70 degrees, is 0.938. It follows that the effect 
of the turns in this coil is less than that of the full pitch coil 
by the ratio of 0.938 to 1. This is of interest in the present 
problem, because it is often possible in making changes to 
change at the same time the span of the coils by one slot, 
more or less, by springing the coil mechanically, and so im- 
prove the performance of the machine under the new con- 
ditions. The point becomes of vital importance, immediately, 
when changing the number of poles without changing the 
throw of the coils. Referring again to the 72- slot motor, 
assume that the coils are wound in slots 1 and 8. For 
an eight-pole connection these coils will have the effect of 
0.938, as explained above. If the connections are changed 
for six poles the effect is entirely different. 72 -r- 6 = 12 and 
180 -:- 12 = 15, or each slot represents 15 electrical degrees. 
A throw of 1 and 8 covers seven complete slots, or 
7 X 15 = 105 degrees; the sine of half of 105 or 52.5 degrees 
= 0.79, which means that when connected for six poles 
the coils have an effect of only 0.79, as against 0.938 when 
connected for eight poles. It is possible to avoid using the 
sine of half the angle and secure a factor which is sufficiently 
accurate practically by using the expression, 

(Number of slots per pole) 2 2 (Number of slots dropped) 2 
(Number of slots per pole) 2 

Using the same eight-pole example above, the number of 
slots per pole is 72 -r- 8 = 9 and the pole pitch is 1 and 10. 
When the coil is wound 1 and 8 it spans seven slots and there 
are 9 7=2 slots dropped. The expression then becomes 



= A ^ = 0.948 



RE-CONNECTING INDUCTION MOTORS 271 

and similarly for the six-pole, 



(12) 2 - 2(5 2 ) 
12 2 

which agrees roughly with the other method. A coil should in 
no case be chorded more than half of the pole pitch, as second- 
ary disturbances of the magnetic field are occasioned by chord- 
ing which become prohibitive at that point. The expression, 
"sine of half the angle spanned by the coil," is given the name 
chord factor, and it should be considered in the work of re-con- 
necting. For example, if the poles are changed from 8 to 6, 
as in the example above, and the chord factor changes from 
0.938 to 0.79, the new line voltage should be 0.79 *- 0.938 
times the old, neglecting the effect of other changes which are 
being made. If nothing else was undergoing change and the 
-normal voltage was 440 in the first place, it should be 370 after 
the change is made or, expressing it another way, if it was still 
operated at 440 volts after the change, the motor should be 
thought of as operating at about 18 per cent, over voltage. 

Phase Insulation. It is the practice of many manufac- 
turers to put heavier insulation on the coils at the ends of the 
polar groups which are mechanically adjacent to one another 
and are also subjected to the voltage between phases, which 
may be the maximum voltage between supply lines. Such 
coils are illustrated at Nos. 3, 6, 9, 12, 15, etc., in Fig. 174. 
By comparing this diagram with Fig. 181 for two-phase 
connection, it appears at once that both the number 
and location of these so-called "phase coils" should be 
changed at the time the machine is re-connected from two- to 
three-phase, or vice versa, assuming that the voltage can be 
changed so that a phase change is permissible. Also in 
changing the number of poles, the number and location of the 
"phase coils" must also be changed. In fact, whatever re- 
connection is attempted the "phase coils" should be checked 
and re-arranged, since this is comparatively easy and adds 
considerably to the protection of the machine from break- 
downs of insulation. 



272 ARMATURE WINDING AND MOTOR REPAIR 

RE-CONNECTING MOTORS TO MEET NEW OPERATING 
CONDITIONS 

Re-connections Frequently Made. The following changes 
are made on account of changes in operating conditions or the 
service conditions on the circuits from which the motors are 
operated. 

1. Changes to operate on a different voltage. 

2. Change for operation on a different phased circuit, three- to two- 
phase, etc. 

3. Changes to operate on a different frequency. 

4. Change in number of poles of the motor. 

In case of change No. 4, the re-connection may be independent of all 
other changes to secure a faster or slower speed or it may follow as a re- 
sult of change No. 3 in order to keep the same speed on a driven 
machine when the motor is operated on the new frequency. 

Procedure when Considering a Re-connection of Windings. 

The procedure in checking up a machine to see whether or 
not it can be re-connected is as follows : First, ascertain the ex- 
isting 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 di- 
rectly from the Table on page 266 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 ar- 
range 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 independ- 
ently or in conjunction with a phase or a voltage change, the 
applied voltage should be changed in the same direction and 
by the same proportional amount as the frequency is changed, 
or 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 volt- 
age increased 20 per cent., or else have the groups re-connected 



RE-CONNECTING INDUCTION MOTORS 273 

so that there will be 20 per cent, less turns in series and run on 
the same voltage. 

If the number of poles is to be changed, and consequently 
the speed, check first the effect of the coil throw or chording 
with the new number of poles. Then think of the motor 
winding as generating counter emf. and bear in mind that 
with a constant field a higher speed will generate more emf. 
and a slower speed less emf. Converted into voltage this 
means that with a higher speed a higher voltage should be 
applied in direct proportion and with a lower speed a lower 
voltage should be applied. If the voltage can not be changed 
try to change the diagram of group connections so as to vary 
the number of turns in series in the right way, that is, if the 
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 conductor which 
counts, just as in a transformer, and a careful consideration 
of the effect of different connections will show whether the 
desired change in voltage per conductor is being accomplished. 

Practical Example for Reconnection. Assume a 25-hp., 
four-pole motor operating on 40 cycles, two-phase, 220 
volts. It is desired to know whether it can be re-connected 
to operate on 60 cycles, three-phase, 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 1 and 15, also that the groups are connected in 
parallel. Since there are 72 -r- 4 = 18 slots per pole, each 
slot is 180 -r- 18 = 10 electrical degrees and 14 slots = 140 
electrical degrees. (The throw of 1 to 15 means spanning 
14 slots.) The sine of one-half of 140 degrees or 70 degrees 
= 0.94 = chord factor, or figured by the formula without trig- 
onometry, since there are 18 slots per pole and a throw of 
1 to 15 means dropping four slots from exact pitch, the chord 

/18 2 2 X 4 2 
factor = -\l ^g 2 - = 0.948. The synchronous speed 

of the motor on 40 cycles as it stands is 4800 -r- 4 = 1200 
rpm. To get this same speed on 60 cycles it is evident the 
motor will have to be connected for 7200 -r- 1200 = six poles. 
If the throw of the coils be left 1 to 15 they will throw two slots 

18 



274 ARMATURE WINDING AND MOTOR REPAIR 

further than full pitch, since 72 -f- 6 = 12 slots per pole and 

1 to 13 would be exact pitch. Throwing the coil over pitch 
has the same effect as throwing it under pitch so the new chord 

/122 2 X 2 2 

factor on six poles = A/ 22 - = 0.97, or sine of one- 
half of 150 degrees = 0.98. Taking into account the changes 
in phase, poles, frequency and chording, the new applied volt- 
age per phase should be ~ X | X ~ x ^? = 305 volts. 

o o 40 0.94 

The explanation of this expression by terms is: The first 
term, (880 -r- 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 2 X 220 
= 440, and the voltage across both phases in series would be 

2 X 440 = 880 volts. If the winding is divided into three 
separate phases not interconnected, the applied voltage on 
each phase would be (880 -f- 3). The next term, (4 -r- 6) is 
due to 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 emf. 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 be- 
cause 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 -4- 
40) is due to the change in frequency and is the application 
of the rule to change the applied voltage directly as the fre- 
quency is changed. The last term, (0.98 -f- 0.94) is due to the 
difference in chord factor. With a throw of 1 to 15, the coils 
are more effective to generate counter emf. 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 emf. 

The value of 305 volts means that if the winding was 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 series star> as in Fig. 175, the applied 
voltage should be 1.73 X 305 = 530 volts. Since this is only 



RE-CONNECTING INDUCTION MOTORS 275 

about 3.5 per cent, off from the 550 volts which is to be used, 
the motor will operate satisfactorily. This calculation for 
voltage so far neglects the difference in the so-called " distri- 
bution factor" between three-phase and two-phase, but this is 
immaterial. This factor acts the same way as the chord 
factor, and is about 0.955 for any normal three-phase windings 
and 0.905 for any normal two-phase winding, so that the ap- 
plied voltage should really be 530 X I QQC) = 5(30 volts, 

which is almost exactly what is required. This motor could 
then have its phase coils re-arranged for six poles and be 
connected series star and would be satisfactory for the new 
conditions. The changes involved do not materially effect 
the slip, so that no change is required in the rotor winding. 
This example illustrates a rough calculation that can be made 
to see what the possibilities of re-connection are. 

Changes in Voltage Only with all Other Conditions Re- 
maining the Same. This is the simplest change which can be 
made in an induction-motor winding and in principle is the 
same as that of a transformer coil in which the number of turns 
of wire in series must be varied in exact proportion to the vol- 
tage applied. Practically all commercial motors are arranged 
so that they can be connected for two voltages, say 110 and 
220, or 220 and 440. This is accomplished by putting the polar 
groups in series, as in Figs. 175 and 182, for the higher voltage, 
and in parallel, as in Figs. 176 and 183, for the lower voltage. 

Assume a case in which the motor as it stands is connected 
for 2200 volts and is connected in series star as in Fig. 175. 
It is desired to re-connect it for 440 volts for the same horse- 
power, phase, cycles and speed. Four hundred and forty 
volts is 20 per cent, of 2200. Refer to the Table on page 266 
and use the horizontal column marked 3-PAase, Series Star. 
Since a re-connection is desired to give 20 per cent, of the 
original voltage, read along the horizontal line until the figure 
20 occurs This is found first under the vertical column 
marked 3-Phase, ^-Parallels. This is obvious, of course, 
because if the number of poles in the machine is divisible by 
five, it could be re-connected in five parallels and operated on 
2200 -T- 5 = 440 volts. But suppose the number of poles 



276 ARMATURE WINDING AND MOTOR REPAIR 

is not divisible by five. Look still further along the hori- 
zontal line and the figure 19 appears under the vertical 
column headed 3-Phase, ^-Parallel Delta. In other words, if 
the number of poles in the machine is divisible by three it can 
be put in three-parallel delta and operated on 2200 -r- (3 X 
1.73) = 424 volts, which is near enough to 440 to give 
perfectly satisfactory operation. 

If the number of poles on the machine is not divisible by five 
or by three, it is evident from the table that it is not possible 
by any ordinary three-phase connection to approach closer than 
550 volts with a four-parallel star connection, or 330 volts using 
a four-parallel delta connection. The relation between 550 volts 
and 330 volts as just given is not quite the theoretical 1.73 
which would be expected, but this is due to the table being 
made up to the nearest integral figure without using frac- 
tions. The error in this instance is three per cent, which is 
immaterial. 

A point which is brought out by the example just cited is 
that, so far as insulation is concerned, a motor may always be 
re-connected for a lower voltage for instance, a 2200-volt 
motor may be re-connected for 440 volts but, on the contrary, 
a motor originally designed for 440 volts may not be run on 2200, 
even if the re-connection is possible so far as number of turns is 
concerned, because the insulation will not stand the dielectric 
strain. It may be stated generally that practically all manu- 
facturers use two classes of insulation up to 2500 volts, one 
class good up to 550 volts and the second good from 600 volts to 
2500. This should be carefully considered and a motor never 
re-connected from the lower into the higher class, although 
the change from the higher to the lower is permissible from 
the insulation standpoint. 

Change of Phase Only. The most common problem which 
presents itself 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. Then, if a three-phase motor 
be re-connected for two-phase at the same voltage and with the 
same coils, it will exhibit all the symptoms of a motor operating 
at 25 per cent over voltage and usually would overheat to a 



RE-CONNECTING INDUCTION MOTORS 277 

dangerous degree after a short period of operation. Con- 
versely, a two-phase motor re-connected and run on three- 
phase at the same voltage with the same coils will show all the 
signs of a motor operating at 20 per cent, under voltage. In 
this case there are too many turns in the machine, One-fifth 
of the total coils might be dead-ended to secure the proper volt- 
age on the remaining 80 per cent. The dead coils should be 
distributed as symmetrically as possible around the machine to 
balance the voltage as nearly as possible on all phases. Parallels 
in the winding should be avoided, as they give a chance for un- 
balanced, circulating local currents, which may cause excessive 
temperatures. Since the normal full-load current on a three- 
phase motor at any given voltage is about 12.5 per cent, greater 
than the two-phase full-load current at the same voltage, it 
follows that the three-phase horsepower will have to be cut 
down about 12.5 per cent, from the two-phase in order to keep 
the current density in the winding as it was on two-phase. 
Unless the current density is kept approximately the same 
greater heating will result. Another makeshift, shown in Fig. 
185, is the so-called "Scott or T connection for operating a two- 
phase motor on three-phase. By this scheme 14 per cent, of 
the coils in one phase of the two-phase machine are omitted as 
symmetrically as possible around the machine. One end, BI 
of this phase, is then connected to the middle of phase Ar~A 2 . 
The resulting voltages between the points Ai-Az-B^ are prac- 
tically 4n a balanced three-phase relation. 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 manner as 
the coils between BI and A 2 . Practically, as motors are 
wound nowadays, this is rarely possible, and if the usual wind- 
ing 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. The efficiency on the T con- 
nection 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 



278 ARMATURE WINDING AND MOTOR REPAIR 

13.5 higher than on the normal three-phase winding. This 
shows that changing from two-phase to three-phase, and 
vice versa, is at best very unsatisfactory. It is better to re- 
wind with normal three-phase coils and avoid the troubles 
which may follow. 

One essential in any phase re-connection is to go over the 
winding and re-arrange 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 Figs. 174 and 181. 

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 volt- 
age on the three-phase cuts down the current, which would 
otherwise be higher than the two-phase circuit. If the phase 
coils are re-arranged there is practically no objection to such a 
re-connection and the motor will give essentially the same per- 
formance on either connection. 

The table on page 266 shows the possibilities of interphase 
connections, as well as the different voltage changes. For 
example, in the case just cited, follow the horizontal line 
marked 2-Phase Series to the first vertical column headed 3- 
Phase Series. The figure is 125. This means that a motor 
originally connected two-phase series, if re-connected 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 1.25 X 440 = 550 volts. The convenience of the 
table is demonstrated for phase changes, as well as voltage 
changes, or for combinations of both. 

Changes in Frequency. The occasion often arises for chang- 
ing 25-cycle motors to 60-cycle and 60 to 25. There is also 
some changing done from 60 cycles to 50 and 50 to 60. Occa- 
sionally 40-cycle motors are changed to 60, but these changes 
are infrequent. 

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 



RE-CONNECTING INDUCTION MOTORS 279 

a few per cent, higher than the full-load speed) is given by the 

Alternations per Minute . 

general expression - Number of Poles This would be 

3000 , oe 7200 , 

XT r -vm f r 25 cycles, ^ u r> ^ f r 60 cycles, 

Number of Poles ' Number of Poles 

etc. If then the frequency is changed and the number of 
poles left the same, the resulting rpm. will vary directly as 
the frequency. This immediately 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 the new speed on the 
motor? 

Consider first the case where the frequency is changed and 
the number of poles remain the same. The resulting change 
in 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 rpm. on the driven machine. The 
next thing that 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 re-connected so as to get the 
proper voltage on each coil? 

The easiest rule to remember is to vary the applied voltage 
on the motor in exactly the same way as the frequency is varied. 
If this be done the magnetic field in t*he iron will remain the 
same and the 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 cur- 
rent. If the torque remains the same, the horsepower de- 
veloped will vary directly as the applied frequency. For 
example, a 60-cycle, 50-hp. motor operated on 25 cycles at 
41.6 per cent, of its original voltage would develop the same 
normal full-load torque, which would be 20.8 hp. 

Changing from 25 to 60 cycles. A change from 25 cycles to 
60 cycles, can often be made by impressing twice the voltage 
on the coils on 60 cycles as on 25 cycles. A 220-volt, 25-cycle 



280 ARMATURE WINDING AND MOTOR REPAIR 

motor operated on 440 volts, 60 cycles, will have about double 
the horsepower. Theoretically, this should be 60 -r- 25 = 2.4 
times the voltage, instead of twice, and the resulting horse- 
power would be 2.4 times. In this case suppose the motor 
was 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 on 5Q-cycle Circuits. Sixty-cycle motors 
are often run on 50 cycles without change. From the rule 
above, that the voltage must vary with the frequency to keep 
the same magnetic densities, it will be noted that the densi- 
ties on 50 cycles at the same voltage will be six-fifths of the 
60 cycles 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 makes the motor hotter, and the 
decreased speed on 50 cycles with the same number of poles 
also makes the ventilation poorer, so that the output of the 
motor in horsepower should be reduced to keep down the cop- 
per losses. 

Another point that should be watched in changing frequency 
if the motor has a squirrel-cage rotor, is to make sure that the 
rotor winding has enough resistane 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 satis- 
factorily no change is necessary, but if trouble is experienced, 
the short-circuiting rings may have to be changed for ones of 
higher resistance. Conversely, when decreasing the frequency 
the resistance can be reduced to advantage, thereby cutting 
down the rotor copper loss and the heating. 

Change in Frequency with Same Speed. In this case the 
number of poles must be changed in the same 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 -f- 4 = 750 rpm. If the 



RE-CONNECTING INDUCTION MOTORS 281 

motor is to have the same speed on 60 cycles, the nearest 
possible pole number is 10 and the synchronous speed will be 
7200 -r- 10 = 720 rpm. It is apparent that in very few cases 
of this kind is it possible to re-connect 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 approximate 
one-fourth of the stator bore, and in the 10-pole winding 
normal coils should span about one-tenth of the stator bore. 
In the paragraph on "chorded windings" (page 269) it was 
pointed out that the coil throw has an effect on the generated 
counter emf proportional to the sine of one-half the electrical 
angle spanned by the coil. This consideration makes hardly 
possible such a condition as connecting a winding for 10 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. This introduces another 
difficulty in the magnetic circuit. In re-connecting the wind- 
ing 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 many 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 10 poles and 
10 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 magnetic flux, if there arc only four poles 
and four magnetic circuits, the same cross-section of core has to 
carry one-fourth of the total magnetic flux, which it is probably 
unable to do. This is the reason why the rotor diameter and 
stator 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. From this it follows 
that a machine may in general be re-wound or re-connected for 
a larger number of poles, but that great caution is required in 
re-connecting for a smaller number of poles. 



282 ARMATURE WINDING AND MOTOR REPAIR 

It is easier to re- wind or re-connect 25-cycle machines for 60 
cycles than it is to re-connect 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 horse- 
power, voltage and rpm. 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 chang- 
ing the number of poles on a squirrel-cage motor is the cur- 
rent in the short-circuiting rings of the rotor winding. These 
rings are in nearly the same relation as regards current that 
the primary core is as regards magnetic flux. That is, the 
total secondary amperes, which remain nearly the same if 
the re-connection is done properly, are divided into as many cir- 
cuits 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 re-con- 
necting for different numbers of poles when changing frequency 
is usually a matter for the designing engineer to investigate. 

Changes in the Number of Poles, all Other Conditions 
Remaining the Same. The need for such changes comes 
from the desire to speed up or slow down the driven machine 
to meet new requirements. It might be broadly stated that 
there are many cases where a change of two poles is permis- 
sible, as for example, changing from four poles to six, or from 
ten to eight and the like. The changes would consist in re- 
arranging the phase coils to agree with the new grouping and 
checking the chord factor, to note its effect on the voltage. 
It is often possible to get a fair operating half speed by con- 
necting for twice the number of poles. Practically all re- 
connections involving pole changes give only a fair operating 
performance. <^- 

Testing a Re-connected Motor. After a motor has been re- 
connected or after any change is made in the winding, it should 
be started up slowly and the load gradually thrown on, ob- 
serving carefully to see if there are any signs of distress, such as 
sudden heating, noise or mechanical vibration. If the motor 
seems to operate normally read the amperes in each phase 
and the voltage across each phase to see that they are balanced 



RE-CONNECTING INDUCTION MOTORS 283 

and are reasonable in amount. The full-load current for three- 
phase 550 volts is somewhere near one ampere per horse- 
power for normal motors of moderate speeds between five and 
200 horsepower. At other voltages this will be inversely as 
the voltage, that is at 440 volts, three-phase, about 1.25 am- 
peres per horsepower. On two-phase the current per phase 
is about 87 per cent, of the corresponding three-phase value. 
If the readings as above look reasonable a thermometer should 
be placed on the stator iron and another on the stator coils and 
read at 15-minute intervals for an hour, and at half -hour in- 
tervals thereafter, until the temperature is constant. The 
speed should be checked at intervals. If the rpm. shows a 
tendency to decrease rapidly or fall below 90 per cent, of syn- 
chronous speed, it may be suspected that the rotor has too 
much resistance and is getting hot. By making all these 
checks, reasonable assurance may be had that the reconnec- 
tion is satisfactory. 

Effects of High and Low Voltage on Motor Operation. 
All changes in alternating-current motors whether of phase, 
voltage, poles or frequency, may be considered as voltage 
changes and reduced to such terms. In making such calcula- 
tions and comparing the results, it is advisable not to apply a 
voltage that differs from the rated voltage by more than plus 
or minus 10 per cent. The general effect of high and low 
voltage may be expressed briefly as follows: 

Effect of High Voltage: 
a. Increases magnetic density. 
6. Increases magnetizing current. 

c. Decreases "leakage current" (leakage reactive component). 

d. Increases starting torque and maximum torque. 

e. Decreases slip or change in speed from no load to full load. 
/. 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 iron loss in the normal 
machine and also the degree of saturation in the iron. 

Effect of Low Voltage: 

a. Decreases magnetic density. 

b. Decreases magnetizing current. 



284 ARMATURE WINDING AND MOTOR REPAIR 

c. Increases leakage current. 

d. Decreases starting and maximum torque. 

e. Increases slip. 

/. Increases secondary copper loss. 

g. Decreases iron loss. 

h. Usually increases power-factor. 

t. 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. 

Operating Standard Alternating-current Motors on Differ- 
ent Voltages and Frequencies. When a motor is to be oper- 
ated on a different voltage or frequency than the motor was 
designed for, there should be a corresponding change made in 
circuit to which the motor will be connected. That is, if there 
is to be an increase in frequency or voltage, an equal decrease 
in voltage or frequency of the circuit will bring about normal 
results. Present-day designs of motors can be used in most 
cases without excessive heating when the variation either up 
or down of frequency or voltage is not more than 10 per cent. 

For an induction motor the following conditions result with 
a change of frequency or voltage: 

1. Pull-out torque and starting torque vary as the square 
of the voltage and inversely as the square of the frequency. 

2. The copper loss in the primary varies as the square of 
the current. The current varies inversely as the voltage, but 
is not affected by a change in frequency, except to the slight 
extent produced by changes in magnetizing current. The 
secondary copper loss and slip tend to vary inversely as 
the square of the voltage, but this tendency is modified by 
the changes in primary IR drop and magnetic leakage. The 
secondary copper loss and slip remain constant with change 
in frequency. 

3. The iron loss is composed of hysteresis and eddy current 
losses. The hysteresis loss varies as the 1.6 power of the flux; 
the eddy current loss varies with the square of the flux; and 
the flux varies directly as the voltage and inversely with the 
frequency. The magnetizing current varies directly with 
the flux except for modifications produced by saturation of the 
magnetic circuit. 

4. The power-factor is usually decreased by an increase in 



RE-CONNECTING INDUCTION MOTORS 



285 



voltage or a decrease in frequency and vice versa, but the total 
change is small. 

5. The efficiency is not materially altered by a change in 
either frequency or voltage. 

The accompanying curve, Fig. 186 shows the operating 
voltage on which a standard motor can be used when con- 



450 
400 

ocn 


























X 


X 






















>x 


X 
























X 










300 
250 
200 
150 
100 
50 
















x 


x' 






















-X 


X 
























X 


X 
























/ 


























/I 


























]/ 


























/ 





























5 10 15 20 25 30 35 40 45 50 55 60 65 70 
Frequency Cycles per Sec. 

FIG. 186. Standard motor frequency and voltage curve. 

This curve indicates the voltage which, if employed in connection with the respective 
corresponding frequencies, will result in the operation of apparatus at approximately 
uniform core densities. By adherence to the relations betweeen frequency and voltage 
indicated, the range of application of standard apparatus can be broadened and the 
required number of different designs minimized. Allowance is made for the use of 
somewhat reduced densities at the higher frequencies, as indicated by the drooping 
ciiaracter of the curve (R. E. Hellmund, Electric Journal, September, 1910, page 691). 

nected to a circuit of a different frequency from that for which 
it was designed. The accompanying table also shows the 
effect on speed and horse-power rating when a motor is used 
on a different voltage and frequency. 

The voltage and frequency of a motor should never be varied 
in opposite directions at the same time. In general any change 
from normal frequency should be accompanied by a change in 
voltage proportional to the square root of the frequency. In 
the case of 400-volt, 60-cycle motor operated on a 66% cycle 
circuit, the voltage should be 422. That is V(66% -5- 60) 
X 400 = 422. In case of decreased frequency, the motor 
should be operated on less than normal voltage on account of 
the increased current and temperature. 



286 ARMATURE WINDING AND MOTOR REPAIR 



EFFECT ON SPEED AND HORSEPOWER WHEN MOTORS ARE OPERATED 
ON DIFFERENT VOLTAGES AND FREQUENCIES 



Rating of motor 


Voltage and fre- 
quency of circuit 


Speed 


Hp. rating 


220 volts, 25 cycles 
440 volts, 25 cycles 
(a) 440 volts, 60 cycles 
440 volts, 60 cycles 
(6) 220 volts, 60 cycles 


250 volts, 33 cycles 
500 volts, 33 cycles 
220 volts, 25 cycles 
220 volts, 33 cycles 
220 volts, 50 cycles 


Increased 33 to 25 
Increased 33 to 25 
Reduced 60 to 25 
Reduced 60 to 33 
Reduced 60 to 50 


Increased 33 to 25 
Increased 33 to 25 
Reduced 60 to '25 
Reduced 60 to 33 
Reduced 60 to 50 



(a) Where good power-factors are essential it may be advisable to use 550-volt, 60- 
cycle motor or a 220-volt, 25-cycle circuit and reduce the rating according to the heating 
between 35 to 45 per cent, of the rating at 60 cycles. 

(6) Standard 60-cycle motors of liberal rating can be used on a 50-cycle circuit with 
the same rating. Best results are secured however, when 220-, 440-, and 550-volt 
motors are operated on 200-, 400- and 500-volt circuits at 50 cycles. 

Factors which Limit a Change in Number of Poles of an 
Induction Motor. The principal factors limiting a change in 
the number of poles of a squirrel-cage induction motor are 
given as follows by a writer in the General Electric Review: 

(a) The number of turns in series per phase. These must remain the 
same since the applied voltage is to be unchanged. 

(6) The insulation between the conductors of different phases. Of 
this there must be sufficient to not reduce the factor of safety against 
breakdowns after the regrouping of the conductors has been carried out. 

(c) The saturation of the iron. It is often inadvisable to use a mag- 
netic density much higher than normal. 

Because the designs of induction motors vary widely with 
different manufacturers and also in the product of each maker 
(for the purpose of supplying motors for various types of 
service), it will be impossible to make other than very general 
statements regarding the expected change in characteristics 
of the motor when running at the higher speed. Furthermore, 
the following statements must not be expected to hold true 
when the number of poles has been decreased sufficiently to 
raise the normal speed more than say 25 per cent. 

After the reconnection, 

(a) The normal speed will be equal to approximately the original 
normal speed times the original number of poles divided by the new 
number of poles. 

(6) There will be a somewhat higher torque per pole exerted, due to 
the slightly increased flux per pole that arises from the shortened pole 
pitch, so that the total motor torque might be expected to be decreased 
but little by the change. 



RE-CONNECTING INDUCTION MOTORS 287 

(c) The running-light current will be slightly lowered. 

(d) The starting torque will probably be slightly decreased. 

(e) The power-factor might be expected to be somewhat higher. 

(/) When the power-factor is higher the rating of the motor can be 
increased about in proportion to the square root of the increase in speed 
with the same heating. 

(flf) The efficiency will be practically the same as before the change. 

Single -circuit Delta and Double-circuit Star Connections. 

The use of a one-circuit delta and a double or two-circuit 
star winding for a three-phase motor, as pointed out by Henry 
Scheril (Electrical Record, March, 1919) depends upon operat- 
ing conditions or design to meet certain requirements. One 
of the best illustrations is offered in the winding of a phase- 
wound rotor of the induction motor. Suppose that the rotor 
has been wound with a two-circuit star winding and the voltage 
between rings on open circuit is 220 volts. Let us assume that 
the full-load current in the rotor is 200 amperes. Since the 
winding has a double circuit, each circuit will take care of 
100 amperes. The voltage per phase will be 127 volts. 

Manufacturers, in general, standardize the brush rigging 
used in connection with machines, using a certain number of 
brushes per ring. If it is found, for instance, that 200 amperes, 
as in this example, brings the current density in the brush to 
too high a value, it would mean that the number of brushes per 
ring must be increased. Since this is not practical nor eco- 
nomical, a change in the connections of the winding may bring 
about the desired results, that is, reduce the current density 
in the brush to within the allowable value. 

If the winding were then reconnected from two-circuit star 
to single-circuit delta, then the voltage between terminals 
will be increased from 220 volts to 254 volts and the current 
per ring would be reduced from 200 amperes to 173 amperes, 
or a reduction of 13.5 per cent. This reduction may just be 
sufficient to decrease the brush density to the desired value. 
Since controllers used in the rotor circuit are made to take care 
of voltages of reasonable variations, an increase in voltage 
from 220 volts to 254 volts will not require a change in the 
controller and therefore the 220 volts controller can be used 
on the 254-volt circuit. 



288 ARMATURE WINDING AND MOTOR REPAIR 

Cutting out Coils of an Induction Motor. In case coils 
burn out in an induction motor, they can be cut out and the 
motor operated for a time or until it can be repaired. It is 
advisable to cut the entire coil and tape up the two ends so 
that they can not come together. There is a limit to the 
number of coils that can be cut out as this is equivalent to 
raising the voltage on the motor which will cause heating. 
Where more than two coils must be cut out the motor should 
be repaired at once or the same number of coils in each phase 
cut out, evenly distributed around the stator. 

PROCEDURE WHEN CONNECTING THE COILS OF AN 
INDUCTION-MOTOR WINDING 

At the end of this chapter typical diagrams for connecting 
the windings of polyphase alternating-current motors are 
shown as prepared by A. M. Dudley. 1 When an armature 
winder or repairman knows the number of poles and phases 
for the winding, and has been supplied with the necessary 
coils of the proper throw, the problem of inserting the coils 
and then connecting them up can be easily understood for 
each of the diagrams referred to if the fundamental procedure 
for any one is understood. This procedure is explained by 
the author of the diagrams as follows: 

The diagrams are not dependent on the total number of 
slots in the machine nor upon the number of coils per group, 
nor upon the throw or pitch of the coils, but are general for 
all machines of the same number of phases and poles. Each 
one of the small arcs in each diagram represents the ends of 
the coils in a single pole-phase-group in the winding. This is 
illustrated in Figs. 187 and 188 showing a stator in three stages 
of being connected. In Fig. 187 (A) a machine is shown in 
which the coils have simply been placed in the slots by the 

1 The diagrams shown have been selected from a series of eighty-one 
devised by A. M. Dudley, Engineer industrial division of the Westing- 
house Electric & Mfg. Co., to be incorporated in an excellent book by 
him on "Connecting Induction Motors." (McGraw-Hill Book Co., 
New York.) Mr. Dudley's diagrams begin with two-pole windings and 
give practically all possible combinations for two- and three-phase, star 
and delta and series-parallel connections. 



RE-CONNECTING INDUCTION MOTORS 



289 



winder and no connections have been made. The wires 
which are the beginnings and endings of the coils are sticking 







I! 

,0 O 



3 



a g^ 

^ rfl CD 

a ra 05 



M 'grt 

c3 8 

o 



OO O O 

"-3 
s +? * 



put at random. In Fig. 187 (5) the coils have been connected 
into several distinct groups and the remaining wires, which 

19 



290 



ARMATURE WINDING AND MOTOR REPAIR 



protrude radially toward and away from the center of the 
machine, form the beginning and the end of each pole-phase- 
group. 

Connecting Pole-phase-groups of a Winding. The opera- 
tion which has been performed between Fig. 187 (A) and Fig. 
187 (B) can be described in this way : Suppose, for example, that 
there are 96 total coils in the winding and that it is to be con- 
nected for three phases and four poles. There will then be 3 X 




FIG. 188. Stator of a 100-hp., 3-phase, 8-pole, 220-volt, 60-cycle, 120- 
coil, 120-slot, induction motor completely wound with pole-phase-groups 
properly cross connected. 

4 = 12 pole-phase-groups, and 96 -f- 12 = 8 coils in each group. 
Starting at any arbitrary point, the winder connects the first 
eight coils in series by connecting the end of coil No. 1 to the 
beginning of coil No. 2, and the end of coil No. 2 to the begin- 
ning of coil No. 3, etc., until eight coils are in series. The 
beginning of coil No. 1 is then bent outward and left long and 
the end of coil No. 8 is bent inward and left long. Between 



RE-CONNECTING INDUCTION MOTORS 



291 



these two are seven short " stubs" or coil-to-coil connections, 
which are shown taped up in Fig. 187 (B) . The winder then 
proceeds to connect coils No. 9 to No. 16 in series in the same 
manner to form pole-phase-group No. 2, and so on around the 
machine until he has completed 12 pole-phase-groups and 
used all the coils. The winding then looks as shown in Fig. 
187 (B). 

In case the winding has certain coils provided with heavier 
insulation on the end turns to take the strain of the full voltage 




FIG. 189. 



FIG. 190. 



FIG. 191. 



FIG. 192. 



FIG. 193. 



FIG. 194. 



FIG. 189. Thirty-two coils connected into 8 pole-phase-groups for a 
2-phase winding. 

FIG. 190. Same as Fig. 189 with pole-phase-groups connected according 
to direction of arrows. 

FIG. 191. Same as Fig. 190 with B-phase reversed. 

FIG. 192. Forty-eight coils connected into 12 pole-phase-groups for a 
3-phase winding. 

FIG. 193. Same as Fig. 192 with pole-phase-groups connected according 
to direction of arrows. 

FIG. 194. Same as Fig. 193 except leads are brought out from different 
groups. 

of the machine where different phases are adjacent, the opera- 
tion is slightly different. In this case the number of coils per 
pole-phase-group must be determined before the coils are 
inserted in the slots, and the specially insulated phase coils 
placed on both ends of each group. In this case the location 



292 ARMATURE WINDING AND MOTOR REPAIR 

of the pole-phase-groups is definitely determined by the winder 
before he starts connecting the coils together. 

The next step is to mark the pole-phase-groups A-B-C-A- 
B-C, etc., around the machine and then to connect all the 
groups together in the proper manner to form a three-phase 
winding as indicated by the diagram of connections. The 
completed winding will then appear as shown in Fig. 188. 

General Theory on which Connection Diagrams are Con- 
structed. Simple methods by which any winding may be 
checked for phase polarity are shown in Figs. 189 to 194, in- 
clusive. In Fig. 189 a winding chosen at random is shown 
"stubbed" into pole-phase-groups for a two-phase connection, 
and in Fig. 192 stubbed for a three-phase connection. To de- 
termine the proper connections for the pole-phase-groups in a 
two-phase winding, the rule is to mark on the groups arrows 
alternating in direction in pairs. That is, on two successive 
groups the arrows are clockwise and on the two immediately 
adjacent, the arrows are counter-clockwise. Such arrows, 
for example, are shown in Fig. 189 just above the windings. 
If now one end of any group in a phase is chosen as a "lead" 
and all the groups are followed through and connected as indi- 
cated by the arrows, the connection will be correct. Such a 
connection is shown in Fig. 190. However, suppose the arrows 
had alternated in pairs, but started with a different group, as 
shown just below the windings in Fig. 190. The result is shown 
in Fig. 191, which is just as correct as Fig. 190, except that the 
motor would run with the opposite direction of rotation. 
Since the rotation can be changed by reversing the two leads 
of either phase outside of the motor, it is evident that the rule 
using the arrows alternating in pairs is correct in all cases. 
It should also be noted that it makes no difference from what 
group the lead is taken, provided all the groups are followed 
through with the arrows. 

In the three-phase machine the method is even more simple. 
The rule in that case is to put arrows on the groups alternating 
in direction from group to group, as shown in Fig. 192. Any 
group may then be chosen as a "lead" group or a "star" 
group so long as the arrows are followed in passing from the 
lead to the star in each phase. Figure 193 shows one arrange- 



RE-CONNECTING INDUCTION MOTORS 



293 



ment and Fig. 194 another equally correct, and there might be 
an indefinite number more, simply by choosing the lead from 
another group and following the arrows through to the star 
in each phase. Although shown for a developed four-pole 
winding only, these diagrams may be considered as strictly 
general, as additional groups may be added to make six, eight, 
or any other number of poles, and the current passed through 
them in any order, so long as the phases are kept in the correct 
rotation and the current in the right direction as indicated 
by the arrows. 

In case of a delta connection instead of a star, check the 
connections through as for a star and then connect the A star 





FIG. 195. Method of checking 
a delta connection from a star 
connection. 



FIG. 196. Winding diagram for 
2-pole, 2-phase motor with series 
connections of coils. 



to the B lead, the B star to the C lead, and the C star to the A 
lead, as shown in Fig. 195; or connect the A lead to the B 
neutral, the B lead to the C neutral, and the C lead to the A 
neutral. The three motor leads will be taken from the corners 
of the delta so formed. 

Determining Number of Poles from Slot Throw of Coils. 
As an example assume a 96-slot stator whose coils span 12 
slots. Then number of slots -t- span of coil = number of 
poles, or 96 -f- 12 = 8 poles. Suppose however, the coils 
span 10 slots. The quotient is then 9.6 which is an impossible 
number of poles. This indicates a chorded winding and the 



294 ARMATURE WINDING AND MOTOR REPAIR 

correct number of poles is probably the next lower even 
number which will again be 8. This is not an invariably 
correct rule. A further check is as follows: Divide the 
number of slots by the number of phases. If this number 
is divisible by the number of poles obtained as above, it may 
be safely assumed that the correct number of poles has been 
determined. 

Typical Circle Diagrams for Connecting Induction Motors. 
On the following pages, 295 to 300 typical winding diagrams 
are shown by which induction motors may be connected for 
a variety of operating conditions. 



RE-CONNECTING INDUCTION MOTORS 



295 




FIG. 197. Winding diagram for 
2-pole, 3-phase motor with series-star 
connection of coils. 



FIG. 198. Winding diagram for 
4-pole, 2-phase motor with series con- 
nections of coils. 




B 



FIG. 199. Winding diagram for 
4-pole, 3-phase motor with series- 
star connections of coils. 




FIG. 200. Winding diagram for 
4-pole, 3-phase motor with series- 
delta connections of coils. 



296 



ARMATURE WINDING AND MOTOR REPAIR 




CAB 



A C 



FIG. 201. Winding diagram for FIG. 202. Winding diagram for 

4-pole, 3-phase motor with 2-parallel 4-pole, 3-phase motor with 2-parallel 
star connections of coils. delta connections of coils. 




FIG. 203. Winding diagram for FIG. 204. Winding diagram for 

4-pole, 2-phase motor with 4-parallel 6-pole, 2-phase motor with series con- 
connections of coils. nections of coils. 



RE-CONNECTING INDUCTION MOTORS 



297 




FIG. 205. Winding diagram for 
6-pole, 3-phase motor with series-star 
connections of coils. 




B C 



FIG. 206. Winding diagram for 
6-pole, 3-phase motor with series- 
delta connections of coils. 




B C 



FIG. 207. Winding diagram for 
6-pole, 3-phase motor with 2-parallel 
star connections of coils. 




ABC 

FIG. 208. Winding diagram for 
6-pole, 3-phase motor with 2-parallel 
delta connections of coils. 



298 ARMATURE WINDING AND MOTOR REPAIR 





ABC 

FIG. 209. Winding diagram for FIG. 210. Winding diagram for 

6-pole, 3-phase motor with 3-parallel 6-pole, 3-phase motor with 6-parallel 

delta connections of coils. delta connections of coils. 





FIG. 211. Winding diagram for FIG. 212. Winding diagram for 

8-pole, 2-phase motor with series con- 8-pole, 2-phase motor with 8-parallel 
nections of coils. connections of coils. 



RE-CONNECTING INDUCTION MOTORS 



299 





ABC ABC 

FIG. 213. Winding diagram for FIG. 214. Winding diagram for 
8-pole, 3-phase motor with series-star 8-pole, 3-phase motor with series- 
connections of coils. delta connections of coils. 



" 



i ... 


i 


13 X v 
.9 
17 11 
15 j 


-1-,-^ 




1 l H - t - f -^~ l 




1 


9 


4 ' 




17 


H? . 


i 
i 


: ; . ^ 




FIG. 215. Winding diagram for FIG. 216. Winding diagram for 

10-pole, 2-phase motor with series 10-pole, 3-phase motor with series- 
connections of coils. star connections of coils. 



300 



ARMATURE WINDING AND MOTOR REPAIR 





ABC 

FIG. 217. Winding diagram for FIG. 218. Winding diagram for 

10-pole, 3-phase motor with series- 10-pole, 3-phase motor with 5-parallel 
delta connections of coils. star connections of coils. 




B C 



FIG. 219. Winding diagram for 
10-pole, 3-phase motor with 5-parallel 
delta connections of coils. 




ABC 

FIG. 220. Winding diagram for 
10-pole, 3-phase motor with 10-par- 
allel star connections of coils. 



CHAPTER XII 
COMMUTATOR REPAIRS 

Commutator troubles are more easily located than faults in 
an armature, but a repair job on an old style motor, many of 
which are still in use, is sometimes a trying and tedious 
operation. 

Causes of Commutator Troubles. Sparking at the brushes 
is generally the first symptom of commutator trouble. One 
of the most frequent causes of sparking is a rough or pitted 
commutator. This may be due to many irregularities, such 
as overload; brushes out of line; not set at neutral points in 
regard to load; poor contacts; current density per square inch 
of brush contact too great; open circuit; weak magnetic 
fields; commutator out of round; high or low bars or high mica. 
The most common cause of sparking is high mica which causes 
the brushes to chatter and to make poor 
contact. This condition results in a 
rapid blackening and burning of the 
bars, sometimes to the extent that the 
copper is eaten away leaving the mica 
segments standing out above the surface 
of the commutator. Some motors seem 
to be particularly subject to this 
trouble, due to the fact that the mica 
is too hard a grade, or the copper too 
soft. When there is no time to turn 
down the commutator the high mica and how they are held by 
can be removed by grinding down the clam P in s rin g s - 
with a piece of sandstone, and using fine sandpaper for 
smoothing. 

Troubles Resulting from High Mica. High mica, while 
it may seem a small matter at first, is often the cause of more 
serious complications. The commutator may become so 

301 



SEGMENT 




FIG. 221. Section of a 



302 ARMATURE WINDING AND MOTOR REPAIR 

hot from the poor brush contact afforded, that the solder 
will be melted and thrown out, resulting in short-circuits 
between bars and open-circuits due to the armature leads 
becoming disconnected. About the only permanent relief for 
sparking at brushes due to high mica is undercutting the mica. 
This remedy is recommended when it is reasonably certain that 
the high mica is caused by the natural condition of the copper 
or mica. If it is not, then the real cause must be found, 
otherwise, undercutting the mica would probably improve 
the running condition somewhat, but would fail to remove the 
cause. 

The mica should not be cut too deeply, a depth of ^ 2 to KG 
inch below the surface of the copper being sufficient. Care 
must be exercised to remove the mica the full width of the 
segment, for any thin slivers left flush with the surface will 
often defeat the purpose of the undercutting. (For details 
for undercutting mica, see page 320.) This method has 
corrected some stubborn cases of sparking and if the job is 
properly done, all that will be necessary to preserve sparkless 
commutation is to keep the slots clean and well below the 
surface of the copper. 

Remedy for High or Low Bars. A new or repaired motor 
may have a commutator that is not " settled." That is, the 
clamping ring has not been drawn up as tightly as it should be. 
When the mica end rings are "cut they are only slightly flexible 
due to the shellac in them and cannot be made to fit perfectly 
when cold. When the commutator is hot, the shellac in the 
mica will soften and allow it to move under the strain of the 
centrifugal force of the bars when the machine is running. 
This movement of the mica allows the bars to move and is 
frequently the cause of high or low bars. To remedy this 
trouble, the machine should be run until its normal operating 
temperature is attained and then shut down. The clamp- 
ing ring can then be tightened. This process may be neces- 
sary several times, or until the commutator is perfectly solid. 
Care must be taken not to tighten the bolts too much while 
the commutator is warm. 

In the case of a high bar, it should be tapped down until it 
rests firmly against the mica end rings. It can then be filed 



COMMUTATOR REPAIRS 



303 



even with the rest of the bars. A low bar can be raised by 
prying up, and inserting a narrow strip of mica beneath it, 
but in the majority of cases this makes a poor job. Usually 
the only alternative is to turn the commutator down to the 
level of the low bar. 

Burn-out Between Bars. Probably the most frequent com- 
mutator trouble is a burn-out between bars. It occurs often 
on the corner of the bars, and is not infrequently caused by 





FIG. 222. Mica segments taken from damaged motors showing the effects 
of short circuits in the commutator. Two tools are also shown made from 
hack saw blades for use in plugging a commutator. 

oil working along the shaft from the bearing and up onto the 
commutator. This oil collects dust and dirt and finally 
causes current to leak from one bar to the other. The mica 
then becomes carbonized, and a short-circuit results. This 
is one of the causes of burned-out armature coils. Sometimes 
the short-circuit will burn itself clear, and no harm will be 
caused except to burn a hole in the mica. However, it may 



304 ARMATURE WINDING AND MOTOR REPAIR 

continue to arc across and burn a good sized hole in the bars 
also. 

Plugging a Commutator. When mica segments are burned 
but not too deep, the holes can be cleaned with a thin knife 
blade and plugged with some kind of filling. If a good filling 
compound is used, and the commutator kept free from oil, it 
will hold for a year or possibly longer. It is always advisable 
to save the wearing surface in this manner whenever it can be 
done, for every time a commutator is turned down in a lathe 
on an average of three years of its useful life is lost. 

A good filling compound for commutators can be made as 
follows: Two parts plaster-of-paris; one part powdered mica; 
and enough glue to make a thick paste. This, when applied, 
will dry quickly, and assume about the same degree of hard- 
ness as the mica segments. 

When a segment becomes burned deep down into the com- 
mutator, a new one must be inserted. Before attempting to 
do this, the armature should be thoroughly blown out with com- 
pressed air in order to remove all dust that may have accu- 
mulated. This is essential, for it is an easy matter for small 
particles of foreign matter to work in under the back end be- 
tween the bars and the sleeve when the commutator is loose. 
Determine just which mica segments must be taken out, and 
number the bars at each burn-out, since the bars may have to 
be removed also in order to get the mica segments out. If 
the segments were not shellaced when the commutator was 
built, the chances are they can be lifted out without disturb- 
ing the bars. Otherwise the bars will have to be taken out 
with the segments. 

Removing Bars and Mica Segments for Repairs. Remove 
the bolts that hold the clamping ring in place. Mark the 
ring so that it may be put back just as it was taken off. Tap 
the end of the ring lightly with a hammer. If the mica ring 
does not loosen from the commutator, it will have to be heated, 
as the ring is probably stuck fast with shellac. Heat the 
commutator with a torch to an even temperature all around. 
This will expand the copper and cause it to bulge out from the 
end ring. Tap the ring again lightly, and it will be found to 
work loose. Then it can be pulled out. 



COMMUTATOR REPAIRS 



305 



Pry the bars apart slightly at one of the burned places to 
see if the mica segments are stuck to the bars. If they are 
not, it is a simple matter to remove them from the commu- 
tator. If they are held fast, the leads from one bar adjoining 



Keep all Dirt and 
Moisture out 



Leave Mica extending 
One-sixteenth Inch 



Seal thoroughly against 
Dirt and Moisture 




FIG. 223. Section of an assembled commutator of a railway motor. 

the burned segment should be unsoldered and the bar lifted 
out. Proceed in the same way with the remaining bad places. 
New segments can be marked off by using one of the old bars 
as a guide. The bars should be scraped and filed clean^ 
and all rough corners rounded off. 

Repairing a Burned Commutator Bar. Frequently it 
happens that there is a good-sized hole burned in the commu- 




Fio. 224. Repair of a burned place on a commutator bar. 

tator bar. This should be repaired before it is used again. 
As there is usually no stock of exactly the proper size on hand 
in a small repair shop out of which to make a new bar, a good 
repair is the next best thing. In Fig. 224 (A) shows the end of 
20 



306 ARMATURE WINDING AND MOTOR REPAIR 

a bar with a burned place and (B) the method of repair. In 
this case the bar was cut down enough to remove the burn, 
and a piece of copper strip carefully squared and soldered in 
the cut. It was then riveted with a small copper rivet, the 
location of which is shown at (X). The rivet was used to 
prevent the patch from flying out should the commutator for 
any reason become hot enough to melt the solder. Such a 
patch should be filed down to the dimensions of the bar. The 
cut for a patch of this kind can be quickly made if a milling 
machine or shaper is handy, otherwise a good sharp fils will serve 
the purpose. 

Replacing a Repaired Commutator Bar. Before replacing 
a commutator bar that has been repaired, an inspection should 
be made of the back mica ring, to be sure that no dust or solder 
has lodged there. The mica segments should be replaced 
first, and then the bar pushed in. Shape the commutator 
as nearly as possible into a circular form and replace the end 
ring. Tighten the clamping nuts as much as possible while 
the commutator is cold. It is a good plan to paint the end 
of the commutator with shellac, in order to fill up any cracks 
that may exist between the bars and the ring. 

Tightening up a Repaired Commutator. When a commu- 
tator has been taken down in a repair shop and assembled 
again, all lock nuts and screws should be first set up hard 
and the commutator baked in an oven. Then the lock nuts 
and screws can be tightened up again since the heating causes 
the copper to expand and put pressure on the mica which, 
combined with the heat, drives out all traces of shellac in the 
mica. Then as the commutator cools and the copper becomes 
normal the bolts can be taken up. This proces's should be 
repeated and finally the commutator cooled quickly by a fan, 
and the nuts tried again. If they seem tight the commutator 
is ready for assembly on the armature shaft. It requires some 
experience to determine just how tight the bolts can be drawn 
on a commutator without injuring the mica end rings. The 
one precaution is not to draw up the bolts finally until the 
commutator has cooled. The commutator surface can now 
be turned down in a lathe. After doing this a short-circuit 
test should be made with a test lamp from bar to bar. Failure. 



COMMUTATOR REPAIRS 



307 




308 ARMATURE WINDING AND MOTOR REPAIR 

of the lamp to light indicates that the commutator is free from 
short-circuits. See also Chapter V, pages 123, 126 and 135. 
A test for grounds should also be made at this time (see pages 
131 and 175). 

Baking Commutator with Electric Heat. In those cases 
where an oven is not available for baking a rebuilt commutator 
and it is too large to heat with a torch, the electrical method 
shown in Fig. 226 has been successfully used (Albert Krause, 
ElectricalWorld, July26, 1919, page 190) A satisfactory heat- 
ing element can be made up by using a layer of ^{Q-UI. asbestos 
paper around the commutator and winding over this about 




FIG. 226. Uniform temperature produced by heating element wound around 

commutator. 

90 ft. of No. 22 Nichrome resistance wire. By applying 
250 volts to the terminals of this resistance wire a uniform 
heat in the commutator can be produced to permit evening up 
the insulating segments and clamping the end rings. Ordi- 
narily it would be advisable to have a variable resistance in 
series with the heating element so its temperature can be kept 
at a desirable value. By covering the armature after the 
commutator is heated in this manner varnish may be applied 
thereto and baked in by the heat conducted to the armature 
through the leads from the commutator. 
Removing and Repairing Grounds in a Commutator. The 




COMMUTATOR REPAIRS 309 

ground frequently occurs between the sleeve ring and the end 
of the bars. A small hole is generally burned through the 
mica ring or taper cone. Some times the mica ring on the rear 
end is punctured. In that event a number of bars in the 
neighborhood of the ground will have to be taken out. The 
burned mica should be cut out 
and a patch put on. When the 
trouble occurs on the front end 
of the commutator, remove the 
ring and cut out the bad mica. 
The patch can be made as shown 

TV rkr*>-r mi . FIG. 227. Patch on a mica end 

in Fig. 227. This new mica must ring of a commutator . 
be a trifle thicker than the original 

mica removed, for it will squeeze together somewhat when 
the ring is drawn up tight and the commutator heated. 

After the repair, test the commutator for grounds with a 
proper voltage (see page 175) . Place one terminal on the shaft, 
and move the other completely around the surface of the com- 
mutator. Freedom from grounds will ,be indicated by no 
sparking at the terminal moved over the surface. A lower 
test voltage should be used on low- voltage commutators and 
a higher voltage for those of high voltage machines. 

On some of the old-style motors, the sleeve nut is on the 
back of the commutator. When this happens, the armature 
leads will have to be disconnected and bent back out of the 
way in order to work on the commutator. The best proce- 
dure in such a case, if there is time, is to remove the com- 
mutator and reverse the sleeve, as both ends are usually bored 
to the same diameter. By reversing the sleeve in this manner 
in order to get the nut where it can be easily reached, much 
time and work will be saved when future repairs must be made 

Turning Down a Commutator without Removing Armature 
from Machine. The method sometimes used in turning down 
a commutator on a repair job where the armature is too large 
to remove, is to leave one or two pairs of brush arms on and 
run the machine from these at as low a speed as the field regu- 
lation will permit or possibly with a water rheostat in the arma- 
ture circuit. This method will do where no other means of 
turning the commutator is available. There is always bad 



310 ARMATURE WINDING AND MOTOR REPAIR 

sparking and burning at the point of the cutting tool due 
to its short-circuiting the bars when it crosses the mica. The 
tool has to be sharpened frequently and a job is seldom good 
even where the greatest care is exercised. 

For these reasons it is always preferable to belt the machine 
to a separate motor and turn down the commutator with 
the fields unexcited. In connecting up a motor for driving 
the armature the speed should be made as low as possible, 
preferably not over 75 revolutions per minute. 

Temporary Cover for Use When Turning Down a Commu- 
tator. In cases where it is necessary to turn down the commu- 
tator of a direct- current machine without removing it from 




ir First Cord on Cover 



2r Cover Stretched up Over Leads. 



a- Lost Tie on Cover 4i- Last End Pulled under 

Cord Winding. 

FIG. 228. Steps in applying a cotton cover over the end of an armature 
before turning down the commutator to prevent copper chips falling behind 
the bars to cause short circuits. 



the frame, H. S. Rich has made use of a muslin cover (Elec- 
trical Record, December, 1918) tied over the open leads so that 
copper chips cannot find a way down behind the bars to cause 
a short-circuit. This cover is made and applied as follows: 
Cut a strip of muslin wide enough to reach across the commu- 
tator and well past the leads, and long enough to go around 
one and one-half times. Tie this very securely with fine 
strong cord, with the muslin laid over the bars as shown in 
Fig. 228. 

Draw the covering and stretch a little at a time all around 
and up over the open leads far enough back to allow of at least 
two separate cords to be tightly wound around and very se- 



COMMUTATOR REPAIRS 311 

jfe 
curely tied on the core body. Turn the armature slowly to 

see that the covering does not interfere with any brush holder. 
If so, the holder should be shifted. The surplus edging, all 
loose strings and threads should now be trimmed off all around. 

By turning back the muslin over the leads the first cord 
tied around the bars is covered neatly. The cords wound 
around the core body can be secured without knots by wind- 
ing over the first end a few turns, and then by winding over 
a short extra loop, the last end can be jerked under all the 
turns and cut short so that a knot is not needed. For a 
permanent armature covering, shellac should be applied all 
over it which seals the cords and stiffens the muslin. 

Refilling a Commutator. When a commutator is to be 
refilled, disconnect the armature leads and remove the com- 
mutator. A simple device for accomplishing this is shown 
in Fig. 229. Two long bolt rods are screwed into holes tapped 
into the sleeve ring, and a bar of heavy iron placed across the 
end of the shaft, with bolt rods coming through, as shown. 
By tightening the nuts evenly, the commutator can be pulled 
off. Next count the number of bars carefully, and enter 
this in a note book for future reference. Remove the sleeve 
and if possible save the mica rings. If these are in good 
condition they can be used again. Carefully caliper the 
diameter of these rings and enter this in the note book also, 
as the new commutator will have to be bored to fit these rings. 

With a micrometer caliper, 
measure the thick and thin 
edges of one of the bars in 
thousands of an inch; also the 

thickness Of the mica Segment. Fl " 2 29.-Device for removing 
It is advisable to Order the a commutator from an armature 

bars and mica segments from 

the motor manufacturer sawed to the proper size, as in all 
probability this can be done cheaper than in an ordinary repair 
shop not equipped for this work. When ordering new bars 
a detailed drawing should be sent, giving all necessary 
dimensions of the old bar for boring and turning purposes. 
Hard drawn copper is usually used as it wears at about the 
same rate as the mica segments (see page 319). 




312 ARMATURE WINDING AND MOTOR REPAIR 

Use of a Commutator Clamp. When assembling the com- 
mutator, a clamp will be necessary to hold the bars to- 
gether while boring. Several makeshift methods are available, 
but it will pay any repair shop to have suitable cast-iron 
clamps, such as shown in Fig. 230. The clamp should be 
smaller than the diameter of the commutator, so that when 
it is drawn tight, there will be a space of about % inch between 
the sections. When using this clamp as shown in Fig. 230 
at the right, wooden blocks (C) can be employed to hold the 
clamp about midway of the commutator. D is an iron face 
plate. The clamp (B) should first be placed on the plate as 
shown, and the bars and mica segments stacked in a circular 
form within it. Care must be exercised to make sure that a 




FIG. 230. At the left, a clamp for holding commutator bars together when 
being assembled. At the right, the use of this clamp is shown. 

mica segment is placed between each copper bar. Count 
the bars carefully, so that their number corresponds with the 
number of bars in the original commutator. 

Take several pieces of copper wire (about No. 9 B. & S. 
gauge) and remove the insulation. Place these around the 
commutator near the top and lower ends to act as band wires, 
and twist them tight. The clamp may then be removed, 
and the commutator straightened. Bring out the mica 
segments even with the surface of the bars by holding the 
fingers against the inside edge of the segments and tapping 
the bars on the outside with a small mallet. Place a square 
or steel scale on the face plate and tap the bars on the outside 
with a small mallet. Place the square or steel scale on the 
face plate and see that the bars line up perpendicularly with 
one edge of the square. If they do not, a gentle pressure one 



COMMUTATOR REPAIRS 313 

way or the other on the top end of the commutator with the 
palm of the hand will bring them in line. See that each bar 
and segment is down flat against the surface of the plate, 
since that end will be fastened to the face plate on the lathe 
when facing off the ends of the bars. Tap each bar and seg- 
ment down solid with a square ended punch, a little narrower 
than the thickness of the bar. When this has been done, the 
band wires can be drawn a little tighter, and the surface of 
the commutator, where the clamp will fit, should be filed 
to remove any protruding mica, and present a smooth surface 
for the clamp. 

Replace each section of the clamp about the commutator 
again using the wooden blocks mentioned before. Draw 
the clamp tight, being sure to leave the same amount of 
space between each clamp section. A small gas burner, 
or some other source of heat should be handy, and the com- 
mutator placed over it and heated. When it is good and 
hot to the hand, tighten the clamp, allow it to cool, and again 
tighten. 

Boring out the End of a Commutator. The next thing to do 
is to bolt the commutator to the face plate of the lathe and 
center it. The same wooden blocks can be used again for 
supporting the clamp. Face off the end of the bars, and then 
groove out the end for the taper rings to the same diameter 
as the mica rings on the sleeve. Take one of the old bars, 
and with a bevel protractor determine exactly the taper used 
on the old commutator. The usual taper employed is shown 
in Fig. 231. A small groove (shown at A) should be cut below 
the intersection of the two tapers to allow room for the edge 
of the mica ring. The other end should be treated in the same 
manner. 

When the boring has been completed, a close inspection 
must be made of all turned surfaces, to make certain that no 
copper has been dragged over the mica to form a short- 
circuit. The corner of the groove A (Fig. 231) should be 
carefully gone over, as it is here that drag-overs most fre- 
quently occur. Scrape and wipe the mica rings on the sleeve 
clean; also wipe out the inside of the commutator with a soft 
rag. Place the sleeve in the commutator again, and draw 



314 



ARMATURE WINDING AND MOTOR REPAIR 



up on the end nut. On a refilling job, when the old mica 
rings are used again, cracks may be found between the band 
ring and the commutator copper due to the irregularity of 
the mica ring caused by the shape of the old commutator. 
These cracks can be filled by pushing in thin sheets of mica. 
Shellac should be used liberally on the ends. 

Bake the commutator in an oven for about one hour until 
it becomes thoroughly hot, and tighten the nuts. Reduce 
the heat somewhat, and bake at a low heat until the shellac 






II 




Ill 

FIG. 231. I shows taper of commutator bars. II is a template for laying 
out mica end rings. Ill shows a scheme for laying out the taper E, of the 
template shown in II. 

becomes hard. Tighten the nuts again and allow the com- 
mutator to cool. When cold give the nuts a final setting up. 
The clamp can now be removed. 

In order to finish the commutator, use a mandrel to fit 
the bore of the sleeve, and take the final finishing cuts. After 
finishing, a bar to bar test with the test lamp outfit should 
be made to be sure there are no short-circuits between bars. 
A test for grounds should also be made by holding one wire 
of the test lamp on the iron sleeve and passing the other from 
bar to bar around the commutator. A high-voltage test for 
grounds should be made also later. 



COMMUTATOR REPAIRS 



315 



The slots for taking the armature leads must next be cut. 
It is well to cut them a little wider than the diameter of the 
lead wires. These slots should be made in a milling machine 
if one is at hand, otherwise a hack saw can be used. Some- 
times two blades fastened together will give the required 
width of slot. The commutator can now be placed on the 
armature shaft. 

Mica Used in Commutators. The selection of mica for the 
insulation of commutator bars and for V-rings is usually based 
upon the type of motor being repaired, but built-up mica has 
been found to be the most satisfactory. India mica is gener- 
ally too hard and domestic mica too brittle for use between 




FIG. 232. Mica segment (B) cut from sheet using bar (A) as pattern. 
Such a segment is cut large at top and at ends so as to turn down evenly 
with copper bars when commutator is finally surfaced. 

bars. Domestic mica also has cracks and fissures which fill 
up with dirt and cause short circuits. Amber mica gives 
good service but it comes in such odd sizes that it is too 
wasteful for the average repair job. Hard-baked built-up 
mica is found best to be used between bars and the unbaked 
best for building up the V-rings. The unbaked mica is 
usually obtained in sheets 0.025 inch and 0.030 inch thick. 
Heavier mica would be difficult to bend and to cut in shape, 
The same sizes of hard-baked mica with the addition of 
0.02 inch and 0.035 inch have been found sufficient stock to 
fill almost any size of commutator. In cutting up the un- 
baked mica for the V-ring insulation it is safer first to fit a 
piece of paper around the ring and then use this as a template. 



316 



ARMATURE WINDING AND MOTOR REPAIR 



Shaping Mica End Rings. When it is impossible to save 
the old mica end rings, new ones must be made. These can 
be made of flexible mica usually 0.030 inch thick. It is 
best to make the rings in three pieces to the layer on small 
commutators; three layers constituting a ring. When heated 
slightly, this mica can be bent around the sleeve to the desired 
shape. Trim the edges square on each piece and fit them 
tightly in the bore of the commutator. The taper rings 
should be fitted in the same manner with a slight space being 
left between abutting ends. 

Templet for Making Mica End Rings. A good shop method 
for making a template to lay off mica end rings is as follows : 




FIG. 233. Parts of a medium sized commutator. 

<.o) and (c) Mica end rings. (6) Iron clamping V-ring. (d) Mica bushing sleeve 
for inside of commutator, (e) Commutator bars and mica segments assembled. 
Sample bars and mica segments are shown at (/) and (0). 

Mark off on a piece of cardboard an arc of a circle, the radius of 
which is equal to the diameter A in Fig. 231. Strike another 
arc from the same center with the radius of the dividers made 
smaller by the distance C. This will give a templet for the 
band D. 

A templet for the taper E can be prepared as illustrated in 
the following example : Assume the diameter B to equal seven 
inches. Then in Fig. 231 lay off one-half this diameter, or 
inches from A on the edge AB as shown at 0. If the 



COMMUTATOR REPAIRS 317 

taper is 30 degrees, take a 30- by 60-degree triangle and square 
its lower edge with the line AB, having its apex, or point 
touching the point at 0. Draw the line OC, which will be 
the radius of the arc of a circle D, shown dotted. Strike 
another arc from the same center with the radius made smaller 
by the distance F, Fig. 231. This will give a templet for the 
taper E. 

Micanite as a Commutator Insulation. For use in insulating 
commutators, the Mica Insulator Company, New York City, 
has developed a mica laying machine which successfully 
constructs large sheets of micanite from thin mica laminae 
with cement uniformly applied between the layers. These 
sheets are finished by drying and baking under high pressure 
after which the sheets are packed through milling machine and 
finished to accurate thickness. The company gives the follow- 
ing directions for using micanite as a commutator insulation. 

If micanite segments for insulation between copper bars 
of a commutator are to be cut from full size sheets of micanite 
plate, a fine tooth band saw with moderate set should be used. 
Bookbinders' shears or foot power cutters are poor tools to 
use as the edge of the micanite is likely to be bruised and this 
may result in dragging of the copper over the micanite when 
the commutator is turned in a lathe. Short circuits, or near 
short-circuits, are often caused in this manner. It is not neces- 
sary to coat the surface of either the copper bars or the mica- 
nite segments with shellac before assembling the commutator. 
After turning the V's for the end rings carefully examine all 
surfaces and scrape clear any places where the lathe tool has 
dragged the copper. Before placing micanite rings, carefully 
remove all dust from both commutator and rings. Short- 
circuits are sometimes caused by overlooking these details. 
After the commutator is assembled on the shells, with the 
rings in place it should receive a baking at a temperature of 
about 150C. (302F.). This baking is for the purpose of 
embedding the end of the segments and setting the cement 
in the rings. 

Precautions when Tightening a Commutator. It is essen- 
tial on account of the expansion of the copper, that the commu- 
tator is not screwed up too tight before it is baked. The best 



318 ARMATURE WINDING AND MOTOR REPAIR 

results are obtained by tightening up the commutator gradu- 
ally after baking as it cools, giving a final tightening when nearly 
cold. Many of the troubles met with in commutators is due 
to excessive tightening. In arch-bound commutators, it is 
important that the assembled segments are not tightened too 
hard in the arch before the V parts are turned, as the result- 
ing stresses from the expanding copper, distort the parts and 
often are the main cause of high bars. Similar care should be 
taken not to exert more pressure than necessary on the end 
rings and whenever possible means should be provided to 
measure the pressure. Cases have been known where the 
angles of the clamp rings in small commutators have been 
changed by excessive pressure. Approximately 1500 Ib. per 
sq. in. on the projected area of the inside cone or taper rings 
is a satisfactory medium pressure. 

Making Micanite End Rings. It is often necessary for 
the commutator builder, particularly the repair man, to make 
up his own rings. For that purpose a different kind of plate 
known as No. 1 micanite or moulding plate is furnished by 
the Mica Insulator Company. This plate is made from white 
mica selected with same care as for micanite commutator seg- 
ment plate and contains the proper proportion of cement to 
produce satisfactory moulding characteristics. Micanite is 
not ductile and will not stretch uniformly like sheet metal. It 
is, therefore, necessary to cut the pattern for micanite com- 
mutator rings into the exact development of the ring before 
moulding. The pattern should be long enough to allow for a 
tapered splice at the ends. For cementing the splice, use a high 
grade shellac of about 2 Ib. to the gallon of solvent. When the 
No. 1 micanite is placed on a steam table or metal plate 
heated to about 140C. (284F.), it becomes plastic and can 
be moulded readily into the desired shape. The micanite 
should not remain on the hot plate any longer than necessary 
as the cement might set, making it less pliable. It is advisable 
to turn the micanite over so as to heat uniformly from both 
sides. After placing the micanite in the mould or in the com- 
mutator, it should be under pressure while cooling. 

Causes of Excessive Commutator Wear. Warren C. Kalb 
has pointed out (Power, March 18, 1919) that commutator 



COMMUTATOR REPAIRS 319 

wear may be caused by the use of abrasive brushes. It may 
also result from the same mechanical or electrical causes 
that produce brush wear. Mechanical conditions of this 
nature are: Rough commutator due to poorly prepared 
surface; rough commutator caused by burning; vibration from 
any source such as the pound of a direct-connected engine, 
belt lacing, improper mounting, loose bearings or commutator 
out of true; high peripheral speed, causing brushes to chatter 
or creating an excessive temperature at the brush faces due 
to friction ; type of brush-holder or angle of operation, resulting 
in the chattering of the brushes. 

Copper Used to Make Commutator Bars. The bars for 
commutators can be made from drop forged, cold rolled, 
hard drawn or cast copper bars. The drop forged copper is 
usually considered the best and cast copper the worst for bars. 
Probably most commutators are made of cold rolled or hard 
drawn copper. Cast copper is poor on account of the many 
pin holes and larger that are liable to be in the structure of 
the bar. These show up at every turning and require filling 
to prevent carbon dust from accumulating. If they are near 
the edge of the bars the carbon is liable to bridge the mica 
between the bars and cause a short-circuit and burn a hole 
in two bars. This calls for a patching of the bars and the 
insulation which adds a weak spot to the commutator and 
simply hastens the time when it must be rebuilt. A good 
grade of copper for bars is economical from points of service 
and reduction of repair costs. 

However, when hard-rolled, or drop-forged copper is not 
available for the repair man, cast copper makes a good substi- 
tute. If the castings are soft they have a minimum conduc- 
tivity of 60 per cent, and are more free from blow holes; also, 
they wear from 75 per cent, to 90 per cent, as long as the hard- 
drawn or drop-forged copper bars. The castings are soft but 
tough, and the low conductivity is due to the fluxes used in 
melting the copper. A reduction in the amount of flux used 
increases the conductivity of the copper, but it also tends to 
make the metal more brittle, more porous and liable to have 
blow holes below the surface. The 60 per cent, soft-copper 
castings do not need filling or machining on the sides. They 



320 ARMATURE WINDING AND MOTOR REPAIR 

can be easily straightened and any projection flattened out on 
a surface plate by the aid of a steel flattener and a hammer. 

Test for Oil -saturated Mica in a Commutator. When the 
mica between commutator bars appears to be saturated with 
oil the following test can be applied. Wet a rag or piece 
of waste with gasoline and wipe the surface of the commutator 
dry. Then with a torch heat the commutator at the point 
where the mica appears to be saturated. If oil has worked 
down between the mica and the bar it will ooze out in small 
bubbles. In such a case new mica segments must be inserted. 

Blackening of a Commutator at Equally Spaced Points. 
This is usually caused by an open circuit in the armature 
winding. The open circuit may be at the commutator necks 
or at the rear-end connections of the winding when two-piece 
coils are used. When these spots are noticed on a commutator 
it should be tested out for open circuits at once before further 
commutator troubles develop. 

UNDERCUTTING MICA OF COMMUTATORS 

The object of undercutting the mica of commutators is to 
clean out the mica between the copper segments to a depth of 
about J^2 to %2 mcn s that the copper bars will wear evenly 
free from the poor commutation that often results when the 
mica is hard and does not wear down evenly with the copper 
segments. The undercutting operation should be done after 
the armature has been rewound, soldered and banded. The 
commutator should also be trued up and all excess solder 
removed from the neck and face. 

Tools for Undercutting Mica. Special motor-driven or 
belt-driven saws are available for doing this work which can be 




FIG. 234. Hand tool for undercutting mica that can be connected to a small 
motor by a flexible shaft. 

used when the armature is centered in a lathe. The saws 
are usually clamped on an arbor which is mounted on a head 



COMMUTATOR REPAIRS 



321 



that moves on slide rails. By means of a hand-operated lever 
or foot pedal controlled by the operator, the revolving saw 
is carried over the face of the commutator. A shaper equipped 
with a special tool can also be used. Frequently a milling 
machine will be found convenient for undercutting the smaller 
sizes of armatures. A less expensive device consists of a motor- 
driven circular saw which is mounted in such a way that the 
saw can be guided over the commutator face by hand. Two 
such motor driven tools are shown in Figs. 234 and 235. 
Size of Circular Saw Required. For the cutting tool 
a circular saw or miller about to 1J inch in diameter 




FIG. 235. Motor operated hand tool that can be connected to a lighting 
circuit for undercutting the mica of a commutator. 

with from 15 to 30 teeth seems to give the best results. A 
small diameter saw must be used in order to cut the slot to 
the proper depth and at the same time not cut into the neck 
of the commutator. The saw should be driven at approxi- 
mately 1500 rpm. and be about 0.005 inch thicker than the 
mica in order to remove the mica completely. If the com- 
mutators are of large diameter, one or two spacers of the same 
thickness as the bars may be used between the saws and two 
or three slots cut at the same time. The cutting edge of the 
saw should revolve in a direction toward the operator while 
cutting the mica. When the hand-operated motor-driven 
21 



322 ARMATURE WINDING AND MOTOR REPAIR 

tools are used they should be drawn toward the operator, 
in order to properly guide the tool. 

It is advisable to have a jet of compressed air or a fan so 
located that the particles of mica and copper will be blown 
away from the armature to prevent this material falling in 
behind the commutator and at the same time make it easy 
for the operator to see the slot when the saw is throwing the 
particles toward him on the face of the commutator. 

Finishing Slots and Commutator Surface after Undercut- 
ting. After the slots are sawed it is advisable to go over them 
with a sharp hand tool to remove remaining particles of mica 
and thin strips along the edges of the slot. This can be done 
with a sharp knife, or V-shaped tool, which can also be used to 
slightly bevel the sharp edges of each bar and remove all 
burrs. Several hand tools that can be used for undercutting 
mica when motor operated tools are now available are described 
in the following paragraphs. 

After the undercutting operation has been completed the 
commutator should be stoned and polished with fine sandpaper 
to remove all burrs of copper. 

Brushes for Use on Undercut Commutators. The removal 
of mica permits the use of brushes without abrasive qualities 
and has resulted in the development of very low friction 
brushes. In fact some of the very lowest friction brushes 
manufactured are hard and do not contain any graphite. 
Such brushes give to a slotted commutator the brown gloss 
that shows perfect operation. The life of a hard non-abra- 
sive brush is several times that of a graphite brush. Some 
criticism has been made of slotted commutators which has 
been traced to the fact that the brushes used were not suitable. 
Often when the same brushes are used on an undercut com- 
mutator that were used before, the commutator will be ridged 
and worn rapidly and the slots filled with copper dust with 
short-circuits and burned out coils the result. The fault in 
such cases is in the brush not in the undercutting process. 

It should also be remembered that the brush tension on 
an undercut commutator can often be much lower than before. 
Brush authorities recommend lj^ to 2J pounds per square 
inch of cross-section on stationary motors, from 3 to 5 pounds 



COMMUTATOR REPAIRS 



323 



per square inch for crane motors and from 4 to 8 pounds per 
square inch on railway motors when the commutators are 
undercut. 

Another factor in good operation of undercut commutators 
is cleaning at regular intervals. If the motor is operated in a 
place where the accumulated dust and dirt is dry, it can be re- 
moved with a blast of compressed air. Even in such cases 
the slots should be scraped occasionally. A lubricant should 
never be used on an undercut commutator nor any other when 
operating conditions are correct. 

Hand Tools for Undercutting Mica of Commutators. An 
easily made hand tool for undercutting the mica of com- 




FIG. 236. 



-Short saw blade clamped in a vulcanized fiber holder for use in 
undercutting mica. 



mutators is shown in Fig. 236 as devised by William H. Watson 
(Power, Oct. 1, 1918). The slot in the holder is made as deep 
or deeper than the width of the hacksaw blade, with small bolts 
to clamp the blades at each end. Any depth of cut desired can 
be made by adjusting the blade in the holder, but there sel- 
dom is occasion to move the blade. The handle part is cut 
away just enough to allow the fingers to pass over the commu- 
tator without rubbing. In undercutting commutators care 
must be taken that no rough edges are left after the mica is 
cut, but it is sometimes hard to avoid this, especially if it is an 
old commutator. These rough corners can be smoothed, after 



324 ARMATURE WINDING AND MOTOR REPAIR 

the mica has been undercut, with a V-shaped tool made of 
hardwood or vulcanized fiber, run between the segments as 
shown at A, and it does not spoil the bar. The front corner 
of the undercutter, which is made of fiber, can be used to 
smooth these raw edges. 

Another hand tool recommended by T. M. Sterling (Power, 
Nov. 5, 1918) for undercutting mica and as useful in other 
ways by the repairman, is illustrated in Fig. 237. In the 
illustration A represents a steel straight-edge about ^ inch 
thick and slightly longer than the length of the commutator 
segments and square at one end to butt up against the head of 
the commutator. B is the undercutting tool, about 12 inches 




FIG. 337. Hand tool for undercutting commutator mica. 

long, made from a piece of %Q hexagonal tool steel forged flat 
for about one-fourth of its length to a thickness at point D 
equal to the thickness of the mica between the segments. The 
"half-heart" shaped lobe C affords a bearing place for the 
fingers in using the tool. 

When using this tool lay the straight-edge on the commu- 
tator with its edge in line with the mica between the segments 
and the square end against the head of the commutator. 
Hold it firmly with the left hand. Then, holding tool B in 
the right hand, draw the point D along the mica the same as 
drawing a pencil along a ruler in ruling paper. This operation 
will start a nice groove in the mica without burring the edges 
of the copper on either side. After one or two passes the tool 
can be turned endwise with point D down and the mica taken 
to any desired depth or a saw blade may be used to finish the 



COMMUTATOR REPAIRS 325 

groove. The disadvantage of the saw blade is that the set 
of the teeth burr the copper more or less, while with this tool 
all chance of a burr is eliminated. 

The tool will also be found handy in raising coil leads out 
of the slots in the head of a commutator when unsoldering 
them preparatory to removing the commutator or making 
coil repairs. After heating the ends of the coils until the solder 
starts to melt, drive the point of the tool into the slot under 
the ends of the wires, with lobe C down, for a fulcrum, on the 
bottom of the slot. The ends of the coils can then be easily 
raised by a downward pressure of the hand. Square edge 
E can be used as a scraper for taking the surplus hot solder 
off the ends of the wires as they are raised. 



CHAPTER XIII 

ADJUSTING BRUSHES AND CORRECTING BRUSH 

TROUBLES 

Many of the troubles which are charged to brushes of motors 
and generators can be traced to improper application and 
adjustment or to other defects of the machine that show 
up in sparking at the commutator. A careful selection of 
brushes is important but no more so than carefulness in ad- 
justment and frequent inspection in operation, for proper 
care of brushes and brush rigging results in good commutation 
and prolonged life of both brushes and commutator. The 
repairman should, therefore, give due consideration to brush 
adjustment when overhauling a machine. 

Fitting or Grinding-in Brushes (Instruction Book, Westing- 
house Electric & Mfg. Co.). When it becomes necessary 
to install a new set of brushes, sand paper or garnet paper 
may be used that is long enough to go around the commutator. 
It should have a lap of several inches and be so mounted on the 
commutator as to preclude the lapped end butting against the 
brushes when the machine is rotated for grinding. With 
many commutators the friction between the commutator bars 
and sandpaper (if the paper is taut) will suffice to keep the 
paper from slipping, especially if, when starting, the paper is 
given a pull in the direction of rotation by the operator. 
If the paper persists in slipping use a little glue to stick the 
under end of the paper at the lap to the commutator. All 
traces of the glue must be removed from the bars before 
putting the machine into service. 

A second method is to remove the middle brush on each arm 
and bind the paper to the commutator by running tape or 
string entirely around the periphery. If necessary, also bind 
the paper at the inner and outer ends of the commutator. 
After the paper is anchored the commutator may be rotated 

326 



ADJUSTING BRUSHES AND CORRECTING TROUBLES 327 

by hand, or in any convenient way. Great care must be 
exercised in " grinding in" brushes in this way, as the cutting 
is very rapid, especially with soft brushes, and much of the life 
of the brush may be ground away in a very few revolutions. 
If after the brushes have been surfaced in the above manner, 




FIG. 238. Grinding-in brushes of a direct-current generator with a strip of 

sandpaper. 

the trailing edge shows a poor seat on account of having had 
to mount the ridge due to the lap in the paper, a final surfacing 
should be done with very fine sandpaper by hand. The 
foregoing method is particularly desirable when a machine 
has very hard brushes, or a large number of soft brushes. 



328 ARMATURE WINDING AND MOTOR REPAIR 

It is a great time-saver after the knack of applying and anchor- 
ing the paper is understood. 

With slip rings, if carbon or graphite brushes are used, the 
same general scheme is applicable, but if metal graphite 
brushes are used, emery cloth, or its equivalent, is preferable. 

There are those who advocate and those who practice 
pulling the grinding paper in the direction of rotation in 
order to more accurately surface the brush. This seems 
logical enough on first thought, but inasmuch as the com- 
mutator is not in motion when the brushes are surfaced in 
this manner there is no assurance that the brush will bear 
the same relation to its holder (and therefore to the com- 
mutator) when the commutator is rotating and, consequently, 
no assurance that the contact will remain fixed. This perhaps 
explains why brushes frequently show perfect contact when 
idle, but poor contact when in service. This method of 
fitting the brushes, therefore, is not necessarily more depend- 
able than other methods, though in some instances it can 
be recommended. 

After the brushes have been fitted all dust should be care- 
fully blown out of the commutator by compressed air. The 
pressure should be about, 50 to 80 pounds per square inch used 
with a J^-inch nozzle. A higher pressure may injure insula- 
tion and is not necessary. 

It is important that the brushholders, whether for direct 
or alternating-current use, be neither too close to nor too far 
from the commutator or slip rings. A suitable distance is 
from three-sixteenths to one-quarter of an inch. 

If the brushes are copper-coated, the coating should be 
not allowed to come in contact with the commutator or slip 
rings. This means that as the brushes wear the copper coating 
should be scraped back and not allowed to extend below the 
brushholder box. With proper shunts there should be no 
need for copper coating from an electrical standpoint, although 
for mechanical reasons it may be desirable as a protection 
for soft or structurally weak brushes. 

If the toe of the brush is very sharp it is good practice to 
"nose off" the knife edge. This reduces breakage and mili- 
tates against gouging into the commutator. 



ADJUSTING BRUSHES AND CORRECTING TROUBLES 329 

Adjustment of Brushholders (Instruction Book, Westing- 
house Electric & Mfg. Co.). If the commutator or slip-ring 
speed is low, a holder having a sluggish spring and slow action 
will give entirely satisfactory results but such a holder on a 
high-speed machine may fail miserably. High speeds require 
quickness of action on the part of the brush. This means 
a sensitive quick-acting spring and a holder designed to aid 
the spring. A high-speed holder, therefore, may give excellent 
results on low-speed machines, but a low-speed holder is not 
likely to approach even fair results on high-speed machines. 
This is a point a repairman must bear in mind when changing 
the speeds of direct-current machines. 

The box, or part holding the brush, should be as nearly a 
full box as possible. Whether the rotation is against the toe 
or against the heel, both the toe and heel sides of the holders 
should have approximately equal areas for the support of the 
brushes. It is obvious that the shorter the box, the greater 
will be the shifting movement of the brush on the commutator 
due to looseness in the holder; and the deeper the box, the 
less will be the shifting. From this it follows that the boxes 
should not be too short, nor should the brushes be too loose 
in the boxes. 

It is an established fact that with even very slight play in the 
holder the changing brush friction due to load, temperatures, 
etc., will result in a changing relation between the brush and 
the commutator, and thereby cause changing and misleading 
brush drops from day to day. This accounts for the difficulty 
in verifying, on different days, brush drops, although the loads 
may be identical. The neutral does not change, but the 
relation of brush contact to commutator does change, and 
the brush drops vary accordingly. Another explanation for 
the changes that take place in brush drops is that when 
brushes have been freshly " sand-papered " or " ground in" 
the surfaces have unglazed or soft faces, the brush drops 
being influenced by the porous condition of the faces, and the 
lubricating values of the particles liberated during the time the 
brush face is seating itself to the commutator. During this 
period the commutation, with certain kinds of graphitized 
brushes, may be much better than after the glaze or permanent 



330 ARMATURE WINDING AND MOTOR REPAIR 

surface forms on the brush. Drops taken during this time may 
not be the same as drops taken after the brush is "faced up" 
and may prove seriously misleading. Definite or reasonably 
permanent drop values are, as a rule, to be had only after 
the brush faces have reached a fixed condition. 

From the foregoing it is obvious that brushes should fit 
snugly, but not tightly, in the holders; also that there are 
certain relative dimensions which are preferable between 
the brushes and holders and which must not be ignored if the 
best results are to be obtained. For instance, the side of the 
holder in the direction of rotation should not have less area 
than the lagging side. In fact, the side against which the 
brush presses should have preferably a greater area than 
the opposite side certainly not less area. With a limited 
area to oppose the brush, movement of the brush in the holder 
due to looseness, oscillation and imperfect relations between 
the brush and commutator, accentuates the change in the 
angle of contact, and results in poor commutation, as well as 
in mechanical and electrical damage to that part of the brush 
in contact with the holder. 

If a brush is thin, its length and holder contact should be 
greater than if it is thick, for the reason that the thinner the 
contact area on the commutator, the less stable will be the 
brush on the commutator, and the greater will be the need 
of support from its holder. To illustrate : If the brush con- 
tact is a knife edge, serious chattering may result and there 
may be a maximum of instability, but if the knife edge be 
gradually removed the brush will become increasingly stable 
as the thickness of the brush contact increases, and if the 
brush angle is correct the maximum stability will be reached 
when the brush has its greatest thickness in contact with the 
commutator. It stands to reason, therefore, that the greater 
the area due to thickness, the more stable will be the brush 
on the commutator, regardless of the holder, and that as a 
consequence a holder for a thick brush might not prove at all 
suitable for a thin brush, the speeds being equal. If, however, 
there is the same amount of lost motion in the holders for a 
given length of thin brush and thick brush, and the brush 
angle is such that the thick brush moves its maximum in the 



ADJUSTING BRUSHES AND CORRECTING TROUBLES 331 

holder, then the thick brush will show a greater change in its 
relation to the commutator than the thin brush. The thick 
brush, however, will not be so likely to shift its maximum on 
account of its bearing surface being greater than the bearing 
surface of the thin brush. In other words, it rests on a 
greater area. 

Causes of Rapid Brush Wear. Warren C. Kalb (Power, 
March 18, 1918) has found that the following electrical con- 
ditions cause rapid brush wear: Sparking, from any cause 
whatever; glowing of brushes; pitting of brush faces. 

Glowing results from excessive current density and may 
be local or may cover the entire contact end of the brush. 
Common causes are unequal collection of current by different 
brushes of the same polarity and very heavy short-circuit 
currents in the coils undergoing commutation, which, added 
to the load current, bring the temperature of the carbon 
at the face of the brush up to the glowing point. 

Pitting may result from glowing in small spots on the brush 
face. It is also caused at times by particles of copper becom- 
ing attached to the brush face. This causes a heavy current 
to localize in a small area, disintegrating the carbon and 
forming a small crater in which the copper embeds itself. 

The increased brush wear caused by increase in current 
density is due to the higher temperature so created at the 
brush face and the consequent more rapid disintegration of 
the carbon at that point. Any factor tending to further 
increase the temperature at the brush face will add to the 
rapidity of wear. Some of the things having this effect are: 
High coefficient of friction ; higher contact drop than is needed 
for commutation, especially on machines of low voltage and 
high current capacity; contact drop too low for sparking 
voltage, permitting heavy currents to flow in the short- 
circuited coil; lack of carrying capacity or very high current 
density. 

Higher current densities are permissible where there is no 
sparking voltage than where there is. This is because the 
heating effect of commutation current is absent and higher 
load currents can be applied before the same temperature 
rise is attained. 



332 ARMATURE WINDING AND MOTOR REPAIR 

Average volts per commutator segment may be as high as 
15 to 20 volts without difficulty being encountered. Or 
the reactance voltage may be that high and still be neutralized 
by interpole flux or fringing field within sufficiently close 
limits to secure good commutation. But the sparking 
voltage that is, the resultant between the reactance voltage 
and the other electromotive forces generated within the coil 
undergoing commutation should be less than the contact 
drop of positive plus negative brush to attain perfect com- 
mutation. Inasmuch as 3 volts is about the highest brush- 
contact drop obtainable, it will be seen that the sparking 
voltages mentioned would not be neutralized within 12 to 17 
volts. Such a voltage would set up excessive currents in the 
short-circuited coils and result in rapid burning away of the 
brush faces even with no-load current whatever carried by 
the machine. 

Methods for Locating the Electrical Neutral in Setting 
Brushes. The following methods have been suggested by 




i: 



V 
FIG. 239. Pilot-brush method of locating brushes on the electrical neutral. 

T. F. Barton (Power, July 16, 1918) for setting brushes on 
the electrical neutral in direct-current machines. 

With the machine running at no load, normal speed and 
voltage, shift the brushes until a low-reading voltmeter shows 
no deflection when connected to points just inside the heel 



ADJUSTING BRUSHES AND CORRECTING TROUBLES 333 

and toe of a pilot brush. The pilot brush must be the full 
size of the brushes used on the machine and can be made of 
fiber or wood with holes drilled through it to allow contact 
on the commutator at the desired point, which should be 
at the center of two adjacent commutator bars as indicated 
in Fig. 239. 

Operate the machine as a shunt-wound commutating-pole 
motor, checking the speed in both directions of rotation, 
holding the same value of armature voltage and shunt-field 
current in each case. The brushes are on neutral when the 
speed is the same in each direction of rotation. 

It is preferable to locate the brushes on the electrical neutral 
and make the adjustments on the commutating- and series- 
field windings to give the desired results. Brush shift can be 
used, provided the final brush position is in a satisfactory 
commutating zone. The capacity of a machine is not re- 
duced by shifting the brushes if the rated voltage and current 
can be successfully obtained. 

Angle at which Brush is Set. When machines are not 
subject to reversals, the proper angle for the brush will depend 
upon whether the rotation is against the toe of the brush or 
against the heel. It also depends upon the friction coefficient 
of the brush. If set against the toe, the angle should approxi- 
mate 35 degrees. Above 35 degrees the toe becomes very 
sharp and mechanically weak. If set against the heel, the 
angle may be from 12 degrees to 25 degrees. In instances 
where the brush touches more bars than required, the sharp 
toe can be beveled off thereby reducing the brush contact 
and the short-circuiting current under the brush. This also 
increases the mechanical strength of the brush. Whether 
running against toe or heel, it is good practice to round off the 
sharp or knife edge of the toe. This reduces the possibility 
of the knife-like edges digging into the commutator in case 
the brushes jam or become tight in their holders and at the 
same time protects against brush breakage. 

The friction coefficient of a brush is a variable one, influenced 
by the characteristics of the brush, by the glazing of the 
contact face of the brush, the varying brush temperature due to 
load and spring pressure and the condition of the commutator. 



334 ARMATURE WINDING AND MOTOR REPAIR 

whether it is smooth or undercut, hot or cold, in good or poor 
mechanical condition. The poor mechanical condition of the 
commutator can be eliminated by proper attention. The 
other influences are permanent except as they may be modified 
by finding the most suitable brush angle. 

Checking Brush Setting. To check the spacing of brushes, 
place a strip of paper completely around the commutator under 
the brushes and mark the position of each set. The strip can 
then be removed and the distance between marks measured. 
If the distance varies the brush holders should be moved into 
the proper position. The degree of accuracy with which 
brushes should be set depend upon the type of machine. 
With interpole machines, the spacing should be more accurate 
than with other types of machines. The brushes on any given 
stud should be staggered with respect to those on adjacent 
studs so that the entire commutator will be covered by the 
brushes except a slight space at each end. This prevents 
grooves wearing in the commutator. 

Brush Pressure. This depends upon the character of the 
brush, the commutator speed in feet per minute, and the 
mechanical condition of the commutator. It varies from two 
to five pounds per square inch, the pressures generally used 
being from 2.5 to 3.5 pounds on commutators and from three to 
five pounds on slip rings. If there is a tendency for the com- 
mutator to burn, this can at times be corrected by increasing 
the pressure, and thereby increasing the abrasive effect of the 
brush sufficiently to scour out the burning and maintain a 
polished surface. When the mica is not undercut the brush 
should have sufficient abrasive effect not only to scour out 
possible burning of the commutator bars but also to wear and 
keep the mica flush with the commutator bars. 

Common Brush Terms. The following terms are frequently 
used in describing brush characteristics and their performance. 

Contact Drop. This refers to the drop in voltage between 
the brush and the commutator. It differs with different 
materials, different brush pressures and different loads, being 
between one and two volts. For a given material and given 
load the contact drop decreases with increased pressure and 
increases with the load. It varies little with changes in speed 



ADJUSTING BRUSHES AND CORRECTING TROUBLES 335 

above 2500 feet per minute and but little with temperature 
changes under normal conditions. 

Brush Friction. The coefficient of friction depends upon 
the brush material, brush angle and commutator speed. With 
a given material and brush angle, the coefficient of friction 
increases with load and brush pressure and in general decreases 
with increased commutator speed. Increased temperatures, in 
actual practice, justify the conclusion that the friction coef- 
ficient increases with increased temperatures. 

Specific Resistance. The ohmic resistance of a cube having 
one-inch sides. 

Current Density or Carrying Capacity. This is based on the 
maximum continuous current capacity per square inch, with- 
out glowing, honeycombing, undue heating or sparking. 

Glowing. A brush is said to "glow" when it becomes red or 
incandescent in spots in proximity to the commutator. This 
is due to the short-circuiting current under the brush, or to the 
short-circuiting current in combination with the working or 
load current. It may also be due to a lack of homogeneity 
of the brush (hard or soft spots of the same or different mate- 
rials having characteristics foreign to the material in the body 
of the brush); to incorrect brush position; to improper brush 
selection; to selective commutation; to the brush covering too 
many bars; to high mica; to the machine having inherently 
poor commutating characteristics due to improperly shaped 
or spaced main poles or commutating poles; to bad commutat- 
ing pole adjustments, or to a poor distribution of the armature 
windings. In the event the glowing can not be stopped by 
correcting such of the brush or mechanical troubles as may 
exist; or by shifting the brushes, or readjusting the commutat- 
ing poles, or both ; a brush having higher current density values 
and less susceptible to the effects of high voltages between the 
commutator bars should be installed. If a suitable brush 
can not be had, the design of the machine itself may be subject 
to modification. 

Honeycombing. When brushes gradually burn away, form- 
ing small craters in their faces, they are said to " honey comb." 
This is due to continuous sparking of a more or less hidden 
nature and may be of either a slow or rapid nature. If the 



336 ARMATURE WINDING AND MOTOR REPAIR 

growth is slow it is sometimes possible to correct it by increas- 
ing the brush pressure, thereby decreasing the contact drop 
and the contact arcing, and at the same time increasing the 
abrasive effect of the brush and grinding away the minute 
craters as they form. Honeycombing is due to the same 
causes, on a reduced scale, as glowing, and at times both may 
take place simultaneously on the same machine. Honey- 
combing has been traced directly to high mica, in combination 
with loose commutators; also to improperly spaced main 
poles ; and unequal air gaps; and in non-commutating-pole 
machines, to an abnormal shifting or distorting of the mag- 
netic flux. The trouble is most frequently corrected by sub- 
stituting a more highly refractory brush possessing abrasive 
characteristics. 

Hardness. Brushes have widely varying physical densities 
some being very hard, others very soft. Very hard brushes 
usually carry a large amount of abrasive and have a low current 
density or carrying capacity. The softer brushes are more 
highly graphite, carry abrasive to a limited extent only and 
have a high current density. The hardness or scleroscope 
reading is, therefore, indicative to a limited degree of the char- 
acter of the brush. 

Brush Inertia. This has to do with the weight of the brush. 
The lighter the brush, the more readily will it follow the irregu- 
larities of the rotating element, and the more promptly will 
it respond to a given spring pressure. Again, the wearing of 
the commutator may not prove so great. If a light and a 
heavy brush prove equally satisfactory electrically on a ma- 
chine, the lighter brush is preferable. If the materials from 
which brushes are made are heavy, it is advisable to have an 
increased number of small brushes, rather than a limited num- 
ber of large brushes; also the spring pressure should be in 
excess of the pressure on lighter brushes having the same 
dimensions, and used for the same service. 

Refractory. Any material which resists the ordinary meth- 
ods of reduction is said to be refractory. A brush, therefore, 
which resists wholly, or to a marked degree, high tempera- 
tures, such as the heat generated by an electric arc, is said to 
be highly refractory. 



ADJUSTING BRUSHES AND CORRECTING TROUBLES 337 

Peripheral Speed. This is the speed in feet per minute of 
the commutator or slip ring. Peripheral speeds which vary 
greatly require brushes having different characteristics. Gen- 
erally speaking, carbon brushes are not as suitable for high 
peripheral speeds as graphite brushes. 

Procedure for Locating Causes of Brush Trouble. Spark- 
ing at the brushes of a motor or generator can usually be traced 
to one or more of the following causes. 

1. Too low brush pressure. 

2. Incorrect spacing of brushes. 

3. Unequal air-gaps or defective fields. 

4. Brushes not operating on electrical neutral. 

5. Incorrect thickness of brushes. 

6. Using brushes of wrong characteristics. 

The above causes are given in the order in which they should 
be checked up in the machine. All but two of these defects 
may cause abnormal short-circuit currents under the face of 
each brush or between two or more studs of the same polarity. 
This point should be kept in mind when searching out the 
trouble. The procedure for investigating each of the causes 
has been outlined by E. H. Martindale of the National Carbon 
Company as follows: 

Before starting an investigation, eliminate the question of 
high mica. If the commutator runs nearly true, stone it 
enough to grind the mica even with the bars. If, however, the 
commutator is not true, or there are flat bars, turn or grind the 
commutator before looking further for the seat of the trouble. 

Too Low Brush Pressure. Low pressure will cause poor con- 
tact between the brushes and the commutator, and force the 
current to pass from one to the other through a small arc. 
This will burn both the brushes and commutator and produce 
high mica. The heat of the arcs between brushes and commu- 
tator will also increase the temperature rise. Further, poor 
pressure produces a high contact drop, which will also heat 
the commutator and brushes. This high contact drop and 
arcing caused by too low pressure frequently heats the brushes 
to a red heat, or, as we say, "to the glowing point, " which is 
always accompanied by pitting or disintegration of the faces 

of the brushes. This again reduces the available contact area 
22 



338 ARMATURE WINDING AND MOTOR REPAIR 

and increases the current density through the balance of the 
brush. The bad effects of too low brush pressure are greatly 
aggravated when the commutator is slightly elliptical or runs 
unbalanced. 

Whether the pressure is high or low, care should be exer- 
cised to get all the brushes on one machine at a uniform pres- 
sure. If the pressure is unequal, the brushes with the highest 
pressure will carry the highest current. In many cases some 
brushes on a machine may carry three or four times as much 
current as other brushes, due to this variation in pressure. 
This excessive current may be enough to burn off the pigtails, 
overheat the brushes, and cause glowing and pitting of the 
brush faces. The best value of brush pressure should be deter- 
mined for any machine by trial, as it is influenced greatly by 
local conditions. A pressure of from 1.75 to 3 pounds per 
square inch of contact surface may be used for motors and gen- 
erators, and from four to seven pounds per square inch for such 
machines as crane motors and railway motors where vibration 
is comparatively severe. However, these values are given only 
as an indication of best average practice, and are not always 
the best to use. See table on page 342, paragraph 6 of head- 
ing No. I. 

Incorrect Spacing of Brushes. By spacing of brushes, is 
meant the distance between brushes on adjacent studs meas- 
ured around the commutator. Thus, in a four-pole machine 
the brushes on each stud should be at a distance exactly one- 
fourth the circumference from the brushes on the adjacent 
studs. This may be checked up by counting the total number 
of commutator bars, then dividing by the number of poles, and 
setting the brushes that number of bars apart. A better way, 
however, is to obtain a piece of wrapping paper of the same 
length as the circumference of the commutator and of the 
same width as the commutator. Measure off the correct 
brush spacings and draw lines across the width of the paper. 
Replace this on the commutator, and with one set of brushes 
set so as to toe one mark, set the brushes on all the other studs 
exactly on their respective marks. If, then, the sparking 
can not be entirely eliminated by shifting the brush yoke either 
forward or backward, examine the field coils and air gaps. 



ADJUSTING BRUSHES AND CORRECTING TROUBLES 339 

Defective Fields. The field coils on the machine are usually 
connected in series so that a part of one coil may become short- 
circuited without becoming apparent except by test. Obtain 
a voltmeter with a suitable scale and test the voltage drop 
across each coil. The drop should be the same for each field. 
If one coil has higher drop than the others, the coil has been 
incorrectly wound. If one coil is lower than the other, the 
coil is either incorrectly wound or a portion of it has become 
short-circuited. Either case will require a new coil. 

Another more common defect in compound machines is a 
reversal of one of the series field poles. For this test use a 
compass and check the shunt as well as the series coil. First 
pass current through the shunt coils. Next, bring the compass 
near one coil; mark it either South or North, depending on 
which end of the needle is attracted to the field pole. If the 
bearings of the compass are not perfectly free, use care not 
to bring the needle suddenly too close to the pole, as it is 
easy to reverse the magnetism in the needles of moderate- 
priced compasses. After marking one pole, proceed around the 
machine and test in the same way each pole. The needle 
should reverse direction at each pole. If two adjacent poles 
are found to attract the same end of the needle, one coil is 
reversed. 

Field coils can also be tested for polarity by two ordinary 
iron nails as described on page 136. 

After all the shunt fields have* been tested, current from 
some source should be passed through the series fields only, 
care being taken to see that this current flows in the same 
direction as when the machine is in operation. Nearly all 
compound machines are cumulative compound, in which case 
each series field should attract the same end of the needle as the 
shunt field attracts. The differential compound winding is 
used only when it is desired to have a rise of speed with an 
increase of load, and is not in extensive use. In such machines, 
however, the series fields produce poles of opposite polarity 
to those produced by the shunt fields. 

Unequal Air Gaps. Unequal air gaps may be the cause 
of serious trouble, particularly in "lap" or "parallel wound" 
machines. Poor centering of the armature when the machine 



340 ARMATURE WINDING AND MOTOR REPAIR 

was assembled, may be the cause or it may result later from 
worn bearings. The wear may be either at the bottom of 
the bearings, due to the weight of the armature, or on a belt- 
connected machine, at the side of the machine, due to the 
pull of the belt. This may be checked mechanically by 
measurement with thin sheets of metal or fiber. For lap 
wound machines a more accurate method, however, is to 
disconnect the bus bars or leads which connect the studs of 
the same polarity. Then, without any load on the machine, 
each stud will be independent, and with a low-reading volt- 
meter any difference between the voltage generated under 
the various poles can be detected. This is valuable on a lap 
wound machine, but on a wave wound machine the variations 
are equalized and the mechanical method is more reliable. 

After having correctly spaced the brushes, checked the 
drop across the field coils, the polarity of the field poles and 
the uniformity of the air gaps, it should be possible to shift 
the brushes to a point where no sparking will appear with 
a steady load on the machine. If the sparking reappears 
with a variation in load, either the brushes are of incorrect 
thickness or the neutral field that is, the distance between 
adjacent pole tips is too small. 

Incorrect Thickness of Brushes. The best way to investigate 
this sparking is with a voltmeter reading about 5 volts. 
Take two ordinary lead pencils and trim down one side of 
each pencil as much as passible without exposing the lead. 
Near the top of each pencil cut a groove deep enough to 
expose the lead and attach a piece of lamp cord or other 
flexible wire to each pencil. Remove one brush and hold 
the pencils in the brushholder with the points on the com- 
mutator and the flat side of one pencil against the front edge 
of the brushholder and the flat side of the other pencil 
against the back edge of the holder. This will give the volt- 
age generated across the brush, or, in other words, the 
"commutation voltage/' Now shift the brushes to a point 
where they do not spark and read the " commutation voltage." 
Increase the load until the brushes spark and again read the 
" commutation voltage." If it has decreased, the brushes are 
too narrow that is, the period of commutation is too brief. 



ADJUSTING BRUSHES AND CORRECTING TROUBLES 341 

The remedy is to use thicker brushes or to shift the brushes with 
each change of load. This condition, however, seldom obtains. 

An increase in the commutation voltage indicates that the 
brushes are too thick that is, they span too many bars or the 
neutral field is too small. This thickness of the brushes may 
easily be changed by cutting down the face of the brush with a 
hack saw or file, leaving the body of the brush the proper size 
to fit the brushholders. This will help the trouble, and if the 
sparking does not entirely cease, it may be necessary to widen 
the neutral field by increasing the distance between adjacent 
pole pieces. This can be done by filing the edges of the pole 
faces. 

Brushes of Wrong Characteristics. If none of above remedies 
cure the sparking, the machine is being operated with brushes 
not adapted to the service. The next step should be to give 
a manufacturer full details regarding the machine and ask 
for recommendation of a brush best adapted to the machine 
and service, since carbon brush manufacturers maintain 
an engineering department capable to give valuable advice 
along this line. 

As a guide in the location of other brush troubles, the ac- 
companying table will be found useful. 

Possible Causes of Brush Troubles on Motors and 

Generators and Their Remedies Compiled by 

E. H. Martindale of the National 

Carbon Company 

I. CAUSES AND REMEDIES FOR SPARKING AT BRUSHES 

1. Brushes off electrical neutral. Shift to neutral by trial, or. set on 
neutral by means of voltmeter. 

2. Brushes spanning too many bars. Trim down faces of brushes for 
short distance back from end or, if holders are clamp type, order thinner 
brushes. 

3. Brush studs not parallel with the commutator bars. Bend the brush 
studs or grind or shim under the bolts which fasten the studs to the yoke. 

4. Incorrect brush spacing. Check the spacing by counting the num- 
ber of bars between studs or by placing a strip of paper around the com- 
mutator with divisions marked off equal to the number of studs, and 
correct the spacing by rotating the brush studs or the brushholders on 
the studs. 



342 ARMATURE WINDING AND MOTOR REPAIR 

5. Brushes tight in brush holders. Clean the holders with gasoline and 
if brushes are still tight, sandpaper them down or file out the holders 
carefully. 

6. Brush pressure too low. Pressure should be 1% to 2% Ib. per square 
inch cross-section for stationary motors and generators, 23^ to 4 Ib. for 
elevator and mill motors, 3 to 5 Ib. for crane motors, 4 to 7 Ib. forrail way 
motors. 

7. Too low contact drop of brush. Consult a brush manufacturer. 

8. Insufficient abrasive action of brushes. Use a commutator stone or 
more abrasive brushes. 

9. High mica. Use abrasive brushes, a commutator stone or undercut 
the mica. 

10. Chattering. See heading No. VII for remedies. 

11. Poor adjustment of interpoles. Consult the manufacturer of the 
machine. 

12. Overloads. Undercut the mica, use low-friction brushes and check 
up all causes for short-circuit currents (see heading No. IV) to reduce 
temperatures as much as possible. 

13. Open circuit in armature coil. Rewind that part of the armature. 

14. Loose end connection. Scrape and resolder all defective connections . 

15. Worn bearings. Shim or renew the bearings. 

16. Unequal air gaps. Shim the short poles or grind off the faces of 
the long poles, or if from worn bearings, see paragraph above. 

17. Short-circuit currents be'ween brush studs caused by unbalanced 
armature winding. Consult the manufacturer of the irachine. 

18. Eccentric commutator on high-speed machine. Turn or grind. 

19. Poor belt lacing. Re-lace or still better use a continuous belt. 

20. Pound of reciprocating engine driving the machine. 

21. Unstable foundation. 

22. Cross currents between generators operated in parallel driven by 
reciprocating engines due to variation in angular speed of engines. Use 
heavier flywheel. 

II. CAUSES AND REMEDIES OF FLAT SPOTS ON 
COMMUTATOR 

1. Any form of sparking. See heading No. 1. 

2. High bar. Tighten the commutator bolts and turn or grind the 
commutator. 

3. Low bar. Use commutator stone or turn or grind the commutator. 

4. Eccentric commutator on high-speed machine causing the brush to 
jump from the commutator at the high spots. Turn or grind the com- 
mutator. 

5. Surges of load current due to short-circuit on the line or an instanta- 
neous high peak load. 

6. Mechanically unbalanced armature. Place on balancing ways and 
add weight at lightest point. 



ADJUSTING BRUSHES AND CORRECTING TROUBLES 343 

7. Difference in hardness of commutator bars. Undercut the mica and 
use non-abrasive brushes. 

8. Difference in hardness of mica. Undercut the mica and use non- 
abrasive brushes. 

III. CAUSES AND REMEDIES FOR BLACKENING OF 
COMMUTATOR 

1. Sparking. See heading No. I for causes and remedies. 

2. Too much lubricant. Clean commutator with gasoline. 

IV. CAUSES OF HEATING IN A MOTOR OR GENERATOR 

WITH REMEDIES 

1. Severe sparking. See heading No. I. 

2. Short-circuit currents. 
(a) Brushes off neutral. 
(6) Faulty brush spacing. 

(c) Too thick brushes. 

(d) Unequal air gaps. 

(e) Crooked brush studs. 

(/) Too low contract drop of brushes. 
(g) Unbalanced armature. 
For remedies of items a to g see same causes under heading No. 1. 

3. Too high or too low brush pressure. See paragraph 6 under heading 
No. I. 

4. High-friction brushes. Undercut mica and use low friction brush. 

5. Commutator too small. Consult manufacturer of machine. 

6. Too high a ratio of brush area to commutator surface. Use fewer 
brushes of higher carrying capacity and lower friction. 

7 Overloads. See paragraph 12 under heading No. I. 
8. Chattering of brushes. See heading No. VII. 

V. CAUSES AND REMEDIES FOR HONEY-COMBING OF 

BRUSH FACES 

1. Short-circuit currents. See paragraph 2 under heading No. IV. 

2. Too low brush pressure. See paragraph 6 under heading No. I. 

3. Brushes of insufficient carrying capacity. Consult a brush manu- 
facturer. 

VI. CAUSES AND REMEDIES FOR BRUSHES PICKING 
UP COPPER 

1. Heavy short-circuit currents. See paragraph 2 under heading No. IV. 

2. Sand under the brush faces. Wipe brush face carefully after sand- 
papermg either brushes or commutator. 

3 Commutator not thoroughly cleaned after turning. Finish the surface 
with a commutator stone after turning. 



344 ARMATURE WINDING AND MOTOR REPAIR 

4. Collection of copper dust by lubricant in abrasive brushes. Undercut 
the mica and use non-abrasive brushes. 

5. Electrolytic action. Change the grade of brush ; better consult a 
brush manufacturer. 

VII. CAUSES AND REMEDIES FOR BRUSHES CHATTERING 

1. High-friction brushes. Change the grade or pressure. 

2. Rough commutator. Use a commutator stone. 

3. Dirty commutator. Clean with gasoline. 

4. High mica. Use a commutator stone or undercut the mica. 

5. Wide slots with thin brushes. Fill the slots with commutator 
cement. 

6. High bars. Tighten the commutator bolts and turn or grind the 
commutator. 

7. Flat spots. Use commutator stone unless the flat spots are too 
large for stoning, in which case turn or grind the commutator. 

8. Brush operating at the wrong bevel, frequently found where brushes are 
operating in a stubbing position with angles of less than 20 degrees. Change 
the grade of brush or angle of operation. Better consult a brush manu- 
facturer or the manufacturer of the machine. 

VIIL CAUSES AND REMEDIES FOR LOOSENING OF 
BRUSH SHUNTS 

1. Poor workmanship in attaching shunts. 

2. Insufficient carrying capacity. Consult a brush manufacturer. 

3. Heating. See heading No. IV. 

4. Vibration. See heading No. VII. 

5. Combination of heating and vibration. 

6. Loose terminal screws causing unequal distribution of load. 

7. Unequal brush pressure causing unequal distribution of load. See 
that all brush pressures are uniform and conform to recommendations 
given in paragraph 6 under heading No. I. 

8. Heavy short-circuit currents between different brushes. See b, d, e, g, 
of paragraph 2, under heading No. IV. 



CHAPTER XIV 

INSPECTION AND REPAIR OF MOTOR STARTERS, 
MOTORS AND GENERATORS 

On account of the variety of troubles that may be corrected 
in a simple way at one time but require more extensive repairs 
at others, it is a very difficult matter to lay down hard and fast 
rules that will always work out on every repair job when fol- 
lowed to the letter. The ability to know when temporary 
repairs will suffice and when a permanent job must be done 
at once, comes largely from experience. The term repairman 
has been used throughout this book as a title that a good engi- 
neer can bear with pride when he measures up to all the quali- 
fications of the man who in the majority of cases knows what 
to do and just how to do it and seldom guesses without a good 
percentage of the probabilities of being right in his favor. 
The main difference between the designer and the repairman 
is that the former must know what to do while the latter must 
know -how to do it. A capable repairman combines both 
qualifications through years of experience. One of the best 
ways of saving time for the young engineer entering the repair 
field, is to serve as an apprentice with a large electrical manu- 
facturing company. In this way through association with 
those who design and those who build and test, a great deal 
of information is absorbed that only years of experience from 
one job at a time will make possible. This fund of information 
is essential in an intelligent discussion of any repair job or the 
presentation of rules or suggestions for looking for the trouble 
and then actually making the repair. 

In the accompanying pages there are presented suggestions 
for the repairman in the inspection and repair of motors and 
generators prepared by H. S. Rich, at the suggestion of the 
author and published in the Electrical Record, October, 1918 
to April, 1919. These suggestions are accompanied with ex- 
ploded views of the device discussed, so that the repairman can 

345 



346 ARMATURE WINDING AND MOTOR REPAIR 

form a mental picture of the work described which will approxi- 
mate as closely as possible the impression that would be se- 
cured by actually doing the work. 

Cost of Repairs for Polyphase Motors. For the purpose 
of furnishing estimates for repairing polyphase motors that 
will guide the repairman who has had a limited experience of 
repairwork, George A. Schneider has compiled the accompany- 
ing table (Journal of Electricity, May 1, 1917) from costs of 
actual repair jobs which he has handled. The table as pre- 
sented has been revised to take into consideration the cost 
of materials entering into repairs for the year 1917. The 
data refers particularly to 60-cycle, two and three-phase, 
squirrel-cage motors wound for the standard voltages of 110 
to 550. 

For most of the sizes listed the costs were arrived at by 
taking the average cost of repairs for a given frame and then 
applying this cost to the various ratings built in that frame. 
This will be apparent by comparing the costs for the different 
ratings. Take for example, frame G. The cost of rewinding 
the stator is $34.75. This figure has been applied to the 
following ratings all of which are built in that frame : 1 horse- 
power, 900 revolutions per minute; 1.5 horsepower, 1200 revo- 
lutions per minute, and 3 horsepower, 1800 revolutions per 
minute. The frame sizes specified do not apply to any par- 
ticular line of motors, but were arbitrarily chosen for the 
purpose of this table. However, the relative output of a 
given frame at the different speeds will be found to agree quite 
closely with several lines of induction motors on the market. 

These estimates may also be used equally well for motors of 
other frequencies by taking the figures applying to a 60-cycle 
rating built in the same frame. This comparison can be 
easily made by referring to the manufacturer's rating and 
dimension sheets for thaf particular line of motors. The 
tables may be further applied to slip-ring or phase-wound 
motors, since the cost of rewinding the rotor of such a machine 
will not differ materially from the cost of rewinding its stator. 
On this basis the cost of completely rewinding a 10 horsepower, 
1800 revolutions per minute slip-ring motor built in frame / 
will be $119, or $59.50 for the rotor or stator separately. 



INSPECTION OF MOTORS AND GENERATORS 347 



COST OF REPAIRS FOR GO-CYCLE POLYPHASE MOTORS BASED ON A LARGE 
NUMBER OF REPAIR JOBS 



Horse- 
power 


Syn- 
chronous 
speed in 
rpm. 


Frame 
size 


Rewinding 
stator 


Re- 
soldering 
rotor 


Bearing 
linings, 
per set of 
two 


Paint- 
ing 


Re- 
crating 


0.50 


1200 


C 


$26.25 


$2.50 


$1.35 


$1.00 


$1.00 


0.50 


1800 


A 


24.25 


2.25 


.35 


1.00 


.00 


0.75 


1200 


E 


28.00 


3.00 


.85 


.00 


.00 


0.75 


1800 , 


B 


24.25 


2.25 


.35 


.00 


.00 


1.00 


900 ' 


G 


34.75 


4.00 


.10 


.50 


.50 


1.00 


1200 


F 


28.50 


3.00 


.85 


.25 


.00 


1.00 


1800 


C 


26.25 


2.50 


.35 


.00 


.00 


1.50 


1200 


G 


34.75 


4.00 


3.10 


.50 


.50 


1.50 


1800 


E 


28.00 


3.00 


1.85 


.00 


.00 


2.00 


1200 


G 


34.75 


4.00 


3.10 


.50 


.50 


2.00 


1800 


F 


28.50 


3.00 


1.85 


.25 


.00 


3.00 


900 


I 


53.50 


6.50 


5.25 


.50 


.50 


3.00 


1200 


H 


48.50 


6.75 


3.55 


.50 


.50 


3.00 


1800 


G 


34.75 


4.00 


3.10 


.50 


.50 


5.00 


900 


K 


73.75 


8.75 


8.05 


.75 


2.00 


5.00 


1200 


I 


53.50 


6.50 


5.25 


.50 


1.50 


5.00 


1800 


H 


48.50 


4.75 


3.55 


.50 


1.50 


7.50 


900 


L 


70.75 


12.00 


7.85 


.00 


2.50 


7.50 


1200 


J 


59.50 


7.00 


6.60 


.75 


2.00 


7.50 


1800 


I 


53 . 50 


6.50 


5.25 


.50 


1.50 


10.00 


900 


M 


75.00 


13.25 


7.85 


2.00 


2.50 


10.00 


1200 


L 


70.75 


12.00 


7.85 


2.00 


2.50 


10.00 


480 


J 


59 . 50 


7.00 


6.60 


1.75 


2.00 


15.00 


720 


P 


93.75 


15.50 


10.25 


3.00 


4.00 


15.00 


900 


N 


71.25 


14.25 


10.25 


3.00 


4.00 


15.00 


1200 


M 


75.00 


13.25 


7.85 


2.00 


2.50 


15.00 


1800 


K 


73.75 


8.75 


8.05 


1.75 


2.00 


20.00 


600 


S 


156.25 


19.00 


12.10 


3.25 


6.00 


20.00 


900 


P 


93.75 


15.50 


10.25 


3.00 


4.00 


20.00 


1200 


N 


71.25 


14.25 


10.25 


3.00 


4.00 


20.00 


1800 


M 


75.00 


13.25 


7.85 


2.00 


2.50 


25.00 


600 


S 


156.25 


19.00 


12.10 


3.25 


6.00 


25.00 


720 


S 


156.25 


19.00 


12.10 


3.25 


6.00 


25.00 


900 


R 


143.75 


17.75 


12.00 


3.25 


6.00 


25.00 


1200 


P 


93.75 


15.50 


10.25 


3.00 


4.00 


35.00 


600 


T 


18.7 . 50 


20.50 


19.95 


3.50 


6.25 


35.00 


720 


S 


156.25 


19. CO 


12.10 


3.25 


6.00 


35.00 


900 


S 


156.25 


19.00 


12.10 


3.25 


6.00 


35.00 


1200 


R 


143.75 


17.75 


12.00 


3.25 


6.00 


50.00 
50.00 


600 
720 


V 
V 


218.75 
218.75 


21.75 
21.75 


30.85 
30.85 


3.50 
3.50 


6.25 
6.25 


50.00 


900 


T 


187.50 


20.50 


19.95 


3.50 


6.25 


50.00 


1200 


S 


156.25 


19.00 


12.10 


3.25 


6.00 



Points to Consider when Estimating Cost of a Motor Repair 
Job. The estimates for rewinding the stator or resoldering 
the rotor given in the accompanying table do not include any 



348 ARMATURE WINDING AND MOTOR REPAIR 

preliminary work required to put the stator structure in fit 
condition to receive the new winding or work required on the 
rotor before the actual resoldering can be started. In other 
words, the figures cover only the actual rewinding or resolder- 
ing, as the case may be. However, this preliminary work is 
frequently necessary and must always be considered in making 
up estimates. It is due to a number of causes. 

For example, the motor bearing linings may have worn 
down sufficiently to allow the rotor to rub against the stator. 
If the motor has operated very long in this condition the lami- 
nations of either or both stator and rotor will probably be 
damaged, which may require considerable work to put them 
into their original condition. Again, a defective or broken 
bearing may injure the shaft. Sometimes this damage will be 
serious enough to require a new shaft. New bearing linings will 
probably be required in either case. Burned-out windings 
may also be accompanied by fusing of parts of the stator lamina- 
tions. These fused portions must necessarily be removed 
before actual replacement of the coils can be commenced. 

In a rotor which has been badly overheated, allowing the 
melted solder to be thrown out, arcing is frequently set up 
between the rotor bars and the end rings causing serious 
burning. When this occurs, new end rings are often needed 
either for one or both ends of the rotor or perhaps part of the 
bars will have to be replaced. With bolted end ring construc- 
tion there is also liability of trouble. The expansion of the 
end rings caused by the excessive heat, tends to snap the bolts 
between the rotor bars and the rings, producing the most 
favorable conditions for arcing. Burnouts of this kind, 
for either soldered or bolted construction, are quite common 
in connection with motors which have been started from 
time to time under loads which have required heavy starting 
torque with long periods of acceleration. Two- or three- 
phase motors allowed to operate single-phase for a considerable 
length of time may also develop troubles of this kind. Very 
often the rotor will be badly damaged while the stator has 
only been only slightly overheated. Conversely, in some 
cases the stator will be burned out while the rotor is uninjured. 

From these points it will be clear that estimates for repairing 



INSPECTION OF MOTORS AND GENERATORS 349 

motors should not be made until after the motor has been 
given a careful inspection for otherwise there is liable to be a 
wide discrepancy between the estimated and the actual cost 
of making the repair. In furnishing an estimate under 
conditions where a detailed inspection is not possible, details 
of what the estimate covers should be given with a notation 
of additional repairs that may be necessary after an inspection. 
The data of the table on page 347, is based on a large number 
of repair jobs -and is conservative for labor and material 
conditions in 1917. 



I. INSPECTION AND OVERHAULING OF DIRECT -CURRENT 
MOTOR STARTERS 

The following points to be considered by the repairman 
when inspecting and repairing a motor starter are given by 
T. H. Reardon (Electrical Review, May 25, 1918). 






I 



FIG. 240. A starting rheostat with no voltage release for use with shunt and 
compound direct-current motors. 

In direct-current motor service the ordinary rheostat 
in which an arm moves over contacts arranged on the arc of a 
circle is the most convenient starting device. All such rheo- 
stats are provided with an electromagnet which holds the 
arm in place after it has been brought up to the last notch 
or full running position. This magnet loses its power if the 



350 ARMATURE WINDING AND MOTOR REPAIR 

current goes off the line and the arm flies back to the starting 
position. The arm is returned to the starting position by a 
spring, which is always acting in opposition to the pull of 
the magnet that tends to keep the arm in last-notch position 
as long as the magnet is energized. If the pull of the spring 
is too great, which somtimes occurs when an inexperienced 
man makes certain changes in the way of correcting things, 
the pull of the magnet will not be sufficient to hold the arm 
in place when it is brought up and the arm will fly back. 
Conversely, if the i^rength of the spring becomes reduced, 
which often happens when motors are placed in damp places, 
or worse still, where dampness and certain bleaching agents 
(such as chlorine and oxides of chlorine) act jointly in bringing 
about metallic deterioration in apparatus, steel springs will 
be found corroded to such an extent that they do not possess 
their original strength and elasticity. As* a preventive 
measure, springs as well as all other metal parts exposed to 
corrosive action should receive a drop or two of a mineral 
oil or be slightly smeared with vaseline occasionally to prevent 
such deterioration. The pull of the spring and the pull of 
the magnet are balanced against each other, but there are 
other conditions that must be taken into account in making 
adjustments. 

After the current ceases to flow in the low-voltage release- 
magnet coils, which will not be until the motor comes to 
rest (the motor acting as a generator will maintain this current 
until the motor stops), the magnet will still exert a considerable 
pull on its armature due to the relatively large amount of 
residual magnetism that remains in the core of an electro- 
magnet after the current has ceased to circulate and before the 
armature is pulled away from the magnet poles. An arm 
that will not fly back to starting point when the motor stops, 
if moved back by hand and then brought up again, will not 
stick. In certain cases, however, it may stick and when it does 
do so it is not due to any pull exerted by the magnet but it will 
probably be found that the sliding contact on the bottom of the 
arm bears so hard against the contacts that the strength 
of the spring is not sufficient to overcome this braking effect 
due to the friction of the shoe moving over the contacts. 



INSPECTION OF MOTORS AND GENERATORS 351 

Sometimes this friction effect may exist only on one contact, 
one contact standing higher than the rest, and if this particular 
contact happens to be next to the magnet, the movement 
of the arm will be retarded before the arm has a chance to 
acquire any momentum that would carry it back provided 
it once got started. 

If the magnet pull due to residual magnetism after current 
circulation ceases should be responsible for the arm sticking 
after the motor stops (this will very rarely be the case, how- 
ever), the keeper can be lightly tinned over with a soldering 
iron, thus placing a certain amount of reluctance in the mag- 
netic circuit of the magnet and its keeper. A drop of oil 
occasionally on the stud on which the arm is pivoted will 
help in securing free movement. 



II. INSPECTION AND OVERHAULING OF AUTO-STARTERS FOR 
A.-C. MOTORS 

When examining and testing an auto-starter to locate 
troubles, T. H. Readron (Electrical Review, May 25, 1918) 
mentions the following points as possible causes of the troubles : 

Auto-starters are generally used for starting alternating- 
current motors of the induction type when the motors are 
above five horsepower. Small motors are usually thrown di- 
rectly on the line without any starting device and, although 
the starting current is four or five times greater than the 
normal current, the fluctuation, as a rule, is not serious enough 
to necessitate the use of an auto-starter. Usual practice is as 
follows: Polyphase induction motors up to 5 hp. are thrown 
directly on the line. Motors of 7.5 to 30 hp. are started 
by means of a star-delta switch which is not an induction 
starter but simply a switch operating in oil and changing the 
connections of the motor from star on starting to delta on 
running. The motor in that case is provided with six termi- 
nals to accommodate the change. Auto-starters or auto- 
transformers are mostly used for squirrel-cage induction 
motors of considerable size or over 30 hp. 

The auto-starter being adapted for alternating-current 
work, differs from the rheostat in that it possesses not only 



352 



ARMATURE WINDING AND MOTOR REPAIR 



some resistance but considerable reactance in damping 
current flow, while the rheostat possesses resistance only. 
Moreover, the rheostat is a step-by-step starting device; 
its arm is to be moved slowly over the contacts, while the 
auto-starter has but one step from starting to running and 
the handle should always be thrown promptly from one posi- 
tion to the other position. 

The accompanying diagram (Fig. 
242) shows the plan of the auto-starter 
as usually constructed. Six wires are 
brought to the rocker cylinder of the 
switch, which is moved by the switch 
handle. One wire is a feed or line 
wire, the wire next goes to the motor, 
the next wire is line and the next one 
motor again, etc. When the switch 
is thrown to the starting position, the 
six contacts on the switch cylinder 
meet six contacts on the starting 
block and the current from a line 
contact on the switch cylinder passes 
to the contact on the starting block 
through one coil of the reactance and 
back to the next contact on the switch 
cylinder, and thence to the motor. 

When the handle is thrown to the 
running position, only three contacts 
on the switch cylinder make contact 

these are the motor contacts on the switch cylinder and they 
meet three contacts on the running block, which contacts are 
directly connected to the line wires either through fuses or 
overload relays. 

The line contacts on the switch cylinder, when the cylinder 
is thrown to running position, do not connect with anything 
they stand clear or dead-ended. These contacts on the switch 
cylinder can be identified by using a lamp bank and making 
contact with them, the switch handle being in off position. 
A light will be obtained between line and line. 

The first trouble to look for is burned or imperfect contacts. 




FIG. 241. An auto- 
starter or compensator for 
use with alternating-current 
squirrel cage induction 
motors. 



INSPECTION OF MOTORS AND GENERATORS 353 

After taking the oil pan off the switch contacts can be inspected 
without any trouble. 

If the contacts are burned or rough, they should be taken 
off and filed smooth or replaced with new ones. The same 
applies to the fingers that meet the contacts. The handle 
should be thrown to one position and then the fingers should 
be tried to see that they press firmly and evenly against the 
contacts on the switch cylinder. If they do not, throw the 
switch handle to off position and they can easily be bent in- 
ward sufficiently to make a firm contact. It will be well not 
to bend them too much at once, for if this is done, they will 
not slide over the switch cylinder contacts properly. 




I |l^Finish 

Side View ^Start Y- Connected 

FIG. 242. Connections for a common form of auto-starter for alternating- 
current motors. 

In regard to broken wires, the wires that are attached to the 
switch cylinder, six in number, are bent slightly every time 
that the switch handle is moved. These wires now and then 
break off. The other six wires that enter the auto-starter 
go to fixed immovable contacts and rarely cause trouble. 
To save labor, test everything out as far as possible with the 
lamp bank or some other way equally good, depending upon 
what kind of testing apparatus there is at hand. 

It will be well to bear in mind that a ring obtained with a 
magneto or a light obtained with a lamp bank is not always 
conclusive. Instances are quite common where a wire 01 
cable breaks inside of the insulation and there afterward re- 
mains a sheath of metallic oxide or smudge that will pass cur- 

23 



354 ARMATURE WINDING AND MOTOR REPAIR 

rent enough to give a dull light on a lamp bank or a feeble 
ring on the magneto. When it is decided that a certain wire 
is broken, take out the two screws that hold that particular 
contact to the switch cylinder and pull down the terminal 
with a pair of pliers. 

If the wire is intact, it will resist a strong pull. If the wire 
is broken and is held by the insulation, it will easily pull 
apart. If it pulls down through the switch cylinder suffi- 
ciently, attach a piece of new wire or cable to it and use it 
for a snake to draw the new wire into place, pulling at the top 
where the wires enter the auto-starter. If this does not work, 
draw the broken wire out at the top at all events and get a 
piece of stiff brass or steel wire, about No. 14 gauge. Bend a 
smooth small loop on the end of it and pass it down from 
above. Have a helper with a small hook wire watch for it and 
hook it above the switch cylinder so that it can be passed 
through the proper hole in the switch cylinder. When this is 
done, the terminal can be soldered on at the lower wire and the 
terminal drawn back and wire spliced at the stop of the auto- 
starter. It should always be the aim to make such a repair 
if possible without disassembling the parts of the auto-starter 
as such parts go back with difficulty. 

It may be advisable or even necessary to use a wire slightly 
smaller than the original one in order to get through tight 
places. There will be no decided objection to doing this as the 
cross-section of copper is always sufficiently ample to justify 
a slight reduction when such a reduction in size is really 
necessary. 

The following method of insulating leads on auto-starters 
will prevent siphoning of oil: 

(a) Remove insulation from each lead just above the highest oil level 
for a distance of two inches. 

(6) Sweat the strands of the cable thoroughly together so as to close 
up all spaces between the conductors for a distance of one inch. 

(c) Insulate the lead with treated cloth tape, wrapping the tape tightly 
around the conductor and brushing each layer with insulating varnish 
while wrapping. 

(d) Extend the wrapping to tape with three overlapping layers at 
least one inch on the insulation at each end of the bare section. 



INSPECTION OF MOTORS AND GENERATORS 



355 



III. INSPECTION AND OVERHAULING OF DRUM TYPE 
CONTROLLERS 

The drum type controller is used with variable speed shop 
motors, on trolley cars, elevated, subway and railway trains, 
on cranes, and on some types of elevators. This controller 
may be mounted in any position so as to permit handy con- 
trol. On trolley and railway cars, it is placed vertically with 
the lever on top. For shop use it is sometimes mounted on a 
machine or on the wall upside down, being well out of the way 
and yet having its lever within reach. On some elevators it is 




2 
RESISTANCE. 6 



CONTROLLER. 

FIG. 243. Gridiron resistance used with drum type controller shown at 
the right for variable speed motors. (Figures refer to numbers of para- 
graphs of text.) 

installed horizontally and operated with a steel cable running 
over a grooved sheave wheel. Some drum type controllers 
have only a single arrangement of segments with the reversing 
connections operated by a separate lever, while others have 
duplicate segments, half of which are arranged to operate on 
reverse. 

1*. The hard usage to which a controller is subjected usually 
will loosen some of its screws or connections, so these should 
be looked over often and kept tight. The line connections 

* The following paragraph numbers refer to the parts in Figs. 243 and 
244. 



356 ARMATURE WINDING AND MOTOR REPAIR 

to the fingers and the segments themselves require the most 
attention. A finger generally carries a copper block of various 
bevelled shapes, riveted to a stiff phosphor bronze strip which 
provides the necessary tension and is regulated by an adjusting 
screw. The fingers should be provided with flexible stranded 
copper pig-tails which carry the current. If the spring carries 
the current, it is liable to heat and loose its tension. Set the 
fingers so that they can not be jammed under a segment instead 
of riding upon it. If any are badly worn renew them, like- 
wise overheated springs and defective pig-tails. 

2. The primary segments for induction motor control and 
the field segments for direct-current motors do not carry as 
much current as the armature or slip ring segments and are 
thus subject to less wear. The ones mostly worn are in series 
with the slip rings or the direct-current brushes, thus if con- 
tacts are very poor the motor will not operate or it may run 
slowly if on alternating current. If the copper segments are 
badly worn, replace them with new ones having the proper 
curvature and with counter sunk screws tightly fastened down. 
A slight lubrication of vaseline is good for segments. 

3. The direct-current controller should supply the motor 
shunt field with current on the first point, likewise the alter- 
nating-current controller should close the circuit to the induc- 
tion motor primary winding either before or at the same instant 
that the armature receives current. If these contacts are not 
properly made at the first point, fuses are liable to blow as the 
result of no field. 

4. Insulating partitions are often provided between the 
segments to prevent arcing over. If these are badly burned, 
renew them. 

5. Controller diagram connections which are generally 
pasted inside of the cover are worth saving. If constant arcing 
is liable to deface the diagram, either provide a duplicate for 
reserve or remove the one from the cover and mark on it the 
controller number, etc., for identification. 

6. Test with a magneto on a dead controller, or with a test 
lamp on a live one for any possible stray ground to the casing. 
It should test free, as a ground would likely bother the operator. 

7. The controller resistance is usually of the gridiron type 



INSPECTION OF MOTORS AND GENERATORS 



357 



which allows any speed continuously. This resistance has no 
connection with the primary winding or line segments. See 
that no grids are broken, as they are quite frail. A few extras 
in stock are handy. Test the resistance for a ground to its 
own frame. A perforated sheet iron hood should be used for 
its protection. 

8. The conductors from the controller may be carried in 
conduit to the motor, and the mo'tor, conduit and controller 




Forward Reverse 

12345678 57 6W I 



13 



" H) Connections of drake 
Coils when Used. If Brake 



Series. 



CONTROLLER CONNECTIONS. 



FIG. 244. Wiring connections for a drum type controller. 

should be well connected to a good ground pipe to protect the 
operator against sneak currents. 

9. The different points or stops on the controller are deter- 
mined by a notched star wheel just underneath the top cover. 
If its spring is broken or very weak, provide the good tension 
needed for positive operation, as placing the drum between 
contacts will cause trouble. 

.10. If the motor fails to run when the controller is on 
the second or third point, shut it off and test for current at the 
line contacts with a test lamp. Further testing of the motor 



. -. 



358 ARMATURE WINDING AND MOTOR REPAIR 

primary connections at the controller contacts may reveal an 
open circuit. If the motor still refuses to run, test the resist- 
ance for open circuit and look carefully over the slip-ring 
brushes for poor contacts. Exposed wiring between the motor, 
resistance and controller may get broken and thus stop its 
operation One open slip ring connection will cause slow 
speed. 

11. The sudden and repeated Operations to which a con- 
troller is subjected 'demands that it be tightly bolted down. 
For this purpose cast lugs are provided to take the bolts. 

12. To keep dust from getting in and sparks from getting 
out, the front case should be kept fastened on by the finger 
nuts provided on both sides. 

13. The control of the speed is accomplished by means of 
the stepped segments which cut out the resistance gradually. 

14. The ON and OFF positions should be plainly marked 
for the operator's guidance. 

15. See that a limit stop is provided either in the shape of a 
cast projection or an extra long tooth on the star wheel, so that 
the segments can not be rotated beyond a whole circle. 

16. It is advisable to provide a circuit breaker in the line 
with the controller so that in case it is left on part speed with a 
dead line, no damage can be done to either the motor or its 
driven machine when the current returns. 

17. One phase connection open will prevent the motor 
from starting, and it may smoke while trying to start. 

IV. OVERHAULING A LARGE COMPOUND D.-C. MOTOR 

All motors should be inspected and given a general over- 
hauling at least once a year. Large sizes should not be slighted 
in any detail just because they look rugged and appear to stand 
everlasting work. Close observation of all parts and con- 
nections will often reveal some surprising defects. However 
small, they should not be allowed to go unattended. 

When repairing a motor, it is advisable to remove all fuses 
and put them into the tool kit or your pocket so that by no 
chance current can be switched onto the motor when not 
expected. 



INSPECTION OF MOTORS AND GENERATORS 



359 



In case of a belt drive, shift the motor, slip off the belt and 
tie the latter up to something near by. If direct-connected, 
open the coupling by removing all the bolts. Replace the 
nuts on them and tie the whole bunch together to save losing 
them. 

If the motor is suspended, it would be well to lower it with 
two cham tackles and disassemble it on the floor, as its parts 
are too heavy to carry down a ladder and rather large to step 
over on a scaffold. If on a shelf or platform, slinging it to 
the floor is often not very difficult. 



- 








-@ 



FIG. 245. Essential parts of a compound direct-current motor. (Figures 
refer to number of paragraphs of text.) 

1 *. Remove the key and pulley, first marking or measuring 
the latter's location,- which will help in replacing it. If 
direct-connected, remove the commutator end shield to allow 
the armature shaft to draw open the coupling. Mark location 
of the coupling on the shaft. If the key is very tight, remove 
the set screws and pour kerosene into the holes, which will 
work around the key, then some motor oil may be dropped 
in to lubricate the key-way. By lightly tapping proper shaped 
drifts, the key can often be started in a few minutes without 
a lot of battering and damage. When the key is removed if 
the pulley is tight on the shaft, more kerosene will help. 
Also by using an 18-inch monkey wrench on the rim, it 

* The following paragraph numbers refer to the parts shown in Fig. 245. 



360 ARMATURE WINDING AND MOTOR REPAIR 

can often be started. Do not hammer on the rim, it may 
crack. If made of paper it will flatten. Strike a hard wood 
block butted against the hub or drive thin wedges behind. Tie 
the key and set screws to the pulley when removed for security. 

2. Before removing commutator end shield, mark the 
position of the rocker arm which may get shifted. 

3. With the pulley removed, try to lift the shaft a trifle 
in the bearing lining to learn if it is worn out of true. A year's 
wear may cause the armature to strike the lower pole shoe. 
It is well to observe this before disassembling. 

4. Mark or tag the armature leads connected to the brush 
rigging, then disconnect and remove the brushes. 

5. Drain the oil wells and remove both end housings also 
the brush rigging. Close up the oil wells before the screw 
plugs get mislaid. Be careful of the glass gauges. 

6. Remove the armature to a pile of blocking straight 
ahead. This can be done with one lift by slipping a large 
iron pipe over the pulley shaft, which will carry the armature 
clear through and two feet beyond without dropping it. 
A suspended motor can be disassembled to best advantage 
on the floor. Set the armature on some burlap and not 
where any metal clips will imbed themselves into the insula- 
tion. Clean the armature thoroughly, especially the air- 
ducts. 

7. If the commutator has enough stock left and is rough or 
grooved, turn it down in a lathe, taking only very fine cuts 
with a diamond pointed tool. Then polish with oil and No. 00 
sandpaper, but not emery. If the" commutator is badly 
worn down, a new one should be on hand and replaced, having 
the same number of segments. 

8. Blow the dust out of the field frame and from around the 
field coils, or scrape with a stick but use no metal bar or knife. 
Wipe out clean with waste and some benzine. Examine the 
insulation of the field coils, shunt and series field leads, and if 
any abrasion is found use tape, thick armature varnish or 
shellac for insulation. If the field coils are loose, tighten 
the bolts usually found outside the frame. If a coil is burned 
out, remove and re- wind Weigh the wire that is stripped off 
and replace the same size and weight carefully wound on and 



INSPECTION OF MOTORS AND GENERATORS 361 

well insulated. In replacing a field coil, its shunt or series 
polarity are liable to be uncertain. To determine this, 
properly connect to the field windings three or four cells of 
dry battery and with a magnetic compass explore the polarity 
of each pole, marking it with chalk for reference. For both 
the series and the shunt windings, the two markings should be 
similar on any one pole, then the fields coincide. The newly 
wound coil should have an opposite sign to the coil on either 
side; if not, change its connections and test repeatedly to 
make sure, for the operation of the motor depends on the 
field polarities. Use armature varnish or shellac on all 
coils and leads. 

9. Do not paint the inner surface of pole shoes. 

10. If the commutator leads are loose solder solidly and 
neatly, being careful not to let hot solder drop down 
behind the commutator as it is liable to cause a short circuit. 

11. If the armature has been striking the pole shoes, the 
banding wires will likely be polished in spots and partly worn 
through. Even if they are only loose, repair them now by 
slipping under plenty of shellaced mica. 

12. If any armature coil is burned out, slip in a new one and 
thus put it in good shape with full power. 

13. Caliper the armature shaft at both ends. If badly 
worn or cut a new one should be fitted in so that new bearing 
linings will fit perfectly. 

14. If the bearing linings are worn enough to allow loose 
play of the shaft, put new ones in the end shields. Extra 
linings should be kept in stock the year round as it will save 
hours delay when one wears out suddenly from poor oil 
feed. Remove the set screw and strike against a hard wood 
block on the outer end of the lining but avoid damaging the oil 
rings. Usually each end has a different sized lining so that the 
proper pair are needed. If the oil rings get damaged true 
them at once. Try the linings on the shaft to see if they fit 
the bore. If linings are split they are easily removed. Fit 
the new ones exactly into place under their set screws no 
matter how long it takes, for if a lining ever turns over out 
of place it will run hot and be ruined. If the old linings are 
re-babbitted on the premises, have oil grooves and channels 



362 ARMATURE WINDING AND MOTOR REPAIR 

cut in for lubrication and scrape the insides for a perfectly 
snug fit. 

15. Flush out the oil wells with gasoline or benzine and wipe 
them out dry with cheese cloth, as waste is liable to leave 
threads to entangle the oil rings. 

16. Clean the brush rigging especially around the insulators 
on the holder bars. Renew short brushes and bevel them 
as near as possible. Renew broken or weak tension springs. 
See that the holder bars are bolted tightly or they may turn. 
Clean out the brush holders and allow the brushes free move- 
ment, with the pig-tails fastened tightly for good current 
connection. 

17. Now replace the brush rigging, put in the armature and 
put on both end shields. Connect the leads to the brushes. 
Set the rocker arm in its original neutral position and oil the 
bearings enough to allow the shaft to be rotated by hand. If 
it binds, look for the cause at once, as it should turn easily 
with the oil rings in their grooves. Turn the armature by 
using a monkey wrench on the shaft pushing against the key 
covered with tape to avoid marring. 

18. Set the brush holders one-eighth inch above the com- 
mutator. Bevel all new and old brushes by drawing under 
them strips of No. 1 sandpaper until a good and full contact 
surface is assured. Have the tension on all springs similar, 
but not too strong. Smear the commutator with a little 
vaseline; it will not carbonize like oil. Replace the pulley 
and key and then rotate the armature and see that no pig-tails 
interfere. 

19. Replace the motor and bolt it down firmly. Oil the 
bearings fully and run the armature with current for a few 
minutes with the belt off. Look sharply for trouble especially 
see that the oil rings are lubricated properly. 

20. Slip on the belt, tighten it up, and run motor under some 
load. If direct-connected have the coupling well insulated 
and securely bolted. Look especially for sparking as the 
brushes may have imperfect contact, too light spring tension 
or be off neutral. Watch the motor for a few hours underload. 
Motor brushes should be set back of neutral point, or given a 
"lag" as it is termed. 



INSPECTION OF MOTORS AND GENERATORS 363 

21. If a compound motor persists in sparking badly under 
nearly a full load, it may be that during the repairing process 
the shunt and series fields were connected opposed to each 
other, whereas their polarities should be similar at each pole. 
Opposed fields really cause a still weaker field at increased 
loads and will result in sparking and flashing, but with the 
series .and shunt fields magnetized in harmony there should be 
no sparking at the brushes if they have good full contacts and 
are neutrally placed. 

22. If the direction of the armature is wrong, reverse the 
brush leads. 

V. OVERHAULING A 60-HORSEPOWER INDUCTION MOTOR 

Although an induction motor does not frequently call for a 
complete overhauling, the details given in what follows cover 
such a case in order to bring out the point which should be 
considered when the motor is completely taken apart and then 
reassembled after repairs. 

1*. Remove all fuses and kill the line. 

2. Slip off the belt and try turning the rotor by hand. If 
it will not turn, then one trouble has been located. 

3. Remove the key and pulley by driving wedges behind the 
latter. 

4. Drain the oil wells and remove both end shields which in 
this size of motor are probably in half sections. 

5. Remove the rotor straight out ahead onto a pile of block- 
ing. Slip a pipe over the short shaft so that the rotor may be 
carried through with one lift. If it seems to be polished on 
its periphery, this is evidence that at least one bearing lining 
is worn badly and has allowed the rotor to drop. Clean the 
rotor of all dust and dirt. 

6. Clean the primary winding with waste and benzine and 
then look for any possible abrasion on the winding. If nec- 
essary apply some thick shellac. Coat all the winding with 
armature varnish, which will dry in a short time. 

7. Remove both bearing linings if they are worn enough to 
allow loose play on the shaft. If they are split they may be 

* The following paragraph numbers refer to the parts shown in Fig. 246. 



364 ARMATURE WINDING AND MOTOR REPAIR 



re-babbitted on the premises, but must fit snugly and have 
oil channels properly cut in them. 

8. Clean out both oil wells as dirt is liable to settle and stop 
the rings. 

9. Replace the rotor and oil the bearings. Try to turn the 
former by hand, and see that the oil rings turn freely. 

10. If the motor seems to be in good shape put on the pulley 
and key it securely. 




Relay Plunger - 

'SetScre*- 

White Mark, .... 

Calibrating Point. 

Piston Rod 




Overload 
Calibrating 
"-Scale ^ 




Dash Pot 
OVERLOAD RELAY 



ROTOR 

FIG. 246. Squirrel cage induction motor installation showing arrange- 
ment of starter and type of relay that can be used instead of fuses. (Figures 
refer to numbers of paragraphs of text.) 

11. Open the compensator case and look for trouble. Most 
compensator cases after being unbolted should be let down care- 
fully as they contain oil in which the contacts are submerged. 

12. Pull the operating handle over to the starting side and 
see that the contact fingers are making good connections. 
If any are burned short renew them. If their cables or leads 
are not all tight, one or more may be found which has the con- 
ductor broken inside of the insulation. These must be re- 



INSPECTION -OF MOTORS AND GENERATORS 365 

connected solidly. Now throw the operating handle sharply 
to the running side and examine the other contact fingers and 
their connections. After all repairs are made to the fingers 
and contacts replace the oil case and put in the fuses for a trial 
spin of the motor. It ought to run nicely now. 

13. Try the no- voltage release by opening the switch. If 
it fails to work instantly see if the solenoid core is free to move. 
If dirt is obstructing it or if the rod is bent, remedy it and 
adjust the stop nut for a certain distance. See that the 
solenoid leads are properly connected to one phase of the 
motor line behind the compensator. This circuit energizes 
the no-voltage release coil. 

14. See that the fuses fit snugly in the clips, as a loose fit 
is liable to disconnect one phase. See that the line wires are 
tightly connected to the cutout and switch. Loose connec- 
tions on a 50-hp. line are liable to heat very quickly. The 
fuses for a 50-hp. compensating starter ought to be not over 
150-amp. size with knife-blade clips. This allows for about 
20 per cent, overload at starting. If there is much load on 
the motor when being started, the fuses may be increased to 
175 amp., but if they are too large there is no overload protec- 
tion afforded to the motor. 

15. For a motor of this size, it is worth the expense of 
installing overload relays in place of fuses, as they can be 
time-set for overloads of dangerous duration. The small 
spring contact on top of each relay is connected in series with 
the no-voltage release at the side of the compensator and also 
in series with one phase from the motor. Thus an overload 
on the motor raises the relay plunger which opens one or both 
of the top contacts, which in turn kills the no-voltage release 
solenoid and by its gravity drop the controlling handle is 
tripped to dead center thus cutting off the power. The coil 
of each overload relay is connected in series with each phase 
of the motor so that both are protected. The oil dash pot 
needs to be filled with relay oil and screwed up tightly. 
The plunger may be set at any amperage and locked with a set 
screw. In the piston cup is a plate having a few holes through 
which the oil is forced when the plunger raises. This plate 
may be set for any time, either slow or fast. It is well not 



366 ARMATURE WINDING AND MOTOR REPAIR 

to have the plunger work too quickly as an induction motor 
may be called upon to carry a heavy load for about a minute, 
when it would be bothersome to have the relays kick. But 
more than a minute allows a large current to heat up the 
motor. These relays are trustworthy, and when encased in a 
steel box they work silently and promptly just when con- 
ditions demand their action. When they kick out and the 
power goes off, the relays will reset themselves, but they will 
act again at the very next overload. 

After all the operations outlined in items 1 to 15 have been 
carefully completed, the job may be considered finished 
and the motor ready to run. 

VI. OVERHAULING A 26-HORSEPOWER SLIP-RING MOTOR 

A machine like a bull-dozer in a forging shop, a crane or a 
dredge is likely to have a motor of the slip-ring type with an 
external resistance control. Such a motor draws current from 
the line with no heavy rushes and will start with not more than 
one and one-quarter times its normal full load demand. This 
is a good feature as it is easy on the generator, fuses and line 
voltage. Otherwise there might be the annoyance of stopping 
and starting every few minutes. These motors, however, 
need repairing at times like all other motors and the following 
procedure is given for such repair : 

1*. Assume a case that upon removing the belt and trying 
to turn the rotor by power, it is found that it runs very slowly, 
suddenly blowing a fuse or the overload release opens the 
line and the motor stops. 

2. Remove all fuses, drain the oil wells and remove both end 
shields. 

3. Remove the bearing linings from the end shields and 
try them on the shaft. If they are worn loose renew or re- 
babbitt them to fit snugly, and cut in oil grooves. 

4. Remove the armature and rest the shaft on some block- 
ing. Examine the slip rings and if they are worn down thin 
or rough, either renew them or turn them down very smoothly 
in a lathe. They are liable to roughen from poor brush 

* The following paragraph numbers refer to the parts shown in Fig. 247. 



INSPECTION OF MOTORS AND GENERATORS 367 



contact. If renewed see that they are properly insulated 
from each other with one-quarter-inch fiber board, and tightly 
bolted up. 

5. See that the band wires are tight on the armature. If 
they are worn thin renew and solder them all around. 

6. Coat all the armature winding with a special black 
varnish, avoiding the core disks, for magnetic conduction. 





BRU5H RIGGING. 



25-HP. SLIP RING MOTOR. 



( Leads -to Armature 
'(Winding Secondary. 




WIRING. 

FIG. 247. Parts and external resistance control for a slip ring alternating- 
current motor. (Figures refer to numbers of paragraphs of text.) 

7. Clean all the dust and dirt from out of the primary 
winding, but use no sharp metal as the insulation is easily 
damaged. Wash with benzine, and coat with armature 
varnish. 

8. Clean the brush rigging of all metal dust and gummed oil. 
Renew the short brushes and the broken and weak tension 
springs. All brushes should have good sized pig-tails to carry 
the current. The brush contacts should be perfect on the ring 
surfaces. 



368 ARMATURE WINDING AND MOTOR REPAIR 

9. Replace the armature, brush rigging and both end shields. 
Smear vaseline on the slip rings and adjust the tension springs 
fairly strong. 

10. See that the leads to the brushes are not loose or dis- 
connected as^ the armature will turn very slowly and heat 
badly if only one conductor is open. It makes no difference 
to which ring any lead is connected coming from the external 
resistance. 

11. Examine the rheostat and see that each lead is con- 
tinuous from its own resistance section and that no coil 
is open or loosely connected. The sliding arms must bear 
simultaneously on successive buttons. If any are worn or 
badly burned they should be renewed. 

12. The motor primary leads must be connected each to the 
proper leg of the three- or four- wire supply line. 

13. See that the line is securely bolted to th^ switch, and have 
the latter's clips clean and capable of a tight contact, as cur- 
rent for a 25-hp. motor is liable to burn any loose connections. 

14. Overload relays or fuses may be used with this motor. 
Relays save time as they can be re-set, and limited to any 
reasonable load. 

15. Oil the bearings fully and run the motor. See that the 
oil rings are turning properly. 

16. When operating a slip-ring motor be careful to move 
the rheostat lever slowly to avoid a rush of current. For 
speed regulation the controller has many speeds forward 
and reverse, with twenty or more contact fingers and blocks 
which have to be renewed at times because of arcing. 

17. If a non-reversing motor runs the wrong way, merely 
exchange the leads of one phase of its line. Changing the slip- 
ring connections has no effect. 

18. A slip-ring motor should have its brushes and rings 
protected from dust and dirt, and the wear on the bearings 
watched just as closely as those of an ordinary induction motor. 

VII. OVERHAULING A SINGLE-PHASE COMMUTATOR MOTOR 

On account of the rather special construction of the single- 
phase commutator motor, there are certain points to which 
the repairman should pay especial attention when it becomes 



INSPECTION OF MOTORS AND GENERATORS 



369 



necessary to overhaul such a motor or make repairs to it. 
These points are outlined in detail in the following paragraphs 
which refer to numbers of Fig. 248. 

1.* Laminated field frame of the motor. 

2. Main field winding which produces the main flux. The 
auxiliary field regulates the power factor and is connected 
permanently to one auxiliary brush, having a switch contact 




FIG. 248. Essential parts of a single-phase commutator motor. (Figures 
refer to numbers of paragraphs of text.) 



for the other auxiliary brush. Both of these windings should 
be coated with black armature varnish to improve their 
insulation. 

3. Commutator end shield. 

4. Oil-well cover. If this cover is missing, a new one should 

* The following paragraph numbers refer to the parts shown in Fig. 248. 
24 



370 ARMATURE WINDING AND MOTOR REPAIR 

be fitted to keep dust from settling where it may cause the oil 
ring to stop. 

5. Oil gauge. When overhauling a motor take apart and 
clean out the gauges as any obstruction may produce a false 
oil level. The bearings should also be drained and flushed 
with gasoline. 

6. Oil plugs. See that they are tight or the oil may drip 
from them and waste. 

7. Brush-holder studs. Bolt all of these up tight to avoid 
tipping the brushes from the commutator by their accidental 
turning. 

8. Carbon brushes with copper pig-tails, in the variable 
speed compensated type of single-phase motor, remain on the 
commutator all the time, similar to direct-current motors. 
With the repulsion type they are short-circuited at the start 
but are removed by a centrifugal governor when up to speed. 
Give all brushes good contact with the commutator by drawing 
under them No. 00 sandpaper against the carbon. A little 
vaseline will stop the squeak of brushes. 

9. Short-circuiting connection on the back of the brush 
yoke for the main brush set or energy brushes. 

10. Brush holders. See that they are cleaned of all gum 
and dirt to allow free action of the brushes. Renew weak 
springs. 

11. The brush yoke is sometimes of moulded composition 
and sometimes of cast iron. This should be well cleaned to 
avoid short circuits. 

12. A fan is used to keep up a circulation of cool air against 
the windings. 

13. Laminated armature with slot windings. The squirrel- 
cage winding is usually placed in the bottom of the slots, and 
the compensating winding near the surface, and between the 
two windings a steel bar separator. All coils should be well 
insulated to be moisture proof. See that band wires are 
tightly soldered, with plenty of mica under them. If the 
bearing linings are worn, the surface of the armature will 
show polished places. 

14. Commutator. To it are connected the leads of the 
commutated type of armature winding. In the repulsion 



INSPECTION OF MOTORS AND GENERATORS 371 

type the commutator bars are all connected at full speed and 
the armature runs like an induction rotor. See that all com- 
mutator leads are well soldered in. If commutator is rough, 
turn it down in a lathe, but ever so little at a time with a 
diamond-point tool. Polish with No. 00 sandpaper and oil. 
Never use emery cloth. 

15. Shaft. In some makes if it becomes worn or damaged 
it can be easily withdrawn and a new one replaced. If its 
bearing surfaces are cut they should be turned down smoothly 
before new bearing linings are fitted on. 

16 and 17. Motor terminals to the line. Some designs allow 
for 110-volt operation by connecting the terminals in parallel, 
and 220-volt operation with them in series. 

18. Terminals from the auxiliary or compensating field 
winding. 

19. Pulley end shield. 

20. Bearing linings. If they are worn enough to allow loose 
play of the shaft, new ones should be put in or the old ones 
re-babbitted with oil channels cut in from end to end. 

21. Oil rings. See that they are not bent out of shape and 
when re-assembling be careful to get them into the slots. 
This is very important and a light might better be used 
than to guess at their location. Also turn the armature 
by hand and see that the rings turn also. Oil well before 
running. 

22. Pulley should be tightly keyed on and fastened with a 
set screw. Only a balanced motor pulley should be used. 

23. Set screw for motor pulley. 

NOTE. No rheostat is needed, although speed controllers are 
used. To reverse the armature rotation, interchange the 
leads to the compensating or auxiliary brushes. 

VIII. OVERHAULING A DIRECT-CURRENT ENGINE TYPE 
GENERATOR 

At least once a year the generators of any manufacturing 
plant should be thoroughly overhauled, cleaned, painted, and 
minor repairs made, even though not badly out of order. 
Such an inspection does no harm and often reveals a weak 
part that may later cause a break-down at a busy time. 



372 



ARMATURE WINDING AND MOTOR REPAIR 



A good job can be done in two or three days by two good 
repairmen on machines up to 150 kilowatts. 

1 . * If belt driven mark the position of the sliding base before 
setting back with the belt screw. 

2. On both belt-driven and direct-connected machines, 
mark the position of the rocker arm, assuring the return 
of the brushes to the neutral point. 

3. Avoid if possible, disturbing the brush holders. 



- 




FIG. 249. Parts of an engine type direct-current generator that should be 
regularly inspected. (Figures refer to paragraphs of the text.) 

4. With a traveling crane or a tripod rigging, remove the 
outboard bearing and pedestal, then the whole brush rigging 
"en masse." Be careful to mark the armature leads for 
proper re-connection. 

5. The armature shaft should be blocked up level to its 
bearing before the latter is removed, otherwise the poles will 
have to support the weight and the other bearing sleeve 
may crack. 

* The following paragraph numbers refer to the parts shown in Fig. 
249. 



INSPECTION OF MOTORS AND GENERATORS 373 

6. Remove either the pulley or the coupling key from the 
other end of the shaft, and by properly slinging the armature 
it can be withdrawn far enough out past the poles to allow 
any small work to be done. No hitch or sling should be made 
around the commutator, even if it is covered with burlap. 
Support all weight by the shaft only. 

7. Blocking under the armature core should be ready to 
support all of the weight when withdrawn, and so arranged as 
to prevent its rolling sideways. 

8. Now with either an air hose, hand bellows or brush 
remove all dust, dirt, and lint from around the poles, in the 
air ducts, and from inside the frame, first covering the engine 
crank, if direct connected. The armature winding should 
be blown out neatly, and where any dirt is inclined to adhere 
use a narrow flat stick to avoid damaging the insulation. 

9. After a good dusting all around a washing with gasoline 
will remove any remaining gummy dirt or oil. 

10. Black armature varnish may be applied to all windings 
and leads, but not to the pole shoes, or armature core disks, 
because the distance between such parts is usually small 
and too much varnish may "freeze" the armature tight 
against the lower poles after a shut-down. 

11. The armature winding and leads may be painted up 
to the commutator bars but not beyond. Plenty of it between 
the commutator open leads helps to insulate them. The 
band winding at the outer end of the commutator and its 
bolted end plate may also be painted. 

12. If the commutator is very rough or badly cut it should 
be turned down either in a lathe or by a turning tool attached 
to the generator frame. After turning off ever so little copper 
No. 00 sandpaper should be applied with oil for a polish, 
but use no emery. If the commutator is badly worn down and 
beyond repair, a new one should be on hand, having been 
ordered weeks in advance, giving the manufacturer all the 
specifications possible from the name plate. Furthermore, 
when the commutator arrives examine the bore, diameter 
length and number of segments, otherwise somebody's mis- 
take may not be discovered till the machine is taken apart. 
Do not pound on the commutator bars as they may sink. 



374 ARMATURE WINDING AND MOTOR REPAIR 

13. If the shaft shows any marks of cutting have its surface 
trued before any bearing sleeve is repaired. 

14. If the bearing sleeves are worn decidedly, have them 
re-babbitted to fit snugly but not too tightly, and grooved 
for oiling. 

15. Be careful not to damage the oil rings, especially when 
replacing the bearings. Also see that all sediment is removed 
from the oil wells, and the glass gauges cleaned out. 

16. If the armature band wires or mica insulation are 
loose, slip under some sheets of mica and re-solder the bands 
securely. 

17. See that no field coils are loose on the poles; if so apply 
more wedging of wood strips. 

18. If the brushes are dirty remove them, one set at a time, 
to avoid any mix up and clean the holders with gasoline, allow- 
ing the brushes to move freely. Replace short brushes with 
new ones; renew weak springs; tighten the pig- tails; or if of 
copper gauze they may be set ahead. Give each brush the 
proper and same tension, not too loose and not so tight as to 
cause cutting. Allow about one-eighth inch under the holders, 
and tip brushes at proper angles. Get the proper bevel 
on each brush to fit the commutator arc by drawing No. 
or No. 1 sandpaper under it, cutting the carbon away while 
it rests on the commutator. Vaseline applied lightly to the 
commutator will help the lubrication of the brush contacts 
and prevent cutting and sparking. 

19. Connect the main leads to their proper bolts tightly, 
and if belt driven slip on the pulley with its key and set screw, 
and with plenty of oil in the bearings turn the armature by 
hand a number of times and watch all parts for interference. 
Especially see that the oil rings are turning. 

20. A timely warning do not coat the generator with 
aluminum paint just for looks, as it is a metallic conductor. 
Leaky voltage and shorts may result. White enamel radiates 
heat the best, although it is some trouble to keep clean. 
Dark green is the usual color. 

A. All these operations take some time when properly done, 
but what a pretty running machine one has when they are 
honestly completed. 



INSPECTION OF MOTORS AND GENERATORS 375 

B. If any insulation is found peeled off or cracked open, 
use a little mica, tape, shellac, or armature varnish. 

C. When ready to assemble the generator, put all bolts 
possible into place before tightening any. Close all oil cocks 
and fill up the bearings. Keep the armature leads away from 
the rotating armature. 

D. When all is ready, slip on the belt, slide back the base 
to its former position and start the engine slowly, watching 
carefully the oil rings. 



CHAPTER XV 
DIAGNOSIS OF MOTOR AND GENERATOR TROUBLES 

The electric motor and generator are sturdy pieces of ap- 
paratus when given the proper attention and operated within 
their normal ratings. The troubles which they develop can 
usually be .traced to two main causes. 1. Faulty operating 
conditions. 2. Mechanical and electrical defects. 

Faulty operating conditions may either bring to light exist- 
ing mechanical and electrical defects or create new ones. 
Under normal conditions, therefore, correct operating con- 
ditions will reduce to a minimum, the troubles due to the 
second cause. The factors in faulty operating conditions may 
be classified as follows: 

1. Lack of proper cleaning. 

2. Operation in damp places. 

3. Exposure to acid fumes and gases. 

4. Lack of frequent inspection and replacement of worn 
parts. 

5. Operating temperatures too high. 

Lack of Proper Cleaning. All machinery having moving 
parts requires lubrication and however well the provisions are 
made to confine the oil to the parts needing the lubrication, it 
will find its way in time to adjacent parts of the machine. 
In motors and generators the oil which is allowed to accumu- 
late on the windings has a detrimental effect on the quality 
of the insulating materials used and is a frequent cause of 
short circuits and grounds. The presence of oil also invites 
the accumulation of dust and dirt which quickly fills up small 
spaces that have been provided for ventilation of windings 
and core. This results in an increase in temperature of the 
parts involved. The only remedy for troubles resulting from 
this cause on a repaired machine, or when a new one is sub- 
stituted, is frequent cleaning at regular intervals. Motors 



MOTOR AND GENERATOR TROUBLES 377 

operated in especially dusty places, as in cement and flour 
mills, should be blown out daily with compressed air or a 
hand bellows. Care must be taken not to blow the dust into 
the windings. An air pressure of not more than 50 pounds 
per square inch with a J^-inch nozzle should be used to avoid 
injury to insulation. 

Operation in Damp Places. The materials employed in 
the insulation of windings of motors and generators consist of 
special papers, cotton tapes and cloth, and mica. With 
the exception of mica, all of these materials absorb moisture 
more or less or are said to be hydroscopic. In damp places 
where the operation of the machine is not continuous enough 
for the generated heat to keep moisture out, the insulation 
may absorb enough moisture to reduce the insulating strength 
to the rupture point resulting in all sorts of troubles. Motors 
and generators should not be installed in damp places except 
they be provided with windings having a special moisture- 
proof insulation which manufacturers can furnish when the 
conditions under which the machine is to operate are known. 
In case a machine has been exposed to dampness or been 
soaked with water in a fire without having the insulation 
damaged by the fire, it should be thoroughly dried out before 
being placed in operation. (For details of drying out a ma- 
chine, see pages 181 to 185.) 

Exposure to Acid Fumes and Gases. As in the cases of oil 
and water, acid fumes destroy the insulating strength of most 
insulating materials. They also attack the metal of the 
machine and thus bring on commutator troubles. In chemi- 
cal works and in industrial plants where acid fumes are present, 
small motors should be enclosed in a fume-tight case which 
can be ventilated. Where this is impossible specially in- 
sulated windings must be provided. The coils needed in the 
repair of such a machine should be secured from the manufac- 
turer and in most cases it will be advisable for the manufac- 
turer to do the repair work. Machines operated should 
be cleaned more frequently than under other conditions and 
the exposed windings painted regularly with an acid-resisting 
paint or a good linseed oil paint. The commutator should be 
wiped daily with a little vaseline. The metal parts of the 



378 ARMATURE WINDING AND MOTOR REPAIR 

machine should also receive a coat of acid-resisting paint at 
frequent intervals. 

Lack of Frequent Inspection and Replacement of Worn 
Parts. Short circuits and grounds are frequently traced to 
rubbing of the motor winding on field poles caused by a bent 
shaft or worn bearings. A worn or bent shaft may also cause 
the core of the armature to strike the pole faces and jam the 
punchings of the core enough to cut the coils and cause a 
ground in the winding. A loose fit of the commutator on the 
shaft, when it has been replaced after a repair job, may result 
in sufficient movement to break off the leads to armature coils 
at the necks of the commutator. The re-soldering of such a 
defect does not cure the trouble. An inspection should be 
made to locate the cause and eliminate it. In this case it- 
will probably call for a new shaft. A mixture of dust and 
foreign material in the bearing oil cups or not enough oil is the 
start of many bearing troubles which can be eliminated by 
frequently draining the bearings and cleaning with kerosene. 
When oil is used that has stood for some time in a receptacle 
or been pumped from barrels to a tank, it is advisable to filter 
it before using it in motor or generator bearings. Wear of 
bearings should be inspected frequently when a large tight 
belt is used on the motor. 

Roughness of the commutator should be guarded against. 
When it occurs and stoning does not remedy the trouble, the 
real cause should be searched out. The same applies to high 
mica. The air gap of the machine should also be frequently 
checked by thickness gauges for this is a good check on the 
condition of the bearings. A record made of all of these tests 
and inspections for reference when other troubles develop 
will save much time and trouble. 

A frequent check for proper end play of the armature shaft 
in its bearings will prevent grooving of the commutator that 
otherwise might be suspected to be caused by unsuitable 
brushes. 

Operating Temperatures Too High. All insulating material 
such as paper and cotton materials have a maximum tempera- 
ture limit above which they will not retain their insulating 
strength and will simply char and eventually crumble, causing 



MOTOR A*ND GENERATOR TROUBLES 379 

electrical faults in the windings. The materials which have 
the highest maximum operating temperature are miea, and 
synthetic resins like bakelite. Where surrounding tempera- 
tures are known to be high as in the case of motors used to 
drive blowers in a boiler room or the scrapers on an economizer, 
special heat-resisting insulation must be provided instead of 
that ordinarily used. 

For the ordinary design of industrial motor the Standardi- 
zation Rules of the American Institute of Electrical Engineers 
recommend the following as maximum temperatures for the 
different kinds of insulating materials specified: 

For cotton, silk, paper and other fibrous materials not so treated as to 
increase the thermal .limit, 95 C. (203F.). When measured by ther- 
mometer, 80C. (176F.). 

For cotton, silk, paper and other fibrous material treated or impreg- 
nated and including enameled wire, 105C. (221F.). When measured 
by thermometer, 90C. (194F.). 

For mica, asbestos, or other material capable of resisting high tempera 
tures, in which the previous materials if used, are for structural purposes 
only and may be destroyed without impairing the insulating or mechan- 
ical qualities, 125C. (257F.). When measured by thermometer, 110C. 
(230F.). 

All parts of electrical machinery other than those whose temperature 
affects the temperature of the insulating material, may be operated at 
such temperatures as may not be injurious in any respect. But no part 
of continuous-duty machinery subject to handling in operation, such as 
brush rigging, shall have a temperature in excess of 100C. (212F.) for 
more than a very brief time. 

NOTE. When a thermometer is used to measure the maximum (hottest 
spot) temperature, a correction of 15C. is added to the thermometer 
reading in order to allow of the impossibility of locating the thermometer 
at the hottest spot, which will be beneath the insulation next to the metal 
of the conductor. 

The hand is a very poor thermometer and should not be 
relied upon except as a general indicator of operating tem- 
peratures. Any temperature above 120F. is very uncomfor- 
table to the touch but is well within the range of safe opera- 
tion. Until a surface temperature of about 176F. is reached 
there is little danger to injury of insulation. The motor 
should however be closely watched at this temperature and 
not allowed to go much beyond it except for a very short 
time. 



380 ARMATURE WINDING AND MOTOR REPAIR 
CAUSES AND REMEDIES FOR TROUBLES IN DIRECT-CURRENT MACHINES 



Faults 


Cause 


How most readily 
detected 


Remedy 


1. Too high 


1. Too high speed 


1. Voltmeter reads 


1. Slow the engine. 


voltage. 


of engine. 


greater than standard 








and lamps burn with 








undue brilliancy. 






2. Too strong mag- 


2. Same. 


2. Introduce more 




netic field. 




resistance in shunt 








field. 


2. Too low 


1. Too low speed of 


1. Voltmeter shows 


1. Increase speed of 


voltage. 


engine. 


lower than standard 


engine. 






and lamps burn dimly. 






2. Too weak mag- 


2. Same. 


2. T a k e out resist- 




netic field. 




ance in shunt field. 




3. Brushes not prop- 


3. Same. 


3. Rock brushes 




erly set. 




sack and forth till 








highest voltage consist- 








ent with sparkless 








commutation is shown. 


3. Excessive 


1. In a generator, 


1. By too high read- 


1. Cut out necessary 


current. 


too many lamps burn- 


ing of ammeter for 


number of lamps. Re- 




ing or motors running 


capacity of machine. 


duce load on motor 






By excessive sparking 


circuits. In this case 






of dynamo brushes 


none of the motors 






and too high reading 


may be doing too 






of dynamo ammeter. 


much work, but there 








may be too many in 








dynamo circuit. 




2. In a motor, too 


2. By excessive 


2. Reduce the load 




much mechanical 


sparking of motor 


on the motor. 




work being done by it. 


brushes nad too high 








reading of motor 








ammeter. 






3. Short circuit; leak 


3. By excessive 


3. Locate and re- 




or ground in external. 


sparking of brushes, 


move leaks or grounds. 




circuit. 


and heating of whole 








armature. 






4. Short circuit in 


4. By heating of 


4. Stop machine. 




armature coil. 


short-circuited coil 


Locate coil. If en- 






more than the others. 


tirely burned out must 








be renewed. 




5. Grounds in arma- 


5. Same as 4. 


5. Locate the 




ture. Two grounds 




grounds. Re-insulate 




to the core amount to 




the coils containing 




a short circuit. 




them. 




6. Due to excessive 


6. By sparking of 


6. File away pole 




friction in bearings or 


brushes. By sound 


pieces or re-center arm- 




by armature striking 


of armature striking 


ature. Clean and oil 




pole pieces. In gen- 


while running. By 


journals, or re-fit bear- 




eral any cause tending 


heating of motor bear- 


ings. 




to slow motor. 


ings. 





MOTOR AND GENERATOR TROUBLES 



381 



CAUSES AND REMEDIES FOR TROUBLES IN DIRECT-CURRENT MACHINES 

(Continued) 



Faults 


Cause 


How most readily 
detected 


Remedy 


4. Excessive 


1. Excessive current; 


1. Same as given un- 


1. Same as given 


sparking at 


therefore due to any 


der "Excessive cur- 


under "Excessive cur- 


brushes. 


of the causes given 


rent." 


rent." 




under that head. 








2. Brushes improp- 


2. By taking brushes 


2. Fit and set ac- 




erly set. 


out of holders and ex- 


curately, then shift 






amining rubbing sur- 


the brushes backward 






face. By measuring 


or forward till spark- 






the peripheral dis- 


ing is reduced to a 






tance between brush 


minimum. 






sets. 






3. Brushes make 


3. By sighting un- 


3. Sandpaper the 




poor contact with 


derneath between 


brushes and adjust the 




commutator. 


brushes and commu- 


spring tension until 






tator. 


they rest evenly on 








commutator with light 








but even pressure. 




4. Rough, non-con- 


4. A rough commu- 


4. Smooth commu- 




centric commutator. 


tator can be detected 


tator with fine sand- 






by lightly touching 


paper. If eccentric- 






finger nail to it whi'e 


ity is due to uneven 


- ', 




running; an eccentric 


wear of bearings, re- 






commutator by the 


new or re-line them. 






regular rise and fall 








of the brushes. 






5. "High" or "flat" 


5. By the jumping 


5. Same as above, or 




bars in commutator. 


or vibrations of the 


turn down the commu- 






brushes. 


tator in lathe. Slot 








out the mica to a 








depth of Ho or \$Q in. 




6. Broken circuit in 


6. Commutator 


6. Locate coil by 




armature or commu- 


flashes, and nearest 


drop of potential 




tator. 


the break is cut and 


method. If in com- 






burnt. Flashing con- 


mutator bridge over 






tinues when armature 


the break. If in arm- 






is slowly turned. 


ature coil, it must be 








renewed. 




7. *Veak fie d mag- 


7. Dynamo fails to 


7. Short circuits or 




netism, caused by 


generate full emf. 


grounds are easily lo- 




broken circuit in field 


, If very weak, motor 


cated and remedied if 




winding or short cir- 


runs very slow, taking 


external to the wind- 




cuit in same; two or 


a current many times 


ings. If internal, 




more grounds in wind- 


full load current. 


faulty coil must be re- 




ings; reversal of one 


'; 


wound or repaired if 




or more field coils. 




only grounded. A re- 








versed coil will lower 








the voltage instead of 








increasing it, and it is 








remedied by reversing 








the connections. 



382 



ARMATURE WINDING AND MOTOR REPAIR 



CAUSES AND REMEDIES FOR TROUBLES IN DIRECT-CURRENT MACHINES 

(Continued) 



Faults 


Cause 


How most readily 
detected 


Remedy 




8. Unequal magnet- 


8. One brush sparks 


8. Only remedied by 




ism. 


more than the other. 


re-shaping pole pieces. 




9. Dirty commuta- 


9. Flashing around 


9. Clean commuta- 




tor, causing brushes 


commutator. 


tor. (Methods given 




to vibrate, particular- 




later.) 




ly if of carbon. 








10. Poor brushes, 


10. By ragged ap- 


10. Renew brushes. 




especially if of his;h- 


pearance of brushes 


Try different grades of 




resistance carbon, 


around edges and for- 


hard and soft brush. 




hard blisters forming 


mation of hard spots. 






on them. 








11. Vibration, espe- 


11. By a humming, 


11. Reduce cause of 




cially of brush hold- 


singing sound of 


vibration or give the 




ers, causing rapid vi- 


brushes. 


brushes a little greater 




bration of brushes. 




pressure on commuta- 








tor. 


4 (a) Excessive 


12. Wrong interpole 


12. With low field 


12. In motor, pro- 


sparking in inter- 


polarity. 


excitation, examine 


gressing in direction of 


pole machines. 




field polarity with a 


armature rotation, po- 






compass, the armature 


larity should be N-n- 






being first removed. 


S-s, etc. In genera- 








tor, progressing in di- 








rection of armature 








rotation, polarity 








should be N-s-S-n, etc. 




13. Interpoles not 


13. By inspection. 


13. Adjustable when 




exactly over commu- 




poles are bolted to the 




tation belt. 




frame. 




14. Brushes not set 


14. Trace out by 


14. Usual setting is 




so that coils under- 


following up coil ends. 


in geometric neutral. 




going commutation 




Set for minimum 




are under interpole. 




sparking under aver- 








age load. 




15. Interpole air gap 


15. See if all inter- 


15. Adjustable when 




too long or too short. 


pole gaps are equal. 


poles are bolted to the 








frame. Weaken inter- 








pole strength by 








shunting the interpole 








winding. 


5. Heating of 


1. Excessive current 


1. Sfeme as given 


1. Same as given 


armature. 


through it and there- 


under "Excessive cur- 


under "Excessive cur- 




fore due to any of the 


rent." 


rent." 




causes given under 








that head. 








2. Eddy currents 


2. Core becomes 


2. This can be cor- 




and hysteresis in core. 


hotter than armature 


rected by improving 






coils after running for 


ventilation by special 






a short time. 


fans or air guides. 



MOTOR AND GENERATOR TROUBLES 



383 



CAUSES AND REMEDIES FOR TROUBLES IN DIRECT-CURRENT MACHINES 

(Continued) 



Faults Cause 


How most readily 
detected 


Remedy 




3. Conduction from 


3. Other parts con- 


3. Locate source of 




other parts as from 


nected to armature, 


heat by thermometer 




commutator or bear- 


as commutator, shaft 


or feel by the hand 




ings, the heat being 


or bearings, hotter 


and correct , it by 




conveyed to armature. 


than the armature. 


cleaning and lubrica- 








tion. 


6. Heating of 


1. Too great pres- 


1. By feeling the 


1. Reduce pressure 


commutator. 


sure of brushes, fric- 


commutator with the 


by adjusting spring. 




tion causing heat. 


hand. 






2. Excessive spark- 


2. Same. 


2. Discover the 




ing. 




cause of sparking and 








correct it according to 








the particular cause 








given under sparking. 




3. Excessive cur- 


3. Same. 


3. Discover cause of 




rent. 




excessive current and 








correct according to 








particular cause al- 








ready given. 




4. Conduction from 


4. Same. 


4. If from bearings, 




other parts. 




lubricate orre-fitthem. 


7. Heating of 


1. Excessive cur- 


1. Too hot to bear 


1. Locate the par- 


field coils. 


rent in field circuit 


the hand. If exceed- 


ticular coil in which 




due to short circuits 


ingly hot, by smell of 


fault lies and repair or 




or grounds. 


burning shellac or var- 


r e-wind. Methods 






nish or charring cotton. 


given later. 




2. Eddy currents in 


2. The pole pieces 


2. Only remedied by 




pole pieces, heat being 


are hotter than the 


better design, use lam- 




conducted to the coils. 


coils after a short run. 


inated pole shoes. 


8. Heating of 


1. Lack of lubrica- 


1. By feeling with 


1. Fill oil cups; clean 


bearings. 


tion. 


hand. Oil cups emp- 


feeding pipes. 






ty or feeding pipes 








clogged. 






2. Dirty or gritty 


2. By feeling with 


2. Remove cap and 




bearings. 


hand. 


thoroughly clean. 




3. Bearings out of 


3. Unequal wear of 


3. Bearings must be 




line. 


bearings, and shaft 


lined up or shells re- 






will not turn freely by 


babbitted. If very 






hand. 


serious, new bearings 








will have to be made. 




4. Rough or cut 


4. Shaft will show 


4. Turn down shaft 




shaft. 


the roughness in the 


in lathe, or scrape the 






bearings. 


bearings. 




5. Shaft bent. 


5. Unequal wear in 


5. Shafts can only 






bearings and arma- 


be straightened by 






ture will wobble. Very 


disconnecting from 






hard to move by hand. 


armature and re-heat- 








ingand re-forging. 



384 



ARMATURE WINDING AND MOTOR REPAIR 



CAUSES AND REMEDIES FOR TROUBLES IN DIRECT-CURRENT MACHINES 

(Continued) 



Faults 


Cause 


How most readily 
detected 


Remedy 




6. Oil rings stuck. 


6. Inspection. 


6. Adjust rings in 








grooves. 


9. Too low 


1. Too much load. 


1. By speed indica- 


1. Reduce the me- 


speed (referring 




tor; heavy sparking, 


chanical load. 


to motors). 




heating of all parts 








and bearings. 






2. Any of the causes 


2. Same, and same 


2. Discover particu- 




given under " Heating 


as given under " Heat- 


lar cause and remedy 




of bearings," causing 


ing of bearings." 


same as given under 




excessive friction. 




" Heating of bearings." 




3. Short circuit or 


3. By motor taking 


3. Same as under 5, 




grounds in armature. 


excessive current 


"Excessive current." 






without load as shown 








by ammeter or heavy 








sparking and heating. 






4. Too low voltage 


4. By motor volt- 


4. By increasing the 




at terminals. 


meter or speed indi- 


line voltage. 






cator. By heavy 








sparking and heating. 




10. Too high 


1. Too light load (in 


1. By noticeable in- 


1. Increase load. 


speed (referring 


series motors). 


crease in speed. 




to motors). 


2. Weak field shunt 


2. Same. 


2. Strengthen field. 




motor. 








3. Too high voltage 


3. Same. 


3. Correct line vol- 




at terminals, due to 




tage by remedies 1 and 




high voltage of dyna- 




2 under "Too high 




mo. 




voltage." 


11. Dynamo 


1. Too weak residual 


1. Very little attrac- 


1. Send a current 


fails to generate 


magnetism, caused by 


tion by the pole pieces 


through field from a 


emf. 


a jar or reversal of 


when tested with a 


few cells or from a 




current not sufficient 


piece of iron. 


running dynamo. 




to reverse magnetism. 








2. Short circuit 


2. Magnetism very 


2. Locate the 




within machine, or 


, weak. 


grounds or short cir- 




grounds in field wind- 




cuits and correct them. 




ings. 








3. Reversed field 


3. All poles should 


3. Make polarity op- 




coils. 


have alternate mag- 


posite by reversing the 






netism; if a coil is re- 


connections of the coil. 






versed it will show 


Each pole should be 






magnetism, but may 


opposite to the one on 






not be of opposite po- 


each side of it. 






larity. 






4. Series and shunt 


4. Voltage falls as 


4. Reverse connec- 




windings connected up 


load is increased, the 


tions of either field, 




opposite to each other 


external circuit being 


but not both 






closed showing that 








they are working 









against one another. 





MOTOR AND GENERATOR TROUBLES 



385 



CAUSES AND REMEDIES FOR TROUBLES IN DIRECT-CURRENT MACHINES 

(Continued) 



Faults 


Cause 


How most readily 
detected 


Remedy 




5. Brushes not prop- 


5. Magnetism and 


5. Find central posi- 




erly placed. 


emf. increased by 


tion by experiment or 






shifting the brushes. 


from drawings of con- 








nections 1 . 




6. Open circuit in 


6. Test circuits with 


6. Set up on all 




field or armature. 


magneto. 


connections. Press 




Brushes not making 




brushes on commuta- 




good contact with the 




tor to start building 




commutator. Loose 




up. 




connections. 








7. Too much resist- 


7. Voltage does not 


7. Cut all resistance 




ance in the shunt field 


exceed that due to 


out of the shunt field 




circuit, i.e., greater 


residual magnetism. 


circuit. Reverse the 




than the " critical " re- 


The voltage due to 


shunt field. 




sistance. Shunt field 


residual magnetism 






bucks the residual 


drops when the shunt 






magnetism. 


field circuit is closed. 




12. Motor fails 


1. Too much load. 


1. No motion and 


1. If motor does not 


to start. 




fuse in circuit melts 


start at once, turn off 






or circuit-breaker acts. 


current and search for 






See if motor runs all 


cause. Reduce load 






right when light. 


on motor. 




2. Excessive friction, 


2. Same, and motor 


2. Remedies same as 




due to any causes giv- 


hard to turn when not 


given under " Heating 




en under heading 


loaded, and with no 


of bearings." 




"Heating of bear- 


current. 






ings." 








3. Short circuit of 


3. Motor refuses to 


3. If connections are 




field or armature or 


revolve, though shows 


made wrong, consult 




among connections. 


signs of strong mag- 


maker's diagram and 






netism. Will turn 


correct them. Test for 






easily by hand if un- 


continuity and short 






loaded and with no 


circuits as given later. 






current. If current is 








very great, it is indi- 








cation of short circuit. 








If fault is in field, 








magnetism will be 








weak. 






4. Open circuits due 


4. Weak magnetism 


4. Turn current from 




to field switch open, 


shows a loose connec- 


motor, and search for 




fuse melted, loose or 


tion in field circuit; no 


cause of discontinuity; 




broken connections, 


magnetism, that field 


examine all switch 




or some fault at gen- 


switch is open. May 


fuses and connections, 




erator. 


be heavy current in 


tightening all. Test 






armature. If there is 


for continuity in ma- 






no armature current 


chine circuits and re- 






there will be no spark 


pair broken or burnt- 






at brushes when 


out coils. 






raised. 





2.1 



386 



ARMATURE WINDING AND MOTOR REPAIR 



CAUSES AND REMEDIES FOR TROUBLES IN DIRECT-CURRENT MACHINES 

(Concluded) 



Faults 



Cause 



How most readily 
detected 



Remedy 



13. Flickering 
of lamps. 



14. Noise. 



1. Uneven running 
of engine, probably 
due to governor fail- 
ing to properly func- 
tion. 

2. Loose connec- 
tions, either on ma- 
chine, switchboard or 
external circuit. 

1. See third fault 
(6), fourth fault (4, 5, 
9, 10, 11), eighth fault 
(3, 4, 5), and tenth 
fault. 

2. Armature running 
against the brushes. 



1. By flickering of 
lamps or vibration of 
voltmeter indicator. 



2. Same. 



2. By unusual noise. 



1. Overhaul engine, 
specially governor. 



2. Examine all con- 
nections and see that 
they are firm and 
make good contact. 
Look for arcs. 



2. Correct direction 
of rotation. 



Electrical Defects. Troubles in motors and generators 
due to electrical defects show up in a variety of Ways. Their 
location, however, is more often a simple than a difficult matter 
and depends upon a few testing devices and a great deal of 
patience and persistence. An experienced operator or re- 
pairman learns to diagnose electrical troubles by a process 
of elimination where there may be several causes. For 
such an analysis of common electrical troubles in motors 
and generators the table on pages 380 to 386 will be found 
useful. It was compiled by the authors of the Naval Elec- 
tricians Text Book and has been taken from that most practical 
work. 

CAUSES AND REMEDIES FOR TROUBLES IN ALTERNATING- 
CURRENT MACHINES 

Induction Motor Troubles. The most common troubles 
with induction motors show up by the machine failing to start 
or by stopping while connected to the line. When the ma- 
chine refuses to start and the fuses are not blown, the cause 
may be in too large a load. The remedy in such cases is to either 
reduce the load or use a clutch for starting since the squirrel- 
cage motor has a limited starting torque but can often carry 



MOTOR *AND GENERATOR TROUBLES 387 

a much larger load while running. When investigating a 
trouble of this kind it is important to see that the voltage is 
normal and that the bearings are in good condition. If none 
of these conditions seem to cause the trouble a test should be 
made for open circuits in the windings. (See Chapter IX.) 

An excessive current at starting may be due to a high volt- 
age applied or too great a load. When the voltage is high 
an auto-transformer with suitable taps must be used for 
starting. When an auto-transformer is used and the starting 
current is excessive, a test should be made for proper connec- 
tions. Too low taps on the starter should be avoided. 

An overload on an induction motor will usually show up 
by the mo tor coming to rest or taking about 10 times more than 
normal current. When fuses and circuit breakers do not 
operate under such heavy current, the machine will burn out. 
It is therefore important that fuses and settings of circuit 
breakers be used to prevent such burn-outs. Since the torque 
of an induction motor varies with the square of the voltage, 
low supply voltage may be the cause of the machine stop- 
ping. Also worn bearings will allow the rotor to drop on the 
stator and thus block the former and stop the machine. For 
other details in testing and inspection of induction motors see 
Chapter XIV. 

Locating Troubles in Winding of Induction Motors. 
When the cause of trouble with an induction motor in starting 
or in operation has been traced to the stator windings, details 
of locating the fault are outlined in Chapter IX. When 
the stator has been repaired and again placed in service its con- 
nections should be carefully looked over. The points to be 
observed when inspecting such a stator are outlined on page 
363. In case the connections of an auto-starter are reversed, 
the starting current will be excessive and insufficient torque 
with the switches in the running position will show up. If 
the motor refuses to start the cause may be due to a defect in 
the change-over switch or to a loose connection or open 
circuit in the auto-starter. 

Mechanical Adjustments. In those cases where induction 
motors are in continuous operation regular inspections should 
be made of the air gap to see that it is not appreciably reduced 



388 ARMATURE WINDING AND MOTOR REPAIR 

at the bottom of the rotor on account of wear of the bearings. 
When the air gap is reduced a test should be made before the 
trouble is attributed definitely to wear of the bearings. It is 
also important that an armature have a small play endwise in 
the bearings. This should be about Me inch. Excessive 
heating or excessive belt tension may be the cause of a great- 
deal of bearing trouble on an induction motor which is belt- 
driven. 

Troubles Due to Electrical Faults. When errors have been 
made in winding an induction motor or in re-connecting the 
coils, trouble will usually show up in heating of the coils, a 
peculiar humming sound for large and unbalanced currents 
in the different phases. Before testing for faults of re-connec- 
tion of coils, it must be determined that the connections of the 
winding to the supply circuit are correct and that none of the 
connections of the phaso coils have been accidentally jammed 
together so as to short-circuit them. Troubles in rotors of 
induction motors are rare, however, in squirrel-cage types 
trouble may be located in soldered joints which have melted 
or corroded. High resistance at joints lowers the efficiency, 
increases the heating and also increases the starting torque of 
the motor. Heating or operation at reduced output or speed 
may be due to a three-phase motor running on one or two 
phases. 

Troubles with Synchronous Motors. When a synchronous 
motor fails to start the trouble is generally due to an overload. 
In testing out for such a condition the motor should be started 
light. If the operation is not satisfactory the load should be 
reduced. Poor starting may also be caused by reduced volt- 
age or open or faulty connections in starting apparatus. An 
open circuit is usually indicated by no current flowing in a 
particular phase. An excessive current usually indicates a 
short circuit but may be due to grounds. An excessive cur- 
rent in a synchronous motor is a dangerous condition and the 
trouble should be located. 

When a synchronous motor is used for power-factor correc- 
tion overheating indicates an excessive current. The machine 
can usually be temporarily operated by reducing the load or 
reducing the amount of leading current. When a synchronous 



MOTOR AND GENERATOR TROUBLES 389 

motor is operating satisfactorily the current in the armature 
phases should be about equal when the rotor is turning slowly. 

Trouble in the field winding such as an open circuit causes a 
shutdown or excessive armature heating. When the field cur- 
rent seems excessive, a test should be made for polarity of the 
armature coils and reversal of connections. When a syn- 
chronous motor fails to show normal starting torque and will 
not carry the load, the trouble will frequently be found in the 
field circuit in the form of an open circuit, short circuit or 
reversal of one or more field windings. 

Causes of A.-C. Motor Fuses Blowing. In addition to 
overload on a motor, many other things cause fuses to blow. 
The following causes and symptoms have been formulated by 
Henry W. Zeuner, Milwaukee (Wis.) Electric Light Company 
(Electrical World, Sept. 20, 1919). 

1. Operator throwing starting switch of compensators 
from starting to running position too quickly. 

2. Operator throwing switch into running position without 
touching the starting position at all. 

3. Motor winding becoming grounded. 

4. Excessive current due to low voltage, short circuits 
in stator windings, single-phase operation, etc. 

5. Starting switch being in running position when service 
comes back on line after interruption. 

Wound-rotor motors and squirrel-cage motors which are 
not protected by a no-voltage release may be shut down at 
any time because of the last-mentioned cause. To overcome 
this trouble a group of such motors is often protected by an 
oil switch equipped with no-voltage release. The arrange- 
ment has the disadvantage, however, that whenever the oil 
switch opens it becomes necessary for some one to go around 
and open each individual motor switch before the oil switch 
is closed again. 

A ground may only blow one fuse ajid leave the motor 
operating from one phase of the line. When a polyphase 
motor is running single-phase it not only gets hot but it 
makes a growling noise which is especially noticeable under 
heavy load. By the time these symptoms indicate the prob- 



390 ARMATURE WINDING AND MOTOR REPAIR 

able source of the trouble and it is decided to shut down the 
motor, a second fuse is sometimes blown by the excessive 
current per phase. If only the first defective fuse found is 
replaced, the motor may be started from the line side of the 
switch which is connected ahead of the fuses. When the 
switch is thrown over into the running position the motor is 
again operating on single phase. Because the motor con- 
tinues to heat up the user often thinks there must be 
something wrong with the motor itself and calls for help, 
whereas if he had only tested all the fuses in the first place and 
replaced all of the burnt-out fuses, instead of only one of 
them, he could have put the motor back in service without 
assistance and with a minimum loss of time. Another source 
of trouble which may be detected at the service fuses is low 
voltage. This causes excessive heating and often burns 
out stator coils. The voltage can be checked roughly with 
test lamps. 

Inspection of Motor-starting Devices. From the fuses it is 
customary to proceed to examine the motor-starting switch, 
compensator or controller. Contact fingers are often found 
bent or burned so that they do not make contact and the 
connecting leads are sometimes burned off. The two-minute 
starting resistance supplied by the manufacturers for starting 
slip-ring motors is often burned out by being used for speed- 
regulating duty which requires a much heavier resistance. 

Testing Motor for Grounds. When testing an alternating- 
current motor for grounds, a magneto or high-voltage testing 
transformer should be used as the line voltage will seldom 
show a ground unless it is making very good contact. 
National Electrical Code rules require that motor frames be 
grounded, but this requirement is not always carried out. 
When the motorframe is insulated from the ground the motor 
can be kept in operation with one phase grounded to the 
motor frame, but tlae defect should be remedied at the first 
opportunity. A second connection between the stationary 
windings and the motor frame will burn out the coils which it 
short circuits, and if not given prompt attention the two 
grounds may result in the burning out of the entire winding. 



MOTOR AND GENERATOR TROUBLES 391 

When both the supply system and the motor frames are 
grounded, as is usually the case, one ground on the stator 
winding will blow one of the motor fuses unless the fuse is 
too heavy, in which case the ground may burn out some of the 
stator coils. However, every ground which blows a fuse 
does not occur in the motor winding. If the motors test 
clear, the wiring leading to the motor may be found grounded 
to the conduit. 

It is a good plan to run the defective motor while testing 
when it is practicable to do so. This makes it possible to 
conduct additional tests and very often to observe symptoms 
which do not appear when the motor is standing still. Under 
such operating conditions the speed should be tested. Low 
speed and inability to pull the load are usually an indication 
of bad connections between the rotor bars and the rotor end 
rings. 

Hot Stator Coils. Another condition to observe while the 
motor is operating is whether the stator coils are hotter in 
one place than in another. This is one of the signs of bearing 
trouble. When the bearings become worn the stator coils 
around the section of reduced air gaps get hotter than the coils 
adjacent to the section where the air gap has been increased. 
If the bearings are not renewed before the rotor begins to 
rub on the stator, the motor windings may be seriously in- 
jured, especially if the fuse is too large and does not blow. 
When it is impracticable to operate the motor the belt should 
be taken off and the bearings examined to find out if there 
is too much play in them. 

Tension of Belts. Much belt and bearing trouble is caused 
by the belt being too tight. This is especially true with 
vertical belt drives from squirrel-cage motors to high-speed 
machines. When bearing trouble is experienced with in- 
stallations of this description it is usually because the pulley 
surface is too small to accelerate the driven machine as fast 
as a squirrel-cage motor comes up to speed, and consequently 
the belt comes off. In attempting to keep the belt on the 
pulley the belt is usually made so tight that the excessive 
tension soon wears down the bearings. This trouble can 



392 ARMATURE WINDING AND MOTOR REPAIR 

sometimes be overcome in installations of small motors by 
using larger pulleys, a larger motor, or by changing from a ver- 
tical belt drive to a horizontal belt drive with the slack side 
of the belt on top. With large belted motors driving high- 
speed machines the slip-ring type of motor should be used in 
order to bring the driven machine up to speed more slowly. 

Troubles in Rotor Windings. A bad contact in the rotor 
winding is not so easily found when the end rings are cast 
on to the end of the rotor bars. This cast construction was 
originally adopted to overcome the bad joints which developed 
with screwed and soldered connections in the rotor windings. 
The change has not entirely eliminated the trouble, however, 
because the molten metal does not always unite with the end 
of the copper bars when the end rings are being cast. When 
this is t"he case trouble due to the poor contact often develops, 
especially if the motor is subject to much dirt and vibration. 
With a little experience a defective joint between the cast 
end rings and the rotor bars can be found by tapping the end 
rings with a hammer and noting the difference in sound at 
various places. A bad joint in this type of construction can 
be repaired by welding the bar and end rings together. Welded 
rotor windings have been very successful, and it is unusual 
to find bad joints in rotor windings which have been put 
together in this manner. 

Examination of Stator Winding. The stator winding 
should also be thoroughly examined while the end shields 
are off the motor. If one or two coils are burned out, the 
other coils can in most instances be raised out of the slot and 
new coils slipped into place provided that the coil insulation 
is flexible and in good condition. Should this be attempted, 
however, after the insulation on the coils has become brittle, 
there is a grave danger of damaging the insulation to such an 
extent that the entire stator winding may have to be renewed. 
When the condition of the insulation is doubtful and only one 
or two coils need replacing, it has been found a good plan 
to cut away the burnt-out coils and thread new wire through 
the slots turn by turn, without disturbing any of the other coils. 

The damage from a local short circuit in the stator winding 



MOTOR AND GENERATOR TROUBLES 393 

can be limited to the one or two coils affected and the motor 
still kept temporarily in service by promptly cutting out 
the damaged coil with a jumper. However, if the trouble 
is neglected it will spread to other coils in the same phase, and 
then the windings of the other phase or phases will also burn 
out. 

Sparking at Slip Rings. This trouble in wound-rotor mo- 
tors occurs when the tension spring has not been reset from 
time to time to keep the brushes in close contact with the slip 
rings. This is the most frequent cause of the trouble, and 
it can usually be eliminated by adjusting the tension of the 
spring and where necessary turning up the slip rings and re- 
newing the brushes. 



CHAPTER XVI 

METHODS USED BY ELECTRICAL REPAIRMAN TO 
SOLVE SPECIAL TROUBLES 

When called upon to locate troubles in electrical apparatus 
many electricians and repairmen find themselves in a state of 
wondering just what to do first. The mystery supposed for 
so long to be associated with the operation of electrical appa- 
ratus seems to discourage many able electrical men to attempt 
to search out an electrical trouble when it seems to be a compli- 
cated one. By the use of the same good sense that makes 
such men successful in the work with which they are familiar, 
most electrical problems in repair can be located and a satis- 
factory remedy applied. In a serious case where expert help 
is reasonable and within easy and prompt access, it may not 
always pay for an inexperienced man to spend the necessary 
time to handle a difficult trouble but in the ordinary run of 
plant operation such troubles are few. What is most needed 
is a little clear thinking aided by a few testing instruments. 

One of the ways by which a repairman can develop con- 
fidence in his own ability to search out trouble and prepare 
himself to properly diagnose troubles, is to read and profit 
by the experiences related by those who have worked out 
puzzling troubles and have told in the columns of electrical 
trade journals how they proceeded. The author has made 
a selection of such experiences covering a wide range of operat- 
ing troubles which are liable to come up at a time or under 
circumstances which make them difficult to handle. The 
details given will furnish suggestions as to methods of pro- 
cedure in many other cases than those described. 

Sparking at Commutator Caused by Poor Belt Joints. In 
operating motors or generators with laced belts, it is important 
that the joints be flexible and the ends neatly laced close 
together to present as nearly as possible a continuous surface 
to the pulley. Where this is not done laced belts very fre- 

394 



METHODS USED TO SOLVE SPECIAL TROUBLES 395 

quently cause sparking due to any one of three causes: (1) 
The laced belt when passing over the pulley may cause a jerk 
or mechanical vibration of the armature which is severe 
enough to throw the brushes from the commutator and cause 
sparking due to the broken circuit. (2) When the bearings are 
loose in their housings or when the armature shaft is loose in 
its bearings, the jerk of the lacing may pull the armature to 
one side. This develops a higher voltage in a generator or a 
higher counter electromotive force in a motor in the part of the 
armature which is pulled closest to the pole faces. This will 
cause sparking due to a pulsating current or in severe cases 
due to a short circuit current between studs of the same 
polarity. (3) The belt may slip while the laced ends are 
passing over the pulley, which will permit a variation in the 
speed of the armature. This will cause a sudden change in 
the current and may produce sparking due to the inductance 
of the armature coils. 

Plugging a Commutator. When commutator bars show a 
tendency to blister and bead at the outer ends, " plugging" 
the commutator often gives relief. To do this, proceed as 
follows: Grind all of the "set" from the teeth of a piece of 
hacksaw blade. With this piece of blade saw the commutator 
side mica to the outline indicated at (a) in the accompanying 
diagram, Fig. 250. Next drive in mica wedges or "plugs" so 
called, to entirely fill and 
perfectly fit the grooves 
made by sawing. The 
best results are to be ob- 
tained by loosening the 
commutator before driv- 
ing in the Wedges, but FlG ' 250.-Method of repairing mica seg- 
; ment by sawing at (a) and plugging. 

this is not absolutely 

necessary. For other details of commutator repair sec 

Chapter XII. 

Knock in a Motor Armature Due to Band Wires Being Too 
High. In the banding of many types of direct-current arma- 
tures, the bands are laid onto a layer of fish paper or of thin 
fiber which affords smooth bedding for the band wires and which 
also provides additional insulation between the band wires and 





396 ARMATURE WINDING AND MOTOR REPAIR 

the armature coils. On many of the later designs of armatures 
the tension used in banding has been greatly increased, and as 
a further precaution against the bands cutting into the insula- 
tion of any coils that may be high in the slot, a layer of thin 
tin is interposed between the band wire and the insulating 
band. That it may not be advisable to adopt this improved 
method on machines that have not been designed for it, is 
illustrated by the following experience (Electrical Record, 
June, 1918). 

A 20-hp. direct-current motor which had just been repaired 
and shipped, was returned within a few days with the com- 
plaint that the armature "had a knock in it." At the time of 
testing the motor, a slight vibration had been noticed, but 
leveling of the motor had eliminated the vibration. Careful 
inspection of the motor this time, however, revealed that one 
of the tin clips of an end band had been striking one of the 
bottom pieces. Investigation disclosed that a contributory 
cause of the knocking was that one of the bearing linings had 
been bored a little out of center, but even after this had been 
corrected a piece of % 4 -inch fiber could not be inserted between 
the armature core and the bottom pole pieces in certain 
positions of the armature The real cause of the trouble was, 
then, that the layers of insulation and tin under the band 
wires, projected the wires and their holding clips too far into 
the air gap. As the air gap was thinner than ordinarily found 
on such motors, the remedy was to remove the tin and install 
band wires of smaller diameter and with less tension, and to 
use thin copper for the band wire clips. These changes re- 
sulted in a %2-inch air g a P- The testers were deceived as to 
the cause of the vibration because the tilting of the machine 
floated the armature to a position where the high clip cleared 
the pole piece it was striking. 

Heating of an Armature Traced to Poor Soldering of Com- 
mutator Connections. E. C. Parham has described (Electrical 
Record, March, 1919) an interesting case where it was necessary 
to replace the armature of an exciter that had been in opera- 
tion so long that the commutator bars had worn so that the 
pressure of the end rings was beginning to buckle the bars 
up in the middle. An extra armature was obtained because 



METHODS USED TO SOLVE SPECIAL TROUBLES 397 

the exciter could not be spared from service. This armature 
was installed in place of the old one and the old one was 
repaired by re-filling its commutator. About a year later a 
switchboard short circuit which started by a stroke of light- 
ning, so badly burned the new exciter commutator that it was 
necessary to take a cut off it. The repaired armature was 
taken from storage and installed but it could not be used, 
because it heated all over as soon as a field was put on the 
exciter. As the armature was the only equipment part 
that had been handled, the trouble evidently was in it. It 
was removed and the commutator disconnected for testing. 
The commutator was found to be perfectly clear but the 
armature leads where they were connected to the commutator 
were found to be a mass of short circuits due to the criminal 
carelessness of the man who had done the soldering. After 
picking out the solder, trimming the leads and re-connecting 
by a workman who knew that class of work, the operation 
was normal. 

One lesson that stands out very prominently from this 
experience is this: When the armature of an indispensable 
machine must be replaced on account of the necessity of 
making repairs, after the repairs have been completed, re-in- 
stall the repaired armature at once for test so that the armature 
that is known to be right, may be kept as the spare and with 
a fair degree of certainty that it will be all right when oc- 
casion arises for using it. Another lesson is that every 
shop or station that pretends to do connecting and soldering, 
should have access to a short-circuit test for locating just 
such trouble as described. 

How a Commutator was Repaired under Difficulties. In 
an instance related by R. L. Hervey (Electrical World, Feb. 
26, 1916) the commutator of a generator had on a number 
of occasions given trouble because of the mica breaking down 
between the segments and at the end rings. After a member 
of the engine-room crew had failed to relieve the trouble, 
an electrician was called in. Two segments were located 
that had been hot enough to melt the solder out of the joint 
between the bars and the risers. This appeared to be the 
seat of the trouble. 




398 ARMATURE WINDING AND MOTOR REPAIR 

Before the commutator was opened it was blown out with 
an air blast at the front and back to remove the copper and 
carbon dust and prevent it from falling into the commutator 
and giving trouble when the latter was opened. One of the 
bars that showed signs of being hot had welded to the rear 
end ring, forming a ground. As this bar passed from one 
brush to the other the ground changed from the positive to the 
negative bar. causing the ground lamps to flicker. Consider- 
able oil had crept along the shaft 
and found its way into the com- 
mutator, starting the trouble by 
FIG. 251. Section of patched causing the binder in the mica ring 

to disintegrate. Four segments were 

removed so that a patch could be put in the mica ring, as shown 
in Fig. 251. The old ring was measured for thickness, and 
two pieces of special ring mica were put in, with a thin piece 
of clear mica covering each of the four joints. A high-grade 
shellac was used to hold the pieces in place until the commutator 
was tightened and also to fill up the small openings in the 
mica and keep oil out. 

The risers used on this machine were poorly attached 
to the commutator bars, being merely soldered in a shallow 
slot in the center of the bar. Before the heated segments 
could be replaced it was found to be nesessary to scrape the 
thin coating of solder from the riser and out of the slot in 
order to make a good joint. If a current is passed through a 
soldered joint while the solder is hot enough to flow, the 
structure of the solder is so changed that the resistance of the 
joint is very much increased and will continue to give trouble 
if worked near its original carrying capacity, although to all 
appearance a good joint has been made. 

When the commutator was built amber mica was used 
between the bars. About every two years it has been neces- 
sary to remove a few pieces, and these had been replaced by 
micanite, which is supposed to be able to withstand the bad 
effects of oil better than the former. At this time 10 pieces 
of mica that showed signs of pitting were taken out. Two 
pieces were of the original amber and the rest were micanite, 
some of which had not been in service more than two years. 



METHODS USED TO SOLVE SPECIAL TROUBLES 399 

This case does not, however, afford a fair comparison between 
the two grades of mica. If the leads had been poorly soldered 
into the riser or the riser into the bar, a high-resistance joint 
would have resulted, causing imperfect commutation with 
creepage of current across the mica between bars. This 
creepage will in time break down the mica segment and thus 
short-circuit the bars. 

In order to test for a short circuit, after the leads were 
soldered in and the commutator drawn tight the engine was 
started and the field switch closed. If a short circuit had 




FIG. 252. Method of attaching turning rig to machine showing its operation 
with tool upside down. 

existed, it would have caused one or more coils to get hot 
in a few minutes, but such a condition was not observed in 
this case. 

Since it had been necessary to turn this commutator so 
often, the owners had purchased a turning rig which was 
supposed to fit this machine. A common difficulty of turning 
devices is that they seldom fit a machine unless made specially 
for it. In this case if the rig had been put on the proper side of 
the pedestal it would have been necessary to leave the bear- 
ing cap off or to have the tool considerably above the center 
of the commutator. In the latter instance, in case the tool 
caught, it would have dug into the commutator. The only 



400 ARMATURE WINDING AND MOTOR REPAIR 

other way by which the rig could be attached was to place it 
so as to cut with the tool upside down. Although the rig 
was not so steady as it would have been if worked in the cor- 
rect position, no difficulty was encountered. 

The turning rig used on this job was very easily attached. 
It was unnecessary to remove a single brushholder in order 
to put it in place. The screws at A in Fig. 252 held the main 
part of the rig to the bearing cap. The two screws B were 
screwed against the side of the pedestal to steady the end of 
the rig. By loosening the screw C the transverse-feed length 
was readily adjusted. The end of the transverse-feed arm 
was supported by the long screw D resting on a brushholder 
arm. To prevent any lateral motion of the armature, the 
screw E was held in the center of the shaft by the adjustable 
arm F. 

Holder for Sandpapering Commutators. In Fig. 253 
there is shown a holder for use in sandpapering commutators 
that makes it unnecessary to shape a block to fit the curved 
surface of the commutator. This device as made by Peter 

J. M. Clute (Popular 
Science Monthly, June, 
1919) consists essentially 
of a handle which is 
broadened at its lower 
extremity, and has two 
blocks, 2 by 2j- in., with 

an arc of a circle on each 

, on, ui i 

face. These blocks are 

pivoted, and will adjust 
and accommodate them- 

FIG. 253. Convenient device for sand- i 

papering a commutator. selves to an y COrnmu- 

tator curvature. Two or 

more of the blocks may be used, depending upon the size of 
the commutator. 

Use of a Portable Electric Drill to Undercut Mica of Com- 
mutator. An interesting method of undercutting a commu- 
tator on which the mica had come to the surface aftter the 
commutator had been turned, has been devised by R. H. N. 
Lockyear, of Trail, B. C. (Electrical Record, June, 1918). 




METHODS USED TO SOLVE SPECIAL TROUBLES 401 

Mr. Lockyear describes as follows the circumstances under 
which the electric drill was successfully used for this purpose. 




FIG. 254. Special arrangement to use a small portable electric drill to 
undercut mica on a commutator. 




FIG. 255. Special rigging for the use of an electric drill shown mounted on a 
generator to undercut the commutator mica. 

"The writer had occasion to undercut a commutator on a 
500-kw. generator, the mica having come to the surface. 



402 ARMATURE WINDING AND MOTOR REPAIR 

after the commutator had been turned. Work had com- 
menced by hand but it was found difficult to make head- 
way. We had a machine for undercutting railway commuta- 
tors having a width of about three inches. This machine was 
rebuilt with longer guide rods and a bracket made for mounting 
it on the brush shifting yoke of the generator. This machine 
was connected with a flexible shaft but soon discarded because 
of the vibration. It then occurred to the writer to direct 
connect a Van Dorn type DA-OO electric drill, which was 
available, to the spindle of a milling machine and undercut the 
mica with this equipment. The scheme proved most effective 
and since that time we have used it on 26 generators with a 
great saving of time, which is an important factor in a power 
plant when repairing large generators. In the tool we employ, 
the cutter has a speed of 1650 rpm., a diameter of %-in., and a 
thickness of 0.003 in." 

Jerky Operation of New Commutator Traced to Burred Com- 
mutator Bars. In some instances where it has been necessary 
to temporarily reduce the speed of a series motor and there 
have not been available suitable resistances for connecting in 
series with the armature, the desired result has been obtained 
by connecting a resistance across the brushes. The resistance 
so disposed not only diverts part of the line current from the 
armature, but also provides an external path through which 
the motor armature can act as a generator, thereby producing a 
braking effect. The extreme of this condition is active when 
a railway motor flashes over from brush to brush. The cur- 
rent generated through the short circuit formed by the arc 
is so great that the suddenly imposed load checks the speed of 
the armature, the sudden checking of the speed constituting 
what railway men call " bucking. " An action which is similar 
in kind but which is milder in degree, occurs when an armature 
coil is short-circuited. The coil becomes 'a short-circuited 
loop moving in a strong field, and the local current of the coil 
is so heavy and the drag imposed by it so great that the re- 
mainder of the armature may be unable to support continuous 
motion. Under this condition the armature will turn in 
jerks, because the resisting drag of the short-circuited coil 
is greater in some positions than in others. 



METHODS USED TO SOLVE SPECIAL TROUBLES 403 

In a certain case (Electrical Record, June, 1918) a new com- 
mutator was installed on a direct-current armature which had 
operated normally up to the time of removing the old com- 
mutator. After installing the new commutator, on trying 
to start in the usual manner, the armature would turn very 
slowly and in jerks, and if the starter were advanced half of 
its travel the breaker would blow. An ammeter was connected 
in series with the armature, and the current was found to be 
of almost full-load value on the first notch of the starter. 
A bar to bar test was made around the commutator and nearly 
all of the armature coils appeared to have zero resistance. 
Just at this time, full-load current being in the armature, which 
was blocked so that it could not return, an explosion accom- 
panied by a flash occurred on the commutator. Investigation 
disclosed a burn between two bars in the groove that is 
turned in the rear end of a commutator. Close inspection 
showed that the groove appeared as a continuous copper 
band extending entirely around the commutator. In cutting 
the groove the tool had been allowed to drag the copper over 
from bar to bar all round the commutator. On clearing 
the mica bodies of the copper bridges, operation became 
normal. 

Why Brush Studs Heated on an Eight-pole Machine. The 
accompanying diagram shows the paths of the current through 
a multiple connected arm- 
ature that is used in a four- 
pole machine, the two brush 
studs of the same polarity 
being connected by means 
of copper busses with ends 
strung onto the studs that 
are to be connected. The 
incoming current arriving 

at brush a divides among FIG. 256. Diagram of paths of cur- 

three paths in order to reach rent through armature * 

brush d to which the other end of the external circuit is con- 
nected. One path, ad, is directly through one-quarter of the 
armature to d; the second path is through one-quarter of the 
armature to brush b and thence by way of bus bd to brush d 




404 ARMATURE WINDING AND MOTOR REPAIR 

and out; the third path carries as much current as the two 
others combined; it runs through bus ac to c where it divides 
equally, half passing through the upper right hand quarter of 
the armature directly to d and out and the other half through 
the upper left hand quarter of the armature to brush b and 
thence by way of bd to d and out. With the busses of negli- 
gible resistance and the sets of brushes equally spaced on the 
commutator all of the studs would carry approximately equal 
currents. 

On an eight-pole machine there are more current ramifica- 
tions because there are more studs and more busses and, there- 
fore, there are more paths for the current. 

It was on such a machine in a particular plant, that two of 
the brush studs would get so hot as to burn the insulating 
bushings by means of which the studs were insulated from the 
yoke. Investigation disclosed that the machine was abusively 
overloaded and that all studs heated above normal but that 
only two got hot enough to burn bushings. An inspector 
^suggested that the heating was due to the generally poor con- 
dition of all contacts and especially to the poor contacts be- 
t"-een the stud threads and the threads of the nuts by means of 
which the busses, the yoke, the washers and the studs were 
held together. 

Acting on this theory, V-shaped pieces of copper strap were 
formed and so installed on the studs as to form a conducting 
bridge 1 from the outside nut and washer over the yoke (with- 
out touching the yoke) to the inside nut and washer. This 
bridge short-circuited all contacts with which the threaded 
parts of the studs were involved. The scheme worked so well 
that similar bridges were installed on all of the studs. After 
giving the whole stud construction a good cleaning there was no 
further trouble. 

An Accident Due to Incorrectly Set Brushes. In a case 
where an isolated plant was furnishing a meat packer with 
electric power at 110 volts for a 10-hp. motor, in order to 
provide emergency service, connections were made to the 
central station's 220-volt mains. The higher voltage required 
the installation of an additional motor with its separate starter. 
When the new 220-volt motor was installed no electrical test 



METHODS USED TO SOLVE SPECIAL TROUBLES 405 

was made until the work had been completed and the belt 
put on the pulleys. With all the machines connected to the 
line shaft, the motor was started with the following results. 
(R. L. Hervey, Electrical World, Jan. 13, 1917). An operator 
observed that the speed was excessive and ran to the switch 
box, missing by about six inches a piece of the rim of a 36-in. 
iron pulley; another piece went through the roof. Before the 
motor stopped the line shaft was bent and the boxes thrown 
out of line by the unbalancing of the pulleys. The dealer who 
sold the motor was held responsible for the damage, and there- 
fore had an examination made. The connections were found 
correct, but the rocker arm was loose and the brushes were 
about 60 electrical degrees back of the neutral position. 

Before the motor was started the brushes were set as closely 
to the neutral as could be by tracing out the armature leads. 
The line and armature wires were taken off the starter to test 
the magnetic switches, which were found to be working prop- 
erly. The line wire was then re-connected and the shunt field 
tested and found complete by observing the flash when the 
switch opened and by holding an iron bolt against the field 
poles to test the pull of each pole. With the belt off, the 
brushes set approximately correct and a current in the shunt- 
field, the armature connection was made and the switches 
closed, the motor started and ran as nicely as could be wished 
for. This little bit of carelessness in not testing the motor 
before putting on the belt caused a shutdown for seven hours, 
an expenditure of $24 in repairs and endangered the life of one 
man. 

Wrong Setting of Brushes for Direction of Rotation Caused 
Motor to Flash. If line voltage be applied to a shunt- wound 
motor without using any starting resistance in the armature 
circuit, the immediate result is likely to be the blowing of a 
fuse or the opening of a circuit breaker, because the armature 
resistance is so low compared to the resistance of the field 
winding that the latter is unable to get current for producing 
the field on which starting depends. In the case of a small 
motor, the fuse may hold and the motor may start without 
demonstration because the maximum current involved is small. 
A large motor is apt to flash and the brushes to burn owing to 



406 ARMATURE WINDING AND MOTOR REPAIR 

their being unable to handle an abnormal current. Shunt- 
wound motors operate better with their brushes set a little 
back of neutral than they do with the brushes set forward of 
neutral or on neutral, because with a slight backward shift 
field distortion helps commutation. 

In one case mentioned by E. C. Parham (Electrical Record, 
October, 1918), a 20-hp. motor had been overhauled and 
was to be given a light running test. As there was no 
starting box available, it was necessary to start the motor 
from full voltage through a switch and also a circuit breaker. 
On closing the breaker and then closing the switch, there was 
a heavy flash at the brushes followed by blowing of the circuit 
breaker, but the motor failed to start. Thinking that the 
flashing might be due to full voltage being applied to armature 
and field connected in parallel and without a starting resist- 
ance, the motor was re-connected so that its field became 
energized on closing the breaker, the armature receiving cur- 
rent on closing the switch after closing the breaker. An 
attempt to start again caused flashing and blowing of the 
breaker, but this time the armature actually began to turn 
and it was observed to have started to turn in the wrong 
direction. On reversing the field connection, there was no 
further demonstration. All of the trouble was due to the 
brushes having a forward shift when the motor was connected 
to run in the wrong direction. 

Proper Adjustment of a Reaction-type Brush-holder. 
The angle at which a reaction-type brush-holder is set on a 
commutator plays an important part in the life of the brushes 
and commutator. When the brush is set against the direction 
of rotation of the commutator, as shown in Fig. 257, the correct 
setting of the brush is of greater importance than when set in 
the opposite direction. If the brush is set too straight, the 
friction will pull it away from the holder and cause a chattering 
and sparking. If it is set too flat, there will be a tendency for 
the brush to wedge between the holder and the commutator. 
Some machine manufacturers using this style of brush-holder 
furnish a templet by which the holder can be accurately set, 
and in such a case it should be carefully used. 

R. L. Hervey (Electrical World, Jan. 22, 1916) relates a case 



METHODS USED TO SOLVE SPECIAL TROUBLES 407 




where he was called upon to locate the cause of sparking on a 
generator using this type of holder. Upon making inquiries 
it was learned that the engineer had purchased a sheet of high- 
grade carbon, so called, and made his own brushes. To save 
carbon and work, both ends were made with the same angle 
and the holder was turned to suit the brush. The brush 
length had been made so great 
that the arm pushed the brush 
away from the holder instead 
of against it. The length L 
(Fig. 257) should never be so 
great that end of arm B will be 
above the line AO drawn per- 
pendicular to the face and 
through the arm center 0. The 
manufacturers of this holder 
recommend that the holder be 
set as shown; nevertheless, there 
are a .great many installations 
working in the opposite direc- 
tion. 

Heating of Brush-holders Traced to Defective Contact 
Springs and Remedied by a Flexible Shunt. R. L. Hervey 
(Electrical World, July 1, 1916) relates the following conditions 
in a case where his attention was called to the discoloring and 
heating of brush-holders on several generators. The holder 
was of the parallel-motion type, as shown in the sketch, 
Fig. 258. To prevent the burning of the hinge joints of 
the parallel motion, phosphor bronze springs 0.010 in. thick 
were used to carry the current around these points. As long 
as the contact between these thin springs and the moving 
arms was perfect no trouble occurred. But as soon as any 
part of the contact became defective an overload was thrown 
on the remaining contacts, causing overheating. The total 
contact area of the springs on the holders of the same polarity 
was 0.360 sq. in. fo'r 250 amp. A properly designed contact to 
carry this amount of current should have 1% sq. in. of surface 
To relieve the trouble, the springs were taken out and replaced 
with a flexible shunt, as shown in the sketch. In such a case 



FIG. 257. Diagram showing 
proper setting for reaction-type 
brush-holder. 



408 



ARMATURE WINDING AND MOTOR REPAIR 



it should be mentioned that unless the soldering of the flexible 
wire into the lug is very carefully done the solder will get into 
the wire and stiffen it. In order to prevent this, the shanks 
of the lugs were flattened with the wire in it, forming a very 
strong electrical joint. 

The box type of construction of this holder afforded a sub- 
stantial contact between the holder and the brush. Due to the 
weight of the moving box and its supporting arms, however, 
this brush-holder was displaced by a stationary holder and 
sliding brush with a light spring. With a high commutator 
speed, the heavy brush and holder does not follow the irregular- 



Woren ff/re Shunf-. 



Sen* 




Spring 



FIG. 258. Substitution of woven wire shunt for springs of parallel motion 

brush-holder. 

ities of the commutator as well as the lighter brush. The 
brush that can move in its holder is better able to follow the 
grooves of the commutator as the armature weaves backward 
and forward in the bearings. Although the brush-holder is 
usually considered a minor part of the motor or generator, it 
has held the attention of the designing engineer for a good 
many years and still needs the attention of the operator and 
repair man. 

Simple Scheme for Banding Armatures. The method 
shown in Fig. 259 has been found convenient in one repair 
shop (G. H. Vescelius, Electrical World, July 19, 1919, page 
132) where a banding lathe and tension device was not avail- 
able and only one man to do the work. In using this scheme 
the free end of the banding wire is soldered to the armature 



METHODS USED TO SOLVE SPECIAL TROUBLES 409 

anywhere and the latter rotated by means of a "dog" and 
pipe as indicated in the illustration until the wire has filled 
all of the banding grooves. In performing this operation no 
tension is exerted on the banding wire. Next, the remaining 
end of the wire is soldered to the armature and a weighted 
pulley hung on a loop of the banding wire nearest this end. 

By selecting a weight twice as heavy in pounds as the de- 
sired tension in pounds per square inch the wire will be drawn 
up to the correct tension by rotating the armature backward. 
At any time when it is necessary to adjust the position of the 
banding wire this can be done easily by the person turning 
the armature because the tension will not be relieved as would 
happen when banding in the ordinary manner. 




FIG. 259. Weighted pulley in place for producing tension in banding wire. 

Use of a Crane to Band an Armature in an Emergency. 

In a large railway substation it was necessary to replace the 
banding wire on the armature of a 1000-kw. rotary con- 
verter. The bands required about 20 turns of steel wire. The 
scheme tried at first was to mount the steel wire reel so as to 
turn- freely and pass the wire through a tension regulator con- 
sisting of two fiber-lined blocks bolted together. The wire 
was then wound on the armature as evenly as possible by 
turning it with bars, a method which was found slow and 
clumsy. The rotary was of the type that is brought up to 
speed with an induction motor with its rotor mounted on the 
shaft of the rotary armature. The idea was therefore con- 
ceived of removing the stator of the induction motor and using 
the rotor as a drum or pulley to rotate the rotary armature. 



410 ARMATURE WINDING AND MOTOR REPAIR 



Three turns of a 1.25-inch rope were wound on this rotor and 
one end fastened to the hook of a traveling crane. The crane 
was then run the full length of the station with one man feeding 
the rope to the drum and keeping it taut. In this way a 
smooth, tight band was wound on the rotary armature with 
considerable saving in time over turning it by hand. 

Method Used to Band a 2000-Hp. Rotor. The banding of 
armatures and rotors, when done in a factory where such jobs 
are an every-day occurrence, has been more or less reduced to a 
science and the methods of procedure are many. When, 
however, the rotor to be banded reaches the 2000-hp. mark, 

Banding 
Wire 



Wire Tens/on Device. Spool. 




- Motor. 

Crane 
Trolley 



'-Flywheel. 



FIG. 260. An arrangement devised to band a 2000-hp. motor armature. 

it is out of the question to think of transporting it to the repair 
shop. Such a problem came up in a steel mill at one time when 
a 2000-hp. motor which operated the rolls became seriously 
grounded. The following method was decided upon (Maurice 
S. Clement, Electrical Record, April,. 1919) : The entire trolley 
of a traveling crane was lowered to the floor and placed in line 
with a huge fly-wheel which was On the same shaft as the rotor. 
The cable drum of the crane trolley was converted into a belt 
drum by covering it with canvas which was held in place by 
means of friction tape. This made a more or less flat surface 
and eliminated both the cable grooves and the grease from the 
face of the belt. 

As the lower half of the fly-wheel was in a pit, it was nee- 



METHODS USED TO SOLVE SPECIAL TROUBLES 411 

essary to place a roller at the edge of this pit as a belt runner. 
Another roller was placed on a frame somewhat resembling 
a large brace, both ends of which were secured to the floor, 
halfway between the crane trolley. The fly-wheel was ar- 
ranged so as to ride the belt and serve as a belt tightener. The 
banding wire tension device used in this case was similar in 
principle to those used on wire nail machines, the sheaves 
being adjustable in the slots so as to tighten or slacken the wire 
at a moment's notice. 

An Improvised Method Used to Turn a Commutator. 
In a case where a rush order was received by a repair shop 
to turn the commutator of a generator at a large country 
house, R. L. Hervey (Electrical World, June 10, 1916) de- 




FIG. 261. Method of using lathe cross-feed in turning a commutator. 

scribes the following method of doing the work when the small 
turning rig which was taken along was found too small. In 
the garage, however, a large engine lathe was found. This 
started an examination to determine whether the armature 
could be taken out and swung in the lathe. This promised 
impossible. While looking around the shop an angle plate 
was uncovered. This plate and the cross-feed of the lathe 
were bolted together by a special screw of the rig. The plate 
was then bolted to the end plate of the generator frame, as 
shown in Fig. 261. There was but one bolt holding the angle 
plate on the generator frame, which made the equipment look 
very unsteady. A hole in the angle plate was selected so that, 
if the plate turned around the bolt, the tool would swing 
away from the commutator and not into it. The screw of the 



412 ARMATURE WINDING AND MOTOR REPAIR 

cross-feed was too short to take a cut across the commutator, 
therefore the tool had to be re-set for each cut. This was a 
little troublesome as the tool had to be tapped into position 
with a hammer. The commutator had grooves in it about 
% inch deep, requiring considerable cutting. However, the 
job was satisfactorily done with this arrangement. 

The generator was direct connected to a low-speed horizontal 
gasoline engine. While working on the commutator the valves 
were taken out to prevent compression, and the turning effort 
provided by a 1-hp. motor belted to the fly-wheel. The elec- 
trical equipment of the plant consisted of the generator and a 
storage battery of 58 lead cells. These supplied current for 
the lights, a motor-driven ice machine, laundry, water-pump 
and machine tools in the garage. 

Cause of Motor Reversing its Direction of Rotation on 
High Speed. R. L. Hervey has described the following pecu- 
liar performance of a motor (Electrical World, Jan. 20, 1917) : 
A 2-hp. 220-volt direct-connected compound motor driving a 
band saw had been in service for approximately 18 months 
without giving the slightest trouble until the operator 
wanted to work the saw at its highest speed, which was ob- 
tained by using a field rheostat. When all of the resistance 
was cut in the field circuit the motor slowly came to a stop and 
started in the opposite direction. This was tried several 
times, and resulted in blowing the fuses most of the times. 
There could be but one condition that would cause this re- 
versal of direction of rotation, namely, the compound winding 
being in opposition to the shunt field and overcoming it when 
the shunt field was weakened. By reversing the compound 
field leads the trouble was removed. This motor had been in 
operation for a year and a half without any indication of 
anything being wrong. The brushes and commutator were 
in excellent condition, which speaks well for the design of the 
machine. 

Checking Connections of an Inter pole Motor. The polarity 
of the interpole field coils should be the same as that of the 
preceding main poles in a direction against the rotation. The 
polarity of the interpole field with relation to the main fields 
for both directions of rotation of motors and generators is 



METHODS USED TO SOLVE SPECIAL TROUBLES 413 

shown m Fig. 262. To determine the proper connections of 
interpoles proceed as follows: 

As a rule one end of the interpole field and one armature 
lead are permanently connected together, and since the correct 




FIG. 262. Polarity of interpole fields with relation to main fields for direc- 
tions of rotation of motors and generators. 

polarity is determined by the manufacturer, there is no oc- 
casion to change it. To reverse the direction of rotation the 
armature and the interpole field must be reversed as a unit. 
Reversed interpole connection is not noticeable at light load. 
As load is applied, a motor with reversed interpoles will spark 



Line 



Line 




Series Ft'efd * 7/tf Field 

Armature Armature 

FIG. 263, Connections for testing out correct connections of interpole 
windings for proper armature rotation. 

badly and drop in speed, while on a generator difficulty will be 
experienced in maintaining rated voltage. Assuming that one 
end of the interpole field and one armature lead are perma- 
nently connected, wire the machine temporarily as a series 
motor with the shunt field disconnected. Care should be taken 



414 ARMATURE WINDING AND MOTOR REPAIR 

to apply the current only momentarily to prevent too high 
an armature speed. 

The connections for this test are shown in Fig. 263. If direc- 
tion of rotation is wrong, interchange terminals 3 and 4 or 1 
and 2. Having secured proper direction of rotation with the 
series connection, insert the shunt field and open the series field 
as shown in Fig. 263. Upon starting up again as a shunt motor, 
the direction of rotation should be the same as it was with the 
series field. If it is not, interchange terminals 5 and 6 and 
make up permanent connections. The correct polarity of the 
interpoles may then be checked with a compass or by bridging 
a main pole and interpole with a piece of iron, bearing in mind 
that unlike poles attract and like poles will repel it and check- 
ing with the diagrams in Fig. 263. 

Heating of Field Coils Traced to Wrong Type of Starting 
Box. On motors that are designed for field control of speed, 
E. C. Parham has pointed out (Electrical Record, February, 
1919) that the shunt winding is likely to be lighter than that 
of a motor where the shunt winding is to be subjected contin- 
uously to full operating voltage. Therefore if the shunt field 
of such a variable speed motor is connected to full voltage con- 
tinuously, it will heat beyond the usual temperature guarantees. 
If the line voltage happens to run well above normal, as it often 
does, the temperature will reach a value liable to injure the 
insulation and ultimately to short-circuit the winding. 

The owner of a machine shop at one time asked that a man 
be sent to find out why the speed of his motor was so low and 
why the motor got so hot after several hours of working. As 
this was all the information obtainable, the inspector took a 
voltmeter and an ammeter that could be used on either direct- 
current or alternating-current circuits, only to find out later 
that no instruments were required. The motor in question 
proved to be of the variable speed, field control, type. There 
was no name plate on the motor, but the type was inferred from 
the type of resistance box which had been furnished with it 
but which was not being used. The motor was being operated 
as a constant speed motor on voltage that was 15 per cent, 
above normal. Of course the maximum speed obtainable was 
that due to full field hot. The field coils heated too much but 



METHODS USED TO SOLVE SPECIAL TROUBLES 415 

did not heat sufficiently for their increased resistance to give 
the speed required. In order to bring the speed to the nec- 
essary value and at the same time relieve the field coils, the 
resistance box, which had many taps, was connected in series 
with the field coils and different tap wires tried until one was 
found that gave approximately the proper speed. As the 
load of the motor was comparatively light there were no ob- 
jectionable features attending the starting of the motor on a 
weakened field, so the connection was made permanent with 
the understanding that the operator would immediately get a 
starter that was adapted to the work. 

Safe Operating Temperature of Portable Desk Fans. 
A well-known manufacturer of fan motors points out that 
warm weather always brings a number of complaints in re- 
gard to heating of small motors. These complaints are usually 
from dealers or users who are possessed with the idea that if 
the motor-body does not feel nice and cool to the hand, the 
windings are in imminent danger of burning out. The latest 
standardization rules of the American Institute of Electrical 
Engineers provide that, in motors with the class of insulation 
used in reliable makes of motors, the temperature as recorded 
by a thermometer, should be within a limit of 80 C. This 
is equivalent to 176F. No one would care to place his 
hand in contact with the frame of a motor at anywhere near 
that limit. 

Let us say, for instance, that a small motor has a tempera- 
ture rise of 40F. when operated continuously. Such a motor 
operating on a day when the normal air temperature is 70, 
would reach a temperature of only 110, which would feel only 
comfortably warm to the hand. The same motor, operating 
on a mid-summer day when the thermometer runs from 90 
to 95, will attain a temperature between 130 and 140 F. 
Any temperature over 120 is quite uncomfortable to the 
touch and gives rise to alarm on the part of the inexperienced 
motor user. 

However, a motor with a surface temperature of 140 
is in no danger of injury from overheating, and motor 
users who make complaint of the heating effect under such 
conditions should be advised to continue operating their 



416 ARMATURE WINDING AND MOTOR REPAIR 

motors until it is apparent that the windings are being dam- 
aged. Unless there is some odor of burning insulation, the 
motor can not be considered in danger. 

An Adjustable Shunt for Series Fields of Exciters. In 
adjusting the compound characteristics of three exciters for 
parallel operation in conjunction with a Tirrill regulator, it 
was found impossible to get the necessary series-field shunt 
adjustment with the shunt taps provided by the manufac- 



CD 



' 0.010 'German Silver 




Boned to Facilitate Ventilation 

Toffacn/ne. To Conduit 
To^Series Field terminal^ \ 



Section cut from one 
Strip to obtain Fine 
Adjustment-.. 




Series Shunt which was Pep faced 

FIG. 264.- Construction of series-field shunt showing bowed formation to 
facilitate ventilation. 

turers. Consequently each shunt was replaced by a very 
simple -and easily adjustable shunt like that shown in Fig. 264, 
(Sydney Fisher, Electrical World, June 10, 1916). By means 
of these the characteristics of the machines were made practi- 
cally identical. The shunt was made of strips of German 
silver slotted to permit the bowed formation shown. Rough 
adjustments were made by adding or removing strips, and 
finer adjustment obtained by varying the cross-section of 



METHODS USED TO SOLVE SPECIAL TROUBLES 417 

one of the strips. The free passage offered to air through the 
shunt permits good ventilation with resulting low temperature 
from no load to full load. The construction of the shunt 
permited using the minimum number of connections, therefore 
they can be made much more readily than where more con- 
nections are required. This is an important consideration 
since the resistance of poor contacts becomes appreciable, 
the series-field resistance being small. 

A Peculiar High-speed Motor Trouble. On one occasion 
R. L. Hervey (Electrical World, Oct. 28, 1916) explains that 
a 500-volt 10-hp. direct-current motor, running at a speed of 
3400 rpm., continued to open its circuit breakers every time 
an attempt was made to start it. The wiring contractor, 
after assuring himself that the connections were correct and 
that the trouble was inside the motor, refused to give more 
time to the job. The local representative of the machine 
company claimed that every machine turned out by his 
factory was given a 24-hour test, therefore the trouble 
could not be in the motor. 

The motor was a shunt-wound, bipolar design, with two 
commutating poles. The three leads were marked A, F, 
and C, which were assumed to mean armature, field and line. 
After testing out the wiring between the starting box and the 
motor, the connections were made as stated above. Since 
the inertia of the machine parts to be started was large and 
the speed quite high, a large starting current was expected, 
so that the circuit breakers were set for their maximum current. 
An effort was made to start the motor to test the connections 
and also to observe its action when the voltage was applied, an 
ammeter having been connected in the circuit so that the 
starting current could be measured. When the starter 
handle reached the third button the current was 50 amp. 
The circuit breakers opened and the starter was smoking 
badly. These results indicated an open-field circuit. The 
internal connections of the motor were checked with consider- 
able difficulty. The lead marked A was found to be the line 
and C the armature. After making this change the motor 
was started again with the same results. The machine 
representative objected to the motor being taken apart, but 

27 



418 ARMATURE WINDING AND MOTOR REPAIR 

yielded to persuasion. A small resistance tube connected in 
the shunt-field circuit was found in the bottom of the frame. 
While this tube could not be reached for examination, it was 
suspected of being the cause of the trouble, so a jumper was 
connected around it. Another effort to start the motor was 
successful. The motor speed was 3350 rpm. with a line 
voltage of 575. The resistance, after being patched, was con- 
nected in the field circuit when the speed measured 3730 rpm. 
As the motor was built for 3400 rpm. at 500 volts, it was 
thought that the factory test was made at that voltage and the 
resistance put in the field to bring up the speed. Since, how- 
ever, the voltage was 575 at the place of installation, the speed 
of 3350 rpm. was high enough and as the resistance had open- 
circuited once, it was decided to leave the resistance out to 
prevent future trouble. 

Ways that End-play Variations Show Up. The principal 
purpose of having end-play in the armature of a motor or of a 
generator is to keep the brushes from tracking in the same 
path and thereby wearing a groove in the commutator or in the 
collector rings as the case may be. To prevent such wearing 
of grooves, the armature end-thrust clearances are so disposed 
that when the armature is running in its normal zone, which is 
governed by the pull of the pole-pieces on the armature core, 
the thrust clearances at the two bearings are equal. Assuming 
the correct gear or pulley alignment to the connected load, the 
armature will never run sideways far enough to knock the 
bearing on either side; because when it has run over a certain 
distance, the field will pull it back toward and past the mag- 
netic center. The same action will then be repeated but in 
the opposite direction. If, however, the end-thrust is un- 
equally distributed for any reason, or if the machine as a 
whole is not level, knocking will occur which in the case of 
small, high-speed machines may take the form of serious 
vibrations that are likely to pound down the bearing 
linings. 

E. C. Parham (General Electric Review, January, 1916) has 
mentioned a case where four motors of fractional horsepower, 
which were direct connected to tool grinding wheels, were 
installed and three of the outfits gave entire satisfaction, but 



METHODS USED TO SOLVE SPECIAL TROUBLES 419 

the fourth one vibrated so badly that it shook the whole 
supporting structure. The rotating elements of two of the 
units were then interchanged and the vibrations remained 
with the same rotor. Spinning the rotating element of 
the troublesome machine in a high-speed lathe proved 
the shaft to be perfectly straight. Close inspection dis- 
closed that the end-play was not distributed on the faulty 
outfit in the same manner as on the faultless ones. All 
vibration was eliminated by loosening the thrust collar so 
that the rotor when running could center itself, then starting 
the motor and letting it slow and stop, and then tightening 
the collar. 

Connections for Two 220-volt Motors When Operated on 
440 Volts. It is practical to operate two 220-volt 7-hp. 



Mofor-pu//ey 



fields are to be separ- 
ate, disconnect at X and 
run f. to field on start i, 




''-Driver 



Driven- 



Fields 



Fields 



FIG. 265. Belt arrangement for two motors operating in series and diagram 
for connecting two shunt motors in series. 



motors in series on 440 volts and secure satisfactory results, 
providing that the motors are approximately the same as 
regards windings and speed. If the drive is by a belt and the 
motors run at the same speed a good plan is to set the motors 
with the pulleys in line and run a short belt from one to the 
other. That is, first belt the motors together by a short belt 
and then put the regular belt over the short belt as shown in 
Fig. 265. Another way is to couple the two armature shafts 
together with a suitable coupling and attach the driver pulley 
on one of the armature shafts. 

The connections for two shunt-wound motors in series are 
shown in Fig. 265. The connections for two compound-wound 
motors in series and the connections for one shunt-wound 



420 ARMATURE WINDING AND MOTOR REPAIR 



motor and one compound-wound motor in series in Fig. 
266. (Frank Hoskinson, Electrical Record, September, 1917.) 
In case one of the motors is higher in speed than the other 
one, it may be advisable to use different sizes of pulleys so as 
to secure the proper speed. 



Starting Box 
Ling Field .Arm. 




Starting Box 

Field\ ^ 7 - 




Shunt Fie/cfs 



, 

Shunt Fields. Series Fields 

Shunt Motor Compound Motor 



FIG. 266. At the left, diagram for connecting two compound motors 
in series and, at the right, series connections for one shunt motor and one 
compound motor. 

Cleaning Motors with Compressed Air. When using com- 
pressed air to clean out dirt and dust from motor windings 
the pressure should not be higher than 100 Ibs. gauge. The air 
can be applied by the use of a rubber hose about J^ inch in 
diameter fitted with a short piece of ^-inch iron pipe to act 
as a nozzle and direct the air into the windings so as to blow 
out accumulated dirt. 

Most motor windings are impregnated or painted with some 
sort of insulating varnish and this usually presents such a 
smooth surface that air, even at extremely high velocity, is 
not likely to lift the edges nor tear the insulating fabric. If 
the dust that accumulates in the motor is of an abrasive char- 
acter, it is by all odds more advisable to use a higher pressure 
than mentioned and get rid of the dust than to let the dust pile 
up until it stops ventilation of the windings and perhaps even 
cuts the insulation. 

Testing out Phase -rotation. Manufacturers of alternating- 
current generators generally have a standard direction of rota- 
tion clockwise, for example and likewise there is adopted 
a standard direction of phase-rotation. This is necessary, 
because alternators that are to be operated in parallel must 
have their phase-rotations the same. The adopted phase- 



METHODS USED TO SOLVE SPECIAL TROUBLES 421 

rotation bears a definite relation to the numbering of the 
alternator terminal blocks. For example, the blocks of a 
three-phase alternator would be numbered 1, 2 and 3 and the 
standard phase-rotation would be in the same order. The 
phase-rotation of every alternator produced is tested by means 
of a phase-rotation tester. The tester is virtually an induction 
motor in fact, a standard motor could be used as a phase- 
rotation indicator. The rotor of the tester, however, is simply 
a vane of iron free to turn under the influence of the stator 
magnetism. The stator winding of a three-phase indicator 
has three terminals, which are marked 1, 2 and 3. These 
terminals are connected to corresponding terminals of the 
alternator the phase-rotation of which is to be tested. Re- 
duced voltage is applied to the stator of the alternator and 
the direction of rotation of the vane of the indicator shows the 
phase-rotation of the alternator. 

When an induction motor is used as a phase-rotation indica- 
tor it should be connected first to the supply end of one ma- 
chine and then to the other. If the motor rotates in the same 
direction it shows the phase-rotation of the two machines to 
be the same. If the rotation reverses, one phase of one of the 
alternators should be reversed. 

An Induction Motor Trouble Due to Wrong Stator Con- 
nections. The importance of checking up connections of 
induction motor windings is shown by the following trouble 
experienced by A. C. Hewitt (Electrical World, Jan. 16, 1916). 
In a cement plant where a 50-hp., three-phase, 440- volt, 
60-cycle squirrel-cage induction motor operating at 450 
rpm. was used to drive a 33-inch Fuller mill for pulverizing 
limestone the motor on several occasions was overloaded 
by feeding the material to the mill too fast and in large sizes. 
This overloading, combined with fluctuations of voltage as 
much as 30 per cent, below normal rating, caused the motor to 
heat and finally called for the replacing of several stator coils. 
The motor was repaired by the plant electrician and again 
placed in operation, but still it ran hot, with the mill showing a 
smaller output than a duplicate installation where the motor 
was running cooler. 

When called to investigate the installation the repairman 



422 ARMATURE WINDING AND MOTOR REPAIR 

noticed that the upper half of the motor was much hotter than 
the lower half. An inspection of the connections seemed to 
show that they were correctly made and that there were no 
grounds or open circuits. The fuses were in good condition 
and the auto-transformer motor starter operated satisfactorily. 
All of the connections for the starter windings checked with the 
diagram furnished with the motor. 

The winding was a two-circuit delta arrangement and 
the motor had 16 poles. In checking over the number of 
poles in each of the two circuits it was finally found that the 
winding was divided in a horizontal plane through the stator, 
and that instead of each circuit having eight poles the upper 
circuit had only seven and the lower had nine poles. This 
explained why the upper half ran hotter than the lower. The 
electrician had made a miscount when connecting up the 
windings, with the result that the load was unevenly distrib- 
uted. The connections were changed so that each circuit 
contained an equal number of poles, and the motor operated 
without the heating trouble previously experienced. 

Stalling of Wound Rotor Induction Motor Explained. 
A complete open circuit in the wound rotor of a polyphase 
induction motor, will prevent its starting because under such 
conditions there can be no secondary current. Such a com- 
plete open circuit may be caused by two of the rotor brushes 
being so stuck in their holders as to make no contact with the 
collector rings. An exception might obtain in the cases of 
fractional horsepower motors which are so small that the 
rotor may start by virtue of the eddy currents induced in 
the rotor laminations. However, the torque due to such re- 
actions could support no load. The function of energy circuit 
of a repulsion induction single-phase motor is similar to that of 
the wound circuits of the rotor of a polyphase induction 
motor, in that both are the seat of the induced current by 
virtue of which the motor is able to do its duty Therefore 
a complete break in the energy circuit of a repulsion induction 
motor, will render the motor unable to start. That an in- 
complete open circuit in the energy circuit, may produce 
different results, is illustrated by the following experience. 

A butcher complained that his motor-driven meat grinder 



METHODS USED TO SOLVE SPECIAL TROUBLES 423 



which had been working satisfactorily for months, was develop- 
ing a tendency to slow down when coarse stock was fed into 
the hopper. With the finer stock, the motor apparently could 
do its work all right. On operating the motor with the end 
cover removed, vicious sparking was seen. At first this was 
thought to be caused by armature trouble but a close inspec- 
tion disclosed that the sparking was due to the energy brushes 
being stuck in the brush-holders. The brushes had worn so 
short that the brush shunts were jammed into the boxes and a 
few more jobs of grinding probably would have burned and 
worn the brushes entirely out of contact with the commutator. 
Even with the arcing, the energy current had been sufficient 
to support the lighter loads, although after installing new 
brushes, it became evident that the motor had been operating 
at reduced speed at all loads. 

Loose Bearing Caused Induction 
Motor to Fail to Start. H. Wilson 
(Power, August 5, 1919, page 231) 
describes the following experience 
after repairing a large three-phase 
induction motor, when an attempt 
was made to put it back into 
service. On closing the compensa- 
tor switch to the starting position, 
the motor failed to start, although 
it was evident from the sound of 
the machine that it was getting 
current through its winding. The 
first thing that suggested itself was 
an open circuit, so the starting com- 
pensator was tested to see if the 
current was coming through single- 
phase only. In order to do this 
quickly, we disconnected the motor 
lead at the machine, and while the 
switch was held on the starting 
position a test lamp was connected across the different leads, 
as in Fig. 267. This showed current on each phase, at a 
reduced voltage, of course. We then put the switch on the 




3-Phase 

Motor 



FIG. 267. Method of testing 
for open-circuit. 



424 



ARMATURE WINDING AND MOTOR REPAIR 




running position and the test 
lamp burned brightly across 
each phase, showing that the 
motor was getting the current 
all right. 

The rotor seemed to be rub- 
bing a little on the stator, as 
it was somewhat hard to turn, 
so the clearance was adjusted 
by means of two draw-bolts 
on the bearing housings. This 
took some time, since when the 
bearings were moved one way 
a little, the rotor would bind; 
then we would shift it back 
slightly until finally getting it 
into a position where the rotor 
turned freely. Another at- 
tempt to start motor met with 
no better result than the first. 
The next step was to trace out 
the winding connections, which 
were found to be apparently 
correct. The winding being 
connected two parallel star 
made it somewhat complicated 
to trace out. However, by 
looking to see if the connec- 
tions went under or over the 
winding, I would mark an 
arrow on the group of coils to 
show the polarity. We went 
around each phase in this way, 
starting with the outside lead 
in each case. The arrows on 
each group pointed alternately 
in opposite directions, show- 
ing the connections to be cor- 
rect, as in Fig. 268. 



METHODS USED TO SOLVE SPECIAL TROUBLES 425 

The next suggestion was to test the polarity of the winding 
with a compass, when direct current was flowing through the 
coils. As a source of direct current an automobile starting 
and lighting battery was pressed into service. The three 
motor leads, A, B and C, Fig. 268, were joined together and 
connected to one terminal of the battery. A wire from the 
other battery terminal was taken to the common or neutral 
point on the winding, where the three phases connected 
together, such as A*, B* and C*, Fig. 268. This connection 
was not attached permanently, as it would have run the 
battery down, owing to the low resistance of the circuit through 
the winding, but was attached each time only while the swing 
of the compass opposite each group was obtained. This test 
proved the connections to be correct, as the compass needle 
pointed in opposite directions on alternate groups as it was 
moved around the winding. 

Since the polarity of the coils was undoubtedly correct, it 
left us all puzzled as to the cause of the trouble, as we had 
tested for grounds in the winding, open circuit, polarity, 
tested for the power at the motor terminal, and also mechanic- 
ally, to see that rotor was free. 

We had about decided to give the job up and send the 
machine to the manufacturers to have it fixed up, when one 
of the engineers came in on the job and after hearing our 
story remarked that the motor had given similar trouble, 
two or three years previous, after being repaired, due to the 
bearing letting the rotor rub on the stator. This gave us an 
idea that there might be a slight looseness in the bearing, 
sufficient to let the rotor lift up and rub on the stator. Acting 
on this, the bearing bolts were adjusted until the rotor would 
just clear the bottom of the stator. This proved to be the 
source of all the trouble; on closing the compensator the motor 
started up and ran satisfactorily, greatly to the relief of every- 
body concerned. 

Three-phase Motors used on Single-phase Lines. It 
often happens that utility customers have a supply of three- 
phase motors when only single-phase service is available, or 
that a customer is asked to purchase single-phase motors until 
the load becomes large enough to justify a three-phase line 



426 



ARMATURE WINDING AND MOTOR REPAIR 



extension. This difficulty can be overcome by a system pat- 
ented by Professor Arno in which the inherent characteristics 
of three-phase motors are developed from a single-phase source 
of supply. 

The system, which is used extensively in Australia, calls for 
the use of a three-phase master motor in addition to the three- 
phase power motors. The master motor is a standard machine 
of either the squirrel-cage or wound-rotor type, and the larger 
the better. The lower size limit of this motor is in practice 
about 10 to 15 per cent, of the total load connected to the sys- 
tem and is at least double the size of the next largest motor. 




MASTZRHOTOR S^IKRIL CA6f HOTOK5 iUPKlNOHOTOK SOUIRXLCAGtJIcm 

FIG. 269. Method of operating 3-phase motors from single-phase line. 
The four motors to the right are power motors, and the one to the left is a motor 
which has its third phase connected to the third phase of all the other motors. The mas- 
ter motor is started up as a split-phase induction motor and runs light. Its function is 
to supply auxiliary current to the third phase of the other motors, and by this means 
the 3-phase power motors are given practically the same characteristics as if they 
were operated from a 3-phase line. 

All motors, including the master motor, are connected to the 
single-phase supply mains, and the third phases, which are 
not connected to the source of supply, are connected together. 
The master motor is started up under no load with special 
starters equipped with an auxiliary winding similar to that 
used for the starting of split-phase induction motors. The 
master motor should as a general rule run unloaded as a phase 
giver. It is permissible, however, to load the master motor 
up to about 25 per cent, of its normal rating in special cases, 
but even when the master motor is running mechanically 
loaded, it may be electrically overloaded. Any of the power 
motors becomes a master motor immediately its load is thrown 



METHODS USED TO SOLVE SPECIAL TROUBLES 427 

off, and if the master motor cannot properly perform its func- 
tion, a small unloaded power motor may become overloaded 
in helping out the master motor. 

Through the voltage induced in the master motor's third 
phase, which is not connected to the supply mains, such motors 
at full speed are able to supply auxiliary current to the loaded 
motors during starting and overload periods. As long as the 
master motor is running the overload capacity of the loaded 
motors is raised, as compared with purely single-phase induc- 
tion motors, and the three-phase motors are able to start 
up from the single-phase line with practically the same start- 
ing torque as they would have when connected to a polyphase 
line. Should all the motors including the master motor be 
loaded to the same extent in proportion to their rated capacity, 
no current would flow in the third-phase connections. After 
the master motor has been started up, the working motors can 
be started one after another as regular standard three-phase 
motors. The gain in efficiency by operating the individual 
motors as three-phase units is practically offset by the losses 
in the master motor, so that the overload efficiency of the 
whole installation is about the same as that of a straight single- 
phase system. The normal output of the power motors when 
operating in this manner is about 75 to 80 per cent, of their 
standard three-phase rating. 

An Apparent Overload Trouble That was Traced to a Defect- 
ive Fuse Block. R. L. Hervey (Electrical World, June 24, 
1916) relates the following trouble at a plating plant where a 
5-hp. single-phase, induction motor, from apparent overload 
continued to blow fuses for several days and then refused to 
start. The motor had been in daily use for six years and the 
repairman, after examining the equipment, said that new 
bearings were needed as the rotor was rubbing the stator 
laminations. The shaft was also cut and grooved, which 
required it to be turned down so that the bearings could be 
properly fitted. After the motor had been replaced and in 
service for a few days the fuses started blowing again. 

The repairman found one of the bearings hot, which was 
taken out and " eased up" a little. A few days later another 
shutdown occurred. This time the repairman reported that 



428 ARMATURE WINDING AND MOTOR REPAIR 

the motor was overloaded, and a larger motor was required. 
As the plant was being operated exactly as it had been for three 
years without a shutdown, the owner of the motor would not 
accept the report, and asked that a thorough examination 
be made to locate the trouble. 

When putting in new reinforced fuses it was noticed that 
the fuse clips and block had been quite hot, for they were 
discolored and loose. The clips were closed up until they 
held the fuse tightly. A close examination of the motor and 
starter showed them to be in first-class condition. An amme- 
ter was connected in the line and the motor started with no 
load. The current taken indicated that the winding was not 
defective. The load of two buffing wheels was then put on 
and the needle of the 50-amp. meter went off the scale. A 
further examination ' of the fuse block showed two loose 
connections. It was the heat generated by this high contact 
resistance that was causing the trouble with the fuses. It is 
very probable that while the rotor was rubbing the stator 
the motor was taking an excessive current and injured the 
fuse block. This instance shows the need of a careful inspec- 
tion of all wiring when a repaired motor is again connected 
in circuit. 

Cause of Noise in a Three-phase Motor Driving an Exhaust 
Fan. The frequency of the current that is applied to the 
stator of an induction motor in commercial operation, ordinar- 
ily is constant within narrow limits if the speed of the prime 
mover to which the current is due, is as constant as it should 
be. The frequency of the current that is generated in the 
rotor of the motor, however, varies with the speed of the rotor, 
being greatest when the rotor is at rest and least when the 
rotor is operating at maximum speed. This is because the 
frequency of the rotor current depends on the difference of the 
speeds of the rotor and of the rotating field. In other words 
the frequency of the rotor current depends on the slip of the 
rotor. The principle of operation of frequency indicators of 
the vibrating type is, that the pendulum properties of each 
reed are such that it will respond synchronously to the im- 
pulses of but one value of frequency. On the same principle 
if a miscellaneous lot of pieces of iron and steel be subjected 



METHODS USED TO SOLVE SPECIAL TROUBLES 429 

to the flux of an alternating current of varied frequency, some 
of the pieces will take up vibration at one value of frequency 
and other pieces will not be effected by that value of frequency 
but will take up vibration at some other value of the frequency. 

In one case a large three-phase induction motor that was 
connected to an exhaust fan, was complained of on account of 
noise emitted and transmitted to all parts of the building when 
the motor was being started. The noise was described as a 
" clatter. " An investigation disclosed that the noise occurred 
between narrow speed limits obtainable on the second notch 
of the controller and the sound was similar to that of rubbing a 
tin can with a piece of sandpaper. By throwing the controller 
back and forth between the first and second notches, there 
was obtained an average speed at which the noise was main- 
tained almost continuously. By listening with the ear applied 
to different parts of the machine, and at the same time touch- 
ing the machine here and there with a lead pencil, the source of 
the trouble was located in one of the slots in which slide the 
bolts that hold the motor frame to the sliding base. On each 
side of the machine extending from one bolt to the other, a 
steel strip was provided in order to keep the head of the bolt 
from turning when there was occasion to turn its nut with a 
wrench. The strip was so " bellied" that there was about J^g 
inch clearance between the middle of the strip and the bottom 
of the machine which effectively constituted a reed supported 
at both ends, and the time element of the construction was 
such that the strip vibrated in response to rotor current fre- 
quency that obtained on the second notch of the controller. 
On removing the strip and re-bending so as to bring the bulged 
part up against the bottom of the rail, no further trouble was 
experienced with noise. 

Cause of a Burned Out Starting Winding in a Single-phase 
Motor. Unless of the commutator type, a single-phase 
motor, in order that it may be started without manual assist- 
ance, must include some phase-splitting device that is effective 
in producing the rotating field so necessary to self -starting 
of such motors. The phase-splitting device usually takes the 
form of an auxiliary starting winding and of an arrangement 
of resistance and inductance or of resistance and reactance, 



430 ARMATURE WINDING AND MOTOR REPAIR 

so disposed that the currents in the main stator winding and 
in the starting winding shall be out of phase with each other. 
As the starting winding is intended to be active for only a 
second or more, it is not proportioned to stand continuous 
application of the voltage. Accordingly, a centrifugal switch 
that turns with the rotor, is provided for automatically cutting 
out the starting winding as soon as the rotor approaches full 
speed. If for any reason the starting winding is left in circuit 
for appreciable time, the winding is likely to be injured. 

In a certain instance (Electrical Record, August, 1918) one 
of these motors stopped a few minutes after having been 
started for the first time and, as the owner expressed it, "the 
motor had no more life in it." The motor was of the resistance- 
inductance self -starting type, in which the phase splitting was 
accomplished without the use of any external resistance or 
reactance. Examination of the motor disclosed that its 
failure had been due to the main stator winding being in 
series with the automatic switch which should have included 
only the starting winding. The motor would start all right, 
but the opening of the centrifugal switch would cut out the 
main winding, leaving the starting winding connected across 
the line. It developed that the motor had been bought at a 
low price because it was in bad condition and had been re- 
wound by a local repair man, who did o good job of winding, 
but a poor job of connecting. 

Cause of One Motor Failing to Start While Another was 
Running on the Same Circuit. At an industrial plant where 
440- volt, three-phase, 60-cycle service was used to operate 
motors of many different sizes, a peculiar trouble was experi- 
enced by A. C. Hewitt (Electrical World, Jan. 8, 1916) when 
one of three fuses was blown on a feeder circuit. One feeder 
circuit supplied some four or five motors, one of which was 
rated at 75 hp. and the others at 10 hp. each. Each motor 
was equipped with a knife switch, fuses and starting compensa- 
tor, and the feeder circuit was fused at the distributing switch- 
board. The 75-hp. motor was delta-connected, while the 
10-hp. motors were connected " Y" (Fig. 270). 

The 75-hp. motor was carrying a load of about 50 hp. Only 
one of the 10-hp. motors was in use, and it was driving a 7-hp. 



METHODS USED TO SOLVE SPECIAL TROUBLES 431 



load at the time the trouble developed. Both motors had 
been running for about four hours when it became necessary 
to stop the small motor for a few minutes. Upon trying to 
operate it again it refused to start. The electrician tested 
the motor fuses with two 250-volt lamps in series, without 
removing the fuses from the circuit, and found them good. 
The lamp test seemed to show, however, that the voltage be- 
tween the middle and either outside wire was not as high 
as it should be. He then tested the fuses on the 75-hp. motor 
while the motor was running and found tho same conditions 



r Leads to Other Motors--* 



Fuses 




Starting 
Compensotor 



lOHp 
Motor 



75 Hp Motor 
FIG. 270. Connections when 10-hp. motor failed to start. 

as with the 10-hp. motor. Evidently something was wrong, 
and with reduced voltage between the middle and outside 
wires it was natural to suppose that there was a high-resistance 
connection somewhere in that feeder. The large motor was 
stopped, and the fuses were tested as before, but this time no 
voltage at all was found between the middle and the outside 
wires. Then the electrician tried to check up the first results 
by starting the large motor, but since it was a squirrel-cage 
three-phase induction motor it would not start on a single- 
phase circuit. 

He next tested the main-feeder fuses and found one of them 
had blown. A new feeder fuse was put in, and everything 
started off all right. The reduced voltage between the middle 
and the outside wires was just one-half of the line voltage, 
or 220 volts, since the middle wire amounted to a 50 per cent. 



432 



ARMATURE WINDING AND MOTOR REPAIR 



tap on the winding of the large motor when it was running 
single-phase. This case seemed quite a puzzler at the time, 
and it was thought at first that the reduced voltage on the 
middle wire was being generated in the large motor. How- 
ever, a little study of the conditions soon made plain the real 
reason as explained. 

Cause of Synchronous Motor Failing to Start. E. C. 
Parham (Power, June 24, 1919) explains the following trouble 
with a synchronous motor in a large mill when it was necessary 
to repair the winding which had been damaged in a fire. 




FIG. 271. Pole-phase group shown reversed at X. 

A 'complete set of coils was available and the rewinding was 
done by local repairmen. After completing the job, however, 
connecting the motor to its source of power failed to turn 
the rotor, although it had started very promptly before being 
damaged. Higher compensator taps were tried, but to no 
avail. An expert alternator winder was called from the 
factory and found that one pole-phase group of coils had been 
reversed as at X, Fig. 271. The interchanging of the leads 
of this group, as in Fig. 272, restored normal starting torque. 
It will be noticed that by tracing each phase through to the 
star connection in Fig. 272, the arrows point in opposite direc- 
tions on adjacent pole-phase groups, which is the correct 
condition. In Fig. 271 the wrong connection is indicated 
by three adjacent arrows pointing in the same direction. 

The existence of such a condition may be proved by con- 
necting ammeters into each phase conductor and observing 
the intake currents while the rotor is turned by hand; a re- 



METHODS USBD TO SOLVE SPECIAL TROUBLES 433 

versed pole-phase group of coils will greatly unbalance the 
currents of the three wires. It is essential that the rotor be 
rotated so as to equalize impedances due to different phases 
including different amounts of iron in their magnetic circuits. 




FIG. 272. Correct connection for a 4-pole series-star winding. 

Effect of Decreased Frequency on Operation of an Induc- 
tion Motor -generator Set. The immediate effect of operating 
a generator below its rated speed is to increase the amount of 
field current required in order to maintain normal voltage. 
This is due mainly to the fact that the maintenance of normal 
voltage under given load conditions requires that certain 
number of field lines of force be cut each second by the arma- 
ture conductors, and if the necessary rate of cutting can not 
be obtained from the existing conductor rate of motion through 
the existing field, the field strength must be increased until 
the increase makes up for the lack of conductor speed. If 
the field strength can not be increased in proportion to the 
speed deficiency, voltage can not be maintained at its normal 
value. 

One effect of decreasing the frequency of the voltage that is 
applied to an induction motor is to decrease proportionally 
the speed of the motor and of its connected load. It follows, 
then, that if an induction motor is the driving member of a 
motor-generator set of which the driven member is a con- 
tinuous-current generator, the effect of low frequency will be 
to reduce the speed of the set. If the speed decrease is too 
great, the generator field will not have sufficient margin to 
permit of maintaining normal voltage at full load the load 

28 



434 ARMATURE WINDING AND MOTOR REPAIR 

itself producing further speed reduction incident to the in- 
crease of the amount of motor slip. 

E. C. Parham (Electrical World, Jan. 1, 1916) has referred to 
such a situation where an operator on a 50-cycle system 
specified, in ordering a small motor-generator set, a 60- 
cycle motor because he knew that ultimately his service would 
require 60-cycle operation. In ordering the 60-cycle set 
for temporary 50-cycle operation he had considered only the 
motor, and had felt safe in assuming that it could stand the 
larger current incident to operating on the lower frequency. 
The safety of such an assumption would largely depend on how 
nearly the motor would operate at its normal rating under the 
prospective load conditions and with the proper frequency. 
The point of interest in the present instance, however, is that 
when the 60-cycle set was put into service it was found 
impossible to maintain the generator voltage at normal value 
even with the generator field rheostat resistance all cut out. 
In order to get out of the difficulty it was necessary to install 
upon the generator a set of shunt-field coils that had a greater 
number of ampere-turns. 

Alternating-current motors are designed and are rated to 
stand a reasonable departure from their voltage, current and 
frequency specifications, but before abusing them with eyes 
wide open it would be advisable for operators to ascertain the 
permissible limits of abnormal use. 

Simple Rules for Re-connecting Alternating-current Motors. 
The repairman should devise simple rules that he can re- 
member easily so as to be able to handle simple changes in 
motors without having to study up the connections or ask his 
neighbor to explain them. A few such suggestions are given 
here as used by Maurice S. Clement (Electrical Record, October, 
1918) for re-connecting a motor to suit changes in voltage, 
speed, etc. For example, if a three-phase, four-pole motor 
is to be changed to two-phase, four-pole, at the same time 
retaining the same speed, the grouping must be changed. 
A three-phase, four-pole connection has 12 groups; that 
is, one group to each pole, of which there are four, and 
four poles to each phase, giving 12 groups. Therefore, the 
numDer of poles multiplied by the number of phases givos 



METHODS USED TO SOLVE SPECIAL TROUBLES 435 

the number of groups. By this rule a two-phase, four- 
pole machine has eight groups. The next element to be 
taken into consideration is the number of coils in each 
group. 

In a three-phase, four-pole machine with 48 coils there 
would be four coils per group; with 36 slots, three coils per 
group. In a 48-coil, four-pole, three-phase machine there are 
12 groups of four coils each. To change this to a two-phas?, 
four-pole winding without change of speed, the following is 
the rule: 

(Number of coils) + (Number of poles X Phases) 

= 48 -T- (4 X 2) - 6. 

Therefore the new grouping will be arranged with six coils 
per group. 

In changing the grouping of a 36-coil machine from four- 
pole, three-phase to four-pole, two-phase, the result becomes 
a trifle puzzling, as it apparently gives four and one-half 
coils per group. Since we know that cutting a coil in half 
to suit a particular grouping can not be done, the grouping of 
coils will be in the following order: 4, 4, 5, 5, 4, 4, 5, 5. This 
method accounts for the half coils and will be found to be 
evenly balanced. If the machine is to be changed from two- 
phase to three-phase, simply reverse the operation that is 
described above. 

For general details for connecting alternating current motors, 
see Chapter XI. 

Changing 440-volt Motor for 220-volt Operation. The 
accompanying illustrations show how a 440-volt, 4-pole, 
2-phase motor was reconnected for 220-volt operation by 
reconnecting the coils in parallel. The diagram of Fig. 273 (a) 
shows the original connections of the motor, and Fig. 273 (6) 
indicates the connection after the necessary alterations had 
been made. The changes indicated in the diagram, Fig. 
273 (6), were made as follows: 

Terminal Xi was joined to the beginning of coil group 
A i and end of group A 4 . Then the beginnings of groups A s 
and A 4 and ends of A\ and A 2 were connected, Terminal X z 



436 



ARMATURE WINDING AND MOTOR REPAIR 



was joined to the beginning and end of groups A 2 and A s 
respectively. The 5-phase coils were connected in a similar 
manner. 

The foregoing alterations give very well-distributed end 
windings, which are considered essential to obtain a uniform 
flux distribution; but the impedance of the parallel circuits 
will be changed if the rotor runs off center, as may happen 
with a worm bearing, thus causing overheating. On account 
of the small air gap usually found in alternating-current 
motors, there is small probability of this happening because 
worn bearings would cause the rotor to rub the stator before 




v, 



x, 




FIG. 273. Motor winding before and after changing from 440 volts to 220 

volts. 



it would bring about overheating. Unequal distribution 
of current between parallel windings due to a change of imped- 
ance may be avoided by connecting the coil groups which are 
diametrically opposite Of course, this cannot be done with 
two parallel windings if one-half the number of poles is an 
odd number. 

Multiple Connection Diagram for A.-C. Motor Windings. 
Maurice S. Clement of Youngstown, Ohio, makes use of the 
diagram shown in Fig. 274 and claims that it saves a great deal 
of time in re-connecting alternating-current motors (Electrical 
Record, February, 1919). This multiple connection winding 
diagram takes the place of four diagrams, since it can be used 
to show connections for single star, single delta, parallel star 



METHODS USED TO SOLVE SPECIAL TROUBLES 437 



and parallel delta windings for alternating-current motors. 

In this instance a four-pole, three-phase diagram is shown, but 

the same plan can be used with different polarity and phasing. 

The following are the formulas for the different connections : 



SINGLE STAR 



7, 9, 11 = Line 

8 & 10 & 12 = Star 

1 & 4 2 & 5 3&6in Series 



PARALLEL STAR 

4 & 7 = Line 
6 & 9 = Line 
2 & 11 = Line 
1&3&5 8&10&12= Stars. 



SINGLE DELTA 

7 & 8 = Line 

9 & 10 = Line 

11 & 12 = Line 

1&4 2&5 3&6in Series 

PARALLEL DELTA 

7& 4 & 8 & 5= Line 
2 & 11 & 3 & 12 = Line 
1&10&9& 6= Line 



When connecting a stator it is always well to look forward 
to the fact that some day the connections may be changed to 
suit other conditions, and that if the long connection is used 
it will be much easier to change. By long connection is meant 
this. If the reader will trace out the first phase of a single 
star on the sketch, he will 
find it to be 1 to 7 to 10 to 
4; whereas, it is sometimes 
connected as follows: 1 to 
4 to 7 to 10. By study- 
ing the sketch closely it will 
be seen that if the long 
connection method is used, 
the connection can be 
changed to any of the other 
three much easier than by 
the latter method, as half 
the connections do not have 
to be opened. Although 
the short connection is per- 
haps a wire saver by a couple of inches, the long connection 
is easier to understand and makes a much neater job. 

Brush and Slip-ring Sparking Traced, to Absence of 
Rotor Balancing Weights. Manufacturing companies take 
particular care that the rotors of alternating-current machines 
and the armatures of direct-current machines are perfectly 




FIG. 274. Diagram that can be used 
for connecting terminals of pole-phase- 
groups for single and parallel star and 
delta connections. 



438 ARMATURE WINDING AND MOTOR REPAIR 

balanced before they are shipped either .in machines or as 
extras. Some go so far as to balance rotors before and after 
installing the coils and the higher speed rotors of the slip-ring 
type may be balanced before and after installing the slip rings. 
Most of the balancing weight is applied by forcing lead into 
pockets provided in the core end plates for that purpose. 
That the absence of these weights may set up vibrations that 
cause roughening of the slip rings and ultimate sparking, is 
proven by the following experience: 

In one shop (Electrical Record, August, 1918) a slip-ring motor 
had given much brush and slip-ring trouble. Turning of the 
rings and changing of the rings, brushes and holders, had given 
no permanent relief. A factory inspector at once diagnosed 
the cause of the trouble as vibration which no one had sus- 
pected because it was in one of those shops where everything 
that moves vibrates. He tested the rotor in a lathe and found 
that the shaft was bent. The shaft was then straightened 
and it was thought that all trouble was at an end; but not so. 
The machine ran better, but there was considerable vibration 
still there. 

On questioning the operator, the statement was obtained 
that the motor had run all right up to the time that it had 
passed through a fire about a year previously. The fire had 
melted out the bearings and had destroyed everything in the 
nature of insulation; also it had distorted the shaft and melted 
the balance weights although no one had considered the pos- 
sibility of the last-named conditions at the time of re-winding 
che machine and installing new bearings. The rotor was sent 
back to the factory with a statement of its history and when 
it was returned and re-installed the motor, in the words of the 
owner, "ran like a new machine. " Probably it was a new 
machine, 

Overheating of an Induction Motor Traced to a Variation of 
Frequency. Provided that an induction motor is not already 
overloaded, it is not considered abusive to operate the motor 
at less than 10 per cent, over voltage or at 10 per cent, under 
frequency. Manufacturers are careful to specify, however, 
that the motor should not be expected to stand without mate- 
rial increase in heating, the continuous application of 10 per 



METHODS USED TO SOLVE SPECIAL TROUBLES 439 

cent, over voltage and 10 per cent, under frequency, because 
both of these variations are in the direction that tends to in- 
crease the heating. If the ammeter of an induction motor 
circuit be observed while the motor is being started, it will 
be seen that when the motor reaches nearly normal speed, the 
current drops suddenly to a value far below what existed imme- 
diately preceding the sudden drop. It is conceivable that 
certain voltage-frequency-load conditions might exist whereby 
the motor would be operating just on the high side of the crit- 
ical point indicated. Under this condition the current would 
be abnormally large. 

In one case a mill operator complained of the heating of one 
of his larger induction motors. He admitted that the motor 
was overloaded, but the overload was constant and this did 
not explain why the heating was so much greater during some 
periods than during others. When taking the motor speed 
during a period of maximum load, the inspector was much 
surprised to find that the speed was five per cent, above syn- 
chronism. The mill operator stated that over speeds had never 
been noticed, but he felt certain that there were times when 
the speed was considerably under normal. Investigation of the 
engine room equipment, disclosed that at times the steam pres- 
sure became so low that it was necessary for the engineer to 
operate the overload valve of the turbine in order to maintain 
its speed, hence frequency. The turbine generator was excited 
from an engine-driven exciter, the speed of which was not 
materially affected by the variations of steam pressure. As 
the frequency of the service applied to the motors depended 
on the speed of the turbine, which speed depended on steam 
conditions, while the voltage ,of the service applied to the 
motors depended on the turbine voltage which in turn de- 
pended not only on the turbine speed but on the turbine excita- 
tion which was independent of steam pressure variations, the 
extreme conditions of high voltage and low frequency, which 
may have obtained at times, can better be imagined than 
estimated. As the engine room conditions could not imme- 
diately be relieved, the duty of the abused motor was lightened 
by removing a few of the connected machines that constituted 
its load. 



440 ARMATURE WINDING AND MOTOR REPAIR 

Relief for a Hot Bearing. In case of a hot bearing, feed 
plenty of heavy oil, loosen the nuts on the bearing cap and 
slacken the belt if one is used. If no relief is shown, take off 
the load and run the machine slowly until the shaft is cool 
so that the bearing will not " freeze. " If the bearing is of 
babbitt examine it to see that the oil grooves are still intact. 
If they are and the surface of the bearing has not been injured, 
renew the oil supply and start the machine again. Watch 
the oil rings to see that they are revolving properly and carry- 
ing plenty of oil to the shaft. A new machine or one in which 
the bearings have been renewed should be run at slow speed 
for an hour or more before the load is applied in order to see 
that the bearings are properly adjusted and worked in. 

Static Sparks from Belts (Instruction Book, Westinghouse 
Electic & Mfg. Co.). It sometimes occurs on belted machines, 
especially in dry weather, that charges of static electricity 
of considerable potential on the belt cause discharges to the 
ground. If the frame of the machine is not grounded, these 
charges may jump to the armature or field winding and then 
to the ground, puncturing the insulation. The belt and frame 
may be discharged by placing close to the belt, at a point near 
the machine pulley, a number of sharp metal points like a 
comb, which are carefully grounded. If the field frame is 
grounded, there should be no danger to the insulation. 

Ratings of A.-C. Generators. In the case of a single-phase 
generator, the rating in kva. is equal to the product of amperes 
and volts. 

For a two-phase generator, the rating in kva. is equal to 
twice the output of one phase when the load is balanced. 

For a three-phase generator the total rating in kva. is equal 
to the output of one phase multiplied by 1.732. That is, the 
readings or amperes and volts for one phase times 1.732 is 
equal to the kva. rating of the machine, when it carries a 
balanced load. 

Alternating-current Motor Phase-rotation (Instruction Book, 
Westinghouse Electric & Mfg. Co.). In order that the 
alternating-current wiring connections between the motor 
and its supply circuit may be correctly made to obtain a given 
direction of rotation, it is necessary to know the phase-rota- 



METHODS USED TO SOLVE SPECIAL TROUBLES 441 

tion of the motor and the supply circuit. By " phase-rotation 
of the motor" is meant the order in which each phase reaches 
its maximum voltage of one polarity. When the machines 
are arranged for clockwise rotation, looking at the commutator 
end, the phase-rotation is given by the order of terminal 
notation. Using letters to designate the terminals of West- 
inghouse machines, the order for three-phase machines is 
B-C-A and for two-phase Bi-Ai B 2 -A 2 . 

The sequence of phase-rotation of the supply system can be 
found by tracing the wiring back to the generating station or 
else by the use of a phase-rotation indicator. When there are 
flexible cable leads on the alternating-current side, the simplest 
method of determining the proper order in which to connect the 
motor leads is to connect the leads in any convenient order and 
start the motor. If the direction of rotation is opposite to 
that desired, reverse any two leads in the case of a three-phase 
motor or interchange the two leads of either phase, in the case 
of a two-phase motor. 

End Bells or Heads. An end bell which is cracked can 
easily be welded, which process makes it just as good as new. 
If a bearing has been worn out or burned out, it must be re- 
lined. This work must be done by a machinist who appre- 
ciates the importance of a good bearing. 

Brushes and Brush Holders. When a brush holder is 
damaged to any great extent, it is advisable to order a new 
set from the factory as it would not pay to make a set in a 
repair shop. Carbon brushes are easily made, however, and 
dimensions should be exact so as to insure their working easily 
in the holder. 

The Rotor. Under certain conditions, a rotor will become 
so heated as to cause the solder to melt from its bars. When 
this happens the affected parts will rattle when the machine 
is run; resoldering the bars will remedy this. If the bars 
become damaged or bent, they should be removed, straight- 
ened, the slots reinsulated and rotor reassembled. 

The Stator. The troubles occurring in an alternating-cur- 
rent stator may be said to be very similar to those of an arma- 
ture and most of the tests described can be used. For details 
of checking up errors in connections see Chapter XI, page 288. 



442 ARMATURE WINDING AND MOTOR REPAIR 



Sizes of Fuses for A.-C. Motors. The National Electrical 
Code does not specify the size of wire which should be used to 
connect up any given motor, nor does it give the sizes of 
starting and running fuses to be used. A committee of the 

SQUIRREL-CAGE THREE-PHASE INDUCTION MOTORS EQUIPPED WITH 
AUTO-STARTERS 



Average 
horsepower 


Full-load amp. 


Starting fuse amp. 


Running fuse amp. 


220 volts 


550 volts 


220 volts 


550 volts 


220 volts 


550 volts 


0.5 
1.0 


1.8 
3.5 
6.5 
9.5 
15.4 
22.4 
29.0 
42.5 
55.0 
68.0 
80.0 
94.0 
105.0 
130.0 
155.0 
192.0 
252.0 
368.0 
484.0 
595.0 
710 


0.7 

1.3 
2.6 
3.8 
6.2 
9.0 
11.8 
17.4 
22.5 
27.0 
32.0 
37.0 
42.0 
52.0 
62.0 
77.0 
101.0 
148.0 
195.0 
240.0 
285.0 


5 
10 

20 
30 
40 
60 
70 
85 
110 
140 
160 
190 
210 
260 
310 
390 
500 
730 
920 
1200 
1420 


5 

5 
10 
10 
15 
25 
30 
40 
55 
65 
70 
75 
85 
110 
125 
160 
200 
300 
390 
480 
570 


5 

5 
10 
15 
20 
25 
35 
45 
60 
75 
90 
110 
115 
145 
170 
210 
280 
410 
530 
650 
780 


5 
5 

5 
5 
10 
15 
15 
20 
25 
30 
35 
40 
45 
60 
70 
85 
110 
160 
215 
265 
315 


2.0 
3 


5.0 
7.5 
10.0 
15.0 
20.0 
25 


30.0 
35 


40.0 
50.0 


60 0... . 


75.0 


100.0 
150.0 
200 


250.0 
300 





Western Association of Electrical Inspectors has compiled the 
accompanying table based on the code rules which give this in- 
formation for single-phase, two-phase and three-phase motors. 
Excerpts from the report and the recommendations for three- 
phase motors follow: 

No consideration was given to limiting the voltage drop at 
the motor. Motors without a starting device and those 
operating at less than 600 r.p.m. require, in the majority of 
cases, one size larger wire or cable than the table calls for. 



METHODS USED TO SOLVE SPECIAL TROUBLES 443 

The wires in the table are calculated for two and one-half 
times the full load current of motors up to 30-amp. rating 
and for twice the full-load current of larger motors. 

Wire or cable sizes for other types of continuous-duty 
induction motors should be based on the following multiples 
of the full-load current: Squirrel-cage motors up to 7.5 hp. 
without starters, three times full-load current; squirrel-cage 
motors with star-delta starting switch, one and a half times 
full-load current; wound-rotor motors with resistance in rotor, 
one and a tenth times full-load current, and single-phase 
repulsion motors up to 15 hp., twice, and single-phase motors 
with split-phase starting, three times full-load current. 



CHAPTER XVII 

MACHINE EQUIPMENT AND TOOLS NEEDED IN A 
REPAIR SHOP 

The work that comes to the average electrical repair shop 
varies from the re-winding a fan motor to the overhauling and 
re- winding of large station- generators. In the latter case the 
work must, in the majority of cases, be done at the location of 
the machine and calls for more portable tools than machine 
equipment. To meet the requirements of work between these 
limits in size of machine there is needed a fairly well equipped 
machine shop. Most motor repair and armature winding 
work can be properly handled if the following equipment is 
available : 

Lathe. 

Coil- winding lathe heads. 

Shaper, or undercutting attachment for lathe. 

Drill press. 

Band saw. 

Emery wheel. 

Portable hand crane. 

Chain block. 

Welding outfit. 

Vices. 

Coil-pulling machine. 

Coil-taping machine. 

Banding machine or tension device for lathe. 

In case only one lathe is available this should be large enough 
to accommodate a good sized armature and have a spread 
between head and tail post of about 60 inches. The shaper 
and band saw are rather special machines for a repair shop 
but very useful. The former serves as a rapid and accurate 
method for undercutting the mica of a commutator. The 

444 



REPAIR SHOP EQUIPMENT AND TOOLS 



445 



latter saves a great deal of time when many coil forms are to 
be cut for several coil winders. 

In the electrical repair shop of a large industrial plant where 
a number of armatures are re-wound and repaired the following 
equipment is provided: 

1 12-inch speed lathe. 

1 24-inch armature-banding lathe. 

1 24-inch commutator grooving lathe. 

1 Commutator turning device. 

1 Double head emery wheel grinder. 

1 3-inch bench vice. 

1 5-inch bench vice. 

2 6-inch bench vices. 

2 Field-winding machines. 

2 Armature coil-taping machines. 

1 One-ton electric hoist and carriage. 




FIG. 275. Armature winders' hand tools. (Numbers refer to their names and 
uses, page 446.) 

Armature Winder's Tools. The experienced armature 
winder usually has a variety of tools either designed according 
to his own ideas or convenience for a particular job or for use 
in connection with armatures of different sizes. In Figs. 275 
and 276 a varied assortment of hand tools and drifts are shown. 
The names of these tools and their uses are given in the 
following tabulations. 



446 ARMATURE WINDING AND MOTOR REPAIR 




FIG. 276. Drifts used by armature winders. (Numbers refer to types and 

uses, page 447.) 

ARMATURE WINDER'S HAND TOOLS (Fio. 275) 



Number 

in 
illustration 



Name of tool 



Uses in winding armatures 



1 
2 

3 
4 

5 and 6 

7 

8 

9 

10 

11 and 12 



Coil hook. . . 
Medium file. 



Knife edge file 

16-oz. machinist's hammer 

No. 1 and No. 2 rawhide mallets , 

12-oz. machinist hammer 

Tinsmith's hammer 

Cold chisel 

Coil lifter and shaper'. 



Metal drifts. 



13 and 
14 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 



15 



Two sizes of wedge drivers . 

Undercutting tool 

Coil scraper 

Diagonal cutters 

Half-round duck bill pliers. 

Round-nose pliers 

Small screw driver 

Shoemaker's knife 

Scissors 

Tinsmith's shears 

6-in. parallel pliers 

8-in. side cutting pliers 

Flat duck bill pliers 

Long metal coil drift 

Monkey wrench 

Large screw driver 



To lift coils out of slots. 

For cleaning commutator necks before 
soldering. 

For trimming edges of mica segments. 

For use on metal surfaces. 

For drifting coils into slots. 

For use on metal surfaces. 

For peening slots of commutator necks. 

For cutting coil leads to commutator. 

For lifting coils out of slots and shaping 
when rewinding. !* 

For driving down coil terminals in com- 
mutator necks. 

For driving wedges into slots. 

For undercutting mica on commutators. 

For scraping cotton insulation off wires. 

Special pliers for rewinding work. 

Special pliers for rewinding work. 

Special pliers for rewinding work. 

For general use. 

For triming and cutting slot insulation. 

For cutting tape, and cotton duck. 

For cutting tin clips for banding wires. 

For gripping wires in winding coils, etc. 

For cutting heavy wire coil terminals. 

For shaping coils. 

For lifting tight coils out of slots. 

For tightening commutator, etc. 

For general use. 



REPAIR SHOP EQUIPMENT AND TOOLS 
DRIFTS USED BY ARMATURE WINDERS (Fie. 276) 



447 



Number 

in 
illustration 



Name of drift 



Particular uses 



3, 4 and 5 



7 
8 

9, 10, 11 
and 12 



13, 14, 15, 
18, 19, 20, 
21 and 22 

16 
17 and 23 



Coil sliders or guides 

Different sizes of fiber slot drifts. 



Fiber coil shapers . 



Fiber coil drift tapered. 
Small metal coil lifter. . 
Center punch 



Lead lifters. 



Metal drifts. 



Ordinary dividers. 
Tee slot drifts... 



For sliding top sides of coils into slots. 
For driving coils into slots. They vary 

in width and length according to size 

of slot. 
The curved surfaces are used to shape 

ends of coils. 

For lifting tight coils in small machines. 
For lifting tight coils out of slots. 
For marking location and pitch of coil 

when stripping armature. 

For driving soldered ends out of com- 
mutator necks when stripping arma- 
ture. 

For seating coil terminals in commutator 
necks. 



For laying out fiber collars. 
For partly closed slots to drive 
down in slots. 



oils 



Device for Shaping Insulating Cells of Armature Slots. 

For shaping the fish paper in making cells for armature and 
stator slots the cell shaper shown in Fig. 277 is very useful. 
It consists of two pieces of wood hinged together so that they 
will make a neat 90-deg. fold. The permanency of the correct- 
fold maker is insured by means of a 
metal strip attached to the wood slot. 
The cell shaper is used by inserting 
a piece of fish paper in the opening 
between the two blocks of wood, which 
is the length of the slot plus twice the 
height and whose width is the width 
of the slots. The metal straight-edge, 
which is adjustable by means of wing nuts, allows the paper 
to be folded so as to be made the height of the slot. 

Tool for Cutting Cell Lining at Top of Slot. For cutting the 
projecting insulation of an armature slot after the coils have 
been assembled, the tool shown in Fig. 279 is recommended by 
Maurice S. Clement (Electrical World, Oct. 12, 1918). This 




CELL SHAPR 

FIG. 277. Device for 
shaping insulating cells of 
armature slots. 



448 ARMATURE WINDING AND MOTOR REPAIR 




S j* 
fl "fl 

,,-. <D 
P 3 



3 



REPAIR SHOP EQUIPMENT AND TOOLS 



449 



riLEHANDLE 




tool as well as those of Figs. 280 and 282 and the insulating cell 
shaping device shown in Fig. 278 are home-made designs he has 
devised. It consists of a piece of forged steel 14 inches long 
by % inch wide by %Q inch thick with a set of beveled knife- 
edges at one end and a file handle at the 
other. 

Special Winding Tools. In Fig. 280 
three convenient tools are shown that can 
bs easily made by any armature winder. 
From left to right in the illustration they FlG 279 Tool for 
are a coil-taping needle, coil raiser, and cutting insulation that 
wire scraper. The coil-taping needle can projec 
be made from one foot of No. 14 banding wire shaped so that 
it can be used for taping coils in closed-slot stators. After 
the user is accustomed to this device high speed may be 
attained. 

The coil raiser consists of a piece of steel, 16 inches by 1 by 
mc h w ^h a four-inch one-sided taper on one end for strip- 



CELL CUTTER 




*tt 


v'/SEnd 

r 








~jj 













) 




* 


Plan 





A 

CO/ 


i 
r V 

LRAISEK 




L \ 





Section 



MEDLE" ^MRESCRAPE* 

FIG. 280. Convenient designs of coil taping needle, coil raiser and wire 

scraper. 

ping open-slot armatures and stators. This also can be used 
to good advantage in removing grounded coils from a newly 
wound armature or in raising coils sufficiently to allow for 
insulating weak spots in the coils, the main object in this case 

33 



450 ARMATURE WINDING AND MOTOR REPAIR 




REPAIR SHOP EQUIPMENT AND TOOLS 



451 



being to lift out a tight-fitting coil without damaging the 
insulation. 

The wire scraper is very simply made and very economical, 
because it eliminates the use of a knife, whose life is short on 
account of the rough treatment accorded it. This device is 
made of spring metal, 12 inches by % inch by Jfe mcn - The 
knife-edges can be sharpened by means of a file and the tool 
used indefinitely. A section the shape of a rectangle is cut 
from the metal at the handle end, greatly increasing the spring 
effect of the device. 

For driving fiber wedges between the top of a coil and the 
lamination overhanging closed-slots, a wedge drift, made of 
a piece of tool steel, eight inches by five inches by ^2 inch, 
over which is fitted a loose-fitting steel sleeve, J^g inch thick, 
is very convenient. This is used by inserting the fiber wedge 
about y inch into the slot; then, with the drift pulled back into 
the sleeve, the sleeve is fitted over the wedge, which is driven 
into the proper place, the sleeve holding the wedge in position. 

Repair Tools that Can be Made from Old Hack-saw Blades. 
Seven tools that have been made from old hack-saw blades 




FIG. 282. Seven armature repair tools made from old hack-saw blades that 
are useful around a motor repair bench. 

and found useful in the re-winding of motors are shown in 
Fig. 282. A small wooden case to stow away these tools can 
be made as follows: Cut out two pieces of any suitable hard 
wood eight inches long, one and one-half inches wide and 
]4, inch thick. Then on the flat side of one of these pieces 
cut several grooves, large enough to allow a hack-saw blade 
to fit in it snugly. The two pieces should then be nailed 



452 



ARMATURE WINDING AND MOTOR REPAIR 



together so that the grooves will be between both pieces. 
When a third piece of wood, eight inches long, one inch wide 
and y inch thick is nailed on as a bottom piece, the 
case is ready for the tools. 

While each of the tools can be used for several different 
operations, a few of the most important uses are as follows: 
Tools one, two and five (from the right) are used mostly in 
taping coils. Tools one and two are used to best advantage in 
digging stray drops of solder out of the winding of a stator 
after the connections have been soldered. Tools four and six, 
which have both edges of the V sharpened to a knife-edge, 
are very well adapted to cutting insulation and scraping wires 
or leads. Number seven is used to drive wedges into tight 
slots. By setting the saw-teeth on the wedge and pounding 
on the beveled end with a hammer, the teeth are made to 
grip the wedge and drive it to its proper place. 

Special Coil-winding Device. For winding coils of practi- 
cally any shape the special device shown in Fig. 283 has been 




Ji Hole off Center 




J^St'd. Thread 
and Washer 

FIG. 283. Framework for use in winding different shapes of armature coils. 

devised by Frank Huskinson (Electrical World, July 27, 1918). 
It consists of an iron framework held together with four bolts 
shown at B. By loosening the nuts of these bolts the two 
outside members can be adjusted for any width of coil needed. 
To give the coil the proper shape, disks such as marked C are 
clamped on the piece D and the latter inserted in the slots 
marked A in the frame. The view at E shows how these 
disks appear when the bolt D is in its proper place in the frame. 
When forming the coils the wire is wound around six of the 
bolts or pegs and between the disks mounted on them. 



REPAIR SHOP EQUIPMENT AND TOOLS 



453 




FIG. 284. Convenient bench device for holding the stator of a small motor 
when repairing windings. 




FIG. 285. At the left a vertical stand is shown for use in mounting the 
commutator on small armatures. At the right, a large armature being wound 
with pushed-through coils. 



464 ARMATURE WINDING AND MOTOR REPAIR 

Steadying Brace for Repairing Small Motors. The device 
shown in Fig. 284 has been devised by Maurice S. Clement 
(Electrical Record, December, 1918) for use in repairing fan 
motors or other small motors when it is necessary to remove 
the winding of the stator from the frame. To remove these 
windings from the frame the laminations must be removed 
with the winding. When once removed it is very difficult 
to work on it as it has a tendency to roll away all the time. 

This brace is constructed as follows: A block is cut as 
shown and bolted to the bench. One end of a length of 
ribbon copper wire about }{$ inch thick and ^ inch wide is 
then screwed to the end of the block and passed over the wind- 
ing which sets in the cut out portion of the block. The other 
end of the copper ribbon is held down by means of a wing 
nut placed between the bolts. A leather strap is arranged 
from one side of the block, over the copper ribbon and fastened 
to a buckle on the other side. This is a tension strap and takes 
the slack out of the copper ribbon. 

Tension Block For Use When Banding Armatures. A tool 
is shown in Fig. 286 which has been devised by Maurice S. 

W* Ifl* 

maatm | "' *W*-W*| , 

-* w^~^ ; 




lii^BRC 



IBREi^rCWWE 




FIG. 286. Tension block for use when winding armatures. 

Clement (Electrical World, Oct. 12, 1918) for banding an arma- 
ture in a small repair shop where a banding lathe is not avail- 
able. By use of the device the armature can be banded on an 
ordinary winding stand which is securely fastened to the floor 
or work bench. When this device is used about one foot of 
stout line with a hook attached to one end is made fast to the 
ring on the tension block and hooked to an eye-bolt which is 
set in the floor for that purpose. The spool of banding wire 
is placed on a small stand beside the eye-bolt and the wire is 
passed between the two blocks at the rear end through the 
hole in the first wire guide over the tension curve and through 



REPAIR SHOP EQUIPMENT AND TOOLS 



455 



the second wire guide hole and then to the armature. The 
tension can be regulated by the wing nut placed at the forward 
upper end of the block. By screwing down the wing nut 
both sides of the block are brought nearer together, thus 






. 






Double ply leather 






%"thick i!o" 




V * 


1 




\1 




M 




7 Brass rivets * 


5^4 




Iron triangle 
FIG. 287. Armature sling made up of double-ply leather belting. 

narrowing the tension curve over which the wire must pass. 
This increases the tightness of the band when a pipe wrench 
can be used to revolve the armature. 




FIG. 288. Method of using a rope sling in handling armatures with a crane. 

Armature Sling. For handling heavy armatures in repair 
shops a sling is sometimes used instead of lifting by means of a 
rope attached to the ends of the shaft as a bale handle for the 



456 ARMATURE WINDING AND MOTOR REPAIR 



crane hook. The sling prevents the possibility of springing 
the shaft. A sling which can be used for handling small 
armatures is shown in Fig. 287. It is constructed of a piece 
of double ply belting about % inch thick, 10 inches wide and 

5% feet long, provided with triangles 
of steel bar for the hook of a shop 
crane. 

Pinion Puller. A device which 
can be made up as shown in Fig. 
289 is convenient for removing 
pinion gears from motor shafts with- 
out injury. By making the head 
sufficiently long and providing holes 
at suitable distances, pinions of 
different sizes can be removed by 
adjusting the end pieces. 

Coil-winding Machines. In Fig. 

290 a device is shown which does not require the use of forms 
when winding armature coils. It consists of an ad j ustable metal 
frame with jaws which may be clamped in various positions so 
as to make a form over which the wire can be wound into the 




FIG. 289. Convenient de- 
vice for removing pinions 
from a motor shaft. 




FIG. 290. Segur armature coil winder for winding hair-pin loop, obtuse loop 
and rectangular loop coils. (Electrical Manufacturers Equipment Company.) 

With this device loops from. 3 inches to 36 inches can be wound. By means of the 
spreader shown in Fig. 291 the coil shapes shown in Fig. 292 can be formed from these 
loops. 

shape of the coil desired. Jaws are provided for forming hair- 
pin loop coils, that is, coils with parallel sides and curved ends. 



REPAIR SHOP EQUIPMENT AND TOOLS 457 

Another set of jaws provided with the machine has four points 
and gives four curves to the coil, the jaws being set so as to 
form a rectangular coil with two angles obtuse and two angles 
acute. These same jaws may be set in another way so as to 
give a triangular or square loop. With these attachments, 
coils of various shapes can be wound, some of the shapes being 
suitable for use directly in various types of motors and gen- 
erators and other coils requiring further operations on a 
machine called a spreader. The purpose of this machine 
(Fig. 291) is to form the coil to the throw required when it is 




FIG. 291. Coil spreader for shaping the coils shown in Fig 292 as wound 
on the machine shown Fig. 290. (Electrical Manufacturers Equipment Com- 
pany.) 

inserted in the armature or stator. The spreader takes the 
coil as it is wound upon the winding machine and opens it 
and twists it at both ends, so that it conforms to the shape 
desired. Some of the forms of coils as wound on these ma- 
chines and as finally shaped on the spreader are shown in the 
accompanying illustrations, Fig. 292. 

Coil-taping Machines. Field and armature coils for me- 
dium and larga-sized motor and generators are wound from 
bare copper wire and are insulated after the winding operation 
has been completed. The material usually employed for 
winding coils is treated or untreated tape which is wound on 
the complete coil by hand using rolls of treated or untreated 
tape or by the use of a taping machine. The taping machine 
consists essentially of a rotating circular element which has an 
opening, by means of which the coil is placed inside of the ring 



458 ARMATURE WINDING AND MOTOR REPAIR 




(e) 



FIG. 292. Coils that can be shaped on the spreader shown in Fig. 291. 
(1) Common loop, 3 wires wide with 5 turns that can be spread as shown in (2)- 
(3) ao-coilgroup 3 wires wide, wound 5 wires vertical and spiral shown f ormed in (4)- 
(5) a rectangular loop 3 wires wide, 2 turns of 6 wires that can be shaped to make a 
3-coil bundle. (7) Obtuse loop, 5 turns, 3 wires wide that can be formed as shown in 
(8) and spread to make an Eickemeyer coil like (9). (10) Ribbon copper coil formed into 
a hair-pin loop by bar bender (Fig. 293) and shaped in coil spreader. 




FIG. 293. Top view of a Segur bar bender for shaping armature coils made 

of copper strip from 0.05 to 0.125 inch thick by any width up to 1H inch. 

(Electrical Manufacturers Equipment Company.) 



REPAIR SHOP EQUIPMENT AND TOOLS 



459 



(Fig. 294). Tape is then attached by hand and the rotating 
element which carries the spool of tape winds it symmetrically 
about the bundle of wires forming the coil. 




FIG. 294. Segur armature coil-taping machine. 
(Electrical Manufacturers Equipment Company.) 




FIG. 295. Portable commutator slotting machine that will handle arma- 
tures up to 18 inch in diameter. (Electric Service Supplies Company.) 

Commutator-slotting and Grinding Machines. In Fig. 295 
is shown a commutator-slotting outfit. This machine consists 
of a frame in which the commutator is held either before or 



460 ARMATURE WINDING AND MOTOR REPAIR 

after the commutator has been placed on the armature, and 
a rotating disk saw mounted on slides which are parallel to the 
direction of rotation of the disk. The rotating saw is driven 
by belts or other suitable means of transmission from the 
source of power. The guide or frame upon which the saw is 
mounted is adjustable according to the diameter of the com- 




Fia. 296. Commutator slotting maching that can be attached to a standard 
engine lathe. (Electric Service Supplies Company.) 

mutator to be worked on and after the correct height and depth 
of the cut have been ascertained, the frame is clamped firmly 
in position. The operator then starts the saw and moves it 
forward on its guide by means of a lever, so that it comes into 
contact with the commutator, thus cutting a slot. The arma- 
ture is moved through a space corresponding to the width of a 
single bar, and the saw is again brought forward cutting 



REPAIR StiOP EQUIPMENT AND TOOLS 



461 



another slot, and so the operation continues until the commu- 
tator has been slotted all around. 

The face of the commutator must be smooth and perfectly 
circular so as to secure sparkless operation and the minimum 
wear of brushes. This result is obtained by grinding the com- 
mutator after it has been assembled or slotted. The com- 




X 



pi 



FIQ. 297. Combination armature banding and tension machine. (Electric 
Service Supplies Company.) 

mutator grinder operates by bringing a stationary grinding 
tool or abrasive material into contact with the rotating com- 
mutator which is mounted in its final position on the armature, 
in this case the entire armature being mounted in a lathe 
and rotated. Another method is to apply a rotating grinding 
wheel to the armature. The commutator grinding or truing 



462 



ARMATURE WINDING AND MOTOR REPAIR 



machine is essentially a lathe and operates in the same manner 
as any other machine for grinding a circular surface. 

Armature-banding Machine. This machine usually con- 
sists of a suitable stand with bearings upon which the arma- 
ture shaft rests, and some means for rotating the armature by 
means of a belt, chain or gear drive from a line shaft or from 
an individual motor. A band wire tension device similar to the 
tension device employed for winding coils is incorporated in the 
machine or is built as a separate machine, which may be at- 
tached to the armature stand. With an armature in position, 
the band wire is attached at several points throughout the length 




FIG. 298. A machine equipped so that coil-winding, banding and grinding 
operations may be performed on it. 

of the armature, one at each end and one or more at equal dis- 
tances between the ends, and the machine is started. The ro- 
tation of the armature causes the wire to be wrapped about the 
armature, the tension device and guide laying the wires evenly, 
to a width which is determined by the setting of the guide. 

Combination Machines. A combination coil- winding, 
banding and grinding machine is shown in Fig. 298. This 
machine combines into one, the following tools: (a) a band- 
ing machine with a self-contained tension carriage for the 
band wire, designed to handle large or small railway, locomo- 
tive or stationary armatures; (6) a commutator slotting ma- 
chine with independent motor drive; (c) a commutator grinding 
machine with independent motor drive; (d) a commutator 



REPAIR SHOP EQUIPMENT AND TOOLS 463 

cutting or turning attachment; (e) a field and armature coil 
plate mounted on the main spindle suitable for all classes of 
heavy form coil winding. This machine closely resembles a 
large power lathe but is provided with the special attachments 
for performing the work above outlined. After an armature 
has had the coils placed in the slot and the leads soldered to the 
commutator, it may be placed in this machine and the work 
completed without removing the armature from the machine. 
The banding attachment is capable of heavy duty, the rotation 
being under control of the operator. When the machine 
is stopped it automatically locks and prevents slack in the 
band wire due to backing motion of the armature. Uniform 
tension is secured by a tension device mounted on the feed 
carriage and traveling with it. The feed carriage is moved 
along by means of a rack and pinion and pilot wheel and is 
adjustable vertically in or out. 

The slotting attachment is supported by a bracket clamped 
to the tail-stock spindle and is removed by loosening two cap 
screws. The commutator truing or grinding attachment 
consists of a traveling grinding wheel supported from the tail 
stock and motor driven. The casting which supports the two 
rods carrying the grinding wheel and the independent motor 
are adjustable along two other steel rods projecting backward 
from the tail stock and bringing the grinding wheel parallel 
to the face of the commutator. These rods are adjustable in 
or out to suit the length or location of the commutator, and 
the grinding wheel is moved along by means of the screw and 
hand wheel. Cutting is done in both directions of travel of 
the wheel. Power is applied either through belt and pulley 
from a countershaft or by an individual motor drive which 
may readily be installed. The control is by means of a clutch 
which is operated by a treadle running the full length of the 
machine. The machine is provided with two changes of 
speed, a low speed for banding and coil winding and a high 
speed for commutator cutting, grinding and truing. The 
head stock is provided with a coil-winding plate to which may 
be attached forms for winding coils of various sizes and shapes, 
upon the same principle as the independent coil-winding 
machines, designed to do this work only. 



APPENDIX 

DATA AND REFERENCE TABLES 

How to Remember the Wire Table. The wire table for B. & S. 
gauge copper wire has simple relations, such that if a few constants 
are remembered the whole table can be constructed mentally with approxi- 
mate accuracy. 

A wire which is three sizes larger than another wire has half the resist- 
ance, twice the weight and twice the area. A wire which is ten sizes 
larger than another wire has one-tenth the resistance, ten times the 
weight and ten times the area. 

No. 10 Wire is 0.10 inch in diameter (more precisely 0.102); it has an 
area of 10,000 circular mils (more precisely 10,380); it has a resistance of 
1 ohm per thousand feet at 20 C. (68F.), and weighs 32 pounds (more 
precisely 31.4 pounds) per thousand feet. 

The weight of one thousand feet of No. 5 wire is 100 pounds. 

The relative values of resistance (for decreasing sizes) and of weight 
and area (for increasing sizes) for consecutive sizes are: 0.50, 0.60, 0.80, 
1.00, 1.25, 1.60, 2.00. 

The relative values of the diameters of alternate sizes of wire are: 
0.50, 0.63, 0.80 1.00, 1.25, 1.60, 2.00. 

The "mil," whose value is one-thousandth (0.001) of an inch, is the 
practical basis for determining the diameters and thereby the areas of 
all wires used as electric conductors. The diameter being given, the area 
is obtained by the well-known rule, "the area of a circle, in circular units, 
is equal to the square of its diameter," hence, the square of the diameter 
of a wire expressed in mils equals the area of its cross-section. D 2 = A, 
which area is expressed in circular mils or CM; hence, D 2 = CM. 

Circular Mils. Conductors of large size are usually specified in cir- 
cular mils as 500,000 circular mils, 750,000 circular mils. 

To find resistance, drop one cypher from the number of circular mils; 
the result is the number of feet per ohm. 

To find weight, drop four cyphers from the number of circular mils 
and multiply by the weight of No. 10 Wire. 

Copper for Various Systems of Distribution. When the power trans- 
mitted, distance, line loss and voltage of lamps is constant and all wires 
30 465 



466 ARMATURE WINDING AND MOTOR REPAIR 

of each system are of the same size, the following is the relationship of 
the copper required. 

Copper 
System. Required 

2 Wire, single-phase or direct current 1 . 000 

3 Wire, single-phase or direct current . 375 

4 Wire, single-phase or direct current 0. 222 

4 Wire, two-phase 1 . 000 

4 Wire, three-phase with neutral . 333 

3 Wire, three-phase Delta ...' 0. 75 



Classification of Wire Gauges. Wire gauges are known under a variety 
of names so that it is important to know the difference when conditions 
require that the values of one gauge shall be converted into those of an- 
other. The following is a classification of gauges by names and uses: 

Brown & Sharpe (B.& S.) = American Wire Gauge (A. W. G.). 

New British Standard (N. B. S.) = British, Imperial, English Legal 
Standard and Standard Wire Gauge, and is variously abbreviated by 
S. W. G. and I. W. G. 

Birmingham Gauge (B. W. G.) = Stubs, Old English Standard and 
Iron Wire Gauge. 

Roebling = Washburn Moen, American Steel and Wire Go's. Iron 
Wire Gauge. 

London = Old English (not Old English Standard). 

As a further complication: 

Birmingham or Stubs' Iron Wire Gauge is not the same as Stubs' 
Steel Wire Gauge. 

Uses of Various Gauges. B. & S. Gauge. All forms of round wire used 
for electrical conductors. Sheet copper, brass and German silver. 

U. S. S. Gauge Sheet iron and steel. Legalized by act of Congress, 
March 3, 1893. 

B. W. Gauge. Galvanized iron wire. Norway iron wire. 

American Screw Co.'s Wire Gauge Numbered sizes of machine and 
wood screws, particularly up to No. 14 (0.2421 inch). 

Stubs' Steel Wire Gauge Drill rod. 

Roebling & Trenton Iron and steel wire. Telephone and tele- 
graph wire. 

N. B. S. Hard drawn copper. Telephone and telegraph wire. 
London Gauge Brass wire. 



APPENDIX 
DIFFERENCE BETWEEN WIRE GAUGES 



467 



Gauge 

No. 


Brown & 
Sharpe's 


Old English or 
London 


Stubs' or 
Birmingham 


New British 
Standard 


0000 


0.460 


0.454 


0.454 


0.400 


000 


0.40964 


0.425 


0.425 


0.372 


00 


. 36480 


0.380 


0.380 


0.348 





0.32495 


0.340 


0.340 


0.324 


1 


0.28930 


0.300 


0.300 


0.300 


2 


0.25763 


0.284 


0.284 


0.276 


3 


0.22942 


0.259 


0.259 


0.252 


4 


0.20431 


0.238 


0.238 


0.232 


5 


0.18194 


0.220 


0.220 


0.212 


6 


0.16202 


0.203 


0.203 


0.192 


7 


0.14428 


0.180 


0.180 


0.176 


8 


0.12849 


0.165 


0.165 


0.160 


9 


0.11443 


0.148 


0.148 


0.144 


10 


0.10189 


0.134 


0.134 


0.128 


11 


0.09074 


0.120 


0.120 


0.116 


12 


0.08081 


0.109 


0.109 


0.104 


13 


0.07196 


0.095 


0.095 


0.092 


14 


0.06408 


0.083 


0.083 


0.080 


15 


0.05706 


0.072 


0.072 


0.072 


16 


0.05082 


0.065 


0.065 


0.064 


17 


0.04525 


0.058 


0.058 


0.056 


18 


0.04030 


0.049 


0.049 


0.048 


19 


0.03589 


0.040 


0.042 


0.040 


20 


0.03196 


0.035 


0.035 


0.036 


21 


0.02846 


0.0315 


0.032 


0.032 


22 


0.025347 


0.0295 


0.028 


0.028 


23 


0.022571 


0.027 


0.025 


0.024 


24 


0.0201 


0.025 


0.022 


0.022 


25 


0.0179 


0.023 


0.020 


0.020 


26 


0.01594 


0.0205 


0.018 


0.018 


27 


0.014195 


0.01875 


0.016 


0.0164 


28 


0.012641 


0.0165 


0.014 


0.0148 


29 


0.011257 


0.0155 


0.013 


0.0136 


30 


0.010025 


0.01375 


0.012 


0.0124 


31 


0.008928 


0.01225 


0.010 


0.0116 


32 


0.00795 


0.01125 


0.009 


0.0108 


33 


0.00708 


0.01025 


0.008 


0.010 


34 


0.0063 


0.0095 


0.007 


0.0092 


35 


0.00561 


0.009 


0.005 


0.0084 


36 


0.005 


0.0075 


0.004 


0.0076 


37 


0.00445 


0.0065 






38 


0.003965 


0.00575 






39 


0.003531 


0.005 






40 


0.003144 


0.0045 







468 



ARMATURE WINDING AND MOTOR REPAIR 



EQUIVALENTS OF WIRE SIZES (B. & S. GAUGE) 



0000 = 2 


No. 0=4 


No. 3 = 8 


No. 6 = 16 


No. 9 = 32 


No. 12 = 64 


No. 15 


000 = 2 


No. 1=4 


No. 4 = 8 


No. 7 = 16 


No. 10 = 32 


No. 13 = 64 


No. 16 


00 = 2 


No. 2=4 


No. 5 = 8 


No. 8 = 16 


No. 11 = 32 


No. 14 = 64 


No. 17 


= 2 


No. 3 = 4 


No. 6 = 8 


No. 9 = 16 


No. 12 = 32 


No. 15 = 64 


No. 18 


1 = 2 


No. 4=4 


No. 7 = 8 


No. 10 = 16 


No. 13 = 32 


No. 16 = 64 


No. 19 


2 = 2 


No. 5 = 4 


No. 8 = 8 


No. 11 = 16 


No. 14 = 32 


No. 17 = 64 


No. 20 


3 = 2 


No. 6 = 4 


No. 9 = 8 


No. 12 = 16 


No. 15 = 32 


No. 18 = 64 


No. 21 


4=2 


No. 7=4 


No. 10 = 8 


No. 13 = 16 


No. 16 = 32 


No. 19 = 64 


No. 22 


5 = 2 


No. 8 = 4 


No. 11 = 8 


No. 14 = 16 


No. 17 = 32 


No. 20 = 64 


No. 23 


6 = 2 


No. 9=4 


No. 12 = 8 


No. 15 = 16 


No. 18 = 32 


No. 21 = 64 


No. 24 


7 = 2 


No. 10 = 4 


No. 13 = 8 


No. 16 = 13 


No. 19 = 32 


No. 22 = 64 


No. 25 


8 = 2 


No. 11 = 4 


No. 14 = 8 


No. 17 = 16 


No. 20 = 32 


No. 23 = 64 


No. 26 


9 = 2 


No. 12 = 4 


No. 15 = 8 


No. 18 = 13 


No. 21 = 32 


No. 24 = 64 


No. 27 


10 = 2 


No. 13 = 4 


No. 16 = 8 


No. 19 = 13 


No. 22 = 32 


No. 25 = 64 


No. 28 


11 = 2 


No. 14 = 4 


No. 17 = 8 


No. 20 = 16 


No. 23 = 32 


.No. 26 = 64 


No. 29 


12 = 2 


No. 15 = 4 


No. 18 = 8 


No. 21 = 16 


No. 24 = 32 


No. 27 = 64 


No. 30 


13 = 2 


No. 16 = 4 


No. 19 = 8 


No. 22 = 16 


No. 25 = 32 


No. 28 




It = 2 


No. 17 = 4 


No. 20 = 8 


No. 23 = 16 


No. 26 = 33 


No. 29 




15 = 2 


No. 18 = 4 


No. 21 = 8 


No. 24 = 16 


No. 27 = 32 


No. 30 




16 = 2 


No. 19 = 4 


No. 22 - 8 


No. 25 = 16 


No. 28 






17 = 2 


No. 20 = 4 


No. 23 = 8 


No. 26 = 16 


No. 29 






18 = 2 


No. 21 = 4 


No. 24 = 8 


No. 27 = 16 


No. 30 






19 = 2 


No. 22 = 4 


No. 25 = 8 


No. 28 








20 = 2 


No. 23 = 4 


No. 26 = 8 


No. 29 








21 = 2 


No. 24 = 4 


No. 27 = 8 


No. 30 









General Wiring Formula For Alternating- and Direct-current Circuits. 
The following general formula may be used to determine the size of 
copper conductors, volts loss in lines, current per conductor, and of cop- 
per per circuit or any system of electrical distribution. 



Area of conductor, circular mils = 



Volts loss in lines 



Current in main conductors = 



Pounds copper 



D X W X C 
P X E* 

PXEXB 
100 

W XT 
E 

D* X W X C X A 
P X #X1,000,000 



W = Total watts delivered. 

D = Distance of transmission (one way) in feet. 

P = Loss in line in per cent, of power delivered, that is of W. 

E = Voltage between main conductors at receiving or consumer's 

end of circuit. 
For continuous current. C = 2160, T = 1, B = 1, and A = 6.04 



APPENDIX 



469 



System 


Value 
of A 


Value of C 


Value cf T 


Per cent., power factor 


Per cent., power factor 


100 


95 


90 


85 


80 


100 


95 


90 


85 


80 


Single-phase 
Two-phase (4 wire) 
Three -phase (3 wire) . . 


6.04 
12.08 
9.06 


2160 
1080 
1030 


2400 
1200 
1200 


2660 
1330 
1330 


3000 
1500 
1500 


3380 
1690 
1690 


1.00 
0.50 
0.58 


1.05 
0.53 
0.61 


1.11 
0.55 
0.64 


0.17 
0.59 
0.68 


1.25 
0.62 
0.72 



APPLICATION OF THE FORMULA 

The value of C for any particular power factor is obtained by dividing 
2160, the value for continuous current, by the square of that power fac- 
tor for single-phase, and by twice the square of that power factor for three- 
wire, three-phase, or four-wire, two-phase. 

The value of B depends on the size of wire, frequency and power fac- 
tor. It is equal to 1 for continuous current, and for alternating current 
with 100 per cent, power factor. For sizes of wire given in the tables of 
wiring constants (pages 470 and 471), and other power factors and cycles 
the values of B are given. 

The figures given are for wires 18 inches apart and are sufficiently 
accurate for all practical purposes provided the displacement in phase 
between current and emf . at the receiving end is not very much greater 
than that at the generator; in other words, provided that the reactance 
of the line is not excessive, or the line loss unusually high. For example, 
the constants should not be applied at 125 cycles if the largest conductors 
are used and the loss 20 per eent. or more of the power delivered. At 
lower frequencies, however, the constants are reasonably correct even 
under such extreme conditions. They represent about the true values 
at 10 per cent, line loss, are close enough at all losses less than 10 per 
cent., and often, at least for frequencies up to 40 cycles, close enough for 
eren much larger losses. When the canductors of a circuit are nearer 
each other than 18 inches, the volts loss will be less than given by the 
formula, and if close together, as with multiple conductor cable, the loss 
will be only that due to resistance. 

The value of T depends on the system and power factor. It is equal 
to 1 for continuous current and for single-phase current of 100 per cent, 
power factor. 

The value of A and the weights of the wires in the table are based on 
0.00000302 pound as the weight of a foot of copper wire of 1 circular mil 
area. 

In using the above formula and constants, it should be particularly 
observed that P stands for the per cent, loss in the line of the delivered 



470 ARMATURE WINDING AND MOTOR REPAIR 

power, not for the per cent, loss in the line of the power at the generator; 
and that E is the potential at the end of the line and not at the generator. 

When the power factor cannot be more accurately determined, it may 
be assumed to be as follows for any alternating system operating under 
average conditions: Incandescent lighting and synchronous motors, 
95 per cent. ; lighting and induction motors together, 85 per cent.; induc- 
tion motors alone, 80 per cent. 

In continuous current, three-wire systems, the neutral wire for feeders 
should be made of ^ the section obtained by the formula for either of the 
outside wires. In both continuous and alternating-current systems, the 
neutral conductor for secondary mains and house wiring should be taken 
as large as the other conductors. 

The three wires of a three-phase circuit and the four wires of a two- 
phase circuit should be made all the same size, and each conductor should 
be of the cross-section given by the first formula. 



GENERAL WIRING DATA FOR FORMULA FOR DIRECT- AND ALTERNATING- 
CURRENT CIRCUITS 25 AND 60 CYCLES 



Size 
of wire 
B. & S. 


Area 
wire 
cir. 
mils 


Wt. Ibs. 
bare 
wire 
per 
1000 ft. 


Resist- 
ance 
ohms 
per 
1000 ft. 

at 2or i 


Value of B for formula (page 468) 


Size 
of 
wire 
B. & 

S. 


25 cycles 


40 cycles 


Per cent, power factor 


Per cent, power factor 








at 4\j iv. 


95 


90 


85 


80 


95 


90 


85 


80 




0000 


211,600 


640.73 


0.04879 


1.23 


.29 


1.33 


.34 


1.52 


1.53 


.61 


1.67 


0000 


000 


167,805 


508.12 


0.06154 


1.18 


.22 


1.24 


.24 


.40 


1.41 


.48 


1.51 


000 


00 


133,079 


402.97 


0.07758 


1.14 


.16 


1.16 


.16 


.25 


1.32 


.35 


1.37 


00 





105,560 


319.00 


0.09775 


1.10 


.11 


1.10 


.09 


.19 


1.24 


.26 


1.26 





1 


83,694 


253.43 


0.1234 


1.07 


.07 


1.05 


.03 


.14 


1.17 


.18 


1.17 


1 


2 


66,373 


200.98 


0.1556 


.05 


.01 


1.02 




.11 


1.12 


.12 


1.10 


2 


3 


52,633 


159.38 


0.1962 


.03 


.02 


1 




.07 


1.08 


.07 


1.05 


3 


4 


41,742 


126.40 


0.2473 


.02 




1 




.05 


1.06 


.03 


1 


4 


5 


33,102 


100.23 


0.3120 




1 


1 




.03 


1.01 




1 


5 


6 


26,250 


79.49 


0.3934 




1 


1 




.02 


1 




1 


6 


7 


20,816 


63.03 


0.4959 




1 


1 


1 


.01 


1 




1 


7 


8 


16,509 


49.99 


0.6250 




1 


1 


1 




1 




I 


8 


9 


13,090 


39.60 


0.7886 




1 




1 




1 




1 


9 


10 


10,382 


31.40 


0.9940 




* 


1 


1 




1 


1 




10 



APPENDIX 



471 



GENERAL WIRING DATA FOR FORMULA FOR DIRECT- AND ALTERNATING- 
CURRENT CIRCUITS 60 AND 125 CYCLES 



Size 

of wire 
B. & S. 


Area 
wire 
cir. 
mils. 


Wt. Ib. 
bare 
wire 

ioo er ft. 


Resist- 
ance 
ohms 

1000 ft. 
at 20C. 


Value of B for formula (page 468) 


Size 
of 
wire 
B. & 

S. 


60 cycles 


125 cycles 


Per cent., power factor 


Per cent., power factor 


95 


90 


85 


80 


95 


90 


85 


80 


0000 


211,600 


640.73 


0.04879 


.62 


.84 


1.99 


2.09 


2.35 


2.86 


3.24 


3.49 


0000 


000 


167,805 


508.12 


0.06154 


.49 


.66 


1.77 


1.95 


2.08 


2.48 


2.77 


2.94 


000 


00 


133,079 


402 . 97 


0.07758 


.34 


.52 


1.60 


.66 


.86 


2.18 


2.40 


2.57 


00 





105,560 


319.00 


0.09775 


.31 


.40 


1.46 


.49 


.71 


.96 


2.13 


2.25 





1 


83,694 


253.43 


0. 1234 


.24 


.30 


1.34 


.36 


.56 


1.75 


1.88 


1.97 


1 


2 


66,373 


200.98 


0. 1556 


.18 


.23 


1.25 


.26 


.45 


.60 


1.70 


1.77 


2 


3 


52,633 


159.38 


0. 1962 


.14 


.17 


1.18 


.17 


.35 


.46 


1.53 


1.57 


3 


4 


41,742 


126.40 


0.2473 


.11 


.12 


1.11 


.10 


.27 


.35 


.40 


1.43 


4 


5 


33,102 


100.23 


0.3120 


.08 


.08 


1.06 


.04 


.21 


.27 


.30 


1.31 


5 


6 


26,250 


79.49 


0.3934 


.05 


.04 


1.02 




.16 


.20 


.21 


1.21 


6 


7 


20,816 


63.03 


0.4958 


.03 


.02 


1 




.12 


.14 


.14 


1.13 


7 


8 


16,509 


49.99 


0.6250 


.02 




1 




.09 


.10 


.09 


1.07 


8 


9 


13,090 


39.60 


0.7886 






1 




.06 


.06 


.04 


1.02 


9 


10 


10,382 


31.40 


0.9940 






1 




.04 


.03 




1 


10 



Amperes in Alternating-current Circuits. The following tables give 
the amperes per lead wire per kilowatt for single-phase and three-phase 
balanced loads. The single-phase table can be used for two-phase 
balanced loads by using a current value corresponding to twice the stated 
potential of the circuit or by dividing the current value at the potential 
of the circuit by two. That is, each wire of a two-phase circuit carries 
one-half of the current indicated at the load specified. These tables 
show the value of the current at power factors varying from unity to 
70 per cent. The power of any circuit in kilowatts can, therefore, be 
computed by dividing the reading of the ammeter by the tabulated value 
corresponding to the measured power factor and voltage of the circuit. 
These values are correct only for a balanced load (and there is generally 
a slight unbalancing of the loads on the phases), but the table is useful in 
computing the sizes of wire required for transmission purposes. 

This table was derived from the following formulas : 

For single-phase circuits: Amperes per wire = watts * (volts X 
power factor). 

For three-phase circuits: Amperes per wire = total watts -r (volts 
between wires X power factor X A/S)- 

For two-phase circuits: Amperes per wire = total watts -i- (volts 
between wires of one phase X power factor X 2). 

In making the computations the number of watts was assumed as 
1000, and the amperes were computed for various values of emf. to a 



472 



ARMATURE WINDING AND MOTOR REPAIR 



sufficient number of decimal places to insure accuracy. The tables were 
then extended by multiplication and division. If desired, these tables 
can be further extended to cover voltages outside of their limits by using 
the tabular values corresponding to potentials of one-tenth (or 10 times) 
the desired potential, care being used to shift the decimal point in the 
proper direction. 

The values for intermediate power factors can be approximated from 
the tables. For lower power factors, the value of the current for unity 
power factor can be divided by actual power factor of the circuit or 
multiplied by the reciprocal of this power factor. 



SINGLE-PHASE CIRCUITS AMPERES FOR ONE KILOWATT AT DIFFERENT 
POWER FACTORS 

Power factor in per cent, 



Volts 


100 


95 


90 


85 


80 


75 


70 


110 


9.0909 


9 . 5693 


10. 1010 


10.6952 


11.3636 


12.1211 


12.9870 


220 


4.5455 


4.7847 


5.0505 


5.3476 


5.6819 


6.0606 


6.4936 


440 


2.2727 


2 . 3923 


2.5252 


2.6738 


2.8409 


3 . 0303 


3 . 2467 


550 


1.8182 


1.9139 


2.0202 


2.1390 


2.2728 


2.4242 


2.5974 


1100 


0.9091 


0.9569 


1.0101 


0.0695 


1 . 1364 


1.2121 


1 . 2987 


2200 


0.4545 


0.4785 


0.5050 


0.5348 


0.5682 


0.6061 


0.6494 


3300 


0.3030 


0.3190 


0.3367 


0.3565 


0.3788 


0.4040 


0.4329 


6600 


0.1515 


0.1595 


0.1684 


0.1783 


0.1894 


0.2020 


0.2165 


11000 


0.0909 


0.0957 


0.1010 


0.1070 


0.1136 


0.1212 


0.1299 



THREE-PHASE CIRCUITS AMPERES PER WIRE FOR ONE KILOWATT AT 
DIFFERENT POWER FACTORS 

Power factor per cent. 



Volts 


100 


95 


90 


85 


80 


75 


70 


110 


5.2486 


5.5249 


5.8319 


6.1749 


6.5608 


6.9982 


7.4980 


220 


2.6243 


2 . 7624 


2.9159 


3 . 0874 


3.2804 


3.4992 


3 . 7490 


225 


2.5660 


2.7010 


2.8511 


3.0188 


3.2075 


3.4213 


3.6657 


440 


1.'3122 


1.3812 


1.4579 


1 . 5437 


1 . 6402 


1 . 7495 


1 . 8745 


550 


1.0497 


1 . 1050 


1 . 1664 


1 . 2350 


1.3121 


1.3996 


1.4996 


1100 


0.5249 


0.5525 


0.5832 


0.6175 


0.6561 


0.6998 


0.7498 


2200 


0.2624 


0.2762 


0.2916 


0.3087 


0.3280 


0.3499 


0.3749 


3300 


0.1749 


0.1842 


0.1944 


0.2058 


0.2187 


0.2333 


0.2499 


6600 


0.0875 


0.0921 


0.0972 


0.1029 


0.1093 


0.1167 


0.1249 


11000 


0.0525 


0.0552 


0.0583 


C.0617 


0.0656 


0.0700 


0.0750 



APPENDIX 



473 



MINIMUM SIZE WIRE FOR MOTOR SERVICES WHEN CONCEALED OR 
PARTLY CONCEALED WIRES ARE USED 



HP. 


Size wire B. & S. gauge 


110 volts 


220 volts 


550 volts 


X 


14 


14 


14 


1 


14 


14 


14 


2 


12 


14 


14 


3 


10 


14 


14 


4 


8 


12 


14 


5 


6 


10 


14 


7K 


4 


8 


14 


10 


3 


6 


12 


15 





5 


10 


20 


00 


3 


8 


25 


000 


1 


6 


30 


0000 





5 


40 




00 


3 


50 


.... 


000 


2 


60 


.... 


0000 


1 


70 


.... 


.... 





80 


.... 


.... 


00 


90 






000 


100 





...- v ; 


0000 



VALUES OF FIELD CURRENT IN DIRECT-CURRENT GENERATORS 

It has been found that a fair average for the field amperes of different 
sizes of generators is as follows : 



Kw.... 


1 


5 


10 


20 


30 


50 


75 


100 


Per cent 


8 


6 


5 


4 


3 5 


3 


3 


2 75 





















The field current (expressed as a percentage of full-load current on 
lines) is determined with all of the resistance cut out, that is, with the 
rheostat on the first notch. 



474 ARMATURE WINDING AND MOTOR REPAIR 



ui % 



ON 8J TA1 



-ON 



.2 a 



JO '0 N 



jo 



edray 



V* V* V* V* V* V* \* \* \* V* \* V* V* V* V* V* V* v* 
W\ \ WN WN N ^ "X *X WX X "X *X WN \ \ rt\ \ cK 






jo 



Bl 



jo 



<* <N O 



.2=8 



sdray 



c.t 



\^|i \* v* \c 

\ M\ w\ \ 



jo 



sdray 



II 



ui ' 



jo -ON 



8 



APPENDIX 



475 



hes 



-HOOOOC5 



OOMOOM OOOOOfN O?OIN(N 

*t>O-*O5 (Nt^<NOOO -*)IOOJ 

II-IT-I ININCOPO-^ "501^00 



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<N 00 CD O CO 00 CO <-* CN 

coec^iocD t^Oii-ieo 



00 <N Tjn Tjl O 00 00 O5 CO 00 t>^ O5 CO 

-Ci-HlNCC ^M<CiCOOO O5-tCOCO 



a3nT?3 -g y -g 



uorjBjnsui 



fOOOOCOOO 
CO <** CO OS W 



W CO O5 CO l^ CO OS CO Hi 

rH *-ii-ie^<NC3 COrtfUS 



'M-f-^OOM 



00 O (N C> 

SS 55 S 



(MCOOOIN 



i-i (NCOiO(N O5OCOO 



-O<Ni-ieO <N<N 
<N CO *' CO 00 O <N 



i- 

V. 

II 

Ja o 

IJ 

gs 

8 

5? 



nd a number equal to or greater than the given hp. 
mber of the smallest wire permissible. For other e 
for 0.75, and 0.78 for 0.70 per cent, efficiency. 



conditions of wiri 
, will be found th 
85; 0.89 for 0.80; 



nder 
the he 
by 1.06 



II 



476 



ARMATURE WINDING AND MOTOR REPAIR 



aximum hp. allowed on wires according to Na 
ities. P. F., 0.85; Efficiency, 0.90. 



Table showing 
and carrying ca 






O -H t- 00 <$< N. OS 

3 S S 2 ?3i <N co 



SS^ggg 



a3ntj3 -g 35, 



ing 
ties 



2- 
og 



uoi^jnsut 

' 



^H ^H 1 (N 



*H rt (N (N CO 



APPENDIX 



477 



rHOO 



,-H rH ^H r-l (N <N CC 



T-iOO 



'-iM>COO(NOi-c 
_ _ ^ ^H IN (N CO 



tfl 



c 2 <B 5 
S3 *< 

I* "a *3 

0> ,0 W Q.I3 



ft* * 



1^1* 

2 Q *& i 



UI4 

2 >> 5 




fl a, 

^^^^^ 
*" "^s *" 

ic o oq S e 



|I|>1 

5 !!i 

> ^ * -a *" 

T3 ^ "O 

te-2. - J 

o > ^ >. ** 

*" - 3 - 



llfll 




478 



ARMATURE WINDING AND MOTOR REPAIR 



General Motor Data. The following table gives the horsepower, vol- 
tage and speed of standard motors at various frequencies for two- and 
three-phase operation: 

VOLTAGE, HORSEPOWER AND SPEEDS OF STANDARD MOTORS 



Windings 


Cycles 


Volts 


H P . 


Rpm. 


2- and 3-phase 


60 


110, 220, 440, 550 


1, 2,3, 5, 7.5, 10, 15 


1800 


2- and 3-phase 


60 


110, 220, 440, 550 


0.75, 1.5, 2, 3, 5, 7.5 


1200 


3-phase 


40 


220, 550 


1,1.5, 2, 3, 5, 


1200 


3-phase 


40 


220, 550 


1,2,3 


800 


3-phase 


25 


110, 220, 440 


1, 2, 3, 5 


1500 


3-phase 


25 


110, 220, 440 


1, 2, 3, 5 


750 



STANDARD POTENTIALS FOR WHICH SKELETON FRAME MOTORS ARE 

WOUND 



Style of winding 


Cycles 


Volts 


Under 50 hp. 


50 hp. 
and above 


3- and 2-phase 


60 
40 
25 


220 
220 
220 


440-2200 
550 
440 


3-phase 


3-phase 





Standard motors will develop considerably more torque than that given 
at the rated speed and voltage, so that there is ample margin to carry full 
load or temporary overloads under ordinary variations of voltage. 

The maximum output of an induction motor varies with the square 
of the voltage at the motor's terminal, but motors will give their rated 
output even with a drop of 10 per cent, in the voltage, as their maximum 
output is greatly in excess of the rated value. At the lower potential 
the efficiency and power factor will be increased at light loads; the full 
load values, however, are usually somewhat lower. 

Transformer Rating for Alternating-current Motors. For the larger 
motors the capacity of the transformers in kilowatts should equal the out- 
put of the motor in hp. Small motors should be supplied with a somewhat 
larger transformer capacity, especially if, as is desirable, they are expected 
to run most of the time near full load, or even at slight overload. Trans- 
formers of less capacity than those given in the following table should 
not be used even when a motor is to be run at only partial load. 

For the operation of industrial motors, from three-phase systems, three 
single-phase units or one three-phase unit are recommended, although, 
if desired, two single-phase transformers may be used. The use of 
the three-phase transformer greatly reduces the space required and makes 



APPENDIX 



479 




Phase B 



Taps 



FIG. 299. Connections for a 2-phase auto-starter for a 4-wire circuit. 



Generator 




Phase A Phase B 



Taps 



FIG. 300. Connections for a 2-phase auto-starter for a 3-wire circuit. 




Tape 



FIG. 301. Connections for a 3-phase auto-starter. 



480 ARMATURE WINDING AND MOTOR REPAIR 



the wiring very simple, while the only advantage gained in using three 
single-phase transformers rather than a three-phase transformer is that 
in the case of one transformer burning out, the other two may be used to 
operate the motor at reduced load. 

RATINGS OF TRANSFORMERS REQUIRED FOR INDUCTION MOTORS 



Size of motor 
hp. 


Kilowatts per transformer 


Two single-phase 
transformers 


Three single-phase 
transformers 


One three-phase 
transformer 


1 


0.6 


0.6 




2 


1.5 


1.0 


2.0 


3 


2.0 


1.5 


3.0 


5 


3.0 


2.0 


5.0 


7^ 


4.0 


3.0 


7.5 


'10 


5.0 


4.0 


10.0 


15 


7.5 


5.0 


15.0 


20 


10.0 


7.5 


20.0 


30 


15.0 


10.0 


30.0 


50 


25.0 


15.0 


50.0 


75 


40.0 


25.0 


75.0 


100 


50.0 


30.0 


100.0 



RATING OF TRANSFORMERS FOR THREE- AND TWO-PHASE INDUCTION 
MOTORS ON VARIOUS CIRCUITS 



Single-phase transformer voltages 



jjeiiverea 
voltage of 
circuit 


110-volt motor % 


220-volt motor 


Primary 


Secondary 


Primary 


Secondary 


1100 

2200 


1100 

2200 


122 

122 


1100 

2200 


244 
244 



SIZE OF WIRES FOR SINGLE-PHASE MOTORS 



220 volts 



Jlp. 


Full-load current-amp. 


Size of wire B. & S. gauge 


1 


6 


14 


2 


11 


12 


3 


16 


10 


4 


22 


8 


5 


26 


6 



APPENDIX 



481 



SIZE OF WIRES OF DIRECT-CURRENT MOTORS 



Horsepower 


220 Volts 


Full load current, 
amp. 


Size of wire, mains, 
B. & S. gauge 


fize of wire, branches, 
B. & 8. gauge 


1.0 


4 


14 


14 


2.0 


8 


14 


14 


3.0 


12 


14 


14 


4.0 


15 


14 


12 


5.0 


19 


12 


10 


7.5 


28 


8 


8 


10.0 


38 


6 


6 


12.5 


47 


6 


4 


15.0 


56 


5 


4 


17.5 


65 


4 , 


3 


20.0 


75 


3 


1 


25.0 


94 


1 





30.0 


113 





2/0 


35.0 


131 


2/0 


3/0 


40.0 


150 


2/0 


4/0 


45.0 


169 


3/0 


4/0 


50.0 


188 


4/0 


250,000 C.M. 


55.0 


206 


4/0 


300,000 C.M. 


60.0 


225 


4/0 


300,000 C.M. 


65.0 


244 


250,000 C.M. 


350,000 C.M. 


70.0 


263 


300,000 C.M. 


400,000 C.M. 


75.0 


281 


300,000 C.M. 


500,000 C.M. 


80.0 


300 


350,000 C.M. 


500,000 C.M. 


85.0 


319 


400,000 C.M. 


500,000 C.M. 


90.0 


338 


500,000 C.M. 


600,000 C.M. 


95.0 


356 


500,000 C.M. 


600,000 C.M. 


100.0 


375 


500,000 C.M. 


700,000 C.M. 


125.0 


463 


700,000 C.M. 


900,000 C.M. 


150.0 


563 


800,000 C.M. 


1,100,000 C.M. 


200.0 


750 


1,300,000 C.M. 


1,700,000 C.M. 


250.0 


938 


1,700,000 C.M. 


2-900,000 C.M. 


300.0 


1,125 


2-800,000 C.M. 


2-1,100,000 C.M. 



Column headed " Size of wire, branches " gives size of wire for branches 
and for mains supplying one motor and is based on 50 per cent, over- 
load. 

Column headed "Size of wire, mains" gives size of wire to be used for 
mains, but in no case must the size of these mains be less than that re- 
quired for the 50 per cent, overload on the largest motor such mains 
supply. 

The question of drop is not taken into consideration in these tables. 

31 



482 ARMATURE WINDING AND MOTOR REPAIR 







aT - M 








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jS 








i 




O10B8 




lit 


N . l ocooc5 22g ,og 3 .. 32 o S o E: 


s 




3 5) * 




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Si 


as 


4 




Sja 


O Q 


K 




O pg CO 


<N CM CO rt< TfH O O 







ffl- 


S8 

(N CO 


03 









| 


1 


ire main 
. gauge 







<N 


|2 


(N CO CO 


S 




r 


'" V 






Oj"fl 








r2 2 a 


X) T _22^coco^io^bIoi2c5^ioc 






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APPENDIX 



483 



000 .00. 00 



d 9 O, Q d 



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o o 



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1-Hr-lOOOOOO 
CO ^^ 



. 

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OOOOOOOOQQOO 



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p o o o o 

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t>.t>.QOOOO5OiO<MOOOO 

i I i I rH C<l O< CO 



484 ARMATURE WINDING AND MOTOR REPAIR 
WIRING DATA FOR DIRECT-CURRENT MOTORS, 115 VOLTS 



Horsepower 


Approx. 
full 
load 
current, 
amp. 


Size of 
fuses, amp. 


Size of 
fused switches, 
amp. 


Size of wire, 
B. & S. gauge 


Size of conduit, 
inches 


H 


2.5 


3 


30 


14 


y 2 


H 


4.4 


5 


30 i 


14 


x 


i 


8.4 


10 


30 


14 


y* 


2 


17.0 


20 


30 


12 


H 


3 


23.6 


30 


30 


8 


H 


5 


38.7 


50 


60 


6 


i 


7K 


57.6 


75 


100 


4 


i 


10 


75.1 


85 


100 


2 


IK 


15 


113 


150 


200 


00 


2 


20 


151 


175 


200 


000 


2 


25 


191 


225 


400 


0000 


2 


30 


226 


275 


400 


300000 


2>i 


35 


264 


325 


400 


400000 


2K 


40 


300 


375 


400 


500000 


3 


50 


375 


*450 


500 


600000 


3 


55 


405 


*500 


500 


700000 


3H 


65 


480 


*600 


600 


900000 


4 


70 


520 


*650 


700 


1000000 


4K 


75 


555 


*700 


700 


1100000 


4K 



*When fuse sizes exceed 600 amperes, circuit breakers of approved 
type should be substituted. 



APPENDIX 485 

WIRING DATA FOR DIRECT-CURRENT MOTORS, 230 VOLTS 



Horsepower 


Approx. 
full 
load 
current, 
amp. 


Size of 
fuse, amp. 


Size of fused 
switches, amp. 


Size of wire, 
B. & S. gauge 


Size of conduit, 
inches 


H 


1.3 


3 


30 


14 


H 


H 


2.3 


3 


30 


14 


H 


i 


4.2 


5 


30 


14 


H 


2 


8.6 


10 


30 


14 


a 


3 


11.8 


15 


30 


12 


H 


5 


19.0 


25 


30 


10 


H 


7K 


28.6 


35 


60 


8 


H 


10 


37.6 


45 


60 


6 


i 


15 


55.0 


70 


100 


4 


i 


20 


73.8 


90 


100 


2 


1M 


25 


95 


125 


200 





in 


30 


113 


150 


200 


00 


2 


35 


130 


175 


200 


000 


2 


40 


150 


175 


200 


000 


2 


50 


185 


225 


400 


0000 


2 


55 


200 


250 


400 


250000 


2 


60 


219 


275 


400 


300000 


2^ 


65 


238 


300 


400 


350000 


^ 


70 


260 


325 


400 


400000 


2K 


75 


274 


350 


400 


450000 


3 


80 


288 


375 


400 


500000 


3 


85 


308 


400 


400 


500000 


3 


90 


328 


*425 


*500 


500000 


3 


100 


368 


*450 


*500 


600000 


3 



*When fuse sizes exceed 600 amperes, circuit breakers of approved 
type should be substituted. 



486 



ARMATURE WINDING AND MOTOR REPAIR 



WIRING DATA FOR DIRECT-CURRENT MOTORS, 550 VOLTS 



Horse- 
power 


Approx. 
full-load 
current-amp. 


Size of 
fuses, amp. 


Size of fused 
switches, amp. 


Size of wire, 
B. & S. gauge 


Size of 
conduit, 
inches 


H 


0.52 


1 


30 


14 


H 


K 


0.94 


2 


30 


14 


H 


i 


1.80 


3 


30 


14 


K 


2 


3.50 


5 


30 


14 


y 2 


3 


4.93 


6 


30 


14 


x 


5 


8.00 


10 


30 


14 


K 


7H 


12.00 


15 


30 


12 


K 


10 


15.60 


20 


30 


12 


H 


15 


22.80 


35 


60 


8 


X 


20 


30.70 


40 


60 


6 


i 


25 


40.00 


50 


60 


6 


i 


30 


47.00 


60 


60 


6 


i 


35 


55.00 


70 


100 


4 


i 


40 


62.00 


75 


100 


2 


1M 


50 


78.00 


95 


100 


2 


IK 


55 


84.00 


100 


100 


1 


IK 


60 


92.00 


125 


200 





IK 


65 


98.00 


125 


200 


00 


IK 


70 


108 . 00 


150 


200 


00 


2 


75 


114.00 


150 


200 


00 


2 


80 


120.00 


150 


200 


00 


2 


85 


129.00 


150 


200 


00 


2 


90 


138.00 


175 


200 


000 


2 


100 


152.00 


200 


200 


0000 


2 



WIRING DATA FOR INDUCTION MOTORS SINGLE-PHASE 110 VOLTS, 
ALL FREQUENCIES, STANDARD SPEEDS 



Horse- 
power 


Approx. 

full current- 
amperes 


Size wire 
B. & S. 
gauge 


Size of 
switches 
in amperes 


Size of 
starting 
fuses, 
amperes 


Size of 
running 
fuses, 
amperes 


Size of 
conduit, 
inches 


1 


16.4 


10 


30 


25 


20 


H 


2 


24.0 


8 


60 


45 


30 


i 


3 


33.6 


6 


100 


70 


45 


1M 


4 


43.6 


4 


100 


85 


60 


IK 


5 


54.0 


4 


100 


100 


70 


IK 


7K 


80.0 


1 


200 


200 


100 


IK 


10 


106.0 





200 


200 


125 


2 



APPENDIX 



487 



WIRING DATA FOB INDUCTION MOTORS SINGLE-PHASE 220 VOLTS, 
ALL FREQUENCIES, STANDARD SPEEDS 



Horse- 
power 


Approx. 

full current- 
amperes 


Size of 
wire, 
B. & S. 
gauge 


Size of 
switches 
in amperes 


Size of 
starting 
fuses, 
amperes 


Size of 
running 
fuses, 
amperes 


Size of 
conduit, 
inches 


1 


8.2 


14 


30 


15 


10 


X 


2 


12.0 


10 


30 


25 


15 


H 


3 


16.8 


8 


60 


35 


20 


i 


4 


21.8 


6 


60 


45 


25 


iK 


5 


27.0 


6 


60 


55 


35 


IK 


7K 


40.0 


4 


100 


75 


50 


IK 


10 


53.0 


2 


100 


100 


65 


*H 



WIRING DATA FOR INDUCTION MOTORS THREE-PHASE 110 VOLTS, 
ALL FREQUENCIES, STANDARD SPEEDS 



Horse- 
power 


Approx. 

full current- 
amperes 


Size of 
wire, 
B. & S. 
gauge 


Size of 
switches in 
amperes 


Size of 
starting 
fuses, 
amperes 


Size of 
running 
fuses, 
amperes 


Size of 
conduit, 
inches 


1 


6 


14 


30 


15 


9 


i* 


2 


12 


12 


30 


25 


18 


x 


3 


18 


' 10 


60 


35 


27 


H 


5 


27 


6 


100 


65 


40 


i 


7K 


39 


4 


100 


80 


58 


1H 


10 


51 


2 


200 


125 


75 


iK 


15 


75 


1 


200 


150 


112 


IK 


20 


101 


00 


400 


225 


150 


2 


25 


125 


000 


400 


250 


187 


2 


30 


150 


0,000 


400 


325 


225 


IK 


35 


175 


300.000 


400 


400 


262 


2K 


40 


210 


400,000 


500 


*500 


315 


3 


50 


246 


450,000 


500 


*500 


369 


3 



* When fuse sizes exceed 600 amperes, circuit breakers of approved 
type should be substituted. 



488 



ARMATURE WINDING AND MOTOR REPAIR 



WIRING DATA FOR INDUCTION MOTORS THREE-PHASE 220 VOLTS, 
ALL FREQUENCIES, STANDARD SPEEDS 



Horsepower 


Approx. 
full 
current- 
amperes 


Size of 
wire, 
B. &. S. 
gauge 


Size of 
switches 
in 
amperes 


Size of 
starting 
fuses, 
amperes 


Size of 
running 
fuses, 
amperes 


Size of 
conduit, 
inches 


u 


1.0 


14 


30 




5 


u 


s* 

K 


2.0 


14 


30 





5 


/ t 

K 


i 


3.0 


14 


30 


10 


5 


H 


2 


6.0 


14 


30 


15 


10 


H 


3 


9.0 


12 


30 


25 


15 


H 


5 


13.3 


10 


60 


35 


20 


H 


7K 


19.5 


8 


60 


45 


30 


i 


10 


25.5 


6 


100 


65 


40 


i 


15 


37.5 


4 


100 


100 


60 


1H 


20 


50.5 


2 


200 


130 


75 


IK 


25 


62 '.5 


1 


200 


150 


95 


IK 


30 


75.0 





200 


200 


100 


IK 


35 


87.5 





200 


200 


125 


IK 


40 


105.0 


00 


400 


225 


150 


2 


50 


123.0 


000 


400 


250 


175 


2 


75 


186.0 


300,000 


400 


400 


275 


2K 


100 


243.0 


450,000 


500 


*550 


350 


3 


150 


362.0 


800,000 


800 


*850 


*550 


3K 


200 


480.0 


1,200,000 


1,000 


*1,150 


*725 


4 



* When fuse sizes exceed 600 amperes, circuit breakers of approved 
type should be substituted. 



APPENDIX 



489 



WIRING DATA FOR INDUCTION MOTORS THREE-PHASE 440 VOLTB, 
ALL FREQUENCIES, STANDARD SPEEDS 



Horse- 
power 


Appro*, 
full current- 
amperes 


Size of 
wire, 
B. & S. 
gauge 


Size of 
switches 
in amperes 


Size of 
starting 
fuses, 
amperes 


Size of 
running 
fuses, 
amperes 


Size of 
conduit, 
inches 


1 


1.5 


14 


30 


5 


3 


K 


2 


3.0 


14 


30 


10 


5 


y 2 


3 


4.5 


14 


30 


15 


6 


H 


5 


i.7 


14 


30 


20 


10 


y* 


7^ 


9.8 


12 


30 


25 


15 


H 


10 


12.8 


10 


60 


35 


20 


H 


15 


18.8 


8 


60 


50 


30 


i 


20 


25.2 


6 


100 


75 


35 


i 


25 


31.3 


6 


100 


75 


50 


i 


30 


37.5 


4 


100 


90 


55 


IX 


35 


43.8 


4 


100 


100 


65 


i'K 


40 


52.5 


2 


200 


135 


75 


tot 


50 


61.5 





200 


175 


90 


w 


75 


93.0 


00 


400 


225 


140 


2 


100 


121.5 


000 


400 


275 


175 


2 


150 


181.0 


300,000 


400 


400 


275 


w 


200 


240.0 


450,000 


400 


*550 


350 


3 



* When fuse sizes exceed 600 amperes, circuit breakers of approved 
type should be substituted. 

Circuit-breakers for Overload Protection of Motors. There are two 
overload conditions that may injure an electric motor. (1) A continu- 
ous load beyond the overload rating of the motor and (2) by an exces- 
sive momentary overload Overloads of the second class can obviously 
be much larger than those of the first class without exceeding safe limits. 
For example, a continuous overload of 50 per cent, may injure a motor, 
while the safe momentary overload may be three or four times the full 
load. If a motor is subjected to overloads of both these classes, ade- 
quate overload protection may be difficult with either a circuit-breaker 
or fuses. Both devices must be set for maximum allowable current, 
usually considerably in excess of the continuous safe carrying capacity 
of the motor, and a continuous overload current inside this limit may 
work injury. Such conditions are unusual, however, and either fuses 
or a properly designed circuit-breaker will ordinarily afford all necessary 
protection. 

For Induction Motors. In case the squirrel-cage induction motors is 
protected by a circuit-breaker, the motor should be so connected as to be 
without overload protection when the starter handle is in the starting 
position; in addition, the circuit-breaker should be provided with a time 



490 ARMATURE WINDING AND MOTOR REPAIR 

element device to prevent opening when the starter handle is thrown to 
the running position. Without this time element the circuit-breaker 
when connected as just described, must be calibrated for two or three 
times the rated full-load current of the motor. If the circuit-breaker 
must be effective in the starting position it must be provided with a time 
element and be calibrated for from three to five times the rated full-load 
current; with this calibration no protection will be afforded from contin- 
uous overloads which might work injury. 

For Motors Carrying a Uniform Load. That is, motors driving line 
shafts, fans, machine tools, or any machines which give them a fairly 
uniform load, carrying maximum continuous rated output at least part 
of the time and possibly full-rated overload for short periods. In general, 
the overload protection for such motors should have a normal calibration 
equal to 125 per cent, of normal full-load motor current. If no standard 
circuit-breaker is listed for this rating, the next larger listed size should 
be used. A circuit-breaker selected in this manner will have a maximum 
calibration at least twice full-load current. Where overloads greater 
than 200 per cent, of normal current are regularly experienced for periods 
not exceeding two seconds, a time element device should be added to the 
circuit-breaker. For heavy overloads frequently recurring and continu- 
ing for periods of five seconds or longer, the circuit-breaker should have 
a maximum calibration of at least 10 per cent, in excess of the maximum 
current. 

For Motors Started and Stopped Frequently. Motors on cranes, ele- 
vators, pumps, air compressors, or other apparatus requiring frequent 
starting and stopping, should have circuit-breakers with a calibration 
of at least 25 per cent, in excess of the maximum current actually required 
by the motor during any continuous period of more than five seconds. 
A time element device should be supplied to care for greater overloads 
lasting less than five seconds. 

Belting. Rubber belts should always be kept free from grease or 
animal oils. If they slip, moisten the inside of the belt with boiled 
linseed oil. Some fine chalk, sprinkled on over the oil, will help the belt. 

Length of Belts. Add the diameter of the two pulleys together, multi- 
ply by three and one-seventh, divide the product by two, add to the 
quotient the distance between the center of the shafts, and the product 
will be the required length. 

ORDINARILY ACCEPTED EQUIVALENTS OF BELTING 

2-ply Rubber Light single leather. 
3-piy Rubber Medium single leather. 
4-ply Rubber Heavy single leather. 
5-ply Rubber Light double leather. 
6-ply Rubber Medium double leather. 
7-ply Rubber Heavy double leather. 
8-ply Rubber Triple leather. 



APPENDIX 



491 



The thickness of rubber belting is usually figured at J-f 6 inch per ply. 

Horsepower Transmitted by Belting. One-inch single belt moving at 
a velocity of 1000 ft. per minute equals one horsepower. One-inch 
double belt moving 700 ft. per minute equals one horsepower. The 
horsepower of any belt equals its velocity in feet per minute, multiplied 
by its width and divided by 1000 for single, and by 700 for double belts. 

The following table is based on single belts running on pulleys of equal 
diameter, revolving at 100 rpm. The power transmitted at other speeds 
is in direct proportion. For double belts, multiply by ten-sevenths. 



HORSEPOWER TRANSMITTED BY BELTING 



Diameter of 
pulley in 
inches 


Width of belt in inches 


2 


3 


4 


5 


6 


8 


10 


12 


14 


16 


18 


20 


22 


6 


.44 


.65 


.87 


1.09 


1.31 


















7 


.51 


.76 


1.01 


1.27 


1.53 


















8 


.58 


.87 


1.16 


1.45 


1.75 


















9 


.65 


.98 


1.31 


1.64 


1.97 


















10 


.73 


1.09 


1.45 


1.81 


2.18 


















11 


.80 


1.20 


1.60 


2.00 


2.40 


















12 


.87'!. 31 


1.75 


2.18 


2.62 


















13 


.95 


1.42 


1.89 


2.36 


2.83 


















14 


1.02 


1.52 


2.02 


2.53 


3.05 


















15 


1.09 


1.64 


2.19 


2.73 


3.29 


















16 


1.16 


1.74 


2.32 


2.91 


3.48 


















17 


1.24 


1.85 


2.47 


3.09 


3.70 


















18 


1.31 


1.96 


2.62 


3.27 


3.92 


















19 


1.39 


2.07 


2.76 


3.45 


4.14 


















20 


1.45 


2.18 


2.91 


3.64 


4.36 


















22 


1.60 


2.40 


3.20 


4.00 


4.80 


















24 






3.50 


4.40 


5.20 


7.0 


8.7 


10.5 


12.2 


14.0 


16.0 


17.0 


19.0 


26 






3.80 


4.70 


5.70 


7.6 


9.5 


11.3 


13.2 


15. 1 








28 






4. 10 


5. 10 


6. 10 


8.1 


10.2 


12.2 


14.3 


16.3 








30 






4.40 


5.40 


6.60 


8.7 


10.9 


13. 1 


15.3 


17.4 


19.0 


22.0 


24.0 


32 






4.70 


5.80 


7.00 


9.3 


11.6 


14.0 


16.3 


18.6 








34 






4.90 


6.20 


7.40 


9.9 


12.4 


14.8 


17.3 


19.8 








36 






5.20 


6.50 


7.80 


10.5 


13.1 


15.7 


18.3 


20.9 


24.0 


26.0 


29.0 


38 






5.50 


6.90 


8.30 


11.0 


13.8 


16.6 


19.3 


22.1 


25.0 


28.0 


30.0 


40 






5.80 


7.30 


8.70 


11.6 


14.6 


17.5 


20.4 


23.3 


26.0 


29.0 


32.0 


42 






6.10 


7.60 


9.20 


12.2 


15.3 


18.2 


21.4 


24.3 


28.0 


31.0 


34.0 


44 






6.40 


8.00 


9.60 


12.8 


16.0 


19.2 


22.4 


25.6 


29.0 


32.0 


35.0 


46 






6.70 


8.40 


10.00 


13.4 


16.8 


20.1 


23.4 


26.8 








48 






7.00 


8.80 


10.40 


14.4 


17.4 


21.0 


24.4 


28.0 


31.0 


35.0 


38.0 


50 






7.20 


9.00 


10.90 


14.6 


18.2 


21.8 


25.4 


29.0 


33.0 


36.0 


40-0 


54 






7.80 


9.80 


11.80 


15.6 


19.6 


23.6 


26.4 


31.2 


35.0 


39.0 


43.0 


60 


.... 




8.80 


10.80 


13.10 


17.4 


21.8 


26.2 


30.6 


34.8 


39.0 


44.0 


48.0 


66 






9.60 


12.00 


14 40 


19. 2 


24 


OQ Q 


oo a 


OQ A 


4. O 


AO n 


rq f) 


72 






10.40 


13.00 


15.60 


21.0 


^^ . u 
26.2 


^O . o 

31.4 


Oo . O 

36.6 


Oo . 4 

41.8 


"0 . U 

47.0 


4O . U 

52.0 


)O U 

58.0 


78 






11.40 


14.20 


17.00 


22.6 


ofi 4 


04 A 


OQ 


4^ 4 


e t f\ 


C'7 f\ 


AO A 


84 


.... 




12.20 


15.20 


19.40 


24.4 


^O . rt 

30.6 


o4 U 

36.4 


oy . o 
42.8 


fcO . 4 

48.6 


51 * U 

55.0 


Of .U 

61.0 


\Z I/ 

67.0 



492 ARMATURE WINDING AND MOTOR REPAIR 

Belting Rules. The power transmitted by a belt depends upon its 
width and its thickness and the speed at which it travels. Hence it is 
customary (as mentioned under "Horsepower Transmitted by Belting") 
to express the transmitting capacity as the speed in icet per minute 
required by a belt one inch wide to transmit one horsepower. For single 
or light double belt, this expression is usually given a value of about 
700, and for heavy double belt approximately 450. Thus a double belt 
four inches wide, running at 2250 ft. per minute will transmit 20 hp. 

Roughly> belt speeds should not exceed one mile per minute ; this speed is 
given when the diameter of either pulley in inches multiplied by its rpm. 
equals 20,000 (D X rpm. = 20,000). 

Minimum diameter of pulleys for long life of heavy belts : 

For double belts ............................... 12 in. 

For double belts, extra flexible ................... 10 in. 

For double 3-ply belts .......................... 18 in. 

hp. 126,500 
Ordinarily, wd = CL- X r~ 



where w width of belt, d = diameter of pulley (both in inches), and 
P = belt stress per inch width. Safe values for P are as follows : 

Single belts, above 4000 ft. per min., 33 lb.; below 4000, 46 lb.; max. 
stress never over 50 lb. Double belts, 40 to 70 lb. ; 3-ply belts, 70 to 95 lb. 

With standard construction and normal belt stress, the stress on the 
shaft and bearings is within safe limits as long as d exceeds w. To find 
length L (length) of belt when the pulley diameters D, d and the distance 
A between centers are known, use the formula 
L = 1.62 (D + d) + 2A 

To determine distance between pulley centers use the formula 

A = K (D - d} 

where the value of the constant K ranges from 3 to 4, 3.5 being a good 
average value when the diameter D is from 4 to 6 times that of d. 

The following precautions should be observed : 

a. The formula does not apply when pulleys are of nearly the same 
diameter; in this case the distance should be great enough to allow some 
belt sag. 

6. As a general rule the distance A should not exceed 25 ft. in any case. 

Rules for Pulley Sizes.---The following formula can be used to calculate 
sizes of pulleys for motor drives : 

, DXN DXN 

d = n = - j 

n d 

D = Diameter of driver. 
d = Diameter of driven pulley. 
N = Revolutions per minute of driver. 
n = Revolutions per minute of driven pulley. 

Speed of Pulleys. When the diameter of the driven pulley is given, 
to find its number of revolutions proceed as follows: Multiply the dia- 



APPENDIX 493 

meter of the driver by the number of its revolutions, and divide the pro- 
duct by the diameter of the driven. The quotient will be the number of 
revolutions of the driven pulley. 

When the diameter and revolutions of driver pulley are given, to find 
the diameter of the driven pulley that will make any given number of 
revolutions in the same time, proceed as follows : Multiply the diameter 
of the driver pulley by its number of revolutions, and divide the product 
by the number of revolutions of the driven pulley. The quotient will 
be the diameter of the driven pulley. 

To ascertain the size of the driver pulley proceed as follows : Multiply 
the diameter of the driven pulley by the number of revolutions you wish 
to make, and divide the product by the revolutions of the driver pulley. 
The quotient will be the diameter of the driven pulley. 

Chain Drives. Chains are now made to transmit power for any pur- 
pose in sizes varying from ^ hp. at 3000 rpm. to 3000 hp. at 50 rpm. 
By the use of a chain instead of a belt or gears, power may be transmitted 
with a positive speed ratio on short centers quietly and with a very high 
efficiency. Chains should be lubricated with a heavy paste grease con- 
taining no solid matter such as automobile transmission grease. 

Approximate Weight of Solid Pinions and Armed Sprockets. When 
T is the number of teeth; F the face in inches; C a constant in pounds per 
inch in face per tooth. Then T X F X C = weight of armed sprocket. 
Add 25 per cent, if the sprocket is split and add 50 per cent, if spring 
sprocket and split. For solid pinions, weight = C X T 2 X (F + 1). 

Points to Consider when Calculating Size of Chain Morse Chain 
Co. When the number of teeth equals T and the exact outside diameter 
is D, then T should be less than 20 when D equals the pitch diameter. 
When T is more than 20 teeth, D is equal to the pitch diameter plus twice 
the addendum. 

The following points should be considered in this connection: 

(1) Use sprockets having an odd number of teeth whenever possible. 

(2) When specially authorized, a larger number of teeth than shown 
may be cut in large sprocket. 

(3) Thickness of sprocket rim, including teeth, should be at least 1.2 
times the chain pitch. 

(4) The number of grooves in the sprocket, their width and distance 
apart, varies according to pitch and width of chain. 

(5) The width of the sprocket should be ^ to K inch greater on small 
drives, and K to ^ inch greater on large drives than nominal width of 
the chain. 

(6) An even number of links in the chain and an odd number of teeth 
in the wheels are desirable. 

(7) Horizontal drives preferred with slack on top strand, but for 
short drives without center adjustment slack thould be on the bottom 
strand. 

(8) Adjustable wheel centers desirable for horizontal drives and neces- 
sary for vertical drives. 



494' 



ARMATURE WINDING AND MOTOR REPAIR 













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of teeth j Small sprocket 
Desirable number of teeth 


sprockets 
Maximum number of teeth i 


ets. (See note 3.) 
Desirable number of teeth 


sprockets 
To find pitch diameter of wh 


ply number of teeth by ( 
Addendum For outside dii 


y 

CO 
i-H 

-2 

8 

3 *2 

II 
1^ 


Maximum rpm 


Tension per 1 ^ 
- u -jxu Small sprock 
inch width > 
, . ,, Small sprocke 
chain, Ibs. } 

Radial clearance beyond 


J 

CJ 

_d 

d 

i 

-5 

s- 

1 

'5 
cr 


Constants for solid pinions . 
Constants for armed sprock 






APPENDIX 



495 



(9) Avoid vertical drives. 

(10) Allow a side clearance for chain (parallel to axis of sprockets and 
measured from nominal width of chain) equal to the pitch. 

(11) Maximum linear velocity for commercial service, 1200 to 1600 
feet per minute. 

HORSEPOWER TRANSMITTED BY STEEL SHAFTING 



Diam- 
eter of 
shaft in 
inches 


Revolutions per minute 


100 


125 


150 


175 


200 


225 


250 


300 


350 


400 


*# 


1.2 


1.4 


1.1 


2.1 


2.4 


2.6 


3.1 


3.6 


4.3 


5.0 


1 Me 


2.4 


3.1 


3.7 


4.3 


4.5 


5.5 


6.1 


7.3 


8.5 


9 7 


1 Me 


4.3 


5.3 


6.4 


7.4 


8.5 


9.5 


10.5 


12.7 


14.8 


16 9 


I 1 He 


6.7 


8.4 


10.1 


11.7 


13.4 


15.1 


16.7 


20.1 


23.4 


26.8 


1'Mft 


10.0 


12.5 


15.0 


17.5 


20.0 


22.5 


25.0 


30.0 


35.0 


40.0 


2 Me 


14.3 


17.8 


21.4 


24.9 


28.5 


32.1 


35.6 


42.7 


49.8 


57.0 


2 ?6 


19.5 


24.4 


29.3 


34.1 


39.0 


44.1 


48.7 


68.5 


68.2 


78.0 


2 1 Me 


26.0 


32.5 


39.0 


43.5 


52.0 


58.5 


65.0 


78.0 


87.0 


104.0 


2'Me 


33.8 


42.2 


50.6 


59.1 


67.5 


75.9 


34.4 


101.3 


118.2 


135.0 


3 Me 


43.0 


53.6 


64.4 


75.1 


85.8 


96.6 


107.3 


128.7 


150.3 


171.6 


3 Y\o 


53.6 


67.0 


79.4 


93.8 


107.2 


120.1 


134.0 


158.8 


187.6 


214.4 


3i He 


65.9 


82.4 


97.9 


115.4 


121.8 


148.3 


164.8 


195.7 


230.7 


243 . t> 


3i Me 


80.0 


100.0 


120.0 


140.0 


160.0 


180.0 


200.0 


240.0 


280.0 


320.0 


4 #6 


113.9 


142.4 


170.8 


199.3 


227.8 


256.2 


284.7 


34J.7 


398.6 


455.6 


4'Me 


153.3 


195.3 


234.4 


273.4 


312.5 


351.5 


390.6 


468.7 


646.8 


625.0 



HORSEPOWER TRANSMITTED BY SINGLE ROPES 
(Working strain = 200e? 2 where d is diameter of rope) 



Diameter of rope in inches 



minute 


H 





1.0 


1.25 


1.50 


1.75 


2.0 


1000 


1.24 


2.25 


3.57 


5.59 


8.02 


10.85 


14.20 


2000 


2.70 


3.84 


6.84 


10.68 


15.39 


20.93 


27.36 


2500 


3.30 


4.71 


8.38 


13.10 


18.86 


25.66 


33.54 


3000 


3.83 


5.46 


9.80 


15.39 


21.87 


29.74 


38.88 


3500 


4.30 


6.23 


10.09 


17.33 


24.94 


34.03 


44.35 


4000 


4.74 


6.83 


12.15 


18.98 


27.33 


37.17 


48.59 


4500 


5.01 


7.24 


12.89 


20.15 


29.00 


39.45 


51.57 


5000 


5.20 


7.47 


13.29 


20.76 


29.89 


40.65 


53.15 


5500 


5.29 


7.60 


13.53 


21.14 


30.43 


41.39 


54.11 


6000 


5.08 


7.32 


13.10 


20.36 


29,32 


39.77 


52.12 


6500 


4.74 


6.83 


12.13 


19.00 


27.34 


37.21 


48.63 


7000 


4.12 


5.93 


10.54 


16.47 


23.72 


32.26 


42.18 


Smallest sheave, diameter, 
















inches 


26.00 


30.00 


42.00 


54.00 


60.00 


72.00 


84.00 


Allowable weight, tension 
















carriage, Ib . . 


80.00 


110.00 


200.00 


300.00 


450.00 


600 . 00 


750.00 



















NOTE. The horsepower decreases when the velocity is above 5500 feet on account 
of centrifugal force. 



496 



ARMATURE WINDING AND MOTOR REPAIR 




s i 

I : 













I 1 5 5 B 
S So o o 



APPENDIX 



497 



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|5 1 1 

3^21 

& ^ 6 o 



498 ARMATURE WINDING AND MOTOR REPAIR 



Some Handy Rules 

Diameter of a circle X 3.1416 = Circumference. 

Radius of a circle X 6.283185 = Circumference. 

Square of the radius of a circle X 3.1416 = Area. 

Square of the diameter of a circle X 0.7854 = Area. 

Square of the circumference of a circle X 0.07958 = Area. 

Half the circumference of a circle X by half its diameter = Area. 

Circumference of a circle X 0.159155 = Radius. 

Sqare root of the area of a circle X 0.56419 = Radius. 

Circumference of a circle X 0.31831 = Diameter. 

Square root a the area of a circle X 1.12838 = Diameter. 

Diameter of a circle X 0.86 = Side of inscribed equilateral triangle 

Diameter of a circle X 0.7071 = Side of an inscribed square. 

Circumference of a circle X 0.225 = Side of an inscribed square. 

Circumference of a circle X 0.282 = Side of an equal square. 

Diameter of a circle X 0.8862 = Side of an equal square. 

Base of a triangle X by ^ the altitude = Area. 

Multiplying both diameters and .7854 together = Area of an ellipse 

Surface of a sphere X by % of its diameter = Solidity. 

Circumference of a sphere X by its diameter = Surface, 

Square of the diameter of a sphere X 3.1416 = Surface. 

Square of the circumference of a sphere X 0.3183 = Surface. 

Cube of the diameter of a sphere X 0.5236 = Solidity. 

Cube of the radius of a sphere X 4.1888 = Solidity. 

Cube of the circumference of a sphere X 0.016887 = Solidity. 

Square root of the surface of a sphere X 0.56419 = Diameter. 

Square root of the surface of a sphere X 1.772454 = Circumference. 

Cube root of the solidity of a sphere X 1.2407 = Diameter. 

Cube root of the solidity of a sphere X 3.8978 = Circumference. 

Radius of a sphere X 1.1547 = Side of inscribed cube. 

Square root of (K of the square of) the diameter of a sphere = Side 
of inscribed cube. 

Area of its base X by ^ of its altitude = Solidity of a cone or pyramid, 
whether round, square or triangular. 

Area of one of its sides X 6 = the surface of a cube. 

Altitude of trapezoid X % the sum of it parallel sides = Area. 






APPENDIX 499 

EQUIVALENT VALUES OF CENTIGRADE AND FAHRENHEIT SCALES 



Temperature 


Temperature 


Centigrade 


Fahrenheit 


Centigrade 


Fahrenheit 





32 


80 


176 


5 


41 


85 


185 


10 


50 


90 


194 


15 


59 


95 


203 


20 


68 


100 


212 


25 


77 


105 


221 


30 


86 


110 


230 


35 


95 


115 


239 


38 


100.4 


120 


248 


40 


104 


125 


257 


42 


107.6 


130 


266 


45 


113 


135 


275 


50 


122 


140 


284 


55 


131 


145 


293 


60 


140 


150 


302 


65 


149 


155 


311 


70 


158 


160 


320 


75 


167 


165 


329 



INDEX 



Angle for setting brushes, 333 
Acid fumes and gases, troubles due 

to, 377 
Alternating current machines, 

causes and remedies of 

troubles in, 386 
Alternating-current windings (see 

also table of contents for 

Chapter II), 26 
coils for, 34 
coil pitch of, 32 
connections for coils of, 43 
connecting a chain winding, 48 
changing star to delta, 41 
checking phase relationship in, 

52 
double-layer, lap connected, 

47 
distributed and concentrated, 

27 
easily remembered rules for, 

51 

full and fractional pitches, 37 
grouping of coils for, 35 
lap and wave, 28 
laying out and connecting, 35 
phase spread of, 32 
polarity of coil groups in, 40 
progressive and retrogressive 

wave, 41 

simple winding diagram for, 38 
single and polyphase, 30 
spiral or chain, 27 
2-phase from 4-phase, 33 
3-phase from 6-phase, 33 
types of, 27 

whole and half-coiled, 30 
Armature slots, 23 



Armature winder's tools, 444 
Auto-starter, overhauling of, 351 



Balancing armatures 

large d. c. sizes, 87 

machine for, 460 

small and medium sizes, 150 
Banding procedure: 

for small d. c. armatures, 65 

for large d. c. armatures, 82 

for railway armatures, 99 

for large rotary armature, 410 

for different armatures, 146 

precautions, 148 

simple scheme, 408 

size of wire for bands, 148 

tension of bands, 147 

tool for applying large bands, 
86 

use of crane for, 409 
Banding wire: 

for large armatures, 84 

for all sizes of armatures, 146 

knock in armature caused by, 
395 

tension of, 147 

tension block, 454 
Bar bender for making coils, 458 
Bars of commutator : 

boring out, 313 

copper for, 319 

end rings for, 316 

excessive wear of, 318 

high and low, 302 

removal of, 304 

repairing of, 305 

repair of burned sections, 303 

replacing of, 306 

tightening, 306 



501 



502 



INDEX 



Basket coils for induction motors, 

195 
Belts, poor joints in cause sparking 

of brushes, 394 
horsepower transmitted by, 

491 

pulley sizes for, 492 
rules for use of, 492 
speed of pulleys for, 492 
static sparks from, 440 
troubles due to tension of, 391 
Bench stand for winding d. c. 

armatures, 447 
Blackening of commutator: 

remedy for, 343 

Boring out end of commutator, 313 
Bracing windings of turbo-genera- 
tors, 228 

of large a. c. generators, 223 
Break-down test for turbo-genera- 
tors, 229 

Bridging commutator bars, 258 

Brushes, adjusting and correcting 

troubles (see also table 

of contents for Chapter 

XIII), 326 

adjustment of brush-holders, 

329 

angle for setting of, 333 
blackening of, 343 
causes of rapid wear of, 331 
causes and remedies of 

troubles, 341 
checking setting, 334 
chattering of, remedy for, 344 
contact drop, 334 
current density, 335 
effect of defective fields, 339 
fitting or grinding-in, 326 
friction of, 335 
glowing, 335 
hardness, 336 
heating of, remedy for, 343 
honey combing, 335 
incorrect thickness of, 340 
incorrect spacing, 338 
inertia of, 336 



Brushes (cont.): 

locating causes of troubles, 337 

locating electrical neutral for, 
332 

picking up copper, 343 

pressure of, 334 

specific resistance, 335 

terms used, 334 

too low brush pressure, 337 

unequal air gaps, 339 

wrong characteristics of, 341 
Brushes for undercut commutator, 

322 

Brush-holder, adjustment of reac- 
tion type, 406 

heating of, 407 

making adjustments of, 329 
Brushes set incorrectly, 404 

wrong setting of, 405 
Brush shunts, loosening of, 344 
Brush studs, heating of, 403 
Burned out starting winding, 429 
Burred commutator bars, troubles 
resulting from, 402 



Calculating wiring circuits, for- 
mula for, 468 

Centigrade and Fahrenheit tem- 
peratures, equivalent 
values, 499 

Chain winding, connecting of, 48 
Chain drives: 

calculating size of chain, 493 
data for, 494 

pinions and sprockets for, 403 

Changes in d. c. motors (see also 

table of contents for 

Chapter X), 237 

Changes in speed of d. c. motor, 

237 

air gaps, 242 
brushes, 243 
connections to commutator, 

243 

operating voltage, 237 
winding wave to lap, 248 



INDEX 



503 



Changes in duplex wave winding, 

252 

connections of coils, 244, 253 
Changes in induction motor wind- 
ings (see heading, proced- 
ure when reconnecting in- 
duction motors): 
factors limiting, 286 
practical methods for, 261 
reconnections frequently 

made, 272 
Changing 440 volt motor for 220 

volt operation, 435 
Changing star to delta connection, 

41 

Characteristics of brushes, 341 
Chord factor, meaning of, 269 
Circuit-breakers for motors, 489 
Cleaning motors with compressed 

air, 420 
troubles due to improper 

cleaning, 376 
Cleaning slots, solutions for, 140 

by filing, 140 
Coil throw, 7, 70 
Coil and commutator insulation, 

test voltage for, 175 
Coils: 

applying insulation on, 163 
connections with dead coils, 98 
classification of, 5 
for large d. c. armatures, 77 
for railway, mill and crane 

motors, 90 

for a. c. windings, 141 
forms for making, 140 
inserting in open slots, 69 
inserting threaded-in type, 60 
insertion in railway armatures, 

94 

insertion in large d. c. arma- 
tures, 81 

insulating for open slots, 67 
insulation for different volt- 
ages, 164, 165, 167, 168 
insulation for d. c. coils, 164, 
165, 168 



Coils (cont.): 

r insulation for induction motor 

coils, 164, 167, 170 
kinds of insulation for, 153 
mica insulation for, 173 
pitches of d. c., 7 
pitches of a. c., 37 
placing on armature, 110 
removing from d. c. armature, 

57 

repairing damages to, 174 
shaping end connections, 70 
table for connecting, 110 
test voltage for insulation of, 

175 

thickness of insulation for, 163 
types and shapes of, 3 
wire, strap and bar wound, 34 
Coil raising tool, 449 
spreader, 457 
taping machine, 457 
taping needle, 449 
winding device, 452 
Commutator: 
action of, 1 

blackening of, remedy for, 343 
bridging bars of, 258 
compound for plugging of, 304 
connections for lap winding, 

102 

connections for wave wind- 
ing, 104 

flat spots on, remedy for, 342 
improvised method to turn, 

411 

insulation of connections to, 76 
locating first connection to 

for lap winding, 101 
locating first connection to 
for wave winding, 105 
making connections to, 101 
number of bars, 24 
plugging of, 395 
poor soldering, troubles re- 
sulting, 396 

reconnecting with dead coil, 
252 



504 



INDEX 



Commutator (cont.): 

reconnecting duplex wave 

windings, 252 
repairs to, 301 

repaired under difficulties, 397 
sandpapering, holder for, 400 
seasoning and grinding, 148 
soldering coil leads to, 144 
sparking at brushes of, 341 
table for connecting coils to, 

111, 246 

trouble from burred bars, 402 
troubles in, 301 
undercutting of, 320 
undercutting with electric drill, 

400 

voltage between bars of, 24 
winding pitch for, 6 
Commutator, repairs to (see also 
table of contents to Chap- 
ter XII), 301 
baking of, 307 
blackening of in spots, 320 
boring out end of, 313 
brushes for undercut commu- 
tator, 322 

burn out between bars, 303 
causes of excessive wear, 318 
causes of troubles in, 301 
copper for bars, 319 
finishing undercut slots, 322 
making micanite end rings, 

318 

mica used in, 315 
micanite insulation for, 317 
precautions when tightening, 

317 

refilling, 311 
remedies for high and low 

bars, 302 

removing bars and mica, 304 
removing grounds, 308 
repairing burned bar, 305 
replacing burned bar, 306 
shaping mica end rings, 316 
templet for making mica 
rings, 316 



Commutator, repairs to (cont.) : 
temporary cover for, 310 
test for oil saturated mica, 

320 

tightening up bars, 306 
tools for undercutting mica. 

320 

troubles from high mica, 301 
turning down surface of, 309 
undercutting mica, 320 
Compressed air pressure in clean- 
ing motors, 420 

Concentrated a. c. winding, 27 
Chain or spiral a. c. winding, 

27 

Conductor or inductor, 3 
Connecting coils of d. c. armature 

in parallel, 244 
Connecting coils of d. c. armature 

in series, 253 

Connections to commutator (see 
table of contents for 
Chapter IV). 

Contact drops for brushes, 334 
Controller, drum type, over haul- 
ing of, 355 
Cost of making repairs, 346 

estimating of, 347 
Cutting out coils of d. c. wave 

winding, 109 

of induction motor winding, 
288 

D 

Dead coils : 

connections of coils with, 98 
wave winding with, 108 
Device for detecting faults in a. c. 

windings, 232 

Diagnosis of motor and generator 

troubles (see also table 

of contents for Chapter 

XV), 376 

Diagrams for connecting induction 

motor windings, 292 
combination scheme for 
several types, 436 



INDEX 



505 



Diagrams for change of connec- 
tions of induction motor 
windings, 262 

different poles and phases, 
295 

2-phase motors, 267 

3-phase motors, 265 
Diamond coils for induction mo- 
tors, 198 

Direct-current armatures (see also 
table, of contents for 
Chapter III) : 

balancing large sizes, 87 

balancing small and medium 
sizes, 150 

banding of, 65 

cleaning slots of, 140 

floor stand for, 75 

loop winding for, 63 

repair shop methods for re- 
winding of, 56 

six steps in winding of, 72 

winding procedure for arma- 
tures having partially 
closed slots, 60 

winding procedure for open 
slots, 66 

winding procedure for large 
armatures, 77 

winding procedure for railway 
and crane armatures, 90 
Direct-current generators : 

speeds and poles of, 25 

winding procedure for 3-wire 
design, 89 

winding procedure for rotary, 

88 

Direct-current machines, causes 
and remedies of troubles 
in, 376 

Direct-current motors changed for 
new operating conditions, 
237 

adjusting air gap, 242 

brush changes for operation 
at lower voltage, 243 

changes in speed, 237 



Direct-current motors (cont.) : 
changes in operating voltage, 

238 

generator used as motor, 241 
motor used as generator, 241 
operating on one-half or 

double voltage, 239 
reconnecting for different 

voltage, 243 

size of wire for coils, 240 
speed when reconnecting wave 

to lap, 242 

Direct-current windings (see also 
table of contents for 
Chapter I), 1 

classification of coils for, 5 
equipotential connectors, 21 
for different number of poles, 

21 

for large armatures, 77 
for rotary converters, 88 
for 3-wire generators, 89 
for railway, mill and crane 

motors, 90 
formulas for lap, 11 
formulas for wave, 17 
lap (multiple or parallel), 9 
lap and wave for large arma- 
tures, 79 
loop windings, 63 
multiplex lap, 14 
multiplex wave, 19 
numbering coil sides in slots, 8 
parts and terms of, 2 
symbols used in formulas for, 8 
symmetrical, 20 
testing of, 122 
types of, 2 

wave (series or two-circuit), 16 
Dismantling a d. c. armature, pro- 
cedure for, 56 

Distributed a. c. winding, 27 
Drifts used by armature winders, 

448 

Drying out insulation: 
of d. c. machines, 181 
synchronous motors, 182 



506 



INDEX 



Drying out insulation of induc- 
tion motors, 183 
insulation test while, 184 
insulation resistance while, 

185 

Duplex wave winding, reconnec- 
tion of, 252 

E 

Electrical neutral, location of, 332 
Element of d. c. winding, 3 
End connections of coils : 

appearance in lap and wave 

d. c. windings, 10 
insulation between, 63 
shaping of in d. c. winding, 70 
thickness of insulation for, 172 
End rings for commutator, 316 
End play of armature, 418 
Engine type generator, inspection 

of, 371 
Equalizer rings for d. c. windings, 

22 
use on multipolar machines, 

120 

Equipotential connectors or equal- 
izers, 21 
Estimating cost of repairs, 347 



Failure of motor to start, 430, 432 
Field coils : 

ad jus table shunt for, 416 

heating of, 414 

insulation of, 176 

test for dead coils, 136 

test for reversed connections, 

136 
Field current in d. c. generators, 

473 
Flat spots on commutator, remedy 

for, 342 
Floor stand for winding d. c. 

armatures, 75 

Form for recording winding data 
of dismantled armature, 
58 



Formulas and rules for d. c. 

windings : 
for calculating wiring circuits, 

468 

for possible symmetrical wind- 
ings, 21 

lap (multiple or parallel), 11 
multiplex lap, 15 
multiplex wave (series-paral- 
lel), 19 

wave (series or two circuit), 17 
Frames for making coils, 140 
Frequency change, reconnection 
of induction motor for, 276 
effect of deer ease on induction- 
motor-generator, 433 
Friction of brushes, 335 
Friction cloth blanket for commu- 
tator end connections, 76 
Fuse block defective, trouble due 

to, 427 

Fuses, sizes for a. c. motors, 442 
d. c. motors, 484 
causes of blowing of, 389 
starting a. c. motors, 487 



G 



Gears, data for, 496 
Generator, engine type, overhaul- 
ing of, 371 

Generator used as motor, 241 
Generators, large a. c. : 

bar and connector winding for, 

217 
coils for partially closed slots, 

210 

coils for open slots, 212 
connecting coils of, 221 
diamond coils for, 219 
double windings for, 221 
glowing of brushes, 335 
insulation of coils for, 214 
inserting shoved through coils, 

215 

lap and wave connections for, 
214 



INDEX 



507 



Generators, large a. c. (cont.): 
bracing heavy windings of, 223 
testing windings of, 221 

Grinding commutator, 148 

Grounds in d. c. windings: 
causes of, 130 
locating dead grounds, 133 
telephone receiver test for, 

135 
tests for, 131 

Grouping coils of induction motor 

winding, 289 

odd number coils per group, 54 
procedure for, 35 



Half-coiled and whole-coiled a. c. 

windings, 30 
Hand tools for armature winders, 

445 

Heating of brush studs, 403 
Heating of motor or generator, 

remedies for, 343 
caused by poor soldering, 396 
caused by variation of fre- 
quency, 438 
hot stator coils, 391 
Holder for sand papering commu- 
tator, 400 
Honey combing of brushes, 335 

remedies for, 343 
Hoods for armatures, 99 

making of, 145 
Hot bearing, relief for, 440 



Impregnating compounds (see also 

varnishes), 176 
Induction motors: 

basket coils for, 195 

coil insulation for, 163, 167, 

170 

connecting coils of, 205 
diamond coils for, 198 
insulation of slots, 195 



Induction motors (cont.): 
inserting new coil in, 203 
open slots, winding of, 201 
operated on different voltages 

and frequencies, 284 
overhauling of, 363 
partially closed slots, winding 

of, 194 

phase wound secondary, 209 
procedure when connecting 

coils of new winding, 288 
reconnecting for changes in 

voltage, frequency, phase 

and speed, 272 
squirrel cage secondary, 20/ 
testing windings of, 202 
2-phase stator, winding of, 202 
winding small sizes, 192 
winding with basket coils, 196 
winding with diamond coils, 

199 

Inspection and overhauling of: 
auto-starters, 351 
compound d. c. motor, 358 
d. c. motor starters, 349 
drum- type controllers, 355 
engine type generator, 371 
induction motor, 363 
single-phase motor, 368 
slip-ring motor, 366 
Inspection and repair of motor 

starters, motors and gen- 
erators, 345 

cost of making repairs, 346 
estimating cost of repairs, 347 
Insulation for: 

a. c. coils, 164, 167, 170 
coils and slots, kinds of, 156 
coils used in open slots, 67 
coils of railway motors, 92 
commutator connections, 76 
core of railway armatures, 93 
d. c. coils, 164, 165, 168 
different voltages, 163, 164, 

165, 166 

electrical protection, 155, 156 
end connections of coils, 63 



508 INDEX 

Insulation for (cont.) : Insulating materials (cont.) : 

field coils, 176 presspahn, 154 

formed coils, 163 rawhide fiber, 160 

high temperatures, 155 , red rope paper, oiled, shel- 

large d. c. armatures, 79 lacked, and varnished, 

mechanical protection, 153 161 

open slots, 69 shellacked bond paper, 1G1 

partially closed slots, 61 test voltage to use, 175 

phase coils, 172 varnishes and compounds, 

slots, thickness needed, 163 176 

Insulation from copper to copper, vulcanized fiber, 154 

between coils, 172 Insulation resistance, measuring 

Insulating materials: of, 185 

treated cloths, 156 \,' . megger test for, 185 

cotton tapes and cloth, 156 voltmeter test for, 184 
Empire cloth, 156 Interpole motor, checking con- 

Kobak cloth, 156 nections of, 412 
oiled canvas, 158 
oiled cotton drill, 158 
oiled muslin, 158 

Japan muslin 157 Knock m armature cauged b 

Japan duck, 157 band wireSj 3% 
varnished cambric, 157 
-"./ varnished silk, 157 

pressboards, fibres and papers, L 

158 

asbestos paper, oiled and Lap winding (multiple or parallel) : 

varnished, 162 commutator connections for, 

drying out, 181 102 

express parchment paper, connecting double layer for 

express paper, shellacked a. c. machine, 47 

and varnished, 161 for d. c. armatures, 9 

fish paper, 154 formulas and rules for d. c. 

horn fibre, Japanned, oiled, armatures, 11 

shellacked and varnished, grouping coils in a. c. machine, 

159 35 

leatheroids, 160 multiplex, formulas for, 14 

manila paper, 155 requirements of, 103 

mica, 155 use on d. c. armatures, 115 

mica for coils, 173 use in a. c. machines, 116 

mica paper and mica cloth, versus wave, 112 

155 vs. multiple wave (series-paral- 

micanite, 155 lei), 119 

micartafolium, 162 wave and lap for a. c. ma- 

pressboard, oiled, Japanned, chines, 28 

shellacked and varnished, Laying out and connecting a. c. 

159 windings, 35 



INDEX 



509 



Lining for slots (see heading of 
"insulation" and table of 
contents for Chapter 
VII). 

Loop winding for small motors, 63 
illustrations of, 64 

Loose bearing, trouble due to, 423 



M 



Machine equipment for repair 

shop: 

banding machine, 460 
bar bender for coils, 458 
coil spreader, 457 
coil tapping machine, 457 
coil winder for lathe, 456 
combination machine, 462 
equipment needed, 444 
slotting and grinding machine, 

459 

Making new coils, 141 
Megger test for insulation resis- 
tance, 185 
Mica insulation: 

built up, 164, 173 
for armature coils, 173 
Mistakes and faults in induction 

motor windings : 
device for detecting faults, 232 
grounds, 231 
improper groups connection, 

234 

open circuits, 234 
reversal of coils or groups, 233 
short-circuits, 231 
use of wrong number of coils, 

234 

wrong number of poles, 235 
Motor circuits, wire size for, 473 
Motor failing to start, 430 
Motor reversed direction of rota- 
tion at high speed, 412 
Motor-starter overhauling of, 349 
Motors, safe temperature for, 415 
Motors operated on double volt- 
age, 419 



N 



Noise in three-phase motor, 428 
Numbering coil sides in slots, 8 



O 



Odd frequencies, winding small 

motors for, 191 
Open circuits in d. c. windings, 

causes of, 127 
tests for, 128 

telephone receiver test for, 135 
transformer testing device, 

125 

Operations before and after winding 
d. c. armatures (see table 
of contents for Chapter 
VI;. 139 

Overhauling and inspection of: 
auto-starters, 351 
compound d. c. motor, 358 
d. c. engine type generator, 

371 

d. c. motor starters, 349 
drum- type controllers, 355 
induction motor, 363 
single-phase motor, 368 
slip-ring motor, 366 



Painting windings, 151 
Phaoe change, reconnection of 
induction motor for, 276 
Phase coils, insulation of, 172 
placing of, 291 
rearranging in induction mo- 

*tor winding, 278 
when reconnecting 2-phase 

to 3-phase, 173 

Phase insulation when reconnect- 
ing induction motors, 271 
Phase rotation, testing of, 420 

of a. c. motor, 440 
Phase spread of a. c. windings, 32 
Pinion puller, 456 



510 



INDEX 



Pitchofd.c. coils: 
definition of, 6 
front and back, 7 
full and fractional, 7 
in winding spaces, coil sides 

and slots, 7 

short-pitch or short cord, 7 
Pitch of a. c. coils : 

effect of fractional pitch, 38 
full and fractional, 37 
Plugging commutator, 395 

filling compound for, 304 
Polarity of a. c. coil groups, 40 
Pole-phase groups of induction 
motor winding, connect- 
ing of, 290 
Polyphase windings, connections 

for, 43 

Potential pitch, 23 
Pressure for brushes, 334 
Procedure when connecting coils 
of induction motor wind- 
ing: 
connecting pole-phase-groups, 

290 

constructing connecting dia- 
grams, 292 
grouping coils, 289 
number poles from coil throw, 

293 

placing phase coils, 291 
winding diagrams for different 
number poles and phase, 
295 

Procedure when reconnecting in- 
duction motors, 272 
changes in voltage only, 275 
change in phase only, 276 
change in frequency, 278 
change in number of poles, 282 
change in speed, 273, 282 
change in resistance of squir- 
rel-cage rotor, 280 
constructing connecting dia- 
grams, 292 
cutting out coils, 288 
dead ending coils, 277 



Procedure when reconnecting 

(cont.) : 
double circuit star to single 

circuit-delta, 287 
effect of high and low voltage 

on operation, 283 
factors limiting change in 

number of poles, 286 
grouping coils, 289 
phase and voltage change, 278 
points to consider, 272 
reconnections frequently 

made, 272 

rearranging phase coils, 278 
Scott or T-connection, 2-phase 

to 3-phase, 269 

testing reconnected motor, 282 
winding diagrams for different 

poles and phases, 25 
Procedure when winding a. c. 

machines : 
induction motor secondaries, 

207 

induction motors with par- 
tially closed slots, 194 
induction motors with open 

slots, 201 

large a. c. stators, 210 
repulsion-start motors, 190 
small single-phase motors, 186 
small polyphase motors, 192 
stator of a. c. turbo-genera- 
tors, 223 

Procedure when winding d. c. 
machines (see also head- 
ing direct current arma- 
tures) : 

banding armatures, 146 
cleaning armature slots, 140 
connecting coils to commuta- 
tor, 101 
cutting out coils of wave 

winding, 109 

insulating coils, 164, 165, 168 
large armatures, 77 
open slots, 66 
partially closed slots, 60 



INDEX 



511 



Procedure when winding (cont.) : 

railway, mill and crane arma- 
tures, 90 

rotary converters, 88 

stripping off old winding, 139 

3-wire generators, 89 
Progressive wave winding, 107 

cutting out coils of, 109 
Pulleys, rules for size of, 492 

speed of, 492 

R 

Rating of a. c. generators, 440 
Reconnecting d. c. winding: 

240 volts lap to 120 volts lap, 

243 
240 volts wave to 120 volts lap, 

248 
bridging commutator bars, 

258 

connecting coils in series, 253 
connecting coils in parallel, 

244 
reconnecting with one dead 

coil, 252 
reconnecting duplex windings, 

252 
table for connecting coils to 

commutator, 246 
table for rewinding coils, 245 
Reconnecting induction motors 
(see also table of contents 
for Chapter XI), 261 
changes possible in existing 

winding, 272 

changes frequently made, 272 
change in voltage only, 275 
change in phase only, 276 
change in frequency, 278 
change in number poles, 282 
change in speed, 273, 282 
change in resistance of squir- 
rel cage rotor, 280 
cutting out coils, 288 
chord factor, meaning of, 269 
diagrams for changes in con- 
nections, 263 



Reconnecting induction motors: 
factors limiting change in 

number of poles, 286 
phase and voltage change, 278 
phase insulation, 271 
points to consider before recon- 
necting, 262 

rearranging phase coils, 278 
Scott or T-connection, 2-phase 

to 3-phase, 269 
table of possible reconnec- 

tions, 266 

testing reconnected motor, 282 
with dead ended coils, 277 
Reentrant, definition of, 14 
Relining split b earings, 151 
Remedies for troubles in d. c. 

machines, 380 
for brush troubles, 341 
Removing old coils from d. c. 

armatures, 57 
Repairing : 

auto-starters, 351 

coils damaged while winding 

armature, 174 
compound d. c. motors, 358 
cost of, 346 

d. c. engine type generator, 371 
d. c. motor starters, 349 
drum-type controllers, 355 
estimating cost of, 347 
induction motor, 363 
single phase motor, 368 
slip ring motor, 366 
Repair shop equipment: 
banding device, 86 
for testing d. c. windings, 138 
floor stand for armatures, 75 
machines and tools, 444 
Repair shop methods for rewinding 
d. c. armatures (see also 
table of contents for 
Chapter III), 56 

Repair shop methods for rewinding 
a. c. machines (see also 
table of contents for 
Chapter VIII), 186 



512 



INDEX 



Repairs to commutator: 

baking of commutator, 307 
blackening of commutator, 

in spots, 320 
boring out end of, 313 
brushes for undercut commu- 
tator, 322 

burn out between bars, 303 
causes of excessive wear, 318 
causes of trouble in, 301 
copper used for bars, 319 
finishing undercut slots, 322 
making micanite end rings, 

318 

mica used in, 315 
micanite insulation for, 317 
precautions when tightening, 

317 

refilling of commutator, 311 
removing bars and mica, 304 
removing grounds, 308 
remedies for high and low bars, 

302 

repairing burned bar, 305 
replacing burned bar, 306 
shaping mica end rings, 316 
templet for making mica rings, 

316 

temporary cover for, 310 
test for oil saturated mica, 320 
tightening up bars, 306 
tools for undercutting mica, 

320 

troubles from high mica, 301 
turning down surface, 309 
undercutting mica, 320 
under difficulties, 397 
Retrogressive wave winding, 107 

cutting out coils of, 109 
Reversal of speed of motor while 

running, 412 

Reversed coils in d. c. winding : 
cause of, 131 

test for with bar magnet, 131 
test for with compass, 132 
Ropes, horsepower transmitted by, 

495 



Rotary converters, winding of, 88 

banding of, 410 
Rules for a. c. windings: 

arrangement of coils, 51 

checking* phase relationship, 
52 

connections for coils of differ- 
ent windings, 43 

grouping coils, 35 

indicating polarity of coil 
groups, 40 

reconnecting induction mo- 
tors, 434 
Rules for d. c. windings: 

for possible symmetrical wind- 
ings, 21 

lap (multiple or parallel), 11 

multiplex lap, 15 

wave (series or two circuit), 17 

multiplex wave (series-paral- 
lel), 19 
Rules, general, 498 



8 



Seasoning and grinding commuta- 
tor, 148 
Shaping insulating cells for slots, 

448 
Single-phase motors: 

connections for windings of, 

191 

distribution of main and start- 
ing windings, 188 
inserting skein coils in slots, 

187 

insulation for slots of, 186 
testing windings of, 192 
winding of by hand, 190 
winding for odd frequencies, 

191 
winding repulsion start type, 

190 

winding skein coils in, 186 
Single-phase motor, overhauling 

of, 368 
Single vs. multiple windings, 118 



INDEX 



513 



Size of switches for: 
d. c. motors, 484 
induction motors, 487 
single-phase motors, 486 
Shaping mica end rings, 316 
Short circuits in d. c. windings: 
causes of, 122 
tests for, 123 
transformer testing device for, 

125 

telephone receiver test for, 135 
Shoved through coils, insertion of, 

215 
Skein coils for single-phase motors, 

186 

Slings for armatures, 455 
Slip-ring motor, overhauling of, 366 
sparking at slip-rings of, 393 
Slots: 

cleaning and filing, 140 
inserting coils in, 61, 69, 81, 
94, 187, 196, 199, 202, 
203, 215, 227 

insulating materials for, 153 
insulation at ends of, 172 
insulation of partially closed 

d. c. type, 61 
insulation of open d. c. type, 

69 

insulation for large d. c. arma- 
tures, 79 
insulation for a. c. windings, 

186, 195, 214, 226 
numbering of coils in, 8 
thickness of insulation for, 163 
Slotting and grinding machine, 459 
Small motors, loop winding for, 63 

winding of, 64 

Solutions for cleaning slots, 140 
Sparking at brushes, remedy, 341 
due to poor belt joints, 394 
due to absence of balancing 

weights, 437 
Speed change, reconnecting a. c. 

motor for, 273, 282 
Speeds of d. c. generators, 25 
safe limit of, 25 



Spiral or chain a. c. winding, 27 
Split bearings, relining of, 151 
Squirrel cage rotor, change in 

resistance of, 280 
Stalling of wound rotor induction 

motor, 422 

Standard motors on different volt- 
ages and frequencies, 284 
Static sparks from belts, 440 
Stator connections, trouble due to, 

421 

Steel shafting, horsepower trans- 
mitted by, 495 

Stripping old d. c. winding, 139 
Symmetrical d. c. windings, 20 
Synchronous motor troubles, 388 
failure to start, 432 



Tables for winding d. c. arma- 
tures, 110 

connecting coils to commu- 
tator, 111 

changing coil connections, 246 
rewinding coils, 245 

Tape made from cotton cloth, 181 

Temperature, safe for motors,415 
CentigradeandFahrenheit, 499 

Terms and parts of d. c. winding, 2 
armature coil, 4 
armature conductor, 3 
concentric coils, 5 
involute coils, 5 
front and back pitch, 7 
full and fractional pitch, 7 
symbols used, 8 
winding element or section, 3 
winding pitch or coil pitch, 6 

Testing d. c. windings (see table 
of contents for Chapter V) 
commutator, 136 
for short circuits, 123, 135 
for open circuits, 125, 128, 135 
for grounds, 131, 133, 135 
for reversed coil, 131, 132 
for reversed or dead field coils, 
136 



514 



INDEX 



Testing, equipment for, 138 

Testing induction motor windings 

for: 

device for detecting faults, 232 
exploring with a compass, 236 
grounds, 231 
improper group connection, 

234 

open circuits, 234 
order in making tests, 235 
reversal of coils or groups, 233 
short-circuits, 231 
use of wrong number coils, 234 
wrong number of poles, 235 

Threaded-in coil, 60 

insertion in slots, 61 

Three-phase motors, reconnection 
for (see reconnection of 
indue tion motors), 261 

Three-phase motors on single- 
phase lines, 425 

Throw of coils, 7, 70 

Tightening commutator bars, 306 

Tools used by armature winder: 
band wire tension block, 454 
banding machine, 460 
bar bender for coils, 458 
coil raiser, 449 
coil spreader, 457 
coil taping needle, 449 
coil taping machine, 457 
coil winding device, 452 
coil winder for lathe, 456 
combination machine, 462 
drifts, 448 

for cutting cell lining, 448 
for shaping insulating cells, 448 
hand tools, 445 
leather sling, 455 
made from hack saw blades, 

451 

pinion puller, 456 
rope sling, 455 
slotting and grinding machine, 

459 

steadying brace, 454 
wire scraper, 449 



Transformer rating for a. c. motors, 

478 
Troubles in a. c. machines, causes 

and remedies: 
due to electrical faults, 388 
due to mechanical adjust- 
ments, 387 
fuses blowing, 389 
hot stator coils, 391 
in windings of, 387 
induction motor troubles, 386 
rotor windings, 302 
sparking at slip rings, 393 
stator windings, 392 
synchronous motors, 388 
tension of belts, 391 
testing for grounds, 390 
Troubles in d. c. motors and genera- 
tors, causes of: 
electrical defects, 379 
exposure to acid fumes and 

gases, 377 

lack of inspection and replace- 
ments, 378 

lack of proper cleaning, 376 
operating temperatures too 

high, 378 

operation in damp places, 377 
remedies for, 380 
Troubles in commutator, causes of, 

307 
with brushes, remedies for, 

341 

with high speed motor, 417 
Turbo-generator : 

bracing windings of, 228 
break-down test for, 229 
coils for, 223 
connecting windings, 228 
inserting coils in, 227 
insulation for coils of, 226 
forming coils for, 225 
testing windings of, 226 

U 

Undercutting mica with standard 
tools, 320 






INDEX 



515 



Undercutting mica with hand tools, 

323 
with portable electric drill, 400 



Varnishes and impregnating com- 
pounds: 

baking and air-drying varn- 
ishes, 178 

characteristics of, 178 
classes of, 176 

clear and black varnishes, 177 
solvent chart for, 178 
table of uses, 177 

Voltage change, reconnection of 
induction motor for, 275 
Voltmeter test for insulation re- 
sistance, 185 

Voltage to use in testing insula- 
tion, 175 

W 

Wave winding (series or two-cir- 
cuit) : 

commutator connections for, 
104 

cutting out coils of, 109 

dead coils in, 108 

duplex winding, 252 

for a. c. machines, 41 

for d. c. armatures, 16 

formulas and rules for d. c. 
armatures, 17 

grouping coils in a. c. ma- 
chines, 35 

lap and wave for a. c. ma- 
chines, 28 

locating first connection to 
commutator for, 105 

multiple wave vs. lap, 119 

multiplex, formulas for, 19 

progressive and retrogressive, 
107 

reconnecting with dead coil, 
252 

reconnecting 240 volts to 120 
volts, 248 

requirements of, 107 



Wave winding (cont.): 

use in a. c. machines, 118 
use on d. c. armatures, 117 
versus lap, 112 
Wedge driver, 62 
Whole-coiled and half-coiled a. c. 

windings, 30 
Winding d. c. machines: 

having partially closed slots, 60 
having open slots, 66 
large armatures, 77 
rotary converters, 88 
3-wire generators, 89 
railway, mill and crane motors, 

90 
Winding a. c. machines: 

induction motors with open 

slots, 201 

induction motors with par- 
tially closed slots, 194 
induction motor secondaries, 

207 

large a. c. stators, 210 
repulsion start designs, 190 
single-phase motors, 186 
small polyphase types, 192 
stator of a. c. turbo-genera- 
tors, 223 
Winding data needed to duplicate 

an old winding, 57 
Winding diagrams for a. c. motors: 
developed diagram, compared 

with circle scheme, 53 
for different poles and phases, 

295 

simple circle diagram, 39 
Winding parts and terms, 2 
Winding procedure for d. c. 
armatures (see heading 
"Direct Current Arma- 
tures")- 

Wire scraper, 449 
Wire size for motor services, 473 
d. c. motors, 481 
motor leads, 474 
3-phase motors, 482 
Wire table, how to remember, 465 
Wire gauges, classification of. 466 



UNIVERSITY OF CALIFORNIA LIBRARY 
BERKELEY 

Return to desk from which borrowed. 
This book i 



MAY 6 1348 

MAR 1 Q 1950 




NOV 2 1 1952 
APR 1 1953^ 



MAY 6 1963 



V 



LD 21-100m-9,'47(A5702sl6)476 



OOUIO 




M184967 



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THE UNIVERSITY OF CALIFORNIA UBRARY