Electric Motors and Drives Fundamentals Types and Applications Third edition Austin Hughes

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Electric Motors and Drives Fundamentals Types and Applications Third edition Austin Hughes

CONTENTS

Preface xvi
1 ELECTRIC MOTORS 1
Introduction 1
Producing Rotation 2
Magnetic field and magnetic flux 3
Magnetic flux density 4
Force on a conductor 6
Magnetic Circuits 7
Magnetomotive force (MMF) 9
Electric circuit analogy 10
The air-gap 11
Reluctance and air-gap flux densities 12
Saturation 14
Magnetic circuits in motors 15
Torque Production 16
Magnitude of torque 18
The beauty of slotting 19
Specific Loadings and Specific Output 21
Specific loadings 21
Torque and motor volume 23
Specific output power – importance of speed 23
Energy Conversion – Motional EMF 25
Elementary motor – stationary conditions 26
Power relationships – conductor moving at
constant speed 28
Equivalent Circuit 30
Motoring condition 32
Behaviour with no mechanical load 32
Behaviour with a mechanical load 35
Relative magnitudes of V and E, and efficiency 37
Analysis of primitive motor – conclusions 38
General Properties of Electric Motors 39
Operating temperature and cooling 39
Torque per unit volume 40
Power per unit volume – importance of speed 41
Size effects – specific torque and efficiency 41
Efficiency and speed 41
Rated voltage 41
Short-term overload 42
Review Questions 42
2 POWER ELECTRONIC CONVERTERS FOR
MOTOR DRIVES 45
Introduction 45
General arrangement of drives 45
Voltage Control – D.C. Output from D.C. Supply 47
Switching control 48
Transistor chopper 49
Chopper with inductive load – overvoltage
protection 52
Features of power electronic converters 54
D.C. from A.C. – Controlled Rectification 55
The thyristor 55
Single-pulse rectifier 56
Single-phase fully controlled converter – output
voltage and control 57
3-phase fully controlled converter 62
Output voltage range 64
Firing circuits 64
A.C. from D.C. SP – SP Inversion 65
Single-phase inverter 65
Output voltage control 67
Sinusoidal PWM 68
3-phase inverter 69
vi Contents
Forced and natural commutation – historical
perspective 69
Matrix converters 70
Inverter Switching Devices 72
Bipolar junction transistor (BJT) 72
Metal oxide semiconductor field effect
transistor (MOSFET) 73
Insulated gate bipolar transistor (IGBT) 74
Gate turn-off thyristor (GTO) 74
Converter Waveforms and Acoustic Noise 75
Cooling of Power Switching Devices 75
Thermal resistance 75
Arrangement of heatsinks and forced air cooling 77
Cooling fans 78
Review Questions 79
3 CONVENTIONAL D.C. MOTORS 82
Introduction 82
Torque Production 84
Function of the commutator 86
Operation of the commutator – interpoles 88
Motional E.M.F. 90
Equivalent circuit 94
D.C. motor – Steady-State Characteristics 95
No-load speed 95
Performance calculation – example 96
Behaviour when loaded 98
Base speed and field weakening 103
Armature reaction 105
Maximum output power 106
Transient Behaviour – Current Surges 107
Dynamic behaviour and time-constants 108
Shunt, Series and Compound Motors 111
Shunt motor – steady-state operating
characteristics 113
Series motor – steady-state operating
characteristics 115
Contents vii
Universal motors 118
Compound motors 119
Four-Quadrant Operation and Regenerative Braking 119
Full speed regenerative reversal 122
Dynamic braking 124
Toy Motors 124
Review Questions 126
4 D.C. MOTOR DRIVES 133
Introduction 133
Thyristor D.C. Drives – General 134
Motor operation with converter supply 136
Motor current waveforms 136
Discontinuous current 139
Converter output impedance: overlap 141
Four-quadrant operation and inversion 143
Single-converter reversing drives 144
Double SP-converter reversing drives 146
Power factor and supply effects 146
Control Arrangements for D.C. Drives 148
Current control 150
Torque control 152
Speed control 152
Overall operating region 154
Armature voltage feedback and IR
compensation 155
Drives without current control 155
Chopper-Fed D.C. Motor Drives 155
Performance of chopper-fed d.c. motor drives 156
Torque–speed characteristics and
control arrangements 159
D.C. Servo Drives 159
Servo motors 160
Position control 162
Digitally Controlled Drives 163
Review Questions 164
viii Contents
5 INDUCTION MOTORS – ROTATING FIELD,
SLIP AND TORQUE 167
Introduction 167
Outline of approach 168
The Rotating Magnetic Field 170
Production of rotating magnetic field 172
Field produced by each phase winding 172
Resultant field 176
Direction of rotation 177
Main (air-gap) flux and leakage flux 177
Magnitude of rotating flux wave 179
Excitation power and VA 182
Summary 183
Torque Production 183
Rotor construction 183
Slip 185
Rotor induced e.m.f., current and torque 185
Rotor currents and torque – small slip 187
Rotor currents and torque – large slip 189
Influence of Rotor Current on Flux 191
Reduction of flux by rotor current 192
Stator Current-Speed Characteristics 193
Review Questions 196
6 OPERATING CHARACTERISTICS OF
INDUCTION MOTORS 198
Methods of Starting Cage Motors 198
Direct Starting – Problems 198
Star/delta (wye/mesh) starter 202
Autotransformer starter 202
Resistance or reactance starter 203
Solid-state soft starting 204
Starting using a variable-frequency
inverter 206
Run-up and Stable Operating Regions 206
Harmonic effects – skewing 208
High inertia loads – overheating 209
Steady-state rotor losses and efficiency 209
Contents ix
Steady-state stability – pullout torque
and stalling 210
Torque–Speed Curves – Influence of Rotor
Parameters 211
Cage rotor 211
Double cage rotors 213
Deep bar rotors 214
Starting and run-up of slipring motors 215
Influence of Supply Voltage on Torque–Speed Curve 217
Generating and Braking 218
Generating region – overhauling loads 219
Plug reversal and plug braking 220
Injection braking 221
Speed Control 221
Pole-changing motors 222
Voltage control of high-resistance cage motors 223
Speed control of wound-rotor motors 224
Power Factor Control and Energy Optimisation 225
Voltage control 225
Slip energy recovery (wound rotor motors) 227
Single-Phase Induction Motors 227
Principle of operation 227
Capacitor-run motors 229
Split-phase motors 230
Shaded-pole motors 231
Size Range 232
Scaling down – the excitation problem 232
Review Questions 233
7 INDUCTION MOTOR EQUIVALENT CIRCUIT 236
Introduction 236
Outline of approach 237
Similarity Between Induction Motor and Transformer 238
The Ideal Transformer 240
Ideal transformer – no-load condition,
flux and magnetising current 240
x Contents
Ideal transformer – no-load condition,
voltage ratio 245
Ideal transformer on load 246
The Real Transformer 248
Real transformer – no-load condition,
flux and magnetising current 248
Real transformer – leakage reactance 251
Real transformer on load – exact
equivalent circuit 252
Real transformer – approximate
equivalent circuit 254
Measurement of parameters 256
Significance of equivalent circuit parameters 257
Development of the Induction Motor Equivalent Circuit 258
Stationary conditions 258
Modelling the electromechanical
energy conversion process 259
Properties of Induction Motors 261
Power balance 262
Torque 262
Performance Prediction – Example 263
Line current 264
Output power 264
Efficiency 265
Phasor diagram 266
Approximate Equivalent Circuits 267
Starting and full-load relationships 268
Dependence of pull out torque on
motor parameters 269
Analysis 270
Graphical interpretation via phasor diagram 271
Measurement of Parameters 274
Equivalent Circuit Under Variable-Frequency
Conditions 274
Review Questions 277
Contents xi
8 INVERTER-FED INDUCTION MOTOR DRIVES 279
Introduction 279
Comparison with d.c. drive 280
Inverter waveforms 282
Steady-state operation – importance of
achieving full flux 284
Torque–Speed Characteristics – Constant
V/f Operation 286
Limitations imposed by the inverter – constant
power and constant torque regions 288
Limitations imposed by motor 289
Control Arrangements for Inverter-Fed Drives 290
Open-loop speed control 291
Closed-loop speed control 293
Vector (Field-Oriented) Control 296
Transient torque control 297
Cycloconverter Drives 300
Review Questions 303
9 STEPPING MOTORS 305
Introduction 305
Open-loop position control 306
Generation of step pulses and motor
response 307
High-speed running and ramping 308
Principle of Motor Operation 311
Variable reluctance motor 312
Hybrid motor 314
Summary 317
Motor Characteristics 318
Static torque–displacement curves 318
Single-stepping 319
Step position error and holding torque 320
Half stepping 321
Step division – mini-stepping 323
xii Contents
Steady-State Characteristics – Ideal
(Constant-Current) Drive 324
Requirements of drive 324
Pull-out torque under constant-current
conditions 326
Drive Circuits and Pull-Out Torque–Speed Curves 328
Constant-voltage drive 328
Current-forced drive 330
Chopper drive 331
Resonances and instability 333
Transient Performance 335
Step response 335
Starting from rest 336
Optimum acceleration and
closed-loop control 337
Review Questions 338
10 SYNCHRONOUS, BRUSHLESS D.C. AND
SWITCHED RELUCTANCE DRIVES 340
Introduction 340
Synchronous Motors 341
Excited-rotor motors 343
Equivalent circuit of excited-rotor
synchronous motor 344
Phasor diagram and Power-factor control 347
Starting 349
Permanent magnet synchronous motors 350
Hysteresis motors 351
Reluctance motors 351
Controlled-Speed Synchronous Motor Drives 352
Open-loop inverter-fed synchronous
motor drives 353
Self-synchronous (closed-loop) operation 354
Operating characteristics and control 355
Brushless D.C. Motors 357
Contents xiii
Switched Reluctance Motor Drives 358
Principle of operation 359
Torque prediction and control 360
Power converter and overall drive
characteristics 363
Review Questions 363
11 MOTOR/DRIVE SELECTION 366
Introduction 366
Power Range for Motors and Drives 366
Maximum speed and speed range 368
Load Requirements – Torque–Speed Characteristics 369
Constant-torque load 369
Inertia matching 374
Fan and pump loads 374
General Application Considerations 375
Regenerative operation and braking 375
Duty cycle and rating 376
Enclosures and cooling 377
Dimensional standards 378
Supply interaction and harmonics 378
Review Questions 379
APPENDIX – INTRODUCTION TO CLOSED–LOOP
CONTROL 381
Reasons for Adopting a Simplified Approach 381
Closed-Loop (Feedback) Systems 382
Error-activated feedback systems 383
Closed-loop systems 384
Steady-State Analysis of Closed-Loop Systems 386
Importance of Loop Gain – Example 390
Steady-State Error – Integral Control 392
PID Controller 394
xiv Contents
Stability 396
Disturbance Rejection – Example Using D.C. Machine 397
Further Reading 400
Answers to Numerical Review Questions 401
Index 404

PREFACE

Like its predecessors, the third edition of this book is intended primarily
for non-specialist users and students of electric motors and drives.
My original aim was to bridge the gap between specialist textbooks
(which are pitched at a level too academic for the average user) and
the more prosaic ‘handbooks’, which are full of useful detail but provide
little opportunity for the development of any real insight or understanding.
The fact that the second edition was reprinted ten times indicated
that there had indeed been a gap in the market, and that a third edition
would be worthwhile. It was also gratifying to learn that although the
original book was not intended as yet another undergraduate textbook,
teachers and students had welcomed the book as a gentle introduction to
the subject.
The aim throughout is to provide the reader with an understanding of
how each motor and drive system works, in the belief that it is only by
knowing what should happen that informed judgements and sound
comparisons can be made. Given that the book is aimed at readers
from a range of disciplines, introductory material on motors and
power electronics is clearly necessary, and this is presented in the first
two chapters. Many of these basic ideas crop up frequently throughout
the book, so unless the reader is well-versed in the fundamentals it
would be wise to absorb the first two chapters before tackling the later
material. In addition, an awareness of the basic ideas underlying
feedback and closed-loop control is necessary in order to follow the
sections dealing with drives, and this has now been provided as an
Appendix.
The book explores most of the widely used modern types of motors
and drives, including conventional and brushless d.c., induction motors
(mains and inverter-fed), stepping motors, synchronous motors (mains
and converter-fed) and reluctance motors. The d.c. motor drive and the
induction motor drive are given most importance, reflecting their dominant
position in terms of numbers. Understanding the d.c. drive is
particularly important because it is still widely used as a frame of
reference for other drives: those who develop a good grasp of the d.c.
drive will find their know-how invaluable in dealing with all other types,
particularly if they can establish a firm grip on the philosophy of the
control scheme.
Younger readers may be unaware of the radical changes that have
taken place over the past 40 years, so perhaps a couple of paragraphs are
appropriate to put the current scene into perspective. For more than a
century, many different types of motors were developed, and each became
closely associated with a particular application. Traction, for example,
was seen as the exclusive preserve of the series d.c. motor, whereas
the shunt d.c. motor, though outwardly indistinguishable, was seen as
being quite unsuited to traction applications. The cage induction motor
was (and still is) the most widely used but was judged as being suited only
for applications that called for constant speed. The reason for the plethora
of motor types was that there was no easy way of varying the supply
voltage and/or frequency to obtain speed control, and designers were
therefore forced to seek ways of providing speed control within the
motor itself. All sorts of ingenious arrangements and interconnections of
motor windings were invented, but even the best motors had a limited
range of operating characteristics, and all of them required bulky control
equipment gear-control, which was manually or electromechanically operated,
making it difficult to arrange automatic or remote control.
All this changed from the early 1960s when power electronics began to
make an impact. The first major breakthrough came with the thyristor,
which provided a relatively cheap, compact and easily controlled
variable-speed drive using the d.c. motor. In the 1970s, the second
major breakthrough resulted from the development of power-electronic
inverters, providing a three-phase variable-frequency supply for the cage
induction motor and thereby enabling its speed to be controlled.
These major developments resulted in the demise of many of the
special motors, leaving the majority of applications in the hands
of comparatively few types, and the emphasis has now shifted from
complexity inside the motor to sophistication in supply and control
arrangements.
From the user’s point of view this is a mixed blessing. Greater flexibility
and superior levels of performance are available, and there are
fewer motor types to consider. But if anything more than constant speed
is called for, the user will be faced with the purchase of a complete drive
system, consisting of a motor together with its associated power electronics
package. To choose wisely requires not only some knowledge of
motors, but also the associated power-electronics and the control options
that are normally provided.
Preface xvii
Development in the world of electrical machines tends to be steady
rather than spectacular, which means that updating the second edition
has called for only modest revision of the material covering the how and
why of motors, though in most areas explanations have been extended,
especially where feedback indicated that more clarity was called for.
After much weighing the pros and cons I decided to add a chapter on the
equivalent circuit of the induction motor, because familiarity with the
terminology of the equivalent circuit is necessary in order to engage in
serious dialogue with induction motor suppliers or experts. However
those who find the circuit emphasis not to their liking can be reassured
that they can skip Chapter 7 without prejudicing their ability to tackle
the subsequent chapter on induction motor drives.
The power electronics area has matured since the 1993 edition of the
book, but although voltage and current ratings of individual switching
devices continue to improve, and there is greater integration of drive
electronics and power devices, there has been no strategic shift that
would call for a radical revision of the material in the second edition.
The majority of drive converters now use IGBT or MOSFET devices,
but the old-fashioned bipolar transistor symbol has been retained
in most of the diagrams because it has the virtue of showing the
direction of current flow, and is therefore helpful in understanding
circuit operation.
The style of the book reflects my own preference for an informal
approach, in which the difficulty of coming to grips with new ideas is
not disguised. Deciding on the level at which to pitch the material was
originally a headache, but experience suggested that a mainly descriptive
approach with physical explanations would be most appropriate, with
mathematics kept to a minimum to assist digestion. The most important
concepts (such as the inherent e.m.f. feedback in motors, or the need for
a switching strategy in converters) are deliberately reiterated to reinforce
understanding, but should not prove too tiresome for readers who have
already ‘got the message’. I had hoped to continue without numbered
headings, as this always seems to me to make the material seem lighter,
but cross referencing is so cumbersome without numbering that in the
end I had to give in.
I have deliberately not included any computed magnetic field plots,
nor any results from the excellent motor simulation packages that are
now available because experience suggests that simplified diagrams are
actually better as learning vehicles. All of the diagrams have been
redrawn, and many new ones have been added.
Review questions have been added at the end of each chapter. The
number of questions broadly reflects my judgement of the relative
xviii Preface
importance of each chapter, and they are intended to help build confidence
and to be used selectively. A drives user might well not bother
with the basic machine-design questions in the first two chapters, but
could benefit by tackling the applications-related questions in subsequent
chapters. Judicious approximations are called for in most of the
questions, and in some cases there is either insufficient explicit information
or redundant data: this is deliberate and designed to reflect reality.
Answers to the numerical questions are printed in the book, with
fully worked and commented solutions on the accompanying website
http://books.elsevier.com/companions/0750647183. The best way to
learn is to make an unaided attempt before consulting a worked solution,
so the extra effort in consulting the website will perhaps encourage
best practice. In any event, my model solution may not be the best!