IET Power and Energy Series 8
This book is intended to explain the technical principles involved in the many AC variable speed drive systems available today. It deals with all the DC link inverter and direct AC to AC converter systems that are in commercial use. The principles of AC motors are considered specifically from the variable frequency point of view, and this chapter concentrates on the effects of harmonics. The different types of power semiconductor switches are considered separately from the drive systems in which they are used. A total of seven separate and technically different drive systems are considered in such a way that their principles can be fully understood and their performance capabilities explained. Square wave and pulse width modulated DC link inverter systems, cycloconverters and slip power recovery drives are all included in this comprehensive book. This book has been written so that it can be understood by general engineers, not just by experts in the field. It should therefore be of great use to any engineer involved with variable speed drives in any capacity. It should also be of interest to university and college electrical engineering departments and students.
David Finney, B.Sc., CEng., FIEE, is division manager and chief engineer, responsible for large variable speed drive systems, at the G.E.C. Industrial Controls plant in Rugby, England. In this position he is responsible for the development, design and manufacture of large drive systems for use in mining, metals, paper, oil, and chemical industries throughout the world. He has been technically involved in the power semiconductor field since 1958, when thyristors were only just emerging, and during this time he has worked on all types of thyristor converters and inverter drives from a few kilowatts up to 10,000 kW using natural and forced commutation techniques and operating in square wave and pulse modulated modes. He has published a number of articles and given lectures around the world in his chosen subject.
Variable Frequency AC Motor Drive Systems
Variable Frequency AC Motor Drive Systems
David Finney Finney
The Institution of Engineering and Technology www.theiet.org 0 86341 114 2 978-0-86341-114-4
Variable Frequency AC Motor Drive Systems
IET Power and Energy Series 8 Series Editors: Prof. A.T. Johns G. Ratcliff J.R. Platts
Variable Frequency AC Motor Drive Systems
Other volumes in this series: Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume
Power circuit breaker theory and design C.H. Flurscheim (Editor) Industrial microwave heating A.C. Metaxas and R.J. Meredith Insulators for high voltages J.S.T. Looms Variable frequency AC motor drive systems D. Finney SF6 switchgear H.M. Ryan and G.R. Jones Conduction and induction heating E.J. Davies Statistical techniques for high voltage engineering W. Hauschild and W. Mosch 14 Uninterruptable power supplies J. Platts and J.D. St Aubyn (Editors) 15 Digital protection for power systems A.T. Johns and S.K. Salman 16 Electricity economics and planning T.W. Berrie 18 Vacuum switchgear A. Greenwood 19 Electrical safety: a guide to causes and prevention of hazards J. Maxwell Adams 21 Electricity distribution network design, 2nd edition E. Lakervi and E.J. Holmes 22 Artificial intelligence techniques in power systems K. Warwick, A.O. Ekwue and R. Aggarwal (Editors) 24 Power system commissioning and maintenance practice K. Harker 25 Engineers’ handbook of industrial microwave heating R.J. Meredith 26 Small electric motors H. Moczala et al. 27 AC-DC power system analysis J. Arrill and B.C. Smith 29 High voltage direct current transmission, 2nd edition J. Arrillaga 30 Flexible AC Transmission Systems (FACTS) Y-H. Song (Editor) 31 Embedded generation N. Jenkins et al. 32 High voltage engineering and testing, 2nd edition H.M. Ryan (Editor) 33 Overvoltage protection of low-voltage systems, revised edition P. Hasse 34 The lightning flash V. Cooray 35 Control techniques drives and controls handbook W. Drury (Editor) 36 Voltage quality in electrical power systems J. Schlabbach et al. 37 Electrical steels for rotating machines P. Beckley 38 The electric car: development and future of battery, hybrid and fuel-cell cars M. Westbrook 39 Power systems electromagnetic transients simulation J. Arrillaga and N. Watson 40 Advances in high voltage engineering M. Haddad and D. Warne 41 Electrical operation of electrostatic precipitators K. Parker 43 Thermal power plant simulation and control D. Flynn 44 Economic evaluation of projects in the electricity supply industry H. Khatib 45 Propulsion systems for hybrid vehicles J. Miller 46 Distribution switchgear S. Stewart 47 Protection of electricity distribution networks, 2nd edition J. Gers and E. Holmes 48 Wood pole overhead lines B. Wareing 49 Electric fuses, 3rd edition A. Wright and G. Newbery 51 Short circuit currents J. Schlabbach 905 Power system protection, 4 volumes 1 4 7 8 10 11 13
Variable Frequency AC Motor Drive Systems David Finney
The Institution of Engineering and Technology
Published by The Institution of Engineering and Technology, London, United Kingdom First edition © 1988 Peter Peregrinus Ltd Reprint with new cover © 2006 The Institution of Engineering and Technology First published 1988 Reprinted 1991, 2006 This publication is copyright under the Berne Convention and the Universal Copyright Convention. All rights reserved. Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act, 1988, this publication may be reproduced, stored or transmitted, in any form or by any means, only with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Inquiries concerning reproduction outside those terms should be sent to the publishers at the undermentioned address: The Institution of Engineering and Technology Michael Faraday House Six Hills Way, Stevenage Herts, SG1 2AY, United Kingdom www.theiet.org While the author and the publishers believe that the information and guidance given in this work are correct, all parties must rely upon their own skill and judgement when making use of them. Neither the author nor the publishers assume any liability to anyone for any loss or damage caused by any error or omission in the work, whether such error or omission is the result of negligence or any other cause. Any and all such liability is disclaimed. The moral rights of the author to be identified as author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988.
British Library Cataloguing in Publication Data Finney, David Variable frequency AC motor drive systems. 1. Alternating current electric motors 2. Variable speed drives I. Title II. Series 621.46’2 ISBN (10 digit) 0 86341 114 2 ISBN (13 digit) 978-0-86341-114-4
Printed in the UK by Short Run Press Ltd, Exeter Reprinted in the UK by Lightning Source UK Ltd, Milton Keynes
Contents
Preface 1
2
Page ix
AC motors 1.1 Introduction 1.2 The induction motor 1.2.1 Induction motor principles 1.2.2 The variable frequency induction motor 1.2.3 The equivalent circuit 1.2.4 The vector diagram 1.2.5 Equations and relationships 1.2.6 Examples of calculations 1.3 The synchronous motor 1.3.1 Synchronous motor principles 1.3.2 Equivalent circuits and vector diagrams 1.3.3 Equations and relationships 1.3.4 Examples of calculations 1.4 Harmonics in AC motors 1.4.1 Harmonic power losses 1.4.2 Torque pulsations 1.4.3 Harmonic equivalent circuits 1.5 Motor power losses 1.6 Motor voltages to earth
1 1 2 3 9 14 20 21 26 32 33 36 40 42 45 46 46 47 49 52
Power switching devices 2.1 Introduction 2.2 The thyristor 2.2.1 Capabilities and performance 2.2.2 The available thyristors 2.2.3 Thyristors in AC motor drive circuits 2.3 The transistor 2.3.1 Capabilities and performance 2.3.2 The available transistors 2.3.3 Transistors in AC motor drive circuits
54 54 55
57 62 65 73 75 83 86
vi
Contents
2.4
The gate turn off thyristor 2.4.1 Capabilities and performance 2.4.2 The available GTO thyristors 2.4.3 GTO's in AC motor drive circuits
91 93 98 100
3
Power switching circuits 3.1 Introduction 3.2 The 3 phase, naturally commutated bridge 3.2.1 As a rectifier 3.2.2 As an inverter — regeneration 3.2.3 Switch voltages 3.2.4 DC voltage harmonics 3.2.5 AC current harmonics 3.3 The three phase bridge inverter 3.3.1 The voltage source bridge inverter 3.3.2 The current source bridge inverter 3.4 Isolation of electronics
104 104 104 104 111 113 115 115 119 120 124 126
4
The 4.1 4.2 4.3
six step voltage source inverter for induction motors Introduction Principles of operation Detailed analysis of the system 4.3.1 Circuit waveforms 4.3.2 Relationships and equations 4.3.3 Examples of calculations 4.4 Practical circuit design considerations 4.4.1 Overcurrent protection 4.4.2 Overvoltage protection 4.4.3 Factors affecting specifications 4.4.4 Circuit variations 4.5 Overall control methods 4.5.1 Supply convertor control 4.5.2 Inverter control 4.5.3 Typical control schemes 4.6 Performance and application 4.6.1 Torque/speed characteristics 4.6.2 Speed control accuracy 4.6.3 Supply power factor and harmonics
131 131 131 135 136 142 148 151 152 154 154 156 158 159 159 160 161 162 164 164
5
The Pulse Width Modulated voltage source inverter for induction motors 5.1 Introduction 5.2 Principles of operation 5.2.1 Pulse width modulation 5.2.2 The PWM drive system
166 166 166 167 174
Contents 5.3
5.4
5.5 5.6
Detailed analysis of the system 5.3.1 Motor waveforms 5.3.2 Inverter circuit waveforms 5.3.3 Circuit relationships and equations 5.3.4 Examples of calculations Practical circuit design considerations 5.4.1 Overcurrent protection 5.4.2 Regeneration 5.4.3 Factors affecting specifications 5.4.4 Typical circuit diagram Overall control methods Performance and application 5.6.1 Torque/speed characteristics 5.6.2 Efficiency 5.6.3 Supply power factor 5.6.4 Motor and supply harmonics 5.6.5 Accuracy and transient performance
vii 177 177 182 185 188 192 192 193 194 194 196 198 199 199 200 200 201
6
The 6.1 6.2 6.3.
six step current source inverter drive Introduction Principles of operation Detailed analysis of the system 6.3.1 Circuit waveforms 6.3.2 The motor vector diagram 6.3.3 Circuit relationships and equations 6.3.4 The standard current source inverter circuit 6.3.5 Examples of calculations 6.4 Practical circuit design considerations 6.4.1 Overcurrent protection 6.4.2 Overvoltage protection 6.4.3 Circuit variations 6.4.4 Factors affecting specifications 6.5 Overall control methods 6.6 Performance and application 6.6.1 Torque/speed characteristics 6.6.2 Efficiency 6.6.3 Supply power factor 6.6.4 Torque pulsations
202 202 203 206 207 213 215 219 223 229 229 230 230 231 232 235 235 236 236 236
7
The six step synchro-convertor system for synchronous motors 7.1 Introduction 7.2 Principles of operation 7.2.1 Starting and low speed operation 7.2.2 Normal running conditions
239 239 241 242 245
viii
Contents
7.3
7.4
7.5
7.6
8
123 Reversing and regeneration 7.2.4 Motor excitation Detailed analysis of the system 7.3.1 Convertor and motor waveforms 7.3.2 Armature reaction 7.3.3 Motor vector diagram 7.3.4 Relationships and equations 7.3.5 Examples of calculations Practical circuit design considerations 7.4.1 Overcurrent protection 7.4.2 Factors affecting specifications 7.4.3 Circuit variations Overall control methods 7.5.1 Supply convertor control 7.5.2 Motor convertor control 7.5.3 Excitation control Performance and application 7.6.1 Torque/speed characteristic 7.6.2 Efficiency 7.6.3 Speed control accuracy 7.6.4 Stability and transient performance 7.6.5 Supply power factor 7.6.6 Torque pulsations
247 248 249 249 251 252 254 257 261 262 263 265 266 268 268 269 269 270 271 272 272 273 274
The current source inverter for the capacitor self-excited induction motor
275
8.1 Introduction 8.2 Principles of operation 8.2.1 High speed running 8.2.2 Lower speed running 8.3 Detailed analysis of the system 8.3.1 Circuit waveforms 8.3.2 The motor vector diagram 8.3.3 Relationships and equations 8.3.4 Examples of calculations 8.4 Practical circuit design considerations 8.4.1 Protection 8.4.2 Commutation methods 8.4.3 Factors affecting specifications 8.5 Overall control methods 8.5.1 Supply convertor control 8.5.2 Motor convertor control 8.5.3 Motor magnetisation control 8.5.4 Typical overall control scheme
275 275 278 279 281 284 286 288 292 295 295 296 299 301 302 302 303 303
Contents 8.6
9
ix
Performance and application 8.6.1 Motor current waveforms 8.6.2 Torque/speed capability 8.6.3 Supply power factor
305 305 306 307
The cycloconvertor 9.1 Introduction 9.2 Principles of operation 9.2.1 The fundamental principles 9.2.2 3 phase systems 9.2.3 Reversal and regeneration 9.2.4 Supply side conditions 9.3 Detailed analysis of the system 9.3.1 Circuit waveforms 9.3.2 Current reversal 9.3.3 The motor vector diagram 9.3.4 Relationships and equations 9.3.5 Examples of calculations 9.4 Practical circuit design considerations 9.4.1 Overcurrent protection 9.4.2 Convertor polarity switching 9.4.3 Alternative power circuits 9.5 Overall control methods 9.5.1 Firing control 9.5.2 Typical control schemes 9.6 Performance and application 9.6.1 Speed range 9.6.2 Dynamic performance 9.6.3 Supply power factor 9.6.4 Harmonics
308 308 309 309 312 313 315 318 318 325 326 327 331 332 333 334 335 338 339 340 343 343 344 344 346
.
10 The slip energy recovery system for wound rotor induction motors 10.1 Introduction 10.2 Principles of operation 10.3 Detailed analysis of the system " 10.3.1 Circuit waveforms 10.3.2 The motor equivalent circuit 10.3.3 The motor vector diagram 10.3.4 Circuit equations and relationships 10.3.5 Examples of calculations 10.4 Practical circuit designs 10.4.1 Overcurrent protection 10.4.2 Overvoltage protection 10.4.3 Circuit variations
349 349 350 354 355 357 360 360 364 367 368 369 370
x
Contents
10.5 Overall control methods 10.6 Performance and application 10.6.1 Efficiency 10.6.2 Power factor 10.6.3 Torque capability 10.6.4 Harmonics in the system
370 373 373 374 376 377
Bibliography
380
Index
390
Preface During recent years there has been a surge of interest in the subject of AC Variable Frequency Motor Drives and this has been mainly due to the many technical and financial benefits which can be derived from being able to vary the speed of a process. The plant can be operated under its optimum condition whatever its loading and in many cases considerable energy savings can be made compared to other drive arrangements. During the same period there has also been considerable technical advance in the capabilities of such drive systems due mainly to the emergence of high quality semiconductor power switches and control microprocessors. This has caused the cost of these drive systems to reduce so that the overall economics of their application can be favourable in an increasing range of potential uses. In writing this book my aim has been to explain the technicalities of these drives in such a way that they can be understood by as wide a range of people as possible so as to encourage the increasing use of these systems. It has not been written just for the technical expert in this area of drives but also for the people who will use, apply and maintain such systems as well as those who only have a general interest in the subject. I have also included information which will be of particular interest to the college and university departments dealing with power electronic equipment and I hope this book helps them widen the scope of their curriculum to include variable speed drives. The preparation of this book was greatly assisted by my developing a set of computer programmes designed to model the individual drive systems. As a result I have decided to complete the development of these programmes and to make them available to others. These programmes model the steady state behaviour of the drive systems and using them it is possible to: a) Model any drive, of any size, of any speed range operating at any voltage level. b) Operate the computer as though it were the drive, using the keyboard to input your requirements and observing the drive operation on the screen.
xii
Preface
c) Establish all the variable parameters of the drive under any condition of operation. All the supply convertor, motor convertor and motor currents, voltages and power factors, etc., are available at any operating speed and torque. d) Observe the switching sequences of the power circuits while controlling the drive model from the computer keyboard. e) Obtain printed graph plots of the variation of all the drive parameters from a printer connected to the computer. f) Carry out experiments on the drive model under a variety of conditions, as though it was a set of laboratory equipment. It is possible to start with a simplified system, e.g. neglecting power losses, etc. and to gradually increase the system complexity until a full practical drive is being modelled and studied. These programmes are a very important aid to the full understanding of these drive systems. Further details can be obtained from ORANGE ENTERPRIZES, 20, BADBY ROAD, DAVENTRY, NORTHANTS. NN11 4AP, ENGLAND. I would like to thank all my colleagues at G.E.C. Industrial Controls, Rugby, for the help they have given me, this book would not have been possible without their help, specifically I would like to thank Mr. David Martin for much expert advice. Special thanks are due to my wife, Lesley, for being patient during the many hours of writing and for the time she spent transferring my untidy handwriting into our word processor and hence into the typed manuscript. Acknowledgement is also given to The General Electric Company of England and to G.E.C. Industrial Controls, Ltd, for permission to publish this book, the contents of which I learned while in their employ. May I hope that all readers find this book interesting, informative and readable. DAVID FINNEY DAVENTRY 1987
