Electric Motors And Drives

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Electric Motors and Drives

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Electric Motors and DrivesFundamentals, Types and ApplicationsThird editionAustin HughesSenior Fellow, School of Electronic and Electrical Engineering,University of LeedsAMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORDPARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYONewnes is an imprint of Elsevier

Newnes is an imprint of ElsevierLinacre House, Jordan Hill, Oxford OX2 8DP30 Corporate Drive, Suite 400, Burlington, MA 01803First edition 1990Second edition 1993Third edition 2006Copyright ß 1990, 1993, 2006, Austin Hughes. Published by Elsevier Ltd. All rights reservedThe right of Austin Hughes to be identified as the author of this workhas been asserted in accordance with the Copyright, Designs andPatents Act 1988.No part of this publication may be reproduced, stored in a retrieval system or transmitted in anyform or by any means electronic, mechanical, photocopying, recording or otherwise without theprior written permission of the publisherPermissions may be sought directly from Elsevier’s Science & Technology Rights Department inOxford, UK: phone ( 44) (0) 1865 843830; fax ( 44) (0) 1865 853333; email: [email protected] Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier materialBritish Library Cataloguing in Publication DataA catalogue record for this book is available from the British LibraryLibrary of Congress Cataloguing in Publication DataA catalogue record for this book is available from the Library of CongressISBN-13: 978-0-7506-4718-2ISBN-10: 0-7506-4718-3For information on all Newnes publicationsvisit our website at http://books.elsevier.com/Printed and bound in Great Britain06 07 08 09 1010 9 8 7 6 5 4 3 2 1

CONTENTSPreface1 ELECTRIC MOTORSxvi1Introduction1Producing RotationMagnetic field and magnetic fluxMagnetic flux densityForce on a conductor2346Magnetic CircuitsMagnetomotive force (MMF)Electric circuit analogyThe air-gapReluctance and air-gap flux densitiesSaturationMagnetic circuits in motors791011121415Torque ProductionMagnitude of torqueThe beauty of slotting161819Specific Loadings and Specific OutputSpecific loadingsTorque and motor volumeSpecific output power – importance of speed21212323Energy Conversion – Motional EMFElementary motor – stationary conditionsPower relationships – conductor moving atconstant speed252628

vi2ContentsEquivalent CircuitMotoring conditionBehaviour with no mechanical loadBehaviour with a mechanical loadRelative magnitudes of V and E, and efficiencyAnalysis of primitive motor – conclusions303232353738General Properties of Electric MotorsOperating temperature and coolingTorque per unit volumePower per unit volume – importance of speedSize effects – specific torque and efficiencyEfficiency and speedRated voltageShort-term overload3939404141414142Review Questions42POWER ELECTRONIC CONVERTERS FORMOTOR DRIVES45IntroductionGeneral arrangement of drives4545Voltage Control – D.C. Output from D.C. SupplySwitching controlTransistor chopperChopper with inductive load – overvoltageprotectionFeatures of power electronic converters474849D.C. from A.C. – Controlled RectificationThe thyristorSingle-pulse rectifierSingle-phase fully controlled converter – outputvoltage and control3-phase fully controlled converterOutput voltage rangeFiring circuits555556A.C. from D.C. SP – SP InversionSingle-phase inverterOutput voltage controlSinusoidal PWM3-phase inverter6565676869525457626464

ContentsForced and natural commutation – historicalperspectiveMatrix convertersvii6970Inverter Switching DevicesBipolar junction transistor (BJT)Metal oxide semiconductor field effecttransistor (MOSFET)Insulated gate bipolar transistor (IGBT)Gate turn-off thyristor (GTO)7272Converter Waveforms and Acoustic Noise75Cooling of Power Switching DevicesThermal resistanceArrangement of heatsinks and forced air coolingCooling fans75757778Review Questions793 CONVENTIONAL D.C. MOTORS73747482Introduction82Torque ProductionFunction of the commutatorOperation of the commutator – interpoles848688Motional E.M.F.Equivalent circuit9094D.C. motor – Steady-State CharacteristicsNo-load speedPerformance calculation – exampleBehaviour when loadedBase speed and field weakeningArmature reactionMaximum output power95959698103105106Transient Behaviour – Current SurgesDynamic behaviour and time-constants107108Shunt, Series and Compound MotorsShunt motor – steady-state operatingcharacteristicsSeries motor – steady-state operatingcharacteristics111113115

viiiContentsUniversal motorsCompound motors4118119Four-Quadrant Operation and Regenerative BrakingFull speed regenerative reversalDynamic braking119122124Toy Motors124Review Questions126D.C. MOTOR DRIVES133Introduction133Thyristor D.C. Drives – GeneralMotor operation with converter supplyMotor current waveformsDiscontinuous currentConverter output impedance: overlapFour-quadrant operation and inversionSingle-converter reversing drivesDouble SP-converter reversing drivesPower factor and supply effects134136136139141143144146146Control Arrangements for D.C. DrivesCurrent controlTorque controlSpeed controlOverall operating regionArmature voltage feedback and IRcompensationDrives without current control148150152152154Chopper-Fed D.C. Motor DrivesPerformance of chopper-fed d.c. motor drivesTorque–speed characteristics andcontrol arrangements155156D.C. Servo DrivesServo motorsPosition control159160162Digitally Controlled Drives163Review Questions164155155159

Contents5 INDUCTION MOTORS – ROTATING FIELD,SLIP AND TORQUEix167IntroductionOutline of approach167168The Rotating Magnetic FieldProduction of rotating magnetic fieldField produced by each phase windingResultant fieldDirection of rotationMain (air-gap) flux and leakage fluxMagnitude of rotating flux waveExcitation power and VASummary170172172176177177179182183Torque ProductionRotor constructionSlipRotor induced e.m.f., current and torqueRotor currents and torque – small slipRotor currents and torque – large slip183183185185187189Influence of Rotor Current on FluxReduction of flux by rotor current191192Stator Current-Speed Characteristics193Review Questions1966 OPERATING CHARACTERISTICS OFINDUCTION MOTORS198Methods of Starting Cage MotorsDirect Starting – ProblemsStar/delta (wye/mesh) starterAutotransformer starterResistance or reactance starterSolid-state soft startingStarting using a variable-frequencyinverter198198202202203204Run-up and Stable Operating RegionsHarmonic effects – skewingHigh inertia loads – overheatingSteady-state rotor losses and efficiency206208209209206

x7ContentsSteady-state stability – pullout torqueand stalling210Torque–Speed Curves – Influence of RotorParametersCage rotorDouble cage rotorsDeep bar rotorsStarting and run-up of slipring motors211211213214215Influence of Supply Voltage on Torque–Speed Curve217Generating and BrakingGenerating region – overhauling loadsPlug reversal and plug brakingInjection braking218219220221Speed ControlPole-changing motorsVoltage control of high-resistance cage motorsSpeed control of wound-rotor motors221222223224Power Factor Control and Energy OptimisationVoltage controlSlip energy recovery (wound rotor motors)225225227Single-Phase Induction MotorsPrinciple of operationCapacitor-run motorsSplit-phase motorsShaded-pole motors227227229230231Size RangeScaling down – the excitation problem232232Review Questions233INDUCTION MOTOR EQUIVALENT CIRCUIT236IntroductionOutline of approach236237Similarity Between Induction Motor and Transformer238The Ideal TransformerIdeal transformer – no-load condition,flux and magnetising current240240

ContentsIdeal transformer – no-load condition,voltage ratioIdeal transformer on loadThe Real TransformerReal transformer – no-load condition,flux and magnetising currentReal transformer – leakage reactanceReal transformer on load – exactequivalent circuitReal transformer – approximateequivalent circuitMeasurement of parametersSignificance of equivalent circuit parametersxi245246248248251252254256257Development of the Induction Motor Equivalent CircuitStationary conditionsModelling the electromechanicalenergy conversion process258258Properties of Induction MotorsPower balanceTorque261262262Performance Prediction – ExampleLine currentOutput powerEfficiencyPhasor diagram263264264265266Approximate Equivalent CircuitsStarting and full-load relationshipsDependence of pull out torque onmotor parametersAnalysisGraphical interpretation via phasor diagram267268269270271Measurement of Parameters274Equivalent Circuit Under Variable-FrequencyConditions274Review Questions277259

xiiContents8INVERTER-FED INDUCTION MOTOR DRIVES279IntroductionComparison with d.c. driveInverter waveformsSteady-state operation – importance ofachieving full flux279280282Torque–Speed Characteristics – ConstantV/f OperationLimitations imposed by the inverter – constantpower and constant torque regionsLimitations imposed by motor9284286288289Control Arrangements for Inverter-Fed DrivesOpen-loop speed controlClosed-loop speed control290291293Vector (Field-Oriented) ControlTransient torque control296297Cycloconverter Drives300Review Questions303STEPPING MOTORS305IntroductionOpen-loop position controlGeneration of step pulses and motorresponseHigh-speed running and ramping305306Principle of Motor OperationVariable reluctance motorHybrid motorSummary311312314317Motor CharacteristicsStatic torque–displacement curvesSingle-steppingStep position error and holding torqueHalf steppingStep division – mini-stepping318318319320321323307308

ContentsxiiiSteady-State Characteristics – Ideal(Constant-Current) DriveRequirements of drivePull-out torque under constant-currentconditions326Drive Circuits and Pull-Out Torque–Speed CurvesConstant-voltage driveCurrent-forced driveChopper driveResonances and instability328328330331333Transient PerformanceStep responseStarting from restOptimum acceleration andclosed-loop control335335336Review Questions10 SYNCHRONOUS, BRUSHLESS D.C. ANDSWITCHED RELUCTANCE DRIVES324324337338340Introduction340Synchronous MotorsExcited-rotor motorsEquivalent circuit of excited-rotorsynchronous motorPhasor diagram and Power-factor controlStartingPermanent magnet synchronous motorsHysteresis motorsReluctance motors341343Controlled-Speed Synchronous Motor DrivesOpen-loop inverter-fed synchronousmotor drivesSelf-synchronous (closed-loop) operationOperating characteristics and control352353354355Brushless D.C. Motors357344347349350351351

xiv11ContentsSwitched Reluctance Motor DrivesPrinciple of operationTorque prediction and controlPower converter and overall drivecharacteristics358359360Review Questions363MOTOR/DRIVE SELECTION366Introduction366Power Range for Motors and DrivesMaximum speed and speed range366368Load Requirements – Torque–Speed CharacteristicsConstant-torque loadInertia matchingFan and pump loads369369374374General Application ConsiderationsRegenerative operation and brakingDuty cycle and ratingEnclosures and coolingDimensional standardsSupply interaction and harmonics375375376377378378Review Questions379APPENDIX – INTRODUCTION TO CLOSED–LOOPCONTROL363381Reasons for Adopting a Simplified Approach381Closed-Loop (Feedback) SystemsError-activated feedback systemsClosed-loop systems382383384Steady-State Analysis of Closed-Loop Systems386Importance of Loop Gain – Example390Steady-State Error – Integral Control392PID Controller394

ContentsxvStability396Disturbance Rejection – Example Using D.C. Machine397Further Reading400Answers to Numerical Review Questions401Index404

PREFACELike its predecessors, the third edition of this book is intended primarilyfor 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) andthe more prosaic ‘handbooks’, which are full of useful detail but providelittle opportunity for the development of any real insight or understanding. The fact that the second edition was reprinted ten times indicatedthat there had indeed been a gap in the market, and that a third editionwould be worthwhile. It was also gratifying to learn that although theoriginal book was not intended as yet another undergraduate textbook,teachers and students had welcomed the book as a gentle introduction tothe subject.The aim throughout is to provide the reader with an understanding ofhow each motor and drive system works, in the belief that it is only byknowing what should happen that informed judgements and soundcomparisons can be made. Given that the book is aimed at readersfrom a range of disciplines, introductory material on motors andpower electronics is clearly necessary, and this is presented in the firsttwo chapters. Many of these basic ideas crop up frequently throughoutthe book, so unless the reader is well-versed in the fundamentals itwould be wise to absorb the first two chapters before tackling the latermaterial. In addition, an awareness of the basic ideas underlyingfeedback and closed-loop control is necessary in order to follow thesections dealing with drives, and this has now been provided as anAppendix.The book explores most of the widely used modern types of motorsand drives, including conventional and brushless d.c., induction motors(mains and inverter-fed), stepping motors, synchronous motors (mainsand converter-fed) and reluctance motors. The d.c. motor drive and theinduction motor drive are given most importance, reflecting their dominant position in terms of numbers. Understanding the d.c. drive isparticularly important because it is still widely used as a frame of

Prefacexviireference 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 thecontrol scheme.Younger readers may be unaware of the radical changes that havetaken place over the past 40 years, so perhaps a couple of paragraphs areappropriate to put the current scene into perspective. For more than acentury, 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, whereasthe shunt d.c. motor, though outwardly indistinguishable, was seen asbeing quite unsuited to traction applications. The cage induction motorwas (and still is) the most widely used but was judged as being suited onlyfor applications that called for constant speed. The reason for the plethora of motor types was that there was no easy way of varying the supplyvoltage and/or frequency to obtain speed control, and designers weretherefore forced to seek ways of providing speed control within themotor itself. All sorts of ingenious arrangements and interconnections ofmotor windings were invented, but even the best motors had a limitedrange of operating characteristics, and all of them required bulky controlequipment 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 tomake an impact. The first major breakthrough came with the thyristor,which provided a relatively cheap, compact and easily controlledvariable-speed drive using the d.c. motor. In the 1970s, the secondmajor breakthrough resulted from the development of power-electronicinverters, providing a three-phase variable-frequency supply for the cageinduction motor and thereby enabling its speed to be controlled.These major developments resulted in the demise of many of thespecial motors, leaving the majority of applications in the handsof comparatively few types, and the emphasis has now shifted fromcomplexity inside the motor to sophistication in supply and controlarrangements.From the user’s point of view this is a mixed blessing. Greater flexibility and superior levels of performance are available, and there arefewer motor types to consider. But if anything more than constant speedis called for, the user will be faced with the purchase of a complete drivesystem, consisting of a motor together with its associated power electronics package. To choose wisely requires not only some knowledge ofmotors, but also the associated power-electronics and the control options that are normally provided.

xviiiPrefaceDevelopment in the world of electrical machines tends to be steadyrather than spectacular, which means that updating the second editionhas called for only modest revision of the material covering the how andwhy 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 theequivalent circuit of the induction motor, because familiarity with theterminology of the equivalent circuit is necessary in order to engage inserious dialogue with induction motor suppliers or experts. Howeverthose who find the circuit emphasis not to their liking can be reassuredthat they can skip Chapter 7 without prejudicing their ability to tacklethe subsequent chapter on induction motor drives.The power electronics area has matured since the 1993 edition of thebook, but although voltage and current ratings of individual switchingdevices continue to improve, and there is greater integration of driveelectronics and power devices, there has been no strategic shift thatwould 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 retainedin most of the diagrams because it has the virtue of showing thedirection of current flow, and is therefore helpful in understandingcircuit operation.The style of the book reflects my own preference for an informalapproach, in which the difficulty of coming to grips with new ideas isnot disguised. Deciding on the level at which to pitch the material wasoriginally a headache, but experience suggested that a mainly descriptiveapproach with physical explanations would be most appropriate, withmathematics kept to a minimum to assist digestion. The most importantconcepts (such as the inherent e.m.f. feedback in motors, or the need fora switching strategy in converters) are deliberately reiterated to reinforceunderstanding, but should not prove too tiresome for readers who havealready ‘got the message’. I had hoped to continue without numberedheadings, as this always seems to me to make the material seem lighter,but cross referencing is so cumbersome without numbering that in theend 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 arenow available because experience suggests that simplified diagrams areactually better as learning vehicles. All of the diagrams have beenredrawn, and many new ones have been added.Review questions have been added at the end of each chapter. Thenumber of questions broadly reflects my judgement of the relative

Prefacexiximportance of each chapter, and they are intended to help build confidence and to be used selectively. A drives user might well not botherwith the basic machine-design questions in the first two chapters, butcould benefit by tackling the applications-related questions in subsequent chapters. Judicious approximations are called for in most of thequestions, 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, withfully worked and commented solutions on the accompanying websitehttp://books

Electric Motors and Drives Fundamentals, Types and Applications Third edition Austin Hughes Senior Fellow, School of Electronic and Electrical Engineering, University of Leeds AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO Newnes is an imprint of .

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