ELECTRIC MOTORS AND DRIVES Fundamentals, Types, and Applications Fourth Edition AUSTIN HUGHES AND BILL DRURY Amsterdam(cid:129)Boston(cid:129)Heidelberg(cid:129)London(cid:129)NewYork Oxford(cid:129)Paris(cid:129)SanDiego(cid:129)SanFrancisco(cid:129)Singapore Sydney(cid:129)Tokyo NewnesisanimprintofElsevier NewnesisanimprintofElsevier TheBoulevard,LangfordLane,Kidlington,Oxford,OX51GB 225WymanStreet,Waltham,MA02451,USA Firstedition1990 Secondedition1993 Thirdedition2006 Reprinted2006,2007,2008(twice),2009 Fourthedition2013 Copyright(cid:1)2013AustinHughesandWilliamDrury.PublishedbyElsevierLtd.Allrightsreserved. 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BritishLibraryCataloguinginPublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary LibraryofCongressCataloging-in-PublicationData AcatalogrecordforthisbookisavailablefromtheLibraryofCongress ISBN:978-0-08-098332-5 ForinformationonallNewnespublications visitourwebsiteatstore.elsevier.com PrintedandboundintheUnitedKingdom 13141516 10987654321 PREFACE Thisfourtheditionisagainintendedprimarilyfornonspecialistusersorstudentsof electric motors and drives. From the outset the aim has been to bridge the gap betweenspecialisttextbooks(whicharepitchedatalevelwhichistooacademicfor the average user) and the more prosaic handbooks which are full of detailed informationbutprovidelittleopportunityforthedevelopmentofanyrealinsight. Weintendtocontinuewhathasbeenasuccessfulformulabyprovidingthereader withanunderstandingofhoweachmotoranddrivesystemworks,inthebeliefthat itisonlybyknowingwhatshouldhappen(andwhy)thatinformedjudgementsand soundcomparisons can bemade. The fact that the book now has joint authors resulted directly from the publisher’s successful reviewing process, which canvassed expert opinions about a prospective fourth edition. It identified several new topics needed to bring the work up to date, but these areas were not ones that the original author (AH) was equipped to address, having long since retired. Fortunately, one of the reviewers (WD) turned out to be a willing co-author: he is not only an industrialist (and author) with vast experience in the field, but, at least as importantly, shares the philosophy that guided the first three versions. We enjoy collaborating and hope and believe that our synergy will prove of benefitto our readers. Giventhatthebookisaimedatreadersfromarangeofdisciplines,sectionsof the book are of necessitydevoted to introductory material. The first two chapters thereforeprovideagentleintroduction toelectromagnetic energyconversionand power electronics. Many of the basic ideas introduced here crop up frequently throughout the book (and indeed are deliberately repeated to emphasize their importance),sounlessthereaderisalreadywellversedinthefundamentalsitwould bewisetoabsorbthefirsttwochaptersbeforetacklingthelatermaterial.Atvarious pointslaterinthebookweincludemoretutorialmaterial,e.g.inChapter7where we prepare the ground for unraveling the mysteries of field-oriented control. A grasp of basic closed-loop principles is also required in order to understand the operation of the various drives, so further introductory material is included in Appendix 1. Thebookcoversallofthemostimportanttypesofmotoranddrive,including conventionalandbrushlessd.c.,inductionmotor,synchronousmotorsofalltypes, switched reluctance, and stepping motors (but not highly customized or applica- tion-specific systems, e.g. digital hard disk drives). The induction motor and induction motor drives are given most weight, reflecting their dominant market position in terms of numbers. Conventional d.c. machines are deliberately intro- duced early on, despite their declining importance: this is partly because under- standing is relatively easy, but primarily because the fundamental principles that j ix x Preface emergecarryforwardtoothermotors.Similarly,d.c.drivesaretackledfirst,because experienceshowsthatreaderswhomanagetograsptheprinciplesofthed.c.drive will findthis knowhow invaluable indealingwith other morechallenging types. The third edition has been completely revised and updated. Major additions include an extensive (but largely non-mathematical) treatment of both field- orientedanddirecttorquecontrolinbothinductionandsynchronousmotordrives; anewchapteronpermanentmagnetbrushlessmachines;newmaterialdealingwith self-excited machines, including wind-power generation; and increased emphasis throughoutontheinherentabilityofelectricalmachinestoacteitherasamotoror agenerator. Younger readers may be unaware of the radical changes that have taken place overthepast50years,soacoupleofparagraphsareappropriatetoputthecurrent sceneintoperspective.Formorethanacentury,manydifferenttypesofmotorwere 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 stillis)themostnumeroustypebutwasjudgedasbeingsuitedonlytoapplications whichcalledforconstantspeed.Thereasonfortheplethoraofmotortypeswasthat 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 for control of speed within the motor itself. All sorts of ingenious arrangements and interconnectionsof motor windings were invented, but even the best motors had a limited operating range, and they all required bulky electromechanical control gear. Allthischangedfromtheearly1960s,whenpowerelectronicsbegantomake an impact.The first major breakthrough camewiththe thyristor,which provided arelativelycheap,compact,andeasilycontrolledvariablespeeddriveusingthed.c. motor.Inthe1970sthesecondmajorbreakthroughresultedfromthedevelopment ofpowerelectronicinverters,providinga3phasevariable-frequencysupplyforthe cageinductionmotorandtherebyenablingitsspeedtobecontrolled.Thesemajor developments resulted in the demise of many of the special motors, leaving the majorityofapplicationsinthehandsofcomparativelyfewtypes.Theswitchfrom analogue to digital control also represented significant progress, but it was the availability of cheap digital processors that sparked the most recent leap forward. Realtimemodelingandsimulationarenowincorporatedasstandardintoinduction andsynchronousmotordrives,therebyallowingthemtoachievelevelsofdynamic performance that had long been consideredimpossible. Theinformalstyleofthebookreflectsourbeliefthatthedifficultyofcomingto grips with new ideas should not be disguised. The level at which to pitch the material was basedon feedback from previouseditionswhich supported our view that a mainly descriptive approach with physical explanations would be most Preface xi appropriate, with mathematics kept to a minimum to assist digestion. The most important concepts (such as the inherent e.m.f. feedback in motors, the need for a switching strategy in converters, and the importance of stored energy) are deliberately reiterated to reinforce understanding, but should not prove too tire- some for readers who have already ‘got the message’. We have deliberately not included any computed magnetic field plots, nor any results from the excellent motorsimulationpackagesthatarenowavailablebecauseexperiencesuggeststhat simplifieddiagrams are actually better aslearning vehicles. Finally, we welcome feedback, either via the publisher, or using the e-mail addresses below. Austin Hughes ([email protected]) Bill Drury ([email protected]) 14 October2012 CHAPTER ONE – Electric Motors The Basics 1. INTRODUCTION Electric motors are so much a part of everyday life that we seldom give them a second thought. When we switch on an ancient electric drill, for example, we confidentlyexpectittorunrapidlyuptothecorrectspeed,andwedon’tquestion howitknowswhatspeedtorunat,orhowitisthatonceenoughenergyhasbeen drawnfromthesupplytobringituptospeed,thepowerdrawnfallstoaverylow level.Whenweputthedrilltoworkitdrawsmorepower,and,whenwefinish,the power drawn from the mains reduces automatically, without intervention on our part. Thehumblemotor,consistingofnothingmorethananarrangementofcopper coils and steel laminations, is clearly rather a clever energy converter, which warrantsseriousconsideration.Bygainingabasicunderstandingofhowthemotor works, we will be able to appreciate its potential and its limitations, and (in later chapters) see how its already remarkable performance is dramatically enhanced by the additionof external electroniccontrols. This chapter deals with the basic mechanisms of motor operation, so readers who are already familiar with such matters as magnetic flux, magnetic and electric circuits, torque, and motional e.m.f. can probably afford to skim over much of it. In the course of the discussion, however, several very important general principles and guidelines emerge. These apply to all types of motor and are summarized in section 9. Experience shows that anyone who has a good grasp of these basic principles will be well equipped to weigh the pros and cons of the different types of motor, so all readers are urged to absorb them before tackling other parts of the book. 2. PRODUCING ROTATION Nearly all motors exploit the force which is exerted on a current-carrying conductor placed in a magnetic field. The force can be demonstrated by placing a bar magnet near a wire carrying current (Figure 1.1), but anyone trying the experimentwillprobablybedisappointedtodiscoverhowfeebletheforceis,and will doubtless be left wondering how such an unpromising effect can be used to make effectivemotors. (cid:1)2013AustinHughesandWilliamDrury. j ElectricMotorsandDrives PublishedbyElsevierLtd. http://dx.doi.org/10.1016/B978-0-08-098332-5.00001-2 Allrightsreserved. 1 2 ElectricMotorsandDrives Figure1.1 Mechanicalforceproducedonacurrent-carryingwireinamagneticfield. We will see that in order to make the most of the mechanism, we need to arrangefortheretobeaverystrongmagneticfield,andforittointeractwithmany conductors, each carrying as much current as possible. We will also see later that althoughthemagneticfield(or‘excitation’)isessentialtotheworkingofthemotor, it acts only as a catalyst, and all of the mechanical output power comes from the electrical supply to the conductors on which theforce isdeveloped. Itwillemergelaterthatinsomemotorsthepartsofthemachineresponsiblefor theexcitationandfortheenergy-convertingfunctionsaredistinctandself-evident. In the d.c. motor, for example, the excitation is provided either by permanent magnetsorbyfieldcoilswrappedaround clearly definedprojectingfieldpoleson the stationary part, while the conductors on which force is developed are on the rotorandsuppliedwithcurrentviaslidingbrushes.Inmanymotors,however,there is no such clear-cut physical distinction between the ‘excitation’ and the ‘energy- converting’ parts of the machine, and a single stationary winding serves both purposes. Nevertheless, we will find that identifying and separating the excitation andenergy-convertingfunctionsisalwayshelpfulinunderstandinghowmotorsof all types operate. Returning to the matter of force on a single conductor, we will look first at whatdeterminesthemagnitudeanddirectionoftheforce,beforeturningtowaysin which the mechanism is exploited to produce rotation. The concept of the magneticcircuitwillhavetobeexplored,sincethisiscentraltounderstandingwhy motors have the shapes they do. Before that, a brief introduction to the magnetic field and magnetic flux and flux density is included for those who are not already familiarwith the ideas involved. 2.1 Magnetic field and magnetic flux When a current-carrying conductor is placed in a magnetic field, it experiences aforce.Experimentshowsthatthemagnitudeoftheforcedependsdirectlyonthe current in the wire and the strength of the magnetic field, and that the force is greatestwhen the magnetic field isperpendicular tothe conductor. ElectricMotors–TheBasics 3 Figure1.2 Magneticfluxlinesproducedbyapermanentmagnet. In the set-up shown in Figure 1.1, the source of the magnetic field is a bar magnet, which producesamagnetic field asshown inFigure 1.2. The notion of a ‘magnetic field’ surrounding a magnet is an abstract idea that helps us to come to grips with the mysterious phenomenon of magnetism: it not only provides us with a convenient pictorial way of visualizing the directional effects,but italsoallows usto quantify the‘strength’ of themagnetismand hence permitsus topredictthe various effectsproduced byit. The dotted lines in Figure 1.2 are referred to as magnetic flux lines, or simply fluxlines.Theyindicatethedirectionalongwhichironfilings(orsmallsteelpins) wouldalignthemselveswhenplacedinthefieldofthebarmagnet.Steelpinshave noinitialmagneticfieldoftheirown,sothereisnoreasonwhyoneendortheother of the pins should pointto aparticularpole of the bar magnet. However,whenweputacompassneedle(whichisitselfapermanentmagnet) inthefieldwefindthatitalignsitselfasshowninFigure1.2.Intheupperhalfof the figure, the S end of the diamond-shaped compass settles closest to the N pole of the magnet, while in the lower half of the figure, the N end of the compass seeks the S of the magnet. This immediately suggests that there is a direction associated with the lines of flux, as shown by the arrows on the flux lines, which conventionallyaretakenaspositivelydirectedfromtheNtotheSpoleofthebar magnet. The sketch in Figure 1.2 might suggest that there is a ‘source’ near the top of the bar magnet, from which flux lines emanate before making their way to a corresponding ‘sink’at thebottom.However,if wewere tolook at thefluxlines inside the magnet, we would find that they were continuous, with no ‘start’ or 4 ElectricMotorsandDrives ‘finish’. (In Figure 1.2 the internal flux lines have been omitted for the sake of clarity,butaverysimilarfieldpatternisproducedbyacircularcoilofwirecarrying a direct current – see Figure 1.7 where the continuity of the flux lines is clear.) Magnetic flux lines always form closed paths, as we will see when we look at the ‘magnetic circuit’, and we draw a parallel with the electric circuit, in which the currentisalsoacontinuousquantity.(Theremustbea‘cause’ofthemagneticflux, of course, and in a permanent magnet this is usually pictured in terms of atomic- levelcirculatingcurrentswithinthemagnetmaterial.Fortunately,discussionatthis physical level isnot necessary for our purposes.) 2.2 Magnetic flux density As well as showing direction, flux plots convey information about the intensity ofthemagneticfield.Toachievethis,weintroducetheideathatbetweenevery pair of flux lines (and for a given depth into the paper) there is the same ‘quantity’ofmagneticflux.Somepeoplehavenodifficultywithsuchaconcept, while others find that the notion of quantifying something so abstract represents aseriousintellectualchallenge.Butwhethertheapproachseemsobviousornot, thereisnodenyingthepracticalutilityofquantifyingthemysteriousstuffwecall magnetic flux, and it leads us next to the very important idea of magneticfl ux density (B). When the flux lines are close together, the ‘tube’ of flux is squashed into asmaller space,whereaswhenthelinesarefurtherapartthesametubeoffluxhas more breathing space. The flux density (B) is simply the flux in the ‘tube’ (F) dividedby thecross-sectional area (A)of the tube, i.e. F B ¼ (1.1) A Thefluxdensityisavectorquantity,andisthereforeoftenwritteninboldtype:its magnitudeisgivenbyequation(1.1),anditsdirectionisthatoftheprevailingflux linesateachpoint.NearthetopofthemagnetinFigure1.2,forexample,theflux density will be large (because the flux is squashed into a small area), and pointing upwards,whereasontheequatorandfaroutfromthebodyofthemagnettheflux density will be small and directed downwards. Wewill see later that in order tocreatehighfluxdensities inmotors,the flux spendsmostofitslifeinsidewell-defined‘magneticcircuits’madeofironorsteel, within which the flux lines spread out uniformly to take full advantage of the availablearea.InthecaseshowninFigure1.3,forexample,thecross-sectionalarea oftheironatbb0istwicethatataa0,butthefluxisconstantsothefluxdensityatbb0 0 ishalf that at aa. Itremainstospecifyunitsforquantityofflux,andfluxdensity.IntheSIsystem, the unit of magnetic flux is the weber (Wb). If one weber of flux is distributed uniformly across an area of one square meter perpendicular to the flux, the flux ElectricMotors–TheBasics 5 Figure 1.3 Magneticfluxlinesinsidepartofanironmagneticcircuit. densityisclearlyoneweberpersquaremeter(Wb/m2).ThiswastheunitofBuntil about 50 years ago, when it was decided that one weber per square meter would henceforth be known as one tesla (T), inhonor of Nikola Tesla, who isgenerally creditedwithinventingtheinductionmotor.ThewidespreaduseofB(measuredin Tesla)inthedesignstageofalltypesofelectromagneticapparatusmeansthatweare constantly reminded of the importance of Tesla; but at the same time one has to acknowledge that the outdated unit did have the advantage of conveying directly what flux densityis, i.e. flux divided byarea. Thefluxina1kWmotorwillbeperhapsafewtensofmilliwebers,andasmall barmagnetwouldprobablyonlyproduceafewmicrowebers.Ontheotherhand, values of flux density are typically around 1 tesla in most motors, which is areflectionofthefactthatalthoughthequantityoffluxinthe1kWmotorissmall, itisalsospread over a small area. 2.3 Force on a conductor We now return to the production of force on a current-carrying wire placed in a magnetic field, asrevealed bythe set-up shown inFigure 1.1. TheforceisshowninFigure1.1:itisatrightanglestoboththecurrentandthe magneticfluxdensity,anditsdirectioncanbefoundusingFleming’sleft-handrule. If we picture the thumb and the first and middle fingers held mutually perpen- dicular,thenthefirstfingerrepresentsthefieldorfluxdensity(B),themIddlefinger representsthecurrent(I),andthethumbthenindicatesthedirectionofmotion,as shown inFigure 1.4. Clearly,ifeitherthefieldorthecurrentisreversed,theforceactsdownwards, and if both are reversed, the direction of theforce remainsthesame. Wefindbyexperimentthatifwedoubleeitherthecurrentorthefluxdensity, wedoubletheforce,whiledoublingbothcausestheforcetoincreasebyafactorof four.Buthowaboutquantifyingtheforce?Weneedtoexpresstheforceintermsof the product of the current and the magnetic flux density, and this turns out to be very straightforward when we work in SIunits.