Electrical Machines and Drives for Electric, Hybri

1ElectricalMachinesandDrivesforElectric,HybridandFuelCellVehiclesZ.Q.ZhuandD.HoweDepartmentofElectronicandElectricalEngineering,UniversityofSheffield,MappinStreetSheffieldS13JD,UKAbstract-Thispaperreviewstherelativemeritsofinduction,switchedreluctanceandpermanentmagnetbrushlessmachinesanddrivesforapplicationinelectric,hybridandfuelcellvehicles,withparticularemphasisonPMbrushlessmachines.Thebasicoperationalcharacteristicsanddesignrequirements,viz.ahightorque/powerdensity,highefficiencyoverawideoperatingrange,andahighmaximumspeedcapability,aswellasthelatestdevelopments,aredescribed.PermanentmagnetbrushlessDCandACmachinesanddrivesarecomparedintermsoftheirconstanttorqueandconstantpowercapabilities,andvariousPMmachinetopologiesandtheirperformancearereviewed.Finally,methodsforenhancingthePMexcitationtorqueandreluctancetorquecomponentsand,thereby,improvingthetorqueandpowercapability,aredescribed.Keywords:Brushlessdrives,electricalmachines,electricvehicles,hybridvehicles,inductionmachines,permanentmagnetmachines,switchedreluctancemachines.1.INTRODUCTIONElectricalmachinesanddrivesareakeyenablingtechnologyforelectric,hybridandfuelcellvehicles.Thebasiccharacteristicswhicharerequiredofanelectricalmachinefortractionapplicationsinclude[1]-[3]:•Hightorquedensityandpowerdensity;•Hightorqueforstarting,atlowspeedsandhillclimbing,andhighpowerforhighspeedcruising;•Widespeedrange,withaconstantpoweroperatingrangeofaround3-4timesthebase-speedbeingagoodcompromisebetweenthepeaktorquerequirementofthemachineandthevolt-ampereratingoftheinverter;•Highefficiencyoverwidespeedandtorqueranges,includinglowtorqueoperation;•Intermittentoverloadcapability,typicallytwicetheratedtorqueforshortdurations;•Highreliabilityandrobustnessappropriatetothevehicleenvironment;•Acceptablecost.Inaddition,lowacousticnoiseandlowtorquerippleareimportantdesignconsiderations.Onanurbandrivingcycleatractionmachineoperatesmostfrequentlyatlightloadsaroundthebase-speed.Therefore,ingeneral,itshouldbedesignedtooperateatmaximumefficiencyandminimumacousticnoiseinthisregion.Typicaltorque/power-speedcharacteristicsrequiredfortractionmachinesareillustratedinFig.1.Inductionmachines(IM),switchedreluctance(SR)andpermanentmagnet(PM)brushlessmachines,Fig.2,haveallbeenemployedintractionapplications,andcanbedesignedtoexhibittorque/power-speedcharacteristicshavingtheformshowninFig.3.IntheconstanttorqueregionI,themaximumtorquecapabilityisdeterminedbythecurrentratingoftheinverter,whileintheconstantpowerregionII,flux-weakeningorcommutationphaseadvancehastobeemployedduetotheinvertervoltageandcurrentlimits.InregionIII,thetorqueandpowerreduceduetotheincreasinginfluenceoftheback-emf.However,thepowercapabilityandthemaximumspeedcanbeenhancedwithoutsacrificingthelowspeedtorquecapabilitybyemployingaDC-DCvoltagebooster[4],atechniquewhichisemployedintheToyotahybridsystem,orbyemployingseries/parallelwindingconnections,i.e.seriesconnectionatlowspeedandparallelconnectionathighspeed,asdemonstratedin[5][6].Ingeneral,however,thedesignconsiderationsandcontrolmethodsforthethreemachinetechnologiesaresignificantlydifferent,aswillbediscussedinthispaper.Electricalmachinedesigncannotbeundertakeninisolation,butmustaccountforthecontrolstrategyandtheapplicationrequirements,bothstaticanddynamic.Hence,asystemleveldesignapproachisessential.Thispaperdescribesthebasicoperationalcharacteristicsanddesignfeaturesassociatedwiththeforegoingelectricalmachinetechnologiesfortractionapplications,andreportsthelatestdevelopments,withparticularreferencetoPMbrushlessmachines,forwhichtherearevarioustopologies.Fig.1.Torque/powerrequirementsfortractionmachines.(a)(b)(c)Fig.2.Maintractionmachinetechnologies.(a)IM–Inductionmachine,(b)SRM–Switchedreluctancemachine,(c)PMM–Permanentmagnetbrushlessmachine.2Fig.3.Idealisedtorque/power-speedcharacteristics.2.INDUCTIONMACHINESOfthethreeelectricalmachinetechnologiesunderconsideration,inductionmachinesarethemostmature.Inthissection,thebasiccharacteristicsofIMsarebrieflyreviewedandspecificdesignfeaturesfortractionapplicationsarehighlighted.Optimalflux,maximumefficiencyandlowacousticnoiseoperationarethendiscussed.2.1BasicCharacteristicsIMsarerobust,relativelylowcost,andhavewellestablishedmanufacturingtechniques.Gooddynamictorquecontrolperformancecanbeachievedbyeithervectorcontrolordirecttorquecontrol.ForconventionalIMs,theconstantpowerrangetypicallyextendsto2-3timesthebase-speed.However,fortractionmachines,thiscanbeextendedto4-5timesthebase-speed,whichisgenerallydesirable[7].Thetorque-speedcharacteristicofanIMismainlycharacterizedbythestartingtorque,thepull-outtorqueandtheassociatedspeed,andthemaximumspeed.Theelectromagnetictorqueisgivenby:⎥⎥⎦⎤⎢⎢⎣⎡+⎟⎟⎠⎞⎜⎜⎝⎛+=22''22krssrsXsRRfsRmpVTπ(1)andthemaximumtorque,i.e.pull-outtorque,is:kskssLXfVT22maxψ∝∝(2)whilethestartingtorqueandcorrespondingphasecurrentaregivenby:()⎥⎦⎤⎢⎣⎡++=22''22krssrsstXRRfRmpVTπ(3)()22'krssstXRRVI++=(4)whereVsandfsarethesupplyvoltageandfrequency,ψsisthestatorflux-linkage,misthenumberofphases,pisthenumberofpole-pairs,sistheslip,Rsan