Halbach-permanent-magnet-machines-and-applications

Halbachpermanentmagnetmachinesandapplications:areviewZ.Q.ZhuandD.HoweAbstract:PermanentmagnetbrushlessmachinesemployingmultipoleHalbachmagnetisedrotorsarebeingdevelopedforvariousapplications,sincetheyofferanumberofattractivefeatures.AlternativeHalbachmachmetopologiesarereviewed,andtherealisationofHalbachmagnetisedmagnetsfrompre-magnetisedsinteredrareearthmagnetsegments(whchapproximatetheHalbachmagnetisationdistributionandtherebycompromisetheirperformance),andasbondedisotropicandanisotropicNdFeBringmagnets(whicharesubsequentlyimpulsemagnetisedwithaHalbachfielddistribution),isconsidered.Radial-andaxial-field,slottedandslotless,rotaryandlinear(tubularandplanar),andsphericalHalbachmagnetisedbrushlessmachinesaredescribed,andpotentialapplications,includingamotor/generatorforahigh-speedflywheelpeakpowerbuffer,hgh-performancelinearandrotaryservomotors,andpassivemagneticbearingsareconsidered.1IntroductionHalbachmagnetisedpermanentmagnetbrushlessmachinesofferanumberofattractivefeatures[l].Forexample,theairgapfielddistributionwhichresultsfromtheHalbachmagnetisationisinherentlysinusoidal,whchmakesfornegligiblecoggingtorqueandanessentiallysinusoidalemfwaveform,withoutrecoursetoconventionaldesignfeaturessuchasskewingofthestatorirotor,optimisationofthemagnetpole-arc,distributedstatorwindings,etc.Itis,therefore,appropriateforbrushlessACmotors.Further,duetotheself-shieldingmagnetisation,rotorback-ironisnotessential,sothatthemassandinertiacanbereduced,withaconsequentimprovementindynamicperformance.Sinceskewisnotrequiredandanon-overlappingstatorwindingcanbeemployed,thecostofmanufacturemaybelowerthanforconventionalbrushlessmotors,particularlyifbondedisotropicoranisotropicHalbachringmagnets,ratherthansegmentedmagnets,areemployed.Inaddition,themagnetcanhaveasignificantlyhigherworkingpointthanequivalentradialorparallelmagnetised,surface-mountedmagnets,whichimprovestheutilisationofthemagnetmaterialandincreasesthepowerdensity.ThismakesHalbachmagnetisedmagnetsparticularlyappropri-ateforslotlessbrushlessmotors.Thispaperreviewsalter-nativeHalbachmachinetopologies,anddescribespotentialapplications.2Halbachcylinders2.IAlternativemanufacturingmethodsTherearetwoalternativeapproachestorealisingaHalbachcylinder:usingpre-magnetisedmagnetsegmentshavingtherequiredmagnetisationorientations,andby0IEE,2001IEEPromdigsonlineno.20010479DOL10.1049/ipepa:20010479Paperfmtreceived6thNovember2000andinrevisedform27thApril2001TheauthorsarewiththeDepartmentofElectronic&El~aicalEngineering,UniversityofShefield,MappinStreet,SheffieldS13JD,UKmouldingorsinteringacompleteringmagnetandsubse-quentlyimpulsemagnetisinginafixture,whichproducesasinusoidallydistributedmagnetisingfield.Inthispaper,HalbachcylindersrealisedfromanisotropicsinteredNdFeBandSmCosegmentsandisotropicandanisotropicinjectionmouldedNdFeBringsareconsidered,theirfeaturesbeing:(i)SegmentedanisotropicHalbachcylinders[2-151:ApproximationtoHalbachfielddistribution,dependentonthenumberofsegmentsperpole;Highmagneticforcesduringassembly;Highresultantmagneticfield;(ii)IsotropicbondedNdFeBHalbachcylinders[1619]:Injectionorcompressionmouldedasringmagnet,andsubsequentlyimpulsemagnetisedinasinusoidallydistrib-utedmagnetisingfield;Relativelyeasytomanufacture;(iii)AnisotropicbondedNdFeBHalbachcylinders[9,201:ProducedbyorientinganisotropicNdFeBmagnetpow-derduringinjectionorcompressionmoulding,andsubse-quentlyimpulsemagnetisinginaHalbachdistributedmagnetisingfield;RequiredaligningfieldforanisotropicNdFeBpowderis1T,whilerequiredmagnetisingfieldis3T.InanidealHalbachmagnetisedmagnet,themagnetisa-tiondistributioninpolar-co-ordinatesexpressedas:whereandiM=M,.z,.+MQ~QM,.=Mcos(pe)MO=fMsin(p8)pbeingthenumberofpole-pairs,‘+’beingforaninternalfieldHalbachcylinder,and‘-’foranexternalfieldHalbachcylinder(Fig.1).Forexample,adiametricallymagnetisedcylindricalmagnetresultsina2-poleexternalfieldHalbachmagnetisation.299IEEProc.-Electr.PowerAppl.,Vol148,No.4,July2001internalfieldexternalfieldIFig.1twnsinairalntemalfieldbExtemalfield4-pokHalbachmagnetisedcylimkrsandresulting@iddistribu-However,whenaHalbachmagnetisedmagnetisrealisedfromdiscretemagnetsegments,theabovemagnetisationdistributionisapproximated,sincethedirectionofmagnet-isationisfnedineachindividualmagnetsegmentby:em=(1fp)&(4)whereOjistheanglebetween8=0andthecentreoftheIthmagnetsegment(Fig.2).Bywayofexample,Fig.3showsthemagnetisationdirectionstogetherwiththefielddistribu-tionswhichresultfroma12-polesegmented,internalfieldHalbachcylinderasthenumberofmagnetsegmentsper.*Mmagnetisationdirection\\*tMmametisation--directionV0=0Fig.2Angularrelationshipbetweene,,,andpoleisvariedfrom2to41151.Fig.4showsthecorrespond-ingfielddistributionsattheinnersurfaceofthecylinder,r=29m,andataradiusof27mmwithinthebore.Aswillbeseen,whileboththeradialandcircumferentialcompo-nentsoffluxdensityexhibitsignificantharmonicsattheinnersurfaceoftheHalbachcylinder,theharmoniccontentdecaysquicklywithradius,andwithin2mmoftheborethefielddistributionisessentiallysinusoidallydistributed,evenwhenonly3segmentsper