Brushless-DC--Motors-直流无刷电机控制基本原理

Brushless DC Motors – Part I: Construction and Operating Principles Electricalequipmentoftenhasatleastonemotorusedtorotateordisplaceanobjectfromitsinitialposition.Thereareavarietyofmotortypesavailableinthemarket,includinginductionmotors,servomotors,DCmotors(brushedandbrushless),etc.Dependingupontheapplicationrequirements,aparticularmotorcanbeselected.However,acurrenttrendisthatmostnewdesignsaremovingtowardsBrushlessDCmotors,popularlyknownasBLDCmotors.ThisarticlewillconcentrateonthefollowingaspectsofBLDCmotordesign:ConstructionoftheBLDCmotorOperationoftheBLDCmotorTorqueandEfficiencyrequirementsComparisonwithInductionandBrushedDCmotorsSelectioncriteriaforaBLDCmotorMotorcontrol–Speed,PositionandTorque,tobecoveredinPartIIofthisarticle.ConstructionBLDCmotorshavemanysimilaritiestoACinductionmotorsandbrushedDCmotorsintermsofconstructionandworkingprinciplesrespectively.Likeallothermotors,BLDCmotorsalsohavearotorandastator.StatorSimilartoanInductionACmotor,theBLDCmotorstatorismadeoutoflaminatedsteelstackeduptocarrythewindings.Windingsinastatorcanbearrangedintwopatterns;i.e.astarpattern(Y)ordeltapattern(∆).ThemajordifferencebetweenthetwopatternsisthattheYpatterngiveshightorqueatlowRPMandthe∆patterngiveslowtorqueatlowRPM.Thisisbecauseinthe∆configuration,halfofthevoltageisappliedacrossthewindingthatisnotdriven,thusincreasinglossesand,inturn,efficiencyandtorque.SteellaminationsinthestatorcanbeslottedorslotlessasshowninFigure2.Aslotlesscorehaslowerinductance,thusitcanrunatveryhighspeeds.Becauseoftheabsenceofteethinthelaminationstack,requirementsforthecoggingtorquealsogodown,thusmakingthemanidealfitforlowspeedstoo(whenpermanentmagnetsonrotorandtoothonthestatoralignwitheachotherthen,becauseoftheinteractionbetweenthetwo,anundesirablecoggingtorquedevelopsandcausesripplesinspeed).Themaindisadvantageofaslotlesscoreishighercostbecauseitrequiresmorewindingtocompensateforthelargerairgap.Properselectionofthelaminatedsteelandwindingsfortheconstructionofstatorarecrucialtomotorperformance.Animproperselectionmayleadtomultipleproblemsduringproduction,resultinginmarketdelaysandincreaseddesigncosts.RotorTherotorofatypicalBLDCmotorismadeoutofpermanentmagnets.Dependingupontheapplicationrequirements,thenumberofpolesintherotormayvary.Increasingthenumberofpolesdoesgivebettertorquebutatthecostofreducingthemaximumpossiblespeed.Anotherrotorparameterthatimpactsthemaximumtorqueisthematerialusedfortheconstructionofpermanentmagnet;thehigherthefluxdensityofthematerial,thehigherthetorque.WorkingPrinciplesandOperationTheunderlyingprinciplesfortheworkingofaBLDCmotorarethesameasforabrushedDCmotor;i.e.,internalshaftpositionfeedback.IncaseofabrushedDCmotor,feedbackisimplementedusingamechanicalcommutatorandbrushes.WithainBLDCmotor,itisachievedusingmultiplefeedbacksensors.Themostcommonlyusedsensorsarehallsensorsandopticalencoders.Note:Hallsensorsworkonthehall-effectprinciplethatwhenacurrent-carryingconductorisexposedtothemagneticfield,chargecarriersexperienceaforcebasedonthevoltagedevelopedacrossthetwosidesoftheconductor.Ifthedirectionofthemagneticfieldisreversed,thevoltagedevelopedwillreverseaswell.ForHall-effectsensorsusedinBLDCmotors,wheneverrotormagneticpoles(NorS)passnearthehallsensor,theygenerateaHIGHorLOWlevelsignal,whichcanbeusedtodeterminethepositionoftheshaft.Inacommutationsystem–onethatisbasedonthepositionofthemotoridentifiedusingfeedbacksensors–twoofthethreeelectricalwindingsareenergizedatatimeasshowninfigure4.Infigure4(A),theGREENwindinglabeled“001”isenergizedastheNORTHpoleandtheBLUEwindinglabeledas“010”isenergizedastheSOUTHpole.Becauseofthisexcitation,theSOUTHpoleoftherotoralignswiththeGREENwindingandtheNORTHpolealignswiththeREDwindinglabeled“100”.Inordertomovetherotor,the“RED”and“BLUE”windingsareenergizedinthedirectionshowninfigure4(B).ThiscausestheREDwindingtobecometheNORTHpoleandtheBLUEwindingtobecometheSOUTHpole.Thisshiftingofthemagneticfieldinthestatorproducestorquebecauseofthedevelopmentofrepulsion(Redwinding–NORTH-NORTHalignment)andattractionforces(BLUEwinding–NORTH-SOUTHalignment),whichmovestherotorintheclockwisedirection. Brushless DC Motors‐‐Part II: Control Principles HavingunderstoodtheconstructionandbasicoperatingprincipleofBLDCmotorinthefirstpartofthisarticle,itbecomesimportanttounderstandthemotorcontroloptionsavailableforthereliableoperationandprotectionofmotors.Basedonthefunctionsserved,motorcontrolcanbeclassifiedintofollowingcategories:SpeedcontrolTorquecontrolMotorprotectionImplementationofthesecontrolfunctionsrequiresmonitoringofoneormoremotorparametersandthentakingcorrespondingactiontoachievetherequiredfunctionality.Beforegettingintothedetailsofthesecontrolfunctionimplementations,itisimportanttounderstandtheimplementationoflogicandhardwarerequiredtobuilduptherotationofthemotorortoestablishcommutation.CommutationimplementationAsdiscussedinthepreviouspartofthisarticle,basedonthepositionofthemotor(identifiedusingfeedbacksensors),twoofthethreeelectricalwindingsareenergizedatatime.Tobeabletoenergizethewindings,externalci