TI-MotorControlCompendium-2010.ppt

TI-MotorControlCompendium-2010.ppt

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“Absolutelyamazing!

IwishIhadatoollikethiswhenIwaslearningaboutmotors.Fivestars!

”,Navigatingthecomplexitiesofmotorcontrol,-NikolaTeslaInventoroftheACInductionMotor,2010-2011,MotorControlCompendium,ByDaveWilsonTIMCUApplicationManagerforMotorControl,CopyrightTexasInstruments2011,MotorGlossary,Flux,isflux,inWebers,Inthemid1800sMichaelFaradayspeculatedthatmagneticfieldsexistaslinesofforce,whichwetodayrefertoas“magneticflux”,whichismeasuredinunitsof“Webers”.Essentially,themorefluxthereis,thestrongerthemagnetwillbe.Weconventionallythinkoffluxleavingthenorthpole,andre-enteringthesouthpoleofamagnet,asshownbythearrowsinthediagram.Ifwemeasurehowmuchfluxcutsthroughasurfaceareawhichisperpendiculartothefluxpath,thenwehaveameasureofthefluxdensityatthatparticularspotinspace.OneWeberoffluxcuttingthroughonesquaremeterofareaconstitutesafluxdensityofone“TESLA”,namedaftertheSerbianengineer,NikolaTesla,whoisALSOtheinventoroftheACinductionmotor.,MotorGlossary,Right-handRule,current,field,force,In1820,HansChristianOersteddiscoveredthatcurrentflowinginawirecreatesitsownmagneticfield,andwhenthisfieldinteractswithasecondmagneticfield,theresultisaforceactingontheconductor.Thisforceisproportionaltotheamountofcurrentflowinginthewire,thestrengthofthesecondmagneticfield,andthelengthofwirethatisaffectedbythesecondmagneticfield.ThedirectionoftheforcecanbedeterminedbyatechniqueknowastheRight-handRule.Ifyourrighthandisconfiguredasshownbelow,whereyourthumbpointsinthedirectionofpositivecurrentflow,andyourindexfingerpointsinthedirectionofthesecondmagneticfieldsflux（i.e.,flowingfromtheNorthpoletotheSouthpole）,thenyourmiddlefingerwillbepointinginthedirectionoftheforceactingonthewire.,MotorGlossary,Left-handRule,S,N,DirectionofMotion,Awiremovinginamagneticfieldwillgenerateavoltageacrossitwhosemagnitudeisproportionaltohowmanyfluxlinesarecutbythewirepersecond.Thepolarityofthisback-EMFvoltagecanbedeterminedbytheLeft-handRule.Ifyouhandisconfiguredasshownbelowwhereyourindexfingerispointinginthedirectionofthemagneticflux（pointingtowardstheSouthpole）,andyourmiddlefingerispointinginthedirectionofmotion,thenyourthumbwillbepointingtowardsthepositiveendofthewire.,MotorGlossary,Torque,Clockwisetorque=r1F1+r2F2-r3F3,Torquehasunitsofdistancetimesforce,i.e.,foot-poundsornewton-meters.,Thesumofalloftheperpendicularmagneticforcesactingonarigidbody（suchastherotorofamotor）,timestheradiusesatwhichtheseforcesareapplied,resultsintorque.Pancakemotorsachievehightorquebymaximizingtheradiusatwhichtheforcesareappliedtotherotor.Ontheotherhand,longermotorswithsmallerradiusescanachievethesametorquebymaximizingtheforcethatisapplied.Thistrade-offallowsamotorcustomertoselectfromawiderangeofmotorformfactorstomeettheirspaceconstraints,andstillhavegoodtorqueperformance.UsingtheRight-handRule,weseethataforceisexertedonawirewhichiscarryingcurrentinamagneticfield.Thisforceisproportionaltotheamountofcurrentflowinginthewire,thestrengthofthemagneticfield,andthelengthofwirethatisaffectedbythemagneticfield.Inmostmotortopologies,thestrengthofthemagneticfieldandthewirelengthsexposedtothemagneticfieldarefixedasafunctionofthemotordesign.Thecurrenthoweverisadjustable.Sowecancombinetheeffectsofthemagneticfieldstrengthandthewirelengthsintoonetermcalledthetorqueconstant.Thisterm,whenmultipliedbythemotorcurrentresultsinmotortorque.Soweseethatinordertocontrolmotortorque,wemustcontrolmotorcurrent.,MotorGlossary,TorqueComponents,Thetorqueproducedbyamotorcanbedividedintotwodifferentcategories.Thefirstoneiscalledreactiontorqueoralignmenttorque.Thisaccountsforthetorquecreatedbythereactionbetweenthemagnetsontherotorandstator.Allmotorswiththeexceptionoftheswitchedreluctancemotorexhibitreactiontorque.Alesserknowntorquecomponentiscalledreluctancetorque.Thistorqueisgeneratedasaresultofamagneticfieldtryingtominimizethereluctanceofitsfluxpath.Forexample,ifyouplaceakitchenknifeonacountertop,andapproachoneendoftheknifewithamagnet,theknifewillmovetowardthemagnetinordertominimizethepathofthemagneticfield,eventhoughtheknifeitselfisnotmagnetized.Sincetheswitchedreluctancemotordoesnotuseamagnetoranelectromagnetontherotor,reluctancetorqueisitsonlytorquecomponent.Bothofthesetorquecomponentsareafunctionofthealignmentbetweentherotorandstator.ConsiderforexampleFigureA,wheretherot