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1、MotorControlPowerSemiconductorApplicationsPhilipsSemiconductorsCHAPTER3MotorControl3.1ACMotorControl3.2DCMotorControl3.3StepperMotorControl241MotorControlPowerSemiconductorApplicationsPhilipsSemiconductorsACMotorControl243MotorControlPowerSemiconductorApplicationsPhilipsS
2、emiconductors3.1.1NoiselessA.C.MotorControl:Introductiontoa20kHzSystemControllingana.c.inductionmotorbythetechniqueofpowerviaalinearamplifiersystemwouldhavelowsinewave-weightedpulse-widthmodulation(PWM)efficiency,atbest64%.Ifinsteadofthelinearcircuitry,fastswitchinggivest
3、hebenefitsofsmoothtorqueatlowspeeds,electronicswitchingdevicesareused,thentheefficiencyandalsocompletespeedcontrolfromzerouptothenominalcanbegreaterthan95%,dependingonthecharacteristicsratedspeedofthemotor,withonlysmalladditionalmotorofthesemiconductorpowerswitch.losses.T
4、raditionalpowerswitchessuchasthyristorsneedswitchingfrequenciesintheaudiblerange,typicallyV/2+between400and1500Hz.Inindustrialenvironments,thesmallamountofacousticnoiseproducedbythemotorwithZthistypeofcontrolcanberegardedasinsignificant.Bycontrast,however,thesameamountofn
5、oiseinadomesticV/2+orofficeapplication,suchasspeedcontrolofaventilationfan,mightprovetobeunacceptable.Now,however,withtheadventofpowerMOSFETs,Fig.1Half-bridgeswitchingcircuitthree-phasePWMinvertersoperatingatultrasonicfrequenciescanbedesigned.Athree-phasemotorusuallymakes
6、evenlessnoisewhenbeingdrivenfromsucha(a)systemthanwhenbeingrundirectlyfromthemainsV/20becausethePWMsynthesisgeneratesapurersinewave-V/2thanisnormallyobtainablefromthemains.(b)Thecarrierfrequencyisgenerallyabout20kHzandsoitisV/2farremovedfromthemodulationfrequency,whichis0
7、typicallylessthan50Hz,makingiteconomictousea-V/2low-passfilterbetweentheinverterandthemotor.ByremovingthecarrierfrequencyanditssidebandsandI(c)harmonics,thewaveformdeliveredviathemotorleadscan0bemadealmostperfectlysinusoidal.RFIradiatedbythemotorleads,orconductedbythewind
8、ing-to-framecapacitanceofthemotor,isthereforealmostentirelyFig.2WaveformsinPWMinvertereliminated
