正弦脉宽调制(中英文)

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1、SinusoidalPulseWidthModulation(SPWM)正弦脉宽调制(SPWM)Theswitchesinthevoltagesourceinverter(SeeFig.1)canbeturnedonandoffasrequired.Inthesimplestapproach,thetopswitchisturnedonIfturnedonandoffonlyonceineachcycle,asquarewavewaveformresults.However,ifturnedonseveraltimesinacycleanimproved

2、harmonicprofilemaybeachieved.Fig1:SimpleVoltageSourcedInthemoststraightforwardimplementation,generationofthedesiredoutputvoltageisachievedbycomparingthedesiredreferencewaveform(modulatingsignal)withahigh-frequencytriangular‘carrier’waveasdepictedschematicallyinFig.2.Dependingonwh

3、etherthesignalvoltageislargerorsmallerthanthecarrierwaveform,eitherthepositiveornegativedcbusvoltageisappliedattheoutput.Notethatovertheperiodofonetrianglewave,theaveragevoltageappliedtotheloadisproportionaltotheamplitudeofthesignal(assumedconstant)duringthisperiod.Theresultingch

4、oppedsquarewaveformcontainsareplicaofthedesiredwaveforminitslowfrequencycomponents,withthehigherfrequencycomponentsbeingatfrequenciesofaclosetothecarrierfrequency.Noticethattherootmeansquarevalueoftheacvoltagewaveformisstillequaltothedcbusvoltage,andhencethetotalharmonicdistortio

5、nisnotaffectedbythePWMprocess.Theharmoniccomponentsaremerelyshiftedintothehigherfrequencyrangeandareautomaticallyfilteredduetoinductancesintheacsystem.WhenthemodulatingsignalisasinusoidofamplitudeAm,andtheamplitudeofthetriangularcarrierisAc,theratiom=Am/Acisknownasthemodulationin

6、dex.Notethatcontrollingthemodulationindexthereforcontrolstheamplitudeoftheappliedoutputvoltage.Withasufficientlyhighcarrierfrequency(seeFig.3drawnforfc/fm=21andt=L/R=T/3;T=periodoffundamental),thehighfrequencycomponentsdonotpropagatesignificantlyintheacnetwork(orload)duethepresen

7、ceoftheinductiveelements.However,ahighercarrierfrequencydoesresultinalargernumberofswitchingspercycleandhenceinanincreasedpowerloss.Typicallyswitchingfrequenciesinthe2-15kHzrangeareconsideredadequateforpowersystemsapplications.Alsointhree-phasesystemsitisadvisabletousessothatallt

8、hreewaveformsaresymmetric.Fig2:Principal