Seite - 257 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2017,10, 616 It is clear that thephasevoltageofmotorchanges followsasinusoidalwaveform;amplitudesof phasevoltage instar, triangleandindependentmodes increased insequence. Fromthe local curveof phaseA’svoltage, therewerefivephasevoltagevalues,−2Vdc/3,−Vdc/3,0,Vdc/3,and2Vdc/3,when themotorwas instarmode. Therewereonly threephasevoltagevalues,−Vdc, 0, andVdc,whenthe motorwas in trianglemode; ifwe ignore thevoltagedifferencebetweenthe twosides’powersources, therewereninephasevoltagevalues,−4Vdc/3,−Vdc,−2Vdc/3,−Vdc/3, 0,Vdc/3, 2Vdc/3,Vdc, and 4Vdc/3whenthemotorwas in independentmode,whichmadethecurrentcontrolmoresmooth. The phasecurrentof themotor changedaccording to thesinusoidal rule; becauseof thezero-sequence currenteffect, theamplitudeof thephasecurrent in trianglemodewasslightlygreater thanthose in starandindependentmodeswith thesameamplitudeofstator’s current. FromFigure8d,wecansee thatafter switchingfromstarmodeto trianglemode, thewaveformofphasecurrentchangedandwas nolongerastandardsinusoidalwaveformdueto thezero-sequencecurrent,which ledtooccupation of the inverter’sextracapacityandincreasingswitchingfrequencyofdevices.At thesametime, the phasecurrentwasrestricted in thehysteresisbandandthecurrent followedaccordingly. Total switching frequencyof inverterdevices (sumofall IGBTdevices’ switching frequencies) of both theproposedand the contrastingmodel is shown inFigure9a. Inverters’ loss is shown in Figure9b. (a) Total switching frequency of inverter devices (b) Inverters’ loss curves Figure9.Curvesof total switchingfrequencyof inverterdevicesandinverter loss. FromFigure9a,wesee that theswitching frequenciesof inverterdevices in trianglemodeare higher thanthose instarmodebecause thephasevoltagevalues in trianglemodeare lowerandtheir amplitudesweregreater, leadingtoahigherchangingrateofphasecurrentandmorefrequentlyhitting thehysteresisboundary; the total switching frequencyof inverterdevices in independentmode is alsohighdueto the twoinvertersworkingtogether in independentmode.However, thereweremore phasevoltagevalues in independentmodeandthecontrolof thephasecurrentcouldbemoresmooth 257