VVC控制模式

CHAPTER2:FREQUENCYCONVERTERS81ControlcircuitThecontrolcircuit,orcontrolcard,isthefourthmaincompo-nentofthefrequencyconverterandhasfouressentialtasks:•controlofthefrequencyconvertersemi-conductors.•dataexchangebetweenthefrequencyconverterandperiph-erals.•gatheringandreportingfaultmessages.•carryingoutofprotectivefunctionsforthefrequencyconvert-erandmotor.Micro-processorshaveincreasedthespeedofthecontrolcircuit,significantlyincreasingthenumberofapplicationssuitablefordrivesandreducingthenumberofnecessarycalculations.Withmicroprocessorstheprocessorisintegratedintothefre-quencyconverterandisalwaysabletodeterminetheoptimumpulsepatternforeachoperatingstate.Fig.2.29showsaPAM-controlledfrequencyconverterwithintermediatecircuitchopper.Thecontrolcircuitcontrolsthechopper(2)andtheinverter(3).Uf123Fig.2.29Theprincipleofacontrolcircuitusedforachopper-controlledintermediatecircuitControlcircuitforchopperfrequencyPIvoltageregulatorSequencegeneratorControlcircuitforPAMfrequencyconverterThisisdoneinaccordancewiththemomentaryvalueoftheintermediatecircuitvoltage.Theintermediatecircuitvoltagecontrolsacircuitthatfunctionsasanaddresscounterinthedatastorage.Thestoragehastheoutputsequencesforthepulsepatternoftheinverter.Whentheintermediatecircuitvoltageincreases,thecountinggoesfaster,thesequenceiscompletedfasterandtheoutputfrequencyincreases.Withrespecttothechoppercontrol,theintermediatecircuitvoltageisfirstcomparedwiththeratedvalueofthereferencesignal–avoltagesignal.Thisvoltagesignalisexpectedtogiveacorrectoutputvoltageandfrequency.Ifthereferenceandintermediatecircuitsignalsvary,aPI-regulatorinformsacir-cuitthatthecycletimemustbechanged.Thisleadstoanadjustmentoftheintermediatecircuitvoltagetothereferencesignal.PAMisthetraditionaltechnologyforfrequencyinvertercontrol.PWMisthemoremoderntechniqueandthefollowingpagesdetailhowDanfosshasadaptedPWMtoprovideparticularandspecificbenefits.DanfosscontrolprincipleFig.2.30givesthecontrolprocedureforDanfossinverters.ThecontrolalgorithmisusedtocalculatetheinverterPWMswitchingandtakestheformofaVoltageVectorControl(VVC)forvoltage-sourcefrequencyconverters.82CHAPTER2:FREQUENCYCONVERTERSFig.2.30ControlprinciplesusedbyDanfossSoftwareHardware(ASIC)InverterVVCSynchronous60°PWMMotorVVCplusAsynchronous•SFAVM•60°PWMControlalgorithmPWMVVCcontrolstheamplitudeandfrequencyofthevoltagevectorusingloadandslipcompensation.Theangleofthevoltagevec-torisdeterminedinrelationtothepresetmotorfrequency(ref-erence)aswellastheswitchingfrequency.Thisprovides:•fullratedmotorvoltageatratedmotorfrequency(sothereisnoneedforpowerreduction)•speedregulationrange:1:25withoutfeedback•speedaccuracy:±1%ofratedspeedwithoutfeedback•robustagainstloadchangesArecentdevelopmentofVVCisVVCplusunderwhich.Theampli-tudeandangleofthevoltagevector,aswellasthefrequency,isdirectlycontrolled.InadditiontothepropertiesofVVC,VVCplusprovides:•improveddynamicpropertiesinthelowspeedrange(0Hz-10Hz).•improvedmotormagnetisation•speedcontrolrange:1:100withoutfeedback•speedaccuracy:±0.5%oftheratedspeedwithoutfeedback•activeresonancedampening•torquecontrol(openloop)•operationatthecurrentlimitCHAPTER2:FREQUENCYCONVERTERS83VVCcontrolprincipleUnderVVCthecontrolcircuitappliesamathematicalmodel,whichcalculatestheoptimummotormagnetisationatvaryingmotorloadsusingcompensationparameters.Inadditionthesynchronous60°PWMprocedure,whichisinte-gratedintoanASICcircuit,determinestheoptimumswitchingtimesforthesemi-conductors(IGBTs)oftheinverter.Theswitchingtimesaredeterminedwhen:•Thenumericallylargestphaseiskeptatitspositiveornega-tivepotentialfor1/6oftheperiodtime(60°).•Thetwootherphasesarevariedproportionallysothattheresultingoutputvoltage(phase-phase)isagainsinusoidalandreachesthedesiredamplitude(Fig.2.32).84CHAPTER2:FREQUENCYCONVERTERS0,00–0,5UDC0,5UDC36006060°120180240300Fig.2.31Synchronous60°PWM(DanfossVVCcontrol)ofonephaseUDC=intermediatecircuitvoltageCHAPTER2:FREQUENCYCONVERTERS85Unlikesine-controlledPWM,VVCisbasedonadigitalgenera-tionoftherequiredoutputvoltage.Thisensuresthatthefre-quencyconverteroutputreachestheratedvalueofthesupplyvoltage,themotorcurrentbecomessinusoidalandthemotoroperationcorrespondstothoseobtainedindirectmainsconnec-tion.Optimummotormagnetisationisobtainedbecausethefre-quencyconvertertakesthemotorconstants(statorresistanceandinductance)intoaccountwhencalculatingtheoptimumoutputvoltage.Asthefrequencyconvertercontinuestomeasuretheloadcur-rent,itcanregulatetheoutputvoltagetomatchtheload,sothemotorvoltageisadaptedtothemotortypeandfollowsloadcon-ditions.0,000,501,00–0,50–1,00U-VV-WW-U360060120180240300Fig.2.32Withthesynchronous60°PWMprinciplethefulloutputvoltageisobtaineddirectlySwitchingpatternofphaseUPhasevoltage(0-point–halftheintermediatecircuitvoltage)CombinedvoltagetomotorVVCpluscontrolprincipleTheVVCpluscontrolprincipleusesavectormodulationprincipleforconstant,voltage-sourcedPWMinverters.Itisbasedonanimprovedmotormodelwhichmakesforbetterloadandslipcompensation,becauseboththeactiveandthereactivecurrentcomponentsareavailabletothecontrolsystemandcon