外文翻译--异步电动机起动的方法和问题.doc

INDUCTIONMOTORSTARTINGMETHODSANDISSUESAbstract-ManymethodscanbeusedtostartlargeACinductionmotors.Choicessuchasfullvoltage,reducedvoltageeitherbyautotransformerorWyes-Delta,asoftstarter,orusageofanadjustablespeeddrivecanallhavepotentialadvantagesandtradeoffs.Reducedvoltagestartingcanlowerthestartingtorqueandhelppreventdamagetotheload.Additionally,powerfactorcorrectioncapacitorscanbeusedtoreducethecurrent,butcaremustbetakentosizethemproperly.Usageofthewrongcapacitorscanleadtosignificantdamage.Choosingtheproperstartingmethodforamotorwillincludeananalysisofthepowersystemaswellasthestartingloadtoensurethatthemotorisdesignedtodelivertheneededperformancewhileminimizingitscost.Thispaperwillexaminethemostcommonstartingmethodsandtheirrecommendedapplications.IndexTerms:motorstarting.Reducedvoltagestartautotransformer,wyes-delta,powerfactorcorrectionI.INTRODUCTIONThereareseveralgeneralmethodsofstartinginductionmotors:fullvoltage,reducedvoltage,wyes-delta,andpartwindingtypes.Thereducedvoltagetypecanincludesolidstatestarters,adjustablefrequencydrives,andautotransformers.These,alongwiththefullvoltage,oracrossthelinestarting,givethepurchaseralargevarietyofautomotiveswhenitcomestospecifyingthemotortobeusedinagivenapplication.Eachmethodhasitsownbenefits,aswellasperformancetradeoffs.Properselectionwillinvolveathoroughinvestigationofanypowersystemconstraints,theloadtobeacceleratedandtheoverallcostoftheequipment.Inorderfortheloadtobeaccelerated,themotormustgenerategreatertorquethantheloadrequirement.Ingeneraltherearethreepointsofinterestonthemotorsspeed-torquecurve.Thefirstislocked-rotortorque(LRT)whichistheminimumtorquewhichthemotorwilldevelopatrestforallangularpositionsoftherotor.Thesecondispull-uptorque(PUT)whichisdefinedastheminimumtorquedevelopedbythemotorduringtheperiodofaccelerationfromresttothespeedatwhichbreakdowntorqueoccurs.Thelastisthebreakdowntorque(BDT)whichisdefinedasthemaximumtorquewhichthemotorwilldevelop.Ifanyofthesepointsarebelowtherequiredloadcurve,thenthemotorwillnotstart.Thetimeittakesforthemotortoacceleratetheloadisdependentontheinertiaoftheloadandthemarginbetweenthetorqueofthemotorandtheloadcurve,sometimescalledacceleratingtorque.Ingeneral,thelongerthetimeittakesforthemotortoacceleratetheload,themoreheatthatwillbegeneratedintherotorbars,shortingringandthestatorwinding.Thisheatleadstoadditionalstressesinthesepartsandcanhaveanimpactionmotorlife.II.FULLVOLTAGEThefullvoltagestartingmethod,alsoknownasacrossthelinestarting,istheeasiestmethodtoemploy,hasthelowestequipmentcosts,andisthemostreliable.Thismethodutilizesacontroltocloseacontactorandapplyfulllinevoltagetothemotorterminals.Thismethodwillallowthemotortogenerateitshigheststartingtorqueandprovidetheshortestaccelerationtimes.Thismethodalsoputsthehigheststrainonthepowersystemduetothehighstartingcurrentsthatcanbetypicallysixtoseventimesthenormalfullloadcurrentofthemotor.Ifthemotorisonaweakpowersystem,thesuddenhighpowerdrawcancauseatemporaryvoltagedrop,notonlyatthemotorterminals,buttheentirepowerbusfeedingthestartingmotor.Thisvoltagedropwillcauseadropinthestartingtorqueofthemotor,andadropinthetorqueofanyothermotorrunningonthepowerbus.Thetorquedevelopedbyaninductionmotorvariesroughlyasthesquareoftheappliedvoltage.Therefore,dependingontheamountofvoltagedrop,motorsrunningonthisweakpowerbuscouldstall.Inaddition,manycontrolsystemsmonitorundervoltageconditions,asecondpotentialproblemthatcouldtakearunningmotorofflineduringafullvoltagestart.Besideselectricalvariationofthepowerbus,apotentialphysicaldisadvantageofanacrossthelinestartingisthesuddenloadingseenbythedrivenequipment.Thisshockloadingduetotransienttorqueswhichcanexceed600%ofthelockedrotortorquecanincreasethewearontheequipment,orevencauseacatastrophicfailureiftheloadcannothandlethetorquesgeneratedbythemotorduringstaring.A.CapacitorsandStartingInductionmotorstypicallyhaveverylowpowerfactorduringstartingandasaresulthaveverylargereactivepowerdraw.SeeFig.2.Thiseffectonthesystemcanbereducedbyaddingcapacitorstothemotorduringstarting.Thelargereactivecurrentsrequiredbythemotorlagtheappliedvoltageby90electricaldegrees.Thisreactivepowerdoesntcreateanymeasurableoutput,butisrathertheenergyrequiredforthemotortofunction.TheproductoftheappliedsystemvoltageandthisreactivepowercomponentcanbemeasuredinVARS(volt-amperereactive).Thecapacitorsacttosupplyacurrentthatleadstheappliedvoltageby90electricaldegrees.Theleadingcurrentssuppliedbythecapacitorscancelthelaggingcurrentdemandedbythemotor,reducingtheamountofreactivepowerrequiredtobedrawnfromthepowersystem.Toavoidovervoltageandmotordamage,greatcareshouldbeusedtomakesurethatthecapacitorsareremovedasthemotorreachesratedspeed,orintheeventofalossofpowersothatthemotorwillnotgointoageneratormodewiththemagnetizingcurrentsprovidedfromthecapacitors.Thiswillbeexpandedoninthenextsectionandintheappendix.B.PowerFactorCorrectionCapacitorscanalsobeleftpermanentlyconnectedtoraisethefullloadpowerfactor.Whenusedinthismannertheyarecalledpowerfactorcorrectioncapacitors.Thecapacitorsshouldneverbesizedlargerthanthemagnetizingcurrentofthemotorunlesstheycanbedisconnectedfromthemotorintheeventofapowerloss.Theadditionofcapacitorswillchangetheeffectiveopencircuittimeconstantofthemotor.Thetimeconstantindicatesthetimerequiredforremainingvoltageinthemotortodecayto36.8%ofratedvoltageafterthelossofpower.Thisistypicallyonetothreesecondswithoutcapacitors.Withcapacitorsconnectedtotheleadsofthemotor,thecapacitorscancontinuetosupplymagnetizingcurrentafterthepowertothemotorhasbeendisconnected.Thisisindicatedbyalongertimeconstantforthesystem.Ifthemotorisdrivingahighinertiaload,themotorcanchangeovertogeneratoractionwiththemagnetizingCurrentfromthecapacitorsandtheshaftdrivenbytheload.Thiscanresultinthevoltageatthemotorterminalsactuallyrisingtonearly50%ofratedvoltageinsomecases.Ifthepowerisreconnectedbeforethisvoltagedecaysseveretransientscanbecreatedwhichcancausesignificantswitchingcurrentsandtorquesthatcanseverelydamagethemotorandthedrivenequipment.Anexampleofthisphenomenonisoutlinedintheappendix.Ill.REDUCEDVOLTAGEEachofthereducedvoltagemethodsareintendedtoreducetheimpactofmotorstartingcurrentonthepowersystembycontrollingthevoltagethatthemotorseesattheterminals.Itisveryimportanttoknowthecharacteristicsoftheloadtobestartedwhenconsideringanyformofreducedvoltagestarting.Themotormanufacturerwillneedtohavethespeedtorquecurveandtheinertiaofthedrivenequipmentwhentheyvalidatetheirdesign.Thecurvecanbebuiltfromaninitial,orbreakawaytorque,asfewasfourotherdatapointsthroughthespeedrange,andthefullspeedtorqueforthestartingcondition.Acentrifugalorsquarecurvecanbeassumedinmanycases,buttherearesomeapplicationswherethiswouldbeproblematic.Anexamplewouldbescrewcompressorswhichhaveamuchhighertorquerequirementatlowerspeedsthanthemorecommoncentrifugalorfanload.SeeFig.3.Byunderstandingthedetailsoftheloadtobestartedthemanufacturercanmakesurethatthemotorwillbeabletogeneratesufficienttorquetostarttheload,withthestartingmethodthatischosen.A.AutotransformerThemotorleadsareconnectedtothelowervoltagesideofthetransformer.Themostcommontapsthatareusedare80%,65%,and50%.At50%voltagethecurrentontheprimaryis25%ofthefullvoltagelockedrotoramps.Themotorisstartedwiththisreducedvoltage,andthenafterapre-setconditionisreachedtheconnectionisswitchedtolinevoltage.Thisconditioncouldbeapresettime,currentlevel,busvolts,