英文文献

INDUCTIONMOTORSTARTINGMETHODSAbstractManymethodscanbeusedtostartlargeACinductionmotors.Choicessuchasfullvoltage,reducedvoltageeitherbyautotransformerorWyes-Delta,asoftstarter,orusageofanadjustablespeeddrivecanallhavepotentialadvantagesandtradeoffs.Reducedvoltagestartingcanlowerthestartingtorqueandhelppreventdamagetotheload.Additionally,powerfactorcorrectioncapacitorscanbeusedtoreducethecurrent,butcaremustbetakentosizethemproperly.Usageofthewrongcapacitorscanleadtosignificantdamage.Choosingtheproperstartingmethodforamotorwillincludeananalysisofthepowersystemaswellasthestartingloadtoensurethatthemotorisdesignedtodelivertheneededperformancewhileminimizingitscost.Thispaperwillexaminethemostcommonstartingmethodsandtheirrecommendedapplications.I.INTRODUCTIONThereareseveralgeneralmethodsofstartinginductionmotors:fullvoltage,reducedvoltage,wyes-delta,andpartwindingtypes.Thereducedvoltagetypecanincludesolidstatestarters,adjustablefrequencydrives,andautotransformers.These,alongwiththefullvoltage,oracrossthelinestarting,givethepurchaseralargevarietyofautomotiveswhenitcomestospecifyingthemotortobeusedinagivenapplication.Eachmethodhasitsownbenefits,aswellasperformancetradeoffs.Properselectionwillinvolveathoroughinvestigationofanypowersystemconstraints,theloadtobeacceleratedandtheoverallcostoftheequipment.Inorderfortheloadtobeaccelerated,themotormustgenerategreatertorquethantheloadrequirement.Ingeneraltherearethreepointsofinterestonthemotor'sspeed-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.Thisreactivepowerdoesn'tcreateanymeasurableoutput,butisrathertheenergyrequiredforthemotortofunction.TheproductoftheappliedsystemvoltageandthisreactivepowercomponentcanbemeasuredinVARS(volt-amperereactive).Thecapacitorsacttosupplyacurrentthatleadstheappliedvoltageby90electricaldegrees.Theleadingcurrentssuppliedbythecapacitorscancelthelaggingcurrentdemandedbythemotor,reducingtheamountofreactivepowerrequiredtobedrawnfromthepowersystem.Toavoidovervoltageandmotordamage,greatcareshouldbeusedtomakesurethatthecapacitorsareremovedasthemotorreachesratedspeed,orintheeventofalossofpowersothatthemotorwillnotgointoageneratormodewiththemagnetizingcurrentsprovidedfromthecapacitors.Thiswillbeexpandedoninthenextsectionandintheappendix.B.PowerFactorCorrectionCapacitorscanalsobeleftpermanentlyconnectedtoraisethefullloadpowerfactor.Whenusedinthismannertheyarecalledpowerfactorcorrectioncapacitors.Thecapacitorsshouldneverbesizedlargerthanthemagnetizingcurrentofthemotorunlesstheycanbedisconnectedfromthemotorintheeventofapowerlo