外文翻译异步电动机起动的方法和问题.docx

外文翻译异步电动机起动的方法和问题.docx

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外文翻译异步电动机起动的方法和问题

INDUCTIONMOTORSTARTINGMETHODSANDISSUES

Abstract-ManymethodscanbeusedtostartlargeACinductionmotors.Choicessuchasfullvoltage,reducedvoltageeitherbyautotransformerorWyes-Delta,asoftstarter,orusageofanadjustablespeeddrivecanallhavepotentialadvantagesandtradeoffs.Reducedvoltagestartingcanlowerthestartingtorqueandhelppreventdamagetotheload.Additionally,powerfactorcorrectioncapacitorscanbeusedtoreducethecurrent,butcaremustbetakentosizethemproperly.Usageofthewrongcapacitorscanleadtosignificantdamage.Choosingtheproperstartingmethodforamotorwillincludeananalysisofthepowersystemaswellasthestartingloadtoensurethatthemotorisdesignedtodelivertheneededperformancewhileminimizingitscost.Thispaperwillexaminethemostcommonstartingmethodsandtheirrecommendedapplications.

IndexTerms:

motorstarting.Reducedvoltagestartautotransformer,wyes-delta,powerfactorcorrection

I.INTRODUCTION

Thereareseveralgeneralmethodsofstartinginductionmotors:

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.FULLVOLTAGE

Thefullvoltagestartingmethod,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.CapacitorsandStarting

Inductionmotorstypicallyhaveverylowpowerfactorduringstartingandasaresulthaveverylargereactivepowerdraw.SeeFig.2.Thiseffectonthesystemcanbereducedbyaddingcapacitorstothemotorduringstarting.

Thelargereactivecurrentsrequiredbythemotorlagtheappliedvoltageby90electricaldegrees.Thisreactivepowerdoesn'tcreateanymeasurableoutput,butisrathertheenergyrequiredforthemotortofunction.TheproductoftheappliedsystemvoltageandthisreactivepowercomponentcanbemeasuredinVARS（volt-amperereactive）.Thecapacitorsacttosupplyacurrentthatleadstheappliedvoltageby90electricaldegrees.Theleadingcurrentssuppliedbythecapacitorscancelthelaggingcurrentdemandedbythemotor,reducingtheamountofreactivepowerrequiredtobedrawnfromthepowersystem.

Toavoidovervoltageandmotordamage,greatcareshouldbeusedtomakesurethatthecapacitorsareremovedasthemotorreachesratedspeed,orintheeventofalossofpowersothatthemotorwillnotgointoageneratormodewiththemagnetizingcurrentsprovidedfromthecapacitors.Thiswillbeexpandedoninthenextsectionandintheappendix.

B.PowerFactorCorrection

Capacitorscanalsobeleftpermanentlyconnectedtoraisethefullloadpowerfactor.Whenusedinthismannertheyarecalledpowerfactorcorrectioncapacitors.Thecapacitorsshouldneverbesizedlargerthanthemagnetizingcurrentofthemotorunlesstheycanbedisconnectedfromthemotorintheeventofapowerloss.

Theadditionofcapacitorswillchangetheeffectiveopencircuittimeconstantofthemotor.Thetimeconstantindicatesthetimerequiredforremainingvoltageinthemotortodecayto36.8%ofratedvoltageafterthelossofpower.Thisistypicallyonetothreesecondswithoutcapacitors.

Withcapacitorsconnectedtotheleadsofthemotor,thecapacitorscancontinuetosupplymagnetizingcurrentafterthepowertothemotorhasbeendisconnected.Thisisindicatedbyalongertimeconstantforthesystem.Ifthemotorisdrivingahighinertiaload,themotorcanchangeovertogeneratoractionwiththemagnetizing

Currentfromthecapacitorsandtheshaftdrivenbytheload.Thiscanresultinthevoltageatthemotorterminalsactuallyrisingtonearly50%ofratedvoltageinsomecases.Ifthepowerisreconnectedbeforethisvoltagedecaysseveretransientscanbecreatedwhichcancausesignificantswitchingcurrentsandtorquesthatcanseverelydamagethemotorandthedrivenequipment.Anexampleofthisphenomenonisoutlinedintheappendix.

Ill.REDUCEDVOLTAGE

Eachofthereducedvoltagemethodsareintendedtoreducetheimpactofmotorstartingcurrentonthepowersystembycontrollingthevoltagethatthemotorseesattheterminals.Itisveryimportanttoknowthecharacteristicsoftheloadtobestartedwhenconsideringanyformofreducedvoltagestarting.Themotormanufacturerwillneedtohavethespeedtorquecurveandtheinertiaofthedrivenequipmentwhentheyvalidatetheirdesign.Thecurvecanbebuiltfromaninitial,orbreakawaytorque,asfewasfourotherdatapointsthroughthespeedrange,andthefullspeedtorqueforthestartingcondition.Acentrifugalorsquarecurvecanbeassumedinmanycases,buttherearesomeapplicationswherethiswouldbeproblematic.Anexamplewouldbescrewcompressorswhichhaveamuchhighertorquerequirementatlowerspeedsthanthemorecommoncentrifugalorfanload.SeeFig.3.Byunderstandingthedetailsoftheloadtobestartedthemanufacturercanmakesurethatthemotorwillbeabletogeneratesufficienttorquetostarttheload,withthestartingmethodthatischosen.

A.Autotransformer

Themotorleadsareconnectedtothelowervoltagesideofthetransformer.Themostcommontapsthatareusedare80%,65%,and50%.At50%voltagethecurrentontheprimaryis25%ofthefullvoltagelockedrotoramps.Themotorisstartedwiththisreducedvoltage,andthenafterapre-setconditionisreachedtheconnectionisswitchedtolinevoltage.Thisconditioncouldbeapresettime,currentlevel,busvolts,ormotorspeed.Thechangeovercanbedoneineitheraclosedcircuittransition,oranopencircuittransitionmethod.Intheopencircuitmethodtheconnectiontothevoltageisseveredasitischangedfromthereducedvoltagetothelinelevel.Careshouldbeusedtomakesurethattherewillnotbeproblemsfromtransientsduetotheswitching.Thispotentialproblemcanbeeliminatedbyusingtheclosedcircuittransition.Withtheclosedcircuitmethodthereisacontinuous

Voltageappliedtothemotor.Anotherbenefitwiththeautotransformerstartingisinpossiblelowervibrationandnoiselevelsduringstarting.

Sincethetorquegeneratedbythemotorwillvaryasthesquareoftheappliedvoltage,greatcareshouldbetakentomakesurethattherewillbesufficientacceleratingtorqueavailablefromthemotor.Aspeedtorquecurveforthedrivenequipmentalongwiththeinertiashouldbeusedtoverifythedesignofthemotor.Agoodruleofthumbistohaveaminimumof10%oftheratedfullloadtorqueofthemotorasamarginatallpointsofthecurve.

Additionally,theaccelerationtimeshouldbeevaluatedtomakesurethatthemot