1、INDUCTIONMOTORSTARTINGMETHODSANDISSUESAbstract-ManymethodscanbeusedtostartlargeACinductionmotors/Choicessuchasfullvoltage/reducedvoltageeitherbyautotransformerorWyes-Delta/asoftstarter/orusageofanadjustablespeeddrivecanallhavepotentialadvantagesandtradeoffs/Reducedvoltagestartingcanlowerthestartingt
2、orqueandhelppreventdamagetotheload/Additionally/powerfactorcorrectioncapacitorscanbeusedtoreducethecurrent/butcaremustbetakentosizethemproperly/Usageofthewrongcapacitorscanleadtosignificantdamage/Choosingtheproperstartingmethodforamotorwillincludeananalysisofthepowersystemaswellasthestartingloadtoen
3、surethatthemotorisdesignedtodelivertheneededperformancewhileminimizingitscost/Thispaperwillexaminethemostcommonstartingmethodsandtheirrecommendedapplications/IndexTerms/motorstarting/Reducedvoltagestartautotransformer/wyes-delta/powerfactorcorrectionI/INTRODUCTIONThereareseveralgeneralmethodsofstart
4、inginductionmotors/fullvoltage/reducedvoltage/wyes-delta/andpartwindingtypes/Thereducedvoltagetypecanincludesolidstatestarters/adjustablefrequencydrives/andautotransformers/These/alongwiththefullvoltage/oracrossthelinestarting/givethepurchaseralargevarietyofautomotiveswhenitcomestospecifyingthemotor
5、tobeusedinagivenapplication/Eachmethodhasitsownbenefits/aswellasperformancetradeoffs/Properselectionwillinvolveathoroughinvestigationofanypowersystemconstraints/theloadtobeacceleratedandtheoverallcostoftheequipment/Inorderfortheloadtobeaccelerated/themotormustgenerategreatertorquethantheloadrequirem
6、ent/Ingeneraltherearethreepointsofinterestonthemotorsspeed-torquecurve/Thefirstislocked-rotortorque(LRT)whichistheminimumtorquewhichthemotorwilldevelopatrestforallangularpositionsoftherotor/Thesecondispull-uptorque(PUT)whichisdefinedastheminimumtorquedevelopedbythemotorduringtheperiodofaccelerationf
7、romresttothespeedatwhichbreakdowntorqueoccurs/Thelastisthebreakdowntorque(BDT)whichisdefinedasthemaximumtorquewhichthemotorwilldevelop/Ifanyofthesepointsarebelowtherequiredloadcurve/thenthemotorwillnotstart/Thetimeittakesforthemotortoacceleratetheloadisdependentontheinertiaoftheloadandthemarginbetwe
8、enthetorqueofthemotorandtheloadcurve/sometimescalledacceleratingtorque/Ingeneral/thelongerthetimeittakesforthemotortoacceleratetheload/themoreheatthatwillbegeneratedintherotorbars/shortingringandthestatorwinding/Thisheatleadstoadditionalstressesinthesepartsandcanhaveanimpactionmotorlife/II/FULLVOLTA
9、GEThefullvoltagestartingmethod/alsoknownasacrossthelinestarting/istheeasiestmethodtoemploy/hasthelowestequipmentcosts/andisthemostreliable/Thismethodutilizesacontroltocloseacontactorandapplyfulllinevoltagetothemotorterminals/Thismethodwillallowthemotortogenerateitshigheststartingtorqueandprovidethes
10、hortestaccelerationtimes/Thismethodalsoputsthehigheststrainonthepowersystemduetothehighstartingcurrentsthatcanbetypicallysixtoseventimesthenormalfullloadcurrentofthemotor/Ifthemotorisonaweakpowersystem/thesuddenhighpowerdrawcancauseatemporaryvoltagedrop/notonlyatthemotorterminals/buttheentirepowerbu
11、sfeedingthestartingmotor/Thisvoltagedropwillcauseadropinthestartingtorqueofthemotor/andadropinthetorqueofanyothermotorrunningonthepowerbus/Thetorquedevelopedbyaninductionmotorvariesroughlyasthesquareoftheappliedvoltage/Therefore/dependingontheamountofvoltagedrop/motorsrunningonthisweakpowerbuscoulds
12、tall/Inaddition/manycontrolsystemsmonitorundervoltageconditions/asecondpotentialproblemthatcouldtakearunningmotorofflineduringafullvoltagestart/Besideselectricalvariationofthepowerbus/apotentialphysicaldisadvantageofanacrossthelinestartingisthesuddenloadingseenbythedrivenequipment/Thisshockloadingdu
13、etotransienttorqueswhichcanexceed600%ofthelockedrotortorquecanincreasethewearontheequipment/orevencauseacatastrophicfailureiftheloadcannothandlethetorquesgeneratedbythemotorduringstaring/A/CapacitorsandStartingInductionmotorstypicallyhaveverylowpowerfactorduringstartingandasaresulthaveverylargereact
14、ivepowerdraw/SeeFig/2/Thiseffectonthesystemcanbereducedbyaddingcapacitorstothemotorduringstarting/Thelargereactivecurrentsrequiredbythemotorlagtheappliedvoltageby90electricaldegrees/Thisreactivepowerdoesntcreateanymeasurableoutput/butisrathertheenergyrequiredforthemotortofunction/Theproductoftheappl
15、iedsystemvoltageandthisreactivepowercomponentcanbemeasuredinVARS(volt-amperereactive)/Thecapacitorsacttosupplyacurrentthatleadstheappliedvoltageby90electricaldegrees/Theleadingcurrentssuppliedbythecapacitorscancelthelaggingcurrentdemandedbythemotor/reducingtheamountofreactivepowerrequiredtobedrawnfr
16、omthepowersystem/Toavoidovervoltageandmotordamage/greatcareshouldbeusedtomakesurethatthecapacitorsareremovedasthemotorreachesratedspeed/orintheeventofalossofpowersothatthemotorwillnotgointoageneratormodewiththemagnetizingcurrentsprovidedfromthecapacitors/Thiswillbeexpandedoninthenextsectionandinthea
17、ppendix/B/PowerFactorCorrectionCapacitorscanalsobeleftpermanentlyconnectedtoraisethefullloadpowerfactor/Whenusedinthismannertheyarecalledpowerfactorcorrectioncapacitors/Thecapacitorsshouldneverbesizedlargerthanthemagnetizingcurrentofthemotorunlesstheycanbedisconnectedfromthemotorintheeventofapowerlo
18、ss/Theadditionofcapacitorswillchangetheeffectiveopencircuittimeconstantofthemotor/Thetimeconstantindicatesthetimerequiredforremainingvoltageinthemotortodecayto36/8%ofratedvoltageafterthelossofpower/Thisistypicallyonetothreesecondswithoutcapacitors/Withcapacitorsconnectedtotheleadsofthemotor/thecapac
19、itorscancontinuetosupplymagnetizingcurrentafterthepowertothemotorhasbeendisconnected/Thisisindicatedbyalongertimeconstantforthesystem/Ifthemotorisdrivingahighinertiaload/themotorcanchangeovertogeneratoractionwiththemagnetizingCurrentfromthecapacitorsandtheshaftdrivenbytheload/Thiscanresultinthevolta
20、geatthemotorterminalsactuallyrisingtonearly50%ofratedvoltageinsomecases/Ifthepowerisreconnectedbeforethisvoltagedecaysseveretransientscanbecreatedwhichcancausesignificantswitchingcurrentsandtorquesthatcanseverelydamagethemotorandthedrivenequipment/Anexampleofthisphenomenonisoutlinedintheappendix/Ill
21、/REDUCEDVOLTAGEEachofthereducedvoltagemethodsareintendedtoreducetheimpactofmotorstartingcurrentonthepowersystembycontrollingthevoltagethatthemotorseesattheterminals/Itisveryimportanttoknowthecharacteristicsoftheloadtobestartedwhenconsideringanyformofreducedvoltagestarting/Themotormanufacturerwillnee
22、dtohavethespeedtorquecurveandtheinertiaofthedrivenequipmentwhentheyvalidatetheirdesign/Thecurvecanbebuiltfromaninitial/orbreakawaytorque/asfewasfourotherdatapointsthroughthespeedrange/andthefullspeedtorqueforthestartingcondition/Acentrifugalorsquarecurvecanbeassumedinmanycases/buttherearesomeapplica
23、tionswherethiswouldbeproblematic/Anexamplewouldbescrewcompressorswhichhaveamuchhighertorquerequirementatlowerspeedsthanthemorecommoncentrifugalorfanload/SeeFig/3/Byunderstandingthedetailsoftheloadtobestartedthemanufacturercanmakesurethatthemotorwillbeabletogeneratesufficienttorquetostarttheload/with
24、thestartingmethodthatischosen/A/AutotransformerThemotorleadsareconnectedtothelowervoltagesideofthetransformer/Themostcommontapsthatareusedare80%/65%/and50%/At50%voltagethecurrentontheprimaryis25%ofthefullvoltagelockedrotoramps/Themotorisstartedwiththisreducedvoltage/andthenafterapre-setconditionisre
25、achedtheconnectionisswitchedtolinevoltage/Thisconditioncouldbeapresettime/currentlevel/busvolts/ormotorspeed/Thechangeovercanbedoneineitheraclosedcircuittransition/oranopencircuittransitionmethod/Intheopencircuitmethodtheconnectiontothevoltageisseveredasitischangedfromthereducedvoltagetothelinelevel
26、/Careshouldbeusedtomakesurethattherewillnotbeproblemsfromtransientsduetotheswitching/Thispotentialproblemcanbeeliminatedbyusingtheclosedcircuittransition/WiththeclosedcircuitmethodthereisacontinuousVoltageappliedtothemotor/Anotherbenefitwiththeautotransformerstartingisinpossiblelowervibrationandnois
27、elevelsduringstarting/Sincethetorquegeneratedbythemotorwillvaryasthesquareoftheappliedvoltage/greatcareshouldbetakentomakesurethattherewillbesufficientacceleratingtorqueavailablefromthemotor/Aspeedtorquecurveforthedrivenequipmentalongwiththeinertiashouldbeusedtoverifythedesignofthemotor/Agoodruleofthumbis
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