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SynchronousMotorExcitationUnderSynchronousMotor

Priortounderstandingthissynchronousmotorexcitation,itshouldberememberedthatanyelectromagneticdevicemustdrawa

magnetizingcurrentfromtheacsourcetoproducetherequiredworkingflux.Thiscurrentlagsbyalmost90tothesupplyvoltage.Inother

words,thefunctionofthismagnetizingcurrentorlaggingVAdrawnbytheelectromagneticdeviceistosetupthefluxinthemagneticcircuitofthedevice.Thesynchronousmotorisdoublyfedelectricalmotori.eitconvertselectricalenergytomechanicalenergyviamagneticcircuit.Henceitcomesunderelectromagneticdevice.Itreceives3phaseacelectricalsupplytoitsarmaturewindingandDCsupplyisprovidedto

rotorwinding.SynchronousmotorexcitationreferstotheDCsupplygiventorotorwhichisusedtoproducetherequiredmagneticflux.

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9/28/2015 SynchronousMotorExcitation|Electrical4u

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Oneofthemajoranduniquecharacteristicsofthismotoristhatitcanbeoperatedatanyelectricalpowerfactorleading,laggingorunityand

thisfeatureisbasedontheexcitationofthesynchronousmotor.

WhenthesynchronousmotorisworkingatconstantappliedvoltageV,theresultantairgapfluxasdemandedbyVremainssubstantially

constant.ThisresultantairgapfluxisestablishedbythecooperationofbothACsupplyofarmaturewindingandDCsupplyofrotorwinding.

CASE1:Whenthefieldcurrentissufficientenoughtoproducetheairgapflux,asdemandedbytheconstantsupplyvoltageV,thenthe

magnetizingcurrentorlaggingreactiveVArequiredfromacsourceiszeroandthemotoroperateatunitypowerfactor.Thefieldcurrent,whichcausesthisunitypowerfactoriscallednormalexcitationor

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DCACGearMotor6to230VAC,.4to24VDCCustom,ValueAdd,BestTotalCost

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normalfieldcurrent.CASE2:Ifthefieldcurrentisnotsufficientenoughtoproducethe

requiredairgapfluxasdemandedbyV,additionalmagnetizingcurrentorlaggingreactiveVAisdrawnfromtheACsource.Thismagnetizingcurrentproducesthedeficientflux(constantfluxfluxsetupbydc

supplyrotorwinding).Henceinthiscasethemotorissaidtooperateunderlaggingpowerfactorandtheissaidtobeunderexcited.

CASE3:Ifthefieldcurrentismorethanthenormalfieldcurrent,motorissaidtobeoverexcited.Thisexcessfieldcurrentproduces

excessflux(fluxsetupbyDCsupplyrotorwindingresultantairgapflux)mustbeneutralizedbythearmaturewinding.HencethearmaturewindingdrawsleadingreactiveVAordemagnetizingcurrentleading

voltagebyalmost90ofromtheACsource.Henceinthiscasethemotoroperateunderleadingpowerfactor.

Thiswholeconceptofexcitationandpowerfactorofsynchronousmotorcanbesummedupinthefollowinggraph.ThisiscalledVcurveof

synchronousmotor.

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ObjectiveQuestionsonSynchronousMotor(MCQs)

Conclusion:Anoverexcitedsynchronousmotoroperateatleadingpowerfactor,underexcitedsynchronousmotoroperateatlaggingpowerfactorandnormalexcitedsynchronousmotor

operateatunitypowerfactor.

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