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外文翻译--负载运行的变压器及直流电机导论.doc

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外文翻译--负载运行的变压器及直流电机导论.doc

TheTransformeronload﹠IntroductiontoDCMachinesTheTransformeronloadIthasbeenshownthataprimaryinputvoltage1Vcanbetransformedtoanydesiredopencircuitsecondaryvoltage2Ebyasuitablechoiceofturnsratio.2Eisavailableforcirculatingaloadcurrentimpedance.Forthemoment,alaggingpowerfactorwillbeconsidered.Thesecondarycurrentandtheresultingampereturns22NIwillchangetheflux,tendingtodemagnetizethecore,reducemandwithit1E.Becausetheprimaryleakageimpedancedropissolow,asmallalterationto1Ewillcauseanappreciableincreaseofprimarycurrentfrom0Itoanewvalueof1IequaltoijXREV111/.Theextraprimarycurrentandampereturnsnearlycancelthewholeofthesecondaryampereturns.Thisbeingso,themutualfluxsuffersonlyaslightmodificationandrequirespracticallythesamenetampereturns10NIasonnoload.Thetotalprimaryampereturnsareincreasedbyanamount22NInecessarytoneutralizethesameamountofsecondaryampereturns.Inthevectorequation,102211NININIalternatively,221011NININI.Atfullload,thecurrent0Iisonlyabout5ofthefullloadcurrentandso1Iisnearlyequalto122/NNI.Becauseinmindthat2121/NNEE,theinputkVAwhichisapproximately11IEisalsoapproximatelyequaltotheoutputkVA,22IE.Thephysicalcurrenthasincreased,andwithintheprimaryleakagefluxtowhichitisproportional.Thetotalfluxlinkingtheprimary,111mp,isshownunchangedbecausethetotalbacke.m.f.,(dtdNE/111)isstillequalandoppositeto1V.However,therehasbeenaredistributionoffluxandthemutualcomponenthasfallenduetotheincreaseof1with1I.Althoughthechangeissmall,thesecondarydemandcouldnotbemetwithoutamutualfluxande.m.f.alterationtopermitprimarycurrenttochange.Thenetfluxslinkingthesecondarywindinghasbeenfurtherreducedbytheestablishmentofsecondaryleakagefluxdueto2I,andthisopposesm.Althoughmand2areindicatedseparately,theycombinetooneresultantinthecorewhichwillbedownwardsattheinstantshown.ThusthesecondaryterminalvoltageisreducedtodtdNVS/22whichcanbeconsideredintwocomponents,i.e.dtdNdtdNVm//2222orvectorially2222IjXEV.Asfortheprimary,2isresponsibleforasubstantiallyconstantsecondaryleakageinductance222222/NiN.Itwillbenoticedthattheprimaryleakagefluxisresponsibleforpartofthechangeinthesecondaryterminalvoltageduetoitseffectsonthemutualflux.Thetwoleakagefluxesarecloselyrelated2,forexample,byitsdemagnetizingactiononmhascausedthechangesontheprimarysidewhichledtotheestablishmentofprimaryleakageflux.Ifalowenoughleadingpowerfactorisconsidered,thetotalsecondaryfluxandthemutualfluxareincreasedcausingthesecondaryterminalvoltagetorisewithload.pisunchangedinmagnitudefromthenoloadconditionsince,neglectingresistance,itstillhastoprovideatotalbacke.m.f.equalto1V.Itisvirtuallythesameas11,thoughnowproducedbythecombinedeffectofprimaryandsecondaryampereturns.Themutualfluxmuststillchangewithloadtogiveachangeof1Eandpermitmoreprimarycurrenttoflow.1Ehasincreasedthistimebutduetothevectorcombinationwith1Vthereisstillanincreaseofprimarycurrent.Twomorepointsshouldbemadeaboutthefigures.Firstly,aunityturnsratiohasbeenassumedforconveniencesothat21EE.Secondly,thephysicalpictureisdrawnforadifferentinstantoftimefromthevectordiagramswhichshow0m,ifthehorizontalaxisistakenasusual,tobethezerotimereference.Thereareinstantsinthecyclewhenprimaryleakagefluxiszero,whenthesecondaryleakagefluxiszero,andwhenprimaryandsecondaryleakagefluxiszero,andwhenprimaryandsecondaryleakagefluxesareinthesamesense.Theequivalentcircuitalreadyderivedforthetransformerwiththesecondaryterminalsopen,caneasilybeextendedtocovertheloadedsecondarybytheadditionofthesecondaryresistanceandleakagereactance.Practicallyalltransformershaveaturnsratiodifferentfromunityalthoughsuchanarrangementissometimesemployedforthepurposesofelectricallyisolatingonecircuitfromanotheroperatingatthesamevoltage.Toexplainthecasewhere21NNthereactionofthesecondarywillbeviewedfromtheprimarywinding.Thereactionisexperiencedonlyintermsofthemagnetizingforceduetothesecondaryampereturns.Thereisnowayofdetectingfromtheprimarysidewhether2Iislargeand2Nsmallorviceversa,itistheproductofcurrentandturnswhichcausesthereaction.Consequently,asecondarywindingcanbereplacedbyanynumberofdifferentequivalentwindingsandloadcircuitswhichwillgiverisetoanidenticalreactionontheprimary.Itisclearlyconvenienttochangethesecondarywindingtoanequivalentwindinghavingthesamenumberofturns1Nastheprimary.With2Nchangesto1N,sincethee.m.f.sareproportionaltoturns,2212/ENNEwhichisthesameas1E.Forcurrent,sincethereactionampereturnsmustbeunchanged1222NINImustbeequalto22NI.i.e.2122/INNI.Forimpedance,sinceanysecondaryvoltageVbecomesVNN/21,andsecondarycurrentIbecomesINN/12,thenanysecondaryimpedance,includingloadimpedance,mustbecomeIVNNIV///221.Consequently,22212/RNNRand22212/XNNX.Iftheprimaryturnsaretakenasreferenceturns,theprocessiscalledreferringtotheprimaryside.Thereareafewcheckswhichcanbemadetoseeiftheprocedureoutlinedisvalid.Forexample,thecopperlossinthereferredsecondarywindingmustbethesameasintheoriginalsecondaryotherwisetheprimarywouldhavetosupplyadifferentlosspower.222RImustbeequalto222RI.)222122122//NNRNNIdoesinfactreduceto222RI.Similarlythestoredmagneticenergyintheleakagefield2/12LIwhichisproportionalto22XIwillbefoundtocheckas22XI.Thereferredsecondary2212221222//IENNINNEIEkVA.Theargumentissound,thoughatfirstitmayhaveseemedsuspect.Infact,iftheactualsecondarywindingwasremovedphysicallyfromthecoreandreplacedbytheequivalentwindingandloadcircuitdesignedtogivetheparameters1N,2R,2Xand2I,measurementsfromtheprimaryterminalswouldbeunabletodetectanydifferenceinsecondaryampereturns,kVAdemandorcopperloss,undernormalpowerfrequencyoperation.Thereisnopointinchoosinganybasisotherthanequalturnsonprimaryandreferredsecondary,butitissometimesconvenienttorefertheprimarytothesecondarywinding.Inthiscase,ifallthesubscript1sareinterchangedforthesubscript2s,thenecessaryreferringconstantsareeasilyfounde.g.21RR,21XXsimilarly12RRand12XX.Theequivalentcircuitforthegeneralcasewhere21NNexceptthatmrhasbeenaddedtoallowforironlossandanideallosslesstransformationhasbeenincludedbeforethesecondaryterminalstoreturn2Vto2V.Allcalculationsofinternalvoltageandpowerlossesaremadebeforethisidealtransformationisapplied.Thebehaviourofatransformerasdetectedatbothsetsofterminalsisthesameasthebehaviourdetectedatthecorrespondingterminalsofthiscircuitwhentheappropriateparametersareinserted.Theslightlydifferentrepresentationshowingthecoils1Nand2Nsidebysidewithacoreinbetweenisonlyusedforconvenience.Onthetransformeritself,thecoilsare,ofcourse,woundroundthesamecore.Verylittleerrorisintroducedifthemagnetisingbranchistransferredtotheprimaryterminals,butafewanomalieswillarise.Forexample,thecurrentshownflowingthroughtheprimaryimpedanceisnolongerthewholeoftheprimarycurrent.Theerrorisquitesmallsince0Iisusuallysuchasmallfractionof1I.Slightlydifferentanswersmaybeobtainedtoaparticularproblemdependingonwhetherornotallowanceismadeforthiserror.Withthissimplifiedcircuit,theprimaryandreferredsecondaryimpedancescanbeaddedtogive221211/ReNNRRand221211/NNXXXeItshouldbepointedoutthattheequivalentcircuitasderivedhereisonlyvalidfornormaloperationatpowerfrequenciescapacitanceeffectsmustbetakenintoaccountwhenevertherateofchangeofvoltagewouldgiverisetoappreciablecapacitancecurrents,dtCdVIc/.Theyareimportantathighvoltagesandatfrequenciesmuchbeyond100cycles/sec.Afurtherpointisnotthe

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