电气工程方面的英汉论文.docx
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电气工程方面的英汉论文.docx
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电气工程方面的英汉论文
ElectricDevicesandSystems
Althoughtransformershavenomovingparts,theyareessentialtoelectromechanicalenergyconversion.Theymakeitpossibletoincreaseordecreasethevoltageleverthatresultsinlowcosts,andcanbedistributedandusedsafely.Inaddition,theycanprovidematchingofimpedances,andregulatetheflowofpowerinanetwork.
Whenweseeatransformeronautilitypoleallweisacylinderwithafewwiresstickingout.Thesewiresenterthetransformerthroughbushingsthatprovideisolationbetweenthewiresandthetank.Insidethetanktheseisanironcorelinkingcoils,mostprobablymadewithcopper,andinsulated.Thesystemofinsulationisalsoassociatedwiththatofcoolingthecore/coilassembly.Oftentheinsulationispaper,andthewholeassemblymaybeimmersedininsulatingoil,usedtobothincreasethedielectricstrengthofthepaperandtotransferbeatfromthecore-coilassemblytotheouterwallsofthetanktoair.Figureshowsthecutoutofatypicaldistributiontransformer.Fewidealversionsofhumanconstructionsexist,andthetransformeroffersnoexception.Anidealtransformerisbasedonverysimpleconcepts,andalargenumberofassumptions.Thisisthetransformeronelearnsaboutinhighschool.
Letustakeanironcorewithinfinitepermeabilityandtwocoilswoundaroundit,onewithN1andtheotherwithN2turns,asshowninfigure.Allthemagneticfluxistoremainintheiron.Weassignsotsatoneterminalofeachcoilinthefollowingfashion:
ifthefluxinthecorechanges,inducingavoltageinthecoils,andthedottedterminalofonecoilispositivewithrespectitsotherterminal,soisthedottedterminaloftheothercoil.Or,thecorollarytothis,currentintodottedterminalsproducesfluxinthesamedirection,
Assumethatsomehowatimevaryingfluxisestablishedintheiron.Thenthefluxlinkagesineachcoilwillbe.Voltageswillbeinducedinthesetwocoil.
Ontheotherhand,currentsflowinginthecoilsarerelatedtothefieldintensityH.ifcurrentsflowinginthedirectionshown,i1intothedottedterminalofcoil1,andi2outofthedottedterminalofcoil2.werecognizethatthisispracticallyimpossible,butsoistheexistenceofanidealtransformer.
Equationsdescribethisidealtransformer,atwoportnetwork.Thesymbolofanetworkthatisdefinedbythesetwoequationsisinthefigure.Anidealtransformerhasaninterestingcharacteristic.Atwo-portnetworkthatcontainsitandimpedancescanbereplacedbyanequivalentother,asdiscussedbelow.Considerthecircuitinfigure.Seenasatwoportnetwork.Generallyacircuitonaside1canbetransferredtoside2bymultiplyingitscomponentimpedances,thevoltagesourcesandthecurrentsources,whilekeepingthetopologythesame.Todeveloptheequivalentforatransformerwe’llgraduallyrelaxtheassumptionsthatwehadfirstimposed.Firstwe’llrelaxtheassumptionthatthepermeabilityoftheironisinfinite.Inthatcaseequationdoesnotrevertto,butratheritbecomeswhereisthereluctanceofthepatharoundthecoreofthetransformerandthefluxonthispath.Topreservetheidealtransformerequationsaspartofournewtransformer,wecanspliti1totwocomponents:
onei1,willsatisfytheidealtransformerequation,andtheother,i1willjustbalancetherighthandside.Thefigureshowsthis.Wecanreplacethecurrentsource,i1,withsomethingsimplerifwerememberthattherateofchangeoffluxisrelatedtotheinducedvoltage.
Sincethecurrenti1flowsthroughsomething,wherethevoltageacrossitIsproportionaltoitsderivative,wecanconsiderthatthissomethingcouldbeaninductance.Thisideagivesrisetotheequivalentcircuitinfigure,.Letusnowrelaxtheassumptionthatallthefluxhastoremainintheironasshowninfigure.Letuscallthefluxintheiron,magnetizingflux,thefluxthatleaksoutofthecoreandlinksonlycoil1.sincelinksonlycoil1,thenitshouldberelatedonlytothecurrentthere,andthesameshouldbetrueforthesecondleakageflux.
Againforagivenfrequency,thepowerlossesinthecoreincreasewiththevoltage.Theselossescannotbeallowedtoexceedlimit,beyondwhichthetemperatureofthehottestspotinthetransformerwillriseabovethepointthatwilldecreasedramaticallythelifeoftheinsulation.LimitsthereforeareputtoE1andE2,andtheselimitsarethevoltagelimitsofthetransformer.Similarly,windingJoulelosseshavetobelimited,resultinginlimitstothecurrentsI1andI2.Typicallyatransformerisdescribedbyitsratedvoltages,thatgiveboththelimitsandturnsradio.Theratiooftheratedcurrentsistheinverseoftheratioofthevoltagesifweneglectthemagnetizingcurrent.Insteadofthetransformerratedcurrents,atransformerisdescribedbyitsratedapparentpower
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