外文翻译变压器Word下载.docx

外文翻译变压器Word下载.docx

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TRANSFORMER

Transformerscomeinmanysizes.Somepowertransformersareasbigasahouse.Electronictransformers,ontheotherhand,canbeassmallasacubeofsugar.Alltransformershaveatleastonecoil.Mosthavetwoalthoughtheymayhavemanymore.

Theusualpurposeoftransformersistochangethelevelofvoltage.Butsometimestheyareusedtoisolatealoadfromthepowersource.

TYPESOFTRANSFORMERS

Standardpowertransformershavetwocoils.ThesecoilsarelabeledPRIMARYandSECONDARY.Theprimarycoilistheoneconnectedtothesource.Thesecondaryistheoneconnectedtotheload.Thereistonoelectricalconnectionbetweentheprimaryandsecondary.Thesecondarygetsitsvoltagebyinduction.

TheonlyplacewhereyouwillseeaSTEP-UPtransformerisatthegeneratingstation.Typically,electricityisgeneratedat13,800volts.Itissteppeddowntodistributionlevels,around15,000volts.Largesubstationtransformershavecoolingfinstokeepthemfromoverheating.Othertransformersarelocatednearpointswheretheelectricpowerisused.

TRANSFORMERCONSTRUCTION

Thecoilsofatransformerareelectricallyinsulatedfromeachother.Thereisamagneticlink,however.Thetwocoilsarewoundonthesamecore.Currentintheprimarymagnetizesthecore.Thisproducesamagneticfieldinthecore.Thecorefieldthenaffectscurrentinbothprimaryandsecondary.

Therearetwomaindesignsforcores:

1.TheCOREtypehasthecoreinsidethewindings.

2.TheSHELLtypehasthecoreoutside.

Smallerpowertransformersareusuallyofthecoretype.Theverylargetransformersareoftheshelltype.Thereisnodifferenceintheiroperation,however.

Coilsarewoundwithcopperwire.Theresistanceiskeptaslowaspossiblekeeplosseslow.

IDEALIZEDTRANSFORMERS

Transformersareveryefficient.Thelossesareoftenlessthan3percent.Thisallowsustoassumethattheyareperfectinmanycomputations.

Perfectmeansthatthewirehasnoresistance.Italsomeansthattherearenopowerlossesinthecore.

Further,weassumethatthereisnofluxleakage.Thatis,allofthemagneticfluxlinksalloftheturnsoneachcoil.

EXCITATIONCURRENT

Togetanideaofjusthowsmallthelossesare,wecantakealookattheEXCITATIONCURRENT.Assumethatnothingisconnectedtothesecondary.Ifyouapplyratedvoltagetotheprimary,asmallcurrentflows.Typically,thisexcitationcurrentislessthan3percentofratedcurrent.

Excitationcurrentismadeupoftwopartisinphasewiththevoltage.Thisisthecurrentthatsuppliesthepowerlostinthecore.CorelossesareduetoEDDYCURRENTSandHYSTERESIS.

Eddycurrentscirculatinginthecoreresultfrominduction.Thecoreis,afterall,aconductorwithinachangingmagneticfield.

Hysteresislossiscausedbytheenergyusedinliningupmagneticdomainsinthecore.Thealignmentgoesoncontinuously,firstinonedirection,thenintheother.

Theotherpartoftheexcitationcurrentmagnetizesthecore.Itisthismagnetizingcurrentthatsuppliesthe“shuttlepower”.Shuttlepowerstoredinthemagneticfieldandreturnedtothesourcetwiceeachcycle.Magnetizingcurrentisquadrature（90degreesoutofphase）withtheappliedvoltage.

1.INTRODUCTION

Thehigh-voltagetransmissionwasneedforthecaseelectricalpoweristobeprovidedatconsiderabledistancefromageneratingstation.Atsomepointthishighvoltagemustbereduced,becauseultimatelyismustsupplyaload.Thetransformermakesitpossibleforvariouspartsofapowersystemtooperateatdifferentvoltagelevels.Inthispaperwediscusspowertransformerprinciplesandapplications.

2.TOW-WINDINGTRANSFORMERS

Atransformerinitssimplestformconsistsoftwostationarycoilscoupledbyamutualmagneticflux.Thecoilsaresaidtobemutuallycoupledbecausetheylinkacommonflux.

Inpowerapplications,laminatedsteelcoretransformers（towhichthispaperisrestricted）areused.Transformersareefficientbecausetherotationallossesnormallyassociatedwithrotatingmachineareabsent,sorelativelylittlepowerislostwhentransformingpowerfromonevoltageleveltoanother.Typicalefficienciesareintherange92to99%,thehighervaluesapplyingtothelargerpowertransformers.

Thecurrentflowinginthecoilconnectedtotheacsourceiscalledtheprimarywindingorsimplytheprimary.Itsetsupthefluxφinthecore,whichvariesperiodicallybothinmagnitudeanddirection.Thefluxlinksthesecondcoil,calledthesecondarywindingorsimplysecondary.Thefluxischanging;

therefore,itinducesavoltageinthesecondarybyelectromagneticinductioninaccordancewithLenz’slaw.Thustheprimaryreceivesitspowerfromthesourcewhilethesecondarysuppliesthispowertotheload.Thisactionisknownastransformeraction.

3.TRANSFORMERPRINCIPLES

Whenasinusoidal