外文翻译变压器Word下载.docx
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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
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