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1、ElectricalMachinesAndElectricalApparatus1•ConstructionandPrinciplesofPowerTransformerTransformerisanindispensablecomponentinmanyenergyconversionsystems.Itmakespossibleelectricgenerationatthemosteconomicalgeneratorvoltage,powertransferatthemosteconomicaltransmissio
2、nvoltage,andpowerutilizationatthemostsuitablevoltagefortheparticularutilizationdevice.Thetransformerisalsowidelyusedinlow-power,low-currentelectronicandcontrolcircuitsforperformingsuchfunctionsasmatchingtheimpedancesofasourceanditsloadformaximumpowertransfer,isola
3、tingonecircuitfromanother,orisolatingdirectcurrentwhilemaintainingalternatingcurrentcontinuitybetweentwocircuits-Essentially,atransformerconsistsoftwoormorewindingscoupledbymutualmagneticflux.Ifoneofthesewindings,theprimaryisconnectedtoanalternatingvoltagesource,a
4、nalternatingfluxwillbeproducedwhoseamplitudewilldependontheprimaryvoltage,thefrequencyoftheappliedvoltage,andthenumberofturns.Themutualfluxwilllinktheotherwinding,thesecondaryandwillinduceavoltageinitwhosevaluewilldependonthenumberofthesecondaryturnsaswellasthemag
5、nitudeofthefluxandthefrequency.Byproperlyproportioningtheprimaryandthesecondaryturns,almostanydesiredvoltageratio,orratiooftransformation,canbeobtained.Theessenceoftransformeractionrequiresonlytheexistenceoftime-varyingmutualfluxlinkingtwowindings.Suchactioncanocc
6、urfortwowindingscoupledthroughair,butcouplingbetweenthewindingscanbemademuchmoreeffectivelyusingacoreofironorotherferromagneticmaterial,becausemostofthefluxisthenconfinedtoadefinite,high-permeabilitypathlinkingthewindings.Suchatransformeriscommonlycalledaniron-cor
7、etransformer.Mosttransformersareofthistype.Thefollowingdiscussionisconcernedalmostwhollywithiron-coretransforme匚Inordertoreducethelosscausedbyeddycurrentinthecore,themagneticcircuitusuallyconsistsofastackofthinlaminations.Twocommontypesofconstructionareshownschema
8、ticallyinFig.1.1•Inthecoretype(Fig.1.1a)thewindingsarewoundaroundtwolegsofarectangularmagneticcore;intheshelltype(Figl.lb)thewindingsarewoundaroundthece
