机电专业英语 教学课件 作者 杨寿智 刘水平 机电专业英语.Lesson 27.ppt

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1、Lesson27TheBasicTransformerTextAbasictransformerisanelectricaldeviceconstructedoftwocoilswithmutualinductance.AschematicdiagramofatransformerisshowninFigure27.1(a).Asshown,onecoiliscalledtheprimarywinding,andtheotheriscalledthesecondarywinding.Thesourcevoltageisap

2、pliedtotheprimaryandtheloadisconnectedtothesecondary,asshowninFigure27.1(b).Typicaltransformersarewoundonacommoncoreinseveralways.Thecorecanbeeitheranaircore,anironcore,oraferritecore.Figure27.2(a)and(b)showtheprimaryandsecondarycoilsonanonferromagneticcylindrical

3、formasexamplesofaircoretransformers①.InPart(a),thewindingsareseparated;inPart(b),theyoverlapfortightercouplingandthusagreatermutualinductance(higherk).Figure27.2(c)illustratesanironcoretransformer.Ironandferritecoresincreasethecoefficientofcoupling.Standardschemat

4、icsymbolsareshownforalltypesinPart(d),andphotosoftypicaltransformersappearinPart(e).Air-coreandferrite-coretransformersgenerallyareusedforhigh-frequencyapplicationsandconsistofwindingsonaninsulatingshellwhichishollow(air)orconstructedofferrite,suchasdepictedinFigu

5、re27.2②.Iron-coretransformersgenerallyareusedforaudiofrequency(af)andpowerapplications.Thesetransformersconsistofwindingsonacoreconstructedfromlaminatedsheetsofferromagneticmaterialinsulatedfromeachother,asshowninthefigure.Thisconstructionprovidesaneasypathforthem

6、agneticfluxandincreasestheamountofcouplingbetweenthewindings③.TURNSRATIOAnimportantparameterofatransformerisitsturnsratio.Theturnsratio(n)istheratioofthenumberofturnsinthesecondarywinding(Ns)tothenumberofturnsintheprimarywinding(Np):Inthefollowingsections,youwills

7、eehowtheturnsratioaffectsthevoltagesandcurrentsinatransformer.EXAMPLE27-1Atransformerprimaryhas100turns,andthesecondaryhas400turns.Whatistheturnsratio?DIRECTIONOFWINDINGSAnotherimportanttransformerparameteristhedirectioninwhichthewindingsareplacedaroundthecore④.As

8、illustratedinFigure27.3,thedirectionofthewindingsdeterminesthepolarityofthesecondaryvoltagewithrespecttotheprimaryvoltage.Phasedotsareusedontheschematic