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1、外文翻譯—原文部分AlternatingCurrentAnalternatingcurrentisonewhichvariesinthecircuitwithtime.Thevalueofthecurrentatanygiveninstantoftimeistheinstantaneousvalueofcurrentsymbolizedby.Theinstantaneousvalueofcurrentisdeemedpositivefo

2、neiidirectionofflowthoughthecrosssectionofaconductnegativeftheoppositedirection.Thedirectionofcurrentflowinwhichinstantaneousvaluesarepositiveiscalledpositive.Acurrentisfullyspecifiedifoneknowsitsinstantaneousvalueasafun

3、ctionoftime=F(t)itspositivedirection.iCurrentwhosevaluerecurfequalincrementsoftimearecalledperiodictheleastincrementoftimefwhichthisrecurrenttakesplaceiscalledtheperiod.TFaperiodiccurrent=F(t)=F(tT)iFig.4.1showsanexample

4、oftherelationship=F(t)faperiodiccurrent.Theiarrowinthediagramindicatesthepositivedirectionofcurrentflow.Thedottedarrowsshowtheactualdirectionofcurrentflowattheinstantsoftimewhen0i0.ThesegmentsofthecurvebetweenpointsabOcc

5、overacompleteicycleofcurrentalternationsoveroneperiod.Thenumberofcycleperiodpersecondisthefrequencyofaperiodiccurrent.Itisreciprocalofitsperiod.Tf1?Itisusuallytospecifythefrequencyofanyperiodicquantityincyclepersecond.Th

6、usthefrequencyofaperiodiccurrentwillbe1cyclepersecondifitsperiodissecond1cyclesec.Adirectcurrentmayberegardedasaspecialcaseofaperiodiccurrentwhoseperiodisinfinitelylongthefrequencyisthuszero.Thetermalternatingcurrentisof

7、tenusedinthenarrowsenseofaperiodiccurrentwhoseconstant(directioncurrent)componentiszerorevolution.Ifthespeedoftherotisrevolutionsperminutes(rmin)thiswksouttocyclesperminutethefrequencyoftheinducede.m.f.is：pn60pnf?Ff=50Hz

8、therotofageneratwithonepairofpolesshouldrunat3000rmin.withtwopolepairsat1500rmin.Withspeedslikethisrotsareusuallyfabricatedwithnonsalientpilesfgreatermechanicalstrength.Highfrequencygeneratsoperatingat800to8000Hzareallof

9、specialdesigns.Theirusesareintheheattreatmentinductionheatingfield.Stillhigherfrequenciesaregeneratedbyvalvesemiconductoscillats.SinusoidalCurrentTheinstantaneousvalueofasinusoidalcurrentisgivenby)2sin(????tTIimwhereisth

10、epeakvalueamplitudeofthecurrentisthemI)2(???tTphaseofthecurrent.Thequantityistheinitialphaseofthecurrent(ft=0)is?termedtheepochangle.Thephaseofacurrentcontinuouslyincreaseswithtime.Whenithasincreasebythewholecycleofchang

11、esisrepeatedexactlyallover?2again.Therefespeakingofthephaseofacurrentitiscustomarytotheintegertoconsiderthephasewithinbetweenzero.?2???2TheabovechangeofphasebyoccursduringtheperiodT.therefetherate?2ofchangeofthephaseisgi

12、venby.ItissymbolizedbytheGreekletterT2??(omega)iscalledthepulsatance.NotingthatwemaywriteTf1?fT???22??Theaboveexpressionrelatinghasbeenresponsiblefthefactthat?fthepulstanceisalsotermedtheangularvelocityfrequency.Itisexpr

13、essedin?radianspersecond.Thusf.IntroduceinEq.(4.1)weHzf50?srad314???obtain)sin(????tIImSinusoidalcurrentofthesamefrequencybutdifferinginamplitudeepochanglesareshowninFig.4.2.theirinstantaneousvaluesare)sin(111????tIim)si

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