电子类文献中英文翻译(发电机)

1ENGLISHORIGINALTEXT①DCGENENRATORS1.INTRODUCTIONForallpracticalpurposes,thedirect-currentgeneratorisonlyusedforspecialapplicationsandlocaldcpowergeneration.Thislimitationisduetothecommutatorrequiredtorectifytheinternalgeneratedacvoltage,therebymakinglargescaledcpowergeneratorsnotfeasible.Consequently,allelectricalenergyproducedcommerciallyisgeneratedanddistributedintheformofthree-phaseacpower.Theuseofsolidstateconvertersnowadaysmakesconversiontodceconomical.However,theoperatingcharacteristicsofdcgeneratorsarestillimportant,becausemostconceptscanbeappliedtoallothermachines.2.FIELDWINDINGCONNECTIONSThegeneralarrangementofbrushesandfieldwindingforafour-polemachineisasshowninFig.1.Thefourbrushesrideonthecommutator.ThepositivebrusherareconnectedtoterminalA1whilethenegativebrushesareconnectedtoterminalA2ofthemachine.Asindicatedinthesketch,thebrushesarepositionedapproximatelymidwayunderthepoles.TheymakecontactwithcoilsthathavelittleornoEMFinducedinthem,sincetheirsidesaresituatedbetweenpoles.2Figure1Sketchoffour-poledcmatchineThefourexcitationorfieldpolesareusuallyjoinedinseriesandtheirendsbroughtouttoterminalsmarkedF1andF2.Theyareconnectedsuchthattheyproducenorthandsouthpolesalternately.Thetypeofdcgeneratorischaracterizedbythemannerinwhichthefieldexcitationisprovided.Ingeneral,themethodemployedtoconnectthefieldandarmaturewindingsfallsintothefollowinggroups(seeFig.2):Figure2Fieldconnectionsfordcgenerators:(a)separatelyexcitedgenerator;(b)self-excited,shuntgenerator;(c)seriesgenerator;(d)compoundgenerator;short-shuntconnection;(e)compoundgenerator,long-shuntconnection.Theshuntfieldcontainsmanyturnsofrelativelyfinewireandcarriesacomparativelysmallcurrent,onlyafewpercentofratedcurrent.Theseriesfieldwinding,ontheotherhand,hasfewturnsofheavywiresinceitisinserieswiththearmatureandthereforecarriestheloadcurrent.Beforediscussingthedcgeneratorterminalcharacteristics,letusexaminetherelationshipbetweenthegeneratedvoltageandexcitationcurrentofageneratoronnoload.ThegeneratedEMFisproportionaltoboththefluxperpoleandthespeedatwhichthegeneratorisdriven,3EG=kn.ByholdingthespeedconstantitcanbeshowntheEGdependsdirectlyontheflux.Totestthisdependencyonactualgeneratorsisnotverypractical,asitinvolvesamagneticfluxmeasurement.Thefluxisproducedbytheampere-turnsofthefieldcoils:inturn,thefluxmustdependontheamountoffieldcurrentflowingsincethenumberofturnsonthefieldwindingisconstant.Thisrelationshipisnotlinearbecauseofmagneticsaturationafterthefieldcurrentreachesacertainvalue.ThevariationofEGversusthefieldcurrentIfmaybeshownbyacurveknownasthemagnetizationcurveoropen-circuitcharacteristic.Forthisagivengeneratorisdrivenataconstantspeed,isnotdeliveringloadcurrent,andhasitsfieldwindingseparatelyexcited.ThevalueofEGappearingatthemachineterminalsismeasuredasIfisprogressivelyincreasedfromzerotoavaluewellaboveratedvoltageofthatmachine.TheresultingcurveisshownisFig.3.WhenIj=0,thatis,withthefieldcircuitopencircuited,asmallvoltageEtismeasured,duetoresidualmagnetism.Asthefieldcurrentincreases,thegeneratedEMFincreaseslinearlyuptothekneeofthemagnetizationcurve.Beyondthispoint,increasingthefieldcurrentstillfurthercausessaturationofthemagneticstructuretosetin.Figure3Magnetizationcurveoropen-circuitcharacteristicofaseparatelyexciteddcmachineThemeansthatalargerincreaseinfieldcurrentisrequiredtoproduceagivenincreaseinvoltage.SincethegeneratedvoltageEGisalsodirectlyproportionaltothespeed,amagnetization4curvecanbedrawnforanyotherspeedoncethecurveisdetermined.ThismerelyrequiresanadjustmentofallpointsonthecurveaccordingtonnxEEGG''wherethequantitiesvaluesatthevariousspeeds.3.VOLTAGEREGULATIONLetusnextconsideraddingaloadongenerator.Theterminalvoltagewillthendecrease(becausethearmaturewindingharesistance)unlesssomeprovisionismadetokeepitconstant.Acurvethatshowsthevalueofterminalvoltageforvariousloadcurrentsiscalledtheloadorcharacteristicofthegenerator.Figure4(a)directscurrentittourgethegeneratorloadcharacteristics;(b)circuitdiagramFig.4showstheexternalcharacteristicofaseparatelyexcitedgenerator.ThedecreaseintheterminalvoltageisduemainlytothearmaturecircuitresistanceRA.Ingeneral,AAGtRIEVwhereVtistheterminalvoltageandIAisthearmaturecurrent(orloadcurrentIL)suppliedbythegeneratortotheload.Anotherfactorthatcontributestothedecreaseinterminalvoltageisthedecreaseinfluxduetoarmaturereaction.ThearmaturecurrentestablishedanMMFthatdistortsthemainflux,5resultinginaweakenedflux,especiallyinnoninterpolemachines.Thiseffectiscalledarmaturereaction.AsFig.4shows,theterminalvoltageversusloadcurrentcurvedoesnotdropofflinearlysincetheironbehavesnonlinear.Becausearmaturereactiondependsonthearmaturecurrentitgivesthecurveitsdroopingcharacteristic.4.SHUNTORSELF-EXCIITEDGENRATORSAshuntgeneratorhasitsshuntfieldwindingconnectedinparallelwiththearmaturesothatthemachineprovidesitsownexcitation,asindicatedinFig.5.Thequestionariseswhetherthemachinewillgenerateavoltageandwhatdeterminesthevoltage.Forvoltageto“buildup”asitiscalled,theremustbesomeremanentmagnetisminthefieldpoles.Ordinarily,ifthegenerator