外文翻译直流发电机Word文件下载.docx

外文翻译直流发电机Word文件下载.docx

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DCGENENRATORS

1.INTRODUCTION

Forallpracticalpurposes,thedirect-currentgeneratorisonlyusedforspecialapplicationsandlocaldcpowergeneration.Thislimitationisduetothecommutatorrequiredtorectifytheinternalgeneratedacvoltage,therebymakinglargescaledcpowergeneratorsnotfeasible.

Consequently,allelectricalenergyproducedcommerciallyisgeneratedanddistributedintheformofthree-phaseacpower.Theuseofsolidstateconvertersnowadaysmakesconversiontodceconomical.However,theoperatingcharacteristicsofdcgeneratorsarestillimportant,becausemostconceptscanbeappliedtoallothermachines.

2.FIELDWINDINGCONNECTIONS

Thegeneralarrangementofbrushesandfieldwindingforafour-polemachineisasshowninFig.1.Thefourbrushesrideonthecommutator.ThepositivebrusherareconnectedtoterminalA1whilethenegativebrushesareconnectedtoterminalA2ofthemachine.Asindicatedinthesketch,thebrushesarepositionedapproximatelymidwayunderthepoles.TheymakecontactwithcoilsthathavelittleornoEMFinducedinthem,sincetheirsidesaresituatedbetweenpoles.

Figure1Sketchoffour-poledcmatchine

ThefourexcitationorfieldpolesareusuallyjoinedinseriesandtheirendsbroughtouttoterminalsmarkedF1andF2.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,EG=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-circuitcharacteristicofaseparatelyexciteddcmachine

Themeansthatalargerincreaseinfieldcurrentisrequiredtoproduceagivenincreaseinvoltage.

SincethegeneratedvoltageEGisalsodirectlyproportionaltothespeed,amagnetizationcurvecanbedrawnforanyotherspeedoncethecurveisdetermined.Thismerelyrequiresanadjustmentofallpointsonthecurveaccordingto

wherethequantitiesvaluesatthevariousspeeds.

3.VOLTAGEREGULATION

Letusnextconsideraddingaloadongenerator.Theterminalvoltagewillthendecrease（becausethearmaturewindingharesistance）unlesssomeprovisionismadetokeepitconstant.Acurvethatshowsthevalueofterminalvoltageforvariousloadcurrentsiscalledtheloadorcharacteristicofthegenerator.

Fig.4showstheexternalcharacteristicofaseparatelyexcitedgenerator.ThedecreaseintheterminalvoltageisduemainlytothearmaturecircuitresistanceRA.Ingeneral,

whereVtistheterminalvoltageandIAisthearmaturecurrent（orloadcurrentIL）suppliedbythegeneratortotheload.

Anotherfactorthatcontributestothedecreaseinterminalvoltageisthedecreaseinfluxduetoarmaturereaction.ThearmaturecurrentestablishedanMMFthatdistortsthemainflux,resultinginaweakenedflux,especiallyinnoninterpolemachines.Thiseffectiscalledarmaturereaction.AsFig.4shows,theterminalvoltageversusloadcurrentcurvedoesnotdropofflinearlysincetheironbehavesnonlinear.Becausearmaturereactiondependsonthearmaturecurrentitgivesthecurveitsdroopingcharacteristic.

4.SHUNTORSELF-EXCIITEDGENRATORS

Ashuntgeneratorhasitsshuntfieldwindingconnectedinparallelwiththearmaturesothatthemachineprovidesitsownexcitation,asindicatedinFig.5.Thequestionariseswhetherthemachinewillgenerateavoltageandwhatdeterminesthevoltage.

Forvoltageto“buildup”asitiscalled,theremustbesomeremanentmagnetisminthefieldpoles.Ordinarily,ifthegeneratorhasbeenusedpreviously,therewillbesomeremanentmagnetism.WehaveseeninSection3thatifthefieldwouldbedisconnected,therewillbesmallvoltageEfgeneratedduetothisremanentmagnetism,providedthatthegeneratorisdrivenatsomespeed.Connectingthefieldforself-excitation,thissmallvoltagewillbeappliedtotheshuntsfieldanddriveasmallcurrentthroughthefieldcircuit.Ifthisresultingsmallcurrentintheshuntfieldisofsuchadirectionthatitweakenstheresidualflux,thevoltageremainsnearzeroandtheterminalvoltagedoesnotbuildup.Inthissituationtheweakmainpolefluxopposestheresidualflux.

Figure5Shuntgenerator:

（a）circuit;

（b）loadcharacteristic

Iftheconnectionissuchthattheweakmainpolefluxaidstheresidualflux,theinducedvoltageincreasesrapidlytoalarge,constantvalue.Thebuild-upprocessisreadilyseentobecumulanve.Thatis,morevoltageincreasesthefieldcurrent,whichinturnincreasesthevoltage,andsoon.Thefactthatthisprocessterminatesatafinitevoltageisduetothenonlinearbehaviorofthemagncticcircuit.InsteadystatethegeneratedvoltageiscausesafieldcurrenttoflowthatisjustsufficienttodevelopafluxrequiredforthegeneratedEMFthatcausesthefieldcurrenttoflow.

Thecircuitcarriesonlydccurrent,sothatthefieldcurrentdependsonlyonthefieldcircuitresistance,Rf.ThismayconsistofthefieldcircuitresistanceRf,thefieldcurrentdependsonthegeneratedvoltageinaccordancewithOhm’slaw.

Itshouldbeevidentthatonanewmachineoronethathaslostitsresidualfluxbecauseofalongidleperiod,somemagnetismmustbecreated.Thisisusuallydonebyconnectingthefieldwindingonlytoaseparatedcsourceforafewseconds.Thisprocedureisgenerallyknownasflashingthefield.

SeriesGenerators

Asmentionedpreviously,thefieldwindingofaseriesgeneratorisinserieswiththearmature.Sinceitcarriestheloadcurrenttheseriesfieldwindingconsistsofonlyafewturnsofthickwire.Atnoload,thegeneratedvoltageissmallduetoresidualfieldfluxonly.Whenaloadisadded,thefluxincreases,andsodoesthegeneratedvoltage.Fig.7showstheloadcharacteristicofaseriesgeneratordrivenatacertainspeed.ThedashedlineindicatesthegeneratedEMFofthesamemachinewiththearmatureopen-circuitedandthefieldseparatelyexcited.ThedifferencebetweenthetwocurvesissimplytheIRdropintheseriesfieldandarmaturewinding,suchthat

whereRSistheseriesfieldwindingresistance.

Figure7Seriesgenerator:

（a）circuitdiagram;

（b）loadcharacteristics

CompoundGenerators

Thecompoundgeneratorhasbothashuntandaseriesfieldwinding,thelatterwindingwoundontopoftheshuntwinding.Fig.8showsthecircuitdiagram.Thetwowindingsareusuallyconnectedsuchthattheirampere-turnsactinthesamedirection.Assuchthegeneratorissaidtobecumulativelycompounded.

TheshuntconnectionillustratedinFig.8iscalledalongshuntconnection.Iftheshuntfieldwindingisdirectlyconnectedacrossthearmatureterminals,theconnectionisreferredtoasashortshunt.Inpracticetheconnectionusedisoflittleconsequence,sincetheshuntfieldwindingcarriesasmallcurrentcomparedtothefull-loadcurrent.Furthermore,thenumberofturnsontheseriesfieldwinding.Thisimpliesithasalowresistancevalueandthecorrespondingvoltagedropacrossitatfullloadisminimal.

CurvesinFig.9representstheterminalcharacteristicoftheshuntfieldwindingalone.Bytheadditionofasmallseriesfieldwindingthedropinterminalvoltagewithincreasedloadingisreducedasindicated.Suchageneratorissaidtobeundercompounded.Byincreasingthenumberofseriesturns,theno-loadandfull-loadterminalvoltagecanbemadeequal;

thegeneratoristhensaidtobeflatcompounded.Ifthenumberofseriesturnsismorethannecessarytocompensateforthevoltagedrop,thegeneratorisovercomepou