外文翻译直流发电机Word文件下载.docx
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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
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