第0页共28页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第0页共28页ENGLISHORIGINALTEXT①DCGENENRATORS1.INTRODUCTIONForallpracticalpurposes,thedirect-currentgeneratorisonlyusedforspecialapplicationsandlocaldcpowergeneration.Thislimitationisduetothecommutatorrequiredtorectifytheinternalgeneratedacvoltage,therebymakinglargescaledcpowergeneratorsnotfeasible.Consequently,allelectricalenergyproducedcommerciallyisgeneratedanddistributedintheformofthree-phaseacpower.Theuseofsolidstateconvertersnowadaysmakesconversiontodceconomical.However,theoperatingcharacteristicsofdcgeneratorsarestillimportant,becausemostconceptscanbeappliedtoallothermachines.2.FIELDWINDINGCONNECTIONSThegeneralarrangementofbrushesandfieldwindingforafour-polemachineisasshowninFig.1.Thefourbrushesrideonthecommutator.ThepositivebrusherareconnectedtoterminalA1whilethenegativebrushesareconnectedtoterminalA2ofthemachine.Asindicatedinthesketch,thebrushesarepositionedapproximatelymidwayunderthepoles.TheymakecontactwithcoilsthathavelittleornoEMFinducedinthem,sincetheirsidesaresituatedbetweenpoles.①第1页共28页第0页共28页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第1页共28页Figure1Sketchoffour-poledcmatchineThefourexcitationorfieldpolesareusuallyjoinedinseriesandtheirendsbroughtouttoterminalsmarkedF1andF2.Theyareconnectedsuchthattheyproducenorthandsouthpolesalternately.Thetypeofdcgeneratorischaracterizedbythemannerinwhichthefieldexcitationisprovided.Ingeneral,themethodemployedtoconnectthefieldandarmaturewindingsfallsintothefollowinggroups(seeFig.2):Figure2Fieldconnectionsfordcgenerators:(a)separatelyexcitedgenerator;(b)self-excited,shuntgenerator;(c)seriesgenerator;(d)compoundgenerator;short-shuntconnection;(e)compoundgenerator,long-shunt第2页共28页第1页共28页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第2页共28页connection.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,thegeneratedEMFincreaseslinearlyuptothekneeofthemagnetizationcurv...
