英文文献 科技类 原文及翻译 （电子 电气 自动化 通信…）27

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1、ENGLISHORIGINALTEXT注：本文来自《科技英语》2004年5期DCGENENRATORS1.INTRODUCTIONForallpracticalpurposes,thedirect-currentgeneratorisonlyusedforspecialapplicationsandlocaldcpowergeneration.Thislimitationisduetothecommutatorrequiredtorectifytheinternalgeneratedacvoltage,therebymakinglargescaledcpowergeneratorsn

2、otfeasible.Consequently,allelectricalenergyproducedcommerciallyisgeneratedanddistributedintheformofthree-phaseacpower.Theuseofsolidstateconvertersnowadaysmakesconversiontodceconomical.However,theoperatingcharacteristicsofdcgeneratorsarestillimportant,becausemostconceptscanbeappliedtoallothermac

3、hines.2.FIELDWINDINGCONNECTIONSThegeneralarrangementofbrushesandfieldwindingforafour-polemachineisasshowninFig.1.Thefourbrushesrideonthecommutator.ThepositivebrusherareconnectedtoterminalA1whilethenegativebrushesareconnectedtoterminalA2ofthemachine.Asindicatedinthesketch,thebrushesarepositioned

4、approximatelymidwayunderthepoles.TheymakecontactwithcoilsthathavelittleornoEMFinducedinthem,sincetheirsidesaresituatedbetweenpoles.27Figure1Sketchoffour-poledcmatchineThefourexcitationorfieldpolesareusuallyjoinedinseriesandtheirendsbroughtouttoterminalsmarkedF1andF2.Theyareconnectedsuchthatthey

5、producenorthandsouthpolesalternately.Thetypeofdcgeneratorischaracterizedbythemannerinwhichthefieldexcitationisprovided.Ingeneral,themethodemployedtoconnectthefieldandarmaturewindingsfallsintothefollowinggroups(seeFig.2):Figure2Fieldconnectionsfordcgenerators:(a)separatelyexcitedgenerator;(b)sel

6、f-excited,shuntgenerator;(c)seriesgenerator;(d)compoundgenerator;short-shuntconnection;(e)compoundgenerator,long-shuntconnection.Theshuntfieldcontainsmanyturnsofrelativelyfinewireandcarriesacomparativelysmallcurrent,onlyafewpercentofratedcurrent.Theseriesfieldwinding,ontheotherhand,hasfewturnso

7、fheavywiresinceitisinserieswiththearmatureandthereforecarriestheloadcurrent.Beforediscussingthedcgeneratorterminalcharacteristics,letusexaminetherelationshipbetweenthegeneratedvoltageandexcitationcurrentofageneratoronnoload.Thegen