直流电动机中英文对照外文翻译文献

外文文献及译文中英文对照外文翻译文献(文档含英文原文和中文翻译)外文文献：DCMotorCalculationsOverviewNowthatwehaveagoodunderstandingofdcgenerators,wecanbeginourstudyofdcmotors.Direct-currentmotorstransformelectricalenergyintomechanicalenergy.Theydrivedevicessuchashoists,fans,pumps,calendars,punch-presses,andcars.Thesedevicesmayhaveadefinitetorque-speedcharacteristic(suchasapumporfan)orahighlyvariableone(suchasahoistorautomobile).Thetorque-speedcharacteristicofthemotormustbeadaptedtothetypeoftheloadithastodrive,andthisrequirementhasgivenrisetothreebasictypesofmotors:1.Shuntmotors2.Seriesmotors3.CompoundmotorsDirect-currentmotorsareseldomusedinordinaryindustrialapplicationsbecauseallelectricutilitysystemsfurnishalternatingcurrent.However,forspecialapplicationssuchasinsteelmills,mines,andelectrictrains,itissometimesadvantageoustotransformthealternatingcurrentintodirectcurrentinordertousedcmotors.Thereasonisthatthetorque-speedcharacteristicsofdcmotorscanbevariedoverawiderangewhileretaininghighefficiency.Today,thisgeneralstatementcanbechallengedbecausetheavailabilityofsophisticatedelectronicdriveshasmadeitpossibletousealternatingcurrentmotorsforvariablespeedapplications.Nevertheless,therearemillionsofdcmotorsstillinserviceandthousandsmorearebeingproducedeveryyear.Counter-electromotiveforce(cemf)Direct-currentmotorsarebuiltthesamewayasgeneratorsare;consequently,adcmachinecanoperateeitherasamotororasagenerator.Toillustrate,consideradcgeneratorinwhichthearmature,initiallyatrest,isconnectedtoadcsourceEsbymeansofaswitch(Fig.5.1).ThearmaturehasaresistanceR,andthemagneticfieldiscreatedbyasetofpermanentmagnets.

Assoonastheswitchisclosed,alargecurrentflowsinthearmaturebecauseitsresistanceisverylow.Theindividualarmatureconductorsareimmediatelysubjectedtoaforcebecausetheyareimmersedinthemagneticfieldcreatedbythepermanentmagnets.Theseforcesadduptoproduceapowerfultorque,causingthearmaturetorotate.

Figure5.1Startingadcmotoracrosstheline.Ontheotherhand,assoonasthearmaturebeginstoturn,asecondphenomenontakesplace:thegeneratoreffect.WeknowthatavoltageEoisinducedinthearmatureconductorsassoonastheycutamagneticfield(Fig.5.2).Thisisalwaystrue,nomatterwhatcausestherotation.Thevalueandpolarityoftheinducedvoltagearethesameasthoseobtainedwhenthemachineoperatesasagenerator.TheinducedvoltageEoisthereforeproportionaltothespeedofrotationnofthemotorandtothefluxFperpole,aspreviouslygivenbyEq.5.1:Eo=ZnF/60(5.1)Asinthecaseofagenerator,Zisaconstantthatdependsuponthenumberofturnsonthearmatureandthetypeofwinding.ForlapwindingsZisequaltothenumberofarmatureconductors.

Inthecaseofamotor,theinducedvoltageEoiscalledcounter-electromotiveforce(cemf)becauseitspolarityalwaysactsagainstthesourcevoltageEs.ItactsagainstthevoltageinthesensethatthenetvoltageactingintheseriescircuitofFig.5.2isequalto(Es-Eo)voltsandnot(Es+Eo)volts.

Figure5.2Counter-electromotiveforce(cemf)inadcmotor.AccelerationofthemotorThenetvoltageactinginthearmaturecircuitinFig.5.2is(Es-Eo)volts.Theresultingarmaturecurrent/islimitedonlybythearmatureresistanceR,andsoI=(Es-Eo)IR(5.2)Whenthemotorisatrest,theinducedvoltageEo=0,andsothestartingcurrentisI=Es/RThestartingcurrentmaybe20to30timesgreaterthanthenominalfull-loadcurrentofthemotor.Inpractice,thiswouldcausethefusestobloworthecircuit-breakerstotrip.However,iftheyareabsent,thelargeforcesactingonthearmatureconductorsproduceapowerfulstartingtorqueandaconsequentrapidaccelerationofthearmature.

Asthespeedincreases,thecounter-emfEoincreases,withtheresultthatthevalueof(Es—Eo)diminishes.ItfollowsfromEq.5.1thatthearmaturecurrent/dropsprogressivelyasthespeedincreases.

Althoughthearmaturecurrentdecreases,themotorcontinuestoaccelerateuntilitreachesadefinite,maximumspeed.Atno-loadthisspeedproducesacounter-emfEoslightlylessthanthesourcevoltageEs.Ineffect,ifEowereequaltoEsthenetvoltage(Es—Eo)wouldbecomezeroandso,too,wouldthecurrent/.Thedrivingforceswouldceasetoactonthearmatureconductors,andthemechanicaldragimposedbythefanandthebearingswouldimmediatelycausethemotortoslowdown.Asthespeeddecreasesthenetvoltage(Es—Eo)increasesandsodoesthecurrent/.Thespeedwillceasetofallassoonasthetorquedevelopedbythearmaturecurrentisequaltotheloadtorque.Thus,whenamotorrunsatno-load,thecounter-emfmustbeslightlylessthanEssoastoenableasmallcurrenttoflow,sufficienttoproducetherequiredtorque.MechanicalpowerandtorqueThepowerandtorqueofadcmotoraretwoofitsmostimportantproperties.Wenowderivetwosimpleequationsthatenableustocalculatethem.

1.AccordingtoEq.5.1thecemfinducedinalap-woundarmatureisgivenbyEo=ZnF/60ReferringtoFig.5.2,theelectricalpowerPasuppliedtothearmatureisequaltothesupplyvoltageEsmultipliedbythearmaturecurrentI:Pa=EsI(5.3)However,EsisequaltothesumofEoplustheIRdropinthearmature:Es=Eo+IR(5.4)ItfollowsthatPa=EsI

=(Eo+IR)I

=EoI+I2R(5.5)TheI2Rtermrepresentsheatdissipatedinthearmature,buttheveryimportanttermEoIistheelectricalpowerthatisconvertedintomechanicalpower.ThemechanicalpowerofthemotoristhereforeexactlyequaltotheproductofthecemfmultipliedbythearmaturecurrentP=EoI(5.6)where

P=mechanicalpowerdevelopedbythemotor[W]

Eo=inducedvoltageinthearmature(cemf)[V]

I=totalcurrentsuppliedtothearmature[A]

2.TurningourattentiontotorqueT,weknowthatthemechanicalpowerPisgivenbytheexpressionP=nT/9.55(5.7)wherenisthespeedofrotation.

CombiningEqs.5.7,5.1,and5.6,weobtainnT/9.55=EoI

=ZnFI/60andsoT=ZFI/6.28Thetorquedevelopedbyalap-woundmotoristhereforegivenbytheexpressionT=ZFI/6.28(5.8)where

T=torque[N×m]

Z=numberofconductorsonthearmature

F=effectivefluxperpole[Wb]*

/=armaturecurrent[A]

6.28=constant,totakecareofunits

[exactvalue=2p]

Eq.5.8showsthatwecanraisethetorqueofamotoreitherbyraisingthearmaturecurrentorbyraisingthefluxcreatedbythepoles.SpeedofrotationWhenadcmotordrivesaloadbetweenno-loadandfull-load,theIRdropduetoarmatureresistanceisalwayssmallcomparedtothesupplyvoltageEs.Thismeansthatthecounter-emfEsisverynearlyequaltoEs.

Ontheotherhand,wehavealreadyseenthatEomaybeexpressedbytheequationEo=ZnF/60ReplacingEobyEsweobtainEs=ZnF/60Thatis,where

n=speedofrotation[r/min]

Es=armaturevoltage[V]

Z=totalnumberofarmatureconductors

Thisimportantequationshowsthatthespeedofthemotorisdirectlyproportionaltothearmaturesupplyvoltageandinverselyproportionaltothefluxperpole.Wewillnowstudyhowthisequationisapplied.ArmaturespeedcontrolAccordingtoEq.5.8,ifthefluxperpoleFiskeptconstant(permanentmagnetfieldorfieldwithfixedexcitation),thespeeddependsonlyuponthearmaturevoltageEs.ByraisingorloweringEsthemotorspeedwillriseandfallinproportion.

Inpractice,wecanvaryEsbyconnectingthemotorarmatureMtoaseparatelyexcitedvariable-voltagedcgeneratorG.Thefieldexcitationofthemotoriskeptconstant,butthegeneratorexcitationIxcanbevariedfromzerotomaximumandevenreversed.ThegeneratoroutputvoltageEscanthereforebevariedfromzerotomaximum,witheitherpositiveornegativepolarity.Consequently,themotorspeedcanbevariedfromzerotomaximumineitherdirection.Notethatthegeneratorisdrivenbyanacmotorconnectedtoa3-phaseline.Thismethodofspeedcontrol,knownastheWard-Leonardsystem,isfoundinsteelmills,high-riseelevators,mines,andpapermills.

Inmodeminstallationsthegeneratorisoftenreplacedbyahigh-powerelectronicconverterthatchangestheacpoweroftheelectricalutilitytodc,byelectronicmeans.

WhathappenstothedcpowerreceivedbygeneratorG?WhenGreceiveselectricpower,itoperatesasamotor,drivingitsownacmotorasanasynchronousgenerator!*Asaresult,acpowerisfedbackintothelinethatnormallyfeedstheacmotor.ThefactthatpowercanberecoveredthiswaymakestheWard-Leonardsystemveryefficient,andconstitutesanotherofitsadvantages.

RheostatSpeedControlAnotherwaytocontrolthespeedofadcmotoristoplacearheostatinserieswiththearmature.ThecurrentintherheostatproducesavoltagedropwhichsubtractsfromthefixedsourcevoltageEs,yieldingasmallersupplyvoltageacrossthearmature.Thismethodenablesustoreducethespeedbelowitsnominalspeed.Itisonlyrecommendedforsmallmotorsbecausealotofpowerandheatiswastedintherheostat,andtheoverallefficiencyislow.Furthermore,thespeedregulationispoor,evenforafixedsettingoftherheostat.Ineffect,theIRdropacrosstherheostatincreasesasthearmaturecurrentincreases.Thisproducesasubstantialdropinspeedwithincreasingmechanicalload.中文译文：直流电动机的计算概述现在，我们对直流发电机有一个很好的了解，我们可以开始对直流电动机的研究了。直流电动机将电能转换成机械能。他们带动着设备运行,如卷扬机、风机及水泵、打孔机、和汽车。这些设备可能有一定的机械特性（如泵或风扇）或其转矩、速度变化很快（如吊重机或汽车）。电机的机械特性必须适应它的负载驱动类型，这就把电机分成了三种类型：1、并励式电动机2、串励式电动机3、复励式电动机。直流电动机很少在一般工业中应用，因为所有的电力系统提供的都是交流电。然而,对于一些特殊应用是有利的,如钢厂、矿山、和电动火车,将交流电转换成直流电就是为了使用直流电机。因为直流电动机的机械特性可在很宽的范围内变化，同时保持高效率。如今这种言论已经受到了质疑，因为有了一种先进的电子驱动器可以让交流电动机实现调速。不过，仍有数以百万计的直流电动机在应用并且每年都会生产数千台直流电动机。反电动势（cemf）直流电动机是根据发电机原理制造的，所以一台直流电机可以作为直流马达或发电机使用。为了说明这一点,假设一个直流发电机的电枢,最初在静止时，通过一个开关连接到一个直流源(Es)(图5.1)。电枢是由电阻R和由一套永磁铁产生的磁场组成。一旦开关闭合，就会有一个很大的电流通过电枢，因为它的阻值很低。由于它身处于永磁体产生的磁场中，电枢绕组就会受力。这些力的合力产生了电磁转矩，使电枢在磁场里转动。图5.1启动电机另外，电枢开始转动的同时，第二个现象出现了：发电机的效果。我们知道当电枢绕组切割磁导线是产生一个感应电动势Eo(图5.2)。这是真理，不管什么原因造成的转动。当作为发电机时会得到平均值和极性相同的感应电动势。感应电动势Eo与转子的转速n每级的磁通量F成正比，5.1式：Eo=ZnF/60(5.1)当只有一台发电机时，Z是一个取决于电枢转动圈数和绕组类型的常数。绕组圈数Z等于电枢导体数。一台电机，感应电压Eo就是所谓的反电动势（cemf），因为其极性与电源E相反。电压回路的串联电路如图5.2，其电压等价于(Es-Eo)伏特,而不是(Es+Eo)伏，从某种意义上说这是违背电压规律的。图5.2直流电机的反电动势电机的加速运转图5.2电路的电枢电压为(Es-Eo)伏特，由此产生的电枢电流/仅限制于电枢电阻R,所以I=(Es-Eo)/R(5.2)当电机处于静止状态，它的感应电动势为Eo=0,所以启动电流为I=Es/R起动电流可能比电机的满载电流大20到30倍。在实践中，这会导致保险丝熔断或断路器跳闸。但是，如果它们没有熔断或跳闸，强大的电流作用于电枢绕组产生很大的转动力矩并且迅速加快电机转速。随着转速的增加,感应电动势Eo增加,其结果就是(Es-Eo)不断减小。从式5.1可知，随着转速的增加电枢电流减小。虽然电枢电流在减小，但电机的转速继续增大直到达到额定最大值。在无负荷转动时，额定速度产生的反电动势Eo略低于电源电动势Es。如果感应电动势Eo等于电源电动势Es那么回来电压(Es-Eo)等于0，所以电枢电流也为0。驱动力将不会作用于电枢绕组，轴承等机械阻力将会使电机转速减慢。随着速度的降低，回路电压(Es-Eo)增加,那么电流也增加。当电枢电流产生的转矩增加到等于负载转矩时，电机的转速将不再减慢。因此，当电机在空载运行时，感应电动势必须比电源电动势小，使有一个小电流足以产生所需的转矩。机械动力和力矩电源和直流电动机的力矩是其最重要的两个属性。现在让我们得出两个简单的公式来计算它们。一个线圈的感应电动势由式5.1给出：Eo=ZnF/60参考图5.2，提