直流電動(dòng)機(jī)中英文對(duì)照外文翻譯文獻(xiàn)

外文文獻(xiàn)及譯文中英文對(duì)照外文翻譯文獻(xiàn)(文檔含英文原文和中文翻譯)外文文獻(xiàn)：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.中文譯文：直流電動(dòng)機(jī)的計(jì)算概述現(xiàn)在，我們對(duì)直流發(fā)電機(jī)有一個(gè)很好的了解，我們可以開始對(duì)直流電動(dòng)機(jī)的研究了。直流電動(dòng)機(jī)將電能轉(zhuǎn)換成機(jī)械能。他們帶動(dòng)著設(shè)備運(yùn)行,如卷?yè)P(yáng)機(jī)、風(fēng)機(jī)及水泵、打孔機(jī)、和汽車。這些設(shè)備可能有一定的機(jī)械特性（如泵或風(fēng)扇）或其轉(zhuǎn)矩、速度變化很快（如吊重機(jī)或汽車）。電機(jī)的機(jī)械特性必須適應(yīng)它的負(fù)載驅(qū)動(dòng)類型，這就把電機(jī)分成了三種類型：1、并勵(lì)式電動(dòng)機(jī)2、串勵(lì)式電動(dòng)機(jī)3、復(fù)勵(lì)式電動(dòng)機(jī)。直流電動(dòng)機(jī)很少在一般工業(yè)中應(yīng)用，因?yàn)樗械碾娏ο到y(tǒng)提供的都是交流電。然而,對(duì)于一些特殊應(yīng)用是有利的,如鋼廠、礦山、和電動(dòng)火車,將交流電轉(zhuǎn)換成直流電就是為了使用直流電機(jī)。因?yàn)橹绷麟妱?dòng)機(jī)的機(jī)械特性可在很寬的范圍內(nèi)變化，同時(shí)保持高效率。如今這種言論已經(jīng)受到了質(zhì)疑，因?yàn)橛辛艘环N先進(jìn)的電子驅(qū)動(dòng)器可以讓交流電動(dòng)機(jī)實(shí)現(xiàn)調(diào)速。不過，仍有數(shù)以百萬計(jì)的直流電動(dòng)機(jī)在應(yīng)用并且每年都會(huì)生產(chǎn)數(shù)千臺(tái)直流電動(dòng)機(jī)。反電動(dòng)勢(shì)（cemf）直流電動(dòng)機(jī)是根據(jù)發(fā)電機(jī)原理制造的，所以一臺(tái)直流電機(jī)可以作為直流馬達(dá)或發(fā)電機(jī)使用。為了說明這一點(diǎn),假設(shè)一個(gè)直流發(fā)電機(jī)的電樞,最初在靜止時(shí)，通過一個(gè)開關(guān)連接到一個(gè)直流源(Es)(圖5.1)。電樞是由電阻R和由一套永磁鐵產(chǎn)生的磁場(chǎng)組成。一旦開關(guān)閉合，就會(huì)有一個(gè)很大的電流通過電樞，因?yàn)樗淖柚岛艿汀Ｓ捎谒硖幱谟来朋w產(chǎn)生的磁場(chǎng)中，電樞繞組就會(huì)受力。這些力的合力產(chǎn)生了電磁轉(zhuǎn)矩，使電樞在磁場(chǎng)里轉(zhuǎn)動(dòng)。圖5.1啟動(dòng)電機(jī)另外，電樞開始轉(zhuǎn)動(dòng)的同時(shí)，第二個(gè)現(xiàn)象出現(xiàn)了：發(fā)電機(jī)的效果。我們知道當(dāng)電樞繞組切割磁導(dǎo)線是產(chǎn)生一個(gè)感應(yīng)電動(dòng)勢(shì)Eo(圖5.2)。這是真理，不管什么原因造成的轉(zhuǎn)動(dòng)。當(dāng)作為發(fā)電機(jī)時(shí)會(huì)得到平均值和極性相同的感應(yīng)電動(dòng)勢(shì)。感應(yīng)電動(dòng)勢(shì)Eo與轉(zhuǎn)子的轉(zhuǎn)速n每級(jí)的磁通量F成正比，5.1式：Eo=ZnF/60(5.1)當(dāng)只有一臺(tái)發(fā)電機(jī)時(shí)，Z是一個(gè)取決于電樞轉(zhuǎn)動(dòng)圈數(shù)和繞組類型的常數(shù)。繞組圈數(shù)Z等于電樞導(dǎo)體數(shù)。一臺(tái)電機(jī)，感應(yīng)電壓Eo就是所謂的反電動(dòng)勢(shì)（cemf），因?yàn)槠錁O性與電源E相反。電壓回路的串聯(lián)電路如圖5.2，其電壓等價(jià)于(Es-Eo)伏特,而不是(Es+Eo)伏，從某種意義上說這是違背電壓規(guī)律的。圖5.2直流電機(jī)的反電動(dòng)勢(shì)電機(jī)的加速運(yùn)轉(zhuǎn)圖5.2電路的電樞電壓為(Es-Eo)伏特，由此產(chǎn)生的電樞電流/僅限制于電樞電阻R,所以I=(Es-Eo)/R(5.2)當(dāng)電機(jī)處于靜止?fàn)顟B(tài)，它的感應(yīng)電動(dòng)勢(shì)為Eo=0,所以啟動(dòng)電流為I=Es/R起動(dòng)電流可能比電機(jī)的滿載電流大20到30倍。在實(shí)踐中，這會(huì)導(dǎo)致保險(xiǎn)絲熔斷或斷路器跳閘。但是，如果它們沒有熔斷或跳閘，強(qiáng)大的電流作用于電樞繞組產(chǎn)生很大的轉(zhuǎn)動(dòng)力矩并且迅速加快電機(jī)轉(zhuǎn)速。隨著轉(zhuǎn)速的增加,感應(yīng)電動(dòng)勢(shì)Eo增加,其結(jié)果就是(Es-Eo)不斷減小。從式5.1可知，隨著轉(zhuǎn)速的增加電樞電流減小。雖然電樞電流在減小，但電機(jī)的轉(zhuǎn)速繼續(xù)增大直到達(dá)到額定最大值。在無負(fù)荷轉(zhuǎn)動(dòng)時(shí)，額定速度產(chǎn)生的反電動(dòng)勢(shì)Eo略低于電源電動(dòng)勢(shì)Es。如果感應(yīng)電動(dòng)勢(shì)Eo等于電源電動(dòng)勢(shì)Es那么回來電壓(Es-Eo)等于0，所以電樞電流也為0。驅(qū)動(dòng)力將不會(huì)作用于電樞繞組，軸承等機(jī)械阻力將會(huì)使電機(jī)轉(zhuǎn)速減慢。隨著速度的降低，回路電壓(Es-Eo)增加,那么電流也增加。當(dāng)電樞電流產(chǎn)生的轉(zhuǎn)矩增加到等于負(fù)載轉(zhuǎn)矩時(shí)，電機(jī)的轉(zhuǎn)速將不再減慢。因此，當(dāng)電機(jī)在空載運(yùn)行時(shí)，感應(yīng)電動(dòng)勢(shì)必須比電源電動(dòng)勢(shì)小，使有一個(gè)小電流足以產(chǎn)生所需的轉(zhuǎn)矩。機(jī)械動(dòng)力和力矩電源和直流電動(dòng)機(jī)的力矩是其最重要的兩個(gè)屬性?，F(xiàn)在讓我們得出兩個(gè)簡(jiǎn)單的公式來計(jì)算它們。一個(gè)線圈的感應(yīng)電動(dòng)勢(shì)由式5.1給出：Eo=ZnF/60參考圖5.2，提