DC Motor直流电机中英文翻译.docx
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DCMotor直流电机中英文翻译
英文原文+中文翻译
(原文:
)IntroductiontoD.C.Machines
D.C.machinesarecharacterizedbytheirversatility.Bymeansofvariouscombinationsofshunt-,series-,andseparatelyexcitedfieldwindingstheycanbedesignedtodisplayawidevarietyofvolt-ampereorspeed-torquecharacteristicsforbothdynamicandsteadystateoperation.Becauseoftheeasewithwhichtheycanbecontrolled,systemsofD.C.machinesareoftenusedinapplicationsrequiringawiderangeofmotorspeedsorprecisecontrolofmotoroutput.
TheessentialfeaturesofaD.C.machineareshownschematically.Thestatorhassalientpolesandisexcitedbyoneormorefieldcoils.Theair-gapfluxdistributioncreatedbythefieldwindingissymmetricalaboutthecenterlineofthefieldpoles.Thisiscalledthefieldaxisordirectaxis.
Asweknow,theA.C.voltagegeneratedineachrotatingarmaturecoilisconvertedtoD.C.intheexternalarmatureterminalsbymeansofarotatingcommutatorandstationarybrushestowhichthearmatureleadsareconnected.Thecommutator-brushcombinationformsamechanicalrectifier,resultinginaD.C.armaturevoltageaswellasanarmaturem.m.f.Wavethenis90electricaldegreesfromtheaxisofthefieldpoles,i.e.inthequadratureaxis.Intheschematicrepresentationthebrushesareshowninquadratureaxisbecausethisisthepositionofthecoilstowhichtheyareconnected.Thearmaturem.m.f.Wavethenisalongthebrushaxisasshown.(Thegeometricalpositionofthebrushesinanactualmachineisapproximately90electricaldegreesfromtheirpositionintheschematicdiagrambecauseoftheshapeoftheendconnectionstothecommutator.)
Themagnetictorqueandthespeedvoltageappearingatthebrushesareindependentofthespatialwaveformofthefluxdistribution;forconvenienceweshallcontinuetoassumeasinusoidalflux-densitywaveintheairgap.Thetorquecanthenbefoundfromthemagneticfieldviewpoint.
Thetorquecanbeexpressedintermsoftheinteractionofthedirect-axisair-gapfluxperpole
andspace-fundamentalcomponent
ofthearmaturem.m.f.wave.Withthebrushesinthequadratureaxistheanglebetweenthesefieldsis90electricaldegrees,anditssineequalsunity.Fora
polemachine
(1-1)
Inwhichtheminussigngasbeendroppedbecausethepositivedirectionofthetorquecanbedeterminedfromphysicalreasoning.Thespacefundamental
ofthesawtootharmaturem.m.f.waveis
timesitspeak.Substitutioninaboveequationthengives
(1-2)
Where,
=currentinexternalarmaturecircuit;
=totalnumberofconductorsinarmaturewinding;
=numberofparallelpathsthroughwinding.
And
(1-3)
isaconstantfixedbythedesignofthewinding.
Therectifiedvoltagegeneratedinthearmaturehasalreadybeendiscussedbeforeforanelementarysingle-coilarmature.Theeffectofdistributingthewindinginseveralslotsisshowninfigure.Inwhicheachoftherectifiedsinewaveisthevoltagegeneratedinoneofthecoils,commutationtakingplaceatthemomentwhenthecoilsidesareintheneutralzone.Thegeneratedvoltageasobservedfromthebrushesandisthesumoftherectifiedvoltagesofallthecoilsinseriesbetweenbrushesandisshownbytheripplinglinelabeled
infigure.Withadozenorsocommutatorsegmentsperpole,theripplebecomesverysmallandtheaveragegeneratedvoltageobservedfromthebrushesequalsthesumoftheaveragevaluesoftherectifiedcoilvoltages.Therectifiedvoltage
betweenbrushes,Knownalsoasthespeedvoltage,is
(1-4)
where
isthedesignconstant.Therectifiedvoltageofadistributedwindinghasthesameaveragevalueasthatofaconcentratedcoil.Thedifferenceisthattherippleisgreatlyreduced.
Fromtheaboveequations,withallvariableexpressedinSIunits,
(1-5)
Thisequationsimplysaysthattheinstantaneouspowerassociatedwiththespeedvoltageequalstheinstantaneousmechanicalpowerwiththemagnetictorque.Thedirectionofpowerflowbeingdeterminedbywhetherthemachineisactingasamotororgenerator.
Thedirect-axisair-gapfluxisproducedbythecombinedm.m.f.
ofthefieldwindings.Theflux-m.m.f.Characteristicbeingthemagnetizationcurvefortheparticularirongeometryofthemachine.Inthemagnetizationcurve,itisassumedthatthearmature–m.m.f.Waveisperpendiculartothefieldaxis.Itwillbenecessarytoreexaminethisassumptionlaterinthischapter,wheretheeffectsofsaturationareinvestigatedmorethoroughly.Becausethearmaturee.m.f.isproportionaltofluxtimesspeed,itisusuallymoreconvenienttoexpressthemagnetizationcurveintermsofthearmaturee.m.f.
ataconstantspeed
.Thevoltage
foragivenfluxatanyotherspeed
isproportionaltothespeed,i.e.
(1-6)
Thereisthemagnetizationcurvewithonlyonefieldwindingexcited.Thiscurvecaneasilybeobtainedbytestmethods,noknowledgeofanydesigndetailsbeingrequired.
Overafairlywiderangeofexcitationthereluctanceoftheironisnegligiblecomparedwiththatoftheairgap.Inthisregionthefluxislinearlyproportionaltothetotalm.m.f.ofthefieldwindings,theconstantofproportionalitybeingthedirect-axisair-gappermeance.
TheoutstandingadvantagesofD.C.machinesarisefromthewidevarietyofoperatingcharacteristicsthatcanbeobtainedbyselectionofthemethodofexcitationofthefieldwindings.ThefieldwindingsmaybeseparatelyexcitedfromanexternalD.C.source,ortheymaybeself-excited;i.e.themachinemaysupplyitsownexcitation.Themethodofexcitationprofoundlyinfluencesnotonlythesteady-statecharacteristics,butalsothedynamicbehaviorofthemachineincontrolsystems.
Theconnectiondiagramofaseparatelyexcitedgeneratorisgiven.Therequiredfieldcurrentisaverysmallfractionoftheratedarmaturecurrent.Asmallamountofpowerinthefieldcircuitmaycontrolarelativelylargeamountofpowerinthearmaturecircuit;i.e.thegeneratorisapoweramplifier.Separatelyexcitedgeneratorsareoftenusedinfeedbackcontrolsystemswhencontrolofthearmaturevoltageoverawiderangeisrequired.Thefieldwindingsofself-excitedgeneratorsmaybesuppliedinthreedifferentways.Thefieldmaybeconnectedinserieswiththearmature,resultinginaseriesgenerator.Thefieldmaybeconnectedinshuntwiththearmature,resultinginashuntgenerator,orthefieldmaybeintwosections,oneofwhichisconnectedinseriesandtheotherinshuntwiththearmature,resultinginacompoundgenerator.Withself-excitedgeneratorsresidualmagnetismmustbepresentinthemachineirontogettheself-excitationprocessstarted.
Inthetypicalsteady-statevolt-amperecharacteristics,constant-speedprimemoversbeingassumed.Therelationbetweenthesteadystategeneratede.m.f.
andtheterminalvoltage
is
(1-7)
where
isthearmaturecurrentoutputand
isthearmaturecircuitresistance.Inagenerator,
islargerthan
andtheelectromagnetictorque
isacountertorqueopposingrotation.
Theterminalvoltageofaseparatelyexcitedgeneratordecreasesslightlywithincreaseintheloadcurrent,principallybecauseofthevoltagedropinthearmatureresistance.Thefieldcurrentofaseriesgeneratoristhesameastheloadcurrent,sothattheair-gapfluxandhencethevoltagevarywidelywithload.Asaconsequence,seriesgeneratorsarenormallyconnectedsothatthem.m.f.oftheserieswindingaidsthatoftheshuntwinding.Theadvantageisthatthroughtheactionoftheserieswindingthefluxperpolecanincreasewithload,resultinginavoltageoutputthatisnearlyusuallycontainsmanyturnsofrelativelysmallwire.Theserieswinding,woundontheoutside,consistsofafewturnsofcomparativelyheavyconductorbecauseitmustcarrythefullarmaturecurrentofthemachine.Thevoltageofbothshuntandcompoundgeneratorscanbecontrolledoverreasonablelimitsbymeansofrheostatsintheshuntfield.
Anyofthemethodsofexcitationusedforgeneratorscanalsobeusedformotors.Inthetypicalsteady-statespeed-torquecharacteristics,itisassumedthatmotorterminalsaresuppliedfromaconstant-voltagesource.Inamotortherelationbetweenthee.m.f.
generatedinthearmatureandterminalvoltage
is
(1-8)
where
isnowthearmaturecurrentinput.Thegeneratede.m.f.
isnowsmallerthantheterminalvoltage
thearmaturecurrentisintheoppositedirectiontothatinagenerator,andtheelectronmagnetictorqueisinthedirectiontosustainrotationofthearmature.
Inshuntandseparatelyexcitedmotorsthefieldfluxisnearlyconstant.Consequentlyincreasedtorquemustbeaccompaniedbyaverynearlyproportionalincreaseinarmaturecurrentandhencebyasmalldecreaseincountere.m.f.toallowthisincreasedcurrentthroughthesmallarmatureresistance.Sincecountere.m.f.isdeterminedbyfluxandspeed,thespeedmustdropslightly.Likethesquirrel-cageinductionmotor,theshuntmotorissubstantiallyaconstant-speedmotorhavingabout5%dropinspeedfromnoloadtofullload.Startingtorqueandmaximumtorquearelimitedbythearmaturecurrentthatcanbecommutatedsuccessfully.
Anoutstandingadvantageoftheshuntmotoriscaseofspeedcontrol.Witharheostatintheshunt-fieldcircuit,thefieldcurrentandfluxperpolecanbevariedatwill,andvariationoffluxcausestheinversevariationofspeedtomaintaincountere.m.f.approximatelyequaltotheimpressedterminalvoltage.Amaximumspeedrangeofabout4or5toIcanbeobtainedbythismethod.Thelimitationagainbeingcommutatingconditions.Byvariationoftheimpressedarmaturevoltage,veryspeedrangescanbeobtained.