直流电机控制系统英文翻译.docx

直流电机控制系统英文翻译.docx

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直流电机控制系统英文翻译

毕业设计（论文）

外文翻译

题目：

直流电机控制系统

专业：

电子信息工程

班级：

08级

学生：

辛国鹏

********

西安理工大学高科学院

2012年

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英文全文

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.approxi