直流电机毕业论文中英文资料对照外文翻译文献.docx

直流电机毕业论文中英文资料对照外文翻译文献.docx

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IntroductiontoD.C.Machines

D.C.machinesarecharacterizedbytheir versatility.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）

is aconstantfixedbythedesignofthewinding.

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. of the fieldwindings. 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）

Thereisthemagnetizationcurvewithonlyone