电气毕业设计用外文翻译(中英文对照)

'TheTransformeronload﹠IntroductiontoDCMachinesTheTransformeronloadIthasbeenshownthataprimaryinputvoltagecanbetransformedtoanydesiredopen-circuitsecondaryvoltagebyasuitablechoiceofturnsratio.isavailableforcirculatingaloadcurrentimpedance.Forthemoment,alaggingpowerfactorwillbeconsidered.Thesecondarycurrentandtheresultingampere-turnswillchangetheflux,tendingtodemagnetizethecore,reduceandwithit.Becausetheprimaryleakageimpedancedropissolow,asmallalterationtowillcauseanappreciableincreaseofprimarycurrentfromtoanewvalueofequalto.Theextraprimarycurrentandampere-turnsnearlycancelthewholeofthesecondaryampere-turns.Thisbeingso,themutualfluxsuffersonlyaslightmodificationandrequirespracticallythesamenetampere-turnsasonnoload.Thetotalprimaryampere-turnsareincreasedbyanamountnecessarytoneutralizethesameamountofsecondaryampere-turns.Inthevectorequation,;alternatively,.Atfullload,thecurrentisonlyabout5%ofthefull-loadcurrentandsoisnearlyequalto.Becauseinmindthat,theinputkVAwhichisapproximatelyisalsoapproximatelyequaltotheoutputkVA,.Thephysicalcurrenthasincreased,andwithintheprimaryleakagefluxtowhichitisproportional.Thetotalfluxlinkingtheprimary,,isshownunchangedbecausethetotalbacke.m.f.,（）isstillequalandoppositeto.However,therehasbeenaredistributionoffluxandthemutualcomponenthasfallenduetotheincreaseofwith.Althoughthechangeissmall,thesecondarydemandcouldnotbemetwithoutamutualfluxande.m.f.alterationtopermitprimarycurrenttochange.Thenetfluxlinkingthesecondarywindinghasbeenfurtherreducedbytheestablishmentofsecondaryleakagefluxdueto,andthisopposes.Althoughand areindicatedseparately,theycombinetooneresultantinthecorewhichwillbedownwardsattheinstantshown.Thusthesecondaryterminalvoltageisreducedtowhichcanbeconsideredintwocomponents,i.e.orvectorially.Asfortheprimary,isresponsibleforasubstantiallyconstantsecondaryleakageinductance.Itwillbenoticedthattheprimaryleakagefluxisresponsibleforpartofthechangeinthesecondaryterminalvoltageduetoitseffectsonthemutualflux.Thetwoleakagefluxesarecloselyrelated;,forexample,byitsdemagnetizingactiononhascausedthechangesontheprimarysidewhichledtotheestablishmentofprimaryleakageflux.Ifalowenoughleadingpowerfactorisconsidered,thetotalsecondaryfluxandthemutualfluxareincreasedcausingthesecondaryterminalvoltagetorisewithload.isunchangedinmagnitudefromthenoloadconditionsince,neglectingresistance,itstillhastoprovideatotalbacke.m.f.equalto.Itisvirtuallythesameas,thoughnowproducedbythecombinedeffectofprimaryandsecondaryampere-turns.Themutualfluxmuststillchangewithloadtogiveachangeofandpermitmoreprimarycurrenttoflow.hasincreasedthistimebutduetothevectorcombinationwiththereisstillanincreaseofprimarycurrent.Twomorepointsshouldbemadeaboutthefigures.Firstly,aunityturnsratiohasbeenassumedforconveniencesothat.Secondly,thephysicalpictureisdrawnforadifferentinstantoftimefromthevectordiagramswhichshow,ifthehorizontalaxisistakenasusual,tobethezerotimereference.Thereareinstantsinthecyclewhenprimaryleakagefluxiszero,whenthesecondaryleakagefluxiszero,andwhenprimaryandsecondaryleakagefluxiszero,andwhenprimaryandsecondaryleakagefluxesareinthesamesense.Theequivalentcircuitalreadyderivedforthetransformerwiththesecondaryterminalsopen,caneasilybeextendedtocovertheloadedsecondarybytheadditionofthesecondaryresistanceandleakagereactance.Practicallyalltransformershaveaturnsratiodifferentfromunityalthoughsuchanarrangementis sometimesemployedforthepurposesofelectricallyisolatingonecircuitfromanotheroperatingatthesamevoltage.Toexplainthecasewherethereactionofthesecondarywillbeviewedfromtheprimarywinding.Thereactionisexperiencedonlyintermsofthemagnetizingforceduetothesecondaryampere-turns.Thereisnowayofdetectingfromtheprimarysidewhetherislargeandsmallorviceversa,itistheproductofcurrentandturnswhichcausesthereaction.Consequently,asecondarywindingcanbereplacedbyanynumberofdifferentequivalentwindingsandloadcircuitswhichwillgiverisetoanidenticalreactionontheprimary.Itisclearlyconvenienttochangethesecondarywindingtoanequivalentwindinghavingthesamenumberofturnsastheprimary.Withchangesto,sincethee.m.f.sareproportionaltoturns,whichisthesameas.Forcurrent,sincethereactionampereturnsmustbeunchangedmustbeequalto.i.e..Forimpedance,sinceanysecondaryvoltagebecomes,andsecondarycurrentbecomes,thenanysecondaryimpedance,includingloadimpedance,mustbecome.Consequently,and.Iftheprimaryturnsaretakenasreferenceturns,theprocessiscalledreferringtotheprimaryside.Thereareafewcheckswhichcanbemadetoseeiftheprocedureoutlinedisvalid.Forexample,thecopperlossinthereferredsecondarywindingmustbethesameasintheoriginalsecondaryotherwisetheprimarywouldhavetosupplyadifferentlosspower.mustbeequalto.doesinfactreduceto.Similarlythestoredmagneticenergyintheleakagefieldwhichisproportionaltowillbefoundtocheckas.Thereferredsecondary.Theargumentissound,thoughatfirstitmayhaveseemedsuspect.Infact,iftheactualsecondary windingwasremovedphysicallyfromthecoreandreplacedbytheequivalentwindingandloadcircuitdesignedtogivetheparameters,,and,measurementsfromtheprimaryterminalswouldbeunabletodetectanydifferenceinsecondaryampere-turns,demandorcopperloss,undernormalpowerfrequencyoperation.Thereisnopointinchoosinganybasisotherthanequalturnsonprimaryandreferredsecondary,butitissometimesconvenienttorefertheprimarytothesecondarywinding.Inthiscase,ifallthesubscript1’sareinterchangedforthesubscript2’s,thenecessaryreferringconstantsareeasilyfound;e.g.,;similarlyand.Theequivalentcircuitforthegeneralcasewhereexceptthathasbeenaddedtoallowforironlossandanideallosslesstransformationhasbeenincludedbeforethesecondaryterminalstoreturnto.Allcalculationsofinternalvoltageandpowerlossesaremadebeforethisidealtransformationisapplied.Thebehaviourofatransformerasdetectedatbothsetsofterminalsisthesameasthebehaviourdetectedatthecorrespondingterminalsofthiscircuitwhentheappropriateparametersareinserted.Theslightlydifferentrepresentationshowingthecoilsandsidebysidewithacoreinbetweenisonlyusedforconvenience.Onthetransformeritself,thecoilsare,ofcourse,woundroundthesamecore.Verylittleerrorisintroducedifthemagnetisingbranchistransferredtotheprimaryterminals,butafewanomalieswillarise.Forexample,thecurrentshownflowingthroughtheprimaryimpedanceisnolongerthewholeoftheprimarycurrent.Theerrorisquitesmallsinceisusuallysuchasmallfractionof.Slightlydifferentanswersmaybeobtainedtoaparticularproblemdependingonwhetherornotallowanceismadeforthiserror.Withthissimplifiedcircuit,theprimaryandreferredsecondaryimpedancescanbeaddedtogive:andItshouldbepointedoutthattheequivalentcircuitasderivedhereisonlyvalidfornormaloperationatpowerfrequencies;capacitanceeffectsmustbetakenintoaccountwhenevertherateofchangeofvoltagewouldgiverisetoappreciablecapacitancecurrents,.Theyareimportantathighvoltagesandatfrequenciesmuchbeyond100cycles/sec.Afurtherpointisnotthe onlypossibleequivalentcircuitevenforpowerfrequencies.Analternative,treatingthetransformerasathree-orfour-terminalnetwork,givesrisetoarepresentationwhichisjustasaccurateandhassomeadvantagesforthecircuitengineerwhotreatsalldevicesascircuitelementswithcertaintransferproperties.Thecircuitonthisbasiswouldhaveaturnsratiohavingaphaseshiftaswellasamagnitudechange,andtheimpedanceswouldnotbethesameasthoseofthewindings.Thecircuitwouldnotexplainthephenomenawithinthedeviceliketheeffectsofsaturation,soforanunderstandingofinternalbehaviour.Therearetwowaysoflookingattheequivalentcircuit:(a)viewedfromtheprimaryasasinkbutthereferredloadimpedanceconnectedacross,or(b)viewedfromthesecondaryasasourceofconstantvoltagewithinternaldropsduetoand.Themagnetizingbranchissometimesomittedinthisrepresentationandsothecircuitreducestoageneratorproducingaconstantvoltage(actuallyequalto)andhavinganinternalimpedance(actuallyequalto).Ineithercase,theparameterscouldbereferredtothesecondarywindingandthismaysavecalculationtime.Theresistancesandreactancescanbeobtainedfromtwosimplelightloadtests.IntroductiontoDCMachinesDCmachinesarecharacterizedbytheirversatility.Bymeansofvariouscombinationofshunt,series,andseparatelyexcitedfieldwindingstheycanbedesignedtodisplayawidevarietyofvolt-ampereorspeed-torquecharacteristicsforbothdynamicandsteadystateoperation.Becauseoftheeasewithwhichtheycanbecontrolled,systemsofDCmachinesareoftenusedinapplicationsrequiringawiderangeofmotorspeedsorprecisecontrolofmotoroutput.TheessentialfeaturesofaDCmachineareshownschematically.Thestatorhassalientpolesandisexcitedbyoneormorefieldcoils.Theair-gapfluxdistributioncreatedbythefieldwindingissymmetricalaboutthecenterlineofthefieldpoles.Thisaxisiscalledthefieldaxisordirectaxis.Asweknow,theACvoltagegeneratedineachrotatingarmaturecoilisconvertedtoDCintheexternalarmatureterminalsbymeansofarotatingcommutatorandstationarybrushestowhichthearmatureleadsareconnected.Thecommutator-brushcombinationformsamechanicalrectifier, resultinginaDCarmaturevoltageaswellasanarmaturem.m.f.wavewhichisfixedinspace.Thebrushesarelocatedsothatcommutationoccurswhenthecoilsidesareintheneutralzone,midwaybetweenthefieldpoles.Theaxisofthearmaturem.m.f.wavethenin90electricaldegreesfromtheaxisofthefieldpoles,i.e.,inthequadratureaxis.Intheschematicrepresentationthebrushesareshowninquaratureaxisbecausethisisthepositionofthecoilstowhichtheyareconnected.Thearmaturem.m.f.wavethenisalongthebrushaxisasshown..(Thegeometricalpositionofthebrushesinanactualmachineisapproximately90electricaldegreesfromtheirpositionintheschematicdiagrambecauseoftheshapeoftheendconnectionstothecommutator.)Themagnetictorqueandthespeedvoltageappearingatthebrushesareindependentofthespatialwaveformofthefluxdistribution;forconvenienceweshallcontinuetoassumeasinusoidalflux-densitywaveintheairgap.Thetorquecanthenbefoundfromthemagneticfieldviewpoint.Thetorquecanbeexpressedintermsoftheinteractionofthedirect-axisair-gapfluxperpoleandthespace-fundamentalcomponentofthearmaturem.m.f.wave.Withthebrushesinthequadratureaxis,theanglebetweenthesefieldsis90electricaldegrees,anditssineequalsunity.ForaPpolemachineInwhichtheminussignhasbeendroppedbecausethepositivedirectionofthetorquecanbedeterminedfromphysicalreasoning.Thespacefundamentalofthesawtootharmaturem.m.f.waveis8/timesitspeak.SubstitutioninaboveequationthengivesWhere=currentinexternalarmaturecircuit;=totalnumberofconductorsinarmaturewinding;=numberofparallelpathsthroughwinding;And Isaconstantfixedbythedesignofthewinding.Therectifiedvoltagegeneratedinthearmaturehasalreadybeendiscussedbeforeforanelementarysingle-coilarmature.Theeffectofdistributingthewindinginseveralslotsisshowninfigure,inwhicheachoftherectifiedsinewavesisthevoltagegeneratedinoneofthecoils,commutationtakingplaceatthemomentwhenthecoilsidesareintheneutralzone.Thegeneratedvoltageasobservedfromthebrushesisthesumoftherectifiedvoltagesofallthecoilsinseriesbetweenbrushesandisshownbytheripplinglinelabeledinfigure.Withadozenorsocommutatorsegmentsperpole,theripplebecomesverysmallandtheaveragegeneratedvoltageobservedfromthebrushesequalsthesumoftheaveragevaluesoftherectifiedcoilvoltages.Therectifiedvoltagebetweenbrushes,knownalsoasthespeedvoltage,isWhereisthedesignconstant.Therectifiedvoltageofadistributedwindinghasthesameaveragevalueasthatofaconcentratedcoil.Thedifferenceisthattherippleisgreatlyreduced.Fromtheaboveequations,withallvariableexpressedinSIunits:Thisequationsimplysaysthattheinstantaneouselectricpowerassociatedwiththespeedvoltageequalstheinstantaneousmechanicalpowerassociatedwiththemagnetictorque,thedirectionofpowerflowbeingdeterminedbywhetherthemachineisactingasamotororgenerator.Thedirect-axisair-gapfluxisproducedbythecombinedm.m.f.ofthefieldwindings,theflux-m.m.f.characteristicbeingthemagnetizationcurvefortheparticularirongeometryofthemachine.Inthemagnetizationcurve,itisassumedthatthearmaturem.m.f.waveisperpendiculartothefieldaxis.Itwillbenecessarytoreexaminethisassumptionlaterinthischapter,wheretheeffectsofsaturationareinvestigatedmorethoroughly.Becausethearmaturee.m.f.isproportionaltofluxtimes speed,itisusuallymoreconvenienttoexpressthemagnetizationcurveintermsofthearmaturee.m.f.ataconstantspeed.Thevoltageforagivenfluxatanyotherspeedisproportionaltothespeed,i.e.Figureshowsthemagnetizationcurvewithonlyonefieldwindingexcited.Thiscurvecaneasilybeobtainedbytestmethods,noknowledgeofanydesigndetailsbeingrequired.Overafairlywiderangeofexcitationthereluctanceoftheironisnegligiblecomparedwiththatoftheairgap.Inthisregionthefluxislinearlyproportionaltothetotalm.m.f.ofthefieldwindings,theconstantofproportionalitybeingthedirect-axisair-gappermeance.TheoutstandingadvantagesofDCmachinesarisefromthewidevarietyofoperatingcharacteristicswhichcanbeobtainedbyselectionofthemethodofexcitationofthefieldwindings.ThefieldwindingsmaybeseparatelyexcitedfromanexternalDCsource,ortheymaybeself-excited;i.e.,themachinemaysupplyitsownexcitation.Themethodofexcitationprofoundlyinfluencesnotonlythesteady-statecharacteristics,butalsothedynamicbehaviorofthemachineincontrolsystems.Theconnectiondiagramofaseparatelyexcitedgeneratorisgiven.Therequiredfieldcurrentisaverysmallfractionoftheratedarmaturecurrent.Asmallamountofpowerinthefieldcircuitmaycontrolarelativelylargeamountofpowerinthearmaturecircuit;i.e.,thegeneratorisapoweramplifier.Separatelyexcitedgeneratorsareoftenusedinfeedbackcontrolsystemswhencontrolofthearmaturevoltageoverawiderangeisrequired.Thefieldwindingsofself-excitedgeneratorsmaybesuppliedinthreedifferentways.Thefieldmaybeconnectedinserieswiththearmature,resultinginashuntgenerator,orthefieldmaybeintwosections,oneofwhichisconnectedinseriesandtheotherinshuntwiththearmature,resultinginacompoundgenerator.Withself-excitedgeneratorsresidualmagnetismmustbepresentinthemachineirontogettheself-excitationprocessstarted.Inthetypicalsteady-statevolt-amperecharacteristics,constant-speedprime moversbeingassumed.Therelationbetweenthesteady-stategeneratede.m.f.andtheterminalvoltageisWhereisthearmaturecurrentoutputandisthearmaturecircuitresistance.Inagenerator,islargethan;andtheelectromagnetictorqueTisacountertorqueopposingrotation.Theterminalvoltageofaseparatelyexcitedgeneratordecreasesslightlywithincreaseintheloadcurrent,principallybecauseofthevoltagedropinthearmatureresistance.Thefieldcurrentofaseriesgeneratoristhesameastheloadcurrent,sothattheair-gapfluxandhencethevoltagevarywidelywithload.Asaconsequence,seriesgeneratorsarenotoftenused.Thevoltageofshuntgeneratorsdropsoffsomewhatwithload.Compoundgeneratorsarenormallyconnectedsothatthem.m.f.oftheserieswindingaidsthatoftheshuntwinding.Theadvantageisthatthroughtheactionoftheserieswindingthefluxperpolecanincreasewithload,resultinginavoltageoutputwhichisnearlyconstant.Usually,shuntwindingcontainsmanyturnsofcomparativelyheavyconductorbecauseitmustcarrythefullarmaturecurrentofthemachine.Thevoltageofbothshuntandcompoundgeneratorscanbecontrolledoverreasonablelimitsbymeansofrheostatsintheshuntfield.Anyofthemethodsofexcitationusedforgeneratorscanalsobeusedformotors.Inthetypicalsteady-statespeed-torquecharacteristics,itisassumedthatthemotorterminalsaresuppliedfromaconstant-voltagesource.Inamotortherelationbetweenthee.m.f.generatedinthearmatureandtheterminalvoltageisWhereisnowthearmaturecurrentinput.Thegeneratede.m.f.isnowsmallerthantheterminalvoltage,thearmaturecurrentisintheoppositedirectiontothatinamotor,andtheelectromagnetictorqueisinthedirectiontosustainrotationof thearmature.Inshuntandseparatelyexcitedmotorsthefieldfluxisnearlyconstant.Consequently,increasedtorquemustbeaccompaniedbyaverynearlyproportionalincreaseinarmaturecurrentandhencebyasmalldecreaseincountere.m.f.toallowthisincreasedcurrentthroughthesmallarmatureresistance.Sincecountere.m.f.isdeterminedbyfluxandspeed,thespeedmustdropslightly.Likethesquirrel-cageinductionmotor,theshuntmotorissubstantiallyaconstant-speedmotorhavingabout5percentdropinspeedfromnoloadtofullload.Startingtorqueandmaximumtorquearelimitedbythearmaturecurrentthatcanbecommutatedsuccessfully.Anoutstandingadvantageoftheshuntmotoriseaseofspeedcontrol.Witharheostatintheshunt-fieldcircuit,thefieldcurrentandfluxperpolecanbevariedatwill,andvariationoffluxcausestheinversevariationofspeedtomaintaincountere.m.f.approximatelyequaltotheimpressedterminalvoltage.Amaximumspeedrangeofabout4or5to1canbeobtainedbythismethod,thelimitationagainbeingcommutatingconditions.Byvariationoftheimpressedarmaturevoltage,verywidespeedrangescanbeobtained.Intheseriesmotor,increaseinloadisaccompaniedbyincreaseinthearmaturecurrentandm.m.f.andthestatorfieldflux(providedtheironisnotcompletelysaturated).Becausefluxincreaseswithload,speedmustdropinordertomaintainthebalancebetweenimpressedvoltageandcountere.m.f.;moreover,theincreaseinarmaturecurrentcausedbyincreasedtorqueissmallerthanintheshuntmotorbecauseoftheincreasedflux.Theseriesmotoristhereforeavarying-speedmotorwithamarkedlydroopingspeed-loadcharacteristic.Forapplicationsrequiringheavytorqueoverloads,thischaracteristicisparticularlyadvantageousbecausethecorrespondingpoweroverloadsareheldtomorereasonablevaluesbytheassociatedspeeddrops.Veryfavorablestartingcharacteristicsalsoresultfromtheincreaseinfluxwithincreasedarmaturecurrent.Inthecompoundmotortheseriesfieldmaybeconnectedeithercumulatively,sothatits.m.m.f.addstothatoftheshuntfield,ordifferentially,sothatitopposes.Thedifferentialconnectionisveryrarelyused.Acumulativelycompoundedmotorhas speed-loadcharacteristicintermediatebetweenthoseofashuntandaseriesmotor,thedropofspeedwithloaddependingontherelativenumberofampere-turnsintheshuntandseriesfields.Itdoesnothavethedisadvantageofveryhighlight-loadspeedassociatedwithaseriesmotor,butitretainstoaconsiderabledegreetheadvantagesofseriesexcitation.TheapplicationadvantagesofDCmachineslieinthevarietyofperformancecharacteristicsofferedbythepossibilitiesofshunt,series,andcompoundexcitation.Someofthesecharacteristicshavebeentoucheduponbrieflyinthisarticle.Stillgreaterpossibilitiesexistifadditionalsetsofbrushesareaddedsothatothervoltagescanbeobtainedfromthecommutator.ThustheversatilityofDCmachinesystemsandtheiradaptabilitytocontrol,bothmanualandautomatic,aretheiroutstandingfeatures. 负载运行的变压器及直流电机导论负载运行的变压器通过选择合适的匝数比，一次侧输入电压可任意转换成所希望的二次侧开路电压。可用于产生负载电流，该电流的幅值和功率因数将由而次侧电路的阻抗决定。现在，我们要讨论一种滞后功率因数。二次侧电流及其总安匝将影响磁通，有一种对铁芯产生去磁、减小和的趋向。因为一次侧漏阻抗压降如此之小，所以的微小变化都将导致一次侧电流增加很大，从增大至一个新值。增加的一次侧电流和磁势近似平衡了全部二次侧磁势。这样的话，互感磁通只经历很小的变化，并且实际上只需要与空载时相同的净磁势。一次侧总磁势增加了，它是平衡同量的二次侧磁势所必需的。在向量方程中，，上式也可变换成。满载时，电流只约占满载电流的5%，因而近似等于。记住，近似等于的输入容量也就近似等于输出容量。一次侧电流已增大，随之与之成正比的一次侧漏磁通也增大。交链一次绕组的总磁通没有变化，这是因为总反电动势仍然与相等且反向。然而此时却存在磁通的重新分配，由于随的增加而增加，互感磁通分量已经减小。尽管变化很小，但是如果没有互感磁通和电动势的变化来允许一次侧电流变化，那么二次侧的需求就无法满足。交链二次绕组的净磁通由于产生的二次侧漏磁通（其与反相）的建立而被进一步削弱。尽管图中和是分开表示的，但它们在铁芯中是一个合成量，该合成量在图示中的瞬时是向下的。这样，二次侧端电压降至，它可被看成两个分量，即，或者向量形式。与一次侧漏磁通一样，的作用也用一个大体为常数的漏电感来表征。要注意的是，由于它对互感磁通的作用，一次侧漏磁通对于二次侧端电压的变化产生部分影响。这两种漏磁通，紧密相关；例如，对的去磁作用引起了一次侧的变化，从而导致了一次侧漏磁通的产生。 如果我们讨论一个足够低的超前功率因数，二次侧总磁通和互感磁通都会增加，从而使得二次侧端电压随负载增加而升高。在空载情形下，如果忽略电阻，幅值大小不变，因为它仍提供一个等于的反总电动势。尽管现在是一次侧和二次侧磁势的共同作用产生的，但它实际上与相同。互感磁通必须仍随负载变化而变化以改变，从而产生更大的一次侧电流。此时的幅值已经增大，但由于与是向量合成，因此一次侧电流仍然是增大的。从上述图中，还应得出两点：首先，为方便起见已假设匝数比为1，这样可使。其次，如果横轴像通常取的话，那么向量图是以为零时间参数的，图中各物理量时间方向并不是该瞬时的。在周期性交变中，有一次侧漏磁通为零的瞬时，也有二次侧漏磁通为零的瞬时，还有它们处于同一方向的瞬时。已经推出的变压器二次侧绕组端开路的等效电路，通过加上二次侧电阻和漏抗便可很容易扩展成二次侧负载时的等效电路。实际中所有的变压器的匝数比都不等于1，尽管有时使其为1也是为了使一个电路与另一个在相同电压下运行的电路实现电气隔离。为了分析时的情况，二次侧的反应得从一次侧来看，这种反应只有通过由二次侧的磁势产生磁场力来反应。我们从一次侧无法判断是大，小，还是小，大，正是电流和匝数的乘积在产生作用。因此，二次侧绕组可用任意个在一次侧产生相同匝数的等效绕组是方便的。当变换成，由于电动势与匝数成正比，所以，与相等。对于电流，由于对一次侧作用的安匝数必须保持不变，因此，即。对于阻抗，由于二次侧电压变成，电流变为，因此阻抗值，包括负载阻抗必然变为。因此，，。如果将一次侧匝数作为参考匝数，那么这种过程称为往一次侧的折算。我们可以用一些方法来验证上述折算过程是否正确。 例如，折算后的二次绕组的铜耗必须与原二次绕组铜耗相等，否则一次侧提供给其损耗的功率就变了。必须等于，而事实上确实简化成了。类似地，与成比例的漏磁场的磁场储能，求出后验证与成正比。折算后的二次侧。尽管看起来似乎不可理解，事实上这种论点是可靠的。实际上，如果我们将实际的二次绕组当真从铁芯上移开，并用一个参数设计成，，，的等效绕组和负载电路替换，在正常电网频率运行时，从一次侧两端无法判断二次侧的磁势、所需容量及铜耗与前有何差别。在选择折算基准时，无非是将一次侧与折算后的二次侧匝数设为相等，除此之外再没有什么更要紧的了。但有时将一次侧折算到二次侧倒是方便的，在这种情况下，如果所有下标“1”的量都变换成了下标“2”的量，那么很容易得到必需的折算系数，例如。值得注意的是，对于一台实际的变压器，，；同样地，。的通常情形时的等效电路，它除了为了考虑铁耗而引入了，且为了将折算回而在二次侧两端引入了一理想的无损耗转换外，其他方面是一样的。在运用这种理想转换之前，内部电压和功率损耗已进行了计算。当在电路中选择了适当的参数时，在一、二次侧两端测得的变压器运行情况与在该电路相应端所测得的请况是完全一致的。将线圈和线圈并排放置在一个铁芯的两边，这一点与实际情况之间的差别仅仅是为了方便。当然，就变压器本身来说，两线圈是绕在同一铁芯柱上的。如果将激磁支路移至一次绕组端口，引起的误差很小，但一些不合理的现象又会发生。例如，流过一次侧阻抗的电流不再是整个一次侧电流。由于通常只是的很小一部分，所有误差相当小。对一个具体问题可否允许有细微差别的回答取决于是否允许这种误差的存在。对于这种简化电路，一次侧和折算后二次侧阻抗可相加，得和需要指出的是，在此得到的等效电路仅仅适用于电网频率下的正常运行；一旦电压变化率产生相当大的电容电流 时必须考虑电容效应。这对于高电压和频率超过100Hz的情形是很重要的。其次，即使是对于电网频率也并非唯一可行的等效电路。另一种形式是将变压器看成一个三端或四端网络，这样便产生一个准确的表达，它对于那些把所有装置看成是具有某种传递性能的电路元件的工程师来说是方便的。以此为分析基础的电路会拥有一个既产生电压大小的变化，也产生相位移的匝比，其阻抗也会与绕组的阻抗不同。这种电路无法解释变压器内类似饱和效应等现象。等效电路有两个入端口形式：（a）从一次侧看为一个U形电路，其折合后的负载阻抗的端电压为；（b）从二次侧看为一其值为，且伴有由和引起内压降的恒压源。在这种电路中有时可省略激磁支路，这样电路简化为一台产生恒值电压（实际上等于）并带有阻抗（实际上等于）的发电机。在上述两种情况下，参数都可折算到二次绕组，这样可减小计算时间。其电阻和电抗值可通过两种简单的轻载试验获得。直流电机导论直流电机以其多功用性而形成了鲜明的特征。通过并励、串励和特励绕组的各种不同组合，直流电机可设计成在动态和稳态运行时呈现出宽广范围变化的伏-安或速度-转矩特性。由于直流电机易于控制，因此该系统用于要求电动机转速变化范围宽或能精确控制电机输出的场合。定子上有凸极，由一个或一个以上励磁线圈励磁。励磁绕组产生的气隙通以磁极中心线为轴线对称分布，这条轴线称为磁场轴线或直轴。我们知道，每个旋转的电枢绕组中产生的交流电压，经由一与电枢连接的旋转的换向器和静止的电刷，在电枢绕组出线端转换成直流电压。换向器一电刷的组合构成机械整流器，它产生一直流电枢电压和一在空间固定的电枢磁势波形。电刷的放置应使换向线圈也处于磁极中性区，即两磁极之间。这样，电枢磁势波形的轴线与磁极轴线相差90°电角度，即位于交轴上。在示意图中，电刷位于交轴上，因为此处正是与其相连的线圈的位置。这样，如图所示电枢磁势波的轴线也是沿着电刷轴线的。（在实际电机中，电刷的几何位置大约偏移图例中所示位置90°电角度，这是因为元件的末端形状构成图示结果与换向器相连。）电刷上的电磁转矩和速度电压与磁通分布的空间波形无关；为了方便起见，我们假设气隙中仍然是正弦磁密波，这样便可以从磁场分析着手求得转矩。转矩可以用直轴每极气隙磁通和电枢磁势波的空间基波分量 相互作用的结果来表示。电刷处于交轴时，磁场间的角度为90°电角度，其正弦值等于1，则对于一台P极电机式中由于转矩的正方向可以根据物理概念的推断确定，因此负号已经去掉。电枢磁势锯齿波的空间基波是峰值的8/。上式变换后有式中=电枢外部电路中的电流；=电枢绕组中的总导体数；=通过绕组的并联支路数；且其为一个由绕组设计而确定的常数。简单的单个线圈的电枢中的整流电压前面已经讨论过了。将绕组分散在几个槽中的效果可用图形表示，图中每一条整流的正弦波形是一个线圈产生的电压，换向线圈边处于磁中性区。从电刷端观察到的电压是电刷间所有串联线圈中整流电压的总和，在图中由标以的波线表示。当每极有十几个换向器片，波线的波动变得非常小，从电刷端观察到的平均电压等于线圈整流电压平均值之和。电刷间的整流电压即速度电压，为式中为设计常数。分布绕组的整流电压与集中线圈有着相同的平均值，其差别只是分布绕组的波形脉动大大减小。将上述几式中的所有变量用SI单位制表达，有这个等式简单地说明与速度电压有关的瞬时功率等于与磁场转矩有关的瞬时机械功率，能量的流向取决于这台电机是电动机还是发电机。直轴气隙通由励磁绕组的合成磁势 产生，其磁通-磁势曲线就是电机的具体铁磁材料的几何尺寸决定的磁化曲线。在磁化曲线中，因为电枢磁势波的轴线与磁场轴线垂直，因此假定电枢磁势对直轴磁通不产生作用。这种假设有必要在后述部分加以验证，届时饱和效应会深入研究。因为电枢电势与磁通成正比，所以通常用恒定转速下的电枢电势来表示磁化曲线更为方便。任意转速时，任一给定磁通下的电压与转速成正比，即图中表示只有一个励磁绕组的磁化曲线，这条曲线可以很容易通过实验方法得到，不需要任何设计步骤的知识。在一个相当宽的励磁范围内，铁磁材料部分的磁阻与气隙磁阻相比可以忽略不计，在此范围内磁通与励磁绕组总磁势呈线性比例，比例常数便是直轴气隙磁导率。直流电机的突出优点是通过选择磁场绕组不同的励磁方法，可以获得变化范围很大的运行特性。励磁绕组可以由外部直流电源单独激磁，或者也可自励，即电机提供自身的励磁。励磁防哪个法不仅极大地影响控制系统中电机的静态特性，而且影响其动态运行。他励发电机的连接图已经给出，所需励磁电流是额定电枢电流的很小一部分。励磁电路中很小数量的功率可以控制电枢电路中相对很大数量的功率，也就是说发电机是一种功率放大器。当需要在很大范围内控制电枢电压时，他励发电机常常用于反馈控制系统中。自励发电机的励磁绕组可以有三种不同的供电方式。励磁绕组可以与电枢串联起来，这便形成了串励发电机；励磁绕组可以与电枢并联在一起，这便形成了并励发电机；或者励磁绕组分成两部分，其中一部分与电枢串联，另一部分与电枢并联，这便形成复励发电机。为了引起自励过程，在自励发电机中必须存在剩磁。在典型的静态伏-安特性中，假定原动机恒速运行，稳态电势和端电压关系为：式中为电枢输出电流，为电枢回路电阻。在发电机中，比大，电磁转矩T是一种阻转矩。 他励发电机的端电压随着负载电流的增加稍有降低，这主要是由于电枢电阻上的压降。串励发电机中的励磁电流与负载电流相同，这样，气隙磁通和电压随负载变化很大，因此很少采用串励发电机。并励发电机电压随负载增加会有所下降，但在许多应用场合，这并不防碍使用。复励发电机的连接通常使串励绕组的磁势与并励绕组磁势相加，其优点是通过串励绕组的作用，每极磁通随着负载增加，从而产生一个随负载增加近似为常数的输出电压。通常，并励绕组匝数多，导线细；而绕在外部的串励绕组由于它必须承载电机的整个电枢电流，所以其构成的导线相对较粗。不论是并励还是复励发电机的电压都可借助并励磁场中的变阻器在适度的范围内得到调节。任何用于发电机的励磁方法都可用于电动机。在电动机典型的静态转速-转矩特性中，假设电动机两端由一个恒压源供电。在电动机电枢中感应的电势与端电压间的关系为式中此时为输入的电枢电流。电势此时比端电压小，电枢电流与发电机中的方向相反，且电磁转矩与电枢旋转方向相同。在并励和他励电动机中磁场磁通近似为常数，因此转矩的增加必须要求电枢电流近似成比例增大，同时为允许增大的电流通过小的电枢电阻，要求反电势稍有减少。由于反电势决定于磁通和转速，因此，转速必须稍稍降低。与鼠笼式感应电动机相类似，并励电动机实际上是一种从空载到满载速降仅约为5%的恒速电动机。起动转矩和最大转矩受到能成功换向的电枢电流的限制。并励电动机的突出优点是易于调速。在并励绕组回路装上变阻器，励磁电流和每极磁通都可任意改变，而磁通的变化导致转速相反的变化以维持反电势大致等于外施端电压。通过这种方法得到最大调速范围为4或5比1，最高转速同样受到换向条件的限制。通过改变外施电枢电压，可以获得很宽的调速范围。在串励电动机中，电枢电流、电枢电势和定子磁场磁通随负载增加而增加（假设铁芯不完全饱和）。因为磁通随负载增大，所以为了维持外施电压与反电势之间的平衡，速度必须下降，此外，由于磁通增加，所以转矩增大所引起的电枢电流的增大比并励电动机中的要小。因此串励电动机是一种具有明显下降的转速-负载特性的变速电动机。对于要求转矩过载很多的应用场合，由于对应的过载功率随相应的转速下降而维持在一个合理的范围内，因此，这种特性具有特别的优越性。磁通随着电枢电流的增大而增大，同时还带来非常有用的起动特性。在复励电动机中，串励磁场可以连接成积复励式，使其磁势与并励磁场相加；也可以连接成差复励式，两磁场方向相反。差复励连接很少使用。积复励电动机具有界于并励和串励电动机之间的速度-负载特性，转速随负载的降低取决于并励磁场和串励磁场的相对安匝数。这种电动机没有像串励电动机那样轻载高转速的缺点，但它在相当的程度上保持着串励方式的优点。 直流电机的应用优势在于可接成并励、串励和复励等各种励磁方式，因而可提供多种性能各异的运行特性。其中有一些特性在本文中已大致提及。如果增加附加的电刷组以至于从换向器上另外可得到一些电压，那么还会存在更多的运用场合，因此直流电机系统的多用性，及其不论对人工还是自动控制的适应性，是它们的显著特性。'