電氣畢業(yè)設(shè)計(jì)外文翻譯 負(fù)載運(yùn)行的變壓器及直流電機(jī)導(dǎo)論

電氣畢業(yè)設(shè)計(jì)外文翻譯負(fù)載運(yùn)行的變壓器及直流電機(jī)導(dǎo)論TheTransformeronload?IntroductiontoDCMachinesIthasbeenshownthataprimaryinputvoltagecanbetransformedtoanydesiredopen-circuitsecondaryvoltagebyasuitablechoiceofturnsratioisavailableforcirculatingaloadcurrentimpedance.Forthemoment,alaggingpowerfactorwillbeconsidered.Thesecondarycurrentandtheresultingampere-turnswillchangetheflux,tendingtodemagnetizethecore,reduceandwithitBecausetheprimaryleakageimpedancedropissolow,asmallalterationtowillcauseanappreciableincreaseofprimarycurrentfromtoanewvalueofequaltoTheextraprimarycurrentandampere-turnsnearlycancelthewholeofthesecondaryampere-turns.Thisbeingso,themutualfluxsuffersonlyaslightmodificationandrequirespracticallythesamenetampere-turnsasonnoload.Thetotalprimaryampere-turnsareincreasedbyanamountnecessarytoneutralizethesameamountofsecondaryampere-turns.Inthevectorequation,;alternatively,Atfullload,thecurrentisonlyabout5%ofthefull-loadcurrentandsoisnearlyequalto.Becauseinmindthat,theinputkVAwhichisapproximatelyisalsoapproximatelyequaltotheoutputkVA,Thephysicalcurrenthasincreased,andwithintheprimaryleakagefluxtowhichitisproportional.Thetotalfluxlinkingtheprimary,,isshownunchangedbecausethetotalbacke.m.f.,()isstillequalandoppositetoHowever,therehasbeenaredistributionoffluxandthemutualcomponenthasfallenduetotheincreaseofwithAlthoughthechangeissmall,thesecondarydemandcouldnotbemetwithoutamutualfluxande.m.f.alterationtopermitprimarycurrenttochange.Thenetfluxlinkingthesecondarywindinghasbeenfurtherreducedbytheestablishmentofsecondaryleakagefluxdueto,andthisopposesAlthoughandareindicatedseparately,theycombinetooneresultantinthecorewhichwillbedownwardsattheinstantshown.Thusthesecondaryterminalvoltageisreducedtowhichcanbeconsideredintwocomponents,i.e.orvectoriallyAsfortheprimary,isresponsibleforasubstantiallyconstantsecondaryleakageinductanceItwillbenoticedthattheprimaryleakagefluxisresponsibleforpartofthechangeinthesecondaryterminalvoltageduetoitseffectsonthemutualflux.Thetwoleakagefluxesarecloselyrelated;,forexample,byitsdemagnetizingactiononhascausedthechangesontheprimarysidewhichledtotheestablishmentofprimaryleakageflux.Ifalowenoughleadingpowerfactorisconsidered,thetotalsecondaryfluxandthemutualfluxareincreasedcausingthesecondaryterminalvoltagetorisewithloadisunchangedinmagnitudefromthenoloadconditionsince,neglectingresistance,itstillhastoprovideatotalbacke.m.f.equaltoItisvirtuallythesameas,thoughnowproducedbythecombinedeffectofprimaryandsecondaryampere-turns.Themutualfluxmuststillchangewithloadtogiveachangeofandpermitmoreprimarycurrenttoflowhasincreasedthistimebutduetothevectorcombinationwiththereisstillanincreaseofprimarycurrent.Twomorepointsshouldbemadeaboutthefigures.Firstly,aunityturnsratiohasbeenassumedforconveniencesothatSecondly,thephysicalpictureisdrawnforadifferentinstantoftimefromthevectordiagramswhichshow,ifthehorizontalaxisistakenasusual,tobethezerotimereference.Thereareinstantsinthecyclewhenprimaryleakagefluxiszero,whenthesecondaryleakagefluxiszero,andwhenprimaryandsecondaryleakagefluxiszero,andwhenprimaryandsecondaryleakagefluxesareinthesamesense.Theequivalentcircuitalreadyderivedforthetransformerwiththesecondaryterminalsopen,caneasilybeextendedtocovertheloadedsecondarybytheadditionofthesecondaryresistanceandleakagereactance.Forimpedance,sinceanysecondaryvoltagebecomes,andsecondarycurrentbecomes,thenanysecondaryimpedance,includingloadimpedance,mustbecomeConsequently,andIftheprimaryturnsaretakenasreferenceturns,theprocessiscalledreferringtotheprimaryside.Thereareafewcheckswhichcanbemadetoseeiftheprocedureoutlinedisvalid.Forexample,thecopperlossinthereferredsecondarywindingmustbethesameasintheoriginalsecondaryotherwisetheprimarywouldhavetosupplyadifferentlosspowermustbeequaltodoesinfactreducetoSimilarlythestoredmagneticenergyintheleakagefieldwhichisproportionaltowillbefoundtocheckasThereferredsecondaryTheargumentissound,thoughatfirstitmayhaveseemedsuspect.Infact,iftheactualsecondarywindingwasremovedphysicallyfromthecoreandreplacedbytheequivalentwindingandloadcircuitdesignedtogivetheparameters,,and,measurementsfromtheprimaryterminalswouldbeunabletodetectanydifferenceinsecondaryampere-turns,demandorcopperloss,undernormalpowerfrequencyoperation.Thereisnopointinchoosinganybasisotherthanequalturnsonprimaryandreferredsecondary,butitissometimesconvenienttorefertheprimarytothesecondarywinding.Inthiscase,ifallthesubscript1’sareinterchangedforthesubscript2’s,thenecessaryreferringconstantsareeasilyfound;e.g.,;similarlyandTheequivalentcircuitforthegeneralcasewhereexceptthathasbeenaddedtoallowforironlossandanideallosslesstransformationhasbeenincludedbeforethesecondaryterminalstoreturnto.Allcalculationsofinternalvoltageandpowerlossesaremadebeforethisidealtransformationisapplied.Thebehaviourofatransformerasdetectedatbothsetsofterminalsisthesameasthebehaviourdetectedatthecorrespondingterminalsofthiscircuitwhentheappropriateparametersareinserted.Theslightlydifferentrepresentationshowingthecoilsandsidebysidewithacoreinbetweenisonlyusedforconvenience.Onthetransformeritself,thecoilsare,ofcourse,woundroundthesamecore.Verylittleerrorisintroducedifthemagnetisingbranchistransferredtotheprimaryterminals,butafewanomalieswillarise.Forexample,thecurrentshownflowingthroughtheprimaryimpedanceisnolongerthewholeoftheprimarycurrent.TheerrorisquitesmallsinceisusuallysuchasmallfractionofSlightlydifferentanswersmaybeobtainedtoaparticularproblemdependingonwhetherornotallowanceismadeforthiserror.Withthissimplifiedcircuit,theprimaryandreferredsecondaryimpedancescanbeaddedtogive:andItshouldbepointedoutthattheequivalentcircuitasderivedhereisonlyvalidfornormaloperationatpowerfrequencies;capacitanceeffectsmustbetakenintoaccountwhenevertherateofchangeofvoltagewouldgiverisetoappreciablecapacitancecurrents,Theyareimportantathighvoltagesandatfrequenciesmuchbeyond100cycles/sec.Afurtherpointisnottheonlypossibleequivalentcircuitevenforpowerfrequencies.Analternative,treatingthetransformerasathree-orfour-terminalnetwork,givesrisetoarepresentationwhichisjustasaccurateandhassomeadvantagesforthecircuitengineerwhotreatsalldevicesascircuitelementswithcertaintransferproperties.Thecircuitonthisbasiswouldhaveaturnsratiohavingaphaseshiftaswellasamagnitudechange,andtheimpedanceswouldnotbethesameasthoseofthewindings.Thecircuitwouldnotexplainthephenomenawithinthedeviceliketheeffectsofsaturation,soforanunderstandingofinternalbehaviourTherearetwowaysoflookingattheequivalentcircuit:viewedfromtheprimaryasasinkbutthereferredloadimpedanceconnectedacross,orviewedfromthesecondaryasasourceofconstantvoltagewithinternaldropsduetoandThemagnetizingbranchissometimesomittedinthisrepresentationandsothecircuitreducestoageneratorproducingaconstantvoltageactuallyequaltoandhavinganinternalimpedanceactuallyequalto.Ineithercase,theparameterscouldbereferredtothesecondarywindingandthismaysavecalculationtimeTheresistancesandreactancescanbeobtainedfromtwosimplelightloadtests.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.wavethenisalongthebrushaxisasshownThegeometricalpositionofthebrushesinanactualmachineisapproximately90electricaldegreesfromtheirpositionintheschematicdiagrambecauseoftheshapeoftheendconnectionstothecommutator.Themagnetictorqueandthespeedvoltageappearingatthebrushesareindependentofthespatialwaveformofthefluxdistribution;forconvenienceweshallcontinuetoassumeasinusoidalflux-densitywaveintheairgap.ThetorquecanthenbefoundfromthemagneticfieldviewpointThetorquecanbeexpressedintermsoftheinteractionofthedirect-axisair-gapfluxperpoleandthespace-fundamentalcomponentofthearmaturem.m.f.waveWiththebrushesinthequadratureaxis,theanglebetweenthesefieldsis90electricaldegrees,anditssineequalsunity.ForaPpolemachineInwhichtheminussignhasbeendroppedbecausethepositivedirectionofthetorquecanbedeterminedfromphysicalreasoning.Thespacefundamentalofthesawtootharmaturem.m.f.waveis8/timesitspeak.SubstitutioninaboveequationthengivesWherecurrentinexternalarmaturecircuit;totalnumberofconductorsinarmaturewinding;numberofparallelpathsthroughwinding;AndIsaconstantfixedbythedesignofthewinding.Therectifiedvoltagegeneratedinthearmaturehasalreadybeendiscussedbeforeforanelementarysingle-coilarmature.Theeffectofdistributingthewindinginseveralslotsisshowninfigure,inwhicheachoftherectifiedsinewavesisthevoltagegeneratedinoneofthecoils,commutationtakingplaceatthemomentwhenthecoilsidesareintheneutralzone.Thegeneratedvoltageasobservedfromthebrushesisthesumoftherectifiedvoltagesofallthecoilsinseriesbetweenbrushesandisshownbytheripplinglinelabeledinfigure.Withadozenorsocommutatorsegmentsperpole,theripplebecomesverysmallandtheaveragegeneratedvoltageobservedfromthebrushesequalsthesumoftheaveragevaluesoftherectifiedcoilvoltages.Therectifiedvoltagebetweenbrushes,knownalsoasthespeedvoltage,isWhereisthedesignconstant.Therectifiedvoltageofadistributedwindinghasthesameaveragevalueasthatofaconcentratedcoil.ThedifferenceisthattherippleisgreatlyreducedFromtheaboveequations,withallvariableexpressedinSIunits:Thisequationsimplysaysthattheinstantaneouselectricpowerassociatedwiththespeedvoltageequalstheinstantaneousmechanicalpowerassociatedwiththemagnetictorque,thedirectionofpowerflowbeingdeterminedbywhetherthemachineisactingasamotororgenerator.Thedirect-axisair-gapfluxisproducedbythecombined//0>.thefieldwindings,theflux-m.m.f.characteristicbeingthemagnetizationcurvefortheparticularirongeometryofthemachine.Inthemagnetizationcurve,itisassumedthatthearmaturem.m.f.waveisperpendiculartothefieldaxis.Itwillbenecessarytoreexaminethisassumptionlaterinthischapter,wheretheeffectsofsaturationareinvestigatedmorethoroughly.Becausethearmaturee.m.f.isproportionaltofluxtimesspeed,itisusuallymoreconvenienttoexpressthemagnetizationcurveintermsofthearmaturee.m.f.ataconstantspeedThevoltageforagivenfluxatanyotherspeedisproportionaltothespeed,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-excitationprocessstarte