电动机转速测定系统的设计外文翻译.docx
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电动机转速测定系统的设计外文翻译
外文文献译文
设计(论文)题目:
电动机转速测定系统的设计
专业与班级:
测控技术与仪器0601班
学生姓名:
杨红伟
指导教师:
周彬
2010年3月18日
Synchronousmotor
Abstract:
Inrecentdecades,withthepowerelectronics,microelectronicstechnologyandthedevelopmentofmoderncontroltheory,mediumandsmallpowermotorsintheindustrialandagriculturalproductionandpeople'sdailylivesareverybroadapplication.Particularlyinthetownshipenterprisesandhouseholdelectricalappliances,needalargenumberofmediumandsmall-powermotors.Becauseofthismotordevelopmentandwideapplication,itsuse,maintenanceandrepairworkhasbecomeincreasinglyimportant,Herearejustontheknowledgeofsynchronousmotor
1.Principleofoperation
Inordertounderstandtheprincipleofoperationofasynchronousmotor,letusexaminewhathappensifweconnectthearmaturewinding(laidoutinthestator)ofa3-phasesynchronousmachinetoasuitablebalanced3-phasesourceandthefieldwindingtoaD.Csourceofappropriatevoltage.ThecurrentflowingthroughthefieldcoilswillsetupstationarymagneticpolesofalternateNorthandSouth.(forconvenienceletusassumeasalientpolerotor,asshowninFig.50).Ontheotherhand,the3-phasecurrentsflowinginthearmaturewindingproducearotatingmagneticfieldrotatingatsynchronousspeed.InotherwordstherewillbemovingNorthandSouthpolesestablishedinthestatorduetothe3-phasecurrentsi.eatanylocationinthestatortherewillbeaNorthpoleatsomeinstantoftimeanditwillbecomeaSouthpoleafteratimeperiodcorrespondingtohalfacycle.(afteratime=1
2f,wheref=frequencyofthesupply).LetusassumethatthestationarySouthpoleintherotorisalignedwiththeNorthpoleinthestatormovinginclockwisedirectionataparticularinstantoftime,asshowninFig50.ThesetwopolesgetattractedandFigure50:
Forceofattractionbetweenstatorpolesandrotorpoles-resultinginproductionoftorqueinclockwisedirectiontrytomaintainthisalignment(asperLenz’slaw)andhencetherotorpoletriestofollowthestatorpoleastheconditionsaresuitablefortheproductionoftorqueintheclockwisedirection.Howevertherotorcannotmoveinstantaneouslyduetoitsmechanicalinertia,andsoitneedssometimetomove.Inthemeantime,thestatorpolewouldquickly(atimedurationcorrespondingtohalfacycle)changeitspolarityandbecomesaSouthpole。
SotheforceofattractionwillnolongerbepresentandinsteadthelikepolesexperienceaforceofrepulsionasshowninFig.51.Inotherwords,theconditionsarenowsuitablefortheFigure51:
Forceofrepulsionbetweenstatorpolesandrotorpoles-resultinginproductionoftorqueinanticlockwisedirectionproductionoftorqueintheanticlockwisedirection.EventhisconditionwillnotlastlongerasthestatorpolewouldagainchangetoNorthpoleafteratimeof1
2f.Thustherotorwillexperienceanalternatingforcewhichtriestomoveitclockwiseandanticlockwiseattwicethefrequencyofthesupply,i.e.atintervalscorrespondingto1
2fseconds.Asthisdurationisquitesmallcomparedtothemechanicaltimeconstantoftherotor,therotorcannotrespondandmoveinanydirection.Therotorcontinuestobestationaryonly.
Onthecontraryiftherotorisbroughttonearsynchronousspeedbysomeexternalmeanssayasmallmotor(knownasponymotor-whichcouldbeaD.CorACinductionrotor)mountedonthesameshaftasthatoftherotor,therotorpolesgetlockedtotheunlikepolesinthestatorandtherotorcontinuestorunatthesynchronousspeedevenifthesupplytotheponymotorisdisconnected.
Thusthesynchronousrotorcannotstartrotatingonitsownorusuallywesaythatthesynchronousrotorhasnostartingtorque.So,somespecialprovisionhastobemadeeitherinsidethemachineoroutsideofthemachinesothattherotorisbroughttonearaboutitssynchronousspeed.Atthattime,ifthearmatureissuppliedwithelectricalpower,therotorcanpullintostepandcontinuetooperateatitssynchronousspeed.Someofthecommonlyusedmethodsforstartingsynchronousrotoraredescribedinthefollowingsection.
2.Methodsofstartingsynchronousmotor
Basicallytherearethreemethodsthatareusedtostartasynchronousmotor:
•Toreducethespeedoftherotatingmagneticfieldofthestatortoalowenoughvaluethattherotorcaneasilyaccelerateandlockinwithitduringonehalf-cycleoftherotatingmagneticfield’srotation.Thisisdonebyreducingthefrequencyoftheappliedelectricpower.Thismethodisusuallyfollowedinthecaseofinverter-fedsynchronousmotoroperatingundervariablespeeddriveapplications.
•Touseanexternalprimemovertoacceleratetherotorofsynchronousmotorneartoitssynchronousspeedandthensupplytherotoraswellasstator.Ofcoursecareshouldbetakentoensurethatthedirectionofrotationoftherotoraswellasthatoftherotatingmagneticfieldofthestatorarethesame.Thismethodisusuallyfollowedinthelaboratory-thesynchronousmachineisstartedasageneratorandisthenconnectedtothesupplymainsbyfollowingthesynchronizationorparallelingprocedure.Thenthepowersupplytotheprimemoverisdisconnectedsothatthesynchronousmachinewillcontinuetooperateasamotor.
•Tousedamperwindingsoramortisseurwindingsiftheseareprovidedinthemachine.Thedamperwindingsoramortisseurwindingsareprovidedinmostofthelargesynchronousmotorsinordertonullifytheoscillationsoftherotorwheneverthesynchronousmachineissubjectedtoaperiodicallyvaryingload.
Eachofthesemethodsofstartingasynchronousmotoraredescribedbelowindetail.
2.1MotorstartingbyreducingthesupplyFrequency
Iftherotatingmagneticfieldofthestatorinasynchronousmotorrotatesatalowenoughspeed,therewillbenoproblemfortherotortoaccelerateandtolockinwiththestator’smagneticfield.Thespeedofthestatormagneticfieldcanthenbeincreasedtoitsratedoperatingspeedbygraduallyincreasingthesupplyfrequencyfuptoitsnormal50or60Hzvalue.
Thisapproachtostartingofsynchronousmotorsmakesalotofsense,butthereisabigproblem:
Wherefromcanwegetthevariablefrequencysupply?
Theusualpowersupplysystemsgenerallyregulatethefrequencytobe50or60Hzasthecasemaybe.However,variable-frequencyvoltagesourcecanbeobtainedfromadedicatedgeneratoronlyintheoldendaysandsuchasituationwasobviouslyimpracticalexceptforveryunusualorspecialdriveapplications.
Butthepresentdaysolidstatepowerconvertersofferaneasysolutiontothis.Wenowhavetherectifier-inverterandcycloconverters,whichcanbeusedtoconvertaconstantfrequencyACsupplytoavariablefrequencyACsupply.Withthedevelopmentofsuchmodernsolid-statevariable-frequencydrivepackages,itisthuspossibletocontinuouslycontrolthefrequencyofthesupplyconnectedtothesynchronousmotorallthewayfromafractionofahertzuptoandevenabovethenormalratedfrequency.Ifsuchavariable-frequencydriveunitisincludedinamotor-controlcircuittoachievespeedcontrol,thenstartingthesynchronousmotorisveryeasy-simplyadjustthefrequencytoaverylowvalueforstarting,andthenraiseituptothedesiredoperatingfrequencyfornormalrunning.
Whenasynchronousmotorisoperatedataspeedlowerthantheratedspeed,itsinternalgeneratedvoltage(usuallycalledthecounterEMF)EA=Kwillbesmallerthannormal.Assuchtheterminalvoltageappliedtothemotormustbereducedproportionallywiththefrequencyinordertokeepthestatorcurrentwithintheratedvalue.Generally,thevoltageinanyvariable-frequencypowersupplyvariesroughlylinearlywiththeoutputfrequency.
2.2MotorStartingwithanExternalMotor
Thesecondmethodofstartingasynchronousmotoristoattachanexternalstartingmotor(ponymotor)toitandbringthesynchronousmachinetonearaboutitsratedspeed(butnotexactlyequaltoit,asthesynchronizationprocessmayfailtoindicatethepointofclosureofthemainswitchconnectingthesynchronousmachinetothesupplysystem)withtheponymotor.Thentheoutputofthesynchronousmachinecanbesynchronizedorparalleledwithitspowersupplysystemasagenerator,andtheponymotorcanbedetachedfromtheshaftofthemachineorthesupplytotheponymotorcanbedisconnected.OncetheponymotoristurnedOFF,theshaftofthemachineslowsdown,thespeedoftherotormagneticfieldBRfallsbehindBnet,momentarilyandthesynchronousmachinecontinuestooperateasamotor.Assoonasitbeginstooperatesasamotorthesynchronousmotorcanbeloadedintheusualmannerjustlikeanymotor.
Thiswholeprocedureisnotascumbersomeasitsounds,sincemanysynchronousmotorsarepartsofmotor-generatorsets,andthesynchronousmachineinthemotor-generatorsetmaybestartedwiththeothermachineservingasthestartingmotor.Moreover,thestartingmotorisrequiredtoovercomeonlythemechanicalinertiaofthesynchronousmachinewithoutanymechanicalload(loadisattachedonlyafterthesynchronousmachineisparalleledtothepowersupplysystem).Sinceonlythemotor’sinertiamustbeovercome,thestartingmotorcanhaveamuchsmallerratingthanthesynchronousmotoritisgoingtostart.Generallymostofthelargesynchronousmotorshavebrushlessexcitationsystemsmountedontheirshafts.Itisthenpossibletousetheseexcitersasthestartingmotors.Formanymedium-sizetolargesynchronousmotors,anexternalstartingmotororstartingbyusingtheexcitermaybetheonlypossiblesolution,becausethepowersystemstheyaretiedtomaynotbeabletoh