电动机转速测定系统的设计外文翻译.docx

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,becausethepowersystemstheyaretiedtomaynotbeableto