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外文翻译感应电机.doc

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外文翻译感应电机.doc

附录A外文资料InductionMotor1StartinganinductionmotorHighinertialoadsputstraininductionmotorsbecausetheyprolongthestartingperiod.Thestartingcurrentinboththestatorandrotorishighduringthisintervalsothatoverheatingbecomesamajorproblem.Formotorsofseveralthousandhorsepower,aprolongedstartingperiodmayevenoverloadthetransmissionlinefeedingtheplantwherethemotorisinstalled.Thelinevoltagemayfallbelownormalformanyseconds,thusaffectingotherconnectedloads.Torelievetheproblem,inductionmotorsareoftenstartedonreducedvoltage.Thislimitsthepowerdrawnbythemotor,andconsequentlyreducesthebrievoltagedropaswellastheheatingrateofthewindings.Reducedvoltagelengthensthestartuptime,butthissusuallynotimportant.However,whetherthestartuptimeislongorshort,itisworthrememberingthefollowingruleforamotorthatisnotloadedmechanicalRuleITheheatdissipatedintherotorduringthestartingperiodfromzerospeedtofinalratedspeedisequaltothefinalkineticenergystoredinalltherevolvingparts.Thisruleholdstrue,irrespectiveofthestatorvoltageorthetorquespeedcurveofthemotor.Thus,ifamotorbringsamassiveflywheeluptospeed,andiftheenergystoredintheflywheelisthen5000joules.therotorwillhavedissipated5000joulesintheformofheat.Dependinguponthesizeoftherotoranditscoolingsystem,thisenergycouldeasilyproduceoverheating.2PlugginganinductionmotorInsomeindustrialapplications,theinductionmotoranditsloadhavetobebroughttoaquickstop.Thiscanbedonebyinterchangingtwostatorleads,sothattherevolvingfieldsuddenlyturnsintheoppositedirectiontotherotor.Duringthispluggingperiod,themotoractsasabrake.Itabsorbskineticenergyfromthestillrevolvingload,causingitsspeedtohill.TheassociatedmechanicalpowerP,isentirelydissipatedasheatintherotor.Unfortunately,therotoralsocontinuestoreceiveelectromagneticpowerPfromthestator,whichisalsodissipatedasheatFig.14.10.Consequently,pluggingproduces12Rlossesintherotorthatevenexceedthosewhentherotorislocked.Motorsshouldnothepluggedtoofrequentlybecausehighrotortemperaturesmaymelttherotorbarsoroverheatthestatorwinding,inthisregarditisworthrememberingtherulefollowingruleforpluggingoperationsforamotorthatisnotloadedmechanicallyRule2Theheatdissipatedintherotorduringthepluggingperiodinitialratedspeedtozerospeedisthreetimestheoriginalkineticenergyofalltherevolvingparts.Fig.14.10Whena3phaseinductionmotorisplugged,therotorlossesareveryhigh.3BrakingwithdirectcurrentAninductionmotoranditshighinertialoadcanalsobebroughttoaquickstopbycirculatingdccurrentinthestatorwinding.Anytwostatorterminalscanbeconnectedtothedcsource.ThedirectcurrentproducesstationaryN,Spolesinthestator.Thenumberofpolescreatedisequaltothenumberofpoleswhichthemotordevelopsnormally.Thus,a3phase,4poleinductionmotorproduces4dcpoles,nomatterhowthemotorterminalsareconnectedtothedcsource.Whentherotorsweepspastthestationaryfield,anacvoltageisinducediiitherotorbars.Thevoltageproducesanaccurrentandtheresultingrotorlossesaredissipatedattheexpenseofthekineticenergystoredintherevolvingparts.Themotorfinallycomestorestwhenallthekineticenergyhasbeendissipatedasheatintherotor.Theadvantageofdcbrakingisthatitproducesfarlessheatthandoesplugging.Ineffect,theenergydissipatedintherotorisonlyequaltotheoriginalkineticenergystoredintherevolvingmasses,andnotthreetimesthatenergy.Theenergydissipatedintherotorisindependentofthemagnitudeofthedccurrent.However,asmallerdccurrentincreasesthebrakingtime.Thedccurrentcanbetwoorthreetimestheratedcurrentofthemotor.Evenlargervaluescanbeused,providedthatthestatorduesnotbecometoohot.Thebrakingtorqueisproportionaltothesquareofthedcbrakingcurrent.4AbnormalconditionsAbnormalmotoroperationmayduetointernalproblemsshortcircuitinthestator,overheatingofthebearing,etc.ortoexternalconditions.Externalproblemsmaybecausedbyanyofthefollowing1.Mechanicaloverload2.Supplyvoltagechanges3.Singlephasing4.FrequencychangesWewillexaminethenatureoftheseproblemsinthesectionsthatfollow.Accordingtonationalstandards,amotorshalloperatesatisfactoryonanyvoltagewithin±10ofthenominalvoltage,andforanyfrequencywithin±5ofthenormalfrequency.Ifthevoltageandfrequencybothvary,thesumofthetwopercentagechangesmustnotexceed10percent.Finally,allmotorsaredesignedtooperatesatisfactorilyataltitudesupto1000mabovesealevel.Athigheraltitudesthetemperaturemayexceedthepermissiblelimitsduetothepoorcoolingaffordedbythethinnerair.5MechanicaloverloadAlthoughstandardinductionmotorscandevelopasmuchastwicetheirratedpowerforshortperiods,theyshouldnotbeallowedtoruncontinuouslybeyondtheirratedcapacity.Overloadscauseoverheating,whichdeterioratestheinsulationandreducestheservicelifeofthemotor.Inpracticetheoverloadcausesthethermaloverloadrelaysinthestarterboxtotrip.bringingthemotortoastopbeforeitstemperaturegetstoohigh.SomedripproofmotorsaredesignedtocarryacontinuousoverloadofI5percent.Thisoverloadcapacityisshownonthenameplatebytheservicefactor1.15.Theallowabletemperatureriseisthen10°Chigherthanthatpermittedfordripproofmotorsoperatingatnormalload.Duringemergenciesadripproofmotorcanbemadetocarryoverloadsasmuchas125percent,aslongassupplementaryexternalventilationisprovided.Thisisnotrecommendedforlongperiodsbecauseeveniftheexternalframeiscool,thetemperatureofthewindingsmaybeexcessive.6LinevoltagechangesThemostimportantconsequenceofalinevoltagechangeisitseffectuponthetorquespeedcurveofthemotor.Ineffect,thetorqueatanyspeedisproportionaltothesquareoftheappliedvoltage.Thus,ifthestatorvoltagedecreasesby10,thetorqueateveryspeedwilldropbyapproximately20.Alinevoltagedropisoftenproducedduringstartup,duetotheheavystartingcurrentdrawnfromtheline.Asaresultofthelowervoltage,thestartingtorquemayhemuchlessthanitsratedvalue.Ontheotherhand,ifthelinevoltageistoohighwhenthemotorisrunning,thefluxperpolewillbeabovenormal.Foramotorrunningatfullload,thisincreasesboththeionlossesandthemagnetizingcurrent,withtheresultthatthetemperatureincreasesslightlyandthepowerfactorisslightlyreduced.Ifthe3phasevoltagesareunbalanced,theycanproduceaseriousunbalanceofthethreelinecurrents.Thisconditionincreasesthestatorandrotorlosses,yieldingahighertemperature.AvoltageUnbalanceofaslittleas3.5cancausethetemperaturetoincreaseby15°C.Theutilitycompanyshouldbenotifiedwheneverthephasetophaselinevoltagesdifferbymorethan2percent.7SinglephasingIfonelineofa3phaselineisaccidentallyopenedorifafuseblowswhilethe3phasemotorisrunning,themachinewiltcontinuetorunasasinglephasemotor.Thecurrentdrawnfromtheremainingtwotineswillalmostdouble,andthemotorwillbegintooverheat.ThethermalrelaysprotectingthemotorwilleventuallytripthecircuithrC1herebydisconnectingthemotorfromtheline.Thetorquespeedcurveisseriouslyaffectedwhena3phasemotoroperatesonsinglephase.Thebreakdowntorquedecreasestoabout40ofitsoriginalvalue,andthemotordevelopsnostartingtorqueatall.Consequently,afullyloaded3phasemotormaysimplystopifoneofitslinesissuddenlyopened.Theresultinglockedrotorcurrentisabout90ofthenormal3phaseLRcurrent.Itisthereforelargeenoughtotripthecircuitbreakerortoblowthefuses.

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