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附录A：外文资料InductionMotor1StartinganinductionmotorHigh-inertialoadsputstraininductionmotorsbecausetheyprolongthestartingperiod.Thestartingcurrentinboththestatorandrotorishighduringthisintervalsothatoverheatingbecomesamajorproblem.Formotorsofseveralthousandhorsepower,aprolongedstartingperiodmayevenoverloadthetransmissionlinefeedingtheplantwherethemotorisinstalled.Thelinevoltagemayfallbelownormalformanyseconds,thusaffectingotherconnectedloads.Torelievetheproblem,inductionmotorsareoftenstartedonreducedvoltage.Thislimitsthepowerdrawnbythemotor,andconsequentlyreducesthebrievoltagedropaswellastheheatingrateofthewindings.Reducedvoltagelengthensthestart-uptime,butthissusuallynotimportant.However,whetherthestart-uptimeislongorshort,itisworthrememberingthefollowingruleforamotorthatisnotloadedmechanical:RuleI-Theheatdissipatedintherotorduringthestartingperiod(fromzerospeedtofinalratedspeed)isequaltothefinalkineticenergystoredinalltherevolvingparts.Thisruleholdstrue,irrespectiveofthestatorvoltageorthetorque-speedcurveofthemotor.Thus,ifamotorbringsamassiveflywheeluptospeed,andiftheenergystoredintheflywheelisthen5000joules.therotorwillhavedissipated5000joulesintheformofheat.Dependinguponthesizeoftherotoranditscoolingsystem,thisenergycouldeasilyproduceoverheating.2PlugginganinductionmotorInsomeindustrialapplications,theinductionmotoranditsloadhavetobebroughttoaquickstop.Thiscanbedonebyinterchangingtwostatorleads,sothattherevolvingfieldsuddenlyturnsintheoppositedirectiontotherotor.Duringthispluggingperiod,themotoractsasabrake.Itabsorbskineticenergyfromthestill-revolvingload,causingitsspeedtohill.TheassociatedmechanicalpowerP,isentirelydissipatedasheatintherotor.Unfortunately,therotoralsocontinuestoreceiveelectromagneticpowerPfromthestator,whichisalsodissipatedasheat(Fig.14.10).Consequently,pluggingproduces12Rlossesintherotorthatevenexceedthosewhentherotorislocked.Motorsshouldnothepluggedtoofrequentlybecausehighrotortemperaturesmaymelttherotorbarsoroverheatthestatorwinding,inthisregarditisworthrememberingtherulefollowingruleforpluggingoperationsforamotorthatisnotloadedmechanically:Rule2-Theheatdissipatedintherotorduringthepluggingperiod(initialratedspeedtozerospeed)isthreetimestheoriginalkineticenergyofalltherevolvingparts.Fig.14.10Whena3-phaseinductionmotorisplugged,therotorlossesareveryhigh.3BrakingwithdirectcurrentAninductionmotoranditshigh-inertialoadcanalsobebroughttoaquickstopbycirculatingdccurrentinthestatorwinding.Anytwostatorterminalscanbeconnectedtothedcsource.ThedirectcurrentproducesstationaryN,Spolesinthestator.Thenumberofpolescreatedisequaltothenumberofpoleswhichthemotordevelopsnormally.Thus,a3-phase,4-poleinductionmotorproduces4dcpoles,nomatterhowthemotorterminalsareconnectedtothedcsource.Whentherotorsweepspastthestationaryfield,anacvoltageisinducediiitherotorbars.Thevoltageproducesanaccurrentandtheresultingrotorlossesaredissipatedattheexpenseofthekineticenergystoredintherevolvingparts.Themotorfinallycomestorestwhenallthekineticenergyhasbeendissipatedasheatintherotor.Theadvantageofdcbrakingisthatitproducesfarlessheatthandoesplugging.Ineffect,theenergydissipatedintherotorisonlyequaltotheoriginalkineticenergystoredintherevolvingmasses,andnotthreetimesthatenergy.Theenergydissipatedintherotorisindependentofthemagnitudeofthedccurrent.However,asmallerdccurrentincreasesthebrakingtime.Thedccurrentcanbetwoorthreetimestheratedcurrentofthemotor.Evenlargervaluescanbeused,providedthatthestatorduesnotbecometoohot.Thebrakingtorqueisproportionaltothesquareofthedcbrakingcurrent.4AbnormalconditionsAbnormalmotoroperationmayduetointernalproblems(short-circuitinthestator,overheatingofthebearing,etc.)ortoexternalconditions.Externalproblemsmaybecausedbyanyofthefollowing:1.Mechanicaloverload2.Supplyvoltagechanges3.Singlephasing4.FrequencychangesWewillexaminethenatureoftheseproblemsinthesectionsthatfollow.Accordingtonationalstandards,amotorshalloperatesatisfactoryonanyvoltagewithin±10%ofthenominalvoltage,andforanyfrequencywithin±5%ofthenormalfrequency.Ifthevoltageandfrequencybothvary,thesumofthetwopercentagechangesmustnotexceed10percent.Finally,allmotorsaredesignedtooperatesatisfactorilyataltitudesupto1000mabovesealevel.Athigheraltitudesthetemperaturemayexceedthepermissiblelimitsduetothepoorcoolingaffordedbythethinnerair.5MechanicaloverloadAlthoughstandardinductionmotorscandevelopasmuchastwicetheirratedpowerforshortperiods,theyshouldnotbeallowedtoruncontinuouslybeyondtheirratedcapacity.Overloadscauseoverheating,whichdeterioratestheinsulationandreducestheservicelifeofthemotor.Inpracticetheoverloadcausesthethermaloverloadrelaysinthestarterboxtotrip.bringingthemotortoastopbeforeitstemperaturegetstoohigh.Somedrip-proofmotorsaredesignedtocarryacontinuousoverloadofI5percent.Thisoverloadcapacityisshownonthenameplatebytheservicefactor1.15.Theallowabletemperatureriseisthen10°Chigherthanthatpermittedfordrip-proofmotorsoperatingatnormalload.Duringemergenciesadrip-proofmotorcanbemadetocarryoverloadsasmuchas125percent,aslongassupplementaryexternalventilationisprovided.Thisisnotrecommendedforlongperiodsbecauseeveniftheexternalframeiscool,thetemperatureofthewindingsmaybeexcessive.6LinevoltagechangesThemostimportantconsequenceofalinevoltagechangeisitseffectuponthetorque-speedcurveofthemotor.Ineffect,thetorqueatanyspeedisproportionaltothesquareoftheappliedvoltage.Thus,ifthestatorvoltagedecreasesby10%,thetorqueateveryspeedwilldropbyapproximately20%.Alinevoltagedropisoftenproducedduringstart-up,duetotheheavystartingcurrentdrawnfromtheline.Asaresultofthelowervoltage,thestartingtorquemayhemuchlessthanitsratedvalue.Ontheotherhand,ifthelinevoltageistoohighwhenthemotorisrunning,thefluxperpolewillbeabovenormal.Foramotorrunningatfull-load,thisincreasesboththeionlossesandthemagnetizingcurrent,withtheresultthatthetemperatureincreasesslightlyandthepowerfactorisslightlyreduced.Ifthe3-phasevoltagesareunbalanced,theycanproduceaseriousunbalanceofthethreelinecurrents.Thisconditionincreasesthestatorandrotorlosses,yieldingahighertemperature.AvoltageUnbalanceofaslittleas3.5%cancausethetemperaturetoincreaseby15°C.Theutilitycompanyshouldbenotifiedwheneverthephase-to-phaselinevoltagesdifferbymorethan2percent.7Single-phasingIfonelineofa3-phaselineisaccidentallyopenedorifafuseblowswhilethe3-phasemotorisrunning,themachinewiltcontinuetorunasasingle-phasemotor.Thecurrentdrawnfromtheremainingtwotineswillalmostdouble,andthemotorwillbegintooverheat.Thethermalrelaysprotectingthemotorwilleventuallytripthecircuit-hrC1’herebydisconnectingthemotorfromtheline.Thetorque-speedcurveisseriouslyaffectedwhena3-phasemotoroperatesonsinglephase.Thebreak-downtorquedecreasestoabout40%ofitsoriginalvalue,andthemotordevelopsnostartingtorqueatall.Consequently,afullyloaded3-phasemotormaysimplystopifoneofitslinesissuddenlyopened.Theresultinglocked-rotorcurrentisabout90%ofthenormal3-phaseLRcurrent.Itisthereforelargeenoughtotripthecircuitbreakerortoblowthefuses.

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