外文翻译感应电机Word文件下载.docx

外文翻译感应电机Word文件下载.docx

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外文资料

InductionMotor

1Startinganinductionmotor

High-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.

2Plugginganinductionmotor

Insomeindustrialapplications,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.14.10

Whena3-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.

4Abnormalconditions

Abnormalmotoroperationmayduetointernalproblems（short-circuitinthestator,overheatingofthebearing,etc.）ortoexternalconditions.Externalproblemsmaybecausedbyanyofthefollowing:

1.Mechanicaloverload

2.Supplyvoltagechanges

3.Singlephasing

4.Frequencychanges

Wewillexaminethenatureoftheseproblemsinthesectionsthatfollow.

Accordingtonationalstandards,amotorshalloperatesatisfactoryonanyvoltagewithin±

10%ofthenominalvoltage,andforanyfrequencywithin±

5%ofthenormalfrequency.Ifthevoltageandfrequencybothvary,thesumofthetwopercentagechangesmustnotexceed10percent.Finally,allmotorsaredesignedtooperatesatisfactor