外文翻译感应电机Word文件下载.docx
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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
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