外文翻译感应电机.docx
《外文翻译感应电机.docx》由会员分享,可在线阅读,更多相关《外文翻译感应电机.docx(8页珍藏版)》请在冰豆网上搜索。
![外文翻译感应电机.docx](https://file1.bdocx.com/fileroot1/2022-11/16/8cd167d6-45a6-4a04-8039-d8b273699706/8cd167d6-45a6-4a04-8039-d8b2736997061.gif)
外文翻译感应电机
附录A:
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