英语学习电工学概论习题答案第四章.docx

英语学习电工学概论习题答案第四章.docx

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英语学习电工学概论习题答案第四章

Howdoes4-1.distinguishthecageformandthewindingpatternfromthestructuralcharacteristicsofthethree-phaseasynchronousmotor?

Answer:

rotorwindinginducedelectromotiveforceisgenerated,currentsandelectromagnetictorque,thestructuretypewithcageandwoundrotortwo,squirrelcagerotorofeachrotorslotinsertapieceofcopperconductingbar,inanextendedatbothendsofthecoreatthenotch,withtwoshortcircuitcopperringsrespectively,theallguidebarendsareweldedtogether.Ifthecoreisremoved,theoverallshapeofthewindingislikeacage,soitiscalledacagerotor.Therotorwindingandthestatorwindingissimilartothatoftheinsulatedconductorembeddedintherotorslotandconnectedintoastarthree-phasesymmetricwinding,windingthreeoutletterminalsarerespectivelyconnectedtotherotorshaftthreeslipring（ringandring,theringandtheshaftareinsulatedfromeachother）,thecarbonbrushtodrawoutthecurrent.

4-2.howtochangethethree-phaseinductionmotortochange?

Answer:

anyofthetwowillbeconnectedwiththree-phasepowersupplyconnectedthreewireswitch,three-phaseasynchronousmotordirectionchange.

4-3.knownathree-phasecageinductionmotorratedpower=3kW,ratedspeed=2880r/min.Findthenumberofpolepairs

（1）;

（2）whentheratedslipratio;（3）ratedtorque.

Solution:

（1）synchronousspeed,sothelogarithmofthemotorpolePis1;

（2）

（3）=9.95

Technicaldataof4-4.KnownY112M-4typeasynchronousmotor=4kw,deltaconnectionmethod,ratedvoltage=380V,=1440r/min,ratedcurrent=8.8a,powerfactor=0.82,efficiency=84.5%.Findthenumberofpolepairs

（1）;

（2）inputpowerduringnormaloperation;（3）whentheratedslipratio;（4）ratedtorque.

Solution:

（1）synchronousspeed,sothelogarithmofthemotorpolePis2;

（2）

（3）

（4）=26.5

4-5.KnownY132M-4typeasynchronousmotorratedpower7.5kw,ratedcurrent=15.4A,ratedspeed=1440r/min,ratedvoltage=380V,ratedpowerfactorof0.85andratedefficiency=0.87,startingtorqueandratedtorque=2.2,thestartingcurrentandratedcurrent=7.0,themaximumtorque/ratedtorque=2.2.Tries

（1）ratedpowerinput;

（2）ratedtorque;（3）ratedslip;（4）thestartingcurrent;（5）startingtorque;（6）themaximumtorque.

Solution:

（1）ratedinputpower

（2）=49.74

（3）

（4）=7=107.8A

（5）=2.2=109.43

（6）=2.2=109.43

4-6.inthethree-phaseasynchronousmotorstartinstant（s=1）,whytherotorcurrentislarge,andthepowerfactoroftherotorcircuitissmall?

A:

theelectricmotorisconnectedtothepowersupply,theinductionelectromotiveforceandtheinducedcurrentoftherotorcircuitarethelargest,whichiscalledthestartingcurrentortheblockingcurrent.Ingeneral,thestartingcurrentofthesmallandmediumsizedthreephaseinductionmotoris5to7timesoftheratedcurrent.Althoughthemotorstartingcurrentisverylarge,butthepowerfactoroftherotorisverylow,infact,thestartingtorqueofthemotorisnotlarge.

4-7.ifthereisalargenumberofasynchronousmotorsinthethree-phasepowergridatthesametime,willbetheimpactofthegridvoltage?

Why?

Alargenumberofasynchronousmotorsstartatthesametime,thepowergridwillhavealargervoltagedrop.Becauseasynchronousmotorstartingcurrentis5~7timestheratedcurrent,ifinthethree-phasepowergridhasalargenumberofasynchronousmotorstartatthesametime,willproducealargestartingcurrent,largestartingcurrentwillinpowerlineproducedlargervoltagelanding,transformerpowersupplywithotherworkload.

4-8.three-phaseinductionmotortorquecharacteristicsoftheshapeofthecurvebywhatfactors?

Answer:

electromagnetictorquecanbeexpressedas

WhentheSisverysmall,canbeneglected,TisproportionaltoS;whentheSiscloserto1,canbeomitted,TisinverselyproportionaltoS.Soonthetorquecharacteristiccurve（T~S）asshowninfigure.

T~Scurve

Atthattime,saidthemotorattheratedoperatingstate,atthistimetherotorspeedfortheratedspeed,torquefortheratedtorque,theoutputofthemechanicalpowerfortheratedpower.

Atthattime,themotorisincriticalcondition.Atthesametime,theelectromagnetictorqueisthelargestvaluecanbycalculatingt~scurveextremummethod,toobtainthecriticalslip.Visibleanddirectlyproportionalto,buthasnothingtodowith;regardlessofisproportionalto.Onlyinthiswaycanbychanging（woundrotorcircuitexternalrheostat）andreducedtochanget~scurveshape,asshownin4.2.3and4.2.4.

FigureTincreasesthe4.2.3~Scurvesof4.2.4toreducetheT~Scurve

4-9.whyshouldadoptthemeasureofreducingbloodpressure?

Whatistheeffectofreducingthestartingtorqueofthemotor?

Underwhatcircumstancescanthebuckstart?

Answer:

reducedvoltagestartingofthepurposeistoreducethemotorstartingwhenstartingtheeffectsofelectriccurrentonthegrid,themethodisinstartinglowervoltageinthestatorwindingofthemotor,tobemotorspeedisclosetothestable,thenthevoltagereturnstonormalvalue.Duetothemotortorqueandvoltageisproportionaltothesquare,soreducedvoltagestartingtorquewillalsodecreaseaccordingly,currentstep-downstartthemainmethodforstar-Delta（Y-delta）changetoconnecttostartanditsapplicableconditionisnormaloperationofthestatorwindingisdeltaconnectedsquirrelcageasynchronousmotor.

Whataresomeofthemainmethodsof4-10.reduction?

Answer:

thestartmethod

（1）directstarting,

（2）y-Deltachangeisconnectedwithastep-downstarting,（3）softstartingmethod.（4）rotorseriesresistancestarting.StartingmainlyY-deltaswitchingstartingandsoftstartmethod.

Whydoestheseriesresistanceintherotorcircuitof4-11.woundrotortypeinductionmotorimprovethestartingperformanceandspeedperformanceoftheinductionmotor?

Answer:

windingtypemotorcanbeusedintherotorcircuitinseriesresistancestartingmethod.Thiscannotonlylimitthestartingcurrent,butalsoincreasethestartingtorque.Soweneedtostartproductionofmechanicaltorqueislarge,suchascranes,forgingmachinesandothercommonlyusedlinearmotordrivearound.Afterthemotorisstarted,thestartingresistanceisremovedbythestepwiththeincreaseoftherotatingspeed.

4-12.three-phaseinductionmotorinthenormaloperation,iftherotorsuddenlystuck,whatwillbetheconsequencesofthemotor?

Answer:

three-phaseasynchronousmotorinnormaloperation,iftherotorsuddenlystuck（stall）,motorcurrentincreasedimmediatelyforseveraltimesoftheratedcurrent,ifnoprotectivemeasurespromptlycutoffthepowersupply,themotorwillbeburnt.

4-13.three-phaseasynchronousmotorinnormaloperation,ifsuddenlythereisaphasepower,themotorcanrun?

Ifthemotorisunderheavyload,whatisthechangeofthemotorcurrent?

Answer:

three-phaseasynchronousmotorinnormaloperation,ifsuddenlythereisaphasepower,themotorcanrun.Ifthemotorisoverloadedincaseofslipincreases,theinducedelectromotiveforceincreases,themotorcurrentincreases.Longrunmayburnthemotor.

4-14.threephaseinductionmotorwhichareseveralspeedcontrolmethod?

Whichmethodisthebestperformanceofthespeedcontrol?

Answer:

（1）thenumberofmagneticpolesofthespeedchange,

（2）changeoftherotatingslipspeed,（3）variablefrequencyspeedregulationarethemainspeedcontrolmethodofthree-phaseasynchronousmotor.InmanyspeedregulationmethodsofACinductionmotor,theperformanceofvariablefrequencyspeedregulationisthebest,whichischaracterizedbyhighspeedrange,goodstabilityandhighefficiency.

4-15.constantsupplyvoltageinthecase,iftheratingfortheerrorconnectedYtypeconnectiondeltaconnection,whataretheconsequences?

AndastheamountoftimeforafalsedeltaconnectiontypeYconnection,andwhataretheconsequences?

Answer:

YtypeequivalentconnectingresistanceofthesamesizethreeresistordeltaafterconnectionfordirectYconnectiontypesize1/3.Therefore,atasupplyvoltageofconstant,ifrateddeltaconnectedmistakenlyconnectedtypeYconnection,actuallargerresistance,smallercurrentinthecircuit,resultinginlackofpower.Iftheamount?

Adeltaconnectionerrorwordtypefastconnection,theactualresistanceissmall,thecurrentinthecircuitbecomeslarger,itispossibletoburnoutresistance.

4-16.thermalrelayinthethree-phaseasynchronousmotorcontrolcircuittoplaywhatrole?

Canitplaytheroleofshortcircuitprotection?

Answer:

thermalrelayisusedforACasynchronousmotoroverloadprotection,itusesthethermaleffectofcurrentandaction.Nowtheproductionofheatrelayhasthreegroupsofheatingelementsandhasaphasefailureprotectionfunction,namelyinthecurrentseriousimbalanceorevendisconnectionofaphase,themechanicalstructurecanalsomakeatripofthegussetplatewillmoveoffcontactdisconnectandprotectsthemotorbecauseofthephaseinterruptioncausedbyphasecurrentoverloadandoffthering.

4-17.testtobeabletodirectlycontrolthecontrolcircuitofathree-phasecageasynchronousmotorattwolocations.

Answer:

Smallcapacitycagetypemotordirectstartingcontrolcircuit,whichusedtheairswitchQ,ACcontactorKM,buttonSB,thermalrelayFRandfuseFUandothercontrolelectricalappliances.WorkfirstairQswitchclosed,intothepower,whenthepressthestartbutton（dynamicclosecontactpoints）,thecoiloftheACcontactorkmenergized,movingcoreisattracted,thethreemaincontacts（contactclosed.Thespeedofthemotorthroughelectricstarting.Whenthereleasebuttoncon