发电机Generator国外经典资料Word格式文档下载.docx

发电机Generator国外经典资料Word格式文档下载.docx

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ACelectricmotorfamilydiagram.

CharlesP.Steinmetzcontributedtosolvingtheseproblemswithhisinvestigationofhysteresislossesinironarmatures.NikolaTeslaenvisionedanentirelynewtypeofmotorwhenhevisualizedaspinningturbine,notspunbywaterorsteam,butbyarotatingmagneticfield.Hisnewtypeofmotor,theACinductionmotor,istheworkhorseofindustrytothisday.Itsruggednessandsimplicity（Figureabove）makeforlonglife,highreliability,andlowmaintenance.YetsmallbrushedACmotors,similartotheDCvariety,persistinsmallappliancesalongwithsmallTeslainductionmotors.Aboveonehorsepower（750W）,theTeslamotorreignssupreme.

ModernsolidstateelectroniccircuitsdrivebrushlessDCmotorswithACwaveformsgeneratedfromaDCsource.ThebrushlessDCmotor,actuallyanACmotor,isreplacingtheconventionalbrushedDCmotorinmanyapplications.And,thesteppermotor,adigitalversionofmotor,isdrivenbyalternatingcurrentsquarewaves,again,generatedbysolidstatecircuitryFigureaboveshowsthefamilytreeoftheACmotorsdescribedinthischapter.

Cruiseshipsandotherlargevesselsreplacereductiongeareddriveshaftswithlargemulti-megawattgeneratorsandmotors.Suchhasbeenthecasewithdiesel-electriclocomotivesonasmallerscaleformanyyears.

Motorsystemleveldiagram.

Atthesystemlevel,（Figureabove）amotortakesinelectricalenergyintermsofapotentialdifferenceandacurrentflow,convertingittomechanicalwork.Alas,electricmotorsarenot100%efficient.Someoftheelectricenergyislosttoheat,anotherformofenergy,duetoI2Rlossesinthemotorwindings.Theheatisanundesiredbyproductoftheconversion.Itmustberemovedfromthemotorandmayadverselyaffectlongevity.Thus,onegoalistomaximizemotorefficiency,reducingtheheatloss.ACmotorsalsohavesomelossesnotencounteredbyDCmotors:

hysteresisandeddycurrents.

HysteresisandEddyCurrent

EarlydesignersofACmotorsencounteredproblemstracedtolossesuniquetoalternatingcurrentmagnetics.TheseproblemswereencounteredwhenadaptingDCmotorstoACoperation.ThoughfewACmotorstodaybearanyresemblancetoDCmotors,theseproblemshadtobesolvedbeforeACmotorsofanytypecouldbeproperlydesignedbeforetheywerebuilt.

BothrotorandstatorcoresofACmotorsarecomposedofastackofinsulatedlaminations.Thelaminationsarecoatedwithinsulatingvarnishbeforestackingandboltingintothefinalform.Eddycurrentsareminimizedbybreakingthepotentialconductiveloopintosmallerlesslossysegments.（Figurebelow）Thecurrentloopslooklikeshortedtransformersecondaryturns.Thethinisolatedlaminationsbreaktheseloops.Also,thesilicon（asemiconductor）addedtothealloyusedinthelaminationsincreaseselectricalresistancewhichdecreasesthemagnitudeofeddycurrents.

Eddycurrentsinironcores.

Ifthelaminationsaremadeofsiliconalloygrainorientedsteel,hysteresislossesareminimized.Magnetichysteresisisalaggingbehindofmagneticfieldstrengthascomparedtomagnetizingforce.Ifasoftironnailistemporarilymagnetizedbyasolenoid,onewouldexpectthenailtolosethemagneticfieldoncethesolenoidisde-energized.However,asmallamountofresidualmagnetization,Brduetohysteresisremains.（Figurebelow）Analternatingcurrenthastoexpendenergy,-Hcthecoerciveforce,inovercomingthisresidualmagnetizationbeforeitcanmagnetizethecorebacktozero,letaloneintheoppositedirection.HysteresislossisencounteredeachtimethepolarityoftheACreverses.ThelossisproportionaltotheareaenclosedbythehysteresisloopontheB-Hcurve.“Soft”ironalloyshavelowerlossesthan“hard”highcarbonsteelalloys.Silicongrainorientedsteel,4%silicon,rolledtopreferentiallyorientthegrainorcrystallinestructure,hasstilllowerlosses.

Hysteresiscurvesforlowandhighlossalloys.

OnceSteinmetz'

sLawsofhysteresiscouldpredictironcorelosses,itwaspossibletodesignACmotorswhichperformedasdesigned.Thiswasakintobeingabletodesignabridgeaheadoftimethatwouldnotcollapseonceitwasactuallybuilt.ThisknowledgeofeddycurrentandhysteresiswasfirstappliedtobuildingACcommutatormotorssimilartotheirDCcounterparts.TodaythisisbutaminorcategoryofACmotors.OthersinventednewtypesofACmotorsbearinglittleresemblancetotheirDCkin.

SynchronousMotors

Singlephasesynchronousmotorsareavailableinsmallsizesforapplicationsrequiringprecisetimingsuchastimekeeping,（clocks）andtapeplayers.Thoughbatterypoweredquartzregulatedclocksarewidelyavailable,theAClineoperatedvarietyhasbetterlongtermaccuracy--overaperiodofmonths.ThisisduetopowerplantoperatorspurposelymaintainingthelongtermaccuracyofthefrequencyoftheACdistributionsystem.Ifitfallsbehindbyafewcycles,theywillmakeupthelostcyclesofACsothatclockslosenotime.

Above10Horsepower（10kW）thehigherefficiencyandleadingpowerfactormakelargesynchronousmotorsusefulinindustry.Largesynchronousmotorsareafewpercentmoreefficientthanthemorecommoninductionmotors.Though,thesynchronousmotorismorecomplex.

Sincemotorsandgeneratorsaresimilarinconstruction,itshouldbepossibletouseageneratorasamotor,conversely,useamotorasagenerator.Asynchronousmotorissimilartoanalternatorwitharotatingfield.Thefigurebelowshowssmallalternatorswithapermanentmagnetrotatingfield.Thisfigurebelowcouldeitherbetwoparalleledandsynchronizedalternatorsdrivenbyamechanicalenergysources,oranalternatordrivingasynchronousmotor.Or,itcouldbetwomotors,ifanexternalpowersourcewereconnected.Thepointisthatineithercasetherotorsmustrunatthesamenominalfrequency,andbeinphasewitheachother.Thatis,theymustbesynchronized.Theprocedureforsynchronizingtwoalternatorsisto

（1）opentheswitch,

（2）drivebothalternatorsatthesamerotationalrate,（3）advanceorretardthephaseofoneunituntilbothACoutputsareinphase,（4）closetheswitchbeforetheydriftoutofphase.Oncesynchronized,thealternatorswillbelockedtoeachother,requiringconsiderabletorquetobreakoneunitloose（outofsynchronization）fromtheother.

Synchronousmotorrunninginstepwithalternator.

Ifmoretorqueinthedirectionofrotationisappliedtotherotorofoneoftheaboverotatingalternators,theangleoftherotorwilladvance（oppositeof（3））withrespecttothemagneticfieldinthestatorcoilswhilestillsynchronizedandtherotorwilldeliverenergytotheAClinelikeanalternator.Therotorwillalsobeadvancedwithrespecttotherotorintheotheralternator.Ifaloadsuchasabrakeisappliedtooneoftheaboveunits,theangleoftherotorwilllagthestatorfieldasat（3）,extractingenergyfromtheACline,likeamotor.Ifexcessivetorqueordragisapplied,therotorwillexceedthemaximumtorqueangleadvancingorlaggingsomuchthatsynchronizationislost.Torqueisdevelopedonlywhensynchronizationofthemotorismaintained.

InthecaseofasmallsynchronousmotorinplaceofthealternatorFigureaboveright,itisnotnecessarytogothroughtheelaboratesynchronizationprocedureforalternators.However,thesynchronousmotorisnotselfstartingandmuststillbebroughtuptotheapproximatealternatorelectricalspeedbeforeitwilllock（synchronize）tothegeneratorrotationalrate.Onceuptospeed,thesynchronousmotorwillmaintainsynchronismwiththeACpowersourceanddeveloptorque.

Sinewavedrivessynchronousmotor.

Assumingthatthemotorisuptosynchronousspeed,asthesinewavechangestopositiveinFigureabove

（1）,thelowernorthcoilpushesthenorthrotorpole,whiletheuppersouthcoilattractsthatrotornorthpole.Inasimilarmannertherotorsouthpoleisrepelledbytheuppersouthcoilandattractedtothelowernorthcoil.Bythetimethatthesinewavereachesapeakat

（2）,thetorqueholdingthenorthpoleoftherotorupisatamaximum.Thistorquedecreasesasthesinewavedecreasesto0VDCat（3）withthetorqueataminimum.

Asthesinewavechangestonegativebetween（3&

4）,thelowersouthcoilpushesthesouthrotorpole,whileattractingrotornorthrotorpole.Inasimilarmannertherotornorthpoleisrepelledbytheuppernorthcoilandattractedtothelowersouthcoil.At（4）thesinewavereachesanegativepeakwithholdingtorqueagainatamaximum.Asthesinewavechangesfromnegativeto0VDCtopositive,Theprocessrepeatsforanewcycleofsinewave.

Note,theabovefigureillustratestherotorpositionforano-loadcondition（α=0o）.Inactualpractice,loadingtherotorwillcausetherotortolagthepositionsshownbyangleα.Thisangleincreaseswithloadinguntilthemaximummotortorqueisreachedatα=90oelectrical.Synchronizationandtorquearelostbeyondthisangle.

Thecurrentinthecoilsofasinglephasesynchronousmotorpulsateswhilealternatingpolarity.Ifthepermanentmagnetrotorspeedisclosetothefrequencyofthisaltern