发电机和电气设备毕业论文外文翻译.docx
- 文档编号:26434209
- 上传时间:2023-06-19
- 格式:DOCX
- 页数:15
- 大小:101.53KB
发电机和电气设备毕业论文外文翻译.docx
《发电机和电气设备毕业论文外文翻译.docx》由会员分享,可在线阅读,更多相关《发电机和电气设备毕业论文外文翻译.docx(15页珍藏版)》请在冰豆网上搜索。
发电机和电气设备毕业论文外文翻译
GeneratorandElectricalEquipments
Generator
Introduction
Electricgeneratorsconvertmechanicalenergytoelectricalenergy,whichismoreeasilytransmittedtoremotelylocatedpointsofapplication.Thefirstlargeelectricgeneratingsystemsuseddirect-current(dc)generators,mainlybecausedirectcurrentwasbetterunderstoodthanalternatingcurrent(ac).However,dcgeneratorsarelimitedtogeneratingpoweratrelativelylowvoltages,largelyduetoproblemsattheircommutates.
Aspowernetworksdeveloped,higherandhighervoltageswererequiredtotransmitlargeblocksofpoweroverlongerandlongerdistances.Electrictransformerscaneasilychangethenormallylowvoltagegeneratedtothehighvoltagesneededforefficientpowertransmission,andofcourse,transformersonlyworkonalternatingcurrent.Acgenerators,oralternatorsastheyarecommonlycalled,aresomuchsimplermechanically,somuchmoreefficient,andrequiresomuchlessmaintenancethandcmachinesthatalllargegeneratingplantsoutputalternatingcurrenttoday.Althoughdetransmissionlinescantransportextremelylargeblocksofpowerveryefficientlyoverlongdistances,thepowerisalwaysgeneratedasalternatingcurrent,transformedtothevoltagerequired,rectifiedandtransmittedasdirectcurrent,andtheninvertedbacktoalternatingcurrentatthepointofapplication.
MechanicalEnergy
Themechanicalenergyfordrivingthegeneratormustbederivedfromasourcewithenoughreliabilityandcapacitytomakeiteconomicallyfeasibletodevelopandtransmittheenergyelectricallytothepointofuse.Asmallwatersupplyrunningonlyduringexceptionallywetyearsorlocatedatagreatdistancefromelectricalconsumerswouldprobablynotbesuitable.Mechanicalenergysourceswhichcannotbemoved,suchashydraulicturbinesorevenwindmachines,musthavethecostoftransportingtheenergyproduced(amongotherfactors)takenintoaccountwhenoverallcostsarecalculated.Steam-turbinepowerplants,however,canbelocatednearacoalseam,lumbermill,orareliablesourceofcoolingwatertosaveontransportationcosts.Somemechanicalpowermaybeobtainedfromsourcesmoreeasilylocatednearthepointofutilization.Gasturbinesandreciprocatinggasordieselenginesfallintothiscategory.Exceptforstandbyemergencypowergenerators,evenhereitmightbemoreeconomicaltoinstalllargeunitsandtransmitthepowertothepointofuse.Largepowerplantswillgenerallyhavebetteroperatingefficienciesthansmallones,anditmaybedesirabletolocatealargeplantnearthecenterofuseandthendistributethepowergeneratedoutward,assumingthefuelsupplyistransportable.
Eachtypeofmechanicaldriverhasitsownpeculiarities,andsomehaveasizableimpactonthegeneratorconfiguration.TherearemarkeddifferencesastotheengineoutputSpeedsavailable,thespeedpulsationspossible,thechancesofoverspeed,etc..Normally,thegeneratorshaftishorizontalanddirect-connectedtothedriver.Sometimesspeed-changinggearboxesareinstalledbetweenahigh-speedturbineandalower–speedgenerator.Theseallowtheturbinetorunatitsmostefficientspeed,aspeedthatmaybetoohighforthegenerator.Smallhydraulicturbinesusuallyhavetheirshaftsmountedhorizontally;largehydraulicmachineshavetheirshaftsdirect-connectedandverticallymounted.Thegeneratormayincludespecialbearingstocarrythethrustimposedbythewaterflowingthroughtheturbine.Criterialiketheseforprovidingmechanicalenergyimposespecialdesignsonthegeneratingmachines.
BasicPrincipleandConstruction
Therearetwoquitedistinctformsofmodemalternator.Whiletheprincipleofoperationofeachisthesame,i.e.,themovementofmagneticpolespaststationarycoils,theirconstructionsareverydifferent.Thereasonforthisisthateachhasbeendesignedto'match'itsprimemover,i.e.,tosuitthemechanicaldevicethatistotapthetwoprincipalnaturalpowerresources-failingwater,ontheonehand,andsteam,generatedbyheatfromfossilfuelsornuclearfuels,ontheother.
Tomatchtheoutputoftheturboalternators,thewaterwheelgeneratorsmustthereforebemulti-polarandhenceoflargediameterandsmallaxialength.Thereisalimittothelengthofaturboalternator,basedlargelyonthemechanicalconsiderationsinvolvedinsupportingalargerotormassbetweenabearingateachend.At3000or3600r/min(50or60Hz)therotormustbeextremelywellbalancedanditssurfacesmooth.Withthelowerspeedwaterpoweredmachine,suchprecautionscanberelaxedwithaviewtomakingthelargerrotorcheapertomake.
Thefundamentaldifferenceinshapebetweentherotorsofthetwotypesofmachineisconsequentupontheaboveconsiderations,butnowasecondarydifferenceisintroducedbywhatcouldbetermedtheexperienceandskillofthedesigner.
Itisnecessarytoproduceasinewaveofinducede..m.f.Thefactorsthataffecttheinstantaneousvalueeofthisarethefluxdensityb,thelengthIoftheconductor,andthevelocityv(theuseofsmalllettersindicatingtheinstantanousvalues).Thus:
е=blv(4.1)
Areasonablycleardefinitionofwhatconstitutesa'pole'canbegivenbydefiningapolepitchratherthanapoleperse.Apolepitchisthedistance(p)betweenpointswherethecurrentflowisamaximum.Thenumberofpolesinthemachineisthentheperiphery(2πr)dividedbyp.Thisdefinitionfitseasilyintolinearmotortechnology,wherethenumberofpolesneedbeneitherevennoraninteger.ThespeedofrotationexpressedinTable4.1as2f/nr/s,wherenwasthenumberofpoles,canalwaysbeconvertedtoalinearspeed,fortheperiphery2~rcontainsnpolepitcheseachoflengthpsothat2πr=np.Hencetherotationalspeed2f/n/r/s'translates'intoalinearspeed,vs,suchthat
vs=(2f/n)(2πr)=(2f/n)(np)=2pf(4.2)
whichissimplythe'commonsense'statementthatatravellingwavemovestwopolepitches(=onewavelength;A)eachcycleofevents.(Thiscorrespondstothewellknownformulav=fλforallwavemotions.)
Three-phasemachinestatorshowninFigure4.l(a).Itdoesnot,asitappearsatfirstsight,havesixpoles,eventhoughithassixobvious'polarprojections'.Thesearetobeseenassix'teeth'inaslottedstatorwithathree-phase'distributed'winding,exceptthatthedistributionhasvirtuallydisappearedexceptinsofarastherearethreephases.Unlesssuchadiagrammakesclearhowthetwocoilsineachphaseareconnected,noonecansaywhetherithastwopolesorfour.ItisworthstudyingFigure4.1carefully,firsttoappreciatethedifferencesbetween(b)and(c),henceto'see'thekindofdifficultythatcanariseinthemindofthestudentbeingconfrontedwiththeproblemforthefirsttime,andfinallytodemolishtheproblemsothatitneverarisesinthefuture.Fortheconnectionsshownat(b),bothred-phasecoilsassisteachotherindrivingfluxdiametricallyacrossthemachine.Sodobothyellow-phasecoilsandbothblue-phasecoils.So,whateverinstantaneouscurrentsflowinthesystemasawhole,theresultantfluxwillbethevectorsumofthreediametralfluxeswhichthereforeisitselfdiametralandthemachinecorrespondstothetwo-polesystem.ButifoppositepairssendopposingfluxesintotherotorthentheonlypossibleresultantfluxpatterncorrespondstothatofFigure4.1(c)andthemachinehasfourpoles.
Inthiscrudeexamplethelackofwinding'distribution'isnowobvious,sinceatwo-pole,three-phasemachinewithonlysixslotshasoneslotperpoleperphase,or,asisnowmore'fashionably'written,oneslotperpoleandphase,Thefour-poleversionhasonlyhalfaslotperpoleandphase,whichgivesavery'lumpy'kindoftravellingfieldtobeavoidedinpracticeifatallpossiblebyhavingalargernumberofslots.Thereaderw-illappreciate,however,thatifamorerealisticexampleoftwo-andfour-polemachineswith,say,twenty-fourslotseachhadbeenchosen,thediagramsmighthavebecometooobscuretomakethepointabout'polecounting'.
(1)Stators
Therapidlyvaryingmagneticfluxinthestatorironcauseshysteresislossesastheironresistschangesinthefluxdensity.Thevaryingmagneticfluxalsocauseselectriccurrents,callededdycurrents,toflowintheironlaminations;lossesalsoresultfromthiscurrentflow.Thestatorisbuiltfromthinlaminationstominimizetheelectricallossesandofspeciallyrolledsiliconsteeltominimizethehysteresislosses.Forsmallmachines,thelaminationsarecircular,intheshapeofthefinishedstator.Forlargemachines,thelaminationsarepunchedassemicirclesandthenassembledintothefinishedcircularstator.Slotsarepunchedforfutureinstallationofthewindings.
Thewindingslotsaresuitablyinsulatedtoprovidebothelectricalinsulationbetweenthewindingsandthegroundedstatorandprotectionfromabrasiondamagetothewindingsbythestatoriron.Windingsarespecifiedwiththeproperspan,wiresize,andamountofinsulationrequiredbythemachinerating.
Forsmallermachines,thewindingsarewoundwithloosecoilsofroundwire',whichareinsertedintotheslotsprovidedinthestator,mmbymm,andfastenedwithslotwedgestopreventmovementofthewindings.Togetasmuchconductorandstatorironaspossibleintothemachine,largeunitsarewoundwithsquareorrectangularwire,whichisformedintorigidcoilswithinsulationbothbetweentheindividualwiresandaroundthecoilsthemselves.Thecoilsareinsertedintothestatorslots,whichhaveparallelwallstoprovideasnugfitbetweenthecoilsandthestatoriron;slotwedgesholdthe
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 发电机和电气设备 毕业论文外文翻译 发电机 电气设备 毕业论文 外文 翻译