电气工程专业英语汉语及翻译.docx
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电气工程专业英语汉语及翻译.docx
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电气工程专业英语汉语及翻译
电气工程
专业英语
姓名:
吕海龙
学号:
20080345
班级:
08电气
专业:
电气工程及其自动化
ElectricDevicesandSystems
Althoughtransformershavenomovingparts,theyareessentialtoelectromechanicalenergyconversion.Theymakeitpossibletoincreaseordecreasethevoltageleverthatresultsinlowcosts,andcanbedistributedandusedsafely.Inaddition,theycanprovidematchingofimpedances,andregulatetheflowofpowerinanetwork.
Whenweseeatransformeronautilitypoleallweisacylinderwithafewwiresstickingout.Thesewiresenterthetransformerthroughbushingsthatprovideisolationbetweenthewiresandthetank.Insidethetanktheseisanironcorelinkingcoils,mostprobablymadewithcopper,andinsulated.Thesystemofinsulationisalsoassociatedwiththatofcoolingthecore/coilassembly.Oftentheinsulationispaper,andthewholeassemblymaybeimmersedininsulatingoil,usedtobothincreasethedielectricstrengthofthepaperandtotransferbeatfromthecore-coilassemblytotheouterwallsofthetanktoair.Figureshowsthecutoutofatypicaldistributiontransformer.
Fewidealversionsofhumanconstructionsexist,andthetransformeroffersnoexception.Anidealtransformerisbasedonverysimpleconcepts,andalargenumberofassumptions.Thisisthetransformeronelearnsaboutinhighschool.
Letustakeanironcorewithinfinitepermeabilityandtwocoilswoundaroundit,onewithN1andtheotherwithN2turns,asshowninfigure.Allthemagneticfluxistoremainintheiron.Weassignsotsatoneterminalofeachcoilinthefollowingfashion:
ifthefluxinthecorechanges,inducingavoltageinthecoils,andthedottedterminalofonecoilispositivewithrespectitsotherterminal,soisthedottedterminaloftheothercoil.Or,thecorollarytothis,currentintodottedterminalsproducesfluxinthesamedirection,
Assumethatsomehowatimevaryingfluxisestablishedintheiron.Thenthefluxlinkagesineachcoilwillbe.Voltageswillbeinducedinthesetwocoil.
Ontheotherhand,currentsflowinginthecoilsarerelatedtothefieldintensityH.ifcurrentsflowinginthedirectionshown,i1intothedottedterminalofcoil1,andi2outofthedottedterminalofcoil2.werecognizethatthisispracticallyimpossible,butsoistheexistenceofanidealtransformer.
Equationsdescribethisidealtransformer,atwoportnetwork.Thesymbolofanetworkthatisdefinedbythesetwoequationsisinthefigure.Anidealtransformerhasaninterestingcharacteristic.Atwo-portnetworkthatcontainsitandimpedancescanbereplacedbyanequivalentother,asdiscussedbelow.Considerthecircuitinfigure.Seenasatwoportnetwork.Generallyacircuitonaside1canbetransferredtoside2bymultiplyingitscomponentimpedances,thevoltagesourcesandthecurrentsources,whilekeepingthetopologythesame.
Todeveloptheequivalentforatransformerwe’llgraduallyrelaxtheassumptionsthatwehadfirstimposed.Firstwe’llrelaxtheassumptionthatthepermeabilityoftheironisinfinite.Inthatcaseequationdoesnotrevertto,butratheritbecomeswhereisthereluctanceofthepatharoundthecoreofthetransformerandthefluxonthispath.Topreservetheidealtransformerequationsaspartofournewtransformer,wecanspliti1totwocomponents:
onei1,willsatisfytheidealtransformerequation,andtheother,i1willjustbalancetherighthandside.Thefigureshowsthis.
Wecanreplacethecurrentsource,i1,withsomethingsimplerifwerememberthattherateofchangeoffluxisrelatedtotheinducedvoltage.
Sincethecurrenti1flowsthroughsomething,wherethevoltageacrossitIsproportionaltoitsderivative,wecanconsiderthatthissomethingcouldbeaninductance.Thisideagivesrisetotheequivalentcircuitinfigure,.Letusnowrelaxtheassumptionthatallthefluxhastoremainintheironasshowninfigure.Letuscallthefluxintheiron,magnetizingflux,thefluxthatleaksoutofthecoreandlinksonlycoil1.sincelinksonlycoil1,thenitshouldberelatedonlytothecurrentthere,andthesameshouldbetrueforthesecondleakageflux.
Againforagivenfrequency,thepowerlossesinthecoreincreasewiththevoltage.Theselossescannotbeallowedtoexceedlimit,beyondwhichthetemperatureofthehottestspotinthetransformerwillriseabovethepointthatwilldecreasedramaticallythelifeoftheinsulation.LimitsthereforeareputtoE1andE2,andtheselimitsarethevoltagelimitsofthetransformer.
Similarly,windingJoulelosseshavetobelimited,resultinginlimitstothecurrentsI1andI2.
Typicallyatransformerisdescribedbyitsratedvoltages,thatgiveboththelimitsandturnsradio.Theratiooftheratedcurrentsistheinverseoftheratioofthevoltagesifweneglectthemagnetizingcurrent.Insteadofthetransformerratedcurrents,atransformerisdescribedbyitsratedapparentpower.
Underratedconditions,maximumcurrentandvoltage,intypicaltransformersthemagnetizingcurrent,doesnotexceed1%ofthecurrentinthetransformer.Itseffectthereforeinthevoltagedropontheleakageinductanceandwindingresistanceisnegligible.
Undermaximumcurrent,totalvoltagedropsonthewindingresistancesandleakageinductancesdonotexceedintypicaltransformer6%oftheratedvoltage.TheeffectthereforeofthewindingcurrentonthevoltagesE1andE2issmall,andtheireffectonthemagnetizingcurrentcanbeneglected.
Theseconsiderationsallowustomodifytheequivalentcircuitinfigure,toobtaintheslightlyinaccuratebutmuchmoreusefulequivalentcircuitsinfigures.
AdjustableSpeedDrives
Bydefinition,adjustablespeeddrivesofanytypeprovideameansofvariablychangingspeedtobettermatchoperatingrequirements.Suchdrivesareavailableinmechanical,fluidandelectricaltyped.
Themostcommonmechanicalversionsusecombinationsofbeltsandsheaves,orchainsandsprockets,toadjustspeedinset,selectableratios-2:
1,4:
1,8:
1andsoforth.Tractiondrives,amoresophisticatedmechanicalcontrolscheme,allowincrementalspeedadjustments.Here,outputspeedisvariedbychangingthecontactpointsbetweenmetallicdisks,orbetweenballsandcones.
Adjustablespeedfluiddrivesprovidesmooth,steplessadjustablespeedcontrol.Therearethreemajortypes.Hydrostaticdrivesuseelectricmotorsorinternalcombustionenginesasprimemoversincombinationwithhydraulicpumps,whichinturndrivehydraulicmotors.Hydrokineticandhydroviscousdrivesdirectlycoupleinputandoutputshafts.Hydrokineticversionsadjustspeedbyvaryingtheamountoffluidinavortexthatservesastheinput-to-outputcoupler.Hydroviscousdrives,alsocalledoilsheardrives,adjustspeedbycontrollingoil-filmthickness,andthereforeslippage,betweenrotatingmetallicdisk.
Aneddycurrentdrive,whiletechnicallyanelectricaldrive,neverthelessfunctionsmuchlikeahydrokineticorhydrovidcousfluiddriveinthatitservesasacouplerbetweenaprimemoveranddrivenload.Inaneddycurrentdrive,thecouplingconsistsofaprimarymagneticfieldandsecondaryfieldscreatedbyinducededdycurrents.Theyamountofmagneticslippageallowedamongthefieldscontrolsthedrivingspeed.
Inmostindustrialapplications,mechanical,fluidoreddycurrentdrivesarepairedwithconstant-speedelectricmotors.Ontheotherhand,solidstateelectricaldrives,createadjustablespeedmotors,allowingspeedsfromzeroRPMtobeyondthemotor’sbasespeed.Controllingthespeedofthemotorhasseveralbenefits,includingincreasedenergyefficiencybyeliminatingenergylossesinmechanicalspeedchangingdevices.Inaddition,byreducing,orofteneliminating,theneedforwear-pronemechanicalcomponents,electricaldrivesfosterincreasedoverallsystemreliability,aswellaslowermaintenancecosts.Fortheseandotherreasons,electricaldrivesarethefastestgrowingtypeofadjustablespeeddrive..
Therearetwobasicdrivetypesrelatedtothetypeofmotorcontrolled-dcandAC.ADCdirectcurrentdrivecontrolsthespeedofaDCmotorbyvaryingthearmaturevoltage(andsometimesalsothefieldvoltage).AnalternatingcurrentdrivecontrolsthespeedofanACmotorbyvaryingthefrequencyandvoltagesuppliedtothemotor.
Directcurrentdrivesareeasytoapplyandtechnologicallystraightforward,TheyworkbyrectifyingACvoltagefromthepowerlinetoDCvoltage,thenfeedingadjustablevoltagetoaDCmotor.WithpermanentmagnetDCmotors,onlythearmaturevoltageiscontrolled.Themorevoltagesupplied,thefasterthearmatureturns.Withwound-fieldmotors,voltagemustbesuppliedtoboththearmatureandthefield.Inindustry,thefollowingthreetypesofDCdrivesaremostcommon,asshowninthefigure.
Drives:
thesearenamedforthesiliconcontrolledrectifiers(alsocalledthyristors)usedtoconvertACtocontrolledvoltageDC.Inexpensiveandeasytouse,thesedrivescomeinavarietyofenclosures,andinunidirectionalorreversingstyles.
RegenerativeSCRDrives:
Alsocalledfourquadrantdrives,theseallowtheDCmotortoprovidebothmotoringandbrakingtorque,Powercomingbackfromthemotorduringbrakingisregeneratedbacktothepowerlineandnotlost.
PulseWidthModulatedDCDrives:
AbbreviatedPWMandalsocalled,generically,transistorizedDCdrives,theseprovidesmootherspeedcontrolwithhigherefficiencyandlessmotorheating,UnlikeSCRdrives,PWMtypeshavethreeelements.ThefirstconvertsACtoDC,thesecondfiltersandregulatesthefixedDCvoltage,andthethirdcontrolsaveragevoltagebycreatingastreamofvariablewidthDCpulses.Thefilteringsectionandhigher
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