电气工程专业英语第5单元.docx
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电气工程专业英语第5单元.docx
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电气工程专业英语第5单元
Part2ElectricalMachine,ControlComponentandSensor
P80~95
IntroductiontoElectricalMachines
5.1BRIEFHISTORYOFELETRICALMACHINES
Electricalmachineryhasbeeninexistenceformanyyears.Theapplicationsofelectricalmachineshaveexpandedrapidlysincetheirfirstusemanyyearsago.Atthepresenttime,applicationscontinuetoincreaseatarapidrate.
ThomasEdisonisgivencreditfordevelopingtheconceptofwidespreadgenerationanddistributionofelectricalpower.Heperformeddevelopmentalworkondirect-current(DC)generatorswhichweredrivenbysteamengines.Edison'sworkwithelectricallightsandpowerproductionledthewaytothedevelopmentofDCmotorsandassociatedcontrolequipment.
Mostearlydiscoveriesrelatedtoelectricalmachineryoperationdealtwithdirect-currentsystems.Alternating-current(AC)powergenerationanddistributionbecamewidespreadashorttimelater.TheprimaryreasonforconvertingtoACpowerproductionanddistributionwasthattransformerscouldbeusedtoincreaseACvoltagelevelsforlong-distancedistributionofelectricalpower.Thusthediscoveryoftransformers(变压器)allowedtheconversionofpowerproductionanddistributionsystemsforDCtoACsystems.Presently,almostallelectricalpowersystemsproduceanddistributethree-phasealternatingcurrent.TransformersallowthevoltageproducedbyanACgeneratortobeincreasedwhiledecreasingthecurrentlevelbyacorrespondingamount.Thisallowslong-distancedistributionatareducedcurrentlevel,reducespowerlosses,andincreasessystemefficiency.
Theuseofelectricalmotors(电机)hasincreasedforhomeappliancesandindustrialandcommercialapplicationsfordrivingmachinesandsophisticatedequipment.Manymachinesandautomatedindustrialequipmentnowrequireprecisecontrol.ThusmotordesignandcomplexityhaschangedsinceearlyDCmotorswhichwereusedprimarilywithrailroadtrains.Motorcontrolmethodshavenowbecomemorecriticaltotheefficientandeffectiveoperationofmachinesandequipment.Suchinnovations(新事物)asservo(伺服)controlsystemsandindustrialrobotshaveledtonewdevelopmentsinmotordesign.
Ourcomplexsystemoftransportationhasalsohadanimpact(冲击)ontheuseofelectricalmachines.Automobilesandothermeansofgroundtransportationuseelectricalmotorsforstaringandgeneratorsfortheirbattery-chargingsystems.Therehasrecentlybeenemphasisinthedevelopmentofelectricmotor-drivenautomobiles.Aircraftuseelectricalmachinesinwayssimilartoautomobiles.However,theyalsousesophisticatedsynchro(同步机)andservo-controlledmachineswhileinoperation.
5.2BASICCONSTRUCTIONOFELECTRICALMACHINES
Rotatingelectricalmachinesaccomplishelectromechanicalenergyconversion.Generatorsconvertmechanicalenergyintoelectricalenergy,whilemotorsconvertanelectricalenergyinputintoamechanicalenergyoutput.Generatorsandmotorshavebasicconstructioncharacteristicswhicharecommonamongmanytypesofmachines.Thefunctionsofvariousmachinesdiffereventhoughtheirconstructionissimilar.Generatorshaverotarymotionsuppliesbyprimemovers(原动机)whichprovidemechanicalenergyinput.Relativemotionbetweentheconductorsandamagneticfieldofgeneratorsproducesanelectricalenergyoutput.Motorshaveelectricalenergysuppliedtotheirwindings(绕组)andamagneticfieldthatdevelopsanelectromagneticinteractiontoproducemechanicalenergyortorque(转矩).
Theconstructionofmostrotatingelectricalmachinesissomewhatsimilar.Mostmachineshaveastationarypartcalledthestatorandarotatingsetofconductorscalledtherotor.Thestatorconsistsofayoke(轭)orframe(框架)whichservesasasupportandametallicpathformagneticfluxdevelopedinamachine.
5.2.1Fieldpolesandwindings
Rotatingmachineshavefieldpoles(磁极)whicharepartofthestator(定子)assembly(组件).Fieldpolesareconstructedoflaminatedsheets(薄片)ofsteelandsecured(固定)tothemachineframe.Theyareusuallycurvedontheportionneartherotortoprovidealow-reluctancepathformagneticflux(磁通,磁流).Thefieldwindings(励磁绕组)orfieldcoils(励磁线圈)areplacedaroundthepoles.Thefieldcoilsareelectromagnetsthatdevelopanelectromagneticfieldinteractionwiththerotortogenerateavoltageortoproducetorqueinamachine.
5.2.2Rotorconstruction
Inthestudyofelectricalmachines,thereisaneedtounderstandtheelectromagneticfieldproducedbytherotatingsectionofamotororagenerator.Thissectioniscalledarmatureorrotor.Sometypesofmachinesusesolidmetalrotorscalledsquirrel-cagerotors.
5.2.3Sliprings,splitrings,andbrushes
Inorderforelectricalenergytobesuppliedtoarotatingdevicesuchasthearmature(电枢),somesortofslidingbrushcontactmustbeestablished.Slidingbrushcontactsareeithersliprings(滑环)orsplitrings(裂环).Slipringsareconstructedofacylinderofinsulating(绝缘的)materialwithtwoseparatesolidmetalringsglued(粘合)toit.Slidingbrushesmadeofcarbonandgraphite(石墨)rideonthemetalringsandpermitapplicationorextractionofelectricalenergyfromtheringsduringrotation.Thesplit-ringcommutatorissimilartotheslipringexceptasolidmetalringiscutintotwoormoreseparatesections.Asageneralrule,slipringsareusedinACmotorsandgenerators,whileDCmachinesemploythesplit-ringcommutatingdevice.Thegap(裂隙)orsplitinthecommutatoriskeptataminimumtoreducesparkingofthebrushes.SlipringsandsplitringsareshowninFig.5.1.
5.2.4Othermachineparts
Thereareseveralotherpartsusedintheconstructionofrotatingmachines.Amongthesearetherotorshaft(轴),whilerotatesbetweenasetofbearings(轴承).Bearingsmaybeeithertheball(滚珠),roller(滚柱),orsleeve(套筒)type.Bearingseals(密封件),oftenmadeoffeltmaterial,areusedtokeeplubricantaroundthebearingandkeepdirtout.Arotorcoreisusuallyconstructedoflaminatedsteeltoprovidealow-reluctancemagneticpathbetweenthefieldpolesofamachineandtoreduceeddycurrents(涡流).Internalandexternalelectricalconnectionsprovideameansofdeliveringorextractingelectricalenergy.
5.3CONSTRUCTIONFEATURES(组件)OFELECTRICALMACHINES
Theenergy-conversionprocessusuallyinvolvesthepresenceoftwoimportantfeaturesinagivenelectromechanicaldevice.Thesearethefieldwinding,whichproducesthefluxdensity(磁通密度),andthearmaturewinding,inwhichthe“working”emf(反电动势)isinduced.Inthissectionthesalientconstructionfeaturesoftheprincipaltypesofelectricmachinesaredescribedtoshowthelocationofthesewindings,aswellastodemonstratethegeneralcompositionofsuchmachines.
5.3.1Three-PhaseInductionMotor
Thisisoneofthemostruggedandmostwidelyusedmachinesinindustry.Itsstatoriscomposedoflaminationsofhigh-gradesheetsteel.Theinnersurfaceisslotted(开槽)toaccommodate(容纳)athree-phasewinding.InFig.5.2(a)thethree-phasewindingisrepresentedbythreecoils,theaxesofwhichare120electricaldegreesapart.Coilaa’representsallthecoilsassignedtophaseaforonepairofpoles.Similarlycoilbb’representsphasebcoils,andcoilcc’representsphaseccoils.Whenoneendofeachphaseistiedtogether,asdepicted(描述)inFig.5.2(b),thethree-phasestatorwindingissaidtobeY-connected.Suchawindingiscalledathree-phasewindingbecausethevoltagesinducedineachofthethreephasesbyarevolvingfluxdensityfield(旋转磁场)areoutofphase(相差,异相)by120electricaldegrees—adistinguishingcharacteristicofabalancedthree-phasesystem.
Therotoralsoconsistsoflaminationsofslottedferromagnetic(铁磁的)material,buttherotorwindingmaybeeitherthesquirrel-cagetypeorthewound-rotor(绕线电机)type.Thelatterisofaformsimilartothatofthestatorwinding.Thewindingterminalsarebroughtoutto(引向,接到)threesliprings.Thisallowsanexternalthree-phaseresistortobeconnectedtotherotorwindingforthepurposeofprovidingspeedcontrol.Asamatteroffact,itistheneedforspeedcontrolwhichislargemeasureaccountsfortheuseofthewound-rotortypeinductionmotor.Otherwisethesquirrel-cageinductionmotorwouldbeused.Thesquirrel-cagewindingconsistsmerelyofanumberofcopperbars(铜导条)embeddedintherotorslotsandconnectedatbothendsbymeansofcopperendrings(铜端条).(Insomeofthesmallersizesaluminumisused.)Thesquirrel-cageconstructionisnotonlysimplerandmoreeconomicalthanthewound-rotortypebutmoreruggedaswell.Therearenotslipringsorcarbonbrushestobebotheredwith.
Innormaloperationathree-phasevoltageisappliedtothestatorwindingatpointa-b-cinFig.5.2.Magnetizingcurrentsflowineachphasewhichtogethercreatearevolvingmagneticfieldhavingtwopoles.Thespeedofthefieldisfixedbythefrequencyofthemagnetizingcurrentsandthenumberofpolesforwhichthestatorwindingisdesigned.Fig.5.2showstheconfigurationfortwopoles.Ifthepatterna-c’-b-a’-c-b’ismadetospan(跨越)only180mechanicaldegreesandthenisrepeatedovertheremaining180mechanicaldegrees,amachinehavingafour-polefielddistributionresults.Forap-polemachinethebasicwindingpatternmustberepeatedp/2timeswithinthecircumferenceoftheinnersurfaceofthestator.
Therevolvingfieldproducedbystatorwindingcutstherotorconductors,therebyinducingvoltages.Sincetherotorwindingisshort-circuitedbytheendrings,theinducedvoltagescausecurrentstoflowwhichinturnreactwiththefieldtoproduceelectromagnetictorque-andsomotoractionresults.
Accordingly,onthebasisoftheforegoingdescription,itshouldbeclearthattheforthethree-phaseinductionmotorthefieldwinding
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