电气工程专业英语第5单元.docx

电气工程专业英语第5单元.docx

《电气工程专业英语第5单元.docx》由会员分享，可在线阅读，更多相关《电气工程专业英语第5单元.docx（15页珍藏版）》请在冰豆网上搜索。

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