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ACMachinesIntroductionTheelectricalmachinethatconvertselectricalenergyintomechanicalenergy,andviceversa,istheworkhorseinadrivesystem.Amachineisacomplexstructureelectrically,mechanically,andthermally.Althoughmachineswereintroducedmorethanonehundredyearsago,theresearchanddevelopmentinthisareaappearstobenever-ending.However,theevolutionofmachineshasbeenslowcomparedtothatofpowersemiconductordevicesandpowerelectronicconverters.Traditionally,ACmachineswithaconstantfrequencysinusoidalpowersupplyhavebeenusedinconstant-speedapplications,whereasDCmachineswerepreferredforvariable-speeddrives.Butinthelasttwoorthreedecades,wehaveseenextensiveresearchanddevelopmenteffortsforvariable-frequency,variable-speedACmachinedrivetechnology,andtheywillprogressivelyreplaceDCdrives.Inmostcases,newapplicationsuseACdrives.ACmachinescangenerallybeclassifiedasfollows:Inductionmachines:Cageorwoundrotor(doubly-fed),Rotatingorlinear;Synchronousmachines:Rotatingorlinear,Reluctance,Woundfieldorpermanentmagnet,Radialoraxialgap(disk),Surfacemagnetorinterior(buried)magnet,Sinusoidalortrapezoidal;Variablereluctancemachines:Switchedreluctance,Stepper.InductionMachinesAmongalltypesofACmachines,theinductionmachine,particularlythecagetype,ismostcommonlyusedinindustry.Thesemachinesareveryeconomical,rugged,andreliable,andareavailableintherangesoffractionalhorsepower(FHP)tomulti-megawattcapacity.Low-powerFHPmachinesareavailableinsingle-phase,butpoly-phase(three-phase)machinesareused,mostofteninvariable-speeddrives.Fig.1-6A-1showsanidealizedthree-phase,two-poleinductionmotorwhereeachphasewindinginthestatorandrotorisrepresentedbyaconcentratedcoil.Thethree-phasewindingsaredistributedsinusoidallyandembeddedinslots.Inawound-rotormachine,therotorwindingissimilartothatofthestator,butinacagemachine,therotorhasasquirrelcage-likestructurewithshortedendrings.Basically,themachinecanbelookeduponasathree-phasetransformerwitharotatingandshort-circuitedsecondary.Bothstatorandrotorcoresaremadewithlaminatedferromagneticsteelsheets.Theairgapinthemachineispracticallyuniform(non-salientpole).PfNee120Oneofthemostfundamentalprinciplesofinductionmachinesisthecreationofarotatingandsinusoidallydistributedmagneticfieldintheairgap.Neglectingtheeffectofslotsandspaceharmonicsduetonon-idealwindingdistribution,itcanbeshownthatasinusoidalthree-phasebalancedpowersupplyinthethree-phasestatorwindingcreatesasynchronouslyrotatingmagneticfield.Therotationalspeedcanbegivenasequation(1-6A-1).Neiscalledsynchronousspeedinrpmandπ2/eewfisthestatorfrequencyinHz.Pisthepolenumbersofamachine.Therotorwindingwillbesubjectedtoasweepingmagneticfield,andhaveinducingcurrentintheshort-circuitedrotor.Theinteractionofairgapfluxandrotormmfproducestorque,maketherotorrotate.Butthespeedoftherotorislessthansynchronousspeed.Soitcalledinductionmachineorasynchronousmachine.Tomeetthevariousstartingandrunningrequirementsofavarietyofindustrialapplications,severalstandarddesignsofsquirrel-cagemotorsareavailablefrommanufacturers’stock.Thetorque-speedcharacteristicsofthemostcommondesigns,readilyavailableandstandardizedinaccordancewiththecriteriaestablishedbytheNationalElectricalbsStatorasaxiscsb'sRotorasStatorc'sc'rbra'rcrb'rarωrRotoraraxisa'sFig.1-6A-1idealizedthree-phase,two-poleinductionmotor(1-6A-1)Manufacturers’Association(NEMA),areshowninFig.1-6A-2.Themostsignificantdesignvariableinthesemotorsistheeffectiveresistanceoftherotorcagecircuits.DCBAT/Nm2003001000nsn/rmin1ClassAMotorsThesemachinesaresuitableforapplicationswheretheloadtorqueislowatstart(suchasfanorpumploads)sothatfullspeedisachievedrapidly,therebyeliminatingtheproblemofoverheatingduringstarting.Inlargemachines,low-voltagestartingisrequiredtolimitthestartingcurrent.ClassBMotorsMotorsofthisclassaregoodgeneral-purposemotorsandhaveawidevarietyofindustrialapplications.Theyareparticularlysuitableforconstant-speeddrives,wherethedemandforstartingtorqueisnotsevere.Examplesaredrivesforfans,pumps,blowers,andmotor-generatorsets.ClassCMotorsClassCmotorsaresuitablefordrivingcompressors,conveyors,andsoforth.ClassDMotorsThesemotorsaresuitablefordrivingintermittentloadsrequiringrapidaccelerationandhigh-impactloadssuchaspunchpressesorshears.Inthecaseofimpactloads,aflywheelisfittedtothesystem.Asthemotorspeedfallsappreciablywithloadimpact,theflywheeldeliverssomeofitskineticenergyduringtheimpact.Fig.1-6A-2torque-speedcharacteristicsfordifferentclassofinductionmotorsSynchronousMachinesAsynchronousmachine,asthenameindicates,mustrotateatsynchronousspeed,thatis,thespeedisuniquelyrelatedtothesupplyfrequency,asindicatedinEquation(1-6A-1).Itisaseriouscompetitortotheinductionmachineinvariable-speeddriveapplications.Fig.1-6A-3showsanidealizedthree-phase,two-polewoundfieldsynchronousmachine.Thestatorwindingofthemachineisidenticaltothatoftheinductionmachine,buttherotorhasawindingthatcarriesDCcurrentandproducesfluxintheairgapthathelpsthestator-inducedrotatingmagneticfieldtodragtherotoralongwithit.TheDCfieldcurrentissuppliedtotherotorfromastaticrectifierthroughslipringsandbrushes,orbybr
本文标题:自动化相关专业英语翻译
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