英文翻译三相感应电动机与自动控制.docx
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英文翻译三相感应电动机与自动控制.docx
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英文翻译三相感应电动机与自动控制
Three-PhaseInductionMotorAndAuto-control
1.Three-PhaseInductionMotor
Thisisoneofthemostruggedandmostwidelyusedmachinesinindustry.Itsstatoriscomposedoflaminationsofhigh-gradesheetsteel.Theinnersurfaceisslottedtoaccommodateathree-phasewinding.InFig.5.2(a)thethree-phasewindingisrepresentedbythreecoils,theaxesofwhichare120electricaldegreesapart.Coilaa'representsallthecoilsassignedtophaseaforonepairofpoles.Similarlycoilbb'representsphasebcoils,andcoilcc'representsphaseccoils.Whenoneendofeachphaseistiedtogether,asdepictedinFig.5.2(b),thethree-phasestatorwindingissaidtobeY-connected.Suchawindingiscalledathree-phasewindingbecausethevoltageinducedineachofthethreephasesbyarevolvingfluxdensityfieldareoutofphaseby120electricaldegrees—adistinguishingcharacteristicofabalancedthree-phasesystem.
Therotoralsoconsistsoflaminationsofslottedferromagneticmaterial,buttherotorwindingmaybeeitherthesquirrel-cagetypeorthewound-rotortype.Thelatterisofaformsimilartothatofthestatorwinding.Thewindingterminalsarebroughtouttothreesliprings.Thisallowsanexternalthree-phaseresistortobeconnectedtotherotorwindingforthepurposeofprovidingspeedcontrol.Asamatteroffact,itistheneedforspeedcontrolwhichinlargemeasureaccountsfortheuseofthewound-rotortypeinductionmotor.Otherwisethesquirrel-cageinductionmotorwouldbeused.Thesquirrel-cagewindingconsistsmerelyofanumberofcopperbarsembeddedintherotorslotsandconnectedatbothendsbymeansofcopperendrings.(Insomeofthesmallersizesaluminumisused.)Thesquirrel-cageconstructionisnotonlysimplerandmoreeconomicalthanthewound-rotortypebutmoreruggedaswell.Therearenoslipringsorcarbonbrushestobebotheredwith.
Innormaloperationathree-phasevoltageisappliedtothestatorwindingatpointsa-b-cinFig.5.2.Magnetizingcurrentsflowineachphasewhichtogethercreatearevolvingmagneticfieldhavingtwopoles.Thespeedoffieldisfixedbythefrequencyofthemagnetizingcurrentsandthenumberofpolesforwhichthestatorwindingisdesigned.Fig.5.2showstheconfigurationfortwopoles.Ifthepatterna-c'-b-a'-c-b'ismadetospanonly180mechanicaldegreesandthenisrepeatedovertheremaining180mechanicaldegrees,amachinehavingafour-polefielddistributionresults.Forap-polemachinethebasicwindingpatternmustberepeatedp/2timeswithinthecircumferenceoftheinnersurfaceofthestator.
Therevolvingfieldproducedbystatorwindingcutstherotorconductors,therebyinducingvoltages.Sincetherotorwindingisshort-circuitedbytheendrings,theinducedvoltagescausecurrentstoflowwhichinturnreactwiththefieldtoproduceelectromagnetictorque—andsomotoractionresults.
Accordingly,onthebasisoftheforegoingdescription,itshouldbeclearthatforthethree-phaseinductionmotorthefieldwindingislocatedonthestatorandthearmaturewindingontherotor.Anotherpointworthnotingisthatthismachineissinglyexcited,i.e.,electricalpowerisappliedonlytothestatorwinding.Currentflowsthroughtherotorwindingbyinduction.Asaconsequenceboththemagnetizingcurrent,whichsetsupthemagneticfield,andthepowercurrentwhichallowsenergytobedeliveredtotheshaftload,flowthroughthestatorwinding.Forthisreason,andintheinterestofkeepingthemagnetizingcurrentassmallaspossibleinorderthatthepowercomponentmaybecorrespondinglylargeforagivencurrentrating,theairgapofinductionmotorsismadeassmallasmechanicalclearancewillallow.Theair-gaplengthsvaryformabout0.02inforsmallermachinesto0.05in.formachinesofhigherratingandspeed.
2.AnalogSensorsforMotionMeasurement
Measurementofplantoutput,isessentialforfeedbackcontrol.Outputmeasurementarealsousefulinperformanceevaluationofaprocess.Furthermore,inlearningsystems(e.g.teach-repeatoperationofroboticmanipulators),measurementsaremadestoredinthecomputerforsubsequentuseinoperatingthesystem,.Inputmeasurementsareneededinfeedforwardcontrol.Itisevident,therefore,thatthemeasurementsubsystemisanimportantpartofacontrolsystem.
Themeasurementsubsysteminacontrolsystemcontainssensorsandtransducersthatdetectmeasurandandconvertthemintoacceptablesignals-typically,voltages.Thesevoltagesignalsarethenappropriatelymodifiedusingsignal-conditioninghardwaresuchasfilters,amplifiers,demodulators,andanalog-to-digitalconverters.Impedancematchingmightbenecessarytoconnectsensorsandtransducerstosignal-conditioninghardware.
Accuracyofsensors,transducers,andassociatedsignal-conditioningdevicesisimportantincontrolsystemapplicationsfortwomainreasons.Themeasurementsysteminafeedbackcontrolsystemissituatedinthefeedbackpathofthecontrolsystem.Eventhoughmeasurementsareusedtocompensateforthepoorperformanceintheopen-loopsystem,anyerrorsinmeasurementsthemselveswillenterdirectlyintothesystemandcannotbecorrectediftheyareunknown.Furthermore,itcanbeshownthatsensitivityofacontrolsystemtoparameterchangesinthemeasurementsystemisdirect.Thissensitivitycannotbereducedbyincreasingtheloopgain,unlikeinthecaseofsensitivitytotheopen-loopcomponents.Accordingly,thedesignstrategyforclosed-loop(feedback)controlistomakethemeasurementsveryaccurateandtoemployasuitablecontrollertoreduceothertypesoferrors.
Mostsensor-transducerdevicesusedinfeedbackcontrolapplicationsareanalogcomponentsthatgenerateanalogoutputsignals.Thisisthecaseeveninreal-timedirectdigitalcontrolsystems.Whenanalogtransducersareusedindigitalcontrolapplications,however,sometypeofanalog-to-digitalconversion(ADC)isneededtoobtainadigitalrepresentationofthemeasuredsignal.Theresultingdigitalsignalissubsequentlyconditionedandprocessedusingdigitalmeans.
Inthesensorstage,thesignalbeingmeasuredisfeltasthe“responseofthesensorelement.”Thisisconvertedbythetransducerintothetransmitted(ormeasured)quantity.Inthisrespect,theoutputofameasuringdevicecanbeinterpretedasthe“responseofthetransducer.”Incontrolsystemapplications,thisoutputistypically(andpreferably)anelectricalsignal.Notethatitissomewhatredundanttoconsiderelectrical-to-electricalsensors-transducersasmeasuringdevices,particularlyincontrolsystemstudies,becauseelectricalsignalsneedconditioningonlybeforetheyarefedintoacontrollerortoadrivesystem.Inthissense,electrical-to-electricaltransductionshouldbeconsidereda“conditing”taskratherthana“measuring”function.
Bymotion,wemeanthefourkinematicvariables:
Displacement(includingposition,distance,proximity,andsizeorgage)
Velocity
Acceleration
Jerk
Notethateachvariableisthetimederivativeoftheprecedingone.Motionmeasurementareextremelyusefulincontrollingmechanicalresponsesandinteractionsindynamicsystems.Numerousexamplescanbecitedofsituationsinwhichmotionmeasurementsareusedforcontrolpurposes.Therotatingspeedofaworkpieceandthefeedrateofatoolaremeasuredincontrollingmachiningoperations.Displacementsandspeeds(bothangularandtranslatory)atjoints(revoluteandprismatic)ofroboticmanipulatorsorkinematiclinkagesareusedincontrollingmanipulatortrajectory.Inhigh-speedgroundtransitvehicles,accelerationandjerkmeasurementscanbeusedforactivesuspensioncontroltoobtainimprovedridequality.Angularspeedisacrucialmeasurementthatisusedinthecontrolofrotatingmachinery,suchasturbines,pumps,compressors,motors,andgeneratorsinpower-generatingplants.Proximitysensors(tomeasuredisplacement)andaccelerometes(tomeasureacceleration)arethetwomostcommontypesofmeasuringdevicesusedinmachineprotectionsystemsforconditionmonitoring,faultdetection,diagnosis,andon-line(oftenreal-time)controloflargeandcomplexmachinery.
3.ApplicationsofGeneral-PurposeInverters
TheapplicationsforinvertersandtheirassociatedACinductiondrivemotorsfallgenerallyintothreebasiccategories:
1)inplaceoforasretrofitsforDCmotordrives,2)insteadoforasretrofitsformechanicalspeedchangers,and3)toregulateorcontrolfluidflow(e.g.,airorliquids)byspeedinsteadofrestrictivedevicessuchasvanes,dampers,andmodulatingvalves.Eachoftheseisdiscussedinthefollowingsections.
ManyexistingDCdrivesarebeingusedbecauseofhistoricpreferenceandperformance.Userswere(andare)willingtoputupwiththespace,costand,maintenanceinaDCdrivebecauseitwas,sotospeak,’theonlygameintown.’Thegreatmajorityofstand-alonesingle-motorDCdriveapplicationscanbesuppliedorreplacedbyaninverterandanACinductionmotor.
Thisisnottosay,however,thatallDCsystemsarereplaceablewithgeneral-purposeinverters.WewilldiscusssomeofthepitfallsthatcanbeencounteredwhenDCdrivesareindiscriminatelyreplacedbyinvertersandACmotors.
WhichDCdrivesarecandidatesforreplacementbygeneral-purposeinvert?
TypicalexampleswouldbethoseapplicationswithperformancerequirementsthatwereavailableonlyfromDCdrives,nowavailablewithgeneral-purposeinverters,suchas:
(1)Operationofseveralmotorsandinvertersfromamasterreference,with
individuallyadjustablevernierspeedsamongthemotors.
(2)Individualstart-stopcontrolofeachmotor.
(3)Independentlyadjustableaccelerationanddecel
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