机械 外文翻译 外文文献 英文文献 直流电动机调速控制.docx

机械 外文翻译 外文文献 英文文献 直流电动机调速控制.docx

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机械外文翻译外文文献英文文献直流电动机调速控制

SpeedControlofDCMotor

AbstractConditioningsystemischaracterizedinthatoutputpowertomaintainstability.Differentspeedcontrolsystemcanuseadifferentbrakesystem,highstartingandbrakingtorque,quickresponseandquickadjustmentrangeofdegreerequirementsofDCdrivesystem,theuseoftheelectricbrakingmode.DependsonthespeedcontrolofDCmotorarmaturevoltageandflux.Tozerospeed,orU=0orΦ=∞.Thelatterisimpossible,itonlychangesthroughthearmaturevoltagetoreducespeed.TospeedtoahighervaluecanincreaseordecreasetheUΦ.

KeywordDCSpeedFeedbackBrake

RegulatorSystems

Aregulatorsystemisonewhichnormallyprovidesoutputpowerinitssteady-stateoperation.

Forexample,amotorspeedregulatormaintainsthemotorspeedataconstantvaluedespitevariationsinloadtorque.Eveniftheloadtorqueisremoved,themotormustprovidesufficienttorquetoovercometheviscousfrictioneffectofthebearings.Otherformsofregulatoralsoprovideoutputpower;Atemperatureregulatormustmaintainthetemperatureof,say,anovenconstantdespitetheheatlossintheoven.Avoltageregulatormustalsomaintaintheoutputvoltageconstantdespitevariationintheloadcurrent.Foranysystemtoprovideanoutput,e.g.,speed,temperature,voltage,etc.,anerrorsignalmustexistundersteady-stateconditions.

ElectricalBraking

Inmanyspeedcontrolsystems,e.g.,rollingmills,minewinders,etc.,theloadhastobefrequentlybroughttoastandstillandreversed.Therateatwhichthespeedreducesfollowingareducedspeeddemandisdependentonthestoredenergyandthebrakingsystemused.Asmallspeedcontrolsystem（sometimesknownasavelodyne）canemploymechanicalbraking,butthisisnotfeasiblewithlargespeedcontrollerssinceitisdifficultandcostlytoremovetheheatgenerated.

Thevariousmethodsofelectricalbrakingavailableare:

（1）Regenerativebraking.

（2）Eddycurrentbraking.

（3）Dynamicbraking.

（4）Reversecurrentbraking（plugging）

Regenerativebrakingisthebestmethod,thoughnotnecessarilythemosteconomic.Thestoredenergyintheloadisconvertedintoelectricalenergybytheworkmotor（actingtemporarilyasagenerator）andisreturnedtothepowersupplysystem.Thesupplysystemthusactsasa”sink”intowhichtheunwantedenergyisdelivered.Providingthesupplysystemhasadequatecapacity,theconsequentriseinterminalvoltagewillbesmallduringtheshortperiodsofregeneration.IntheWard-LeonardmethodofspeedcontrolofDCmotors,regenerativebrakingisinherent,butthyristordriveshavetobearrangedtoinverttoregenerate.Inductionmotordrivescanregenerateiftherotorshaftisdrivenfasterthanspeedoftherotatingfield.Theadventoflow-costvariable-frequencysuppliesfromthyristorinvertershavebroughtaboutconsiderablechangesintheuseofinductionmotorsinvariablespeeddrives.

Eddycurrentbrakingcanbeappliedtoanymachine,simplybymountingacopperoraluminumdiscontheshaftandrotatingitinamagneticfield.Theproblemofremovingtheheatgeneratedissevereinlargesystemasthetemperatureoftheshaft,bearings,andmotorwillberaisedifprolongedbrakingisapplied.

Indynamicbraking,thestoredenergyisdissipatedinaresistorinthecircuit.WhenappliedtosmallDCmachines,thearmaturesupplyisdisconnectedandaresistorisconnectedacrossthearmature（usuallybyarelay,contactor,orthyristor）.Thefieldvoltageismaintained,andbrakingisapplieddowntothelowestspeed.Inductionmotorsrequireasomewhatmorecomplexarrangement,thestatorwindingsbeingdisconnectedfromtheACsupplyandreconnectedtoaDCsupply.Theelectricalenergygeneratedisthendissipatedintherotorcircuit.DynamicbrakingisappliedtomanylargeAChoistsystemswherethebrakingdutyisbothsevereandprolonged.

DCMotorSpeedControl

ThebasisofallmethodsofDCmotorspeedcontrolisderivedfromtheequations:

thetermshavingtheirusualmeanings.IftheIaRadropissmall,theequationsapproximateto

or

。

Thus,controlofarmaturevoltageandfieldfluxinfluencesthemotorspeed.Toreducethespeedtozero,eitherU=0orΦ=∞.Thelatterisinadmissible;hencecontrolatlowspeedisbyarmaturevoltagevariation.Toincreasethespeedtoahighvalue,eitherUismadeverylargeorΦisreduced.Thelatteristhemostpracticalwayandisknownasfieldweakening.Combinationsofthetwoareusedwhereawiderangeofspeedisrequired.

ASingle-QuadrantSpeedControlSystemUsingThyristors

Asingle-quadrantthyristorconvertersystemisshowninFig.1.ForthemomentthereadershouldignoretherectifierBR2anditsassociatedcircuitry（includingresistorRintheACcircuit）,sincethisisneededonlyasaprotectivefeatureandisdescribedinnextsection.

Fig.1ThyristorspeedcontrolsystemwithcurrentlimitationontheACside

Sincethecircuitisasingle-quadrantconverter,thespeedofthemotorshaft（whichistheoutputfromthesystem）canbecontrolledinonedirectionofrotationonly.Moreover,regenerativebrakingcannotbeappliedtothemotor;inthistypeofsystem,themotorarmaturecansuddenlybebroughttorestbydynamicbraking（i.e.whenthethyristorgatepulsesarephasedbackto180o,aresistercanbeconnectedacrossthearmaturebyarelayorsomeothermeans）.

RectifierBR1providesaconstantvoltageacrosstheshuntfieldwinding,givingaconstantfieldflux.Thearmaturecurrentiscontrolledbyathyristorwhichis,inturn,controlledbythepulsesappliedtoitsgate.Thearmaturespeedincreasesasthepulsesarephasedforward（whichreducesthedelayangleoffiring）,andthearmaturespeedreducesasthegatepulsesarephasedback.

Thespeedreferencesignalisderivedfromamanuallyoperatedpotentiometer（shownattheright-handsideofFig.23.1）,andthefeedbacksignaloroutputspeedsignalisderivedfromtheresistorchainR1R2,whichisconnectedacrossthearmature.（Strictlyspeaking,thefeedbacksignalinthesysteminFig.23.1isproportionaltothearmaturevoltage,whichisproportionaltotheshaftspeedonlyifthearmatureresistancedrop,IaRa,issmall.MethodsusedtocompensatefortheIaRadroparediscussedinReadingMaterial.）Sincethearmaturevoltageisobtainedfromathyristor,thevoltageconsistsofaseriesofpulses;thesepulsesaresmoothedbycapacitorC.Thespeedreferencesignalisoftheoppositepolaritytothearmaturevoltagesignaltoensurethatoverallnegativefeedbackisapplied.

AfeatureofDCmotordrivesisthattheloadpresentedtothesupplyisamixtureofresistance,inductance,andbackEMFDiodeDinFig.1ensuresthatthethyristorcurrentcommutatestozerowhenitsanodepotentialfallsbelowthepotentialoftheupperarmatureconnection,inthemanneroutlinedbefore.Inthedriveshown,thepotentialofthethyristorcathodeisequaltothebackEMFofthemotorwhileitisinablockingstate.ConductioncanonlytakeplaceduringthetimeintervalwhentheinstantaneoussupplyvoltageisgreaterthanthebackEMF.InspectionofFig.2showsthatwhenthemotorisrunning,thepeakinversevoltageappliedtothethyristorismushgreaterthanthepeakforwardvoltage.Byconnectingadiodeinserieswiththethyristor,asshown,thereverseblockingcapabilityofthecircuitisincreasedtoallowlow-voltagethyristortobeused.

References:

Fig.2IllustratingtheeffectofmotorbackEMFonthe

Peakinversevoltageappliedtothethyristor

Fig.3Armaturevoltagewaveforms

ThewaveformsshowninFig.2areidealizedwaveformsasmuchastheyignoretheeffectsofarmatureinductance,commutatorripple,etc.TypicalarmaturevoltagewaveformsareshowninFig.3.InthiswaveformthethyristoristriggeredatpointA,andconductioncontinuestopointBwhenthesupplyvoltagefallsbelowthearmaturebackEMF.TheeffectofarmatureinductanceistoforcethethyristortocontinuetoconductuntilpointC,whenthefly-wheeldiodepreventsthearmaturevoltagefromreversing.Whentheinductiveenergyhasdissipated（pointD）,thearmaturecurrentiszeroandthevoltagereturnstoitsnormallevel,thetransientshavingsettledoutbypointE.TheundulationsonthewaveformbetweenEandFareduetocommentatorripple.

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直流电动机调速控制

摘要调节系统的特征在于