外文翻译直流电动机调速控制Word格式.docx
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外文翻译直流电动机调速控制Word格式.docx
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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,theequationsapproximatetoor。
Thus,controlofarmaturevoltageandfieldfluxinfluencesthemotorspeed.Toreducethespeedtozero,eitherU=0orΦ=∞.Thelatterisinadmissible;
hencecontrolatlowspeedisbyarmaturevoltagevariation.Toincreasethespeedtoahighvalue,eitherUismadeverylargeorΦisreduced.Thelatteristhemostpracticalwayandisknownasfieldweakening.Combinationsofthetwoareusedwhereawiderangeofspeedisrequired.
ASingle-QuadrantSpeedControlSystemUsingThyristors
Asingle-quadrantthyristorconvertersystemisshowninFig.1.ForthemomentthereadershouldignoretherectifierBR2anditsassociatedcircuitry(includingresistorRintheACcircuit),sincethisisneededonlyasaprotectivefeatureandisdescribedinnextsection.
Fig.1Thyristorspeed
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