外文翻译--直流电机速度控制-精品.doc

外文翻译--直流电机速度控制-精品.doc

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SpeedControlofDCMotor

RegulatorSystems

Aregulatorsystemisonewhichnormallyprovidesoutputpowerinitssteady-stateoperation.

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

ElectricalBraking

Inmanyspeedcontrolsystem,e.g.,rollingmillsminewinders,etc.,theloadhastobefrequentlybroughttoastandstillandreversed.Therateatwhichthespeedreducesfollowingareducedspeeddemandisdependentonthestoredenergyandthebrakingsystemused.Asmallspeedcontrolsystem（sometimesknownasavelodyne）canemploymechanicalbraking,butthisisnotfeasiblewithlargespeedcontrollerssinceitisdifficultandcostlytoremovetheheatgenerated.

Thevariousmethodsofelectricalbrakingavaiableare:

（1）Regenerativebraking.

（2）Eddycurrentbraking.

（3）Dynamicbraking.

（4）Reversecurrentbraking（plugging）.

Regenerativebrakingisthebestmethod,thoughnotnecessarilythemosteconomic.Thestoredenergyintheloadisconvertedintoelectricalenergybytheworkmotor（actingtemporarilyasagenerator）andisreturnedtothepowersupplysystem.Thesupplysystemthusactsasa“sink”intowhichtheunwantedenergyisdelivered.Providingthesupplysystemhasadequatecapacity,theconsequentriseinterminalvoltagewillbesmallduringtheshortperiodsofregeneration.IntheWard-LeonardmethodofspeedcontrolofDCmotors,regenerativebrakingisinherent,butthyristordriveshavetobearrangedtoinverttoregenerate.Inductionmotordrivercanregenerateiftherotorshaftisdrivenfasterthanspeedoftherotatingfield.Theadventoflow-costvariablevariable-frequencysuppliesfromthyristorinvertershavebroughtaboutconsiderablechargesintheuseofinductionmotorsinvariablespeeddrives.

Eddycurrentbrakingcanbeappliedtoanymachine,simplybymountingacopperoraluminiumdiscontheshaftandrotatingitinamagneticfield.Theproblemofremovingtheheatgeneratedissevereinlargesystemasthetemperatureoftheshaft,bearing,andmotorwillberaisedifprolongedbrakingisapplied.

Indynamicbreaking,thestoredenergyisaresistorinthecircuit.WhenappliedtosmallDCmachines,thearmaturesupplyisdisconnectedandaresistorisconnectedacrossthearmature（usuallybyarelay,contactor,orthyristor）.Thefieldvoltageismaintained,andbrakingisapplieddowntothelowestspeed.Inductionmotorsrequireasomewhatmorecomplexarrangement,thestatorwindingsbeingdisconnectedfromtheACsupplyandreconnectedtoaDCsupply.Theelectricalenergygeneratedisthendissipatedintherotorcircuit.DynamicbrakingisappliedtomanylargeAChoistsystemwherethebrakingdutyisbothsevereandprolonged.

Anyelectricalmotorcanbebroughttoastandstillbysuddenlyreconnectingthesupplytoreversethedirectionofrotation（reversecurrentbraking）.Appliedundercontrolledconditions,thismethodofbrakingissatisfactoryforalldrivers.Itsmajordisadvantageisthattheelectricalenergyconsumedbythemachinewhenbrakingisequaltothestoredenergyintheload.Thisincreasestherunningcostsignificantlyinlargedrives.

EqualpulsewidthPWMlaw

VVVF（VariableVoltageVariableFrequency）installsintheearlytimeisusesPAM（PulseAmplitudeModulation）tocontrol,itsinventorpartwhichthetechnologyrealizesbutonlycanoutputthefrequencyadjustablesquare-wavevoltagenottobeabletoadjustthepressure.ThepulsewidthPWMlawarepreciselyinordertoovercome,isinthePWMlawwhichthePAMlawthisshortcomingdevelopmentcomesthesimplestonekind.ItiseachpulsewidthequalpulserowtookthePWMwave,throughachangepulserowcyclemaythefrequencymodulation,thechangepulsewidthorthedutyfactormayadjustthepressure,usesthesuitablecontrolmethodthentocausethevoltageandthefrequencycoordinationchange.IsoppositeinthePAMlaw,thismethodmeritsimplifiedtheelectriccircuitstructure,enhancedtheinputendpowerfactor,butsimultaneouslyalsohasintheoutputvoltagebesidesthefundamentalwave,butalsocontainsthebigharmoniccomponent.

StochasticPWM

ThattimethehighefficiencytransistormainlyforthebipolarityDaringtontriode,thecarrierfrequencygenerallydidnotsurpass5kHz,thevibration