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1、SpeedControlofDCMotorRegulatorSystemsAregulatorsystemisonewhichnormallyprovidesoutputpowerinitssteady-stateoperation.Forexample,amotorspeedregulatormaintainsthemotorspeedataconstantvaluedespitevariationsinloadtorque.Eveniftheloadtorqueisremoved,themotormustprovidesufficie
2、nttorquetoovercometheviscousfrictioneffectofthebearings.Otherformsofregulatoralsoprovideoutputpower;Atemperatureregulatormustmaintainthetemperatureof,say,anovenconstantdespitetheheatlossintheoven.Avoltageregulatormustalsomaintainmustoutputvoltageconstantdespitevariationin
3、theloadcurrent.Foranysystemtoprovideanoutput,e.g.,speed,temperature,voltage,etc,anerrorsignalmustexistundersteady-stateconditions.ElectricalBrakingInmanyspeedcontrolsystem,e.g.,rollingmillsminewinders,etc.,theloadhastobefrequentlybroughttoastandstillandreversed.Therateatw
4、hichthespeedreducesfollowingareducedspeeddemandisdependentonthestoredenergyandthebrakingsystemused.Asmallspeedcontrolsystem(sometimesknownasavelodyne)canemploymechanicalbraking,butthisisnotfeasiblewithlargespeedcontrollerssinceitisdifficultandcostlytoremovetheheatgenerate
5、d.Thevariousmethodsofelectricalbrakingavaiableare:(1)Regenerativebraking.(2)Eddycurrentbraking.(3)Dynamicbraking.(4)Reversecurrentbraking(plugging).Regenerativebrakingisthebestmethod,thoughnotnecessarilythemosteconomic.Thestoredenergyintheloadisconvertedintoelectricalener
6、gybytheworkmotor(actingtemporarilyasagenerator)andisreturnedtothepowersupplysystem.Thesupplysystemthusactsasa“sink”intowhichtheunwantedenergyisdelivered.Providingthesupplysystemhasadequatecapacity,theconsequentriseinterminalvoltagewillbesmallduringtheshortperiodsofregener
7、ation.IntheWard-LeonardmethodofspeedcontrolofDCmotors,regenerativebrakingisinherent,butthyristordriveshavetobearrangedtoinverttoregenerate.Inductionmotordrivercanregenerateiftherotorshaftisdrivenfasterthanspeedoftherotatingfield.Theadventoflow-costvariablevariable-frequen
8、cysuppliesfromthyristorinvertershavebroughtaboutconsiderablechargesintheuseofinductionmotorsinva
