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1、SpeedControlofDCMotorRegulatorSystemsAregulatorsystemisonewhichnormallyprovidesoutputpowerinitssteady-stateoperation.Forexample,amotorspeedregulatormaintainsthemotorspeedataconstantvaluedespitevariationsinloadtorque.Eveniftheloadtorqueisremoved,themoto
2、rmustprovidesufficienttorquetoovercometheviscousfrictioneffectofthebearings.Otherformsofregulatoralsoprovideoutputpower;Atemperatureregulatormustmaintainthetemperatureof,say,anovenconstantdespitetheheatlossintheoven.Avoltageregulatormustalsomaintainmus
3、toutputvoltageconstantdespitevariationintheloadcurrent.Foranysystemtoprovideanoutput,e.g.,speed,temperature,voltage,etc,anerrorsignalmustexistundersteady-stateconditions.ElectricalBrakingInmanyspeedcontrolsystem,e.g.,rollingmillsminewinders,etc.,theloa
4、dhastobefrequentlybroughttoastandstillandreversed.Therateatwhichthespeedreducesfollowingareducedspeeddemandisdependentonthestoredenergyandthebrakingsystemused.Asmallspeedcontrolsystem(sometimesknownasavelodyne)canemploymechanicalbraking,butthisisnotfea
5、siblewithlargespeedcontrollerssinceitisdifficultandcostlytoremovetheheatgenerated.Thevariousmethodsofelectricalbrakingavaiableare:(1)Regenerativebraking.(2)Eddycurrentbraking.(3)Dynamicbraking.(4)Reversecurrentbraking(plugging).Regenerativebrakingisthe
6、bestmethod,thoughnotnecessarilythemosteconomic.Thestoredenergyintheloadisconvertedintoelectricalenergybytheworkmotor(actingtemporarilyasagenerator)andisreturnedtothepowersupplysystem.Thesupplysystemthusactsasa“sink”intowhichtheunwantedenergyisdelivered
7、.Providingthesupplysystemhasadequatecapacity,theconsequentriseinterminalvoltagewillbesmallduringtheshortperiodsofregeneration.IntheWard-LeonardmethodofspeedcontrolofDCmotors,regenerativebrakingisinherent,butthyristordriveshavetobearrangedtoinverttorege
8、nerate.Inductionmotordrivercanregenerateiftherotorshaftisdrivenfasterthanspeedoftherotatingfield.Theadventoflow-costvariablevariable-frequencysuppliesfromthyristorinvertershavebroughtaboutconsiderablechargesintheuseofinductionmotorsinva
