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1、英文资料ThecontraltechniquesdrivesandcontralshandbookChapterA4Torque,speedandpositioncontrolA4.1GeneralprinciplesA4.1.1TheidealcontrolsystemManyapplicationsexistwheresomethinghastobecontrolledtofollowareferencequantity.Forexample,thespeedofalargemotormay
2、besetfromalow-powercontrolsignal.Thiscanbeachievedusingavariable-speeddriveasdescribedinthefollowing.Ideally,therelationshipbetweenthereferenceandthemotorspeedshouldbelinear,andthespeedshouldchangeinstantlywithchangesinthereference.Anycontrolsystemca
3、nberepresentedasinFigureA4.1b,withaninputreferencesignal,atransferfunctionFandanoutput.Forthesystemtobeideal,thetransferfunctionFwouldbeasimpleconstant,sothattheoutputwouldbeproportionaltothereferencewithnodelay.FigureA4.1Variable-speeddriveandmotorA
4、4.1.2Open-loopcontrolUnfortunately,thetransferfunctionofmanypracticalsystemsisnotaconstant,andsowithoutanyformoffeedbackfromtheoutputtocorrectforthenon-idealnatureofthetransferfunction,theoutputdoesnotfollowthedemandasrequired.Usinganinductionmotorsu
5、ppliedbyasimpleopen-loopvariable-speeddriveasanexample,thefollowingillustratessomeunwantedeffectsthatcanoccurinpracticalsystems:Speedregulation.Theoutputofasimpleopen-loopdriveisafixedfrequencythatisproportionaltothespeedreference,andsothefrequencyap
6、pliedtothemotorremainsconstantforaconstantspeedreference.Thespeedofthemotordropsasloadisappliedbecauseoftheslipcharacteristicofthemotor,andsothespeeddoesnotremainattherequiredlevel.Instability.Itispossibleundercertainloadconditionsandatcertainfrequen
7、ciesforthemotorspeedtooscillatearoundtherequiredspeed,eventhoughtheappliedfrequencyisconstant.Anothermajorsourceofinstabilityinrotatingmechanicalsystemsislow-losselasticcouplingsandshafts.Non-linearity.Therearemanypossiblesourcesofnon-linearity.If,fo
8、rexample,themotorisconnectedtoagearbox,thespeedattheoutputofthegearboxcouldbeaffectedbybacklashbetweenthegears.Variationswithtemperature.Someaspectsofthesystemtransferfunctionmayvarywithtemperature.Forexample,theslipofaninductionmotorincreasesasthemo
