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    外文翻译--电力绞车的控制.doc

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    外文翻译--电力绞车的控制.doc

    英文原文ElectricalWinchControlsbyTomYoungTheformofmotorcontrolweallknowbestisthesimplemanualstationwithupanddownpushbuttons.Whilethesestationsmaystillbetheperfectchoiceforcertainapplications,adizzyingarrayofmoresophisticatedcontrolsisalsoavailable.Thisarticleaddressesthebasicelectricalrequirementsofthemotorsanduserinterfaceissuesyouwillneedtoaddressbeforespecifying,buildingorbuyingwinchcontrols.Tobeginwith,themanualcontrolstationsshouldbeofthehold-to-runtype,sothatifyoutakeyourfingeroffofthebuttonthewinchstops.Additionally,everycontrolstationneedsanemergencystop(E-stop)thatkillsallpowertothewinch,notjustthecontrolcircuit.Thinkaboutitifthewinchisntstoppingwhenitshould,youreallyneedafailsafewaytokillthelinepower.Itsalsoagreatideatohaveakeyoperatedswitchoncontrolstations,especiallywhereaccesstothestationsisnotcontrolled.Safeoperationbyauthorizedpersonnelmustbeconsideredwhendesigningeventhesimplestmanualcontrols.ControllingFixedSpeedMotorsTheactualcontrollingdeviceforafixedspeedwinchisathreephasereversingstarter.ThemotorisreversedbysimplyswitchingthephasesequencefromABCtoCBA.Thisisaccomplishedbytwothree-polecontactors,interlocked,sotheycantbothbeclosedatthesametime.TheNECrequiresbothoverloadandshortcircuitprotection.Toprotectthemotorfromoverheatingduetomechanicaloverloadsathermaloverloadrelayisbuiltintothestarter.Thishasbi-metallicstripsthatmatchtheheatingpatternofthemotorandtripscontactswhentheyoverheat.Alternatively,athermistorcanbemountedinthemotorwindingtomonitorthemotortemperature.Shortcircuitprotectionisgenerallyprovidedbyfusesratedforusewithmotors.Aseparatelinecontactorshouldbeprovidedaheadofthereversingcontactorforredundancy.Thiscontactoriscontrolledbythesafetycircuits:E-stopandovertravellimits.Thisbringsustolimitswitches.Whenyougettothenormalendoftravellimitthewinchstopsandyoucanonlymoveitintheoppositedirection(awayfromthelimit).Therealsoneedstobeanovertravellimitincase,duetoanelectricalormechanicalproblem,thewinchrunspastthenormallimit.Ifyouhitanovertravellimitthelinecontactoropenssothereisnowaytodriveoffofthelimits.Ifthisoccurs,acompetenttechnicianneedstofixtheproblemthatresultedinhittingtheovertravellimit.Then,youcanoverridetheovertravelsusingthespringreturntoggleswitchinsidethestarterasopposedtousingjumpersorhandshootingthecontactors.VariableSpeedRequirementsOfcourse,thesimplefixedspeedstartergetsreplacedwithavariablespeeddrive.Hereswherethingsstarttogetinteresting!Attheveryleastyouneedtoaddaspeedpottothecontrolstation.Ajoystickisabetteroperatorinterface,asitgivesyouamoreintuitivecontrolofthemovingpiece.Unfortunately,youcantjustorderanyoldvariablespeeddrivefromyourlocalsupplierandexpectittoraiseandlowerequipmentsafelyandreliablyoverkidsonstage.Mostvariablespeeddriveswont,astheyarentdesignedforlifting.Thedriveneedstobesetupsothattorqueisdevelopedatthemotorbeforethebrakeisreleased,and(whenstopping)thebrakeissetbeforetorqueistakenaway.FormanyyearsDCmotorsanddrivesprovidedapopularsolutionastheyallowedforgoodtorqueatallspeeds.ThelargeDCmotorsrequiredformostwinchesareexpensive,costingmanytimeswhatacomparableACmotorcosts.However,theearlyACdriveswerenotveryuseful,astheyhadaverylimitedspeedrangeandproducedlowtorqueatlowspeeds.Morerecently,astheACdrivesimproved,thelowcostandplentifulavailabilityofACmotorsresultedinatransitiontoACdrives.TherearetwofamiliesofvariablespeedACdrives.Variablefrequencyinvertersarewellknownandreadilyavailable.ThesedrivesconvertACtoDC,thenconvertitbacktoACwithadifferentfrequency.Ifthedriveproduces30Hz,anormal60Hzmotorwillrunathalfspeed.Intheorythisisgreat,butinrealitythereareacoupleofproblems.First,atypical60Hzmotorgetsconfusedatalinefrequencybelow2or3Hz,andstartstocog(jerkandsputter),orjuststops.Thislimitsyoutoaspeedrangeofaslowas20:1hardlysuitableforsubtleeffectsonstage!Second,manylowercostinvertersarealsoincapableofprovidingfulltorqueatlowspeeds.Employingsuchdrivescanresultinjerkymoves,oracompletefailuretoliftthepieceexactlywhatyoudontwanttoseewhenyouaretryingtostartsmoothlyliftingascenicelement.Someofthenewerinvertersareclosedloop(obtainfeedbackfromthemotortoprovidemoreaccuratespeedcontrol)andwillworkquitewell.TheotherfamilyofACdrivesisfluxvectordrives.Theseunitsrequireanencodermountedonthemotorshaftallowingthedrivetopreciselymonitortherotationofthearmature.Aprocessordeterminestheexactvectorofmagneticflux(thusfluxvectordrive)requiredtorotatethearmaturethenextfewdegreesatagivenspeed.Thesedrivesallowaninfinitespeedrange,asyoucanactuallyproducefulltorqueatzerospeed.Theprecisespeedandpositioncontrolofferedbythesedrivesmakethemafavoriteinhighperformanceapplications.PLC-basedcontrolsprovidesystemstatusaswellascontroloptions.ThisscreengivetheoperatorfullaccesstoCarnegieHallsninestagefloorlifts.PLCBasedSystemsAPLCisaprogrammablelogiccontroller.Firstdevelopedtoreplacetherelaybasedindustrialcontrolsystemsofthe50sand60s,thesecontrolsareathomeinrugged,industrialenvironments.ThesearemodularsystemswithagreatvarietyofI/Omodulesallowingsemi-customhardwareconfigurationstobeassembledeasilyatareasonableprice.Theseincludepositioncontrolmodules,counters,A/DandD/Aconvertersandallsortsofsolidstateorhardcontactclosureoutputs.ThegreatvarietyofI/OcomponentsandthemodularnatureofthePLCmakethisaneffectivewaytobuildcustomandsemi-customcontrolsystems.ThegreatestdrawbacktoPLCsystemsisthelackofreallygreatdisplaystotellyouwhattheyaredoingortohelpyouprogramthem.Monochromeandmediumresolutioncolordisplaysarethenorm,astheprimaryuseforthesecomponentsinonafactoryfloor.OneofthefirstmajorPLCsystemsusedinalargeentertainmentvenueisthecomplexliftandwagonsystemattheoriginalMGMGrand(nowBallys)inLasVegas.SeveralmanufacturersofferstandardPLC-basedsystemsandahostofsemi-customacousticbanner,shell,andliftcontrolsystemsisalsoavailable.TheabilitytobuildcustomsystemsfromstandardbuildingblocksisthegreateststrengthofPLC-basedcontrols.HighEndControllersThemostsophisticatedriggingcontrollersgowellbeyondspeed,time,andpositioncontrol.Theyincludetheabilitytowritecomplexcues,recordprofiledmoves,andmanagemultiplecuesrunningatonce.Manyofthelargeroperahousesaremovingtowardpointhoistsystems,wherethereisaseparatewinchforeachliftline(theriggingequivalentofdimmerpercircuit).Whenmultiplewinchesareusedtocarryasinglepiece,thewinchesmustbeperfectlysynchronized,ortheloadcanshiftsothatanindividualwinchcanbecomedangerouslyoverloaded.Thecontrolsystemmustbeabletokeepselectedwinchesinsynchorprovidearapid,coordinatedstopifawinchisunabletostayinsynchwiththeothers.Withatypicaltopspeedof240fpmandarequirementtokeepthewinches,youhavelessthanthreemillisecondstorecognizeaproblem,attempttocorrecttheerrantwinchsspeed,determinethatyouvefailedandinitiateacoordinatedstopofallthewinchesinthegroup.Thistakesalotofcomputing,fastI/O,andwell-writtensoftware.Therearetwoverydifferentapproachestolargeriggingcontrolsystems.Originally,asingleconsolewasused,withtheusualproblemofwhereitshouldbelocatedfortheoperatorsoptimumview.Unfortunatelythiscanchangenotonlyfromshowtoshow,butalsofromonecuetothenext.Thisdilemmahasbeenpartiallyaddressedbyusingvideocamerasatdifferentlocationsinconjunctionwith3Dscreengraphicsthatallowtheoperatortoviewtheexpectedriggingmotionthreedimensionallyfromanyviewpoint.Thisallowstheoperatortoviewtheonscreenmovementoftheriggingfromaviewpointthatmatcheshisactualviewofthestage,ortheactualviewofaclosedcircuitcamera.Forcomplexmoveswithinter-relatedpiecesthismakesthecontrolandunderstandingofwhatishappeningmuchsimpler.Theotherapproachisadistributedsystem,withseveralportableconsoles.Thisallowsdifferentoperatorstocontroldifferentaspectsoftherigging,inthesamemannerwehavedonewithmanualsets.AdramaticexampleofthisapproachisusedbytheRoyalOperaatCoventGarden,wheretherearetenconsolescontrollingatotalof240motors.Eachconsolehasfiveplaybacks,andissetupsothateachmotorisassignedtoasingleconsole.Oneoperatorandconsolecouldcontroleverything,butfrequentlyoneconsolemayberunningstagelifts,anothertheonstagerigging,andathirdisbeingusedbackstagetomovestoreddrops.

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