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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,themanualcontrolstationsshouldbeoftheholdtoruntype,sothatifyoutakeyourfingeroffofthebuttonthewinchstops.Additionally,everycontrolstationneedsanemergencystopEstopthatkillsallpowertothewinch,notjustthecontrolcircuit.Thinkaboutitifthewinchisntstoppingwhenitshould,youreallyneedafailsafewaytokillthelinepower.Itsalsoagreatideatohaveakeyoperatedswitchoncontrolstations,especiallywhereaccesstothestationsisnotcontrolled.Safeoperationbyauthorizedpersonnelmustbeconsideredwhendesigningeventhesimplestmanualcontrols.ControllingFixedSpeedMotorsTheactualcontrollingdeviceforafixedspeedwinchisathreephasereversingstarter.ThemotorisreversedbysimplyswitchingthephasesequencefromABCtoCBA.Thisisaccomplishedbytwothreepolecontactors,interlocked,sotheycantbothbeclosedatthesametime.TheNECrequiresbothoverloadandshortcircuitprotection.Toprotectthemotorfromoverheatingduetomechanicaloverloadsathermaloverloadrelayisbuiltintothestarter.Thishasbimetallicstripsthatmatchtheheatingpatternofthemotorandtripscontactswhentheyoverheat.Alternatively,athermistorcanbemountedinthemotorwindingtomonitorthemotortemperature.Shortcircuitprotectionisgenerallyprovidedbyfusesratedforusewithmotors.Aseparatelinecontactorshouldbeprovidedaheadofthereversingcontactorforredundancy.ThiscontactoriscontrolledbythesafetycircuitsEstopandovertravellimits.Thisbringsustolimitswitches.Whenyougettothenormalendoftravellimitthewinchstopsandyoucanonlymoveitintheoppositedirectionawayfromthelimit.Therealsoneedstobeanovertravellimitincase,duetoanelectricalormechanicalproblem,thewinchrunspastthenormallimit.Ifyouhitanovertravellimitthelinecontactoropenssothereisnowaytodriveoffofthelimits.Ifthisoccurs,acompetenttechnicianneedstofixtheproblemthatresultedinhittingtheovertravellimit.Then,youcanoverridetheovertravelsusingthespringreturntoggleswitchinsidethestarterasopposedtousingjumpersorhandshootingthecontactors.VariableSpeedRequirementsOfcourse,thesimplefixedspeedstartergetsreplacedwithavariablespeeddrive.HereswherethingsstarttogetinterestingAttheveryleastyouneedtoaddaspeedpottothecontrolstation.Ajoystickisabetteroperatorinterface,asitgivesyouamoreintuitivecontrolofthemovingpiece.Unfortunately,youcantjustorderanyoldvariablespeeddrivefromyourlocalsupplierandexpectittoraiseandlowerequipmentsafelyandreliablyoverkidsonstage.Mostvariablespeeddriveswont,astheyarentdesignedforlifting.Thedriveneedstobesetupsothattorqueisdevelopedatthemotorbeforethebrakeisreleased,andwhenstoppingthebrakeissetbeforetorqueistakenaway.FormanyyearsDCmotorsanddrivesprovidedapopularsolutionastheyallowedforgoodtorqueatallspeeds.ThelargeDCmotorsrequiredformostwinchesareexpensive,costingmanytimeswhatacomparableACmotorcosts.However,theearlyACdriveswerenotveryuseful,astheyhadaverylimitedspeedrangeandproducedlowtorqueatlowspeeds.Morerecently,astheACdrivesimproved,thelowcostandplentifulavailabilityofACmotorsresultedinatransitiontoACdrives.TherearetwofamiliesofvariablespeedACdrives.Variablefrequencyinvertersarewellknownandreadilyavailable.ThesedrivesconvertACtoDC,thenconvertitbacktoACwithadifferentfrequency.Ifthedriveproduces30Hz,anormal60Hzmotorwillrunathalfspeed.Intheorythisisgreat,butinrealitythereareacoupleofproblems.First,atypical60Hzmotorgetsconfusedatalinefrequencybelow2or3Hz,andstartstocogjerkandsputter,orjuststops.Thislimitsyoutoaspeedrangeofaslowas201hardlysuitableforsubtleeffectsonstageSecond,manylowercostinvertersarealsoincapableofprovidingfulltorqueatlowspeeds.Employingsuchdrivescanresultinjerkymoves,oracompletefailuretoliftthepieceexactlywhatyoudontwanttoseewhenyouaretryingtostartsmoothlyliftingascenicelement.Someofthenewerinvertersareclosedloopobtainfeedbackfromthemotortoprovidemoreaccuratespeedcontrolandwillworkquitewell.TheotherfamilyofACdrivesisfluxvectordrives.Theseunitsrequireanencodermountedonthemotorshaftallowingthedrivetopreciselymonitortherotationofthearmature.Aprocessordeterminestheexactvectorofmagneticfluxthusfluxvectordriverequiredtorotatethearmaturethenextfewdegreesatagivenspeed.Thesedrivesallowaninfinitespeedrange,asyoucanactuallyproducefulltorqueatzerospeed.Theprecisespeedandpositioncontrolofferedbythesedrivesmakethemafavoriteinhighperformanceapplications.PLCbasedcontrolsprovidesystemstatusaswellascontroloptions.ThisscreengivetheoperatorfullaccesstoCarnegieHallsninestagefloorlifts.PLCBasedSystemsAPLCisaprogrammablelogiccontroller.Firstdevelopedtoreplacetherelaybasedindustrialcontrolsystemsofthe50sand60s,thesecontrolsareathomeinrugged,industrialenvironments.ThesearemodularsystemswithagreatvarietyofI/Omodulesallowingsemicustomhardwareconfigurationstobeassembledeasilyatareasonableprice.Theseincludepositioncontrolmodules,counters,A/DandD/Aconvertersandallsortsofsolidstateorhardcontactclosureoutputs.ThegreatvarietyofI/OcomponentsandthemodularnatureofthePLCmakethisaneffectivewaytobuildcustomandsemicustomcontrolsystems.ThegreatestdrawbacktoPLCsystemsisthelackofreallygreatdisplaystotellyouwhattheyaredoingortohelpyouprogramthem.Monochromeandmediumresolutioncolordisplaysarethenorm,astheprimaryuseforthesecomponentsinonafactoryfloor.OneofthefirstmajorPLCsystemsusedinalargeentertainmentvenueisthecomplexliftandwagonsystemattheoriginalMGMGrandnowBallysinLasVegas.SeveralmanufacturersofferstandardPLCbasedsystemsandahostofsemicustomacousticbanner,shell,andliftcontrolsystemsisalsoavailable.TheabilitytobuildcustomsystemsfromstandardbuildingblocksisthegreateststrengthofPLCbasedcontrols.HighEndControllersThemostsophisticatedriggingcontrollersgowellbeyondspeed,time,andpositioncontrol.Theyincludetheabilitytowritecomplexcues,recordprofiledmoves,andmanagemultiplecuesrunningatonce.Manyofthelargeroperahousesaremovingtowardpointhoistsystems,wherethereisaseparatewinchforeachliftlinetheriggingequivalentofdimmerpercircuit.Whenmultiplewinchesareusedtocarryasinglepiece,thewinchesmustbeperfectlysynchronized,ortheloadcanshiftsothatanindividualwinchcanbecomedangerouslyoverloaded.Thecontrolsystemmustbeabletokeepselectedwinchesinsynchorprovidearapid,coordinatedstopifawinchisunabletostayinsynchwiththeothers.Withatypicaltopspeedof240fpmandarequirementtokeepthewinches,youhavelessthanthreemillisecondstorecognizeaproblem,attempttocorrecttheerrantwinchsspeed,determinethatyouvefailedandinitiateacoordinatedstopofallthewinchesinthegroup.Thistakesalotofcomputing,fastI/O,andwellwrittensoftware.Therearetwoverydifferentapproachestolargeriggingcontrolsystems.Originally,asingleconsolewasused,withtheusualproblemofwhereitshouldbelocatedfortheoperatorsoptimumview.Unfortunatelythiscanchangenotonlyfromshowtoshow,butalsofromonecuetothenext.Thisdilemmahasbeenpartiallyaddressedbyusingvideocamerasatdifferentlocationsinconjunctionwith3Dscreengraphicsthatallowtheoperatortoviewtheexpectedriggingmotionthreedimensionallyfromanyviewpoint.Thisallowstheoperatortoviewtheonscreenmovementoftheriggingfromaviewpointthatmatcheshisactualviewofthestage,ortheactualviewofaclosedcircuitcamera.Forcomplexmoveswithinterrelatedpiecesthismakesthecontrolandunderstandingofwhatishappeningmuchsimpler.Theotherapproachisadistributedsystem,withseveralportableconsoles.Thisallowsdifferentoperatorstocontroldifferentaspectsoftherigging,inthesamemannerwehavedonewithmanualsets.AdramaticexampleofthisapproachisusedbytheRoyalOperaatCoventGarden,wheretherearetenconsolescontrollingatotalof240motors.Eachconsolehasfiveplaybacks,andissetupsothateachmotorisassignedtoasingleconsole.Oneoperatorandconsolecouldcontroleverything,butfrequentlyoneconsolemayberunningstagelifts,anothertheonstagerigging,andathirdisbeingusedbackstagetomovestoreddrops.

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