外文翻譯--電力絞車的控制.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.Thisdilemmahasbeenpartiallyaddressedbyusingvideocamerasatdifferentlocationsinconjunctionwith3Dscreengraphicsthatallowtheoperatortoviewtheexpectedriggingmotionthreedimensionallyfrom