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附录英文原文AComparisonofSoftStartMechanismsforMiningBeltConveyorsMichaelL.Nave,P.E.CONSOLInc.1800WashingtonRoadPittsburgh,PA15241BeltConveyorsareanimportantmethodfortransportationofbulkmaterialsintheminingindustry.Thecontroloftheapplicationofthestartingtorquefromthebeltdrivesystemtothebeltfabricaffectstheperformance,lifecost,andreliabilityoftheconveyor.Thispaperexaminesapplicationsofeachstartingmethodwithinthecoalminingindustry.INTRODUCTIONTheforcerequiredtomoveabeltconveyormustbetransmittedbythedrivepulleyviafrictionbetweenthedrivepulleyandthebeltfabric.Inordertotransmitpowertheremustbeadifferenceinthebelttensionasitapproachesandleavesthedrivepulley.Theseconditionsaretrueforsteadystaterunning,starting,andstopping.Traditionally,beltdesignsarebasedonstaticcalculationsofrunningforces.Sincestartingandstoppingarenotexaminedindetail,safetyfactorsareappliedtostaticloadingsHarrison,1987.Thispaperwillprimarilyaddressthestartingoraccelerationdutyoftheconveyor.ThebeltdesignermustcontrolstartingaccelerationtopreventexcessivetensioninthebeltfabricandforcesinthebeltdrivesystemSuttees,1986.Highaccelerationforcescanadverselyaffectthebeltfabric,beltsplices,drivepulleys,idlerpulleys,shafts,bearings,speedreducers,andcouplings.Uncontrolledaccelerationforcescancausebeltconveyorsystemperformanceproblemswithverticalcurves,excessivebelttakeupmovement,lossofdrivepulleyfriction,spillageofmaterials,andfestooningofthebeltfabric.Thebeltdesignerisconfrontedwithtwoproblems,Thebeltdrivesystemmustproduceaminimumtorquepowerfulenoughtostarttheconveyor,andcontrolledsuchthattheaccelerationforcesarewithinsafelimits.Smoothstartingoftheconveyorcanbeaccomplishedbytheuseofdrivetorquecontrolequipment,eithermechanicalorelectrical,oracombinationofthetwoCEM,1979.SOFTSTARTMECHANISMEVALUATIONCRITERIONWhatisthebestbeltconveyordrivesystemTheanswerdependsonmanyvariables.Thebestsystemisonethatprovidesacceptablecontrolforstarting,running,andstoppingatareasonablecostandwithhighreliabilityLewdlyandSugarcane,1978.BeltDriveSystemForthepurposesofthispaperwewillassumethatbeltconveyorsarealmostalwaysdrivenbyelectricalprimemoversGoodyearTireandRubber,1982.Thebeltdrivesystemshallconsistofmultiplecomponentsincludingtheelectricalprimemover,theelectricalmotorstarterwithcontrolsystem,themotorcoupling,thespeedreducer,thelowspeedcoupling,thebeltdrivepulley,andthepulleybrakeorholdbackCur,1986.Itisimportantthatthebeltdesignerexaminetheapplicabilityofeachsystemcomponenttotheparticularapplication.Forthepurposeofthispaper,wewillassumethatalldrivesystemcomponentsarelocatedinthefreshair,nonpermissible,areasofthemine,orinnonhazardous,NationalElectricalCode,Article500explosionproof,areasofthesurfaceofthemine.BeltDriveComponentAttributesSize.Certaindrivecomponentsareavailableandpracticalindifferentsizeranges.Forthisdiscussion,wewillassumethatbeltdrivesystemsrangefromfractionalhorsepowertomultiplesofthousandsofhorsepower.Smalldrivesystemsareoftenbelow50horsepower.Mediumsystemsrangefrom50to1000horsepower.Largesystemscanbeconsideredabove1000horsepower.Divisionofsizesintothesegroupsisentirelyarbitrary.Caremustbetakentoresistthetemptationtoovermotororundermotorabeltflighttoenhancestandardization.Anovermotoreddriveresultsinpoorefficiencyandthepotentialforhightorques,whileanundermotoreddrivecouldresultindestructiveoverspendingonregeneration,oroverheatingwithshortenedmotorlifeLords,etal.,1978.TorqueControl.Beltdesignerstrytolimitthestartingtorquetonomorethan150oftherunningtorqueCEMA,1979Goodyear,1982.Thelimitontheappliedstartingtorqueisoftenthelimitofratingofthebeltcarcass,beltsplice,pulleylagging,orshaftdeflections.Onlargerbeltsandbeltswithoptimizedsizedcomponents,torquelimitsof110through125arecommonElberton,1986.Inadditiontoatorquelimit,thebeltstartermayberequiredtolimittorqueincrementsthatwouldstretchbeltingandcausetravelingwaves.Anidealstartingcontrolsystemwouldapplyapretensiontorquetothebeltatrestuptothepointofbreakaway,ormovementoftheentirebelt,thenatorqueequaltothemovementrequirementsofthebeltwithloadplusaconstanttorquetoacceleratetheinertiaofthesystemcomponentsfromresttofinalrunningspeed.ThiswouldminimizesystemtransientforcesandbeltstretchShultz,1992.Differentdrivesystemsexhibitvaryingabilitytocontroltheapplicationoftorquestothebeltatrestandatdifferentspeeds.Also,theconveyoritselfexhibitstwoextremesofloading.Anemptybeltnormallypresentsthesmallestrequiredtorqueforbreakawayandacceleration,whileafullyloadedbeltpresentsthehighestrequiredtorque.Aminingdrivesystemmustbecapableofscalingtheappliedtorquefroma2/1ratioforahorizontalsimplebeltarrangement,toa10/1rangesforaninclinedorcomplexbeltprofile.ThermalRating.Duringstartingandrunning,eachdrivesystemmaydissipatewasteheat.Thewasteheatmaybeliberatedintheelectricalmotor,theelectricalcontrols,,thecouplings,thespeedreducer,orthebeltbrakingsystem.ThethermalloadofeachstartIsdependentontheamountofbeltloadandthedurationofthestart.Thedesignermustfulfilltheapplicationrequirementsforrepeatedstartsafterrunningtheconveyoratfullload.Typicalminingbeltstartingdutiesvaryfrom3to10startsperhourequallyspaced,or2to4startsinsuccession.Repeatedstartingmayrequirethedreadingoroversizingofsystemcomponents.Thereisadirectrelationshipbetweenthermalratingforrepeatedstartsandcosts.VariableSpeed.Somebeltdrivesystemsaresuitableforcontrollingthestartingtorqueandspeed,butonlyrunatconstantspeed.Somebeltapplicationswouldrequireadrivesystemcapableofrunningforextendedperiodsatlessthanfullspeed.Thisisusefulwhenthedriveloadmustbesharedwithotherdrives,thebeltisusedasaprocessfeederforratecontroloftheconveyedmaterial,thebeltspeedisoptimizedforthehaulagerate,thebeltisusedatslowerspeedstotransportmenormaterials,orthebeltisrunaslowinspectionorinchingspeedformaintenancepurposesHager,1991.Thevariablespeedbeltdrivewillrequireacontrolsystembasedonsomealgorithmtoregulateoperatingspeed.RegenerationorOverhaulingLoad.Somebeltprofilespresentthepotentialforoverhaulingloadswherethebeltsystemsuppliesenergytothedrivesystem.Notalldrivesystemshavetheabilitytoacceptregeneratedenergyfromtheload.Somedrivescanacceptenergyfromtheloadandreturnittothepowerlineforusebyotherloads.Otherdrivesacceptenergyfromtheloadanddissipateitintodesignateddynamicormechanicalbrakingelements.Somebeltprofilesswitchfrommotoringtoregenerationduringoperation.CanthedrivesystemacceptregeneratedenergyofacertainmagnitudefortheapplicationDoesthedrivesystemhavetocontrolormodulatetheamountofretardingforceduringoverhaulingDoestheoverhaulingoccurwhenrunningandstartingMaintenanceandSupportingSystems.Eachdrivesystemwillrequireperiodicpreventativemaintenance.Replaceableitemswouldincludemotorbrushes,bearings,brakepads,dissipationresistors,oils,andcoolingwater.Ifthedrivesystemisconservativelyengineeredandoperated,thelowerstressonconsumableswillresultinlowermaintenancecosts.Somedrivesrequiresupportingsystemssuchascirculatingoilforlubrication,coolingairorwater,environmentaldustfiltering,orcomputerinstrumentation.Themaintenanceofthesupportingsystemscanaffectthereliabilityofthedrivesystem.Cost.Thedrivedesignerwillexaminethecostofeachdrivesystem.Thetotalcostisthesumofthefirstcapitalcosttoacquirethedrive,thecosttoinstallandcommissionthedrive,thecosttooperatethedrive,andthecosttomaintainthedrive.Thecostforpowertooperatethedrivemayvarywidelywithdifferentlocations.Thedesignerstrivestomeetallsystemperformancerequirementsatlowesttotalcost.Oftenmorethanonedrivesystemmaysatisfyallsystemperformancecriterionsatcompetitivecosts.Complexity.Thepreferreddrivearrangementisthesimplest,suchasasinglemotordrivingthroughasingleheadpulley.However,mechanical,economic,andfunctionalrequirementsoftennecessitatetheuseofcomplexdrives.Thebeltdesignermustbalancetheneedforsophisticationagainsttheproblemsthataccompanycomplexsystems.Complexsystemsrequireadditionaldesignengineeringforsuccessfuldeployment.Anoftenoverlookedcostinacomplexsystemisthecostoftrainingonsitepersonnel,orthecostofdowntimeasaresultofinsufficienttraining.SOFTSTARTDRIVECONTROLLOGICEachdrivesystemwillrequireacontrolsystemtoregulatethestartingmechanism.ThemostcommontypeofcontrolusedonsmallertomediumsizeddriveswithsimpleprofilesistermedOpenLoopAccelerationControl.Inopenloop,thecontrolsystemispreviouslyconfiguredtosequencethestartingmechanisminaprescribedmanner,usuallybasedontime.Inopenloopcontrol,driveoperatingparameterssuchascurrent,torque,orspeeddonotinfluencesequenceoperation.Thismethodpresumesthatthecontroldesignerhasadequatelymodeleddrivesystemperformanceontheconveyor.Forlargerormorecomplexbelts,ClosedLooporFeedbackcontrolmayheutilized.Inclosedloopcontrol,duringstarting,thecontrolsystemmonitorsviasensorsdriveoperatingparameterssuchascurrentlevelofthemotor,speedofthebelt,orforceonthebelt,andmodifiesthestartingsequencetocontrol,limit,oroptimizeoneorworeparameters.Closedloopcontrolsystemsmodifythestartingappliedforcebetweenanemptyandfullyloadedconveyor.Theconstantsinthemathematicalmodelrelatedtothemeasuredvariableversusthesystemdriveresponsearetermedthetuningconstants.Theseconstantsmustbeproperlyadjustedforsuccessfulapplicationtoeachconveyor.Themostcommonschemesforclosedloopcontrolofconveyorstartsaretachometerfeedbackforspeedcontrolandloadcellforceordriveforcefeedbackfortorquecontrol.Onsomecomplexsystems,Itisdesirabletohavetheclosedloopcontrolsystemadjustitselfforvariousencounteredconveyorconditions.ThisistermedAdaptiveControl.Theseextremescaninvolvevastvariationsinloadings,temperatureofthebelting,locationoftheloadingontheprofile,ormultipledriveoptionsontheconveyor.Therearethreecommonadaptivemethods.Thefirstinvolvesdecisionsmadebeforethestart,orRestartConditioning.Ifthecontrolsystemcouldknowthatthebeltisempty,itwouldreduceinitialforceandlengthentheapplicationofaccelerationforcetofullspeed.Ifthebeltisloaded,thecontrolsystemwouldapplypretensionforcesunderstallforlesstimeandsupplysufficienttorquetoadequately
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