附錄英文原文AComparisonofSoftStartMechanismsforMiningBeltConveyorsMichaelL.Nave,P.E.CONSOLInc.1800WashingtonRoadPittsburgh,PA15241BeltConveyorsareanimportantmethodfortransportationofbulkmaterialsintheminingindustry.Thecontroloftheapplicationofthestartingtorquefromthebeltdrivesystemtothebeltfabricaffectstheperformance,lifecost,andreliabilityoftheconveyor.Thispaperexaminesapplicationsofeachstartingmethodwithinthecoalminingindustry.INTRODUCTIONTheforcerequiredtomoveabeltconveyormustbetransmittedbythedrivepulleyviafrictionbetweenthedrivepulleyandthebeltfabric.Inordertotransmitpowertheremustbeadifferenceinthebelttensionasitapproachesandleavesthedrivepulley.Theseconditionsaretrueforsteadystaterunning,starting,andstopping.Traditionally,beltdesignsarebasedonstaticcalculationsofrunningforces.Sincestartingandstoppingarenotexaminedindetail,safetyfactorsareappliedtostaticloadings(Harrison,1987).Thispaperwillprimarilyaddressthestartingoraccelerationdutyoftheconveyor.Thebeltdesignermustcontrolstartingaccelerationtopreventexcessivetensioninthebeltfabricandforcesinthebeltdrivesystem(Suttees,1986).Highaccelerationforcescanadverselyaffectthebeltfabric,beltsplices,drivepulleys,idlerpulleys,shafts,bearings,speedreducers,andcouplings.Uncontrolledaccelerationforcescancausebeltconveyorsystemperformanceproblemswithverticalcurves,excessivebelttake-upmovement,lossofdrivepulleyfriction,spillageofmaterials,andfestooningofthebeltfabric.Thebeltdesignerisconfrontedwithtwoproblems,Thebeltdrivesystemmustproduceaminimumtorquepowerfulenoughtostarttheconveyor,andcontrolledsuchthattheaccelerationforcesarewithinsafelimits.Smoothstartingoftheconveyorcanbeaccomplishedbytheuseofdrivetorquecontrolequipment,eithermechanicalorelectrical,oracombinationofthetwo(CEM,1979).SOFTSTARTMECHANISMEVALUATIONCRITERIONWhatisthebestbeltconveyordrivesystem?Theanswerdependsonmanyvariables.Thebestsystemisonethatprovidesacceptablecontrolforstarting,running,andstoppingatareasonablecostandwithhighreliability(LewdlyandSugarcane,1978).BeltDriveSystemForthepurposesofthispaperwewillassumethatbeltconveyorsarealmostalwaysdrivenbyelectricalprimemovers(GoodyearTireandRubber,1982).Thebeltdrivesystemshallconsistofmultiplecomponentsincludingtheelectricalprimemover,theelectricalmotorstarterwithcontrolsystem,themotorcoupling,thespeedreducer,thelowspeedcoupling,thebeltdrivepulley,andthepulleybrakeorholdback(Cur,1986).Itisimportantthatthebeltdesignerexaminetheapplicabilityofeachsystemcomponenttotheparticularapplication.Forthepurposeofthispaper,wewillassumethatalldrivesystemcomponentsarelocatedinthefreshair,non-permissible,areasofthemine,orinnon-hazardous,NationalElectricalCode,Article500explosion-proof,areasofthesurfaceofthemine.BeltDriveComponentAttributesSize.Certaindrivecomponentsareavailableandpracticalindifferentsizeranges.Forthisdiscussion,wewillassumethatbeltdrivesystemsrangefromfractionalhorsepowertomultiplesofthousandsofhorsepower.Smalldrivesystemsareoftenbelow50horsepower.Mediumsystemsrangefrom50to1000horsepower.Largesystemscanbeconsideredabove1000horsepower.Divisionofsizesintothesegroupsisentirelyarbitrary.Caremustbetakentoresistthetemptationtoovermotororundermotorabeltflighttoenhancestandardization.Anovermotoreddriveresultsinpoorefficiencyandthepotentialforhightorques,whileanundermotoreddrivecouldresultindestructiveoverspendingonregeneration,oroverheatingwithshortenedmotorlife(Lords,etal.,1978).TorqueControl.Beltdesignerstrytolimitthestartingtorquetonomorethan150%oftherunningtorque(CEMA,1979；Goodyear,1982).Thelimitontheappliedstartingtorqueisoftenthelimitofratingofthebeltcarcass,beltsplice,pulleylagging,orshaftdeflections.Onlargerbeltsandbeltswithoptimizedsizedcomponents,torquelimitsof110%through125%arecommon(Elberton,1986).Inadditiontoatorquelimit,thebeltstartermayberequiredtolimittorqueincrementsthatwouldstretchbeltingandcausetravelingwaves.Anidealstartingcontrolsystemwouldapplyapretensiontorquetothebeltatrestuptothepointofbreakaway,ormovementoftheentirebelt,thenatorqueequaltothemovementrequirementsofthebeltwithloadplusaconstanttorquetoacceleratetheinertiaofthesystemcomponentsfromresttofinalrunningspeed.Thiswouldminimizesystemtransientforcesandbeltstretch(Shultz,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,orthebeltisrunaslowinspectionorinchingspeedformaintenancepurposes(Hager,1991).Thevariablespeedbeltdrivewillrequireacontrolsystembasedonsomealgorithmtoregulateoperatingspeed.RegenerationorOverhaulingLoad.Somebeltprofilespresentthepotentialforoverhaulingloadswherethebeltsystemsuppliesenergytothedrivesystem.Notalldrivesystemshavetheabilitytoacceptregeneratedenergyfromtheload.Somedrivescanacceptenergyfromtheloadandreturnittothepowerlineforusebyotherloads.Otherdrivesacceptenergyfromtheloadanddissipateitintodesignateddynamicormechanicalbrakingelements.Somebeltprofilesswitchfrommotoringtoregenerationduringoperation.Canthedrivesystemacceptregeneratedenergyofacertainmagnitudefortheapplication?Doesthedrivesystemhavetocontrolormodulatetheamountofretardingforceduringoverhauling?Doestheoverhaulingoccurwhenrunningandstarting?MaintenanceandSupportingSystems.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.Anoften-overlookedcostinacomplexsystemisthecostoftrainingonsitepersonnel,orthecostofdowntimeasaresultofinsufficienttraining.SOFTSTARTDRIVECONTROLLOGICEachdrivesystemwillrequireacontrolsystemtoregulatethestartingmechanism.ThemostcommontypeofcontrolusedonsmallertomediumsizeddriveswithsimpleprofilesistermedOpenLoopAccelerationControl.Inopenloop,thecontrolsystemispreviouslyconfiguredtosequencethestartingmechanisminaprescribedmanner,usuallybasedontime.Inopenloopcontrol,drive-operatingparameterssuchascurrent,torque,orspeeddonotinfluencesequenceoperation.Thismethodpresumesthatthecontroldesignerhasadequatelymodeleddrivesystemperformanceontheconveyor.Forlargerormorecomplexbelts,ClosedLooporFeedbackcontrolmayheutilized.Inclosedloopcontrol,duringstarting,thecontrolsystemmonitorsviasensorsdriveoperatingparameterssuchascurrentlevelofthemotor,speedofthebelt,orforceonthebelt,andmodifiesthestartingsequencetocontrol,limit,oroptimizeoneorworeparameters.Closedloopcontrolsystemsmodifythestartingappliedforcebetweenanemptyandfullyloade