带式输送机论文中英文资料外文翻译文献Word文档下载推荐.docx
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带式输送机论文中英文资料外文翻译文献Word文档下载推荐.docx
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AComparisonofDriveStartingMechanismsfor
AggregateBeltConveyors
Abstract
Thepurposeofthispaperistodescribethetorque/speedcharacteristics,duringstartingconditions,ofthemostcommondrivesusedonbeltconveyorstoday.
RequirementsofaBeltConveyorDrive
Abeltconveyorisconsideredtobeaconstanttorquedevice.Inotherwords,therequireddrivingtorqueisapproximatelyconstantatvaryingspeeds(seefigurel).otherapplications,suchasapumpdrive,havevariabletorquerequirements(seefigure2).
However,toincreasethespeedofaconveyoradditionaltorquemustbeaddeduntilthedesiredspeedisobtained.Newton’sSecondLawofMotiongovernsthisrelationship.
Themoststraightforwardexamplewouldbeaconstantaccelerationtorque(seefigure3).Inrealitytheaccelerationtorqueisrarelyconstant.However,staticcalculationmodelsasoutlinedintheConveyorEquipmentManufacturersAssociationhandbook(CEMA)makethisassumption.Whenusingstaticmodelstheaverageaccelerationtorqueisestimatedovertheentireaccelerationtimeandassumedtobelinear.Dynamicmodels,whicharebeyondthescopeofthispaper,allowaccelerationtorquevaluestovaryinmagnitudeduringtheacceleration(ordeceleration)Period.
Itshouldbenotedthat,givenaconstantload,alargeraccelerationtorqueresultsinafasteraccelerationtimeandalsohigherPeakbelttensions.Conversely,asmalleraccelerationtorqueresultsinalongerstarttimeandsmallerPeakbelttensions.Across-The-LineACMotorStart
Technicallythisisthesimplesttypeofdriveusedonabeltconveyor.InthisdrivetypeanACsquirrelcageinductionmotorisstartedbysimplythrowingthecontactorandenergizingthemotor.Theresultingoutputtorque,assumingthatratedvoltageismaintained,isstrictlyafunctionofthemotordesign.NEMAhasProvideddesignstandardsthatdefinetheoutputtorquecharacteristicsofthemostcommonlyused3Phasemotorsuptoapproximately250hp(figure4).Insizeslargerthan250hpmanufacturersgenerallyusetheNEMAdesigncodesinarelativemanner(i.e.,NEMAChasagreaterlockedrotortorquethanaNEMABmotor).
ThemostcriticallocationsontheACmotorspeed/torquecurvehavebeennamedfordefinitionpurposes.Thesecommonnamesareprovidedinfigure5.
Themostrigorousmethodofdeterminingaverageaccelerationtorque,forstaticcalculations,istobreakthecurveintoseveralverticalsections,thensumtheindividualareasunderthecurveandfinallydividebythenumberofsections.
Themorecommonwayistoapplythefollowingsimplifiedequation:
Thesestaticapproximationmethodsworkformostbeltconveyorsbutcangetthedesignerintotroublefromtimetotime,especiallyonlongand/orsteepand/orfastconveyors.Oneitemthatneedstobeexaminedisbreakawaytorque.Justbecausethedriveprovidesenoughaveragetorquetoacceleratetheloaddoesn’tmeanthatitprovidesenoughtorquetobreakitawayfromzerospeedandgetitmoving.
CEMAdefinesbreakawaytorqueastwicethetorquerequiredtoovercomethetotalfrictionplusthetorquerequiredtolifttheloadvertically.Lockedrotortorque(LRT)needstobegreaterthanbreakawaytorque!
AgoodstaticProgrammakesthischeck.
Inadditiontoexaminingtheeffectthataveragetorquehasontheconveyorcomponentsthebeltdesignerneedstodeterminetheeffectofpeaktorque.Itisnotuncommonforthebreakdowntorque(BDT)ofaNEMACmotortobegreaterthan
2.5timesfullloadtorque(FLT).GenerallythebeltingandPulleymanufacturersallowatransientoverloadof1.5timesfullloadoperatingload.Anacross-the-linestartcaneasilycausetensionstoexceedthesemaximums.Thesehigherthannormalloadscanbedesignedintotheconveyoriftheyareknownupfront.
Consideringonlyaveragestartingtorquecancausetheconveyordesignertoundersizethetake-upweight.Itisnotuncommonforconveyorswithacross-the-linestarterstoexperienceintermittentdriveslip.ThisgenerallyhappenswhenPeaktorque
(BDT)isinputbythedriveandthetake-uphasbeensizedforaveragetorquebutnotpeaktorque.Theresultcanbedevastating.Whenthedrivepulleyslipsduringthiscondition,thetensionontheTlandT2sides(highandlow)ofthedrivePulleytriestoequalize.Thiscansubjectalowtensionbendortake-uppulley,justbehindthedrivepulley,totensionsthatapproachTltension.ThesePulleysarerarely,ifever,designedforthisloadconditionandtheresultislowtensionPulleyfailure.Thisconditioniseasilydemonstratedwithdynamicanalysis.
AnothercommonProblemwithacross-the-linestartsiscausedbyvoltagedipsduringstarting.Ifthepowerdistributionsystemisnotstiffenoughtohandlethehugeinrushcurrentsofanacross-the-1inestart,thestartingtorqueofthemotorscanbereducedtoaPoint
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