机械外文文献.docx

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机械外文文献

机械外文文献

AComparisonofDriveStartingMechanismsfor

AggregateBeltConveyors

Abstract

Thepurposeofthispaperistodescribethetorque/speedcharacteristics，during

startingconditions，ofthemostcommondrivesusedonbeltconveyorstoday.RequirementsofaBeltConveyorDrive

Abeltconveyorisconsideredtobeaconstanttorquedevice.Inotherwords，the

requireddrivingtorqueisapproximatelyconstantatvaryingspeeds（seefigurel）.otherapplications，suchasapumpdrive，havevariabletorquerequirements（see

figure2）.

However，toincreasethespeedofaconveyoradditionaltorquemustbeaddeduntil

thedesiredspeedisobtained.Newton’sSecondLawofMotiongovernsthis

relationship.

Fma,,

Themoststraightforwardexamplewouldbeaconstantaccelerationtorque（seefigure3）.Inrealitytheaccelerationtorqueisrarelyconstant.However，static

calculationmodelsasoutlinedintheConveyorEquipmentManufacturersAssociationhandbook（CEMA）makethisassumption.Whenusingstaticmodelstheaverageaccelerationtorqueisestimatedovertheentireaccelerationtimeandassumedtobelinear.Dynamicmodels，whicharebeyondthescopeofthispaper，allowacceleration

torquevaluestovaryinmagnitudeduringtheacceleration（ordeceleration）Period.

Itshouldbenotedthat，givenaconstantload，alargeraccelerationtorqueresultsina

fasteraccelerationtimeandalsohigherPeakbelttensions.Conversely，asmaller

accelerationtorqueresultsinalongerstarttimeandsmallerPeakbelttensions.Across-The-LineACMotorStart

Technicallythisisthesimplesttypeofdriveusedonabeltconveyor.InthisdrivetypeanACsquirrelcageinductionmotorisstartedbysimplythrowingthecontactorandenergizingthemotor.Theresultingoutputtorque，assumingthatratedvoltageis

maintained，isstrictlyafunctionofthemotordesign.NEMAhasProvideddesignstandardsthatdefinetheoutputtorquecharacteristicsofthemostcommonlyused3Phasemotorsuptoapproximately250hp（figure4）.Insizeslargerthan250hpmanufacturersgenerallyusetheNEMAdesigncodesinarelativemanner（i.e.，NEMA

ChasagreaterlockedrotortorquethanaNEMABmotor）.

ThemostcriticallocationsontheACmotorspeed/torquecurvehavebeennamedfordefinitionpurposes.Thesecommonnamesareprovidedinfigure5.

Themostrigorousmethodofdeterminingaverageaccelerationtorque，forstatic

calculations，istobreakthecurveintoseveralverticalsections，thensumthe

individualareasunderthecurveandfinallydividebythenumberofsections.

Themorecommonwayistoapplythefollowingsimplifiedequation:

Thesestaticapproximationmethodsworkformostbeltconveyorsbutcangetthedesignerintotroublefromtimetotime，especiallyonlongand/orsteepand/orfast

conveyors.Oneitemthatneedstobeexaminedisbreakawaytorque.Justbecausethedriveprovidesenoughaveragetorquetoacceleratetheloaddoesn’tmeanthatit

providesenoughtorquetobreakitawayfromzerospeedandgetitmoving.CEMAdefinesbreakawaytorqueastwicethetorquerequiredtoovercomethetotalfrictionplusthetorquerequiredtolifttheloadvertically.Lockedrotortorque（LRT）needstobegreaterthanbreakawaytorque!

AgoodstaticProgrammakesthischeck.

Inadditiontoexaminingtheeffectthataveragetorquehasontheconveyorcomponentsthebeltdesignerneedstodeterminetheeffectofpeaktorque.Itisnotuncommonforthebreakdowntorque（BDT）ofaNEMACmotortobegreaterthan2.5timesfullloadtorque（FLT）.GenerallythebeltingandPulleymanufacturersallowatransientoverloadof1.5timesfullloadoperatingload.Anacross-the-linestartcaneasilycausetensionstoexceedthesemaximums.Thesehigherthannormalloadscanbedesignedintotheconveyoriftheyareknownupfront.

Consideringonlyaveragestartingtorquecancausetheconveyordesignertoundersizethetake-upweight.Itisnotuncommonforconveyorswithacross-the-linestarterstoexperienceintermittentdriveslip.ThisgenerallyhappenswhenPeaktorque（BDT）isinputbythedriveandthetake-uphasbeensizedforaveragetorquebutnotpeaktorque.Theresultcanbedevastating.Whenthedrivepulleyslipsduringthiscondition，thetensionontheTlandT2sides（highandlow）ofthedrivePulleytriestoequalize.Thiscansubjectalowtensionbendortake-uppulley，justbehindthedrive

pulley，totensionsthatapproachTltension.ThesePulleysarerarely，ifever，designed

forthisloadconditionandtheresultislowtensionPulleyfailure.Thisconditioniseasilydemonstratedwithdynamicanalysis.

AnothercommonProblemwithacross-the-linestartsiscausedbyvoltagedipsduringstarting.Ifthepowerdistributionsystemisnotstiffenoughtohandlethehugeinrushcurrentsofanacross-the-1inestart，thestartingtorqueofthemotorscanbereducedto

aPointthattheconveyorwillnotstart.Thisisduetothefactthattheoutputtorqueof

anACsquirrelcageinductionmotorisreducedbythesquareoftheappliedvoltage.Inotherwords，avoltagedropof10%wouldequatetoatorquereductionof19%.ReducedVoltageStarting

ThereducedvoltagestartingofanACsquirrelcageinductionmotorisdonefortwobasicreasons:

1.ToreducetheinrushcurrentthatnaturallyoccurswhenamotorisStarted

across-the-1ine.Atypicalcurrent/speedgraphisshowninfigure6.Itisnot

uncommonfortheinrushcurrenttobe6timesormorethanitisatfullload

torque.Asstatedabovehighinrushcurrentscausethevoltageinapower

distributionsystemtosag.Thecostofelectricalpowerdistributionequipment

canbecomeveryhighifitneedstobedesignedtohandlethehighinrush

currents.

2.ToreducePeakmotortorqueduringstartingconditions，whichsubsequently

increasesaccelerationtime.ByreducingthePeaktorquestheconveyor

componentscanbedesignedforlowertensionloads.Thisprimarilyincludes

belting，Pulleysandexternalsupportstructure.Thiscanresultinsignificantcost

savings.

TwocommontypesofreducedvoltagestartersaretheCurrentLimitingandtheConstantTorquedevices.

Graphsareincludedabove（figures7through8）thatdepictthesamemotor/conveyorapplicationwithanAcross-The-Line，alimitdCurren,andaconstantTorquestart.

Afterstudyingthegraphsitbecomesapparentthatthebestuseofthelimitedtorquestartistoprotectthepowerdistributionsystemfromhighinrushcurrents.TheconstanttorquestartreducesthehightorquePeaksandProtectstheconveyor’s

mechanicalcomponents.InbothcasestheStarttimeisincreasedbecausetheoverallmagnitudeofacceleratingtorqueisreduced.However，neithermethodwillmakeit

easiertostarta“hard-to-startconveyor.”Correctingahardstartingconveyorisnota

reasontouseareducedvoltagestarter!