负荷分配 AB.docx

负荷分配 AB.docx

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负荷分配AB

LoadSharingApplicationswithACDrives

Overview

Loadsharingisatermusedbymanytodescribeasystemwheremultipledrivemotorsetsarephysicallycoupledtogetherandusedtopoweranoverallmechanicalload.Inthestrictestsense,load-sharingmeansthattheamountoftorqueappliedtotheloadfromeachmotorisprescribedandcarriedoutbyeachdrive/motorset.Eachdriveperformsitsshareaccordingtoaspecificcontrolmethoddesignedforparticularapplicationcharacteristics.

IdentifyingLoadSharingApplications

Thefirststepinaddressingtheneedsofanapplicationwithmultiplemotorsistodetermineifthemotorsarerequiredtoworktogethertosharetheload.Loadsharinginvolvesmultiplemotorsanddrivespoweringthesameprocess.Eachdrive/motorsetmustcontributeits’proportionalshareofpowertothedrivenload.

Multiplemotorsthatarepoweredbyasingledriveisacrudeformofloadsharingrelyingentirelyonthegiveandtakerangeofthenaturalslipcharacteristicsofthemotor.Thisisnottheformofloadsharingaddressedbythisoverview.

Processandequipmentapplicationsinvolvingmultiplemotorsmechanicallyengagedtocontributetotheoverallpowernecessarytofunctionisthesubjectofthisloadsharingoverview.Eachmotorhasitspowercontributionregulatedandcontrolledwithrespecttotheothermotors.

ControlTopologies

Threecategoriesofloadsharingtechniqueswillbepresented,eachhavinguniquecharacteristics.Thesubtledifferenceswillbeaddressedtobetteridentifyhowtoimplementeachtoensureasuccessfulapplication.ThecategoriesareDroop,TorqueFollower,andSpeedTrimFollower.

Droop

Droopisthesimplestformaloadsharingtosetupandthereforetheleastpreciseandlessflexible.Theprecisionofthiscontrolisdependentonthreefactors;thedrivescontrolalgorithm,themotorcharacteristics,andthetypeofloadbeingcontrolled.

Sinceavolts/hertzdrivedoesnothavetheabilitytorunin“torquemode”,amorelooseinterpretationoftheterm“loadsharing”issometimesused.Loadsharingusingavolts/hertzdriveismuchlesscontrollableandtoalargeextentdependentonthemotorslipandtypeofloadcoupling.Sincealoadedinductionmotorhas“slip”asaninherentcharacteristic,loadsharingisanaturalresponsetoloading.

Whentwomotorsarecoupledtothesameload,eachwillslipaccordingtoits’portionofload,forcingloadtobepickedupbytheothermotor.Inthisregardahighslipmotorisbetterforloadsharingapplications.However,inageneralsensetheslipofamotorisrepresentativeofit’s’efficiencyrating.Highslipmotorstendtohavehighstartingtorquebuthavelowefficiency.Thoughhighefficiencymotorswillslipwhenloaded,theymaynothaveenoughslipatfullloadtoachieveareasonabledegreeofloadsharing.Forthisreasonmanydrivesofferafeaturecalled“Droop”.

TheDroopfeatureenhancesthenaturalslipofthemotorbyreducingtheoutputfrequencyinproportiontoloadcurrent.Ideallyitisonlythetorque-producingcomponentoftheloadcurrentthatdeterminesthedroop.Theamountofdroopcanusuallybeprogrammedinthedriveinhertzor%ofspeed.

TorqueFollower

Thistypeofloadsharingrequiresadrivehavingthecapabilityofoperatingin“torquemode”suchasthe1336IMPACT,PowerFlex700VCandPowerFlex700Sdrives.Ifspeedregulationisrequired,oneofthedrivesmaybein“speedmode”（calledthe“Master”）.Keepinmindthatadriveinspeedregulatormodeisreallystillintorquemodewithitstorquereferenceorcommandcomingfromitsouterloopspeedregulator.Theouterloopspeedregulatorfunctioncanalsobeperformedoutsidethedrivebyaseparatecontroller/PLC/computeretc.aslongastheoutputtorquecommandisperunitizedtothedrive/motorcapacityrating.

Thissametorquecommandoutputcanbedistributedtotheotherdrivesonthesystem（calledthe“Slaves”or“TorqueFollowers”）.Thistorquesignalmaybescaledatthemasteroutputortheslaveinputtodividetheloadsharinginanyratiodesired.Inthiscaseitispossibletohavedrive/motorsetsofvaryinghorsepowersizeseachpullingloadtotheextentoftheircapability.Forexampleasystemrequiring150horsepowercouldbepoweredbya100horsepoweranda75horsepowerdrive/motorseteachrunningat86%ofits’powerrating.Theratioofloadsharingcanbeadjustedsuchthateachmotor/drivesetpullsaproportionalamountofloadunderallconditions.

SpeedTrimFollower

Thisconfigurationrequiresdrivesthatcanproduceaninternaltorquereferenceorestimatetorquecurrent.Masterandfollowerdrivesareoperatedinspeedregulationmodeandreceivethesamespeedreference.Interconnectionofthedrivesisnecessarysothattorquereferencesmaybecompared.

Ina“SingleSource”configuration,thetorquereferenceofthemasterdriveissenttothefollowerdrives.Eachfollowerdrivecomparesits’owntorquereferencewiththatofthemasterdrive.Theoutputofthecomparatorisanerrorsignalthattrimsthespeedofthefollower.

Ina“Cascade”configuration,thetorquereferenceofthemasterissenttothefirstfollowerdrive.Consecutivelythetorquereferenceofthefirstfollowerdriveissenttothenextfollowerdriveandsoon.Eachfollowerdrivecomparesits’owntorquereferencewiththatofthedriveaheadofitself.Theoutputofthecomparatorisanerrorsignalthattrimsthespeedofthatfollower.

Applicationcharacteristicsvschoiceofloadsharingcontroltechnique

ApplicationCharacteristic

LoadSharingControlTechnique

Droop

TorqueFollower

SpeedTrimFollower

Nodirectmechanicalloadcoupling

X

Elasticorrubberbandeffect,involvingstretch,expansionandcontractionofmediamechanicallycoupled（belt,monorail,chainconveyorsetc.）

X

X

Parallelfansmovinggaseousmediafromsameplenum

X

Parallelpumpingofliquids

X

Mechanicallyandrigidlycoupledequipmentwithmulti-motors;

▪coupledinlinedriveshafts

▪rackandpinion

▪engagingthesameaxleorshaft

▪engagingabullorringgear

▪niprollers

X

Serpentineconveyorbeltaroundcloseproximitydrives

X

Anyrigidmechanicalcouplingofmultiplemotorsteamedupasonelargemotor.

X

weblineswithmulti-stagetensionregulation

X

Configurations

Droop

Themasterandfollowerdrive（s）receivethesamespeedreferenceandnointerconnectionofthedrivesisrequired.Thespeedofthemotorswillbedependentontheloadandamountofdroopthatisprogrammedatfullloadcurrent.

DroopConfiguration

TheDroopfeatureenhancesthenaturalslipofthemotorbyreducingtheoutputfrequencyinproportiontoloadcurrent.Ideallyitisonlythetorque-producingcomponentoftheloadcurrentthatdeterminesthedroop.Theamountofdroopcanusuallybeprogrammedinthedriveinhertzor%speed.

Driveswiththecapabilityoftorqueregulation,suchasthe1336IMPACT,PF700VCPF700S,havebothatorqueregulatorandavelocityregulatorbuiltin.Thevelocityregulatorcontrolsmotorspeedinengineeringunitsofrpm.TheDroopfeatureisprogrammedin%orRPMdependingonthedrive.Standardvolts/hertzdrivesalsooffertheDroopfeatureandareusuallyprogrammedinHz.Principleofoperationisthesame.

Bothfollowerdrives（theretechnicallyisnoleadormasterdrivewhenusingDroop）areoperatedinSpeedRegulatormodewithDroopfeatureenabled.Theprincipleofoperationisastheloaddemandshighercurrent,theDroopfeatureproportionallyreducesspeedperthedroopvaluechosen,allowingthedrivetoshedaportionofitsload.Theoppositeoccurswhencurrentdemandisreduced.Thedrivewillincreasespeedproportionallyinherentlytakingonagreatershareoftheload.

Forbetterspeedregulationitmaybenecessarytooperatingoneofthedriveswitharigidspeedregulator（nodroop）.Iftheleadormasterdriveisnotallowedtodroop,allfollowerdriveswillbe“pulled”bythemaster.Onepotentialworkaroundforutilizingdroopwithamasterspeedregulatorwithoutdroop,istoscalethereferenceinputtothefollowerdrives.Thefollowersneedtohaveahigherreferencevaluesincetheywillbeloweringtheoutputduringloadedconditions.

Droopresolvestheissuewhenmultipledrivesaresetuptooperateasindependentspeedregulatorswithoutanygiveandtakecompensation.Thispreventsmultipledrivesonasystemfrom“fighting”againsteachothertocontroltheload.Typicallytheresultisdriveswillgravitatetotakingondisproportionatepercentagesoftheload.Onemaytakeonasignificantpercentagewhileanothersettlestodoingverylittle.Iftherearemoredrivestherecouldbeanycombinationofdisproportionatesharingoftheload.Severeoscillationcanalsodevelop.

Akeynegativeresultofloadsharingviadroop,issacrificingspeedregulation.Thegreatertheloadthemorethecommandedspeedisreducedorcompromisedandhastobemadeupbythecontrollercommandingthespeed.

DroopConfiguration

SpeedPIRegulators

Theoutputofthevelocityloopisatorquecommandusedbythetorquetocurrentconverterregulator.Thespeedregulatorincreasesordecreasestheoutputtorquecommandinordertoreachthedesiredmotorspeed,byconstantlytryingtoreducetheerrorbetweencommandedspeedandthefeedback.

Thetorquereferenceoutputofthespeedregulatorisalsofedbackthroughthedroopcircuitrytosubtractaspecificamountofrpmfromthespeedreferenceatfullloadoralsoalinearproportionatlesserlevelsofload.Thevalueprogrammedinto[DroopPercent]isbasedonratedmotorrpm.Fora1750rpmmotoratfullload,a2%droopvaluewouldredu