Siemens LMV52.440B1 - Types of Vfds: Vector and Volt;Hz; Centrifugal Blower Fundamentals

LMVSeriesTechnicalInstructions 
DocumentNo.LV5‐1000
SCCInc. Page7 Section5
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TypesofVFDs:VectorandVolt/Hz
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AlthoughthereareoverahundreddifferentmanufacturersofVFDs,twomaintypesofVFDsare
producedbythesemanufacturersforuseonblowermotors.ThesetwotypesareVectorandVolt/Hz.
VectorVFDscanusuallyberunineitherVectormodeorVolt/Hzmode.VectorVFDsarealsotypically
slightlymoreexpensivethanVolt/HzVFDsforagivensize.
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TheadvantageofVectorVFDsisthattheyprovidemoreaccuratetorquecontro
lofthemotor.This
accuratetorquecontrolenablesmuchmoreaccuratespeedcontrolofthemotor,especiallyatlower
motorspeeds.Moreaccuratespeedcontrolofthemotorenablesmoreaccurate,repeatablecontrolof
theairflow.
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Asmentioned
earlier,theLMV52employsasafety‐relatedspeedfeedbackontheblowershaft,thus
continuouslycheckingandadjusting(ifnecessary)thesignaltotheVFDtoachievethedesiredblower
speedwithinacertainband.TheLMV52canlockoutandshutdowntheburnerifblowerspeed
deviationsarelargeandpersistfortoolong.Duetotheirinc
reasedaccuracy,VectorVFDsprovide
trouble‐freeoperationonalmostallLMV52VFDblowerapplications.Volt/HzVFDscanwork
satisfactorilyinsomeapplications,butarenotpreferredduetotheirdecreasedaccuracy.
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VectorVFDsaretypicallyruninOpenLoopVector(OLV)mode.Inth
ismode,theVFDusesa
mathematicalmodelofthemotorcombinedwithextremelyaccurate,fastscanningofthecurrentand
otherdatatakenfromtherotatingmotor.Inreality,OpenLoopVectormodedoeshavefeedback,but
theVectorVFDitselfdoesnotrequireaseparateencodertoachievethis.
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SinceVectorVFDsuseamathem
aticmodelofthemotor,andthedesignofmotorsdifferssomewhat
betweenmotorOEMs,astaticordynamicauto‐tuneissometimesrequiredsothattheVectorVFD
"learns"keyaspectsofthemotoritisconnectedto.Astaticauto‐tune(motorisnotspun)doesnot
requirethattheload(blowerwheel)be
de‐coupledfromthemotor.Adynamicauto‐tune(motoris
spun)typicallyrequiresthattheload(blowerwheel)bede‐coupledfromthemotor,whichisnot
possibleorpracticalinmanysituations.Adynamicaut
o‐tunetypicallygeneratesthebest"learning"of
themotorproperties.Astaticauto‐tuneistypicallyallthatisnecessaryifspeedcontrolissuesare
encounteredonaVectorVFD.
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CentrifugalBlowerFundamentals
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SinceacentrifugalbloweristhepieceofmachinerybeingcontrolledbytheLMV52andtheVFD,abrief
mentionofitsbasiccharacteristicsiswarranted.Spe cifically,therearethreefundamental"fanlaws"
thatapersonworkingonsuchequipmentshouldbeawareof.Theseare:
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1. Airflowvarieslinearlywiththespeedoftheblower.Inotherwords,theCFMofthebloweris
directlyproportionaltotheRPMoftheblower.
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