Siemens LMV52.240B1 - Page 158

TechnicalInstructions  LMVSeries
DocumentNo.LV5‐1000
Section5 Page6 SCCInc.
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TheDCbuscanabsorbasmallamountofenergyintheDCbuscapacitors.However,ifthemotor
generatesmorethanwhatthesecapacitorscanabsorb,theDCbusvoltagewillrisetocriticallevelsand
oneoftwoactionswillbetakenbytheVFD.DependingontheparametersettingoftheVFD,theVFD
willeitherstopdecelerating(stallprevention)ortheVFDwillalarmandshutdown.Eit
heroneofthe
actionsisnotadesirableresultonacombustionairapplication.
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ToavoidDCbusovervoltageissues,abrakingresistorcanbeaddedtotheVFDsothattheexcess
electricalenergygeneratedbydeceleratingtheblow
erwheelcanbeturnedtoheat.Thisprocess
happensseamlesslysothattheVFDcandeceleratetheblowersmoothly.
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Duetoanumberofvariables,itisdifficulttodetermi neifa brakingresistorwillbeneededona
particularapplicationunlessthatapplicationhasbeentested.Theonlydisadvantageofhavingabraking
resistorandnotnee
dingitiscostandpossiblythespacefortheresistor.Burnershavingthefollowing
characteristicswilltypicallyneedabrakingresistor:
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1. Aheavyblowerwheel‐Kineticenergyisstoredinaspinningwheel.Theheaviertheblowerwheel,
thegreaterthestoredenergy.Whenthiswheelissloweddown,thekineticenergymustgo
somewhere,anditisusually"pushed"backtotheVFDaselectricalenergy.
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2. Fastramptimes‐Thefastertheramptimes,thefastertheblowerwheelmustbeacceleratedand
decelerated.Justlikeacar,moreenergyisrequiredtoacceleratequickly(biggerengine)andmore
energyisrequiredtobedissipatedwhendeceleratingquickly(biggerbrakes).Deceleratingagiven
blowerwheelmorequicklywillpushmoreelectricalenergybacktotheVFD.
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3. Mostlyclosedairdamper‐Amotorspinningat3600RPMdrawsfewerampswithaclosedornearly
closedairdamperascomparedtoawideopenairdamper.Thus,thehorsepowerusedbythe
motorandthedrag(braking)ontheblower wheelwillbemuchlesswitha closedornearlyclosed
airdamper.Deceleratingagivenblowerwheelwithreduceddragwillalsopushmoreelectrical
energybacktotheVFD.
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Asonemightexpect,theabovepointscompoundoneanother.Deceler
atingaheavyblowerwheel
withafastramptimeandamostlyclosedairdamperwillpushalargeamountofelectricalenergyback
attheVFDandwilllikelycauseDCbusovervoltageissuesifabrakingresistorisnotinstalled.
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Incontrast,alightblowerwheel(shee
tmetalinsteadofcastiron),aslowerramptime(90seconds
insteadof30seconds),andslowingtheblowerdownonamoreopenairdamperarecharacteristicsthat
willgreatlyreducetheamountofelectricalenergypushedbacktotheVFDandshouldallowthebraking 
resistortobeomittedinmostcases.
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OnsomemodelsofVFDs,brakingresistorscanbeaddedaftertheVFDisinstalledifnecessary.Thisisa
point
toconsiderwheninstallingVFDsforcombustionairapplications.
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