Siemens LMV51.140C1 Technical Instructions

TechnicalInstructions  LMVSeries
DocumentNo.LV5‐1000
Section5 Page6 SCCInc.


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.

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.

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:

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.

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.

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.

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.

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.

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.






HOME

Siemens LMV51.140C1 Specifications

General

Brand	Siemens
Model	LMV51.140C1
Category	Control Systems
Language	English