Siemens LMV52.240B1 - Blower Speed Monitoring

LMVSeriesTechnicalInstructions 
DocumentNo.LV5‐1000
SCCInc. Page13 Section5
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IftheanalogsignalisdecreasedtotheminimumallowablesignalandtheblowerRPMisstillhigh,a
messagewillbedisplayedontheAZLthatstates“ControlrangelimitationVSDModule”.
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Thereasonthatthestandardizationisdoneat19.5mAinsteadofat20mAistogivetheLMV52some
additional"room"toincreasetheanalogsignalforalowRPMconditionathighfire.Becausethe
standardizationisdoneat19.5mA,theanaloginputontheVFDisspannedsothat20mA=62Hz.Thisis
donesothattheblowerwillstillachievefull60Hzblowerspeedathighfireonjobswherethebloweris
justlargeeno
ugh.
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NOTE:MostVFDscanbescaledtooutput400Hzormore.Consulttheblowerand/ormotor
manufacturerbeforeover‐speedingthemotorandblower,sinceblowerwheelsandmotorrotorscan
catastrophicallyfailifRPMlimitsareexceeded.
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Inadditiontolimitsonhowmuchthe4‐20mAsignalcanbecompensated,theLMV52alsohaslimitson
howfartheblowerspeedcandeviatefromthestandardizedspeedline.Thenextsectionexplainshow
theblowerspeedismonitoredwhentheburnerisinoperation.
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BlowerSpeedMonitoring
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Blowerspeedhasalargeimpactontheairflowdeliveredtotheburnerandthusthefuel‐airratio.Fuel‐
airratiosmustbekeptinasaferangewhileaflameispresentintheboiler.Ifthefuel‐airratiocannot
bekeptinasaferange,theburnermustbeshutdown.
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Tohelpensurethatthebu rneriseitheroperatedatasafefuel‐airratioorisshutdown,theblower
speedisconstantlymonitoredwhileaflameispresentintheboiler.Thespeedismonitoredsothat
nuisanceshutdownsareeliminated,butfastshutdownswilloccurifthespeeddeviationislarge.Todo
this,theLMV52evaluatesthemagnitudeofthespeeddeviationincombin
ationwithhowlongthe
speeddeviationexists.Toaccomplishthis,threedistinctbandsandonelimitcenteredaboutthe
standardizedspeedlineareused.Thesebandsare:
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1. NeutralBand‐ifthespeediswithinthisband,itisconsidered tobeOKandnoactionistaken.The
widthofthisbandis+/‐0.5%ofthestandardizedspeed.
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2. LowRisk Band‐ifthespeediswithinthisband,theanalogsignaltotheVFDisadjustedtobringthe
speedbackintotheNeutralBand.IftheNeutralBandspeedcannotbeachievedin8seconds,a
lockoutwilloccur.Thewidthofthisbandis+/‐2.0%ofthestandardizedspeed.
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3. OutsideLowRiskBand‐ifthespeedisoutsidetheLowRiskBandbutdoesnot exceed theHighRisk
Limit,theanalogsignaltotheVFDisadjustedtobringthespeedbackintotheLowRiskBandand
thenultimatelyintotheNeutralBand.ThewidthofthisbandvarieswiththesettingoftheHighRisk
Limit(parameterTolQuickShutdown).IftheLowRiskBand
speedcannotbeachievedin3seconds,
alockoutwilloccur.
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4. HighRiskLimit(parameterTolQuickShutdown)‐ifthespeedexceedstheHighRiskLimitthreshold
formorethan1second,alockoutwilloccur.Thislimitisdeterminedbyparameter
TolQuickShutdown,whichcanbesetbytheburnerOEM.Thedefaultsettingis+/‐10%ofthe
standardizedspeed.
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