Siemens LMV52.240B1 - ; Introduction; VFD and AC Induction Motor Fundamentals

TechnicalInstructions  LMVSeries
DocumentNo.LV5‐1000
Section5 Page2 SCCInc.
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Introduction
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TheLMV52featuresanintegrated,closed‐loopVariableSpeedDrive(VSD)controlthatistypicallyused
torampthespeed ofthecombustionairblowerwithfiringrate.Thisisaccomplishedbytransmittinga
4‐20mAsignalfromtheLMV52totheVSD,andthenreadingthespeedofthemotorviaamotor‐
mounted,safety‐ratedencoderwheel(speedwheel)andspeedsensor.Byusingthespeedwheeland
sensor,theLM
V52canpositivelyverifyboththespeedanddire ctionoftheblower,thusensuring
properVSDoperation.
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Blowerspeedanddirectionofrotationhavealargeimpactontheairflowdeliveredtotheburner,and
thusthefuel‐airratio.ThemostcommontypeofVSD,aVariableFr
equencyDrive(VFD),istypicallynot
safety‐ratedandwilltypicallynotfailinasafemanner(aVFDfailurewilltypicallycausethecombustion
airblowertoslowdownorstop,causingtheburnertogorich).Thecombustionairpressureswitch
offersonlyasmallamountofprotectioninaVFDapplication,sincetheswitchmustbesettoallowlow
fireoperatio
nwhentheblowerisspinningslowlyandthebloweroutputpressureislow.Themotoror
blowershaft‐mounted,safety‐ratedspeedwheelandsensorensurethataVFDfa
ilurewillbequickly
detectedandtheburnerwillshutdowninasafeway.
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VFDandACInductionMotorFundamentals
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VFDsaretypicallyconnectedtoathree‐phasealternatingcurrent(AC)inductionmotorthatisusedto
powerthecombustionairblower.ModernVFDsoperatebytakingsingle‐orthree‐phaseACand
rectifyingthispowertohighvoltagedirectcurrent(DC)fortheDCbus.TheACpoweristypically
rectifiedtoDCwithbanksofdiodes.TheDCbusfeedsabankofInsulatedGat
eBipolarTransistors
(IGBTs),andamicroprocessorisusedtofiretheIGBTsinawaythatthevoltageandfrequencyofthe
modifiedsinewavescanbecontrolled.ThisisdoneforeachofthethreephasesontheVFDoutput.
Themicroprocessorvariesthevoltageandfrequencyofthemodifiedsinewavesin
responsetoasignal;
inthiscase,the4‐20mAinput.
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Bydesign,athree‐phaseACinductionmotorwillattempttoapproximatelysynchronizeitsspeedwith
thefrequencyofthree‐phasepowerthatitisbeingfed.Thus,ifthefrequencycanbeadjusted,socan
thespeedofthemotor.Astheirnamesuggests,three‐phasei
nductionmotorsgeneratemagneticfields
intherotorofthemotorbyusinginductionratherthanbyusingslipringsorbrushes.Theadvantageof
thistypeofconstructionisverylowmaintenance,andasmalldisadvantageisaphenom
enoncalledslip.
Slipisdefinedasthedifferencebetweenthetheoreticalspeedatagiven ACfrequencyandtheactual
speedatagivenACfrequency.Slipincreasesastheloadonthemotor(torqueoutput)increases.
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Three‐phaseACmotorsthatdonothavesliparereferredtoassynchronousmo
tors,sincethesemotors
exactlysynchronizetheirspeedtothefrequencyoftheincomingACpower.Thistypeofmotorisnot
typicallyusedonblowers,butismentionedasacomparisontotheACinductionmotor.Atruly
synchronous2‐polemotorwillspinatexactly3600RP
Mifitisfedexactly60Hz.Atrulysynchronous4‐
polemotorwillspinatexactly1800RPMifitisfedexactly60Hz.Incontrast,a2‐pole,three‐phaseAC
inductionmotorfed60Hzwillspinlessthan3600RPM,andhowmuchlessisdeterminedbyhow
heavilythemoto
risloadedandhowmuchslipthatloadingcauses.
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