Siemens LMV51.140C1 - Shaft Current; Braking Resistors

LMVSeriesTechnicalInstructions 
DocumentNo.LV5‐1000
SCCInc. Page5 Section5
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NOTE:TheDCbusrunsatvoltagessubstantiallyhigherthantheincomingvoltagetothedrive(about
35%higher)andtypicallyemployslargecapacitors.Thesecapacitorsremainchargedforaperiodof
timeaftertheincomingpowertothedriveisde‐energized,andareashockhazarduntilthey
discharge.SeetheVFDmanufacturer'srecommendationsforminimumwa
itingtimetoworkonthe
driveafterthedriveisde‐energized.
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Ifwirelengthcannotbekepttolessthan150feetonthedriveoutput,correctionoptionsareavailable.
ThesearelistedinFigure5‐2:
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WireLength‐upto(ft) CorrectionOption
150 NoneRequired
300 LoadReactoratVFDOutpu t
650 LoadReactoratMotorInput
2000 dV/dTFilteronVFDOutput
ConsultMotorOEM InverterDutyMotor
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Figure5‐2:CorrectionOptionsforLongWireLengthbetweenVFDandMotor
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ShaftCurrent
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Aswasmentionedearlier,thefastswitchingor"firing"oftheIGBTsenabletheVFDtoprod ucemodified
sinewavesofdifferentfrequenciesanddifferentvoltagesinordertospeeduporslowdownamotor.
ThefastswitchingoftheIGBTsdoeshaveelectricalsideeffects,someofwhicharedetailed
onthe
previouspages.
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ThisfastswitchingoftheIGBTscanalsocause"shaftcurrent"onthemotor.Whenthishappens,a
voltagechargebuildsuponthemotor'sshaft.Whenthisvoltagegetshighenough,itwillarctoground
throughthepathofleastresistance.Thepathofleastresistanceistypicallytheballbearing
sthat
supporttherotorofthemotor.Whenthisarcingoccursinthebearings,damageoccurstothebearings.
Overtime,thebearingswillbedestroyed,andthemotorwillfail.
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Shaftcurrentcanbemitigatedbyusingagroundingring,whichistypicallybolte
dtothemotorhousing
andhassometypeofconductivefilamentthatcontactstheshaft,thusgroundingtheshaft.Some
motorOEMshavegroundingringsbuiltintothemotor,soanexternalringis notnecessary.
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BrakingResistors
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Three‐phaseACinductionmotorscanalsofunctionasthree‐phaseACgenerato
rsiftheybecomedriven
bywhattheytypicallydrive.Inthecaseofablower,themotordrivestheblowerwheelwhenthespeed
ofthewheelisincreased(accelerated).Conversely,theblowerwheelcandrivethemotorwhenthe
speedoftheblowerwheelisdecreased(decelerated
)withaclosedairdamper.Whenthemotoris
drivenbytheblowerwheel,itwillactasageneratorand"push"electricalenergybacktotheVFD.This
energywillbeseenasavoltageincreaseontheVFD’sDCbus.
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