Siemens LMV52.240B1 - Line Reactors

LMVSeriesTechnicalInstructions 
DocumentNo.LV5‐1000
SCCInc. Page3 Section5
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Asmentionedabove,VFDsswitchmultipleIGBTsonandoffveryrapidlytogeneratea"modified"sine
waveonallthreephasesgoingtothemotor.Doingthishassometradeoffs,oneofwhichiselectrical
noise,orharmonics.Thisnoiseistypically"wireborne"insteadofairborne,andcancauseissueswith
electronicsinsomesituations.Thankfully,electricalnoiseassociatedwithVFDscanbe
mitigatedusing
properwiringtechniques(connectingshieldsandgroundscorrectly)andbytheproperapplicationof
linereactorsand/orloadreactorsforsomeapplications.Fordifficultapplications,EMCfiltersforthe
VFDarealsoavailable.
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LineReactors
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Linereactors,or"chokes",aretypicallyusedwhentheimped
anceontheinputsideofthedriveislow.
ImpedanceontheinputsideofthedriveistypicallylowwhenarelativelysmallVFDisbeingfedbya
relativelylargetransformer.Inthissituation,thesupplysideofthedriveis"s
tiff",meaningthatan
instantaneouscurrentdrawbythedrivewillbemet veryquicklybythelargetransformer(thinksquare
waveform),causingvoltageandcurrentdistortionsinthepowerdistributionsystemfeedi
ngthedrive.
Inthissituation,addingalinereactorwilladdreactancewhichopposesinstantaneouscurrentdrawand
"softens"theinputsideofthedrive.
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Conversely,ifthetransformerfeedingthedriveisnotlargerelativetothedrive
,theimpedanceonthe
inputsideofthedriveishigherandthesystemis"softer".Inthissituation,aninstantaneouscurrent
drawbythedrivewillnotbemetasquickly,andtheresultingvoltageandcurrentdistortionsinthe
powerdistributionsystemfeedingthedrivewillbesm
aller.Anadditionallinereactorinthissituationis
notneeded.
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Figure5‐1:LineReactorRecommendation‐VFD(HP)vsTransformer(kVA)
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