Siemens LMV52.240B1 - Output Wiring; Load Reactors

TechnicalInstructions  LMVSeries
DocumentNo.LV5‐1000
Section5 Page4 SCCInc.
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Ingeneral,alinereactorisrecommendedifthesupplycapacity(kVA)ofthetransformerfeedingthe
driveisgreaterthanorequalto10timesthecapacity(kVA)ofthedrivefortransformers600kVAand
larger.
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Figure5‐1notes:
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1. DrivepowerisshowninHPratherthankVA.Thisconversioncanbedoneassumingapowerfactor
ofunity(1)andnegligib
lelossesduetoefficiency.
2. Transformerslessthan600kVAhavehighenoughimpedance(“soft”enough)sothatlinereactors
aretypicallynotnecessary.
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Example1:A25HPdriveisbeingfedbyan800kVAtransformer.Isalinereactorrequired?
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Assumptions:
Thepowerfactorisunity(powerfactor=1)
Lossesduetoefficiencyandwiringarenegligibl e
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1. Converthorsepowertokil
owatts:25HPx0.745HP/kW=18.63kW
2. ConvertkilowattstokVA:kW=kVA*Pf(Pfispowerfactor,whichisassumedtobe1inthis
example
)
Thus,a25HPdriveis18.63kVA.
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3. CalculatethekVAratio:800kVA/18
.63kVA=42.94
Sincearatioof42.94isgreaterthan10,andthetransformerislargerthan600kVA,aline
reactorwillbenecessaryforthisapplication.Thesameconclusioncanalsobearrivedatby
usingFigure5‐1.
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Example2:A10
HPdriveisfedbya400kVAtransformer.Isalinereactorrequired?
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Usingthesameassumptionsandcalculationasexample1,thekVAratiois53.7,butthetransformeris
smallerthan600kVA,soalinereactorisnotnecessary.Th
esameconclusioncanalsobearrivedatby
usingFigure5‐1.
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OutputWiring/LoadReactors
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WhentheVFD/motorarerunning,highlevelsofelectricalnoiseareproducedonthewiringbetween
theVFDandthemotor.Thisisduetothefactthatmodifiedsinewavesprodu
cedbythedriveIGBTsare
basicallyhighfrequency/highvoltageDCpulses.Theseoutputwiresmustbeenclosedinsometypeof
shielding(metallicconduitormetal‐shielded cable)tomitigateradiatedelectricalnoise.
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WirelengthbetweentheVFDandthemotorshouldbekepttoless than 150
feetifpossibleduetothe
reflectedwave/standingwavephenomenonandvoltageovershootphenomenon.Bothofthese
phenomenaarerathercomplex,andareafunctionofthewirelengthfromtheVFDtothemotor.The
reflectedwave/standingwavephenomenonandvoltageovershootphenom
enoncandamagenon‐
inverterdutymotorwindingsovertimeduetothehighpeakvoltagesthatthesephenomenacan
produce.
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