Vfd input output current relationship
Vfd input output current relationship
(OP)
I have noticed that the input current 3ph to a vfd drive is lower that the output, can someone please explain why this is.
I know the input is converted for the dc bus.
I know the input is converted for the dc bus.





RE: Vfd input output current relationship
Neglecting the losses, the input power is delivered at a certain voltage and current. The output power is delivered at a different voltage and current. The output voltage is reduced linearly as frequency drops, so at 25Hz (30Hz if you are in the US) the output voltage is half of the value it would have at rated speed. If the power input and output are assumed to be approximately equal, the output current must be double that of the input current because the output voltage is halved for this case. Similar logic applies at 25% speed, 75%, and so on.
This explanation neglects losses, power factor, harmonics and a whole lot of other things, but you hopefully get the idea.
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RE: Vfd input output current relationship
RE: Vfd input output current relationship
Pr =1.732*I*Vll * PF
Pr = Effective or Real Power ( watts or kW) (consumption billed by the utility)
I = Average RMS Line current (Amps)
Vll = Line to line average voltage (Volts)
PF = Circuit Power Factor
You could do measurements of real power (watts or kW) for both the driver input and the motor input, then compare the results and you will get the driver’s efficiency.
Do not be confused with the apparent power (VA) which does not consider the PF.
VA = I * V11
RE: Vfd input output current relationship