VFD front end current

[thinker]

Please advise how to estimate a current consumed by VFD (front end)from the grid when spinning the motor (no-load). Motor magnetizing current is known. This is 4-quadrant VFD with AFE, DC bus and PWM inverter/converter. We understand that at no-load condition motor consumes reactive current (from DC bus), but how to estimate the front end input current?

 

[djs]

Power in = Power out / Efficiency

 

[mikekilroy]

Since the magnetizing current is all supplied by the caps in the DC bus, only thing that comes from front end of VFD is current required to rotate the motor against the load.

So if load is just the motor, measure the torque of the motor rotating with a torque wrench, and just ratio this torque to the nameplate rated torque. then use that ratio against the motor nameplate current rating and you will be close. (it will be off a tad since the nameplate is vector sum of both currents).

Or put an ammeter/clampon around vfd input and measure it.

Or assume it is about 1/2 amp for a motor upto 10hp, .75a for 11-20hp, 1a for 21-50hp, etc. It really is very low.

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