I have a site that used to have an across the line starter where we have installed a VFD (15hp). When the motor is direct wired it is pulling well within the range of the FLA of the motor. We installed a VFD and on the load side the power is normal going to the motor but the line side now has a HUGE imbalance. This imbalance is not there with the motor direct wired and we replaced the VFD in case it was bad out of the box. The second VFD is doing the same thing. I am really at a loss???

 

Hello,

I'm excluding two on two items with rectifier damaged, so, the next way is your instrument: are you using a TRMS A-meter?

regards

Gabriele Corrieri

 

Yes it is a True RMS meter. The motor was running about 10ºC above rated temp even though the amperage was normal. Could have been a motor issue but hard to get past the line side issues of the VFD which shouldn't have anything to do with the load sides draw.

 

When you hae a polyphase rectifier charging a capacitor bank, there is no diode conduction until the capacitor voltage is exceeded, then heavy conduction for a narrow angle. just at the peak of the line voltage. So, if one phase is just a little low, it's diodes may not conduct at all. A motor load is much more forgiving and will draw current more or less proportional to the line voltage. So I would expect that you have a slightly lower voltage on the phase that is drawing low current or the inverse.

Regards
cww

 

Trinity... has the problem existed since initial start-up?

Regards, Phil Corso

 

Trinity... assuming you haven't found the cause of the problem can you supply input phase and line-voltages and line-currents to the VFD?

Phil Corso