Unbalancing of 3-Phases

Please post make and model of VFD, voltage and amps.

This is a question that requires details for a good answer

drive is actually old one....it is of make - mitsubishi, 37 kw..

there isn't any problem in cable...drive is taking current 15.4,17.1,19.3 in r,y,b respectively.

It should be fairly well balanced within a few percents. It might be a motor winding failure or cabling imbalance. It could also be the failure (open) of one transistor or gate driver in the drives' inverter.

If you can, switch the drive or motor with another device. You may be able to test the motor and its cable with a megger once the drive is disconnected.

Good luck.

Also, make sure that all the connections are tight. These problems can have simple causes.

Or rewire the motor and see if the values of the current readings stay the same and follow the same leg of the rotor.

If say the A leg was the leg that was the one with a lower current than the other two, when you rewire the motor does the A leg remain the one with the low reading or does it move to one of the other two legs? Did the leg that the low reading move to the same lug on the motor? If so the motor would be suspect.

Travis

Unbalances of the three lines in an application such as this can be improved sometimes buy rotating the phases. If the lines, R, Y & B are changed with each other in the same rotation, it sometimes imoproves the balance. Take the wires from left to right and move each wire to the right. If they are in that order, R-Y-B, then put them in the order Y-B-R and then check the load. Then move them to B-R-Y and check again. Choose the one that yields the best balance. Don't change the ORDER of the wires as this will reverse the motor rotation. Doesn't always have a large effect, but sometimes does. No guarantee.

Good answer. And if the unbalance stays with the cable rotation very closely, then the problem is in the drive or the motor cables. If the unbalance moves to the new cable rotation, then the problem is in the motor windings.

But before you start all of that, describe how you are measuring the output current. It is very very difficult to attain accurate and meaningful current measurements on the output of a VFD without very expensive and sophisticated meters that compensate for the extremely rich current harmonics. The average clamp-on ammeter is virtually useless for this and the current it tells you could be off by a factor of 50%. Just because a meter says "True RMS Current" does not mean that it can handle reading the OUTPUT of a VFD, and some cannot even be trusted on the input side.

The reason I bring this up is because of your mentioning that you read an unbalanced current on the input of the VFD. There should be no reason for this unless the diode bridge is compromised, in which case the drive would not likely function at all! It makes me suspect you are using a meter that is incapable of measuring accurately even on the input side, which makes it totally useless on the output.

Most VFD's have current snesoing display on them. Maybe this one does. You are right about the lack of reliability of the clamp-on with a VFD output.

DEAR ALL

information given is very usefull for me....ya i m agree with Mr. JRaef. clamp meter i m using is not very expensive...its simple one...Drives input currents are differnce in R,Y,B also.