VFD Overvoltage Alarm

[oceanobob]

Recently a motor (480vac, 900 rpm, 8.6kw, 12 hp, 17.7a, delta connected, 160L frame) installed with a AB VFD. VFD connected to bus using line reactor and motor connected to VFD with 25lf of shielded VFD cable etc. Motor operation is typically loaded to about one third of the rated voltage and about 45% max rpm.

Motor load end drives a coupling to a helical gear that is reaction torque restrained; coupling alignment AOK. Motor position is vertical with load end up.

Motor hours about 4000.

The machine shut off with the VFD in overvoltage alarm. Meggered the motor and it failed right off. Pulled the motor and at the rebuild shop they found the load end outer bearing race in the cast iron housing had spun which allowed the rotor to contact the windings and the windings were failed as shorted to ground.

With this background information - here be the general inquiry:

How would this condition cause an overvoltage alarm on the DC bus at the VFD? or does the overvoltage alarm also indicate other malfunctions??

 

[petersonra]

The over voltage on DC bus alarm most of the time means that there is some kind of energy in the system that is being fed back through the motor and being regenerated to the DC bus.

If there is no way to bleed this energy off like through a braking resistor it builds up on the DC bus and eventually trips the VFD.

It can also happen if you have a high input voltage but that's not real common because it has to be pretty high and the utilities are pretty good at keeping them within spec.

Once in a while you get this alarm on some VFDs and you never really figure out what caused it.

 

[ptonsparky]

Utility Capacitor Bank switching will cause the overvoltage fault but line reactors always fixed that.