VFD Overvoltage Problem

[GordS]

Recently we had an incident on a bus feeding multiple PWM style VFDs, motor loads and small transformers. The bus is high resistance grounded, limited to 2 A. Over a span of about 3 hrs, 3 drives tripped and a transformer shorted to ground on the primary. Several of the drives would not restart. They would immediately trip on overvoltage. Finally, another drive tripped. The drives that would not start due to overvoltage then started. We found the last drive that tripped was feeding a motor that was faulted to ground.

Is ther any way a groundfault on the output of a VFD would show up as an overvoltage trip on other drives on the same bus?

 

[davidbeach]

If the output ground fault pulled a phase voltage to ground then yes.

 

[LionelHutz]

The motor shorted will cause the DC buss in the VFD to have the positive side and then negative side of the buss alternatively connected to ground at the output carrier frequency of the VFD. Then, the buss being grounded will ground whatever phase is currently providing power to that buss. So, all 3 phases will be switching to ground at the carrier frequency.

So, to sum it up, this switching of the phases to ground is ugly and who knows what other problems could arise from it unless you a detailed investigation of what exactly is happening at the other drives.

 

[sreid]

I once had a drive do this but I never really explained the exact mechanism. The Bus voltage pumped up like the PWM was acting as a boost switching power supply. Fixing the motor lead ground fixed the problem.