VFD TRIPPING
VFD TRIPPING
(OP)
We have had tripping problems on a particular VFD (old 400hp Emerson drive system) for a few months now. Unfortunately, the trip condition shows no diagnostics to help identify the problem.
I used a datalogger yesterday to monitor and record various electrical parameters and found that the current in the "A" phase 5% of that in the other two phases and the ground cable is carrying a current equal to that of the other two phases as well.
I am surprised that our clamp on ammeter did not pick up this unbalance. Without further analysis I suspect that the ground fault relay on the ground conductor senses the imbalance, resulting in the trip condition.
Aren't those clamp on meters true RMS?
I used a datalogger yesterday to monitor and record various electrical parameters and found that the current in the "A" phase 5% of that in the other two phases and the ground cable is carrying a current equal to that of the other two phases as well.
I am surprised that our clamp on ammeter did not pick up this unbalance. Without further analysis I suspect that the ground fault relay on the ground conductor senses the imbalance, resulting in the trip condition.
Aren't those clamp on meters true RMS?





RE: VFD TRIPPING
Clip around all three source-phase leads and see if you get an above-the-noise reading. In that case, there is likely damage to the drive—possibly an internal ground fault.
5% is 5%, but so-called true RMS ammeters are sometimes misunderstood—see that yours is not “RMS calibrated/average responding.”
RE: VFD TRIPPING
RE: VFD TRIPPING
I was referring to the current at the output of the drive to the motor. We made some progress today in identifying sources of ground fault. Additionally, there were a couple bad SCR's, discovered when the drive was run independent of the motor.
We are about to return the pump to service with the datalogger installed and monitor electrical parameters. Hopefully we have addressed the problem, if not back to square one!
RE: VFD TRIPPING
On tripping the fuses are bypassed and the fault seems to trip the breaker within the drive. I have checked the breaker settings and fuse sizes which are identical to three other drive units which are in service.
Any comments?
RE: VFD TRIPPING
I believe the high current in the neutral could be due to high harmonic content, especially if the drive is an older one. Just some suggestions.
RE: VFD TRIPPING
RE: VFD TRIPPING
how is your DC bus voltage...is it dipping once you
run the motor...'could be a bad power semiconductor or bad capacitor...
dydt
RE: VFD TRIPPING
RE: VFD TRIPPING
I think I have found the problem.
Initially the breaker had been tripping on thermal overload. An infrared scan demonstrated significant temperature rise on this particular unit. Furthermore,a t full load current it was hot to the touch.
I installed a minisplit AC unit to compliment the central unit that is already in the substation. Since then the tripping problem has not occured.
I guess more science should be applied to this theory to see if this is actually the case but troubleshooting revealed no issues. DC bus voltage was perfect. Output waveforms could not have been better and the ground current had been addressed.
Any comments?
RE: VFD TRIPPING
What voltage?
RE: VFD TRIPPING