Now that the local temp is getting consistently warm the guys at work are trying to start-up the cooling towers for a hospital. One of the motors for a pump runs for about 10 mins. and then shuts down on ground fault as seen in the display on the Danfoss VFD that is controlling it. Since it takes roughly ten minutes after the pump motor starts for this ground fault to occur I suspect it must be the motor insulation. So far we've tested the conductors between the motor and the VFD with a Fluke 1507 megohmmeter and they appeared to be fine. Tomorrow we will test the motor. Anyone have other suggests or experience troubleshooting Danfoss VFD's? Thank-you.

It appears the problem has been resolved. After meggering the conductors between the motor and the drive and then the motor leads themselves we did not discover any potential issues. Since there is an identical drive and motor right next to the the ones we were having problems with we decided to try running the motor with the second VFD. Using a temporary feeder of 3C #6 Teck cable we ran it from the second drive to the motor. The motor ran without shutting down for 30 mins. Since this temp. cable was in place, and despite meggering the existing conductors, we thought we'd use the temp. cable to feed the motor in case our megger test didn't find an issue the existing. While connecting the temp. Teck cable we discovered the ground terminal on the VFD was loose - maybe the real problem? Ran the motor without issue with the temp. cable for 30 mins. and then reconnected the old cable and ran the motor again for 30 mins. without any shutdowns. A loose ground terminal at the drive appears to have been the original problem.

A ground terminal that was loose could create a situation where common mode noise in the motor leads has nowhere to go until it suddenly does, which to the residual ground detection system within the drive briefly appears to be a current flow mismatch. Good find.