Ground Faults on Drives

A PowerFlex AC drive detects a ground fault when the output currents between two phases differ by more than 25% of the drive’s rated current.

 Common causes include:

  • Loose wire
  • Wire in the motor junction box/conduit box may have rubbed through the insulation

  • Motor data not entered correctly

  • The Motor is not wired correctly

  • The lead length is too long for the motor with no solution on the output (reactor, RWR)

  • Moisture in the motor conduit box

  • Moisture in a Quick Connect Cable

  • The Drive has been tuned to the motor properly. PowerFlex 525 recommends having V5.xx firmware

  • Bad Encoder card or bad cable

The Troubleshooting steps:

  1. Disconnect the motor leads in the motor junction box.
  2. Insulate the motor leads at the motor junction box.
  3. Run the drive to see if the fault still occurs.
    1. If the drive no longer faults, the ground is either in the motor or within the motor junction box connections. Replace the motor or provide more insulation in the motor leads.
    2. To check the motor for insulation breakdown, megger the motor (at least 1000V minimum) on each output phase to the ground. It is recommended to use a megger voltage of 1500V on a 460V system and 2000V on a 575V system. The megger measurements should be at least 10M Ohm minimum on each phase. If 1500V or 2000V is not used, then the megger test may not reveal that the motor is bad.
  4. If the fault still occurs with the motor disconnected at the motor junction box, then the problem could be in the wires between the drive and the motor or within the drive.
  5. Disconnect the motor leads at the drive terminals and run the drive again.
    1. If the fault still occurs with the motor leads disconnected at the drive, the drive needs to be repaired or replaced.
    2. If the fault does not appear, the problem is within the motor leads between the drive and the motor and needs to be replaced.

Note: If the drive incurs repeated ground faults while idle (i.e. after being jostled or vibrated), then replace the drive.

Note: If PowerFlex 525 drive uses encoder feedback, the fault may be caused by a damaged encoder card (25-ENC-1 or 25-ENC-2) or a motor rotating in opposite direction to motor feedback.