How to Troubleshoot a VFD Drive

1. Set the VOM to the Diode Check mode by turning the dial on the VOM so the line on the dial is pointing at the diode setting.
2. Place the negative VOM lead on the positive side of the DC bus. The DC bus is where the wires terminate in the VFD. Different-sized VFDs have the DC bus in different locations, but if you follow the wiring it will lead you to the DC bus. There will be five points of contacts for the wires on the DC bus: two positive contacts, two negative contacts and one in the middle for a groundwire. If you are uncertain which side is the positive side, look for the '+' sign at the end of the DC bus.
3. Refer to the VFD schematic to locate which of the terminals on the DC bus control the incoming phases. Place the positive VOM lead on each of the incoming phases to the DC bus. The reading on the VOM should drop between .3 to .6 volts.
4. Place the positive VOM lead on the negative DC bus port and place the negative VOM lead on each of the input phases that were identified from the VFD schematic. The reading on the VOM should drop between .3 to .6 volts. If the reading on the VOM does not change, then there is a short in the input bridge of the VFD.
5. Place the positive lead of the VOM on the negative bus and touch the negative VOM lead to the output terminals of the motor. The reading on the VOM should show a drop. If the reading does not drop, there is a short in the output device.
6. Inspect the capacitors for damage. If a capacitor has a cracked casing or dislodged pressure plug, you will need to remove and replace it.