A customer installed a new conveyor. The motor turned backward. They swapped two motor leads at the VFD output. The motor ran forward but hummed loudly. The VFD faulted with “output phase loss.” They swapped back. The motor ran backward. They swapped the VFD input phases instead. The VFD faulted with “input phase loss.” They called us.
The issue was simple but easy to miss. The customer had swapped two output phases to reverse the motor. On a VFD, swapping output phases does reverse the motor — but it also breaks the phase rotation that the VFD expects for current sensing. The VFD uses current transformers on the output to sense load and faults. Swapping phases makes the current sensing read incorrectly. The VFD hums and eventually faults.
The correct way to reverse a VFD-driven motor is in the parameter set, not by swapping wires. Set parameter “motor rotation direction” to reverse (or swap the command from forward to reverse in the PLC). The customer changed the parameter. The motor ran forward, no hum, no faults. The wiring was untouched. They had spent two hours swapping wires. The parameter was one setting.
This is a common mistake. With a line-powered motor, you swap two leads to reverse. With a VFD, you don’t. The VFD controls the phase rotation electronically. Swapping output wires confuses the drive. If you need to reverse a VFD motor, use the parameter. If you need to swap a VFD between two machines, document the parameter change. Don’t touch the wiring.
The customer was relieved. They had been ready to call an electrician. The fix was a single parameter. The moral: when you have a VFD, it is not a contactor. It does things electronically that a contactor does with wires. Read the manual before swapping wires. The wiring is the last thing to change, not the first.
