We replaced the motor three times in eight months on a 7.5 kW fan. Each time the motor winding failed to ground. Each time the motor shop said the insulation was burnt — VFD spikes, they said. We went to a VFD-rated motor (inverter-duty, Class F insulation). It lasted four months. The cable between the VFD and motor was 35 meters of unshielded 4-core. That was the clue.
The VFD switches at 8 kHz. Each pulse rises in about 50 nanoseconds. The cable looks like a capacitor at that speed. The reflected wave travels down the cable, bounces off the motor impedance, and comes back. At 35 meters, the round-trip time is about 350 nanoseconds — shorter than the pulse rise time. The reflected wave adds to the outgoing pulse. At the motor terminals, the voltage can hit twice the DC bus. A 400V VFD has a 565V DC bus. The motor sees 1100V. The winding insulation, rated for 690V, breaks down. The first few turns of the winding take the hit. That’s why it always fails on the end turns.
The motor shop kept saying “inverter duty” motor. But an inverter-duty motor has stronger insulation on the wire, not on the end turns. The real problem was the cable. We replaced the 35 m unshielded cable with a shielded VFD cable (three symmetrical conductors, 360-degree shield). The capacitance dropped by a factor of three. The reflected wave was absorbed by the shield. The motor voltage dropped to 600V at the terminals. The motor didn’t fail again. That was two years ago.
The VFD was also running at 12 kHz carrier. We dropped it to 4 kHz. The motor got a bit louder (you could hear a low whine at low speed) but the pulses were slower. The stress on the insulation dropped. The shielded cable was the bigger fix, but the lower carrier helped. We didn’t need a dV/dt filter because the cable was now short enough in electrical terms (under 50 m shielded) that the reflection didn’t build up.
If you’re putting a VFD more than 15 meters from the motor, use shielded VFD cable. If it’s over 50 meters, add a dV/dt filter at the VFD. Don’t buy a more expensive motor until you fix the cable. The motor shop was happy to sell us expensive inverter-duty motors. They didn’t ask about the cable. The cable was the problem all along.
