A motor bearing that failed after 6 months on a VFD-driven fan. The motor was a 15 kW, 4-pole, TEFC, 400V. The VFD ran at 10 kHz carrier frequency. The bearing was a 6308 on the drive end. The customer replaced the bearing. It failed again after 4 months. The motor was new. The VFD was new. The issue wasn’t the bearing quality — it was electrical discharge machining (EDM) through the bearing. The VFD’s high-frequency PWM voltage creates shaft voltage. The voltage arcs through the bearing lubricant. The bearing race is pitted. This is about VFD bearing currents and how to prevent them.
The shaft voltage mechanism
A VFD outputs PWM voltage at the carrier frequency (2-16 kHz). The voltage jumps from 0 to 650V in nanoseconds. The fast switching creates a capacitive coupling between the stator winding and the motor shaft. The shaft charges to a voltage relative to the frame. When the voltage exceeds the dielectric strength of the bearing grease (about 500V), the grease breaks down. An electric current arcs through the bearing. The arc melts tiny pits in the raceway. Over time, the pits form a washboard pattern. The bearing runs noisy. Then it fails.
The shaft voltage is present on every VFD-driven motor. It’s worse on larger motors (over 10 kW) and longer cables (over 30 m). The 15 kW motor on a 30 m cable had enough shaft voltage to arc. The bearing failed in months.
What was changed
1. Installed a shaft grounding ring. A grounding ring (a ring of conductive microfibers) mounts around the shaft at the drive end. The fibers contact the shaft, providing a low-resistance path to ground. The shaft voltage bleeds off through the ring instead of arcing through the bearing. The bearing life extended from 6 months to 5 years. The grounding ring costs $80. The bearing replacement costs $200 in labor. The ring pays for itself in one bearing replacement.
2. Reduced the carrier frequency. The VFD was set to 10 kHz. Lowering to 4 kHz reduces the capacitive coupling. The shaft voltage drops by 50%. The motor runs slightly louder (the carrier frequency is in the audible range) but the bearing survives. For fans and pumps where noise isn’t critical, 4 kHz is fine. For gear motors where noise matters, a grounding ring is preferred.
3. Used a shielded cable. The original motor cable was unshielded. The shielded cable contains the PWM voltage. The capacitive coupling to the shaft drops. The cable shield is grounded at both ends (VFD and motor). The shield provides a return path for high-frequency currents. The shaft voltage drops by 70%. The shielded cable costs 2x the unshielded one. For VFD installations over 10 m, it’s mandatory.
The prevention methods comparison
| Method | Effectiveness | Cost | Use for |
|---|---|---|---|
| Shaft grounding ring | Excellent | $80 | All VFD motors over 10 kW |
| Insulated drive-end bearing | Good (redirects current) | $100 (special motor) | Large motors, long cable runs |
| Sinusoidal output filter | Excellent (eliminates dV/dt) | $500+ | Long cables, quiet motors |
| Lower carrier frequency | Moderate | $0 | Fans, pumps, noise-tolerant |
| Shielded cable | Good | 2x cable cost | All VFD installations |
The cable length effect
The shaft voltage increases with cable length. A 5 m cable has minimal effect. A 30 m cable doubles the shaft voltage. A 100 m cable triples it. The reflected wave effect (PWM pulses bouncing off the motor impedance) doubles the voltage at the motor terminals. The 650V bus becomes 1300V at the motor. The bearing sees this voltage. For cables over 30 m, a dV/dt filter or sinusoidal output filter is required at the VFD end. The filter reduces the pulse rise time. The capacitive coupling drops. The bearing survives.
For the failed fan motor, the cable was 30 m. The grounding ring solved the immediate problem. The next fan installation uses a shielded cable and a grounding ring. The bearing life is 5 years.
The bearing rule: every VFD motor over 10 kW gets a shaft grounding ring. The failed bearing wasn’t defective — the PWM voltage arced through the race. Lower the carrier frequency to 4 kHz for noisy environments. Use shielded cable grounded at both ends. For cables over 30 m, add a dV/dt filter. VFD bearing failure is predictable and preventable — don’t wait for it to happen.
