Shaft Grounding Rings Explained: How They Work and Why Your Motor Needs One

1. A Hidden Threat in Modern Motor Drive Systems

With the widespread use of variable frequency drives (VFDs) and PWM-controlled inverters, electric motors are operating more efficiently than ever. However, these modern drive systems introduce an often-overlooked problem: shaft voltage and shaft current.

If not properly managed, this electrical phenomenon can lead to premature bearing failure, unexpected downtime, and increased maintenance costs—especially in industrial motors, pumps, fans, compressors, and wind turbines.

This is where a shaft grounding ring becomes a critical reliability component rather than an optional accessory.

2. How Shaft Voltage and Shaft Current Are Generated

In VFD-driven motors, high-speed switching of insulated gate bipolar transistors (IGBTs) creates high-frequency common-mode voltage. This voltage couples capacitively between:

• Stator and rotor

• Rotor and shaft

• Shaft and bearings

As a result, shaft voltage accumulates on the motor shaft. Once the voltage exceeds the dielectric strength of the bearing lubricant film, it discharges abruptly in the form of shaft current.

This discharge does not occur smoothly—it behaves like a microscopic lightning strike.

3. Bearing Damage Path: Electrical Discharge Machining (EDM)

When shaft current passes through motor bearings, it causes Electrical Discharge Machining (EDM). Each discharge creates tiny craters on the bearing raceway. Over time, these craters form:

• Frosted or fluted bearing surfaces

• Increased vibration and noise

• Accelerated grease degradation

• Premature bearing failure

Importantly, lubrication alone cannot stop EDM damage, because the electrical discharge penetrates the lubricant film.

4. VOLSUN Engineering Solution

VOLSUN shaft grounding rings are engineered using high-density conductive microfibers arranged circumferentially around the shaft.

This design ensures:

• 360-degree continuous grounding

• Ultra-low contact resistance

• Stable performance under high-speed rotation

• Effective dissipation of high-frequency common-mode currents

By preventing voltage buildup at the source, the grounding ring eliminates the root cause of EDM damage, not just the symptoms.

5. Selection Recommendations

• To select the right shaft grounding ring, consider:

• Motor type (AC induction, permanent magnet, servo)

• Drive method (VFD/PWM, inverter switching frequency)

• Shaft diameter and surface condition

• Installation location (drive end or non-drive end)

• Environmental factors (dust, oil, humidity)

For inverter-driven motors above a few kilowatts, installing at least one shaft grounding ring is strongly recommended.

6. Application Limits and Risk Boundaries

• Shaft grounding rings are highly effective, but they are not universal solutions:

• They do not replace proper system grounding

• Incorrect installation may reduce effectiveness

• Extremely contaminated environments may require additional protection

• In some applications, combined solutions (grounding ring + insulated bearing) are necessary

Conclusion

A shaft grounding ring is a simple yet powerful solution to a complex electrical problem introduced by modern motor drives.

By safely discharging shaft voltage before it becomes destructive, shaft grounding rings protect bearings, extend motor life, and reduce unplanned downtime.

In today’s inverter-driven world, shaft grounding is no longer optional—it is essential.