What Is Shaft Voltage and How to Prevent Bearing Damage in VFD Motors

VFDs improve control and efficiency, but their switching can induce shaft voltage that discharges through motor bearings. Learn the causes, warning signs, and how shaft grounding rings help.

Variable Frequency Drives (VFDs) have become standard in industrial motors and electric vehicles. While they improve efficiency and control, they can also create a hidden problem: shaft voltage.

When shaft voltage discharges through motor bearings, it causes electrical erosion, fluting, noise, and premature bearing failure.

What Causes Shaft Voltage?

High-frequency switching in VFDs induces common-mode currents. These currents seek the path of least resistance and often pass through motor bearings.

Common causes include:

PWM switching from VFDs
Poor grounding
High-speed motors
Insulated couplings
Long motor cables

Signs of Bearing Electrical Damage

Typical symptoms include:

Bearing fluting
Frosted raceways
Unusual noise
Increased vibration
Shortened bearing life

Bearing fluting and electrical erosion damage on a motor bearing race

Why Bearing Failure Is Expensive

Bearing damage can lead to:

Unexpected downtime
High maintenance costs
Production losses
Motor replacement

How Shaft Grounding Rings Work

A shaft grounding ring creates a low-resistance path from the shaft to ground.

VOLSUN Shaft Grounding Rings use metallized carbon fiber microfilaments to safely divert harmful currents without damaging the shaft.

Shaft grounding ring providing a low-resistance current path from motor shaft to ground

Advantages of VOLSUN Solutions

Low contact resistance
Long service life
Suitable for oil-cooled and high-speed motors
Maintenance-free design
Customizable dimensions

Applications

EV traction motors
Industrial motors
Pumps
Wind turbines
Railway motors

Conclusion

If your motors are driven by VFDs, shaft voltage is a serious reliability risk. Installing a shaft grounding ring is one of the most effective and economical solutions.

Contact VOLSUN for technical support and product recommendations.