Shaft Grounding Rings - The Necessary Accessory for Your Motor and VFD

Why do you need a shaft grounding ring (SGR)?

Motors that are operated with a variable frequency drive (VFD) are more susceptible to bearing failure. In standard generated sine waves, the voltage is balanced; this means that the amplitudes of the wave form on both the positive and negative sides are equal. In PWM (pulse wave modulated) generated sine waves, produced by VFD's, the voltages are not balanced. This unbalance can cause voltage potential in the motor structure, leading to voltage passing between the rotor and stator to ground through the motor bearings causing damage

As the voltage arcs between the bearings and the housing, it leaves microscopic weld splatters. These splatters make the bearings' rolling elements; ball or roller, and internal raceways rough; which could lead to the bearing failure.

What does bearing failure mean to you?

Bearing replacement with motor removal and reinstallation expenses

Downtime

Possible bearing fit and winding failures

What causes current running through the bearings?

Parasitic capacitance builds between the motor stator and rotor irons; resulting in a buildup of voltage, which is passed into the motor shaft. This current moves through the shaft looking for the easiest path to ground, that path usually happens to be through the bearings.

How do we protect the bearings?

The shaft grounding ring is placed near either end of the shaft . The carbon fiber brushes attached to the ring contact the shaft. The brushes now become the easiest path to ground for current running through the shaft. The current no longer passes through the bearings, which eliminates bearing failure caused by the unbalanced PWM wave.