Shaft Currents and Bearing Defects: Expert Solutions to Extend Electric Motor Life

Shaft voltages and currents that can reduce bearing life have been known for more than a century. Early research identified several culprits: accidental potential applied to a motor frame, electrostatic charging, shaft magnetization, electrical dissymmetry, and common‑mode voltage. Each of these factors can create a damaging electrical path through the rotor, bearings, and frame.

When the rotor and the iron on the stator frame are at different potentials, a secondary loop forms that allows induced currents to flow through the bearings or via a capacitive network. This establishes a voltage between the rotor and the stator frame, creating a path across the bearing to ground. As the voltage rises, it can arc over the bearing in an attempt to reach ground. A slow‑running motor keeps the balls in contact with the race, which continuously shunts the current and prevents voltage buildup. In contrast, a faster motor lets a thin film of grease separate the balls from the outer race, permitting the voltage to accumulate and potentially produce a damaging arc through the conductive grease.

Practical remedies include isolated bearings, shaft grounding, and non‑conductive grease. Less common fixes are Faraday shields, individually tuned filters, and highly specialized inverter designs, which can be costly.

For additional guidance on motor maintenance and reliability, visit the PdMA Corporation website at www.pdma.com.