How Shaft Currents Damage Motor Bearings — Causes, Effects & Solutions

Shaft Currents — The Silent Hacker of Motor Bearing Systems

Shaft current is a major quality killer for variable frequency motors, large motors, high-voltage motors, and generators, posing serious damage to motor bearing systems. Failures due to inadequate shaft current prevention measures are all too common. Characterized by low voltage and high current, shaft currents can cause harm that is difficult to detect and prevent.

Shaft currents originate from shaft voltages and closed-loop paths. To address this issue, one can either eliminate the shaft voltage or break the circuit loop. Causes of shaft voltage include magnetic circuit imbalance, inverter power supply, electrostatic induction, static charges, and external electrical interference. Although shaft voltage is usually small in magnitude, once it exceeds a critical threshold, it can break down the lubricant film in bearings, forming a closed loop through the bearing surfaces. The resulting high shaft current can lead to severe thermal damage to bearings in a short period.

Bearings damaged by shaft current often show washboard-like marks on the surface of the inner race. To mitigate shaft current issues, appropriate design and manufacturing measures must be taken. For instance, insulation measures can be added to the end shields and bearing housings. For smaller motors, insulated bearings are a practical solution. Additionally, carbon grounding brushes can be installed during use to safely discharge stray currents.

From a usage standpoint, breaking the circuit within the motor components is a more permanent solution. In contrast, using conductive paths like carbon brushes may require maintenance and replacement, so it’s essential to ensure brush systems remain reliable throughout the motor’s maintenance cycle.

01. Insulated Bearings

Insulated bearings share the same dimensions and load ratings as standard bearings, but their key advantage lies in their ability to block electrical current. They effectively prevent electrical erosion, ensuring more reliable motor operation. Insulated bearings avoid damage from induced currents, protecting the lubricant, rolling elements, and raceways from electrical wear.

02. Why Do Inverter-Fed Motors Produce Shaft Voltage?

When a motor is powered by an inverter, the supply voltage contains high-frequency harmonics. These harmonics induce electromagnetic effects between the stator coil ends, terminal connections, and rotor shaft, leading to shaft voltage.

The stator windings of asynchronous motors are embedded in the stator core slots, where distributed capacitance exists between winding turns and between the stator windings and motor frame. Under high switching frequencies from a general-purpose inverter, the rapid changes in common-mode voltage can cause leakage current through these distributed capacitances, flowing from the motor housing to the ground.

These leakage currents may result in both radiated and conducted electromagnetic interference. Additionally, magnetic circuit imbalance, electrostatic induction, and common-mode voltages are key contributors to shaft voltage and shaft current generation.