Managing Bearing Temperature and Shaft Current Challenges in Coreless Motor Systems

Bearing heating is an inherent aspect of their operation. Typically, a bearing will achieve a state of thermal equilibrium where the heat generated is equal to the heat dissipated, thus maintaining a stable temperature within the bearing system.

The maximum allowable temperature for motor bearings is capped at 95°C, considering the material quality and grease used. This limit ensures that the bearing system remains stable without causing significant temperature increases in the windings of the coreless motor.

The primary sources of heat generation in bearings are inadequate lubrication and insufficient heat dissipation. In practice, the bearing lubrication system may falter due to various operational or manufacturing missteps.

Issues such as insufficient bearing clearance, loose fits between the bearing and the shaft or housing, can lead to erratic motion; severe misalignment due to axial forces; and improper fits with related components that disrupt lubrication, can all lead to excessive bearing temperatures during motor operation. The grease may break down and fail at high temperatures, leading to rapid catastrophic failure of the motor’s bearing system. Therefore, precise control over the fit and clearance of parts is crucial in the design, manufacturing, and maintenance phases of the motor.

Shaft current is an inescapable risk for large motors, particularly for high-voltage and variable-frequency motors. It poses a significant threat to the bearing system of coreless motors. Without proper mitigation, the bearing system can suffer damage within seconds due to shaft current, leading to disintegration within hours. Early signs of this issue include increased bearing noise and heat, followed by grease failure and, shortly thereafter, bearing wear that can cause the shaft to seize. To address this, high-voltage, variable-frequency, and low-voltage high-power motors implement preventative measures at the design, manufacturing, or operational stages. Common strategies include circuit interruption (using insulated bearings, insulating end caps, etc.) and current diversion (using grounded carbon brushes to conduct the current away from the bearing system).