Why are insulating bearings used in high-voltage motors? Please explain

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Axial current: When a motor is in operation, the potential difference that arises between the two ends of the rotating shaft or between the shaft and the bearings is called shaft voltage. If a circuit is formed between the two ends of the shaft through elements such as the motor frame, then the shaft voltage results in an axial current. Axial current is generated when axial voltage forms a closed circuit through the motor shaft, bearings, stator frame, or auxiliary devices. Under normal conditions, the voltage on the motor shaft is low, and the lubricating oil film in the bearings can serve as an insulator, preventing the generation of shaft current. However, when the shaft voltage is high, or when the oil film has not yet stabilized at the moment of motor startup, the shaft voltage will cause the lubricating oil film to discharge and break down, creating a circuit that generates a shaft current. The high temperature generated by the energy release from shaft current partial discharges can melt many small areas on the inner ring, outer ring, or balls of the bearing, creating grooves that lead to noise and vibration. If this is not detected and addressed in a timely manner, it can result in bearing failure, having a significant impact on production. The most prominent characteristic of electrical erosion of bearings caused by high-frequency shaft current in variable frequency speed control systems is the formation of dense, ‘scrubboard-like’ grooved patterns on the inner and outer rings of the motor bearings as well as on the balls. Therefore, the motor needs insulation at one of its bearings. Especially for variable-frequency motors, when the frame size is 280 or above, insulation at one end is required