Why is the rear bearing of a large electric motor insulated

Why is the rear bearing of a large electric motor insulated

Shaft current: The potential difference generated between the two ends of the shaft or between the shaft and the bearing during the operation of the motor is called 

shaft voltage. If the two ends of the shaft form a circuit through the motor base, the shaft voltage forms the shaft current. Shaft current is generated by the shaft voltage 

passing through the motor shaft, bearings, stator frame, or auxiliary devices to form a closed circuit. Under normal circumstances, the shaft voltage of the electric motor 

is low, and the lubricating film inside the bearing can provide insulation without generating shaft current. But when the shaft voltage is high or the oil film is not stable 

at the moment of motor starting, the shaft voltage will cause the lubricating oil film to discharge and break through, forming a circuit and generating shaft current. The 

high temperature generated by the partial discharge energy release of shaft current can melt many small areas on the inner and outer rings or balls of bearings, forming 

grooves and generating noise and vibration. If not detected and treated in a timely manner, it will lead to bearing failure and have a great impact on production. 

The most significant feature of the high-frequency shaft current in the variable frequency speed regulation system causing electrical corrosion to the bearings is the 

formation of "washboard like" dense groove stripes on the inner and outer rings and balls of the motor bearings. So the motor needs to have one end bearing insulated.

Especially for variable frequency motors, one end of the base generally needs to be insulated above 280.

The main reason for the insulation of the rear bearings of large electric motors is to prevent damage to the bearings caused by shaft currents. ‌The generation and harm 

of shaft current.

When an electric motor is running, there will be a potential difference between the two ends of the shaft or between the shaft and the bearing, which is called shaft voltage. 

If the shaft voltage forms a closed loop through the motor shaft, bearings, stator frame, or auxiliary devices, it will generate shaft current. Axial current can cause breakdown 

of the lubricating oil film inside the bearing, resulting in partial discharge energy release and the formation of high temperature, which melts small areas on the inner and 

outer rings or balls of the bearing, forming grooves and generating noise and vibration. If not dealt with in a timely manner, it can lead to bearing failure and have a sig--

nificant impact on production.The function of insulated bearings.

Blocking shaft current: Insulated bearings use insulation materials on the bearing sleeve to isolate the bearing chamber, thereby cutting off the shaft current circuit caused 

by shaft voltage, protecting the bearing, and extending its service life.

Reduce vibration and noise: Vibration is one of the main causes of bearing noise, and insulation treatment can effectively reduce vibration, thereby reducing noise.

Principles and application scenarios of insulated bearings.

The insulated bearing sleeve is formed by adding an insulation filling layer (with a thickness of 2-4mm) on the bearing sleeve to separate the inner and outer sleeves, 

thereby blocking the shaft current. This insulation treatment is suitable for large-sized motors, high-voltage motors, and variable frequency speed control motors, 

especially in high-voltage and variable frequency speed control systems where the effect is significant.

Through the above measures, it is possible to effectively prevent damage to the rear bearings of large motors caused by shaft current, extend their service life, and ensure 

the stable operation of the motor.