Firstly, we need to understand that there is current flowing through the bearing, and this current is in an intermittent state. Intermittent circuits can generate arcs, which can burn bearings.

There are two main reasons for the flow of current in AC motor bearings. One is the induced voltage caused by the imbalance of the internal electromagnetic field; Secondly, the high-frequency current path caused by stray capacitance.

The magnetic field in an ideal AC induction motor is symmetrical. When the currents of the three-phase windings are equal and the phase difference is 120 °, no voltage will be induced on the motor shaft. When the output PWM voltage of the frequency converter causes an asymmetric magnetic field inside the motor, a voltage will be induced on the shaft. The amplitude of the voltage is 10-30V, which is related to the driving voltage. The higher the driving voltage, the higher the voltage on the shaft.

When this voltage value exceeds the insulation strength of the lubricating oil in the bearing, a current path will be formed. During the rotation of the shaft, at a certain point, the insulation of the lubricating oil blocks the current. This process is similar to the on-off process of a mechanical switch, in which an arc is generated, burning the surface of the shaft, ball, and bowl, forming pits. If there is no external vibration, small pits will not have a significant impact, but if there is external vibration, grooves will appear, which will have a great impact on the operation of the motor.

In addition, experiments have shown that the voltage on the axis is also related to the fundamental frequency of the output voltage. The lower the fundamental frequency, the higher the voltage on the shaft, and the more severe the bearing damage.

In the early stage of motor operation, when the lubricating oil temperature is low, the current amplitude is 5-200mA, so this small current will not cause any damage to the bearings. But after the motor runs for a period of time, as the temperature of the lubricating oil increases, the peak current will reach 5-10A, causing flashover and forming small pits on the surface of the bearing parts.