There is a current flowing through the bearing, and this current is in a state of intermittent communication. The circuit that is intermittently connected generates an arc, and the arc burns the bearing.

There are two main reasons for the current flowing through the bearings of the AC motor. First, the induced voltage generated by the internal electromagnetic field imbalance, and the second, the high-frequency current path caused by the stray capacitance.

The magnetic field inside the ideal AC induction motor is symmetrical. When the currents of the three-phase windings are equal and the phase difference is 120, no voltage is induced on the shaft of the motor. When the PWM voltage output from the inverter causes the magnetic field inside the motor to be asymmetrical, the voltage is induced on the shaft. The voltage amplitude is 10~30V, which is related to the driving voltage. The higher the driving voltage, the higher the voltage on the shaft. high.

When the value of this voltage exceeds the insulation strength of the lubricating oil in the bearing, a current path is formed. During the rotation of the shaft, at some 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, ablation of the shaft, balls, and the surface of the shaft bowl to form a dimple. If there is no external vibration, the small pit will not have an excessive influence, but if there is external vibration, a groove will be generated, which has a great influence on the operation of the motor.

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

At the beginning of the motor operation, when the temperature of the lubricating oil is low, the current amplitude is 5-200 mA, and such a small current will not cause any damage to the bearing. However, when the motor is running for a period of time, as the temperature of the lubricating oil rises, the peak current will reach 5-10A, which will cause arcing and form small pits on the surface of the bearing component.