Why does the frequency converter damage the motor bearing?

Date:2022-06-02 09:26

The damage of the inverter to the motor includes two aspects, stator winding damage and bearing damage, which usually occurs within a few weeks to ten months, and the specific time is related to the inverter brand, motor brand, motor power, inverter carrier frequency, cable length between the inverter and the motor, ambient temperature and other factors.


The mechanism of motor bearing damage caused by the frequency converter:

The reason why the inverter damages the motor bearing is that there is a current flowing through the bearing, and this current is in a state of intermittent communication, and the intermittent communication circuit will produce an arc and burn out the bearing.


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; The second is the high-frequency current path caused by stray capacitance.

When the currents of the three-phase windings are equal and the phase difference is 120°, the magnetic field inside the ideal AC induction motor is symmetrical? The voltage will not be induced on the motor shaft, and when the PWM voltage output by the inverter causes the internal magnetic field of the motor to be asymmetrical, the voltage will be induced on the shaft, and its amplitude is between 10-30V, which is related to the driving voltage. The higher the drive voltage, the higher the voltage on the shaft, and when this voltage value exceeds the dielectric strength of the lubricating oil in the bearing, a current path is formed. In the process of shaft rotation, at a certain moment, the insulation of the lubricating oil blocks the current again, this process is similar to the on-off process of the mechanical switch, an arc will be generated during the on-off process, the arc will ablate the shaft rod, ball and shaft bowl surface, forming a pit, if there is no external vibration, the small pit will not have much impact, but if there is an external vibration, it will produce a groove, which has a great impact 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 output voltage of the inverter, the lower the fundamental frequency, the higher the voltage on the shaft, and the more serious the bearing damage. At the beginning of motor operation, when the lubricating oil temperature is low, the current amplitude is 5-200 mA, such a small current will not cause any damage to the bearing, but after the motor runs for a period of time, as the lubricating oil temperature rises, the peak current will reach 5-10A, which will cause an arc and form a small pit on the surface of the bearing part.