The reason why the variable frequency power supply damages the motor bearings is that there is current flowing through the bearings, and this current is in a state of intermittent connection. The intermittently connected circuit will generate an arc, and the arc will burn the bearings.
There are two main reasons why current flows through the bearings of AC motors. First, the induced voltage generated by the imbalance of the internal electromagnetic field, and second, the high-frequency current path caused by stray capacitance.
The magnetic field inside 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 PWM voltage output by the inverter causes the magnetic field inside the motor to be asymmetrical, a voltage will be induced on the shaft. The voltage amplitude is between 10 and 30V , which is related to the drive voltage. The higher the drive voltage, the higher the voltage on the shaft. When the value of this voltage 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 moment, the insulation of the lubricating oil blocks the current. This process is similar to the on-off process of a mechanical switch. In this process, an arc will be generated, burning the surface of the shaft, ball bearing, and bowl to form pits. If there is no external vibration, small pits will not have too much impact, but if there is external vibration, grooves will be generated, which will have a great impact on the operation of the motor.
In addition, experiments show 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.
In the early stage of motor operation, when the lubricating oil temperature is low, the current amplitude is 5-200mA , and such a small current will not cause any damage to the bearing. However, 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 arcing and form small pits on the surface of the bearing components.
Post time: Feb-05-2025