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Why does the frequency converter damage the motor bearing?
Release time:
2022-10-14
First of all, we have to understand that there is a current flowing through the bearing, and this current is in an intermittent state. Intermittent circuits can produce arcs that burn the bearings. There are two main reasons for current flow in
AC motor bearings. One is the induced voltage caused by the imbalance of the internal electromagnetic field; second, the high-frequency current path caused by the 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 is induced on the motor shaft. When the output PWM voltage of the inverter causes the internal magnetic field of the motor to be asymmetrical, a voltage will be induced on the shaft. The amplitude of the voltage is 10 to 30V, which is related to the driving voltage. The higher the drive 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, which ablates the surface of the shaft, ball, and bowl to form a pit. If there is no external vibration, the small pits will not have much 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 shaft 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 serious the bearing damage. At the beginning of
motor operation, when the lubricating oil temperature is low, the current amplitude is 5-200mA, so that the small current will not cause any damage to the bearing. However, when the motor runs for a period of time, as the temperature of the lubricating oil increases, the peak current will reach 5-10A, which will cause flashover and form small pits on the surface of the bearing parts.
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