Why does the variable frequency motor run with shaft current

Update:11 Jun 2021

Shaft current occurs when a variable frequency motor is running, and it is common in high-power variable-frequency motors. Because when a high-power variable-frequency motor is running, when the stator core of the motor is unbalanced in the reluctance of the outer circumference and upwards, the difference will occur. A shaft voltage occurs, and this voltage occurs along the axial direction.

The shaft voltage generated by static charge is intermittent and has long and short periods. During the operation of the motor, the load-side fluid will rub against the running rotating body to generate static charge on the rotating body, and the electric charge gradually accumulates to generate a shaft current.

Under normal circumstances, the alternating electric potential felt by the rotor of the variable frequency motor cutting magnetic field, the alternating current generated is also symmetrical, so there will be no difference in the nominal voltage between the two ends of the rotor under normal conditions.

In the operation of large and medium-sized variable frequency motors, once the rotor shaft voltage forms a loop, shaft current will occur, which is a typical low-voltage high-current mode. The oil is recovered between the shaft and the bearing bush for lubrication, and the motor bearing is pressed on the oil film. Due to the low shaft voltage amplitude, the insulation of the oil film will generally not be broken down.

During the high-speed operation of the rotor, if the lubricating oil quality is unsatisfactory, or when there is an environment such as oil shortage, the oil film will be split and broken down, resulting in a metallic war between the shaft and the bearing bush. At the moment of the war, the shaft voltage will be closed. Loop, forming a low voltage breakdown. At this time, the shaft current generated is quite large, reaching hundreds of amperes or even thousands of amperes in an instant, which is enough to burn out the journal and bearing bush.

Machinery and equipment manufacturers should pay special attention when using high-power variable frequency motors. Regular maintenance and inspection of the motor can also effectively prevent the motor shaft current from being generated.



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