The shaft current of the three phase motor has two kinds: low frequency and high frequency. Due to alternating magnetic flux, electrostatic charge accumulation and asymmetry in motor manufacturing, the generated shaft current is often not high frequency and belongs to low frequency shaft current. In general, the AC drive system will produce shaft current, and the shaft current amplitude of most systems is small and does not reach the degree of harm. If the shaft current of the system is large, it may damage the motor bearings.

The oil film of the rolling bearing is generally thin and sensitive to the shaft voltage. When the shaft current flows through the tiny contact point between the rolling body and the inner and outer ring, if the voltage is relatively high, the contact surface of the inner and outer ring will break down. Generally, the shaft voltage of rolling bearings is required to be less than 300mV.

Modern 3 phase induction motor design and manufacturing processes add many measures to eliminate low frequency shaft current, but modern AC drive systems produce high frequency current pulses in bearings due to the frequency conversion power supply method and the use of fast switching elements. If the energy of these pulses is high enough, it will also cause damage to the bearing.

The main basis for judging whether the shaft current causes the bearing to burn is to check the bearing surface marks. All the burns caused by shaft current will appear like washboard strip burn marks on the bearing inner and outer ring track, which is a typical feature of shaft current damage to rolling bearings. At the same time, its surface is also accompanied by pitting, scars, and some even cracks appear; The temperature of the motor bearing rises quickly and is accompanied by the outflow of lubricating grease.

When the roller or ball is rolling and rolling on the track of the bearing ring, the contact resistance at the contact point of the rolling ring is very small, and the grease is squeezed to both sides, and a small gap is generated when the rolling body is about to leave the original position. The shaft current discharges on the surface of the shaft and bearing inner ring, producing a lot of corrosion points, burning the runway surface into line marks.

The number of lines is related to shaft current frequency, motor speed and bearing conditions. When the later rolling body continues to rotate forward, the burn marks are flattened and smoothed by rolling, so the surface of the runway will appear bright.