Research Progress on the Mechanism and Mitigation Measure of Electrical Corrosion Damage in Rotating Motor Bearings

The bearing voltage and bearing current caused by variable frequency drive system and their negative effect of the bearing electrical erosion have become the main bearing failure modes. The exact mechanism of electrical-erosion failure of rotating motor bearings is unclear, and it is difficult to predict the electric life of bearings effectively. The electrical fault of bearings has become the key problem limiting the reliability of the motor system. This paper mainly summarizes the progress of research on the mechanism and mitigation measures for motor bearings’ electrical erosion damage. Firstly, the source and classification of bearing voltage and current are introduced. The results show that electrical discharge machining (EDM) current and high-frequency circulating bearing current are the main causes of electric-erosion damage and shortening of bearing life. Bearing damage caused by electrical erosion mainly includes frosting, pitting, fluting, white etching cracks (WEC), and lubricating oil degradation. Frosting and pitting damage are early failures with little influence on bearing operation. The appearance of fluting and WEC leads to greater vibration and noise of bearings, and the bearing life decreases sharply. The factors influencing bearing corrosion damage are analyzed, including speed, lubricating oil, power supply excitation, motor structure parameter, temperature, load, etc. According to the current research, with the increase in the speed, the bearing current presents a trend of first increasing and then decreasing. The discharge activity of the bearing is kept at a high value when the motor is started. The variable speed operation is more harmful to the bearing life than the constant speed operation. Power excitation is an important factor affecting bearing breakdown. Both DC and AC power supplies cause breakdown and discharge of bearing. The influence of power switching frequency on bearing discharge behavior is not uniform. In the design phase of the motor, the impact of the motor impedance on bearing voltage should be considered. The bearing voltage can be reduced by adjusting the motor impedance under the typical motor operation. According to the current literature, there are differences in the influence of temperature on bearing discharge activity. Lubricating oil is the premise of rolling bearings' reliable operation. Different types of lubricating oil have different bearing discharge levels under the same working conditions. Therefore, when selecting the bearing lubrication grease, its performance should be considered to reduce the risk of electrical erosion. Load is an important parameter for motor operation. Compared with radial load, axial load may increase the amplitude of EDM current. Bearing discharge activity is more intense when the motor runs in light load conditions for a long time. Bearing electrical models and bearing capacitance calculation methods are analyzed. Compared with analytical methods, bearing capacitance parameters calculated by finite element analysis are closer to the measured values. The current bearing model and capacitance calculation methods must comprehensively consider influencing factors like speed, temperature, and load. Mitigation techniques can be divided into: on the inverter, on the connection, and on the motor. The improved modulation strategy and the new inverter topology eliminate or reduce the amplitude of the common-mode voltage and reduce the bearing current at the source. Other mitigation methods are achieved by increasing the bearing current loop impedance or providing a low-impedance path for the bearing current. The mitigation technology for bearings mainly involves using insulated bearings and conductive grease. However, bearing damage mitigation techniques are only effective for some types of bearing current. Understanding the bearing current mechanism is needed before selecting the appropriate mitigation method. Most mitigation techniques impose increased costs and need to be evaluated. Finally, the possible future research direction of bearing electrical erosion is prospected. © 2024 China Machine Press. All rights reserved.