Shaft current in AC induction machine. An online monitoring system and prediction rules

This paper looks at a system for online shaft current monitoring. The proposed method of shaft current monitoring is noninvasive, reliable, portable, and low cost. Causes of the shaft current are described and prediction rules introduced. Fault conditions such as air-gap eccentricity, slot harmonics and broken rotor bars or end rings result in an asymmetrical flux pattern around the shaft, hence, resulting in shaft voltage. Bearing failures can occur when the voltage level exceeds the threshold of the oil film. The importance of the oil film is described.