Shaft current in AC induction machine: an on line monitoring system and prediction rules

This paper looks at a method of on-line shaft current monitoring system. Fault conditions such as air gap eccentricity, slot harmonics and rotor broken bars or end ring result in 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. The proposed method of shaft current is noninvasive, reliable, portable and low cost. Causes of the shaft current are described and prediction rules introduced.