Eliminating Load Oscillation Effects for Rotor Eccentricity Detection in Closed-Loop Drive-Connected Induction Motors

Effectively detecting rotor eccentricity faults in the presence of a load torque oscillation is an important concern for some types of industrial loads such as reciprocating compressors. In a closed-loop induction motor drive, eccentricity related fault characteristic harmonics are introduced into both stator current and voltage spectra due to the low-pass filtering property in PI controllers. Therefore, both stator current and voltage harmonics need to be monitored simultaneously. However, a rotor position dependent load torque oscillation has exactly the same effect as a rotor eccentricity fault on the spectrum. Harmonics in stator current and voltage spectra resulting from a load oscillation usually have larger magnitudes, which leads to an ambiguity in the detection of rotor eccentricities. This paper develops an effective method to eliminate load oscillation effects for rotor eccentricity detection in a closed-loop vector-controlled induction motor by monitoring eccentricity induced negative sequence information in both stator currents and voltages. A significant advantage of this scheme is its independence on motor parameters. Experimental results validate the efficacy of using the negative sequence indicator. The practical issues involved in separating the eccentricity related negative sequence information from those resulting from a stator inter-turn fault are also addressed.