Classification of magnetic circuit asymmetry due to source supply unbalance, load variations and stator turn fault in induction motors using observer coil technique

This paper presents a simplified approach for detecting faults in induction motors using an economic and easily implementable technique. The proposed technique uses an observer coil (OC) located into stator slots, and laid over the stator coil parallel to it. A sinusoidal voltage is induced in the OC due to the interaction with the stationary stator and the dynamic rotor. The frequency components analysis of the e.m.f. induced in the OC was carried out. THD and third harmonic components were found giving accurate and identifiable variations corresponding to source unbalance, loading conditions and stator turn fault. These parameters also exhibited sensitivity to severity of the fault. The proposed simplified, economical technique for detection and diagnosis of magnetic circuit asymmetry has yet not been reported in literature. Experiment for confirming the performance of the proposed method was done on 0.746 kW, three phase, 4 poles, 440 V squirrel cage induction motor. Experimental data were recorded under varying load conditions from no load to full load, and increasing severity of turn fault. The mathematical theory of the technique is provided and supported by an experimental validation.