Analysis of a structural failure mode arising in cage rotors of induction machines

This study deals with the analysis of a typical failure mode of the induction machines used in high-speed motor drives, consisting of cracks appearing at the junction between the bars and end-rings of a cage rotor. The cracks are generated by the fatigue stress due to the end-ring oscillations, excited by tractive torque harmonics. The problem is investigated here by establishing a simple model of a typical cage rotor, which is validated using experimental results. The model is employed for identifying the eigenmodes and eigenfrequencies of the rotor and for evaluating the stress arising in the bars during service, either for an undamaged or a damaged rotor, yielding physical insight on the failure mechanism. A practical and simple design solution is presented and validated using the proposed model, with the goal of increasing the torsional stiffness of the rotor, thus reducing the amplitude of the torsional vibrations. As a consequence, the fatigue stress arising at the junction between the bars and end-rings during service are reduced as well, and the operative life of the rotor is prolonged.