Failure root-cause analysis of end-ring torsional resonances and bar breakages in fabricated-cage induction motors

This work shows experimental evidences of abnormal end-ring torsional vibrations in fabricated cage induction motors and their role as root cause of bar breakages. A case-study of cage torsional resonances in railway traction motors is presented, which led to bar breakages with a peculiar failure pattern. A particular cage resonant mode (the second natural mode with both the rings vibrating in phase-opposition with respect to the rotor iron stack) is shown as responsible of excessive bar bending, fatigue, and bar fractures, through direct measurements of ring-stack oscillations in working motors. Cage torsions were excited by large harmonic torques due to inverter supply in the case considered. The mechanism of resonance-induced bar solicitations and fatigue is discussed, and the difference of failure pattern with respect to more common bar breakage mechanisms (overloads, heavy start-ups, etc.) is evidenced. Some theoretical models for modal frequency calculation are presented, which may help to identify the origin of vibrations and the fault root-cause in faulted drives. Dynamic simulations of the drive have been carried out for airgap torque calculation and bar length optimization.