Lateral and Torsional Vibration Monitoring of Multistack Rotor Induction Motors

In this article, both the lateral and torsional vibrations in multistack rotor induction motors (IM) are modeled in the same framework, and the corresponding precursors in the current waveform are analyzed in detail. Due to the very large rotor length-to-diameter ratio, rotor vibrations can become a major issue in these types of high-torque motors. In order to monitor the vibration components, the lumped mass-spring model for the rotors and couplings is developed to calculate the natural frequencies and torsional-mode shapes of the mechanical system. To model the electrical part, stator windings are split up into several identical windings that are connected in series and correspond to each rotor stack. Since the torsional vibration does not remarkably affect the winding symmetry, the dq model for each section is sufficient to describe the motor behavior during torsional oscillation. Similarly, for the lateral vibration, the mechanical system is modeled in all lateral directions and all modes. Due to the loss of symmetry, the abc model is used for each rotor and stator. In this modeling approach, all inductances (self and mutual of stator and rotor and the mutual inductances between the stator and rotor) are needed. To mimic the effect of lateral vibrations on the motor inductances, a new approach is developed using winding function theory. In this model, all inductances are formulated as a function of eccentricity severity and angle, and a new analytical IM model is developed to show the effects of lateral vibration. Finally, the experimental results are presented and compared with the analytical results.