Bearing fault model for induction motor with externally induced vibration

This paper investigates the relationship between vibration and current in induction motors operated under external vibrations. Two approaches are usually available to define this relationship. The former is based on airgap variations, the latter on torque perturbation. This paper is focused on the airgap variations model. The ball bearing fault is modeled by contact mechanics. External vibrations often occur in many industrial applications where external induced vibrations of suitable amplitude cause cyclic radial loading on the machine shaft. The model is validated by experiments, thanks to a dedicated test setup, where an external vibration source (shaker) was employed, together with ball bearing alterations in order to decrease the stiffness of the support along radial direction. To maximize the effects of externally induced vibrations, the frequency chosen was near the flexural resonance of the rotor (determined by FEM analysis). The direction of the external vibration is radial with respect to the axis of the electric machine under test. During tests, both stator phase currents and vibration of the machine were sampled. The test setup allowed to vary machine speed and load, vibration amplitude and bearing stiffness (damage level). Radial effects are usually visible only in case of large failures that result in significant airgap variations, as confirmed by experiment