Electrical monitoring of mechanical looseness for induction motors with sleeve bearings

Mechanical looseness in induction motors is a common problem caused by loose bolts, structural damage, improper fit, or increased clearance between components, and results in increased vibration and wear. Detection of mechanical looseness in the sleeve bearing of industrial motors mainly relies on vibration analysis. However, there are many motor applications in the field where vibration analysis cannot be applied due to cost, environmental, and/or sensitivity limitations. The feasibility of using existing voltage/current measurements available in the motor control center for remote, low-cost monitoring of mechanical looseness due to improper fit between the bearing-housing and bearing-shaft is evaluated in this paper. It is shown through experimental testing on a custom built sleeve bearing test setup and on a 6.6 kV, 2250 hp motor that monitoring of the one-half and one-third subsynchronous components in the instantaneous power spectrum can serve as a reliable indicator of mechanical looseness.