Performances of dielectric greases for rolling bearings employed in high power induction motors fed by PWM inverters

The steep voltage pulses associated to modern Adjustable Speed Drives (ASDs) stimulate the capacitive coupling between the stator and the rotor, resulting in stray shaft and bearings voltages. These latter act on the dielectric grease employed for lubricating the bearings and may cause the breakdown of the film. The associated Electric Discharge Machining (EDM) currents deteriorates the bearings performance and, by reducing their lifetime, negatively affects the overall reliability of the ASD. In this paper a simple characterization aimed at elucidating an aspect which seems not sufficiently covered in either manufacturer's data sheets or the scientific literature and concerning the performances of commercially available lubricating greases subjected to unconventional stresses is presented. The breakdown characteristics are evaluated in a point-to-plane configuration allowing a non uniform field to be applied to the materials. The breakdown performances appear to be related to the dielectric losses.