Breakdown voltages in thrust bearings: Behavior and Measurement

This paper investigates breakdown characteristics in the bearings of variable speed electric drive trains. The analyses reveal that breakdown phenomena previously known from high-voltage technology appear to also occur in the bearing contact, though at significantly lower voltages. By loading the bearing with AC voltage of different frequencies at various mechanical load points, five states, which range from partial discharge through arc discharge to a completely conductive channel, emerge. Consistent with the concept that the magnitude of the breakdown voltage depends on the lubricant film height, the contact force, and the frequency of the AC voltage, a thicker lubricant film results in a higher breakdown voltage, analogously to a capacitor with a larger plate spacing, which requires a higher potential difference to be punctured. Furthermore, the sinusoidal frequency of the AC load correlates with the breakdown voltage. The breakdown tends to occur earlier at low frequencies, which is illustrated by the fact that at low frequencies, capacitive behaviour can only be observed at higher speeds, i.e., at a larger lubrication gap, while no voltage can be built up over the bearing at lower speeds. An increase of the contact force for the same lubricant film height lowers the breakdown voltages. The reduction of the breakdown voltage by the contact force may result from an increase of the relative permittivity of the lubricant with higher pressure and likewise of the contact area; the increased area may also increase the number of surface-roughness peaks where an increase in the electric field occurs.