Electrical characterization of bearing lubricants

In this paper, commercial bearing lubricants have been characterized with respect to their electrical properties as function of lubricant type, additives and usage. In a wider perspective, the research aims at studying, characterizing and modeling bearing currents and the associated premature failure of bearings within electrical generators of wind turbines. In parallel to the work presented here, the phenomenon is also investigated through experiments on bearings under running conditions in test rigs under different conditions. As a complement to these investigations, this paper presents the results from characterization of electrical properties of lubricants such as relative permittivity and conductivity, by means of frequency domain spectroscopy, and breakdown voltage. The relative permittivity at 40 Hz was found to be 2.1 for a lubricant oil without additive, 2.2 for a lubricant with additive for wear protection, 3.8 for fresh grease and 4.7 for a used one. The conductivity at 50 °C for lubricant oil without additives was 2.9 pS/m, with additives 114 pS/m and for grease 4200 pS/m. The breakdown voltage for a 100 μm gap for oil without additives was 24.4 kV/mm and for oil with additives 30.8 kV/mm.