Inverter-Driven Electric Motors and Generators: Accurate Analysis of Bearing Capacitance and Identification of Contributors

Previously considered a rather simple element in motors or generators and widely ignored, bearings have become a key topic in electric cars, wind turbines, and variable-speed industrial drives. Their not fully understood degradation under pulse-width-modulated (PWM) control is harming the reliability of electric motors and increasing maintenance costs. Though a mechanical component, the formation of voltages and displacement as well as break-down currents across a bearing strongly relies on its electrical parameters. Whereas earlier research failed describing the impedance of bearings and particularly the capacitive component correctly, requiring double-digit correction factors, this paper couples several models to quantify and explain the capacitance of bearings in various operating points. Our solution combines a macroscopic finite-element electrostatic model of the bearing with a microscopic elastohydrodynamic model for the area where the balls touch the raceway as well as the immediate vicinity, estimating the material deformation and the spacing between the components and the pressure conditions, which in turn feeds a microscopic electrostatic model of this area. The separation of the problem into distinct models with weak coupling could reproduce measurements with errors in the low percent range, whereas previous approaches underestimated the capacitance by factors up to 30. Despite the small area in the micrometer range, the small spacing between raceways and rolling element filled by the lubricant lets the contacts of the rolling elements in the presented example of a thrust bearing be responsible for about 60 % part of the bearing capacitance, while the remaining 40 % result from the macroscopic conditions.