Normalization-Based Approach to Electric Motor BVR Related Capacitances Computation

Abstract

Electrical discharge machining (EDM) bearing currents that may occur within electric machines of variablespeed-drive motor systems have been recognized for a long time.One key influential factor, the machine's capacitive voltage divider "bearing-voltage-ratio" BVR strongly depends on the rotor-to-frame and the stator winding-to-rotor capacitances; these are, in turn, affected by the design of the machine's stator slot. This paper presents an approach to improve the accuracy with which these capacitances can be estimated. It is based on the well-known plate capacitance equation which is then corrected by normalization functions. The functions are defined by extensive parameter studies using electrostatic FEM simulations. The final expressions not only allow for the prediction of the statorwinding-to-rotor and rotor-to-frame capacitances, they are also readily applicable. Thereby, they facilitate, for example, the clear-cut study of the sensitivity of the BVR towards changes in the different stator slot parameters.

Analysis of Impact of Design Solutions of an Electric Machine with Permanent Magnets for Bearing Voltages with Inverter Power Supply

“…The equivalent diagram (Figure 6) shows that the common-mode voltage uCM is the source of the voltage between the bearing raceways ub. The ratio of these voltages (BVR-Bearing Voltage Ratio) is given by the Equation (2) [16]: The values of these parameters depend on the bearing structure, temperature, and the operating condition of the machine. During the normal operation of the machine, an oil film forms dielectric insulation between the rolling elements and raceways of the bearing.…”

Modified Slot Opening for Reducing Shaft-to-Frame Voltage of AC Motors

“…The approximation is slightly inaccurate because it assumes all electric fields between the windings to the rotor surface are vertical and orthogonal to the rotor surface, ignoring the effect of slot teeth attracting the fields. Although more accurate capacitance calculation can be found [11,12], at least we can use Equation (3) to predict the overall tendency of the winding-to-rotor capacitance with changed slot dimensions.…”

Analytical Determination of the Slot and the End-Winding Portion of the Winding-to-Rotor Capacitance for the Prediction of Shaft Voltage in Electrical Machines

“…Grey frosting of the raceway and the rolling element surfaces, and the periodic raceway corrugation structures can be the result of EDM bearing currents [3]. The winding-to-rotor capacitance required to predict the shaft voltage is composed of the slot portion and the two end-winding portions [4,5]. The slot portion corresponds to the capacitive coupling in the active part of an electrical machine.…”

“…The analytical determination of the slot portion of the winding-to-rotor capacitance is mainly based on calculating a plate capacitor [3,5,[7][8][9][10]. However, the derivation of the calculation rule of a plate capacitor is based on the assumption of a one-dimensional field area.…”

“…Hence, the influence of the stator lamination is not taken into account. Therefore, correction functions are introduced in References [3,5]. In Reference [11], the electric field area between the stator winding and the rotor lamination in the active part of an electrical machine is divided into a slot area and an air-gap area.…”