Normalization-based approach to electric motor BVR related capacitances computation

Electrical discharge machining (EDM) bearing currents that may occur within electric machines of variable-speed-driven motor systems have been recognized for a long time. One important factor of influence, the capacitive voltage divider 'bearing-voltage-ratio' BVR of the machine strongly depends on the rotor-to-frame and the stator winding-to-rotor capacitances which 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 relatively accurately predicting the stator-winding-to-rotor and rotor-to-frame capacitances. They are also easily applicable, so as to, for example, easily study the sensitivity of the BVR towards changes of the different stator slot parameters.