Influence of the air gap on bearing current threat in electrical machines

Abstract

With the widespread application of variable frequency drive technology, bearing current caused by parasitic capacitances has become one of the main causes of motor failure. Bearing voltage ratio (BVR) is a key indicator for assessing the risk of non-circulating bearing current, and its value is closely related to the distribution of parasitic capacitance within the motor. This thesis aims to explore the influence of motor air gap length on BVR from an electrostatic field perspective. The thesis employs two-dimensional finite element analysis (FEA) to simulate a series of motor models with identical slot geometry but different air gap length. Key parasitic capacitances within the motor are obtained, and the bearing voltage ratio under different air gap conditions is calculated. The thesis also evaluated that the influence of air gap length variation on BVR is systematic. The results show that changes in air gap length significantly affect the capacitance between the stator and rotor, and between the stator and winding, leading to fluctuations of BVR. This thesis quantitatively analyses the dependence between air gap parameters and BVR and discusses the technical feasibility of suppressing bearing current by adjusting the air gap length. Based on simulation results, this thesis further discusses the potential effect of air gap optimization in suppressing bearing current and its feasibility in practical motor design and application. This study provides a new analytical perspective for understanding and assessing bearing current risks from a geometric design standpoint, offering a theoretical basis and design reference for preventing bearing current risks during the motor structure design phase.