Electrostatic Shield to Mitigate the High Frequency Circulating Bearing Current - A Study for Design Guidelines

In a previous work, the authors addressed electrostatic shielding to reduce the bearing discharge currents (Electrostatic discharge machining currents - EDM). In order to expand the application of such a shielding device, this paper presents an analysis of its use to reduce the inverter-induced circulating bearing currents on induction machines. Employing finite element analysis (FEA), 2D and 3D models were developed to clarify the tradeoff between the shield effectiveness and the associated additional heat losses and motor efficiency reduction, considering different geometric arrangements and materials for the shielding device construction. The results provide very good guidelines for the shield design, contributing to its practical application viability. It is shown that it is possible to design a shielding configuration that achieves a high mitigating level with a negligible impact on the motor efficiency and developed torque. Furthermore, comparing calculation models of different complexities, the paper also shows the importance of representing all capacitive couplings inside the motor and the non-uniform current distribution in the shield to determine its effectiveness. Since a large fraction of the motor failures are caused by bearing currents, this paper can provide benefits to key topics for industry, such as equipment reliability and availability.