Novel Stator Slot Opening to Reduce Electrical Machine Bearing Currents

Abstract

Ball bearings are largely used in electrical machines, where the high frequency common mode currents affect their lifetime. This document presents a novel Zig-Zag stator slot opening geometry to reduce the winding-to-rotor capacitance <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_wr</i> with the consequent reduction of Bearing Voltage Ratio and electrical discharge machining bearing currents. An improved analytical model to permit the calculation of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_wr</i> for unusual slot opening geometries is implemented. Electrostatic and electromagnetic Finite Element Analysis comparisons between the Zig-Zag, oblique and original (classical slot opening configuration) machines are performed. These comparisons suggest that the Zig-Zag model presents the best performance in terms of Bearing Voltage Ratio reduction, while keeping torque performance close to the original machine. An additional comparison between the novel Zig-Zag model and the classical one with reduced slot opening to have the same <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_wr</i> values is proposed, showing an higher degradation in terms of torque performance for the latter. The study shows that the Zig-Zag model could be a good candidate to reduce effectively electrical machine bearing currents. A validation test is proposed on two motorette prototypes having classical and Zig-Zag slot opening configurations, respectively. Moreover, the improved analytical method can be helpful for engineers and researchers to calculate <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C_wr</i> with a good accuracy for unusual slot opening geometries.