Novel Stator Slot Opening to Reduce Electrical Machine Bearing Currents

Ball bearings are largely used in electrical machines, where high-frequency common-mode currents affect their lifetime. This document presents a novel zig-zag stator slot opening (SO) geometry to reduce the winding-to-rotor capacitance
$C_{\text {wr}}$
with the consequent reduction of bearing voltage ratio (BVR) and electrical discharge machining bearing currents. An improved analytical model to permit the calculation of
$C_{\text {wr}}$
for unusual SO geometries is implemented. Electrostatic and electromagnetic finite-element analysis (FEA) comparisons between the zig-zag, oblique, and original (classical SO configuration) machines are performed. These comparisons suggest that the zig-zag model presents the best performance in terms of BVR 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 SO to have the same
$C_{\text {wr}}$
values is proposed, showing a 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 SO configurations, respectively. Moreover, the improved analytical method can be helpful for engineers and researchers to calculate
$C_{\text {wr}}$
with good accuracy for unusual SO geometries.