The converter switching in variable-frequency drives can generate high frequency common mode voltage between the machine winding and the converter ground, leading to high frequency parasitic currents which can flow through the machine bearings unless precautions are taken in design and installation. These parasitic and unintended currents in the bearings cause deterioration of the lubrication film and surface damage to the rolling elements of the bearings. These problems will be exacerbated as wide-bandgap semiconductors with faster switching rise times start becoming more widespread in variable-frequency drives. This paper reviews the modelling and mitigation techniques of high frequency bearing currents in inverter fed AC drives. It aims to provide a solid base for the research community to further understanding the bearing currents phenomenon and helping to find novel improved technique for their mitigation and measurement.

This is a repository copy of Experimental ball bearing impedance analysis under different speed and electrical conditions.

No 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 Cwr with the consequent reduction of Bearing Voltage Ratio and electrical discharge machining bearing currents. An improved analytical model to permit the calculation of Cwr 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 Cwr 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 Cwr with a good accuracy for unusual slot opening geometries.