Localized asymmetric behavior of common-mode current through bearing balls

The high-frequency pulse width modulation in voltage source inverters generates common-mode voltage and current in inverter-fed electrical machine drive systems, leading to high-frequency bearing currents and potential bearing damage. Due to the enclosed structure of bearings and the distributed nature of bearing currents, accurate measurement remains challenging. This paper reviews existing measurement methods and identifies a previously unreported bearing current component caused by uneven potential distribution on the end-caps, termed the “end-cap bearing current.” The “end-cap bearing current” occurs exclusively when the bearings are in a conducting state, which can happen during low-speed operation or bearing breakdown. Experimental results from a modified end-cap setup with multiple connection wires reveal asymmetric radial current flow through bearing balls, which is not captured by conventional single-wire measurement methods. A 3D finite element model and an enhanced circuit model are developed to illustrate the underlying mechanism. The study demonstrates that up to 25% of the high-frequency common-mode current in the end-cap may flow through the bearing balls under conductive conditions, highlighting the limitations of existing measurement techniques and the need for more accurate bearing current assessment.