Calculation of influence of insulated bearings and insulated inner bearing seats on circulating bearing currents in machines of inverter-based drive systems

The high-frequency (HF) circulating bearing current that may occur in the machines of the inverter-based drive systems can be reduced or even completely suppressed by the use of the electrically insulated bearings or the insulated inner bearing seats. This paper proposes a method to calculate the influence of the thickness of the insulating coat on the obtained current reduction as a function of the several machine-design parameters. The parameters of the equivalent circuit are derived from an eddy-current model. The calculation considers the transient nature of the current flow. For the standard fan-cooled motors with up to 500-kW rated power, a minimum thickness of the insulating coat of 250 mum is required to reduce the circulating bearing currents to values where the apparent bearing-current density is less than 0.1 A/mm2, which is commonly taken as the threshold to the bearing failure