Brushless Mitigation of Bearing Currents in Electric Machines Via Capacitively Coupled Shunting

The overwhelming trend within the industry today is to pair electric machines with variable-frequency drives. It is well known that the use of these drives can introduce high-frequency bearing currents during semiconductor switching events. This paper focuses on bearing discharge currents that gradually degrade the bearings by pitting their surfaces and can lead to premature failure. Common solutions include insulating the bearings, installing common-mode filters, or using a brush to ground the rotor shaft. These solutions are not universally scalable and may require periodic maintenance. This paper presents a new approach using noncontact (brushless) capacitive coupling. Here, a rotating capacitor is placed electrically in parallel with the bearing. At high frequencies, the impedance of the rotating capacitor is far lower than the bearing impedance, thereby shunting current around the bearings and reducing shaft voltage. The technique is experimentally demonstrated on a 3-hp (2.2-kW) induction machine. The experiment demonstrates a bearing current reduction factor of at least 8x.