Design of an active EMI filter for bearing current elimination in VFD

High-frequency currents flow in the bearing of variable frequency induction motors due to parasitic couplings in the motor drive system. An active filter to eliminate the bearing current in a 5 Hp induction motor is simulated in this paper. At higher frequencies, parasitic couplings between stator and rotor windings and the rotor and motor frame are prominent which can cause circulating currents in the bearing when shaft voltage exceeds the breakdown voltage level of the bearing lubricant. This circulating current damages the motor bearing intensely and shortens the life of the motor drastically. Properly designed passive Electromagnetic Interference (EMI) filters can mitigate the high-frequency common-mode voltage across the motor terminals and thus nullify the adverse effects of PWM inverters. However, the components of a passive filter account for a large portion of converter volume, and weight making it expensive. In this paper, an active filter is designed to eliminate the bearing current and to bring down the passive component size.