A Passive EMI Filter with Access to the Ungrounded Motor Neutral Line

This paper describes an effect of a passive EMI filter on preventing bearing current from flowing inside an inverter-driven motor. Motor-bearing damages are often caused by bearing currents resulting from breakdown of grease films in the motor bearing. The high-frequency common-mode voltage generated by the PWM inverter induces a shaft voltage between the rotor and the frame. When the shaft voltage exceeds a breakdown voltage of the grease films, a destructive instantaneous discharge current with a peak value of about 1A flows through the motor bearing. The passive EMI filter, that is unique in access to the motor neutral line, can reduce the shaft voltage as a result of eliminating the high-frequency common-mode voltage from the motor terminals. Hence, no breakdown occurs in the grease film, so that no bearing current flows. The viability and effectiveness of the passive EMI filter is verified by experimental results obtained from a 400-V, 3.7-kW laboratory system.