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Recently, attention has been paid to bearing failure in an inverterdriven motor. It is well-known that most of the bearing failure may be caused by bearing currents flowing inside the motor. A three-phase voltage-source PWM inverter using IGBTs produces a common-mode voltage including high-frequency components, which is considered as a zero-sequence voltage. The common-mode voltage induces a shaft voltage between the motor shaft and the motor frame. When the shaft voltage exceeds a threshold voltage ranging from 10 to 15 V, breakdown of the grease film occurs in the motor bearing. At the instant, a short circuit is formed between the rotor and the frame, so that the shaft voltage suddenly goes down to zero. As a result, the bearing current with a peak value of about 1 A flows through the motor bearing in a 400-V 3.7-kW induction motor. This may cause damages such as fluting and pitting on the motor bearing, finally leading to bearing failure. This paper focuses on a passive EMI filter that is unique in access to the motor neutral line. This filter consists of a common-mode filter and a dv/dt reduction circuit. The common-mode filter can eliminate the 8-kHz (switching frequency) component generated by a three-level neutral-point-clamped (NPC) PWM inverter from the motor terminals. Thereby, the shaft voltage can be reduced so that no breakdown of the grease film occurs. This means that no bearing current flows inside the motor. On the other hand, the 150-Hz component generated by the diode rectifier still appears at the motor terminals because the common-mode filter has no capability of attenuating such a low-frequency component. However, since it is a low-frequency component with a small amplitude, it produces little effect on the bearing and leakage currents. The dv/dt reduction circuit can make a value of dv/dt as small as one-tenth, thus producing good effects on conducted EMI and stator-winding insulation. An experimental system consists of a 400-V three-level NPC PWM inverter with a switching frequency of 8 kHz, a 400-V, 3.7kW, 4-pole three-phase induction motor, and the passive EMI filter designed and fabricated in this paper. Fig. 1 shows the experimental waveforms when no EMI filter is connected. The waveform of the shaft voltage suddenly went down to zero at the moment it exceeded about 10 V. The bearing current with a peak value of about 1 A was observed in this experimental system. The bearing current iB has no relationship to the ground leakage current iG because the bearing current circulates inside the motor. The peak value of the leakage current reaches 500 mA in Fig. 1(a). This current may cause conducted and/or radiated EMI. Fig. 2 shows the experimental waveforms when both dv/dt reduction circuit and the common-mode filter are connected between the inverter and the motor. The waveform of the shaft voltage is reduced to a peak value of about 1 V as a result of having eliminated the common-mode voltage from the motor terminals. Therefore, no breakdown occurs in the bearing because the shaft voltage is lower than the breakdown voltage. As a result, it is concluded that the passive EMI filter can suppress the bearing current as well as ground leakage current, as shown in Fig. 2(b).

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