A Passive EMI Filter Requiring Access to the Ungrounded Motor-Neutral Line

The authors have already proposed a passive EMI filter that does require access to the motor-neutral point. The EMI filter has been applied to an induction motor driven by a voltage-source PWM inverter with a three-phase diode rectifier used as the front end. Experimental results have shown that the EMI filter can mitigate leakage currents flowing out of the motor and the heat sink, as well as bearing currents circulating inside the motor. This paper deals with a passive EMI filter for preventing high-frequency leakage currents from flowing through the heat sink of a voltage-source PWM inverter together with a voltage-source PWM rectifier used as the front end. Both rectifier and inverter have the same carrier frequency as 14kHz. The tested motor equipped with the motor-neutral line is an interior permanent magnet synchronous motor (IPM-SM) rated at 190V, 5.5kW, 6 poles and 1,800rpm. This paper designs, constructs and tests two types of EMI filters. These filters consist mainly of two 10-mH identical common-mode inductors that are installed on the ac sides of the rectifier and the inverter, respectively. However, the two filters are slightly different in that one accesses to the dc-link mid-point, and the other does not access to it. Experimental waveforms verify that the former is superior to the latter in eliminating the high-frequency leakage currents from both the heat sink and the motor.