Novel method of mitigation of conducted EMI in PWM inverter fed induction motor adjustable speed drives

Mitigation of Electro Magnetic interference (EMI) in PWM inverter fed a.c. induction motor adjustable speed drives is becoming an important issue. In this paper the power converters are viewed totally in a different way from that of the normal viewpoint of energy conversion. It has been found that the converter itself is a common mode voltage generator. As the switching speeds of the converters have gone up due to the advent of modern power electronic devices like the IGBT and at the same time the parasitic coupling becomes the dominant side effect. In addition, the harmonics and the commonly unwanted noise signals produced by the pulse width modulation also produce the EMI. This paper presents the design, implementation and measurement of a novel method of sensing and mitigation of unwanted conducted EMI noise voltages produced by the PWM inverter and inverter devices. The LISN, current probe and other associated circuits are used to mitigate the unwanted noise voltages. The proposed system is a new method based on sensing the individual phase current, by the current transformer (CT). The individual voltage from the current transformer secondary is amplified and summed to get the common mode voltage for mitigating the EMI from the output terminals of the PWM inverter within the acceptable limits. The standard LISN, the standard current probes and the necessary circuits are used for filtering, measurement and mitigation of the unwanted signals. This system can be used in any application regardless of its working voltage. Experimental results are presented to verify the feasibility of the proposed system.