Reduction of EMI in AC drives through dithering within limited switching frequency range

In this paper, a method is presented to reduce electromagnetic (EM) emissions in ac drives. This method is applicable to many ac drives. It is proposed to dither the switching frequency of the power devices of an inverter in a pseudorandom way to spread out the emitted RF energy over a larger frequency range. The proposed method requires only software changes. Practical constraints to implement the frequency dithering approach are discussed. Simulation and measurement results show an improvement of more than 10 dB. The proposed method also converts specific spikes in the long-wave range from narrowband to broadband noise, which results in less stringent EM emission requirements for many applications.