Reduction of EMI through switching frequency dithering

In this paper, a method is presented to reduce electro-magnetic 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 the inverter in a pseudo-random way to spread out the emitted radio- frequency (RF) energy over larger frequency range. The proposed method requires only software changes. Practical constraints to implement the frequency dithering approach are discussed. Simulation & measurements shows an improvement of more than 10 dB. The proposed method also converts certain spikes at integer harmonics of the switching frequency from narrow band to broadband noise.