An active filter for eliminating harmonics and interharmonics in the input current to an AC voltage controller for refrigeration and air-conditioning applications

Energy saving motor controllers for refrigeration and air-conditioning applications operate at the minimum input power point by finding the optimum firing angle of the thyristors in the motor controller. Consequently, the distorted input current contains harmonics, sub-harmonics, and interharmonics whose frequencies change with the motor speed. Therefore passive filters are not suitable and the best solution is to use an active filter. A single phase active filter with simple constant frequency switching is able to reduce waveform distortion by eliminating a sufficient range of frequencies and offers a low cost solution. Theoretical analysis of the full-bridge and half-bridge topologies led to the development of design equations and curves. Simulation of a half-bridge active filter confirmed the design equations