A Comparative Study of Single-Phase Modulated AC Choppers

With the increased availability of power MOSFET's and GTO's, a new generation of simple choppers for small and medium ac loads is foreseen. These new power semiconductors ease the use of forced commutation to improve the supply power factor even with highly reactive loads. Asymmetrically-and symmetrically modulated ac choppers designed to operate with reactive loads are described. The ability of the asymmetric chopper to hold a unity power displacement factor for a large range of reactive loads presents interest for induction machine controllers. On the other hand, the supply current produced by the symmetrical chopper has a less objectionable harmonic content than the asymmetrical chopper over a large range of operating conditions. Both choppers are compared to the common ac phase controller, with or without freewheeling switch. Experimental results confirm the analytical analysis. The practical limits of operation for the new choppers are also discussed.