PWM buck-boost ac choppers for safe commutation

Novel pulse-width modulated (PWM) buck-boost ac choppers solving the commutation problem are proposed for single-phase and three-phase systems. The power circuits of the single-phase and three-phase buck-boost ac choppers use only two and three standard switch modules, respectively, and regenerative de snubbers are attached directly to standard switch modules to absorb energy stored in line stray inductance. The switching policy for solving the commutation problem is achieved. Compared with previous buck-boost ac choppers, the switching loss is reduced and RC snubbers causing power loss are eliminated. The equivalent circuits of the proposed ac choppers are obtained and the power factor is derived from the equivalent circuits. The proposed ac chopper has the following advantages; improved power factor, low harmonics, sinusoidal input current, fast dynamics, high efficiency, high reliability, and significant reduction of the filter size. Experimental results show that the proposed scheme gives good steady-state performances of the ac chopper and that they coincide with the theoretical results.