Control of transistors in AC voltage PWM controller with elimination of commutation overvoltages

Method of controlling transistors in ac voltage PWM controller aimed at eliminating commutation overvoltages in RL-type load circuit is proposed in the paper. The method is based upon keeping continuity of load current; this is achieved by appropriate control of transistors on the basis of detecting voltage sign (polarity) at the supply terminal and detecting load current sign (polarity). This type of control does not depend on character of the load and it makes possible increase of converter´s efficiency by elimination of RC circuits protecting transistors from overvoltages. The scheme of main circuit is shown as well as waveforms demonstrating the control principle. Four different characteristic operating conditions are discussed and illustrated with simulation results. Measurements have been run to verify the method with real 3-phase ac controller and results of these tests are presented.