Two-phase hard commutation chopper is suggested as suitable system for safe operation of subway electric railway traction. This thesis proposes a optimal design method of chopper main circuit which is safely operated in worst condition. Hence, chopper main circuit can be consistently designed if input condition ($E_{d,min}$), load condition ($I_{o,max}$), device condition ($t_q$) is provided on designer. More exact thyristor turn off model makes it possible to simulate exact voltage and current transient of thyristor power circuit with R-L pair snubber, stray inductance of snubber circuit and series operation, etc.. And, using this model, optimum thyristor snubber design method is newly proposed.
