PURPOSE: To reduce by half the number of saturable reactors by passing a pair of upper and lower arm conductors through a common core so that the directions of short-circuiting currents flowing out from a DC power source become the same.

CONSTITUTION: A core 30 is passed from a DC positive bus P, and connected to the collector of a transistor 11. The emitter of the transistor 11 is connected to the collector of a transistor 12 through an R-phase output bus R. An output from the emitter of the transistor 12 is connected to a DC negative bus N again through the core 30. When the transistor 11 is turned ON and then turned OFF, the magnetic flux of the core 30 is reset by the current. Again, when the transistor 11 is turned ON, the direction of the short-circuiting current becomes reverse to a load current, the core 30 is operated as a reactor for a predetermined period to limit the short-circuiting current.