PURPOSE: To reduce the capacitance of a snubber capacitor and lessen the restrictions on the arrangement of the half bridges by wiring inductance, in short, the structure of an inverter, in an RCD-type PN snubber circuit being provided for every half bridge so as to suppress the overcurrent at interruption of the arm short circuit current of an inverter circuit.

CONSTITUTION: The voltage across each RCD PN snubber circuit 41(42) is applied to the capacitor C2(C1) of the adjacent RCD-type PN snubber circuit 42(41) through a diode D5(D6), and the adjacent RCD PN snubber circuit is used jointly as a low-speed type of overvoltage control means.