To start retry operation early upon occurrence of an overvoltage by operating a power generating/discharging circuit (hereinafter, referred to DBR) upon occurrence of an overvoltage thereby lowering the voltage of a DC circuit quickly down to a normal level.

An inverter body 5 having a capacitor 2 in the DC circuit generates an AC power having frequency and voltage subjected to gate control through a control circuit 6 and drives a motor 7 with variable speed. A series circuit of a resistor 8 and a semiconductor switch 9 is provided, as a DBR, in parallel with the capacitor 2. The control circuit 6 monitors the voltage of the capacitor 2 and when occurrence of an overvoltage is detected from that voltage, gate of the inverter body 5 is interrupted and a switch 9 is turned on continuously or intermittently to discharge the capacitor 2 through the resistor 8 thus lowering the voltage thereof close to a normal level. Since retry operation conditions are ensured, retry operation can be started early upon occurrence of an overvoltage.