A converter and an inverter are connected via a clamp circuit. The converter performs commutation in accordance with any of a first commutation mode in which trapezoidal waves are compared with a carrier and a 120-degree conduction mode. A diode of the clamp circuit is short-circuited by a shorting switch. The shorting switch is rendered conductive when a power factor reduces or a power supply voltage reduces, and capacitors of the clamp circuit are connected in series between DC power supply lines. The converter performs commutation in accordance with the 120-degree conduction mode, not in accordance with the first commutation mode, while the shorting switch is conductive.