Abstract: A main unit 2 and a subsidiary unit 4 include two inverters 6m, 8m and 6s, 8s, and PWM control circuits 26m, 26s, respectively. A changeover switch 20 operates to simultaneously connect the inputs of the two inverters of the main unit 2 and the inputs of the two inverters of the subsidiary unit 4 in series or in parallel. An imbalance detecting circuit 28 detects imbalance in the input voltages to the two inverters of the main unit 2, and provides the detection result to the PWM control circuits 26m and 26s. The PWM control circuit 26m controls the inverters 6m and 8m of the main unit 2 in accordance with the detection result supplied from the imbalance detecting circuit 26m, and the PWM control circuit 26s controls the subsidiary unit 4 in accordance with the detection result supplied from the imbalance detecting circuit 26m. The input current to the main unit 2 is larger than the input current to the subsidiary unit 4.