Solving Means: A field coil current and a stator current are controlled by a controller. The stator current is controlled under vector control and is controlled so that the phase keeps an efficient motor zone. In a case of rated power generation, when the motor speed is low, the phase current and field coil current are increased to reserve the generated power. And, as the speed is increased, the phase current is decreased to reduce the copper loss, while in place of decreasing the phase current, the field coil current is kept high to reserve the generated power. Thereafter, as the speed is increased more, the field coil current is decreased to reduce the iron loss, while in place of decreasing the field coil current, the phase current is increased to reserve the generated power.