A dynamotor capable of operating as either a motor or a generator is used with both the armature portion and the field portion thereof capable of being rotated. In the case where a pulley operatively interlocked with the output shaft of the prime mover is mounted on the rotary shaft of the armature portion, the drive shaft of the compressor is mounted on the rotating field portion. Once the dynamotor is operated in motor mode, the rotational speed of the compressor is increased to the sum of the input rotational speed and the rotational speed of the dynamotor. The compressor is stopped by disconnecting a power feed circuit. When the input rotational speed is too high, the dynamotor is operated in generator mode. In this way, the rotational speed is reduced in accordance with the generated electric energy.