An inverter is capable of converting a direct current from a direct current power supply into an alternating current and supplying it to an armature winding. A rectifier circuit is capable of rectifying the current, that has been converted to an alternating current by the inverter, into a direct current and supplying it to the field winding. The amount of the alternating current supplied to the armature winding and the amount of the direct current supplied to the field winding are in a proportional relationship, and implementation of switching control of the inverter controls both the amount of the alternating current supplied to the armature winding and the amount of the direct current supplied to the field winding while this proportional relationship therebetween is maintained.