In a plug-in operation, a first winding which functions as an armature winding for supplying an R-phase current of a three-phase rotary electric machine, a third winding, and a second winding are connected respectively to terminals for supplying R-phase, S-phase, and T-phase currents of a three-phase power source. Thus, the S-phase current and the T-phase current are interchanged as compared with in a normal operation. Thereby, reverse rotation of a field element is intentionally caused. A reverse-rotation prevention mechanism is provided in the field element.