PURPOSE: To reduce the capacity of a motor generator by separating a part of a load of the generator from the generator when the output of the generator becomes higher than the prescribed value.

CONSTITUTION: A current which is supplied to the separately-excited coil 6 of a motor from a motor generator 1 is detected by a detector 5. A controlling device 7 opens a contactor 21 when the detected value of the detector 7 exceeds the set value to stop supply of current to the part of a load such as a cooler or the like. When the detected value becomes lower than the set value, the contactor 21 is closed to reopen the supply of the current to the load. In this manner, the capacity of the generator can be reduced.