There is provided a control unit (40) applied to a motor (10) that includes a rotor (12) having a field winding (11) and a rotor (13) having armature winding groups (10a, 10b) to control a field current passed through the field winding. Each of the armature winding groups is applied with a prescribed voltage. The field current is controlled so as to be a minimum field current value If_min with which a deviation between an amplitude of an induced voltage generated in the armature winding groups by rotation of the rotor, and an amplitude of the voltage applied to the armature winding groups becomes equal to or smaller than a prescribed value.