A rotary electrical machine includes a switch for supplying power to a field winding and a controller. A ratio of an on-time to a switching cycle of the switch, i.e., a duty ratio which is larger than the duty ratio corresponding to the field current that gives the maximum reduction amount of the inductance of the field winding with respect to an increasing amount of the field current in a range that the field current can take and which has a predetermined value less than 100%. The controller calculates the duty ratio on the condition that an upper limit of the duty ratio is set as the predetermined value and turns on/off the switch based on the calculated duty ratio, and sets the predetermined value to be larger as a rotation speed of a rotor is higher, or as a d-axis current flowing through an armature winding is larger.