A rotary electrical machine including an annular stator having a stator core around which an armature winding is wound, and a rotor arranged on an inner circumference of the stator, a permeance of a q-axis magnetic circuit is made larger than a permeance of a d-axis magnetic circuit. The machine includes a switch for supplying power to a field winding and controller. The controller calculates the duty ratio on the condition that an upper limit of the duty ratio of the switch is a predetermined value and turns on/off the switch based on the calculated ratio. The predetermined value is set to a value 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 current can take and is less than 100%.