According to one embodiment, a control device detects a field current in a rotary electrical machine, estimates a rate of rotation of a rotor of the machine based on the detected current, obtains a field voltage in the machine based on a difference between the estimated rate of rotation and a target rate of rotation, and controls the switching of an inverter based on the field voltage such that the rate of rotation follows the target rate of rotation. A permanent magnet using the machine is an R—Co permanent magnet containing 25 to 40 at % iron. The control device performs field-weakening control by increasing and decreasing the field voltage based on a negative-field current in accordance with the rate of rotation by a material of the permanent magnet.