A rotary electrical machine comprises a stator and a rotor having an axis of rotation, wherein, on a plane radial to the axis of rotation has a rotor tooth with an arc of a circle. The arc is defined by a centre situated on the axis of symmetry of the tooth, a first point at an intersection between the axis of symmetry and a circle centred on the axis of rotation, and an outer radius. A second point is situated on an axis which passes via the axis of rotation, and forms an angle of 360° divided by a number equal to twice the number of magnetic poles. The second point is situated from the axis of rotation an amount equal to the outer radius decreased by an allowance. A ratio between the allowance and the outer radius is between 0.015 and 0.07.