A method of balancing a permanent magnet rotor for an electrical machine, characterised in that the method is carried out whilst the rotor is assembled in the electrical machine of which it forms a part and the method comprises the steps of: a) performing a first balancing operation at a first rotational speed to detect a perceived imbalance in the rotor made up of a mechanical imbalance and a magnetic imbalance, and to ascertain first information indicative of the magnitude and direction of a first force needed to balance the perceived imbalance detected at the first speed; b) performing a second balancing operation at a second rotational speed to detect a perceived imbalance in the rotor made up of a mechanical imbalance and a magnetic imbalance, and to ascertain second information indicative of the magnitude and direction of a second force needed to balance the perceived imbalance detected at the second speed, wherein, at each speed, the perceived vector imbalance force is the vector sum of the vector imbalance force due to magnetic imbalance and the vector imbalance force due to mechanical imbalance; c) operating on the first information and the second information to arrive at balancing information indicative of the mechanical imbalance of the rotor and the magnetic imbalance of the rotor, thereby allowing the mechanical imbalance only to be corrected, wherein the operating step comprises calculating from the first and second information (S1, S2) and the first and second speeds the vector of the correction force that would counteract the imbalance of the rotor due to magnetic effects, and calculating, from the vector of the correction force and the first and second information (S1, S2), the respective force vectors (F1, F2) that are necessary to cancel out the mechanical imbalance at each of the first speed and the second speeds; and (d) calculating, from the force vectors (F1, F2), the mass and the position of such a mass that must be applied to the rotor to mechanically balance the rotor. a) performing a first balancing operation at a first rotational speed to detect a perceived imbalance in the rotor made up of a mechanical imbalance and a magnetic imbalance, and to ascertain first information indicative of the magnitude and direction of a first force needed to balance the perceived imbalance detected at the first speed; b) performing a second balancing operation at a second rotational speed to detect a perceived imbalance in the rotor made up of a mechanical imbalance and a magnetic imbalance, and to ascertain second information indicative of the magnitude and direction of a second force needed to balance the perceived imbalance detected at the second speed, wherein, at each speed, the perceived vector imbalance force is the vector sum of the vector imbalance force due to magnetic imbalance and the vector imbalance force due to mechanical imbalance; c) operating on the first information and the second information to arrive at balancing information indicative of the mechanical imbalance of the rotor and the magnetic imbalance of the rotor, thereby allowing the mechanical imbalance only to be corrected, wherein the operating step comprises calculating from the first and second information (S1, S2) and the first and second speeds the vector of the correction force that would counteract the imbalance of the rotor due to magnetic effects, and calculating, from the vector of the correction force and the first and second information (S1, S2), the respective force vectors (F1, F2) that are necessary to cancel out the mechanical imbalance at each of the first speed and the second speeds; and (d) calculating, from the force vectors (F1, F2), the mass and the position of such a mass that must be applied to the rotor to mechanically balance the rotor.