Rotor loss in permanent-magnet brushless AC machines

The eddy-current loss in the permanent magnets of brushless ac machines is usually neglected, since the fundamental air-gap held usually rotates in sy nchronism with the rotor, and time harmonics in the current waveform and sp ace harmonics in the winding distribution are generally smell, However, an important category of brushless ac machine design is emerging in which the fundamental component of the stator MMF has fewer poles than the rotor, the torque being developed by a higher order MMF harmonic. The fundamental and lower order MMF harmonics can then give rise to significant rotor eddy cur rents. An analytical model is developed to predict rotor-induced eddy curre nts in such machines, and to quantify the effectiveness of circumferentiall y segmenting the permanent magnets in reducing the rotor loss.