OPTIMIZATION OF MAGNETIC-POLE GEOMETRY FOR FIELD HARMONIC CONTROL IN ELECTRIC MOTORS

A principal source of vibration in permanent magnet motors and generat ors is the induced stress from the rotating permanent magnets. The har monic content of this forcing function may excite resonant modes of vi bration in the motor or surrounding structure. Thus attenuation of spe cific harmonics is of considerable interest. This paper describes a me thod for optimal shaping of the permanent magnets to eliminate one or more of these harmonics. The analytical model for an optimized 4-pole motor consisted of segmented PMs and a solid ring stator. The permanen t magnets were modeled as a number of thin radially cut annular layers with specific sector angles. Changing the shape of the PMs resulted i n a different flux density field and thus a different frequency spectr um of the forcing function. Attenuation of specified higher harmonics could be achieved at the expense of increasing other harmonics. For a 4-pole motor, the optimization algorithm was fairly successful at elim inating any one of the 8th, 12th or 16th harmonics. The algorithm used was developed to solve combinatorial optimization problems, and drew heavily upon principles from statistical mechanics. The final pole geo metry is dependent upon the choice of the cost function used in the op timization algorithm.