Performance of a brushless DC motor due to the axial geometry of the permanent magnet

This paper investigated the skewing effect of the permanent magnet through three dimensional finite element analysis of a brushless DC motor, the principle of virtual work and Maxwell stress tensor by analyzing the mechanism and the characteristics of the production of the torque and the unbalanced magnetic force in the axial direction. It also suggested the novel shape of a herringbone magnet which increases the torque efficiency when rotated only in one direction. Skew magnets reduce the cogging torque by canceling the tangential force from the upper and the lower magnet because it has an anti-symmetric structure of permanent magnet with respect to the mid-plane. They produce the unbalanced magnetic force in the axial direction. However, herringbone magnets produce the increased cogging torque in any position due to the symmetry of the permanent magnet with respect to the mid-plane, which results in the large commutation torque. They do not produce the unbalanced magnetic force in an axial direction.