Operation Principle and Torque Component Quantification of Short-Pitched Flux-Bidirectional-Modulation Machine

This paper analyzes the operation principle of a short-pitched fiux-bidirectional-modulation machine, with especial efforts in terms of its torque component quantification and back EMF harmonics reduction. With the bidirectional-fiux-modulation effect, increased equivalent permeance of the air gap is constructed to enhance the machine torque production. Meanwhile, with the shorted-pitched distributed winding design, the electrical gear ratio of machine is increased to realize boosted torque production, and a variety of back-EMF harmonics are eliminated, thus the torque ripple is reduced. In this paper, the machine operation principle is analyzed based on a simple magnetomotive force-permeance analytical model. The torque components and back EMF harmonics are deduced by winding factor analysis. It is revealed that in the proposed machine, the excitation fields with pole pairs of 2, 22 and 46 have generated the effective backEMF and constant torque and their contribution percentages are 77%, 20% and 3% respectively. Rotor-PM and stator-PM contribute 59% and 41% of the torque respectively. With the 5/6 short-pitch design, the 3rd, 5th and 7th back-EMF harmonics are reduced by 55%, 88% and 88% respectively and the THD of back-EMF is reduced 56.8%. A prototype is fabricated and tested for experimental verification.