Permanent-magnet brushless machines with unequal tooth widths and similar slot and pole numbers

This paper presents a comparative study of three-phase permanent-magnet brushless machines in which the slot and pole numbers are similar, with reference to conventional brushless dc machines in which the ratio of the slot number to pole number is usually 3 : 2. Three different motor designs are considered. Two have equal tooth widths, with one having a coil wound on every tooth and the other only having a coil wound on alternate teeth, while the third machine also has coils wound on alternate teeth but these are wider than the unwound teeth while the width of their tooth tips is almost equal to the rotor pole pitch in order to maximize the flux linkage and torque. Analytical and finite-element methods are employed to predict the flux-linkage and back-electromotive-force waveforms, and the self- and mutual-inductances, and these are shown to be in good agreement with measured results. It is also shown that the third machine is eminently appropriate for brushless dc operation. Index Terms--Back electromotive force (EMF), brushless machines, flux linkage, fractional slot, permanent magnet.