Design considerations on fractional-slot fault-tolerant synchronous motors

This paper presents some design considerations of fault-tolerant synchronous motors, characterized by a fractional number of slots per pole per phase. The first advantage of this configuration is a smooth torque, because of the elimination of the periodicity between slots and poles. The second one is a higher fault-tolerant capability making the machine able to work even in faulty conditions. However, the fractional-slot configuration presents a high contents of M.M.F. harmonics that may cause an unbalanced saturation and thus an unbearable torque ripple. A method to design fractional-slot motors is illustrated in the paper, including double-layer and single-layer winding. The analytical computation is extended to determine the harmonics of M.M.F. distribution. Their effect is highlighted in isotropic as well as anisotropic motors. Finally some considerations are reported to avoid unsuitable configurations