Fractional slot windings based on concentrated tooth coils are frequently used in today's electric machines. A drawback of these windings is the occurrence of large space harmonics in the air gap, which can cause torque ripple issues and rotor additional losses. Various provisions have been proposed in the literature to improve the air-gap harmonic content due to concentrated-winding armature reaction. This paper proposes an enhancement based on a multiple-layer design characterized by tooth coils with different number of turns. The improvement is proposed for 12-slot 10-pole permanent magnet machines but can be generalized to other possible topologies. It is shown that the proposed approach leads to the same air-gap field harmonic content as for a dual-three-phase dual-layer winding with all space harmonics reduced by the same coefficient (equal to 0.928). Pros and contras of the proposed design with respect to other existing improved solutions are highlighted in the paper. Comparisons and assessments are provided based on Finite Element Analysis (FEA).
