Fractional-slot concentrated windings (FSCWs) are characterized with many advantages. However, their magnetic fields have rich space harmonics. These harmonics, especially tooth harmonics, have negative effect on
machine performances. This paper proposes a new method to reduce the tooth harmonics in FSCW permanent- magnet (PM) machine, in which the magnetic distribution in air-gap will be affected by winding span
radian. Also, a new stator structure is designed on the method. By using the new stator design, each order tooth harmonic can be reduced significantly and the working harmonic can also be increased slightly. Simultaneously, the unbalanced radial magnetic force can be significantly weakened. The basic principle of tooth harmonic reduction method can be analyzed by using the stator structure of the FSCW machine with single layer windings. Then, a three phase 18-slot/16-pole FSCW machine with double layer windings is newly designed, which can effectively weaken the tooth harmonic components of FSCW machines. It is shown that, under the fixed electrical and geometrical constrains, the designed FSCW machine has better performances, such as loss, vibration and noise, than the conventional ones.