Cogging analysis for fractional slot/pole permanent magnet synchronous motors

Analytical and FEA predictions of cogging torque are compared with experimental results for a range of non-skewed PMSMs of similar size but with slot/pole ratios of 18/8, 12/10 and 30/8 (fractional) together with 30/10 and 18/6 (integer). The two analytical methods and the experimental data show close agreement and predict that stator or rotor skewing would be required to reduce the cogging torque to acceptable levels for the motors with the integer ratios, but the cogging torques for the fractional ratios are sufficiently small to obviate the need for skewing, thereby reducing the manufacturing operations. Finally the 12/10 ratio is recommended since this yields a higher stall torque than the 18/6 or 30/8 ratios. The explanation of this is that the 12/10 ratio enables a higher slot fill to be obtained than the 30/8 ratio. Also the winding overhang can be significantly reduced for the 12-10 design with a segmented stator in contrast with all the other slot pole combinations.