The Evaluation of Optimal Ratio of Outer Diameter to Axial Length on Torque Density for the External Rotor PMSM

The high torque density external rotor permanent magnet synchronous motors (ER-PMSMs) are desired in numerous applications today. The ratio of rotor diameter to axial length and rotor diameter to stator diameter are key parameters which has great impact on the torque density. This paper investigated the optimum ratio of outer diameter to axial length for maximising the torque density of the ER-PMSMS. It was found that when the ratios of outer diameter to axial length and rotor diameter to stator diameter are 3.1 and 1.11, respectively, the torque of model motor reaches the maximum. It is also found that the volume of the motor has little effect on the selection of the optimum ratio within the proposed torque range.