Study on the ratio of outer diameter to stack length suitable for high torque and high power in IPMSMs

In many applications that use IPMSMs, the miniaturization of the motor is desired from the viewpoint of space saving and manufacturing cost reduction. To achieve this miniaturization, it is necessary to make the motor's torque density and power density high. In this paper, the influence of the ratio of the outer diameter to the stack length (D/L) on the maximum torque and power in IPMSMs is examined. In models that change the cross section of the motor structure in proportion to D, the maximum torque increases as D/L increases for all types. However, the power decreases as D/L increases for all types. Moreover, the spoke-type model has high torque when D/L is small, and the V-shaped model has high torque and high power when D/L is large. In the models where the motor volume, magnet volume, and electric loading are equal regardless of D/L, the eight-pole models have a high torque density and high power density at D/L=2.42–4.01. Moreover, the eight-pole models at D/L=3.05 have high efficiency at 3000 min−1 and 6000 min−1. Therefore, in comparing the models, the eight-pole model with D/L=3.05 has a high torque density, high power density, and high efficiency in a wide speed range.