Motor constant of rectangular and skew windings in a high-speed slotless PM motor

Abstract

A slotless motor, generally used in high speed and small sized applications, has several winding patterns such as rectangular, rhombic, skew, and hexagonal windings. Skew and rectangular windings are mostly applied to slotless motors because they have high stiffness due to their self-supporting structure and because they have smaller resistance due to their short winding length. In this paper, the characteristics of a slotless motor with a parallelly magnetized permanent magnet composed of two poles are investigated by comparing the motor constant of rectangular windings with that of skew windings. The torque and resistance of the slotless motor are mathematically derived for each winding pattern. The height to diameter ratio of a motor with a specific volume is a crucial factor in the initial design because it affects the motor performance. Therefore, the motor constant, proportional to the efficiency, is optimized with different height to diameter ratios for each winding pattern. The results show that a slotless motor with skew windings has higher efficiency than that with rectangular windings. The results also showed that skew windings are more appropriate when the height to diameter ratio is low while rectangular windings are optimal when the height to diameter ratio is high. Additionally, within the height to diameter ratio range where both windings are applicable, skew windings have higher efficiency than rectangular windings. The proposed design method was validated by comparing the numerical results of the proposed method with the results obtained with three-dimensional finite element analysis. This research will contribute to the creation of a selection guide for winding patterns in the initial design process for slotless motors.