Performance comparison of a spoke-type PM motor with different permanent magnet shapes and the same magnet volume

Abstract

This study investigates the characteristics of a spoke-type permanent magnet (PM) motor with one of three magnet shapes, rectangular magnet (RM), trapezoidal magnet (TM), or inverted TM (ITM) in order to compare motor performance for the same magnet volume. The three magnet shapes vary in terms of upper thickness, lower thickness, and width of the magnet. Two-dimensional (2D) finite element method is performed to analyse torque, inductance, core loss, efficiency, and demagnetisation reliability. Advantages and disadvantages of each magnet shape are presented based on the analysis results. The ITM motor shows higher efficiency at its rated load due to the reduced core loss resulting from the decreased armature reaction field coupled with the increased magnetic reluctance circuit of the rotor near the airgap. However, the ITM motor shows lower maximum torque due to the reduced reluctance torque and demagnetisation reliability with a heavy over-current. ITM is thus suitable for applications that require low core loss with small starting torque, e.g. fans, TM is helpful for increasing maximum torque, and RM is appropriate for low-cost applications that require high demagnetisation reliability. Therefore, magnet shape should be carefully considered based on the required specifications of the motor application.