Analytical magnetic field distribution of slotless brushless PM motors. Part 2: Open-circuit field and torque calculations

Abstract

This study presents the open-circuit analytical magnetic field distribution of slotless brushless permanent magnet motors equipped with surface-mounted magnets. Although the focus of the study is on open-circuit magnetic field analysis, by augmenting the open-circuit magnetic field distribution with the armature reaction field distribution obtained from the first part of the series of two studies, the electromagnetic torque is calculated. The electromagnetic torque is calculated for both internal and external rotor motors with six different magnetisation patterns and three different armature current waveforms. The analysis domain consists of eight annular concentric regions: motor shaft, rotor back-iron, magnets, retaining sleeve, air-gap, winding, stator back-iron and exterior regions. Based on the open-circuit magnetic field distribution, the induced back-electromotive force (EMF) is also calculated. The validity of the proposed model is confirmed by comparing the analytical results with those obtained from the finite element method and an experimental set-up.