Investigation of a stator-ironless brushless DC motor with non-ideal back-EMF

This paper deals with the effects of permanent magnets (PMs) shapes and segmentation types on the trapezoidal degrees of the back electromagnetic force (EMF) and torque characteristics and rotor losses. The back-EMF waveforms with different PMs shapes and segmentation types are calculated through transient finite element method (FEM). It is found that the effects of PMs inward eccentricity and curvature have no influence on the shape of the back-EMF waveforms. The PMs asymmetrical segmentation with the middle sub-PMs applied with low magnetic energy product sintered ferrite magnet can obtain better trapezoidal back-EMF waveforms. Then, the torque characteristics and rotor losses of PMs' symmetrical and asymmetrical segmentations are investigated and evaluated. The results show that the PMs segmentation and applying low magnet energy product PMs of the middle sub-PMs can further improve the torque characteristics. Moreover, the outer PMs eddy current loss and rotor iron loss exhibit a great extent of decrement. Finally, a stator-ironless PM BLDC motor without the outer rotor PMs segmentation is manufactured and tested. The experimental results show good agreement with the FEM ones.