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Flat Magnets in Surface-Mounted Permanent Magnet Machines

The influence of flat magnets (FM) in surface- mounted permanent magnet machines (PMM) is analyzed taking the conventional single curved magnet (SCM) configuration as reference. The flat shape of magnets and the air gap between them increase the magnetic circuit reluctance and distort air gap flux density. As a consequence, the back electromotive force (BEMF) decreases due to a reduction in the fundamental harmonic of the air gap flux density. This reduction depends on air gap geometry (pole arc, rotor external diameter, magnets span, number of FM...) and it can be analytically calculated by means of the magnetic circuit approach. Air gap flux density reduction factor is used as design criterion to select the optimum FM number. The analytical study is contrasted with finite element method simulations of different PMM geometries with both SCM and FM configurations. The results show that the error in the analytical calculation is smaller than 5%. A particular FEM methodology is used in order to reduce time computation. A PMM prototype with four FM per pole is tested to validate the study.