Influence of magnet shape on cogging torque and back-emf waveform in permanent magnet machines

This paper reports the influence of the magnet shape on the cogging torque and the back-emf waveform of permanent magnet machines. Two prototype stators, together with several rotors, are employed. The first stator has 6 slots and a concentrated coil wound on every tooth, while the second stator has 12 slots and a full-pitched overlapping winding. The 4-pole rotors are equipped with surface-mounted magnets. The influence of magnetization and sinusoidal outer surface is investigated. In particular, the sinusoidal magnet shaping of arc magnets (i.e. magnet with circular bottom) and bread-loaf shaped magnets (i.e. magnet with flat bottom) are compared. It shows that the sinusoidally shaped magnets result in not only low cogging torque, but also more sinusoidal back-emf waveform. However, shaping the magnets with circular bottom, the peak of back-emf may be reduced, which can be avoided by employing either an overlapping winding stator or a bread-loaf shaped magnets.