Slot Harmonic Impact on Rotor Losses in Fractional-Slot Permanent-Magnet Machines

Fractional-slot permanent-magnet (PM) machines have several advantages, but they suffer from the high content of space harmonics in the air-gap magnetomotive force (MMF) distribution. The MMF space harmonic amplitude and frequency depend on the particular combination of numbers of slots and poles. Such harmonics induce losses in the rotor since they are not synchronous with the rotor speed. This paper compares several kinds of machines with the same stator lamination but with different numbers of phases; three-, five-, and seven-phase PM machines are considered. Several winding configurations are compared, changing the number of poles but maintaining the same number of slots. Although in each case there is a reduction in terms of harmonic content, it is shown that rotor loss reduction is lower than expected. Some considerations about the harmonic impact are given. Although no experimental results are given, this paper refers to previous works in which rotor losses have been investigated and measured experimentally.