Rotor losses in fractional-slot three-phase and five-phase PM machines

Fractional-slot PM machines have several advantages, but they suffer from the high contents of space harmonics in the air-gap MMF distribution. The MMF space harmonic amplitude and frequency depend on the particular combination of number of slots and poles. Such harmonics induce losses in the rotor since they are not synchronous with the rotor speed. This paper compares two kind of machines with the same stator lamination but with different number of phases, a three phase and a five-phase PM machine. 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 contents, it is shown that rotor losses reduction is lower than expected. Some considerations about the harmonic impact are given.