Influence of Magnet Imperfections on Torque Pulsation of PM Machines Having Different Pole and Slot Combinations

This paper investigates the influence of magnet imperfections on the torque pulsation of permanent magnet machines having different pole/slot combinations (2p/Ns), including fractional-slot as well as integer-slot topologies. The magnet imperfections result in changing in the cogging period, which in turn might reduce the effectiveness of skewing, and also introduce new harmonic orders that could lead to more noise and vibrations. It is found that such influence is reduced significantly with the number of structure cyclic symmetry, which is equal to the greatest common divisor between the pole and slot numbers. Hence, the imperfection is more severe in machines having similar numbers of poles and slots, i.e. 2p=N-s +/- 1 followed by 2p=N-s +/- 2, and the influence is relatively smaller in other fractional slot combinations as well as integral-slot machines.