In all machine stator and rotor magnetic field rotates (stationary with respect to each other) at synchronous speed with respect to stationary observer.
For reduction of harmonics method is given below:
integral slot winding
larger air gap
Sr≠Ss : avoid cogging
Sr-Ss≠P,2P,5P avoid synchronous cusps
Sr -Ss ≠ 3P avoid magnetic locking
here, Sr is number of rotor slot, Ss is number of stator slot