Effect of Increasing Pole Number on the Harmonic Content of Air-gap Flux Density Waveforms in Electric Machines

Abstract

This paper studies the effects of increasing the number of rotor poles on air gap flux density waveforms. The aim is to reveal the harmonic behavior of the working flux in the air gap, which controls the performance, and the operational behavior of the whole machine. A permanent magnet motor is used to perform the analysis and study the effects. Its original design contains 6 rotor poles and 36 stator slots with three phase currents embedded. For the comparison purpose, the rotor is redesigned to have 4, 8, and 12 poles mounted on the same rotor iron surface and the excitation currents were rearranged so as to produce 4, 8, and 12 poles respectively without changing the stator structure. FEM analysis is adopted to obtain the working flux waveform, and the Fourier filter is used to extract its harmonic components. The result demonstrates that there are more harmonic components contained in the working flux as the number of slots per pole per phase decreases.