Analysis of Dominant Electromagnetic Force Harmonics in External-Rotor SPM Machine Considering Tangential Force

The radial electromagnetic vibration of the rotor is critical for the electromagnetic noise of external-rotor (ER) surface-mounted permanent magnet (SPM) (ER-SPM) machines. Due to the similar permeability of SPM and airgap, it is proven that both radial and tangential electromagnetic force density (EFD) can induce the radial electromagnetic vibration of ER-SPM machines. However, it is still challenging yet to determine the dominant EFD harmonics that contribute to the radial vibrations of ER-SPM machines, which is essential for the design of ER-SPM machines affected by abundant EFD harmonics. This article proposes a method for identifying the dominant EFD harmonics in ER-SPM machines. The principles and implemental procedures of the proposed method are detailed. Then, the method is applied to an ER-SPM machine to identify the dominant EFD harmonics at rated speed under open-circuit condition. Moreover, its practical feasibility and universal applicability are further confirmed. The effectiveness of the method is validated through couple-field finite element analysis (FEA) and experiments.