A novel radial magnetic-field-modulated brushless double-rotor machine (MFM-BDRM), which is the key part of compound-structure permanent-magnet synchronous motor (CS-PMSM), is proposed in this paper. The speed and torque relations of the first-stator magnetic field, the first PM rotor and the modulating ring rotor are investigated by analytical method. The harmonic components of magnetic field in the inner and outer air gap are analyzed by finite-element method (FEM), when the PM magnetic field and armature field work independently. Furthermore, electromagnetic performances of two typical schemes are analyzed, which shows that torque ripples are the major problem of the radial MFM-BDRM. Some methods are proposed to reduce the torque ripples and validated by FEM.
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