Static Analysis and Eccentric Design Combining Permanent Magnets With Inner Rotor on Drag-Cup Motor

Precision attitude control in aerospace systems requires drag-cup permanent magnet brushless dc motors with extremely low torque ripple. Conventional permanent magnet sinusoidal shaping (PMSS) can reduce air-gap field total harmonic distortion (THD) but typically relies on a single eccentricity configuration, which can approach process or magnetization limits and produce suboptimal flux waveforms. This article introduces a novel combined eccentricity design approach that integrates permanent magnets (PMs) and inner rotor sinusoidal shaping (PMIRSS), guided by the equivalent surface current method and finite element analysis (FEA). Various eccentricity combinations are evaluated through static simulations and validated experimentally. The proposed design reduces air-gap THD to 2.26%, compared to 29.62% for the non-eccentric baseline, a reduction of 92.4% while maintaining a competitive maximum flux density (B_max). These results demonstrate a strong synergistic effect between rotor and PM eccentricity. Based on the results, this article proposes recommendations for normalized eccentricity and an appropriate B_max interval to improve cross-scale compatibility. The results show that simultaneously optimizing the eccentricity of the rotor and PMs can produce uniform air-gap flux, suppress high-order harmonics, and avoid local saturation, resulting in smoother torque and minimized ripple. The combined eccentric motor is ideal for high-precision aerospace applications such as control torque gyroscopes, magnetically levitated flywheels, and high-power precision drives.