Torque Adaptive Hexagon Voltage Extension Method For PMSM Flux-Weakening Control Based on Dual PI Cascade Structure

Hexagon voltage extension is an effective method to improve the maximum output torque of permanent magnet synchronous motors (PMSMs) in flux-weakening (FW) region. But, the hexagon voltage extension can cause the voltage harmonic inevitably, leading to torque ripple. In this article, it is proved that the increasing rate of voltage harmonic gradually increases during the extension from the inscribed circle voltage to the hexagon voltage. To avoid the voltage harmonic at low torque demand conditions, it is expected that the hexagon voltage extension is only employed at the maximum torque demand conditions. However, the conventional methods employ the nonadjustable hexagon voltage in the whole torque range. To realize the torque-adaptive hexagon voltage extension and suppress the voltage harmonic at low torque demand conditions, this article proposed the dual PI cascade voltage control structure. Two PI controller are designed for adjustments of the extended voltage trajectory and the flux amplitude, respectively. By establishing the Rouse table, the stable conditions of the proposed method are calculated. The effect of suppressing voltage harmonic is theoretically demonstrated. Finally, the comparative experiments are conducted to verify the effectiveness of the proposed method.