Accurate two-degree-of-freedom discrete-time current controller design for PMSM using complex vectors

Properties of the current controller are essential for permanent magnet synchronous machine (PMSM) drives, but the conventional continuous-time current controller cannot fully decouple the cross-coupling terms when applied in the digital processor. Its performance is related closely to the rotational speed. To improve the performance of the current loop, the direct design method in the discrete-time domain is adopted using the accurate discrete-time complex vector model. An integrated accurate hold-equivalent discrete model for PMSM is derived considering the difference between the output of the voltage source inverter and the back electro-motive force. Then an accurate two-degree-of-freedom (2DOF) current controller with a third-order closed-loop transfer function is designed. The 2DOF controller has more freedom in pole placement, and two schemes with a different cancelled pole-zero pair are investigated. Analysis is conducted by the robust root locus method via the complex vector root locus and sensitivity functions, showing properties in disturbance rejection and sensitivity to parameter variation of two schemes. Both schemes have their own advantages. Finally, the dynamic performance and flexibility of the proposed current controller is verified on a 2.5-kW PMSM test bench.