An Improved Observer-Based Current Decoupling Method for Mitigating Speed Fluctuation of Variable Flux Memory Machines

Variable flux memory machines (VFMMs) usually need to inject d-axis current pulses in the stator windings to conduct magnetization state (MS) manipulations, which inevitably causes large torque pulsation and speed fluctuation especially under load condition. In this article, an improved observer-based current decoupling method is proposed to calculate the q-axis current (i(q)) reference for speed fluctuation mitigation based on the estimated dq-axis flux linkages. Specifically, an improve stator current observer with a supertwisting sliding mode regulator is structured to more accurately estimate the voltage disturbances caused by parameter mismatches. Then, the dq-axis flux linkages are decoupled from the voltage disturbances estimated by the current observer, considering dynamic changes of PM flux linkage and inductances during MS manipulations. Based on the estimated dq-axis flux linkages, an i(q) reference can be calculated according to the torque equation and the divergence of i(q) can be avoided by using a piecewise function when the denominator is close to 0. Finally, the effectiveness and feasibility of the proposed method are validated through both simulation and experimental measurements on a separated series-parallel VFMM.