Field weakening in flatness-based torque control of saturated surface-mounted permanent magnet synchronous machines

This paper presents an expansion of a flatness-based torque control strategy for saturated surface-mounted permanent magnet synchronous machines (PMSMs) to systematically account for the voltage limits of the voltage source inverter in the higher speed range of the machine. Field weakening operation is incorporated in a feedforward manner by solving a constrained nonlinear optimization problem. Different to existing works, magnetic saturation is systematically considered in the controller design to track the reference trajectories. The proposed torque controller exhibits a two-degrees-of freedom control structure with a flatness-based feedforward controller and a time-variant feedback controller for the trajectory error system. The torque controller is formulated in the stator-fixed reference frame and hence does not need any coordinate transformation. Furthermore, the proposed control structure covers the whole maximum torque per ampere (MTPA) and field weakening (FW) region without any switching in between. Implemented on an experimental test bench, an accelerated run-up of the PMSM with approximately four times the rated torque and up to twice the rated speed is performed in all four quadrants of operation to demonstrate the feasibility of the proposed torque controller in the entire operating range including significant magnetic saturation.