Model Predictive Direct Torque Control With Finite Control Set for PMSM Drive Systems, Part 2: Field Weakening Operation

The direct torque control approach called model predictive direct torque control (MP-DTC) is extended to field-weakening operation in this research. The controller is of the finite control set (FCS) type, which takes the discrete states of the voltage source inverter (VSI) into account. Each sampling period, the future behavior of the plant is predicted and inputs (voltage vectors) are selected using an optimization criterion, i.e., a cost function. Optimization, however, depend on the operation region. Maximum torque per ampere (MTPA) tracking for high electrical efficiency is aimed below rated speed, operation off the MTPA trajectory is necessary for obtaining field-weakening. In this work, a cost function is designed, which is suitable for operation at high speeds without penalizing operation below rated speed. MP-DTC is applied to permanent magnet synchronous machine (PMSM) drive systems. High control performances, i.e., dynamics, is obtained without increasing the switching frequency nor reducing significantly the torque quality. This feature is interesting above all for high power applications. The proposed control strategy has been evaluated on a small-scale (PMSM) drive system with two-level VSI for demonstration showing promising results.