Predictive direct torque control for a synchronous reluctance machine with predetermined stator flux compensation down to zero speed

This paper presents enhancements of a predictive direct torque control for synchronous reluctance machines down to zero speed. The proposed control scheme is used when high-performance torque control is desired with the advantages of constant switching frequency and relative long sampling time. To get that, the control algorithm calculates one step in advance the switching instants of two possible active voltage space phasors that build the demanding torque. Next, the trajectory of the stator flux is predicted with these two pre-selected voltage space phasors and the optimum of them, which leads to the best trajectory of the stator flux at the end of the cycle, will be applied to the machine. At very low speeds a second active voltage space phasor is used to counteract the drop of the stator flux. In this paper it is shown, how the switching instant of this second voltage space phasor can be successfully predetermined before the switching instants of the other two voltage space phasors for torque and flux control as well as the criteria for its appropriate selection. The proposed algorithm has been implemented by using a digital signal processor (DSP) and a field programmable gate array (FPGA) embedded in the same board. Experimental results at very low and zero speed using a commercially available machine verify the effectiveness of the enhanced control scheme.