Encoderless predictive direct torque control of the synchronous reluctance machine at low and zero speed

This work deals with a Predictive Direct Torque Control (PDTC) for the Synchronous Reluctance Machine (SynRM) with and without encoder at very low speeds including zero speed. This method to control stator flux and electromagnetic torque of the machine combines the good dynamic of direct torque control (DTC) with the advantages of constant switching frequency and relative long sampling time. PDTC is a model-based, predictive control, which calculates the switching instants of two possible active voltage space phasors to build the demanding torque one step in advance. 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, is applied to the machine. At very low speeds a second active voltage space phasor is used to counteract the drop of the stator flux. The optimum switching instant of this second active voltage space phasor as well as the criteria for its appropriate selection are explained in detail.