Direct torque control of induction motors: Stability analysis and performance improvement

Practitioners in the electric drives community have recently introduced a novel torque control strategy for high power induction motor applications called direct torque control (DTC), which has three distinctive features: 1) it focuses on stator (instead of rotor) flux regulation; 2) in contrast to classical field-oriented control, it does not aim at an asymptotic system inversion (hence it does not require additional current loops); and 3) it explicitly takes into account the discrete nature of the control actions, which are simply points of a finite set obtained from a switching logic. Besides its simplicity, it is claimed that the achieved performance is (in some instances) superior to field oriented strategies because of the digital form of the control structure and the reduced dependence on parameter variations of the stator flux calculations. Our objective in this paper is twofold, first to carry out a mathematical analysis of the stabilization mechanism of DTC, which helps us assess the achievable performance of the current scheme and provide guidelines for its tuning. Second, to propose a modified DTC to improve its dynamical behavior. Experimental results are presented to illustrate the main points of our paper.