Stator flux optimised Direct Torque Control system for induction motors

Abstract

Direct Torque Control (DTC) has, over the years, become one of the most popular methods of control for induction motors (IM) drive systems. There still remain, however, many disadvantages with this method of control, which have to be overcome. In the present paper we, therefore, consider a method of improving conventional DTC by reducing the undesirable torque ripple that can occur in the output torque. This new method, which is fully described in the paper, also has the additional advantage of maintaining the switching frequency of the power switching devices constant. The average torque ripple reduction achieved by this new method of control can be greater than 50%. This significant reduction in torque ripple is realised by the introduction of a two state modulation process between the active selected state from conventional DTC and an additional null state. The control of the two state modulation process is achieved by the application of an adaptive Fuzzy Logic controller (FLC). Additionally, a novel flux optimiser strategy is also introduced into the new DTC system, which together with the two state modulation process further reduces the torque ripple. Simulation and experimental results for the drive system presented are included in the paper and it is shown that not only good agreement exists between both sets of results but they also support the claims made in the paper.