Electromotive force compensation in direct torque control with discretized voltage intensities

This paper describes a way of electromotive force compensation in direct torque control algorithm with discretized voltage intensities. Theoretical background of DVI-DTC algorithm with discretized voltage intensities is presented. The main advantage of this algorithm is the change in number of available voltage intensities in order to reduce the torque ripple as required. The effect of induced EMF on torque increments within a single inverter switching cycle is analytically evaluated. The method for compensation of EMF effect with the goal of elimination of torque error in steady state is proposed. Experimental results are shown in the paper to illustrate the response speed of torque control with and without induced EMF compensation.

I. Introduction

Direct torque control, form the moment of its appearance until today, has undergone many modifications with the goal of eliminating the disadvantages of the method, the most important of which is the large torque ripple. In algorithms based on discrete voltage vectors and switching tables (ST-DTC) the torque ripple can be reduced by application of voltage vectors with different intensities. The method is described in many published papers. The authors in [1], [2] suggest the DSVM-DTC algorithm based on three basic voltage vectors during a single calculation cycle. In this way it possible to define several resulting voltage vectors with different intensity and direction. With the increase in number of voltage vectors, the switching table becomes more complicated, and the duration of calculation cycle needs to become longer to suppress high number of switching (four or more voltage vectors during one calculation cycle). The development of multilevel and polyphase (more than three phases) inverters has kept the switching table DTC algorithms in focus of the research community [3]. Although the algorithms are simple for implementation, higher number of available voltage vectors could lead to problems regarding the complexity of switching tables. The switching tables for inverters with more than three voltage levels become very complicated, therefore, the practical applications are scarce. The structure itself of the multilevel inverters introduces challenges in design and control of such inverters.