Two Approaches For Direct Torque Control Using a Three-Level Voltage Source Inverter R. Zaimeddine 1 - E.M. Berkouk 2 Department of Electrical Engineering University of Mouloud Mammeri ,Hasnaoua II ,Tizi Ouzou ALGERIA Abstract: - The object of this paper is to study a new control structure for sensorless induction machine dedicated to electrical drives using a three-level voltage source inverter (VSI). The output voltages of the three- level VSI can be represented by four groups : the zero voltage vectors, the small voltage vectors, the middle voltage vectors and the large voltage vectors in (d , q) plane. Then, the amplitude and the rotating velocity of the flux vector can be controlled freely. Both fast torque and optimal switching logic can be obtained. The selection is based on the value of the stator flux and the torque. Both approaches, classical DTC and fuzzy logic controller, are simulated for a induction motor. The results obtained show superior performances over the FOC one without need to any mechanical sensor. Key Words: - Direct Torque Control, Field Oriented Control, Induction motor, Sensorless Vector Control, Flux Estimators, Switching Strategy Optimisation, Multi-level inverter, Neural-point clamped, fuzzy control. 1 Introduction The rapid development of the capacity and switching frequency of the power semiconductor devices and the continuous advance of the power electronics technology have made many changes in static power converter systems and industrial motor drive areas. The conventional GTO inverters have limitation of their dc-link voltage. Hence, the series connections of the existing GTO thyristors have been essential in realizing high voltage and large capacity inverter configurations with the dc-link voltage [1]. The vector control of induction motor drive has made it possible to be used in applications requiring fast torque control such as traction [2]. In a perfect field oriented control, the decoupling characteristics of the flux and torque are affected highly by the parameter variation in the machine. This paper describes a control scheme for direct torque and flux control of induction machines fed by a three-level inverter using a switching table. In this method, the output voltage is selected and applied sequentially to the machine through a look –up table so that the flux is kept constant and the torque is controlled by the rotating speed of the stator flux. The direct torque control (DTC) is one of the actively researched control scheme which is based on the decoupled control of flux and torque providing a very quick and robust response with a simple control construction in ac drives [3,4]. 2 Three-Level Inverter Topology Fig . 1 shows the schematic diagram of neutral point clamped (NPC) three-level VSI. Each phase of this inverter consists of two clamping diodes, four GTO thyristors and four freewheeling diodes. Table. 1 shows the switching states of this inverter. Since three kinds of switching states exist in each phase, a three level inverter has 27 switching states. Fig. 1. Schematic diagram of a three-level GTO inverter Proceedings of the 6th WSEAS Int. Conf. on FUZZY SYSTEMS, Lisbon, Portugal, June 16-18, 2005 (pp104-110)