An Experimental and Analytical Investigation of the Direct Torque Control Method of a Three-Phase Induction Motor

The Direct Torque Control (DTC) method is an important technique used to control the torque and flux of AC motors. This method is attractive due to both its simplicity, robustness, and its wide range of applications, such as robotics and electric cars. Three-phase motors have been extensively used in various industrial applications. The basic DTC algorithm is based on the direct drive of the Voltage Source Inverter (VSI), i.e., the appropriate voltage vector (VE) is applied directly without a Pulse Width Modulation (PWM) stage. This vector is applied as long as the stator flux vector moves in the same sector. In fact, researches that interested in studying the effect of the VEs with the change of operating conditions (the speed and load) is almost limited. In this paper, a full analysis of the VEs is presented to determine the accurate effect on both the torque and the stator flux throughout the entire sector with the change of the speed and the load. The results were validated by means of laboratory measurements using a platform with dSPACE-DS1103 card. The analytical analysis showed that the application of specific VEs results in false switching states in a specific range of the sector with the change of speed, load, or both. More specifically, some switching states are not fully defined. These wrong states are called uncontrollable angles. Throughout this research, It has been possible to accurately determine the values of these angles within a wide speed range with the change of load. In fact, the results from this research are a starting point for researchers to devise new analytical methods to reduce the values of these angles as much as possible, which will positively affect the response of both flux and torque in this algorithm.