ABSTRACT
In this, an improved Direct Torque Control (DTC) strategy for a Squirrel Cage Induction Machine (SCIM) powered by two level voltage source inverter (VSI). This strategy is based on the fuzzy logic controller. The main objective is to improve the performance of the system by reducing electromagnetic torque ripples and improving the currents shape by optimization of the total harmonic distortion (THD). The PI controller gains are tuned with the help of Fuzzy control in the speed control loop. The hysteresis regulators and voltage vectors selection table of the conventional DTC are replaced by fuzzy logic blocks to realize fuzzy DTC control. These two control strategies are simulated in the MATLAB/SIMULINK environment followed by a comparative analysis to validate the effectiveness of the proposed strategy. Many improvements in term of rise time, torque ripples, flux ripples and current harmonics have been done, namely stator and rotor flux ripple and torque ripple have been reduced. The stator currents THD have been also reduced.
INTRODUCTION
Over the previous decades, DC machines were utilized broadly for variable speed applications because of the decoupled control of torque and flux that can be accomplished by armature and field current control individually. DC drives are worthwhile in numerous perspectives as in conveying high starting torque, simplicity of control and non linear performance. But due to major drawbacks of DC machine like presence of mechanical commutator and brush assembly, DC machine drives are using rarely for industrial applications. The robustness, low cost, the better performance and simple maintenance create the asynchronous motors advantageous in several general or industrial applications. Squirrel cage induction motors (SCIM) are most generally used than remainder of electrical motors as they need all benefits of AC Motors and are cheaper in price compared to slip ring induction motors. Also, it requires less maintenance and has got a rugged construction. As slip rings are absent in SCIM, brushes maintenance duration and price associated with the wear and tear of brushes are reduced. Due to these advantages, the induction motors have been the execution element of most of the electrical drive system for all connected aspects: starting, braking, speed change and speed reversal etc. The basic concept of direct torque control (DTC) of inductionmotor is to regulate each stator flux-linkage and electromagnetic torque of the machine at the same time by employing a switching vector look-up table. As voltage, current and speed are the key issue in the direct torque control, these are the solely governable factors within the induction motor. The basic concept of direct torque control (DTC) of IM drives is to regulate each stator flux-linkage and electromagnetic torque of the machine at the same time by employing a switching vector look-up table. The DTC structure is simple compared to a vector control rule, because it will not need coordinate transformation and voltage modulation block, and thus, can be implemented comparatively simply In addition, it has fast dynamic performance. While DTC is widely used as a result of these benefits, it has disadvantages like high ripples in torque and attainable problems throughout start or low speed operation and through changes in torque command. To overcome these shortcomings, various approaches for flux and torque ripple reduction. The basic scheme of DTC is preferred in high power range applications, where a lower inverter switching frequency will justify higher current distortion. DTC allows a sensible torque control in steady state and transient operating condition

Block Diagram