الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Many industrial applications reqUIre variable speed motor drives.
Traditional1y, dc motors have been used in such applications. On the other side, ac machines, especial1y induction types, exhibit highly coupled, nonlinear, multivariable structures as opposed to dc machines. However, recent advancements in semiconductor power electronics and micro-controllers have made it possible to use ac motors in many variable speed drive applications.
More than decade ago, direct torque control (DTC) was introduced to give fast and good dynamic torque response. DTC can be considered as an alternative to the field oriented control (FOC) technique. The DTC was proved as a powerful method for control1ing induction motors because its fast behavior (due to the closed-loop control of the flux and torque by hysteresis control1ers) and its simple structure (due to the lack of current loops and shaft sensor as its basic configuration consists of a pair of hysteresis comparators, Torque / Flux estimator and a lookup table) [25].
However, The DTC has some drawbacks. First, switching frequency varies according to the motor speed and the hysteresis bands of the torque and flux. Second, large torque ripple is generated especial1y in low speed region because of small back EMF of the induction machine, and third, high control sampling time is required to achieve good performance[8].
DTC technique, in its basic construction, depends on applying the voltage command (the active voltage vector) for the whole sampling period which pushes the torque error to get out of the torque upper band-limit. So, the system applying a successive zero voltage vector to reduce the torque error which pushes the torque to get down below the lower band-limit.
Based on the above, many researches were done to identify the on duration in which the active voltage vector selected from the DTC switching table is applied during the sampling time in order to minimize the P-P torque error.
This thesis presents the conventional Direct Torque Control ”DTC” versus different methods to highlight the improvement we gain in the P-P torque error with the new methods. A complete comparison was derived using Matlab-Simulink package under the same circumstances of the motor parameters, reference values and the motor load.