الفهرس 
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Abstract
Many industrial applications require variable speed motor drives. The AC machines, especially induction types, have been widely used in such applications. In certain applications such as traction applications, speed is not the state of interest to be controlled. Instead, the torque is directly controlled, as it is seen of more importance from the load point of view.
The direct torque control (DTC) is a new scheme of induction motor torque contro~ and is considered nowadays one of the superior strategies compared to other different schemes such as scalar or vector control techniques.
The aim of this study is to present a new DTC scheme for asynchronous motors using the classical two-level six-switch voltage source inverter (VSI) when controlled by an innovated 1500 conduction-mode. This switching-mode results in a novel space-vector¬diagram (SVD) for the two-level inverters, consequently enables new switching-tables for the DTC scheme. The new DTC scheme quietly improves the motor performance compared to the classical method.
To ignore a practical limitation in the new DTC strategy, several understudy solutions are outlined. The implementation of an economic solution is an important item in our future work. Furthermore, this limitation has been completely overcame by applying a similar DTC strategy to the three-level (3L-DTC) instead of to two-level inverter. The novel 3L-DTC technique also shows a quite improved response.
New three-level voltage source inverter, that combines some desirable features of two- and three-Ievel inverters, is introduced. Also, a novel switching strategy referred as; open-leg switching strategy (OLSS) is introduced. It is shown that, controlling the two¬level VSIs by the proposedOLSS synthesizes a new 1500 conduction-mode resulting in a better system performance. Furthermore, controlling the three-Ievel VSIs by the OLSS adds new six space-voltage-vectors to the literature, which improves the system performance-at low modulation indices. For high modulation indices, another conduction-mode is proposed.
The systems are simulated using MA 11.ABlSIMULINK software. Comparisons are carried-out and recorded under the same operating conditions. Prototypes for two, three. and new three -level VSIs were constructed using MOSFET transistors and controlled by micro-controllers. The experimental and analytical results are figured out showing a good matching.