Thesis (M.Sc.) - Cairo University - Faculty of Engineering - Department of Electrical Power and Machines

In this thesis a direct torque control (DTC) system of three - phase induction motor (IM) using three - phase three - level (3L) neutral-point-clamped (NPC) inverter is proposed. An analytical study is performed to construct a novel switching table (ST) which is mandatory for 3L - DTC operation. Based on the analytical study, the ST is subdivided into three parts according to motor speed where different groups of voltage space vectors (VSVs) are assumed for each part to ensure best performance. The proposed 3L-DTC system is modeled and built in MATLAB/Simulink. Simulation analysis for 3L - DTC is held against two - level DTC (2L - DTC) in the three operating ranges. Results include speed response, torque and flux ripples, voltage and current quality, common-mode voltage and switching frequency. A simple solution for flux drooping problem related to DTC at low speed is also introduced. The problem of neutral-point (NP) voltage deviation related to the NPC inverter is also solved. Comparison results show the effectiveness of the proposed system with respect to 2L - DTC. A 3L - NPC inverter prototype is implemented in the laboratory. The advantages of 3L - NPC inverter over 2L inverter are verified experimentally

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