Research On Direct Torque Control Of Three - Level Inverter For Asynchronous Motor

High-voltage and large-capacity AC drive technology fed by the multi-level inverter is one of hot research points in the electric drivetrain. Because the Diode-clamped three-level inverter has advantages of low voltage requirements for devices, small harmonic contents, and good control performance, so it is one of the most widely used multi-level topologies. This paper presents the direct torque control systems of induction motor which fed by the two-level inverter and diode-clamped three-level inverter respectively, comparing the results of simulations.The paper analyses the mathematical model of the motor and introduces the idea of vector decomposition based on ISR method of direct torque control. In the dual-loop DTC system consisting of a flux loop and torque loop, the space vector can be obtained directly by simple mathematical calculations in the ?? coordinate. This method avoids the complex transformation of synchronous rotating and makes the system controller simple. In this paper, this approach is called the simplified ISR method of direct torque control.This paper analyses the principle of the three-level inverter and the reason of neutral-point voltage imbalance, then introduces a method which can control the fluctuation of neutral-point voltage by selecting and discarding the small vectors. This method can calculate switch status and action time by using space vector pulse width modulation(SVPWM) in a non-orthogonal g-h coordinate system.This paper gives direct torque control system of induction motor fed by three-level inverter, which is on the basis of simplified ISR method of direct torque control, neutral-point voltage control and SVPWM modulation strategy. This method can effectively reduce the ripple of flux and torque, simplify the controller of system and enhance the robustness of the system. The paper builds the two-level and three-level direct torque control system in Matlab/Simulink. The simulation results demonstrate that the simplified ISR methods and three-level inverter neutral-point voltage control method are effective.