Direct Torque Control Of Induction Motor Of NPC Three-level Inverter

In the field of power electronics and power transmission,the use of multi-level inverters for large-capacity variable-frequency speed control systems has attracted attention.Compared with two-level inverters,multi-level inverters have the characteristics that output voltage is closer to sine wave,less harmonic content,and less voltage jump,which is very suitable for high-voltage and large-capacity AC variable frequency speed control systems.However,as a new type of power conversion technology,high-performance speed control based on induction motor powered by multi-level inverter still has a lot of problems worthy of study and discussion in theory and practice.Based on the diode-clamped(NPC)three-level inverter,this thesis conducts a comprehensive study of the control method of the inverter,and analyzes the direct torque control(DTC)of the induction motor and studies the direct torque.The NPC three-level inverter in the control system focuses on the two issues of midpoint potential fluctuation and generation of specific higher harmonics,and uses a hybrid control method to control the three-level inverter.This thesis first introduces the background and research significance of the multi-level inverter,and analyzes its research status,and then briefly describes the development status of the direct torque control system.Aiming at the problem that the midpoint potential of the NPC three-level inverter will fluctuate,a solution is proposed,and the neutral point potential balance control is implemented using a spatial pulse width vector modulation(SVPWM)control method.Because the NPC three-level inverter generates certain higher harmonics and increases the equipment loss when the switch is in operation,in order to solve this problem,an improved special harmonic elimination pulse width modulation(SHEPWM)technology is used to eliminate Specific harmonics.On the basis of ensuring the neutral point potential balance control of the three-level inverter,the output phase voltage waveform of the three-level inverter is decomposed into a Fourier series,and the fundamental wave modulation degree is set to m.The wave lists a set of nonlinear transcendental equations.The improved genetic algorithm is used to solve the equations,and the switch angle is used to control the three-level inverter.The two control methods are mixed and controlled to achieve smooth switching.Using MATLAB/Simulink simulation analysis,the results show that the specific higher harmonics of the NPC three-level inverter are eliminated,the waveform quality is significantly improved,and a high-quality three-level voltage waveform is output.In the end,the three-level inverter is added to the direct torque control system of the induction motor,and the inverter is controlled by the improved hybrid control method.The PI adjustment idea is introduced in the direct torque control,and the control mode of the three-level inverter is separated from the direct torque control system,and the control is effectively reduced.The complexity is easy for the system to be implemented.The simulation analysis is carried out through MATLAB/Simulink and compared with the direct torque control method using two-level inverters.The results show that the torque and speed pulsation of the motor have been significantly reduced.