Research On Direct Torque Control Strategy Of Asynchronous Motor Fed By Three-level Inverter

Compared with two-level inverters,the multilevel inverters have the characteristics of sinusoidal output voltage,small harmonic,low voltage stress on switches and low switching frequency,which is suitable for high-voltage high-power variable frequency speed-regulation control.An excellent AC drive technique--direct torque control technology has been widely used in industrial and agricultural production,because of its simple structure,less parameters and strong robustness.This paper focuses on the further study of the diode-clamped three-level inverter direct torque control system.In this paper,the development and research status of the multi-level inverter and direct torque control technology are briefly introduced and the significance of this research is pointed out.The working principle and topology of the three-level inverter are analyzed in detail and the mathematical model of the three-level inverter is deduced.The unified mathematical model of the asynchronous motor supplied by three-level inverter under α-β coordinate is established,which becomes the foundation of three-level direct torque control.The traditional direct torque control scheme has the phenomenon that the stator flux observation is not accurate,the neutral point potential waveform and the output torque ripple are too large.Considering the inaccuracy of the stator flux observation,an improved integrator--amplitude-limited low-pass filter is applied in this paper.After deeply analyzing the mechanism of the neutral-point potential fluctuation of the three-level inverter,the neutral-point current model is established,which provides the theoretical basis for the neutral-point control.Considering the large torque ripple caused by the complexity of the vector selection and the complex selection of the vector in the traditional DTC,based on the analysis of variable control strategies,a fast space voltage vector control method with negative vector as negative vector is proposed and the torque ripple is reduced,which is beneficial to the digital realization.The simulation model of DTC is built in MATLAB/SIMULINK.The hardware of the three-level direct torque control system is established by TMS320F28335 DSP.And the three-level PWM open-loop is completed on the platform,which is stable and reliable.The software programming process of the direct torque control scheme is briefly described,and the experimental results are analyzed.The simulation and experimental results demonstrate the validity of the theoretical research and the effectiveness of the control strategy.