Study On Direct Torque Control Strategy Of Induction Motor Drive With Duty Ratio Regulator

Induction motor drive fed by two-level voltage source inverter is studied in this thesis. Based on switching-based direct torque controlled (ST-DTC) induction motor drive, the research is carried out from three aspects, including theoretical analysis, computer simulation and experimental verification, and wide speed operation of high performance frequency control of induction motor adjustable speed drive is obtained.The traditional switching-based DTC scheme uses two hysteresis regulators and a heuristic switching table for the control of both stator flux magnitude and electromagnetic torque in two-phase stationary frame. Hysteresis comparison only consider the direction of the error, without the error size into consideration, thus inevitably cause the flux and torque ripples, especially at low speed range torque ripple is bigger. In order to reduce the flux and torque ripples, various modified DTC-based schemes have been proposed, considering in low speed range and steady state. Although there are various methods in obtaining the duty ratio, so far to induction motor no comparative study of these methods in terms of torque ripple has been reported. In addition, how to reduce the high starting current of direct torque controlled induction motor drive, also need to pay attention to research. This paper is proposed by the technology of pre-excitation to reduce the amplitude of starting current, effectively solved the problem of the DTC starting current is too large.Based on dynamic mathematical model for induction motor. Luenberger observer is deeply studied. To solve the problem of gain matrix selection, analytical method is used to find multiple sets of gain matrix solution under the traditional pole assignment method. Also the relationship between different gain matrixes when the state variables are different is given. A novel simple gain matrix is proposed to improve the stability for Luenberger observer, and the stability performance is enhanced by reducing the amount of calculation of the control system and the influence of the motor speed error.After comparing research of three typical kinds of duty ratio control methods for DTC in large number of simulations and experiments, based on dead-beat control by adopting the combination of quantitative and qualitative methods, this paper proposes a very simple but effective method to obtain the duty ratio. For acquiring the duty ratio by optimizing torque and flux linkage errors, the proposed scheme is able to reduce the torque and flux ripples significantly while maintaining the simplicity and robustness of the conventional DTC at the most. Simulations and presented experimental results validate the effectiveness of the proposed schemes in this paper.Based on analytical deducing the instantaneous variation rates of stator flux and torque of each inverter output voltage vector for dynamic mathematical model of induction motor, deeply analyzing the relationship in the variation rates of stator flux and torque with motor load, motor velocity and output voltage vector is obtained. And applying torque ripple minimization DTC method, a simple and effective scheme considering the effect of motor velocity is proposed to acquire the duty ratio, and have access to the key control parameters using the mathematical analytical method. Simulation and experimental results show that the proposed scheme is able to reduce the torque ripple and steady-state error of torque significantly, while maintaining the simplicity and robustness of the traditional DTC at the most, for in the duty ratio calculation considering the influence of motor velocity, can effectively improve the control performance of the traditional DTC.