Speed ​​sensorless Direct Torque Control System

Direct torque control(DTC) technique which was put forth after Vector control technique is a novel control method for motor drives. With its simple control structure and excellent dynamic response, DTC technique has been a research focus in the AC drive fields. In this thesis, the improvement of low speed performance, the design of speed-sensorless schemes and the digital realization of DTC system are studied detailedly.Firstly, based on detailed analysis of the mathematical model of induction motor, the principle of the conventional DTC is studied deeply, and the deficiency in the conventional DTC scheme is analyzed. Secondly, different improved schemes are discussed and analyzed to improve the performance of the DTC system. Instead of hysteresis controllers of torque and flux,the Fuzzy logic controller is firstly adopted. It optimizes the selection of voltage vector, so the better dynamic and static performance can be obtained. The other technique applied into DTC is discrete space vector modulation(DSVM). This technique increases the number of voltage vector by synthesizing three voltage vector at one sampling period and combines with a new switching table, so the torque ripple is reduced. By systematic study and analysis of the principle of space vector modulation(SVM), a novel DTC strategy based on the increment of reference stator flux with SVM is proposed. Simulation results show that this system can effectively reduce the ripple of motor torque and stator flux , especially in lower speed range.Thirdly, an adaptive speed observer was studied based on the full order flux observer. With better accuracy of flux estimation and speed identification and higher robustness to the motor parameters , it is regarded as one of the most promising and effective method to improve the system performance . According to the dissimilarity of state variables of the observer, two schemes of speed sensorless DTC with high performance are proposed. In both schemes, the switching table is replaced with the above-mentioned fuzzy logic controller and the flux observer is directly used to estimate the stator flux. Simulation results show the schemes can improve the dynamic and static performance and have bright application prospect.