Improvement Study Of Direct Torque Control For Permanent Magnetic Synchronous Motor

Direct torque control (DTC) of Permanent Magnetic Synchronous Motor (PMSM) is anther high performance control technique. It discard the decouple idea of vector control, and directly control the stator flux and torque of PMSM. It has the advantage of simple control structure, directly control, and rapid torque response. But, as a kind of new control technique, it has the disadvantage of too big torque and flux ripple and bad low-speed performance. This paper is based on the trait of PMSM, and studies the cause of torque and speed ripple of DTC and then gives the methods to improve it.By studying the math models of PMSM and the basic principles of DTC, we set up the simulation models of DTC based on MATLA/Simulink. The results of simulation indicate that the direct torque control of PMSM exist torque and speed ripple. In this paper, summary to the observation error of stator flux caused by the change of the stator resistance, we develop the online compensator with PI regulator based on the observation. And we also develop the new flux observer algorithm based on the low-pass filter and the flux amplitude feedback to avoid the impact of stator flux pure integral model. Then, we study the effect of zero-voltage vector to direct torque of PMSM. The results of simulation and the analysis of theory shows that the zero-voltage vector can be used into the direct torque control of PMSM to maintain the torque. But the use of zero-voltage will result in the attenuation of current. After that, we refine the hysteresis comparators and switch tables of classical torque control, introduce the zero-voltage vector, design the double hysteresis comparators of torque and flux, and develop a new voltage-selected table. The results of simulation show that the new table can reduce the ripple of torque when keeping stator flux not attenuates.For fundamentally inhibiting the ripple of torque and speed in direct torque control of PMSM, we develop a new torque control method based on space vector modulation (SVM), named SVM-DTC. After introducing the basic principles of SVM-DTC algorithm, we set up the models of simulation. It uses PI regulators to control the size and direction of errors, and estimate the expected voltage vector in using of the increment of flux amplitude and phase. And in the end, the expected voltage vector is modulated by SVM algorithm. This method does not have discrete hysteresis output and make full use of the whole voltage space. The results also show that this system can reduce the ripple of torque and speed efficiently. At last, we simulate the SVM-DTC system when speed or load usually changes. The results of simulation indicate that the SVM-DTC system has a better robustness than traditional DTC system.