High Performance Direct Torque Control For Permanent Magnet Synchronous Motors

This thesis presents the recent and new developments of the direct torque control (DTC) techniques for permanent magnet synchronous motors (PMSM). Many important issues about DTC for PMSM, from the basic theory to new control algorithms, have been investigated both analytically and numerically. The theory has been implemented and verified experimentally on two high performance PMSM DTC drive systems in the laboratories of Department of Electrical Engineering, Zhejiang University, China, and the Centre for Electrical Machines and Power Electronics, University of Technology, Sydney, Australia.The correction, compensation and improvement methods are investigated in this thesis for the conventional DTC PMSM drive systems according to the problems associated with this method. Specific issues examined are: compensation for the variation of the stator resistance, the offset error of the DC bus voltage, the voltage error generated by the forward voltage drop the dead time of the switches, improvement of the steady state performance, and the speed sensorless control for the PMSM DTC drive system are of major concern in this thesis.Fast torque control is an excellent feature of the DTC method. Proper space voltage vectors are selected in the conventional PMSM DTC according to the errors between the reference torque, stator flux linkage and their estimated values, and the position of the stator flux linkage. The torque control algorithm is simply implemented by an optimal switching table, while the current control loop, which is commonly used in other control algorithms, is not required.The problems associated with the conventional PMSM DTC have been discussed in this thesis. In the conventional PMSM DTC system, the stator flux linkage is calculated from the integration of the difference between the machine input voltage and voltage drop across the stator resistance, and the input voltage is obtained by synthesizing the DC-link voltage and the switching mode of the inverter. The accurate calculation of the input voltage and the compensation for the DC-offset error and the variation of the stator resistance are important factors in practical implementation of the integration since they can cause a drift in the stator flux linkage trajectory and furthermore deteriorate the quality of torque control.Only one space voltage vector is applied in each sampling period in the implementation of the conventional PMSM DTC system. This results in large ripples in the torque and the stator flux linkage, and nonconstant switching frequency. A fuzzy logic method is introduced to the conventional PMSM DTC system in this thesis to improve the steady state performance of the system. An appropriate fuzzy controller is designed for the DTC system to replace the switching table and twohysteresis controllers. To reduce the computation time, a mapping technique is used in the fuzzy PMSM DTC system.None of the available vectors in the conventional DTC system can offer precise torque and flux linkage control. To further improve the performance, a space vector modulation (SVM) method is proposed in this thesis to offer the desired space voltage vector for accurate control of the torque and the stator flux linkage. A space voltage vector used to compensate the error of the torque and stator flux linkage is calculated based on the error vector between the reference and the actual stator flux linkage. Thus the ripples of the torque and the stator flux linkage can be eliminated completely.Sensorless is a major feature of the PMSM DTC system. While in a high performance speed closed-loop control system, the speed signal is essential for the feedback. A sensorless method is helpful for reducing the cost and improving the reliability of the system. A rotor flux linkage vector based speed sensorless method is proposed for the PMSM DTC system in this thesis. This theory is equally applicable to fuzzy DTC and SVM DTC. A filter is used to smooth the noise in the estimated rotor speed signal in order to obtain a high performance sensorle...