A Simple Method to Reduce Torque Ripple in Direct Torque-Controlled Permanent-Magnet Synchronous Motor by Using Vectors With Variable Amplitude and Angle

In this paper, a modified direct torque control (DTC) for permanent-magnet synchronous machines, which enables important torque- and flux-ripple reduction by using voltage vectors with variable amplitude and angle, is proposed. In the proposed DTC, the amplitudes of torque and flux errors are differentiated and employed to regulate the amplitude and angle of the output voltage vectors online, which are finally synthesized by space-vector modulation (SVM). Two simple formulas are developed to derive the amplitude and angle of the commanding voltage vectors from the errors of torque and flux only. The conventional switching table and hysteresis controllers are eliminated, and a fixed switching frequency is obtained with the help of SVM. Stator flux is estimated from an improved voltage model, which is based on a low-pass filter with compensations of the amplitude and phase. The proposed DTC is comparatively investigated with the existing SVM-DTC from the aspects of theory analysis, computer simulation, and experimental validation. The simulation and experimental results prove that the proposed DTC is very simple and provides excellent steady-state response, quick dynamic performance, and strong robustness against external disturbance and control-parameter variations.