Research On The Direct Torque Control Strategy For Permanent Magnet Synchronous Motors

Permanent Magnet Synchronous Motor (PMSM) is well-known as such advantages: the simply structure, high power factor, high efficiency, low inertia and so on. AC servo systems which use PMSM as power units will be the trend.The Direct Torque Control (DTC) strategy has achieved great success in the control of induction motor. However, the DTC drive of PMSM was proposed only a few years ago, there are many basic theoretical problems that must be clarified, and the strategy can be modified to improve on the torque control performances for PMSM.Direct Torque Control (DTC) and Vector Control (VC) are widely used as two high performance control strategies for PMSM.To catch the difference of system performance between VC and DTC for torque control of PMSM, both principles are analyzed and their characteristics are presented. The simulating models of these two schemes are built. The results of simulation show: DTC has better torque dynamic performance than VC, but DTC suffer from high ripples of torque and stator flux linkage.Then the conclusion is declared: the essential which causes the ripples of torque and stator flux linkage in DTC is that there are only electively eight discrete space voltage vector and the only one space voltage vector is applied in each sampling period.In order to reduce the ripple of torque and stator flux linkage in conventional DTC, the Space Vector Modulatong (SVM) is combined with DTC and a novel direct torque control scheme of PMSM which is called PMSM SVM-DTC is proposed. Aiming at eliminating the error of torque and stator flux linkage completely, a reference voltage vector is constructed. The reference voltage vector is then realized using a votage vector modulator. The scheme can realize continuous voltage vector modulation and decrease the ripple of torque and flux linkage .This scheme is validated by simulating results of PMSM DTC-SVM system.