Research Of Direct Torque Predictive Control In PMSM Based On State Observer

Without decoupling control, direct torque control (DTC) has been widely used inpermanent magnet synchronous motor due to the simple structure and fast dynamicresponse speed. In the practical process of production, influenced by the traditionalStator Flux Observer’s performance and time-delay, the torque and flux ripple ofPMSM DTC system are large which have serious impact on stability of the system.Therefore, the researches on stator flux observation methods and DTC system withtime-delay have very important theoretical and practical value.The major works and creations of this paper are summarized as follows:1. Full-Order stator current and stator flux observer and Reduced-Order statorcurrent and stator flux observer of PMSM are studied. The performance of the DTCsystem is affected by the accuracy of the Stator Flux Observer. While the DC offsetand motor parameter variations and so on factors will lead to unsatisfactory StatorFlux Observer’s performance in low speed region. To solve this problem, firstly thestator current equations containing a rotor location signal are transformed intoequations without rotor position signal by doing some simple variable substitution.Secondly, a Full-order Observer whose state variables are the stator current andstator flux is designed to eliminate the influence of parameter variations on the statorflux observation precision. Then, a Reduced-Order Observers, in which the statorcurrent and stator flux are treated as state variables, is further constructed for thepurpose of simplifying the system structure. Finally simulation results verify that theDTC systems using these two Observers have better robustness against motorparameters, and torque and flux ripple is small too. Compared with the traditionaldirect torque control system, the control performances of the new two systems areimproved obviously in low speeds.2. Two predictive direct torque control methods are studied. In general, theexistence of time-delay is prone to great torque ripple of the system. Two types of predictive DTC control strategies are presented to compensate time-delay. The firstpredictive method is a Full-Order State predictive strategy. It directly predicts thestator current value and stator flux value of the next cycle based on Full-orderobserver, calculates the torque value for the next period, and then uses the samelook-up table with traditional DTC to find the control voltage. The second predictivemethod is a method that directly forecast torque and flux amplitude square.Combining the advantages of vector control and DTC, this method presents a novelpredictive model by decoupling the torque and flux amplitude, then calculates therequired control voltage through assuming that the given torque value and stator fluxvalue is the next cycle of predictive values. Simulation results illustrate that these twopredictive methods both can reduce the torque ripple and stator flux ripple torque, andthe control system’s performance is effectively improved in the whole range.Meanwhile the second predictive method dynamic response speed is faster than thefirst one, since it uses SVPWM method to control the motor.