SVM Direct Torque Control Of Synchronous Machine Based On Sliding Mode Variable Structure

One of the trend in modern electric drive controls is developing new type alternating current (AC) speed regulation and servo system. While in the AC speed regulation system, no matter low power permanent magnet synchronous motor and switched reluctance motor, or high power excitation synchronous motor and permanent magnet synchronous motor, both of them show outstanding advantage. For instance, compared with direct current (DC) motor, synchronous motor has high overloaded capacity, high efficiency, small volume, little weight, simple maintenance, good reliability, etc; and compared with induction motor, it has high power factor, small capacity of inverter, low inertia, etc.Direct torque control (DTC), which has been developed after vector control in recent years, is a new technology in domain of AC drives. DTC method has many merits, its structure is very simple, the physical concepts is clear, the response of torque is rapid, the method is little dependent on interior parameters of the motor and easy to digitization. So more and more people pay their attention to this method.Therefore, applying DTC technology to synchronous machine has huge theory value and wide application prospect.This thesis takes DTC method as center to make theoretical research and simulation analysis. Firstly, the mathematical model of synchronous motor is analyzed in different coordinate, and DTC principle is studied based on it and the rules about choosing voltage vector is determined. The simulation result indicates that although the rotational speed of traditional DTC responds rapidly, the flux linkage and torque pulsation are quite high. So the thesis introduces a method of space voltage vector PWM modulation (SVM) to make the improvement to traditional DTC. The simulation result indicates that the rotational speed of this method responds rapidly and it can reduce the flux linkage and torque pulsation effectively, but its design principle decides that the system can not control promptly to the errors caused by uncertain factor and external disturbance. So the self-adaptability and robustness of this method are not strong. In allusion to the defects of the former two DTC method, the thesis proposes sliding mode variable structure control method for the flux linkage and torque, which is introduced to SVM DTC system of synchronous machine. Sliding mode variable structure control is one of the control strategy of variable structure system. The fundamental difference between this method and others is the discontinuity in control, namely a switching characteristic which causes the system "structure" to change with time. According to the variation of operation parameters, the controlled quantities of the system are forced to move at the sliding surface s = 0, while this sliding mode can be desighed and it is independent of system parameters and disturbance. So the system under sliding mode has good robustness that can make the actual flux linkage and torque track to the given flux linkage and torque better.Using the equations of synchronous motor model and the principle of sliding mode variable structure control, the thesis designs a sliding mode controller of the flux linkage and torque of synchronous machine, and proposes a reasonable solution to the chattering problem in sliding mode control. The simulation result indicates that this method has removed the flaws of the former two system, it has weak flux linkage and torque pulsation, stable speed regulation, rapid response, strong self-adaptability and robustness. So it is an advanced control method of synchronous motor.