| The technology of modern AC speed regulation is usually divided into scalar control and vector control. The traditional control of slip frequency is scalar control, but the implementation of control systems make it difficult to ensure good dynamic characteristics. Rotor field oriented control and direct torque control are both vector control. Among them, the rotor field oriented vector control depends on the accuracy of field-oriented and is high sensitive to changes in system parameters. It also requires the accuracy of system parameters identification, so it will be difficult to achieve. The DTC based on simple physical processes, without complex coordinate transformation, obtain speed regulation performance with high dynamic response.In this paper, the theory of DTC is analyzed, and the simulation of basic DTC system is carried on. After the analysis of torque ripple and excessive starting current existed in the basic DTC system, the following improvements are made. First of all, the conventional torque control uses hysteresis control method with bad control effects of torque and flux, this paper presents the three-value torque errors controller which improved torque ripple. In this control, the direction that torque errors change has been ignored and only its existing values been considered. Secondly, the startup current is related to the protection of the power grid. In practice, it does not allow too much startup current. In this paper, by adding the appropriate zero-voltage vector based on the direct startup, limits the startup current well. This method has a better torque and speed characteristics than the method of current feedback regulation.To achieve the operation in full speed range, this paper adds flux weakening control for constant power operation above base speed. The part below the base speed is divided into two parts, low speed with the regulation of circular flux, high speed with the regulation of hexagonal flux. Operation in full speed range needs to control the reference torque and flux. This paper adds the reference torque controller and reference flux controller based on the improved basic DTC system, to output appropriate reference torque and reference flux in accordance with a given speed in full speed range. By this way, it can take full advantage of the characteristics of the DTC and be able to meet the characteristic of motor electromagnetic torque and output power.Finally, the simulation of DTC system operating in full speed range, achieved a good control effect. |
