Research On Direct Torque Control Of Doubly-Fed Induction Motors

Doubly-fed electric machine is a new type of eletric machine which combines the advantages of the traditinoal induction machine and synchronous machine. Not only it can fulfill the need of general mechanical variable speed system,but also can supply reactive power to power system by modulating the frequency and phase of the rotor current. Besides, it can be used in variable speed constant frequency power generation. Doubly-fed electric machine can be operated as super-synchronous speed generator, super-synchronous speed motor, sub-synchronous speed generator and sub-synchronous speed motor. It has better behavior and higher efficiency, which is provided with wide prospects of industrial application.This paper processes some research on the direct torque control system for the doubly- fed machine. Firstly, based on the mathematical model of doubly-fed machine, the model in the d-q reference frame is built by the vector transform technique. Then, the characteristics of DTC (direct torque control) for doubly-fed machine are obtained through comparing with DTC for asynchronous motor, the method of how to adjust the speed and reactive power is discussed. And then principle of choosing suitable rotor voltage vector both in the least rotor current mode and power factor adjustable mode are analyzed, the switch-tables are given, based on which a flexible active and reactive power control strategy can be developed. Its feasibility is proved through computer simulation. Following, the effect produced by a given voltage vector on torque variations is analyzed, the low number of voltage vectors may cause undesired torque ripple. An improvement of the drive performance can be obtained using a new DTC algorithm based on the application of the space vector modulation for prefixed time intervals. Numerical simulations under various conditions have been carried out showing the advantages of the new DTC scheme with respect to the basic one. In the end, the whole hardware configuration and partial software flow on the doubly-fed machine direct torque control system is set forth.