Asynchronous Motor Direct Torque Control Of Stator Flux Estimation Method

The direct torque control (DTC) of induction motors is one of thehigh performance control strategies, which was proposed after the vectorcontrol scheme. It became one of the hot topics in the field of AC speedcontrol because of its novel control method, simple structure, high staticand dynamic performance. The accurate estimation of the stator flux isone of the most important parts to realize the DTC system. Usingadvanced control theory, the stator flux estimation methods of DTC arestudied in the thesis. The main research works are as follows:The basic principle of DTC is introduced in this thesis. The DTCsystem that has the flux track of hexagon and approximate circle isanalyzed in details. The flux track of approximate circle control systemcan reduce the torque ripple and stator current distortion when the speedis low; the flux track of hexagon control system can reduce the switchingfrequency of the inverter. By integrating the two models and judging thespeed, a model of DTC for full-speed region is built, which can realizethe translation between the two kinds of control methods.The accurate estimation of the stator flux is important to the choice ofvoltage space vector and the torque estimation. It is the key part forimproving the performance of DTC. Stator flux estimation methods arediscussed in this thesis. The open loop stator flux estimation method forclassical DTC, modified u-i integrator method, and stator flux observermethod are analyzed in depth. Because of the limitations of thosemethods, a stator flux observer based on the modified integrator withvariable cutoff frequency is proposed in this thesis. Using a fuzzycontroller, the cutoff frequency of low-pass filter can be adjusted in realtime according to the online operating states of induction motor.Consequently, it can eliminate the errors caused by the initial value andDC drift of the integrator and improve the precision of stator fluxestimation efficiently. This observer can be used for stator flux estimationover full speed region.By the use of Matlab6.5/Simulink simulation tool, the simulation onthe two methods of DTC, which are the hexagon and approximate circle flux track control systems, are achieved in this thesis. A simulation modelfor full speed region is built, which realizes the smooth translationbetween the two kinds of control methods. Based on these models ofDTC, stator flux estimation methods are studied. The simulation resultsaccord with the analysis of theory. The stator flux observer based on themodified integrator with variable cutoff frequency improves the precisionof stator flux estimation efficiently' during its full speed region.