Research On Direct Torque Control Of Induction Motor Based On Field Oriention

Since the field-oriented control scheme for induction machine has been developed from 1970s, it has gained huge success in perfecting control theory and improving control algorithm etc. It is widely used in industry with advancing of microelectronic technology, modern control theory, and high-performance power electronic devices. Although the vector control of AC machines has many excellent properties, the researchers have been attempting to simplify the calculating steps and the structure of the control system to improve the accuration of filed-oriented and the performance of AC machine drives. Among these, Germany and Japan scientists firstly developed a novel AC machines control scheme named as direct torque control in the midst time of 1980s. This scheme is characterized by concise control structure and robust to the variance of an AC motors parameters. It is also characterized by the rapid response of torque and flux. Then this method is evoking the great interest of many researchers. However, the applications of this method are limited due to some disadvantages, such as, the large ripple of electromagnetic torque, the great harmonic components, and the distortion of the stator flux track.How to remain good performance and to eliminate the disadvantages of conventional direct torque control causes greate interest of many researchers. The key discussiones of the paper are described as follows.As we know, the performance of the rotor field-oriented vector control is easily affected by the variance of rotor parameters of an AC machines. This paper firstly highlights the basic facts about traditional theory of fteld-oriented vector control. An improved algorithm for d-axis component of current is proposed, by which the stator flux amplitude can be controlled accurately. Then a new approach of stator field-oriented control of AC motors is presented, in which the rapid dynamic response of flux and torque is archieved. In addition, its performance is not affected by the variance of rotor parameters.A modified method of direct torque control is proposed based on the analysis of convetional scheme. The calculational method of stator voltage as well as the relationship to stator flux and synchronous angle velocity are analyzed. A simplified method of caculating the stator synchronous angle velocity is obtainedby using the rotor flux component and compensation of proportional regulator of torque error control. The control signals of invertor are produced by space vector modulation to achieve a small torque ripple and constant switch frequency.The theory of space vector modulation and harmonics are discussed in cietail, and the analytical formation of voltage to space vector modulation is obtained. We can evenly allocate symmetrically the zero voltage vector between the active voltage vectors during one regulating period to reduce the torque ripple and the current distortion.In the condition of keeping the constant stator flux amplitude, the rapid torque response can be obtained by maintaining the reference angle velocity of the stator flux to the sum of the stable state electrical synchronous angle speed aad the product of the rotor current amplitude dericative and a constant. According to this, we develop the method of direct torque control based on the field-oriente4 and discuss respectively the direct torque control based on the stator field-oriented and the rotor field-oriented in detail. This new method reserves the advantage of conventional DTC and overcomes its disadvantage.On the basis of the theory analysis, The simulation of these new methods above are presented respectively. The simulating results verify that the presened methods are valid. The experiment system has been built by using the digital signal processor of TMS320F2812. The compariable experiments of the conventional DTC method and the field-oriented DTC method is doae at the various work conditions. The experiment results also show that the proposed method is feasible.