The Research On Control Technology Of Three-phase Four-switch Inverter For Induction Motor

With the continuous development of power electronics and motor control theory, AC motor speed control system or servo system has been used more widely. Among the AC motor drive systems, the inducton motor has man y advantages, making it one of the most widely used motors for high precision speed control system or servo system. The researches fucusing on the induction motor driving has become one of the hottest topics. The traditional three-phase six-switch inverter fed AC motor drive system has been very mature. However, in some certain applications, people want to further strengthen the cost savings. And at the same time, the driving system is desired to have the fault-tolerant ability. Under the circumstance, the motor operation mode based on the three-phase four-switch inverter came into being. In this dissertation, the theory of four-switch motor operation mode and its control performance is deeply analyzed. The model predictive direct torque control algorithm is proposed in order to obtain a better static and dynamic characteristics.Firstly, this dissertation describes the priciple, merits and faults of the of d irect torque control algorithm based on the mathematical model of induction motor and the voltage space vector of the conventional three-phase six-switch inverter. Through theoretical analysis, the implementation process of t he direct torque control based on the six-switch operation mode is analyzed. The simulation model is builded to varify the correctness and effectiveness of the control strategy, but pointing out the contradiction between dynamic and static performance at the same time.Based on the implementation process of DTC, this dissertation rebuild the direct torque control algorithm for four-switch inverter fed inducton motor drive system. including the recalculation of the mathematical model, redefinition ofthe voltage space vector, reallocation of the sectors and the reselection o f the switching state. Several simulations are done to show that the algorithem is reasonable and feasible. However, due to the deterioration of the power supply condition, coupled with the inherent shortcomings of DTC, the system shows up a poor steady-state performance and the torque ripples are serious.To solve these problem, the model predictive control theory is introduced into the four-switch inverter control system. The basic priciples and the operation process of this strategy is analyzed in detail, including the prediction model, objective fuction and the rolling optimization. The control performance is compared with conventional DTC through simulation. It could be easily seen that the propo sed algorithm maintains a fast transient behavior, and effectively improves steady-state performance of the system. The simulation results are in accord with the theoretical analysis.At last, the experimental platform of the induction motor drive system based on the three-phse four-switch inverter is designed. And the control algorithm is written based on the TMS320F28335 DSP from TI company. The control strategies are tested respectively through practicality experiments. The control system is proved feasible and effective in practical applications and could achieve a good control effect.