Translated Abstract
This dissertation is within the research field of permanent magnet synchronous motor control. Permanent magnet synchronous motor with its power density, low-speed output torque, high efficiency, easy maintenance, etc. in electric drives, wind power generation, ships and other industries are driving more and more attention. This dissertation will be applied to direct torque control of permanent magnet synchronous motor drive system, and the torque and flux control, the stator flux observer, speed observations on issues such as specific in-depth study. Background of this study is based on the project "1.5MW vertical axis wind power generation system with permanent magnet motor " in the follow-up study of motor control strategies. This study deepens the permanent magnet synchronous motor direct torque control of knowledge and gives important theoretical significance and value in engineering.
Traditional permanent magnet synchronous motor direct torque control strategy is based on the torque and stator flux hysteresis controller output and the stator flux position signal, through the switch menu, select the appropriate voltage space vector. As compared with vector control, the flux and torque ripples are more serious. The switching frequency is not constant, it varies with the speed, load torque and the bandwidth of the flux and torque hysteresis controller. So a deadbeat direct torque control scheme is proposed. Firstly with the use of the torque equation, the expected torque angle is calculated . Then the expected stator flux angle is obtained with the expected torque angle, the actual stator flux angle and the rotor speed. Then the desired stator flux vector is obtained with the expected stator flux angle and the reference amplitude of stator flux. Then according to the desired stator flux vector , the actual stator flux vector and the stator current vector, the desired space voltage vector can be obtained. This desired space voltage vector can accurately compensate the error between the reference and actual flux amplitudes, and also the error between the reference and actual torques. So the flux and the torque can be controled in a deadbeat fashion. Finally, the inverter switching state sequence is obtained using the space voltage vector modulation strategy. Theoretical analysis, computer simulation and experimental validation show that the system with this method can achieve better torque and flux control effect, such as the smaller flux and torque ripples, fixed switching frequency with the traditional direct torque control method. And the advantages of quick torque response are also maintained.
For the permanent magnet synchronous motor, since the permanent magnet rotor body, so the rotor initial position decides the initial value of the stator flux. For the direct torque control of permanent magnet synchronous motor, it needs to know the initial position of the rotor in order to determine the initial stator flux values,which brings new problem to the stator flux observation. According to the knowledge of modern control theory, the state observer can eliminate the initial error. This dissertation uses state observer method to observe the stator flux. And the reduced-order observer compared with other state observers,the algorithm is relatively simple,so a reduced-order state observer is used to achieve stator flux observations. In this method, based on the mathematical model of permanent magnet synchronous motor, the reduced-order stator flux observer model and structure are derived. Then the designed state observer is used in the traditional DTC system and the proposed improved DTC system. Through theoretical analysis, computer simulation and experimental validation,the system can use this method in a relatively short period of time to eliminate the initial flux errors, so the initial rotor position information can be not needed.
For the conventional direct torque control system, the torque and flux control loop does not need speed signal. As this dissertation, the speed signal is required in the improved direct torque control algorithm and the reduced-order stator flux observer.A speed identification method is proposed based on the rotor flux estimation. Firstly based on the relationship of the rotor flux and stator flux, the rotor flux vector is obtained. And then the rotor flux angle can be obtained from the rotor flux vector. Then the rotor electrical angular velocity can be obtained by the differentiation of the the rotor flux angle. The implementation steps of this method is derivated firstly. And then this speed identification algorithm is applied to the modified direct torque control algorithm and the reduced-order state observer of stator flux observer algorithms separatly. Finally the speed identification algorithm is applied to both the modified direct torque control algorithm and the reduced-order state observer of stator flux observer algorithm, and the improved permanent magnet synchronous motor direct torque control system is composited completely. Through theoretical analysis, computer simulation and experimental systems, it is verified that the motor speed can be obtained quickly using the speed identification algorithms. The speed identification algorithms can be integrated in the proposed direct torque control algorithm and the prposed stator flux observer algorithm, and does not affect system performance; so that the whole system can operate without speed sensor.
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