Research On Several Questions About AC Drive Variable Frequency Vector Control Based On The Three-level Inverter

The AC drive variable frequency vector control system based on three-level inverter isresearched in this thesis. From the motor control theory, this paper analysed the principle of the motordrive. After building the mathematic model of the three phase induction motor, a control method isobtained by using coordinate transformation technology which the rotor flux and electromagnetictorque can be controlled separately under the two phase rotating coordinate system, so as to realize theflux and torque of decoupling control.The multilevel technology is widely used in high-voltage and high-power occasions in recentyears. In this paper, it studied the Neutral-Point-Clamped (NPC) three-level inverter, analyzed theworking principle and mathematical model of the inverter. The related problems of the three-levelSVPWM algorithm are studied. Neutral point voltage bias is the basic problem of the three levelinverter. The cause of neutral point voltage bias and the possible adverse influence are analyzed, andfinally the fuzzy design neutral point voltage balance control strategy which has good robustness andadaptability is designed, thus the balance of neutral point voltage can be achieved.In the closed-loop drive systems, rotor speed is a very important variable. In the traditionalmethod, a shaft encoder or photoelectric digital pulse coder is used which increases the cost andinconvenient of the control system,it also can’t be used in a harsh environment This paper studies theopen loop speed estimation based on the mathematical model and Model Reference Adaptive System(MRAS) methods include rotor flux model and the back emf model, and finally the researches arefocused on full-order state observer method, and establish the full-order state observer of the threelevel speed-sensorless vector control system model by using MATLAB/SIMULINK simulation.The motor parameters would be changed during the running process, especially the changes ofrotor resistance affect speed observation and even the whole vector control system. It is known thatthe simultaneous estimation of the rotor speed and rotor resistance is impossible in vector control.This thesis presented a method of estimating simultaneously the motor speed and rotor resistance inthe transient state base on direct field oriented vector control system without speed sensor, anddesigns the modified integration of voltage flux observer which has no relationship with full-orderobserver and adaptive recursive least squares algorithm is used. Using the method, it is possible toestimate motor speed and rotor resistance simultaneously. Then considering the long-term operationof motor in steady state, a single harmonic is injected to identify the real value of the rotor resistance. At last, all of the theories are proved by systematic experiments based on the TI TMS320F28335’s platform.This paper also introduces the hardware structure design and software realizationmethod of the500kW three-level inverter system.