Sensorless Vector Control Of Induction Motor Under Low Modulation Ratio

Reducing the switching frequency of the switching device will increase the harmonic content of the current, resulting in the PWM waveform disorder, affecting the stator current decoupling control. Suitable motor control strategy should be adopted in the large capacity power transmission system, which can reduce the current harmonic content and improve the ability to respond quickly of the system under low modulation ratio. In this paper, the sensorless vector control scheme of induction motor under low modulation ratio is studied in depth, main contents include:Firstly, the principle of vector control of induction motor is analyzed. Based on the analysis of the mathematical model of the induction motor in three coordinate systems, the rotor flux observation method and the rotor speed estimation method are discussed. The flux observer is analyzed in detail based on the current model flux observer and based on voltage model flux observer, giving the current model contains the information of rotor speed, so current model of rotor flux apply to the sensor vector control system.In addition, the current model contains the rotor inductance, resistance information, with the running of the motor, the above parameters will change, these changes will affect the accuracy of the flux observer. Therefore, flux observer based on current model is not suitable for sensorless control system. The voltage model does not contain rotor resistance, so the motor does not depend too much on the rotor parameters at running time, which is only related to the stator parameters of the motor. Moreover, the voltage model does not contain motor speed, therefore, the rotor flux observer based on voltage model is suitable for the sensorless vector control system. However, the voltage model contains an integration, which generates integral drift, resulting in the cumulative error of the initial value accumulating obviously. These questions will lead to the system can not be stable operation. In view of the influence of the pure integral in voltage model. The paper proposes that using the first order inertia link instead of the pure integral not only achieving the goal of control, but also solving the problem of pure integral.The basic principle and structure of MRAS speed identification system are analyzed in detail, and the selection scheme of adaptive rate of MRAS identification system is discussed in this paper. Finally, a speed observer based on MRAS is designed. The simulation waveforms show that the speed estimation is accurate.Under low modulation ratio, the stator current harmonic content of induction motor increasing, the traditional PWM modulation strategy is not suitable. The optimized PWM method must be used, the current harmonic minimum pulse width modulation method is used in this paper. Firstly, the basic principle of CHMPWM is analyzed in detail, and the calculation method of switching angle is discussed. Subsequently, the induction motor under the load current harmonics and torque ripple were in-depth analysis, analysis shows that the load can significantly reduce the content of harmonic current of induction motor and greatly suppress the torque ripple of the motor. Finally, the CHMPWM strategy is simulated, and the simulation results show that the effect of CHMPWM strategy is very good.A simulation model of the sensorless vector control system under low modulation ratio is built, which simulate in four cases. No-load starting simulation results show that speed response is fast and the starting torque is large; starting with full load simulation show that the load capacity of motor is strong; accelerating and decelerating simulation results show that the system can accurately track the given speed, the speed estimation is accuracy and robustness of system is strong.Then, the test platform to verify the is system carried out. The experimental results show that the control method can ensure the induction motor under low modulation operate stably, the motor has good speed regulation performance.