Research And Design Of Direct Torque Control Method For Three-Level Four-Quadrant Speed Sensorless Induction Motor

Field Oriented Control(FOC)and Direct Torque Control(DTC)are the research hotspots of high performance AC variable frequency drive technology.Compared with the field-oriented control,the Direct Torque Control(DTC)has the advantages of simple structure,less dependence on the motor parameters,fast torque and speed response.But the traditional DTC uses two hysteresis controllers to control the AC motor stator flux linkage amplitude and electromagnetic torque,which will increase the ripple of the AC motor electromagnetic torque.A direct torque control method for decoupling the voltage vector of a fixed switching frequency is studied.In order to eliminate the influence of the direct current component of the stator current on the direct-axis component of the voltage vector,a new method to determine the direct current feed-forward compensation value from the torque reference value is proposed.The speed sensor will not only increase the hardware complexity of the system and the difficulty of installation and debugging,but also reduce the reliability of the whole system.A current development trend of AC speed control system is speed sensorless.The induction motor stator flux voltage model is used in conjunction with the current model.The PI regulator is used to amplify the deviations of the estimated values of the two models so that the two flux observer models can compensate each other to improve the estimation accuracy of the stator flux.The flux observer is applied to the direct torque control system.To achieve speed sensorless AC speed control system,it is necessary to solve the problem of flux and speed estimation.The accuracy of flux linkage and speed estimation depends on the accuracy of the motor parameters.A new method of off-line identification of asynchronous machine parameters is proposed for three-level inverter.The pulse voltage vector method for measuring short-circuit impedance is proposed,and a three-level SVPWM based on the simplified method of compression mapping is specially designed for this purpose.The modulation method improves the switching trajectory of the three-level voltage vector and effectively avoids the switching operation of more than one phase of the inverter when the traditional three-level SVPWM expresses the pulse voltage vector.The four-quadrant inverter has the advantages of adjustable power factor at the network side and bidirectional flow of energy between the motor and the power grid.Four-quadrant inverter PWM rectifier as the front-end rectifier,the network side of the current waveform close to the sine wave,greatly reducing the power electronics harmonic pollution on the grid.Aiming at the traditional software phase-locked loop technique,an improved approximation function of estimation angle deviation is proposed.The improved software PLL is applied to the estimation of synchronous speed and voltage vector angle estimation of PWM rectifier.The simulation model of four-quadrant speed sensor-less direct torque control system of induction motor is built by using PLECS software,and the improved control method is verified by simulation.The simulation results show that in order to avoid the shock when the sensorless system starts,the speed ring should be blocked until the flux observer starts and reaches a steady state.