An Optimized Strategy Of Direct Torque Control System For Hybrid Vehicle Motor

As environmental problem and energy crisis become increasingly serious, hybrid electric vehicle becomes the most important trend of international automotive industry. For the driving system is the most significant part of the hybrid electric vehicle, it has important effects on the vehicle control performance. Most of hybrid electric vehicles use induction motor as their driving system because of the obvious advantages of induction motor. Induction motor has high efficiency, good stability, and low cost.etc. As a rising technology in the control strategies of induction motor, direct torque control has several features such as novel control process, clear objective, simple structure, etc. Therefore, DTC share great advantages comparing with other control strategies of induction motor. This paper studies the direct torque control system of hybrid electric vehicle induction motor, and analysis the defects of the control system. Main research is as follows.1. In traditional DTC system, voltage vector is selected in vector table according to the deviations from hysteresis comparator. So the traditional control system is easily influenced by hysteresis width and the voltage vector is limited in numbers. Besides, the switching frequency of inverter is not fixed. This paper introduces Space Voltage PWM Modulation technique into DTC control system. In SVPWM technique, the desired voltage vector can be forecast by the deviations in last cycle and constructed by zero voltage vector and two adjacent non-zero voltage vector in its sector. The SVPWM technique has no limitation of the number of voltage vector. And the switching frequency of inverter is constant. Therefore, this method can smooth locus of stator flux s and decrease the ripple of DTC system.2. In DTC control system, accurate motor speed is needed to achieve closed-loop control. However, speed sensor will not only increase the expenses, but hard to install in some industrial conditions. In this paper, speed estimator is designed using model reference adaptive system principle according to mathematical model of induction motor and DTC principle. In this way, accurate motor speed is obtained.3. This paper combines sliding mode control strategy with DTC system. According to the theory of sliding mode control, sliding mode controller is designed to replace the PI controller and produce reference voltage vector. And sliding mode observer is designed to improve the observation accuracy of stator flux. Lyapunov stability principle is used to verify the stability of sliding mode controller and observer. Sliding mode structure can protect the system from the change of motor parameters and other disturbance, improving the robust of the DTC system.Simulation model of control system is established in MATLAB/Simulink environment. By comparative analyzing, the simulation results show that the novel DTC control system improves the ripple of torque and stator flux and obtains accurate motor speed. In conclusion, the scheme this paper proposed is effective.