Research On Theory And Application Of Direct Torque Control Of Asynchronous Electrical Machine

Direct torque control (DTC) technology has been widely used in electrical locomotive, automotive industry, household appliances and other industrial control areas. In the motion control systems direct torque control, as a new strategy of AC drive, is playing an enormous role in the fields of motion control because of its novel idea, simple structure, and fast dynamic response to torque. Although DTC has many advantages over flux-oriented vector control, there are many needful improvements existed in DTC. DTC adopts two torque and flux hysteresis controllers, which serve to control torque and flux within a certain value of given torque and flux respectively. This control strategy inevitably brings some problems such as undesirable torque ripple and unconstant inverter switching frequency. DTC adopts stator flux orientation and uses stator resistance to estimate stator flux, both of which will bring flux error in low-speed operation. While estimating stator flux within full-speed adopts a compound voltage-speed model composed by current-velocity model at low-speed and voltage-current model at high-speed, the smooth handoff between two models is a problem needed to be solved. DTC under base-frequency has been achieved; however, its theory and application can be extended to above base-frequency realm.In order to solve these problems, this paper presents the traditional DTC system and two improved DTC systems aiming at the mathematical model of asynchronous machine on two-phase standstill axis. In the traditional DTC system, three estimation models of the stator flux are discussed in detail and the weighted model is designed, both of which serve to get precise stator flux in full-speed operation. In order to cope with undesirable torque ripple and unconstant switching frequency this paper designs two improved DTC systems. In the DTC system based on duty cycle control, an optimized duty cycle of a non-zero voltage vector is adopted to solve the problem of large torque ripple. There is only one switching opportunity from the non-zero voltage vector to zero one in a total sample cycle, which realizes the constant inverter switching frequency. In the DTC system based on sliding mode control, this paper designs torque and flux sliding mode controllers to replace the traditional ones, and realizes constant switching frequency by handling SVPWM. This paper extends the traditional DTC and two improved DTC methods to above base-frequency realm.In order to verify the effectiveness of the control systems mentioned above, simulations are made using Matlab/Simulink. A stator flux estimated weighted model is constructed in the traditional DTC system. Simulation shows a good result within all speeds. As for the DTC system based on duty cycle control and the DTC system based on sliding mode control, this paper discusses four instances of the load torque with disturbance and no disturbance, the constant and variable assigned rev. Simulation shows that both improved DTC methods can reduce torque ripple and the steady-state error of rev effectively. For the opptunity above base-frequency, three different DTC methods are utilized. Simulation shows that both improved DTC systems are superior to the traditional one in above base-frequency operation.