Research Of Torque Control Of AC Motors And EV's Drive

With development of power electronics and microprocessors, AC drives have been improved greatly, which are steadily taking the place of DC drives in traditional fields. It is now developing towards digital, intelligent and integrated implementation, which is focused on field orient control (FOC), direct torque control (DTC) and space vector pulse width modulation (SVPWM) etc. Since DTC is a newly developed method, recently it is an important and challenging area in the field of AC drives. In this thesis, attention is focused on DTC of squirrel cage motors and electric vehicles (EV) driven by AC motors.Based on an overall review of related papers, the state of the arts and development trends of AC drives and EV's AC drives are summarized. Since there are some disadvantages among FOC, DTC and SVPWM, it is necessary to mix all these methods and syntheses them to achieve linear characteristics. And observation of the stator flux is the most important one for AC drives.After analysis and experiments on DTC system, the disadvantages of DTC are summarized. Also sensitivities between parameters and control character are analyzed based on each type of flux model. In the end of this part, a DTC method with phase tuned (PT-DTC) is proposed to improve character at low speed and its feasibility is proved by experiments.Based upon analysis and experiments in the fore part, stabilities of DTC systems are further discussed using Lyapunov theory, zero and pole method of transfer function of AC systems. It is very important that stability of observation for stator flux and current has serious effect for vibration at low speed.To improve DTC system, an improved method of vector torque control (IVTC), based on analysis of effects between vectors and variations of flux and torque, is proposed which utilizes an improved flux model at low speed. Since it sets up a function, it is possible for researchers to use more kinds of strategies to control flux and magnetic torque respectively in DTC systems. During the research, a simple PWM method with direct flux control (DFC-PWM) and a novel identification method (NIM) for parameters using least square theory are proposed. Filter is designed to simplex calculations without decreasing correctness of results.For EV application, except traction methods for AC drives above, some new schemes are discussed. After analysis the state of regenerative brake, a new regenerative brake method of slip ratio control (RBSRC) is presented here. Since zero vector can not limit currents allthe time in the experiments, a new current limit mode is introduced. For utilization of the batteries, a method for over modulation with two novel modes is analyzed and realized in EV system.Finaly, a prototype of test vehicle, namely GREENSCIENCE, is designed for the application of these methods. In the appendix of this thesis some specified papers and photos are introduced also.