PERFORMANCE EVALUATION OF INDUCTION MOTOR DRIVE FOR ELECTRIC TRACTION

Abstract

At present majority of the electric railway traction use DC motors supplied from a 25 kV single-phase at 50 or 60 Hz mainline which is further rectified into dc supply in locomotive. At the turn of the 20th century, it was recognised that the steep torque-speed characteristics and inherent regenerative capability of three-phase induction motors (IMs) would be ideal for electric railway traction. But the main technical challenges were the supply of three-phase IMs from a dc or single-phase ac traction line and effective traction motor speed control. The development of the large power GTO/IGBT, made the voltage-source inverter (VSI) a practical proposition, and the VSI has now become the standard traction inverter to supply three phase to IMs. Since the traction load uses high power induction motor, three level inverter is used. From the speed torque characteristics of IM at different frequencies of operation at constant flux (i.e. maintaining v/f ratio constant), it is understood that even at starting, maximum torque can be produced by IM which is required in traction to accelerate the train. Hence, speed control is achieved in the voltage source inverter-driven induction motor by means of variable frequency along with the variation in applied voltage, to keep the air gap flux constant and not let it saturate. In vector control, the field and torque components of stator current are decoupled, which in turn results in fast dynamic response. In vector control technique, coordinated control of stator current magnitudes, frequencies, and their phases to obtain the control of both flux and torque of IM is possible. According to the acquisition of field angle, vector control schemes are classified as direct vector control (DVC) method and indirect vector control (NC) method. Both these methods use PWM switching technique for switching the inverter. Under direct vector control category, a method which uses Space Vector Modulation (SVM) for switching the inverter is called Direct Torque Control (DTC). All these three methods are used for evaluating the induction motor drive for electric traction. However, a high starting torque of the induction motor alone does not guarantee a high acceleration of the train which also depends upon the wheel-rail interaction. Senini, et al offer a valuable insight into wheel-rail interaction and provide a mathematical formulation for this complex interaction by dividing entire mechanics of traction load into wheel dynamics and train dynamics which is used for modelling the traction load. In this dissertation, the traction load coupled with a vector controlled (all 3 methods viz. DVC, IVC and DTC) induction motor drive with wheel slip control strategy has been simulated to evaluate the performance of the induction motor drive for electric traction.