COMPARATIVE EVALUATION OF VECTOR CONTROL AND DIRECT TORQUE CONTROL INDUCTION MOTOR DRIVES

Abstract

Vector Control and Direct Torque Control are becoming the industrial standards for induction motor control. The vector control technique decouples the two components of stator current: one providing the control of flux and the other providing the control of torque. The flux as well as the torque level of the machine is controlled with perpendicular components of the stator current vector in the synchronously rotating reference frame (SRRF). Thus a current control loop is usually realized which controls the stator current and calculates the necessary inverter switching states. In Direct torque control to achieve the decoupling of flux and torque components, the flux and the torque errors are processed in hysteresis controller. The inverter switching states necessary to achieve the flux and torque levels are then directly obtained from a switching table with the stator flux angle as additional input. The motor torque and flux become direct controlled variables and hence, the name — Direct Torque Control. This work gives a contribution for a detailed comparison between the two control techniques, emphasizing upon the advantages and disadvantages respectively. The performance of the two control schemes is evaluated based on various criteria including the complexity of control and the dynamic performance. The dynamic response of vector control and direct torque control under various operating conditions such as starting, speed reversal and load perturbation (load application and load removal) is simulated and examined in MATLAB 7.3 environment using simulink and power system blockset toolboxes. The analysis has been carried out on the basis of the results obtained by numerical simulations. The simulation and evaluation of both control strategies are performed using voltage source inverter fed three phase squirrel cage induction motor for 1HP and 30HP ratings respectively. iii