Induction Motor Operated In Vector Control Mode and Direct Torque Control Mode: A Comparative Study

Abstract

Variable speed operation of IM drives can be generally classified as scalar control and vector control. Scalar control is used for low performance drives where only the magnitude and frequency of stator voltage or current is regulated. The most commonly used scalar control technique is the constant volts/Hertz (V/f) control, which offers moderate dynamics performance and is therefore used in application where high speed precision is not required such as fans, pumps and elevators. These control methods results in poor torque and flux response. High performance induction motor drive can be implemented by using vector control or direct torque control. For the induction motor controlling to achieve variable speed, Vector Control (VC) or Field Oriented Control (FOC) and Direct Torque Control (DTC) have become the industrial standard. The vector control technique decouples the two component of stator current: one is responsible for controlling torque and other is controlling flux independently as in the case of separately excited fully compensated DC motor. Direct torque control technique also controlling torque and machine flux independently, but not same as DC motor or Vector control technique. In DTC, torque is controlled by controlling the angle between stator flux and rotor flux as torque is proportional to sine of that angle and flux is controlled by the stator voltage space vector. The closed loop control of Vector control and Direct Torque Control of IM drives required accurate information of speed or rotor position. This information is provided by Techo-generator or encoder. However the use of speed sensors has many drawbacks such as, higher cost, lower reliability, increase in weight and size and difficulty to use in harsh environment. To overcome this difficulty, sensorless speed estimation algorithm for induction motor drive can be used to eliminate speed sensor for both the schemes. This report work aimed to give a contribution for detailed comparison between the two control technique (FOC and DTC) based upon the various criteria including basic control strategy, steady state and dynamic performance and implementation complexity. Steady state and dynamic response of vector control and direct torque control under various operating condition such as starting, speed reversal, load application and load removal for both with and without speed sensor is simulated and examined in MATLAB environment using Simulink and Sim Power System toolboxes. The analysis has been carried out on the basis of result obtained by numerical simulations. The simulation and evaluation of both control technique are performed using voltage source inverter fed three phase squirrel cage induction motor for 30HP, 5HP and 2HP ratings.