Nonlinear observer based sensorless direct torque control of induction motor

Induction motor speed control is an area of research that has been in prominence for some time now. Recent advances in this field have made it possible to replace the DC motor by induction machines, even in applications that demand a fast dynamic response. Many industrial applications demand speed sensorless operations, due to various reasons. It is also required to strictly maintain the speed of the motor within certain permissible tolerance, irrespective of the load changes that occur in the system. Unless prior knowledge of the load characteristics is known, it is very difficult to compensate for the same. Direct torque control (DTC) of an induction motor is a popular method because of the resulting fast dynamic response of the motor, lower sensitivity to motor parameter variations and relatively low switching harmonics in the inverter. However, the present DTC approach is unsuitable for high performance applications because of the need of a speed sensor for increased accuracy, the absence of any error decay mechanism, and the requirement of prior knowledge of the load or disturbance characteristics. In this paper, a nonlinear observer is designed for the stator flux, speed and load torque estimation that will take care of the above limitations. The estimated values along with other measured states are used for the closed loop speed sensorless control operation of the induction motor. Simulations are done and the results discussed.