SENSORLESS VECTOR CONTROLLED INDUCTION MOTOR DRIVE

Abstract

Vector control is becoming the industrial standard for induction motor control. The vector control technique decouples the two components of stator current space vector: one providing the control of flux and the other providing the control of torque. The two components are defined in the synchronously rotating reference frame. With the help of this control technique the induction motor can replace a separately excited dc motor. The DC motor needs time to time maintenance of commutators, brushes and brush holders. The main effort is to replace DC motor by an induction motor and merge the advantages of both the motors together into variable speed brushless motor drive and eliminate the associated problems. The squirrel cage induction motor being simple, rugged, and cheap and requiring less maintenance, has been widely used motor for fixed speed applications. So with the implementation of vector control, induction motor replaces the separately excited do motor. The vector control technique is therefore a better solution so that the control on flux and torque become independent from each other and the induction motor is transformed from a non-linear to linear control plant. However the prerequisite for the correct operation of the vector controller is an accurate knowledge of the rotor velocity which requires speed sensors such as shaft mounted tacho-generators, resolvers or digital shaft position encoders. These degrade system's reliability and reduce the advantage of an induction motor drive system. In some cases it is difficult (e.g. submarine applications) to use sensors for speed measurement. This has led to speed sensorless vector control in which the speed is estimated using some of the estimation methods rather than using speed sensors. In this dissertation work the three schemes of the open loop speed estimators using monitored stator voltages and currents are used and compared. The performance of these schemes is evaluated based on various operating conditions such as starting ,speed reversal , load perturbation (load application and load removal) and is simulated in MATLAB 7.6 using simulink and simpower system block set toolboxes.