Due to power electronic switches, variable speed of motor drive system using various control system have been generally used in many applications, such as direct torque control.
DIRECT TORQUE CONTROL:
Due to it efficiency and low sensitive to parameter variation it have been generally accepted in the control of motor speed widely in all industrial applications because of its technique.
Despite its importance, it has a major setback associated with it. That is the large torque and flux ripple at steady state operation of the motor. These ripples can affect the accuracy of speed consideration of motor.
Effort have been made using the space vector modulation and the multi-level inverter methods to reduce these ripples. These methods when used though, achieved some degree of success in reducing the ripples but they are difficult and costly to implement.
In this chapter, a lot of control techniques are discussed, the work done in reducing the torque and flux ripples using direct torque control method is highlighted. The proposed fuzzy logicwith duty ratio control is equally treated in detail.
In DTC drives, the uncoupling of the torque and flux components are Source Inverter (VSI).achieved by using hysteresis comparators which compares the actual and consideredvalues of the electromagnetic torque and stator flux. The DTC drive consists of DTCcontroller, torque and flux calculator, and a Voltage
2.2 Principle of direct torque control of induction motor:
In a direct torque controlled induction motor drive, it is possible to control directly the stator flux linkage (s?) or the rotor flux (r?)or the magnetizing flux (m?) and the electromagnetic torque by the selection of an optimal inverter voltage vector. The selection of the voltage vector of the voltage source inverter is made to restrict the flux and torque error within their respective flux and torque hysteresis bands and to get the fastest torque response and highest efficiency at every instant. DTC enables both quick torque response in the transient operation and reduction of the harmonic losses and acoustic noise.