Due to advances in power electronic switches, and microprocessors, variable speed drive system using various control system have been generally used in many applications, some of them include field oriented control, or vector control, sensor less vector control and direct torque control.
DIRECT TORQUE CONTROL:
Due to its 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, the 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 logic with duty ratio control is equally treated in detail.
In DTC drives, the uncoupling of the torque and flux components are achieved by using hysteresis comparators which compares the actual and considered values of the electromagnetic torque and stator flux. The DTC drive consists of DTC controller, torque and flux calculator, and a Voltage Source Inverter (VSI).
2.1METHOD OF OPERATION
Direct torque control (DTC) is one way of using variable frequency drives to control the torque and finally the speed of three phase AC electric motors. This involves calculating an estimate of the motors magnetic flux and torque based on the evaluated voltage and current of the motor.
Stator flux linkage is considered by integrating the stator voltages. Torque is considered as a cross product of considered stator flux linkage vector and evaluated motor current vector. The considered flux or torque deviates from the reference motor than permitted tolerance, the transistors of the variable frequency drive are switched OFF and ON in such a manner that the flux and torque errors will come back in their tolerant bands as fast as possible. Thus, direct torque control is one form of the hysteresis control. The direct torque method performs very well even without speed sensors. However, the flux consideration is usually based on the integration of the motor phase voltages. Due to the inevitable errors in the voltage measurement and stator resistance estimated integrals tend to become erroneous at low speed. Thus, it is not possible to control the motor if the output frequency or the variable frequency drive is zero. However, by careful design of the control system, it is possible to have the minimum frequency in the range of 0.5Hz to 1Hz which is enough to make it possible to start an induction motor with full torque from a standstill situation.