Due to advances in power electronic switches
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).
2.2 Principle of direct torque control of induction motor:
In a direct torquecontrolled (DTC) induction motor drive, it is possible tocontrol 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. Theselection of the voltage vector of the voltage source inverter is made to restrict the flux andtorque error within their respective flux and torque hysteresis bands and to get the fastesttorque 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.
The Benefits of using DTC include the following:
1 No need for motor speed or position feedback in 95% of applications. Thus, installation of costly encoders or other feedback devices can be avoided.
2DTC control is available for different types of motor including permanent magnet and synchronous reluctance motors.
3Accurate torque and speed control down to low speeds, as well as full startup torque down to zero speed.
4 Excellent torque linearity.
5 High static and dynamic speed accuracy.
6 No preset switching frequency optimal transistor switching is determined
2.2.1 Voltage Source Inverter
A six step voltage source inverter provides the variablefrequency AC voltage input to the induction motor in DTC method. The DC supply to theinverter is provided either by a DC source like a battery, or a rectifier supplied from a three phase or single phase AC source. Fig. 2.2 shows a six step voltage source inverter. Theinductor L is inserted to limit short circuit through fault current. A large electrolytic capacitor C isinserted to stiffen the DC link voltage.
The switching devices in the voltage source inverter bridge must be capable of being turned OFF and ON. Insulated gate bipolar transistors (IGBT) are used because they can offer high switching speed with enough power rating. Each IGBT has an inverse parallel-connected diode. This diode provide alternate path for the motor current afterthe IGBT, is turned off.
Figure 2.2 Voltage Source Inverter
Each leg of the inverter has two switches one connected to the high side (+) of the DC link and the other to the low side (-); only one of the two can be ON at anymoment. When the high side gate signal is ON the phase is assigned the binary number 1, andassigned the binary number 0 when the low side gate signal is ON. Considering thecombinations of status of phases a, b and c the inverter has eight switching modes(Va,Vb,Vc=000-111) V2 (000) are zero voltage vectors V0 (000) and V7 (111) where the motor terminals are short circuited and the others are nonzero voltage vectors V1 to V6
The six nonzero voltages space vectors will have the orientation, and also shows the possible dynamic locus of the stator flux, and its differentvariation depending on the VSI states chosen. The possible global locus is divided into sixdifferent sectors signaled by the discontinuous line. Each vector lies in the center of a sectorof width named S1 to S6 according to the voltage vector it contains.
It can be seen that the inverter voltage directlyforce the stator flux, the required stator flux locus will be obtained by choosing theappropriate inverter switching state. Thus the stator flux linkage move in space in thedirection of the stator voltage space vector at a speed that is proportional to the magnitude of the stator voltage space vector. By selecting one after another the appropriate stator voltage vector, is then possible to change the