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 induction 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 induction motor. These ripples can affect the accuracy of speed consideration of induction motor.
In the past, 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 various control techniques are discussed, the work done in reducing the torque and flux ripples using directtorque 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 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 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. (Ludike and Jayne, 2012).
Figure 2.1 Control method
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 stator flux in the required method. If an increase of the torque is required then the torque is controlled by applying voltage vectors that advance the flux linkage space vector in the direction of rotation. If a decrease in torque is required then zero switching vector is applied, the zero vector that minimize inverter switching is selected.
In summary if the stator flux vector lies in the k sector and the motor is running anticlockwise then torque can be increased by applying stator voltage vectors Vk+1 or Vk-1, and decreased by applying a zero voltage vector V0 or V7. Decoupled control of the torque and stator flux is achieved by acting on the radial and tangential components of the stator voltage space vector in the same directions, and thus can be controlled by the appropriate inverter switching. In general, if the stator flux linkage vector lies in the “k” sector its magnitude can be increased by using switching vectors Vk+1 for clockwise rotation or Vk-1 for anticlockwise rotation and can be decreased by applying voltage vectors Vk+2 for clockwise rotation or Vk-2 for anticlockwise rotation. (Luder and Jayne, 2012). In Accordance with figure 2.1, the general table 2.I can be written. It can be seen from table 2.I, that the states Vk and Vk+3 , are not considered in the torque because they can both increase or decrease the torque at the same sector depending on the stator flux position.