Page 127 - Industrial Power Engineering and Applications Handbook

6/108 Industrial Power Engineering and Applications Handbook
which are the controlling variables, in terms of correction This technology was introduced by Ws ABB of Finland
required and feeds these to the machine through a VSI or to achieve an extremely fast and highly accurate speed
CSI to achieve the required controls. control in an a.c. machine. This is also based on phasor
Mathematical modelling of the machine is a complex control, but the field orientation is now obtained without
subject and is not discussed here. For this, research and using a modulation (PWM) control circuit. The manu-
development works carried out by engineers and the facturer makes use of only motor theory, through a highly
textbooks available on the subject may be consulted. A accurate mathematical motor model, to calculate the motor
few references are provided in the Further reading at the torque directly. There is no need to measure and feed
end of this chapter. In the above analysis we have back the rotor speed and its angular position through an
considered the rotor flux as the reference frame. In fact encoder to the motor model. The controllable variables
any of the following may be fixed as the reference frame now are only 4 and T. It is therefore called a direct torque
and accordingly the motor’s mathematical model can be control (DTC) technique. There is no need to control the
developed: primary control variables V or I and N now as in a flux
phasor control. This scheme eliminates the signal
Rotor flux-oriented control - when the rotor is considered processing time in the absence of a PWM and also an
as the reference frame. encoder circuit, both of which introduce an element of
Stator flux-oriented control - when the stator is considered delay. As these drives control @and Tdirectly, they respond
as the reference frame and extremely quickly and it is possible to achieve a response
Magnetic field-oriented control - when the field is time as low as 1-2 ms. They are thus a corollary to the
considered as the reference frame. sensor-less flux control drives. For reference, a rough
comparison between the various types of drives is given
The rotor flux-oriented control is more popular among in Table 6.2.
different manufacturers to achieve high prccision of speed
control in an induction machine. With this technology Basic scheme of a DTC drive
(any of the three methods noted above), it is now possible A simple block diagram as shown in Figure 6.13 illustrates
to obtain a high performance of the machine, i.e. torque the operation of a DTC drive. It contains two basic sections,
up to 100% of T, at speeds down to zcro. one a torque control loop and the other a speed control
Since the motor’s fixed parameters can now be varied
to suit a particular load requirement, there is no need to loop. The main functions of these two control circuits
are as follows:
pre-match a motor with the load. Now any motor can be
set to achieve the required characteristics to match with
the load and its process needs. Full-rated torque (T,) at 1 Torque control loop
zero speed (during start) should be able to pick up most
of the loads smoothly and softly. Where, however, a higher Section 1
Tst than T, is necessary, a voltage boost can also be provided This measures
during a start to meet this requirement. (See also Section the current in any two phases of the motor
6.16.1 on soft starting.) The application of phasor (vector) the d.c. bus voltage, which is a measure of the motor
control in the speed control of an a.c. motor is shown in voltage
a block diagram in Figure 6.12. the switching position of the inverter unit.

Application Section 2
This is a highly advanced motor model, which is first
FOC drives are capable of providing precise speed control made to read and store the machine’s vital parameters
and are used for applications calling for high performance such as R,, L,, saturation coefficients and its moment
and precision (e.g. machine tools, high-speed elevators, of inertia during an autocalibration run. The motor is
mine winders, rolling mills, etc.). These drives are capable run under a locked rotor condition and the mathematical
of regulating a number of variables at the same instant model is capable of computing its basic characteristics
such as speed. position, acceleration and torque. in terms of these parameters or any data that may be of
use to actuate the control logistics. The block diagram
6.4.3 Direct torque control (DTC) is drawn in an open-loop condition, which would suffice
This is an alternative to FOC and can provide a very fast for most process lines. For still higher accuracy in speed
response. The choice of a static drive. whether through a control, an encoder may be introduced into this circuit,
as shown by the dotted line in Figure 6.13. The quantities
simple Vlfcontrol, field-oriented phasor control or direct measured in Section 1 are also fed into this section,
torque control with open or closed-loop control and
feedback schemes, would depend upon the size of the which are able to compute the actual operating values
machine, the range of speed control (whether required to of T and @ about every 25 p s, i.e. around 40 000 times
every second. These are the output control signals of
operate at very low speeds, 5% N, and below), the accuracy this section.
of speed control and the speed of correction (response
time). The manufacturers of such drives will be the best
guide for the most appropriate and economical drive for Section 3
a particular application or process line. The above actual operating data are fed into a torque