|
79
Chapter 3
Industrial Automation
Synthesis
3.1 Introductory Principles in Designing Automation Circuits
3.1.1 The Latch Principle
Let’s consider a machine operating with the help of an electric motor. The motor is a direct start
type and is being controlled by the utilization of a relay C. In Section 1.1 it has been analyzed that
the automation circuits are actuating on the power relays, supplying subsequently the motors or
other industrial devices with the appropriate electrical power. In this case, the automation problem
can be stated as follows: What automation circuit can we select to energize or de-energize the relay C
and thus directly control the operation or non-operation of the motor?
A straightforward solution to this problem is the electrical circuit in Figure 3.1a. In this design,
with the help of the RS switch having two switching positions, we can achieve the permanent
operation of the motor in position 1 and the permanent non-operation in position 0. The term
“permanent” means that if the operator leaves this control panel, the motor will continue to oper-
ate or shut down according to the selected state. This does not happen in the case of the automation
circuits shown in Figure 3.1b and 3.1c, where the pressing of a button energizes the relay C. In the
case that an NO button is utilized (Figure 3.1b), there is the option of a permanent stop but instant
operation of the motor, as long as the button is pressed. In contrast, if an NC button is utilized
(Figure 3.1c) there is the option of a permanent operation, but with an instant stop, as long as the
button is pressed. The term “instant operation” does not only mean a moment in time but also con-
tains the meaning of the time duration. This property comes from the construction of the button
and the mechanism of the (spring based) automatic return in the relaxation position and depends
on how long the operator keeps pressing the button. In the most common cases, the buttons are
being pressed instantly and for a very short time interval. If for any reason the button (e.g., the NO
button in Figure 3.1b) remains pressed for a long time interval, then the operation of the motor
will last for a corresponding amount of time, but for simplicity this is characterized as “instant”.
Therefore, the circuits in Figures 3.1b and 3.1c are not suitable for the usual and practical cases
of machine operation in general. As a result, the following problem is formulated: Is it possible to
80
◾
Introduction to Industrial Automation
utilize only buttons to achieve the same operation depicted in Figure 3.1a, which means the potential of
a permanent stop or permanent operation of a machine in general?
The solution to this problem is provided by the automation circuit in Figure 3.1d, which oper-
ates as follows. When the circuit is idle (there is no voltage among the power lines R and N), the
relay C is de-energized. By pressing the START button, the voltage R-N is applied on the relay’s
coil, and thus the relay C is energized. After this, the NO contact of relay C (which is connected
parallel to the START button) closes, and thus an alternative route to the current is provided. In
this way, when the START button has been released and its contact is open again; the relay C
still continues to be energized, based on the previously formulated alternative route of the current.
Due to the fact that the relay C remains constantly energized from the flow of current through
its own contact, this situation is called as “self-latch” for relay C, while the NO contact of C is
called a “self-latching contact”. In this energized state, the relay C remains as long as it is desired
from the automation specifications. By pressing the button STOP, the application of voltage to
the relay’s coil is interrupted, and thus the relay is de-energized. After releasing the button STOP,
the relay C is not energized again, since the contact of the button START and the self-latching
contact remain open. Thus, the electric circuit shown in Figure 3.1d is able to provide a permanent
STOP or START operation of the machine. However, it should be highlighted that the electric
circuits in Figure 3.1a and 3.1d do not have the same operation, since they are characterized by a
very important difference, which is going to be explained in the following.
Let’s assume the switch RS in Figure 3.1a is in the 1 position, which means that the machine
is operating. If a general shutdown of the power supply network is assumed, then the automation
circuit (and the power circuit also) will be de-energized and the machine will stop. In the case that
the power supply network is restored, the machine will operate by itself, without any intervention
by a human operator, which is a case that could be totally catastrophic or dangerous for specific
kinds of machines or industrial processes. For example, the smashing machines of various materials
are not supposed to start when filled with the materials for smashing. In this case, these machines
should initially start operating and subsequently the materials to be smashed should be supplied
to the machine, otherwise the machine will be blocked and may be destroyed by the overload. In
another case, it could be assumed that the multiple machines are operating sequentially due to
230 V
R
N
STOP
START
C
C
Self-latching
contact
230 V
R
N
b
C
230 V
R
N
C
230 V
R
N
RS
C
b
(a)
(b)
(c)
(d)
The two circuits are not identical
Do'stlaringiz bilan baham: | 
