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Introduction to Industrial Automation
pieces of metal that, when contacted together, complete a circuit and allow the circuit’s current to
flow, just like a simple switch. The operation of the relay is achieved by forming two independent
and isolated circuits, the power circuit and the auxiliary or the control one. The power circuit
refers always to the electrical device usually called “load”, which is powered through the relay
contact, while the auxiliary circuit refers always to the coil of the relay. Since the auxiliary circuit,
which is explained in Chapters 3 and 4, performs the automated operation of a machine, it is also
called the “automation circuit”. The very simple form of the auxiliary circuit shown in Figure 2.6
should not lead the reader to false conclusions. As is explained in Chapter 3, the complicated and
interdependent operation of many relays in a complex machine requires usually very complicated
automation circuits. As a kind of definition, the auxiliary circuits of a large number of relays in a
complex industrial machine or production line, embedded all in one common circuit, constitute
the automation circuit.
The power circuit consists of the relay’s electric contact, the electric source that provides the
electric power and the load. Generally, in the considered industrial applications, the electric source
will be always the public electric power network and the powered load a motor that, in this case,
is a single-phase motor. The auxiliary circuit consists of the coil, the electric power source (usu-
ally different and independent from the corresponding source of the power circuit), and the hand
switch RS. When the switch RS closes, 230 V is applied on the coil and the generated magnetic
field attracts the movable arm due to the generated force F to the direction shown in Figure 2.6.
The movable arm carrying on the insulation material and the movable contact part, causes the
closing of the electric contact, thus permitting the electric current to flow in the power circuit
and hence the motor to operate. If the hand switch RS opens, the magnetic field is nulled and the
attraction of the movable arm stops. The return spring then brings back the movable arm, the
relay’s electric contact opens and the motor stops. The main result is briefly summarized as follows:
RS is closed
Relay is energized
Motor operates
→
→
RS is open
Relay is de-energized
Motor does not operat
→
→
ee
Therefore, the possible operational states of the relay are two: the first one corresponds to the
voltage applied on the relay coil; then it is said that the relay is energized or simply the relay is ON;
and the second one corresponds to the voltage not applied on the relay coil, then it is said the relay
is de-energized or simply the relay is OFF. These two states of the relay operation ON and OFF
can be corresponded to the digital logic signals 1 and 0, a property that will be invoked later in
Chapter 4 concerning the logic design of the automation circuits.
The current I
C
in the auxiliary circuit is generally low, of the order of a few tens of mA. Instead,
the current I
P
of the power circuit may be very high, depending on the durability of the electric
contact and is the basic parameter that determines the size of the relay. Various types of power
relays exist in the market where their electric contacts have the ability to rate up to 2000 A. In
general, the hand switch RS could be far from the physical position of the relay, even at a distance
of a few tens of meters, while the distance limit is introduced by the permitted voltage drop across
the lines of the auxiliary circuit. This voltage drop, and hence the maximum permitted remote
control distance, depends on the characteristics of the wires of the auxiliary circuit, the nominal
coil voltage, and the kind of supplied voltage (AC or DC). The nominal coil voltage of a relay is
selected from a set of standardized voltages that are the most usual in the market, as shown in
Table 2.1. The power absorbed by a coil (self-consumed power of a relay) depends on the relay size
Hardware Components for Automation and Process Control
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27
and is usually of the order of a few watts. This absorbed power is greater during the activation of
the relay, when the inertia torque of the movable arm should be overcome and is reduced when the
activated state of the relay has been achieved. The time required for the activation and deactiva-
tion phases of the relay depends on the type of relay and is usually rated up to 10–50 msec. Since
the described relays include mechanical and electrical components, they are also called “electro-
mechanical relays” in order to be distinguished from the “solid state relays” consisting only of
electronic and semiconductor components (transistors, thyristors, and triacs) and circuits.
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