Introduction to Industrial Automation

Figure 2.5  The pneumatic cylinder as linear motion actuator.

---

Hardware Components for Automation and Process Control 

◾

 

25

2.1.4   Relays

Since electric motors are supplied with electric power through the utilization of relays or contac-

tors, their operational principles and characteristics are presented in this chapter as a specific kind 

of actuator. Another reason for describing relays in this chapter is the need to know their structure 

and operation in order to utilize them in designing and synthesizing automation circuits, which 

will be presented in Chapter 3.

In general, a relay is a binary actuator as it has two stable states, either energized and latched 

or de-energized and unlatched, while a servomotor, for example, is a continuous actuator because 

it can rotate through a full 360° of motion.

Some synonymous terms, such as “magnetic contactor”, “relay”, “remote operated switch”, and 

“tele-operated switch” are used for an electromechanical device that opens and closes electrical 

contacts, and should not create confusion to the reader, since all of these terms refer to an electri-

cally operated switch that has the same principle of operation, and these terms are often used inter-

changeably. A soft distinguishing between the terms “relay” and “contactor” is that the second one 

is used for powering large electric motors. In the rest of the book, the term “relay” will be adopted 

as it is the most widely used one. There are many types and categories of relays, each one developed 

to satisfy the various specific needs of several applications. From a construction point of view there 

are two types of relays, the power relays and the general purpose or control relays, both having the 

same principle of operation described below.

2.1.4.1   Relays’ Operation Principle

Relays consist of two parts; a first one containing the various main or auxiliary electric contacts, 

and a second one containing the electromagnet-based mechanism that creates the motion required 

for the operation of the electric contacts. Figure 2.6 shows a simplified form of a relay, including 

the coil-core electromagnet, the movable arm, the electric contact consisting of two parts (the 

fixed and the movable one), and the return spring. The relay’s contacts are electrically conductive 

230 V 

230 V 

F

Remote

control

distance

1-phase motor

Power circuit

Auxiliary circuit

Electric contact

Wire terminal

RS

Core

Coil

Return

spring

Insulation

I

C

I

P