Introduction to Industrial Automation

1.4   Industrial Automation Circuits

In Section 1.1 it has been mentioned that the automation of an industrial system is being achieved 

through the utilization of automation circuits. These automation circuits will be analyzed in detail 

in Chapters 3 and 4, where their design principles and operations will be presented and through 

the utilization of numerous realistic examples. In Chapter 7, which focuses on PLCs, the main 

effort will be in transforming these automation circuits into proper software programs for PLCs. 

The design of the automation circuits, from the perspective of a functional design, consists of 

fundamental knowledge for all industrial automation engineers in order to understand the indus-

trial operation; identify the needs; design and simulate the solution to the industrial automation 

problem; and, of course, produce the optimal solution from a cost-related perspective, either in the 

form of classical industrial automation or in the form of software for PLCs. In Chapter 1, we will 

refer only to the scope of automation circuits, their general form, and their alteration from other 

types of industrial circuits.

In the broad discipline of electrical engineering, many types of electrical circuits are involved, 

such as, for example, the electrical circuits of basic electrical components (R, L, and C), electronic 

circuits, power circuits, telephone circuits, integrated circuits, and many more. However, the elec-

trical circuits being utilized for the study and implementation of an industrial manufacturing 

plant can be divided into the following categories:

 

◾

Power circuits

 

◾

Automation circuits

 

◾

Wiring diagrams

Power circuits (also called “main circuits”) indicate the type of power supply for the utilized 

motors and all other related power devices. As an example, Figure 1.10 depicts: (a) the single-line, 

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Industrial Automation 

◾

 

11

three-phase circuit of a motor with two directions of rotation; (b) the complete multi-line circuit of 

the same motor; and (c) the power circuit of a direct starting motor. More details on the different 

types of motor starting, rotation inversion, etc., will be presented in Chapter 3.

Automation circuits (which can also be referred to as control circuits, auxiliary circuits, sec-

ondary circuits, or schematic circuits) represent the operational logic and control of the power 

devices, as indicated in Figure 1.11, for a start/stop operation of the previously depicted motor in 

Figure 1.10c.

Wiring diagrams are circuits representing both the power circuit and the automation circuit 

while, at the same time, representing the actual positioning of all the devices and components 

in the industrial installation, which is ideal information for the technician executing the wiring 

and overall installation. In Figure 1.12, the wiring diagram which is produced from the synthesis 

of the power circuit shown in Figure 1.10c, and the automation circuit shown in Figure 1.11, is 

displayed.

3~ 50 Hz 280 V

50 Hz 380 V

50 Hz 380 V

Switch

R

S

T

R

S

T

Fuse

Contactor

Overload

relay

Motor

M

3 ~

M

3 ~

M

3 ~

(b)

(c)

(a)