Introduction to Industrial Automation

Example 4.1:  START/STOP Operation of a Motor

Let’s consider a machine operating with a direct on line starting motor. From the analysis in 

Chapters 2 and 3, it was clear that the automation circuit will contain a power relay C, which will 

control the power supply of the motor, and inherently has two states: C=0 where the motor is not 

in operation and C=1 where the motor is in operation. These states, being the possible operating 

states of the machine, can be represented by two corresponding circles, indicated in Figure 4.5. 

It is also defined from the desired industrial application that the transition between these states 

will take place through the utilization of two buttons b

1

 and b

0

 for the START and STOP opera-

tions, correspondingly. At this point it should be highlighted that the type of the devices causing 

the transitions are completely dependent on our selections, e.g., it can be an SPDT contact of 

a sensor, the contact of a button, a relay, etc. In these cases, even if we have selected the type of 

the transition device, we do not need to define the type of the contacts (e.g., one contact will be 

NO or NC), since this is something that will be produced from the logical design that we are 

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Logical Design of Automation Circuits 

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123

introducing in Chapter 4 directly. Thus, for this example, we can define state C based on Step 3, 

so that:

 

Turn OFF of C b

Turn On of C b

and

=

=

0

1

Subsequently, the logical formula (1) of Step 4 should be calculated, in this case, for the C state as 

follows:

 

C b b C

=

+

⋅

0

1

(

)

where for this expression, the digital implementation is the one depicted in Figure 4.6.