Introduction to Industrial Automation

Example 4.2:  START/STOP Operation of a Motor with Thermal Overload Protection

In Example 4.1, the issue of thermal protection for the motor has not been taken under consider-

ation. If we would like the automation to consider this option (as we already know, the activation of 

a thermal relay [e] will stop the motor) using the same approach so that the STOP button b

0

 would 

stop the motor, only the logical expression for Turn OFF would change into:

 

Turn OFF of C b

e

=

+

0

Subsequently, the START-STOP operation of the motor from a secondary control position will 

need to be controlled through the buttons f

0

 (for STOP) and f

1

 (for START). In this case, the state 

diagram will be altered to the following one, presented in Figure 4.7, where we have:

 

Turn OFF of C e b

f

Turn On of C b f

and

= + +

= +

0

0

1

1

C=0

C=1

b

1

b

0

C=0

The power relay is not energized

or the machine does not operate

C=1

The power relay is energized

or the machine operates