Introduction to Industrial Automation

Figure 3.5  Machine operation with a start-up delay (circuit No.1).

50 Hz 230 V

R

N

RS

T

C

T

T

OFF

ON

RS

C

OPEN

CLOSED

t

t

Circuit features:

•Permanent operation or stop of machine C

•Reoperation 

C

C

t

V

T

Figure 3.6  Machine operation with a start-up delay (circuit No.2).

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  Introduction to Industrial Automation

functionality. As can be observed, in this case, the voltage V

T

 is set to zero after the elapse of 

time T.

The automation circuit shown i

which can be found in many cases, which we will consider subsequently. This problem consists 

of the synchronization of two or more switching contacts of the same relay, during the change of 

their state. For further comprehension of this problem, let’s consider that the NO contact of C 

delays to change its switching state, when compared with the NC contact of C. This means that 

the NC contact will open first and before the closing of the NO contact. In this case, the timer will 

be deactivated, the opening of the timer’s contact T will happen before the creation of the alterna-

tive current route, through the NO contact of C, and finally this will result in the de-energizing of 

relay C. Such an operation of the automation circuit is wrong, and thus a proper redesign should 

be provided for dealing with this issue. In normal operations, such as a problematic delay of one 

contact with respect to the others does not exist. However, the problem still remains, even in the 

cases of simple automation circuits with usual relays and thus the question “will the NO contact 

of a relay close before the opening of the corresponding NC contact?” arises. In this question, a 

clear and generic answer cannot be provided. Due to the inertia of the relay’s mechanical parts 

and their acceleration during the relay operation, usually the synchronization of the contacts’ 

state change is satisfied. In the general purpose miniature relays, the printed circuit board (PCB) 

relays, or in the case of low-cost relays where the moving parts are of a low mass and with a short 

translation of the movable contacts, this problematic synchronization might be evident. In these 

cases, the automation circuit should be redesigned in order to satisfy the proper sequential logic 

of instances and states.

In the case that it is desirable that the automation circuits o

ent the feature of reoperation, the switch RS must be replaced by a couple of START and STOP 

buttons. The corresponding automation circuits are presented i

•Permanent operation or stop of machine C

•Non reoperation

•Timer continuously under voltage during operation 

 

N

T

C

T

50 Hz  230 V

R

Stop

Start

d

d

e

C

C

N

T

C

T

50 Hz 230 V

R

Stop

Start

d

d

e

•Permanent operation or stop of machine C

•Non reoperation

Features of the two circuits: