Introduction to Industrial Automation

Figure 3.22  Example of a pump which stops if there is no fluid flow.

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Industrial Automation Synthesis 

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fault of the pump itself, the absence of liquid, the blocking of pipe, etc. The necessary automation 

circuit is presented in Figure 3.23a. The time responses of the most characteristic components are 

presented in the same figure and correspond to the two potential cases of presence or absence of 

flow in the pipes, after the operation of the pump. It should be additionally noted that this circuit 

covers also the case where the flow will stop in an unforeseen time after a proper operation of the 

pump.

In this specific case, the pump will stop operating after a time T from the time instance that 

the flow has stopped. Thus, the operator would like to put the pump in operation again, without 

being able to recognize the reason for the pump having stopped in order to resolve the previous 

fault situation. For this reason, a more complete automation circuit should energize and retain 

an indication of operation canceling due to a flow termination case, in order to be further dis-

tinguished from the case of stopping due to a thermal protection error. The automation circuit 

shown in Figure 3.24 is fully equipped with all the necessary indications (operation, thermal fault, 

(a)

T

OFF

ON

C

t

t

t

t

Start

T

Closed

Open

T

t

Flow

switch

Closed

Open

OFF

ON

C

t

t

t

t

Start

T

Closed

Open

T

t

Flow

switch

N

C

50 Hz 230 V

R

Stop

Start

e

C

T

C

Flow switch

NC contact 

T

or

(b)

(c)