Introduction to Industrial Automation

3.8.3   Operation of Two Pumps with a Cyclic Alteration, 

Based on a Low-High Demand

Let’s consider a small watering network being supplied from two similar pumps according to the 

demand. This demand is measured directly through the utilization of two pressure sensors, due 

XFMR

Aeration

Aeration channel

Disconnector

switch

Insulators

Buchholz

relay

Conservator

Thermometer

Medium

voltage

switch

Low voltage

main switch

Cable

junction

box 

Figure 3.33  General view of a transformer power station.

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  Introduction to Industrial Automation

0V

+24 V DC

Lamps test

Buchholz

alarm

Thermometer

alarm

d

2

d

3

h

1

d

3

1

2

d

4

h

2

d

4

3

4

d

5

h

3

d

5

5

6

d

7

h

4

d

6

7

8

9

10

11

12

d

2

d

2

d

2

d

2

d

5

d

6

d

3

d

4

d

7

d

7

d

7

Sire

n

tes

t

Sire

n

stop

Buchholz

trip

Thermometer

trip

1–2 = Buchholz alarm signal 3–4 = Thermometer alarm signal 5–6 = Buchholz trip signal 7–8 = Thermometer trip signal 9–10 = Trip coil of medium voltage circuit breaker 11–12 = Trip coil of low voltage circuit breaker

(a)

R

N

d

1

Public power supply

24 V DC 

Power supply of the automation circui

t

+

–

Battery

Transformer

Rectifier

(b)

Fi

gu

re 

3.

34

 

(a

) A

ut

om

at

io

n 

ci

rc

ui

ts 

fo

r 

a 

tr

an

sf

or

m

er 

st

at

io

n 

sa

fe

ty 

op

er

at

io

n, 

an

d 

(b

) c

ha

ng

e 

ov

er 

of 

po

w

er 

su

pp

ly 

of 

th

e 

au

to

m

at

io

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ci

rc

ui

t.

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Industrial Automation Synthesis 

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to the simple fact that a high demand will create a pressure drop. These two pressure sensors are 

tuned into two different nominal pressures, which correspond to a low and a high demand. Since 

the low demand usually lasts longer than the high one, within an operational cycle (e.g., 24 hrs), 

the pump that covers the low demand is called a base pump. Based on a similar approach, the 

pump that covers the high demand is called a peak pump and it is able to operate (in parallel to 

the base pump) for smaller time durations and only for a few times within the operational cycle. If 

the base and peak pumps were always the same, then after a large time window of operation (e.g., 

one year), the base pump would have completed a very large number of operating hours, especially 

when compared to the peak pump. This is a situation that might result in a fault in the base pump 

and thus should be properly avoided through cyclic operation of the base and peak pumps.

The operation circuit for these two pumps is desired to have the following functionalities:

  1. The automation circuit should provide the ability to choose between the manual and the 

automatic operation of each pump separately.

  2. If the base pressure sensor is energized, the based pump should be activated.

  3. If the base pump is in operation and the peak pressure sensor is energized, then the peak 

pump should be set into operation. If the peak pressure sensor is de-activated, then the 

operation of the peak pump should also be stopped.

  4. If the base sensor pressure is de-activated, the based pump should stop. However, as soon 

as the base pressure sensor is activated again, the peak pump should be put in operation 

and considered as the new base pump, and the previously considered base pump should be 

considered now as the new peak pump (cyclic operation). The same procedure of changing 

the consideration among the current base and peak pumps should be followed for every 

operational cycle, until the end of the overall operation where we would desire the complete 

operation stop for both pumps.

The overall automation circuit is presented in Figure 3.35, and contains four auxiliary relays, 

with the relay d

3

 responsible for the cyclic operation of the two pumps.

N

C

1

50 Hz 230 V

R

e

1

C

2

e

2

RS

1

‘H’

‘A’

RS

2

‘H’

‘A’

d

1

d

1

d

4

d

2

d

2

d

4

d

1

d

2

d

3

d

2

d

1

d

3

d

1

d

2

d

1

d

3

d

3

d

2

d

4

1

2

3

4

1–2 = SPST (NO) output contact of the low-pressure sensor

3–4 = SPST (NO) output contact of the high-pressure sensor

Figure 3.35  Automation circuit for a two pump operation with cyclic alternation.

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  Introduction to Industrial Automation