Introduction to Industrial Automation

Figure 7.62  The conventional Y/

Δ

 automation circuit and its implementation in a PLC. (a) Power 

circuit of star-delta (Y/

Δ

) motor, (b) conventional Y/

Δ 

automation circuit and (c) implementation 

of Y/Δ automation with PLC.

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  Introduction to Industrial Automation

implement the Y/

Δ

 operation. However, in a few cases it is possible to experience an operating fault 

and a three-phase short circuit if for any reason, mechanical or other, the relay C

3

 is delayed in deac-

tivation with respect to C

2

 at the time of change. This operation can be easily observed by inspecting 

the power circuit of Figure 7.62a, so if this does occur, then the star node is transferred from the ends 

of the motor windings to the three phases of the network, thus causing the three-phase short circuit. 

Although the probability that this fault exists is small, in a conventional automation circuit it is not 

taken into account, and the automation is implemented with one timer. In the case of programmable 

automation, where the PLCs have a plurality of timers, two timers can be used, one to measure the 

time of change from the star to delta configuration and the second to enter a delay of 2–3 s between the 

deactivation of the C

3

 relay and the activation of C

2

. In this way, any possibility of a three-phase fault is 

eliminated. Figure 7.63 shows the classic automation circuit that implements the above logic with two 

timers. The timer T

1

 introduces the delay between the relays Y and 

Δ

, while the timer T

2

 determines 

the time instance of change from Y to 

Δ

. For the same I/O connections to the PLC of the Figure 7.62c, 

the required Boolean language program will be in the following form:

A

I0.3

A(

O

I0.0

O

M20.0

)

=

M20.0

A

M20.0

A

T2

L

‘2  sec’

SD

T1

A

M20.0

L

‘10 sec’

SD

T2

A

M20.0

AN T2

AN I0.5

=

Q2.6

A

M20.0

A

T1

AN I0.7

=

Q2.3

A

M20.0

A(

O

I0.7

O

I0.5

)

=

Q2.0

BE

Parking fullness check. In Figure 7.64, there is a parking lot whose entry and exit of cars are elec-

tronically controlled by the photocells PC

1

 and PC

2

, respectively. In this case, it is desired for the 

counter to “monitor” the number of cars inside the parking lot so that the PLC will notify drivers 

whether or not the lot is full. In particular, if the number of cars within the lot exceeds 950, it is 

desired to activate a warning light (h1), and if the number of cars reaches the lot limit, a siren will 

sound to prevent other cars from entering. With the set button, the counter is set at an initial value 

+24 V DC

STOP

START

C

3

d

d

C

3

e

0 V

C

3

C

2

d

T

1

T

2

C

1

C

2

T

2

T

1

C

2

T

2