Introduction to Industrial Automation

tion in case of overload relay trip-reset.

R S T

Μ

3 ~

R΄  S΄ T΄

e

2

C

2

C

1

e

1

N

50 Hz 230 V

R

Stop

Start

1

e

2

C

2

C

1

e

1

C

2

Start

2

C

1

C

2

C

1

(a)

(b)

Figure 3.18  (a) Power and (b) automation circuits of a motor connected in two different supply 

networks.

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Industrial Automation Synthesis 

◾

 

95

3.4   Automation Circuits for Motors

3.4.1   Motor with Inversion in Rotation

For every kind of motor, the theory of electrical machines provides the schematics for both the 

power circuit and the functional operation of the machine, or the operation sequence of the 

relays that supply the electrical motor with power. For a motor with two rotation directions (with 

inversion), the power circuit is presented in Figure 3.19. When the relay C

1

 is energized, the coils 

of the motor are connected to the R, S, and T phases; while when the relay C

2

 is energized, the 

coils are connected to the T, S, and R phases. In the first case, the motor is rotating clockwise, 

while in the second case the motor is rotating counterclockwise. Additionally, in the presented 

case, the motor is being protected by the thermal overload protection (e), while three buttons 

are needed for the manual operation of the motor; one for the clockwise rotation, one for the 

counterclockwise rotation, and one for pausing the motor operation (STOP). The energizing of 

one of the power relays should have a mutual exclusion with the other relay, since synchronal 

energizing of the two relays, even for a very short time, will cause a direct short circuit. All these 

desired operations are satisfied through the automation circuit shown in Figure 3.20a, which 

also includes a light indication for every direction of rotation and a mechanical latch. With this 

circuit, it is not possible to change the motor’s direction of rotation, while it is in operation, since 

it is needed initially to stop the motor and afterwards to press the button corresponding to the 

desired direction of rotation. Moreover, the circuit provides the ability for constant operation. In 

cases where an instant operation is needed (as long as the buttons are being pressed) e.g., in the 

case of the cranes, a simpler automation circuit can be utilized, as shown in Figure 3.20b. The 

buttons with double actuation contacts for latching during start could also be utilized for both 

presented automation circuits.

R S T

Μ

3 ~

C

2

C

1

e

R

S

T

R

S

T