Introduction to Industrial Automation

3.2   Step-by-Step Basic Automation Examples Synthesis

3.2.1   Motor Operation with Thermal Overload Protection

In Section 2.2.1, the thermal overload relay has been described, which has a specific utilization in 

protecting the motor form an overcurrent. As has been explained, the thermal overload relay is not 

directly breaking the power circuit, but it is happening indirectly and through the utilization of a 

supporting switching contact in the automation circuit. Today, all the motors are being supplied 

through an overcurrent thermal protection relay and thus the utilization of the corresponding 

thermal protection auxiliary contact in the automation circuit will be considered an obligatory 

component. The corresponding automation circuits shown in Figures 3.1a and 3.1d with thermal 

overload protection are depicted in Figure 3.3. The NC contact of the thermal relay is always 

included in the automation circuit. As long as the motor is consuming the nominal current (in 

which the thermal relay has been tuned), the automation circuit allows the operation of the motor. 

In the case that the consuming current becomes bigger than the nominal one, the thermal over-

load protection relay will react and will open the contact “e”. Since contact “e” is connected in 

series to the coil of the power relay, its opening will cause the deactivation of the relay and thus it 

will lead to the overall stop of the motor.

50 Hz 230 V

R

N

RS

C

50 Hz 230 V

R

N

C

Sensor

Command

Control

panel

Automatic operation

Hand operation

(a)

(b)