Introduction to Industrial Automation

Figure 2.18  General purpose solenoid actuators with/without a return spring.

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  Introduction to Industrial Automation

physical quantity. In this sense, digital sensors with discrete outputs will be mainly covered in this 

section, but some transducers will also be included.

2.2.1   Thermal Overload Relay

Thermal overload relays are devices for protecting motors from overcurrent situations that may 

cause them damage. Before proceeding to their detailed description, let’s discriminate some issues 

regarding their operation. Unlike their widely used name, these devices are not relays like the ones 

described in Section 2.1.1, but sensors detecting electric current. Furthermore, these relays could 

be considered or called “switches”, but are not switches although they perform indirectly the action 

of switching. In fact, thermal overload relays can interrupt the auxiliary or control circuit of a 

power relay—that is, the real switch—causing its deactivation. From this point of view, a thermal 

overload relay is an integral part of any power relay supplying an electric motor. It is mechanically 

and electrically coupled with the power relay, and both of them constitute together a unified power 

device that is inserted in the power circuit of an electric motor. The relay acts then as a switching 

mechanism and the overload relay as a sensor detecting the motor current. For this reason, all the 

power relays manufacturers produce also the corresponding thermal overload relays.

Every motor is characterized by the nominal current I

nominal

 absorbed during the motor’s opera-

tion and under normal conditions. The thermal overload relay detects the possible overloading 

situation of the motor, expressed by the condition I

real

 > I

nominal

, where I

real

 is the actual current of 

the motor. If the motor draws more current than I

nominal

 for an extended period of time, then it will 

be damaged. In order to avoid such a fault, the thermal overload relay protects the operation of the 

motor indirectly by deactivating the power relay or the contactor, as is described subsequently. The 

output of the thermal overload relay is, in most cases, a DPST contact, as shown in Figure 2.19, 

which helps stop the motor operation.

The principle of the thermal overload relay operation is based on the known behavior of the 

bimetallic strip, which consists of two dissimilar metals by means of two oblong metallic pieces 

bonded together. The two metals have different thermal expansion characteristics, as for example 

the brass and the nickel-iron alloy, and therefore the bimetallic strip bends at a given rate when 

heated. In Figure 2.20a the simplified form of a bimetallic strip is shown, where the motor current 

flows through it and hence it is directly heated, while in Figure 2.20b the strip is indirectly heated 

through insulation winding around the strip. When the motor current has its nominal value 

or lower, the bimetallic strip doesn’t bend, and the contacts have their normal status. As motor 

current rises for any reason, the bimetallic strip bends, pushes the trip lever, and mechanically 

changes the state of the two contacts, as presented in Figure 2.20c. The NC contact is inserted 

in the control circuit of the power relay supplying the motor and, in such an overload condition, 

Thermal overload

relay symbol

L

1

L

2

L

3

DPST output of

thermal overload relay

Power lines

Control lines