Introduction to Industrial Automation

Figure 2.19  Schematic symbol of a thermal overload relay.

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Hardware Components for Automation and Process Control 

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39

causes the deactivation of the power relay and hence the overall stopping of the motor. The cor-

responding automation circuit, with a thermal overload protection of a motor, was given in Figure 

1.11 as an example of control circuits. Its operation explanation and other examples are described 

in more detail in Chapter 3. The NO contact of the thermal overload relay is used to activate the 

fault indication of the condition “overload relay tripped”.

Furthermore, Figure 2.20 shows a single bimetallic strip, while the thermal overload protection 

relays are of a three-pole type, and hence it contains three bimetal strips mounted together with a 

uniform tripping mechanism in a housing made of insulating material, as shown in Figure 2.21. 

Each bimetallic strip detects the motor current of the corresponding phase, making the overload 

relay also sensitive to the phase imbalance or losses. Each thermal overload relay covers a standard 

range of the motor current, e.g., 4–6 A or 6–8 A (there are about 20 such ranges), and is equipped 

with a small dial for the ampere constraint setting into the standard range by the displacement of 

the trip mechanism relative to the bimetal strips. After an overload situation, the bimetallic strip 

Bimetallic

strip

Wire

I

real

nominal

I

real

nominal

I

real

>I

nominal

(a)

NO NC

Auxiliary

contacts

(b)

(c)

NO NC

Closed

Open

Trip

lever