The document discusses thermal overload relays used in motor control to protect against excessive current. It covers the definition, operation principles, connection methods, and selection/testing procedures for the relays. Key characteristics include allowing temporary overloads, tripping during dangerous conditions, and the ability to reset after overload removal.

1. Ministry of Technical Education and
Vocational Training (Yemen)
One Target Institute
Relay Logic Control
And
Programmable Logic Controller
(LECTURE4)
THERMAL OVERLOAD RELAY
Eng.Waleed A.S Al-Akwa’a Mon-Oct 29

3. • Agenda
 Definition.
 Structure.
 Idea of operation.
 Way of connection.
 Selection.
 Testing.

4. Thermal overload relay is an electrical component
used in motor control applications which provide
protection for motor against excessive current.
Definition

5. Structure
Line
terminals
reset
Set value dial
Auxiliary contact(s)

7. Principles of operation:
This component has a
bimetal strip that is made of
two dissimilar metals bonded
together. The two metals
have different thermal
expansion characteristics, so
the bimetal strip bends at a
given rate when heated.
As current rises, heat also rises. The hotter
the bimetal strip becomes, the more it
bends. In an overload condition, the heat
generated from the heater will cause the
bimetal strip to bend until the mechanism is
tripped, stopping the motor.
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8. Way Of Connection:
Typical set-up of contactor and overload relay
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9. Characteristics:
1. Allow harmless temporary overloads (such as motor
starting) without disrupting the circuit.
2. Trip and open a circuit if current is high enough to
cause motor damage over a period of time.
3. Can be reset once the overload is removed.
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10. • Methods of overload selection:
-Using catalogs:
Catalogs methods
 Important note :set the overload current to the actual pulled value of the load not to full load
current.

12. Testing
-How to test the contactor:
1-By using Avometer(switch it to the alarm).
2-You must test it before you begin using it.

13. • Homework
-Complete your design by adding(selecting) suitable
overload.
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Phone774275408
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