Thermal Relay

 Thermal Relay

These relays are based on the effects of heat, which means – the rise in the ambient temperature from the limit, directs the contacts to switch from one position to another.

These are mainly used in protecting motor and other inductive loads against single phasing, overload and unbalanced voltages. 

This relay consists of bimetallic elements like temperature sensors as well as control elements. 

Thermal overload relays are the best examples of these relays.

Bimetallic Thermal Relays

When a bimetallic strip heats up, it expands or bends as shown in the figure due to the inequality of linear

expansion of two dissimilar metals. 

For any reason, if the circuit draws heavy or excessive currents, this in turn causes to increase the temperature of winding or conductor so this bimetallic strip also gets expanded, and then finally the circuit contacts become open.

These relays can be used to operate either in automatic or manual modes.

Solid State Thermal Relays

This relay is built with solidstate electronic components. 

It continuously monitors the starting and running currents to correspondingly calculates the average motor or load temperature. 

If the limits of the temperature exceed, then it trips the load.