- Thermal Relay Definition: A thermal relay is defined as a device that uses the unequal expansion rates of metals in a bimetallic strip to detect overcurrent conditions.
- Working Principle: The thermal relay operates by heating a bimetallic strip, causing it to bend and close normally open contacts, triggering a circuit breaker.
- Construction of Thermal Relay: It consists of a bimetallic strip with metals having different coefficients of expansion, a heating coil, and contacts.
- Time Delay Function: The relay’s heating effect follows Joule’s law, causing a delay in operation that allows temporary overloads without tripping.
- Application: Thermal relays are used for overload protection, especially in electric motors, where they prevent tripping from short-term overloads.
A bimetal strip joins two metals with different coefficients of thermal expansion. Heating makes one layer lengthen more than the other, so the bonded strip bends. A thermal overload relay uses this repeatable movement to operate a trip mechanism.
Working Principle of Thermal Relay
A bimetallic thermal relay works by passing load current through a heater or through the bimetal assembly itself. The working principle of thermal relay links the resulting temperature rise to sustained over current. As the bimetal deflects, a trip bar changes the state of auxiliary contacts. In a common motor starter, the normally closed auxiliary contact opens the contactor control circuit; a normally open contact may provide an alarm. The external circuit determines the contact action, so closing a normally open contact to trip a circuit breaker is only one possible arrangement.
Construction of Thermal Relay
A bimetal thermal overload relay contains one or more bimetal strips, heater elements, a current-setting mechanism, a trip linkage, auxiliary contacts and a reset mechanism. In the simplified drawing, metal A has a lower expansion coefficient than metal B.
Load current heats the element directly or through a separate heater. An overload or phase loss increases the thermal input.
Metal B expands more than metal A, so the bonded strip bends towards metal A. The movement accumulates until the trip linkage crosses its calibrated point and changes the auxiliary contacts.
In a typical contactor starter, the normally closed overload contact opens and de-energises the contactor coil. Another design may use an auxiliary contact in the trip circuit of a circuit breaker.
Heating has thermal inertia. Joule’s law of heating gives the electrical energy produced in a resistive heater as:
I is the load or over current through the heater.
R is the heater’s electrical resistance, and t is the heating time. This I²Rt relation explains the thermal input, but it does not by itself calculate trip time. Heat loss, ambient compensation, current multiple, prior loading, trip class and the relay’s calibrated current-time curve also affect operation.
The inverse thermal delay allows normal motor starting current while tripping before a sustained over current overheats the motor. Set current from the motor nameplate and select the trip class for the motor’s starting time and thermal capability. A thermal overload relay does not replace coordinated short-circuit protection. After a trip, allow the relay and motor to cool before reset, and use automatic reset only where an unexpected restart cannot create a hazard.
This is why thermal relays are widely used for overload protection of electric motors, including protection against sustained overload and phase-loss heating.
