- 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.
The coefficient of expansion is a fundamental property of materials. Different metals have varying rates of linear expansion. A bimetallic strip bends when heated because of the different expansion rates of the two metals.
Working Principle of Thermal Relay
A thermal relay works depending upon the above mentioned property of metals. The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying over current of the system, it bends and makes normally open contacts.
Construction of Thermal Relay
The construction of thermal relay is quite simple. As shown in the figure above the bimetallic strip has two metals – metal A and metal B. Metal A has lower coefficient of expansion and metal B has higher coefficient of expansion.
When overcurrent flows through the heating coil, it heats the bimetallic strip.
The heat generated by the coil causes both metals to expand. Metal B expands more than metal A. This unequal expansion bends the bimetallic strip towards metal A.
The strip bends, the NO contact is closed which ultimately energizes the trip coil of a circuit breaker.
The heating effect is not instantaneous. As per Joule’s law of heating, the amount of heat generated is
Here, I represents the over current flowing through the heating coil of the thermal relay.
R is the electrical resistance of the heating coil, t is the time for which the current I flows through the heating coil. Hence from the above equation it is clear that, heat generator by the coil is directly proportional to the time during which the over current flows through the coil. Hence there is a prolonged time delay in the operation of thermal relay.
That is why this type of relay is generally used where over load is allowed to flow for a predetermined amount of time before it trips. If overload or over current falls down to normal value before this predetermined time, the relay will not be operated to trip the protected equipment.
A common use of thermal relays is for overload protection of electric motors.
