What is Thermal Relay? – Construction, Working | New Topic [2024]

In this note, we are going to learn about a topic called “What is Thermal Relay?“, also its construction, working principles, advantages, and applications. Welcome to Poly Notes Hub, a leading destination for engineering notes.

Author Name: Arun Paul.

What is Thermal Relay?

A thermal relay is an electromechanical device that prevents electrical circuits from overheating. It operates by measuring the circuit’s temperature and triggering a switch to block current flow if it surpasses a particular threshold. Thermal relays are widely employed in situations where motors or other electrical devices may overheat, such as industrial machinery or HVAC systems.

Construction of Thermal Relay

Below shows the Thermal Relay Diagram and explain its every section –

• Bimetallic Strip: The bimetallic strip is a critical component made up of two different metals bonded together with different coefficients of thermal expansion. When exposed to temperature fluctuations, the strip bends due to the different expansion rates of the metals.
• Spring: The spring is attached to one end of the bimetallic strip. It provides the force required to return the bimetallic strip to its original position as the temperature drops.
• Relay Contact: The relay contact is an electrical switch that opens or closes based on the movement of the bimetallic strip. When the strip bends due to increased temperature, it either makes or breaks the electrical contact, interrupting the circuit.
• Heating Coil: When an excessive amount of current runs through the circuit, the heating coil generates heat. As the current increases, the heating coil generates more heat, which raises the temperature of the bimetallic strip.
• Insulated Contact Arm: The insulated contact arm connects the relay contact to the heating coil. It guarantees that electrical current travels through the heating coil without interfering with the operation of the relay contact.

Thermal Relay Working Principle

Below we describe that how a thermal relay works –

• The electrical resistance of the relay causes it to heat up as current goes through it.
• As the temperature rises, the bimetallic strip in the relay expands unevenly, forcing it to bend.
• The bending of the strip activates a switch mechanism, interrupting the current flow.
• The activation threshold is set based on the relay’s calibration settings.
• Once the temperature drops and the strip returns to its original position, the switch reconnects, allowing current to flow again.

Advantages of Thermal Relay

Below we listed some merits of thermal relay –

1. Detects and reacts to high current flow, preventing damage to electrical circuits.
2. Minimizes downtime by automatically resetting when an overload condition is resolved.
3. Provides cost-effective protection at a lower price point than certain competing products.

Applications of Thermal Relay

Below we listed some important uses of thermal relay –

1. Use for Motor Protection: Thermal relays are widely utilized in motor protection applications to prevent overheating caused by extended overload circumstances. They prolong the motor’s life and assist avoid motor burnout.
2. Use for Transformer Protection: Thermal relays protect transformers in electrical distribution networks from damage brought on by high current flow. To avoid transformer failure in the event of an overload, they trip the circuit.
3. Use in Industrial Machinery: Industrial machinery, including pumps, compressors, and conveyor systems, use thermal relays to prevent overheating while they are in use. This guarantees continuous production procedures and equipment durability.
4. Use in Heating Systems: In order to guard against overheating and fire threats, thermal relays are incorporated into heating systems, such as electric furnaces, ovens, and boilers. They provide essential safety precautions for heating applications in both homes and businesses.
5. Also use in Power Distribution Networks: Power distribution networks are shielded from overloads and short circuits by thermal relays. They assist in preserving system dependability and averting extensive disruptions.