What Are Thermal Relays?

Thermal relays are devices that protect electric motors and electrical circuits against overcurrent and overload. In electrical systems, they are used to prevent damage when motors are overloaded or draw excessive current while operating. These relays operate by utilizing thermal effects. When excessive current enters the circuit or when motors overheat beyond normal levels, the relay cuts the circuit, preventing the motor from burning out and protecting other equipment from damage.

Working Principle of Thermal Relays

Thermal relays operate through bimetallic elements contained within them. A bimetal is an element created by combining two different metals with distinct expansion coefficients. When exposed to heat, these metals exhibit bending behavior. When electric motors or circuits draw excessive current, this current heats the bimetal element inside the thermal relay. The heated bimetal bends and opens the contact in the circuit, disconnecting the motor’s electrical connection and thereby preventing damage to the motor.

Thermal relays are commonly used in industrial facilities for the protection of electric motors, conveyor belts, pumps, and compressors. They are especially preferred in systems where motors need to run for long periods and where there is a risk of overcurrent in the circuit.

Protection of Electric Motors

Electric motors are at risk of overheating when operated continuously or under overload. Thermal relays protect motors in such situations, extending the motor's operational life.

Protection in Industrial Machines

Machines used on production lines in factories are also protected with thermal relays. When these machines operate under overload, the thermal relay activates and stops the machine.

Differences Between Thermal Relays and Magnetic Relays

Thermal relays are often confused with magnetic relays, but their operating principles are different. Magnetic relays operate based on the electromagnetic principle, disconnecting the circuit when a fault occurs. In contrast, thermal relays disconnect the circuit due to the heat caused by overcurrent. Therefore, thermal relays are generally suitable for long-term overcurrent situations, while magnetic relays are used for protection against sudden current spikes like short circuits.

Adjustment and Usage of Thermal Relays

Thermal relays can be adjusted according to the motor’s rated current. This setting is slightly above the motor's nominal current, so the motor does not shut down during short-term overloads, but the relay still cuts the circuit during prolonged overloads. Correct adjustment ensures the motor operates efficiently and remains protected. The adjustment is typically made using a screwdriver on the setting screws located on the relay.

Adjustment Range

The adjustment range is generally selected based on the motor's operating current. This ensures that the motor does not shut down during temporary overloads, but is protected during long-term overloads.

Advantages of Thermal Relays

• High Protection: Thermal relays provide excellent overload protection for motors, preventing damage to both motors and circuit elements.

• Economical Solution: One of the most cost-effective solutions for protection against overcurrent.

• Easy Installation and Use: Due to their simple structure and easy adjustability, they can be quickly integrated into electrical circuits.

Types of Thermal Relays

Thermal relays are produced in various types according to application areas and needs. The most common types of thermal relays are:

• Standard: The most commonly used type, typically used for the protection of AC motors. With a bimetal structure, it disconnects the circuit in case of overcurrent, protecting the motor.

• Electronic: These relays use electronic components to protect motors more precisely. Preferred in applications requiring high sensitivity and usually offer more adjustment options.

• Integrated Protection Relays: These types provide both thermal and magnetic protection, offering comprehensive coverage. They protect against both overcurrent and sudden current spikes like short circuits.

Thermal Relay Selection Criteria

There are several important factors to consider when selecting a thermal relay for a system:

• Motor’s Rated Current: The thermal relay is adjusted according to the motor’s rated current. If the wrong relay is selected, the motor may not be adequately protected or may experience unnecessary disconnections.

• Environmental Conditions: Conditions such as ambient temperature and humidity are important in relay selection. Some relays are produced to withstand extreme temperatures and harsh environments.

• Intended Use: If the relay will be used in a system where the motor is frequently started and stopped, it should be suitable for such load changes. While standard relays provide effective protection during continuous operation, more sensitive models should be selected for frequent start-stop movements.

Maintenance and Testing of Thermal Relays

To ensure that thermal relays function correctly and protect motors effectively, regular maintenance is essential. Relays should be tested periodically to check whether they disconnect the circuit correctly during overload conditions.

• Visual Inspection: Thermal relays should be visually inspected regularly. Signs of physical damage, overheating, or loosened connections should be observed.

• Check Settings: Ensure the relay is set correctly. A relay set too high or too low above the nominal current may fail to protect the motor effectively.

• Testing and Simulation: The functionality of thermal relays should be tested under simulated overload conditions to verify that the circuit disconnects when exceeding set limits.

Common Problems with Thermal Relays

Although thermal relays are durable and reliable devices, several issues may arise from time to time:

• Incorrect Settings: Incorrectly adjusted thermal relays may either fail to protect the motor or cause unnecessary circuit disconnections. Therefore, proper adjustment is crucial.

• Overheating: When ambient temperature is very high, the bimetal element may expand excessively and cause the relay to open the circuit unnecessarily. To prevent this, thermal relays resistant to appropriate temperature levels should be used.

• Worn Relays: The mechanical components of thermal relays can wear out over time and lose sensitivity. In such cases, the relay may fail to protect the motor. Regular maintenance and replacement when necessary can prevent this issue.

The Future of Thermal Relays

With the rapid development of electronic systems, more advanced and integrated models of thermal relays have begun to emerge. Especially with the widespread adoption of IoT (Internet of Things) technology, work is underway on remotely monitorable and controllable thermal relays. These smart relays can detect overcurrent situations more precisely and send instant notifications. This enhances the safety not only of motors but also of entire electrical systems.

Conclusion

Thermal relays are essential devices for protecting electric motors against overcurrent and overload. Working based on the thermal expansion principle of bimetal structures, these relays prevent motors from overheating, ensuring the safety of both the motor and the circuit. Widely used from industrial plants to household appliances, they provide simple yet effective protection and contribute to the long-lasting and safe operation of electrical systems.