What is a thermal overload relay?

What is a thermal overload relay?

Thermal overload relays are designed to provide electromechanical protection to motors against overloading from drawing excessive input currents from the main circuit. These electronic components can provide great protection against permanent electrical damages during electrical anomalies like phase failure and over voltages.

What Is the Function of Thermal Overload Relay? 

The electrical industry is huge with umpteen equipment for several purposes. Each product is defined with a certain motive and eases human labour and work. One such device is the thermal overload relay. The thermal overload relay serves as a crucial protective device in electrical systems, primarily used to safeguard motors from overheating and potential damage due to excessive current. It operates based on the principle of thermal sensing, monitoring the heat generated by the motor during operation.

When the motor draws excessive current over an extended period, indicating a potential overload, the thermal overload relay responds by tripping and disconnecting the power supply to the motor. This prevents the motor from overheating, reducing the risk of damage and enhancing operational safety.  

Different Kinds of Relays

There are various types of relays, each designed for specific applications within electrical and electronic systems:

  1. Electromagnetic Relays: Utilize electromagnetic coils to open or close contacts when the coil is energized.
  2. Solid-State Relays: Employ semiconductor devices, such as transistors and thyristors, for switching without moving parts, offering faster response times and longer lifespans.
  3. Overload Relays: Protect motors from excessive currents, preventing overheating and damage.
  4. Time Delay Relays: Introduce a time delay before the relay responds to a change in input conditions, useful in applications where delayed activation or deactivation is required.
  5. Protective Relays: Monitor electrical parameters and initiate protective actions in case of faults, playing a crucial role in ensuring system reliability and safety.
  6. Latching Relays: Maintain their state (either open or closed) even after the input power is removed, reducing energy consumption.

These diverse relay types cater to specific needs in controlling and protecting electrical circuits across various industries and applications.

Auto Reset vs Manual Reset on Thermal Overload Relays

Auto-reset and manual-reset are two reset mechanisms used in thermal overload relays, each serving specific purposes in motor protection.

Auto-Reset: In auto-reset thermal overload relays, once the relay trips due to overheating, it automatically resets itself after a cooling-off period. This feature is suitable for applications where temporary overloads are expected, and the motor can resume operation once it cools down. However, continuous or severe overloads may lead to repeated tripping, indicating the need for investigation into the cause.

Manual Reset: Contrastingly, manual-reset thermal overload relays require manual intervention to reset after a trip. This offers a more deliberate approach to restarting the motor, ensuring that an operator assesses the situation, identifies and addresses the cause of the overload, and approves the reset. Manual-reset relays are beneficial when safety and a careful evaluation of the system are paramount.

The choice between auto-reset and manual reset depends on the specific application, the nature of the loads, and the level of control and oversight desired in the motor protection system.

How does a thermal overload relay work?

The thermal overload relays allow harmless, temporary overloads to pass through the circuit without disrupting the electrical circuit and only trip the open circuit when they detect any flow of current at unnaturally high levels, thus protecting the connected motor from any electrical damage.

The thermal overload relay is equipped with electrical contacts that are sensitive to heat and can close or open a circuit depending on the temperature across the relay's coil. In case the motor draws current at dangerously high voltages, the electrical coil of the relay heats up, tripping the electrical contacts and disrupting the flow of electricity through the circuit. These electrical contacts can be manually or automatically operated to resume the electrical flow once the relay’s coil has cooled down sufficiently.

Auto Reset vs manual reset on thermal overload relays

Thermal overload relays that are manufactured for commercial use are mostly available in two types of resets - automatic and manual. Depending on the type of controls, you can distinguish between them as:

Manual reset thermal overload relay

Thermal overload relays with manual reset require the user to physically intervene and restart the motor once the relay has been tripped due to overheating. The user must be physically present to operate the start button or the external switch that sends power to the contactor coil to start the motor.

However, thermal overload relays can only be restarted once they have adequately cooled down. The user must be able to manually monitor the relay for it to cool down while also considering external factors like sunlight, ambient temperature, and airflow before the motor can be restarted.

Automatic reset thermal overload relay

Unlike manual reset thermal overload relays, the automatic ones with auto-reset function are equipped with a bi-metallic strip that can detect whether the relay has sufficiently cooled down before it returns to its original state, before making contact to resume the relay’s function and automatically restarting the motor.

The automatic reset thermal overload relay does not require any manual intervention. However, despite its obvious advantage, auto-reset thermal overload relays can often be unpredictable as the cooling rate of the overload is not always constant, and it can restart the motor abruptly without any warning.

Advantages and disadvantages of thermal overload relay

There are several advantages and disadvantages associated with the use of thermal overload relays for electrical protection, and some of them are as follows:

Advantages of thermal overload relays

- Thermal protective relays follow an easy but effective operating principle that ensures greater accuracy and support to connected motors.

- Thermal overload relays are primarily used to prevent electrical motors from getting overheated over time. These relays can be conveniently used in 1 and 3 phase electrical motors.

- Some thermal overload relay models are designed to ensure protection from internal phase losses.

- Thermal overload relays are known for their easy installation. Most thermal overload relay models can be directly mounted to contactors, or they can be conveniently mounted to the operation panel using rail adaptors.

- Certain thermal overload relay models are equipped with internal trip class selection buttons.

- Thermal overload relay models come with automatic as well as manual reset functions for easy operations.

- They have an internal test button for troubleshooting options.

- Thermal overload relays are active over a wide and adjustable current range.

- These relay models have a trip-free mechanism for optimal operation.

- They have temperature compensation features for accurate functioning.

- Thermal overload relays are cost-effective devices and can be readily used anywhere.

Disadvantages of thermal overload relays

- Though thermal overload relays offer electrical protection, they do not come with short circuit protection.

-  Thermal overload relays are not designed with direct breaking functions. Instead, they require to be used with other switching and electrical protection devices for disconnecting a live circuit.

- Most thermal overload protection devices are slow in operation.

- Thermal overload protection devices work optimally against low-resistance circuits. They do not always perform as well when used in heavy-duty circuits.

- Other than the most sophisticated models, thermal overload relays are rarely able to withstand vibrations and electrical shocks.

- As thermal overload relays do not come with a high switching frequency, they often require time to cool down once they have overheated and tripped.

Thermal overload relays from the Schneider Electric eShop

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Frequently Asked Questions

Q1. How do thermal overload relays work?

A. Thermal overload relays work by monitoring the heat generated by an electric motor during operation. They contain a bimetallic strip that expands with increased temperature. When the motor draws excessive current, indicating a potential overload, the strip activates the relay, disconnecting power to the motor. This protective measure prevents overheating, safeguarding the motor from damage. The relay resets either automatically after a cooling-off period or manually, depending on the type of relay and application requirements.

Q2. How does an overload relay thermal unit work?

A. The thermal unit in an overload relay works by utilizing a bimetallic strip or other heat-sensitive elements. When current flows through the relay, it generates heat. The bimetallic strip, sensitive to temperature changes, bends as the temperature rises. If the current exceeds the preset limit for a sustained period, the strip activates the relay, tripping the contacts and interrupting power to the motor. This mechanism prevents prolonged exposure to excessive current, protecting the motor from overheating and potential damage.

Q3. What causes a thermal overload to trip?

A. A thermal overload relay trips when a connected motor experiences excessive current over a prolonged period, leading to overheating. This can occur due to factors such as mechanical issues in the motor, voltage fluctuations, or increased load demand. The relay senses the elevated temperature through its thermal unit, activating the protective mechanism to disconnect power and prevent further damage. Continuous or severe overloads necessitate investigation into the root cause before resetting the relay.

Q4. What are two types of thermal overload relays?

A. Two common types of thermal overload relays are auto-reset and manual-reset. Auto-reset relays automatically restore power to the motor after a cooling-off period when an overload condition is detected. Manual-reset relays, on the other hand, require manual intervention to reset after a trip. The choice between these types depends on the application's requirements, with auto-reset being suitable for situations where temporary overloads are expected, and manual-reset offering a more deliberate approach for careful evaluation and intervention.

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