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.

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 up 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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