Thermal overload relay

The thermal relay is an essential component in commercial and industrial electrical systems, designed to provide protection against the risks of excessive electrical currents and high loads.

In this article, we will discuss thermal relays, their main function, their working principle, different types, and highlight their key advantages and disadvantages.

What is a Thermal Relay?

A thermal relay is a device used to activate protection for electrical systems from the dangers of excessive electrical currents. It helps protect equipment and devices from damage.

When the relay detects a noticeable increase in temperature, it disconnects the electrical circuit to avoid potential risks from excessive current, such as damage or malfunction.

The relay consists of several components, each playing a significant role in responding to electrical currents and disconnecting excess current when needed.

The main components of the relay include:

• Bimetallic strips: These detect abnormal temperature rises caused by current.

• Electrical conductors: These deliver current to the relay.

• Disconnecting mechanism: Responsible for cutting off the current when necessary.

• Visual indicators: These clarify the cause of disconnection, whether due to a fault or high temperatures.

• Manual or automatic current adjustment settings: These control the electrical current levels.

How Does a Thermal Relay Work?

A thermal relay is crucial for ensuring safety and protecting devices when temperatures rise excessively by disconnecting the electrical current.

The working principle relies on a PTC (Positive Temperature Coefficient) disk, made from a material with a thermal coefficient. This material has relatively low resistance when not connected to electricity.

When the compressor is powered, the resistance may increase significantly, raising the relay's temperature due to the heating of the PTC disk caused by the current.

As a result, the current will no longer reach the starter windings, leaving the running windings operational. This disconnects the starter windings from the circuit as the compressor starts, causing the relay to heat up further.

While the continues running, the relay remains hot and will only cool down after disconnecting the electrical current for approximately 30 seconds, allowing it to gradually cool.

In short, the thermal relay operates by disconnecting the starter windings from the circuit through the PTC disk, ensuring device protection during compressor operation.

Functions of a Thermal Relay

The thermal relay serves several purposes across residential, commercial, and industrial electrical systems, including:

• Protecting household appliances from the dangers of excessive loads, often used in air conditioners or washing machines.

• Safeguarding electric motors in industrial systems from potential risks caused by overloading by disconnecting the circuit when excess current is detected.

• Integration into commercial electrical circuits to maintain system efficiency and reduce overloading risks.

• Protecting equipment and machinery in factories and workshops from overloads that could damage devices or halt operations.

If a thermal relay disconnects the electrical current, it is essential to identify the root cause before restarting to avoid recurrence or device damage.

The circuit may disconnect due to high temperatures caused by mechanical malfunctions in motors or voltage fluctuations, which the relay detects and reacts to by cutting off the current.

Different Types of Thermal Relays

There are various types of thermal relays, each performing specific functions to control electrical circuits and protect electrical and electronic systems.

1. Magnetic Relay

This type uses magnetic coils to open and close contacts when the compressor connects to or disconnects from the electrical current.

2. Solid-State Relay

This type offers fast response times, relying on semiconductor devices like transistors and thyristors for switching without moving parts.

3. Overload Relay

A popular type of thermal relay used in electrical and electronic devices, primarily for protecting them from overheating and ensuring their safety.

4. Protection Relay

Designed to ensure smooth operation of the electrical system by monitoring faults. If any potential issue arises, it takes preventive measures.

5. Latching Relay

Its primary function is energy efficiency by maintaining its state (open or closed) regardless of input power changes, conserving energy.

6. Time Delay Relay

This type has the ability to set a delay time for activation or deactivation before the relay responds to the electrical current.

Advantages and Disadvantages of Thermal Relays

Advantages:

• Operates with a simple, flexible mechanism to ensure necessary protection for connected motors.

• Suitable for a variety of applications.

• Protects motors from overheating risks, especially single-phase and three-phase motors.

• Easy installation, either directly on conductors or on control panels.

• Some types feature automatic and manual resetting for ease of use.

• Includes a built-in test button for error diagnosis.

• Provides accurate performance by compensating for temperature changes.

• Cost-effective compared to other electronic devices.

Disadvantages:

• Slow response times, sometimes taking a long time to disconnect the current.

• May not provide effective protection against short circuits.

• Performance may be influenced by environmental factors like high temperatures.

• Performance and efficiency may vary due to reliance on thermal characteristics.

• Not ideal for high-load circuits despite good performance with low-resistance circuits.

• Requires significant cooling time after overheating and shutdown.

To maintain optimal performance and extend the lifespan of a thermal relay, regular maintenance is recommended. This includes inspecting for potential faults, repairing damaged parts, cleaning internal components, and removing dust and debris that may affect efficiency.