What is the Function of a Thermal Overload Relay?

A motor รีเลย์โอเวอร์โหลดความร้อน is a protective device that prevents electric motors from overheating by detecting excessive current flow and disconnecting power when necessary. 

This ensures the motor runs safely and efficiently while reducing the risk of damage.

Understanding Thermal Overload Relays

A motor thermal overload relay is a crucial component in electrical systems, particularly in motor protection. 

It works by monitoring the current flowing through a motor and reacting when the current exceeds safe limits for a prolonged period. 

This device prevents overheating, prolongs motor life, and minimizes electrical hazards.

How a Thermal Overload Relay Works

The relay consists of bimetallic strips that bend when exposed to excessive heat caused by high current. 

When the bending reaches a certain point, the relay trips, cutting off power to the motor and preventing further overheating. 

Once the temperature normalizes, the relay resets automatically or manually.

Thermal Overload Relay Symbol

Credits to Siemens

Key Features of Thermal Overload Relays

• Current Sensing Mechanism – Detects excessive current and activates protection.
• การตั้งค่าปรับได้ – Allows users to set overload limits based on motor specifications.
• Auto/Manual Reset Options – Provides flexibility in restoring motor function.
• Trip Indicators – Displays a signal when the relay has tripped.
• Compatibility with A Overload Contactor – Works alongside motor starters for complete protection.

Importance of Thermal Overload Protection

Motor thermal overload protection is essential for ensuring motor longevity and electrical safety. 

Without it, motors can suffer from:

• Overheating – Leading to insulation damage and component failure.
• Excessive Energy Consumption – Causing inefficiency and higher electricity costs.
• Increased Maintenance Costs – Due to frequent motor breakdowns.
• Electrical Hazards – Raising the risk of short circuits and fire.

การใช้งานของรีเลย์โอเวอร์โหลดความร้อน

• Industrial Machinery – Used in conveyor systems, pumps, and compressors.
• ระบบปรับอากาศและระบายอากาศ – Protects motors in air conditioning and ventilation units.
• โรงงานผลิต – Prevents motor damage in automated equipment.
• Agricultural Equipment – Ensures protection for irrigation and processing machinery.

Difference Between Thermal Overload Relay and Overcurrent Relay

• รีเลย์โอเวอร์โหลดความร้อน – Reacts to gradual overheating due to prolonged overcurrent conditions.
• Overcurrent Relay – Responds instantly to sudden high currents, such as short circuits.

How to Select the Right Thermal Overload Relay for Your Motor

Choosing the correct thermal overload relay is essential for ensuring motor protection and system efficiency. 

Selecting the wrong relay can lead to frequent tripping or insufficient protection, both of which can cause damage to the motor over time. Here are key factors to consider:

Motor Full-Load Current (FLC)

The relay should be rated based on the motor’s FLC to prevent premature tripping or failure to operate when needed. 

Most relays have an adjustable range, so it’s important to set them according to the motor’s specifications.

Trip Class

Different applications require different trip times. Trip classes (e.g., Class 5, 10, 20, or 30) determine how long the relay will take to disconnect the motor when an overload is detected. 

Faster trip classes protect motors from short bursts of excessive current, while slower ones accommodate high-inertia loads.

Ambient Temperature Conditions

Since thermal overload relays rely on bimetallic strips, their performance can be affected by surrounding temperatures. 

If the motor operates in extreme conditions, consider a relay with temperature compensation.

Reset Type

Overload relays come with manual or automatic reset options. 

Manual reset is ideal for applications requiring human intervention to restart, while automatic reset is better for systems where continuous operation is preferred after cooling down.

Proper selection helps maximize motor efficiency, reduce downtime, and prevent unnecessary maintenance costs.

Common Signs of a Faulty Thermal Overload Relay

A failing thermal overload relay can lead to motor failures, unexpected shutdowns, and operational inefficiencies. Here are common symptoms indicating a faulty relay:

• การสะดุดล้มบ่อยครั้ง: If the relay trips too often, even under normal operating conditions, it may be set incorrectly or have internal damage.
• Failure to Trip: When a motor overheats, but the relay does not cut power, it could indicate a stuck bimetallic strip or a malfunctioning sensing mechanism.
• Burn Marks or Physical Damage: Over time, excessive heating can cause burn marks on the relay housing, signifying internal component failure.
• Motor Overheating: If the motor runs hotter than usual without overloading, the relay may not be providing proper protection.

Identifying and replacing faulty overload relays in time prevents costly motor failures and downtime.

How to Test and Maintain a Thermal Overload Relay

Regular maintenance and testing of thermal overload relays ensure their reliability and effectiveness. Here’s how:

Current Injection Testing

This method simulates an overload condition by passing a controlled overcurrent through the relay to observe if it trips within the expected time.

Visual Inspection

Check for dust buildup, corrosion, or damaged connections. Dirty contacts can reduce efficiency and cause erratic relay operation.

Relay Trip Adjustment Verification

Ensure the relay is set within the correct amperage range. Adjusting it too low can cause nuisance tripping, while too high a setting may fail to protect the motor.

Manual Testing

Many relays have a test button to manually simulate a trip condition. This should be checked periodically to confirm the relay is functioning properly.

Routine maintenance ensures that thermal overload protection remains effective, extending motor life and reducing unexpected failures.

Resources:

What is a thermal overload relay?

A Guide on Thermal Overload Relay Working Principle

Thermal Overload Relay Working Principle Explained

Thermal Overload Relay: Definition, Function, Price