A thermal overload relay is an electromechanical protection device used in electrical motor circuits to prevent damage caused by excessive current and overheating. It is an essential component in motor control systems, providing a safeguard against overloads that could lead to motor burnout or damage.

What Is Thermal Overload Relay  

A thermal overload relay is an essential safety device found in most motor-driven systems. It doesn’t just trip randomly; it responds to actual heating caused by excess current. This means it works directly with real-time temperature changes, not just electrical flow. When the current exceeds the rated load for a certain time, the relay cuts off the power. The function of the thermal overload relay is to protect motors by detecting excessive heat due to prolonged current flow, keeping the system safe and secure.

Types of Thermal Overload Relays and Related Motor Protection Devices 

Thermal overload relays and motor protection devices are used to protect electric motors from overheating, overload, and electrical faults. Below are the main types, explained clearly and simply.

Traditional Thermal Overload Relays 

Traditional thermal overload relays use a bimetallic strip or heating element to detect motor overloads. They respond to the temperature rise caused by current flow and protect motors by interrupting the electrical circuit.

Electronic Overload Relays

Electronic overload relays use solid-state components to monitor motor currents and provide more precise and adjustable protection. They often offer additional features such as communication capabilities and advanced settings.

Magnetic Overload Relays 

Magnetic overload relays rely on the magnetic force generated by the current passing through a coil to trip the relay and disconnect power in the event of an overload. They are suitable for applications with high starting currents.

Solid-State Overload Relays 

Solid-state overload relays use electronic components like transistors and integrated circuits for current sensing. They offer accurate and adjustable protection and may include features like digital displays and communication interfaces.

Differential Overload Relays 

Differential overload relays compare the current in different phases of a motor and trip if an imbalance is detected. This protects against phase imbalances or faults that could lead to motor damage.

Time Delay Overload Relays 

Time delay overload relays introduce a delay before tripping in response to overloads. This delay allows for temporary high-current conditions during motor starting or specific operations without triggering a trip.

Bimetallic Overload Relays 

Bimetallic overload relays use a bimetallic strip that bends with temperature changes. When exposed to prolonged overload conditions, the strip bends to trip the relay and disconnect power.

Inverse Time Overload Relays  

Inverse time overload relays trip faster for higher overloads and slower for lower overloads. The tripping time is inversely proportional to the magnitude of the overload, providing adaptability to various conditions.

Motor Protection Relay  

Motor protection relays are comprehensive devices that often incorporate multiple protection functions. These may include overload protection, phase imbalance protection, thermal protection, and other features. Motor protection relays are more advanced and suitable for critical motor applications.

Thermal Overload Relay Working Principle 

A thermal overload relay works by sensing excess heat caused by current overload. When the temperature exceeds a safe limit, the relay trips and disconnects the motor to prevent damage. It uses a simple heat-sensing and mechanical mechanism, making it a reliable and widely used motor protection method.

• Heat Generation: As current flows through the motor, it naturally produces heat. This heat builds up especially when the motor is overloaded. A bimetallic strip inside the relay absorbs this heat. Over time, if the load doesn’t reduce, the strip starts to bend.

• Bimetallic Strip Response: The bending of the bimetallic strip is the core part. It bends more as the temperature rises. When it bends to a set point, it pushes a contact open. This action cuts off the power supply to the motor, stopping further heat build-up.

• Reset Mechanism: Once the motor cools down, the strip goes back to its original shape. Some relays  reset automatically while others need manual resetting. This allows users to inspect the motor and fix the problem before restarting. Knowing how to test a thermal overload relay is important during reset and maintenance.

• Trip Time Variation: The relay doesn’t trip instantly. It allows short overcurrent for a brief time to avoid false trips. This time delay lets the motor start up properly but still protects it from long-term damage. The trip curve is set by design.

• Current and Temperature Link: More current means more heat. This direct link helps the relay function accurately. The relay only reacts if the overload continues long enough to heat the strip. That’s how it ensures precise motor protection.

Application Of Thermal Overload Relay 

Thermal overload relays are used across many industries. They help stop motors from burning out and reduce the chances of breakdowns. These devices ensure that operations continue smoothly even during load variations or system faults. Their use increases the lifespan of motors. Now, let’s look at what is thermal overload relay and some specific areas where they are applied.

• Industrial Machinery: In factories, motors run machines for hours. Any sudden spike in load can overheat the motor. A thermal overload relay senses this rise and shuts the motor down in time. This protects machinery from heat damage and long stoppages. The application of a thermal overload relay ensures equipment runs safely during peak hours.

• Agricultural Motors: Farming needs pumps and motors to operate in tough outdoor conditions. These motors often run non-stop, especially in irrigation. A thermal overload relay helps avoid burnouts due to overloads or low voltage. It reacts quickly to unusual heat levels.

• Water Pump Systems: In water supply systems, especially in residential or industrial sectors, pump motors face a heavy load. Relays prevent overheating caused by long run times. This protects the motor and reduces repair costs. Regular use of relays in these systems keeps things running smoothly.

• Commercial Buildings: Buildings rely on motors for elevators, air conditioning, and ventilation. These systems cannot afford downtime. Overload relays stop the motors from failing during rush hours or heavy use. They act silently behind the scenes.

• Mining and Construction Sites: Machines in mining and construction run under extreme loads. Motors often work in dusty and humid conditions. Thermal overload protection is essential here. It helps the motor stop when things heat up and start again safely after cooling.

How do Thermal Overload Relays work? 

Thermal overload relays are commonly used in conjunction with motor starters or contactors in industrial applications. Here's how a thermal overload relay typically functions:

• Sensing Temperature: The thermal overload relay is equipped with a bimetallic strip or a heating element that is sensitive to temperature changes. This element is placed in proximity to the windings of the motor. The element must warm up slowly and respond steadily. This part of the thermal overload relay's working principle ensures smooth performance in various load conditions.

• Current Monitoring: As current flows through the motor during normal operation, the bimetallic strip or heating element also heats up due to the I2R losses (current squared times resistance) in the motor windings. This happens gradually. The relay doesn’t react immediately to spikes, which prevents false trips and keeps operations steady.

• Overload Detection: In the event of an overload or a sustained increase in current beyond the motor's rated capacity, the bimetallic strip or heating element heats up at an accelerated rate. This steady heating helps detect real issues, not momentary current increases. The thermal overload relay's working principle relies on this slow and accurate detection.

• Relay Activation: When the temperature surpasses a predetermined threshold, the bimetallic strip or heating element triggers a mechanical mechanism within the thermal overload relay. This trigger isn’t just reactive, it’s based on how long the overcurrent lasts. That timing makes the relay very reliable and safer for long-term motor use..

• Contact Opening: The triggered mechanism causes the contacts of the thermal overload relay to open, interrupting the electrical circuit to the motor. Once open, the circuit stays cut until the fault is cleared and the relay resets. The opening process happens cleanly, with no arcing, making it durable and repeatable.

• Motor Shutdown: With the contacts open, the power supply to the motor is cut off, preventing further overheating and damage. This results in the automatic shutdown of the motor. The function of the thermal overload relay in this step is to isolate the motor before any internal part breaks or catches fire.

• Manual Reset (Optional): Some thermal overload relays feature a manual reset button that allows the relay to be reset after the motor has cooled down. This prevents immediate restarting of the motor before addressing the cause of the overload. If reset too early, the motor might trip again, so this step needs attention

Use cases of Thermal Overload Relays with Examples 

Thermal overload relays are critical components in motor protection systems, finding application in various industries and settings. Here are some common use cases with examples:

Industrial Motor Protection 

Used to protect motors in factories from overheating during continuous or heavy-duty operation.

Example: In a manufacturing facility with multiple motors driving conveyor belts and machinery, thermal overload relays protect the motors from sustained overloads during heavy production cycles.

Pump Systems  

Applied in water supply, wastewater, and process pumps to prevent motor damage due to overload, blockage, or prolonged running.

Example: In a water treatment plant, thermal overload relays safeguard pumps that move water through the filtration and distribution processes. They prevent motor damage due to overloads or prolonged running.

HVAC Systems  

Protect fan and compressor motors from overheating caused by high demand, airflow restrictions, or mechanical faults.

Example: In a large commercial building, thermal overload relays protect the motors of HVAC (Heating, Ventilation, and Air Conditioning) systems. They respond to increased loads during peak usage or mechanical issues.

Agricultural Equipment  

Used in irrigation pumps and farm machinery to safeguard motors during long operating hours and voltage fluctuations.

Example: On a farm, thermal overload relays ensure the protection of motors in agricultural equipment, such as irrigation pumps. They prevent overheating during extended operation.

Conveyor Systems  

Prevent motor burnout when conveyors experience excess load, material buildup, or mechanical jamming.

Example: In a distribution centre, thermal overload relays are used to protect the motors of conveyor systems. They respond to varying loads caused by changes in product weight or blockages.

Mining Operations  

Protect motors in crushers, conveyors, and hoists operating under extreme load and harsh environmental conditions.

Example: In mining equipment like crushers and conveyors, thermal overload relays provide crucial protection. They prevent motor damage due to excessive loads during the extraction and processing of minerals.

Oil and Gas Industry  

Used to protect motors driving pumps, compressors, and drilling equipment in continuous and hazardous operations.

Example: In the oil and gas sector, thermal overload relays protect motors driving pumps, compressors, and drilling equipment. They respond to varying loads and conditions in harsh environments.

Wastewater Treatment Plants 

Safeguard motors used in pumps and agitators against overload caused by clogs or variable fluid flow.

Example: In a wastewater treatment facility, thermal overload relays safeguard the motors of pumps and agitators. They prevent damage caused by clogs, blockages, or variations in liquid flow.

Marine Applications  

Protect motors used in propulsion and auxiliary systems from overheating due to changing load conditions.

Example: On ships and vessels, thermal overload relays protect motors powering propulsion systems and auxiliary equipment. They respond to changes in load conditions during navigation.

Manufacturing Machinery  

Applied in presses, injection moulding machines, and production equipment to prevent damage during overload situations.

Example: In a factory with diverse manufacturing machinery, thermal overload relays protect motors in injection moulding machines, presses, and other equipment, preventing damage due to overloads.

Electric Vehicles  

Help protect traction motors from overheating during acceleration, hill climbing, or high-load operation.

Example: In electric vehicles, thermal overload relays protect the motors that drive propulsion systems. They respond to variations in load during acceleration or when climbing steep gradients.

Commercial Kitchens  

Used in conveyor ovens and food-processing machines to protect motors during continuous or high-load operation

Example: In commercial kitchens with conveyor ovens and food processing equipment, thermal overload relays protect motors from overheating due to continuous operation or sudden increases in load.

Also Read: What is an Overload Relay? Types, Functions & Applications

Lauritz Knudsen Thermal Overload Relays 

Upgrade your motor protection with Lauritz Knudsen Thermal Overload Relays, designed for precision and reliability. Safeguard your industrial and commercial operations from motor damage caused by overloads. Explore the range of advanced features, including accurate current sensing and adjustable settings. Ensure the longevity of your electrical systems. Don't compromise on quality; choose Lauritz Knudsen Electrical & Automation for trusted motor protection.

FAQs About Thermal Overload Relay 

Q1. Why is a thermal overload relay used in motor protection?

Ans. It is used to stop motors from running when they get overloaded. Motors heat up if they’re used too much or if there’s a fault. The function of the thermal overload relay is to break the circuit and stop power before this heat causes damage. It keeps the motor safe and running longer.

Q2. Can a thermal overload relay be reset after tripping?

Ans. Yes, it can be reset manually or automatically depending on the type. Manual reset gives time to inspect the motor. Automatic reset starts it again after cooling down. Knowing how to test a thermal overload relay before resetting is also important to avoid repeated trips due to an existing fault.

Q3. What is the function of the “manual” and “automatic” reset modes in thermal relays?

Ans. Manual mode needs someone to press the reset button. Automatic mode resets the relay by itself after a cooldown. Automatic mode is faster but might be risky if the fault is still present. Both modes follow the same thermal overload relay working principle, just differ in how they resume function.

Q4. Can thermal overload relays be used with both single-phase and three-phase motors?

Ans. Yes, they work with both types of motors. The relay just needs to match the motor’s rated current and voltage. In single-phase systems, fewer contacts are used, but the application of thermal overload relay stays the same, cutting power when there's too much heat or current.

Q5. What should I check before replacing a thermal overload relay?

Ans. Check if the motor is overheating often or if the relay is tripping too frequently. You should also inspect wiring, settings, and whether the relay is correctly sized. If unsure, learn how to test the thermal overload relay to see if it still works properly before replacing it.

Q6. What causes a thermal overload to trip?

Ans. Overloads can happen due to long motor use, jammed parts, blocked airflow, or incorrect settings. Too much current builds heat and makes the relay trip. This thermal overload protection system avoids total failure by reacting to temperature rise, not just current spikes.