Bimetallic Thermal Overload Relay – Types, Construction, Working and Applications

What is a Bimetallic Thermal Overload Relay?

A Bimetallic Thermal Overload Relay is a protective device that protects the motor from damage caused by overload or excessive current (overcurrent).
It works on the principle of thermal expansion of metals, where the strip bends when heated and breaks the circuit.

🏗️ 2. Construction of Bimetallic Thermal Overload Relay

It consists of a bimetallic strip, heater coil, contacts (NO and NC), reset mechanism, and adjustment knob.
All these parts together detect the overload and safely shut down the motor.

⚙️ 3. Working Principle of Bimetallic Thermal Overload Relay

When excess current flows through the motor, the heater coil produces heat, causing the bimetallic strip to bend.
This action activates the tripping mechanism, which opens the contacts and disconnects the motor.

Applications

It is used in motor control panels, pumps, compressors, HVAC systems, and industrial machines to protect motors from overload and overheating.

⚡ 5. Difference Between Thermal and Magnetic Overload Relay

A thermal relay operates on heat (bending of the bimetallic strip) and provides overload protection,
whereas a magnetic relay operates on magnetic force and provides instant protection against short circuits.

types of Bimetallic Thermal Overload Relay

Thermal overload relays are classified by reset mechanism (manual, automatic), mounting (separate, contactor-mounted), number of phases (1-phase, 3-phase), and tripping design (differential, non-differential).

Introduction

A Bimetallic Thermal Overload Relay is a protective device used in electric motor circuits to safeguard the motor from damage due to overcurrent or overload conditions.

It operates on the principle of thermal expansion of metals — when excessive current flows through the motor, the relay heats up and opens the circuit, protecting the motor from burnout or thermal damage.

In simple terms, it acts as a temperature-sensitive safety guard that protects your motor from overheating.

⚙️ Construction of Bimetallic Thermal Overload Relay

This relay is composed of several key parts that help in detecting overloads and safely interrupting the current flow:

1️⃣ Bimetallic Strip:
Made of two different metals (such as brass and steel) joined together.
When heated, both metals expand at different rates, causing the strip to bend.

2️⃣ Heater Coil:
Connected in series with the motor circuit.
As current flows through the motor, the coil generates heat proportional to the current, which in turn heats the bimetallic strip.

3️⃣ Contacts (NO and NC):

• The NC (Normally Closed) contact opens when overload occurs, disconnecting the motor.
• The NO (Normally Open) contact is often used for signal indication or alarm circuits.

4️⃣ Reset Mechanism:
After the overload is cleared and the relay cools down, it can be reset manually or automatically.

5️⃣ Adjustment Knob:
Used to set the tripping current according to the motor’s rated current, allowing accurate overload protection.

Working of a Bimetallic Thermal Overload Relay

The Bimetallic Thermal Overload Relay is one of the most commonly used protective devices in electrical systems.
It is highly popular due to its simple construction, reliable operation, and low cost.

This relay contains bimetallic strips — two different metals joined together — with current-carrying coils wound around them (as shown in Figure 1.30).
The upper ends of these strips are fixed, while the lower ends are free to move, allowing them to bend when heated.

The relay operates on the principle of converting motor current into heat.
As current flows through the motor, the heater coil generates heat, which is transferred to the bimetallic strip.

When the motor experiences an overload current, the temperature of the strip rises.
As a result, the strip bends in one direction (usually to the right), and this mechanical movement activates the tripping mechanism.
Once tripped, the relay contacts open, disconnecting the motor circuit and thereby protecting the motor from damage.

A notable feature of this relay is its inverse time characteristic — meaning, the higher the current, the faster the relay trips.
The tripping current can be adjusted using the adjustment knob, allowing the relay to be set according to the motor’s rated current.

Characteristics of Bimetallic Thermal Overload Relay

1️⃣ Time-Delay Operation:
This relay can withstand temporary overloads without tripping.
If the current increases only for a short duration, it does not immediately disconnect the circuit, allowing the motor to continue operating under normal conditions once the current stabilizes.

2️⃣ Current-Dependent Action:
The heating of the relay and the bending of the bimetallic strip depend directly on the magnitude of the current.
The higher the current, the faster the strip heats up and bends.

3️⃣ Resettable:
After the overload condition is cleared and the relay cools down, it can be reset manually or automatically, allowing the system to resume normal operation.

4️⃣ Inverse Time Characteristic:
The relay exhibits an inverse time feature — meaning, the greater the overload current, the faster the tripping occurs.
This ensures enhanced protection for the motor by responding proportionally to the severity of the overload.

Types of Thermal Overload Relay (Bimetallic Thermal Overload Relay Types)

Thermal overload relays are classified based on various criteria such as reset mechanism, mounting arrangement, number of phases, and tripping design.
Each type is explained in detail below 👇

🔸 1️⃣ Based on Reset Mechanism

This classification depends on how the relay is reset after tripping.

🔹 (a) Manual Reset Type
After tripping, the relay remains in the OFF position until it is manually reset.
It is used in applications where safety is critical and human inspection is required before restarting.
Example: Motor control panels, pumps, and industrial machines.

🔹 (b) Automatic Reset Type
This relay automatically resets itself after cooling down.
It is used where continuous operation is essential and manual resetting is inconvenient.
Example: Small compressors or unattended machines.

🔹 (c) Manual–Automatic Combination Type
This type provides both manual and automatic reset options.
A selector switch allows the user to choose between modes for greater flexibility.
Example: Modern motor control circuits.

⚙️ 2️⃣ Based on Mounting Arrangement

This classification depends on how the relay is installed.

🔹 (a) Separate Mounting Type
The relay is mounted independently on a panel or base plate.
It is suitable for systems where easy replacement or servicing is needed.
Example: Standalone motor starter circuits.

🔹 (b) Contactor-Mounted Type
This relay is mounted directly on the contactor base, making it compact and space-saving.
It often comes with mechanical interlocking, ensuring both the contactor and relay trip simultaneously.
Example: Modern motor starter units.

⚙️ 3️⃣ Based on Number of Phases

Thermal overload relays are also classified according to the number of power lines (phases) they monitor.

🔹 (a) Single-Phase Overload Relay
Used for single-phase motors and small electrical equipment.
It features a simple design, low cost, and easy installation.

🔹 (b) Three-Phase Overload Relay
Used for three-phase induction motors.
Provides balanced protection across all phases and trips when any phase experiences overload or imbalance.

⚙️ 4️⃣ Based on Tripping Mechanism Design

🔹 (a) Differential Type
This relay detects both phase imbalance and overload conditions.
If any phase current becomes abnormal, the tripping mechanism is activated.

🔹 (b) Non-Differential Type
This relay trips only when the total current exceeds the preset limit.
It is simpler and used in balanced or small-load circuits.

Advantages of Bimetallic Thermal Overload Relay

✅ 1️⃣ Simple Construction:
This relay has a very simple design, consisting of only a few mechanical components — such as the bimetallic strip, heater coil, and contacts.
Because of its simplicity, it provides reliable protection at a low cost.

✅ 2️⃣ Reliable Operation:
It works on the thermal principle, ensuring long-term stability and dependable performance.
No external power supply or complex circuitry is required for its operation.

✅ 3️⃣ Inverse Time Characteristic:
The relay trips faster for higher overload currents and slower for smaller overloads, providing accurate and proportional protection to the motor.

✅ 4️⃣ Resettable:
It can be reset manually or automatically, allowing repeated use after tripping.
This makes it a durable and long-lasting protective device.

✅ 5️⃣ Low Maintenance:
Since it contains no complex electronic components, it requires minimal maintenance and is cost-effective.

⚠️ Limitations of Bimetallic Thermal Overload Relay

❌ 1️⃣ Slow Response:
Because it depends on heat generation, it cannot respond quickly to sudden short circuits or rapid current surges.

❌ 2️⃣ Temperature Dependency:
Its accuracy is affected by ambient temperature changes.
In hot conditions, it may trip too quickly, while in cold environments, it may trip too slowly.

❌ 3️⃣ Overload Protection Only:
It provides protection only against overloads, not against short circuits.
Hence, it is usually used in combination with circuit breakers or fuses.

❌ 4️⃣ Limited Precision:
Compared to electronic or digital relays, its accuracy and precision are relatively lower.

Applications of Bimetallic Thermal Overload Relay

The Bimetallic Thermal Overload Relay is widely used in applications where electric motors and industrial control systems need protection from overload currents.
It is a highly reliable device that safeguards motors from overheating and long-duration overloads.

Below are the key applications 👇

🔹 1️⃣ Motor Protection:
This relay protects induction motors, synchronous motors, and DC motors from overload conditions.
When the motor current exceeds the rated limit, the relay automatically trips and disconnects the circuit, preventing damage.

🔹 2️⃣ Industrial Control Panels:
It is used in control panels and starter circuits such as DOL (Direct-On-Line), Star-Delta, and Auto Transformer Starters to protect motors from abnormal load conditions.

🔹 3️⃣ Pump and Compressor Circuits:
In pumps and compressors, the load can fluctuate suddenly.
The relay prevents the motor from drawing excessive current and safely shuts it off in overload conditions.

🔹 4️⃣ Conveyor Systems and Machinery:
In applications where machinery operates under continuous mechanical load, this relay prevents the motor from overheating or burning out.
Hence, it is widely used in conveyor belts, crushers, and mill drives.

🔹 5️⃣ HVAC and Fan Control Systems:
In HVAC units and large fans, motors often run for extended periods.
This relay ensures reliable thermal protection, thereby extending equipment life and maintaining operational safety.

🔹 6️⃣ Agricultural Motor Control:
It is highly useful for agricultural pump motors.
When a pump runs in a dry or jammed condition, the relay detects the overload current and shuts down the motor, preventing possible damage.

Difference Between Thermal and Magnetic Overload Relay

The table below highlights the key differences between the Thermal Overload Relay and the Magnetic Overload Relay 👇

Real-Life Example

In an industrial motor control panel, a bimetallic thermal overload relay is connected in series with the motor’s supply line.

If the motor experiences a mechanical jam or an overload condition, it begins to draw excessive current (overcurrent).
As this happens, the bimetallic strip inside the relay heats up and bends, activating the tripping mechanism, which automatically disconnects the motor circuit.

Thus, the relay effectively protects the motor from burning, overheating, or potential fire hazards, ensuring safe and reliable operation.

Conclusion

The Bimetallic Thermal Overload Relay is a simple, cost-effective, and highly efficient protective device essential for safeguarding motors against overload and overheating.
Its features — time delay, reset capability, and inverse time characteristic — make it an ideal choice for both industrial and domestic motor protection systems.