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How Does a Thermal Overload Relay Work?

A thermal overload relay protects motors by monitoring the heating effect caused by excessive current.

Traditional thermal overload relays use bimetallic elements. When a motor operates under overload conditions, increased current generates heat through the relay’s heating elements. The bimetallic strips inside the relay are made from two different metals bonded together. Because the metals expand at different rates, the strips bend when heated. Once the bending reaches the designed operating point, the trip mechanism activates.

In a motor starter circuit, the overload relay normally does not disconnect the main power circuit directly. Instead, it operates through the contactor control circuit. When the relay trips, the normally closed auxiliary contact opens and removes power from the contactor coil, causing the contactor to disconnect the motor.

Modern electronic overload relays use current sensing technology instead of traditional thermal elements. They provide improved accuracy and may include additional functions such as phase imbalance detection, communication functions, and advanced motor monitoring.

How a Thermal Overload Relay Works with a Contactor

A thermal overload relay is usually installed together with a contactor to form a complete motor starter system.

The typical power circuit arrangement is:

Power Supply → Circuit Breaker → Contactor → Thermal Overload Relay → Motor

The contactor controls motor operation, while the overload relay provides protection against prolonged overload conditions.

During normal operation, the overload relay continuously monitors the motor current. If the motor experiences abnormal conditions, such as mechanical overload or phase imbalance, the relay trips and opens the control circuit.

How to Set the Thermal Overload Relay Current

The overload relay current setting should be based on the motor nameplate full-load current (FLA). The motor manufacturer specifies the rated operating current, and this value should be used as the primary reference when adjusting the relay.

For a standard direct-on-line (DOL) motor starter, the overload relay is normally set close to the motor nameplate current.

For example, if a motor nameplate indicates a full-load current of 18A, the overload relay should typically be adjusted to approximately 18A.

The overload relay should not be adjusted higher simply to prevent nuisance tripping. Frequent trips usually indicate another problem, such as:

• Excessive mechanical load
• Voltage imbalance
• Poor motor ventilation
• Bearing or mechanical issues
• Incorrect motor selection

The correct approach is to identify and solve the underlying cause instead of increasing the protection setting.

Thermal Overload Relay Setting for Star-Delta Starters

Star-delta motor starters require special consideration because the overload relay setting depends on its installation position.

In many star-delta applications, the overload relay is installed inside the delta circuit. In this arrangement, the relay measures the motor phase current rather than the line current.

The approximate setting calculation is:

For example:

• A motor with a full-load current of 30A:
• 30A × 0.58 = approximately 17.4A

However, some star-delta starters install the overload relay in the motor supply line. In this case, the setting should normally match the motor nameplate current.

Therefore, before adjusting the overload relay, always confirm the starter wiring configuration and follow the relay manufacturer’s recommendations.

Understanding Thermal Overload Relay Trip Class

Trip class defines the response time of an overload relay under motor starting and overload conditions.

The correct trip class depends on the motor starting characteristics and application requirements.

Class 10 overload relays are commonly used for general industrial applications such as pumps, fans, and standard machinery.

For applications with higher starting torque or longer acceleration periods, such as large compressors or heavy-duty equipment, a higher trip class may be required.

Selecting the correct trip class helps prevent unnecessary tripping during motor starting while maintaining effective overload protection.

Manual Reset and Automatic Reset Selection

After an overload trip, the relay must return to its normal operating condition before the motor can restart.

The reset mode should always be selected according to the equipment safety requirements and operating environment.

Effect of Ambient Temperature on Overload Relay Setting

Because thermal overload relays respond to temperature, ambient conditions can influence their operating characteristics.

A relay installed in a high-temperature environment may trip earlier because the internal temperature rises faster. In very cold environments, the response may be slower.

Most modern overload relays include temperature compensation within their specified operating range. If the installation environment exceeds the recommended range, the manufacturer’s correction factors should be followed.

Thermal Overload Relay Wiring and Testing

Correct wiring is essential for reliable motor protection.

The most common auxiliary terminals are:

The 95-96 contact is normally connected in series with the contactor coil. When the overload relay trips, this contact opens and disconnects the contactor control circuit.

After installation, the protection system should be tested. Many overload relays include a test button that simulates a trip condition. This allows technicians to confirm that the contactor releases correctly and that the motor protection circuit operates properly.

Common Thermal Overload Relay Setting Mistakes

Incorrect overload relay adjustment can reduce motor protection effectiveness.

Common mistakes include:

Thermal Overload Relay vs Circuit Breaker

Although both devices are used in motor protection systems, they provide different types of protection.

A reliable motor protection system requires both overload protection and short-circuit protection.

Frequently Asked Questions