I. Basic Structure

A thermal relay mainly consists of the following components:

1.Thermal Element: Generates heat when current flows through.

2.Contacts: Includes normally closed (NC) and normally open (NO) contacts to control circuits.

3.Reset Mechanism: Allows manual or automatic resetting after operation.

Below is a schematic diagram illustrating the structure and principle of a thermal relay:

II. Working Principle

1.Under Normal Operating Conditions
The thermal element is connected in series with the motor's main circuit, while the NC contact is connected in series with the control circuit of the AC contactor. When the motor operates normally, the current stays within its rated value, and the relay contacts remain unaffected.

2.Under Overload Conditions

If the motor experiences an overload and the current exceeds the rated value:

The thermal element generates excessive heat, causing the bimetal strip to bend.

The bimetal strip pushes the outer guide plate, moving the inner guide plate to the left.

This action separates the relay’s contacts, cutting off the control circuit of the AC contactor.

The contactor is de-energized, disconnecting the main circuit and isolating the motor from the power source, thereby providing protection.

3.Under Phase Failure (Single-Phase) Faults
If a single-phase failure occurs:

The bimetal strip in the faulted phase cools and moves back to the right.

The bimetal strip in the remaining active phases continues to heat and bend, driving the outer guide plate to the left.

The differential mechanism (via the lever) ensures faster operation, allowing the relay to trip promptly during a phase failure, protecting the motor.

4.Reset Mechanism

After tripping, the thermal relay requires a cooling period before the motor can restart.

Pressing the reset button resets the relay's contacts.

For automatic reset, turn the adjustment screw clockwise; for manual reset, turn it counterclockwise.

Compensation for Ambient Temperature

Thermal relays equipped with compensation bimetal strips maintain stable operation within an ambient temperature range of -30°C to +40°C.

The compensation bimetal strip should bend in the same direction as the main bimetal strip to effectively offset ambient temperature variations.

By incorporating these features, thermal relays ensure reliable protection for motors against overloads and phase failures, while adapting to different environmental conditions.