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    Thermal Overload Relay Basics

    Thermal overload relays are overload relays that apply the principle of current thermal effect and use electrical thermal bimetallic sheets as sensitive components. The electrical thermal bimetal sheet is made by heating and rolling two alloys with large difference in linear expansion coefficient. When heated, the bimetallic sheet bends from the high expansion layer to the low expansion layer. When the current is too large, the element acts due to "heat", so that its linked moving contact cuts off the power supply of the controlled circuit and the protected equipment.

    Thermal overload relays are used for motor overload protection, and there are many types. The thermal overload relay is easy to install and use, has complete functions and low cost. It has been proved by practice that it can reliably protect the motor, so it has always played an important role.


    • 3 Phase thermal overload relayThere is a current adjustment cam to adjust the set current.
    • There is a temperature compensation device to ensure that the operating characteristics are basically unchanged within the temperature range of the surrounding medium from -20C to 60C.
    • There is a reset adjustment knob to adjust the reset mode, and there are two reset states: manual and automatic.
    • There is a tension spring flipping and jumping mechanism to ensure that the contact moves quickly and reliably.
    • There is a differential phase failure/three-phase unbalance protection device.
    • There is a detection slider/switch position indicator to simulate the tripping of the thermal relay and display the action status. Check and make sure the auxiliary circuits are wired correctly by this simulation. When the marking line on the slider is at the "0" mark, it shows tripping, and when it is at the "l" mark, it shows work.
    • There is a disconnect button. When the disconnect button is pressed, the normally closed contact opens the series contactor and disconnects the load. After releasing the disconnect button, the load restarts through the contactor.


    Thermal overload relay is used in circuits with AC 50Hz, rated insulation voltage of 660V, and current of 0.1 to 630A. It is mainly used for overload and phase failure protection of three-phase AC motors, and can be combined with suitable AC contactors to form a starter.

    Working principle

    Thermal relay is a protective device that uses the principle of current thermal effect to cut off the motor circuit when the motor cannot withstand overload, and provide overload protection for the motor.

    When the thermal relay protects the motor from overload, connect the thermal element in series with the stator winding of the motor, connect the normally closed contact of the thermal relay in series with the control circuit of the electromagnetic coil of the AC contactor, and adjust the setting current adjustment knob to make the herringbone shape. There is an appropriate distance between the lever and the push lever. When the motor is working normally, the current passing through the heating element is the rated current of the motor. The heating element heats up, and the bimetallic sheet bends after being heated, so that the push rod just contacts the herringbone lever, but cannot push the herringbone lever.

    The normally closed contact is in a closed state, the AC contactor keeps pulling in, and the motor runs normally. If the motor is overloaded, the current in the winding increases, the current in the thermal relay element increases, the temperature of the bimetal sheet rises higher, and the degree of bending increases, pushing the herringbone lever, and the herringbone lever pushes the normally closed contact, so that the contact is disconnected and the coil circuit of the AC contactor is disconnected, the contactor is released, the power supply of the motor is cut off, and the motor is stopped and protected.


    • Under normal circumstances, turn on and off the no-load and load current in the high-voltage circuit.
    • When the system fails, it can cooperate with the protection device and the automatic device to quickly cut off the fault current to prevent the accident from expanding, thereby ensuring the safe operation of the system.
    • The thermal relay generates heat by the current flowing into the thermal element, which deforms the bimetallic sheets with different expansion coefficients. When the deformation reaches a certain distance, it pushes the connecting rod to act to disconnect the control circuit, thereby causing the contactor to fail. The main circuit is disconnected to realize the overload protection of the motor.

    Tips for using the thermal overload relay

    1. The thermal overload relay can only be used as overload protection for controlling the motor, not as short-circuit protection.
    2. The rated current of the protected object must be known during installation, and the selected specification must be +20% of the rated current.
    3. It is usually installed directly after the starting contactor of the motor.
    4. When the current is too high or the load current exceeds the setting, the overload relay will be activated to break the circuit. Although the reset button can be used to restore the circuit, if the overload relay of the device is activated, the set current cannot be arbitrarily increased. The cause of the overload should be found out, otherwise the protected device will be easily burned.
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