The role of thermal relay and overload relay’s installation wiring

what is a thermal overload relay ?

A thermal relay is a protector that uses the thermal effect of current to cut off the circuit. When the thermal relay is used for overload protection in the motor, it often work with ac contactor to form an electromagnetic starter, and then to be used in the control and protection circuit of the three-phase asynchronous motor. If the motor is overloaded for a long period of time, under-voltage or phase-failed during operation, the current of the motor will exceed the rated value. At this time, the thermal overload relay will cut off the control circuit of the motor, thus to effectively protecting the motor.

The working principle of the thermal relay is that the current flowing into the heating element generates heat to make the bimetal of different expansion coefficients to deform. When the deformation reaches a certain degree, the link is pushed to break the control circuit, thereby making the contactor lose power. And the main circuit is disconnected to achieve the overload protection of the motor.

As an overload protection component of the motor, the thermal relay has been widely used in production due to its small size, simple structure and low cost.

What is the role of the thermal relay?

It is mainly used for overload protection of asynchronous motor. His working principle is that after the overload current passes through the heat element, the bimetal is heated and bent to push the action mechanism to drive the contact action, thereby disconnecting the motor control circuit to realize the motor power off for overload protection. In view of the fact that the transfer of heat takes a long time during the bending of the bimetal, the thermal relay cannot be used as a short circuit protection, but can only be used as an overload protection. The symbol is FR.

motor overload relay

When the current through the thermal relay to the motor exceeds the set value for a certain period of time (the internal thermal differential element, the larger the excess value, the shorter the operating time), the auxiliary contact actions to disconnect the contactor coil  which controls the motor for the overload protection.

The thermal relay is mainly used for protecting the overload of the motor in the electric drive system and controlling the heating state of other electrical equipment. The purpose is to prevent the equipment such as the motor from being seriously overloaded for a long time which will accelerate the aging of insulation elements,shortening service life and even burning of the motor.

thermal overload protector

When the motor is overloaded for a period of time (there is an adjustable value on the thermal relay to set the current value), a certain amount of heat is accumulated inside the thermal relay to cause the relay to operate. After confirming the cause of the overload, press a button on the relay to resume.

The overload protection is not over-current protection, and fuses are generally used for over-current.

Thermal relay installation and wiring

overload protection switch

The thermal element of the relay shall be connected in the main circuit, and its breaking control contact shall be connected in series in the auxiliary circuit.

The wires which to be connected to the output and input terminals of the thermal relay should be correctly selected according to the rated current of the motor. Try to use copper wire as much as possible. If the aluminum wire to be connected to the copper terminal of the thermal relay, the contact resistance is large, the connection is likely to be heated and the thermal relay may move in advance or malfunction. The cross-sectional area of the copper wire has a great influence on the accuracy of the operation of the thermal relay. If the selected wire is too thin,the axial heat conduction is poor and may make the thermal relay action in advance; if the selected wire is too thick, the axial heat conduction is fast and the thermal relay may lag in operation.

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By | 2018-12-08T07:05:23+00:00 November 23rd, 2018|relay|