How does a thermal overload relay work? How to connect? What are the precautions?

Working principle of thermal overload relay: Thermal overload relay is a protective electrical component that uses the thermal effect of current to drive the action mechanism, thereby cutting off the control circuit, and then cutting off the main circuit. It is generally used as overload protection for motors.

Thermal overload relays are also called thermal relays. There are many models and styles. Some thermal relays have phase loss protection and three-phase current unbalance protection. When the motor is working, overload or failure occurs, resulting in a rapid increase in operating current, and the thermal element of the thermal relay heats up rapidly. The higher the temperature, the more obvious the bending of the bimetallic strip. The curved bimetal pushes the action mechanism to move, the normally closed contact opens and the normally open contact closes, the greater the overload current, the faster the action time, thus protecting the motor from being burned out.

The rational use of thermal relays directly affects the safe operation of motor equipment, and the points for attention are as follows.

1. The conditions of use should be clear, frequent start and stop and reverse connection braking circuit should not heat the relay.

2. The setting value of the current should be set accurately. If the setting value is too large, it will not work. If the setting value is too small, the motor cannot run. Generally, it is 0.95-1.15 times the rated current of the motor. Select small for light load and large for heavy load. Generally, the setting current value is equal to the rated current of the motor.

3. The connection contacts should be tight to prevent malfunction due to heating caused by poor contact.

4. After an overload fault, check to ensure that the heat relay is in good condition before it can be used again.

5. The reset method should be reasonable. When there is no one on site, it can be manually reset to prevent automatic reset after failure and the motor to start again.

The thermal relay has a set of normally closed contacts 95 and 96, and a set of normally open contacts 97 and 98. The normally closed contacts are connected in series to the control circuit, usually the coil of the AC contactor is connected in series. When the motor is overloaded, the thermal relay trips, the normally closed contacts 95 and 96 are disconnected, the coil of the AC contactor loses power, and the motor stops running. The normally open contact can be connected to the alarm or fault indication, or not connected.