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Classification and Common Types of Thermal Relays

Thermal relay is used for motor or other electrical equipment, electrical circuit overload protection of electrical appliances. In the actual operation of the motor, for example, in the process of driving the production machinery, if the abnormal condition of the machinery or the abnormal circuit causes the motor to encounter overload, the motor speed will decrease, the current in the winding will increase, and the winding temperature of the motor will increase.

If the overload current is not large and the overload time is short, the motor winding does not exceed the allowable temperature rise. This overload is allowed. However, if the overload time is long and the overload current is large, the temperature rise of the motor winding will exceed the allowable value, which will cause the motor winding to age, shorten the service life of the motor, and even burn the motor winding in severe cases. Therefore, this overload is unacceptable to the motor. The thermal relay is a protection device that uses the principle of the thermal effect of the current to cut off the motor circuit in the event of an overload that the motor cannot withstand, providing overload protection for the motor.

Classification of thermal relays

There are many types of thermal relays, among which are commonly used:

  • Bimetal type: Using a bimetal to use two metals with different expansion coefficients, usually manganese-nickel copper plates, rolled into a heated bend to push the lever to move the contacts.
  • Thermistor type: A thermal relay made by the characteristics that the resistance value changes with temperature.
  • Fusible alloy type: When the fusible alloy reaches a certain temperature value by the overheating current, the alloy melts and the relay acts.

As electrical equipment, there are many types of thermal relays for motor overload protection, but the most common ones are bimetal thermal relays. The utility model has the advantages of simple structure, small volume, low cost and good anti-time limit protection characteristics on the basis of selecting appropriate heat elements. At present, the thermal relays produced in China are bimetal type, which is often combined with a contactor to form an electromagnetic starter. It can be classified as follows.

According to the number of poles: there are unipolar bipolar and three poles. Among them, the three poles include the protection device with phase failure and the protection device without phase failure.

According to the reset mode: After the automatic reset contact is disconnected, it can automatically return to the original position and manual reset.

According to the current regulation method: current regulation and currentless adjustment can be achieved by replacing the thermal element to change the setting current.

According to the temperature compensation points: temperature compensation and no temperature compensation.

According to the control contact points: with normally closed contact contacts before the action is closed with normally closed and normally open contacts. The structure of the contacts is: switching contact bridge double breakpoints and the like.

Common types of thermal relays

  1. Bimetal type–Bimetals with different expansion coefficients (such as manganese nickel and copper sheets) are heated and bent to push the lever to make the contacts move.
  2. Thermistor type – the use of metal resistance value changes with temperature changes this feature made of thermal relay.
  3. Fusible alloy type – the use of overload current heating to make the fusible alloy to a certain temperature value will melt this feature, and make the relay action.

Among the above three types of thermal relays, the most used is a bimetal thermal relay, which is usually combined with a contactor to form an electromagnetic actuator.

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