Technical Guides

What is a Thermal Relay?

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Thermal relays are a type of electrical device used to protect motors and electrical circuits from overloading, often used with contactors. Thermal relays have the function of automatically switching contacts by the thermal expansion and contraction of metal rods.

Application of Thermal Relays

Thermal relays are fitted with Contactor to protect electrical equipment, especially electric motors when overcurrent, overload during operation.

Note: Thermal relays only work to change the contact state, but not to disconnect the power so it must be combined with another switch.

The characteristic of thermal relays is that it takes a certain amount of time to operate based on the thermal expansion mechanism and not act as fast (instantaneously) as electromagnetic switches. Therefore, thermal relays are only used for overload protection, not for short circuit protection. To protect against short circuit, it must be used with aptomat, fuse.

Thermal relays operating at alternating voltages up to 500V, frequency 50Hz, have a range of effects from a few hundred mA to a few hundred A. Thermal relays of Mitsubishi, LS, Schneider have a range of 0.1A to 800A.

Structure of thermal relay


  1. Leverage
  2. The contact is normally closed
  3. The normally open contact
  4. Screw adjusting the impact current
  5. Bimetallic bar
  6. Heating wire
  7. The lever
  8. Reset button

It can be said that the thermal relay is not too complicated and the usage is very simple.

The working principle of thermal relays

True to its name, thermal relays operate based on the change of temperature of the current. When the current is overloaded, a huge amount of heat is produced which causes the metal plate of the relay to heat up, leading to the expansion. In the composition of the thermal relay, the double metal plate plays an extremely important role for the device to work effectively. This dual metal plate is made of two metal bars with different elongation index.

Typically, the first metal bar will have less expansion coefficient and often invar (including 36% Ni + 64% Fe). The second metal bar is usually made from brass or chromium – nickel steel because of its expansion index about 20 times greater than the invar. The two slabs are assembled into one sheet by hot rolling or welding.

When the current has a sudden change, the temperature will impact on the double steel rod so that it bends in the direction of the metal rod with less expansion coefficient now that we can use directly for the current or surrounding resistance wire. . The amount of bend is more or less depending on the length and thickness of the metal rod.

Classification of Thermal Relays

According to the structure of thermal relays divided into two types: open type and closed type.

– On request use: Single and double pole type.

– By heating method:

+ Direct heating: Electricity flows directly through the double metal plate. This type has a simple construction, but when changing the rated current, the plate must be changed Double metal, this type is not convenient.

+ Indirect heating: The electric current flows through the independent heating element, the heat radiated indirectly causes the metal plate to bend. This type has the advantage of wanting to change the rated current we only need to change the heating element. The disadvantage of this type is that when there is a large overload, the heating element can reach quite high temperatures, but because of poor heat transfer air, the metal sheet has not been able to toxic and the heating element has been burnt off.

+ Combined heating: This type is relatively good because it burns directly and indirectly. It has relatively high thermal stability and can work at multiple overloads

How to choose a thermal relay

Thermal relays are used to protect the motor overload, so when choosing a thermal relay, the right motor type must be selected for protection. In many cases, the user selects a thermal relay according to the current of the contactor or the aptomat which is incorrect and leads to the motor to be burnt when overloaded.

Below is the Table of thermal relays according to motor power:

Some notes when selecting thermal relays:

+ Choose a thermal relay with an adjustable threshold corresponding to the operating range of the motor or slightly higher. The lowest regulating threshold of a thermal relay should be lower than the middle of the motor operating range. The highest adjustable threshold of the thermal relay must be higher than the upper limit of the motor operating range.

+ Some types of thermal relays have a pin for contactor (usually small thermal relays). Therefore it can only install the right type of contactor compatible with it.

+ Some high-end thermal relays have built-in phase failure protection.

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