Do you know the difference between thermal relay and motor protection circuit breaker?

Thermal relay is one of the most common electrical control components.It mainly plays a role in protecting the motor and preventing the motor from overloading.Most thermal relays are cheap, economical and practical.The motor protection circuit breaker (which can also be called motor protection switch) can be used as overload, short circuit, phase failure protection and infrequent start and stop control of three-phase cage asynchronous motor, and can be used for distribution line protection and infrequent load conversion, as well as isolator, and can also be used in conjunction with alarm contacts, shunt trip, undervoltage trip and other accessories.

The main difference between the two is their different working methods and principles, although they can protect the motor and prevent overload.However, the principle of thermal relay is that the current flowing through the thermal element generates heat, which causes bimetallic strips with different expansion coefficients to deform.When the deformation reaches a certain distance, the connecting rod is pushed to act, and the normally closed contact on the thermal relay is connected in series to the control circuit of the motor contactor.After the normally closed contact is disconnected, the coil of the contactor loses power, and the main contact of the contactor is disconnected, thus realizing the overload protection of the motor.

The principle of motor protection circuit breaker is:when the motor protection circuit breaker works normally, press the handle to turn on the power supply, at this time, the tripping mechanism is locked, and the contact cannot act.When the current is too high, the bimetallic strip deforms and pushes the lock catch.When the instantaneous current is too large, the iron core is sucked, and the latch is pulled by the pulling pin.When the internal temperature rise is too high, the bimetallic strip is compensated for deformation, and the contact is disconnected under the action of the release spring, thus completing the breaking protection function of the circuit breaker for motor protection.We can see that the motor protection circuit breaker is a comprehensive electrical switch which directly integrates the characteristics of thermal relay and circuit breaker.Just because it is directly installed on the main power supply circuit of the motor, it will disconnect the main circuit directly after it feels overcurrent, instead of disconnecting the main circuit through contactor.

The main circuit of thermal relay can not be used alone, but must be used together with contactor.The motor protection circuit breaker is more advanced than the thermal relay, and can directly make and break the main circuit.It can be directly connected to the motor without some control circuits such as start-stop switches.Another point is that the motor protection circuit breaker integrates the functions of circuit breaker and thermal relay, which reduces the occupied space and is more suitable when many motors need to be controlled in one control cabinet.