Thermal relay is one of the most common electrical control components. It mainly plays the role of protecting motor and preventing motor overload. Most thermal relays are cheap, economical and practical. It can also be used to protect the motor from overload and tripping, and can also be used to protect the motor.

The main difference between the two is the working mode and principle, although both can protect the motor and prevent overload. But the principle of the thermal relay is: the current flowing through the thermal element generates heat, which makes the bimetallic sheet with different expansion coefficient deform. When the deformation reaches a certain distance, the connecting rod is pushed to open the normally closed contact on the thermal relay. The normally closed contact is connected in series to the control circuit of the electrical contactor. After the normally closed contact is disconnected, the coil of the contactor will lose power The main contact of the contactor is disconnected, thus realizing the overload protection of the motor.

The principle of the motor protection circuit breaker is: press the handle to connect the power supply under the normal working condition of the circuit breaker for motor protection. At this time, the tripping mechanism is locked and the contact cannot act. When the current is too high, the bimetallic sheet will deform and push the latch. When the instantaneous current is too large, the iron core is sucked and the latch is pulled by the broach. When the internal temperature rise is too high, the deformation of bimetallic sheet is compensated, and the contact is disconnected under the action of release spring, thus completing the breaking protection function of circuit breaker for motor protection. We can see that in fact, the motor protection circuit breaker is a comprehensive electrical switch which directly integrates thermal relay and circuit breaker. It is only because it is directly installed on the main power supply circuit of the motor. When it feels the over-current and other conditions, it will directly disconnect the main circuit, rather than through the contactor to disconnect the main circuit.

Therefore, according to the above, it is concluded that the main circuit of the thermal relay can not be used alone, but must be used together with the contactor. Motor protection circuit breaker is more advanced than thermal relay, which can directly turn on and off the main circuit. It can be directly connected to the motor without some control circuits such as start stop switch. Another point is that the motor protection circuit breaker integrates the functions of circuit breaker and thermal relay, which reduces the occupied space. It is more applicable when many motors need to be controlled in a control cabinet.