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The difference and application of motor protection circuit breaker and thermal relay


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

The main difference between the two is that the working methods and principles are different, although they can both protect the motor and prevent overload. However, the principle of the thermal relay is: the current flowing through the heating element generates heat, which deforms the bimetal sheets with different expansion coefficients. When the deformation reaches a certain distance, the connecting rod is pushed to open the normally closed contact on the thermal relay. , and the normally closed contact is connected in series to the control circuit of the motor contactor. After the normally closed contact is disconnected, the coil of the contactor will lose power, and the main contact of the contactor will be disconnected to realize the overload protection of the motor. .

The principle of the motor protection circuit breaker is: when the motor protection circuit breaker is working normally, press the handle to turn on the power, at this time the tripping mechanism is locked and the contacts cannot move. When the current is too large, the bimetal strip deforms and pushes the lock. When the instantaneous current is too large, the iron core is attracted, and the lock is pulled through the pull pin. When the internal temperature rises too high, the deformation of the bimetal sheet is compensated, and the contacts are disconnected under the action of the release spring to complete the breaking protection function of the circuit breaker for motor protection. We have seen that the motor protective circuit breaker is actually a comprehensive electrical switch that directly integrates the thermal relay and the circuit breaker. Just because it is directly installed on the main power supply circuit of the motor, when it senses overcurrent and other conditions, it will directly disconnect the main circuit instead of disconnecting the main circuit through the contactor.

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