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Question:

Why can not thermal relays be used for short circuit protection?

Such as the title, I need is the explanation, thank you!

Answer:

By the thermal relay structure and working principle we can see that it is the use of the thermal effect of the current work. When the current flowing through the heat generating element connected to the main circuit exceeds the allowable value and after a certain period of time (thermal inertia), the bimetal sheet having different linear expansion coefficients is thermally deformed to deflect the buckle so that its normally closed contact In series with the control circuit), the contactor coil is de-energized, and the motor main circuit is disconnected to achieve overload protection. As the thermal relay has a thermal inertia, the motor starts or short time overload, it will not move, you can avoid unnecessary motor parking. When the motor short-circuit fault occurs, the short-circuit current in a very short period of time to achieve a large number of requirements, quickly cut off the power to protect the motor. But the heat accumulation of the thermal element in such a short period of time is not sufficient to deform the bimetallic sheet to drive the contact action. So the thermal relay can not meet the circuit when a short circuit failure instantaneous power requirements immediately, and therefore can not be used for short circuit protection. The working principle of the thermal relay is caused by the heat flowing into the heat element, so that the bimetal with different expansion coefficient is deformed. When the deformation reaches a certain distance, the connecting rod is pushed to make the control circuit open, so that the contactor Power failure, the main circuit off, to achieve the motor overload protection. Relay as the motor overload protection components, with its small size, simple structure, low cost advantages in the production has been widely used.
May 9, 2017
In simple terms, the thermal relay is detected when the temperature rises to a certain extent. At this point the current has been relatively large, after the components of the device is likely to have been burned. Generally only a few to tens of milliseconds is enough to burn equipment, thermal relay sensitivity is not so high.
May 9, 2017
Because the thermal relay works by heating the metal sheet to shorten the connection but it takes time to accumulate
May 9, 2017

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