LU7-275-2000

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LU7-275-2000 is a limit unit type switch frequently used in the thermal overload protection device of high-end electric equipment. Specifically, its application field mainly include transformers, generators and electric motors. The operating principle of the LU7-275-2000 switch is to leverage the Joule-Thomson effect, which is an endothermic cooling effect observed when a high-pressure gas expands adiabatically. Early limit unit switches worked by using law pressure nitrogen gas, which created small differences in pressure between two sides of a calibrated spring. As the pressure differential increased, the product would trip and open the switch contact. However, LU7-275-2000 switches is more efficient and reliable than other limit unit switches.

Instead of using pressurised nitrogen gas like its predecessors, this switch relies on calibrated and hermetically sealed temperature-responsive membrane (TRM) to detect temperature. The membrane is a thin film of a particular elastomer material with a ceramic glaze. Its physical property is the ability to expand and contract depending on the differential pressure created by a heated fluid. When the temperature of the fluid rises and causes the TRM to expand, an axial movement corresponding to the ambient temperature is created. The entire system is designed to close the circuit when the threshold temperature is exceeded, effectively shutting off the device for protection.

The LU7-275-2000 switch is distinct for its robust and reliable performance. Its temperature ranges from –15°C to +75°C and its break point temperature can be set in the field with an easy to use knob. The knob adjusts the calibrated spring resistance and, thus, controls the break point temperature. A special feature of this switch is the clear-off position feature. This allows for manual resetting before power is restored if the motor was stopped due to an overload or excessive heat.

The operation of the LU7-275-2000 switch is fairly straight forward. Its main components consist of the hermetically sealed membrane, the calibrated spring, the terminal block and the contact. The calibrated spring is designed to have an exact torque-to-temperature coefficient. It is the combination of spring force and ambient heating which generates force feedback and regulates the switch’s temperature response. When the TRM reaches a certain temperature, its expansion causes the switch to trip and open the contact, thus stopping the application.

The LU7-275-2000 switch is an ideal solution for temperature and overload protection in most electric devices. Its reliable performance, clear-off position feature and adjustable break point temperature make it an attractive choice for a variety of applications. Furthermore, its easy installation and low power consumption offer convenience, flexibility and cost savings to the user.

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