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What is a normally closed contact? Connecting a thermal relay. Main function and operating principle

Normally closed contacts

Normally closed contacts- a term characterizing the state of the main and additional contacts of relays, buttons and other switching electrical devices that have two asymmetrical states. One state is working, the other is not working. For example, for a button, the non-working state is not pressed, but the working state is pressed; for relay contacts, the operating state is not when the winding is de-energized, but the operating state is when current is applied to the winding.

There are timing relays which cause a signal in the output circuit after a certain period of time from the moment the input signal is applied and time output relays which cause a signal in the output circuit after a certain period of time when the input signal is interrupted.

Figure Representation mode of time relays in electrical circuits. The mode of representation in the electrical circuits of these relays is shown in the drawing. Classification of time relays. In accordance with the operating principle of the synchronization unit, temporary relays can be. With electromagnetic time, using an electromagnet that acts clockwise with gears.

In technology

Normally closed contacts are a device design that has closed contacts when not in operation. For example, a switch with normally closed contacts is used as a switching button for safety reasons, which, when not pressed, provides power supply. electrical voltage. When you press the button, the voltage is switched off briefly, which turns off the device; the same thing happens when the wires connecting the button break. Using a normally open button to turn off is unreliable, since if the connecting wires are broken, it is impossible to turn off the device.

With time induction relays which, due to the time dependence of these relays, can also perform the function of a timer relay in addition to overcurrent or directional relays. With electronic time, using semiconductor elements, diodes and transistors.

With an electrothermal delay, which exploits the effect of thermobimetallic deformation over time. With synchronization of electric motors using synchronous reluctance micromotors. With pneumatic timing, etc. These relays are used in automation and power systems where they understand the time required for selective protection. They are usually controlled by a current or voltage relay and transmit the signal to an intermediate relay.

In programming

Normally closed contact is also used as a metaphor ladder logic programming language for programmable logic controllers. In this case, each contact is assigned a logical variable equivalent to an active or passive state; when this variable is set to FALSE (passive state), the contacts are considered closed, and when the variable is set to TRUE (active state), the contacts are open. Image of normally closed contacts in the program:

Design versions of electromagnetic time relays. Thus, a time relay is formed from an input circuit, a synchronization unit and an output circuit. Electromagnetic time relays have an electromagnetic drive element. They consist of electromagnetic electromagnet, which synchronizes the mechanism clockwise and closes the normally open contact. Schematic diagram such a relay is shown in the figure.

Schematic diagram of a time synchronizing relay. After receiving the input signal, the electromagnet 1 pulls the movable armature 2, and the mechanism moves and closes the contacts in the output circuit after some time. The operating mode of these relays in the figure is represented by the clockwise time principle, which realizes the synchronization of the relays. The toothed sprocket acts on the gear sector. The handling of the gear sector causes the gears 6, 7, 8 and Wheel 7 to rotate in the direction shown in the drawing, but is stopped because its inclined teeth are blocked by the slider 8, fixed by wheel 6, and which does not allow free movement only in the opposite direction .

Literature

  • Michel J. Programmable controllers: architecture and application. - M.: Mechanical Engineering, 1986

See also

  • Ladder Logic Language

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, buttons and other switching electrical devices that have two asymmetrical states. One state is working, the other is non-working. For example, for a button, the working state is pressed, and the non-working state is unpressed; for relay contacts, the non-working state is when the winding is de-energized, and the working state is when current is applied to the winding.

Since both electrical relays and analog relays have low accuracy at high timings, digital relays are preferred. These digital time relays are based on splitting the pilot oscillator frequency or mains frequency and have a relaxing oscillator, flip-flop, frequency dividers and capacitors.

Electronic relays have the advantage of low level, low power consumption and high adjustment range. Instead, electronic relays are sensitive to temperature changes, switch overvoltages, and because they have very low output current, they require output amplifiers or intermediate relays.

In technology

Normally closed contacts are a device design that has closed contacts when not in use.

Such use, for example for safety reasons, at a button that turns off equipment (stop): a button with normally closed contacts is used, which, when not pressed, provides the supply electric current through closed contacts. When you press the stop button, the current is briefly interrupted, which gives a command to turn off the device; the same thing happens when the wires connecting the button break. Using a button with normally open contacts to turn off the equipment is unreliable, since if the connecting wires are broken, it is impossible to use electrical circuit and is guaranteed to turn off the switched on equipment.

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In programming

Normally closed contact is also used as a metaphor ladder logic programming language for programmable logic controllers. In this case, each contact is assigned a Boolean variable equivalent to an active or passive state, with this variable set to FALSE (passive state) the contacts are considered closed, and when set to TRUE (active state) the contacts are considered open. Image of normally closed contacts in the program:

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Literature

  • Michel J. Programmable controllers: architecture and application. - M.: Mechanical Engineering, 1986

See also

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