As we all know: in the low voltage control circuit, the stop and emergency stop buttons are basically closed contacts. In PLC control, many colleagues don’t think so. The idea of “whether it is normally open or closed contact can be realized” often leads to great losses.

Example of simple PLC control motor: the stop button uses normally closed contact. If the stop button is not in good contact, it can ensure that the motor is in the stop state, which is safe for both equipment and people; on the contrary, if the stop button uses normally open contact, in case the button is not in good contact, it can not guarantee to stop immediately. Isn’t it a potential safety hazard.

This is the input point. Let’s talk about the output point.

A dust removal control system has been made for a metallurgical industry. The dust removal fan is driven by a 10KV rated voltage motor through a hydraulic coupler. The protection of the motor is natural: vibration, front and rear bearing temperature, stator temperature, cooling water temperature.. .。。 Our control system is to collect these signals and compare them with the set alarm value. If the requirements are not met, the motor will not be sent a “allowed operation” signal, and the signal will be sent when it reaches the requirements. In fact, the PLC output controls a relay, and the relay contacts are connected in the control circuit of the motor. At that time, for the convenience of debugging, the relay normally closed contact was used for the signal, which thought that even if the PLC did not work, it would not affect the motor start-up (because at that time, the PLC programming had not been completed, and the owner had called the motor manufacturer to the site to require the motor test). As a result, he was discovered by the technicians of the motor manufacturer (this is a motor factory in Dalian). He told us something he had just met. He went to another factory for debugging. They found that the bearing bush had overtemperature on the field instrument, but the motor was still running, causing the bearing bush worth hundreds of thousands to be burnt out. After inspection, it was found that the contact was normally closed, and the PLC was not put into operation at all. So the owner and the technician asked us to use normally open contact!

If the normally open contact is selected, the PLC will not allow the motor to start if it is not put into operation. When the PLC is put into operation, if the collected data meet the requirements, the relay will be powered on, the normally open contact will act, and the motor control circuit will be connected to allow the motor to run; if the collected data can not meet the requirements, the relay will lose power, and the normally open contact will remain disconnected to cut off the control circuit of the motor, so as to ensure that the motor stops running.

This event impressed me very deeply, so I dare not choose to open and close normally in the future project any more!

Selection of normally open and normally closed contacts in electrical design

Initial contact with electrical control partners are always confused about the choice of normally open and normally closed contacts. Today, we will popularize this knowledge.

In normal (no power) condition, the contact in the open state is called the normally open contact. When the coil is powered on, the open contact will close, and the normally closed contact will open. For the auxiliary switch linked with the main switch: when the main switch is closed, the auxiliary switch is also closed.

Selection of normally open and normally closed contacts in electrical design?


Normally open and normally closed contacts are connected to the input point of PLC. Generally, normally closed contacts should be selected for safety related contacts, such as emergency stop button, stop button, travel switch, safety grating, safety door, etc., because once there is a problem with these contacts, the circuit will be disconnected, and PLC will detect it. If you use the normally open contact here, once the line is broken, you can press the button again Play any role, PLC can not tell whether you have no operation button or disconnection, although in PLC no matter what contact, will be in accordance with your program, but for safety considerations, must use normally closed contact to wire.

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