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Relay output type PLC contact failure case
Case description:
There is a PLC system that controls an industrial production line, which is used to control the start and stop of multiple motors. The contactor of one of the motors cannot be properly attracted, but the output indicator of the PLC is on.
Troubleshooting process:
First, use the resistance range of the multimeter to check the PLC relay output contacts connected to the motor contactor. It is found that when the PLC output signal causes the relay to operate, the resistance value of its normally open contact is very large, which should be close to zero ohms under normal circumstances, indicating that the contact is not closed normally.
Open the PLC output module housing (while ensuring safety) and check the relay itself. It is found that the relay contacts have obvious burn marks, which is due to long-term frequent operation and possible arcing, resulting in contact damage.
Solution:
Replace the damaged relay. At the same time, in order to avoid similar problems from happening again, the control program is optimized to reduce unnecessary frequent operation of the relay. And in terms of hardware, consider adding an arc extinguishing device to protect the relay contacts.
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Transistor output type PLC short circuit fault case
Case description:
The PLC in a certain automation equipment is used to control the opening and closing of multiple solenoid valves. During the operation of the equipment, the PLC suddenly alarmed, some solenoid valves could not work properly, and the power indicator light of the PLC output module flashed.
Troubleshooting process:
Check the output indicator lights of the PLC output module and find that several indicator lights are not on. Use a multimeter to measure the voltage between these output points and the common end, and find that the voltage is abnormal, and the voltage of some output points is almost zero.
Then check the output wiring and find that the insulation layer of two of the output wires is damaged. The two wires came into contact during the vibration of the equipment, causing a short circuit. The short circuit caused the protection circuit of the output module to operate and cut off part of the output signal.
Solution:
Repair the damaged wires and re-insulate them. In addition, check and organize all output wiring, add fixtures, and reduce the shaking of wires during the operation of the equipment. At the same time, add output fault detection and alarm functions to the PLC program so that similar situations can be discovered in time when they occur again.
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Case of equipment failure caused by loose output wiring
Case description:
A PLC system is used to control the cargo handling equipment in the warehouse, including the conveyor belt motor and the lifting motor of the stacker. During the operation of the equipment, the conveyor belt motor suddenly stopped working, but the PLC program showed that the motor should be in operation and the PLC output indicator light was on.
Troubleshooting process:
Checking the motor’s terminal box, it was found that the wire connecting the PLC output terminal to the motor contactor coil was loose and had poor contact. Due to the poor contact, the motor could not receive the normal start signal.
Solution:
Reconnect the loose wires firmly to the terminal of the motor contactor coil. To prevent looseness from occurring again in the future, spring washers were added to the terminal, and the equipment wiring was checked and tightened regularly.
