The PLC should be kept away from strong disturbance sources such as electric welding machines, high-power silicon rectifier equipment and large-scale power equipment, and cannot be installed in the same switch cabinet as high-voltage electrical appliances.
The power line, control line and the power line and I/O line of the PLC should be separated from the wiring, and the double glue line should be used between the barrier transformer and the PLC and I/O. Separate the IO line and the high-power line of the PLC, if it must be in the same trunking, separate and bundle the communication line and DC line, if the conditions permit, the slot is the best, which can not only make it as large as possible space distance, but also reduce the disturbance to the minimum limit.
The PLC should be kept away from strong disturbance sources such as electric welding machines, high-power silicon rectifier equipment and large-scale power equipment, and cannot be installed in the same switch cabinet as high-voltage electrical appliances. In the cabinet, the PLC should be far away from the power line (the distance between the two should be greater than 200mm). Electrophysical loads installed in the same cabinet as the PLC, such as relays with large power and coils of contactors, should be connected in parallel with RC arc suppression circuits.
It is best to separate the input and output of the PLC, and the switching and imitation should also be separated. The transmission of the imitation signal should be shielded wire, and the shielding layer should be grounded at one end or both ends, and the grounding resistance should be less than 1/10 of the resistance of the shielding layer.
Do not use the same cable for the communication output line and the DC output line, and the output line should be as far away as possible from the high-voltage line and the power line to avoid parallelism.
1. Wiring at the I/O end
Enter the wiring
The input wiring should generally not be too long. However, if the environmental disturbance is small and the voltage drop is not large, the input wiring can be appropriately longer.
The input/output lines cannot be used with the same cable, and the input/output lines should be separated.
As far as possible, the normally open contact method is used to connect to the input terminal, so that the prepared ladder diagram is consistent with the relay schematic diagram and is easy to read.
Output articulation
The output terminal wiring is divided into independent output and common output. In different groups, different types and voltage levels of output voltages can be used. However, the output in the same group can only use the same type of power supply of the same voltage level.
Since the output components of the PLC are encapsulated on the printed circuit board and connected to the terminal board, if the load of the connected output components is short-circuited, the printed circuit board will be incinerated.
When selecting the relay output, the size of the electrorational load accepted will affect the service life of the relay, therefore, the use of electrorational load should be reasonably selected, or the barrier relay should be added.
The output load of the PLC may be disturbed, so measures should be taken to control it, such as the maintenance of the freewheeling tube of the DC output, the resistance and capacitance absorption circuit of the communication output, and the bypass resistance maintenance of the output of the transistor and bidirectional thyristor.
2. Correctly select the grounding address and improve the grounding system
Good grounding is an important condition to ensure the reliable operation of PLC, which can avoid the accidental occurrence of voltage shock hazards. There are usually two purposes for grounding, one is for safety and the other is to suppress disturbances. The perfect grounding system is one of the important measures to resist electromagnetic disturbance in the PLC control system.
The ground wire of the PLC control system includes the system ground, the shielding ground, the communication ground and the maintenance ground. The disturbance of the grounding system to the PLC system is mainly due to the uneven distribution of the potential of each grounding address, and the ground potential difference between different grounding addresses, which causes the ground loop current and affects the normal operation of the system. For example, the cable shield must be grounded at one point, if the cable shield is grounded at both ends A and B, there is a ground potential difference, and there is a current flowing through the shield, when an abnormal situation such as lightning strikes, the ground wire current will be greater. In addition, the shield, the ground wire, and the ground may form a closed loop, and under the effect of changing the magnetic field, an induced current will appear in the shield, and the coupling between the shield and the core wire will disturb the signal loop. If the system is disordered with other grounding treatments, the unequal potential dispersion may occur on the ground line, which will affect the normal operation of the logic circuit and analog circuit in the PLC. The logic voltage disturbance tolerance of PLC operation is low, and the dispersion disturbance of logical ground potential can easily affect the logic operation and data storage of PLC, resulting in data disorder, program flight or crash. The dispersion of imitating ground potential will lead to a decrease in measurement accuracy, resulting in severe distortion and malfunction of signal measurement and control.
Safely or mains grounded
Grounding the power cord and the cabinet connection to the ground are safety grounding. If the power supply leaks or the cabinet is electrified, it can be imported underground from the safety grounding and will not cause harm to people.
System grounding
The PLC manipulator is grounded in order to be at the same potential as the various equipment under control, which is called system grounding. The grounding resistance value shall not be greater than 4Ω, and it is generally necessary to connect the PLC equipment system with the negative end of the switching power supply in the control cabinet as the control system.
The signal is grounded with the shield
Generally, it is required that the signal line must have the only reference ground, and the shielded cable should only be grounded in the local or control room when there is a possibility of conduction disturbance, so as to avoid the formation of a "ground loop". When the signal source is grounded, the shield should be grounded on the signal side; When it is not grounded, it should be grounded on the PLC side; When there is a joint in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multi-point grounding must be avoided; When the shielded twisted pair of multiple measurement point signals is connected with the multi-core twisted pair total shielded cable, each shielding layer should be well connected with each other, and the appropriate grounding should be selected as a single-point contact after insulation treatment.
3. Suppression of inverter disturbance
There are generally the following ways to deal with the disturbance of the inverter:
Barrier transformer:
It is mainly aimed at the conduction disturbance from the power supply, and can block most of the conduction disturbance before the barrier transformer. The use of filters, the filter has a strong anti-disturbance ability, but also has the ability to avoid the equipment itself disturbance to the power supply, and some also have the function of spike voltage absorption.
Use of output reactors:
The increase of communication reactor between the inverter and the motor is mainly to reduce the output of the inverter, and the electromagnetic radiation occurs on the line during the energy transmission process, which affects the normal operation of other equipment.
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