The PLC should be kept away from strong disturbance sources such as electric welding machines, high-power silicon rectifiers and large-scale power equipment, and should not 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 necessary, in the same trunking, separate the bundled communication line and DC line, if the conditions permit, the best slotted trace, which can not only make it as large as possible space spacing, but also minimize the disturbance.
The PLC should be kept away from strong disturbance sources such as electric welding machines, high-power silicon rectifiers and large-scale power equipment, and should not be installed in the same switch cabinet as high-voltage electrical appliances. The PLC in the cabinet 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 separated by the same cable, and the input/output lines should be separated.
● As far as possible, the normally open contact form is connected 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.
● Because the output component of the PLC is encapsulated on the printed circuit board and connected to the terminal board, if the load of the output component is short-circuited, the printed circuit board will be incinerated.
● When selecting relay output, the size of the electrophysical load 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 it is necessary to adopt measures 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 docking address and improve the grounding system
Excellent grounding is an important condition to ensure the reliable operation of the PLC, which can avoid the accidental occurrence of voltage surges. There are generally two purposes of grounding, one is for safety, and the other is for pressing and disturbing. 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, it is necessary for the cable shield to 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, and when an abnormal state such as lightning strikes occurs, 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, which will disturb the signal loop through the coupling between the shield and the core. If the system and other grounding processes are disordered, the ground circulation may have an unequal potential distribution on the ground, which will affect the normal operation of the logic circuit and the analog circuit in the PLC. The logic voltage disturbance tolerance of PLC operation is low, and the distribution disturbance of logical ground potential simply affects the logic operation and data storage of PLC, resulting in data disorder, program flight or crash. Mimicking the distribution of ground potential will lead to the decrease of measurement accuracy, and cause serious distortion and malfunction of signal measurement and control.
Safely or mains grounded
●The grounding end of the power cord and the connection of the cabinet are grounded for safety grounding. If the power supply leaks or the cabinet is electrified, it can be imported from the safety ground into the ground, and it will not cause damage to people.
System grounding
●The PLC controller is grounded in order to have the same potential as the 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 necessary for the signal line to have the only reference ground, and the shielded cable should only be grounded in the local or control room when it encounters the occasion 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 center 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 connected to each other, and the appropriate grounding should be selected as a single-point contact after insulation treatment.
3. Press on the inverter disturbance
There are generally the following methods for the disturbance treatment 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 disturbance of the device itself to the power supply, and some also have the function of spike voltage absorption.
Use of output reactors:
The main purpose of adding a communication reactor between the inverter and the motor is to reduce the electromagnetic radiation of the inverter output in the process of energy transmission, which affects the normal operation of other equipment.
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