In order to reduce the electromagnetic interference radiated by the power cable, especially the feeder cable of the frequency conversion device. In a project, the author used copper tape armored shielded power cable, thereby reducing the electromagnetic interference of power line production. After the project was put into production, satisfactory results were achieved.
Different types of signals are transmitted by different cables. Signal cables should be arranged in layers according to the type of transmission signals. It is strictly forbidden to use different wires of the same cable to transmit power supply and signals at the same time. interference.
Hardware filtering and software anti-if measures
Before the signal is connected to the computer, a capacitor should be connected in parallel between the signal line and the ground to reduce the common mode interference; adding a filter between the two poles of the signal can reduce the differential mode interference.
Due to the complexity of electromagnetic interference, it is impossible to fundamentally eliminate the influence of the interference. Therefore, in the software design and configuration of the PLC control system, anti-interference processing should be carried out in the software to further improve the reliability of the system. Some commonly used measures: digital filtering and power frequency shaping sampling, which can effectively eliminate periodic interference; regularly correct the reference point potential, and use dynamic zero point, which can effectively prevent potential drift; use information redundancy technology to design corresponding software flags; Use indirect jumps, set software traps, etc. to improve the reliability of software structure.
Correctly select the grounding point and improve the grounding system
The purpose of grounding is usually two, one is for safety, and the other is to suppress interference. A perfect grounding system is one of the important measures for PLC control system to resist electromagnetic interference.
There are three types of system grounding methods: floating grounding, direct grounding and capacitive grounding. For the PLC control system, it is a high-speed low-level control device and should be directly grounded. Due to the influence of the distributed capacitance of the signal cable and the filtering of the input device, the signal exchange frequency between the devices is generally lower than 1MHz, so the grounding wire of the PLC control system adopts one point grounding and one point grounding in series. The centrally arranged PLC system is suitable for one-point grounding in parallel. If the distance between the devices is large, the grounding method should be adopted in series. Use a large-section copper busbar (or insulated cable) to connect the center grounding point of the cabinet of each device, and then connect the grounding busbar directly to the grounding electrode. The ground wire adopts copper wire with a cross-section greater than 22mm2, and the general busbar uses a copper bar with a cross-section greater than 60mm2. The grounding resistance of the grounding electrode is less than 2Ω, the grounding electrode is preferably buried 10~15m away from the building, and the grounding point of the PLC system must be more than 10m away from the grounding point of the strong electrical equipment.
When the signal source is grounded, the shielding layer should be grounded on the signal side; when it is not grounded, it should be grounded on the PLC side; when there is a connector 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 the point signal is connected with the multi-core twisted pair total shielded cable, the shielding layers should be well connected to each other and treated with insulation. Select a single point contact at the appropriate ground.
