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In order to protect the system from or reduce internal and external electromagnetic interference in the industrial electromagnetic environment, three suppression measures must be taken from the design stage: suppression of interference sources; Cut off or attenuate the transmission path of electromagnetic interference; Improve the anti-interference ability of devices and systems. These three points are the basic principles of suppressing electromagnetic interference.
The anti-interference of PLC control system is a system engineering, requiring the manufacturing unit to design and produce products with strong anti-interference ability, and depends on the use of the department in the engineering design, installation and construction and operation and maintenance to be fully considered, and combined with the comprehensive design of the situation, in order to the electromagnetic compatibility and reliable operation of the system. The anti-interference design of specific projects should be mainly in the following two aspects.
In the selection of equipment, we must first choose products with high anti-interference ability, including electromagnetic compatibility (EMC), especially the ability to resist external interference, such as the use of floating technology, good isolation PLC system; Secondly, we should also understand the anti-interference indicators given by the manufacturer, such as common simulation ratio, differential simulation ratio, voltage resistance, how much electric field strength and how high frequency magnetic field strength environment is allowed to work; In addition, it is based on the examination of its application in similar work.
In the selection of foreign imported products should pay attention to: China is the use of 220V high internal resistance power grid system, while Europe and the United States is 110V low internal resistance power grid. Due to the large internal resistance of China’s power grid, the zero potential drift is large, the ground potential changes are large, the electromagnetic interference on the site of industrial enterprises is at least 4 times higher than that in Europe and the United States, the system anti-interference ability is high, and the PLC products that can work normally in foreign countries may not be reliable in domestic industry, which is necessary when using foreign products. According to China’s standard (GB/T13926) reasonable choice.
3.2 Integrated anti-interference design
Several suppression measures from outside the system are mainly considered. The main contents include: shielding the PLC system and the outer lead to prevent electromagnetic interference from space radiation; Isolation and filtering of external leads, especially the principle power cable, layered arrangement to prevent the introduction of conducted electromagnetic interference through the external lead; Correctly design the ground point and grounding device to improve the grounding system. In addition, software means must be used to further improve the system.
4 Main anti-interference measures
4.1 The use of excellent power supply to suppress the interference introduced by the power grid
In the PLC control system, the power supply occupies a very important position. Grid interference into the PLC control system mainly through the PLC system’s power supply (e.g
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