On-site electromagnetic interference is one of the most common and most susceptible factors affecting system reliability in plc control systems.
(1) Interference sources and general classification
The interference sources that affect the PLC control system are mostly generated in places where the current or voltage changes drastically. The reason is that the current changes to generate a magnetic field, which generates electromagnetic radiation to the device; the magnetic field changes to generate current, and the electromagnetic high-speed generates electromagnetic waves. Generally, electromagnetic interference is classified into common mode interference and differential mode interference according to different interference modes. Common mode interference is the potential difference of the signal to the ground, which is mainly formed by the superposition of the common state (same direction) voltage induced by the power grid, the ground potential difference and the space electromagnetic radiation on the signal line. The common mode voltage can be converted into a differential mode voltage through an asymmetric circuit, which directly affects the measurement and control signal. (Source: http://) causes component damage (this is the damage rate of some system I/O modules). The higher main cause), this common mode interference can be DC or AC. Differential mode interference refers to the interference voltage acting between the two poles of the signal. It is mainly caused by the coupling of the space electromagnetic field between the signals and the common mode interference caused by the unbalanced circuit. This interference is superimposed on the signal and directly affects the measurement and control. Precision.
(2) Main sources and ways of interference in PLC system
Strong electrical interference: The normal power supply of the PLC system is powered by the grid. Due to the wide coverage of the grid, it will be subject to electromagnetic interference from all spaces and induce voltage on the line. In particular, internal changes in the power grid, surges in knife switch operation, start and stop of large-scale power equipment, harmonics caused by AC and DC transmissions, and short-circuit transients of power grids are transmitted to the primary side of the power supply through transmission lines.
Interference in the cabinet: high-voltage electrical appliances in the control cabinet, large inductive loads, and chaotic wiring are all likely to cause a certain degree of interference to the PLC.
Interference from signal line introduction: Various types of signal transmission lines connected to the PLC control system, in addition to transmitting various types of information, there will always be external interference signals. There are two main ways of this interference: one is the power grid interference through the power supply of the transmitter or the power supply of the shared signal meter, which is often neglected; the second is that the signal line is interfered by the space electromagnetic radiation, that is, the signal line The external inductive interference, which is very serious. The introduction of interference from the signal will cause the I/O signal to work abnormally and the measurement accuracy will be greatly reduced. In severe cases, the component will be damaged.
Interference from chaotic grounding systems: Grounding is one of the effective means of improving electromagnetic compatibility (EMC) of electronic equipment. The correct grounding can not only suppress the influence of electromagnetic interference, but also suppress the external interference of the equipment; and the wrong grounding will introduce serious interference signals, so that the PLC system will not work normally.
Interference from the PLC system: mainly caused by mutual electromagnetic radiation between internal components and circuits of the system, such as the mutual radiation of logic circuits and their influence on analog circuits, the interaction between analog ground and logic ground and the mutual interaction between components Match usage, etc.
Inverter interference: First, the harmonics generated during the startup and operation of the inverter cause conduction interference to the power grid, causing grid voltage distortion and affecting the power supply quality of the grid; second, the output of the inverter will generate strong electromagnetic radiation interference, affecting the surrounding The device works normally.
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