PLC control system, what are the main source of electromagnetic interference

( 1) Radiation electromagnetic field from space radiation interference space ( EMI) Mainly by the electric power network, the transient process of electrical equipment, lightning, radio, television, radar, high frequency induction heating equipment, etc, often referred to as radiated interference, its distribution is very complex. If PLC systems in the rf field, recycling to the radiated interference, its influence mainly by two paths; It is directly on the PLC internal radiation, generated by the circuit induction interference; But the PLC communication within the network of radiation, introduced by communication lines of inductive interference. Radiated interference and electromagnetic fields generated by the field equipment layout and equipment size, especially the frequency, usually by setting the shielded cable and PLC local block and high-pressure discharge components for their own protection. ( 2) Interference from outside the system lead mainly introduced through power supply and signal lines, usually referred to as the conducted interference. This kind of interference in industrial field more serious in our country. ( 3) Interference from the power practice proved that the PLC control system failure caused by the interference of power supply is introduced into the situation a lot, the author encountered in some engineering commissioning, after the replacement higher PLC power supply isolation performance, problems to be solved. PLC system are both normal power supply by the power supply. Due to the grid coverage range, it will have all the space electromagnetic interference and online induction voltage and circuit on the road. Internal changes, especially the grid into the switching surge, large power equipment start-stop, ac/dc rotating device caused by harmonic, power grid short-circuit transient impact, etc. , through the transmission lines to the power supply side. PLC power supply usually adopt isolated power supply, but its mechanism and manufacturing process factors make its isolation is not ideal. In fact, due to the distribution parameters, especially the existence of the distributed capacitance, absolute isolation is impossible. ( 4) Interference from signal is introduced into all kinds of signal transmission lines connected to the PLC control system, in addition to all kinds of signal transmission effectively, there will always be the external interference signal. There are two main ways: the interference is an instrument or Shared via the transmitter signal interference power supply series of the power grid, which tend to be ignored; Secondly, signal interference of electromagnetic radiation induced by space, namely the external induction interference on line, this is very serious. Introduced by signal interference can cause I/O signal abnormal work and measurement accuracy is greatly reduced, serious when will cause components damage. For the worse performance of isolation systems, but also will cause mutual interference between signals and caused a total system bus back, cause logical data change, misoperation and crash. PLC controller control system for damage caused by I/O module signal into several serious cases, the resulting system failure is also a lot. ( 5) Interference from grounding system chaos when the grounding is to improve the electronic equipment electromagnetic compatibility ( EMC) One of the effective means. Proper grounding, can restrain the influence of electromagnetic interference, and inhibit the equipment sent outside interference; And the wrong ground, on the contrary will introduce serious disturbance signal, make the PLC controller system will not be able to work properly. PLC control system of earth system, shielding, interact and conservation, etc. Grounding system chaos disturbance to the PLC system are mainly the ground potential distribution, between different ground potential difference, causes to the loop current, normal operation system. Must point grounding cable shielding layer, for example, if A and B are grounded on both ends of the cable shielding layer, there is A potential difference, has A current flows through the shielding layer, when the abnormal state and lightning occurs, the ground current will be larger. Besides, shield, ground wire, and the earth could constitute a closed loop, in under the action of magnetic field change, shielding layer inside will be induced current, by shielding layer between the core and coupling, jamming signal circuits. If chaos system and other ground handling, the land circulation may cause different potential distribution on the ground, affect the normal work of the PLC logic circuit and analog circuit. PLC logic voltage interference tolerance is low, logically potential distribution of the interference easily affect the PLC logic operation and data storage, data confusion, fly or crash program to run. Analog ground potential distribution will be led to the decrease of the accuracy of measurement, cause serious distortion of signal measurement and control and false action. ( 6) Interference from the PLC system is mainly composed of the internal components and circuit between electromagnetic radiation against each other, mutual radiation such as logic circuit and its influence on analog circuits, analog ground and logic to influence each other and don't match each other between the components of use, etc. Which belong to the PLC manufacturer to the content of the electromagnetic compatibility design of internal system, is more complex, the application department is unable to change, but don't consider too much, but should choose to have more application performance or tested system.
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