Site electromagnetic interference is the PLC control system is the most common and most likely to affect the reliability of the system is one of the factors, the so-called treatment of the symptoms of the first, to find out where the problem is, in order to put forward solutions to the problem. Therefore, it is necessary to know the source of field interference. (1) the source of interference and general classification
The source of interference affecting the PLC control system, most of which are generated in the current or voltage drastic changes in the parts, the reason is that the current changes in the generation of magnetic fields, electromagnetic radiation on the equipment; magnetic field changes in the generation of electric current, electromagnetic high-speed generation of electromagnetic waves. Usually electromagnetic interference according to the different modes of interference, divided into *** mode interference and differential mode interference. *** mode interference is the signal to the ground potential difference, mainly by the grid string into the ground potential difference and space electromagnetic radiation in the signal line induced by the *** state (same direction) voltage superposition is formed. *** mode voltage through the asymmetrical circuit can be converted to differential mode voltage, directly affecting the measurement and control signals, resulting in component damage (this is the main reason for the high damage rate of some system I / O module), this **** mode interference can be DC, can also be AC. Differential mode interference refers to the role of the signal between the poles of the interference voltage, mainly by the electromagnetic field in the space between the signal coupling inductance and by the unbalanced circuitry conversion *** mode interference formed by the voltage, this interference is superimposed on the signal, which directly affects the measurement and control accuracy.
(2) the main sources of interference in the PLC system and the way
Strong electrical interference
PLC system of the normal power supply are powered by the grid. Due to the wide coverage of the grid, it will be subject to all space electromagnetic interference and the induction of voltage on the line. In particular, changes within the grid, knife switch operation surge, large power equipment start and stop, AC and DC drives caused by harmonics, grid short circuit transient shock, etc., are transmitted through the transmission line to the original side of the power supply.
Cabinet interference
Control cabinet of high-voltage electrical appliances, large inductive loads, chaotic wiring are prone to a certain degree of interference with the PLC.
Interference from the introduction of the signal line
and PLC control system connected to the various types of signal transmission lines, in addition to the transmission of all kinds of effective information, there will always be an external interference signal intrusion. This interference mainly has two ways: one is through the transmitter power supply or *** with the signal instrumentation power supply string into the grid interference, which is often ignored; the second is the signal line by the space electromagnetic radiation induction of interference, that is, the signal line on the external induction interference, which is very serious. Interference introduced by the signal will cause the I / O signal work abnormally and greatly reduce the accuracy of the measurement, and in serious cases will cause damage to the components.
Interference from grounding system confusion
Grounding is one of the effective means to improve the electromagnetic compatibility of electronic equipment (EMC). Correct grounding can suppress both the effects of electromagnetic interference and the equipment to the outside of the interference; while the wrong grounding, on the contrary, will introduce serious interference signals, so that the PLC system will not be able to work properly.
Interference from within the PLC system
Mainly generated by the system's internal components and circuits between the mutual electromagnetic radiation, such as logic circuits radiate each other and their impact on the analog circuitry, analog ground and logic ground and the mutual impact of components and the use of mutual mismatch.
Inverter interference
One is the frequency converter start and run the process of generating harmonics on the grid to produce conduction interference, causing grid voltage distortion, affecting the quality of power supply grid; the second is the output of the frequency converter will produce strong electromagnetic radiation interference, affecting the normal work of the peripheral equipment.
3. The main anti-interference measures
(1) reasonable treatment of power supply, inhibit the introduction of grid interference
For the introduction of grid interference power supply can be installed with a shielding layer of the ratio of 1:1 of the isolation transformer, in order to reduce the interference between the equipment and the ground, but also in the power supply input side of the series connection LC filter circuit. As shown in Figure 1
(2) Installation and wiring
● Power lines, control lines, and PLC power lines and I/O lines should be wired separately, and the isolation transformer and PLC and I/O should be connected with a double-glue line. Separate wiring of PLC's IO line and high power line, if it must be in the same wire trough, separate bundling of AC lines, DC lines, if conditions allow, separate trough wiring is best, which not only makes it possible to have the largest possible spatial distance, and can minimize interference.
● PLC should be far away from strong interference sources such as welding machines, high-power silicon rectifier and large power equipment, can not be installed in the same switchgear with high-voltage appliances. In the cabinet PLC should be away from the power line (the distance between the two should be greater than 200mm). Inductive loads installed in the same cabinet with the PLC, such as larger power relays, contactor coils, should be connected in parallel with the RC arc elimination circuit.
● PLC inputs and outputs should be routed separately, and switching and analog should be routed separately. Analog signal transmission should be shielded wire, the shield should be grounded at one or both ends, and the grounding resistance should be less than 1/10 of the shield resistance.
● AC output line and DC output line do not use the same cable, the output line should be as far as possible from high-voltage lines and power lines, to avoid parallelism.