Do you really understand the ins and outs of PLC interference?

Many people know about PLC, but do you know where its interference comes from? On-site electromagnetic interference is one of the most common factors in PLC control systems and the most likely to affect system reliability. It is very necessary to think about the question "where do you come from and where do you go?" Today, the editor will talk about the classification and common sources of PLC interference sources, hoping to help with interference management.

1. Interference sources and general classification

Interference sources that affect PLC control systems mostly occur in locations where current or voltage changes drastically. The reasons are:

Changes in current create a magnetic field

Produce electromagnetic radiation to equipment

Changes in the magnetic field produce electric current

Electromagnetism generates electromagnetic waves at high speed

Generally, electromagnetic interference is divided into common mode interference and differential mode interference according to different interference modes.

Common mode interference: The potential difference between the signal and the ground is mainly formed by the superposition of common-mode (same direction) voltage induced on the signal line by the power grid in series, the ground potential difference and space electromagnetic radiation. Common mode voltage can be converted into differential mode voltage through asymmetric circuits, which directly affects measurement and control signals and causes component damage (this is the main reason why some system I/O module damage rates are high). This common mode interference can be DC , can also be used for communication.

Differential mode interference: refers to the interference voltage acting between the two poles of the signal. It is mainly caused by the coupling induction of the space electromagnetic field between the signals and the voltage formed by the conversion of common mode interference by the unbalanced circuit. This interference is superimposed on the signal and directly affects the measurement and control. Accuracy.

2.Main sources and pathways of interference in PLC systems

Strong electrical interference: The normal power supply of the PLC system is powered by the power grid. Due to the wide coverage of the power grid, it will be subject to electromagnetic interference from all spaces and induce voltages on the lines. In particular, changes within the power grid, such as switching operation surges, starting and stopping of large power equipment, harmonics caused by AC and DC transmission devices, grid short-circuit transient impacts, etc., are all transmitted to the primary side of the power supply through transmission lines.

Interference in the cabinet: High-voltage electrical appliances, large inductive loads, and chaotic wiring in the control cabinet can easily cause a certain degree of interference to the PLC.

Interference introduced by signal lines: In addition to transmitting various types of valid information, various signal transmission lines connected to the PLC control system will always have external interference signals intruding. There are two main ways of this interference: one is the interference from the power grid connected in series through the power supply of the transmitter or the power supply of the shared signal instrument, which is often ignored; the other is the interference of the signal line by space electromagnetic radiation induction, that is, the signal line External induced interference is very serious. Interference introduced by the signal will cause abnormal operation of the I/O signal and greatly reduce the measurement accuracy. In severe cases, it will cause damage to components.

Interference when the grounding system is chaotic: Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Correct grounding can not only suppress the influence of electromagnetic interference, but also inhibit the interference from the equipment; while incorrect grounding will introduce serious interference signals, making the PLC system unable to work normally.

Interference within the PLC system: mainly caused by mutual electromagnetic radiation between components and circuits within the system, such as mutual radiation of logic circuits and their impact on analog circuits, the interaction between analog ground and logic ground, and mutual mismatch between components Use etc. Frequency converter interference: First, the harmonics generated during the startup and operation of the frequency converter cause conductive interference to the power grid, causing grid voltage distortion and affecting the power supply quality of the power grid; second, the output of the frequency converter will produce strong electromagnetic radiation interference, affecting surrounding areas. normal operation of the equipment. The above is the source analysis of PLC interference, I hope it can help you.