1. Radiated electromagnetic fields from the surrounding environment are typically generated by power networks, transient electrical equipment, lightning, radio, TV, radar, and high-frequency induction heating systems. These sources can cause radiation interference, which is often complex in distribution. If a PLC system is placed within an RF field, the effects of this radiation occur through two main paths: direct radiation to the PLC, inducing interference in the circuit, or radiating into the communication network within the PLC, causing disturbance through the communication lines. Radiation interference is closely related to the size of the electromagnetic field produced by the equipment and its frequency. To mitigate this, shielded cables, partial shielding of the PLC, and high-voltage bleeder components are commonly used for protection.
2. Interference from external leads of the system:
(1) Power supply interference: The normal power supply for a PLC system comes from the grid. Since the grid covers a wide area, it is susceptible to electromagnetic interference that can induce voltage and current on the lines. Changes within the power grid, such as switching surges, the start and stop of large equipment, harmonics from AC/DC transmission, and short-circuit conditions, can all be transmitted through the power lines to the PLC’s power supply. Although isolated power supplies are commonly used, their design and manufacturing processes may not provide perfect isolation. Due to distributed parameters, especially distributed capacitance, absolute isolation is not possible.
(2) Signal line interference: Signal transmission lines connected to the PLC control system not only carry information but also act as pathways for external interference. Two common sources include interference from the power grid via the transmitter's power supply or shared signal meters, which are often overlooked, and interference from space electromagnetic radiation, leading to inductive coupling on the signal lines. This type of interference can cause abnormal operation of the UO signal and significantly reduce measurement accuracy. In severe cases, it can damage components. Poor isolation can also lead to signal crosstalk, causing data corruption, malfunctions, or system crashes.
(3) Grounding system interference: Proper grounding is one of the most effective ways to improve electromagnetic compatibility in electronic devices. Correct grounding helps suppress both external electromagnetic interference and internal grounding issues. However, improper grounding can introduce serious interference signals, making the system unstable or even non-functional.
3. Internal interference within the PLC system arises from mutual electromagnetic radiation between its components, such as the interaction between logic circuits and analog circuits, and the mismatch between analog and logic grounds. These issues are part of the manufacturer's internal electromagnetic compatibility (EMC) design and are more complex to address. As a result, end users cannot modify them directly. It is recommended to select modules with better performance or conduct thorough testing to ensure reliability under various conditions.
Overall, understanding and mitigating these interference sources is crucial for ensuring the stability and accuracy of PLC systems in real-world applications. Whether it's through proper shielding, grounding, or component selection, each step plays a vital role in maintaining system integrity and performance.
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