In the field of electrical engineering, the layout, wiring, and cabling of PLC (Programmable Logic Controller) cabinets may seem straightforward, but improper handling can lead to a cascade of issues, ranging from system interference to equipment damage. To ensure the stable and efficient operation of PLC systems, attention to detail is crucial to avoid common pitfalls.
Today, we will discuss five indispensable considerations when it comes to the internal layout, wiring, and cabling of PLC cabinets to help you steer clear of these "traps"!
一. Proper Power Distribution: Avoid Overloading
In a PLC system, power sources are divided into external and internal. External power drives output devices and provides input signals, while internal power supplies the energy for the PLC's operation.
The impact of external power on PLC performance is relatively minor, so there's no need to be overly selective; however, internal power is different as it directly determines the reliability of the PLC's operation. Therefore, it's advisable to opt for high-quality, regulated power supplies within the PLC to ensure stable operation. If the PLC system is installed in an environment with strong interference, using an isolated transformer with a shielding layer is a good idea.
A special note: power lines should be sufficiently thick to prevent voltage drops caused by the startup of high-capacity equipment, which can affect the PLC's operation. If an external DC power supply is used to power the PLC's input circuit, a regulated power supply is the safest choice to prevent the PLC from receiving erroneous signals.
二. Keep Away from Interference Sources: Provide a Clean Environment for PLC
PLCs, like quiet "programmers," detest interference! Electromagnetic interference from high-power equipment such as welders and frequency converters is a nightmare for PLCs. Therefore, whether it's power lines, I/O lines, or power and control lines, they should all be routed separately, ideally in different channels, to ensure sufficient spatial distance. If it's unavoidable to route them in the same channel, AC and DC lines should be bundled separately to minimize the risk of interference.
Additionally, never install PLCs and high-voltage equipment in the same switchgear cabinet, maintaining a distance of at least 200mm between them. If a PLC coexists with inductive loads (such as high-power relays) in the same cabinet, paralleling an RC snubber circuit for the relay is a good choice to avoid electromagnetic interference.
三. I/O Wiring Matters: Don't Let Slack Affect Signal Quality
I/O wiring for PLCs is a "technical job," and improper handling can affect the quality of signal transmission. First, the input and output lines of the PLC should be routed separately, and switch quantities should not be mixed with analog quantities. Particularly for analog signals, shielded cables should be used for transmission, ensuring that the shield is grounded.
The length of the input lines is also crucial; shorter is better, but if the environmental interference is not significant, it's acceptable to extend them appropriately. When wiring the output end, independent outputs and common outputs should be clearly separated, and outputs of different voltage levels should only be mixed within different groups, avoiding confusion. If the output load generates interference, measures such as DC output freewheeling diode protection or AC output RC snubber circuits can be taken to suppress it.
四. Grounding Treatment: No Room for Slack
Grounding is not a trivial matter; improper grounding can not only affect the normal operation of the PLC but also pose safety hazards. Types of grounding in PLC systems include system ground, shield ground, AC ground, and protective ground. Good grounding can effectively suppress electromagnetic interference and ensure the stable operation of the system.
System grounding should ensure a resistance value of less than 4Ω, and the PLC equipment system ground and the negative terminal of the switch power supply in the control cabinet should be connected together to form a control system ground. Additionally, the shielding layer of the signal lines should also be grounded to avoid forming "ground loops." Especially for shielded twisted pairs with multiple measurement point signals, insulation must be properly handled, and appropriate single-point grounding should be chosen to avoid ground potential difference interference caused by multi-point grounding.
五. Be Aware of Converter Interference: Keep Your Feet Firm
Interference from converters is often an issue that cannot be ignored in PLC systems. Common countermeasures include using isolated transformers to block conducted interference from the power source; adding AC reactors between the converter and the motor to reduce electromagnetic radiation generated during the converter's output process. Filters are also a good choice, as they not only enhance anti-interference capabilities but also prevent interference from devices from being conducted back to the power source.
Furthermore, if your PLC system works in conjunction with converters, ensuring reasonable routing of signal and control lines between the two can effectively reduce the troubles brought by interference.
The layout, wiring, and cabling inside a PLC cabinet are a "delicate task," and any negligence in any link may lead to a decline in system performance or even equipment damage. Therefore, during the installation process, always be meticulous, properly distribute power, stay away from interference sources, standardize I/O wiring, ensure correct grounding, and effectively suppress converter interference.
Only in this way can the PLC system perform at its best as expected and avoid unnecessary troubles.
