Preface: This is a detailed discussion, so I've categorized it into sections for easier navigation. Introduction: I am seeking assistance from those who may have encountered a similar issue. A PLC5-40 Series C Enhanced has been functioning smoothly for a long time. Recently, one specific output, O:012/11, which controls a relay for the run contacts of a PowerFlex 40, has started flickering a few times per minute. This anomaly has not been observed with any other outputs. Could it be a faulty triac or supporting component? System Overview: In this setup, O/0 is not in use. Outputs O/1 to O/5 are utilized for three starters and two PowerFlex 40 run relays that operate continuously during production. Outputs O/6 to O/12 are responsible for controlling the run relays for discharge belts of five material supply hoppers. Each hopper holds approximately 2.5 hours of material and discharges until empty before moving to the next hopper. Outputs O/13 to O/17 control reverse relays for unused hopper drives. The system consists of 7 inputs, 3 outputs, 1 analog input, and 3 analog outputs. The Issue: Recently, a discharge process began experiencing sudden stops and restarts. Upon investigation, it was found that the run relay was intermittently flickering off every few minutes. After inspecting the wiring and scoping the output, it was discovered that truncated half waves were randomly occurring, causing the run relay to drop out. This led to disruptions in the production process. Attempts to Resolve: In an attempt to address the issue, a coworker replaced the problematic module during a brief downtime. However, the first replacement had a backplane connection issue, causing the entire module to flicker unless physically held in place. A second replacement initially appeared to function properly but was later found to have a stuck output. Subsequent testing with a used module seemed to temporarily resolve the problem, but the flickering issue persisted during production. Next Steps: Considering the ongoing issue, the next course of action would be to replace the backplane during the next available opportunity. This situation presents a unique challenge that is both frustrating and intriguing. Has anyone encountered a similar problem or have any alternative solutions to suggest? Appendix: Attached is an example of the issue and a picture of the equipment in question, which is nearing retirement. The scope reading shows 118V during a full wave, dropping to 108V during the occurrence of the flicker.
Try forcing the output to determine if the flickering persists, in order to eliminate the possibility of a program race condition causing the output to pulse. Have you attempted relocating the device connected to that output to a different unused output? If the issue remains, it is likely due to the load on that output. Don't forget to also check the total load on the output module.
It's rather unusual. Triac output modules are designed to switch on and off close to the zero crossing point. Is it possible that the PLC program or an external device is turning off the output for a scan, causing the waveform delay? It's unlikely that the relay coil is causing a waveform like that, but it's worth considering replacing it. Expect to miss at least half a cycle before it resumes normal operation.
It appears that the Triac may be on the verge of failure, so it is advisable to replace the module before it malfunctions. Unlike SCRs, both Triacs and SCRs can turn on at any point in the cycle but can only turn off when the current passing through them becomes negative. Triac outputs are typically designed for zero-crossing, turning on near the zero point to reduce electrical noise in the circuit. Triacs consist of 2 parallel SCRs mounted in opposite directions.
Your issue certainly seems like a tricky one, but I've experienced something similar before with a PLC5 system. Your hunch about a faulty triac or supporting component could be spot on. Given the age of the system and the difficulty you've had replacing the module, it might be an issue with some of the aging components. The backplane replacement can eliminate or confirm whether it is a backplane issue. However, before going down that route, have you tried isolating the problematic output from other system components? There might be some interference from a nearby component that's causing these halved waveforms. Also, considering the age of the equipment, it might be worthwhile to evaluate if an upgrade to a newer system could be a more long-term solution. Of course, I understand that downtime is crucial to limit in your case, so these are just possible pointers to contemplate.
It seems you've done thorough investigative work on this issue! Bang on with also considering the backplane as the potential problem, that's astute observation. I’ve heard of a similar issue in the past in which flickering was due to a failing power supply, you might want to look into that as well. Generally, as you've noted this system is nearing retirement, this could potentially be a systemic issue owing to end-of-life hardware - the age of the equipment and the symptoms you mention bring to mind a lot of issues related with aging hardware. However, ensure you rule out any software glitches as well as these can sometimes present as hardware problems. Consider upgrading the entire system if budget allows, as infrastructure failures tend to get more frequent as the equipment ages. Not to mention, newer systems are often more efficient and easier to troubleshoot.
The first thing that came to my mind while reading about your issue is power quality. The fact that it's not happening to all outputs and you're witnessing drops in voltage makes me wonder if there's something going on with your power supply, or perhaps the quality of the power feeding the PLC. Therefore, instead of replacing the backplane, I would initially recommend thoroughly checking out your power supply. Also, consider placing a line reactor or isolation transformer between the supply and your device, which may help with power quality issues. Finally, you may need to use an oscilloscope to measure the voltage on either side of your power supply to verify the quality and consistency of the power. If there's nothing wrong with your power supply and the issue still persists, only then would I consider attributing it to a backplane fault. Just my two cents as an electrical engineer.
Given your thorough investigation and replacement efforts, this indeed seems like a puzzling issue. My first thought was a faulty output module but you've already replaced that. Although most symptoms point towards hardware issues, you also might want to rule out any software glitches. Since PLC5-40 Series C Enhanced has been in operation for a long time, it could inevitably suffer from software-related system errors. Plus, even though it's a single output acting up, it's worth checking whether there's any surplus load on your PLC which shouldn't be there. Consider updating your PLC software firmware, and look for signs of possible firmware glitches that could cause such anomalies. The backplane replacement of course seems like a plan too, but if your system nears retirement, maybe it's worth thinking about upgrading the whole setup, if budget and resources allow.
It sounds like you're dealing with quite the frustrating situation! Given that you've already replaced the module and are still seeing issues, I’d definitely recommend taking a hard look at the backplane connections as you mentioned. Intermittent flickering can sometimes be traced back to poor connections or interference. Also, it might be worth checking if there are any grounding issues or nearby devices causing noise on the line. Since the equipment is nearing retirement, if all else fails, consider whether a temporary workaround could be implemented while you prepare for a more permanent solution. Good luck, and I hope the next steps clarify the issue!
It sounds like you've done quite a bit of troubleshooting already! Flickering outputs can definitely be frustrating, especially when you've ruled out physical connections and replacements. A couple of thoughts come to mind: have you checked for any potential grounding issues or voltage drops that might be impacting that specific output? Additionally, intermittent issues can sometimes stem from age-related degradation in components—perhaps inspecting the capacitors around the output circuitry could reveal something. If it is indeed the backplane causing the problem, ensuring that all connections are secure during replacements will be key. Good luck with the next steps!
✅ Work Order Management
✅ Asset Tracking
✅ Preventive Maintenance
✅ Inspection Report
We have received your information. We will share Schedule Demo details on your Mail Id.
Answer: Answer: The flickering issue could potentially be caused by a faulty triac or supporting component within the output module.
Answer: Answer: The PLC5 Output Module 1771-OAD controls the run relay for the PowerFlex 40 and is part of a setup where various outputs are used for different functions such as controlling starters, run relays for discharge belts, and reverse relays for unused hopper drives.
Answer: Answer: So far, attempts to resolve the issue included replacing the problematic module with new and used ones, but the problem persisted. The next step planned is to replace the backplane during the next available opportunity.
Answer: Answer: The discussion seeks input from others who may have encountered a similar problem or have alternative solutions to suggest for addressing the ongoing flickering issue with the output module.
Join hundreds of satisfied customers who have transformed their maintenance processes.
Sign up today and start optimizing your workflow.