Power supplies: just another detail?

March 24, 2017
Careful specification can prevent startup problems and maintenance headaches.

As a chemical engineer by training, one aspect of automation that I had to pick up post-graduation was a basic understanding of electrical engineering. Although it wasn't a natural fit for me (my alma mater makes Chem Es take an electrical engineering course and the EEs a thermodynamics course to separate the wheat from the chaff), I managed to get by, but not without learning a few lessons the hard way.

One of the more challenging electrical problems I faced was with the power supply for a continuous emission monitoring (CEM) analyzer system installed at the 200 foot level of a main stack. Fortunately, we had an elevator, and the stack itself was a 26-foot-diameter pipe inside the concrete stack, so we were out of the elements. All the equipment was installed in the annular space between the stack and tower, including the existing step-down transformer from the original analyzer installation.

After installing the system, we had all sorts of noise problems. After some investigation and help from the manufacturer and my electrical engineering colleagues, we ended up having to add a small UPS system as a way to isolate the analyzers from the transformer. Doing so also gave us the added benefit of a more reliable power supply.

What I learned is that the device needed to be connected to a delta versus wye transformer. The wye system is best when there are mostly line-to-neutral loads, and the delta is best with line-to-line loads. Both systems can supply both types of loads, but in my case, because of location and other loads on the same feeder, it made a difference.

Another time when it’s important to pay attention to power supplies is with any sort of fieldbus system, especially one where the signal and power are on the same wire pair, because the frequency, which is how data is transmitted, matters. For Foundation Fieldbus (FF) and Profibus PA, this is a 31.25 kbps signal. Because a 24-V bulk power supply’s objective is to provide a linear (i.e. steady) voltage, when it senses these variations, it works to remove them—the opposite of what we're trying to achieve. This is why such systems have a fieldbus power supply, which basically puts an inductive circuit between the bulk power supply and the network.

I did, however, once have someone approach me after a fieldbus presentation to tell me he was having difficulties with his FF network operating sporadically. When I asked what power supply was being used, the response was that he was only using a conventional bulk power supply, and the system, which was small and short, still worked. This is an indication of how robust the system can be.

My final story is about laptop power supplies, which are surprisingly electrically noisy. We discovered this while using a USB-based oscilloscope to capture some FF signals, and noticed that the baseline had a significant low-amplitude, high-frequency noise level. After some head scratching, we made the same measurement running the laptop on its battery, and the noise disappeared. As a result, now, whenever I use a laptop-based instrument, I always do so using the battery, just to be sure I get the best readings I can.

This brings up the last and most challenging item: ground loops. Though not actually part of the power supply, ground loops do affect the signal quality. About 80% of the time there is a network problem, it’s a signal problem. Most signal problems are because the physical layer has a problem, and the root cause is related to improper grounding somewhere in the system. As a result, I no longer take grounding for granted.

One other practice I now follow whenever specifying terminal block strips is to always have a way to isolate the circuit. By putting in a fuse or, at minimum, knife-gate terminal blocks, when necessary I can quickly and easily break the connection between the field device and control system for testing to help resolve problems like those described above. It’s just one preemptive action that can be taken to protect the control system, while also making troubleshooting that much easier.

Many automation professionals tend to take the physical layer in their design for granted, however, as I and many others have learned the hard way, when we blithely say, “It’s just a power supply,” or “just terminal blocks,” it’s far better that you “just” be sure you check them anyway.

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