Its Effect on Reliability
How is your control system cooled? If it’s a single cabinet, does it have fans and filters? External ventilation and the dust it draws in are bad for your controls. Here we explain why, and what to do about it.
All control systems generate heat. The normal way to remove excess heat is with fans. But even filtered fans let in fine dust, and the finest dust is the worst for your electronics. As long as you don’t disturb anything, it all keeps on working. When a fault occurs, someone is assigned to fix it and he, or she, digs around, measuring voltages and disturbing wiring. The dust cascades down onto everything and some of it gets around the connectors. If these connectors aren’t well sealed, the dust enters and when the connector is unplugged, then re-plugged, the fun begins.
The dust gets in between the contact surfaces and contact is lost. Pin and socket connectors are not bad, but circuit board edge connectors are the worst. The contact fingers apply only light pressure, so as not to destroy the printed contacts over many insertions, and this lets dust get trapped between the surfaces. because the voltage difference is small, the current cannot arc across and bridge the gap. The contact is lost. PC computers are particularly prone to this effect because their add-on boards use edge connectors. However, many industrial PLCs now use circuit board edge connectors for their connecting cables and these are prone to the same problem. Dust is the primary cause of faults in industrial electronics. Its cost in downtime is millions of dollars every year.
Keep the dust out of the cabinet. At Scale-Tron, we have a policy of never using external fans; we cool our control systems in other ways. If you don’t have any holes in the cabinet, no dust can get in. When we visit a customer after many years of operation, our cabinets still look like new.
On that note, I can remember one customer whose cabinet interior was in terrible condition. Curious, I asked the operator why this had happened. He said “I don’t know. I keep it closed all the time, but every day I bring my lunch and leave it in the cabinet to keep it safe”. He didn’t think about it, but the act of opening it sucked any dust around the door into the cabinet each time he opened it. And in concrete plants, even in what they call a “clean control room” there is always a lot of dust.
Cleaning a Dusty Cabinet
If you have a control cabinet with ventilation and dust, how can you clean it up? It’s not easy and it’s time consuming but the best way is to use a vacuum cleaner to remove the obvious dust. Then look for all the connectors, disconnect them one by one and clean them. Use the vacuum first, to remove the obvious dust. If it is a circuit board edge connector, like the two in the picture, clean the circuit board part using alcohol or soap and water on a rag. Cut a piece of cardboard with suitable thickness and, using the same cleaning solution, plug it into the edge connector repeatedly until it comes out clean after you cut off the mating edge. Wait until it is dry before applying power.
Do you need Extra Cooling?
If you have a PLC or a small electronic board mounted inside a large cabinet, you probably don’t need any extra cooling. Typically a PLC with its wiring, power supplies and connection terminals, inside a cabinet of 24″ square or larger, will not need extra cooling. You can download a nice calculator for determination of cooling requirements for any size of cabinet here. It tells you when you need extra cooling and how much. Another method, for manual calculation, is here.
Or you can do it yourself by taking the appropriate cabinet, heating it and measuring the temperature rise. Wire a light bulb or two, place them towards one side of the cabinet for best circulation, above the floor (but not on a cardboard box!) and measure the temperature in the center of the cabinet. Monitor the current and voltage of the lamps to calculate power (Volts x Amps) and after a few hours, when everything has reached stability, measure the ambient and internal temperature. The temperature rise is the important parameter. This is roughly proportional to the power dissipated in the cabinet.
Now calculate your power dissipation in the cabinet. Take the power of each device, in Watts, and add them all together. If a device is a motor starter, soft starter, VFD or PLC control system, don’t forget that external loads such as motors, starters, solenoid valves etc. should not be included in your total. Just the power dissipated inside the cabinet. Apply this to your cabinet temperature rise and determine if you need any extra cooling.
Most cabinets for control applications will not require extra cooling. If you do need extra cooling, however, here comes the interesting part:
Cool the electronics, not the whole thing
When you have specific electronic devices that dissipate heat, you should cool just those things and let the cabinet warm up. For example, we use embedded computers in a lot of our systems. These are complete PC computers inside small packages with external cooling fins. The reliability of an electronic device is inversely proportional to its temperature. As it gets hotter, its reliability goes downhill. You can cool just this device by using a small fan blowing onto its cooling fins. It will warm up the whole cabinet, but that’s fine. The heat is then dissipated by convection currents around the cabinet. In this way, you don’t need to use external air to cool your control system and you keep your connectors clean. All Scale-Tron systems use this method and as a result, we have an exceptional reputation for reliability.
The achilles heel
Desktop PC computers are an exception and create a big problem when used as parts of control systems. In the early days of PC computers in the 1980s and ’90s, we used a PC computer together with a PLC in a combination control system. In those days, we bought the parts and made our own computer, with our own chassis and cooling fan. We then mounted it inside a large cabinet, along with the PLC, wiring and terminals. No external ventilation and no dust; great reliability as a result. Nowadays, it doesn’t make sense to do this, since ready-made PC computers are so cheap.
Inevitably, the computer sits outside the cabinet on a desk or table. Also inevitably, it is the first thing to fail because of ventilation and dust. A large contributing factor is dust in its connectors, brought in by the cooling fan. The solution to this problem is either to use an expensive industrial computer, an embedded computer inside a cabinet (also expensive) or just replace the computer every 2 years.