Proactive attention to this often-overlooked element of UPS architecture, the capacitor, can extend the value of the UPS system that protects critical electronic systems.
Capacitors are fairly simple devices, ranging in size from a thimble to a soda can, that smooth out fluctuations in electrical voltage. A typical UPS contains a dozen or more different types and sizes of capacitors: small ones that smooth out the power supplied to the UPS processor (its on-board intelligence), and large ones to regulate the power that flows to protected equipment.
Like batteries, capacitors degrade over time. A typical capacitor might be rated by the manufacturer for, say, 6 years of round-the-clock use, but could potentially deliver up to 8 to 10 years of useful life under favourable operating conditions. When a capacitor fails, you might not see any visible effects, but other capacitors will have to take over the workload, which will shorten their useful lives. In many cases, a capacitor failure triggers the UPS to switch to bypass mode, during which it can´t protect downstream loads.
To maximise the performance and reliability of your UPS, treat capacitors as the perishable commodity they are, and plan on replacing them at or near the end of their rated service life. Proactive attention to this often-over looked element of UPS architecture - the humble capacitor - can extend the value of the UPS system that protects your critical electronic systems.
As lifespan is a function of rated voltage and operating temperature, it logically follows that you can shorten the useful life of a capacitor by overstepping its design thresholds in either of those areas.
Take, for example, a capacitor that has a theoretical life of 32 years at 405 V and 20 degrees Celsius. If you increase its workload to 540 V and twice as much heat, the capacitor will probably last only about 8 years - a significant compromise. The good news is that good management - operating the UPS within its rated capacity in a clean, cool environment - can mitigate these risk factors.
Do capacitors degrade?
Yes they do, just as batteries do, and for similar reasons. Wet capacitors, when properly designed and fabricated, show very gradual changes in essential characteristics over time, when operated at normal rated voltages. However, the paper, aluminum foil, and electrolyte inside the capacitor are subject to normal breakdown. As those materials age and start to degrade physically and chemically, they lose capacitance. Ultimately, the capacitor no longer performs its job. Adverse operating conditions such as excessive current and heat can hasten the demise of capacitors.
How long does it last?
The theoretical operational lifetime of a capacitor is a mathematical function of the rated voltage, applied voltage, current through the capacitor, ambient temperature and thermal resistance. In practice, you can estimate expected service life based on the manufacturer´s rated lifespan and the operating temperature of the device. The hotter the operating temperature, the shorter the life.
AC capacitors have a different useful life expectancy than DC capacitors because they are manufactured from different materials and are under different stress levels.
Based on the manufacturer´s rated service life, it is recommended to schedule a full replacement of capacitors which are used in the primary power-train portion of the UPS after 5 to 7 years of operation. However, newer UPSs typically have oil-filled DC capacitors that are rated for the full life of the UPS. When a capacitor fails, you might not see any visible effects, but other capacitors will have to take over the workload, which will, in turn, shorten their remaining lives. In many cases, a capacitor failure triggers the UPS to switch to bypass mode, during which it can´t protect downstream loads.
If a capacitor fails?
A capacitor can fail in an ´open´ condition - simply stop doing the work for which it was intended. This kind of failure is quiet and can pass unnoticed. Or a capacitor can fail in a ´short´ condition - developing a leak of the dielectric medium or even venting with a loud pop that sounds like a firecracker.
The electrolyte is conductive, naturally, so spilled electrolyte can make unintended connections that could possibly disrupt UPS performance. The electrolyte is also corrosive and could interact with other components inside the equipment. For these reasons, many electrolytic capacitors have a scoring mark at one end, which is designed to rupture and leak gently, rather than allowing the container to vent suddenly and spread electrolyte across a wide area.
Does it hamper UPS functionality?
This depends on where the capacitor is located, the number of capacitors working in series (tandem) or parallel to perform an equivalent task and the overall health of the other capacitors. For instance, if capacitors are operating well below their voltage rating, one or two capacitors might readily be able to take over for a failed capacitor without significantly affecting UPS operation.
A failed capacitor in the power train can diminish the unit´s overall filtering ability, but it won´t render the unit nonfunctional. However, when a capacitor in the power train fails, a typical three-phase UPS transfers to bypass mode, whereby the power stream bypasses the UPS´s filtering electronics. During this time, the UPS - although operational - isn´t actually protecting downstream equipment.
In rare cases, a failed capacitor can disrupt power to the on-board computer (the logic processor), but leading UPSs are designed to execute a safe and orderly shutdown in that unlikely event.
Extend the life of a capacitor
Probably the biggest factors under your control are to maintain the recommended ambient temperature, humidity, and cleanliness. Keep air filters clean, so air can flow freely to keep the unit cool. And when replacing capacitors on old equipment, don´t install capacitors that are below the voltage rating of the original parts.
Good datacenter management can be a powerful testament in prolonging equipment life. Complete power capacitor replacements are recommended to proactively insure the reliability of the UPS against a catastrophic failure and reduce the risk of unscheduled downtime.
To maximise the performance and reliability of your UPS, treat capacitors as the perishable commodity that they are, and plan on replacing them at or near the end of their rated service life. Be wary of choosing a third-party service organisation over a UPS manufacturer´s certified technicians to replace capacitors. If the manufacturing company designs, engineers and manufactures your UPS, they will know it better than anyone else, and the technicians are specialists. Technicians should be trained, certified and will likely have access to proprietary diagnostic software and the latest engineering updates as well as complete access to to-level technical support (all documentation, engineering data and drawings, technical data and updates). You can be assured that only factory-authorised parts with appropriate specifications and the latest firmware or revisions will be installed in your UPS, not after-market parts of unknown quality or specs.
(The article has been authored by Ed Spears, Product Marketing Manager, Eaton´s Critical Power Solutions Division in Raleigh, North Carolina).
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