The information in this document applies to a variety of UPS models. Replacement recommendations will vary based on the specific model and capacitor type.

Q: What do DC capacitors do?

A: The primary use for DC capacitors in UPS applications is to smooth out fluctuations in voltage—a process also known as “supply voltage filtering.” If there’s a change in voltage input, the capacitor dampens the voltage change, eliminating the peaks and filling in the valleys to help maintain a constant voltage level.

Q: What do AC capacitors do?

A: They smooth out input transients and reduce the harmonic distortion on the utility input to the UPS. More importantly, they connect directly to the critical load output and help control the waveform shape of the UPS output voltage.

Q: Are there particular UPS kVA sizes where capacitor replacements are more applicable?

A: Capacitor replacement is not kVA-dependent. Any UPSs with fully removable capacitor assemblies should have the capacitors replaced to avoid failure.

Q: Do capacitors degrade over time under normal use?

A: Like batteries, capacitors degrade over time. A typical capacitor might be rated by the manufacturer for seven years of around-the-clock use, and could potentially deliver up to 10 years of useful life under favorable operating conditions. When a capacitor fails, you might not see any visible effects, but other capacitors will have to take over the workload, shortening 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.

UPS DC and AC capacitor replacementFrequently asked questions

Eaton field engineers handle all capacitor replacements. Unlike a third party service provider, they’re also aware of the latest engineering updates and have access to 24x7 technical support.

Service FAQ

Q: Where are DC capacitors found in my UPS?

A: These capacitors are found at the rectifier(s) outputs and/or the inverter(s) inputs. This area is known as the DC link, which is also where backup batteries are often connected. Working as filtering elements, these capacitors are essential contributors to the overall function of the UPS, which is to take unpredictable or erratic supply voltage and turn it into a filtered voltage for the inverter and battery. The capacitors also play a role in allowing a UPS to recover from load steps and helping prevent nuisance transfers to bypass.

Q: Do DC capacitors ever completely wear out?

A: Yes they do, just as batteries do, and for similar reasons. Capacitors, when properly designed and fabricated, still show very gradual changes in essential characteristics over time, even when operated at normal rated voltages and temperatures. 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, the unit loses overall capacitance. Ultimately, the capacitor no longer performs its job. Adverse operating conditions such as excessive current and heat can also hasten the demise of capacitors.

Q: How are AC capacitors different than DC capacitors?

A: AC capacitors have much of the same characteristics as DC capacitors in that they have an expected period of useful service and should be considered as perishable commodities. AC capacitors are found primarily near the input and output filters.

Q: How long do capacitors usually last?

A: In practice, you can estimate expected service life based on the manufacturer’s rated lifespan. This assumes that the device is maintained under nominal rated operating temperature and does not endure a number of charge and discharge cycles outside of its specified parameters over a specified period of time. Based on the manufacturer’s rated service life, it is our recommendation to replace AC and DC oil filled capacitors for most Eaton UPS models at or before seven years of service. Failures are certainly possible any time after the manufacturer’s rated service life, and increase in likelihood as the capacitor gets older. Below is the estimated capacitor service life for AC and DC capacitors across various Eaton UPS models.

Q: If the UPS keeps working, why would I worry about replacing capacitors?

A: When an individual capacitor fails, that can be a sign that other capacitors are not doing their jobs. There may be evidence of one or two failed capacitors that have split or leaked, but visible inspection would not reveal other capacitors that have failed “open” and perhaps actually caused the other failures. This condition could trigger the failure of still more capacitors and ultimately diminish the UPS’s overall performance. Replacing single capacitors (which have lower impedances) may lead to quicker failure. The best strategy is to keep the impedance of all capacitors working together in the same circuit consistent.

Q: Why aren’t capacitor replacements covered under my service contract?

A: Individual failures are covered under Eaton service contracts with full parts and labor coverage. Full capacitor replacements, like batteries, are excluded as they are high wear consumable parts. 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.

Q: What is the recommended strategy for testing and replacing capacitors?

A: This depends a lot on the age and size of the system. For instance, if your large UPS system is seven to 10 years old, piecemeal replacement of capacitors might appear to be more cost-effective in the short run. Ultimately, though, it’s more costly and risky for the reasons stated earlier.

Eaton recommends that power capacitors be inspected for deformation, leaks and other apparent problems during annual preventive maintenance visits. If some capacitors are visibly failing, their failures may have been triggered by the failures of others, and it can be difficult or time consuming to determine which ones. Consider also that testing and replacing individual capacitors can take five or six hours for a large three-phase system. In contrast, a technician can replace an entire bank of capacitors in less than an hour. If the capacitor failure rate has been on the rise, you may soon find that one-by-one replacement has led to replacing most of the capacitors anyway, but at greater cost and inconvenience than a wholesale replacement.

Here’s an overview of the estimated capacitor service life and types of capacitors inside Eaton UPSs. When doing a full capacitor replacement, we also recommend performing a full fan replacement at the same time.

Capacitor type 9330 9355 93E 93PM 9390 9315 93959395 High Performance

DC (Electrolytic) Life of UPS Life of UPS Life of UPS 10 years Life of UPS 6 years Life of UPS Life of UPS

DC (Oil filled) N/A N/A N/A N/A 7 years N/A 7 years 10 years

AC (Oil filled) 7 years N/A N/A N/A 7 years 7 years 7 years 10 years

2 EATON Capacitor replacement frequently asked questions

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Q: My UPS is a very old model. Can it accept today’s capacitors?

A: Yes, unless otherwise specified as incompatible by Eaton’s engineering team.

Q: Who actually replaces the capacitors? Can I use a third-party service provider?

A: Eaton field engineers provide this service, and have been doing so reliably for many years. They are supported by our latest engineering updates and have immediate access to 24x7 technical support, including product documentation, engineering data and technical updates or bulletins. Third party service providers do not have consistent support in these areas.

Eaton 9390 and 9395 UPS model capacitor replacement policy

Rated capacitor life for 9390 and 9395 UPS models:

• AC input/output capacitors have an expected life of seven years

• DC electrolytic capacitors have an expected useful life equal to the product life (20+ years)

• DC (oil filled) capacitors have an expected life of seven years

Unlike other models with DC electrolytic capacitors, 9390 and 9395 UPS models have capacitors soldered to the power modules and are not field-repairable. (In the rare event that a DC electrolytic capacitor failure is diagnosed, the power module will need to be replaced.) Depending on the failed number of DC capacitors, model and rating there may be multiple power modules requiring replacement.

Eaton full parts coverage support agreement customers are fully covered for DC electrolytic capacitor replacement in the event of a failure. Proactive DC electrolytic capacitor replacement is not required based on engineering data, and Eaton does not recommend it. Customers requesting proactive DC electrolytic capacitor replacement will need to replace all power modules as a billable (non-covered) event.

Engineering explanation for 9390 and 9395 capacitor recommendations

Life expectancy of capacitors is affected by temperature more than anything else—primarily internal heat from ripple current resistive dissipation. Manufacturer published data is given at rated voltage and ripple current, so at a frequency of 120 Hz and either 85°C or 105°C, the rated life is usually in the vicinity of 2,000 or 3,000 hours (depending on the capacitor maximum temperature rating). Every 10°C lower, we anticipate the expected life to double. At 25°C, that would mean a multiplier of 32 for an 85°C rated capacitor.

With regards to current, every UPS designed in the last 20 years has increasingly used higher frequencies. Further, the 9390 and 9395 use interleaving, which effectively doubles the frequency of the ripple current as seen by these capacitors. This is important as the higher the frequency, the higher the current the capacitor can handle. Or conversely, the longer it will last for a given set of conditions.

Eaton’s designs do not use capacitors (or any component for that matter) to its maximum ratings. Worst case scenarios are typically 80 to 85 percent voltage rating, perhaps less with respect to current. These “stacked” margins were used to determine that a capacitor Eaton solders to a board will last “forever” (or the life of the equipment).

Capacitor lifetimes vary by model and capacitor type. For most models, we recommend a full replacement of capacitors in the UPS’s primary power-train after every six to seven years of operation.