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Everything you need to know about UPS capacitors — and their replacement!

While much attention has been given to the importance of regularly replacing uninterruptible power system (UPS) batteries, there is also a lesser-known, often-overlooked component that cannot be ignored: the capacitors.

After batteries, capacitors rank among the most common UPS elements susceptible to failure.

Proactive attention to these components can extend the life of the UPS system and optimize the protection of your critical equipment.

What is a capacitor?

Devices designed to store and release electrical energy, capacitors are responsible for smoothing and filtering voltage fluctuations.

Ranging in size and type, the number of capacitors inside a UPS varies greatly depending on the kVA rating of the unit.

For instance, even the smallest UPSs have dozens of capacitors, while a 750 kVA three-phase UPS will likely have hundreds.

Like batteries, capacitors degrade over time and for similar reasons, such as excessive current, overwork and heat.  

As the materials within a capacitor age and degrade, the part loses its ability to effectively perform its job.

While a typical capacitor might be rated by the manufacturer for seven years of round-the-clock operation, it could potentially deliver up to 10 years of useful life under favorable operating conditions.

Conversely, a capacitor may operate for a much shorter time under unfavorable circumstances.

Unless a PM suggest otherwise, most UPS capacitors need to be replaced every seven to ten years.

What happens when capacitors fail?

While there may be no visible signs, when one capacitor fails, others must assume the workload, which will in turn shorten their lifespan.

The consequences of capacitor failure vary, in large part based on where the capacitor is located, the number that are working in tandem or parallel, and the overall health of the other capacitors.

For example, if capacitors are operating well below their voltage rating, one or two might be able to take over for a failed capacitor without significantly affecting UPS operation.

In many cases, a capacitor failure will trigger the UPS to switch to bypass mode.

When this occurs, the power stream bypasses the UPS’s filtering electronics and although the UPS remains operational, it isn’t protecting downstream equipment.

When should you replace capacitors?

Don’t wait until your UPS capacitors reach the end of their rated service life to start preparing for their replacement.

Instead, request replacement quotes as they approach year 7 or 8.

This ensures you have one or more proposals on hand when replacement time arrives.

In addition, track UPS preventive maintenance calls and take the time to read service reports closely.

If there are any possible impending signs of failure, take steps to replace the capacitors immediately.

To maximize the performance and reliability of your UPS, be sure to replace capacitors at or near the end of their rated service life.

A qualified service engineer can diagnose the condition of the capacitors in the UPSs and perform a full or partial replacement of capacitor banks.

It is recommended to schedule a full replacement of capacitors used in the primary power-train portion of the UPS around 7-10 years of operation.

However, some newer UPSs may have oil-filled DC capacitors that are rated for the full life of the UPS. At the minimum, an annual inspection will help optimize operation and extend their lifespan.

How do you replace capacitors?

Regardless of your current UPS Maintenance provider, consider requesting a capacitor replacement proposal from Unified Power and keep it on file.

Share the proposal with your management for review and budgeting purposes.

And if you aren’t currently engaging in regular service with a preferred maintenance provider, now is the perfect time to consider protecting your equipment — and your overall organization — with a UPS maintenance contract from Unified Power.

You’ll not only receive an update on the overall health of your unit and its capacitors, but when replacement time arrives, you’ll already have an established relationship with a provider you trust and who has worked on your equipment.

If you are currently working with Unified Power, please reach out to your account manager, and they can review your last PM/service report with you and discuss what to expect in the future.

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