Capacitors in an uninterruptible power supply help to smooth, filter and store energy. A UPS includes dozens of different capacitors in both the power section and the printed circuit board level (PCB). 

Capacitors contain a pair of conducting surfaces, usually electrodes or metallic plates, enclosed in aluminium or chromium-plated cylinders ranging in size from a miniature drink can through to a tube of Pringles. A third element – the dielectric medium – separates the conducting surfaces.  

The charge a UPS capacitor can store is measured in farads – named after the famous physicist Michael Faraday – which is determined by the thinness of the dielectric layer and the surface area of the aluminium. 

What Are The Different Types Of UPS Capacitor? 

In the main power section of a UPS system, capacitors are divided into the following categories: 

• AC input capacitors: form part of the UPS input filter and/or the power factor correction stage. These capacitors smooth out input transients and reduce harmonic distortion 
• AC output capacitors: form part of the UPS’s output filter. These connect to the critical load output, controlling the waveform of the UPS output voltage 
• DC capacitors: form part of the rectification system and energy storage, smoothing out any voltage fluctuations (also known as supply voltage filtering).  

As well as batteries, capacitors are the UPS components most prone to failure. They age over time, with the electrolyte, paper and aluminium foil inside degrading over time. Factors such as excessive heat or high currents can speed up the rate of deterioration.  

Depending on the manufacturer rating, capacitors can deliver up to 10 years of service life in the most favourable operating conditions. However, generally accepted industry best practice recommends capacitors are proactively replaced between years 4-8 of service life to reduce the risk of serious failure and optimise performance. 

Proactively replacing the capacitors and fans is often referred to as a UPS Overhaul.

How Does A Failing Capacitor Affect A UPS? 

A single failure may not have too much of an impact as the remainder will be able to pick up the slack, although this does place them under increased strain and increases the likelihood of wider problems.  

Ultimately, capacitor failure has a negative bearing on a UPS’s performance. Filtering ability will suffer and there will be more issues with harmonics and electrical noise. In addition, energy storage volume will decrease, and it can even damage battery strings.   

The worst-case scenario of a serious capacitor failure will trigger the UPS to bypass mode, leaving the critical load unprotected.  

Can I Reduce The Risk Of Capacitor Failure? 

The most effective way to minimise the threat is by proactive replacement ahead of the end of their rated service life as part of a UPS Overhaul.  

In addition, ensure the UPS sticks to recommended ambient operating temperatures and humidity levels, as well as keeping air filters clean so cooling air can flow freely. 

Further Reading: 

• What are supercapacitors?  
• Whitepaper: Supercapacitors and UPS Systems
• What’s in an onsite UPS crash kit?