Uninterruptible Power Supplies

What are Uninterruptible Power Supplies?

Uninterruptible Power Supplies (or UPS for short) are devices that allow the continuation of power. Usually used to protect against a power cut but they can be used to solve otherĀ Power Quality Problems as well.

There are 3 types of UPS. The most simple type is termed offline. These tend to be the cheapest devices. In normal operation the voltage which is coming into the UPS goes straight out of the UPS. When the supplied voltage goes out of limits the unit switches from mains power to battery power via the unit inverter. There is a very small break in power when the UPS switches from mains power to battery power. In most offline UPS the inverter will be square wave.

The next UPS type is termed line interactive. These systems are similar to offline but have the benefit of extending the input voltage range before the UPS goes to battery operation. It does this by introducing what is called “buck and boost” modes. If the voltage goes to high, the system “bucks” it down. If too low, it “boosts” it up. Line interactive units are available with square wave and sine wave inverters.

The UPS technology which provides the highest levels of power protection are called “online” or “online double conversion“. Here the output is provided from the inverter at all times. This has the benefit that the connected equipment does not see any change in voltage or any gap in power regardless of what happens to the mains supply. Online units will nearly always be sine wave.

Choice of UPS Technology

Your choice of unit depends upon the load type and the environment. Square wave systems, for example, should only really be used on switch mode power supplies. These are like the typical “wall wart” power supplies and also used in computers and monitors and the like. However, some computer power supplies (typically higher powered) do have problems when confronted with a square wave, so if in doubt a sine wave is required.

Sine wave systems are suitable for all connected loads, so you can’t go wrong with these systems.

Online systems provide the highest levels of protection, but they are the most inefficient and also the noisiest as they have always on cooling fans.

UPS Runtime

Uninterruptible Power Supplies require stored energy to provide power to the inverter. This stored energy is usually in the form of VRLA batteries, although Lithium Ion UPS are now available. Regardless of the technology, the more power you need you will need more battery capacity for the same runtime, or for the same power you will need more battery capacity to extend the runtime. People are often surprised at just how many batteries are required when they ask for days worth of runtime! A good tool is our UPS Selector which will calculate this for you.

The more runtime you want the technology options start to become limited. For example, we do not recommend using square wave systems for long runtime applications. This is because square waves have fast rising edges and these can be damaging to electronic components over time. Sinewave systems are always preferred.

Although line interactive units can in some instances be extended run, this should be limited. The reason is that line interactive units are not designed to run from the inverter at all times. So if you need runtime of over 30 minutes or so, particularly at higher power levels then you are limited to online double conversion technology.

Battery Recharge

Another consideration is recharge time. Most standard UPS contain internal batteries with a charger to cope. If you add additional battery packs to this the charger will struggle to charge them following an outage. To get around this you will need to either invest in an extended run UPS System, or use battery cabinets with in-built chargers (such as our VFI-B series).

UPS Classification

In this page we’ve used the general terms for UPS technologies but they have since been defined in EN62040. This is a harmonised European standard which defines offline as VFD, which is Voltage, Frequency Dependent. Line Interactive as VI – Voltage Independent. Online as VFI – Voltage Frequency Independent.