Eximpower | UPS | Uninterruptible Power Supply

What is a UPS?

An Uninterruptible Power Supply is a device that sits between a power supply (e.g. a wall outlet) and a device (e.g. a computer) to prevent undesired features of the power source (outages, sags, surges, bad harmonics, etc.) from the supply from adversely affecting the performance of the device.

UPS Technologies

There are basically three different types of Technologies, all of which are occasionally passed off as UPSes.
1 - Stand by : The standby UPS is the simplest and least expensive UPS design. In fact, some don't even consider a standby UPS to really be a UPS, calling it instead a standby power supply (SPS). However, many of the most common consumer-grade devices marketed as UPSes, particularly on the lower end of the budget scale, in fact use this general design. They are sometimes also called offline UPSes to distinguish them from Online Ups In this type of UPS, the primary power source is line power from the utility, and the secondary power source is the battery. It is called a standby UPS because the battery and inverter are normally not supplying power to the equipment. The battery charger is using line power to charge the battery, and the battery and inverter are waiting "on standby" until they are needed. When the AC power goes out, the transfer switch changes to the secondary power source. When line power is restored, the UPS switches back.

2 - Line Interactive The line-interactive UPS uses a totally different design than any type of standby UPS. In this type of unit, the separate battery charger, inverter and source selection switch have all been replaced by a combination inverter/converter, which both charges the battery and converts its energy to AC for the output as required. AC line power is still the primary power source, and the battery is the secondary. When the line power is operating, the inverter/converter charges the battery; when the power fails, it operates in reverse. The main advantage of this design is that the inverter/converter unit is always connected to the output, powering the equipment. This design allows for faster response to a power failure than a standby UPS. The inverter/converter is also normally fitted with circuitry to filter out noise and spikes, and to regulate the power output, providing additional power during brownouts and curtailing output during surges. The line-interactive UPS is an improved design that is commonly used in units for home and business use, available in sizes up to 3,000 VA or so. It is superior to the standby UPS, but it still has a transfer time, and thus does not provide protection as good as the Online Ups

3 - Online The online UPS, sometimes called a true UPS, is the best type you can buy. Paradoxically, it is both very similar to, and totally opposite to, the least-expensive type, the standby UPS. It is very similar to it in that it has the same two power sources, and a transfer switch that selects between them. It is the exact opposite from the standby UPS because it has reversed its sources: in the online UPS the primary power source is the UPS's battery, and utility power is the secondary power source! Of course, while seeming small, this change is a very significant one. Under normal operation the online UPS is always running off the battery, using its inverter, while the line power runs the battery charger. For this reason, this type of UPS is sometimes also called a double-conversion or double-conversion online UPS. This design means that there is no transfer time in the event of a power failure--if the power goes out, the inverter (and its load) keeps chugging along and only the battery charger fails. A computer powered by an online UPS responds to a power failure in the same way that a plugged-in laptop PC does: it keeps running without interruption, and all that happens is that the battery starts to run down because there is no line power to charge it. You may ask yourself, why bother having the secondary power path (the dashed line in the diagram above) if you are always running off the battery anyway? The reason is that this provides backup in the event that the inverter fails or stutters due to some sort of internal problem. While unusual, this can happen, and if it does, the unit will switch to the filtered, surge-suppressed line power. In this event, the matter of transfer time comes into play again, just as it does when a standby UPS reacts to a power failure. Of course, power failures are much more common than inverter failures. There is another key advantage to having the equipment running off the battery most of the time: the double-conversion process totally isolates the output power from the input power. Any nasty surprises coming from the wall affect only the battery charger, and not the output loads.

How can it help ?

A UPS has internal batteries to guarantee that continuous power is provided to the equipment even if the power source stops providing power. Of course the UPS can only provide power for a while, typically a few minutes, but that is often enough to ride out power company glitches or short outages. Even if the outage is longer than the battery lifetime of the UPS, this provides the opportunity to execute an orderly shutdown of the equipment. Advantages:

1. Computer jobs don't stop because the power fails.
2. Users not inconvenienced by computer shutting down.
3. Equipment does not incur the stress of another (hard) power cycle.
4. Data isn't lost because a machine shut down without doing a "sync" or equivalent to flush cached or real time data.

What sort of stuff does a UPS do?

A UPS traditionally can perform the following functions:

Absorb relatively small power surges.
Smooth out noisy power sources.
Continue to provide power to equipment during line sags.
Provide power for some time after a blackout has occurred.

In addition, some UPS or UPS/software combinations provide the following functions:

Automatic shutdown of equipment during long power outages.
Monitoring and logging of the status of the power supply.
Display the Voltage/Current draw of the equipment.
Restart equipment after a long power outage.
Display the voltage currently on the line.
Provide alarms on certain error conditions.
Provide short circuit protection.

How long can equipment on a UPS keep running after the power goes?

That depends on how big a UPS do you have and what kind of equipment it protects. For most typical computer workstations, one might have a UPS that was rated to keep the machine alive through a 15 minute power loss. If it is important for a machine to survive hours without power, one should probably look at a more robust power backup solution that includes a generator and other components. Even if a UPS powers a very small load, it must still operate its DC (battery) to AC converter (the inverter), which costs power. A rough extrapolation from APC's documentation, leads me to guess that its 2000 VA UPS can operate its own inverter (with no extra load) for just over 8 hours. A 1250 VA UPS could run its converter for about 5. These are very rough guesses based on information provided by one vendor for one vendor.

How are the "sizes" of a UPS determined?

Typically, a UPS has a VA rating. The VA rating is the maximum number of Volts * Amps it can deliver. The VA rating is not the same as the power drain (in Watts) of the equipment. (This would be true if the load were only resistive or the circuit were DC, not AC). Computers are notoriously non-resistive. A typical PF (power factor: Watts/VA) for some computers may be as low as 0.6, which means that if you record a drain of 100 Watts, you need a power source with a VA rating of 167. Some literature suggests that 0.7 may be a good conversion factor, but this will depend heavily on the specific equipment. Moreover, there's really no way to determine these numbers besides measuring them.

Electrical Waveform Output

A quality consideration that can be important in some applications is the output waveform of the UPS. This refers to the shape of the alternating current signal produced by the UPS. The quality and cost of the inverter that is within the UPS is the primary determinant of the shape of the AC signal that is produced by most UPSes, especially ones on the lower end. The ideal situation is for the UPS to produce a clean output waveform that is close to what would be produced by the electrical utility, but this is not always the case.
There are three main waveform types produced by UPSes:

Sine Wave: This is the best waveform, as it is the shape of an (ideal) AC electrical signal from the wall. The highest-quality UPSes produce a true sine wave output, which requires fairly expensive components in the inverter. This is especially important for online UPSes, since their loads are always running off the inverter. True sine wave UPSes are normally found only in higher-end models.
Square Wave: The least desirable output waveform type, a square wave is sort of a "flattened-out" version of a sine wave. Instead of the voltage smoothly increasing from the negative maximum to the positive maximum and back again, it shifts suddenly from negative to positive, stays there for half a cycle, and then jumps to full negative and stays there for half a cycle, then repeats. Cheaper inverters are designed produce a square wave output primarily because the components required to do this are cheap. It wouldn't surprise you to learn that some equipment doesn't really like running on a square wave (it may be more surprising to learn that many types of equipment will run on it!) There are several reasons why square waves cause problems. For starters, the peak voltage of a square wave is substantially lower than the peak voltage of a sine wave, which causes issues with some types of equipment. In addition, while a sine wave has a single frequency in it--60 Hz in North America--a square wave contains many higher frequencies as well, called harmonics, which can cause buzzing or other problems with some equipment. Square wave output is found only in the cheapest equipment and should be avoided if possible.
Modified Square Wave: This waveform is a compromise between the sine wave and the square wave. The positive and negative pulses of the square wave are thinned, separated and made taller, so the peak voltage is much closer to that of a sine wave, and the overall shape of the wave more closely resembles that of a sine wave. At the same time, the cost of the circuitry to produce a modified square wave output is much closer to the cost of a square wave's circuitry than that of a sine wave unit. (In fact, you can create a modified square wave by adding together two square waves that are shifted in phase slightly from each other.) Many fewer pieces of equipment have problems with modified square wave power than with straight square wave. Modified square wave output is used on many lower- to middle-range UPSes, and is also sometimes called "stepped approximation to a sine wave", "pulse-width modified square wave", or even "modified sine wave". The last term is marketing cutesy-speak, since the output form isn't really a sine wave, modified or otherwise