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What Are The Different Uninterruptible Power Supply Types?

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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 UPS, particularly on the lower end of the budget scale, in fact use this general design. They are sometimes also called offline UPS 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.

While the least desirable type of UPS, a standby unit is still a UPS and will serve well for most users. For a very critical function, however, such as an important server, they are not generally used. The issue with a standby UPS is that when the line power goes out, the switch to battery power happens very quickly, but not instantly. There is a delay of a fraction of a second while the switch occurs, which is called the switch time or transfer time of the UPS. While rare, it is possible for the UPS to not make the switch fast enough for the PC's power supply to continue operation uninterrupted. Standby UPS are usually available in a size range of up to about 1000 VA.

A ferroresonant standby UPS is an improvement on the design of the standby UPS. As with a standby UPS, the primary power source is line power from the utility, and the secondary power source is the battery. The battery and inverter are still waiting on standby until needed. The big difference is that the standby UPS's transfer switch that selects between power sources has been replaced by a ferroresonant transformer.

A transformer is a core of metal around which are normally wrapped two coils of wire. When current is applied to one coil, it is magnetically coupled to the other coil and a current is induced in the second coil. Transformers are normally used to change one AC voltage into another. However, the other effect of a transformer is that it tends to smooth out power problems in its input, and not pass them on to the output. This makes them well suited for use as a line-filtering component in a UPS.

The transformer used in this type of UPS is not a regular transformer however, because it has three windings. Two of the windings are input, from the primary and secondary power sources, and the third is the output. This allows the transformer itself to act as a sort of switch, because whichever input source is operating, the output will take its energy.

Ferroresonant standby UPS are usually available in a size range of up to about 15,000 VA, making them suitable for even the largest servers. Once one of the most common designs, the ferroresonant UPS is still made by some manufacturers, while others have abandoned this design, claiming that it is inefficient, in favor of the online UPS for models in the range of 1,000 VA or higher.

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.

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 battery, and utility power is the secondary power source.

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. 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.

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