UPS

IDG Online Staff (Good Gear Guide)
— 22 June, 2005 16:59

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Types of UPS

Most types of UPS devices fit into the following three categories: Offline (Standby) UPS; Online UPS; and Line Interactive UPS.

Offline (Standby) UPS Standby UPS

With this type of UPS, the AC line is the primary power source. Any drop in voltage or current is detected by the UPS, which switches over to the backup battery automatically. When AC power is restored, the UPS switches back. The switchover (transfer) time should be no more than about four milliseconds -- which is the tolerance limit of a typical computer power unit. Any longer and the unit will shut down.

If you are technically inclined, you can do a simple calculation to be sure this will work. Compare the unit's transfer time to the hold (or holdup) time of your computer's PSU (which indicates how long the PSU can tolerate having its input cut off before stopping. If the transfer time is less than the hold time, you can feel confident the PC won't shut down unexpectedly.

A Ferroresonant UPS is an improvement on this basic offline/standby design. It stores energy in the core of the ferroresonant transformer itself, providing an "electricity buffer" to help tide the PSU over during the transfer to battery power.

Line-Interactive UPS

In this type of system, the separate battery charger, inverter and source selection switch of an offline UPS have been replaced by a combination inverter/converter. This inverter/converter both charges the battery and converts its DC current to AC for the output to protected devices. AC line power is still the primary power source, and the battery is 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 gives a faster response to a power failure than an offline/standby UPS. Usually, the inverter/converter also filters out line noise and spikes, and regulates the power output to supply extra power in sags/brownouts and avoiding surges/spikes.

The line-interactive UPS gives better protection than the standby UPS, but it still has a transfer time, so is not as good as the online/continuous UPS.

Online UPS

Sometimes called a True UPS, the Online UPS is the best type you can buy. Also known as double conversion, or continuous, this is a top-end UPS which operates with the battery backup unit as the primary power source, rather than the secondary source (as with the Offline and Line-Interactive UPS). The big advantage is that power comes via the battery, which is constantly being charged. The double-conversion method converts AC power from the mains to DC for the battery and then through an inverter to transform the DC back to AC for the external devices. So, there's no transfer time (hence, no interruptions in the flow of electricity) in the event of power failure.

Being more complex, it generally costs more than an offline UPS. It is also less efficient and tends to have higher running costs and higher operating temperature. It is generally used only in larger and mission-critical installations.

A Delta-conversion online UPS is an improvement on the standard online UPS, which has lower running costs. In this design, the battery charger is replaced with a "delta converter". Instead of the battery providing all the output (which is relatively inefficient), some of it is also provided directly by the delta converter from the AC input. In an outage, the delta converter stops and the UPS works like a normal online/double-conversion unit.

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