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Uninterruptible power supplies


Uninterruptible power supplies (UPSs) are devices that maintain the supply of power to a load even when the AC input power is interrupted or disturbed. This is typically accomplished by drawing the necessary power from a stored energy source, such as a battery. UPSs may also convert unregulated input power to voltage and frequency-filtered AC power. Thus, the UPS will provide stable power and minimize the effects of electric power supply disturbances and variations.

UPSs are currently found in commercial, industrial, medical and residential markets. Applications include:

  • individual computers and computer systems
  • shipboard systems
  • automated manufacturing
  • microprocessor- and microcontroller-based equipment
  • medical applications
  • laboratories
  • analytical systems
  • robotics
  • precision motor-speed applications
  • military applications
  • mission-critical fields such as telecommunications and Internet nodes
  • finance
  • public health
  • air traffic control
  • transport

Sizes of UPSs vary, from approximately 250 VA to 1000 kVA. Small UPSs are used for single personal computers and workstations where down time is tolerable but data loss must be avoided. These UPSs provide enough backup time for reliable equipment shutdown. Large UPSs support mission-critical applications where large-scale protection is essential.

Types of Uninterruptible Power Supplies

The three main types of UPSs are:

  • standby (offline)
  • online
  • line interactive

In the following, "input power" refers to power obtained from the utility, and "load power" refers to power supplied by the UPS to the load.

Offline (or standby) UPSs are the simplest and most efficient. Normally, power reaches loads directly from its source. During a power failure, a switch connects a backup battery to the load, with a short, distinct power interruption. During unstable conditions such as when input power frequency deviates from the required range, the same switching occurs, connecting the backup battery. In persistently unstable conditions, the battery may be drained, making it inadequate during a blackout.

Since offline UPSs provide only partial protection from many common power problems, they are most often used to shield single-user personal computers and other less critical applications.Offline UPSs are smaller and lower-priced than online UPSs.

Online UPSs provide load power at all times through a battery that is continuously charged by input power. The battery is always online; therefore, no switching is called for during power failures.

Online UPSs provide complete protection and isolation from almost all types of power problems and provide digital-quality power that is not possible with offline systems. For these reasons, they are typically used for mission-critical applications that demand high productivity and systems availability.

"Double-converter system" is another name for an online UPS since it must convert AC input power to DC for charging the battery and afterward convert DC to AC for use by the load. Double conversion makes this UPS less efficient than other types.

Online systems provide the same benefits of an offline UPS combined with a line conditioner, at a price lower than the cost of both components purchased separately.

Line-interactive UPSs retain some of the efficiency of offline UPSs while providing the voltage regulation features of online systems. Instead of converting the input power to DC and storing it in a battery, the UPS sends the power to the load through a ferroresonant transformer that provides voltage regulation and power conditioning for disturbances such as electrical line noise. In addition, when a power outage occurs, the transformer maintains an energy reserve that is usually sufficient to power most personal computers briefly during the switchover to the UPS's battery power. In general, these UPSs work best with linear loads such as motors, heaters and lights.

Line-interactive UPSs are very efficient, highly reliable and, unlike offline systems, offer voltage regulation features.


Performance test methods for UPSs are provided by:

None of these standards, however, specify minimum energy efficiencies. High-efficiency programs such as ENERGY STAR® do not include UPSs.

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