An uninterruptible power supply, or UPS, is a device or system that maintains a continuous supply of electric power to certain essential equipment that cannot be shut down unexpectedly. The equipment is inserted between a primary power source, such as a commercial utility, and the primary power input of equipment to be protected, for the purpose of eliminating the effects of a temporary power outage and transient anomalies.

They are generally associated with telecommunications equipment, computer system, and other facilities such as airport landing systems and air traffic control systems where even brief commercial power interruptions could cause injuries or fatalties, serious business disruption or data loss.

Most uninterruptible power supply designs for telecommunications equipment use one or more rectifiers to convert the incoming commercial AC power into a low voltage DC supply, typically in the range of 12 to 50 volts. One or more rechargable batteries are connected in parallel with the rectifiers to maintain the voltage should the power fail. Various arrangements exist to ensure that the batteries, which are on a continuous trickle charge, can be maintained at an appropriate voltage and state of charge, as well as be given a boost charge should the charge state become too low. Because a battery backed DC output is used, this type of uninterruptable power supply is only suitable in specialised telecommunications applications where the equipment does not require a commercial AC power feed.

Early uninterruptible power supply designs that supplied commercial quality AC power to equipment often involved a motor-generator system with a large flywheel that kept the generator rotating and producing electric power while an auxiliary motor was started at the moment of power interruption. Sometimes the flywheel itself was used to start the motor. These systems could typically cover a 30 second interruption until the auxiliary motor started.

Modern uninterruptible power supply systems used with commercially available computer equipment consist of a static (electronic) rectifier that rectifies the AC power from the incoming power line fed by the utility. The rectified power keeps a battery trickle-charged and also powers an static (electronic) inverter. In the event of a power interruption or transient anomaly, the inverter draws its power from the battery and takes over without the loss of even a fraction of a cycle in the AC output of the UPS. The inverter/battery arrangement normally requires an output filter which also provides additional protection against transients. The duration of the longest outage for which protection is ensured depends on the battery capacity, and to a certain degree, on the rate at which the battery is drained.

Often, especially in larger installations, a static (electronic) switch connects the incoming commercial power directly to the equipment. This arrangement permits the inverter to run in hot standby, synchronised with the AC power but under no load and allows the rectifier(s), inverter(s) or battery to be removed from service for maintenance or in the event of a fault.

Output sizes from under 1kW to several hundred kilowatts are commercially available. While most UPS equipment will only operate for about 10 minutes after an outage occurs, some telecommunications systems are designed to operate for over 24 hours without power.

Some modern uninterruptible power supplies are specially designed Switched-mode power supplies that have an integrated battery backup system. Depending on the design there may be one or two Switched-mode power supplies.

  • NOTE: Do not confuse a UPS with a standby generator, which does not provide protection from a momentary power interruption, or which may result in a momentary power interruption when it is switched into service, whether manually or automatically.

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Part of this article was originally taken from a public-domain entry in Federal Standard 1037C