An uninterruptible power source (UPS), also known as an uninterruptible power supply, is a piece of equipment situated between electronics and the main power source and used to provide continuous and predictable power. Unlike a backup generator or other emergency power system, an uninterruptible power source provides immediate power with no noticeable interruption when the main power goes out, and may protect against variation in voltage from the main power supply. In contrast, other emergency backup systems may take several minutes to begin operation after a power outage. This difference is most important when dealing with some computers or high-powered electronics that need to be properly shut down, because sudden breaks in power while the equipment is running may physically damage them. It is important for users to check the limits of their equipment and not to overload the uninterruptible power source.
The most basic uninterruptible power source consists of a battery backup. Technically, the battery in a laptop computer could be characterized as an uninterruptible power source. The smaller external versions marketed to home-users may look like very large surge protectors and will have sockets on them for plugging in the electronic equipment to be protected. They may also provide the features of a surge protector. These units usually only provide power for a few minutes, but allow users to save data or properly shut down small servers or other equipment that could be damaged by arching from a sudden power interruption.
Aside from surge and interruption protection, an uninterruptible power source may have features that provide protection against sustained spikes in voltage and brown-outs — temporary dips in voltage. In smaller uninterruptible power sources, this is accomplished by switching to battery power, but larger versions may actually include transformers to step up or step down voltage. In higher-end units, this ability may allow the uninterruptible power source to also function as a line or power conditioner, which protects against variations in power that may be harmful or simply not ideal for sensitive equipment.
Larger, industrial uninterruptible power sources may be the size of a small room. Such uninterruptible power sources may simply be larger versions of those marketed for home use, essentially a “battery room,” or they may employ a combination of a flywheel — which kinetically stores energy to prevent power interruption — and generators. Both the battery-powered and mechanical uninterruptible power sources of this size are likely to require qualified electricians to install, and their design and operation may be subject to specific legal requirements.
The mechanical or rotary uninterruptible power sources that use flywheels and generators are one of the oldest types of uninterruptible power sources. The main power supply is converted to kinetic energy by powering a motor that spins the flywheel, which then powers a generator that converts the energy back into electricity. In the case of power loss, the high mass of the flywheel causes it to keep spinning and creating electricity until power returns or backup generators can be started. Additionally, the inertial qualities of the flywheel mean that small spikes or drops in voltage have little effect on its speed, thus increasing the consistency of the output voltage.