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The uninterruptible power supply (UPS) is a device that provides emergency power to a load when the input power source, normally mains power, fails. A UPS differs from an auxiliary or emergency power system or standby generator in that it will provide near-instantaneous protection from input power interruptions, and will also continue to provide protection for a period of time after power is restored.
With a typical UPS, the load is protected from a power failure by connecting it to the UPS before the power fails. When the power resumes, the load will be powered by the UPS until the utility power resumes. The UPS will then switch to utility power. Some UPS systems allow the load to be powered directly from the UPS, bypassing the utility power, for a limited time after a power failure.
The first UPS was invented by Walter Liebert in 1928. The modern version was introduced in 1957 by Eaton Corporation. Liebert’s UPS had a 12-volt DC motor, contained within the unit, that spun a generator that produced 110 V AC. The modern UPS, as invented by Eaton, uses a motor-driven, variable-speed reversible electric motor, which is often referred to as a double-conversion UPS, and which is typically powered by a large lead-acid battery. The motor runs at a constant speed and frequency and can either be directly connected to the system load, or to a large capacitor. When the power fails, the motor spins the generator, which provides power to the load. After an adjustable delay, the motor stops, and the UPS switches to its battery.
UPS units range from small devices that protect a single computer or other electronic equipment from brief power outages to large units that can provide protection for an entire building. The smallest units are designed for devices such as personal computers and monitors, while larger units such as those used for computer server rooms can cost thousands of dollars and weigh hundreds of pounds.
A UPS consists of three main components:
In addition, most UPS systems use a battery or batteries to provide power to the load after the utility power has failed. In some cases, the UPS may also provide limited power to the load during a brief power outage.
The isolator is a device that allows the load to be powered by the utility power or the UPS, but not both at the same time. If the utility power fails, the isolator will open, and the UPS will take over power to the load. If the UPS begins to fail, the isolator will close, and the load will be powered by the utility power.
The UPS provides power to the load by either connecting to the utility power directly or by connecting to a large capacitor. If the UPS connects to the utility power, it will transfer power to the load through a transformer. When the power fails, the UPS switches to its battery or batteries. If the UPS connects to a large capacitor, it keeps the capacitor charged. When the power fails, the UPS switches to its battery or batteries, and the capacitor releases its charge to run the load. In both cases, the UPS regulates the voltage and frequency of the power to the load, and provides protection from overvoltage, undervoltage, overcurrent, and over-temperature.