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An uninterruptible power supply (UPS) is a device that allows a computer to keep running for at least a short time when incoming power is interrupted. Provided utility power is flowing, it also replenishes and maintains energy storage.
A UPS protects equipment from damage in the event of a power failure. It is used in any situation where electrical equipment is sensitive to power loss or issues with power quality, for example, if a system experiences unsafe changes in voltage output. UPSes are typically used in settings pertaining to computer systems, data servers or industrial devices, or in settings with mission-critical equipment, such as medical and laboratory systems.
Energy can be stored in different ways. Rechargeable batteries are the most common. For simplicity, the examples and illustrations here are based on that technology. However, kinetic energy can also be stored in heavy, rotating flywheels or energy can be stored as fuel.
The more energy stored, the longer backup power can be maintained, with practical limitations that will be discussed later. The differences among UPS systems lie in the technology that enables them to do their jobs.
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According to Verified Market Research, the UPS market is expected to grow to over $11.95 billion by 2031.
A battery-powered UPS typically contains the following components:
A UPS is typically plugged into an AC outlet and activates when it detects a power outage, voltage drop, surge or frequency variation. In the case of an outage, the UPS almost immediately switches to the battery -- fast enough so no electrical equipment is damaged. From there, the battery acts as a continual emergency power source until it uses all its power.
Depending on its size, a UPS can protect a single computer or a whole data center. Its stored power might last from a few minutes to several hours. The goal is to either last until generators are turned on or until protected devices are properly shut down.
The most common type of UPS is also the most effective, generally called a full-time or full double conversion UPS. For any UPS, incoming utility power is AC, which is also required by most IT equipment (ITE).
Batteries, on the other hand, are DC devices, so all battery-type UPSes must convert -- or rectify -- the incoming AC power to DC to charge the batteries. The UPS must also still deliver AC to the ITE, so DC power must be converted back to AC through a device known as an inverter.
In a double-conversion UPS, power flows continuously through the rectifier and then through the inverter to the ITE. The output voltage and frequency are completely isolated from, and independent of, the input voltage and frequency. They can even be completely different than the input, so this system is technically classified as voltage and frequency independent (VFI).
Voltage and frequency independent. Figure 1 below shows a VFI system in normal operation. Anomalies in the input power are dealt with in two ways. A surge suppression device (SPD) absorbs particularly bad voltage spikes. These can be caused by lightning strikes on power lines, large motors used on elevators, medical electronics equipment, welders or numerous other sources. But even the smallest variations -- including voltage sags or brownouts -- never make it through a VFI UPS to the output.
Batteries are excellent electrical shock absorbers, but they also maintain a steady and constant voltage to the inverter, which completely resynthesizes the voltage and current so that the power delivered to the ITE is clean and steady. Connecting air conditioners or other motors to the UPS serving the ITE could contaminate this clean output power, so it''s not recommended.
The battery is always in the circuit in normal operation, delivering small amounts of power when needed, such as during brownouts, so there isn''t the slightest interruption in output power.
When utility power fails, as shown in Figure 2 below, the battery continues to deliver stored energy to the inverter, which continues to deliver clean power to the ITE. When utility power is restored, power flows back through the rectifier, feeds the inverter and recharges the batteries.
UPS static and maintenance bypass. UPSes aren''t uninterruptible. They''re electrical or mechanical devices, so they not only require routine maintenance, but also are subject to component failures. For these reasons, all UPS systems have a built-in bypass to route incoming power around the system and directly to the ITE when necessary.
The high-quality SPD is still in the circuit but is only slightly better than running your home electronics on a power strip with surge protection. It won''t stop utility power interruptions or deal with voltage sags or brownouts. If the UPS fails, the bypass operates immediately as a static switch.
When a technician must work on the system, the bypass is operated manually to render the internal components safe. If utility power fails while the UPS is in bypass, power to the ITE is interrupted. Any installation with only one UPS has this vulnerability. Figure 3 below shows the UPS in bypass mode.
Note that major spikes have been removed, but the voltage drop continues through.
Economy mode operation. The first law of thermodynamics, conservation of energy, states that energy can be neither created nor destroyed. No electrical or mechanical device is 100% efficient, so every conversion incurs a loss, which escapes as heat.
UPS systems are far more efficient than a decade ago, and they maintain close to the same efficiency from low to high load. But there''s still loss in both the rectifier and the inverter, which are eliminated when the UPS is in bypass mode. Many VFI UPSes now offer a sophisticated version of bypass known as economy mode, or eco mode, as shown in Figure 4 below. An eco-mode UPS can return to full VFI operation when needed.
When rectifier and inverter losses are eliminated, power and cost are saved until power fails and full UPS operation is needed. Some users set the system for VFI operation during the day, and have it automatically switched to eco mode at night if those operations are considered less critical. Eco mode is generally highly dependable, but many users are leery of switching the modes back and forth. Further, new VFI UPS efficiencies are within 1% or less of what can be achieved in eco mode, so many users now consider this alternate operating mode unnecessary.
Note that eco-mode UPSes incorporate high-quality filters, which also incur a small loss, and that there''s usually a short instability when switching modes. Eco mode efficiency is statistical, but it can be 99% if power failures rarely occur and are short-term.
Line interactive UPS. A true line interactive UPS, also known as voltage independent (VI), has the same output frequency as the input. They look virtually identical to VFI UPSes in eco mode, except for the size of their rectifiers and the inability to switch to VFI mode.
The smaller rectifier needs only to charge the batteries, which helps absorb anomalies and boost power when there are voltage sags. The batteries take over completely when power fails. Figure 5 below shows how the battery and inverter help compensate for incoming voltage variations by running in parallel with the output.
Figure 6 below shows a line interactive UPS when incoming service fails. The battery takes over, just as it would in a double conversion UPS, but the bypass switches the utility out of the circuit. Because the ITE runs on utility power most of the time, the second conversion through the inverter is avoided until power fails, eliminating one of the efficiency loss components.
A decade ago, VI UPSes could have an efficiency advantage of 5% or more over VFI units, but improvements in VFI UPSes have reduced that to 1% or less.
As shown in Figure 8 below, when power fails, the utility is switched out of the circuit, and the battery and inverter are switched in. There''s some switching instability, but the delay is short enough for most computer power supplies to ride through.
When power is restored -- either through the utility or a generator -- the inverter is disconnected, line power is switched back on and the batteries are recharged by the rectifier, which is much smaller than in a VFI or VI UPS.
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