UPS Systems The uninterruptible power supply.
Commonly called the UPS, this device is a clever threefold package--a
set of batteries, an inverter that transforms the low-voltage
direct current of the batteries into the standard alternating current used by
your computer, and a battery charger that assures that reserve power is
always available inside the batteries. In other words, it is a power storage
system (the batteries) with interfaces to match it to utility power and your
computer system. The UPS is designed to protect against
changes, specifically a temporary loss of electrical supply. It keeps your
computer and its support equipment operating without interruption--hence the
name--so that you don't lose your work. As such, the UPS is meant to
supply power only for as long as it takes you to save your work and shut down
your computer. It might also bridge the gap between the loss of
utility-supplied electricity and the acquisition of power from another source,
such as a generator, which may take several seconds to start and stabilize. A
general-purpose UPS is not meant to keep your computer running for more than a
short while. A growing trend today, however, is the extended run UPS. Equipped
with extra battery reserves, the extended run UPS is meant to keep you going
for the duration of a rolling blackout, typically about two hours. Full power protection means more than just buying a UPS. The UPS must
be integrated with your computer and network via control and monitoring
software. Moreover, in major installations, the UPS is part of a larger
picture that may include a standby generator and special heating and cooling
arrangements. In fact, some UPS makers like American Power Conversion are
adding complete turn-key power services that embrace overall wiring, power
systems engineering, and even HVAC (heating, ventilation, and air conditioning)
design to their offerings. Although the traditional UPS might not seem a hot spot for engineering
development--after all, electrical power hasn't changed much in the last 100
years--the last few years have seen major changes in the UPS industry. Most of
the effort has been concentrated on fulfilling the needs of LAN and Internet
servers. Today's processor-intensive rack packaging of thin servers is driving
the need for ever more compact rack-mount UPSs. Tripp
Lite, for example, now packs a 3 KVA unit into a
3.5-inch high rack-mount package. Such dense packaging has focused UPS maker's
efforts on high-capacity cooling. In addition, demands for high availability have led to the creation of modular and redundant UPSs, units that will keep functioning--and continue to
protect your data--even if a critical component such as the inverter fails.
Further, the threat of power-line pollution has focused awareness on the
quality of UPS power in addition to the quantity each UPS makes available. You'll find a wide range between inexpensive UPSs listing for around $100 and professional products
targeted at the server market, which cost from $700 to more than $2000. The connection method, usually reflected in
the number of electrical receptacles on the unit--determines how convenient the
UPS is to hook up and use. The general tendency in modern UPS design is to provide a combination of
battery-protected and unprotected outlets. When utility power fails, only the
protected outlets will provide battery power. The unprotected outlets serve
only as a convenience function, giving you a place to plug in your printer and
other non-critical peripherals. The largest UPSs are
meant to be permanently wired in place and may connect directly to your fuse
box or power distribution system. The
UPS core electronics serve two basic functions. The power circuitry
handles the heavy-duty electricity that powers your computer and its
peripherals. It takes care of the power conversions--from AC to DC to charge
batteries, and from DC to AC to convert battery power to computer-compatible
voltages. The control circuitry tells the power circuitry what to
do--when to switch from line power to the battery supply, how much current to
use to charge the batteries, when to limit power or otherwise protect the
electronics of the UPS and your computer. In addition, the control circuitry
lets you monitor the operation of the UPS. The control circuitry also links
with your computer so you can remotely monitor your UPS and the UPS can warn
your computer of power failures. Off-line UPSs Line-interactive UPS The next step up the UPS ladder is the line-interactive UPS. The
chief change between the off-line and line-interactive designs is that the
line-interactive UPS adds some degree of voltage regulation. That is, the line
interactive UPS attempts to correct for voltage fluctuations on your power line
and deliver electricity that is as close as possible to 120 or 240 volts
(depending on what your computer uses). On-line
UPS
The term on-line UPS is used differently by
various manufacturers. Most UPS makers reserve the
term for true double-conversion UPSs because they
keep their inverter circuitry constantly on-line. Some manufacturers, however,
also use the term for line-interactive products The
double-conversion UPS is the only truly uninterruptible system. It never
switches because its inverter is always connected to your computer. But the
double-conversion design does more than eliminate the brief switching
interruption. The battery reservoir completely isolates your computer from the
vagaries of the power line. It protects against all forms of power irregularity
including surges, spikes, overvoltages, sags,
brownouts, and blackouts. The double-conversion UPS doesn't switch transformer
taps when line voltage surges or sags, but adjusts its own solid-state voltage
regulators to cope with under- and overvoltages.
Consequently the double-conversion UPS maintains a closer tolerance on its
output voltage. Typically, a double-conversion UPS delivers an output within 2%
of its nominal rating.
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