Q. What is an uninterruptable power supply?
A. An
uninterruptable power supply (or UPS) is essentially a back-up battery for your computer or
other electronics system in the event of a power failure (blackout) or other electrical line
problems. In the occurrence of a blackout, for example, your computer would continue to draw
power from the UPS thus allowing you to save your work and shutdown your system properly
without data loss or interruption of service.
Q. What are the types of uninterruptable power supplies?
A. The common type is AC/AC, where the energy source is the AC line (120VAC or 240 VAC),
and the output is the same voltage through an inverter. The more efficient type is AC/DC, where
the UPS delivers the DC voltages needed by the system.
Q. How about some examples of AC/DC UPS ?
A. Custom UPS
are often designed at PowerStream to deliver the DC voltages required by the system, thus
eliminating inefficiencies in the power conversion, and simplifying the equipment. For example,
a telecommunications unit designed to run on a -48V bus gets the -48V directly from the UPS. A
mobile cart containing a computer, printer, and scanner gets +12, -12, +5, and +3.3 V directly
from the UPS for the computer, printer, and scanner, and the monitor gets 120VAC from an
inverter circuit also included in the UPS. Thus the battery energy is used more efficiently,
giving longer service between charges.
Q: What provides the power when the mains supply fails?
A: A battery inside the UPS takes over and produces fresh alternating current via an inverter.
Though these are usually lead-acid batteries, the acid is in the form of a gel and the
batteries are sealed and require no maintenance.
Q. What is a brownout?
A. A brownout is more or less the
opposite of a power surge, where voltage falls below normal levels.
Q: How long can I expect the batteries to last?
A:
Depends upon the specification of the batteries used. On larger equipment (above around 5kVA)
you can often specify 10-year design life batteries. Unless you do so, the manufacturer will
fit standard batteries. These should be renewed at around four years - by then even if they
haven't failed the runtime will be deteriorating.
Battery life depends upon a number of issues, particularly
temperature and the number of 'deep discharges' experienced. Battery life is halved for every
10 degrees (C) temperature increase above it's specified operating temperature (usually 25
degrees (C). UPS batteries are designed for maximum life in 'typical' use, which means long
periods at continuous low charge and occasional minor discharges. So-called 'deep' discharges
on a repeated basis will reduce the life of the battery.
Q. What is a power surge?
A. A power surge is a rise in
voltage that goes above normal levels, potentially overloading any active equipment.
Q: What is meant by 'On-line' and Off-line' UPS?
There
are two basic types of UPS in modern use. The first, known as 'Off-line', passes through
incoming mains power to the output sockets while the supply is good. It monitors the line
constantly for a major voltage drop that signifies power failure. It then switches on it's
inverter (the part that produces output alternating current from it's inverter) and supplies
output power from the battery or batteries until the mains voltage has been restored and is
stable for a period of time. There is by definition a momentary power loss during switchover,
but typically of around five milliseconds, which is insignificant to most loads. According to
how much you pay for your UPS, you may get some conditioning of the mains power that is passed
through under normal conditions, and you may get a genuine sine-wave output as opposed to the
square-wave or trapezoidal wave that you will find on the cheaper products.
The second type is known as an 'On-line' design because it's
inverter is 'on-line' permanently. In this design, all the incoming power is always converted
to direct current, which both tops up the batteries and feeds the inverter. The inverter is
constantly producing fresh alternating current to supply the load. When the mains goes off, the
batteries are still holding up the incoming DC supply to the inverter, so the load continues to
be supplied with absolutely no break whatsoever. The inverter is a constant-duty inverter, so
this costs a little more. In any reputable on-line UPS, there is also a fail-safe mechanism
known as a 'Static Bypass' which on detecting failure in the inverter will throw the load on
the mercy of the mains supply, rather than letting it drop altogether. This design inherently
cleans the supply by virtue of reducing it to DC, then reconverting it to fresh AC.
