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THE BASIC TYPES OF UPS SYSTEMS
Public utility
supplied AC voltage is seldom a steady 120V AC. Power can dip down to around
100V and peak into the 130V region without major problems. However, if power
falls outside this range, you can lose computer data and cause damage your
hardware.
To prevent
under-voltage “brownouts” and over-voltage “power surges”, some on-line UPS
products can correct utility power ranging from 85V into the mid-130Vs. They
also keep you safe from spikes, those brief moments when lightning or other
energy sources send mega-voltage flowing through the power line. The UPS's Radio
Frequency Interference (RFI) filter prevents line-noise interference that might
come from other electrical products in the office (fans, microwave ovens,
fluorescent lights, etc.)
There are
three basic types of UPS products:
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On-line UPS products are
ideally suited to networks running mission-critical applications. On-line UPSs
take in all types of voltages and frequencies and continuously create clean,
regulated AC power. Some on-line UPSs can compensate for under-voltage and
over-voltage without using the battery, saving battery time for use only in
complete blackouts.
-
Standby, or "off-line,"
UPS products provide utility power during normal operation. When voltage (and
sometimes frequency) changes become severe, the standby UPS starts an internal
inverter that takes DC power from the battery and turns it into AC power to
run your system. The standby UPS does not regulate incoming power and is not
recommended for networking systems.
-
Line-interactive UPSs
provide a measure of voltage regulation by "boosting" utility voltage up or
"bucking" it down before passing it to your system. For some types of
line-interactive models, the UPS uses the battery for this type of voltage
regulation, thus its battery life is shorter than that of on-line models.
The UPS's
power rating is measured in VA or kVA. Here's how to determine the power rating
you'll need:
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List all the equipment you
need to protect.
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Multiply the nameplate
voltage and amperage for each piece of equipment to obtain your VA (the
product of volts and amperes). If the equipment is specified only in watts,
divide the watts by 0.7 to obtain VA in Root Mean Square (RMS) values, which
is how the UPS’s capacity is calculated.
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Add the individual VA
requirements to get your total system's needs. If your VA requirements are,
for example, 770VA and you purchase an 800VA UPS, remember that your full-load
runtime--the amount of battery run time you can expect when running at full
load--is likely to be only five to 20 minutes at best. For extended runtime,
you can purchase add-on battery packs from the vendor. When sizing the UPS,
be sure to allow for start-up surges. If you are not sure, ask the equipment
supplier, or your IT professional.
Alternatively,
you can purchase a higher rated UPS when you need more run time. This is
usually more desirable for many reasons. These include:
-
A UPS with a higher rating
will allow for load expansion in the future.
-
A UPS with a higher rating
is usually less costly than one with an add-on battery pack. This is true
because you now have the cost of two enclosures, and the interconnections
associated with this.
-
The load characteristics
associated with computer type loads is such that the current waveform (the
shape of the current that the typical computer power supply consumes) is very
non-linear. This results in a typical crest factor (the ratio of peak to RMS
current values) in the order of 3:1. In order to properly supply this type of
load without undue stress on the UPS, and without causing undue AC output
harmonic distortion, it is advisable to derate the UPS by 50% anyway. This
can translate into the longer run time desired, without any significant price
increases due to over sizing the UPS.
In addition to
the UPS rating, one should consider the application. If the UPS will be rack
mounted, or free standing are important considerations. The environmental
characteristics are also important factors. Temperature will severely effect
the battery operating time, and the battery life. Dirt, shock, vibration, and
humidity are other important environmental issues to be aware of. Nova Electric
has special UPS systems for many of these unique applications.
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