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Sine Wave: This is the best waveform, as it is the shape
of an (ideal) AC
electrical signal from the wall. The highest-quality
UPSes produce a true sine wave output, which requires fairly expensive
components in the inverter. This is especially important for online UPSes,
since their loads are always running off the inverter. True sine wave UPSes
are normally found only in higher-end models.
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Square Wave: The least desirable output waveform type, a
square wave is sort of a "flattened-out" version of a sine wave.
Instead of the voltage smoothly increasing from the negative maximum to the
positive maximum and back again, it shifts suddenly from negative to
positive, stays there for half a cycle, and then jumps to full negative and
stays there for half a cycle, then repeats. Cheaper inverters are designed
produce a square wave output primarily because the components required to do
this are cheap. It wouldn't surprise you to learn that some equipment
doesn't really like running on a square wave (it may be more surprising to
learn that many types of equipment will run on it!) There are
several reasons why square waves cause problems. For starters, the peak
voltage of a square wave is substantially lower than the peak voltage of a
sine wave, which causes issues with some types of equipment. In addition,
while a sine wave has a single frequency in it--60 Hz in North America--a
square wave contains many higher frequencies as well, called harmonics,
which can cause buzzing or other problems with some equipment. Square wave
output is found only in the cheapest equipment and should be avoided if
possible.
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Modified Square Wave: This waveform is a compromise
between the sine wave and the square wave. The positive and negative pulses
of the square wave are thinned, separated and made taller, so the peak
voltage is much closer to that of a sine wave, and the overall shape of the
wave more closely resembles that of a sine wave. At the same time, the cost
of the circuitry to produce a modified square wave output is much closer to
the cost of a square wave's circuitry than that of a sine wave unit. (In
fact, you can create a modified square wave by adding together two square
waves that are shifted in phase slightly from each other.) Many fewer pieces
of equipment have problems with modified square wave power than with
straight square wave. Modified square wave output is used on many lower- to
middle-range UPSes, and is also sometimes called "stepped approximation
to a sine wave", "pulse-width modified square wave", or even
"modified sine wave". The last term is marketing cutesy-speak,
since the output form isn't really a sine wave, modified or otherwise.
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