Recently, a new type of computer power supply has been developed, called a
"power factor corrected power supply". This new type of supply exhibits an input
power factor is equal to one. It is likely that an international regulation
(called IEC 555) may force most computers to use a power supply of this type at
some point in the future. This is specially true for computers with power
supplies rated at greater than 500Watts output. However, today power supplies of
this type remain rare.
American Power Conversion specifies both the Watt and the VA rating for all UPS
products.
For further discussion of the meaning and application of power factor, consult
the references at the end of this note.
Crest factor
In addition to a low power factor, computer loads are also unusual in that they
exhibit a very high crest factor. Crest factor is the ratio between the
instantaneous peak current required by the load and the RMS current (RMS stands
for Root Mean Square, which is a type of average). Most common electrical
appliances exhibit a crest factor of 1.4 (1.4 is the ratio of the peak value of
a sine wave to its RMS value). When a load exhibits a crest factor of more than
1.4, the source (UPS) must supply the peak current desired by the load. If the
source does not supply the current, then the source voltage will become deformed
(distorted) by the excess peak current. Therefore, if a UPS is not sized to
supply the crest factor desired by the load, the output voltage waveform of the
UPS will be distorted.
The crest factor requirement of a computer load will vary depending on the
source which it is supplied from. The crest factor may even vary when the
computer load is moved from one AC receptacle to another in the same room. It is
widely believed that the crest factor is an inherent characteristic of a
computer load, when in fact crest factor results from an interaction between the
load and the AC source. The crest factor required by a computer load depends on
the AC source waveform. For a sine wave source, a computer will typically
exhibit a crest factor of 2 to 3. For a source waveform which is a stepped
approximation to a sine wave, a computer will exhibit a crest factor of 1.4 to
1.9.
It is widely but mistakenly believed that it is desirable to operate a computer
at as high a crest factor as possible. In fact, computer manufacturers go to
great lengths to reduce the crest factor of the computer because high crest
factor causes overheating of power supply components.
The reduction in crest factor which occurs when a computer load is operated from
a UPS, surge suppressor, or power conditioner is a positive side benefit, except
if the reduction is accompanied by excessive distortion of the input voltage
waveform to the computer load. Such distortion may result in a significantly
reduced peak voltage being supplied to the load, which is equivalent to a
brownout condition. The UPS or line conditioner must be designed to maintain the
proper peak voltage.
sine wave UPS systems have a very high crest factor
capability of approximately 3 when operated at full load, 4 when operated at 1/2
load, and 8 when operated at 1/4 load. The smaller stepped wave models have a
crest factor capability of 1.6 at full load and 2 at 1/2 load. This is much
better performance than a typical power conditioner or UPS. PowerStar UPS systems are
designed to maintain the proper peak voltage into the computer load for
computers with any input crest factor specification.
In all computers tested, it was found that operating the computer from an
PowerStar UPS caused the efficiency of the computer power supply
to increase by a small but measurable amount, resulting in decreased operating
