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
temperature of the computer.
Surge factor
This term is often mistakenly related to the surge suppression characteristic of
a UPS or surge suppressor, which is a different and unrelated characteristic.
Surge factor relates to the momentary overload capacity of the UPS and is a
measure of the ability of the UPS to start-up loads which temporarily require
extra power when they start-up. Examples of loads which have a high surge factor
requirement include motors and disk drives.
For typical computer systems with hard disk drives, the surge factor required is
approximately 1.15 times the steady-state power consumption. For large systems
with disk arrays, the surge factor required can be approximately 1.5 times the
steady-state power consumption.
All PowerStar UPS systems have enough surge factor capability to
start typical hard disk loads even when the UPS is operated at the full power
rating.
11.
Computer Interface Port Specifications for the PS-1501vi Smart-UPS family
