11. Power, Crest, And Surge Factors
Introduction
This Technical Note explains the technical terms of Power Factor, Crest Factor,
and Surge Factor. The use of these terms in specifying UPS is explained.
Power factor
Power factor is a quantity which has important implications when sizing a UPS
system. Power is a measure of the delivery rate of energy and in DC (direct
current) electrical circuits is expressed as the mathematical product of Volts
and Amps (Power = Volts x Amps). However, in AC (alternating current) power
system, a complication is introduced; namely that some AC current (Amps) may
flow into and back out of the load without delivering energy. This current,
called reactive or harmonic current, gives rise to an apparent power (Volt x
Amps) which is larger than the actual power. This difference between the
apparent power and the actual power gives rise to the power factor. The power
factor is equal to the ratio of the actual power to the apparent power. The
apparent power is expressed as the Volt-Amp or VA rating. Therefore, the actual
power in any AC system is the VA rating multiplied by the power factor.
For many types of electrical equipment the difference between apparent power
(VA) and actual power (Watts) is very slight and can be ignored, but for
virtually all computers the difference is very large and important. In a study
done by PC Magazine, it was found that typical computer systems exhibit a power
factor of .65 which means that the apparent power (VA) was 50% larger than the
actual power (Watts)!
To size a UPS and insure that the UPS output capacity is sufficient, the VA
rating of the UPS should be larger than the VA requirement of the load. The
power or Watt rating of the load is lower and should not be used because this
rating does not include the extra reactive or harmonic current that the UPS will
actually be required to supply during service.
Many UPS manufacturers do not include both Watt and VA specifications for their
UPS products. When the VA rating of a UPS is not furnished, it can be very
difficult to determine if the UPS is capable of supplying a specific load. In
fact, many UPS systems which are specified for operation at a specific Watt
rating will not actually run rated power if the load is a typical computer
system with a .65 power factor! When the Watt rating is the only rating
specified for a UPS, we have found that a general rule is to assume that a UPS
Watt rating is equal to the VA rating.
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.
We specify both the Watt and the VA rating for all UPS
products. The ps1501vi has a 1000w capability and the isotran is 1575watts and va. So the total on all plugs including the oconus surge plugs is
about 1800watts not counting the ups.
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
