1. Cooling and Fan Operation of -UPS
INFORMATION: All PS-1501vi -UPS , have cooling fans.
Fan cooling runs during four normal, though in most cases infrequent, modes of
operation. These modes are listed as follows:
1 ) The fan is on whenever the UPS is operating on-battery.
2 ) The fan is on when the UPS' internal ambient temperature exceeds 60
!
C
(140
!
F). When cooled to below 50
!
C (122
!
F), the fan is shut off.
3 ) The fan is on when the output load rises to above 75% of full rated load.
When the load is reduced to below 70% of full rated load, the fan is shut off.
4 ) The fan is on when the battery charger is delivering power subsequent to a
utility outage.
Note: ps-1501vi -UPS must be allowed 1" (2.5 cm) of clearance on all sides for proper
ventilation.
2. 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.
