PROPERTY OF SOLIDSTATE CONTROLS, INC. 7/1998
unit to maintain a constant output voltage with a varying input voltage. A longer conduction period
increases the DC output voltage and, conversely, a shorter period lowers the output voltage.
When the input voltage is high, the thyristors are gated on later during each half cycle. As a result,
in order to provide constant power, we will necessarily see a high current spike at this point which
we will refer to as form factor or crest factor. Consequently even though the kilowatts (KW), which
you pay for at the electrical meter, are what you might expect based upon 92-94% efficiency, this
“form factor,” or “crest factor,” causes a higher-than-normal peak current demand which must be
considered when selecting all upstream devices. (Note: Although not true power factor, this form
factor or crest factor, is many times expressed in those terms when discussing input requirements
for rectifier/chargers.)
In general, a good rule of thumb for estimating the input power requirements for a UPS system is to
use 2.5 times the output kVA rating for our Ferroresonant units and 2 times the output rating for our
PWM units.
All wiring, transformers, distribution breakers, etc., must be sized for the maximum currents, as
dictated by the NEC, even though, under normal operating conditions, the input current can be
expected to be much lower.
The following formula can be used to derive the rectifier/charger input power requirements
expressed in (VA) volt amperes.
CIP = Ve x Ar x C1
PF x EF
Ve = Charger equalize voltage
Ar = Charger rated output in amperes
C1 = Charge current limit point (typically 1.2 to 1.25)
PF = Charger input power factor (typically 0.8)
EF = Charger efficiency (typically 92-94%)
CIP = Charger input power in (VA) volt amperes
The following formula can then be used to derive the rectifier/charger maximum input current. This
information is necessary in determining the UPS system input electrical service requirements.
Ac = CIP
V1 x 3
V1 = 3 phase AC input voltage at low line (typically nominal voltage –10 or –15%)
3 = 1.732
Ac = charger input current in amperes
Note: SCI can provide adjustable current limiting from 100% to 125% to aid in limiting the input
current requirements under conditions where maximum current limiting is not desirable.
The bypass source input must be supplied with an input voltage and frequency which exactly
matches the output of the UPS. For example, if the UPS system output is 208Y/120 Vac, 60 Hertz,
then the bypass source input must be 208Y/120 Vac, 60 Hertz. If this matching voltage is not
available, an appropriate transformer must be installed to provide the proper voltage. SCI can
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