16 Theoryof Operation
fullload with optional input filter. For modules500 kVA and larger,the optional 12-pulse rectifier reduces
reflectedTHD to less than 4% with optional filter, or 9% without.
Thefiltered output of the rectifier/charger providesregulated DC power to drive the inverter and charge thebat-
tery.
Input Circuit Breaker
Theinput circuit breaker (CB1) is sized to allow enough current to recharge the batteryand supply a full rated
loadat the same time. The circuit breaker contains a thermal magnetictrip mechanism and an undervoltage
releasethat interrupts power, preventing damage to the system,if there is an internal AC over-current condition
ora short circuit. The circuit breakeralso opens during certain alarmed faults.This circuit breaker must be closed
manually.
Isolation Transformer
Theoptional input isolation transformer has a dry type core, and copper windingswith class H insulation. The
transformeron modules with standard 6-pulse rectifier haswye primary and delta secondary windings. For mod-
elswith the optional 12-pulse rectifier, the transformerhas a delta primary, with delta and wye secondaryw ind-
ings.The isolation transformer provides the initial stepof critical bus isolation. This reduces the AC shock
hazardat the battery and at other DC components, and prevents a DC fault from disruptingupstream AC cir-
cuit ry.
Thetransformer has a nominal tap and one tap 6% below the rated nominal input voltage (normallyused for 460
VACinput).
Input Current Limit
ACinput current-sensing transformers (CTs) are used to measure currentlevels. Control circuitry connected to
theCTs restricts the AC current to lessthan 125% of the full input current rating by reducing the batterycharging
voltage.This current limit is adjustable from 100 to 125% and is factory set at 115%.A second (optional) level
ofinput current limit can be initiated by an external contact closure (field suppliedfor use with back-up genera-
tor),and is adjustable from 85 to 100% (factory set at 100%).
Duringa start-up procedure, the current slowly ramps up (walks-in) from 20% of the rated input currentto 100%
overa 15 to 21 second period. The maximum rate of change of the AC input current is 15% per second. The
inputcurrent walk-in reduces the start-up surge distortione ffectson all other equipment connected to the same
sourceand prolongs the service life of internal components.
Input Current Inrush
Themaximum sub-cycle of inrush current due to the optional input isolationtransformer is less than 500-800%
ofthe rated input current for the first 1/2 cycle. Without the optionaltransformer, inrush current is typically less
than2-3 times normal.
Input Power Factor
Therated input power factor is no less than 0.85 laggingat the nominal input voltage and the full rated UPS load.
Theoptional input filter will improve the powerfactor to better than 0.92 lagging at fullload. Refer to published
specificationsfor your UPS model.
2.2.3 Battery Charging Circuit
TheUPS module charging circuit is capable of rechargingthe battery plant to 95% of full capacity within 10
timesthe discharge time. Recharging the last 5% takeslonger because of characteristics inherent in the battery.
DCripple voltage is limited to less than 0.5% RMS to preserve batteryl ifeduring long-term float charging while
theUPS system is operating on utility source power.
MultipleUPS module rectifier/chargers can share a common battery plant,if that configuration is preferred for
yourapplication.
Operation After Discharge
Whencommercial power is interrupted, the battery continues to supply DC power to the inverter withoutinter-
ruptionto the critical load. If the AC sourcepower is restored before the battery has fully discharged,the rectifier
automaticallyrestarts and resumes carrying the inverter and batteryrecharge load requirements.
DISCONTINUED
PROD U C T