18 Theoryof Operation
2.2.4 Inverter
Theinverter is a solid state device that converts the DC output ofthe rectifier/charger or the battery to AC power.
Operation
Theinverter converts DC power—from either the battery or the rectifier/charger—intothree pulse-width-modu-
lated/six-stepwaveforms. These waveforms are filtered into low-distortion sinew avepower. The inverter is con-
trolledby an Application-Specific Integrated Circuit(AS IC).This integrated circuit controls the precise
synchronization,amplitude, and frequency of the output voltage.
Inaddition to the inverter efficiently supplyinga regulated AC output from a DC source, the output isolation
transformeracts as a second stageof isolation between thecritical load bus and the commercialsource power.
Theinverter is configured to handle most criticalload inrush surges. It maintains outputvoltage Total Harmonic
Distortion(THD) within specifications even when handling non-linear computerloads. Refer to non-linear load
characteristics.
Output Regulation and OverloadPerformance
Theinverter is capable of sustaining full output voltage±2% of the nominal voltage) for up to 150% overload at
theoutput for as long as 30 seconds without reducing the output voltage.It can also handle at least 125% of the
ratedcurrent for up to 10 minutes and 104% of the rated current continuously.The System Control Cabinet
(SCC)will transfer the critical load to thebypass source if the overload exceeds the system capacity orthe capac-
ityof the modules on-line. The multi-module overload capacity (in termsof pe rcentof rated full load current of
moduleson-line) is 140% for 20 seconds, 108% for 10 minutes,and 104% continuously.
Notethat in normal operation the SCC will transfer the loadto bypass before a UPS module exceeds its overload
capacity.If a module overloadcondition is sustained (beyond inverter capacity)the UPS module is automatically
takenoff-line (disconnected from the criticalbus) andis shut down. If the UPS system is operating in the redun-
dantmode, the load may remain on the UPS system. Refer to 3.4 - Automatic Operations.
Non-Linear Load Characteristics
Computersand computer equipment with switching power supplies generate non-linear currentsrich in fifth and
seventhharmonics.
Theinverter pulse-width-modulated/six-step waveform, coupled with the output filter,provides a natural path
forreducing the fifth and seventh harmonic currents produced by the load. The inverter/filterlimits the output
voltageTHD to less than 5% with up to 100% typical electronic data processing (EDP) loads. EDP equipment
characteristicallyincludes both non-linear and linear loadcomponents.
Unbalanced Load Characteristics
Unbalancedloads are actively regulated. The phase-to-phase voltagebalance is maintained to within 2%, even
witha 50% load imbalance.
Thethree-phase, root mean square (RMS—approximately 70% of the peak)average voltage is also regulated
througha separate control circuit (phase-to-phase sensing).
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