Understanding UPS OperationEaton 93PM UPS (60–100 kW, 480V) Installation and Operation Manual P-164000227—Rev 2 www.eaton.com/powerquality 5-3Figure 5-2. Path of Current Through the UPS in Energy Saver System Mode5.2.3 Normal ModeDuring Normal mode, power for the system is derived from a utility input source through the rectifierswitchgear. Three-phase AC input power is converted to DC using IGBT devices to produce a regulated DCvoltage to the inverter. When the battery switchgear is closed the battery is charged directly from the regulatedrectifier output through a buck or boost DC converter, depending on the system voltage and the size of thebattery string attached to the unit.Figure 5-3 shows the path of electrical power through the UPS system when the UPS is operating in Normalmode.The battery converter derives its input from the regulated DC output of the rectifier and provides either aboosted or bucked regulated DC voltage charge current to the battery. The battery is always connected to theUPS and ready to support the inverter should the utility input become unavailable.The inverter produces a three-phase AC output to a customer's load without the use of a transformer. Theinverter derives regulated DC from the rectifier and uses IGBT devices and pulse-width modulation (PWM) toproduce a regulated and filtered AC output. The AC output of the inverter is delivered to the system outputthrough the inverter switchgear.If the utility AC power is interrupted or is out of specification, the UPS automatically switches to Battery modeto support the critical load without interruption. When utility power returns, the UPS returns to Normal mode.If the UPS becomes overloaded or unavailable, the UPS switches to Bypass mode. The UPS automaticallyreturns to Normal mode when the overload condition is cleared and system operation is restored withinspecified limits.If the UPS suffers an internal failure, it switches automatically to Bypass mode and remains in that mode untilthe failure is corrected and the UPS is back in service.StaticSwitchBypass Input StaticSwitchBypass SwitchgearRectifier InverterInverterSwitchgearRectifierSwitchgearRectifier Input OutputBattery ConverterBatterySwitchgearMain Power FlowSwitchgearClosedOpenBatteryTrickle CurrentDe-EnergizedEnergized