Understanding UPS OperationEaton 93PM UPS (20–100 kW, 480V – 100 kW Frame) Installation and Operation Manual P-164000227—Rev 6 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 a buckedregulated DC voltage charge current to the battery. The battery is always connected to the UPS and ready tosupport 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 and is configured for capacity, it switches automatically to Bypass modeand remains in that mode until the failure is corrected and the UPS is back in service.StaticSwitchBypass Input StaticSwitchBypass SwitchgearRectifier InverterInverterSwitchgearRectifierSwitchgearRectifier Input OutputBattery ConverterBatterySwitchgearMain Power FlowSwitchgearClosedOpenBatteryTrickle CurrentDe-EnergizedEnergized
