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Understanding UPS Operation
5.2
Single UPS
A single UPS operates independently to support an applied load from the inverter, providing conditioned and
uninterruptible AC power to the critical load from the output of the module. During an outage, the inverter
continues to operate, supporting power to the load from the battery supply. If the unit requires service, applied
loads are transferred to the internal bypass either automatically or manually. With the exception of an optional
external battery cabinet, no other cabinets or equipment are required for the single UPS to successfully support
its applied loads.
5.2.1
Modes
The Eaton 93PMUPS supports a critical load in four standard modes of operation and one optional mode:
In Energy Saver System (ESS) mode, commercial AC power is supplied directly to the critical load through
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the internal bypass and transfers automatically to Normal mode (double-conversion on demand) if an
abnormal condition is detected. The battery charger also provides charging current for the battery, if needed.
In Normal (double-conversion) mode, the critical load is supplied by the inverter, which derives its power
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from rectified utility AC power. In this mode, the battery charger also provides charging current for the
battery, if needed.
In Bypass mode, the critical load is directly supported by utility power. The battery charger also provides
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charging current for the battery, if needed.
In Battery mode, the battery provides DC power, which maintains inverter operation. The battery supports
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the critical load.
The following paragraphs describe the differences in the UPS operating modes, using block diagrams to show
the power flow during each mode of operation.
5.2.2
Energy Saver System Mode
When the UPS is operating in ESS mode, commercial AC power is supplied directly to the critical load through
the internal bypass with the power module in a standby state. Surge protection and some filtering is also
provided, ensuring that clean power is delivered to the load equipment. If a commercial power brownout,
blackout, overvoltage, undervoltage, out-of-tolerance frequency condition occurs, or any other disturbance is
detected in the incoming power, the system forward transfers to normal mode (double-conversion on demand).
In case of a complete utility outage or if input power is outside the tolerances of the system, the UPS transfers
to battery mode and continues to supply conditioned, clean power to the critical load. When the input line
returns to normal operating range, the UPS returns to ESS operation. For charging batteries, the UPS will not
enter ESS mode until charger is 25% or less of maximum charger current.
When operating in ESS, the UPS superior detection and control algorithms continuously monitor incoming
power quality and allow the fast engagement of power converters. Typical transition time to double conversion
is less than two milliseconds, which is practically seamless. If the UPS detects a fault condition while operating
in ESS, it is also able to detect and determine whether the fault is caused by the load or if it is upstream from
the UPS. A fault at the bypass source results in an immediate switch over to the inverter; a fault in the load
keeps the UPS in Energy Saver System (ESS) mode.
When power conditions are within acceptable limits, the UPS operates as a high-efficiency, energy-saving
system — providing surge protection for IT equipment and ensuring clean power is delivered to the facility.
Energy saver system increases system efficiency to 99% at Minislot100% linear load and nominal Vin.
Figure 5-2 shows the path of electrical power through the UPS system when the UPS is operating in Normal
mode.
ESS mode is a normal operating mode, and not an alarm condition. While the UPS is in this mode, the NORMAL
light on the front display will illuminate.
5-2
Eaton 93PM UPS (20–50 kW, 480V Four Wire – 50 kW Frame) Installation and Operation Manual P-164000540—Rev 4
www.eaton.com/powerquality
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