Chapter 1 Overview 5
Liebert APM 90 Integrated Modular UPS User Manual
Normal mode
As shown in Figure 1-3, the UPS rectifiers derive power from the AC mains input source and supply DC power
to the inverters, which continuously supply the AC load. Simultaneously, the charger, which derives power
from the rectifiers, float or boost charges the associated backup battery of the UPS.
Rectifier input switch
Mains input
Battery charger
Output switch
Inverter
Rectifier
UPS output
Battery
Automatic inverter switch
Mains input
Rectifier input switch
Rectifier
Battery
Battery charger
Inverter
Automatic inverter switch
Output switch
UPS output
Input switch
Figure 1-3
Schematic diagram of normal mode
Battery mode
As shown in Figure 1-4, the UPS is operating in battery mode when the battery is supplying backup power to
the load through the inverters. Upon mains failure, the UPS automatically transfers to battery mode without
power interruption to the load. Upon restoration of the AC mains, the UPS automatically transfers back to
normal mode without the necessity of user intervention, without power interruption to the load.
Note: Battery start function is available for switching the UPS on into Battery (charged) mode directly during
mains failure. Thus, the battery power can be used independently to increase the UPS utility.
Output switch
Inverter
Rectifier
UPS output
Battery
Automatic inverter switch
Rectifier
Inverter
Automatic inverter switch
Output switch
UPS output
Battery
Figure 1-4
Schematic diagram of battery mode
Bypass mode
As shown in Figure 1-5, during normal mode operation, if the inverters fail, are overloaded or turned off, the
static switch will perform a transfer of the load from the inverters to the bypass source, with no interruption in
power to the load. Should the inverters be asynchronous with the bypass, the static switch will perform a
transfer of the load from the inverters to the bypass, with interruption in power to the load. This is to avoid
paralleling of unsynchronised AC sources. This interruption is programmable but typically set to be less than
3/4 of an electrical cycle, for example, less than 15ms (50Hz) or less than 12.5ms (60Hz).
Input switch
Mains input
Static switch
Output switch
UPS output
Input switch
Mains input
Static switch
Output switch
UPS output
Figure 1-5
Schematic diagram of bypass mode
