System Control Principle - Emerson Liebert APM 90 User Manual

electronics, which make the inverter frequency track that of the static bypass supply, provided that the bypass
remains within an acceptable frequency window.
A manually controlled, maintenance bypass supply is incorporated into the UPS design. It enables the critical
load to be powered from the maintenance bypass supply while the UPS is shut down for routine maintenance
and repair.
When the UPS is operating in bypass mode or on maintenance bypass, the connected equipment is not protected from
power failures.

1.2.3 System Control Principle

Normal operation
Normal mode: It means that the UPS has normal input mains, the rectifier and inverter operate normally, the
load is supplied by the inverter, the BCB is closed, and the battery is in stable floating charge state.
Mains abnormal
When the mains fails or is abnormal, the rectifier will stop working automatically, and the system will transfer
to battery output (through inverter). The length of the operation time in battery mode depends on the load
and the battery capacity. During this period, if the battery voltage falls to the EOD voltage and the mains still
has not been recovered, the inverter will stop working automatically, and the UPS operation control panel will
display corresponding alarm message.
Mains recovery
When the mains resumes normal within allowable time, the rectifier will start automatically (at this time its
output power will increase gradually) and supply the load and charge the battery again. Therefore, the power
supply to the load will not be interrupted.
Battery disconnection
To disconnect the battery from the UPS system for maintenance, the external isolating switch can be used. At
this time, except for the mains failure battery backup function, other functions and all the steady state
performance of the UPS will not be affected.
UPS Single Module failure
In case of an inverter failure, rectifier fault or output fuse blow-out, the load will be automatically transferred
to the bypass, and the output power supply will not be interrupted. In this case, please contact the local
customer service center of Emerson for technical support.
Overload
If the inverter is overloaded or the inverter current remains outside the specifications (refer to Table 11-6)
longer than the specified time, the load will be automatically transferred to the bypass without power
interruption. If both the overload and the current are reduced to a level within the specified range, then the
load will be transferred back to the inverter. In case of output short circuit, the load will be transferred to the
bypass, and the inverter will shut down. Five minutes later, the inverter will start up automatically. If at this
point the short circuit is removed, the load will be transferred back to the inverter. The transfer is determined
first of all by the features of the protection device of the system.
In the above two situations, the UPS operation control panel will display alarm message.
Liebert APM 90 Integrated Modular UPS User Manual
Note
Chapter 1 Overview 3