Range of UPS Products from APC by Schneider Electric

An online or double conversion UPS takes the input AC (Alternating Current) mains and supplies a rectifier which then converts this AC source into DC (Direct Current). The batteries are then usually connected to the DC bus created by the rectifier. Charging of the batteries can be done directly via the DC bus or via separate DC Charging circuit.

The DC supply from the rectifier supplies the Inverter section so that the load is always protected by the inverter and thus maintains a constant output supply regardless of the mains input voltage. Whilst the inverter tracks and synchronises its output waveform to that of the incoming supply the output voltage and frequency are maintained at a constant preset value. The ability to synchronise with the mains allows the UPS to switch from inverter to mains supply seamlessly in the event of a fault, overload or for maintenance purposes.

In the event of a mains outage the DC Bus and inverter are supplied directly from the batteries without any break in output supply.

The conversion from AC to DC and back to AC offers the best protection from all mains deviations and is where the term double conversion is derived.

A line-interactive UPS keeps the inverter online at all times and simply switches from charging mode to battery power in the event of a power failure.

With this topology, the UPS inverter is always online and thus the protected load benefits from a constant preset output voltage. When the input AC power (mains supply) is within tolerance, the UPS inverter acts in reverse and is able to charge the batteries. If the input power fails, the transfer switch will open and the power will flow from the battery, through the inverter to the UPS load. A line interactive UPS also incorporates an automatic voltage regulator (AVR) which utilises  transformers to either boost or reduce the incoming supply should it fall just outside of the preset input tolerances. This feature prevents the batteries from being discharged when the input source is unsteady or unreliable but only slightly outside of an acceptable voltage window. There will also be additional filtering and there is no break in output supply in the event of a mains disturbance as the inverter is always connected to the critical load.

In an off-line or standby UPS system, the critical load is powered directly by the utility or mains power during normal operation.

The UPS monitors the incoming supply and the inverter, which is supported by a battery system, is only activated once the input voltage falls outside of preset voltage or frequency tolerance windows.

There is a very short break in supply (typically 2-3 mS) while the inverter transfers online. This break in power is fast enough that almost all electrical and electronic equipment supported will continue to operate unaffected. The UPS incorporates a surge suppressor and in-line filter to condition the mains supply fed to the load although any variations in input voltage and frequency are not adjusted.