There are essentially 3 main types of UPS available, Standby, Online and Line Interactive.
This type of UPS is the simplest, with its alternative name being an offline UPS. It provides basic surge protection and limited battery backup when the mains voltage drops below an acceptable level, but will not protect against extended periods of mains power loss.
This is a sophisticated UPS which can react to variations in input voltage by adding or subtracting coils to ensure the output voltage is maintained at a constant level. An inverter converter is employed to firstly charge the battery and secondly to convert the DC output from the batteries to AC to supply the load when mains power fails. Because this system is permanently connected to the output load it can respond extremely quickly to an fluctuations or loss of mains power. There are two types of line interactive UPS, namely Modified Sine Wave and True Sine Wave.
Modified Sine Wave systems are generally small in size with fairly small outputs up to 3Kva, which are suited to PC and Server installations in an office or data centre.
True Sine Wave systems are high end systems where the load is serviced by the mains supply at the same time as the batteries are being charged. There is a voltage stabiliser which is correcting fluctuations in voltage without the need to switch over to battery power. Batter power is only used when there is a total brownout.
These systems tend to be Dual Conversion Online UPS which have near zero transfer time and also provide isolation for the load devices from the mains supply. Just like the Line Interactive systems, they correct fluctuations in power without drawing power from the batteries. The batteries are permanently connected through the inverter to the load so that when power fails the converter is disconnected from the circuit and the batteries take over instantaneously. When the utility power is restored the converter is re-connected to the load devices and charging of the batteries is also commenced.
Uninterrupted Power Supplies are discussed in our Networking and Systems 2-Day Instructor Led Training Course.