The Line - Interactive UPS is similar in operation to a Standby UPS, but with the addition of a multi-tap variable – voltage autotransformer. This is a special type of Transformer that can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field and the Output Voltage of the Transformer. This is also known as Buck-Boost Transformer.
This type of UPS is able to tolerate continuous Under Voltage brownouts and Over Voltage surges without consuming the limited reserve battery power. It instead compensates by automatically selecting different power taps on the autotransformer. Depending on the design, changing the autotransformer tap can cause a very brief output power disruption, which may cause UPSs equipped with a power – loss alarm to “chirp” for a moment.