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Ferroresonant Standby UPS
A ferroresonant standby UPS is an improvement on the design of the standby UPS. As with a standby UPS, the primary power source is line power from the utility, and the secondary power source is the battery. The battery and inverter are still waiting "on standby" until needed. The big difference is that the standby UPS's transfer switch that selects between power sources has been replaced by a ferroresonant transformer.
A transformer is a core of metal around which are normally wrapped two coils of wire. When current is applied to one coil, it is magnetically coupled to the other coil and a current is induced in the second coil. Transformers are normally used to change one AC voltage into another. However, the other effect of a transformer is that it tends to smooth out power problems in its input, and not pass them on to the output. This makes them well-suited for use as a line filtering component in a UPS.
The transformer used in this type of UPS is not a regular transformer however, because it has three windings, as shown in the diagram above. Two of the windings are input, from the primary and secondary power sources, and the third is the output. This allows the transformer itself to act as a sort of switch, because whichever input source is operating, the output will take its energy (the transfer switch in the diagram above just isolates the line power connection when the power goes out). Even better, however, is that since the core of the ferroresonant transformer stores energy in its magnetic field, this acts a buffer in the event that the primary power cuts off and the switch must be made to the secondary (battery) power source. For a fraction of a second, the output will function off the energy in the transformer itself. This stored energy greatly reduces the chances of the protected equipment being affected during the transfer time from the primary to secondary sources.
Ferroresonant standby UPSes are usually available in a size range of up to about 15,000 VA, making them suitable for even the largest servers. Once one of the most common designs, the ferroresonant UPS is still made by some manufacturers, while others have abandoned this design, claiming that it is inefficient, in favor of the online UPS for models in the range of 1,000 VA or higher.
Next: Line-Interactive UPS