|
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.
|
