Constant Voltage Transformers

Ferro Resonant type Constant Voltage Transformers - CVT, the AC mains power the input winding, which is widely separated physically from the isolated output winding. The input winding normally runs at very moderate Flux linkage levels. The output winding exhibits an intrinsic energy characteristic and this energy storage operate in conjunction with mains capacitor to produce self-generated AC flux Field which is indirectly extracted from the Input Winding.
The ferroresonant action is a flux limiter rather than a voltage regulator, but with a fixed supply frequency it can maintain an almost constant average output voltage even as the input voltage varies widely. 
All problems related to variation / fluctuation in Voltages is effectively handled because of this principle and a constant voltage output of ± 1% is given.