UPS Bypass Transformer

A ferroresonant transformer runs with the core always in saturation which produces a very precise regulated output. Since it is saturated, the output will be a square wave. That works great for DC power generating circuits but generally is very undesirable for normal AC applications. Almost everything AC is designed for sign waves which are very expensive to produce from a square wave source. Most motors will not run on square waves. Most dimmers, etc won't either. It overheats most computer power supplies if they will work at all.It does however do a pretty good job of filtering most higher frequency noise coming from the input.

Faraday shielded transformers run as a normal transformer in all respects. The Faraday shield keeps high frequency noise (especially common mode) from coupling from the primary to the secondary. Most isolation transformers are Faraday shielded for this reason.

Bottom line is stay away from ferroresonant transformers if you're not sure.