The different types of UPS systems
There is a sub-type of the common Standby design, the Ferrous-Resonant type of UPS. Although it is very similar to the Standby design, the Ferrous-Resonant UPS is using (as the name suggests) a Ferro-resonant transformer to filter the output. This transformer turns the Standby UPS into an "online" type of unit and has limited voltage regulation and waveform shaping.
Although this type of UPS used to be dominant a few years ago, it is no longer being used to power high technology systems because it is relatively unstable and creates severe voltage distortions and extreme transients. The output quality of such types of UPS systems can be worse than very poor utility AC grids. Other disadvantages include the large and very heavy construction compared to other types of systems and the generation of great deals of heat because Ferro-resonant transformers are innately inefficient.
Even though this type of UPS has disadvantages which prevent it from becoming a widely used home / small business product, it has its own set of advantages which makes it relatively popular for small industrial applications. It is a very robust design which also is considerably cheaper than any other online UPS type and offers excellent electrical isolation, making ideal for loads which are not easily affected by voltage fluctuations and/or for loads which need to be isolated by the main utility power supply; however this kind of UPS did not expand a lot and even the biggest units are limited to 100-150kVA applications because it exhibits major problems while cooperating with power factor correction equipment.