Editing Uninterruptible power supply (section)

===Ferroresonant===

Ferroresonant units operate in the same way as a standby UPS unit; however, they are online with the exception that a [[Voltage regulator#Constant-voltage transformer|ferroresonant transformer]], is used to filter the output. This transformer is designed to hold energy long enough to cover the time between switching from line power to battery power and effectively eliminates the transfer time. Many ferroresonant UPSs are 82–88% efficient (AC/DC-AC) and offer excellent isolation.

The transformer has three windings, one for ordinary mains power, the second for rectified battery power, and the third for output AC power to the load.

This once was the dominant type of UPS and is limited to around the {{nowrap|150 kVA}} range. These units are still mainly used in some industrial settings (oil and gas, petrochemical, chemical, utility, and heavy industry markets) due to the robust nature of the UPS. Many ferroresonant UPSs utilizing controlled ferro technology may interact with power-factor-correcting equipment. This will result in fluctuating output voltage of the UPS, but may be corrected by reducing the load levels or adding other linear type loads.{{Elucidate|date=May 2012}}