Adjustable frequency controller or AFC
A device that is used to control the speed of an AC motor by varying the frequency of the voltage supplied to it. These are also known as adjustable speed drives, variable frequency drives, adjustable frequency drives and other, similar, names.
Source: API RP 14F, Design, Installation, and Maintenance of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class 1, Division 1 and Division 2 Locations, Fifth Edition, July 2008. Global Standards