commercial electric utility power stations are usually constructed on a large scale and designed for continuous operation. virtually all electric power stations use three-phase electrical generators to produce alternating current (ac) electric power at a frequency of 50 hz or 60 hz. large companies or institutions may have their own power stations to supply heating or electricity to their facilities, especially if steam is created anyway for other purposes. steam-driven power stations have been used to drive most ships in most of the 20th century until recently. steam power stations are now only used in large nuclear naval ships. shipboard power stations usually directly couple the turbine to the ship's propellers through gearboxes. power stations in such ships also provide steam to smaller turbines driving electric generators to supply electricity. nuclear marine propulsion is, with few exceptions, used only in naval vessels. there have been many turbo-electric ships in which a steam-driven turbine drives an electric generator which powers an electric motor for propulsion.