Almost all three-phase power is generated by three-phase synchronous machines operated as generators. Synchronous generators are also called alternators and are normally large machines producing electrical power at hydro, nuclear, or thermal power plants. Ef™ciency and economy of scale dictate the use of very large generators. Because of this, synchronous generators rated in excess of 1000 MVA (mega-volt-amperes) are quite commonly used in generating stations. Large synchronous generators have a high ef™ciency which at ratings greater than 50 MVA usually exceeds 98%. The term synchronous refers to the fact that these machines operate at constant speeds and frequencies under steady-state operations.