An alternating-current generator that consists of one or more coils that are made to rotate in the magnetic fields produced by several electromagnets excited by a direct-current source. The frequency, f, of the alternating currents and emfs induced in the coils is equal to the product of the speed, n_s, at which the coils rotate and the number of pairs of magnetic poles, p. This type of generator can operate and produce electrical power independently of any other source of alternating current and is the type most commonly employed in power stations.