An AC generator is a machine that converts mechanical energy into electrical energy, generated as an alternating current sinusoidal output waveform. The mechanical energy is usually supplied by steam turbines, gas turbines or internal combustion engines; however, with current interest in more sustainable power, the drive is increasingly likely to come instead from wind, hydropower or tidal resources.
AC generators rely on Faraday’s law of electromagnetic induction, which says that electromotive force -EMF or voltage – is generated in a conductor that cuts a uniform magnetic field. This can be achieved either by rotating a conducting coil in a static magnetic field, or by rotating the magnetic field that contains the stationary conductor. The normal arrangement is to keep the coil stationary while rotating the magnetic field, because drawing induced alternating current is easier from a stationary armature coil than from one that is rotating.
The EMF generated depends on the magnetic field strength, the number of armature coil turns and the speed of the rotating field.
AC generators are also known as alternators, and are classified according to their rotor construction. Salient pole or projected pole rotors are used for medium speed alternators, operating at around 300 RPM, as found in hydropower plants. Cylindrical type rotors are more suitable for higher-speed alternators, operating at around 3000 RPM, as used in steam or nuclear power stations.