A prime mover (an engine or turbine) spins a magnet inside a coil of wire. Most of the time electro magnets are used instead of permanent magnets as the strenght of the electomagnet can be controlled electronically. Controlling the strenght of the magnetic field allows control over how much voltage is generated. This method is used to regulate the AC voltage produced so as to keep it a constant voltage.

The speed at which the magnet spins determines the frequency of the AC produced.

by cutting the magnetic flux produce by the magnets usually 2 magnets that will produced opposite poles (north and south), cutting magnetic flux by rotating object like turbines.. cutting magnetic flux produce current ( amperes).... AC gens.composed of coil or magnets, rotating object that wil cut the magnetic flux made by the coil or magnet..... this principle is used in some hydroelectric power plant like dam....