How are ac voltage generated into waveform?

A.C. (alternating current) is typically generated by converting mechanical, rotational energy, into electrical energy, with an alternator. The alternator has a rotating electrical field (rotor) which has two or more magnetic poles. The magnetic field of the rotor is produced by d.c. current through coils wound on the rotating iron core of the rotor. At minimum, the rotor has two magnetic poles, positive and negative.

The rotating magnetic field is magnetically coupled to coils that are wound inside the outer case of the alternator. These coils are called the stator, and they too are wound on an iron core that is built up of many thin laminations. There are two stator coils in a single-phase alternator and three in a three phase alternator. When the positive and negative poles of the rotor sweep past the stator coils, they cause electrical current in the stator coils to move in a particular direction through each coil and connected electrical circuit. The direction in each coil reverses when it becomes magnetically coupled to the opposite magnetic pole of the stator.

To produce a generated waveform of the proper frequency (60 Hz for the U.S. power grid), the alternator has to be rotated at a precise speed, 3600 rpm for an alternator with two poles, and 1800 rpm for an alternator with four poles. Alternators have been built with 20 or more poles for very low rpm machines.