The Three-phase alternator
In this case, three separate sets of coils are wound on the stator and connected to produce three separate outputs. Because of the physical displacement of the coils around the stator, the outputs will have different phases with respect to each other. Three-phase systems are common even when DC outputs are required (e.g. electric alternators to charge a car battery) because of the torque reaction problem mentioned above. The AC output is rectified with a set of six diodes mounted in a single pack (two diodes are needed to rectify each phase).
A two-pole, three-phase alternator is shown in the diagram below. Stator coils AB belong to phase 1, coils CD to phase 2 and coils EF to phase 3. Each pair of coils are connected in series and their outputs add (e.g. A and B are connected in series to produce phase 1 output). Notice it is two-pole because there are only two poles (i.e. one pole pair) on the rotor. The frequency of each phase output is therefore equal to the rotational speed of the rotor.
Unlike the single-phase alternator, it turns out that the torque required to drive a three-phase alternator is constant, with no pulsating component. This is an important advantage.The figure below shows the arrangement for a multipole three-phase alternator. Again, coils A&B, C&D and E&F are connected in series to three differnet outputs, each with a phase shift of 120° apart (because of the physical arrangement of the coils on the stator).