Q.How are the three different emfs generated in a three-phase AC generator? Show the graphical representation of these three emfs.

Construction. In the simplified three-phase design, six armature conductors sit in six slots spaced $60^{\circ}$ apart around the core; conductors 1 & 4 form coil 1, 3 & 6 form coil 2, and 5 & 2 form coil 3, so the three resulting coils end up mechanically $120^{\circ}$ apart from one another around the core.

The three emfs. As the field magnet rotates, coil 1's emf $\varepsilon_1$ begins its cycle at t = 0; because coil 2 sits $120^{\circ}$ around the core, its identical emf $\varepsilon_2$ begins its cycle only after the field magnet rotates a further $120^{\circ}$; and coil 3's emf $\varepsilon_3$, a further $120^{\circ}$ around, begins after $240^{\circ}$ total rotation. All three are identical sine waves in amplitude and frequency, simply shifted by a fixed $120^{\circ}$ phase from one another -- this fixed $120^{\circ}$ separation between three otherwise-identical alternating emfs is the defining feature of a three-phase system.

Poly-phase generators, generally. A generator carrying more than one armature coil, each producing its own separate emf, is called a poly-phase generator; two coils give a two-phase generator, three coils give the three-phase generator used almost universally in commercial power systems. …