AC Generators or Alternators
A 3 phase ac generator is called an "Alternator". Like a DC generator, it also consists of two windings namely, (1) Field Winding (2) Armature Winding.
However, unlike in a DC Generator, its poles are on rotor and armature winding is on stator.
The field winding on poles requires DC current which is supplied by using a pair of slip rings on the shaft and a pair of stationary carbon brushes. The number of poles is always even. All the poles are identical and symmetrically fitted on the rotor with alternate polarities i.e., a "North" pole is followed by a "South" pole which in turn is followed by another "North" pole and so on.
The stator is made up of thin laminations of alloy. Laminated structure is used to reduce magnetic losses and also to obtain convenience in the construction. These laminations (also known as stampings) have been punched around the internal periphery to produce slots. Copper windings are placed in these slots. It is known as "Armature Winding" and "Stator Winding". It is a three phase winding generally in 'star'. All the three terminals of the star connection, including the fourth terminal the neutral are brought out in the terminal box of the machine.
The generator is driven by its engine in the correct direction at its rated speed (RPM). DC Field current is then supplied to its field winding through the brush and slip ring arrangements. The poles produce sufficiently strong magnetic flux. Since the poles are rotating, the magnetic field is also rotating. The armature windings in the stator windings cut the flux of the rotating poles. Therefore electric power is produced in the stator winding. Note that the frequency of the electric power is kept constant at 50 Hz (or at 60 Hz) by keeping the generator speed constant.
"Basic Marine Engineering" by J.K. Dhar