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.
Marine Engineering" by J.K. Dhar
ęCopyright Jabelu Firoz - 2010
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Last updated: April 2010.