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A typical AC generator consists of
a stationary stator and a rotor mounted within the
stator (see below: Typical AC Generator). The stator contains a specific
number of coils, each with a specific number of
windings. Similarly, the rotor consists of a specific
number of field poles, each with a specific number of
windings. In addition to the rotor and stator, a generator
has a collector assembly (usually consisting
of collector slip rings, brushes, and brush holders).
DC
flows from the exciter, through the negative brush
and slip ring, to the rotor field poles. The return
path to the exciter is through the positive brush and
slip ring.
Typical AC Generator
Rotor - The rotor contains magnetic fields which
are established and fed by the exciter. When the
rotor is rotated, AC is induced in the stator. The
changing polarity of the rotor produces the alternating
characteristics of the current. The generated
voltage is proportional to the strength of the magnetic
field, the number of coils (and number of windings
of each coil), and the speed at which the rotor
turns.
Stator - The frame assembly is the main component of the stator. Insulated windings (or coils) are
placed in slots near an air gap in the stator core.
There is a fixed relationship between the unit’s
number of phases and the way the coils are connected. The stator in a four-wire, three-phase unit
has three sets of armature coils which are spaced
120 electrical degrees apart. One end of each coil is
connected to a common neutral terminal. The other end of each coil is connected to separate terminals.
Conductors attached to the four terminals carry the
current to the system’s switchgear and on to the
load.
Collector slip rings - Slip rings are usually
made of nonferrous metal (brass, bronze or copper);
iron or steel is sometimes used. Slip rings usually
do not require much servicing. The wearing of
grooves or ridges in the slip rings is retarded by
designing the machine with limited end-play and by
staggering the brushes. Surfaces of the slip rings
should be bright and smooth, polishing can be performed
with fine sandpaper and honing stone. Electrolytic
action can occur at slip ring surfaces producing
formation of verdigris. Verdigris is a
greenish coating that forms on nonferrous metals. Electrolytic deterioration can be prevented by reversing
the polarity of the slip rings once or twice a
year. The stator of the three-wire, three-phase unit
also has three sets of armature coils spaced 120
electrical degrees apart. The ends of the coils are
connected together in a delta configuration. Conductors
are attached to the three connecting points.
References: “Joint Departments of the Army and the
Navy, Operation Maintenance and Repair
of Auxiliary Generators, 26 August 1996”
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