AC Generators:

        In order to take the ac output from a generator, the ends of
the coil must be attached to Slip Rings which rotate along with
the armature. Stationary conductors called Brushes are placed in 
contact with the slip rings and allowed to slide along the surface as
the rings rotate. In this way, brushes and slip rings provide a connection
between the armature coil and an external load which uses the power
supplied by the generator. Brushes usually contain carbon or graphite
to maintain low friction without cutting into the metal slip rings. 
As the brushes ride on the slip rings, they slowly wear away and must
be replaced periodically. Since the brushes wear to the shape of the 
slip rings, the resistance between them and the slip rings can be
very low.

        AC generators are also called Alternators. Alternators
provide most of the electric power used in homes and industry. These
machines are very large and generate large amounts of power. As pointed
out previously, a voltage may be induced in a conductor regardless of
whether the conductor moves or the field moves. The field winding is
called the Stator, and the output voltage is taken from the 
armature or rotor. However, with larger machines where large amounts
of power are generated, it is impractical to take the output power
through the slip rings and brushes. The slip rings and brushes are
better suited to low power due to the special nature of the materials
used. We know that a small magnetic field will produce a large voltage
if a large number of conductors cut the lines of force, or if the lines
of force are cut at great speed. Therefore, many large alternators
reverse the roles of the stator and rotor. In these machines, a small
dc voltage is applied through the brushes and slip rings to a rotating
Electromagnet. The DC voltage is known as DC Excitation.
The electromagnet produces the field, and, instead of turning the 
armature winding, the rotor turns the field winding. The field coil
is wound on a soft iron core and, with a steady dc voltage applied to
the slip rings, a steady electromagnetic field is produced. The 
Armature coils are then wound on the Stator. The rotor
can be driven at high speed, or a large number of armature windings
can be used, and the high voltage can be conveniently taken from
the stationary armature windings. Since relative motion is produced
between the rotating field and the stationary armature in the same
way as for the reverse arrangement, the output is ac just as in the
previous example.

        Generators which operate on the principles of electromagnetic
induction operate with a steady field which is normally provided by
DC excitation from an external source. The shaft is driven by a 
mechanical force at a constant speed of, 1,800 or 3,600 rpm. A 
synchronous alternator is the type used to produce electricity for
ordinary household purposes.

        The output ac frequency of an alternator is determined by the
speed of the rotor and the number of poles in the alternator. The
formula for determining the ac frequency is:

                     P x rpm
                F = ----------
                       120
                F = the frequency in hertz
                
                P = the number of magnetic poles in the alternator

                rpm = the rotor speed of the alternator in 
                       revolutions per minute.
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