BASIC AC GENERATORS
Regardless
of size, all electrical generators, whether dc or ac, depend upon the principle
of magnetic induction. An emf is induced in a coil as a result of (1) a coil
cutting through a magnetic field, or (2) a magnetic field cutting through a
coil. As long as there is relative motion between a conductor and a magnetic
field, a voltage will be induced in the conductor. That part of a generator
that produces the magnetic field is called the field. That part in which the
voltage is induced is called the armature. For relative motion to take place
between the conductor and the magnetic field, all generators must have two mechanical
parts — a rotor and a stator. The ROTor is the part that ROTates; the STATor is
the part that remains STATionary. In a dc generator, the armature is always the
rotor. In alternators, the armature may be either the rotor or stator.
ROTATING-ARMATURE
ALTERNATORS
The
rotating-armature alternator is similar in construction to the dc generator in
that the armature rotates in a stationary magnetic field as shown in figure
3-1, view A. In the dc generator, the emf
generated
in the armature windings is converted from ac to dc by means of the commutator.
In the alternator, the generated ac is brought to the load unchanged by means
of slip rings. The rotating armature is found only in alternators of low power
rating and generally is not used to supply electric power in large
quantities.
fig(1) |
ROTATING-FIELD
ALTERNATORS
The
rotating-field alternator has a stationary armature winding and a
rotating-field winding as shown in figure (1), view B The advantage of having a
stationary armature winding is that the generated voltage can be connected directly
to the load.
A
rotating armature requires slip rings and brushes to conduct the current from
the armature to the load. The armature, brushes, and slip rings are difficult
to insulate, and arc-overs and short circuits can result at high voltages. For
this reason, high-voltage alternators are usually of the rotating-field type. Since
the voltage applied to the rotating field is low voltage dc, the problem of
high voltage arc-over at the slip rings does not exist.
The
stationary armature, or stator, of this type of alternator holds the windings
that are cut by the rotating magnetic field. The voltage generated in the
armature as a result of this cutting action is the ac power that will be
applied to the load.
The
stators of all rotating-field alternators are about the same. The stator
consists of a laminated iron core with the armature windings embedded in this
core as shown in figure (2). The core is secured to the stator frame.
figure (2) |
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