AC GENERATORS
Alternating-current generators produce most
electric power used today. Ac generators are also used
in aircraft and automobiles.
Ac generators come in many different sizes,
depending on their intended use. For example, any one
of the huge generators at Boulder Dam can produce
millions of volt-amperes, while the small generators
used on aircraft produce only a few thousand
volt-amperes.
Regardless of their size, all generators operate on
the same basic principle-a magnetic field cutting
through conductors, or conductors passing through a
magnetic field.
All generators have at least two distinct sets of
conductors:
A group of conductors in which the output
voltage is generated known as the armature
winding.
A second group of conductors through which
direct current is passed to obtain an
electromagnetic field of fixed polarity known as
the field winding.
Since relative motion is needed between the
armature and field flux, ac generators are built in two
major assemblies—the stator and the rotor (fig. 3-11).
Figure 3-11.-An ac generator and schematic.
The rotor rotates inside the stator. It is driven by several
commonly used power sources: gas or steam turbines,
electric motors, and internal-combustion engines.
THREE-PHASE GENERATORS
A three-phase ac generator, as the name implies, has
three single-phase windings spaced so that the voltage
induced in each winding is 120° out of phase with the
voltages in the other two windings. A schematic
diagram of a three-phase stator showing all the coils
becomes complex, and it is difficult to see what is
actually happening. A simplified schematic diagram
showing all the windings of a single phase lumped
together as one winding is illustrated in figure 3-12,
view A.
The rotor is omitted for simplicity. The
waveforms of voltage are shown to the right of the
schematic. The three voltages are 120° apart and are
similar to the voltages that would be generated by three
single-phase ac generators whose voltages are out of
phase by angles of 120°. The three phases are
independent of each other.
Wye Connection
Rather than have six leads come out of the
three-phase ac generator, one of the leads from each
phase may be connected to form a common junction.
The stator is then said to be wye, or star, connected. The
common lead may or may not be brought out of the
machine. If it is brought out, it is called the neutral. The
simplified schematic (fig. 3-12, view B) shows a
wye-connected stator with the common lead not brought
out. Each load is connected across two phases in series.
is connected across phases A and B in series;
is connected across phases A and C in series; and
Figure 3-12.—Three-phase ac generator: A. Simplified
schematic and wave forms; B. Wye connection; C. Delta
connection.
3-8
