generator breaker and takes the generator off the line
when power is fed from the line to the generator instead
of from the generator to the line.
A voltage regulator is mounted on each switchboard
and operates automatically to vary the field excitation
to maintain the generator voltage constant throughout
normal changes in load. In all installations, a means is
provided to manually adjust the voltage if the automatic
regulator fails.
Figure 3-13.--An ac ground detector lamp circuit.
Ground Detector Circuits
through conductors, or conductors passing through a
magnetic field.
A set of three ground detector lamps (fig. 3-13) is
All generators have at least two distinct sets of
connected through transformers to the main bus of each
conductors.
ship's service switchgear group. It provides you with a
means to check for grounds on any phase of the
1. The armature winding, which consists of a
three-phase system. To check for a ground, turn switch
group of conductors in which the output voltage
Son and observe the brilliancy of the three lights, and
is generated.
look for the conditions shown below.
2. The field winding, which consists of a group of
conductors through which dc is passed to obtain
an electromagnetic field of fixed polarity.
Since relative motion is needed between the
armature and field flux, ac generators are built in two
major assemblies-the stator and the rotor. The rotor
rotates inside the stator. It is driven by several
commonly used power sources, such as gas or steam
turbines, electric motors, and internal-combustion
engines.
TYPES OF AC GENERATORS
There are various types of ac generators used today.
They all perform the same basic function. The types
discussed in this chapter are typical of the ones used in
shipboard electrical systems.
Revolving Armature
AC GENERATORS
In the revolving-armature ac generator, the stator
provides a stationary electromagnetic field. The rotor,
Alternating-current generators produce most
acting as the armature, revolves in the field, cutting the
electric power used today. AC generators are also used
lines of force, thereby producing the desired output
in aircraft and automobiles.
voltage. In this generator, the armature output is taken
from slip rings, retaining its alternating characteristic.
AC generators come in many different sizes,
depending on their intended use. For example, anyone
The use of the revolving-armature ac generator is
of the huge generators at Boulder Dam can produce
limited to low-power, low-voltage applications. The
millions of volt-amperes, while the small generators
primary reason for this limitation is its output power is
used on aircraft produce only a few thousand
conducted through sliding contacts (slip rings and
volt-amperes.
brushes). These contacts are subject to frictional wear
and sparking. In addition, they are exposed and liable
Regardless of their size, all generators operate on
to arc-over at high voltages.
the same basic principle-a magnetic field cutting
3-11
