water, food, and other services. Communications
between the various parts of a ship also depend
on the availability of electric power. The generator
is the power source for the ships electrical
A generator operates most efficiently at its
full-rated power output, and it is not practical to
have one large generator operating constantly at
reduced load. Therefore, two or more smaller
generators that are operated at high load are
installed aboard ship.
Two or more generators are usually installed
aboard ship for another reason. If one generator
is shut down because of damage or scheduled
maintenance, there is still a source of power for
lighting until the defective generator has been
repaired. In addition, generators are widely spaced
in the engineering spaces to decrease the chance
that all electrical plants would be disabled by
Most generators used aboard ships are ac
generators. However, since some dc generators
are still in service, we will briefly discuss dc
generators before moving on to ac generators.
DC GENERATORS AND EXCITERS
A dc generator is a rotating machine that
changes mechanical energy to electrical energy.
In the past, ships service generators produced
direct current. At present, practically all ships
have 450-volt, 60-hertz (Hz), ac ships service and
emergency generators. The dc generators used in
Navy installations for ships service or for exciters
operate at either 120 volts or 240 volts. The power
output depends on the size and design of the dc
generator. A typical dc generator is shown in
AC generators are also called alternators. In
an ac generator, the field rotates, and the
armature is stationary. To avoid confusion, the
rotating members of dc generators are called
armatures; in ac generators, they are called rotors.
The general construction of ac generators is
somewhat simpler than that of dc generators.
An ac generator, like a dc generator, has
magnetic fields and an armature. In a small
ac generator the armature revolves, the field
is stationary, and no commutator is required.
In a large ac generator, the field revolves
and the armature is wound on the stationary
member or stator.
The principal advantages of the revolving-field
generators over the revolving-armature generators
are two essential parts of a dc generator:
are as follows:
The yoke and field windings, which are
l The load current from the stator is
connected directly to the external circuit
the armature, which rotates.
without using a commutator.
Figure 12-1.A dc generator.