A large amount of electricity is required aboard ship to power machinery that supplies air, 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 ship's electrical system.

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 enemy shells.

Most generators used aboard ships are ac generators. However, since some do generators are still in service, we will briefly discuss do generators before moving on to ac generators.

DC GENERATORS AND EXCITERS

A do generator is a rotating machine that changes mechanical energy to electrical energy. There are two essential parts of a do generator:

1. The yoke and field windings, which are stationary, and

2. the armature, which rotates.

In the past, ship's service generators produced direct current. At present, practically all ships have 450-volt, 60-hertz (Hz), ac ship's service and emergency generators. The do generators used in Navy installations for ship's service or for exciters operate at either 120 volts or 240 volts. The power output depends on the size and design of the do generator. A typical do generator is shown in figure 12-1.

AC GENERATORS

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 do generators are called armatures; in ac generators, they are called rotors.

The general construction of ac generators is somewhat simpler than that of do generators. An ac generator, like a do 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 as follows:

The load current from the stator is connected directly to the external circuit without using a commutator.

Figure 12-1.-A do generator.

Figure 12-2.-Low-speed, engine-driven alternator.

Only two slip rings are necessary to supply excitation to the revolving field.

The stator winding is not subjected to mechanical stresses that are due to centrifugal force.

The ac generators (alternators) used by the Navy are divided into two classes: (1) low-speed, engine-driven alternators and (2) high-speed, turbine-driven alternators.

The low-speed, engine driven alternator fig. 12-2 has a large diameter revolving field, with many poles, and a stationary armature. The stator (view A) contains the armature windings. The rotor (view B) consists of protruding poles on which the do field windings are mounted.

The high-speed alternator may be either steamer gas-turbine driven. The high-speed, turbinedriven alternator is connected either directly or through gears to a steam turbine. The

Figure 12-3.-High-speed, turbine-driven alternator.

enclosed metal structure is part of a forced ventilation system that carries away the heat by circulating air through the stator (view A) and rotor (view B).