Figure 12-2.—Low-speed, engine-driven alternator.l Only two slip rings are necessary to supplyexcitation to the revolving field.l The stator winding is not subjectedto mechanical stresses that are due tocentrifugal force.The ac generators (alternators) used by theNavy are divided into two classes: (1) low-speed,engine-driven alternators and (2) high-speed,turbine-driven alternators.The low-speed,enginedrivenalternator(fig. 12-2) has a large diameter revolving field,with many poles, and a stationary armature. Thestator (view A) contains the armature windings.The rotor (view B) consists of protruding poleson which the dc field windings are mounted.The high-speedalternator may be either steam-er gas-turbine driven. The high-speed, turbine-driven alternator (fig. 12-3) is connected eitherdirectly or through gears to a steam turbine. TheFigure 12-3.—High-speed, turbine-driven alternator.enclosed metal structure is part of a forcedventilation system that carries away the heat bycirculating air through the stator (view A) androtor (view B).SHIP’S SERVICE TURBINE-DRIVENGENERATORSShip’s service generators furnish electricityfor the service of the ship. Aboard most steam-driven ships of the Navy, these generatorsare driven by turbines. Large ships may haveas many as six or eight ship’s service generatorsand from one to three emergency diesel-drivenalternators.New cruisers and destroyers have three gas-turbine-driven ship’s service generators andsmaller diesel-driven emergency generators. Thesegenerators are located in three different compart-ments and separated by at least 15 percent of thelength between perpendiculars to make sure theysurvive.12-3
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