Principles of naval engineering - Historic Naval Ships Association

Chapter 20. -SHIPBOARD ELECTRICAL SYSTEMSand Ni and N2 are the number of turns in theprimary and secondary windings, respectively.In ordinary transformers, the induced primaryvoltage is almost equal to the applied primaryvoltage; hence, the applied primary voltage andthe secondary induced voltage are approximatelyproportional to the respectivenumber of turns inthe two windings.A-C MOTORSA-c motors are manufactured in many differentsizes, shapes, and ratings for use in awide variety of applications. Since this discussioncannot possibly cover all aspects of allkinds of a-c motors, it will be limited to thepolyphase induction motor. Information on othertypes of motors may be found in Basic Electricity, NavPers 10086-A, and in various manufacturers'technical manuals.The induction motor is a widely used type ofa-c motor because it is simple, rugged, andinexpensive. It consists essentially of a statorand a rotor; it can be designed to suit mostapplications requiring constant speed and variabletorque.The stator of a polyphase induction motorconsists of a laminated steel ring with slots onthe inside circumference. The stator windingis similar to the a-c generator stator windingand is generally of the two-layer distributedpreformed type. Stator phase windings are symmetricallyplaced on the stator and may be eitherwye connected or delta connected.Most induction motors used by the Navy havea cage-type rotor (fig. 20-22) consisting of alaminated cylindrical core with parallel slots inthe outside circumference to hold the windingsin place. The rotor winding is constructed ofindividual short circuited bars connected to endrings.In induction motors, the rotor currents aresupplied by electromagnetic induction. Thestator windings contain two or more out-oftime-phasecurrents which produce correspondingmagnemotive forces which establish a rotatingmagnetic field across the air gap. Thismagnetic field rotates continuously at constantspeed, regardless of the load on the motor. Thestator winding corresponds tothe primary windingof a transformer.The induction motor derives its name fromthe fact that mutual induction (transformeraction) takes place between the stator and therotor under operating conditions. The magnetic77.77Figure 20-22.— Cage-type induction motor rotor.revolving field produced by the stator cutsacross the rotor conductors, thus inducing avoltage in the conductors which causes rotorcurrent to flow. Hence, motor torque is developedby the interaction of the rotor currentand the magnetic revolving field.POWER DISTRIBUTION SYSTEMThe power distribution system is the connectinglink between the generators that supplyelectric power and the electrical equipment thatutilizes this power to furnish the various servicesnecessary to operate the ship. The powerdistribution system includes the ship's servicepower distribution system, the emergency powerdistribution system, and the casualty power distributionsystem.Most a-c power distribution systems on navalships are 450-volt, three-phase, 60-cycle,three-wire systems. The lighting distributionsystems are 115-volt, three-phase, 60-cycle,three-wire systems supplied from the powercircuits through transformer banks. On someships, the weapons systems, some I.C. circuits,and aircraft starting circuits receive electricalpower from a 400-cps system.SHIP'S SERVICE POWERThe ship's service power distribution systemis the electrical system that normally supplieselectric power to the ship's equipment and machinery.The switchboards and associated generatorsare located in separate engineeringspaces to minimize the possibility that a singlehit will damage more than one switchboard.509