Chapter 20. -SHIPBOARD ELECTRICAL SYSTEMS 



and Ni and N2 are the number of turns in the 

 primary and secondary windings, respectively. 

 In ordinary transformers, the induced primary 

 voltage is almost equal to the applied primary 

 voltage; hence, the applied primary voltage and 

 the secondary induced voltage are approximately 

 proportional to the respectivenumber of turns in 

 the two windings. 



A-C MOTORS 



A-c motors are manufactured in many dif- 

 ferent sizes, shapes, and ratings for use in a 

 wide variety of applications. Since this discus- 

 sion cannot possibly cover all aspects of all 

 kinds of a-c motors, it will be limited to the 

 polyphase induction motor. Information on other 

 types of motors may be found in Basic Elec- 

 tricity , NavPers 10086-A, and in various manu- 

 facturers' technical manuals. 



The induction motor is a widely used type of 

 a-c motor because it is simple, rugged, and 

 inexpensive. It consists essentially of a stator 

 and a rotor; it can be designed to suit most 

 applications requiring constant speed and vari- 

 able torque. 



The stator of a polyphase induction motor 

 consists of a laminated steel ring with slots on 

 the inside circumference. The stator winding 

 is similar to the a-c generator stator winding 

 and is generally of the two-layer distributed 

 preformed type. Stator phase windings are sym- 

 metrically placed on the stator and may be either 

 wye connected or delta connected. 



Most induction motors used by the Navy have 

 a cage-type rotor (fig. 20-22) consisting of a 

 laminated cylindrical core with parallel slots in 

 the outside circumference to hold the windings 

 in place. The rotor winding is constructed of 

 individual short circuited bars connected to end 

 rings. 



In induction motors, the rotor currents are 

 supplied by electromagnetic induction. The 

 stator windings contain two or more out-of- 

 time-phase currents which produce correspond- 

 ing magnemotive forces which establish a ro- 

 tating magnetic field across the air gap. This 

 magnetic field rotates continuously at constant 

 speed, regardless of the load on the motor. The 

 stator winding corresponds tothe primary wind- 

 ing of a transformer. 



The induction motor derives its name from 

 the fact that mutual induction (transformer 

 action) takes place between the stator and the 

 rotor under operating conditions. The magnetic 



77.77 

 Figure 20-22.— Cage-type induction motor rotor. 



revolving field produced by the stator cuts 

 across the rotor conductors, thus inducing a 

 voltage in the conductors which causes rotor 

 current to flow. Hence, motor torque is de- 

 veloped by the interaction of the rotor current 

 and the magnetic revolving field. 



POWER DISTRIBUTION SYSTEM 



The power distribution system is the con- 

 necting link between the generators that supply 

 electric power and the electrical equipment that 

 utilizes this power to furnish the various serv- 

 ices necessary to operate the ship. The power 

 distribution system includes the ship's service 

 power distribution system, the emergency power 

 distribution system, and the casualty power dis- 

 tribution system. 



Most a-c power distribution systems on naval 

 ships are 450-volt, three-phase, 60-cycle, 

 three-wire systems. The lighting distribution 

 systems are 115-volt, three-phase, 60-cycle, 

 three-wire systems supplied from the power 

 circuits through transformer banks. On some 

 ships, the weapons systems, some I.C. circuits, 

 and aircraft starting circuits receive electrical 

 power from a 400-cps system. 



SHIP'S SERVICE POWER 



The ship's service power distribution system 

 is the electrical system that normally supplies 

 electric power to the ship's equipment and ma- 

 chinery. The switchboards and associated gen- 

 erators are located in separate engineering 

 spaces to minimize the possibility that a single 

 hit will damage more than one switchboard. 



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