ELECTRIC PROPULSION OF SHIPS. Ill 



The instant the screws commence to turn over in the reverse direction, the system must 

 supply tiie energy and all further stopping energy is dissipated at the propellers. There are 

 two factors to be considered in stopping, namely, the energy returned through the propellers 

 and the stored energy in the propellers and the motor armatures. Analysis shows that re- 

 versing even at full speed is not a serious problem and that it does not place as severe require- 

 ments upon the electrical machinery as does turning with hard-over rudder. The details of 

 the distribution and absorption of the energy of reversal would occupy too much space to 

 be discussed at this time. Suffice it for the present to say that the inherent facilities af- 

 forded by electrical systems for dissipating energy and for giving full power in either direc- 

 tion are what make the electric drives ideally suited for stopping. The time required to stop 

 an electric ship is the same as, or less than, that of other drives. 



DESCRIPTION OF ELECTRICAL PROPULSIVE EQUIPMENTS. 



General Classification. — Electrical systems for ship propulsion may be classified into two 

 general types from the standpoint of the prime movers, namely, turbine electric and Diesel 

 electric. Due to the inherent performance of these two types of prime movers, both kinds of 

 electric machinery are used — A. C. machinery with turbine electric because the A. C. gen- 

 erator is inherently suitable for direct connection to the economical high-speed turbine; and 

 D. C. machinery with Diesel-electric drive because the inherent characteristics of the D. C. 

 generators are ideally suited to Diesel engine performance. 



The electrical equipment for turbine-electric drive may be further subdivided as fol- 

 lows, in regard to the type of motor : 



rWound secondary JOi'dina^' ^^ with 



1. Induction. ] Squirrel cage. (P°^^ ^^^^or correction. 



[Combined squirrel cage and wound secondary. 



2. Synchronous. 



In the case of the Diesel-electric drive there is no broad subdivision. As a minor classi- 

 fication, this type of drive might be subdivided with respect to the method of generator 

 operation, i. e., series and parallel. However, series operation is so vastly superior that 

 parallel operation can be disregarded except for the purpose of comparison. 



Units Involved. — A complete turbine-electric drive involves the following apparatus : 

 Boiler plant, evaporation plant, condenser plant, turbine, generator, motor, oiling system, 

 exciter set, control. 



A complete Diesel-electric drive involves the following apparatus : Diesel engines, gen- 

 erators, motors, small auxiliary air compressors, air bottles, exciters, fuel pumps (if not 

 attached to engine), lubricating pumps (if not attached to engine), control. 



Turbine Electric. — With turbine-electric drive, the electrical equipment is of the A. C. 

 type, principally for the reason that A. C. turbo-generators are inherently better suited for 

 the high economical speeds of turbines. The speed at which the turbine operates is influ- 

 enced by the propeller speed because of the limitation in the number of poles of the motor. 

 Theoretically, an A. C. motor can be built for any even number of poles, but in practice such 

 factors as power factor (induction motor), diameter and assembly fix the upper limit in the 

 neighborhood of 60 to 72 poles. As the motor speed and number of poles fix the generator 

 and turbine speed, there is consequently an approximately fixed limit to turbine speeds. Inci- 



