116 ELECTRIC PROPULSION OF SHIPS. 



high-speed generating sets for supplying power to single or double-ixnit direct-connected 

 propelling motors. Not only must the generated power divide evenly or proportionately 

 between the generating units, but the system must also be such as will conveniently and eco- 

 nomically lend itself to speed control. 



In the case of alternating current it would be necessary to operate the generators in 

 parallel. To operate A. C. generators in parallel necessitates the very closest speed regula- 

 tion and practically identical angular velocities of all prime movers. To properly visualize 

 this exacting requirement it must be remembered that satisfactory parallel operation of A. C. 

 generators necessitates that the angular displacement of the field poles of one machine with 

 respect to another must not vary more than approximately ± 3 electrical degrees, or a total 

 of 6 electrical degrees. Since 360 electrical degrees constitute the space between adjacent 

 like poles, the total variation in mechanical degrees, for example, in the case of a 20-pole ma- 

 chine must not exceed 0.6 degree. While succesful operation under such requirements is 

 carried out in several land installations where the prime movers operate at constant speed, 

 it is not considered safe practice on board ship where the necessity for varying the speed 

 of all sets simtiltaneously introduces another very serious difficulty. To overcome this con- 

 dition successfully would require absolutely perfect engine governors which would fimction 

 100 per cent perfect at any speed setting. The speed of the motor could be varied by the 

 rheostatic method, thus allowing the engines to operate at constant speed; however, this 

 method is extremely wasteful at reduced speed operation and at best offers a solution for only 

 one of the many difficulties. It is for these reasons that alternating current is not suitable 

 for Diesel electric propulsion. 



Direct current not only obviates all of the above difficulties but possesses many advan- 

 tages in the way of operation, control and reserve power. With direct current we have the 

 choice between two methods of generator operation, i. e., parallel and series. From the 

 standpoint of engine performance only, parallel operation of D. C. generators is entirely 

 feasible and easily accomplished. However, when considering economical methods of speed 

 control of the propeller motors, another factor enters which makes parallel operation diffi- 

 cult, even with D. C. machines. This is explained below. 



With direct current we have a choice between two methods of motor speed control, 

 i. e., armature rheostatic and generator voltage control. Armature control is not only un- 

 justifiably wasteful at reduced speed operation but also adds complication to the control. 

 On the other hand, the voltage control method is practically 100 per cent economical and 

 provides an ease and a flexibility of control unapproachable by other systems. In the case 

 of very small drives, armature rheostatic control might be selected because of factors not 

 related to the propulsive equipment making it preferable to have a constant voltage system 

 which is common to the propulsion and auxiliary circuits. For a drive of any appreciable size, 

 the best results are obtained by isolating the propulsive equipment so that immediate maneu- 

 vering can be done without affecting non-related circuits. Therefore, having an isolated 

 plant for propulsion only, voltage control is obviously the method to use. 



With parallel operation of generators, voltage control is not simple of accomplishment. 

 To satisfactorily vary the voltage of two or more generators simultaneously over the full 

 range from zero to maximimi necessitates very closely and very carefully adjusted field 

 rheostats, generators with practically identical saturation curves and engines with practi- 

 cally identical regulation, or some complicated and delicate automatic voltage balancing 

 instrument. 



The series arrangement of generators, however, is ideal from every standpoint such 



