ELECTRIC PROPULSION OF SHIPS. 115 



of fact some of the steps are combined so as to reduce the actual number of levers and oper- 

 ations to a reasonable amount. 



Although the synchronous motor requires special design and introduces additional com- 

 plications in the control, it has the important inherent characteristics of unity power factor. 

 The unity power factor results in slightly decreased weight and size, and consequently less cost 

 of the motor and generator. Taking tlie drive as a whole, and considering that both motor 

 and generator must be excited from a separate source at a cost of at least twice the steam per 

 KW. hour as the main turbine, the efficiency improvement over the induction motor drive 

 is more apparent than real. Therefore the principal advantages of the synchronous motor 

 drive as compared with the ordinary induction motor drive are a slight saving in cost, 

 weight and space, with a possible inappreciable margin in efficiency. A somewhat larger 

 air gap and greater ease of repair are claimed by some as additional advantages. 



Turbine Electric {Induction with Power Factor Correction). — The unity power factor 

 advantage of the synchronous motor system is also available with a wound secondary induc- 

 tion motor system in which the low lagging power factor is corrected to unity. This arrange- 

 ment not only provides a system of the same weight, space and cost of the synchronous sys- 

 tan, but in addition possesses the superior torque characteristics and simplicity of control of 

 the wound secondary induction system. 



The diagrammatic scheme of connections is shown in Fig. 4, Plate 14. It will be noted 

 that this is the same as the wound secondary induction motor system shown in Fig. 1, ex- 

 cept that power factor corrective apparatus has been added. The scheme functions in the 

 same manner and sequence as that shown in Fig. 1 until the motor is in step with the gen- 

 erator, and at this point the power factor corrective apparatus is caused to function. The 

 corrective apparatus consists of a Le Blanc phase advancer driven by a small D. C. motor, 

 the speed of which may be varied to suit the power factor correction desired. The function 

 of the phase advancer is somewhat similar to that of the D. C. exciter in the case of the syn- 

 chronous motor in that it supplies excitation to the induction motor. The cormection is made 

 to the secondary or rotor winding, and thus excitation is supplied through the secondary in- 

 stead of through the primary, with the result that the primary current is all "power" current, 

 which means "unity" power factor. 



The phase advancer is a small commutating machine of very simple construction. The 

 losses in the phase advancer consist of the internal copper loss due to the secondary current, 

 almost negligible iron losses, friction, windage and brush drop. The total of these losses is 

 very little and compares very favorably with the excitation power of the synchronous motor. 



The induction motor used with this system is simple in construction, smaller, lighter and 

 consequently less expensive than the motor used with the ordinary wound secondary induc- 

 tion motor system. The generator is the same as that for the synchronous motor drive. 

 Thus the induction motor drive with the phase advancer not only provides the advantages 

 resulting from unity power factor, but in addition provides the superior torque characteristics 

 of the wound secondary induction motor for maneuvering, and also obviates the necessity for 

 synchronous operation with the generator. In fact, it combines all the really desirable fea- 

 tures and characteristics of both systems, without complications. 



Diesel Electric. — For practical reasons, direct current only is feasible with Diesel elec- 

 tric drive. This will be evident by a proper analysis of the performance of Diesel en- 

 gines in connection with the characteristics involved in applying A. C. and D. C. machinery, 

 due consideration of ship requirements being taken into account. To obtain the best results 

 with Diesel electric drive, it is necessary to provide several relatively small and moderately 



