324 SYNCHRONOUS MACHINES. [Exr. 



verier depends upon armature heating and may be based upon 

 several assumptions. Based upon average armature heating and 

 the assumption of unity power factor, the relative capacities of a 

 converter are 



single-phase, 0.85; 3-phase, 1.33; 6-phase, 1.93; 



the capacity as a direct current generator being unity. 



The advantage of the 6-phase converter is obvious (2/a, Exp. 

 6-A), but there is little advantage in more than six phases, the 

 capacity for infinite phases being only 2.3 as compared with 1.93 

 for six phases. 



8. Voltage Control.* The D.C. voltage of a converter is 

 usually controlled by altering the A.C. voltage supplied to the 

 collector rings, and this is commonly done : ( I ) by means of an 

 induction regulator (or some other form of potential regulator, 

 Exp. 7~B) ; (2) by means of reactance placed in series with the 

 converter on the A.C. side, as discussed later; or, (3) less com- 

 monly by a synchronous booster.^ Another method, used in the 

 split-pole:): converter, controls the D.C, voltage without altering 

 the A.C. voltage. 



* (8a). For a discussion of converter construction and operation, with 

 particular reference to voltage control, see the following papers and their 

 discussions: A. I. E. E., Vol. XXVII., C. W. Stone, p. 181 ; J. E. Wood- 

 bridge, p. 191; C. A. Adams, p. 959; also Elect. Journal, Vol. V., F. D. 

 Newbury, pp. 615, 616. 



f(8b). Synchronous Booster. A small auxiliary alternator, mounted 

 on the same shaft as the converter is connected in series with it as a 

 booster on the A.C. side. The A.C. voltage supplied to the converter 

 depends, therefore, upon the excitation of the booster, which may be 

 controlled by a suitable regulator. Some of the field windings of the 

 booster may be put in series with the D.C. load, thus giving an increasing 

 excitation with load. 



$(8c). Split-pole Converter. In this converter each pole is divided 

 into sections which can be given different excitations so as to vary the 

 flux distribution. This shifts the flux, which is practically the same as 

 shifting the brushes, and so changes the D.C. voltage ; or, it alters the wave 

 form of electromotive force and so changes the ratio of the D.C. to the 

 A.C. voltage; or, it both shifts the flux and alters the wave form. 





