8o ALTERNATING CURRENTS 



The p.d. reaches a zero value in the position shown in diagram (e), 

 the points C and D having now come halfway between A and B. 

 It is evident that since (a) differs from (e) only in that C and D have 

 now changed places, a similar series of changes to that just considered 

 will take place in the p.d. across CD during the next half-revolution; 

 the p.d. being now, however, directed from D to C instead of from 

 C to D as during the first half-revolution. We thus get an alternating 

 p.d. between the slip-rings connected to the diametrically opposite 

 points C and D in the winding. Such an arrangement would, there- 

 fore, constitute a single-phase generator, or single-phaser. 



It is clear that by selecting two pairs of diametrically opposite 

 points, such as C, D and E, F in Fig. 60 (a), so arranged that the 



diameter CD is perpendicu- 

 lar to the diameter EF, we 

 get, during the rotation of 

 the armature, an alternating 

 p.d. between C and D which 

 differs 90 in phase from 

 the alternating p.d. between 

 E and F, the p.d.'s being 

 (a) () necessarily of the same fre- 



FIG. GO.-Slip-ring Connections for Two- and - 



Three-phase Currents. rings connected to the 



points C, D and E, F, and 



connecting one external circuit to C, D, and another to E, F, we 

 obtain a two-phase system. The machine now becomes a two-phase 

 generator, or two-phaser. 



Lastly, by taking three points, G, H, and K, in the winding, 120 

 apart, as in Fig. 60 (6), and attaching them to three slip-rings, we get, 

 between any pair of slip-rings, an alternating p.d. which differs by 

 120 in phase from the p.d.'s between the two remaining pairs of 

 rings. Three conductors connected to the slip-rings would thus form 

 the mains of a three-phase system, the machine now acting as a three- 

 phase generator, or three-phaser. The arrangement of the armature 

 winding in this case would, it may be noted, correspond to a delta 

 or mesh connection ( 16). 



It will now be evident that, by the simple addition of a suitable 

 number of slip-rings, any ordinary continuous-current dynamo may 

 be converted into an alternator. Such a mode of construction is used 

 in practice when either (1) the same machine is required to generate 

 both continuous and alternating current, the former being obtained 

 from the commutator and the latter from the slip-rings ; or (2) when 

 a transformation from alternating to continuous current, or vice versa, 

 has to be effected. In the first case, the machine forms a double- 

 current generator ; in the second, a rotary converter. 



