252 



ALTERNATING CURRENTS 



147. Six=phase Converter supplied from Three- 



phase Mains 



The arrangement of connections for supplying a six-phase con- 

 verter (i.e. one having six slip-rings) will be understood by reference 

 to Fig. 154. Let the circle in Fig. 154 (a) represent diagram- 

 matically the winding of a 2-pole armature, fitted with six slip-rings 

 connected to the points A, B, C, A', B', and C' of the winding. 

 Remembering the rule ( 141) for finding the magnitude and phase 

 of the e.m.f. in any section of the winding, we see that the e.m.f.'s 

 between the slip-rings A, B, C form a three-phase system, as also 

 do those between the slip-rings A', B', and C'. We thus get a 

 double A of voltages; and since AB is in phase with A'B', BC in 



(8) 



FIG. 154. Connections of Six-phase Converter. 



phase with B'C', and CA in phase with C'A', it is evident that 

 both A's may be obtained from the same set of three-phase trans- 

 formers. For this purpose, the transformers are wound as shown 

 in Fig. 154 (5), a mesh connection for both primaries and secondaries 

 being used by preference.! PiP 2 , PaPa, and P 3 Pi are the high- 

 voltage primaries. Each transformer is provided with two equal 

 secondaries, which are connected to form the A's, ABC, and A'B'C'. 

 The junctions A, B, C ; A', B', and C' of the two sets of secondary 

 windings are connected to the points A, B, C ; A', B', C' of the 

 armature winding respectively. 



t With a mesh connection of transformers, continuity of supply will be maintained 

 even if one of the transformers should break down ( 57). 



