370 



ELECTRICAL MACHINERY 



alternating e.m.fs. must be added vectorially (instead of 

 arithmetically) to give the resultant e.m.f., hence, if we 



tap the winding of Fig. 241 

 X at A and B (diametrical 



points), the vector giving 

 the voltage A-B will be 

 found by adding vectorially 

 the e.m.fs. of coils 1 to 6 

 inclusive. But this vector 

 will evidently be the diam- 

 eter of a regular polygon 

 having for its sides the 



FIG. 241.-Elementary Winding for Vectors g ivcn in Fi S' 242 ' 



a Synchronous Converter, Single- 

 phase Taps at A and B. 



Voltage for Single-phase 

 Taps. This polygon is 

 shown in Fig. 243 and the 



voltage between the points A and B, Fig. 241, is shown 

 by the line AB in Fig. 243. The c-c. brushes, by means 

 of the commutator, are continually maintained at a differ- 

 ence of potential equal to AB, 

 Fig. 243, because the brushes 

 continually make contact with 

 the winding on the line XY, 

 and so there are continually 

 six coils adding their e.m.f. 

 between the c-c. brushes. 



But suppose that the points 

 A and B are connected to a 

 pair of slip rings; the maxi- 

 mum e.m.f. between the rings 

 will be AB, Fig. 243, and 

 this is the same as the e.m.f. 

 between the c-c. brushes. 



Hence we have the relation that, on a single-phase 

 converter, the maximum alternating voltage is the same as 

 the continuous voltage, and hence the virtual alternating 



11 



FIG. 242. Vector Diagram of 

 the e.m.fs. Generated in the 

 Different Coils of Fig. 241. 



