2 ;o ALTERNATING CURRENTS 



Let us assume, for a moment, that all the windings of the two 

 transformers employed for supplying current to the field coils are 

 star-connected. Then it follows (since a wattless current lags 90 

 behind the p.d.) that the second set of brushes (which supply the 

 compounding current) must be displaced 90 ahead of the first set. 

 Now, if we replace the star connection of the secondaries of the first 

 transformer by a mesh connection, we accelerate the phase of the 

 secondary p.d., and hence also that of the current supplied by it, by 

 30.* In order to maintain the first set of brushes in their proper 

 position relatively to the commutator, we therefore have to shift them 

 backwards through 30. But this will make the angle between 

 the two sets 90 + 30 = 120 which means that the two sets of 

 brushes become coincident. 



Thus, by adopting a star connection for the secondaries of one 

 transformer, and a mesh connection for those of the other, we do away 

 with the necessity of using two sets of brushes, a single set being 

 sufficient. The complete arrangement of connections is shown in 

 Fig. 163. The three armature circuits, it will be noticed, are con- 

 nected to a neutral point not directly, but through the primaries of 

 the compounding transformer. Exact compounding is obtained by 

 varying the number of turns in the compounding transformer, and 

 adjustable resistances are introduced into the circuits of the exciting 

 transformer. 



The non-inductive resistances bridged across the ends of the field 

 coils perform three important functions : (1) they provide a path for 

 the load component of the generator current ; (2) they serve to dis- 

 tribute the current more or less equally among the six parallel 

 circuits ; and (3) they prevent sparking. 



The Heyland self-excited and compounded alternator has already 

 achieved a considerable amount of success, and some large machines 

 of this type have been constructed. In addition to its satisfactory 

 self-regulating qualities, it is characterized by its small size as com- 

 pared with machines of ordinary construction; this is due to the 

 possibility of allowing armature reaction to exceed very considerably 

 the limits admissible in alternators of the ordinary type. 



* Since there is a phase difference of 30 between the phase p.d. and the line p.d. 

 (sec 28). 



