258 



ALTERNATORS. 



current sent by the leading machine must be sufficient to 

 prevent the displacement becoming greater. 



Synchroniser. In order to ascertain when two machines 

 are running at the same speed and are in phase, so that they 

 may be switched into parallel, a synchroniser is employed. 

 The principle of its action is now to be discussed. 



Fig. 122 represents two alternators G l and 6r connected 

 in parallel to an external circuit A B, of which A represents 

 one main conductor and B the other. 



FIG. 12^. Two ALTERNATORS COUPLED IN PARALLEL. 



Obviously, in order that the machines may both supply 

 current to the circuit A B, the brushes, which are simul- 

 taneously of the same sign, must be joined to the same 

 conductor. This is shown in the diagram, where, at the 

 instant represented, the upper brushes are both -)-, and 

 are connected to A. 



The conductors joining the alternators form a second 

 closed circuit distinct from the mam external circuit. In this 

 circuit the two machines generate electromotive forces tending 

 to send currents in opposite directions round this smaller 

 circuit. At the instant represented on the diagram the 

 directions of these electromotive forces will be those represented 

 by the arrows. Consequently, when the machines are in 

 correct phase for working in parallel, there will be equal 

 and opposite electromotive forces acting in this local circuit, 

 having a resultant electromotive force equal to zero. If the 

 machines were wrongly "connected together (i.e., -j- to - 

 instead of -f- to -f- and to ) the resultant electromotive 



