234 ALTERNATORS. 



voltage and D B shows the phase of the current in the external 

 circuit (since the external current and terminal voltage will be 

 in phase wilh a non-inductive load circuit). Thus the angles 

 D B A in Fig. 1 08 show the angle of phase difference between 

 load current and total volts for various loads. The angles 

 B D A show the angle of phase difference between current 

 and voltage in the armature itself. 



The reason for this varying phase difference may be 

 repeated in the following general terms. 



The voltage generated in the armature has to send current 

 through a circuit composed partly of reactance (due to the 

 armature self-induction) and partly of non-inductive 

 resistance (in external circuit and conductors of the 

 armature). The angle of lag of the current behind the total 

 generated voltage will depend entirely on the relative magni- 

 tude of the reactance and resistance of the circuit. If the load 

 current increases this must be due to a decrease of the non- 

 inductive resistance in the external circuit. Since the arma- 

 ture self-induction remains nearly constant, this change must 

 correspond to a smaller proportion of non-inductive 

 resistance in the whole circuit and a greater angle of lag. 



The general result of an increase of load must therefore 

 be an increase in the angle of lag between current and total 

 volts.* 



From the characteristic curve of an alternator on load 

 or from the short-circuit characteristic, the angle of lag 

 between current and total volts can be approximately 

 determined by the construction given on p. 205. This con- 

 struction assumes, however, that the total voltage generated 

 remains constant at all loads. This is by no means an 

 accurate assumption, on account of the weakening of the field 

 due to armature reactions. 



A nearer approximation to the true voltage relations in 

 the circuit and armature may be made by the following 

 modification of the method already given. 



Determine first the variation in excitation required to 

 maintain a constant terminal voltage, when the alternator 

 runs at normal speed, and supplies a non-inductive circuit. 



* The student must note the distinction between " total volts generated 

 in the armature " and " terminal volts." The terminal volts will, of course, 

 always be in phase with the current in the case of a non-inductive circuit, 



