ALTERNATORS. 



243 



The chief interest of the preceding experiment lies in the 

 conclusions which may be drawn from it regarding the 

 voltage drop of the alternator at various loads, and with 

 various power factors. The usefulness of this form of 

 measurement is increased by the fact that the machine 

 requires only a small amount of power to drive it, although 

 the currents taken from the armature may be large. The 



4O 

 35 

 3O 

 25 

 2O 

 15 

 1O 



O -1 -2 -3 -4 -5 '6 



Amperes, Excitation. 

 FIG. 115. SHORT CIRCUIT CHARACTERISTIC. 



experiment may thus be applied to a large machine without 

 great expenditure of power. 



As already pointed out, the voltage measured at the 

 terminals of the alternator with the circuit open represents 

 the voltage generated in the armature at the particular 

 speed and excitation employed. On closing the switch 

 in the armature circuit the voltage generated falls, 

 owing to the weakening of the main field by the armature 

 magnetic reactions. For the purposes of calculation it is 

 simpler to assume that the total voltage generated remains 

 the same on short circuit as on open circuit, but that the 

 portion which is actually lost due to magnetic reaction 

 disappears as part of the idle voltage lost in the armature. 

 Making this assumption, therefore, we consider the whole 

 of the no-load voltage as being spent in sending current 

 through the armature and ammeter, and overcoming the self- 

 induction and resistance of the circuit composed of them. 



