ALTERNATORS. 231 



resultant of the voltage A D spent in overcoming 

 armature impedance, and the voltage D B measured 

 at the terminals of the machine. When the current 

 is small, the line A D is short, since the voltage 

 overcoming armature impedance is small. Under these 

 conditions the measured voltage B D and the total 

 voltage A B are nearly equal, since A D is nearly perpen- 

 dicular to both of them. As the current increases the 

 voltage A B remains the same, but the voltage D B decreases 

 at first slowly, but afterwards more rapidly. The drop in 

 armature impedance increases in proportion to the current, 

 and consequently the sides of the triangle AC D all increase 

 in the same proportion. The angle A G B is,- however, 

 always a right angle and consequently the point C will 

 always lie on a semi-circle described on the line A B, and 

 must move round this semi-circle towards B as the current 

 (and consequently the length of A C) increases. It thus 

 follows that the line C B will continuously decrease as the 

 current increases on account of the altered position of the 

 point C. It will further decrease owing to the fact that the 

 voltage represented by G D (= armature ohmic drop) 

 increases also in proportion to the current. It would then 

 be easy to draw a characteristic curve on the assumption 

 that the armature impedance, and total voltage generated, 

 both remained constant. 



Fig. 108 shows the form which the diagram in Fig. 95 

 would take when the current in the circuit has the values 

 10, 20 30, and 40 amps. In each case the length of D B 

 represents the terminal voltage. The external circuit in 



C D = Armature and line drop. 

 D B = Terminal volts. 

 A B = Total volts generated. 

 A C = Reactance voltage. 

 FIG. 108. CALCULATION oy EXJERNAI, CHARACTERISTIC. NON-INDUCTIVE LOAD. 



