ALTERNATORS. 



251 



with the speed. The loss of voltage due to eddy currents is 

 probably too small at moderate speeds to show much effect 

 upon the curve, and this would increase in approximately 

 direct proportion to the speed. The increase in the voltage 

 drop thus varies directly as the speed, and the speed-volt 

 curve for a constant load remains a straight line. 



The curve shown in Fig. 120 shows the result of the test 

 upon the same alternator when short-circuited. The excita- 

 tion in this case was -55 amp. This curve shows the 



200 



4OO 



16OO 



6OO 8OO 1OOO 12OO 14OO 



Revolutions per minute. 



FIG. 120. VARIATION OF SHORT CIRCUIT CURRENT WITH SPEEDS. 

 Excitation '55 ampere. 



interesting fact that after normal speed has been reached, the 

 short-circuit current is practically independent of speed. 

 This illustrates in another way the statement just made, 

 that the armature impedance increases in almost direct 

 proportion with the speed. The voltage generated also 

 increases directly as the speed, and consequently the current 

 remains constant, since it is the quotient of the volts divided 

 by the impedance. 



