ALTERNATORS. 



235 



(See Experiment XXXVII.) Then connect the armature 

 to a supply of alternating current, and pass a series of in- 

 creasing currents through the stationary armature, at the 

 same time exciting the magnets with the excitation previously 

 determined to be that required to maintain the armature 

 voltage constant when rotating and supplying each value of 

 the current. In this way the excitation will always be so 

 adjusted that its effect upon the armature impedance will be 

 the same as when the alternator is working under load with 

 constant terminal voltage. 



After adjusting current and voltage as just described, take 

 readings on a wattmeter of the watts supplied to the armature 

 and of the current, and the volts applied to the armature 

 terminals. 



Calculate for each set of readings the value of cos < by 

 dividing the true watts by the apparent watts supplied to the 

 armature. The value and also the phase of the armature drop 

 is thus determined in relation to the terminal voltage for any 

 load. 



As an illustration, the following readings are given, taken 

 on a small 4-pole ring- wound Crompton alternator having an 

 output of 5 amperes at 100 volts. 

 TEST or VOLTAGE DROP WITH STATIONARY ARMATURE. 



From the values of < given in this table the lower curve of 

 $ in Fig. Ill has been plotted. It is seen that the angle of 

 lag between current and volts in the armature varies between 

 56 and 65 for the range of loads taken. 



-From the readings thus taken the total voltage which 

 would have to be generated in the armature when the machine 

 works at a constant terminal voltage of 103 was calculated 

 in the manner described below, and the results were plotted 

 as shown in the top curve in Fig. Ill marked " Total volts." 



