196 ALTERNATORS. 



voltage and speed by substituting in the following formula 

 already given above : 



NnZ 

 = 10 8 x 60 X 



e x 10 s x 60 



Nnk 



k being 2-22 for an open circuit alternator giving a sine wave 

 form. 



EXPERIMENT XXXIV. DETERMINATION OF MAGNETISA- 

 TION CURVE OF AN ALTERNATOR AT No -LOAD. 



DIAGRAM OF CONNECTIONS. 



Fie. 91. 



M j M g Source of direct current. 



Alternator armature. 



F Alternator field windings. 



R Resistance for varying exciting current. 



A Ammeter for measuring exciting current. 



V Voltmeter for measuring alternator voltage. 



Instructions. Connect the field windings to a supply of 

 direct current in series with a variable resistance and ammeter. 



Connect the alternator armature terminals to a voltmeter. 



Run the alternator at constant speed. Read the armature 

 voltage first without excitation, and then with exciting 

 current gradually increased to its full value. Then take a 

 similar series of readings with gradually decreasing values 

 of the current. 



Enter the readings under headings as shown in the follow- 

 ing table and plot a curve in which exciting current is plotted 

 horizontally and armature voltage vertically. 



If the speed cannot be kept absolutely constant a cor- 

 rection in the observed voltage must be made. If the speed 

 is too low, the voltage observed will be too low in exactly 

 the same proportion. 



