ALTERNATORS. 



223 



Variation in Exciting Current. Another method of 

 showing the effect of the armature current on the field is to 

 trace the variation of the exciting current by the method 

 described in Experiment XV. 



K 



Mi 



Fu;. 103. DIAGRAM OP CONNECTIONS FOR DETERMINING VARIATION OF 

 FIELD CURRENT. 



The connections required for this determination are 

 shown in Fig. 103, where the symbols employed have the 

 following meaning : 



O Alternator armature. 

 F Alternator field winding. 

 R Load resistance. 

 A, A l Ammeters. 

 M l M., Source of direct current for excitation.. 



K Revolving contact-maker on alternator shaft or 

 driven synchronously Avith the alternator. (See 

 page 84). 



V Electrostatic voltmeter. 

 C Condenser. 

 DPS Two-pole throw-over switch for enabling readings 



of armature voltage and exciting current to be taken 

 alternately. 



.7?! Non-inductive resistance of known value, in field 



circuit enabling the excitation to be calculated from 



the observed voltage at its terminals. 



The connections will be clear from the above diagram. 



The method of carrying out the experiment, is to run the 



alternator at constant speed and constant load. The fields 



are excited through an ammeter and non-inductive resistance 



from a source of constant continuous voltage. The electro- 



