268 



ELECTRICAL ENGINEERING 



With inductive load the demagnetizing effect is increased and 

 the terminal e.m.f. falls off more, curve (2). With a capacity 

 load in which the current leads the terminal e.m.f. the armature 

 m.m.f. is magnetizing and so raises the terminal e.m.f., curve (3). 



These voltage characteristics are calculated from equation 255 

 on page 263. The value of field current // is chosen and the cor- 

 responding value of EQ obtained from Fig. 248. Any required 

 power factor cos <j> is taken, the current / is varied and the values 

 of E obtained and plotted as ordinates. 



1200 



1100 



1000 



900 



800 



700 



i 



, 600 

 500 



a 400 



H 

 300 



100 



103 200 300 400 500 600 700 800 900 1000 

 Amperes Load Current 



FIG. 246. Voltage characteristics of an alternator. 



159. Compounding Curves. The " compounding curves" or 

 "field characteristics " show the relation between the field current 

 and armature current for a constant terminal e.m.f. at any re- 

 quired power factor. 



Fig. 247 shows the compounding curves for unity power factor, 

 curve (1), 80 per cent power factor lagging, curve (2), and 80 per 

 cent power factor leading, curve (3). 



At non-inductive load an increase of field current is required as 

 the load current increases to maintain a constant terminal e.m.f. 



