THE ALTERNATING CURRENT GENERATOR 255 



The terminal voltage is obtained by subtracting 

 (vectorially) the armature IR and IX drops from the 

 generated e.m.f. These combined effects give external 

 characteristics similar in form to those given in Fig. 155. 



Field Compounding Curve. The armature characteristic, 

 or field compounding curve, shows the relation between the 

 load current and the field current, the latter being so 

 varied that the terminal voltage of the alternator remains 

 constant as the load is changed. The field current must 

 be increased to a considerable extent when the load is 

 changed from zero to rated capacity and over and this 

 increase is greater for an inductve than for a non-inductive 

 load. 



Experimentally obtained results for a small alternator 

 are given by the curves KIN and KHM of Fig. 155. The 

 first curve is for a non-inductive load and the second for a 

 load of cos <|> = .7, lagging current. 



Capacity of the Exciter. The exciter for this alternator 

 must be designed to carry safely the current required under 

 the worst condition, i.e., overload with a lagging current. 

 Thus the exciter for the alternator whose curves are given 

 in Fig. 155 should be designed to carry the current OS, 

 although with a normal non-inductive full-load current, 

 the required field current is only OT. 



63. The Voltage Regulator. Commercial loads always 

 have a lagging current and so if the excitation of an alter- 

 nator were left unaltered as the load increased, the terminal 

 voltage would fall to a considerable extent. It is desirable 

 with most pieces of apparatus to which power is supplied 

 (Jamps, motors, transformers, etc.), to maintain a constant 

 voltage at the piece of apparatus. 



This necessitates an increase in the terminal voltage 

 of the generator supplying the power, to overcome the 

 increased line drop with increased load, but if left to itself 

 the alternator would give a decrease in terminal voltage. 

 Hence it is necessary either to have the operator vary the 



