VOLTAGE REGULATION OF THE ALTERNATOR. 125 



case being taken as 100, the number in the second case would 

 be 75, and the electromotive forces would be in the ratio of 



Fig. 111. 



loo x 0.637 to 75 X 0.784, that is in the ratio of 63.7 to 58.8. 

 It is therefore evident that only a slight increase of electromotive 

 force would be obtained by filling up the whole armature surface 

 of a single-phase alternator with windings, whereas the armature 

 resistance, the loss of power due to armature resistance, and the 

 cost of copper would all be increased in the ratio of 75 to 100 in 

 the case cited. A single-phase alternator giving, say, 1 ,000 volts 

 and having an armature wound with wire large enough to carry 

 100 amperes, and therefore able to deliver 100 kilowatts to a 

 noninductive receiving circuit, must be larger and more costly 

 than a two-phase or three-phase alternator capable of delivering 

 the same total amount of power. 



62. Voltage regulation of an alternator. An alternator, driven 

 at a constant speed and having a constant field excitation, gives 

 a certain terminal voltage when its current output is zero, and its 

 terminal voltage generally falls off in value with increase of cur- 

 rent output ; or, if the machine is adjusted to give a certain ter- 



