ALTERNATORS. 225 



in the field circuit. The currents as registered on the ammeters 

 in the two circuits were 1-5 amperes in the armature, and 

 1-6 amperes excitation. 



The most important result illustrated by the curves is 

 that the excitation of an alternator, although supplied from 

 a uniform source of electromotive force is subject to regular 

 pulsations. These pulsations have the peculiarity that they 

 occur with exactly twice the frequency of the fluctuations of 

 the armature current. It is a general rule that if the wave 

 form of the voltage generated by an alternator is composed 

 of a number of waves of harmonic form, the frequency of these 

 waves will be 1, 3, 5, &c., times the frequency of the funda- 

 mental wave. The frequency of the variations in the excita- 

 tion will be 2, 4, &c., times the frequency of the fundamental 

 wave. This may be stated briefly by saying that the alternator 

 voltage contains only odd harmonies, while the field current 

 contains only even harmonies. 



The double frequency of the induced currents is due to the 

 combined action of the rotation of the armature and the 

 alternating character of the armature current. Thus if the 

 armature were supplied with a direct current a flux would be 

 induced in the field windings due to the armature rotation. 

 This would have a frequency equal to the frequency of the 

 alternating currents actually produced by the machine. 

 Similarly if the armature were stationary and supplied with 

 an alternating current there would be a flux at the poles 

 equal in frequency to the alternation of the current. In 

 the case of the loaded alternator both of these effects occur 

 at once, and produce a variation in the field current which 

 has double the frequency of the main armature current. 

 The variations thus represent those due to rotation, and to 

 the alternations of the current superposed. The double 

 frequency of the field variations is well shown in Fig. 104, 

 where thevariations of the armature electromotive force 

 are plotted below for comparison. 



Characteristic Curves with Leading and Lagging Current. 



Since alternators have frequently to supply inductive 

 circuits, and since the power-factor of the circuit affects the 

 voltage, it is sometimes necessary to determine characteristic 

 curves with the alternator working on inductive or capacity 

 load. 



The difficulties already referred to in connection with 

 magnetisation curves on inductive load arise in the case of 



