288 ELECTRICAL EQUIPMENT 



The vector E t represents the terminal e.m.f. and / the current 

 which in this case is in phase with the terminal e.m.f., the load 

 being non-inductive. The e.m.f. consumed by the resistance is 

 equal to RI, in phase with /, and E a is the e.m.f. which must be 

 induced to obtain a terminal e.m.f. E t and overcome the effects 

 of the resistance and reactance, thus causing a current to flow. 



The flux required to produce E g is 90 ahead of this e.m.f., the 

 magneto-motive force or ampere-turns to produce the same being 

 represented by F g . Due to the demagnetizing effect of the arma- 

 ture current, i.e., the armature reaction, the vector F is the 

 resultant of the m.m.f. of the armature current F a , and the total 

 impressed m.m.f. or field excitation F e . The m.m.f. F a is in 

 phase with the current, and after having determined the value of 

 F and F a , the necessary field excitation F e is obtained by com- 

 pleting the parallelogram. 



The effect of a lagging inductive load is shown in Fig. 158 and 

 of a leading inductive load in Fig. 159. For the same terminal 

 voltage E h it is seen that, as compared with a non-inductive load, 

 a much higher field excitation is required with a lagging inductive 

 load, and a lower field excitation with a leading inductive load. 

 The field excitation required to produce the terminal voltage E t 

 at open-circuit would be obviously less than the field excitation 

 with non-inductive load. 



Field Excitation. The excitation or filed ampere-turns 

 required to produce the magnetic flux which is necessary in order 

 to induce a desired e.m.f. depends on the character of the mag- 

 netic circuit, i.e., on its dimensions and on the material of which it 

 is made up. The values are readily obtained by reference to 

 standard saturation curves, similar to the ones shown in Fig. 160, 

 these curves, of course, depending upon the qualities of the iron 

 or steel which is used. The total magneto-motive force per mag- 

 netic circuit is equal to the sum of the m.m.f's. necessary for 

 establishing the required flux in the separate parts of the circuit 

 which are in series; viz., the pole pieces, the field spider, the air 

 gaps, the teeth and the armature core. 



The relation of the e.m.f. produced by an alternator at no-load, 

 i.e., open circuit, to the field current when the alternator is driven 

 at constant speed is represented by the no-load saturation curve. 

 Such a curve is shown by curve A, Fig. 161, and it is seen that this 

 curve is almost a straight line for small exciting currents. At low 



