ALTERNATORS. 207 



zero. In this case the whole of the voltage generated is 

 spent in overcoming the armature impedance. 



The voltage thus spent is apparently 46 volts in the case 

 of the machine under consideration. The armature self- 

 induction is lower at this low excitation, and consequently 

 the voltage spent in overcoming the armature impedance 

 is greater, but this is probably not sufficient to account 

 even for the greater part of the difference between the voltage 

 46 and 26-3 obtained by calculation above. With very 

 weak excitation and relatively strong armature current the 

 magnetic re-actions are very strong and have a strongly 

 demagnetising action, since the circuit (consisting of the 

 armature alone) is highly inductive, and lagging currents 

 tend to weaken the main field, as is explained below. 



A comparison of the full-load and no-load magnetisa- 

 tion curves shows the loss in terminal voltage between 

 no-load and full-load when the excitation remains at 

 a constant value. It further enables a determination 

 to be made of the change in excitation necessary, to 

 overcome this loss of voltage and to maintain the same 

 voltage at full-load as at no-load. Thus, on referring 

 to Fig. 94, if 93 volts is required, it will be seen that the 

 difference in exciting current between the points corresponding 

 to 93 volts on the two curves is -38 amperes, so that the range 

 of a field-regulating resistance to maintain this voltage at 

 ah 1 non-inductive loads would have to be sufficient to produce 

 this variation in the excitation. It must, however, be 

 remembered that the loss of voltage will be greater on 

 inductive loads, and consequently the curves shown in Fig. 

 97 would have to be employed in cases where the machine 

 supplies inductive circuits. 



A further important point, which is more likely to escape 

 notice, is that the magnetisation curve enables the magnitude 

 of the steps of the regulating resistance to be determined in 

 order that regulation may be accomplished with any desired 

 degree of sensitiveness. Thus, if it is desired to regulate 

 the Voltage within one volt, the variation of exciting current 

 corresponding to the volt difference of pressure may be at 

 once obtained by an inspection of the curve. From this 

 the exact value of each step of the regulator can be calculated 

 for a fixed voltage of excitation. For instance, at the upper 

 part of the curve at 2-4 amperes excitation in Fig. 94 each 

 volt corresponds to an increased excitation of -092 amperes. 



