218 ALTERNATORS. 



The construction for showing graphically the resultant 

 drop due to both causes has already been given in connection 

 with Figs. 94 and 95, page 205. 



On non-inductive loads, the ohmic drop is the most impor- 

 tant factor, since it is in phase with the terminal volts. On 

 inductive loads the reactance voltage has a greater effect in 

 lowering the voltage. 



It will be found that the loss of voltage in overcoming the 

 impedance of the armature accounts for the greater part of 

 the total loss shown in the characteristic curve of the 

 machine. Practically the whole of the remaining loss of 

 voltage is due to the third of the causes enumerated above, 

 viz., weakening of main field. 



(3) The weakening of the main field, due to magnetic 

 reactions of the armature currents, is due to two causes : 

 (a) The direct weakening effect of the armature field acting 

 in opposition to the main field, and (6) the " screening " 

 effect of eddy currents induced in the pole faces of the field 

 magnets, and possibly also in the magnet windings them- 

 selves. The direct weakening effect is small when the 

 armature is working on non-inductive load, as the action of 

 the armature field is chiefly to distort and not to oppose 

 the main field. There will be a slight weakening effect due 

 to the fact that the lines will be more concentrated at one 

 pole tip owing to the distortion, and that this will decrease 

 the permeability of the pole in consequence. The direct 

 weakening effect will be increased by a lagging current and 

 diminished by a leading current. The screening action of 

 the eddy currents is due to the induction of local currents in 

 the pole faces and any other metal parts which are not 

 laminated, caused by the pulsations of the field. The direc- 

 tion of these currents is always such that the lines of force 

 due to them opposes the field inducing the currents, and the 

 formation of the currents thus has the effect of a " screen " 

 hindering the penetration of magnetic effects. 



These eddy currents will be more readily produced in 

 cases when the self-induction of the armature varies con- 

 siderably in passing from pole to pole, i.e., with armature 

 winding concentrated in few open slots. In order to lessen 

 the eddy currents, the poles or pole faces of large alternators 

 are frequently made of laminated iron. 



(4) Alteration of wave-form producing different virtual 

 value of voltage. This cause would be too slight on a non- 



