222 ALTERNATORS. 



may, consequently, assume that the 260 watts supplied with 

 the armature in this position is practically that due to the 

 losses in the armature itself. The resistance of the armature 

 was 2 ohms, so that 200 watts were lost in the armature 

 winding and about 60 watts in armature hysteresis and eddy 

 currents. It is interesting to notice that while the short- 

 circuiting of the magnet windings diminishes the impedance 

 of the armature winding on account of the transformer action 

 which produces currents in the magnet winding, the power 

 absorbed is less, owing to the more effectual magnetic screening 

 of the currents in the winding as compared with the eddy 

 currents in the iron of the poles to which the losses are due 

 when the magnet winding is open. 



The effect of excitation of the magnets is to diminish the 

 armature impedance owing to increased saturation of the 

 core added to the production of alternating currents in the 

 magnet windings super-posed on the normal excitation. 



The power curve (II.) in this case shows the influence of 

 the field distortion resulting from both armature and exciting 

 currents. There is thus no position when the fields are 

 unaffected by the armature flux. In Fig. 102 the areas 

 enclosed by curves I. and II., and the horizontal axis are 

 almost identical, showing that the average effect of eddy 

 currents for all positions of the armature is about the same 

 with the field coils excited and open. 



When the alternator works under normal conditions, the 

 variations of the current will occur with the same frequency 

 as the variation in position of the armature. Thus if the 

 current is in phase with the induced electromotive force, the 

 current would have its maximum value when the conductors 

 are exactly under the centre of the poles, and the centre of 

 the coil is midway between the poles, i.e., the position corre- 

 sponding to 90 on the curves Figs. 101 and 102. The 

 maximum rate of change of current will consequently occur 

 when the coil is at the point on the curve, where its effect 

 on the poles will be greatest. We should thus obtain the 

 maximum loss due to eddy currents with armature current 

 and induced volts exactly in phase. This condition is, 

 however, scarcely ever found, since, even with a non-inductive 

 load circuit the current lags behind the induced voltage. It 

 is thus easy to see that the action of the armature currents 

 in inducing currents in the field windings and pole faces 

 depends on the power factor of the circuit. 



