ALTERNATORS . 219 



inductive load under ordinary circumstances to be appreci- 

 able. 



Effect of Armature Currents upon the Field. In Experiment 

 VII. , page 52, was given the method of determining the 

 impedance of the armature while stationary. It was further 

 stated that currents might be induced in the pole faces, 

 poles and field windings, by the alternating armature flux. 

 It is now desirable to consider in greater detail the nature 

 and measurement of these effects. 



The winding of the armature acts like the primary 

 winding of a transformer in producing an alternating mag- 

 netic flux. The path of this flux will vary with the position 

 of the armature winding relative to the field. In certain 

 positions the flux will pass across the air-gap from the arma- 

 ture core to one pole tip, then through the iron of the pole 

 to the other pole tip of the same pole, and across the air-gap 

 into the armature core. In the position occupied by the 

 armature cycle later, the flux has to complete its path 

 external to the armature core between a pair of poles, instead 

 of through the iron of one pole. In this case the effect of 

 the flux on the poles is comparatively slight. With an 

 alternating current flowing in the stationary armature it 

 will therefore depend on the position of the armature relative 

 to the poles how much these will be affected by the 

 alternating flux of the armature. 



If the poles are not laminated, the effect of the alternating 

 field will be to produce eddy currents in the iron of which 

 they are composed. If they are surrounded by a winding, 

 an alternating voltage will be induced in the winding which 

 will produce an alternating current if the winding is closed. 



The effect of such induced currents upon the armature 

 winding will be similar to the effect of currents in the secondary 

 of a transformer upon the primary winding. That is, the 

 self-induction of the winding is reduced and some of the 

 power of the primary circuit is lost in producing these 

 secondary currents. 



The actual extent to which these losses occur in actual 

 working is not easily measured, but the following description 

 gives some idea of the way in which they are formed and 

 the extent to which they may exist. 



By making connections exactly as described in Experiment 

 VII. for the determination of amature impedance while 

 stationary, but including a wattmeter in the supply circuit, 



