54 



IMPEDANCE. 



The self-induction of the armature can be easily calculated 

 from the results obtained above, since 

 Impedance = *J~R*~+~(2 IT n Lf 



When R = resistance of armature. 

 L = self-induction armature. 

 n = periodicity of current. 



The self-induction should be calculated for two values 

 obtained with the magnets excited, and the load current 

 respectively, at its maximum and minimum values. 



Fig. 24 gives two curves obtained in the manner just 

 described, the experiment being carried out on the armature 

 of a small Pyke & Harris inductor alternator. The lower curve 



30 



120 



150 



60 90 



Degrees. 



Fi. 24. CUKVKS OF IMPEDANCE OF ARMATURE OF INDUCTOR ALTERNATOR. 



shows the results with the field fully excited with 3-8 amps., 

 and the armature carrying its normal current of 5 amperes. 

 The upper dotted curve shows the results obtained with the 

 field unexcited, and the same armature current. 



In this type of machine the armature is wound in two sets 

 of windings between which the soft-iron inductors rotate ; 

 the variation of self-induction is therefore specially large in 

 this case. The armature resistance was high, being 3 -65 ohms. 

 The effect of the field excitation is to partially saturate the 

 iron cores of the armature and of the inductors. This reduces 

 the permeability of the magnetic path, and consequently also 



180 



