CHARACTERISTIC CURVES. 



389 



The drooping of the characteristic of the series generator. It 

 would seem as though the useful flux <I> through the armature of 

 a series generator should increase more and more with the current 

 output of the machine, but in fact the effect of magnetic leakage 

 around the armature is to cause < to actually decrease in value 

 when the current output is excessive, especially if the field iron 

 becomes saturated before the armature iron. This decrease of <l> 

 means an actual decrease of E a and of course E x falls off more 

 than E a on account of the armature drop R a f a . 



Fig. 22 shows the external characteristic (full line curve), and 

 the total characteristic (dotted curve) of a Wood arc lighting 

 series generator, with its governing device disconnected. 



,2500 



1300 



500 



Volts 



46 

 Fig. 22. 



Explanation of drooping of series characteristic. When a series generator has 

 a large coefficient of magnetic leakage, and when the field yoke and cores are nearly 

 saturated magnetically, the armature core being far below saturation, then the total 

 electromotive force induced in the armature decreases with increase of current. This 



