84 ELEMENTS OF ELECTRICAL ENGINEERING. 



flows before it reaches the terminals of the machine. Electro- 

 motive force lost in this way is called internal drop, and because 

 of this internal drop, the electromotive force between the terminals 

 of the generator is less than the electromotive force actually 

 induced in the armature. 



The effect of internal drop upon the terminal voltage of a 

 generator may be easily formulated as follows : 



Let 7 a be the current flowing through the armature ; 



/ the current flowing through the shunt field winding ; 

 / the current flowing through the series field winding of 



a series or compound machine ; 

 /. the current flowing through the external circuit ; 

 E the total electromotive force induced in the armature ; 



GL 



E x the electromotive force between the terminals of a 



generator ; 



R a the resistance of the armature, including resistance of 

 brushes and of contacts between the brushes and the 

 commutator ; 



R 8 the resistance of the shunt field winding ; and 

 R c the resistance of the series field winding ; 

 R the resistance of the external circuit. 



The loss of electromotive force in the armature due to armature 

 resistance is RJ a , and the loss of electromotive force in the series 

 field winding on account of its resistance is RJ c , so that the total 

 internal drop is RJ a -f RJ C and the relation between E x and E a 



E, = E,-RJ a -RJ, (22) 



A voltmeter connected to the terminals of a generator indicates 

 E x . The value of E a cannot be directly measured by a voltmeter 

 except when 7 n and / are negligibly small, in which case E a E x . 



In case of the shunt generator there is no series field winding, 

 so that R c is zero .and the term RJ c drops out of equation (22). 



In case of the^ series generator 7 a = / = /. since there is but 

 one electric circuit. 



