CHAPTER VII 

 THE ALTERNATING CURRENT GENERATOR 



56. General Construction. The alternating current gen- 

 erator, or alternator, as it is frequently called, consists of 

 a field structure, magnetized by electro-magnets, and an 

 armature, in the winding of which the electromotive force 

 is generated. In three respects the alternator differs greatly 

 from the continuous current generator; it has no commu- 

 tator, the field structure rotates instead of the armature as 

 in the case of c-c. machines and an alternator is practically 

 never self-exciting. Small alternators sometimes have a 

 stationary field and a rotating armature, but all large 

 alternators are of the revolving field type. 



Slip Rings Instead of Commutator. In case the armature 

 rotates the ends of its winding are connected to slip rings 

 instead of to a commutator; the number of rings used 

 depends upon the type of armature winding but generally 

 there are either two or three. Bfushes (generally made of 

 blocks of some special metal) bear on these rings and 

 serve to carry the current to the external circuit. When 

 the field revolves, the continuous current for its excitation 

 is led into the field winding by means of slip rings and 

 brushes. 



Alternator Not Self -exciting. The current for 'the field 

 winding must be continuous and as the armature of the 

 alternator furnishes only alternating current it is evident 

 that some additional source of power must be available 

 for obtaining the field excitation. 



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