PARTS OF A DYNAMO-ELECTRIC MACHINE 43 



that the joints in one lamination do not come opposite 

 those in the adjacent one. 



Smooth and Slotted Cores. In the early types of dynamo- 

 electric machines the periphery (outside surface) of the 

 armature core was smooth but in all modern machines the 

 armature core is toothed (or slotted). In such a core there 

 are a series of slots in the outer surface, running parallel 

 to the direction of the shaft. These slots serve to hold 

 the armature winding safely in place and also to reduce 

 the effective/length of the air gap of the machine. 



By reference to Fig. 18 it is seen that the minimum 

 length of/air gap is fixed by the distance required for rnechan- 



FIG. 18. Smooth Core and Slotted Core. 



ical clearance between the armature and the pole face plus 

 the depth of the winding. In the toothed armature the 

 effectiv length of the air gap is determined by the mechanical 

 clearance distance only; as the armature teeth are good 

 conductors for the magnetic lines, the only distance the 

 magnetic lines have to go through air is that between 

 the outside of the teeth and the inside of the pole face. 



Armature Spider. The ring-shaped armature core must 

 be fastened to the shaft in some way and for this purpose 

 the armature spider is used. The spider generally consists 

 of a cast-iron hub through which the shaft is fitted and from 

 which radiates a set of spokes; on the ends of the spokes 

 are lugs which are fastened by dovetailing keys to the 



