ELEMENTS OF ELECTRICAL ENGINEERING. 



Pole pieces are 

 sometimes lami- 



the armature core has projecting teeth, inasmuch as the lines of 

 force between the pole face and the armature core are concentrated 

 in the form of tufts or brushes, as shown at bbb in Fig. 31, and 

 as the armature rotates these tufts sweep across the pole face and 



thus tend to produce eddy 

 currents, 

 therefore 

 nated. 



The prevention of eddy 

 currents is the more com- 

 plete the thinner the lami- 

 nations and the better the 

 insulation between them. 

 When a mass of iron is sub- 

 ject to very rapid and fre- 

 quent magnetic reversals the 

 laminations are made very 

 thin and insulated by a coat- 

 ing of hard varnish before 

 being assembled on the arma- 

 ture shaft. For example, in an experimental alternator recently 

 constructed by the Westinghouse Electric & Manufacturing Com- 

 pany * the armature core is subject to 20,000 magnetic reversals 

 per second and the laminations are only o.oo I inch in thickness. 

 In direct-current dynamos of moderate size the armature core is 

 subject to from 15 to 60 or more magnetic reversals per second, 

 and the laminations are usually made from 0.020 to 0.035 mcn m 

 thickness. Large copper wires on smooth core armatures have 

 eddy currents induced in them on account of the difference of 

 intensity of the magnetic field in the air gap at the two edges of 

 the moving conductor. These eddy currents flow up along one 

 side and down along the other side of the conductor. The result 

 is that the actual total current is crowded over to one side of the 



Fig. 31. 



* Trans. A. I. E. ., B. G. Lamme, 1904. 



