76 



ARMATURK CONSTRUCTION 



A practice to which recourse is often permissible, consists 

 in employing two or three extra thick laminations on each side 

 of the duct, to prevent the teeth from spreading towards the 

 duct, and to decrease the liability of breaking off at the roots, 

 in which case comparatively few fingers need be employed. 

 These fingers may in such cases project but part way to the 

 surface, or every other finger may project to the surface, 

 the alternate fingers stopping off at or, preferably, quite a 

 little below the bottom of the slots. Cast distance-pieces, in 

 spite of their cheapness, are gradually being abandoned, as they 



FIG. 77. Ventilating Spacing-piece. Dick, Kerr & Co. 



occupy too much of the space which ought to be left free. In Fig. 

 76, however, we have given a typical cast spacing-piece which 

 has been employed on a 550 k.w. 10-pole traction generator, of 

 which a number are in use in Great Britain. 



The ventilation of armatures with a comparatively small 

 number of teeth per pole may more readily be made effective, 

 since each tooth will be comparatively quite wide at the root, and 

 hence the openings between the coils will be more free for the 

 discharge of air. Other instances of good designs of ventilating 

 pieces are shown in Figs. 77 to 83. Fig. 77 shows a method 

 which has been used by Messrs Dick, Kerr & Co., where the 

 stampings are spaced at the teeth by twisting the teeth of one of 

 the laminations at right angles. 



