ARMATURE LAMINATIONS 



45 



FIG. 36. German Type Die. 



it is particularly appropriate for induction motors, which, owing to 

 the small radial depth of the air-gap, require very exact die-work. 

 He further points out that when such ___ m ^__^_^^^ 

 work is done on circular shears, with 

 the consequent lesser exactness, diffi- 

 culties are sometimes encountered in 

 subsequently turning down the core on 

 a lathe to the precise required dimen- 

 sions. These difficulties relate to the 

 liability of bending the teeth, and, in 



the case of nearly closed slots, tearing the tips of the teeth. The 

 bending of the teeth may be avoided by inserting temporary keys 

 completely filling the slots, prior to the turning. The tearing of 

 the teeth may be avoided by first punching completely closed 

 slots, then turning the core to the precise dimensions, and finally 

 milling out the slot openings. This last step in the operation 



introduces the lia- 

 bility to increased 

 eddy current loss. 

 This is of minor 

 importance on the 

 rotors of induction 

 motors, but is to be 

 avoided in the 

 stators. In fact, in 

 the case of the 

 stators of induction 

 motors, compound 

 dies are greatly to 

 be preferred. 



Frucht, however, 

 has obtained excel- 

 lent results with the 

 process employing 

 the punch shown in 

 Fig. 39. He states that by using good, smooth plates, and by 

 ensuring that slots of different plates but corresponding to a given 

 position of the index of the punching machine shall be over one 

 another in the assembled core, he has for years, without doing any 

 tool-work or filing on the assembled core, successfully employed 

 the following scale of radial depths of air-gap and diameter at 

 air-gap: 



FIG. 37. Disc with Slots in Inner Periphery. 



