46 



ARMATURE CONSTRUCTION 



TABLE IX. 

 FRUCHT'S VALUES FOR AIR-GAP DEPTHS. 



Radial Depth of Air-gap in mm. Diameter at Air-gap. 



0-4 Up to 200 mm. 



0-5 400 



075 600 



1-00 800 



The writers do not, however, care to employ so small a radial 



depth of air-gap, and their own 

 experience has shown them that 

 it is by no means essential to 

 good results. 1 



In the case of the rotors of in- 

 duction motors, plates consider- 

 ably thicker than *5 mm. might 

 quite properly be used ; and this 

 would not only decrease their first 



FIG. 38. Punching extra deep Slot. , , 



cost, but, inasmuch as it would 



decrease the number to be handled, the labour cost would also be 



less. But since the rotor core plates can be made out of the 



otherwise wasted centres punched out from the stator core plates, 



this practice is generally followed, and the same thickness of sheet 



is generally employed for stator and rotor ; in 



fact, the very best results are obtained by 



making a compound die to punch out both 



stator and rotor discs, complete with slots, at 



a single operation. It is also sometimes the 



practice to punch out both stator and rotor 



slots at a single operation, and then to cut 



first the external diameter of the stator ; 



secondly, the internal diameter ; and finally 



the internal diameter of the rotor, on circular 



shears. Some discs, thus prepared, are shown 



at the foot of the index press in Fig. 40. 



Obviously, there still remains the further l ^ oi '~~^ periphery!"* 

 operation of turning down the rotor core on a 

 lathe subsequent to assembling. Index presses of various firms 

 are illustrated in Figs. 40, 41, 42, and 43. While varying greatly 

 in detail, the underlying principle is that each stroke of the punch 

 is followed by an angular motion of the clamps holding the disc or 



1 A Table of the writers' values for induction motor air-gaps will be found 

 on p. 105, Chap. VI. 



