12 



ARMATURE CONSTRUCTION 



kept in the stores; but from the strictly technical standpoint, 

 sheets of very different thickness should be employed in designs 

 for various purposes. The choice should be dependent upon the 

 periodicity and speed of the machine, and also upon the induced 

 volts per cm. length of armature core in a direction parallel to the 

 shaft. There are many cases where sheets much thicker than 0*5 

 mm. could be employed with advantage such, for instance, as for 

 the cores of the rotors of induction motors. 



On the other hand, not much, if anything, can be gained by the 

 use of sheets of less than 0*5 mm. thickness ; for this entails a 

 high percentage of total thickness of insulating varnish. As the 

 thickness of insulating varnish per sheet cannot be reduced much 

 below 0'03 mm., it is obvious that the total percentage lost space 

 will be greater the greater the subdivision of the core. Thus, 

 taking 0'03 mm. as a representative thickness of Japan varnish, 

 and 0'02 mm. as the lost space through lack of uniformity of the 

 sheets as pressed up between the armature core flanges, then the 

 total core length per lamination (exclusive of ventilating ducts) 

 may, for various thicknesses of laminations, be derived as set forth 

 in Table I. 



TABLE T. 



SPACE OCCUPIED BY COKE PLATE INSULATION. 



The thinner the plate, moreover, the greater will be the per- 

 centage which the skin of inferior magnetic quality bears to the 

 total thickness of the plate. The price also increases \vith 

 decreasing thickness. To sum up, it may be said that experience 

 has shown 0*5 mm. (20 mils.) to be a satisfactory value for most 

 purposes, and in the interests of standardisation it is preferable 

 to empl< value in almost all armature coree. Departures 



