96 



PRINCIPLES OF ELECTRICAL DESIGN 



(Slot Lining of Press- 

 Npahn about 0.010" 

 (Thick 



. Cotton Tape; Empire 

 XJloth or Micanite to 

 Total Radial Thick- 

 ness about 0.030" 

 Cotton Covering; 

 Braiding or Tape on 

 Conductors, of 0.007" 

 to0.0138ingle 



\ Preaapahn or Fish 

 Paper Divi'ding Strip, 

 of about O.Oao'Tblck 



need not at present concern himself with practical details of 

 manufacture. It is obvious that the insulation should be so 

 arranged as to leave the greatest possible amount of space for 

 Hard wood wedge the copper. The insulation 

 may be placed around the in- 

 dividual coils, or in the slot 

 before the coils are inserted. 

 If preferred, part of the insu- 

 lation may be put around the 

 coils and the remainder in 

 the form of a slot lining. The 

 essential thing is to have 

 sufficient thickness of insula- 

 tion between the cotton-cov- 

 > ap er at Bottom ered wires and the sides of the 



~of Slot about 0.020 i , mi j? n r> 



Thick slot. The following figures 



FIG. 32. Insulation of conductors in may be used in determining 



the necessary thickness of slot 



lining. These figures give the thickness, in inches, of one side 

 only, and this is also the thickness that should be provided be- 

 tween the upper and lower coil sides in the slot. 



For machines up to 250 volts . 035 in. 



For machines up to 500 volts . 045 in. 



For machines up to 1,000 volts . 06 in. 



t For machines up to 1,500 volts 0.075 in. 



In high-voltage machines an air space is sometimes allowed 

 between the end connections, i.e., the portions of the coils not 

 included in the slots. This air clearance would be from % to 

 K in. for a difference of potential of 1,000 volts, with an addition 

 of J-6 in. for every 500 volts. In regard to the surface leakage 

 where the coils pass out from the slots; a breakdown of insulation 

 at this point is usually guarded against by allowing the slot 

 lining to project at least % in. beyond the end of the slot. For 

 working pressures above 500 volts, add Y in. for every additional 

 500 volts. 



The finished armature should withstand certain test pressures 

 to ensure that the insulation is adequate. The standardization 

 rules of the A.I.E.E. call for a test pressure of twice the normal 

 voltage plus 1,000 volts. 



29. Current Density in Armature Conductors. The per- 

 missible current density in the armature coils is limited by tern- 



