208 PRINCIPLES OF ELECTRICAL DESIGN 



thick (see Art, 28, page 96). The space left for copper is 

 therefore 0.5 - (0.07 + 0.048) = 0.382; and the width of each 

 conductor is one-third of this amount, or 0.127. Let us make 

 this J in. (0.125). The depth of the (rectangular) conductor 



0402 

 will be 25^ = 0.322, or, say, % 6 in. (0.312). These are the 



dimensions called for under item (26); and the corrected value 

 for item (25) is 0.312 X 0.125 = 0.039 sq. in. 

 The required slot depth is made up as follows: 



Hard- wood wedge, which should be about . 200 in. 



Insulation above, below, and between the coils 



= 3 X 0.035 0. 105 in. 



Cotton covering on wires (twice 0.016) . 032 in. 



Copper 0.624 in. 



0.961 in. 

 or, say, 1 in. for the dimension d in Fig. 77. 



Before finally adopting these dimensions, it will be necessary 



to see that the flux density 

 in the teeth is not excessive 

 (item (39)). 



Items (32) to (34) : Tooth Di- 

 mensions. The width of tooth 

 at the top is t = X - s = 0.576. 

 The circumference of the 

 circle through the bottom of 

 the slots is IT X 17.5; and since 

 the slots have parallel sides, the 

 FIG. 77. Arrangement of conduc- width of tooth at the root is 



*-*ff - 0.5 = 0.466. 



Items (35) and (36) : Cooling Ducts. Refer Art. 33, page 105, 

 Not more than three ducts should be necessary in an armature 

 11 in. long. Each duct might be 0.4 in. wide. 

 Item (37) : Net Length of Armature. Refer Art. 31, page 103. 



l n = 0.92 (11.1 - 1.2) = 9.1 in. 



Item (38) : Net Cross-section of Teeth under Pole. The cross- 

 section of iron in the teeth under one pole, at a point halfway 

 up the tooth, is, 



9.1 X 0.521 X X 0.72 = 48.6 sq. in. 



