350 PRINCIPLES OF ELECTRICAL DESIGN 



flux lines into the air gap above the wedge (tooth top leakage), is 

 approximately, 



0.47T X 12,300 X (L75 X * 9 g 5) X 2>54 = 1,900,000 maxwells, 



where the figure 1.75 is the assumed radial depth, in inches, of 

 the flux path, and 1.8 is the assumed average length of the flux 

 lines (somewhat greater than the width of slot below the wedge). 

 The sum of the two flux components is 4,000,000 maxwells, 

 making the total slot flux for both sides of the pole face equal to 

 twice this amount, or 8,000,000 maxwells. As a rough estimate, 

 we may assume the end leakage to be about one-sixth of this, 

 making a total of 9,300,000 maxwells. The full-load leakage 



65 21 93 

 coefficient is therefore ' = 1.142 or (say) 1.15. 



The maximum number of flux lines in the rotor, which cross 

 the section below the slots (represented by 28 lines in Fig. 145) 



The cross-section of iron below the vent ducts is 12J- X 

 600 sq. in.; which makes the average flux density 62,500 lines 

 per square inch. The section of iron below the slots is therefore 

 sufficient, and the reluctance of the body of the rotor is a neg- 

 ligible quantity in comparison with that of the teeth and air gap. 



Items (62) to (68). With a density of 8,500 gausses in the 

 stator core (item 28), and ample iron section in the rotor, the 

 additional ampere-turns required to overcome reluctance of 

 armature and field cores will probably not exceed 200; and since 

 this is a very small percentage of the excitation for air gap and 

 teeth, we shall not need to draw a new curve for item (62): 

 the curves of Fig. 139 may be thought of as applying to the 

 machine as a whole. The ampere-turns at no load and at full 

 load (items 63 and 64) will therefore be taken at 27,000 and 

 37,000 respectively, as previously calculated. 



The slot insulation should be about J^ in. thick, and the field 

 winding might be in the form of copper strip \Y in. wide laid 

 flat in the slot. Allowing % in. total depth of insulation prefer- 

 ably of mica or asbestos fabric between the layers of the wind- 

 ing, the cross-section of copper will be 2% X 1J4 = 3.6 sq. in. 



12 300 

 making the current density at full load, A = ' - = 3,430 



O.v) 





PROPERTY OF ELECTRICAL oTv 



FACULTY OF APPLIED SCIENCE. 



