PROCEDURE IN DESIGN OF D.C. GENERATOR 227 



current; but this is usually an unnecessary refinement; and when 

 calculating brush losses, great accuracy in results is not attainable 

 because the resistance at the brush-contact surface is always a 

 quantity of rather doubtful value. 



Item (113): Brush-friction Loss. Refer Art. 54. Assuming a 

 coefficient of friction of 0.25, the brush-friction loss by formula 

 (89) is 



w 0.25 X 1.5 X 18 X 600 X 13.5 X TT X 746 00yl 

 Wf=s ~ 12X33,000 =324 watts. 



Items (115) otyZ (116): Cooling Surface and Temperature Rise 

 of Commutator. Refer Art. 54 and Fig. 70, page 183. The radial 

 height of the risers will be about 2 in., making D r = 17^ in* 

 The radial depth of the exposed ends of the copper bars might 

 amount to % in., making D e = 12 in.; and the total cooling 

 surface considered, worked out as explained on page 182, amounts 

 to 492 sq. in. 



The radiating coefficient, as given in formula (90), is 



O 1 Qf) 



- 025 + = - 0463 ' 



1 054 

 whence T = 400 y n Q463 = 46.3 C., which is permissible. 



Had this calculated temperature rise exceeded 60, it might 

 have been necessary to increase the axial length of the com- 

 mutator, or reduce the losses by using a soft quality of carbon 

 brush and perhaps a lighter pressure at the contact surface. 

 In some cases special ventilating ducts are provided inside the 

 commutator; but these should not be necessary in a machine of 

 the type and size considered. 



Item (117): Leakage Coefficient. Refer Art. 56, page 186. 

 The value of this item was estimated at 1.2 in connection with 

 the shaping of the pole shoe (item (68)). 



Items (118) to (121): Flux Density in Pole Core. A cylindrical 



pole core 10 in. in diameter has been decided upon (item (68)). 



The area of cross-section is therefore 78.54 sq. in., and the 



full-load flux density in the pole core near the yoke ring is 



6,970,000 X 1.2 



78.54 X 6.45 



Item (122): Radial Length of Pole. Refer Art. 55, page 186. 

 The full-load ampere-turns per pole for air gap, teeth and slots 

 amount to about 6,000 (see Fig. 83). Then, on the basis of 



