PROCEDURE IN DESIGN OF D.C. GENERATOR 233 



termination of the temperature rise, it would be necessary either 

 to increase the weight of copper in the coils, or to sectionalize 

 the windings so as to improve the ventilation. The latter course 

 would be the right one in this case since the copper loss is not by 

 any means excessive, and it would be desirable to reduce rather 

 than increase the amount of copper in the field windings. 



Items (145) and (146) : Resistance of Diverter. Assuming the 

 "long shunt" connection, the series current passing through the 

 diverter will be 30.56 amp., and the resistance of the diverter 

 must therefore be 



OQQ 



0.0028 X ^TT^ = - 0275 ohm - 



oO.OD 



A resistance slightly greater than this should be provided, of a 

 material and cross-section capable of carrying at least 40 amp. 

 without undue heating. The final adjustment can then be made 

 when the machine is on the test floor. 



Item (147) : New Calculation of Losses in Teeth. Refer Art. 60, 

 Chap. IX. The maximum value of the air-gap density under 

 full-load conditions may be read off curve C of Fig. 84 (page 221) 

 where it is seen to be 10,800 gausses. The corresponding tooth 

 density, as read off Fig. 81 (page 216), is 23,100. This is the 

 density at the narrowest part of the tooth. On the assumption 

 that flux neither enters nor leaves the tooth up to a distance d e 

 from the bottom of the slot (see Fig. 38, page 122), the tooth 

 densities at the three sections considered are B w 18,870, B m = 

 21,000, and B n = 23,100 gausses. Referring to Fig. 34 (page 

 102), we find the watts per pound corresponding to these den- 

 sities to be 4.1, 4.8, and 5.5, respectively, the mean value being 

 4.8. The total weight of iron in the teeth (item (57)) is 75 lb., and 

 the corrected total loss in the teeth is 75 X 4.8 = 360 watts. 



Items (148) and (149): Efficiency at Any Output. Refer Art. 

 60, Chap. IX. The efficiency table on page 235 requires but 

 little explanation. Each column stands for a particular output, 

 expressed as a fraction of rated full load. The terminal voltage 

 is calculated on the assumption that it conforms to a straight- 

 line law based on the known full-load and no-load voltages. 



The windage and friction loss is taken as 1.8 per cent, of the 

 full-load output (see page 196). 



The core loss which includes the corrected tooth loss is the 

 calculated full-load value. It will actually vary somewhat with 



