EXAMPLE OF ALTERNATOR DESIGN 357 



is a refinement which need not be applied in the case of a turbo- 

 alternator, in which the pole pitch is always large, causing 4>. 

 to be small in relation to <l>; but in machines with a small pole 

 pitch especially if there is only one slot per pole per phase 

 the correction should be made. 



The full-load flux per pole is 65.2 X 10 6 maxwells (item (40)), 

 as against 62.2 X 10 6 (item (17)) on open circuit. With the in- 

 crease of flux density, the loss per pound of iron will be about 

 3.5 instead of 3.2 watts, and the full-load iron loss will, therefore, 

 be 0.3 X 28,000 = 8.4 kw. more than on open circuit; thus 

 bringing the total iron loss up to (say) 130 kw. 



The loss at the slip rings may be calculated by the approximate 

 formula given in Art. 111. The diameter of the slip rings will 

 probably not be less than 15 in., so that the rubbing velocity 



will be T * 2 15 X 1,800 = 7,100 ft. per minute. The contact 



area of the two sets of brushes (to carry 514 amp.) might be 5 



7 100 X 5 

 sq. in., making the loss from this cause ? ir ^ = 355 watts, 



which is negligible in comparison with the other losses. 

 Adding up the separate losses, we have: 



Windage and friction ................................. 88 kw. 



Stator iron .......................................... 130 kw. 



Stator copper ........................................ 21 kw. 



Rotor copper ........................................ 66 kw. 



Total .................................. 305 kw. 



The kw. output is 0.8 X 8,000 = 6,400 and the efficiency, ex- 

 cluding losses in exciter and in air blower external to the gener- 



ator, is therefore . ' , Qg = 0.955. 



Items (74) and (75). On the basis of 290 kw. to be carried 

 away by the circulating air with a mean increase of temperature 

 of about 20C., the quantity of air required will be 29,000 cu. ft. 

 per minute. The cross-section of the ducts (item (69)) is 6.6 sq. 

 ft., and the average velocity in the ducts (item (75)) is, therefore, 



' _ = 4,400 ft. per minute. 

 b.o 



In the design of large generators there are many matters of 

 detail to be considered which have received but little attention 

 here. The question of temperature rise, for instance, is one that 

 would receive more attention from the practical designer than 



