222 PRINCIPLES OF ELECTRICAL DESIGN 



values of SI corresponding to each known value of the air-gap 

 density, B g . 



The average ordinate of the curve A in Fig. 84 as obtained 

 by dividing the area under the curve by the length of the base 

 is found to check within 1 per cent, of the required amount 

 (average B g = 5,910 gausses). Had there been an appreciable 

 difference between the calculated and measured areas of the 

 flux curve, it would have been necessary to correct the m.m.f 

 curve of Fig. 83, and re-plot the flux curve A of Fig. 84. 



Items (78) and (79): Flux Distribution under Load. Refer 

 Art. 43, page 137. The curve of armature m.m.f., of which the 

 maximum value is 3,feO ampere-turns (item (17)), may now be 

 drawn in Fig. 83. The point k has been selected for the brush 

 position because the (positive) field m.m.f. has here about the 

 same value as the (negative) armature m.m.f. From the re- 

 sultant m.m.f. curve, the flux curve B of Fig. 84 is plotted, the 

 area of which measured between brush and brush is found 

 to be 99.5 sq. cm. while curve A measures 107 sq. cm. The 

 average air-gap density is therefore less than before current 

 was taken from the armature, and the loss of flux is due partly 

 to distortion (tooth saturation) and partly to the demagnetizing 

 ampere-turns (brush shift). 



Items (80) to (83): Corrected Full-load Flux Distribution. 

 Refer Art. 43. The e.m.f. to be developed at full load is 238.3 

 volts, obtained by adding the numerical values of items (45), 

 (46), and (48), to the full-load terminal voltage. The final flux 



, 106.3 X 238.3 

 curve C should therefore have an area of - 220 - = 115 



sq. cm.; the number 106.3 being item (74). In order to estimate 

 the probable increase in field excitation to obtain this increase 

 of flux, we may follow the method outlined on page 138. The 

 ampere-turns necessary to bring up the flux from the reduced 

 value under curve B to the original value under curve A, are 

 calculated by assuming that the air-gap density under the pole 



99 5 

 has changed from 7,800 gausses to 7,800 X T = 7,260 gausses; 



and the ampere-turns necessary to increase the developed volts 

 from 220 to 238.3 are calculated by assuming that the air-gap 

 density under the pole must be raised from 7,800 to 7,800 X 



ooo o 



= 8,450 gausses. The total additional excitation is indi- 



cated by the distance SS' in Fig. 82, its value being about 900 





