344 PRINCIPLE'S OF ELECTRICAL DESIGN 



The m.m.f. curve for flux curve A has been re-drawn in Fig. 

 142, the stepped curve being replaced by a smooth curve. In 

 this connection it should be noted that the " fringing 7 ' of flux at 

 the tooth tops tends- to round off the sharp corners of the flux- 

 distribution curves, and so justifies the use of smooth curves in 

 any graphical method of study. At the same time, it will gen- 

 erally be possible to detect in oscillograph records of the e.m.f. 

 waves the irregularities or " ripples" due to the tufting of the 

 flux at the tooth tops; but these minor effects will not be con- 

 sidered, either here or later when calculating the form factor of 

 the e.m.f. wave. 



10 20 30 40 50 



FIG. 142. M.m.f. curves for 8000 k.v.a. turbo-generator. 



70 80 90 100 110 120 130 140 150 160 170 180 10 20 30 

 Electrical Decrees 



Items (52) to (54). The area of the flux curve A of Fig. 141, 

 on the assumption that it is a true sine curve similar to the one of 

 Fig. 140, and on the basis of unit squares with sides equal to 

 1 cm., is 6.01 X 18 =" 108 sq. cm., where 6.01 is the average 

 density in kilogausses (item (18)). The required area of the full- 

 load flux curve (7, at the specified power factor of 0.8, is therefore 



4 000 

 108 X ^77; = 113.5, where the figure 4,000 is the length of the 



o,olU 



vector OE'g of Fig. 137, as calculated under item (40), and the 

 figure 3,810 is the open-circuit star voltage (vector OE t ). 



In order to determine the field excitation necessary to provide 

 the required flux with full-load current taken from the machine 



