PROCEDURE IN DESIGN OF D.C. GENERATOR 225 



O 



and this corresponds to a density of t ., 



J.J. 



= 126 gausses. 



The ideal flux density at beginning of commutation is therefore 

 712 126 = 586 gausses, and at the end of commutation, 712+ 

 126 = 838 gausses. 



Item (107): Sparking Limits of Flux Density in Zone of Com- 

 mutation. Refer Art. 52. The time of commutation, in seconds, 



6000 - 



3000- 



2000 



1000- 



1000 - 



I k j i h g 



FIG. 85. Flux distribution in zone of commutation. 



sec -5 an( *> 



is approximately t c = ^ = 2 130 X 12 = 

 formula (88) page 177; 



$ = M X 0.00176 X 10 8 

 = 132,000 maxwells. 



The variation in flux density which is permissible when carbon 

 brushes of medium hardness are used, will therefore be 



132,000 



1.083X11X6.45 



gausses ' 



A portion of the full-load flux curve C of Fig. 84 has been re- 

 drawn in Fig. 85 to a larger scale, and the flux density required 



15 



