FLUX DISTRIBUTION 



137 



the curves of Fig. 49 and plotting the values of air-gap density 

 corresponding to the ampere-turns read off the curve R of Fig. 53. 

 The procedure is exactly the same as when obtaining the open- 

 circuit flux curve (Fig. 51) by using 'the values of Figs. 49 and 

 50, but the new curve of flux distribution which may be called 

 curve B, to distinguish it from the open-circuit flux curve A 

 gives the distribution over armature surface for a given brush 

 position and a specified output. The difference in area of 

 curves A and B is a measure of the flux lost through armature 

 reaction; it includes not only direct demagnetization, but also 

 cross-magnetization which by producing distortion of the flux 



Max.Field SI per Pole 



Resultant M.M.F.under 

 Load Condi tons 



Brush Shift 



Fia. 53. Addition of field and armature m.m.fs. 



distribution leads sometimes to local concentration of high 

 densities and reduction of total flux owing to saturation of the 

 iron in the armature teeth. 



The final flux curve for the loaded machine is generally similar 

 to curve B, except that its area must be such as to indicate that 

 the desired voltage will be generated. This increased area is, of 

 course, obtained by increasing the field ampere-turns. In other 

 words, the curve F of Fig. 53 has to be replaced by a new open- 

 circuit m.m.f. curve such that the new R curve resulting from its 

 combination with the existing curve A will produce a new flux curve 

 similar to B, but of the required area. This new flux-distribution 

 curve may be called curve (7, to distinguish it from the open- 

 circuit curve A and also from the flux curve B, which, although a 

 load curve, has too small an area to generate the required volt- 

 age. The amount by which* the ordinates of curve F should be 

 increased may be found by trial, but it is generally possible to 

 estimate the necessary correction to give the required result. 



