FLUX DISTRIBUTION 



131 



shown in Fig. 44. This curve, which represents the permeance 

 per square centimeter of armature surface between pole and 

 armature, can evidently be thought of as a flux-distribution curve 

 on the assumption that one pole acts alone without interference 

 from neighboring poles. 



In regard to the actual flux distribution for no-load conditions, 

 it may be argued that if two neighboring poles each acting alone 

 would produce a flux distribution as shown respectively by the 

 full-line and dotted curves of Fig. 46, then the flux at any point 

 p will be pm pn. This method of plotting the resulting flux- 

 distribution curve should give satisfactory results in the space 



Or 



FIG. 47. Practical construction for deriving flux curve from permeance 



curve. 



between pole tips, but it does not provide for the gradual change 

 in the flux distribution near the pole tips where the shading effect 

 of the masses of iron becomes important. For the practical 

 designer the writer recommends the approximation indicated in 

 Fig. 47 where P is the permeance curve previously obtained. 

 The flux curve is derived therefrom by drawing the straight 

 line RS, connecting the point on the permeance curve directly 

 over the geometric neutral to the point S immediately under the 

 pole tip. By subtracting from the ordinates of the curve P 

 the corresponding ordinates of the triangle ORS, the curve OA'N 

 is obtained, representing the flux distribution on open circuit. 

 This curve has yet to be calibrated, because the value of its 

 ordinates cannot be determined unless either the m.m.f. or the 

 total flux per pole is known. In designing a machine, the total 



