290 PRINCIPLES OF ELECTRICAL DESIGN 



100). This must be balanced by the field component MM oy 

 giving OM as the required field excitation at full load. If the 

 load is now thrown off, the developed voltage will be OE , where 

 the point E is the intersection of OM and the prolongation of 



f)W O1\/T 



the line PE' 0} because this satisfies the condition 77^7- = ^,,- 



(Jrj g UM. 



The maximum value of the e.m.f. OE will be generated in the 

 conductors immediately opposite the center of the pole face. 

 The required angle of displacement, ft between center line of 

 pole and position of conductor carrying the maximum current 

 may thus be calculated, and the full-load flux curves plotted as 

 in the case of the D.C. machine, where the displacement of the 

 curve of armature m.m.f. is determined by the movement of the 

 brushes. It must not be overlooked that this method is not 

 strictly accurate, since it is based on assumptions that are rarely 

 justified in practice. 



99. Air-gap Flux Distribution under Load. Having deter- 

 mined the value of the angle & (Fig. 108, page 277), the curve of 



I 



FIG. 116. Vector diagram of alternator m.m.fs. 



resultant m.m.f. for any condition of loading can be obtained by 

 adding the ordinates of the field and armature m.m.f. curves. 

 The procedure is then the same as was followed in the D.C. de- 

 sign to obtain the load flux curve C (Art. 43, Chap. VII), except 

 that the drawing of the flux curve B as an intermediate step will 

 not now be necessary, seeing that the effect of armature distor- 

 tion and demagnetization has been taken account of in the vector 

 construction of Fig. 116. The final check is obtained by measur- 

 ing the area of flux curve C, which must satisfy the condition 



Area of full-load flux curve C _ OE' g 

 Area of open-circuit flux curve A OEt 



