THE ALTERNATING CURRENT GENERATOR 245 



and hence there is no m.m.f. generated by the coil. At 

 the position 2-2' the current has started to build up and 

 the m.m.f. generated by the coil is shown by the vector 

 Ob, which is proportional to the value of the current when 

 the coil is in the position 2-2' and is at right angles to the 

 coil in this position. The m.m.fs. for the other positions 

 of the coil given in Fig. 150 are shown by Oc, Oe and Of. 



Locus of Armature M.M.F. is a Circle. The vectors, 

 if carefully constructed, are found to be on a circular locus, 

 which locus is given in Fig. 150 by the dotted circle. As 

 the armature continues to rotate (only 180 of rotation 



FIG. 150. Locus of Armature Reaction of a Single-phase Generator, 

 Current in Phase with Generated e.m.f. 



is considered in Fig. 150) the m.m.f. vector of the armature 

 starts again over this same circular locus. 



Division oj Armature M.M.F. into Components. If 

 we consider any m.m.f. vector (as for example 06), it is 

 seen that this may be divided into two components, OA 

 which tends to directly magnetize the main field, and OB 

 which tends to cross-magnetize the main field. For every 

 vector 06 there will be one symmetrically placed with 

 respect to Od as -an axis of symmetry. In the case of 06 

 the symmetrical vector is Of. This vector Of may be 

 resolved into its two components, OA' and OB; the com- 



