AIR-GAP FLUX DISTRIBUTION 



281 



be a sine curve, of which the maximum ordinate is about half the 

 instantaneous maximum m.m.f. per pole of the single-phase 

 winding, and it may be used exactly in the same manner as the 

 curve M in Fig. 108 (representing armature m.m.f. of a poly- 

 phase machine) ; that is to say, it can be combined with the field 

 pole m.m.f. curve to obtain the resultant m.m.f. at armature 

 surface from which can be derived the flux distribution curves 

 under loaded conditions. 



The maximum value of the resultant ampere-turns per pole is, 

 therefore, 



X A 



X 2p 



X 



where Z is the total number of armature face conductors, 

 pressed in gilberts, the formula is, 



9 



Maximum ordinate of armature m.m.f. 

 curve in single-phase alternator. 



Ex- 



2 X2p 



(103) 



which, together with formula (102) may be compared with the 

 formulas (100) and (101) for polyphase generators. 1 



95. Slot Leakage Flux. Referrring again to Fig. 108, if we 

 wish to derive a curve of resultant m.m.f. over the armature 

 periphery for any condition of loading, it will be necessary, 

 before combining the curves of armature and field pole m.m.f., 

 to determine the relative positions of these two curves. In the 

 direct-current machine, the position of maximum armature 

 m.m.f. coincides with the brush position; but the point B in 

 Fig. 108 is not so easily determined. Its distance from the 

 center of the pole is /? + 90, a displacement which depends 

 not only on the power factor of the load (i.e., on the lag of the 

 current behind the terminal potential difference), but also on the 

 strength of the field relatively to the armature, because this 

 relation determines the position (relatively to the center of the 

 pole) of the maximum e.m.f. developed in the conductors. 



1 If the " spread " of the winding is different from the 60 per cent, assumed 

 in this calculation, modified formulas should be derived in a similar 

 manner. 



