AIR-GAP FLUX DISTRIBUTION 291 



If the approximate value of the field ampere-turns, as given 

 by the vector OM of Fig. 116, does not produce the proper 

 amount of flux in the air gap, a correction must be made, and a 

 new curve of resultant m.m.f. obtained, from which the correct 

 full-load flux curve is plotted. 



100. Form of Developed E.m.f. Wave. Having plotted the 

 curve of air-gap flux distribution for any given condition of 

 loading, it is an easy matter to obtain a curve of e.m.f . due to the 

 cutting of the flux by the armature conductors. It may be 

 argued that it is not quite correct to derive the e.m.f. wave from 

 the curve of air-gap flux distribution, because the flux actually 

 cut by each armature conductor at a given instant depends not 

 only upon the value of the air-gap density, but also on the amount 

 of the slot leakage flux which is not cut by the conductor. By 

 referring to Fig. Ill (page 283) it will be seen that, although the 

 slot leakage appears at first sight to pass between the pole and 

 the conductor, it actually enters the armature core through the 

 teeth, and, with the exception of the slot flux in the neutral zone, 

 it all links with the armature winding. The shape of the e.m.f. 

 wave is therefore not modified to any great extent by the slot 

 leakage flux; but, unless the armature current is zero in the con- 

 ductors passing through the neutral zone, the average value of 

 the developed voltage must be less than it would be if all the 

 flux entering the tops of the teeth were cut by the conductors. 

 This is shown in the diagram, Fig. 115, where OE' g is the "appar- 

 ent" developed voltage (assuming all the flux lines in the air 

 gap to be cut), and OE g is the actual developed voltage. It is 

 unnecessary to introduce refinements with a view to determining 

 the exact wave shape of the e.m.f. actually developed in the con- 

 ductors because, by using the flux curve C of air-gap distribution, 

 the wave shape of the " apparent " developed e.m.f. is obtained, 

 and with the aid of equivalent sine-waves (to be explained later) 

 the terminal voltage can be calculated with sufficient accuracy 

 for practical purposes. It is important to bear in mind that the 

 e.m.f. wave-shape obtained at the terminals of a Y-connected 

 three-phrase generator is not necessarily the same as the wave 

 shape developed in each phase-winding by the cutting of the 

 flux in the air gap. This was explained in Art. 71 (page 246), 

 and in order to obtain the wave-form of e.m.f. at the terminals 

 of a Y-connected generator, it is necessary to add the corres- 

 ponding ordinates of two star-voltage waves plotted with a 



