CHAP. VI] EXCITING AMPERE-TURNS 113 



current, because the leakage fliix increases the more rapidly than 

 the useful flux. This increase is partly offset by the fact that the 

 pole-tips also become gradually saturated by the leakage flux, so 

 that the leakage factor does not increase as rapidly as it would 

 otherwise. The practical point to be observed is, that for the 

 higher flux densities, if accuracy is desired, the leakage should 

 be estimated separately for a few points on the no-load saturation 

 curve. 



For a given terminal voltage, the leakage factor of a machine is 

 somewhat higher at full-load than at no-load, because the required 

 m.m.f. between the pole-faces is higher, due to the armature reac- 

 tion and to the voltage drop in the armature. In comparatively 

 rare cases, when the armature reaction assists the field m.m.f., for 

 instance, in the case of an alternator supplying a leading current, 

 the leakage factor decreases with the increasing load. The fol- 

 lowing example illustrates the influence of the load upon the value 

 of the leakage factor. 



Let the useful flux per pole in an alternator, at the rated 

 voltage and at no-load, be 5 megalines, and let 6000 amp.-turns 

 per pole be required for the air-gap and the armature core. Let 

 t lie permeance of the leakage paths between a pole and the neutral 

 planes be 120 perms, so that the leakage flux is 0.72 megaline, and 

 akage factor is (5.00 + 0.72) /5.00= 1.14. Let a useful flux 

 <>f :..") megalines be required at the same voltage and at full load, 

 in increase of 10 per cent being necessary to compensate for the 

 internal drop of voltage due to the armature impedance. If the 

 teeth and the armature core were not saturated at all, an m.m.f. 

 <0 amp.-turns would be required. In reality, the m.m.f. is 

 higher, say 7500 amp.-turns. Let the armature reaction be equal 

 to 1500 demagnetizing ampere-turns per pole. To compensate 

 a action, 1500 additional ampere-turns are required on each 

 field coil. Thus, the difference of magnetic potential between 

 a pole-tip and the adjacent plane of symmetry MN (Fig. 29) 

 is now 9000 am)). -turns, and the leakage flux is increased to 

 -lines. Then-fore, the leakage factor at full load is 

 1.0S)/5.50-1.20. Similar relations hold for the din-n- 

 ut machi 



In < ah ulat iiiL r the performance of a synchronous or n di; 



t machine >ne has to use the relation l>ei \\eni the field cur- 

 rent and the voltage induced in the armature. Ordinarily, the 



