282 PRINCIPLES OF ELECTRICAL DESIGN 



The field m.m.f. will depend upon the flux in the air gap, and 

 since this includes the slot leakage flux, it will be necessary to 

 consider the meaning, and determine the value, of the slot flux 

 before attempting to calculate the angle of Fig. 108. 



Apart from the action of the armature winding as a whole, 

 causing a reduction of the total flux crossing the air gap from 

 pole face to armature teeth, the current in the individual con- 

 ductors, by producing a leakage of flux in the slots themselves, 

 still further reduces the useful flux when the machine is loaded. 

 The whole of the flux entering the tops of the teeth is not cut 



Fia. 110. Flux entering armature of A.C. generator under open circuit 



conditions. 



by the conductors buried in the slots, and the voltage actually 

 developed in the "active" portion of the armature windings will 

 be reduced in proportion to the amount of flux which, instead of 

 entering the armature core, is diverted from tooth to tooth. 

 This loss of voltage is usually attributed to the reactance of the 

 embedded portion of the windings, and is referred to as a react- 

 ance voltage. This term, however, although very convenient, is 

 liable to lead to confusion when an attempt is made to realize 

 the physical meaning of armature reactance. It suggests that a 

 certain electromotive force is generated in the conductors, thus 

 causing a flow of current which, in turn, produces the flux of 

 self-induction and a reactive electromotive force. This is 

 incorrect and leads to a mistaken estimate of the actual amount of 

 flux in the armature core a mistake of little practical import 



