286 PRINCIPLES OF ELECTRICAL DESIGN 



The permeances of the air paths of the component fluxes 

 $ 2 and $3 can be calculated fairly accurately (See Art. 5, Chap. 

 II). Let them be P z and P 3 respectively. Then, if l a is the axial 

 length of the armature core in centimeters, the total " equivalent " 

 slot flux in the neutral zone is 



+ P 2 + P 3 ) (104) 



97. Effect of Slot Leakage on Full-load Air-gap Flux. Before 

 considering a method of drawing the curve of air-gap flux dis- 

 tribution under load, it will be advisable to determine what 

 should be the area of this curve. The area of the required flux 

 distribution curve is a measure of the total flux per pole in the air 

 gap, and it would be possible to express this in terms of the open- 

 circuit flux distribution curve if we knew the e.m.f. that would 

 have to be developed in the armature windings on the assumption 

 of all the flux passing from pole face to armature teeth being 

 actually cut by all the conductors. It is therefore proposed to 

 determine what may be called the "apparent" developed e.m.f., 

 that is to say, the e.m.f. that would be developed in the armature 

 windings under load conditions if it were not for the fact that 

 some of the flux in the air gap leaks across from tooth to tooth in 

 the neutral zone, and is not actually cut by the conductors. 



Consider first the condition of zero power factor. The current 

 then lags 90 degrees behind the e.m.f., and reaches its maximum 

 value in the conductors situated midway between poles. The 

 slot leakage flux, and the demagnetizing effect of the armature 

 winding, will then both have reached their maximum value. 



In the vector diagram, Fig. 114, let OE t represent the required 

 terminal voltage if the machine is mesh-connected, or the cor- 

 responding potential difference per phase winding if the machine 

 is star-connected. (In a three-phase Y-connected generator OE t 



would be 7= times the terminal voltage.) The vector 07, drawn 

 v 3 



90 degrees behind OE t , is the armature current on a load of zero 

 power factor. The impedance-drop triangle is constructed by 

 drawing E t P parallel to 01 of such a length as to represent the 

 IR drop per phase, and PE g at right angles to 01 to represent the 

 reactive pressure drop per phase in the end connections. OE g is 

 therefore the voltage actually developed in the slot conductors, 

 because it contains the component PE g to balance the voltage 



