308 



PRINCIPLES OF ELECTRICAL DESIGN 



magnetic circuit of the machine has been designed. The distance 

 DR in Fig. 125 is the loss of voltage corresponding to E'gP of 

 Fig. 126, and is approximately constant for a given armature 

 current. The portion SDj however, of the total difference of 

 voltage depends on the slope of the line MS, and is thus some 

 function of the degree of saturation of the iron in the magnetic 

 circuit. It is far from being constant (except over the linear part 

 of the open-circuit saturation curve) and must be measured off 

 the diagram for each different value of the field excitation. This 

 diagram (Fig. 125) shows very clearly the advantage of high flux 



densities (magnetic saturation) in some 

 portion of the magnetic circuit, if good 

 regulation is aimed at. 



107. Short-circuit Current. The 

 amount of the short-circuit current is in- 

 timately connected with the regulating 

 qualities of a machine, and in large gen- 

 erators becomes a matter of importance. 

 The maximum value of the armature 

 current at the instant a short-circuit 

 occurs depends mainly on the induc- 

 tance of the armature windings; but 



when the armature magnetomotive force has had time to react 

 on the field and has actually reduced the flux of induction in the 

 air gap, the resulting current may be fairly accurately calculated 

 by using the construction indicated in Figs. 127 and 128. 



The vector triangle Fig. 127 is constructed for any assumed 

 value, I C y of the armature current. It shows that when the 

 terminal voltage is zero, the machine being short-circuited, the 

 flux in the air gap must be such that the pressure OE' g would be 

 developed in the armature conductors on open circuit. The 

 value OF (Fig. 128) of the ampere-turns necessary to produce this 

 flux in the air gap is thus obtained, the ordinate OE' g being the 

 generated voltage as determined by the vector diagram. Now 

 since the magnetomotive force of the armature windings will be 

 almost wholly demagnetizing, it is correct to assume that the field 

 excitation must be increased by an amount equal to the maxi- 

 mum armature ampere-turns per pole in order that the resultant 

 excitation may be OF. Thus FG in Fig. 128 is made equal to the 

 maximum armature ampere-turns, and by drawing, to a suitable 

 scale, the ordinate GJ equal to the assumed armature current 7 C , 



(slots) 



IX(ends) 



IE drop 



FIG. 127 Vector diagram of 

 short-circuited armature. 



