THE ARMATURE REACTIONS OF ALTERNATORS 245 



in the armature + the back ampere-turns of the armature + the fall 

 of potential in the air-gap-(-the fall of potential in the field magnets 

 corresponding to the total flux. 



When the alternator is unsaturated or but slightly saturated, this 

 latter fall of potential corresponding to the total flux may be admitted 

 proportional to the flux a +/i+/2+-4A 



Thus the flux J/i plays a part entirely similar to the flux /2, and it 

 may therefore be united with the latter in the coefficient of self- 

 induction of the armature. It must, however, be remarked that 

 the flux Jfi only follows the magnetic circuit to the point of emer- 

 gence of the flux from the field magnets, and only absorbs consequently 



D 



the fraction -^ - of the ampere-turns which would be necessary 



Kfotal 



to make it traverse the entire magnetic circuit. The virtual self- 

 induction s which may be advantageously assumed will then have 

 an approximate expression 



and this should be employed in the determination of the segment 

 BD, as above. 



In order to get the total necessary field-magnet ampere-turns 

 NI!I", it is no longer necessary to add any excitation upon the 

 field magnets except the ampere-turns neutralizing those of the 



armature - . The total ampere-turns XQ" are thus ob- 



tained. If the armature current were suddenly suppressed, an E.M.F. 

 E^=Q"c would appear in the armature on open circuit, which is that 

 appearing with the same notation in Fig. 2. 



The same construction may serve reciprocally to calcuate the 

 fall of potential produced in an alternator having the excitation XQ" 

 for the reactive or wattless current Id in the armature. 



Remark No. i, Upon the Case of an Unsaturated Armature. 

 When the armature and the pole pieces are not saturated, the diagram 

 of E.M.F. 's (Fig. i) is also a diagram of the flux, to a different scale, 

 if care be taken to divide the values of the E.M.F. 's by the coefficient 

 kajN-2 



2-X/2 ' 



