THE ARMATURE REACTIONS OF ALTERNATORS 241 



Diagram of Ampere-Turns in the Case of Unsaturated Armature. 

 The consideration of ampere-turns does not need to appear in the 

 method, as is evident in the case of an unsaturated armature, until 

 after having traced the diagram of E.M.F.'s. The excitation ampere- 

 turns are drawn, if desired, along the direction of the vector OC, 

 in order to facilitate certain comparisons; but the calculation of ampere- 

 turns is no longer in this method a vectorial, geometric calculation, but 

 a scalar calculation, and may be made upon the saturation curve as 

 calculated or observed, which represents the induced E.M.F. (or the 

 useful flux traversing the armature) as a function of the total ampere- 

 turns applied on the field magnets. 



Their determination is based on the following facts: 

 (i) The reactive counter-ampere-turns of the armature are pro- 

 portional to the number of peripheral conductors on the armature per 



Nz 

 pair of poles, 2Vjj, which supply a number of turns equal to ; but 



these turns do not act in unison, partly because they belong to different 

 phases, and partly because they are not in the same slots. For this 

 reason it is necessary to apply a reduction-coefficient K. The effect- 

 ive reactive current /2 sin thus gives rise to an M.M.F. exactly 

 opposite to that of the field magnets, and having for its magnitude 

 in counter-ampere-turns 



2 \/2 



(2) These counter ampere-turns act, on the one hand, upon the 

 circuit common to the field magnets and the armature; and on th e 

 other hand, upon the circuit of the arma- ., T 



T> * .ZVl It _ 



ture and of the stray magnetic fields. This 

 may be represented diagrammatically as in 



Fig. 3, in which the full lines ABCDEF 

 represent the principal magnetic circuit with 

 its two M.M.F.'s acting in the opposite 



KN 2 I d 



directions N\Ii and -, and the dotted 



V 



Field 



Field leakage 



Armature leakage/ 2 



D 



^- Air Gap 



D' 



lines AD AD indicate the stray circuits in | 



which the lines of force escape, either be- ~*#o E 



tween the teeth of the armature /2 or between j? IG _ 3 . 



the polar horns t. In reality the stray fields 



fi of the field magnet are not concentrated along any single path, but 



