136 ALTERNATING CURRENTS 



motive force reaches its maximum value when the coil sides are 

 directly under the pole centers, position (2), Fig. 141 (a). The 

 current, however, reaches its maximum value 90 electrical space- 

 degrees ahead of this position or at (1). It will be observed that 

 the ampere-turns of the coil now assist or strengthen the main 

 field, as they are acting directly in conjunction with it. The coil 

 again is in the most favorable position so far as its effect upon the 

 magnetic circuit of the machine is concerned. This condition 

 may be represented by the vector dia- 

 . - s gram, Fig. 142. The magnetomotive 

 A~~ F! F force of the field ampere-turns is FI, 



FIG. 142. Vector dia- that of the armature is A, and the re- 

 gram showing effect of su it a nt magnetomotive force is their sum 



armature reaction with 



current leading by 90. F, because the two are acting in the 

 same direction. 



It should be noted that for a given value of armature current, 

 the effect of armature reaction varies with the power-factor 

 in a salient-pole type of machine, due to the varying reluctance 

 caused by the salient poles. In a non-salient-pole generator, 

 the air-gap is uniform around the periphery of the armature, 

 so that the armature magnetomotive force acts on a path of 

 uniform permeance. 



If the power-factor has other than one of the three values just 

 illustrated, the armature mmf. will add vectorially to the im- 

 pressed mmf. according to the phase angle existing between the 



FIG. 143. Vector diagram of armature reaction with current lagging 6 degrees. 



current and the induced voltage. This is illustrated in Fig. 143. 

 The direction of the armature reaction is shown at a power-factor 

 cos 0, the current lagging, 6 being the angle between the current 

 and the terminal voltage. F\ is the field mmf. When the cur- 

 rent is in phase with the induced emf. the direction of the arma- 

 ture reaction is along A\ t 90 behind the resultant mmf., F. 



