252 ELECTRICAL ENGINEERING 



In practice an alternator is given the same rating two- and three- 

 phase and 65 per cent of that rating single-phase. 



151. Armature Reaction. The flux distribution in the air gap 

 of an alternator at no load is symmetrical about the centre line of 

 the pole and usually follows approximately a sine wave. The 

 e.m.f. generated in the armature is also a sine wave. (See Fig. 

 224.) 



When current flows in the armature winding, the m.m.f. of the 

 armature combines with the m.m.f. of the field and changes both 

 the magnitude and distribution of the flux crossing the air gap 

 and cut by the armature conductors. It thus changes both the 

 magnitude and the wave form of the e.m.f. generated. These 

 results are termed the armature reaction. 



Armature reaction depends not only on the intensity of the 

 current in the armature but also on its phase relation with the 

 generated e.m.f. Fig. 229 illustrates armature reaction in a 

 machine with a single-phase concentrated winding. 



In (a) the armature coil is shown in the position of zero e.m.f.; 

 if the current is in phase it is also zero. 



In (6) the e.m.f. is maximum and the current is maximum. 

 The m.m.f. of the armature is cross magnetizing, that is, it de- 

 creases the flux over one half of the pole and increases it over the 

 other half. The useful flux is only decreased by the small amount 

 lost due to the higher saturation, and, therefore, decreased perme- 

 ability over the half of the pole where the density is increased. 

 The flux distribution no longer follows a sine wave and the e.m.f. 

 will not be a sine wave. 



In (c) the current is maximum but lags 90 degrees behind the 

 generated e.m.f. The m.m.f. of the armature acts directly against 

 the m.m.f. of the field. It is, therefore, demagnetizing and 

 decreases the flux but does not distort it. 



In (d) the current is maximum and leads the e.m.f. by 90 degrees. 

 The m.m.f. of the armature acts directly with the field and mag- 

 netizes it. The useful flux is increased and is not distorted. 



The following results have been. obtained: 



(1) A current in phase with the generated e.m.f. is cross 

 magnetizing and only decreases the flux to a very slight extent. 



(2) A current lagging 90 degrees behind the generated e.m.f. 

 demagnetizes the field and decreases the flux and decreases the 

 generated e.m.f. 



