246 ELECTRICAL MACHINERY 



ponent OA' tends to directly demagnetize the main field 

 and OB is a cross-magnetizing effect. 



Now every other vector has its mate and the resultant 

 direct magnetizing or demagnetizing effect of any such 

 pair is zero, because the magnetizing component of one is 

 just equal to the demagnetizing component of the other. 

 The cross-magnetizing effect of the two vectors (06 and Of 

 for example) is in the same direction so that the cross-magnet- 

 izing effect is not neutralized. 



Hence we conclude that in the single-phase alternator, 

 the armature of which is carrying current in phase with its 

 generated e.m.f., the armature reaction produces no reiultant 

 demagnetizing effect but does cross magnetize the main field 

 so that it is concentrated in the trailing pole tips. 



Armature M.M.F. Produces Pulsations in the Strength 

 of the Main Field. But although there is no net magnetizing 

 effect in suoh a machine it is to be noticed that at some 

 instants the main field is stronger than normal and at 

 others, weaker (because of the magnetizing effect OA and 

 demagnetizing effect of OA', for example). As a result 

 the main field pulsates in strength, and a study of this 

 effect shows that this pulsation takes place with a frequency 

 just twice as great as that of the generated e.m.f. of the 

 armature. It may also be seen that the cross-magnetizing 

 action is also pulsating in its effect, being a maximum when 

 the armature current is a maximum and 90 later, being 

 zero. 



Effects of Single-phase Armature Reaction. The total 

 effect of the armature reaction in a single-phase generator 

 carrying current in phase with its generated e.m.f., is, 

 therefore, a periodic weakening and strengthening of the 

 main field combined with a periodic oscillation of the field 

 back and forth across the pole face. Both of these effects 

 produce eddy currents in the pole face so that a single-phase 

 generator, unless special precautions have been taken in 

 the design, is likely to develop a great amount of heat in 



