Define armature reaction in an alternator. Explain the effect of armature reaction at various P.F. of loads of alternator.
1 Answer
Answer :
Armature Reaction: The effect of armature flux on main flux is called as armature reaction.
Armature Reaction at Various Power Factors: When armature is loaded, the armature flux modifies the air gap flux & its angle (electrical) with respect to main flux depends on the load power factor. This is illustrated in waveform diagram as shown below :
a) When the load p.f. is unity, the effect of armature reaction is wholly distorting. In other words, flux in air gap is distorted but not weakened. As shown in figure, at unity p.f., armature flux is 90° electrically behind the main flux. The result is that the total flux is strengthened at trailing pole tips and weakened at leading pole tips. However the average flux in the air gap practically remains unaltered. b) When the load p.f. is zero lagging, the effect of armature reaction is wholly demagnetizing. In other words the flux in air gap is weakened. As shown in figure, at zero p.f. lagging, the armature flux and main flux are in direct opposition with each other resulting in considerably reduction in air gap flux and hence generated emf. c) When the load p.f. is zero leading, the effect of armature reaction is wholly magnetizing. In other words the flux in air gap is increased. As shown in figure at zero p.f. leading the armature flux and main flux are in phase with each other resulting in considerably increased in air gap flux. d) For intermediate values of load p.f. the effect of armature reaction is partly distorting and partly weakening for inductive loads which is shown in figure for 0.7 lagging p.f. For capacitive loads the effect of flux is partly distorting and partly strengthening.