What is armature reaction? Explain briefly the cross magnetizing and de-magnetizing effect of a loaded generator

    

 Armature reaction: The current flow in the armature conductors of a dynamo has an important and undesirable effect upon the dynamo’s operation. The current in the armature produces a flux and it is the interaction between this flux and the field flux that is called  armature reaction.

     Cross magnetizing and demagnetizing effect:  The effects of the armature flux  my be analyzed by considering the flux to consists of two components that are at right angle to each other as shown in fig. One component is at right angle to the field, and because this component crosses the main –field  flux, it is known as the cross –magnetizing component of the armature flux. The second component is in the same plane as the main field flux . The direction of this component is opposite to the direction of the main field flux, with the result the it tends to reduce the effect of the main field flux. This component of the armature flux , is known as demagnetizing component of the armature flux. 

Elimination: A method to reduce the armature flux on large machines is by the use of compensating windings .These windings are placed in the pole faces of the field pole can run parallel to the armature conductors. A connection is made  from one of the brushes to one end of this winding, so that current from the armature must first pass through this winding before going to the load. The direction of current through the winding is opposite to that of the current in the armature conductors under the pole. The location of the compensating winding and of the connection of the winding is shown in fig. indicates that the current in the armature conductors located under the north pole carry current away from the observer , therefore the direction of current following in the part of the compensating windings situated in the north pole is toward the observer. The direction of flux due to the current in the armature conductor shown in fig. will be from the brush to the bottom brush. Since the current in the compensating winding is opposite in direction to the current in the armature conductors, the flux product by the current in the compensating winding will be opposite in direction to the armature flux. The compensating flux , being opposite in direction to the armature flux, tends to cancel the armature flux. If the armature current increases, the compensating current increases, so that the armature flux is cancel for all load conditions.