All current-carrying conductors produce magnetic fields. The magnetic field produced by current in the armature of a dc generator affects the flux pattern and distorts the main field. This distortion causes a shift in the neutral plane, which affects commutation. This change in the neutral plane and the reaction of the magnetic field is called armature reaction.
You know that for proper commutation, the coil short-circuited by the brushes must be in the neutral plane. Consider the operation of a simple two-pole dc generator, shown in figure. View A of the figure shows the field poles and the main magnetic field. In this the flux is distributed symmetrically with respect to polar axis,which is the line joining the centres of NS poles.The Magnetic neutral axis or plane (M.N.A)coincides with the geometrical neutral axis or plane (G.N.A).M.N.A may be defined as the axis along which no e.m.f is produced in the armature conductors or the axis which is perpendicular to the flux passing through armature.
The armature is shown in a simplified view in views B and C with the cross section of its coil represented as little circles. The symbols within the circles represent arrows. The dot (0)represents the point of the arrow coming toward you, and the cross (+) represents the tail, or feathered end, going away from you. When the armature rotates clockwise, the sides of the coil to the left will have current flowing toward you, as indicated by the dot. The side of the coil to the right will have current flowing away from you, as indicated by the cross. The field generated around each side of the coil is shown in view B of figure. This field increases in strength for each wire in the armature coil, and sets up a magnetic field almost perpendicular to the main field.
Now you have two fields — the main field, view A, and the field around the armature coil, view B. View C of figure shows how the armature field distorts the main field and how the neutral plane is shifted in the direction of rotation. If the brushes remain in the old neutral plane, they will be short- circuiting coils that have voltage induced in them. Consequently, there will be arcing between the brushes and commutator. To prevent arcing, the brushes must be shifted to the new neutral plane.Due to this brush shift,the armature conductors and hence armature current is redistribued. Some armature conductors which were earlier under the influence of N-pole come under the influence of S-pole and vice-versa.
Compensating Windings and InterPolesShifting the brushes to the advanced position (the new neutral plane) does not completely solve the problems of armature reaction. The effect of armature reaction varies with the load current. Therefore, each time the load current varies, the neutral plane shifts. This means the brush position must be changed each time the load current varies.
In small generators, the effects of armature reaction are reduced by actually mechanically shifting the position of the brushes. The practice of shifting the brush position for each current variation is not practiced except in small generators. In larger generators, other means are taken to eliminate armature reaction. COMPENSATING WINDINGS or INTERPOLES are used for this purpose.Their function is to neutralize the cross magnetizing effect of armature reaction. The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature in such a way that the current in them flows in opposite direction to that flowing in armature conductors directly below the pole shoes.
The series-connected compensating windings produce a magnetic field, which varies directly with armature current. Because the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the effects of the armature magnetic field. The neutral plane will remain stationary and in its original position for all values of armature current. Because of this, once the brushes have been set correctly, they do not have to be moved again.
It should be carefully noted that compensating winding must provide sufficient m.m.f so as to counter balance the armature m.m.f. Let
Zc = No.of compensating conuctors/pole face
Za = No.of active armature conductors/pole
Ia = Total armature current
Ia/A = Current/armature conductor
ZcIa = Za (Ia/A) or Zc = Za/A
Another way to reduce the effects of armature reaction is to place small auxiliary poles called "interpoles" between the main field poles. The interpoles have a few turns of large wire and are connected in series with the armature. Interpoles are wound and placed so that each interpole has the same magnetic polarity as the main pole ahead of it, in the direction of rotation. The field generated by the interpoles produces the same effect as the compensating winding. This field, in effect, cancels the armature reaction for all values of load current by causing a shift in the neutral plane opposite to the shift caused by armature reaction. The amount of shift caused by the interpoles will equal the shift caused by armature reaction since both shifts are a result of armature current.
