Q. What is meant by armature reaction. Explain briefly the bad effects of armature reactions.

Sol. By armature reaction is meant the effect of magnetic field set up by armature current on the distribution of flux under the main poles of a generator.

Effect of armature reaction:

1. Cross - magnetizing effect. The armature flux path is normal to the field of main poles. Therfore, the armature reaction is basically cross magnetizing.

2. Distortion of the main field. Cross - magnetizing effect of armature reaction distorts the main field in the air - gap. The two principle effect of the distortion of main field are :

 (a) the creation of magnetic field in the interpolar region, where the brushes are placed for providing the commutation and

 (b) the increased flux density under one half of the pole arc and decreasd under the other half.

  The above two effect, caused by the distortion of the main field under load, lead to more iron losses, poor commutation or even sparking at the commutator surface. These are briefly described below :

    (i) Iron losses : Iron losses depend on the maximum value of flux density. The cross - magnetizing armature reaction increase the flux density under one - half of the pole arc and decreases it under the other half of the pole arc. The increase in iron losses due to large flux density. Consequently the iron losses at load are more than at no load.

    (ii) Commutation : At no load, zero crossing of the flux density wave is along the interpolar axis. Under loaded condition of the d.c. machine, zero crossing of the flux density wave is shifted by an angle θ, which depends on the magnitude of armature current.

         For good commutation, the coils short - circuited by the brushes shouls have zero e.m.f. induced in them. The brushes are usually placed along the interpolar axis. Since zero crossing of the flux density wave is shifted from the interpolar axis, the coils undergoing commutation do not have zero e.m.f. induced in them. In the coil under the brush, the e.m.f. and therefore the direction of current is opposite to what it would be under the approaching pole, there is thus a delay in the reversal of current in the short circuit coil and this may result in sparking at the brushes.

  (iii) Sparking : Under heavy load, the flux density wave is distorted considerably. If the two coil - sides of a coil are under the maximum flux density points, a much greater voltage may be generated in this coil. If this voltage b/w adjacent commutator segments exceeds 30 or 40 volts, a spark may occur b/w these adjacent segments. Sometims this spark may spread around the commutator in the form of a ring fire.

3.   Demagnetizing effect. The demagnetizing effect of cross - magnetizing armature m.m.f. is to reduce the total flux per pole from its no - load value due to magnetic saturation. The reduction in the total flux per pole depends upon the degree of magnetic saturation. In a generator, the magnitude of the e.m.f. generated in the armature decreases with the increase in load. In a motor, electro - magnetic torque is decresed as the flux per pole is reduced under load.

4.  The effect tof armature reaction on the main field flux are of opposite sign for a generator and a motor. This is because the same direction of armature current under the main poles require one direction for a generator and opposite direction for a motor.