Armature reaction and commutationArmature reaction:The effect of armature flux produced by armature current on the distribution of main flux is called armature reaction.(or) The effect of armature flux produced by armature current on the distribution of main flux under the poles of DC machine is called armature reaction.The concept of armature reaction in DC machine (generator) explained as fallow:When load is applied on the generator ,current is flows in armature windings. This armature current causes magnetic flux(armature flux). These armature flux distrub the distribution of main flux in the air gap.Case1: When the generator field wdg is excited and no-load on the generatorThe distribution of main flux under the poles is shown below.

GNA

MNANSPOLE AXISoABrush axis

The direction of flux along the pole axis and these are symmetrically distributed above the pole axis and below the pole axis. Pole axis is perpendicular to the M.N.A(magnetic neutral axis)and M.N.A is coinsides with G.N.A (geomentrical neutral axis). The brushes will make contact with conductors lying in magnetis field.M.N.A: The axis along which no e.m.f induced in the armature conductors is called magnetic neutral axis. The vector OA represents the magnitude and direction of main m.m.f.Case2: field winding is not excited and load is applied on the generator.When load is applied on the generator current flows in armature wdg. The conductors under N-pole carries current inwards direction(shown by )and conductors under S-pole carreis current outword direction(shown by ).these current carrying conductor causes magnetic flux.The distribution of armature flux are shown below NSPOLE AXISoBThe vector OB represents magnitude and direction of armature m.m.f. here armature flux acts right angles to the main flux. This magnetizing effect of armature current is called cross magnetization. cross magnetization

NSPOLE AXISoBNew M.N.AACNew M.N.A

NSPOLE AXISoBNew M.N.AA

NSPOLE AXISNew M.N.A

NSPOLE AXIS0New M.N.A