Short note: Armature reaction in dc machine

Armature reaction is the effect of the magnetic field set up by the armature current on the main field flux of the machine.

When the generator is loaded, the armature carries current and produces its own mmf. This acts on the main field and has two effects.

1. Distortion (cross-magnetising effect): the armature field is at right angles to the main field. It strengthens the flux under one half of each pole and weakens it under the other half. The flux distribution becomes uneven, and the magnetic neutral axis (MNA) shifts away from the geometrical neutral axis (GNA), forward in the direction of rotation for a generator (backward for a motor).
2. Demagnetising effect: if the brushes are shifted to the new MNA, the armature mmf has a component directly opposing the main field. This reduces the net flux per pole.

   Main flux  -->   |  Armature flux (cross)  |
   Resultant: crowded at one pole tip, weak at other
   MNA shifted from GNA by angle theta

Harmful effects:

• Reduced total flux, so the generated emf is lower.
• Shifted neutral axis, so brushes spark if kept on the GNA.
• Flux crowding at pole tips causes saturation and extra iron loss.

Remedies:

• Shift brushes to the MNA (small machines).
• Use interpoles (commutating poles) to neutralise cross-magnetisation in the commutation zone.
• Use compensating windings in the pole faces on large and fast-varying machines.
• Increase the air gap or use a laminated pole tip with slots.