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.

Cause

On no load the main flux is symmetrical about the pole axis and the magnetic neutral axis (MNA) coincides with the geometrical neutral axis (GNA). When the armature carries current, it produces its own flux, acting at right angles to the main flux (along the interpolar axis).

 main flux:     --->       armature flux:    |
                                             v
 resultant flux is distorted: weakened at one pole tip,
 strengthened at the other

Effects (generator)

1. Cross-magnetising effect (distortion): the flux is weakened at the leading pole tip and strengthened at the trailing tip (for a generator). The flux density becomes non-uniform.
2. Shift of MNA: the neutral axis shifts in the direction of rotation for a generator (opposite to rotation for a motor). The brushes are then no longer at zero-emf position and sparking results.
3. Demagnetising effect: when brushes are shifted forward, part of the armature mmf directly opposes the main field. This reduces the total flux and the generated emf.
4. In saturated machines, strengthening at one tip is less than weakening at the other, so a net reduction of flux occurs, which lowers the terminal voltage.

Remedies

• Brush shifting to the new MNA (not practical for varying load).
• Interpoles (commutating poles) in series with the armature, to neutralise the cross-magnetising mmf in the commutation zone.
• Compensating winding in pole-face slots, in series with the armature, with opposite current direction, for large machines.
• Using a higher air-gap or high reluctance pole tips.