Armature reaction in DC machines and its remedies

Armature reaction

Armature reaction is the effect of the magnetic field set up by the armature current on the distribution of the main field flux in a DC machine. On no load only the main field acts; on load the armature conductors carry current and produce a cross field that distorts and weakens the main field.

   main flux (N to S) --->
       N |  ____  | S
         | /    \ |        GNA: geometric neutral axis
  --------(  arm. )------  MNA shifts in the direction of
         | \____/ |        rotation (generator) or against
                           rotation (motor)
   armature flux acts along the brush axis (90 deg elec.)

Effects

1. Cross-magnetising effect: armature MMF is at 90° to the main field. It strengthens flux at one pole tip and weakens it at the other, distorting the flux.
2. Shift of magnetic neutral axis (MNA): the MNA shifts forward in the direction of rotation for a generator (backward for a motor). If brushes stay on the geometric neutral axis, coils undergoing commutation have EMF induced in them, causing sparking.
3. Demagnetising effect: if brushes are shifted to the new MNA, part of the armature MMF directly opposes the main field, reducing net flux. This reduces the generated EMF (generator) or increases speed (motor).
4. Because of saturation at the strengthened pole tip, the increase there is less than the decrease at the other tip, so the net flux per pole falls even without brush shift.
5. Uneven flux raises voltage between adjacent commutator segments, risking flashover, and increases iron loss in teeth.

Measures to reduce armature reaction

• Compensating winding: placed in slots in the pole faces, connected in series with the armature, with current opposite to the adjacent armature conductors. It cancels the cross field under the poles (used in large machines).
• Interpoles (commutating poles): small poles between main poles, in series with the armature. They neutralise the cross field in the commutating zone and induce the reversing EMF for sparkless commutation.
• High reluctance in the cross-flux path: pole tips chamfered or made with laminations having slots, or a larger air gap at pole tips.
• Strong main field: design the main field MMF to be large compared with the armature MMF.
• Brush shift: moving brushes to the new MNA reduces sparking (only works for a fixed load; not used in modern machines).