In a DC machine, the main field is produced by field coils. In both the generating and motoring modes, the armature carries current and a magnetic field is established, which is called the armature flux. The effect of armature flux on the main field is called the armature reaction.
The armature reaction:
demagnetizes the main field, and
cross magnetizes the main field
The demagnetizing effect can be overcome by adding extra ampere-turns on the main field. The cross magnetizing effect can be reduced by having common poles.