The various effects armature reaction can be summarised as,
1) The armature reaction always results in reduction of generated e.m.f. due to decrease in value of flux per pole.
2) The iron losses in the teeth and pole shoes are determined by the maximum value of flux density at which they work. Due to distortion in main field flux the maximum density at load increase above no load. Thus iron losses are observed to be more on load than on no load.
3) Due to the armature reaction the maximum value of gap flux density increases. This will increase the maximum voltage between adjacent commutator segments at load. If this voltage exceeds beyond 30 V the sparking may take place between adjacent commutator segments.
4) The armature reaction shifts brush axis from GNA. Thus flux density in the interpolar axis is not zero but having some value. Thus there will be an induced e.m.f. in the coil undergoing commutation which will try to maintain the current in original direction. This will make commutation difficult and will cause delayed commutation.