If the brushes are left on GNA, then emf collected would reduce and coil undergoing commutation would no longer have zero rotational voltage leading to serious commutation problems. The immediate solution therefore, appears to shift the brushes in the new MNA.
The brush shift results into improved commutation but reduce thus resultant flux resulting into reduction in emf in generator action and increase in speed in motor action.
Brush shift has serious limitations. Since, shift in MNA is proportional to Armature current the brush has to be shifted in a new position every time the load changes, direction of rotation changes or mode of operation changes.
Therefore, brush shift is limited to various small machines and there too the brushes are fixed at a position corresponding to expected load, direction of rotation and mode of operation.
In practice the brushes are moved slightly further to counter the effect of reactance voltage that would further improve commutation.
Brushes are shifted in direction of rotation for a generator and opposite to direction of rotation of a motor.
In large machines, inter poles also called commutating poles are used to overcome commutation problems.
Armature reaction waveforms are shown below :
Armature MMF from the above figure is triangular in shape and due to large air gap along q-axis the armature flux density is reduced along q-axis.
The point where total flux density is zero is defined as Magnetic Neutral Axis and where flux density due to poles is zero is defined as Geometric Neutral Axis. From above figure due to Armature Reaction there is a shift between the two.
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