To achieve sparkless commutation brushes of a D.C. generator are rockedm ahead so as to bring them

To achieve sparkless commutation, the brushes of a DC generator are rocked ahead so as to bring them under the influence of the trailing pole tip.

Here's why:

Commutation: Commutation is the process of transferring current from one set of brushes to the next as the armature conductors pass under the influence of the magnetic poles.
Sparkling: If this transfer of current happens abruptly, it can cause sparking at the brushes, which can damage the brushes and commutator.
Role of Trailing Pole Tip:
The trailing pole tip of a pole piece has a demagnetizing effect on the armature conductors.
By shifting the brushes ahead, the conductors carrying current are brought under the influence of this demagnetizing effect just before they leave the influence of the main pole.
This demagnetizing effect helps to gradually reduce the current in the conductors, leading to a smoother and sparkless commutation.
Visual Representation:
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DC generator with brushes shifted ahead under the influence of the trailing pole tip

Key Points:

The exact amount of brush shifting required depends on various factors, including the load current, speed, and the design of the generator.
Improper brush shifting can lead to increased sparking, reduced efficiency, and even damage to the commutator.