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Excessive sparking at the brushes of a DC motor or generator can be attributed to a variety of mechanical and electrical factors that interfere with the motor's commutation process. Option D, which encompasses all the possible causes listed, is the most comprehensive and accurate.

When brushes are not properly set at the neutral point, it can lead to arcing and sparking as the brushes make contact with the commutator from an incorrect position. This misalignment results in a loss of control over the currents, contributing to instability in operation. Furthermore, dirt or contamination on the commutator can form a poor electrical connection, exacerbating the sparking issue.

Additionally, weak spring tension on the brushes can prevent them from maintaining sufficient contact pressure with the commutator, leading to inadequate electrical transfer and additional sparking. Worn brushes also contribute to excessive sparking as they may not take on the shape necessary for smooth contact, while a worn or eccentric commutator can further aggravate the situation by producing inconsistent contact surfaces.

High mica insulation between the commutator bars or uneven heights of the commutator bars can initiate similar issues since these conditions can disrupt the smooth transition of electrical current, leading to increased difficulties in proper commutation.

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