In addition to improper brush pressure or seating, what can result in excessive sparking at the brushes of a DC propulsion motor?

Correct Answer: Option D — improper positioning of brush rigging outside the neutral plane

DC motor commutation relies on brushes being precisely positioned in the neutral plane where armature coil voltages are zero. Displacement of the brush rigging outside this plane causes severe sparking, rapid brush wear, and potential flashover, violating IEEE 45 operational limits.