USCG Exam PracticeElectrical basics — generators, motors, and safety
Which phenomenon in a DC generator causes the magnetic neutral plane to shift in the direction of rotation, leading to sparking at the brushes?
- A. Electromagnetic induction from the rotating armature coils
- B. Residual magnetism in the field poles
- ✓ Armature reaction caused by the magnetic field of load current in the armature windingsCorrect
- D. Commutator segment wear reducing contact area with the brushes
Why C is correct
NEETS Mod. 5 §1-2 explains that load current flowing in the armature windings creates its own magnetic field that distorts the main field — this is armature reaction. It shifts the magnetic neutral plane in the direction of rotation, causing brushes to short coils that still carry induced voltage, producing sparking. Brush wear and residual magnetism are separate issues.
Cited:NEETS Mod. 5 §1-2