Definition
In a generator or motor, the position in the rotation of the armature where the armature coils are moving parallel to the magnetic field lines, so no voltage is being induced in them. Brushes are positioned at the neutral plane so that commutation occurs at the moment current in the coil is at or near zero, minimizing arcing at the brushes.
Plain English
It is the spot in each rotation where a coil is sliding alongside the magnetic field instead of cutting across it. Because the coil is not cutting field lines at that instant, no voltage is being produced in it, and that is the safe moment to switch the brush from one commutator segment to the next without sparking.
Context Anchor
Seen in aircraft structural repair and stress discussions, especially when describing how airframe parts behave when they bend under load.
Derivation
Neutral comes from the Latin neuter, meaning 'neither one nor the other.' Plane here means a flat reference position in the rotation. Together it names the position where the coil is doing neither — neither building positive voltage nor negative voltage — because it is not cutting through the field.
Why Pilots Care
Brushes that are not aligned with the neutral plane will arc heavily during operation. That arcing pits the commutator, shortens brush life, and can cause generator output problems. Maintenance technicians shift brush rigging to the neutral plane to keep DC machines running cleanly.
Analogy
When you bend a soft eraser, the outside of the bend stretches and the inside of the bend squeezes. Somewhere between them is a layer that stays about the same length; that is the idea of the neutral plane.
Intuition Check
Neutral does not mean unimportant or inactive here. It means the layer in the bent material where the length is not changing.
Example Sentence 1
When servicing the DC generator, the technician rotated the brush rigging until the brushes sat on the neutral plane to reduce arcing.
Example Sentence 2
Bending loads place the upper surface in compression and the lower surface in tension, with the neutral plane remaining unaffected between them.