Armature Reaction in a DC Generator

Drag the load slider and watch the field distort in real time

Developed by : ElectricalNotebook.com
Youtube : Pranab's Lab

What is armature reaction?

When a DC generator is on no load, the armature carries no current and the main field flux from the N and S poles stays symmetric. As soon as the generator supplies load current, that current flowing through the armature conductors creates its own magnetic field — the armature flux. This field reacts with the main field and distorts it. That interaction is called armature reaction, and it produces two effects:

1. Cross-magnetizing effect

Flux is weakened under one pole tip and strengthened under the other. This bends the neutral plane and shifts the MNA away from the GNA, in the direction of rotation.

2. Demagnetizing effect

Because iron saturates faster than it strengthens, the gain under the trailing tip never quite matches the loss under the leading tip — so net main flux drops slightly and the generated EMF falls a little.

In short: When the load is increased, the armature current generates its own magnetic field, which weakens the main field on one side and strengthens it on the other. Consequently, the neutral plane (MNA) shifts in the direction of rotation, and since the total flux decreases slightly, the generated EMF also drops marginally.

Generator load control

0% (no load) 100% (full load)
No load — flux is symmetric, MNA coincides with GNA.
Generator load
0%
Armature current Ia
0 A
Main field flux Φf
100.0
Armature flux Φa
0.0
Resultant flux Φr
100.0
MNA shift
0.0°
DC generator armature reaction diagram Two-pole DC generator showing the N and S pole faces, a rotating armature, the main field flux, the armature reaction field, and the magnetic neutral axis shifting away from the geometric neutral axis as load increases. Armature reaction — magnetic field distortion N Trailing Pole Tip Leading Pole Tip S Leading Pole Tip Trailing Pole Tip GNA Φr Φa Φf MNA MNA shift = 0.0°
Main field flux Armature MMF / reaction Resultant flux MNA

Simplified teaching diagram — proportions and shift angle are exaggerated for clarity, not to scale.

Key terms

GNA — Geometrical Neutral Axis
The axis exactly midway between adjacent pole tips. Brushes sit here at no load, where no EMF is induced in the shorted coil.
MNA — Magnetic Neutral Axis
The axis of zero resultant flux under load. It shifts away from the GNA, in the direction of rotation for a generator.
Leading pole tip
The tip a conductor meets first as it sweeps under a pole. Flux density falls here under load.
Trailing pole tip
The tip a conductor leaves last. Flux density rises here under load — though not quite enough to offset the drop at the leading tip.

Live calculation

1. Armature current

Scales linearly with load, up to the rated full-load current.

Ia = (load / 100) × Ia(max)

2. Armature (cross-magnetizing) flux

The field set up by the armature current itself, at 90° to the main field.

Φa = (Ia / Ia(max)) × Φa(max)

3. Demagnetizing effect on Φf

Saturation means the trailing tip can't fully make up for the leading tip's loss, so net main flux droops a little as load rises.

Φf' = Φf × (1 − demag%)

4. Resultant flux

Vector sum of the (reduced) main flux and the perpendicular armature flux.

Φr = √(Φf'² + Φa²)

5. MNA shift angle

The angle the resultant flux — and so the neutral plane — makes with the GNA.

θ = tan⁻¹(Φa / Φf')

ElectricalNotebook.com Pranab's Lab — Subscribe for Technical Videos