MC-B2 · Armature reaction

Armature Reaction and the Shift of the Neutral Axis

Under load the armature becomes a magnet too and twists the main flux. See where the resultant flux tilts and where the neutral axis for commutation is pushed, then choose for yourself where the brushes must sit for sparkless commutation.

Where must the brushes sit for no sparking?

The main flux (vertical) and the armature cross flux (horizontal) add into a resultant that tilts toward rotation. The magnetic neutral axis where flux is zero (gold dashes) is pushed the same way. Commutation must happen where the flux is zero. Try three brush positions and find the one aligned with the magnetic neutral.

Tap to pick the brush position. Gold dashes = magnetic neutral.

Brush vs magnetic-neutral mismatch

β

Leaving the brushes at the geometric neutral leaves them off by the amount the magnetic neutral has shifted toward rotation. Some EMF remains in the commutating coil and it sparks.

The armature becomes a magnet too

Under load the armature winding carries current, and the axis of the armature magnetomotive force it creates points along the brush axis, the geometric neutral. If the main flux runs vertically from pole to armature, the armature flux is horizontal, at right angles to it. This cross-magnetising flux is the start of armature reaction.

ObserveΦa ⊥ Φf

The armature flux is a cross axis, perpendicular to the main flux.

The vector sum pushes the neutral axis

Because the main flux and the armature cross flux are perpendicular, the resultant tilts toward rotation as their vector sum. The magnetic neutral, where flux density is zero, is perpendicular to the resultant, so it shifts from the geometric neutral toward rotation by the same angle. The shift grows with armature current. One pole tip crowds with flux and saturates while the other weakens, slightly reducing the total flux (the demagnetising effect).

ChooseΦr = Φf + ?

The resultant flux is the vector sum of main and armature flux.

Fill inβ ∝ ?

The neutral-axis shift grows with armature current.

Three fixes that protect commutation

Moving the brushes to the new magnetic neutral cleans commutation at that load, but the shift differs with load, which is awkward. Interpoles are small poles placed at the neutral that induce just the right opposing EMF in the commutating coil to cancel the spark. A compensating winding set in slots in the pole face cancels the armature cross flux right where it forms, preventing the distortion itself. Larger machines use both interpoles and a compensating winding.

On your ownfix: ?

Interpoles and a compensating winding stop the spark and distortion.

Back to the first screen

The spark vanished only when the brushes moved with rotation onto the magnetic neutral. The armature cross flux adds vectorially with the main flux, tilting the resultant toward rotation and pushing the zero-flux neutral by that much. Commutation must happen where the flux is zero, so fixed brushes fall off the mark and spark. Armature reaction is what unsettles B1’s promise that the commutator switches at the zero crossing. Hence the brush shift, and the interpoles and compensating winding that hold the zero crossing in place regardless of load.

The armature reaction: the armature cross flux from load current adds vectorially with the main flux, tilting the resultant toward rotation and pushing the zero-flux neutral axis by that much (the shift is proportional to armature current). Fixed brushes fall off and spark, so a brush shift, interpoles and a compensating winding hold the commutating zero crossing.

Once you hold this distortion

Armature reaction is both the headache and the craft of DC machine design. With the neutral axis and flux in hand, we turn to running the same machine as a motor: the torque and speed that flux and armature current produce. The next unit covers torque T ∝ Φ·Ia and speed N ∝ (V - IaRa)/Φ, and how changing the field controls the speed (MC-B3).