DC armature reaction and commutation

Reference notes

Use Next → on the narrator above to step through how armature current distorts the main field and how interpoles cure the resulting commutation problem.

Armature reaction — what the load current does to the field

With no armature current, only the field winding's MMF exists in the air gap. The flux density wave under each pole is roughly flat (in the pole-face region) and drops to zero at the gaps between poles. The magnetic neutral axis (MNA) is the q-axis — perpendicular to the field axis.

Switch on the armature current. From the previous lessons, the commutator pins the armature MMF along the brush axis — which sits on the q-axis. So we now have two MMFs at 90° to each other: the field's main MMF along the d-axis, and the armature's reaction MMF along the q-axis.

Two effects of armature reaction

• Cross-magnetising: the q-axis armature MMF adds to the main flux on one tip of each pole and subtracts on the other tip. The flux density wave gets tilted — one pole edge saturates while the other goes weak.
• Demagnetising (only if brushes are shifted off the GNA): if the operator deliberately shifts the brushes to the new shifted MNA (older practice — improves commutation), a component of the armature MMF lies along the d-axis, opposing the field. The total flux per pole drops, weakening the machine.

Why the MNA shifts

With the flux wave tilted, the actual zero-flux line (the magnetic neutral axis) is no longer at the geometric mid-point between poles — it has rotated in the direction of rotation (for generator) or opposite (for motor) by an angle proportional to the armature current. This matters because brushes should sit on the MNA for sparkless commutation. As load varies, the MNA moves — and you can't keep manually shifting brushes. That's where interpoles come in.

Commutation — the second armature-reaction problem

As a coil moves past a brush, its current must reverse within the very short time the coil is short-circuited by the brush. The coil's own inductance produces a reactance EMF that opposes the change in current — exactly what we don't want. Without help, the current doesn't quite reverse in time → sparking at the brush → commutator wear.

Interpoles — the elegant fix

Interpoles (also called "commutating poles") are small auxiliary poles placed BETWEEN the main poles, in the gap region. They:

• Are wired in series with the armature — so their MMF is automatically proportional to armature current.
• Have polarity arranged so they cancel the q-axis armature MMF locally in the commutation zone.
• Produce a small rotational EMF in the commutating coil that exactly cancels the reactance EMF — automatic compensation at any load. Sparkless commutation across the operating range.

This is why every modern DC machine above a fraction of a kW has interpoles.

Compensating windings — for severe cases

For very large machines (rolling mills, traction motors with rapidly varying loads), the armature MMF along the pole face is so strong that cross-magnetisation under the pole face matters too — not just at the brushes. A compensating winding is buried in slots in the pole face itself, in series with the armature, with polarity arranged to cancel the q-axis MMF along the ENTIRE pole face. Expensive, but essential for the highest-performance DC drives.

Summary of mitigations

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