Armature reaction is the effect that the magnetic field produced by current flowing through the armature windings has on the main magnetic field in a DC machine. When a DC generator or motor operates under load, the armature carries current that creates its own magnetic field. This armature field interacts with the main field from the poles, weakening it and distorting its distribution. The result is reduced voltage output in generators, altered speed in motors, and sparking at the brushes.
How Armature Reaction Works
Every DC machine has two sources of magnetic flux. The first is the main field flux, produced by the field windings wrapped around the poles. The second is the armature flux, created by current flowing through the armature conductors. When the machine runs with no load, armature current is negligible and the main field is symmetrical. The moment the machine picks up a load, armature current increases, and its magnetic field becomes strong enough to interfere with the main field.
The armature’s magnetic field acts at a right angle to the main field. When these two fields combine, the resulting flux distribution is no longer symmetrical under the poles. Instead, it becomes crowded on one side and weakened on the other. Think of it like two people pushing a cart: if one pushes straight ahead and the other pushes from the side, the cart drifts off course. The main field gets “pushed” sideways by the armature field.
Demagnetizing and Cross-Magnetizing Effects
The armature’s magnetic effect can be broken into two components. The first, called the demagnetizing component, acts directly against the main field. It weakens the total flux in the machine. In a generator, less flux means less generated voltage. In a motor, reduced flux can cause the speed to increase unexpectedly under heavy load.
The second component, called the cross-magnetizing component, acts at right angles to the main field. Rather than weakening the flux overall, it redistributes it. Flux density increases under one half of each pole face and decreases under the other half. This distortion is what shifts the magnetic neutral axis, the imaginary line where the flux density is zero, away from its original geometric position. In a generator, this shift happens in the direction of rotation. In a motor, it shifts opposite to the direction of rotation.
You might expect the strengthening on one pole tip to cancel out the weakening on the other, leaving the total flux unchanged. In practice, that doesn’t happen. The iron in the pole tips saturates magnetically on the strengthened side, so the gain there is smaller than the loss on the weakened side. The net effect is always a reduction in total flux.
Why It Causes Sparking at the Brushes
Commutation is the process of reversing the direction of current in each armature coil as it passes through the neutral zone. The brushes are positioned along the magnetic neutral axis so that the coils being switched carry zero induced voltage at that instant, allowing a smooth current reversal. This entire reversal happens in roughly 1/500 of a second.
When armature reaction shifts the magnetic neutral axis away from the brushes’ fixed position, the coils undergoing commutation are no longer in a zero-flux zone. They now sit in a region with some magnetic field, which induces a voltage in them during the switching process. On top of that, each coil embedded in the high-permeability armature iron has significant self-inductance. As the current tries to reverse, this inductance produces a self-induced voltage (called reactance voltage) that opposes the change.
If the current in a coil hasn’t fully reversed by the time the brush breaks contact with its commutator segment, the remaining current is forced to jump across the gap as a spark. Repeated sparking progressively damages both the brush surface and the commutator, shortening the machine’s lifespan and degrading performance.
Generators vs. Motors
The direction of the neutral axis shift depends on whether the machine operates as a generator or a motor. In a generator, armature current flows in the same direction as the induced voltage, and the neutral axis shifts forward in the direction of rotation. In a motor, armature current flows against the back-EMF, and the neutral axis shifts backward, opposite to the direction of rotation.
This distinction matters for any corrective measures. If you shift the brushes to follow the displaced neutral axis in a generator, you move them forward along the direction of rotation. In a motor, you move them backward. However, brush shifting is only a partial fix because the amount of shift depends on the load, which constantly changes.
How Interpoles Correct the Problem
The most common solution is to install interpoles, also called commutating poles. These are small auxiliary poles mounted between the main poles, directly in the commutation zone where the brushes sit. Interpoles are wired in series with the armature, so the magnetic field they produce is automatically proportional to the armature current. As the load increases and armature reaction gets worse, the interpoles get stronger in exactly the right proportion.
An interpole has the same polarity as the next main pole in the direction of rotation (in a generator). Its field counteracts the armature field right in the commutation zone, effectively pushing the neutral axis back to its original position. Because the interpole strength scales with load current, the correction is self-adjusting. This is why nearly all modern DC machines of any significant size include interpoles.
Compensating Windings for Heavy Loads
Interpoles only correct the flux in the narrow commutation zone between the poles. They don’t fix the distorted flux distribution across the entire pole face. For large machines that operate under rapidly changing or very heavy loads, that remaining distortion can still cause problems, including voltage fluctuations and localized saturation in the pole tips.
Compensating windings address this. These are conductors embedded in slots cut into the faces of the main poles themselves, connected in series with the armature. Their current produces a magnetic field that directly opposes the armature’s cross-magnetizing effect across the full width of each pole face. The result is a nearly uniform flux distribution under the poles regardless of load. Compensating windings are expensive to manufacture and are typically reserved for machines in demanding applications like steel rolling mills or large elevator drives where load swings are severe and voltage regulation must be tight.
Practical Effects on Performance
In a DC generator without compensation, armature reaction causes the terminal voltage to drop more than you’d expect from resistance losses alone. The weakened main flux means less voltage is generated, and the distorted field makes commutation rougher. Under heavy overloads, the flux distortion can become severe enough that the voltage collapses.
In a DC motor, the reduced flux from armature reaction can cause the motor to speed up under load, which is the opposite of what most applications need. This is particularly problematic in series motors, where the field and armature currents are the same, and the interaction between armature reaction and field weakening can lead to dangerous runaway speeds if the mechanical load is suddenly removed.
For machines with interpoles and proper design, armature reaction is well-managed under normal operating conditions. It becomes a practical concern mainly during overloads, during rapid load changes, or in older machines where the interpoles or compensating windings have degraded. Understanding the effect helps explain why DC machines have load-dependent voltage characteristics and why brush maintenance remains important for long machine life.