The armature reaction in a DC motor refers to the influence of the magnetic field (armature magnetic field) generated by energizing the armature winding on the main magnetic field (magnetic field generated by the main magnetic pole) of the motor during operation.
This influence will change the distribution, strength, and direction of the main magnetic field, thereby affecting the performance of the motor (such as commutation, speed, output torque, etc.).

1、 Core background: Two magnetic fields of DC motor
To understand armature reaction, it is necessary to first clarify the two independent magnetic fields that exist during the operation of a DC motor:
Main magnetic field (main pole magnetic field)

The main magnetic pole on the stator of the motor (usually a permanent magnet or a DC excitation winding) generates the “basic magnetic field” for energy conversion in the motor.

In an ideal state, the main magnetic field is symmetrically distributed in a trapezoidal shape along the air gap of the motor (the gap between the stator and rotor), with the magnetic field direction pointing from the N pole to the S pole.
Armature magnetic field (rotor magnetic field)The armature is the rotor part of a motor, on which the armature winding is wound.

When direct current is applied to the armature winding (during motor operation) or induced current is generated by cutting the main magnetic field due to rotor rotation (during generator operation), the armature winding will generate its own magnetic field, namely the armature magnetic field.
The direction of the armature magnetic field can be determined by the right-hand screw rule: the bending direction of the four fingers is the direction of the current, and the thumb points in the direction of the magnetic field’s N pole.
Its distribution characteristics are: the magnetic field axis is perpendicular to the main magnetic field axis (known as the “cross axis magnetic field”), and it is symmetrically distributed along the circumference of the armature.

2、 The core influence of armature reaction: distortion and weakening of the main magnetic field
The superposition of armature magnetic field and main magnetic field in the air gap of the motor results in the destruction of the original distribution of the main magnetic field. The core influence can be divided into two points:

1. The main magnetic field undergoes “distortion” (distortion)
The ideal main magnetic field is symmetrical, but the armature magnetic field will have a “push-pull” effect on the main magnetic field:
At the front pole tip of the main magnetic pole (the magnetic pole tip in the direction of motor rotation), the armature magnetic field is in the same direction as the main magnetic field, resulting in an increase in the air gap magnetic field at this location;

At the rear pole tip of the main magnetic pole (the end of the rotating magnetic pole), the armature magnetic field is in the opposite direction to the main magnetic field, resulting in a weakening of the air gap magnetic field at this location.
In the end, the symmetrical distribution of the main magnetic field is broken, and the magnetic field axis deviates (deviates from the main magnetic pole axis), a phenomenon known as “magnetic field distortion”.

2. The overall main magnetic field is “weakened” (only existing in DC motors)
For DC motors, the direction of armature current is opposite to the direction of induced electromotive force, and the armature magnetic field not only generates an orthogonal component, but also a “direct axis demagnetization component” opposite to the direction of the main magnetic field, resulting in a slight weakening of the overall strength of the main magnetic field.

3、 The hazards of armature reaction: affecting motor performance and reliability
Armature reaction is not a “beneficial phenomenon”, and its direct harm is mainly reflected in two aspects:
Difficulty reversing, sparks generated
Magnetic field distortion can cause induced electromotive force (referred to as reactive electromotive force) in the “commutation element” of the armature winding (the winding coil that is switching from one branch to another), resulting in electric sparks between the brushes and the commutator during commutation.
Sparks not only wear down the brushes and commutators, shorten the lifespan of the motor, but in severe cases may also cause ring fires (sparks forming arcs), burning out the armature windings.
The performance of the motor operation decreases
Magnetic field distortion leads to the generation of “additional torque” during motor operation, causing speed fluctuations, vibrations, and noise;
Weakening the main magnetic field can lead to a decrease in the output torque of the motor, an increase in speed (softening of characteristics), or a decrease in the output voltage of the generator.

4、 Key measures to weaken armature reaction
To reduce the harm of armature reaction, the following technical measures are usually adopted in industry:
Install ‘compensation winding’
A set of compensation windings connected in series with the armature winding is wound around the pole shoes of the main magnetic pole, which generates a magnetic field in the opposite direction to the armature magnetic field, directly canceling out the influence of the armature magnetic field and suppressing magnetic field distortion and demagnetization effects from the root (mostly used in large capacity, high-speed DC motors).
Set the “reversing pole” (intermediate pole)

Install a set of small magnetic poles (reversing poles) between the two main magnetic poles, with their windings connected in series with the armature winding, to generate a magnetic field direction opposite to that of the armature magnetic field. This is specifically designed to counteract the armature magnetic field at the commutation element, eliminate reactive electromotive force, and improve commutation conditions (standard configuration for medium and large capacity DC motors).

Using “wide brush” or “slotted armature”
Wide brushes can cover more commutator segments, reduce the current change rate of individual components, and lower sparks;
The armature core adopts a “skewed groove” design (the groove forms a certain angle with the axis), which can weaken the harmonic components of the armature magnetic field and reduce magnetic field distortion.