Armature reaction in a DC motor refers to the magnetic field produced by the armature current, which affects the main magnetic field produced by the field windings. This interaction between the armature current and the main magnetic field can have significant effects on the performance of the motor.

When the armature current increases, the armature reaction is increased. This can be explained by the Fleming's left-hand rule, which states that when the current flows through a conductor, a magnetic field is produced around it. The direction of this magnetic field can be determined by pointing the thumb of the left hand in the direction of the current and the fingers will point in the direction of the magnetic field.

- When the armature current increases, the magnetic field produced by the armature windings also increases. This magnetic field interacts with the main magnetic field produced by the field windings.
- The armature magnetic field can distort the main magnetic field, causing it to shift. This shift can result in uneven distribution of magnetic flux in the air gap, leading to various effects on the motor's performance.
- One of the effects is the demagnetizing effect, where the armature magnetic field weakens the main magnetic field in the pole shoes. This reduces the overall magnetic flux in the motor, resulting in decreased motor performance.
- Another effect is the cross-magnetizing effect, where the armature magnetic field produces a magnetic field perpendicular to the main magnetic field. This can cause the motor to develop a torque in the opposite direction, leading to motor instability and increased power losses.
- Additionally, armature reaction can also cause motor sparking at the brushes, leading to brush wear and reduced motor efficiency.

Therefore, when the armature current increases, the armature reaction is increased, resulting in various negative effects on the motor's performance. It is important to properly account for armature reaction in the design and operation of DC motors to ensure optimal performance and efficiency.