How should be the value of field mmf to prevent excessive distortion of field form by armature reaction in an electric machine?

The value of the field mmf (magnetomotive force) in an electric machine should be adjusted in such a way that it adequately compensates for the armature reaction to prevent excessive distortion of the magnetic field form. This adjustment depends on the specific operating conditions of the machine, including its load and speed. To achieve this balance, the following measures can be considered:

1. Increasing Field MMF: By increasing the field current to raise the field mmf, the effects of armature reaction can be counteracted. This is necessary because the armature reaction tends to demagnetize the main field, especially under heavy loads, reducing the machine’s overall performance. By increasing the field mmf, the machine can maintain a stable magnetic field, thereby reducing distortion.

2. Employing Compensating Windings: Compensating windings, placed in slots on the stator of a machine, generate a mmf that opposes and neutralizes the armature mmf. This technique is especially effective in large machines where armature reaction can significantly affect performance.

3. Using Interpoles or Commutating Poles: In DC machines, interpoles are placed between the main poles. They carry a winding connected in series with the armature and are specifically designed to produce a magnetic field that neutralizes the armature field in the commutation zone. This not only helps in reducing the field form distortion caused by armature reaction but also assists in achieving spark-free commutation