Introduction:
In a DC machine, the magnetic field is produced by the field winding placed on the stator. The field winding can be either connected in series with the armature winding (series-wound machine) or connected in parallel with the armature winding (shunt-wound machine). The mmf (magnetomotive force) is the driving force for the magnetic field, and it determines the strength of the magnetic field.

Explanation:
The mmf produced by the field winding is known as the interpole mmf, while the mmf produced by the armature winding is called the armature mmf. The interpole mmf is provided by the interpole windings, which are placed between the main field poles. These interpole windings are connected in series with the armature winding.

1. Purpose of Interpoles:
The main purpose of interpole windings is to counteract the cross-magnetizing effect of armature reaction. When current flows through the armature winding, it produces a magnetic field that interacts with the main magnetic field produced by the field winding. This interaction causes distortion in the main magnetic field, leading to various undesirable effects such as sparking, flux weakening, and poor commutation. The interpole windings are designed to produce a mmf that neutralizes the effect of armature reaction and maintains a uniform magnetic field.

2. MMF Comparison:
The interpole mmf is designed to be slightly greater than the armature mmf. This is done to ensure that the total mmf produced by the armature and interpole windings is sufficient to overcome the reluctance of the magnetic circuit and establish a strong magnetic field. If the interpole mmf is less than the armature mmf, there may not be enough mmf to establish a strong magnetic field, resulting in reduced performance of the DC machine.

3. Resultant MMF:
The resultant mmf is the vector sum of the interpole mmf and the armature mmf. Since the interpole mmf is designed to be greater than the armature mmf, the resultant mmf will be more influenced by the interpole mmf. This means that the interpole mmf contributes more to the establishment of the magnetic field compared to the armature mmf.

Conclusion:
In conclusion, the interpole windings provide a mmf that is slightly greater than the armature mmf in a DC machine. This is done to counteract the cross-magnetizing effect of armature reaction and establish a strong and uniform magnetic field. The interpole mmf plays a significant role in maintaining the performance and efficiency of the DC machine.