Question

The effect of "Armature Reaction" in a DC generator mainly results in:

• A. Cross-magnetization and demagnetization
• B. Increased terminal voltage
• C. Improved commutation
• D. Reduction in mechanical friction

Hint:

Armature reaction always degrades generator performance by distorting the field and reducing the terminal voltage.
To combat these negative effects, compensating windings are placed in the pole faces to counter the cross-magnetizing MMF, and brushes are shifted to the new MNA, or interpoles are utilized.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question relates to the phenomenon of armature reaction in DC machines.
Armature reaction refers to the magnetic effect of the armature current on the main magnetic field flux produced by the field poles.

Step 2: Key Formula or Approach:
The total magnetic field in a DC machine under load is the vector sum of the main field MMF ($F_f$) and the armature MMF ($F_a$).
Analyzing this vector interaction reveals two main components of the armature field relative to the main field axis.

Step 3: Detailed Explanation:

• Under no-load conditions, only the main field winding is excited, establishing a symmetrical magnetic field distribution along the polar axis.

• The magnetic neutral axis (MNA) is coincident with the geometrical neutral axis (GNA).

• When a load is connected, current flows through the armature conductors, generating its own armature magnetic field.

• The armature MMF is perpendicular to the main field MMF, resulting in a cross-magnetizing effect.

• This cross-magnetization distorts the main field flux distribution, crowding the flux toward the trailing pole tips in a generator and shifting the MNA in the direction of rotation.

• Due to magnetic saturation in the pole tips, the increase in flux on one side is less than the decrease on the other side, leading to a net reduction in the main field flux.

• This reduction in net flux is known as the demagnetizing effect of the armature reaction, which consequently lowers the generated EMF and terminal voltage.

Step 4: Final Answer
Hence, the primary consequences of armature reaction are the distortion (cross-magnetization) and the weakening (demagnetization) of the main field flux.