When armature conductors carry a lower load current, the armature's MMF (magnetomotive force) causes:

Question

Question

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RRB JE EE 22 Apr 2025 Shift 1 CBT 2 Official Paper

Answer 1

Nature of armature reaction on alternator:

Power factor	Nature
Unity	Cross-magnetizing
Zero PF lagging	Purely demagnetizing
Zero PF leading	Purely magnetizing
0.8 lagging	Partially cross-magnetizing and partially demagnetizing
0.8 leading	Partially cross-magnetizing and partially magnetizing

Explanation

Even at lower load currents, the armature current still produces an MMF, though it is smaller in magnitude. This MMF interacts with the main field primarily in the quadrature axis (perpendicular) to the main field, which is termed as a cross-magnetising effect.
The demagnetizing effect of armature reaction in a DC machine refers to the reduction in the main field flux due to the armature flux. This effect occurs because the armature flux, generated by the current flowing through the armature windings, opposes the main field flux, leading to a weakening of the overall magnetic field.