Question

Due to the effect of armature reaction in DC machine, the flux per pole and generated voltage ______ and ________ respectively.

(a) Increases, decreases

(b) Decreases, decreases

(c) Decreases, increases

(d) Increases, increases

The question was asked in final exam.

This intriguing question originated from Armature Reaction -1 in section Armature Reaction of DC Machines

Answer 1

Correct option is (b) Decreases, decreases

The explanation is: The nature of armature reaction in a dc machine is cross-magnetizing with its axis (stationary) along the q-axis (at 90° elect. to the main pole axis). It causes no change in flux/pole if the iron is unsaturated but causes reduction in flux/pole (demagnetizing effect) in presence of iron saturation.