Question

In armature characteristic, nature of If at lower values of armature current is ______

(a) Very large

(b) Almost constant

(c) Decreasing rapidly

(d) Increasing then decreasing

This question was posed to me in a job interview.

The doubt is from External and Armature Characteristics of DC Generator in division DC Generator of DC Machines

Answer 1

The correct option is (b) Almost constant

The explanation: It is seen from the characteristic that at low values of Ia, the increase in If is very small to provide for increasing Ia *Ra drop. At large values of Ia there is a sharp increase in If to compensate for voltage drop caused by armature reaction.