Question

Simple equation of DC machine operating in motoring mode, with non-zero Ra value is________

(a) Vt = Ea – Ia/Ra

(b) Vt = Ea + Ia*Ra

(c) Vt = – Ea + Ia*Ra

(d) Vt = Ea – Ia*Ra

This question was addressed to me during an interview.

This is a very interesting question from Electromagnetic Power and Circuit Models topic in chapter Circuit Model, Emf and Torque of DC Machines

Answer 1

The correct answer is (b) Vt = Ea + Ia*Ra

To explain I would say: The machine operates in motoring mode (puts in electrical power) when Ia is in the direction opposite of induced emf Ea, also Ea<Vt. For the given armature circuit, with non-zero value of Ra, Vt= armature terminal voltage= Ea+ Ia*Ra.