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Simple equation of DC machine operating in generating 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

I have been asked this question in an internship interview.

This intriguing question originated from Electromagnetic Power and Circuit Models in division Circuit Model, Emf and Torque of DC Machines

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The correct answer is (d) Vt = Ea – Ia*Ra

Easy explanation: The machine operates in generating mode (puts out electrical power) when Ia is in the direction 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.

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