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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*RaI 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

Answer»

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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