Question

Emf and torque produced in a DC machine are proportional to ________ and _________ respectively.

(a) Armature speed and armature emf

(b) Armature emf and armature speed

(c) Armature current and armature emf

(d) Armature speed and armature current

I had been asked this question during an interview.

This interesting question is from EMF and Torque Production topic in portion Circuit Model, Emf and Torque of DC Machines

Answer 1

Right answer is (d) Armature speed and armature current

Explanation: Average coil emf generated= ∅ωNP/π. Machine torque = ka*∅*Ia. Thus, average coil emf generated can also be represented as ka*∅*ω. So, average coil emf is directly proportional to ω (armature speed) and average torque is directly proportional to Ia (armature current).