Question

A moving iron ammeter produces a full-scale torque of 240 μN-m with a deflection of 120° at a current of 10 A. the rate of change of self-inductance (μH/rad) of the instrument at full scale is?

(a) 2.0 μH/rad

(b) 4.8 μH/rad

(c) 12.0 μH/rad

(d) 114.6 μH/rad

This question was addressed to me in an international level competition.

I would like to ask this question from Advanced Problems on Magnetically Coupled Circuits in chapter Resonance & Magnetically Coupled Circuit of Network Theory

Answer 1

The correct choice is (b) 4.8 μH/rad

Explanation: At full scale position, \(\frac{1}{2} I^2 \frac{dL}{dθ}\) = TC

 \(\frac{1}{2} 10^2 \frac{dL}{dθ}\) = 240 × 10^-6

∴ \(\frac{dL}{dθ}\) = 4.8 μH/rad.