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

The question was asked in an online quiz.

The origin of the question is Advanced Problems on Indicating Instruments topic in chapter Extension of Instrument Ranges of Electrical Measurements

Answer 1

The correct answer is (b) 4.8 μH/rad

Best 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.