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A moving-iron instrument provides accurate readings under what frequency conditions?

Difficulty: Easy

Correct Answer: All frequencies up to a certain maximum value

Explanation:


Introduction / Context:
Moving-iron (MI) instruments operate by the deflection caused due to the magnetic field created by current. Their response depends on inductance and coil impedance, making them frequency-dependent for AC measurement.


Given Data / Assumptions:

  • MI instruments can measure both AC and DC.
  • Inductance of the coil creates reactance that increases with frequency.


Concept / Approach:
At low frequencies (including DC), the impedance is nearly resistive. As frequency increases, inductive reactance grows and the current for a given applied RMS voltage decreases, causing errors. Thus, MI instruments remain accurate only up to a certain limiting frequency.


Step-by-Step Solution:

Impedance = √(R^2 + (ωL)^2).At small ω, Z ≈ R, hence accurate.At higher ω, inductive effect increases → error grows.


Verification / Alternative check:

Practical MI voltmeters are specified up to ~100–200 Hz for accuracy.


Why Other Options Are Wrong:

Low or high frequencies alone are misleading; ”one frequency” is irrelevant; resonance does not apply here.


Common Pitfalls:

Assuming MI meters are broadband instruments; they are not.


Final Answer:

All frequencies up to a certain maximum value
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