A moving-iron instrument provides accurate readings under what frequency conditions?
Electronics and Communication Engineering
Measurements and Instrumentation
Difficulty: Easy
Choose an option
-
ALow frequencies only
-
BHigh frequencies only
-
COnly one frequency
-
DAll frequencies up to a certain maximum value
-
EAt resonance only
Answer
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