Which of the following types of instrument can be used for D.C only?

Question

Examoogle · Accepted Answer

Option C: Permanent magnet type

Explanation & Concepts:
Permanent Magnet Moving Coil (PMMC) instruments are designed exclusively for D.C. measurement because the torque direction depends on the polarity of the current flowing through the coil. If an A.C. signal is applied, the torque reverses rapidly with the cycle, resulting in an average deflection of zero due to the inertia of the moving system.

Working Principle: The principle relies on the interaction between a permanent magnetic field and a current-carrying conductor (Lorentz force). The force $F = BIl \sin(	heta)$ produces a deflecting torque $T_d = N B I l d$. Since the magnetic field $B$ is constant in both magnitude and direction due to the permanent magnet, the torque $T_d$ is directly proportional to the current $I$. Changing the polarity of $I$ reverses the direction of motion, preventing its use for A.C.

Key Formulas: $T_d = N B I l d$ — Deflecting torque in a PMMC instrument; $T_c = K 	heta$ — Controlling torque provided by springs

Key Points: PMMC instruments utilize a permanent magnet to provide a constant magnetic field.; The scale of a PMMC instrument is uniform (linear) because deflection $	heta \propto I$.; These instruments have high sensitivity and high torque-to-weight ratio.; They are inherently unsuitable for A.C. measurement unless used with a rectifier.

Advantages: Uniform scale provides easy reading.; Low power consumption compared to other types.; Very high accuracy and excellent damping characteristics.

Disadvantages: Can only be used for D.C. measurements.; Moving parts are susceptible to wear and tear over long durations.; More expensive to manufacture than moving-iron instruments.

Applications: D.C. Voltmeters and Ammeters; Galvanometers in laboratory measurements

Additional Info: Moving iron (MI) and hotwire instruments are 'deflection-type' and respond to the square of the current, allowing them to measure both A.C. and D.C.. Option A and B (Moving Iron) work on the principle of magnetic attraction or repulsion, which remains independent of current polarity.

Answer: C is correct — Permanent magnet moving coil (PMMC) instruments depend on the direction of current for torque production, making them functional for D.C. circuits only.

Exam Tip: Always remember that 'square-law' instruments like Moving Iron and Hotwire respond to $I^2_{RMS}$, making them suitable for both A.C. and D.C., whereas PMMC responds to $I_{avg}$ and requires specific polarity.

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Option A: Moving iron attraction type

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Option B: Moving Iron repulsion type

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Option D: Hotwire type

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