Which of the following types of instrument can’t be used fo D.C

Question

Examoogle · Accepted Answer

The correct answer is Option D: Induction type. Induction type instruments operate on the principle of electromagnetic induction, which requires an alternating magnetic field to produce eddy currents in a rotating disc. Since a D.C. source produces a steady, unidirectional magnetic field, it cannot induce the necessary currents to create a torque, making these instruments suitable only for A.C. measurements.

Step-by-Step Solution & Concepts:
Induction type instruments operate on the principle of electromagnetic induction, which requires an alternating magnetic field to produce eddy currents in a rotating disc. Since a D.C. source produces a steady, unidirectional magnetic field, it cannot induce the necessary currents to create a torque, making these instruments suitable only for A.C. measurements.

Working Principle: The instrument consists of two electromagnets energized by the source. These magnets produce alternating fluxes $\Phi_1$ and $\Phi_2$ that differ in phase. These fluxes induce eddy currents in a metallic disc, which interact with the fluxes to produce a torque $T_d \propto \Phi_1 \Phi_2 \sin \alpha$. In D.C., the rate of change of flux is zero, meaning no eddy currents are induced and $T_d = 0$.

Key Formulas: $T_d \propto \Phi_1 \Phi_2 \sin \alpha$ — Torque produced by induction type instruments; $I_{eddy} \propto \frac{d\Phi}{dt}$ — Eddy current induction principle

Key Points: Induction instruments rely on electromagnetic induction (Faraday's Law).; They require a changing magnetic field to generate an eddy current.; D.C. results in a constant flux, yielding zero torque.; These instruments are widely used as watt-hour meters in A.C. systems.

Advantages: Long scale of deflection (up to 300°).; High operating torque-to-weight ratio.

Disadvantages: Operates only on A.C.; Errors occur due to frequency and temperature variations.

Applications: A.C. Energy meters; A.C. Wattmeters

Additional Info: Moving Iron instruments work on both A.C. and D.C. because torque depends on $I^2$.. Moving Coil (PMMC) instruments are specifically for D.C. because they use a permanent magnet and polarity sensitive torque.

Answer: D is correct — Induction type instruments require a changing magnetic field (A.C.) to produce torque via eddy currents, which is absent in D.C. systems.

Exam Tip: Always remember that induction-based components (like transformers and induction motors) fail on D.C. due to the lack of flux change, leading to zero reactance and excessive current.

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Option A: Moving iron-attraction type

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Option B: Moving coil permanent magnets type

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Option C: Hotwire type

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