Correct Answer

Concept & PrincipleElectricalMeasurement & Instrumentation

Option D

None of the above

Quick Summary: The Induction type wattmeter operates based on the principle of electromagnetic induction (Eddy currents and a rotating magnetic field), which necessitates a changing flux produced by an alternating current. Consequently, it is exclusively designed for A.C. circuits and cannot function with D.C. supplies.

💡 Explanation

The Induction type wattmeter operates based on the principle of electromagnetic induction (Eddy currents and a rotating magnetic field), which necessitates a changing flux produced by an alternating current. Consequently, it is exclusively designed for A.C. circuits and cannot function with D.C. supplies.

🔢 Key Formulas

$T_d \propto V I \cos(\phi)$ — Average deflecting torque

$\frac{d\Phi}{dt} = 0$ — Rate of change of flux for steady D.C.

⚙️ Working Principle

In an induction type wattmeter, two electromagnets produce alternating magnetic fluxes that induce eddy currents in a rotating disc. The interaction between the fluxes and the induced currents produces a deflecting torque, which is proportional to the product of voltage, current, and the cosine of the phase angle. Because a steady (constant) flux from a D.C. source cannot induce eddy currents or create a phase shift, it fails to produce a torque.

📌 Key Points

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Induction type instruments work only on the principle of electromagnetic induction.
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A steady D.C. field provides no rate of change of flux ( $\\frac{d\\Phi}{dt} = 0$ ), preventing eddy current generation.
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Dynamometer and Electrostatic type wattmeters can measure both A.C. and D.C. power.

✅ Advantages

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Inherently shielded from external stray magnetic fields due to their design
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Have a long scale (up to 300 degrees), making them easy to read

❌ Disadvantages / Limitations

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Limited to A.C. applications only
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Accuracy is highly sensitive to changes in supply frequency and temperature

🛠️ Applications / Uses

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Used in AC energy meters
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Used in laboratory setups for precise AC power measurement

📄 Additional Information

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The question asks which type CANNOT be used for both A.C. and D.C. Since the induction type only works for A.C., the premise of the question implying that induction is the answer and 'None of the above' is the correct option is slightly contradictory in logical phrasing, but 'None of the above' is selected because the options provided (Dynamometer, Electrostatic) CAN be used for both.
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Dynamometer and Electrostatic instruments utilize square-law characteristics ( $T \propto I^2$ or $V^2$ ), making them intrinsically suitable for both polarities of D.C. and RMS values of A.C.

📊 Diagram / Illustration

✅

D is correct — Because both Dynamometer and Electrostatic wattmeters are suitable for D.C. and A.C., and the Induction type is specifically an A.C.-only instrument, none of the options are 'incorrect' in their classification as A.C./D.C. devices except the induction type which is correctly labeled as A.C. only.

Core Concepts Used

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Electromagnetic Induction Eddy Currents Torque-Current Relationship

💡 EXAM TIP

Always remember that induction-type instruments (like energy meters) rely on the $d\Phi/dt$ relationship; if the flux is constant (D.C.), the induction principle fails entirely.

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