Correct Answer

Concept & PrincipleElectricalMeasurement & Instrumentation

Option C

Permanent magnet

Quick Summary: The braking system in an induction-type single-phase energy meter consists of a permanent magnet, commonly known as a braking magnet. It is positioned near the rotating aluminum disc to exert a retarding torque that opposes the driving torque, ensuring the disc speed is proportional to the power being measured.

💡 Explanation

The braking system in an induction-type single-phase energy meter consists of a permanent magnet, commonly known as a braking magnet. It is positioned near the rotating aluminum disc to exert a retarding torque that opposes the driving torque, ensuring the disc speed is proportional to the power being measured.

🔢 Key Formulas

$T_b = K \cdot \phi^2 \cdot N$ — where $K$ is a constant, $\phi$ is the magnetic flux, and $N$ is the rotational speed of the disc.

$T_d = T_b$ — condition for steady-state speed where driving torque equals braking torque.

⚙️ Working Principle

When the aluminum disc rotates through the field of the permanent magnet, eddy currents are induced in the disc. According to Lenz's law, these currents produce a magnetic field that interacts with the permanent magnet's field to create a braking torque. The retarding torque $T_b$ is directly proportional to the speed of the disc, such that $T_b \propto \phi^2 N$ , where $\phi$ is the flux of the permanent magnet and $N$ is the rotational speed.

📌 Key Points

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The permanent magnet is typically a C-shaped Alnico magnet.
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Braking torque is adjusted by changing the radial position of the magnet relative to the disc center.
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Eddy currents are the primary mechanism for generating the retarding force.
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The braking system prevents the disc from spinning indefinitely when the load is disconnected (prevents 'creeping' to an extent).

✅ Advantages

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Provides linear braking characteristic relative to speed.
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Stable over long periods of operation due to high coercivity of permanent magnets.

❌ Disadvantages / Limitations

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Susceptible to weakening if subjected to extreme heat or vibration.
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Sensitivity to stray magnetic fields if not shielded properly.

🛠️ Applications / Uses

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Electromechanical induction-type watt-hour meters.
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Rotating disc-type speed control systems.

📄 Additional Information

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If the permanent magnet is shifted towards the center of the disc, braking torque decreases; shifting it outward increases braking torque.
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Option B (Temporary magnet) is incorrect because a temporary magnet's flux would vary with current, leading to non-linear braking.

📊 Diagram / Illustration

✅

C is correct — The braking system in a single-phase energy meter utilizes a permanent magnet to induce eddy currents in the disc, creating a retarding torque.

Core Concepts Used

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Electromagnetic Induction Lenz's Law Eddy Current Braking

💡 EXAM TIP

Always remember that in moving-coil and induction-type instruments, braking is almost exclusively achieved via eddy current effects generated by permanent magnets.

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