In eddy-current damping systems, the disc is usually made of

Question

Examoogle · Accepted Answer

Conducting and Non-magnetic material

Explanation: In eddy-current damping systems, the disc is constructed from a conducting, non-magnetic material such as aluminum or copper. This configuration allows for the generation of induced eddy currents that interact with a stationary magnetic field to produce a braking torque without causing permanent magnetization effects.

Working Principle: When the conducting disc moves through a magnetic field, the changing magnetic flux induces eddy currents in the disc as per Faraday's Law. These currents create their own magnetic field that opposes the original motion (Lenz's Law), effectively providing a damping force. A non-magnetic material is essential to ensure that the braking torque is strictly a result of eddy-current induction rather than magnetic hysteresis or attraction.

Key Formulas: $F_d = BIl$ — Force on a conductor in a magnetic field; $T_d = k\omega$ — Damping torque is proportional to angular velocity

Key Points: Conducting materials like Aluminum are preferred due to their high conductivity-to-weight ratio.; Non-magnetic materials prevent magnetic 'stickiness' (hysteresis/cogging), ensuring smooth damping.; The damping force is proportional to the velocity of the disc, which is ideal for steady-state stability.; Eddy current damping is widely used in induction-type energy meters.

Advantages: Linear relationship between damping torque and velocity; Contactless operation results in zero wear and tear; Simple and rugged construction

Disadvantages: Power loss occurs due to $I^2R$ heating in the disc; Requires a constant and stable magnetic field source

Applications: Induction-type energy meters; Speedometers and tachometers; Moving coil galvanometers

Additional Info: Aluminum is the standard industry choice due to being non-magnetic, lightweight, and highly conductive.. Option C is incorrect because magnetic materials would lead to undesirable magnetic attraction forces, hindering the precise damping required.

Answer: D is correct — The disc must be a non-magnetic conductor to ensure eddy currents are generated without introducing magnetic drag forces.

Exam Tip: Always remember that 'damping' in electrical meters requires a material that facilitates eddy current flow while being magnetically 'invisible' to avoid permanent magnet interaction.

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