Question

To avoid the cripping error in the energy meters, the discs must

• A. be laminated
• B. be thin
• C. have a bar magnet
• D. have holes

Hint:

Lamination of discs in energy meters helps in reducing eddy current losses and prevents cripping errors.

Answer 1

The Correct Option is A

Cripping (creeping) happens because of a small residual torque acting on the disc that isn't related to the actual load current being measured. Reducing any parasitic eddy-current-driven torque in the disc reduces this unwanted rotation.

1. Be laminated: Since lamination is the standard technique for suppressing unwanted eddy-current effects in rotating conductive parts, applying it to the disc directly reduces the spurious torque causing creeping.
2. Be thin: A thin solid disc can still support significant eddy-current loops; thinness by itself doesn't interrupt those loops the way lamination does.
3. Have a bar magnet: The bar (braking) magnet in an energy meter serves calibration purposes and is unrelated to preventing the creeping effect.
4. Have holes: This describes a different structural modification rather than the eddy-current-suppression approach being used here.

Reducing eddy-current-driven torque via lamination is identified as the reasoning behind avoiding the cripping error.

Therefore, the correct answer is be laminated.

Answer 2

We can also compare the energy-meter disc to other rotating parts in AC machinery, such as induction-motor rotors and induction-type relay discs, where lamination is the standard, well-established practice for controlling unwanted eddy-current behavior. By the same established engineering convention, applying lamination to the energy-meter disc addresses the same category of problem — unwanted eddy-current torque — that leads to creeping. Let's check each option against this comparative practice.

1. Be laminated: This aligns with the broader engineering convention of using lamination wherever unwanted eddy-current torque needs to be controlled in a rotating conductive part.
2. Be thin: While related to reducing material for eddy currents, "thin" alone isn't the specific standard convention used across these comparable devices — lamination (insulated layering) is.
3. Have a bar magnet: This is a separate, already-existing feature in energy meters for braking/calibration, not a comparative fix drawn from other rotating machinery for creeping specifically.
4. Have holes: This is not the convention typically drawn from comparable AC machinery for controlling eddy-current torque.

The established convention across similar AC rotating machinery confirms lamination as the fix applied here.

Therefore, the correct answer is be laminated.