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

Step 1: Understand cripping error.
Cripping error is a phenomenon in energy meters where the disc comes to rest prematurely due to hysteresis losses in the iron core, leading to inaccurate readings.
Step 2: Preventing cripping error.
To reduce this error, the discs must be laminated to minimize eddy current losses and reduce hysteresis in the magnetic field.
Step 3: Conclusion.
Thus, the discs must be laminated to avoid cripping error, corresponding to option (A).

Answer 2

The cripping (creeping) error in an energy meter refers to unwanted, spurious rotation of the aluminium disc caused by stray eddy-current torques, even when no proper load current is being measured. This is analogous to unwanted eddy-current losses in transformer cores, which are conventionally reduced by laminating the core into thin, insulated sheets that break up the paths available for eddy currents to circulate. Applying the same principle to the energy-meter disc, let's check each option.

1. Be laminated: Laminating the disc restricts the loops available for stray eddy currents to form, cutting down the spurious torque responsible for creeping — matching this option directly.
2. Be thin: Thinness alone, without lamination (i.e., without insulating layers), does not by itself interrupt eddy-current loops in the same targeted way.
3. Have a bar magnet: A permanent magnet is already used in energy meters, but for a different purpose — providing braking torque for calibration — not specifically to fix the creeping error.
4. Have holes: This is a different design change altogether and is not the fix reflected in the reasoning applied here.

Following the eddy-current-suppression principle used elsewhere in AC machines, laminating the disc is identified as the fix for the cripping error.

Therefore, the correct answer is be laminated.