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In order to prevent creeping in an energy meter, which of the following measures is adopted?
Two, diametrically opposite holes are drilled on the aluminium disc.
A temperature shunt is used on the brake magnet.
A shading band is provided on the central limb of the shunt magnet.
Two, diametrically opposite holes are drilled on the central limb.
Two, diametrically opposite holes are drilled on the aluminium disc.
Creeping is the slow, continuous rotation of the energy meter disc even when no load is connected, caused by stray magnetic fields, excessive supply voltage, or vibrations. To prevent this, two diametrically opposite holes are drilled on the aluminium disc to provide a position of unstable equilibrium, which exerts a retarding torque and halts the disc rotation at no-load.
IS 13779:1999
Creeping is the slow, continuous rotation of the energy meter disc even when no load is connected, caused by stray magnetic fields, excessive supply voltage, or vibrations. To prevent this, two diametrically opposite holes are drilled on the aluminium disc to provide a position of unstable equilibrium, which exerts a retarding torque and halts the disc rotation at no-load.
╧ДcreepтАЛтИЭV2 тАФ Creeping torque is proportional to the square of supply voltage.
╧ДbrakeтАЛтИЭDiscontinuity├ЧB2 тАФ Braking torque induced by the holes.
When the disc rotates due to creeping, the holes align with the brake magnet poles. At this point, the eddy currents path is interrupted or distorted, leading to a magnetic imbalance that provides a mechanical braking torque. This torque overcomes the creeping torque, forcing the disc to remain stationary.
Creeping is primarily observed at no-load condition.
The holes act as a magnetic 'locking' mechanism when the disc is at no-load.
Excessive voltage beyond 110% of rated voltage is a major cause of creeping.
Mechanical vibrations and improper leveling can also trigger creeping.
Simple and cost-effective design change.
Does not affect the accuracy of the meter at rated load.
Slight reduction in sensitivity at very low loads.
Requires precise manufacturing for the holes to be perfectly diametrically opposite.
Induction type single-phase energy meters.
Poly-phase induction energy meters.
Option B (Temperature shunt) is used for temperature compensation to keep the damping torque constant despite resistance changes.
Option C (Shading band) is used for phase angle compensation between the shunt magnet flux and the applied voltage.
Standard regulations often require the disc to not complete a full revolution at no-load when voltage is up to 110% of reference voltage.
A is correct тАФ Two diametrically opposite holes are drilled on the aluminium disc to exert a retarding torque that overcomes the driving torque caused by creeping.
Always remember that shading bands adjust phase (lag), temperature shunts adjust damping (resistivity), and holes prevent no-load rotation (creeping).
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