Energy meter creeps

Question

Energy meter creeps

Examoogle · Accepted Answer

Due to asymmetry in magnetic circuit

Explanation: Creeping in an energy meter is a phenomenon where the aluminum disc rotates continuously even when there is no load current, but only the voltage supply is present. This error is primarily caused by an imbalance in the magnetic circuit or excessive friction compensation.

Working Principle: The driving torque in an induction energy meter is proportional to the product of flux $\Phi_v$ (from voltage coil) and $\Phi_i$ (from current coil). When load current is zero, driving torque should be zero. However, due to asymmetry in the magnetic circuit or mechanical vibrations, a small unwanted torque is produced. If this torque exceeds the control of the braking magnet, the disc creeps. To prevent this, two small diametrically opposite holes are drilled in the disc to provide a 'parking' position.

Key Formulas: $T_d \propto \Phi_v \Phi_i \sin(\delta)$ — Torque equation where $\delta$ is the phase angle; $T_{creep} > T_{friction}$ — Condition for the onset of creeping

Key Points: Creeping is considered a major fault in electromechanical induction-type energy meters.; The anti-creeping holes ensure the disc comes to rest when the potential circuit is energized but the load current is zero.; Friction compensation is often done by placing a shading loop on the pressure coil core, which if over-adjusted, can lead to creeping.; Creeping results in false energy registration, leading to charges for energy not consumed.

Disadvantages: Causes inaccurate energy billing for consumers.; Indicates improper calibration of the meter.

Applications: Calibration of induction type energy meters.; Testing of meter sensitivity and friction balance.

Additional Info: Modern static (digital) meters do not have moving parts like discs and are therefore immune to the creeping phenomenon.. Option A is incorrect as supply changes affect torque magnitude but do not specifically cause creeping. Option B is incorrect as reversal of voltage polarity would reverse the torque direction, not cause creeping. Option D is incorrect as turns ratio determines the voltage step-down/step-up, not the mechanical rotation of the disc.

Answer: C is correct — Creeping is an error caused by asymmetry in the magnetic circuit that creates a small unintended driving torque on the meter disc.

Exam Tip: In competitive exams, always link 'Creeping' to the 'Anti-creeping holes' on the disc and 'Over-compensation of friction' as common theoretical questions.

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