Energy Meters Questions and Answers

Answer: A

Both over-generous friction compensation and a high voltage can produce a creeping torque. The remedy is the pair of holes in the disc; moving the braking magnet would spoil the full-load calibration and would not remove the torque causing the creep.

Answer: B

There is nothing to zero: the register counts from wherever it stands and only the difference between two readings is used. An indicating instrument needs a zero adjustment because its pointer must rest at the mark when nothing is applied.

Answer: C

Correct answer: over-compensation of the friction adjustment

Over-compensation adds more torque than is needed to overcome friction, so the disc runs fast and the meter registers *high* - and at no load it creeps, which is exactly what the holes in the disc exist to prevent. The other three all slow the disc: moving the braking magnet outward increases the braking torque, excessive friction opposes rotation directly, and saturation of the series magnet makes the driving torque less than proportional at heavy current.

Answer: D

Registration with no load points either to creeping or to a load the consumer has overlooked, such as a standby circuit. Applying voltage with the load side disconnected and watching the disc distinguishes the two at once.

Answer: A

An error that appears only when the power factor departs from unity is by definition an error in the quadrature angle of the shunt magnet flux. The braking magnet affects all power factors alike and so cannot be the cause.

Answer: B

Beyond a certain current the series flux stops rising in proportion and the driving torque falls behind the power, so the meter reads low. The overload compensation, usually a magnetic shunt across the series magnet, is provided for exactly this.

Answer: C

Below a small fraction of rated current the signal is lost in the noise and offsets of the electronics, so the meter registers nothing. It is the static equivalent of the induction meter's difficulty at light load, though the threshold is usually a good deal lower.

Answer: D

Remote reading removes the cost of visiting every premises, and the same channel allows tariff switching, demand recording and disconnection. Accuracy at light load and freedom from creeping are real gains too, but the commercial case rests on communication.

Answer: A

A wattmeter needs a control spring so its pointer settles at a deflection proportional to power. The meter must instead run continuously, so there is no control spring and the braking torque sets the speed. Both have a deflecting torque and a pressure coil.

Answer: B

Correct answer: 475 kWh

\[ 04987 - 04512 = 475\ \mathrm{kWh}. \] The register is cumulative, so consumption is always the difference between two readings and never a reading itself - which is what the 4987 offered would be. The leading zero is part of the dial layout and carries no significance.