Energy Meters Questions and Answers

Answer: A

The order is deliberate: the full-load speed is set first at unity power factor, then the power-factor-dependent error is removed at 0.5, and finally the small-load error is corrected at five per cent, each step being a smaller correction than the last.

Answer: B

The shunt magnet supplies the voltage-proportional flux lagging ninety degrees, the permanent magnet supplies the speed-proportional braking that makes speed follow power, and the gear train converts revolutions into a reading in kilowatt hours.

Answer: C

Each step is a smaller correction than the one before, so the largest is made first: full-load speed, then the power factor error, then the light-load error. The creep test comes last because the friction adjustment is the very thing that can cause creeping.

Answer: D

Correct answer: 1 and 2 only are correct

Statements 1 and 2 are correct: an energy meter has no controlling torque, since it is intended to keep turning rather than to come to rest at a deflection, and its steady speed is proportional to the power because the braking torque rises with speed until it balances the driving torque. Statement 3 is wrong, for the reason the direct current item gives - a steady flux induces nothing.

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.