Energy Meters Questions and Answers

Answer: C

Correct answer: register nothing while current continues to be drawn

The register is driven by the disc through a gear train, so a seized disc advances nothing at all while the load continues to be supplied. The consumer is billed for nothing, which is why meter tampering often takes exactly this form and why a reading that stops changing while the premises are plainly occupied is the first thing an inspection looks for.

Answer: D

Reactive current heats conductors and occupies the capacity of transformers and cables while transferring no net energy, so the supply charges for it or penalises a poor power factor. An active energy meter reads nothing from it, which is why a separate meter is needed.

Answer: A

Shifting the pressure circuit voltage by ninety degrees turns the cosine of the load angle into its sine, so the disc speed follows the reactive power. On a three-phase system the shift is obtained simply by connecting to the other two lines.

Answer: B

Standards allow a wider error at five per cent of rated current than at full load, because the friction correction cannot be perfect over the whole range. Accuracy classes are quoted as a figure such as class 1 or class 2, with a table of limits against load.

Answer: C

The torque comes from the flux of one magnet acting on the eddy currents produced by the other, and that requires the current of one to flow through the field of the other. Coincident fluxes, however displaced in phase, would produce no net rotation.

Answer: D

Reversing one of the two fluxes reverses the sign of the torque, so the disc runs the other way. Meters intended for one direction of energy flow are fitted with a ratchet to prevent the register running backwards, which also makes reverse connection a detectable fault.

Answer: A

Import and export are usually valued differently, so the two must be accumulated separately rather than netted by a single register. A static meter does this by detecting the sign of the power and directing the accumulated energy to the appropriate register.

Answer: B

The register must advance by one kilowatt hour for every K revolutions of the disc, so the gear ratio follows directly from the constant. A meter of a different constant needs a different train, which is why the constant is stamped on the nameplate.

Answer: C

Their current coils carry the same current and their pressure coils see nearly the same voltage, so each measures the same power and registers the same energy. The tiny drop across the first coil makes an immaterial difference, and this is a standard way of comparing two meters.

Answer: D

The wattmeter multiplies two currents directly, so its pressure coil current must be in phase with the voltage. The meter multiplies two fluxes through a sine, so it needs one of them in quadrature with the voltage. The requirements are opposite for the same reason: to obtain the cosine.