Energy Meters Questions and Answers

Answer: C

An error in the quadrature angle shows itself as a power factor dependent error, so it is invisible at unity power factor and plain at 0.5. The routine is to set the full-load speed at unity power factor with the braking magnet, then correct the lag at 0.5.

Answer: D

Moving the magnet outward increases the radius at which the eddy currents act and so the braking torque, which slows the disc and reduces the registration. This is the coarse speed adjustment and it is made at rated current and unity power factor.

Answer: A

Friction is roughly constant while the driving torque falls with the load, so at five per cent of rated current it may be an appreciable fraction of the torque and the meter would run slow. A small extra torque, obtained from a shading loop near the shunt magnet, makes up for it.

Answer: B

A copper loop or vane placed asymmetrically under the shunt magnet pole splits its flux into two components slightly out of phase, and those two produce a small torque proportional to the square of the voltage. That torque is present whatever the load and offsets the friction.

Answer: C

It is the classic consequence of over-generous friction compensation, though vibration, stray fields and an excessive supply voltage can also produce it. The meter then registers energy that has not been consumed, which is why it must be prevented.

Answer: D

As a hole approaches the shunt magnet pole the eddy current pattern is distorted and a torque arises that holds the disc there. Any creeping motion therefore stops within half a revolution, while at normal speeds the effect is negligible.

Answer: A

Saturation makes the series flux rise less than in proportion to the current, so the meter would register low on overload. A magnetic shunt across the series magnet, or a saturable flux diverter, restores the proportionality over the working range.

Answer: B

The full-load speed is set first with the braking magnet at rated current and unity power factor, because everything else is a correction to it. The lag adjustment follows at 0.5 power factor and the friction adjustment at about five per cent of rated current.

Answer: C

It is marked on the nameplate as a figure such as 1200 rev/kWh, and dividing the revolutions counted by that figure gives the energy. The reciprocal is sometimes quoted as the disc constant in watt hours per revolution, but the usual marking is the first form.

Answer: D

The pressure circuit is fed at rated voltage from one source while the current circuit is fed at rated current from a separate low-voltage source. The power drawn is then only the losses of the two circuits, which for a large meter is a great saving.