Energy Meters Questions and Answers

Answer: C

The load power is 200 W.

At 1200 revolutions per kWh, 5 revolutions record 5/1200 kWh. Power is that energy divided by the time, 75 s.

\[ \begin{aligned} E &= \frac{5}{1200}\ \text{kWh} = \frac{5 \times 3.6 \times 10^{6}}{1200}\ \text{J} \\ &= 15\,000\ \text{J} \\ P &= \frac{15\,000}{75} = 200\ \text{W} \end{aligned} \]

The other options come from mishandling the hour-to-second conversion; a 1000 W load, for example, would turn the disc 25 times in 75 seconds.

Answer: D

The holes are drilled to avoid creep on no load.

Creep is the slow, continuous rotation of the disc of an induction energy meter when no load current flows, caused by over-compensation for friction or by a supply voltage above normal. Two holes are drilled on opposite sides of the disc. When a hole comes under the pole of the shunt magnet, the eddy current pattern is distorted and a small torque acts that stops the disc there, ending the creep.

The holes do not balance the disc, they do not dissipate eddy current energy, and they do not increase the driving torque.

Answer: B

The watt-hour meter is an integrating instrument.

An energy meter adds up power over time, so its register shows the total energy consumed since it was installed rather than the value at any one moment. Instruments of that kind are called integrating instruments; ampere-hour meters are another example.

An indicating instrument, such as an ammeter or voltmeter, shows the present value with a pointer. A recording instrument draws a continuous graph of a quantity on a chart. A transfer instrument is one calibrated on dc and then used to calibrate ac instruments, such as the electrodynamometer.

Answer: D

The integrating instruments are ampere-hour and watt-hour meters.

Instruments are classed as indicating, recording or integrating. Integrating instruments add up a quantity over time and show the total. A watt-hour (energy) meter integrates power over time and records the energy consumed, as in a household electricity meter, and an ampere-hour meter integrates current over time to give the total charge, as used with batteries.

Ammeters, voltmeters and wattmeters are indicating instruments: they show the instantaneous or average value of current, voltage or power at the moment of reading, not an accumulated total.