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Solved Problems on Moving & dynamometer type instruments
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By Prof Sunil Kumar, Electrical Engineering, IEC College, Greater Noida, Uttar Pradesh
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Q4. Explain in brief why energy meter reads energy while wattmeter does not. An energy meter has a registration constant of 100rev/kwh if the meter is connected
to a load drawing 20A at 230V and 0.8 power factor for 5 hours. Find the number of revolution should be made by it of it is actually made 1800 revolutions.
find the % age error and explain it from consumer point of view?
Answer.
An energy meter is fitted with some type of registration mechanism whereby all the instantaneous reading of power are summed over a definite
period of time.
ENERGY = POWER X TIME
So, an energy meter can read energy.
Whereas, a wattmeter indicates the value of power at a particular instant when it is read and hence it can not read energy.
Registration constant= 100 rev/Kwh
I = 20 A, V = 230 V, cos φ = 0.8
Energy in 5 hours = VI Cos φ x 5 / 1000
= 20 x 230 x 0.8 x 5 / 1000
= 18.4 Kwh
The number of revolution of the meter should make it is correct = 18.4 x 100 = 1840 revolution.
Number of revolution actually made = 1800 revolution
% error = 1840 - 1800 x 100
1840
= 0.0217%
It would be better from the customer point of view % age error is very less.
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