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Type 7: Energy Meter
For Concept, refer to Measurement K-Notes, Electro- mechanical Instruments.
Point to remember:
The measured value of energy in an energy meter is calculated in terms of meter constant
and number of revolutions and true value of energy is derived from Power and time. Using
these two values we can compute error in energy meter.
Sample Problem 7:
ww
A dc A-h meter is rated for 15 A, 250 V. The meter constant is 14.4 A-sec/rev. The meter
constant at rated voltage may be expressed as
w.E
(A) 3750 rev/kWh
(C) 1000 rev/kWh
(B) 3600 rev/kWh
(D) 960 rev/kWh
asy
Solution: (C) is correct option
Meter constant (A-sec/rev) is given by
14.4
1
speed En
14.4
1 gin
K Power
Where ‘K’ is the meter constant in rev/kWh. eer
14.4
1
K VI ing
14.4
15
K 15 250
K
1
14.4 250 .ne
K
1
14.4 250
3600 1000
1000 3600
3600
1000 rev/kWh t
Q.1 The current and flux produced by series magnet of an induction watt-hour energy Meter
are in phase, but there is an angular departure of 30 from quadrature between voltage and
shunt magnet flux. The speed of the disc at full load and unity power factor is 40 rpm.
Assuming the meter to read correctly under this condition, calculate it’s speed at 1 /4 full load
and 0.5 P.F. lagging?
(A) 4.3 rpm (B) 4.4 rpm (C) 4.1 rpm (D) 4.5 rpm
Q.2 Single phase wattmeter operating on 230 V and 5 A for 5 hour makes 1940 Revolutions.
Meter constant in revolutions is 400. The power factor of the load will be
(A) 1 (B) 0.8 (C) 0.7 (D) 0.6
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