Measurement Of Power In 3-Ph A.C. Circuit https://sah786.wordpress.com www.slideshare.net/SaHussain1 www.facebook.com/Sah92786 https://www.linkedin.com/in/arshad-hussain-8b0a2613b Measurement of Power in 3-Phase Circuit

1. Measurement Of Power In
3-Ph A.C. Circuit
Department of Electrical Engineering

2. Gujarat Technological
University INDIA
DepartmentofelectricalEngineering
of
GovernmentEngineeringCollageDahod
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3. Prepared By:
• Arshad Hussain 9019
• Deokar Kamlesh 9002
• Patel Ramesh 9013
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4. Introduction
• In almost all cases the power in a d.c. circuit is best measured by
separately measuring quantities, V and I and by computing P=VI
• In case of a.c. circuits the instantaneous power varies continuously
as the current and voltage go through a cycle of values
• The fact that the power factor is involved in the expression for the
power means that a wattmeter must be used instead of merely an
ammeter and voltmeter.
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5. Wattmeter
• A wattmeter is essentially an inherent combination of an ammeter
and a voltmeter and, therefore , consists of two coils known as
current coil and pressure coil.
• Wattmeter connection:
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6. Wattmeter Errors
• Error due to inductance of pressure coil
• Error due to pressure coil capacitance
• Error due to Eddy currents : soild metal parts are removed as far away from
the current coil as possible
( )
wattmeterofadingpowerTrue Re
coscos
cos
×
−
=
θϕθ
ϕ
ϕθ
θ
cotsin
sin
−
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7. • Error due to power loss in pressure coil or current coil
• There are two method of connecting wattmeters in the circuit for
measurement of power, as shown in figure below (a) and (b).
• Fig (a) :
• Fig (b) :
crIWreadingWattmeter 2
+=
prR
V
WreadingWattmeter
+
+=
2
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8. Measurement of Power in Single Phase A.C. Circuit
• 3-voltmeter method
• Disadvantages : (i) Even small errors in measurement of voltages may
cause serious errors in the value of power, (ii) Supply voltage higher than
normal voltage is required
R
VVV
P
2
2
2
2
1
2
3 −−
=
21
2
2
2
1
2
3
2
cos
VV
VVV −−
=ϕ
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9. • 3-Ammeter method
• The disadvantages of measurement of power by 3 voltmeters are overcome
in this method
( )2
2
2
1
2
3
2
III
R
P −−=
21
2
2
2
1
2
3
2
cos
II
III −−
=ϕ
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10. Measurement of power in conjuction with instrument
transformers
• This method is used when the currents and voltages of the circuits
to be measured are high
• Figure below shows a measurement of power with wattmeter in
conjunction with instrument transformers in single phase A.C.
circuits
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11. • Vector diagram for inductive load
• Vector diagram for capasitive load
δβθϕγ ±−−=
( )δβθϕθ
ϕ
±−−
=
coscos
cos
K
δβθϕγ ±++=
( )δβθϕθ
ϕ
±++
=
coscos
cos
K
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12. Measurement of Power in 3-Phase Circuit
• Measurement of power in 3-
phase, 4-wire circuits-----------
• P=W1+W2+W3
• Measurement of power in 3-
phase, 3-wire circuits-------------
• P=W1+W2+W3
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13. • 3-wattmeter method of measuring
3-phase power of delta connected
• P=W1+W2+W3
• 1-wattmeter method of measuring
balanced 3-phase power (a) star
connected, (b) delta connected
• P=3W
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14. • 2-wattmeter method of measuring
3-phase 3-wire power :
– (a) star connected,
– P=W1+W2
– (b) delta connected
– P=W1+W2
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15. Determination of P.F. from Wattmeter Reading
• If load is balanced, then p.f. of the
load can be determined from the
wattmeter readings
• Vector diagram for balanced star
connected inductive load -----
• The watt-ratio Curve ----------
• p.f. can be determined from
reading of two wattmeters
( )
21
211 3
tancoscos
WW
WW
+
−
= −
ϕ
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