Post on 05-Dec-2014
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POWER MEASUREMENT
IN 3-PHASE CIRCUITS
METHODS
•Three-Watter Method
•Two-Watter Method
•One-Watter Method
THREE-WATTER METHOD
3 wattmeter's are connected in such a way that each has its current coil in one line and its potential coil between that line and some common point.
N
B
R
Y
Z1
Z3Z3
W1
W2
W3
±
±
±
I3
I2
I1
±
±
±
X
THE TRICK
Total power : w1 + w2 + w3
• If neutral wire available, the common point X should be the neutral wire.
• Note: Total power does not depends upon whether the load is balanced or not.
WHERE TO PLACE ‘X’
The point X may be specified
point in the three phase system or it may be merely a paint in space at which the three potential coils have a common junction
TWO-WATTER METHOD
Current coils of two wattmeter's are connected in any two lines and the potential coil of each
joined to the third line
N
B
R
Y
Z1
Z3Z3
W1
W2±
±
I3
I2
I1
±
±
±
THE TRICK
Total power : w1 + w2 + w3
• If the neutral wire is available, it should carry no current.
• Or else the neutral of the load should be isolated from the neutral of the source.
TRIVIA
The two-wattmeter method is essentially the three-wattmeter method with common point shifted to one of the lines
ON A NOTE
Two-watter method is a universal method of
measuring power in a 3-phase circuit.
TWO-WATTER METHOD BALANCED LOAD
300
300
300
300ɸ
ɸ ɸ
VBY
VRN
-VYN
IR
VYN
IY
VRY
VBN IB
VRN
VYN
VBN
R.M.SIR
IY
IB
R.M.S
LOAD PHASE VOLTAGES
PHASE CURRENTS
POWER FACTOR
•Since, Total power = W1+ W2
•Power Factor :
(w2-w1)(w2+w1)
tan ɸ = √3
A BRAIN TEASER
Which is greater W1 or W2 ?
W1=VLILCOS(300+ ɸ)W2=VLILCOS(300- ɸ)
ANSWER
• Since ɸ can vary between 900 to 00
• So, tan ɸ vary between 0 to 1.
• Therefore, whose reading is proportional to (300- ɸ) is greater and positive
(w2-w1)
(w2+w1)tan ɸ = √3 PF :
EFFECT OF LOAD PF ON WATTMETER READINGS
ɸ 00 600 More than 600
900
Cos ɸ 1 o.5 <0.5 0
W2 +ve +ve +ve +ve
W1 +ve 0 -ve -ve
Conclusion W1 = W2
Total Power: W1+W2
W1 =0Total Power: W2
Total Power:W1-W2
W2=-W1
Total Power:0
(w1-w2)(w1+w2)
tan ɸ = √3
(w2+w1)tan ɸ = √3 Lagging p.f
Leading p.f
(w2-w1)
(Higher reading - Lower Reading)
(Higher reading + Lower Reading)
TRICK :tan ɸ = √3
REACTIVE POWER
√3 (w2-w1)
Difference Of Readings Of Two
Wattmeter’s
ONE-WATTER METHOD
If the load (3-phse) Y or is )balanced, the power in any phase can be measured by a single wattmeter.
N
B
R
Y
Z
ZZ
W
IB
IY
IR
VYB
THE TRICK
• Two wattmeter’s readings are taken with the help of one.
• The current coil is connected in any one line and the pressure coil is connected in any one line and the pressure coil is connected alternately between this and the other two lines.
• Algebraic sum of the two readings give the total power drawn by the balanced 3-phase load.
REACTIVE POWER
√3 (Wattmeter
Reading)
HAPPY LEARNING
Tell me and I forget. Teach me and I remember. Involve me and I learn.
PRESENTED BY:
Abhisek Padhihttp://TechniKaly.com
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