All the three methods of measurement of power in 3-phase ac circuits. 3,2,1--watter methods explained.

1. POWER
MEASUREMENT
IN 3-PHASE CIRCUITS

2. METHODS
• Three-Watter Method
• Two-Watter Method
• One-Watter Method

3. 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.

4. ±
R
Z1
B
I1
±
W2
N
Z3
W1
±
Z3
X
I2
Y
±
W3
±
I3
±

5. 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.

6. 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

7. 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

8. ±
R
Z1
W1
I1
±
N
Z3
B
Z3
I2
Y
±
W2
±
I3
±

9. 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.

10. TRIVIA
The two-wattmeter method is essentially
the three-wattmeter method with common
point shifted to one of the lines

11. ON A NOTE
Two-watter method is a universal
method of measuring power in a
3-phase circuit.

12. TWO-WATTER METHOD BALANCED LOAD
VBN
VRN
IB
VBY
R.M.S
R.M.S
IY
IR
VYN
VBN
300
IB
VRY
-VYN
300
LOAD
PHASE
VOLTAGES
ɸ
300
ɸ
IY
VYN
PHASE
CURRENTS
300
ɸ
VRN
IR

13. POWER FACTOR
• Since, Total power = W1+ W2
• Power Factor :
tan ɸ = √3
(w2-w1)
(w2+w1)

14. A BRAIN TEASER
W2=VLILCOS(300- ɸ)
W1=VLILCOS(300+ ɸ)
Which is greater W1 or W2 ?

15. ANSWER
PF :
tan ɸ = √3
(w2-w1)
(w2+w1)
• 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

16. 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 =0
Total Power:
W2
Total Power:
W1-W2
W2=-W1
Total Power:0

17. Lagging p.f
TRICK
tan ɸ = √3
: tan ɸ = √3
Leading p.f
(w2-w1)
(w2+w1)
(Higher reading - Lower Reading)
(Higher reading + Lower Reading)
tan ɸ = √3
(w1-w2)
(w1+w2)

18. REACTIVE POWER
√3 (w2-w1)
Difference Of Readings
Of Two Wattmeter’s

19. ONE-WATTER METHOD
If the load (3-phse) Y or is )balanced,
the power in any phase can be
measured by a single wattmeter.

20. W
R
IR
Z
N
Z
B
VYB
Z
IY
Y
IB

21. 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.

22. REACTIVE POWER
√3
(Wattmeter Reading)

23. HAPPY LEARNING
Tell me and I forget. Teach me and I remember. Involve
me and I learn.

24. PRESENTED BY:
Abhisek Padhi
http://TechniKaly.com
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