Page 188 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Therefore, the Total Power Measured by the Two Wattmeter W and W will be obtained by adding the equation
2
1
(1) and (2).

It is used to measure the power in star or delta connected load in balanced or unbalanced condition
Current coil of two meters are connected in any two lines and the potential coil of each are connected to the third
line. Pressure coils of all the three wattmeter are connected to common point at neutral line. The resultant sum of
all the readings of wattmeter will give the total power of the circuit.
Total Power = P +P 2
1
Consider the instantaneous power in the system
Instantaneous current through W =I 1 R
1
Instantaneous P.D across W =E =E -E B
R
1
RB
Instantaneous Power read by W =I (E -E )
1 R R B
Instantaneous current through W =I Y
2
Instantaneous P.D across W = E = E -E B
1
YB
Y
Instantaneous Power read by W = I (E -E )
1 Y Y B
W +W = I (E -E )+ I (E -E )
R
R
2
B
Y
B
1
Y
= IRER-IREB+IYEY-IYEB
Rearrange the values then = IRER+IYEY-IREB-IYEB
= IRER+IYEY-EB(IR+IY)
Now IR+IY+IB = 0 (Acc. Kirchhoff’s Point law)
IR+IY = -IB
= IRER+IYEY-EB(-IB)
= IRER+IYEY+IBEB
0 Degree 0-59 Degree 60 Degree 60-90 Degree 90 Degree

PF 1 <1 but >0.5 0.5 <0.5 but >0 0

W1 +Ve +Ve +Ve +Ve +Ve

W2 +Ve +Ve 0 -Ve -Ve
W1 &W2 are W1 &W2 are W1 &W2 are equal but
equal W1 &W2 are W1 only different and opposite
W1=W2 different W1=W2
opposite

175
CITS : Power - Electrician & Wireman - Lesson 26-29