power measurement in 3-phase ac circuits

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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