Revised in 2008

DEPARTMENT OF ELECTRICAL ENGINEERING FIRST YEAR E.T. LAB EXP. NO. – 4

THREE – PHASE POWER MEASUREMENT OBJECTIVES: To measure power in a three phase circuit under

i) Balanced resistive load condition. ii) Unbalanced resistive load condition.

BALANCED LOAD

CIRCUIT DIAGRAM (Fig: 1)

PROCEDURE:

1. Connect the circuit as shown in Fig.- 1 2. Adjust the ganged rheostat for the maximum resistance. 3. Switch on the supply. 4. Close switch S1. 5. Read the meters to obtain VL, I1, I2 and I3. Note the wattmeter reading W1 and W2

(Note the multiplying factor on the wattmeter). 6. Vary the load resistance and obtain at least five sets of observations, the current

should not exceed the limit (4.1 A). 7. Tabulate the readings and check the results by completing the calculations

indicated in the table.

1

Revised in 2008

OBSERVATION TABLE – 1 (Three phase power in a balanced load) Sl. No.

VL

(V)

I1

(A)

I2

(A)

I3

(A)

W1

(W)

W2

(W)

Calculated power (Wc)= (VL / √3) (I1+I2+I3)

(W)m = (W1 + W2)

Error W - Wc Wc X 100%

UNBALANCED LOAD

CIRCUIT DIAGRAM (Fig: 2)

PROCEDURE:

1. Connect the circuit as shown in Fig.-2. 2. Replace the ganged rheostat by three separate rheostats of 26 Ω, 4.1 A and

connect in a star. 3. Adjust the three rheostats at the maximum values. 4. Switch on the supply and set the autotransformer to110 V. 5. Close switch S1 and take five sets of observation for different rheostat settings

such that the reading of I1, I2 and I3 in each set is appreciably different to create unbalanced loading condition. The current should not exceed the limits in each arm.

6. Tabulate and check the result by completing the computations indicated in Table:-2.

2

Revised in 2008

OBSERVATION TABLE – 2- (Three phase power in an unbalanced load)

Sl. No.

V1

(V)

V2

(V)

V3

(V)

I1

(A)

I2

(A)

I3

(A)

W1

(W)

W2

(W)

Calculated Power = Wc

=V1I1+V2I2+V3I3

Wm = (W1+W2)

Error W - Wc Wc X 100%

DISCUSSION:

What do you understand by a balanced three-phase load? How would you measure power using a) Three watt meters and b) One wattmeter

for balanced/unbalanced loads? Is it possible to measure power factor of the balanced (three –phase load by two-

wattmeter method)?

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