Measurement of 3-Phase Power by 2 Wattmeter Method

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Aim: To measure the total 3-phase power of 3-phase balanced load using 2-wattmeter method and verify the result with its theoretical value.

Apparatus Required

SL No.ApparatusSpecificationQTY
1Voltmeter0 – 600V1
2Ammeter0 – 5A1
3Wattmeter600V, 5A, UPF2
43-Phase balanced resistive load10KW1

Theory

Two wattmeter method can be employed to measure the power in a 3 phase, three wire star and delta connected balanced load.

In two wattmeter method the current coils of the wattmeter are connected in series with two lines, say R and B and the potential coil of each wattmeter are joined to the third line i.e. Y as shown in the figure below.

Procedure

a) Star Connection

  • Connect the circuit as shown in the circuit diagram 1
  • Switch on the 3-phase supply using TPST switch
  • Note down the line voltage, line current and readings of the voltmeter
  • Compare the total power observed by the two wattmeter method with its theoretical value

b) Delta Connection

  • Connect the circuit as shown in the circuit diagram 2
  • Repeat steps 2 to 4 of the star connection procedure

Observations and Calculations

Circuit diagram

Star Connection

Delta Connection

Tabular Column

3 Phase ConnectionLine Voltage VLLine Current ILPower factor cosϕ W1W2Total 3 phase power
(Practical)
P=W1 + W2 (watts)
Total 3 phase power (Theoretical)
P= √ 3 VLIL cosϕ
(watts)
STAR1
STAR1
STAR1
DELTA1
DELTA1
DELTA1

Before writing wattmeter reading multiply with wattmeter constant :

wattmeter constant K = VI cosϕ / full scale deflection
where ‘cosϕ‘ is power factor of the wattmeter

Result

Star connection

Theoretically Calculated PowerPractically Measured Power

Delta connection

Theoretically Calculated PowerPractically Measured Power

Inference

By conducting the experiment we found out practically that two wattmeters are sufficient to measure the power of 3 phase load.

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