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Choose the correct statement regarding two watt-meter method for power measurements in 3-phase circuit.(a) When power factor is unity, one of the wattmeters reads zero(b) When the readings of the two watt-meters are equal but opposite sign, the power factor is zero(c) Power can be measured using two watt-meter method only for star connected 3-phase circuit(d) When two watt-meters show identical readings, the power factor is 0.5The question was asked by my school principal while I was bunking the class.I would like to ask this question from Advanced Problems on Measurement of Power in Polyphase Circuit topic in section Measurement of Power and Related Parameters of Electrical Measurements

Answer»

Right option is (b) When the readings of the two watt-meters are EQUAL but opposite sign, the POWER factor is ZERO

The explanation is: When power factor is 0, we have ∅ = 90°

P1 = (3)^0.5 VIcos (30° – ∅) = (3)^0.5 VIcos (30° – 90°) = \(\frac{(3)^{0.5}}{2}\) × VI

P2 = (3)^0.5 VIcos (30° + ∅) = (3)^0.5 VIcos (30° + 90°) = \(– \frac{(3)^{0.5}}{2}\) × VI

∴Total powerP = 0

So, with zero power factor the readings of the two watt-meters are equal but of opposite sign.


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