Question

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 0.5

The 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 1

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 power  P = 0

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