In a two wattmeter method, if one wattmeter reads negative then
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The relationship between the total three-phase power factor and the two wattmeter readings can be computed directly using this formula:
\[
\tan(\phi) = \sqrt{3} \left( \frac{W_1 - W_2}{W_1 + W_2} \right)
\]
- If $W_2 = 0 \implies \tan(\phi) = \sqrt{3} \implies \phi = 60^\circ \implies \text{PF} = 0.5$.
- If $W_2 < 0 \implies$ the denominator decreases while the numerator increases, meaning $\phi > 60^\circ \implies \text{PF} < 0.5$.