Question

Two wattmeter method is used for measurement of power in balanced three-phase load supplied from a balanced three-phase system. If one of the wattmeters reads half of the other (other positive), then the power factor of the load is

• A. 0.532
• B. 0.632
• C. 0.707
• D. 0.866

Answer 1

The correct option is D 0.866

In two Wattmeter method,
$tan\varphi =\frac{\sqrt{3}(W_1-W_2)}{(W_1+W_2)}$
Given,
$W_2=\frac{W_1}{2}$
$\tan \varphi =\frac{\sqrt{3}(W_1-\frac{W_1}{2})}{(W_1+\frac{W_1}{2})}$
$\varphi ={30}^{\circ }$
$cos\varphi =cos{30}^{\circ }=0.866$