# when calculating apparent power, how do you know if its leading or lagging?

Relevance

Well, if the pf is given and you know nothing else, then you cannot know.

If YOU are determining the pf in a circuit, it just depends on if it is ultimately inductive or capacitive reactance that dominates:

Recall the equation for average power: ½ VmIm cos (θ)

The Power Factor is simply cos(θ): pf = cos (θ)

Therefore we can rewrite average power as: ½ VmIm (pf)

Recall that θ is the angle of the impedance Z

In a purely resistive load, θ=0, therefore the pf=1.

In a purely reactive load (all capacitance or all inductance), θ=±90°, therefore the pf=0.

Note that the angle of Z must be between +90° and -90° because if there is any resistance,

resistance can not be negative. Only reactance can be negative, as is the case with a capacitor.

Since pf = cos(θ), then the pf will always be some number between 0 and 1.

The power factor is never negative because resistance is never negative.

If the pf = ½, how do you know if θ= +60° or -60°? You don't unless

there is some way of specifying that the reactance is either inductive or capacitive.

Source(s): 30 years engineering
• Login to reply the answers
• Login to reply the answers
• Login to reply the answers