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lead lag power factor calculation

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raman00084

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in general watts = v x I x pf

if power factor is leading then sign is - (minus)

for example v=230, I=4a, pf=-.9(leading)


then watts= 230 x 4 x (-0.9) this is correct?
 

if power factor is leading then sign is - (minus)
Click to expand...
No. A negative power factor means that power is flowing to the grid, it's not related to leading or lagging phase angle.
 

if power factor is leading then sign is - (minus)
Click to expand...
That's new! Technically, there is no negative Power Factor because power factor is cos(Phi), in which phi is angle between voltage and current.
Pf is negative if phi is larger than 90 and smaller than 270, (generating mode). However I never heard someone refer to generating mode by using negative pf term.
 

a negative power factor means current is leading voltage, not power flowing from your site to the grid..! you can have a leading power factor (-) by having lots of caps across the mains and still draw plenty of real power from the grid... the neg sign is just convention, a lagging power factor (inductive) is by default positive. With modern loads we have total power factor which takes into account harmonic currents (and voltages) and their phasing to give true power factor, watts = True PF x VA, displacement power factor refers to leading or lagging, in the old days caused by motors being inductive, if you think about it lagging should be minus and leading (capacitive) should be +ve, but convention is otherwise....
 

a negative power factor means current is leading voltage, not power flowing from your site to the grid..! you can have a leading power factor (-) by having lots of caps across the mains and still draw plenty of real power from the grid... the neg sign is just convention, a lagging power factor (inductive) is by default positive.
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Where do you see this definition? It means that power factor is no longer cos phi rather than cos phi * sign (S). Cos does not jump from +1 to -1 around zero degree. This would be of course a legal definition, but does it make sense?

- - - Updated - - -

The definition as cos phi (or P/S), involving a positive PF number for lagging and leading power factor can be found in many electrical engineering text books, e.g. Electrical Machines, Drives and Power Systems by Theodor Wildi. In so far I would consider it as standard. Some sources suggest that PF should be a positive number for generating mode, too. In this case it's calculated as absolute value of cos phi.
 

cos phi is always positive; reactive power may be positive (inductive circuits) or negative (capacitive).
By convention when is positive - "send" reactive power to grid and when negative, "consume" reactive power.
 

there is a misunderstanding. negative or positive reactive power is totally different from the definition negative or positive PF. you can express negative or positive reactive power (by convention) but not negative/positive pf. However, definition of reactive power is different. it has many difinition in time and frequency domain.
I am power quality expert, but never heard of kind of definition you referring to.
If you want more info please study Akagi book about instantaneous reactive power , chapter 1.

Good luck
 

you mean power factor is always positive within the range of 0-1 ?
 

As long as your device does not generate active power.
 

if my device is capacitive load then what is the case?
 

Leading or lagging is always restricted to +/- 90 degrees (capacitors and inductors). It is possible to have phase differences in the 2nd and 3rd quadrant but they are interpreted differently. For example, a phase shift of 180 will be considered as a two phase system (the voltages are equal and opposite). In all these cases (phase restricted in the range of +/-90 degree) the power factor is positive and in the range of 0-1.

Phase differences can be more than +/-90 degrees. In such cases, the system is source of power.
 

Well this has turned into an interesting discussion.
kappa_am said:
As long as your device does not generate active power.
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I think this is basically the key point. If real power is negative because the "load" is sourcing power to the generator, then PF will be negative. Or you could just redefine real power as being positive in the opposite direction and then your PF would be positive again.

Then consider what happens when a load has negative resistance...
 

That is the reason I always think in the terms of leading or lagging power factor to prevent confusion.

Meaning that the current leads the voltage (in a capacitive load) or the current lags the voltage (in an inductive load).
 

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