# POWER FACTOR

• posted
On a balanced 3 phase given kw and V:
To find Current:
(kW x 1000) / (E x PF) = amps
I have a 208v, 50/60Hz, 9kW, 3 PH
kw=9
E=208
PF=???
I know PF = cos(theta)
or PF (3 PH) = Input Watts / (E x I x 1.732)
I am trying find the current??
I'm I doing something wrong? Because I thought PF has to be between 0
and 1.
-----
• posted
----------------------------
Power factor is a single phase relationship between phase voltage and phase current . You are looking at line to line voltage for what appears to be a Y system. There is an extra 30 degree shift between line voltage and line current. For a balanced system it can be found from the KW/KVA ratio (your last equation below which is the first expression re-ordered) as it would be the same for each phase.
In this case the voltage is 208 line to line for a Y system(assumed because of the voltage) and 120V line to neutral. The line current is 9000/(1.73*208*pf) =25/pf A and there is no information to find the pf. Alternatively use 3000/120*pf =25*pf A (line and phase current the same for a Y)and still no info for the power factor. If you knew either the current or the pf then you could find the other but otherwise you are stuck.
With the expressions that you give, you are stuck with an unknown angle between phase voltage and phase current so you can't find the pf and without the pf you can't find the current.
And, yes, pf is between 0 and 1. --
Don Kelly snipped-for-privacy@shawcross.ca remove the X to answer
• posted
For motor, transformer, and other inductive laods assume a power factor of about 0.7 to 0.8 For resistive loads assume a power factor of 1.0 Now you can find the amperes.
Power (in watts) = E x I x 1.732 x PF for three phase.
• posted
Let us know when the cos (x) of any angle is greater than 1.
• posted
So to correct a 240v 3600W isolation transformer at say 0.8, I should use a capacitor of ??? ufd??
TVMIA
• posted
Power factor correction for a lagging power factor is seldom done in for normal field installations, especially for small loads. Transformers are usually rated in volt ameres because the power factor is not know at time of manufacturer.
• posted
Transformers are rated in VA because that determines the maximum current through the winding. The load watts don't matter to the transformer. In other words, a one kVA transformer could deliver one kVA to a one kVAR capacitor that draws zero watts, or to a heater that draws one kW. It is fully loaded either way.
Ben Miller

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.