# Calculation of power fector.

Discussion in 'General Electronics Discussion' started by devgorkha, Nov 13, 2011.

1. ### devgorkha

5
0
May 2, 2011
The multiplication of current and voltage is VA.

But i have to calculate true power in Watt that can obtain by multiplying with power factor. the problem is to calculate power factor.
Is there any pf calculation circuit ?

2. ### davelectronic

1,087
12
Dec 13, 2010
Hi devgorkha.
Yes there is,i cant remember off hand but its fairly simple, power factor is a rather loose term, i googled the equation, my findings where motors and transformers should go for a power factor of the worst case scenario, and for the two items above i found 0.6 but as i say i googled it as i wanted better VA descriptions and the formula.

All this 100 % power factor is a myth. if you google it you should find the equation.
Dave.

1,087
12
Dec 13, 2010
4. ### devgorkha

5
0
May 2, 2011
thank u dacelectronic,

i know how to calculate PF in paper if watt and VA is given. but the problem is to calculate Energy (watt) , i don't know pf of every devices .. first i have to calculate pf by sensing Current and Voltage. By the way i am going to design energy meter.

5. ### davelectronic

1,087
12
Dec 13, 2010
Hi again devgorkha.
I am not sure what you mean, a power factor for every device ?.
You can use ohms law to calculate power in watts for a given current reading, ie: current x volts = watts, you might be able to do a power factor assessment from that, other than this i dont understand what you are looking for in every device.
Dave.D

Last edited: Nov 16, 2011
6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,397
2,777
Jan 21, 2010
The power factor is essentially a term describing the mathematical equivalent phase shift between voltage and current.

If you have a simple resistive circuit, we know that they're going to both be in phase, but also that the current will track the voltage -- it will be a nice smooth sine wave (just like the voltage).

In a capacitive or inductive circuit some phase shift is introduced, but the current still follows (or leading or lagging) the voltage in a nice sine wave.

In these cases, if you can calculate the phase shift, you can calculate the power factor.

However, in real life, many devices have non-linear relationships between voltage and current. In these cases, not only is the current leading or lagging the voltage, but the current waveform is distorted. In this case the determination of power factor is no longer such a trivial problem.

If you're designing a power factor meter, then you could do worse than look at some documented designs.