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Signode
Power-User

Join Date: Aug 2007
Location: Lahore
Posts: 369

Some confusion about power factor

02/10/2008 2:55 PM

Dear All,

As you know that power factor is a very complicated subject in the field of AC power. I too have confusion for understanding it. Can anyone direct me through a DEMO which may help me for understanding the PF?

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TVP45
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Join Date: Jul 2007
Posts: 4448
Good Answers: 143
#1

Re: Some confusion about power factor

02/10/2008 3:48 PM

Yes, here's an applet

http://www.falstad.com/circuit/directions.html

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Blink
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Join Date: Aug 2006
Posts: 4484
Good Answers: 246
#3
In reply to #1

Re: Some confusion about power factor

02/11/2008 1:30 AM

Great applet! (Along with others at falstad too.)

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indel
Guru
Engineering Fields - Electrical Engineering - New Member

Join Date: Oct 2007
Location: Sour Lake, TX 30°08'59.68"N 94°19'42.81"W
Posts: 675
Good Answers: 13
#2

Re: Some confusion about power factor

02/10/2008 4:42 PM

When we talk electric power, we think in terms of load. We exercise energy on a load, which might be: pure resistive (like in a boiler resistance) inductive (as in an electric motor) and capacitive (some ultrasound generators).

Applying the driving voltage on the load, there will be a certain current, determined by the voltage and the impedance of the load (resistive, inductive or capacitive).

Considering an ac voltage, say the good old 120V, a sinusoidal wave. It swings from zero to a max positive value, back to zero, to a max negative value and back to zero in one period, or 360°). There are, in the USA, 60 such periods for the residential power. In a resistive load, as the voltage increases (the first quarter of one period, or 90°), the current increases the same way, in the same time. The voltage peaks at the 90° and so the current. We know that the power is the product between voltage and current (P=UxI) so, at 90° the power is maximum.

An inductive load will oppose to the increase of the current, so when voltage reaches its maximum value at 90°, the current has yet to come to its maximum value (and this depends on the magnitude of the inductance, the residual resistance of the inductance, etc), So, if we take again, the product UxI, it is obvious that the power is not the maximum one, voltage and current do not peak in the same moment. The difference, in degrees, of the voltage and current peaking, called φ will be the subject of the power factor definition, Cos φ. If the difference is φ=0 (resistive load), cosφ=1, power is maximum

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simon2007@webmail.co.za
Participant

Join Date: Feb 2008
Posts: 2
#4

Re: Some confusion about power factor

02/11/2008 5:10 AM

Power factor is the cosine angle between the voltage and current. Idealy the sinusodial waveform of the current and the voltage must be in sequence, at zero phase shift. But that is not possible in reality because of the usage of inductive loads and capacitive loads. If a load is inductive this causes the the current to lagg the voltage and if it's capacitive it leadsthe voltage. meaning there will be phase shift between the waveform and that angle will be formed.
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Anonymous Poster
#5

Re: Some confusion about power factor

02/11/2008 8:32 AM

Check out my web site regarding your confusion about power factor. I have written a very good tutorial on the subject of power factor and harmonics. The web site is www.stacoenergy.com look in the power factor section and you can download the book.
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JohnBob
Associate

Join Date: Jan 2008
Location: Bath, UK
Posts: 53
Good Answers: 2
#6

Re: Some confusion about power factor

02/11/2008 11:30 AM

This might help as it is looking at p.f. from a slightly different angle (no pun intended ;-)

http://www.froment.co.uk/Data/TheRoleofLoadBanksinGen-setTesting.pdf
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Ydobon
Associate

Join Date: May 2007
Posts: 47
#7

Re: Some confusion about power factor

02/11/2008 12:12 PM

Keep it simple:

ELI the ICE man: ELI, voltage leads current in an inductive circuit. ICE, current leads voltage in an capacitive circuit.

It is the ratio of the above: In a balanced circuit the power factor would be unit (ie 1). In reality, it only comes very close, with in meter measurements.

SC
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Anonymous Poster
#8

Re: Some confusion about power factor

02/11/2008 2:24 PM

While not exactly what you are looking for this demo illustrates how power factor (a measure of the phase relationship of current and voltage) correction brings the phase of the voltage and current flowing closer to being in sync – i.e., reducing the phase angle.

http://www.youtube.com/watch?v=O5v-ozjbrq0

The pdf describes how it can be done – most loads are that distort the power factor are inductive and are corrected using capacitive devices.

http://www.pacificpower.net/File/File3873.pdf
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