# Determining leading or lagging power factor

Discussion in 'Math' started by raz, Jul 18, 2011.

1. ### raz Thread Starter New Member

Jun 25, 2011
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I have a system with three power supplies and two shaded pole fans supplied by 115VAC. How do I determine if the input power factor is leading or lagging?

2. ### praondevou AAC Fanatic!

Jul 9, 2011
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Could you please provide more details? 3 power supplies, do you mean common 3-phase power supply (120 degree phase shift)?

Maybe you can attach a schematic?

3. ### raz Thread Starter New Member

Jun 25, 2011
10
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Sorry about that. It is a single phase 115VAC input to the three power supplies and two fans.

4. ### praondevou AAC Fanatic!

Jul 9, 2011
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Ok, got it. I don't think there is a way you could guess the PF. Either you have to consult the datashees/spec sheets of the fans and power supplies (if you are lucky the PF might be indicated) or you have to measure it with power meter.
You can also estimate it measuring voltage and current with an oscilloscope, but since the input current won't be a sine wave (if your power supplies don't have power factor correction), this might be difficult to achieve.

5. ### raz Thread Starter New Member

Jun 25, 2011
10
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Sorry it took so long to reply and thank you for your replies. My computer decided to give me trouble. In the mean time the power supply vendor finally sent me the pf and said it was leading. Two supplies have a pf of 56 and the third is 60. How do you determine what the total pf is since they are all on the same circuit?

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Presumably the manufacturer's data would also state that the quoted power factors are for a known load condition. Were they given for no-load or rated load or some other condition?

You would most likely have variable pf as load varies so you could probably only state the combined power factor based on the defined load conditions which related to the quoted pf values.

In that case you would calculate the individual real and reactive powers for each supply and then add the real and reactive terms to give total real and reactive power terms. From these you would obtain the overall pf.

7. ### raz Thread Starter New Member

Jun 25, 2011
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I told the vendor what I had for a load on each supply. They then responded with what I stated before. So do I add the three pf's together and then divide by 3 to get the total pf?

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Not necessarily.

That would only be true if the supplies were identical and operating at exactly the same load.

I explained how the value would be found otherwise in my previous post.

raz likes this.
9. ### Wendy Moderator

Mar 24, 2008
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Eliminating thubab2 from the main index page. He has been deleted, but traces remain. No more.

10. ### raz Thread Starter New Member

Jun 25, 2011
10
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Thank you, t n k.