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Confusion About Pfc Capacitors


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#1 AB2005

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Posted 17 November 2006 - 03:31 AM

Dear All,

There is a question in my mind which always confuses to me i.e. when we connect a PFC capacitor with system; it improves the power factor if lagging. We also know that each electric equipment draw current from supply when it connects and converts it in the other form of energy. What happen in capacitor? Is it also increase a load on system?
Sorry if this is a goofy idea.

"Don't assume any thing, always check/ask and clear yourself".

#2 jraef

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Posted 17 November 2006 - 10:00 PM

The capacitor does not convert anything, it just stores and releases. So in the instance that you energize a capacitor there is technically a flow of current that charges the capacitor that is not doing any physical work, but that happens so fast that it doesn't affect overall power consumption. After that it is just being charged by the flow of current through the system and is discharging into the inductive load when necessary to keep the core magnetized. Other than an insignificant amount of resistance in the capacitor itself, there is no power consumed in the process. Think of it as a rechargeable battery capable of charging and discharging instantaneously.
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#3 AB2005

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Posted 18 November 2006 - 02:03 AM

Dear Mr. Jraef,

Thanks for reply. I have understood your point.
We have 12 PFC capacitors in main distribution panel each 100kvar, 400V. Each capacitor draws 140-150A current at 0.95-0.97pf when all machines work. Are these capacitors current dependent at anything or this is a constant current?

"Don't assume any thing, always check/ask and clear yourself".

#4 jraef

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Posted 18 November 2006 - 07:15 AM

It will be fairly constant, but remember, that current is not "flowing" in the conventional sense. In other words, it is not being recorded on your energy meter, which only reads the kWh pulled by the motors associated with those caps. Hopefully you have the capacitors controlled so that they only come on when their associated motors are running. If not you may end up over-correcting, which can lead to other system problems.
"He's not dead, he's just pinin' for the fjords!"

#5 AB2005

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Posted 20 November 2006 - 02:04 AM

Thank you Mr. Jraef.
"Don't assume any thing, always check/ask and clear yourself".

#6 joshua

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Posted 21 November 2006 - 03:27 AM

QUOTE(jraef @ Nov 18 2006, 03:15 PM) View Post

It will be fairly constant, but remember, that current is not "flowing" in the conventional sense. In other words, it is not being recorded on your energy meter, which only reads the kWh pulled by the motors associated with those caps. Hopefully you have the capacitors controlled so that they only come on when their associated motors are running. If not you may end up over-correcting, which can lead to other system problems.


What would happen if there were only resistive loads connected in a system ? Would you be paying more when the capacitor is installed ? (ex: in the day time when everyone's away at work, might only use the microwave oven or kettle to boil water or etc.) So what would happen in this cases ?

Thanks.

#7 jraef

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Posted 21 November 2006 - 04:50 AM

If your capacitors are connected all of the time and you are not running inductive loads, you can get high voltage surges that can damage your equipment, especially when a large load is switched on while the over-unity PF exists.

If you haven't already done so joshua, go to this webpage and read up on PFC Capacitors, especially the part near the bottom on Supply Resonance.

http://www.lmphotonics.com/pwrfact.htm
"He's not dead, he's just pinin' for the fjords!"

#8 joshua

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Posted 21 November 2006 - 06:48 AM

QUOTE(jraef @ Nov 21 2006, 12:50 PM) View Post

If your capacitors are connected all of the time and you are not running inductive loads, you can get high voltage surges that can damage your equipment, especially when a large load is switched on while the over-unity PF exists.

If you haven't already done so joshua, go to this webpage and read up on PFC Capacitors, especially the part near the bottom on Supply Resonance.

http://www.lmphotonics.com/pwrfact.htm


Is there any effect to the KWH ? I might be wrong but just for an example sake : a electric kettle rated at 240V , 6.0A. meanwhile the capacitor is 240V, 1.7A(small capacitor). Would it multiply in meaning 240*6.0A*1.7A = 2448W ? ( maybe it would not multiply by that much) but could it mean it would register a higher value in the KWH meter ?

thanks.

#9 kens

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Posted 21 November 2006 - 07:15 AM

QUOTE(joshua @ Nov 21 2006, 07:48 PM) View Post

Is there any effect to the KWH ? I might be wrong but just for an example sake : a electric kettle rated at 240V , 6.0A. meanwhile the capacitor is 240V, 1.7A(small capacitor). Would it multiply in meaning 240*6.0A*1.7A = 2448W ? ( maybe it would not multiply by that much) but could it mean it would register a higher value in the KWH meter ?

thanks.


Hi Joshua, no offence intended but you should go and read the link that jraef posted as I think that you lack a fundamental understanding of the subject. We are willing to help but you must help yourself by studying a little more theory. Have a look at this site also as it is may help http://www.powerstan...ingToyIndex.htm
Ken
An expert is one who knows more and more about less and less until he knows absolutely everything about nothing

#10 joshua

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Posted 21 November 2006 - 09:25 AM

QUOTE(kens @ Nov 21 2006, 03:15 PM) View Post

Hi Joshua, no offence intended but you should go and read the link that jraef posted as I think that you lack a fundamental understanding of the subject. We are willing to help but you must help yourself by studying a little more theory. Have a look at this site also as it is may help http://www.powerstan...ingToyIndex.htm
Ken


No offence taken at all as I'm here to learn. I just wanted to create a leading power factor by attaching a resistive load with a capacitor to see whether there would be any changes in the KWH meter.(someone told me that there would be) Anyway I did read the paper by marke. Thanks alot for the referal to the powerstands site. I appreciate it alot. Looking forward to learn alot more from you guys.

Thanks

P/s: could the ratings of a capacitor be known in farad by only looking at the supply voltage and ampere reading ? How to convert from Farad to VAr ?

Sorry if it sounds silly. And a another thing to check; If I have the KW and KVA readings(using a power clamp meter), could I calculate KVAr ? any formula for it ?

#11 AB2005

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Posted 22 November 2006 - 02:37 AM

QUOTE(Joshua @ Nov 21 2006, 07:37 AM) View Post

P/s: could the ratings of a capacitor be known in farad by only looking at the supply voltage and ampere reading ? How to convert from Farad to VAr ?

Sorry if it sounds silly. And a another thing to check; If I have the KW and KVA readings(using a power clamp meter), could I calculate KVAr ? any formula for it ?

Dear Joshua

I have asked almost same question before and found no answer. Then I consulted with an engineer from siemens and he found my calculations correct. Here is the link for that question. (http://www.lmpforum.com/inforum/index.php?showtopic=1387&st=0&p=4796&#entry4796)
Anyhow, here is the answer of your question.

QC = Vē*W*C
Where
V = Rms value of supply voltage
W = 2πf (f = Line Frequency)
C = Capacitor rating in fared
For Example
C = 3.5uf
V = 220V
F = 50HZ
QC =?
QC = 220*220*(2*3.14*50)* 0.0000035
QC = 53VAr
"Don't assume any thing, always check/ask and clear yourself".




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