 # Capacitance And Micro Farads

7 replies to this topic

### #1 edogro

edogro

Posted 22 July 2008 - 04:58 AM

can anyone tell me how to calculate the micro farads necessary to generate the capacitance to be added to a motor to increase PF to near unity? I know how to calculate kVAR and how to calculate how much it has to decrease in order to approach unity but I have been unable to find how to calculate the micro farad levels necessary to provide the capacitance. Thanks!

### #2 marke

marke
• Gender:Male
• Location:Christchurch, New Zealand

Posted 22 July 2008 - 08:38 AM

Hello edogro

Welcome to the forum.

Power factor correction capacitors are rated in KVAR at a particular voltage and frequency, so it is not necessary to calculate the microfarads required.

For a given capacitance, at a given voltage and frequency, the current through the capacitor can easily be calculated. If you then multiply the voltage across the capacitor by the current through it, you get the VAR of the capacitor. Divide this by one thousand, and you have the KVAR of the capacitor.

You can calculate the current by the voltage divided by the reactance where the reactance is 1/(2 x pi x f x C).
pi =3.142....
f = frequency
C = capacitance.

A bit of algebra and you can come up with the formula to calculate the capacitance from the KVAR.

Note for a three phase capacitor, assume three equal capacitors star connected with phase - neutral voltage across the capacitor.
If the final capacitor is made up of three delta connected capacitors, you can use star to delta transformation formulae to calculate the equivalent delta connected capacitors.

Best regards,
Mark

### #3 edogro

edogro

Posted 04 August 2008 - 11:36 PM

QUOTE (marke @ Jul 22 2008, 08:38 AM) <{POST_SNAPBACK}>
Hello edogro

Welcome to the forum.

Power factor correction capacitors are rated in KVAR at a particular voltage and frequency, so it is not necessary to calculate the microfarads required.

For a given capacitance, at a given voltage and frequency, the current through the capacitor can easily be calculated. If you then multiply the voltage across the capacitor by the current through it, you get the VAR of the capacitor. Divide this by one thousand, and you have the KVAR of the capacitor.

You can calculate the current by the voltage divided by the reactance where the reactance is 1/(2 x pi x f x C).
pi =3.142....
f = frequency
C = capacitance.

A bit of algebra and you can come up with the formula to calculate the capacitance from the KVAR.

Note for a three phase capacitor, assume three equal capacitors star connected with phase - neutral voltage across the capacitor.
If the final capacitor is made up of three delta connected capacitors, you can use star to delta transformation formulae to calculate the equivalent delta connected capacitors.

Best regards,
Mark

### #4 edogro

edogro

Posted 04 August 2008 - 11:56 PM

Hello Mark:

Thanks for the reply, I will run some calculations and let you know how it works out. I am not sure I followed your explanation or if I understood correctly. If motor run capacitors connected in a delta configuration for power factor correction create a specific amount of KVAC to offset the KVAR of the installation, is determined by voltage and frequency across the capacitors how then is the total amount of capacitance effected by the size of the capacitors?

You mentioned the star to delta formula, where can I find this?

Best regards!!

### #5 Sharon

Posted 02 January 2010 - 02:14 PM

QUOTE (edogro @ Jul 22 2008, 04:58 AM) <{POST_SNAPBACK}>
can anyone tell me how to calculate the micro farads necessary to generate the capacitance to be added to a motor to increase PF to near unity? I know how to calculate kVAR and how to calculate how much it has to decrease in order to approach unity but I have been unable to find how to calculate the micro farad levels necessary to provide the capacitance. Thanks!

Thanks you for the post.
Hi guys, Im a newbie. Nice to join this forum.

### #6 Bysikmode

Bysikmode

Posted 20 April 2014 - 01:11 PM

What I read I really liked it. Thank you for your information!

dubli

### #7 Eurybates

Eurybates

Posted 11 June 2014 - 08:15 AM

Really, If you don't agree with Mark, then the best thing to do is to find the inter-cable-phase-adapter, Make sure the termination is nice and tight. Swing on it a few times and go research the power triangle for a while before dedicating your life to three phase theory. If this does not work, then get a job talking as much *beep* your body can handle without imploding into approx. 10 micro-farads.

Best of luck

PS.. If this post is not deleted, I will assume that LMP forum is dead and gone. ) .. Was good talking

### #8 marke

marke
• Gender:Male
• Location:Christchurch, New Zealand

Posted 16 June 2014 - 12:16 AM

Definitely not dead, just pounded by spam for a while and so filtered quite hard at present.

BR

Mark

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