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Capacitance And Micro Farads, how to calculate |
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 Jul 22 2008, 04:58 AM
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#1
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Junior Member  Group: Members Posts: 3 Joined: 22-July 08 Member No.: 4,696  |
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!
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Jul 22 2008, 08:38 AM
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#2
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Posting Freak Group: Moderator Posts: 2,383 Joined: 24-April 02 From: Christchurch, New Zealand Member No.: 1 |
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 -------------------- Mark Empson administrator
Skype Contact = markempson | phone +64 274 363 067 LMPForum | LMP Tech Warehouse | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | LMP Software | Mark Empson Website |
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Aug 4 2008, 11:36 PM
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#3
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Junior Member Group: Members Posts: 3 Joined: 22-July 08 Member No.: 4,696 |
QUOTE (marke @ Jul 22 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 |
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Aug 4 2008, 11:56 PM
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#4
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Junior Member Group: Members Posts: 3 Joined: 22-July 08 Member No.: 4,696 |
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!! |
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Jan 2 2010, 02:14 PM
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#5
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Group: Members Posts: 1 Joined: 2-January 10 Member No.: 7,464 |
QUOTE (edogro @ Jul 22 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! Thanks you for the post. Hi guys, Im a newbie. Nice to join this forum. -------------------- |
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