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Thread: capacitor calculation

  1. #1
    Join Date
    Aug 2009
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    Hudsonville Michigan
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    Thumbs up capacitor calculation

    Im trying to figure out what size capacitor i need. not sure how to symbols, but i will do my best here.

    Reactive power (Q) = (sq. rt.)of (apparent power(s))2 - (true power(P))2

    so then:

    Q=E2/X
    or
    X=E2/Q
    this is the part i need help with. When im working with 480 motors, I would just use the (480)2, not (480 x 1.73)2?

    Seems clear as mud to me. lol.

    so continuing on this it what i went with

    X=(480)2/107kvar

    X = 2.153 ohms

    then Xc= 1/2(pi)C...............or C=1/2(pi)Xc
    so....

    C= 1/2(pi)(60hz)(2.153 ohms)

    C=123.2 uf

    am I correct in my math? I am trying to size my motor capacitors to bring my powerfactor closer to 1.0

  2. #2
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    Aug 2009
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    Hudsonville Michigan
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    [QUOTE=VoltzElectric;1088315]

    Reactive power (Q) = (sq. rt.)of (apparent power(s))2 - (true power(P))2

    forgot to add some stuff, where my first numbers came from

    107kvar (Q) = (127.9kva(S))2 - (70kw(P))2

  3. #3
    Join Date
    Feb 2003
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    Quote Originally Posted by VoltzElectric View Post
    Reactive power (Q) = (sq. rt.)of (apparent power(s))2 - (true power(P))2
    Reactive power (Q) = (sq. rt.)of (KVA)2 - (KW)2
    Your formula is correct. I assume your effort is to correct the existing power factor to a higher value. If so, then you need to determine the KVAR necessary to bring the PF to this higher value. You do not need the capacitor value. Once you determine the total required KVAR you can then determine the values to be installed on your motors. The motor MFG can give you the correct value for each motor.

  4. #4
    Join Date
    Sep 2009
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    Condensing unit pf

    I have a 4ton A/C unit and I would like to improve the PF?.... Any help is appreciated

  5. #5
    Join Date
    Nov 2007
    Posts
    306

    PFC for 4 Tons A/C

    Quote Originally Posted by xsman View Post
    I have a 4ton A/C unit and I would like to improve the PF?.... Any help is appreciated
    What final PF do you want to achieve? 1.0? or just 0.90?
    Here it is (assuming a final PF of 0.9 lagging):

    4 Tons A/C with EER of 10 (assumed) will be 4.8 kW electrical (4 X 12,000 BTU/Hr /10 BTU/W-Hr)
    With an initial PF of 0.8:
    The kVA = 4.8/0.8 = 6 kVA
    The kVars = sqrt(6^2 - 4.8^2) = 3.6 kVars

    If you correct the PF to 0.9:
    The kVA = 4.8/0.9 = 5.333 kVA
    The new kVar = sqrt(5.33^2-4.8^2) = 2.325 kVars
    The difference in kVars will be the kVar of the capacitor:
    Capacitor total kVars = 3.6 - 2.325 = 1.275 kVars
    Per phase kVars = 1.275/3 = 425 Vars
    If the supply voltage is 480V 3-phase and you will connect the capacitors in delta:

    Xc = E^2/Qc = 480^2/425 = 542.12 ohms
    C = 1/(2 X pi X f X Xc) = 1/(2 X 3.1416 X 60 X 542.12) = 4.89 uF
    That's all.

  6. #6
    Join Date
    May 2005
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    Barboursville, West Virginia
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    Quote Originally Posted by xsman View Post
    I have a 4ton A/C unit and I would like to improve the PF?.... Any help is appreciated
    If you have an entirely separate question than the original poster, you need to start your own thread please.

    Quote Originally Posted by VoltzElectric
    I am trying to size my motor capacitors to bring my powerfactor closer to 1.0
    You just need to run through the power factor correction calculation (which you can probably find on the internet) and as bob said you don't really need to know the capacitance value, just the reactive power you need. I think you'll also actually need to know the existing power factor of the motor that you're trying to change. I'll look in my old textbook later. It's been a little while since I've done it.
    Engineers are always honest in matters of technology and human relationships. That's why it's a good idea to keep engineers away from customers, romantic interests, and anyone else who can't handle the truth.

  7. #7
    Join Date
    Sep 2009
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    2
    Thanks for your reply, I have a 4 ton 12 seer unit I tried to install 2 80uF caps with 22k resistors in parrallel with the load(compressor) side of the contactor. I have checked the load side and the feed side for change in amp draw and still nothing, is there any way that I can make trhis work. I wanted to upload a diagram but it will not allow me

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