3 phase power factor calculations, capacitors

Discussion in 'The Projects Forum' started by DanD, May 29, 2008.

  1. DanD

    DanD Thread Starter New Member

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    I used the power factor information here to calculate the power factor and calculate the capacitor size required to correct the power factor. It cut my electric bill by 20%!

    So, now I want to help a friend who has 3 phase on his shop. Can anyone tell me how to practically calculate the capacitor size needed when using 3 phase?

    I assume it only requires 2 caps, one between each of the three line wires. Is that correct?
  2. mik3

    mik3 Senior Member

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    Well,i am not an expert on three phase circuits but i think you have to use 3 capacitors. Take each pair of phases as a single phase system and calculate the capacitor needed but be aware that the voltage is higher so you have to use capacitors with higher voltage rating.
  3. gerty

    gerty Well-Known Member

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    mik3 is correct 3 capacitors. phase A-B, phase B-C, and phase A-C.
  4. DanD

    DanD Thread Starter New Member

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    Ok, but how do I calculate the capacitor size for 3 phase using a clamp meter and the power meter?
  5. dhanrajbiradar

    dhanrajbiradar New Member

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    but why in Delta why not star connection of Capacitors?
  6. GetDeviceInfo

    GetDeviceInfo Senior Member

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    Just out of curiosity, what value of capacitors did you install to save you 20% of your electrical bill, and what loads are you running to get such a poor power factor?
  7. t_n_k

    t_n_k AAC Fanatic!

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    Firstly, GetDeviceInfo asks a very good question - this is worth knowing.

    The three phase system loading may not be balanced across all phases and the individual phase power factors may be quite different. You might have to individually compensate each phase. If the loads vary appreciably then the power factor compensation may have a detrimental effect by actually overcompensating the load.

    Remember you can only compensate a lagging power factor with a capacitor - which is usually the case anyway. I'm guessing you knew that anyway.

    There are also issues with adding capacitors to your installation which requires due consideration to safety and ciruit protection. Presumably you have allowed for this in your friend's case.

    In principle you can compensate each of the 3 phases individually as you did in your single phase installation. You can place the capacitors between each phase line conductor and the neutral - if neutral is connected in the service. You can use lower voltage rating for the capacitors in this mode but the currents will be higher than with a line-to-line connection mode - there's always a trade-off.

    Using the power meter and ammeter you can determine the required compensating capacitor (line-to-neutral connection) for each phase as follows.

    1. Measure the phase power P
    2. Measure the phase current I
    3. Calculate the phase reactive power Q = √((V*I)^2-P^2) where V is the known phase-to-neutral voltage.
    4. The required compensating (for unity power factor) capacitance C=Q/(ω*V^2) where ω=2*\pi*f and C is in Farads
    5. Capacitor ratings - Vmin=line-to-neutral voltage and Imin=ωCV Amps

    For instance if P=1kW, V=110V, I = 12A and f=60Hz

    Q=√((110*12)^2-1000^2) = 861.6VAR

    C=861.6/(2*\pi*60*110^2)=1.89e-4F or 189uF (rated for 110V and 7.84 A continuous duty). This was a 'bad' case with an original power factor of 0.76 - so you end up with a mighty large capacitor to get back to unity power factor.
  8. t_n_k

    t_n_k AAC Fanatic!

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    As a supplementry comment I'm also puzzled that your electric power provider charges you for reactive power. In my country domestic consumers are only charged for the real power they consume - and only fairly substantial customer installations attract a benefit for installing PFC at their mains connection point. Hence my interest in GetDeviceInfo's comment / question. Your reactive power load must have been fairly high for the conductor I^2R losses in your dwelling to have been so significant. I'm assuming (perhaps incorrectly?) that your are a domestic consumer.
  9. t_n_k

    t_n_k AAC Fanatic!

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    Sorry DanD, this part is not well expressed - ignore that bit. Of course you are only charged for the real power consumed!

    The remaining part of my last post is still relevant and your further comments re the details of your installation improvements are certainly of interest.
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