sparkytools

Fourth year Power quality · tool

Power factor correction calculator

Enter the real power and the power factor now, and the power factor you want. The tool gives the capacitor kVAR needed; add the voltage for the current saved and the capacitance per phase.

Supply
Frequency
  • Capacitor kVAR needed—
  • kVA before → after—
  • kVAR before → after—
  • Line current before → after—
  • Current the capacitors draw—
  • Capacitance per phase, delta bank—
  • Capacitance per phase, wye bank—
  • Capacitance—
Show the working

    Arithmetic on the numbers you entered. Capacitor selection, switching, protection and placement follow the maker’s instructions and your code book.

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    How it works

    The real power (kW) stays the same; correction only shrinks the reactive power (kVAR) the supply has to carry. Capacitors supply kVAR locally, so the kVAR you need is the load’s kVAR now minus the kVAR it would have at the target power factor. With the angle φ whose cosine is the power factor, kVAR = kW × tan φ.

    • kVAR = kW × (tan φ1 − tan φ2)φ = arccos(PF)
    • kVA = kW ÷ PFbefore and after
    • C = Q ÷ (2π f V²)per capacitor, Q in VAR it carries, V across it

    Worked: 100 kW from 0.70 to 0.95 at 480 V

    1. tan(arccos 0.70) = 1.0202; tan(arccos 0.95) = 0.3287.
    2. kVAR needed = 100 × (1.0202 − 0.3287) = 69.2 kVAR.
    3. kVA drops from 100 ÷ 0.70 = 142.9 to 100 ÷ 0.95 = 105.3. Current at 480 V drops from 171.8 A to 126.6 A — the same work for about 26% less current.
    4. A delta bank: each capacitor carries 69,150 ÷ 3 = 23,050 VAR at 480 V → C = 23,050 ÷ (377 × 480²) = 265 µF.

    Questions people ask

    Why not correct all the way to 1.0?

    The last few points cost the most capacitance for the least gain, the load’s power factor changes as it loads and unloads, and a bank sized for 1.0 at full load can push the plant leading at light load — which can raise the voltage and upset some equipment. Targets in the mid-0.9s are common; what your utility bills against is in its rate schedule.

    Where should the capacitors go?

    Close to the load corrects the current in more of the wiring, but switches with the load; one bank at the service is cheaper and easier to control but only corrects what the utility sees. Capacitors on a motor’s load side are sized against the motor and its overload setting, and on a VFD’s output they are never used. The motor or capacitor maker’s instructions and your code book decide the details.

    Do capacitors and harmonics mix?

    Carefully. A capacitor bank and the system’s inductance can resonate near a harmonic that drives or other non-linear loads produce, magnifying it. Plants with a lot of drives often use detuned (filtered) banks. If the plant has many drives, it is an engineering question before it is a purchase.

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