How to use it
- Enter the real power of the load in kW (from a meter or the nameplate).
- Enter the present power factor and the target, for example 0.75 and 0.95.
- Choose three phase or single phase, the voltage (line-to-line for three phase) and the frequency.
- Read the capacitor kvar, the capacitance per phase for a delta or star bank (or the single capacitor), and the current before and after.
Formulas
- φ = arccos(PF); Qc = P × (tan φ₁ − tan φ₂) kvar
- Apparent power S = P / PF kVA
- Current: I = S / V single phase, I = S / (√3 × V) three phase
- Capacitance (ω = 2πf): single phase C = Qc / (ωV²); three phase delta C = Qc / (3ωV²) per phase; star C = Qc / (ωV²) per phase
Worked example
A 100 kW load at 0.75 power factor on 400 V, 50 Hz three phase, corrected to 0.95. tan φ₁ = √(1 − 0.75²) / 0.75 = 0.8819 and tan φ₂ = 0.3287, so the bank must supply 100 × (0.8819 − 0.3287) = 55.32 kvar. With ω = 314.16, a delta bank needs 55,323 / (3 × 314.16 × 400²) = 366.9 µF per phase (a star bank 1,100.6 µF).
The apparent power falls from 133.33 to 105.26 kVA and the line current from 192.45 A to 151.93 A, 21.05 % less.
Tips
- Capacitors are sold in kvar at a rated voltage; choose a rating at or above your supply voltage and the nearest step at or below the calculated kvar.
- Correct at the motor or centrally. Individual correction cuts current all the way back to the board; a central automatic bank follows a changing load.
- Harmonics change the picture. With variable-speed drives or other non-linear loads, capacitors can resonate with the supply; such cases need detuned banks and a harmonic study.