Power Factor Calculator
Power Triangle — Before & After
Before Correction
After Correction
Resonance Analysis
Detuning Reactor Recommended: 5.67% (5.7 kVAR)
A poor power factor means a facility draws more apparent power (kVA) than the real work (kW) it performs, inflating demand charges and conductor losses. This calculator sizes the capacitor bank needed to raise power factor from its present value to a target, checks the resulting bank for harmonic resonance, recommends a detuning reactor if needed, and estimates the annual savings and payback.
Formula
Qc = P × (tan(arccos PF1) − tan(arccos PF2))
- P
- Real load power (kW)
- PF1
- Present (uncorrected) power factor
- PF2
- Target power factor
- Qc
- Required capacitor reactive power (kVAR)
How it works
- Enter the load in kW, current and target power factor, system voltage and frequency, the supply transformer kVA and impedance, and whether harmonic sources are present.
- Required reactive power is Qc = P × (tan φ1 − tan φ2); the tool rounds up to the nearest standard kVAR bank and recomputes the corrected power factor from the new power triangle.
- It then estimates the bank's resonant frequency against the system, flags resonance risk near characteristic harmonics (when harmonic sources exist), suggests a detuning reactor percentage, and projects demand-charge plus loss savings to compute payback in months.
Worked example
A 200 kW load at 0.75 PF to be raised to 0.95 PF on a 480 V, 60 Hz system fed by a 1000 kVA, 5.75% transformer with harmonic sources present.
- φ1 = arccos(0.75) = 41.41°, φ2 = arccos(0.95) = 18.19°.
- Qc = 200 × (tan 41.41° − tan 18.19°) = 200 × (0.8819 − 0.3287) ≈ 110.65 kVAR.
- Round up to the nearest standard bank: 125 kVAR. Corrected power factor ≈ 0.97.
- Resonant frequency ≈ 708 Hz (near the 12th harmonic); with harmonic sources this flags a resonance risk and a 5.67% detuning reactor.
Required ≈ 110.65 kVAR → 125 kVAR standard bank, corrected PF ≈ 0.97, resonance risk flagged with a 5.67% detuning reactor (~7.1 kVAR), annual savings ≈ $4178, payback ≈ 13 months.
Frequently asked questions
- Why does improving power factor save money?
- Many utilities bill industrial customers on kVA demand or apply a penalty when power factor drops below about 0.90. Correcting power factor lowers the kVA drawn for the same kW of work, reducing demand charges and the I²R losses in transformers and conductors.
- What is harmonic resonance and why is it dangerous?
- A capacitor bank and the system inductance form a resonant circuit. If its natural frequency lands near a harmonic produced by drives or rectifiers, currents and voltages at that frequency are amplified, which can overheat or destroy the capacitors and other equipment.
- What does a detuning reactor do?
- A detuning reactor is an inductor placed in series with the capacitor bank to shift the system's resonant frequency below the lowest troublesome harmonic. Common ratings are 7% (tuned below the 5th harmonic) and 14% (below the 3rd), so the bank can correct power factor without exciting resonance.
- Can I over-correct the power factor?
- Yes. Adding too much capacitance pushes the power factor leading, which can raise voltage and cause its own problems. This tool targets your specified power factor and selects the nearest standard bank at or above the requirement rather than grossly overshooting it.