In short. Low power factor means you draw more current than needed for the real power, which raises losses and utility charges. Capacitors supply reactive power locally. The kVAr needed depends on the real load, the present and target power factor. On generators and with harmonics, correction needs extra care.
The calculation
where φ = arccos (power factor)
Example (illustrative): 500 kW load at 0.80 power factor to be raised to 0.95. tan φ1 = tan(arccos 0.80) = 0.75; tan φ2 = tan(arccos 0.95) = 0.329. Required = 500 × (0.75 − 0.329) ≈ 210 kVAr. A 200 to 225 kVAr bank would be chosen.
Why improve power factor
- Lower current gives lower cable and transformer losses.
- More capacity is released in transformers and generators.
- Utility tariffs may penalise low power factor or reward high.
- Better voltage regulation.
Fixed or automatic
| Type | Use |
|---|---|
| Fixed capacitor | Steady loads, for example at a large motor |
| Automatic (APFC) panel | Variable loads; steps switch in and out under a controller |
| Detuned (reactor) banks | Where harmonics are present, to avoid resonance |
| Active compensation | Fast-changing loads and harmonics |
See APFC and power-factor correction.
Cautions with generators
- A generator may be unable to absorb leading reactive power. Overcorrection, where capacitors push the power factor leading, can cause over-voltage and loss of control.
- On generator supply the capacitors should usually be disconnected or controlled so the generator sees lagging power factor.
- Capacitors switching on can produce transients that stress the generator.
- Check the alternator’s reactive capability curve.
Harmonics and resonance
Capacitors can resonate with the system inductance at a harmonic frequency, amplifying harmonic current. Where drives, UPS or rectifiers are present, use detuned reactors or filters, and check with a harmonic study. See harmonic screening.
Where to place capacitors
- At individual large motors: most effective for cable losses but cannot follow load.
- At the main board: simple and common.
- Distributed groups: compromise.
Motor capacitors on drives, soft starters or star-delta need special care, and should not be connected in a way that causes self-excitation.
Worked check on a generator (illustrative)
A site’s 500 kW load at 0.8 would need about 625 kVA from the generator. After correction to 0.95, about 526 kVA. The generator’s kVA loading falls by about 16 per cent, but its kW remains the same. Because the engine’s kW limit is unchanged, the benefit is on alternator and cable loading. If the capacitor bank remained connected at light load, the power factor could lead, and the generator voltage rise. Automatic control avoids that.
Capacitor sizing arithmetic
Required kVAr = kW × (tan φ1 − tan φ2), where cos φ1 is the present and cos φ2 the target power factor
Worked example (illustrative)
A 400 kW load at 0.78 power factor: tan φ1 = 0.802. Target 0.95: tan φ2 = 0.329. Required = 400 × (0.802 − 0.329) = 189 kVAr. Choose a 200 kVAr automatic bank in stages (for example 4 × 50 kVAr).
| Item | Consideration |
|---|---|
| Harmonics | Use detuned reactors (such as 7 percent) where drives or UPS exist |
| Stages | Enough steps to follow load without overcorrection |
| Generator supply | Disconnect or control the bank when on generator |
| Switching | Contactors rated for capacitor inrush |
| Protection | Fuses or breakers, discharge devices |
Why generator supply is different
Leading current from capacitors can raise generator voltage, destabilise the AVR and trip the loss-of-excitation protection. With light load and a full bank, the generator can see a net leading power factor. Interlock the bank to switch off on generator operation unless the study approves.
Check
Measure present power factor over a representative period, since the average may hide low-load periods that need fewer stages.
See reactive power studies and harmonic risk screening.
Common mistakes
- Fixed capacitors left connected at light load.
- No detuning where harmonics exist.
- Using the same bank on mains and generator without checks.
What to send us
Send the load, present power factor and any power-quality data. Send your application details. See also reactive power and power-factor studies.
Questions engineers ask
What target power factor should I use?
Commonly 0.95 to 0.99, depending on tariffs and system. Avoid going leading.
Can I use capacitors on a generator supply?
With care. Control them so the generator is not forced to absorb leading power.
Why are reactors added to capacitors?
To detune the circuit so it does not resonate with system harmonics.
Does power-factor correction reduce the generator size?
It reduces kVA loading but not kW, so the engine limit is unchanged.
Figures are illustrative. Use measured data and a harmonic check for design.