Power Factor Correction: When You Need It and How to Calculate

What is reactive power and cos φ

In AC circuits, power has three components: active P (kW), reactive Q (kVAR), and apparent S (kVA). Power factor cos φ = P / S.

Typical cos φ values:

• Incandescent lamps, heaters: cos φ ≈ 1.0
• LED lights: cos φ ≈ 0.92-0.98
• Induction motors: cos φ ≈ 0.7-0.85
• Welding machines: cos φ ≈ 0.5-0.7
• UPS systems: cos φ ≈ 0.8-0.9

Why low cos φ is bad

1. Utility penalties — up to 30% of active energy cost
2. Cable overheating — current increases as 1/cos φ
3. Oversized cables required
4. Voltage drop increases
5. Transformer underutilization

Calculation example: 100 kW office

Measured cos φ = 0.75, target cos φ = 0.95.

Qc = P × (tan φ1 — tan φ2) Qc = 100 × (0.882 — 0.329) = 55.3 kVAR

Select automatic capacitor bank 60 kVAR.

Compensator types

1. Automatic capacitor banks (3-8 RUB/VAR)
2. Active harmonic filters (15-30 RUB/VAR)
3. Synchronous compensators (>5 MVAR)

Payback

Penalties: ~$150/month. Cable losses: ~$30/month. CAPEX 60 kVAR bank: ~$2,500. Payback: ~14 months.