Power triangle
| Real (active) P | Watts (W) — does actual work |
|---|---|
| Reactive Q | VAR — stored/returned, no work |
| Apparent S | VA = √(P² + Q²) — what the supply must provide |
| Power factor | PF = P / S = cos φ (for sinusoidal) |
| Leading vs lagging | Capacitive load leads; inductive load lags |
Typical PF values
| Load | PF |
|---|---|
| Incandescent lamp | 1.00 |
| Resistive heater | 1.00 |
| Induction motor (full load) | 0.80–0.90 |
| Induction motor (no load) | 0.10–0.30 |
| Uncorrected fluorescent | 0.40–0.60 |
| Switching PSU (cheap) | 0.50–0.70 |
| Switching PSU (with PFC) | 0.90–0.99 |
| LED driver (good) | 0.90–0.99 |
Why it matters
- Poor PF forces utility to deliver extra current — commercial customers are billed for it (kVA or PF penalties).
- Cable / transformer sizing uses apparent power S, not real power P.
- Power factor correction (PFC): add capacitors for inductive loads, inductors for capacitive — or active PFC for electronic loads.
- Non-sinusoidal loads (rectifiers, PC power supplies) have harmonics — PF involves total harmonic distortion: PF = cos φ · (1 / √(1 + THD²)).
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