Power Factor Correction
Low power factor wastes energy and incurs penalty charges. Learn what power factor is, how to correct it, and the economic benefits.
What is Power Factor?
Power factor is the ratio of real power (kW) to apparent power (kVA) in an AC circuit. It measures how effectively electrical power is being used. A power factor of 1.0 means all the power is doing useful work. A power factor of 0.7 means 30% of the apparent power is wasted.
Inductive loads (motors, transformers, welders, fluorescent lighting ballasts) draw reactive power (kVAR) to maintain magnetic fields. This reactive power doesn't do useful work but increases the current drawn from the supply, causing higher apparent power and lower power factor.
Why Power Factor Matters
- Penalty charges: Most electricity retailers charge a penalty for power factor below 0.9 or 0.95. The penalty increases the kVA demand charge.
- Higher current: Low power factor means higher current for the same useful power. Higher current means larger cables, larger transformers, and more I2R losses.
- Reduced capacity: Transformers and switchgear are rated in kVA. Low power factor means more kVA for the same kW, reducing available capacity.
- Voltage drop: Higher current causes greater voltage drop in cables and transformers, reducing voltage at the equipment.
How to Correct Power Factor
Power factor correction involves adding capacitors to the electrical system. Capacitors supply the reactive power locally, so it doesn't need to be drawn from the supply. This reduces the apparent power and improves the power factor.
- Bulk correction: Capacitors installed at the main switchboard. Corrects the overall plant power factor. Simple but doesn't help individual circuits.
- Individual correction: Capacitors installed at each motor. Corrects at the point of use. Reduces current in the motor circuit, improving efficiency.
- Automatic correction: A controller switches capacitor banks in and out based on the measured power factor. Maintains a target power factor under varying load conditions.
| Original PF | Corrected PF | Current Reduction | Annual Saving (100 kW load) |
|---|---|---|---|
| 0.70 | 0.95 | 26% | $2,000 to $5,000 |
| 0.80 | 0.95 | 16% | $1,000 to $3,000 |
| 0.85 | 0.95 | 11% | $500 to $2,000 |
| 0.90 | 0.95 | 5% | $200 to $1,000 |
Don't over-correct
Installing too much capacitance can cause a leading power factor, which is also undesirable. Leading power factor can cause voltage rise at the equipment, resonance with transformer inductance, and problems with VFDs and other electronic equipment. Always size correction to achieve a power factor of 0.95 to 0.98, not 1.0.
Tip: Check your electricity bill for power factor charges. If you see kVA demand charges or power factor penalty line items, you are being charged for low power factor. A power factor correction audit and capacitor installation typically pays for itself in 1 to 2 years.