Power factor is defined as the ratio of the real power to the apparent power where real power is the rate of energy transfer and apparent power is the voltage multiplied by current. Power factor is usually expressed as a number between 0 and 1 or as a percentage.

For a purely resistive source & load the power factor will be 1, this is the optimum value since it minimises losses in the supply cables. The power factor is reduced with reactive (capacitive or inductive) loads and those that only draw current for parts of the AC cycle (so called non-linear loads).

Reactive loads

In inductive loads the current lags the voltage as such they are said to produce a lagging power factor or positive volt-amperes reactive (var). Common inductive loads include motors and some welding equipment.

In capacitive loads the current leads the voltage and they are said to have a leading power factor or negative var. In practice there are few very capacitive loads.

The power factor in linear loads is equal to the absolute value of the cosine of the phase angle (φ) between the voltage and current.

That is:

PF = |cos φ|

Power factor correction

Power factor correction (PFC) is often employed to bring the power factor closer to 1. In this case of inductive loads this is done by connecting a capacitor in parallel with the load. In practice it is rarely cost effective to improve the power factor beyond about 0.95

Non-linear loads

Non sinusoidal current draw can also reduce power factor. Common non-linear loads include dimmers, switch mode power supplies and electronic ballasts.

The power factor in non-linear loads where the current and voltage are mostly in phase can be calculated using the following equation where THD is the total harmonic distortion.

Power Factor Correction

The EU has brought in limits on the amount of harmonic distortion items can produce and most now incorporate some kind of filtering, this can range from simple inductors to more complicated IC controlled active filters.

The Practical Importance of Power Factor

" The Practical Importance of Power Factor. If an AC generator is rated to give say, 2000 A at a voltage of 400V, it means that these are the highest current and voltage values the machine can give without the temperature exceeding a safe value. Consequently the rating of the generator is given as 400x2000/1000=800kVA. The phase difference between the voltage and the current depends upon the nature of the load and not upon the generator. Thus if the power factor of the load is unity, the 800 kVA are also 800kW, and the engine driving the generator has to be capable of developing this power together with the losses in the generator. But if the power factor of the load is say, 0.5, the power is only 400kW, so that engine is developing only about one-half of the power of which it is capable, though the generator is supplying it's rated output of 800kVA. Similarly the conductors connecting the load to the generator have to be capable of carrying 2000A without excessive temperature rise." - Extract from; Hughes Electrical Technology 1995 edition published by Prentice Hall

See Also

• Wikipedia article on power factor
• Impedance