Updating our AC power equation to include non-resistive loads, we can immediately
see the significance of the power factor:
Pavg is formula for real power. AC Power is composed of 3 components: apparent
power, reactive power, and real power.
Apparent power (S), measured in Volt-Amps (VA), is power a utility "appears"
to deliver to a load. If the load is ideal, then the apparent power equals real power.
Reactive power (Q), measured in reactive Volt-Amps (Var), is the inductive
- capacitive energy reflected back onto the AC mains. Reactive power is undesirable and causes the
power factor to become less than 1.
Real power (P), measured in Watts (W), that is actually delivered to a load.
Also known as true power.
Apparent power is a combination of reactive and real power. The actual
calculations involve complex mathematics due to the reactive component. If reactive power is zero,
then apparent power equals real power. The basic point to remember is apparent power, i.e.,
power that has traveled all the way down an electrical grid is not the same as the real power
consumed at a load. This disconnect is wasted energy and a source of concern for utilities.
The power factor is calculated as the cosine of phase angle between voltage
and current. It is defined as Real power (P) ÷ Apparent power (S), a dimensionless value which
never can exceed 1.
To their credit, utility power meters record and charge only for real power. That
said, industrial customers with poor power factors are assessed surcharges.
Power Factor Correction
Now we have a good understanding of the "power
factor" and how it influences electrical grid efficiency. PFC involves techniques to cancel out
undesirable reactive power in electronic loads. Power supplies are uniquely positioned for power
factor correction, situated directly between the load and incoming AC power.
Computer power supplies with PFC are sometimes misrepresented in the marketplace.
A power supply with PFC is not more efficient than a power supply without PFC. PFC does nothing to
improve power supply efficiency and thus will not save consumers energy costs. PFC improves the
efficiency of the electrical grid and can be thought of as a common good. It directly benefits
utilities by saving energy on the grid. Theoretically, the cumulative effect of individual power
corrected loads has a positive trickle down effect to end users.
Computer power supplies employ two types of power factor correction: Passive PFC
and Active PFC.
Passive PFC has fallen out of favor due to superior Active PFC. Passive designs
use fixed inductive and capacitive elements to counter act reactive power. Although better than
nothing, passive PFC has several drawbacks, including requiring a switchable AC input
(115VAC / 230VAC) and potential increased electromagnetic noise.
Active PFC, as the name implies, employs continuous monitoring & correction of
power factor. This is accomplished with an IC and supporting analog components. The IC acts as the
control center making adjustments on the fly and ensuring a power factor > .95 is almost all
cases. Another nice feature of typical Active PFC packages is automatic adjustment for AC input
voltage, which lends itself nicely to full range (100-265VAC) AC input. Active PFC is a great way
to globalize a power supply product.