Tech Tips: Power Factor Correction

» Power factor correction (PFC) explained

Power Factor Correction (PFC) is mandated in several parts of the world and encouraged in all others. Despite universal agreement on the importance of PFC, it is often misunderstood. Let's demystify the "power factor", answer why it needs correction, how, and who benefits.

Power Factor
Electrical power is delivered by utilities in sinesodal form, known as AC power. All electrical power is calculated by multiplying voltage and current. Thus, AC power is the alternating voltage multiplied by the alternating current. Because AC power alternates a constant is applied to both current and voltage to obtain an average. The constant is known as RMS (root mean square).

I_p and V_p are the peak current at voltage of the sinesoldal waveform. RMS values used to calculate average power are obtained by dividing peak by the square root of 2.

In this figure voltage and current are perfectly aligned or "in phase". Electrical utilities deliver power to a diverse set of consumer and industrial loads. These loads are not ideal or purely resistive in nature. They contain stored inductive and capacitive energy that reflects back onto the incoming utility power. This reflection causes the AC voltage and current to become "out of phase". The power factor measures the degree of alignment between AC voltage and current.

A power factor of 1 (unity) is the ideal, perfectly aligned voltage and current, the result of a purely resistive load. In the real world power factors are < 1. The power factor is calculated from the cosine of the phase angle between voltage and current as an absolute value. A power factor may be given followed by the term Leading or Lagging. For example, power factor is .7 lagging. Lagging means an inductive load, leading a capacitive load.

Updating our AC power equation to include non-resistive loads, we can immediately see the significance of the power factor:

P_avg 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.

tip: 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.