Testing – Power Factor (PF)

Power factor (PF) is one of those mysterious properties of AC that even most electrical engineers have a hard time explaining.  A thorough technical discussion goes beyond the scope of this review (not to mention this author’s understanding). 

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Power factor is defined as the ratio of true power (measured in watts) to apparent power (measured in Volt Amps).  It measures how effectively AC power is being used by a device.  The difference between true power and apparent power is expressed as the power factor and results from the way true power and apparent power are measured.  Ideally we would like to have true power and apparent power equal to one another, which would result in a PF of 1.00 or 100% effective power utilization. 

True power (also referred to as working power) defines power that produces work, heat and light.  We see true power in DC circuits and in AC circuits that are powering purely resistive loads like a resistance heater or light bulb.

Apparent power occurs only in AC circuits. Along with true power it also takes into account the extra power needed to create the alternating magnetic fields inside inductors (transformers and motors) and to charge capacitors.  Devices that incorporate inductance and capacitance in an AC circuit are referred to as reactive loads.  Reactive power does not produce any work.  Nearly all AC devices include some form of reactive load, which causes the PF to be less than 1 and the apparent power to always be greater than the true power.

Apparent power can easily be found by multiplying the AC voltage times the AC current using the RMS (Root Mean Square) values.  Apparent Power = Volts (RMS) x Amps (RMS) = VA

The basic formula for true power includes the power factor (the power factor compensates for the extra reactive power).  True Power = Volts (RMS) x Amps (RMS) x PF = Watts

When AC is applied to a purely resistive load, the current rises and falls in almost perfect harmony with the voltage.  Plotting a graph of the AC voltage and current will result in two classic sine waves that are in alignment with each other.  But power supplies are not resistive loads.  They include inductors and capacitors among other things, which result in a complex reactive load.  A reactive load causes the alternating current to become out of phase with the alternating voltage and can distort the shape of the AC waveform.  A load with predominantly inductive reactance will cause the current sine wave to lag behind the voltage sine wave by a certain amount (phase angle).  When the two sine waves are in perfect alignment (purely resistive load) the Power Factor is 1.00.  The more the waveform is distorted and the current lags behind the voltage, the smaller the power factor value becomes. 

For example, when AC is used to power a light bulb (resistive load), electricity flows thru the filament and is converted into heat and light.  Under these conditions the true power and apparent power are essentially the same so the PF ~ 1 and the waveform is not distorted.  All of the incoming AC power is being effectively used.  If that same AC has to first go thru a transformer or power supply (reactive load) before reaching the light bulb filament, extra current is required to create the magnetic fields inside the inductors and keep the PSU’s capacitors charged. 

The added reactive power causes the apparent power to now be greater than the true power, which in turn decreases the PF to something less than 1.00.  This extra reactive power does no real work so is factored out (power factor) when the true power of the circuit is measured.

Switching mode power supplies incorporate power transistors in a switching circuit to create a high frequency, pulsed DC, which is transformed into the various low voltage DC outputs.  This circuit creates harmonics (multiples of the 60 Hz input) that can have a detrimental affect on the AC mains because of the harmonic distortions they generate.  A switching mode power supply can draw highly distorted current from the AC mains, which will significantly reduce the power factor and may even adversely affect other equipment on the same line.

Reactance and harmonic distortions work together causing the PSU to draw more power than is actually converted into DC power and heat.  With a PF of 0.66, a power supply will use 50% more power (100 watts / 150 VA = 0.66 PF) than it converts into DC power and heat.

Some PC switching mode power supplies contain Power Factor Correction (PFC) circuits.  Passive PFC typically adds a capacitor onto the AC input while active PFC incorporates more sophisticated circuitry that smoothes out the AC current waveform and brings it back closer to a sinusoidal wave.  Active PFC is usually only found in higher wattage and more expensive units and is required in many European Union countries.  A power supply that does not have PFC will normally exhibit a PF of <0.70 and will generate significant harmonics, which can distort the AC current waveform and induce harmonic distortions back onto the power lines.  Power supplies with active PFC will have power factors >0.95 with minimal harmonic distortions. 

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Here are three screen shots from a digital oscilloscope, showing how the AC current (blue) follows the applied AC voltage (green).  Without any power factor correction, the current waveform on the right is distorted and lags behind the voltage.  Applying passive PF correction (middle) helps restore the waveform a little but in a power supply with active PF correction (left), the current waveform is almost perfectly restored to a sinusoidal wave that is aligned with the voltage.

What this really means to the end user is that a PSU with PFC will pull less current from the AC mains to generate the same amount of DC power as a similar non-PFC unit.  For commercial users who are billed based on VA usage and PF, this may save you money.  However, most residential users are billed per kilowatt-hour, which factors out the load’s reactive power component.  Because of this a PSU with active PFC won’t save the typical residential user any money on their electric bills.  However if you have a room full of computers operating on a single circuit, equipping them with PFC power supplies will draw significantly less current (therefore allowing more computers 🙂

I measured the AC Power Factor with a WattsUp? Pro power analyzer.  All three of the Seasonic units incorporate active power factor correction circuits, which resulted in a PF reading of 0.99. 

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The average AC current draw for the three Seasonic units was approximately 2.9 Amps.  This is considerably lower than the 4.2~4.5 Amps average AC current drawn by most non-PFC power supplies in the same size range operating under the same load.  So our tests support the theory: incorporating active PFC does significantly reduce a power supply’s AC current draw.

One final note on power factor: a power supply that incorporates active PFC circuitry might seem to be more efficient than one that does not have PFC. It’s true a PFC power supply will pull less current than one without PFC but because of the way power supply efficiency is calculated, it is technically not more efficient than a comparable unit without PFC. This is because true power (watts) is used in the efficiency calculation, not apparent power (VA).  A power supply with active PFC is more effective at converting electrical power but is not necessarily more efficient.

A power supply with active PFC is also more environmentally friendly (doesn’t pollute the AC transmission grid with harmonic distortions) and will use less current, but it won’t save you money unless you are a commercial user whose bill is based on PF and usage.  As we will see in the next section, the three Seasonic power supplies with active PFC were only slightly more efficient than average non-PFC units even though they pulled substantially less current while operating under the same load.  I suspect this increased efficiency (~5%) is due more to Seasonic’s high efficiency circuit design and has little to do with active PFC. 


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