Mathematically, power factor can be expressed as the following:
Power Factor = Real Power (Watts) / Apparent Power (VA)
Looks harmless, doesn’t it! So, why is this an important ENERGY STAR and IEC 61000-3-2 requirement?
Power distribution and transmission corporations are severely penalized for low power factor in their systems, and these expenses are usually passed on to the end user. This makes designing for a high power factor very important in the context of IEC 61000-3-2 and ENERGY STAR. Apart from non-compliance with regulatory standards and increasing utility bills, low power factors can result in wear and tear of motors, transformers and other costly equipment due to overheating.
In AC power conversion systems, loads aren’t always our forgiving resistive kinds. A reactive load (think coils, capacitors, transformers, motors), in reality, dissipates or absorbs energy resulting in a less efficient power system. Ideally, the power factor of a power system would be 1 (or 100%), and some switch mode power supplies (SMPS) have succeeded at meeting or exceeding the 0.9 (or 90%+) mark.
Inductive loads, for example, cause the load current to lag the load voltage by a certain phase angle. If the voltage and current waveforms are sinusoidal, the cosine of that phase angle is another representation of the power factor. Since switch mode power supplies typically produce waveforms that are not sinusoids (see Fig. 1), an accurate measurement of power factor would require a measurement system to compute real and apparent power.
Fig.1 – Load voltage and current waveforms of a switch mode power supply measured by the MSO5104B oscilloscope
The low power factor caused by an inductive load is compensated for by adding a capacitive load, and vice-versa. However, a typical switch mode power supply requires a more complex power factor correction circuit. This process is known as power factor correction.
Power factor also manifests itself in the current harmonics measurements. For example, the distortion due to low power factor will result in poor current harmonics. Some oscilloscopes can provide insight into whether the harmonics meet standards. Shown below in Fig. 2 is the power analysis software (DPOPWR) on a Tektronix MSO5104B oscilloscope indicating that the Harmonics “pass” IEC 61000-3-2 requirements. The gray bars display the standard limits, and the green bars indicate the actual measurements.
Fig. 2 – Current harmonics measurements using DPOPWR on an MSO5104B oscilloscope
In fact, this scope can also measure the power factor of an SMPS. Fig. 3 below indicates a power factor of 0.7715 or 77.15%. You can verify that the scope measures the true and apparent powers before making this calculation.
Fig.3 – Power quality analysis using DPOPWR on an MSO5104B oscilloscope
Also notice that Crest Factor, True Power and Apparent Power and several other measurement options are readily available on DPOPWR. Feel the need to brush up on some of these power supply related measurements? Refer to our Fundamentals of AC Power Measurements Primer or watch our webinar on Testing Power Supplies for Energy Efficiency. As always, be sure to post up any questions or thoughts in the comments section below.