Editing Power factor (section)

== Measurement techniques ==
The power factor in a single-phase circuit (or balanced three-phase circuit) can be measured with the wattmeter-ammeter-voltmeter method, where the power in watts is divided by the product of measured voltage and current. The power factor of a balanced [[Polyphase system|polyphase circuit]] is the same as that of any phase. The power factor of an unbalanced polyphase circuit is not uniquely defined.

A direct reading power factor meter can be made with a [[moving coil meter]] of the electrodynamic type, carrying two perpendicular coils on the moving part of the instrument. The field of the instrument is energized by the circuit current flow. The two moving coils, A and B, are connected in parallel with the circuit load.  One coil, A, will be connected through a resistor and the second coil, B, through an inductor, so that the current in coil B is delayed with respect to current in A. At unity power factor, the current in A is in phase with the circuit current, and coil A provides maximum torque, driving the instrument pointer toward the 1.0 mark on the scale. At zero power factor, the current in coil B is in phase with circuit current, and coil B provides torque to drive the pointer towards 0. At intermediate values of power factor, the torques provided by the two coils add and the pointer takes up intermediate positions.<ref>{{Citation |first1=Donald G. |last1=Fink |author1-link=Donald G. Fink |first2=H. Wayne |last2=Beaty |title=Standard Handbook for Electrical Engineers |edition=11 |publisher=McGraw-Hill |place=New York |year=1978 |isbn=978-0-07-020974-9 |page=3-29 paragraph 80}}</ref>

Another electromechanical instrument is the polarized-vane type.<ref>{{Citation |title=Manual of Electric Instruments Construction and Operating Principles |id=GET-1087A |publisher=General Electric, Meter and Instrument Department |place=Schenectady, New York |year=1949 |pages=66–68}}</ref> In this instrument a stationary field coil produces a rotating magnetic field, just like a polyphase motor. The field coils are connected either directly to polyphase voltage sources or to a phase-shifting reactor if a single-phase application.  A second stationary field coil, perpendicular to the voltage coils, carries a current proportional to current in one phase of the circuit. The moving system of the instrument consists of two vanes that are magnetized by the current coil. In operation, the moving vanes take up a physical angle equivalent to the electrical angle between the voltage source and the current source. This type of instrument can be made to register for currents in both directions, giving a four-quadrant display of power factor or phase angle.

Digital instruments exist that directly measure the time lag between voltage and current waveforms. Low-cost instruments of this type measure the peak of the waveforms. More sophisticated versions measure the peak of the fundamental harmonic only, thus giving a more accurate reading for phase angle on distorted waveforms. Calculating power factor from voltage and current phases is only accurate if both waveforms are sinusoidal.<ref name=ni_white_paper>{{cite web|url=http://www.ni.com/white-paper/4278/en/|title=The Fundamentals of FFT-Based Signal Analysis and Measurement in LabVIEW and LabWindows/CVI|work=National Instruments Corporation|access-date=6 November 2017}}</ref>

Power Quality Analyzers, often referred to as Power Analyzers, make a digital recording of the voltage and current waveform (typically either one phase or three phase) and accurately calculate true power (watts), apparent power (VA) power factor, AC voltage, AC current, DC voltage, DC current, frequency, IEC61000-3-2/3-12 Harmonic measurement, IEC61000-3-3/3-11 flicker measurement, individual phase voltages in delta applications where there is no neutral line, total harmonic distortion, phase and amplitude of individual voltage or current harmonics, etc.<ref name=Yokogawa_WT3000E>{{cite web|url=http://www.yokogawa.co.jp/ftp/dist/ks/catalog/en/BUWT3000E-01EN_020.pdf|title=WT3000E Series Precision Power Analyzers|work=Yokogawa Corporation|access-date=6 November 2017|archive-url=https://web.archive.org/web/20171107112155/http://www.yokogawa.co.jp/ftp/dist/ks/catalog/en/BUWT3000E-01EN_020.pdf|archive-date=7 November 2017|url-status=dead}}</ref><ref name=Fluke_1760>{{cite web|url=https://cdn.testequity.com/documents/pdf/1760-ds.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://cdn.testequity.com/documents/pdf/1760-ds.pdf |archive-date=2022-10-09 |url-status=live|title=Fluke 1760 Three-Phase Power Quality Recorder |work=Fluke Corporation|access-date=6 November 2017}}</ref>