Why do I care about Power Factor?

Bad power factors cost you money in the form of extra wire losses and increased strain on the power infrastructure.  They reduce the amount of usable energy a circuit can provide but are often overlooked because they can be difficult to measure.  This post talks about what power factor does to a system and how to measure it.

Define: Power Factor

Power factor is a measure of how effectively the power provided is being used; it is the ratio of real power to apparent power.  When the PF (Power Factor) is 1:

  • The power efficiency is maximized
  • The wiring losses are minimized
  • Circuit breakers are less likely to trip

As that figure drops, the generator and infrastructure have to push more energy into the system to provide the same amount of useful energy.  If it hits zero, no useful work is being done, but the power plant is still paying to push current back and forth.  Don’t worry, they’ll still charge you for their trouble.

For a moment, let’s assume a world of perfect sinusoids.  This world exists:

  1. Back in time, before diodes and transistors wreaked their non-linear havoc
  2. On paper and whiteboards
  3. In systems dominated by big linear things, like motors and transformers

In this world, the power factor is phase relationship between voltage and current.  If they are not perfectly aligned, your load is actually pushing power back onto the grid.  As they become misaligned, less useful work is done as energy uselessly sloshes back and forth between your load and the grid.  You may be paying the power company to give power back to them!

Power Factor = cos(ΦV – ΦI)

A system with PF = 0.  Note that no power is transferred on average.  All of the power drawn in quadrants I and III (Sink) is pushed back to the grid in quadrants II and IV (Source).

A system with PF >0.98.  Current and Voltage are nearly in phase, and  very little time is spent in the wasteful quadrants.

Why does this happen?

Resistive loads, like incandescent light bulbs or electric heaters, have perfect power factors because their current is in phase – they have negligible imaginary resistance.  Inductive loads, like motors or transformer leakages, cause the current to lag behind voltage and drop the power factor.  In most industrial settings, a lagging power factor is common.

This lag can be corrected by adding capacitors, which cause the current to lead before the voltage.  Banks of capacitors can be switched in or out to optimize power factor by counter-acting the inductors’ lag.

Remember, keep it CIVIL: C I before V, V before I in L.  Capacitors current (I) leads their voltage.  Voltage leads current in inductors (L).  Resistors are in phase.

Measuring Power Factor:

Measuring power factor requires simultaneous measurement of both current and voltage, which unfortunately means that traditional single channel meters cannot make this measurement.  Power Factor lends itself nicely to using the existing wiring as a quick’n’dirty current shunt, with no impedance calibration required.  This is because the measurement is unitless and only depends on the relationship between voltage and current; the actual amplitudes are irrelevant.  The Mooshimeter can take this type of measurement several ways:

1: The Three Point Connection

This is the easiest, cheapest, and most flexible option.  It also has none of the inherent sources the other options do.  I recommend the 3 point connection method if you are just measuring PF.

Make the following connections:

  1. Meter Common to one wire of your single phase connection.
  2. Meter Precision Input to the same wire a little bit away.   You want a burden  in the 50uV to 50mV range, but this value is not critical.
  3. Meter High Voltage to the opposite wire.

Will It Blend: PF Setup

Will It Blend? No, but the blender will.  3 Point setup for PF measurements with a power strip as a current shunt.

2: Current Clamp

Using a current clamp is a more traditional PF measurement method, but look out for phase shift!  Many current clamps will have phase shifts of several degrees at 50-60Hz (worse at higher frequencies), and some phase shifts are so bad they refuse to specify them in the datasheets!

  1. Measure current with the current clamp between common and the precision input.
  2. Measure voltage with the high voltage input and the “A” input as an auxiliary common.

3: Internal Current Shunt

This method uses the current shunt internal to the Mooshimeter and requires no additional equipment.  However, it does require re-routing the power through the device.  Also, since the common is shared, there is a small coupled impedance between voltage and current, which tends to result in slightly more positive readings.  At 120V this effect is not noticeable, but it may be of note in very low voltage readings (e.g. after the transformer in a wall-wart).

  1. Turn off and disconnect the system under test.
  2. Route the phase through the internal current shunt with connections “A” and “Common”.
  3. Connect the “V” input to the opposite wire.

Too Long; Didn’t Read:

This post covers power factor in purely linear systems.  Stay tuned for power factor’s non-linear cousin, Total Harmonic Distortion.

A non-linear load's VI curve with AC excitation

A load that isn't an ellipsoid in the VI plane ?!  Non-linear loads and Harmonic Distorion.

The first two diagrams were generated in LTSpiceIV.  I’ve included the schematic and plot definition.  Modify L1 and R1 to adjust reactive and real impedance.  Note V1 starts 90 degrees off so the simulation hits steady state quickly.

3 Responses to “Why do I care about Power Factor?”

  1. Chuck April 9, 2014 at 10:51 am #

    it’s “wreaked” not “wrecked” their non-linear havoc

  2. Richard Nelson May 9, 2016 at 3:00 pm #

    I need to measure PF. Will a Mooshimeter work on a Win10 PC? I can buy a Bluetooth dongle.


    X y,


    • James May 10, 2016 at 1:59 pm #

      Hi Richard,

      The Mooshimeter doesn’t have a Windows 10 application, sorry. Only iOS and Android are supported. There’s a python API based around the BLED112 dongle if that helps.

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