Power Factor

Wheelman55 · September 2015

Would someone please school me on power factor? My kill-a-watt meter (plugged into the new fridge) is showing 3.5 watts and 11.0 va.

How do I interpret this for use in an off grid battery based system?

Thanks.

waynefromnscanada · September 2015

Hi Weelman,
Probably the simplest way of looking at it is to consider the shape of the pure sine wave of voltage in an AC system. Ideally the current flow will increase and decrease in exact step with the changing voltage, so if you were to look on a dual trace oscilloscope at the waveform of the voltage and the current, they would be exactly one on top of the other. Some devices however, such as electric motors, cause the current flow to lag behind the voltage swings, so that max current flow occurs later than the max voltage. Thus you could for example have max current flow only after the voltage has reached it's peak and is half way down to zero again. This can cause problems as in cases like this, the max current flow happens at a much lower voltage than would be ideal, increasing transmission losses and losses in transformers etc. AND in cases like this, for a given wattage, the current will be higher and sometimes much higher than if the power factor was better and the current and voltage were in step. Other load circuits using capacitors for example, can cause current flow to lead, happen ahead of the voltage swings with similar results. Such loads don't take more power out of the batteries, a watt is a watt, but it can stress the inverter trying to produce higher than otherwise needed current.
Hope this helps.

BB. · September 2015

Another anology---Think of peddling a bicycle. If you peddle evenly, you get most of the force from your legs going to the wheels.

If you back peddle, you can actually slow down the bike. And if you jump on the peddles for only part of the cycle, the average power to the wheel is less (but the crank system needs to be strong enough to stand up to jumping on the peddles).

Sort of similar with Power Factor... The math (or at least one of the maths):

Power = Voltage * Current * Cosine of the angle between voltage and current = Voltage * Current * Power Factor (where PF ranges from 0.0 to 1.0 -- And typically from 0.5 to 0.95)

How does it matter... There is another formula:

VA = Voltage * Current (no PF/Cosine factor)

Power--We use that to size the battery bank, solar array, genset for charging, etc...

VA -- Use that to size the AC wiring, switches, Transformers, etc....

Roughly, poor PF (less than 0.90 or so) does not really take more energy from the battery bank. However, on the AC side, it does take more current to obtain the same amount of power. So--You have to "up size" the AC wiring, Transformers, Switches, Circuit Breakers for the "same" amount of power.

And for the AC output of an AC inverter or Genset--Generally for residential use, they are size for Watts=VA output... You need to size the AC inverter (and AC genset) output to reflect the higher current usage when PF is poor (motors, CFL lighting, etc. can run PF in the range of 0.5 to 0.66). So poor PF does "cost you money".

Of course, because there is more current flow in poor PF circuits, there are some more wasted energy. Plus, the larger than "needed" AC inverter/AC Genset can waste more energy too.

-Bill

oil pan 4 · September 2015

Low power factor comes from inductive loads such as motors, coils and solid state switching.

Problem comes from amps rushing into the load just as the AC current starts to flow. With a low power factor load the current is already dropping off as the voltage builds to peak.

Unlike a purely resistive load with a power factor of 1 where current trickles through the load as voltage starts to build. Current peaks when voltage peaks.

I power factor corrected my air compressors and welding machines. Mainly so that the 120 volt machine will work better on a 15 amp circuit, a generator or long run of extension cord. I power factor corrected the 220 volt machines mostly so they wouldn't over load my generator.

inetdog · September 2015

All of the discussion from Bill and others has concentrated on what is called the displacement power factor, an offset between two sinusoidal waveforms. oil pan 4 also alluded to the effects of rectifier power supplies. That is not a phase displacement and is usually referred to a distortion PF. It can also be a problem of high peak current for inverters or generators, but usually cannot be improved with a capacitor bank the way displacement PF can be. In fact a capacitor bank can increase the peak current into a rectifier at turn on and potentially cause damage.

And do not try to blindly apply your newfound PF knowledge to the case of a Modified Square Wave inverter!
Put a capacitor bank on the output of one of those and the high frequency harmonics will kill either the inverter or the capacitor bank itself.

Bill discusses distortion power factor in a lot more detail in this thread.

oil pan 4 · September 2015

Solid state switching found in LED and CFLs typically produce phase distortion which can not be corrected just by adding capacitance to the circuit.

Rectifiers don't produce phase distortion, its usually what they supply power too.

inetdog · September 2015

oil pan 4 said:

Solid state switching found in LED and CFLs typically produce phase distortion which can not be corrected just by adding capacitance to the circuit.
Rectifiers don't produce phase distortion, its usually what they supply power too.

I prefer not to call this phase distortion, since the shape of the waveform rather than the phase of the waveform is what causes problems.

A rectifier without any load will not draw any current at all, so in that sense your second statement is correct. But the load on a rectifier system can be perfectly resistive and the current into to the rectifier will still be non-linear. But since you use the term phase distortion, it is not clear what characteristic of the current into a rectifier you are talking about.

Power Factor

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