Ok, I'll bite.
First, some diffnitions.. from Delmer's Standard (ISBN-13: 978-1-4180-6580-5):
Apparent Power (VA) is the value found by multiplying the applied voltage by the total current of an AC circuit. Apparent power is measured in volt-amps (VA) and should not be confused with true power, measured in watts.
"Power Factor (PF) is a comparison of the true power (Resistive, or Watts) to the Apparent power (Volt Amps) in an AC circuit."
"Reactive power (VARs or Volt Amps Reactive... sometimes called "wattless power"), is calculated by multiplying reactive voltage by reactive current. Which can be produced by inductive or capacitive loads.".
(The key concept here is the notion that there is funny reactive stuff going on opposing input stuff)..
there's also a few different resistances to get your head around.... (I've tried to attach a cheat Sheet from E2.. but I guess the file was too big, so it got kicked back... contact me off list and I can send it along)..
anyway, Assuming you're in single phase parallel land (AC).. :
First, there's a pure resistance measured in ohms (Greek letter omega), opposing current flow across any purely resistive loads.. (power in this part of the circuit is measured in watts.. current is in phase with voltage)...
there's also a current limiting property opposing current flow across any inductive loads.. it's also measured in ohms, but is called XL.. or Inductive Reactance... funny thing about this stuff is that it's not really a resistance.. it's more of a reaction to magnetic fields.. so the current wave form lags the voltage wave form by 90 degrees... Power in this part of the circuit is tabbed VARsL...
similarly there's a resistance opposing current flow (or something like it) in capacitive loads.. also measured in ohms. this resistance is given the handle of Xc... or Capacitive Reactance. Power in this part of the circuit is tabbed VARsc .. this current leads the voltage wave form by 90 degrees, and is 180 degrees out of phase with any inductive current .. so you can subtract the smaller from the larger, and come up with a total VARs for the circuit.. which might be either inductive or capacitive.. and because the relationship of either wave form is 90 degrees out of phase from resistive power, we can use right angle trig. to keep tabs. Chuck Thompson (E1) taught us the phrase "ELI the ICE man" to help us keep things straight.
Finally, there's Z (or total circuit impedance or "total opposition to current flow in an AC circuit"), and is calculated from ohms, XL and Xc. Z can be substituted for R total circuit calculations... I think..
--So--
The way I like to make sense of it.. when you stuff AC power into a purely resistive circuit, it consumes watts..... the current wave form is in phase with the voltage wave form so the resistive force pushes straight back on applied power... (twinkle, twinkle little star, power = I squared R).
When you stuff AC power into a combination AC circuit (resistive, inductive, and maybe capacitive.. like a motor), really funny things happen.. you can still walk up and measure stuff with your meter (and you still see Volts and Amps ), but you're seeing "apparent power".. (VA).. the current wave form is likely not in phase with the voltage wave form, so rather than there being a resistance opposing current flow directly, there's now a "funny power" pushing back the on the applied power.. a reactive power.... (VARs). which can be either inductive (lagging), or capacitive (leading). that means VARs is oppositional to VA... And Power Factor (PF) is the ratio of Watts/ Volt Amps (VA).
So, the way I see it, VA is total apparent power.. stuff you can see... VARs is reactive power.. (funny power pushing back on applied power)... So that means that if there is no VARs, (XL=Xc), there is no out of phase (funny power) opposing input power... which would make the Power Factor (PF= W/VA)=1.
Now I have been known to be full of... beans, so if I've oopsied some, please chime in.. thanks. hope this helps. db
Comments?
Dan Brown
Foxfire Energy Corp.
Renewable Energy Systems
(802)-483-2564
www.Foxfire-Energy.com
NABCEP #092907-44
Dan Brown
President
Foxfire Energy Corp.
Renewable Energy Systems
(802)-483-2564
www.Foxfire-Energy.com
NABCEP #092907-44
-------- Original Message --------
Subject: Re: [RE-wrenches] var/ power factor
From: Kent Osterberg <k...@coveoregon.com>
Date: Fri, May 14, 2010 1:35 pm
To: RE-wrenches <re-wrenches@lists.re-wrenches.org>
Todd,
You are getting credit for all the energy your system can produce. Turn off the humidify and you'll see the power factor go up to nearly 1.0. That's what the inverter is operating at, regardless of whether the humidifier is on or off.
Say the PF is 80% without the inverter running. That means the utility is providing some reactive power or VARs. With the inverter running, the real power delivered by the utility is reduced, but the reactive power is not changed. To see it mathematically: the VA delivered by the utility is the square root of the sum of the squares of VAR and Watts. PF = W/VA = W/sqrt(W^2+VAR^2). Reduce W without changing VAR and the power factor goes down, possibly even to zero.
Kent Osterberg
Blue Mountain Solar
toddc...@finestplanet.com wrote:I have a question about my personal home system. It is an Outback battery based grid tie. My digital utility meter is capable of reading var and power factor. So often (like just now) when I read this it is not even close to unity. Today's reading was 1.57 VAR, 56 PF. What is going on here? Am I not being credited for everything I am selling? The only significant load I have running is a dehumidifier... so an inductive load.
Todd
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