Power factor is a bit difficult to explain, but here goes:

Ohm's law: Volts = Amps * resistance
Watts = Volts * Amps or substituting, Watts = Volts squared / resistance.


Power factor is the ratio of "real" power divided by the apparent power. Under 
what circumstances do real and apparent power differ from each other? There are 
two reasons for this:


1.       A nonlinear load (like something with rectifiers) will present a 
resistance that appears to vary with the instantaneous voltage. For a 
sinusoidal alternating voltage, this leads to a non-sinusoidal current, which 
creates a non-sinusoidal power. This non sinusoidal power will deliver less 
average power than the equivalent sinusoidal power.

2.       A reactive load (capacitive and inductive) will have a sinusoidal 
current, but it will lead or lag the voltage. This means that any instantaneous 
power (voltage * current) will be less than the average voltage times the 
average current.

This means that with a power factor, you get less "work" (power) for a given 
voltage and current. In other words, a load with a power factor less than 1 
requires more "push" (current) to get the same amount of work done as a load 
with a power factor of one.

A real world example:

Let's say you have a UPS running on batteries that produces 120V AC. This feeds 
a reactive load that draws 60 Watts at a power factor of .5. This means that 
your battery is supply 60 Watts of power (assuming 100% UPS efficiency), BUT 
your UPS is delivering a current of 1 Amp (not 60/120 or .5 Amps) because of 
the power factor. This is why UPS are rated in VA instead of Watts.

Ken Cornetet 812.482.8499
To err is human - to moo, bovine.

From: James Kerr [mailto:cluster...@gmail.com]
Sent: Thursday, October 20, 2011 12:58 PM
To: NT System Admin Issues
Subject: Re: ( OT ) Convert mA to Watts

You lost me at "factor".
On Tue, Oct 18, 2011 at 12:37 PM, Ben Scott 
<mailvor...@gmail.com<mailto:mailvor...@gmail.com>> wrote:
On Tue, Oct 18, 2011 at 12:15 PM, Stefan Jafs 
<stefan.j...@gmail.com<mailto:stefan.j...@gmail.com>> wrote:
> I guess the big unknown is the PF, I assumed 60% (based on Googeling), the
> is a power supply, would it not be higher for a transformer load?
 To continue the water analogy, power factor is like a big reserve
tank right before the water tap.  It can cause your water demand to be
out-of-sync with the apparent water usage (coming out of the tap).
You run the faucet for a bit, and the tank starts to drain, but the
supply pipe isn't touched.  Then the tank starts to fill, pulling from
the supply pipe.  Then you shut the faucet off, but the tank keeps
filling.

 Or so I'm given to understand; the actual mechanism behind power
factor is magic to me.  I know a purely resistive load -- like a space
heater -- has a power factor of 1.0.  "Inductive loads" are
"reactive", whatever that means.  :)  Apparently AC motors are
"inductive".  Rectifiers -- like in an AC->DC power supply, such as in
a PC -- are also apparently "reactive".  "Power factor correction"
helps turn equipment with a lower power factor into something with a
higher power factor.

 The numbers I usually see pulled out of the air for PC power supply
units are 0.6 PF for a standard PSU, and 0.9 PF for a PFC PSU.  I have
no idea how much things vary in practice.

-- Ben

~ Finally, powerful endpoint security that ISN'T a resource hog! ~
~ <http://www.sunbeltsoftware.com/Business/VIPRE-Enterprise/>  ~

---
To manage subscriptions click here: 
http://lyris.sunbelt-software.com/read/my_forums/
or send an email to 
listmana...@lyris.sunbeltsoftware.com<mailto:listmana...@lyris.sunbeltsoftware.com>
with the body: unsubscribe ntsysadmin


~ Finally, powerful endpoint security that ISN'T a resource hog! ~
~ <http://www.sunbeltsoftware.com/Business/VIPRE-Enterprise/>  ~

---
To manage subscriptions click here: 
http://lyris.sunbelt-software.com/read/my_forums/
or send an email to 
listmana...@lyris.sunbeltsoftware.com<mailto:listmana...@lyris.sunbeltsoftware.com>
with the body: unsubscribe ntsysadmin

~ Finally, powerful endpoint security that ISN'T a resource hog! ~
~ <http://www.sunbeltsoftware.com/Business/VIPRE-Enterprise/>  ~

---
To manage subscriptions click here: 
http://lyris.sunbelt-software.com/read/my_forums/
or send an email to listmana...@lyris.sunbeltsoftware.com
with the body: unsubscribe ntsysadmin

Reply via email to