Re: Please help with buying a UPS for my system

From: Todd H. (comphelp_at_toddh.net)
Date: 10/15/05 

Date: 15 Oct 2005 12:58:10 -0500

Unruh <unruh-spam@physics.ubc.ca> writes:
> comphelp@toddh.net (Todd H.) writes:
> >"amanda" <amanda772005@yahoo.com> writes:
> >> Walter Roberson wrote:
> >> [..]
> >>
> >> > UPS's are rated either in VA (Volt-Amps), or in W (Watts).
> >> > The conversion between VA and W is not entirely straight forward.
> >>
> >> Thanks for that info. I was told that Watt = Volt * Amps.
>
> Watts is volt*Amp averaged over time.

For what it's worth, this really isn't mathematically accurate.

The original poster can ignore this though, as we're out on a tech
geek tangent. :-)

> If volts are sometimes positive and sometimes negative when Amps are
> positive, the averge will cancel out and watts will be much less
> than max Volts times max amps.

That's one way to look at it.

Perhaps a more accurate model to think of is in terms of phase. AC
voltage and currents are sinusoidal (non-geeks: that just means they
look like a sine wave going positve, and negative, one cycle every
1/60 of a second here in the US where we have 60Hz power).

Now, if you're turning on a lightbulb (which is electrically pure
resistive), and you had a scope on voltage and current, those
waveforms would be 100% in phase. This means a "unity power factor"
and your 60W light bulb is going to require only 60VA of generator
output to do it's thing.

Now if voltage and current waveforms are NOT in-phase (as happens when
the load--in this case a computer with a very non-linear switching
power supply--is not purely resisitve, but is slightly inductive or
slightly capacitive in nature) you have to make a distinction between
Watts and VA because your "250W computer power supply" might demand as
much as, say 350VA out of the wall. How much it demands depends on
its design. Switching power supplies are electrically far more yucky
than a simple resistive little lightbulb.

In this case where voltage and current waveforms are not in phase
because of a "reactive" load, the power source has to "work" harder
(the "electrical effort" is measured in Volt-Amperes) than the actual
real power (measured in Watts) that ends up doing useful work.

As an aside, this is why electrical distribution systems go to great
lengths to balance the "power factor" and make it as close to 1.
Electric power lines are inductive in nature (as are motor loads for
all those HVAC blower motors in our homes), so peridically, up on your
electric poles you'll see square gray thingees that are capacitor
banks designed to balance out the power factor to cancel out some of
that inductance out there plugged into the electric grid. The power
company's generators need to handle the load in Volt-Amperes, but they
can only charge customers for real energy use (measured in
Watt-hours).

Circling back to the present discussion of UPS's, like any other power
source, the capacity of it is best reflected in Volt-Amperes. APC
being a geeky engineering company accurately portrays that.

All the consumer needs to know is that they're going to need more VA
of UPS than they have a total of Watts in use on their computer. It's
best to trust APC's sizing program though.

Another take on the Watts vs VA is here:
        http://www.marvac.com/fun/power.aspx

The math is pretty wacky. Your choices are either a) dealing in
time-domain differential equations, or b) frequency-domain fun with
algebra that uses complex numbers with real and imaginary parts using
phasor notation. These are the tools to _really_ explain what's going
on in AC circuits. It's the stuff of 2nd year electrical engineering
study.

Best Regards, 

-- 
Todd H.
http://www.toddh.net/

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