James.P.Lux at jpl.nasa.gov
Sat Jun 7 19:23:51 EDT 2003
----- Original Message -----
From: "Robert G. Brown" <rgb at phy.duke.edu>
To: "Jim Lux" <James.P.Lux at jpl.nasa.gov>
Cc: "Glen Kaukola" <glen at mail.cert.ucr.edu>; "Beowulf" <beowulf at beowulf.org>
Sent: Saturday, June 07, 2003 11:47 AM
Subject: Re: ups units
> On Sat, 7 Jun 2003, Jim Lux wrote:
> > First off...
> > VA is not watts... Watts is active power ( integral of instantaneous
> time average
> (to pick nits -- VA and Watts >>are<< identical units. But Jim knew
> that; he meant time integral divided by the time, or average:-)
Indeed.. from a dimensional analysis standpoint, both VA and Watts (and VARs
for that matter) are all rate of energy.. Joules/sec
And, yes, it's really the time average of the product of the instantaneous
power (i.e. volts*amps)
> > * instantaneous current). VA is RMS volts * RMS amps.
> Everything else Jim said was just peachy. I'd amplify a couple of
> points about PF -- if all your systems have a PF of 0.8 to 0.9 (typical
> for a non-PFC switching power supply) then as he noted a system that
> draws 120 watts (average power) on a 120 volt line needs to draw perhaps
> 120 Amps, not the 100 Amps you might expect on the basis of the average.
And, watch out for low line voltage... Switching supplies are kind of
interesting.. they're "constant power" devices, so as the line voltage
drops, the current increases. This is unlike most conventional
resistive/inductive loads, where as voltage drops, so does the current.
Gang up a raft o' PCs so that your line voltage gets sucked down to 110 or
105 volts, and the current goes up correspondingly.. increasing the IR
losses in the distribution circuitry.
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