Robert G. Brown
rgb at phy.duke.edu
Sat Jun 7 14:47:29 EDT 2003
On Sat, 7 Jun 2003, Jim Lux wrote:
> First off...
> VA is not watts... Watts is active power ( integral of instantaneous voltage
(to pick nits -- VA and Watts >>are<< identical units. But Jim knew
that; he meant time integral divided by the time, or average:-)
> * 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.
However, power companies have to be able to deliver PEAK currents, not
average currents, and usually charge you on the basis of those peaks as
it limits their ability to distribute power elsewhere on their grid.
These higher, off phase currents also have negative implications for your
primary supply transformers, which can heat and actually become damaged
or have a shorted lifetime if continuously operated near their nominal
Finally, voltage brownouts during the peak draw period can limit the
systems' ability to draw current during precisely the period they need
to be drawing current to provide sustained power. This, in turn, can
cause systems to lose some of their innate ability to buffer and ride
out line surges caused by (for example) lots of systems turning on and
off at once. Which in turn means that there can be more downtime, more
expense and lost work associated with downtime, more hassle.
The moral of all which is: a) Consider using harmonic mitigating
transformers and other line conditioning on new "large" facilities
intended to hold lots of cluster nodes with switching power supplies and
get the wiring done by competent people who understand the issues
involved and not just joe electrician off of the street; b) Otherwise
leave a healthy margin of surplus capacity on each line when considering
what you can run on old circuits or even new circuits with no special
power factor/harmonic correction measures taken.
Robert G. Brown http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567 Fax: 919-660-2525 email:rgb at phy.duke.edu
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