josip at lanl.gov
Mon Jun 9 11:17:12 EDT 2003
Jim Lux wrote:
> First off...
> VA is not watts... Watts is active power ( integral of instantaneous voltage
> * instantaneous current). VA is RMS volts * RMS amps
Exactly! VA rating is only an upper bound on Watts...
> The ratio between the "watts" and the "VA" is called the power factor. So,
> if you have a device that runs at 120V, and draws 10 amps, it has a VA of
> 1200. But, maybe the current is out of phase a bit, or non-sinusoidal, so
> if you actually measure the power (i.e. how fast the wheel spins on the
> meter), (maybe you put the load in a sealed, insulated box and measure the
> temperature rise?)... you find that it dissipates, say 1000 Watts. The
> power factor, in this case, would be 1000/1200 or 0.83 (which, by the way,
> is a typical kind of pf for a switching power supply).
Kill-a-Watt (available at Radio Shack for $30) can measure VA, Watts,
and power factor. At the input to my UPS unit (supporting a dual CPU
server, a fast ethernet switch, etc.), it reports about 185 VA or 130 W
or power factor 0.7 (somewhat lower than Jim's example).
> Now we come to the other big problem.. inrush current.
This server draws less than 1 A RMS, but when it is powered up it draws
almost 2 A RMS briefly, which matches Glen's 2:1 inrush estimate. Since
the power-on phase is brief, a slow acting 20 A circuit breaker can
support 12 such machines. However, as Jim pointed out, one needs to be
more careful in observing VA limits of commodity UPS units whose
electronics are sensitive to instantaneous load.
Finally, the power supply rating printed on your PC is only an upper
bound. While your PC may require a 300-400 W power supply (mainly to
supply sufficient inrush current to the PC's components), under normal
circumstances the PC will dissipate less than half this maximum power
rating. So, if you have a room with X PCs, here is an estimate of the
sufficient air conditioner capacity: you will have to remove roughly
X*150W of heat (i.e. roughly X*500 BTU/h) plus a safety margin.
P.S. This "half the power supply rating" power dissipation estimate is
very crude, but can be useful when more accurate figures are hard to
obtain. BTW, in sizing power supplies, the aggregate Watt rating does
not tell the whole story, because a PS supplies multiple voltages, each
with its own current limit.
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