# [Beowulf] power usage, Intel 5160 vs. AMD 2216

Bruce Allen ballen at gravity.phys.uwm.edu
Fri Jul 13 21:00:22 EDT 2007

>> That's interesting.  Where does the PDU store 1 second of power?
>
> I don't know about "PDU" per se -- but units that do significant power
> conditioning (up to the extreme of dual isolation transformers) usually
> have big capacitors to buffer surges and load variations.  I wouldn't
> have expected them to make it through a whole second unless they were
> designed to do so, but at this point they may be so designed.

A PDU is a power distribution unit.  Typically it contains a 3-phase
transformer 480v input 208v output, one or more panels of circuit
breakers, and some metering for voltage, current and power usage on each
phase.

There are no (energy storage) capacitors in these units -- they are AC not
DC.

An isolation transformer can not store energy for more than one cycle.
Since you are a physicist I don't need to explain to you that in a
transformer the magnetic field changes direction 60 times per second.  If
you take away the intput power, the only energy stored is that in \vec
B^2.  It's gone after 16 msec.

Regarding the comments on batteries, the major factor influencing battery
life is their temperature.  In our UWM cluster room, the expected battery
life is more than 7 years based on temperature and load.  The cost of
replacing all of them (3 cabinets of 40 batteries) is about 80 dollars per
battery, or $10k. That's about$1.5k per year, which is in the noise
campared with other operating costs such as the electric bill.

Cheers,
Bruce
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