[Discuss] Server Room Power

[tmetro-blu at vl.com (Tom Metro)]

Edward Ned Harvey wrote:
> Hold it.  P=VI is a DC rule.  Power is more complex in AC.
> What's the difference between VA and W?
> 
> If you have inefficient power supplies, you might be overpaying 30% 
> for power.

You're referring to power factor:
http://en.wikipedia.org/wiki/Power_factor

  The power factor of an AC electric power system is defined as the
  ratio of the real power flowing to the load over the apparent power in
  the circuit,[1][2] and is a dimensionless number between 0 and 1
  (frequently expressed as a percentage, e.g. 0.5 pf = 50% pf).
  [...]
  Circuits containing purely resistive [loads] have a power factor of
  1.0. Circuits containing inductive or capacitive elements (electric
  motors, solenoid valves, lamp ballasts, and others ) often have a
  power factor below 1.0.

So when PF=1.0, VA==Watts. The better the quality of your power supply,
the closer its PF will be to 1.0. In the last decade it has become
common for name brand computer power supplies to specify a PF as a
selling point.

See also:
http://en.wikipedia.org/wiki/Switching_regulator#Power_factor

for discussion of PF with respect to computer power supplies.


> When you're talking about 208, you're talking 3-phase.

You can attach single phase loads to a multi-phase supply, as long as
they are balanced:
http://en.wikipedia.org/wiki/Three-phase_electric_power#Single-phase_loads


> If you want to use 3-phase 208, you need a special power supply in the
> server.  Generally you don't have such a thing...

Old power supplies used to have a 120V/240V mechanical switch. Most
modern switching supplies will work fine with any input voltage from
like 90V up to 250V (check your supply specifications). The ability to
handle a wide input range is a byproduct of the switching regulator design:

http://en.wikipedia.org/wiki/Switching_regulator

 -Tom

-- 
Tom Metro
Venture Logic, Newton, MA, USA
"Enterprise solutions through open source."
Professional Profile: http://tmetro.venturelogic.com/