OT: Continuous mode UPSes

[Ric Werme]

Ben Scott wrote:

>   The power factor on PC power supplies is often as bad as 0.6, so
> this is a big difference.  More recently, PFC (Power Factor
> Correction) supplies have improved that to around 0.8 or 0.9.

Power factor originally referred to motors and other inductive loads
that have a peak current draw sometime after the peak voltage.  Resistive
loads like lamps and squirrels have both peaks at the same time.

Swicthing power supplies like to draw a steady power, but do it in sips
several thousand times a second.  When the line voltage is low, they
sip longer, when the line voltage is high, short sips do fine.  Again,
current draw and voltage don't line up.

The "Kill-A-Watt" device (see Newegg, Amazon, etc) measures PF and
lotsa other stuff.  Don't design a UPS installation without one!

>   It's a UPS industry convention to spec real power in volt-amps, and
> apparent power (at some assumed power factor) in watts.  This is why
> the VA ratings are higher than the watt ratings.  The watt ratings
> reflect the apparent power, which is more than the real power the
> equipment actually needs.

Real power is watts, and is V x R for resistive loads.  IIRC, VA is
peak volts x peak amps and for non-zero PFs (and all inductive loads)
is more than watts.

Some of the drivers for the high PF power supplies is coming from the
European environmental community trying to make the power system more
efficient.