# [Beowulf] UPS & power supply instability

Jim Lux James.P.Lux at jpl.nasa.gov
Wed Sep 28 11:45:37 PDT 2005

```At 10:18 AM 9/28/2005, Andrew Piskorski wrote:
>On Wed, Sep 28, 2005 at 09:08:01AM -0400, Robert G. Brown wrote:
>
> > Don't assume that Dell's power supplies are really PFC just because they
> > say that they are, also.  Believe it when you put a dual input scope on
> > it and measure the current and voltage simultaneously as a function of
> > time on a triggered scope and see two perfectly sinusoidal waves, in
> > phase.
>
>Is the "PFC" readout of a Kill-a-Watt sufficient for verifying that a
>power supply is really truly doing PFC?

An interesting question... I'll ask some people who have reverse engineered
them.

I will say that they show a poor PFC on non PFC corrected switching power
supplies (like the one for my laptop).

They also show poor PFC (which they should) on a capacitor input filter
power supply that is lightly loaded.

I won't say they detect ALL forms of bad waveforms or PFC, but certainly,
it's a good start.  If your computer seller claims the PF is >0.95 and your
Kill-A-Watt shows PF = .75, then you can ask them to start explaining why.

But, what you're really concerned about is harmonic content in the load
current, not the phase, per se.  If you hooked a big inductive load up, the
current would lag the voltage but still be sinusoidal, and you might not
have any problems.

Just as an example.. if the third harmonic current were equal to the
fundamental (i.e. equal power in the harmonic and the fundamental), the PF
would be .707.  (and, in a three phase system, the neutral current would be
3*.71 * leg current)

>--
>Andrew Piskorski <atp at piskorski.com>
>http://www.piskorski.com/
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James Lux, P.E.