[Beowulf] UPS & power supply instability

[Jim Lux]

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.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875