[Beowulf] Re: UPS & power supply instability

David Kewley kewley at gps.caltech.edu
Fri Sep 30 10:53:16 PDT 2005

On Friday 30 September 2005 09:50, David Mathog wrote:
> RGB mentioned that "the HVAC is a nasty load".  That got me
> thinking - when you increase the number of nodes running the
> HVAC servicing that rack(s) is going to sense the increased
> heat and compensate, presumably by running its motors more.
> So you could have an indirect problem such as:
>  increased load ->
>  increased HVAC activity ->
>  UPS instability
> Where the load correlates with the instability
> but is not the direct cause.
> Does the HVAC cooling the test rack run off the same UPS
> as the rack itself???

Maybe I wasn't clear enough.  We have two UPSes, one for computers only, and 
one for HVAC only.  Here is more detail of the feeds into the room:

substation A -> panel A --> UPS B -> HVAC control, fan, condensate pumps
                        \-> HVAC heaters, humidifiers

substation C -> UPS A --> PDU1 --(45 whips)--> 15 racks
                      \-> PDU2 --(48 whips)--> 16 racks

(leaving out some breaker panels)

We also have a feed, likely from substation A, into some wall outlets that 
we don't use normally (used occasionally for vacuum cleaners, power tools,  
rarely & temporarily for terminals, etc.).

> If so,  it might be interesting to shut off that one HVAC, crank
> up the others in the room, maybe put a few fans in (on different
> circuits) to push the hot air over to them, and then try
> increasing the number of nodes again.
> That should at least rule out "lots of HVAC activity" as being
> the (sole) source of the problem.

The HVAC and the computers have only the following circuit elements in 
common, to the best of my knowedge:

a) the main building feeder (upstream of substations A and C)
b) ground connections, in a loopy ;) topology

I think it's unlikely that there's significant feedback between the power to 
the HVAC and the power to the computers, but feel free to point out 
possibilities that I'm missing.


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