[Beowulf] UPS & power supply instability

Jim Lux James.P.Lux at jpl.nasa.gov
Wed Sep 28 07:54:42 PDT 2005

At 06:08 AM 9/28/2005, Robert G. Brown wrote:
>David Kewley writes:
>I don't know what you can do to positively diagnose the situation, but I
>expect that it will involve a dual trace oscilloscope rigged so it can
>function as a line voltmeter and ammeter at the same time, in a test
>circuit you'll probably have to hand wire so you can insert the one
>(ammeter) and run the other (voltmeter) across all three wire pair
>combos, and a handful of nodes to load the test circuit with.  If you
>aren't comfortable with wiring, and don't know why you NEVER put an
>ammeter across two voltage lines and ALWAYS but a voltmeter across the
>voltage lines and which wire is hot and which is not and which is
>neutral, DON'T TRY THIS YOURSELF.  Dying is such a drag.  Be sure to rig
>a scope to measure current (safely, without starting fires or injuring
>living things including yourself), which isn't horribly easy but can be

Tektronix has a nice clamp on current probe for oscilloscopes.

In fact, Tek has a kit specifically designed for looking at 3 phase power, 
as does Fluke.  3 or 4 clamp on probes, good voltage isolation, etc.

You should be able to rent the whole shebang from one of the big equipment 
rental companies (TestEquity, ElectroRents, etc.) for under $1000/month.

>Look at the shape of the neutral line current, compared to the line
>voltage, when a single Dell is on the system and compared to the
>pictures in Mirus FAQ #7.  This should give you a quick-and-dirty
>picture of whether the Dell power supplies are really PFC or if they're
>just ordinary switching power supplies that are supposedly more
>efficient or something so Dell claims that they are "PFC".  Dell may in
>ignorance interpret "PFC" as having current and voltage "in phase" for
>the primary draws but ignore the presence of third harmonics.  Honestly,
>from your reported neutral current I expect that this is the case (and
>I'm assuming since you REPORT the neutral current that you do indeed
>know how to measure it -- but looking at it is better).  Look for
>voltage distortion on the supply lines as well.

>If you discover that -- surprise -- the "PFC" supplies aren't, you can
>a) Bug Dell for "real" PFC supplies, directing them to the Mirus FAQ in
>case they are clueless about what that means and telling them that when
>you slap the aforementioned scope on them under load you'd better see
>nearly perfect 60 Hz sinusoids, in phase, in both power and current with
>"no" odd-phase harmonics -- once you can find an engineer somewhere you
>understands a word of this;
>SO, you might want to put your dual scope on the UPS supply lines
>themselves under various loads, looking for ripple and harmonics on both
>Some of this stuff you can check for on your own, but really you may
>need to find a COMPETENT electrician -- one that specializes in server
>room wiring and is e.g. union trained -- to help you out.  My
>brother-in-law is a journeyman electrician in the Detroit area, and I
>know what he went through in his journeyman training -- serious physics,
>actually.  I also know what the local electricians who wired our server
>room had as training -- think "How to Wire Your Own Home" from Home
>Depot (well, maybe a BIT more than this, but you get the idea...:-).
>There exist competent people but you'll have to look for them and
>probably pay for their knowledge.

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

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