Andreas,
The power distribution of a VAX 11/780 is much simpler than that. There is
only one three-phase transformer in the entire machine, and it is used to
power the shutdown interlocks.
The three phases are not combined into any "wye" or "delta" arrangement
using a transformer (other than the interlock). The front side of the power
distribution panel consists of a bunch of standard 120V three-prong
household outlets, and various portions of the VAX are connected to these
outlets.
It's very easy to convert an 11/780 to run from single-phase power, the
problem is the high current draw, which is more than the typical 15A feed
in our houses. I've been running mine for short periods of time, which each
phase plugged into a different branch circuit in my house. This really
makes GFI protected circuits go bonkers (for obvious reasons), but I've
been very careful not to exceed ampacity of any of the branches.
I wouldn't recommend this to everyone, though. I brought home a Fluke 41
Power Harmonics Analyzer and made a simple test harness so I can actually
measure what is going on with my modification to the 11/780. When Gunther
was visiting, we ran it for about twenty minutes, and that was the longest
run of the machine in my house to date. I was also very cautious to make
sure that none of the "phases" were causing any excessive heating in the
wires, however, this is not a good safety test of the modification.
After safety checks using the 41, my next step is to install those small
inline "Power Planners" to the cooling fans. That should cut the power
requirements of the 780 by a few hundred watts. I've tested those Power
Planners, and for typical AC motors that do not run under a heavy load,
they really do work and cut electrical consumption. They do generate
harmonics, but not any worse than your typical PC switching power supply.
My eventual goal is to run the 780 from a 240V source.
Any yes, my wife thinks it nuts.
- Matt
At 01:11 PM 2/1/2002 +0100, you wrote:
>Gunther,
>
>be careful with the return line: if you are running three different
>phases, these are phase-shifted 120 degree against each other, so the
>current maximum on each phase will occur at a different time, and the
>center ("ground") line will not need to carry any current at all (in a
>perfectly balanced circuit). If balance is not perfect, the ground wire
>will have to bear at most the same load as any of the live wires.
>
>If you feed a single phase into the same circuit on all three lines, you
>will have peak current on all three "live" lines at the same time, and
>the "ground" line will need to handle that. If it's not made to that
>spec, you'll overload it by a factor of 3...
>
>I don't think this is true for VAX power supplies, but I have heard
>about process automation computers that are fed with three phases just
>to make the period shorter and to use smaller capacitors. With this kind
>of power supply, you'd get a lot of ripple on the DC output.
>
>Regards,
>Andreas
>
>Gunther Schadow wrote:
> >
> > Hi,
> >
> > just to be sure, I would simply put all three phases on the
> > same single phase. Are there any problems with that? The
> > VAX 6000 is much pickier, but the VAX 11 and everything
> > having the simple power distribution box should be fine,
> > right?
> >
> > thanks,
> > -Gunther
> >
> > --
> > Gunther Schadow, M.D., Ph.D. gschadow_at_regenstrief.org
> > Medical Information Scientist Regenstrief Institute for Health Care
> > Adjunct Assistant Professor Indiana University School of Medicine
> > tel:1(317)630-7960 http://aurora.regenstrief.org
>
>--
>Andreas Freiherr
>Vishay Semiconductor GmbH, Heilbronn, Germany
>http://www.vishay.com
Matthew Sell
Programmer
On Time Support, Inc.
www.ontimesupport.com
(281) 296-6066
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Received on Fri Feb 01 2002 - 10:07:11 GMT