1 Feb
2002
1 Feb
'02
6:21 a.m.
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.
This sounds similar to the VAX 6k power supply. There are three circuits
tapped off of the three-phase power to feed various pieces-parts, and the
configuration of the DC power supply is such that two legs at 240V is a
close match to three legs at 208V
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.
A moderm dryer (30A) or range (50A) actually has all the connections
required: 2 lines, one neutral, and one ground. If the power supply
neutral is replaced with a properly-sized conductor, it shouldn't be any
big deal to change the cord and run it off of a standard dryer/range
recpt. Make sure you have both the neutral and equipment ground
avaliable-- the older style of electric dryer recpt didn't provide a
neutral, and the ground was often used to provide 120v to the timer!
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.
The longest run on my VAX 6320 & TU81+ was 24+ hours, on 240V 20A single
phase and 120V 30A respectively, with no adverse reactions except to the
electric meter. I feel confident enough that when I start it up For Real,
it'll run unattended 24/7 until such time as I can't afford to pay the
electric bill.
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.
details?
My eventual goal is to run the 780 from a 240V source.
Any yes, my wife thinks it nuts.
^_^ My wife didn't quie realize just how big a 6K machine was...
- Matt
Bob
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:
Matthew Sell
Programmer
On Time Support, Inc.
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(281) 296-6066
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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(a)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