Running 3 phase 780s on single phase power

[Rich Alderson]

> From: Johnny Billquist
> Sent: Monday, January 19, 2009 1:20 PM

> What you need to understand here, is that there is nothing in a 11/780
> that is actually driven by 3-phase power. And the power controller is
> nothing like what you are trying to describe.

  [ snip ]

> And with this kind of construction, it really isn't any problem at all
> in using the same phase for all three phases. There are very few things
> DEC did which really required 3-phase power.

> Off my head, I know that the 86x0 machines really require 3-phase (the
> fans are AC motors), the RP06 drives use AC motors (probably RP04 and
> RP05 as well). I'm not sure, but it might be that KL-10 machines
> actually used 3-phase. It's been a while since I looked inside one.

The KL-10, like other DEC gear, uses 3-phase input power as the simplest
way to get lots of amps into the system:  60A @ 208 3-phase.  All of
this feeds into a single gigantic boat-anchor grade transformer which
forms the basis of a linear power supply.  2/3 of that power goes back
out as simple heat.

> But as always, if you try do run a machine with 3-phase power off a
> single phase, you need to know how that machine is built, and if it
> actually is possible do "rewire" it. Don't play around with this if you
> don't really know, because it can be lethal both to the machine and you.

Some here seem to think that we are amateurs--if not idiots--working on
a home system, and unaware of the precautions necessary before making
changes to the power system of any large computer.  The electrical
engineer on our team very carefully located three phase-isolated outlets
for us, working from the power panel outwards, before we did anything
else.  We tested the plug box for phase isolation before connecting
either VAX, and we had every internal breaker off and every internal
plug disconnected for several hours while we checked the power coming
through the main breaker.

We're paid for our ability to do this work.  The fact that we love doing
it does not make us any less professional in how we do it.  Knowing that
poor design choices on our parts could injure us or our guests, or
destroy equipment that may be impossible to replace (VAXen are not
uncommon, but PDP-7 systems are!), makes us very cautious in how we
approach things.  We share our occasional triumphs with the likeminded
not in a game of oneupsmanship but so that knowledge is not lost if we
get hit by a train on our way to lunch.


Rich Alderson
Server Engineer, PDPplanet Project
Vulcan, Inc.
505 5th Avenue S, Suite 900
Seattle, WA 98104

mailto:RichA at vulcan.com
(206) 342-2239
(206) 465-2916 cell