3phase power for VAXen [was Re: VAX 780 on eBay]

[Charles Dickman]

On Tue, Jan 4, 2022 at 5:17 PM Rich Alderson via cctalk <
cctalk at classiccmp.org> wrote:

>
> We did not have a 3phase outlet on the second floor of the building where
> the
> collection was housed, and there was no room for the first VAX in the small
> computer room on the third floor, so Ian and Keith came up with an
> alternative:
> They tested all the outlets on the second floor and determined that there
> were
> three within reach of the room in which Ian was going to work on the 785
> which
> were fed from 3 different phases off the big honking breaker panel (200A
> service, IIRC).  Keith put together a box with the appropriate NEMA socket
> and
> three heavy duty cords feeding into it, which in turn were plugged into
> three
> outlets on the walls around what eventually became the vintage exhibition
> hall
> at Living Computers: Museum + Labs (the eventual name of the place after
> the
> modern exhibit space on the first floor was built).
>
> So it's possible to power a 780 or 785 without a power supply rebuild if
> you
> simply have the right (industrial) breaker panel in your building...
>
> Happy New Year, everybody!
>
>                                                                 Rich
>

After looking at the 869-D diagrams and the 11/780 power diagram, this
makes sense. The power diagrams show 3 120VAC circuits. There is no phase
relationship required between the phases, EXCEPT in the 869-D itself. The 4
pole 30A circuit breaker will trip if any (L1, L2, L3, N) is overloaded. If
L1, L2, L3 are out of phase then the neutral doesn't carry much current,
but if they are all the same phase then the total limit will be 30A.

There are 4 connections L1, L2, L3 and N.

In the US you would connect the highest current draw leg to one side of the
120VAC and connect the other two legs to the other 120VAC phase and then
HOPE that it works.

The 869-D outlet are:
unswitched L1
switched L1
switched L2
switched L3

For me, if I can buy a Shelby Cobra, I buy it first and figure out where to
park it later.

Chuck D.