Anything special about converting VAX 11/780 to single phase?

From: Matthew Sell <msell_at_ontimesupport.com>
Date: Fri Feb 1 18:33:41 2002

Gunther,



>All right. I need to respond to a few assumptions that had
>been made and of course ask Matt Sell a question.

At your service... : )

>Question first: that shutdown interlock transformer you said
>is the only thing that really has anything to do with all 3
>phases together. Do I have to take any action with this or
>will it be fine?

Remove the fuses from F2 and F3 on the power distribution panel. Leave F1
fuse in place.


>Comments: the VAX 6000 is really VERY different. It has a

COMPLETELY different.... : )

>The only concern I had was about something like this
>interlock transformer that Matt was talking about. Will
>I get all my panels powered, the unswitched and the switched
>ones?

Yes.


>About this ground alias neutral wire overload, I think that's
>just scare without real substance behind it. Given that I
>have a 25 to 50 A 110 V single phase circuit to devote, my
>wiring will simply be like this:
>
> +--- P1
> |
>---phase-----------+--- P2
>
> |
> +--- P3
>
>
>---neutral---------+--- N
>
>---ground----------+--- GND


If you have that kind of "ampacity", then I don't think you'll need those
three extension cords and reel that I have to use....

Just wire it all together. I would be careful about the gauge of wire used
for the neutral - one amp on each phase equals three on the neutral. It may
be too much of a load on the neutral wire.

>since the phase, neutral (and ground, not that it matters)
>wires are all same gauge, the 30A that go through the phase
>line come right out of the neutral line.

On three phase, if you had a 1 amp load on each phase, you would have a
little more than 1 amp through the neutral since they are 120 degrees
apart. With all three phases on one circuit, there is no phase difference
and the neutral bears the same current as all three phases together. It's
been too long since my power distribution classes in college for me to
remember the exact formulas off the top of my head. RF communications and
digital logic were my specialties......

> If neutral overloads
>phase will too. That's what you have circuit breakers and
>the right gauged wireing for.

What size circuit breaker is connected inline with the neutral? That would
determine your total capacity - if connected this way. For example, if you
had a 30 amp breaker on the neutral, then each phase could draw 10 amps
constant.

> I can see no risk. The only
>theoretical risk I could possibly see is that the N line
>would be underdimensioned internal to the power distribution
>box. But hey, it's DEC, had big DEC ever built anything
>under-dimensioned :-).

The neutrals are separate inside of the distribution box.


>Matt had a different problem, which was he didn't have
>a 30 or 50 A line to begin with.

Tell me about it. I'll need an electrician to make a house call when the
6660 arrives.

>Finally, there is another thought: I might actually want
>to use a 220 V 2 phase (4 wires) line that can do 60 A.

That was my plan - to move to attempting to "balance" as best as possible
two of the three phases and use 240 V.

>It's the same line that goes into my computer room in the
>basement that also runs by my computer room in the
>garage.

Your house was made for this......

> I am thinking of just splicing this wire. It's
>certainly not NEC professional, but it will work and the
>circuit is overall protected 60 A with big enough wires
>everywhere and no problem if power fails (only old
>computers who suddenly stop working.)

Careful where you make these modifications. I don't know what your local
building codes are.....

> Is there any problem
>as far as this interlock transformer is concerned that
>suggests I not use the 2x110 V (180 degree) line? I
>would do something like this:
>
>---phase1----------+--- P1
>
>---phase2----------+--- P2
> |
> +--- P3
>
>---neutral------------- N
>
>---ground-------------- GND
>

That's what I'm hoping to do for mine.

>Now, there may be a risk. Remember, when I did this same
>trick on my 6000 the power would shut down after about
>5 minutes run time? It might be a preexisting problem
>in the PSU, but it might also be that the interlock stuff
>was being fried because of the 180 degree phase difference
>i.e. U(P1,P2) being 220 V when it wanted a 120 degree
>phase difference with a U(Px,Py) = 110 V * 1.5 = 165 V.
>Any idea what it would do, Matt?

I don't think you'll have a problem with the 780. I don't know much about
the 6000's, but the 780 is different as far as the power requirements are.
It seems as if three-phase was used just for the convenience of hooking
this beast up. Once you open that power distribution panel you'll see just
how simple it is. I couldn't believe my luck when I saw the internal wiring.

Hope this helps.....



         - Matt
Received on Fri Feb 01 2002 - 18:33:41 GMT