Here is a note from my friend about how he converted his VAX to run on single
phase. I am sending it to the list, as a few people where intereseted and I am 
sure others may be too.

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>From HEINKELN@firga.sun.ac.za Wed Mar 27 17:22:14 1996
>From: "Leon Heinkelein" <HEINKELN@firga.sun.ac.za>
To: andre@capes1.vector.co.za (Andre Skarzynski)
Date: Wed, 27 Mar 1996 17:18:44 GMT+200
Subject: Re: Converting 3-pahse VAX Power.

> Hi Leon,
> 
> I hope I find you well. I had a great long weekend at the Buff'. Anyway,
> the reason I called/wrote, is that I would like to please have the details
> of converting a 3-phase VAX power to single like you did at home. BTW, what
> model VAXen are you running? and what current are they drawing? 
> 
> Cheers,
> 
> Andre'


Hello Andre

Looking to upgrade , eh ? The machine I have at home is a VAX 6220. 2 
cpu's , 96 MB of RAM. Used to need three phase power , as you 
correctly observe.

The power system architecture of the VAX is arranged so that each 
power regulator needs a DC supply of 300V. There are various ways of 
obtaining this. They use a single phase bridge on the smaller 
machines (just like a PC PSU) , off 220V this gives about 311V. (220 
* sqrt (2)) . However , on the larger machines , it gets a little 
more involved. On the 6000 series they use a 3-phase controlled 
bridge , so they can actually regulate the 300V line. It's laid out 
like this (here goes nothing) :
              
              
            ____________________    +300V
            |    |    |
            -__  -__  -__  thyristors
            ^    ^    ^
            |    |    |
  L1  ------*    |    |
  L2  -----------*    |
  L3  ----------------*
            |    |    |
            -    -    -    diodes
            ^    ^    ^
            |    |    |
            --------------------- Return


 The three-phase controlled bridge ( as you can see ) uses the Phase 
to Phase voltage ( NOT the phase to neutral voltage ). With a 3-phase 
220V supply thats about 380V AC. Multiply that by sqrt(2) and you 
have about 520V DC . KABOOM ! However , in america , on a 115V 3-
phase system , phase to phase is 208V , multiply by sqrt(2) and you 
get about 300V DC. All 6000 series machines for 380V therefore came 
equipped with a 3-phase xformer which takes the 380 V phase/phase to 
208V phase/phase. Which of course is very close to our neutral/phase 
, which is 220V. The way to do it , then , is to bypass the xformer, 
connect L1 in the drawing to LIVE , say , and connect L2 to NEUTRAL , 
and leave L3 open. The circuit then works just like a single phase 
controlled bridge rectifier , and works very well , giving about 305 
V on the DC power bus.

Total current is ( I'm guesstimating now) propably about 4A , 
measured on the AC side , around 0.8-1.0 KVA.

I you get a 6000 (or similar) machine let me know. I have worked out 
a very elegant way to do the above mod which entails no soldering , 
no disconnecting of joints etc , and can be put back to factory 
standard in about 30 secs.

Good luck

Leon
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