VAX 780 on eBay

Grant Taylor cctalk at
Sat Jan 1 14:08:18 CST 2022

On 1/1/22 12:44 PM, Paul Koning via cctalk wrote:
> Could be.  For the CDC case, I see actual 3-phase DC supplies, i.e., 
> 3 phase bridge rectifiers fed by 3-phase transformers.

Interesting.  That brings to mind 6ɸ DC supplies.  --  Two typical 3ɸ 
rectifier networks (making DC pulses be 60° apart) separated by a ∆ / Y 
or Y / ∆ transformer inducing a 30° phase shift between them.  Thereby 
causing a net effect of 12 x DC pulses 30° apart.

> If you run those off one phase the ripple would be a lot larger.


> And of course 400 Hz gave them an additional advantage in reduced 
> transformer size and reduced ripple (for a given filter capacitor).

I was going to ask how the frequency altered the size of the ripple, but 
"for a given filter capacitor" makes sense.  You are messing with the 
ratio and having a fixed value "for a given filter capacitor".  --  If 
I'm even remotely understanding correctly.

> They'd do this even in devices that don't draw large amounts of power, 
> for example the DD60 console display gets its power (other than fans 
> which are single phase mains power) from the 400 Hz 3-phase supply 
> feeding the mainframe.  I'd be surprised if it uses even a kW, so 
> this was probably a case of "why not since it's there to be used".

My understanding is that some of the special power requirements had more 
to do with isolation and cleanliness / quality of the power.  Meaning 
don't try to power the equipment off of the same circuit(s) that motors 
and the likes are on which will induce lots of noise on the line, some 
of which might make it into the computer and adversely effect things.

The motor / generator provides a quite strong isolation for things like 
that.  Probably more so than just a normal transformer.

The frequency change means that a simple isolation transformer won't 
suffice.  The higher frequency also has benefits of reducing the size of 
other down stream components.

I find this to be interesting to learn, even if I'll almost certainly 
never actually work near anything that uses it.

Grant. . . .
unix || die

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