VAX 780 on eBay

[Paul Koning]

> On Jan 1, 2022, at 2:38 PM, Grant Taylor via cctalk <cctalk at classiccmp.org> wrote:
> 
> On 1/1/22 12:33 PM, Paul Koning via cctalk wrote:
>> And mainframes may use 3-phase power supplies; CDC 6000 mainframes do, though with those it's 3 phase 400 Hz produced by motor-generators (which in turn run off 3-phase mains power).
> 
> I know that some IBM mainframes prefer 3ɸ power but I know of multiple people that have re-wired their CECs to use 1ɸ power.  They too are largely independent 1ɸ supplies on different phases to jointly power a common internal bus.  So re-wiring the necessary supplies to the same phase is mostly not a big deal.
> 
> Apparently even IBM sold and supported some systems in this configuration.  The only down side is that it put an upper cap on the amount of power that you could draw and thus things that you could run in this configuration.  But for smaller installations, it's perfectly fine.

Could be.  For the CDC case, I see actual 3-phase DC supplies, i.e., 3 phase bridge rectifiers fed by 3-phase transformers.  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).

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".

	paul